Fill - Free fillable +0)26-4 Learning Objectives PDF form

TRANSFORMER

➣➣

➣ Working Principle of

Transformer

➣➣

➣ Transformer Construction

➣➣

➣ Core-type Transformers

➣➣

➣ Shell-type Transformers

➣➣

➣ E.M.F. Equation of

Transformer

➣➣

➣ Voltage Transformation

Ratio

➣➣

➣ Transformer with losses

➣➣

➣ Equivalent Resistance

➣➣

➣ Magnetic Leakage

➣➣

➣ Transformer with Resistance

and Leakage Reactance

➣➣

➣ Total Approximate Voltage

Drop in Transformer

➣➣

➣ Exact Voltage Drop

➣➣

➣ Separation of Core Losses

➣➣

➣ Short-Circuit or Impedance

Test

➣➣

➣ Why Transformer Rating in

KVA?

➣➣

➣ Regulation of a Transformer

➣➣

➣ Percentage Resistance,

Reactance and Impedance

➣➣

➣ Kapp Regulation Diagram

➣➣

➣ Sumpner or Back-to-back-

Test

➣➣

➣ Efficiency of a Transformer

➣➣

➣ Auto-transformer

+0)26-4

Learning Objectives

To overcome losses, the electricity from a

generator is passed through a step-up

transformer, which increases the voltage.

Throughout the distribution system, the voltages

are changed using step-down transformers to

voltages suitable to the applications at industry

and homes.

CONTENTS

1116

Electrical Technology

Fig. 32.1

Lam

inated Core

Prim

ary

Secondary

32.1. Working Principle of a Transformer

A transformer is a static (or stationary) piece of apparatus by means of which electric power in

one circuit is transformed into electric power of the same frequency in another circuit. It can raise or

lower the voltage in a circuit but with a correspond-

ing decrease or increase in current. The physical

basis of a transformer is mutual induction between

two circuits linked by a common magnetic flux. In

its simplest form, it consists of two inductive coils

which are electrically separated but magnetically

linked through a path of low reluctance as shown

in Fig. 32.1. The two coils possess high mutual

inductance. If one coil is connected to a source of

alternating voltage, an alternating flux is set up in

the laminated core, most of which is linked with

the other coil in which it produces mutually-in-

duced e.m.f. (according to Faraday’s Laws of Electromagnetic Induction e = MdI/dt). If the second

coil circuit is closed, a current flows in it and so electric energy is transferred (entirely magnetically)

from the first coil to the second coil. The first coil, in which electric energy is fed from the a.c. supply

mains, is called primary winding and the other from which energy is drawn out, is called secondary

winding. In brief, a transformer is a device that

1. transfers electric power from one circuit to another

2. it does so without a change of frequency

3. it accomplishes this by electromagnetic induction and

4. where the two electric circuits are in mutual inductive influence of each other.

32.2. Transformer Construction

The simple elements of a transformer

consist of two coils having mutual

inductance and a laminated steel core. The

two coils are insulated from each other and

the steel core. Other necessary parts are :

some suitable container for assembled core

and windings ; a suitable medium for

insulating the core and its windings from

its container ; suitable bushings (either of

porcelain, oil-filled or capacitor-type) for

insulating and bringing out the terminals

of windings

from the

tank.

In all

types of

transformers,

the core is constructed of transformer sheet steel laminations assembled

to provide a continuous magnetic path with a minimum of air-gap

included. The steel used is of high silicon content, sometimes heat

treated to produce a high permeability and a low hysteresis loss at the

iron core

secondary

coil

secondary

coil

110/120

volts

220/240

volts

110/120

volts

primary

coil

220/240

volts

Principle of transformer

Fig. 32.2

Transformer

1117

usual operating flux densities. The eddy current loss is minimised by

laminating the core, the laminations being insulated from each other by a

light coat of core-plate varnish or by an oxide layer on the surface. The

thickness of laminations varies from 0.35 mm for a frequency of 50 Hz to

0.5 mm for a frequency of 25 Hz. The core laminations (in the form of

strips) are joined as shown in Fig. 32.2. It is seen that the joints in the

alternate layers are staggered in order to avoid the presence of narrow

gaps right through the cross-section of the core. Such staggered joints

are said to be ‘imbricated’.

Constructionally, the transformers are of two

general types, distinguished from each other

merely by the manner in which the primary

and secondary coils are placed around the

laminated core. The two types are known as (i) core-type and (ii) shell-

type. Another recent development is spiral-core or wound-core type, the

trade name being spirakore transformer.

In the so-called core type transformers, the windings surround a

considerable part of the core whereas in shell-type transformers, the core

surrounds a considerable portion of the windings as shown schematically

in Fig. 32.3 (a) and (b) respectively.

Fig. 32.3 Fig. 32.4

In the simplified diagram for the core type transformers [Fig. 32.3 (a)], the primary and secondary

winding are shown located on the opposite legs (or limbs) of the core, but in actual construction,

these are always interleaved to reduce leakage flux. As shown in Fig. 32.4, half the primary and half the

secondary winding have been placed side by side or concentrically on each limb, not primary on one

limb (or leg) and the secondary on the other.

Lam

ination

Coils

Butt

Joint

Fig. 32.5 Fig. 32.6

In both core and shell-type transformers, the individual laminations are cut in the form of long

strips of L’s, E’s and I’s as shown in Fig. 32.5. The assembly of the complete core for the two types of

transformers is shown in Fig. 32.6 and Fig. 32.7.

Low

voltage

High

voltage

Normal Operation

Oil

Iron Core

Magnetic Flux

Ground

Core-type transformer

Leads

Primary

Winding

Secondary Winding

Paper Insulation

Laminated

core

Shell-Type transformer

1118

Electrical Technology

As said above, in order to avoid high reluctance at the joints where the laminations are butted against

each other, the alternate layers are stacked differently to eliminate these joints as shown in Fig. 32.6 and

Fig. 32.7.

Butt Joint

Fig. 32.7

32.3.32.3.

32.3.

CorCor

Cor

e-type e-type

e-type

ransfransf

ransf

oror

mermer

mer

The coils used are form-wound and

are of the cylindrical type. The general

form of these coils may be circular or oval

or rectangular. In small size core-type

transformers, a simple rectangular core is

used with cylindrical coils which are either

circular or rectangular in form. But for

large-size core-type transformers, round

or circular cylindrical coils are used which are so

wound as to fit over a cruciform core section as

shown in Fig. 32.8(a). The circular cylindrical coils

are used in most of the core-type transformers

because of their mechanical strength. Such

cylindrical coils are wound in helical layers with the

different layers insulated from each other by paper,

cloth, micarta board or cooling ducts. Fig. 32.8(c)

shows the general arrangement of these coils with

respect to the core. Insulating cylinders of fuller

board are used to separate the cylindrical windings

from the core and from each other. Since the low-

voltage (LV) winding is easiest to insulate, it is placed

nearest to the core (Fig. 32.8).

Core

L.V.H.V.

H.V.

L.V

. L.V.

H.V

()b

()c

Insulating

Cylinder

H.V. Winding

L.V. Winding

Fig. 32.8

Coil Coil

Coil

Coil Coil

Single-Phase Transformer Cores

2-leg core 3-leg core 4-leg core

Fig. 32.8 (a)

Transformer

1119

Because of laminations and insulation, the net or effective core area is reduced, due allowance for

which has to be made (Ex. 32.6). It is found that, in general, the reduction in core sectional area due to the

presence of paper, surface oxide etc. is of the order of 10% approximately.

As pointed out above, rectangular cores with rectangular cylindrical coils can be used for small-size

core-type transformers as shown in Fig. 32.9 (a) but for large-sized transformers, it becomes wasteful to

use rectangular cylindrical coils and so circular cylindrical coils are preferred. For such purposes, square

cores may be used as shown in Fig. 32.9 (b) where circles represent the tubular former carrying the coils.

Obviously, a considerable amount of useful space is still wasted. A common improvement on square core

is to employ cruciform core as in Fig. 32.9 (c) which demands, at least, two sizes of core strips. For very

large transformers, further core-stepping is done as in Fig. 32.9 (d) where at least three sizes of core plates

are necessary. Core-stepping not only gives high space factor but also results in reduced length of the mean

turn and the consequent I

R loss. Three stepped core is the one most commonly used although more steps

may be used for very large transformers as in Fig. 32.9 (e). From the geometry of Fig. 32.9, it can be shown

that maximum gross core section for Fig. 32.9 (b) is 0.5 d

and for Fig. 32.9 (c) it is 0.616 d

where d is the

diameter of the cylindrical coil.

Fig. 32.9

32.4. Shell-type Transformers

In these case also, the coils are form-would but are multi-layer disc type usually wound in the

form of pancakes. The different layers of such multi-layer discs are insulated from each other by

paper. The complete winding consists of stacked discs with insulation space between the coils–the

spaces forming horizontal cooling and insulating ducts. A shell-type transformer may have a simple

rectangular form as shown in Fig. 32.10 or it may have distributed form as shown in Fig. 32.11.

Fig. 32.10

A very commonly-used shell-type transformer is the one known as Berry Transformer–so called

after the name of its designer and is cylindrical in form. The transformer core consists of laminations

arranged in groups which radiate out from the centre as shown in section in Fig. 32.12.

1120

Electrical Technology

It may be pointed out that cores and coils of transformers must be provided with rigid mechanical

bracing in order to prevent movement and possible insulation damage. Good bracing reduces vibration and

the objectionable noise–a humming sound–during operation.

The spiral-core transformer employs the newest development in core construction. The core is as-

sembled of a continuous strip or ribbon of transformer steel wound in the form of a circular or elliptical

cylinder. Such construction allows the core flux to follow the grain of the iron. Cold-rolled steel of high

silicon content enables the designer to use considerably higher operating flux densities with lower loss per

kg. The use of higher flux density reduces the weight per kVA. Hence, the advantages of such construction

are (i) a relatively more rigid core (ii) lesser weight and size per kVA rating (iii) lower iron losses at higher

operating flux densities and (iv) lower cost of manufacture.

()b

()a

Cylindrical

Winding

agnetic

Core

Fig. 32.11 Fig. 32.12

Transformers are generally housed in tightly-fitted sheet-metal ; tanks filled with special insulating oil*.

This oil has been highly developed and its function is two-fold. By circulation, it not only keeps the coils

reasonably cool, but also provides the transformer with additional insulation not obtainable when the

transformer is left in the air.

In cases where a smooth tank surface does not provide sufficient cooling area, the sides of the tank are

corrugated or provided with radiators mounted on the sides. Good transformer oil should be absolutely free

from alkalies, sulphur and particularly from moisture. The presence of even an

extremely small percentage of moisture in the oil is highly detrimental from the insulation viewpoint

because it lowers the dielectric strength of the oil considerably. The importance of avoiding moisture

in the transformer oil is clear from the fact that even an addition of 8 parts of water in 1,000,000 reduces

the insulating quality of the oil to a value generally recognized as below standard. Hence, the tanks

are sealed air-tight in smaller units. In the case of large-sized transformers where complete air-tight

construction is impossible, chambers known as breathers are provided to permit the oil inside the tank

to expand and contract as its temperature increases or decreases. The atmospheric moisture is

entrapped in these breathers and is not allowed to pass on to the oil. Another thing to avoid in the oil

is sledging which is simply the decomposition of oil with long and continued use. Sledging is caused

principally by exposure to oxygen during heating and results in the formation of large deposits of dark

and heavy matter that eventually clogs the cooling ducts in the transformer.

No other feature in the construction of a transformer is given more attention and care than the

insulating materials, because the life on the unit almost solely depends on the quality, durability and

handling of these materials. All the insulating materials are selected on the basis of their high quality

and ability to preserve high quality even after many years of normal use.

* Instead of natural mineral oil, now-a-days synthetic insulating fluids known as ASKARELS (trade name) are

used. They are non-inflammable and, under the influence of an electric arc, do not decompose to produce

inflammable gases. One such fluid commercially known as PYROCLOR is being extensively used because

it possesses remarkable stability as a dielectric and even after long service shows no deterioration through

sledging, oxidation, acid or moisture formation. Unlike mineral oil, it shows no rapid burning.

Transformer

1121

All the transformer leads are brought out of their cases through suitable bushings. There are many

designs of these, their size and construction depending on the voltage of the leads. For moderate voltages,

porcelain bushings are used to insulate the leads as they come out through the tank. In general, they look

almost like the insulators used on the transmission lines. In high voltage installations, oil-filled or capacitor-

type bushings are employed.

The choice of core or shell-type construction is usually determined by cost, because similar character-

istics can be obtained with both types. For very high-voltage transformers or for multiwinding design, shell-

type construction is preferred by many manufacturers. In this type, usually the mean length of coil turn is

longer than in a comparable core-type design. Both core and shell forms are used and the selection is

decided by many factors such as voltage rating, kVA rating, weight, insulation stress, heat distribution etc.

Another means of classifying the transformers is according to the type of cooling employed. The

following types are in common use :

(a) oil-filled self-cooled (b) oil-filled water-cooled (c) air-blast type

Small and medium size distribution transformers–so called because of their use on distribution

systems as distinguished from line transmission–are of type (a). The assembled windings and cores

of such transformers are mounted in a welded, oil-tight steel tank provided with steel cover. After

putting the core at its proper place, the tank is filled with purified, high quality insulating oil. The oil

serves to convey the heat from the core and the windings to the case from where it is radiated out to

the surroundings. For small size, the tanks are usually smooth-surfaced, but for larger sizes, the cases

are frequently corrugated or fluted to get greater heat radiation area without increasing the cubical

capacity of the tank. Still larger sizes are provided with radiators or pipes.

Construction of very large self-cooled transformers is expensive, a more economical form of

construction for such large transformers is provided in the oil-immersed, water-cooled type. As

before, the windings and the core are immersed in the oil, but there is mounted near the surface of oil,

a cooling coil through which cold water is kept circulating. The heat is carried away by this water. The

largest transformers such as those used with high-voltage transmission lines, are constructed in this

manner.

Oil-filled transformers are built for outdoor duty and as these require no housing other than their

own, a great saving is thereby effected. These transformers require only periodic inspection.

For voltages below 25,000 V, transformers can be built for cooling by means of an air-blast. The

transformer is not immersed in oil, but is housed in a thin sheet-metal box open at both ends through

which air is blown from the bottom to the top by means of a fan or blower.

32.5. Elementary Theory of an Ideal Transformer

An ideal transformer is one which has no losses i.e. its windings have no ohmic resistance, there is no

magnetic leakage and hence which has no I

R and core losses. In other words, an ideal transformer consists

of two purely inductive coils wound on a loss-free core. It may, however, be noted that it is impossible

to realize such a transformer in practice, yet for convenience, we will start with such a trans-

former and step by step approach an actual transformer.

()b

Prim

ary

Secondary

()a

Fig. 32.13

1122

Electrical Technology

Consider an ideal transformer [Fig. 32.13 (a)] whose secondary is open and whose primary is con-

nected to sinusoidal alternating voltage V

. This potential difference causes an alternating current to flow in

the primary. Since the primary coil is purely inductive and there is no output (secondary being open) the

primary draws the magnetising current I

only. The function of this current is

merely to magnetise the core, it is small in magnitude and lags V

by 90°. This alternating current I

produces an alternating flux φ which is, at all

times, proportional to the current (assuming

permeability of the magnetic circuit to be

constant) and, hence, is in phase with it. This

changing flux is linked both with the primary

and the secondary windings. Therefore, it

produces self-induced e.m.f. in the primary.

This self-induced e.m.f. E

is, at every in-

stant, equal to and in opposition to V

. It is

also known as counter e.m.f. or back e.m.f.

of the primary.

Similarly, there is produced in the sec-

ondary an induced e.m.f. E

which is

known as mutually induced e.m.f. This

e.m.f. is antiphase with V

and its magni-

tude is proportional to the rate of change

of flux and the number of secondary turns.

The instantaneous values of applied

voltage, induced e.m.fs, flux and

magnetising current are shown by sinu-

soidal waves in Fig. 32.13 (b). Fig. 32.13

32.6. E.M.F. Equation of a Transformer

Let N

= No. of turns in primary

= No. of turns in secondary

= Maximum flux in core in webers

= B

× A

f = Frequency of a.c. input in Hz

As shown in Fig. 32.14, flux increases from its zero value to

maximum value Φ

in one quarter of the cycle i.e. in 1/4 f second.

∴ Average rate of change of flux =

1/4

=4 f Φ

Wb/s or volt

Now, rate of change of flux per turn means induced e.m.f. in volts.

∴ Average e.m.f./turn = 4 f Φ

volt

If flux Φ varies sinusoidally, then r.m.s. value of induced e.m.f. is obtained by multiplying the average

value with form factor.

Form factor =

r.m.s. value

=1.11

average value

∴ r.m.s. value of e.m.f./turn = 1.11 × 4 f Φ

= 4.44 f Φ

volt

Now, r.m.s. value of the induced e.m.f. in the whole of primary winding

= (induced e.m.f/turn) × No. of primary turns

=4.44 f N

= 4.44 f N

A ...(i)

Fig. 32.14

Cycle

Tim



Step-up transformer

Primary coil

Secondary coil

If the primary coil

has 3 loops and the

secondary coil has

30, the voltage is

stepped up 10

times.

Primary coil

Secondary coil

If the primary coil

has 30 loops and

the secondary coil

has 3, the voltage

is stepped down 10

times.

Step-down transformer

Step-up transformer

Transformer

1123

Similarly, r.m.s. value of the e.m.f. induced in secondary is,

= 4.44 f N

A ...(ii)

It is seen from (i) and (ii) that E

= E

= 4.44 f Φ

. It means that e.m.f./turn is the same in both

the primary and secondary windings.

In an ideal transformer on no-load, V

= E

and E

= V

where V

is the terminal voltage

(Fig. 32.15).

32.7 Voltage Transformation Ratio (K)

From equations (i) and (ii), we get

This constant K is known as voltage transformation

ratio.

(i) If N

> N

i.e. K > 1, then transformer is called step-up

transformer.

(ii) If N

< N

i.e. K < 1, then transformer is known as

step-down transformer.

Again, for an ideal transformer, input VA = output VA .

= V

IVK

Hence, currents are in the inverse ratio of the (voltage) transformation ratio.

Example 32.1. The maximum flux density in the core of a 250/3000-volts, 50-Hz single-phase

transformer is 1.2 Wb/m

. If the e.m.f. per turn is 8 volt, determine

(i) primary and secondary turns (ii) area of the core.

(Electrical Engg.-I, Nagpur Univ. 1991)

Solution. (i) E

= N

× e.m.f. induced/turn

= 250/8 = 32; N

= 3000/8 = 375

(ii) We may use E

= − 4.44 f N

∴ 3000 = 4.44 × 50 × 375 × 1.2 × A; A = 0.03m

Example 32.2. The core of a 100-kVA, 11000/550 V, 50-Hz, 1-ph, core type transformer has a

cross-section of 20 cm × 20 cm. Find (i) the number of H.V. and L.V. turns per phase and (ii) the e.m.f.

per turn if the maximum core density is not to exceed 1.3 Tesla. Assume a stacking factor of 0.9.

What will happen if its primary voltage is increased by 10% on no-load ?

(Elect. Machines, A.M.I.E. Sec. B, 1991)

Solution. (i) B

=1.3 T, A = (0.2 × 0.2) × 0.9 = 0.036 m

∴ 11,000 = 4.44 × 50 × N

× 1.3 × 0.036, N

= 1060

550 = 4.44 × 50 × N

× 1.3 × 0.036; N

= 53

or, N

= KN

= (550/11,000) × 1060 = 53

(ii) e.m.f./turn = 11,000/1060 = 10.4 V or 550/53 = 10.4 V

Keeping supply frequency constant, if primary voltage is increased by 10%, magnetising current will

increase by much more than 10%. However, due to saturation, flux density will increase only marginally and

so will the eddy current and hysteresis losses.

Example 32.3. A single-phase transformer has 400 primary and 1000 secondary turns. The

net cross-sectional area of the core is 60 cm

. If the primary winding be connected to a 50-Hz supply

at 520 V, calculate (i) the peak value of flux density in the core (ii) the voltage induced in the

secondary winding. (Elect. Engg-I, Pune Univ. 1989)



Fig. 32.15

1124

Electrical Technology

Solution. K = N

= 1000/400 = 2.5

(i) E

= K ∴

ΚΕ

= 2.5 × 520 = 1300 V

(ii) E

= 4.44 f N

or 520 = 4.44 × 50 × 400 × B

× (60 × 10

−4

) ∴ B

= 0.976 Wb/m

Example 32.4. A 25-kVA transformer has 500 turns on the primary and 50 turns on the second-

ary winding. The primary is connected to 3000-V, 50-Hz supply. Find the full-load primary and

secondary currents, the secondary e.m.f. and the maximum flux in the core. Neglect leakage drops

and no-load primary current. (Elect. & Electronic Engg., Madras Univ. 1985)

Solution. K = N

= 50/500 = 1/10

Now, full-load I

= 25,000/3000 = 8.33 A. F.L. I

= I

/K = 10 × 8.33 = 83.3 A

e.m.f. per turn on primary side = 3000/500 = 6 V

∴ secondary e.m.f. = 6 × 50 = 300 V (or E

= KE

= 3000 × 1/10 =300 V)

Also, E

=4.44 f N

; 3000 = 4.44 × 50 × 500 × Φ

∴ Φ

= 27 mWb

Example 32.5. The core of a three phase, 50 Hz, 11000/550 V delta/star, 300 kVA, core-type

transformer operates with a flux of 0.05 Wb. Find

(i) number of H.V. and L.V. turns per phase. (ii) e.m.f. per turn

(iii) full load H.V. and L.V. phase-currents. (Bharathithasan Univ. April 1997)

Solution. Maximum value of flux has been given as 0.05 Wb.

(ii) e.m.f. per turn = 4.44 f φ

=4.44 × 50 × 0.05 = 11.1 volts

(i) Calculations for number of turns on two sides :

Voltage per phase on delta-connected primary winding = 11000 volts

Voltage per phase on star-connected secondary winding = 550/1.732 = 317.5 volts

= number of turns on primary, per phase

= voltage per phase/e.m.f. per turn

= 11000/11.1 = 991

= number of turns on secondary, per phase

= voltage per phase/e.m.f. per turn

= 317.5/11.1 = 28.6

Note : (i) Generally, Low-voltage-turns are calculated first, the figure is rounded off to next higher even

integer. In this case, it will be 30. Then, number of turns on primary side is calculated by turns-ratio.

In this case, T

= T

) = 30 × 11000/317.5 = 1040

This, however, reduces the flux and results into less saturation. This, in fact, is an elementary

aspect in Design-calculations for transformers. (Explanation is added here only to overcome a doubt

whether a fraction is acceptable as a number of L.V. turns).

(ii) Full load H.V. and L.V. phase currents :

Output per phase = (300/3) = 100 kVA

H.V. phase-current =

100 1000

9.1 Amp

11,000

L.V. phase-current = (100 × 1000/317.5) = 315 Amp

Example 32.6. A single phase transformer has 500 turns in the primary and 1200 turns in the

secondary. The cross-sectional area of the core is 80 sq. cm. If the primary winding is connected to

a 50 Hz supply at 500 V, calculate (i) Peak flux-density, and (ii) Voltage induced in the secondary.

(Bharathiar University November 1997)

Transformer

1125

Solution. From the e.m.f. equation for transformer,

500 = 4.44 × 50 × φ

× 500

= 1/222 Wb

(i) Peak flux density, B

= φ

/ (80 × 10

−4

) = 0.563 wb/m

(ii) Voltage induced in secondary is obtained from transformation ratio or turns ratio

or V

= 500 × 1200/500 = 1200 volts

Example 32.7. A 25 kVA, single-phase transformer has 250 turns on the primary and 40 turns on

the secondary winding. The primary is connected to 1500-volt, 50 Hz mains. Calculate

(i) Primary and Secondary currents on full-load, (ii) Secondary e.m.f., (iii) maximum flux in the core.

(Bharathiar Univ. April 1998)

Solution. (i) If V

= Secondary voltage rating, = secondary e.m.f.,

1500

250

, giving V

= 240 volts

(ii) Primary current = 25000/1500 = 16.67 amp

Secondary current = 25000/240 = 104.2 amp

(iii) If φ

is the maximum core-flux in Wb,

1500 = 4.44 × 50 × φ

× 250, giving φ

= 0.027 Wb or 27 mWb

Example 32.8. A single-phase, 50 Hz, core-type transformer has square cores of 20 cm side.

Permissible maximum flux-density is 1 Wb/m

. Calculate the number of turns per Limb on the High

and Low-voltage sides for a 3000/220 V ratio. (Manonmaniam Sundaranar Univ. April 1998)

Solution. E.M.F. equation gives the number of turns required on the two sides. We shall first

calculate the L.V.-turns, round the figure off to the next higher even number, so that given maximum

flux density is not exceeded. With the corrected number of L.V. turns, calculate H.V.-turns by

transformation ratio. Further, there are two Limbs. Each Limb accommodates half-L.V. and half H.V.

winding from the view-point of reducing leakage reactance.

Starting with calculation for L.V. turns, T

4.44 × 50 × [(20 × 20 × 10

−4

) × 1] ×

= 220

= 220/8.88 = 24.77

Select T

=26

= V

= 26 × 3000/220 = 354, selecting the nearest even integer.

Number of H.V. turns on each Limb = 177

Number of L.V. turns on each Limb = 13

32.8. Transformer with Losses but no Magnetic Leakage

We will consider two cases (i) when such a transformer is on no load and (ii) when it is loaded.

32.9. Transformer on No-load

In the above discussion, we assumed an ideal transformer i.e. one in which there were no core losses

and copper losses. But practical conditions require that certain modifications be made in the foregoing

1126

Electrical Technology

theory. When an actual transformer is put on load, there is iron loss in the core and copper loss in the

windings (both primary and secondary) and these losses are not entirely negligible.

Even when the transformer is on no-load, the primary input current is not wholly reactive. The primary

input current under no-load conditions has to supply (i) iron losses in the core i.e. hysteresis loss and eddy

current loss and (ii) a very small amount of copper loss in primary (there being no Cu loss in secondary as

it is open). Hence, the no-load primary input current I

is not at 90° behind V

but lags it by an angle φ

90°. No-load input power

= V

cos φ

where cos φ

is primary power factor under no-load conditions. No-load

condition of an actual transformer is shown vectorially in Fig. 32.16.

As seen from Fig. 32.16, primary current I

has two components :

(i) One in phase with V

. This is known as active or working or

iron loss component I

because it mainly supplies the iron loss plus

small quantity of primary Cu loss.

= I

cos φ

(ii) The other component is in quadrature with V

and is known as

magnetising component I

because its function is to sustain the

alternating flux in the core. It is wattless.

= I

sin φ

Obviously, I

is the vector sum of I

and I

, hence I

= (I

+ I

The following points should be noted carefully :

1. The no-load primary current I

is very small as compared to the full-load primary current. It is

about 1 per cent of the full-load current.

2. Owing to the fact that the permeability of the core varies with the instantaneous value of the

exciting current, the wave of the exciting or magnetising current is not truly sinusoidal. As such it

should not be represented by a vector because only sinusoidally varying quantities are represented

by rotating vectors. But, in practice, it makes no appreciable difference.

3. As I

is very small, the no-load primary Cu loss is negligibly small which means that no-load

primary input is practically equal to the iron loss in the transformer.

4. As it is principally the core-loss which is responsible for shift in the current vector, angle φ

known as hysteresis angle of advance.

Example 32.9. (a) A 2,200/200-V transformer draws a no-load primary current of 0.6 A and

absorbs 400 watts. Find the magnetising and iron loss currents.

(b) A 2,200/250-V transformer takes 0.5 A at a p.f. of 0.3 on open circuit. Find magnetising

and working components of no-load primary current.

Solution. (a) Iron-loss current

no-load input in watts

400

primary voltage 2,200

0.182 A

Now I

= I

+ I

Magnetising component I

(0.6 0.182)

−=

0.572 A

The two components are shown in Fig. 29.15.

Fig. 32.16





I

Transformer

1127

(b) I

= 0.5 A, cos φ

= 0.3 ∴ I

= I

cos φ

= 0.5 × 0.3 = 0.15 A

0.5 0.15 0.476 A

−=

Example 32.10. A single-phase transformer has 500 turns on the primary and 40 turns on the

secondary winding. The mean length of the magnetic path in the iron core is 150 cm and the joints

are equivalent to an air-gap of 0.1 mm. When a p.d. of 3,000 V is applied to the primary, maximum

flux density is 1.2 Wb/m

. Calculate (a) the cross-sectional area of the core (b) no-load secondary

voltage (c) the no-load current drawn by the primary (d) power factor on no-load. Given that

AT/cm for a flux density of 1.2 Wb/m

in iron to be 5, the corresponding iron loss to be 2 watt/kg at

50 Hz and the density of iron as 7.8 gram/cm

Solution. (a) 3,000 = 4.44 × 50 × 500 × 1.2 × A ∴ A = 0.0225 m

= 225 cm

This is the net cross-sectional area. However, the gross area would be about 10% more to allow

for the insulation between laminations.

(b) K = N

= 40/500 = 4/50

∴ N.L. secondary voltage = KE

= (4/50) × 3000 = 240 V

AT for air-gap =

1.2

0.0001 95.5

410

Hl l

−

=×= × =

π×

Total AT for given B

max

= 750 + 95.5 = 845.5

Max. value of magnetising current drawn by primary = 845.5/500 = 1.691 A

Assuming this current to be sinusoidal, its r.m.s. value is I

1.691/ 2 1.196 A

Volume of iron = length × area = 150 × 225 = 33,750 cm

Density = 7.8 gram/cm

∴ Mass of iron = 33,750 × 7.8/1000 = 263.25 kg

Total iron loss = 263.25 × 2 = 526.5 W

Iron loss component of no-load primary current I

is I

= 526.5/3000 = 0.176 A

22 2 2

1.196 0.176

+= + =

0.208 A

(d) Power factor,cos φ

= I

= 0.176/1.208 = 0.1457

Example 32.11. A single-phase transformer has 1000 turns on the primary and 200 turns on the

secondary. The no load current is 3 amp. at a p.f. of 0.2 lagging. Calculate the primary current and

power-factor when the secondary current is 280 Amp at a p.f. of 0.80 lagging.

(Nagpur University, November 1997)

Solution. V

is taken as reference. cos

−1

0.80 = 36.87°

= 280 ∠−36.87º amp

I’

= (280/5) ∠−36.87º amp

φ = cos

−1

0.20 = 78.5°, sin φ = 0.98

= I

+ I’

= 3(0.20 − j 0.98) + 56 (0.80 − j 0.60)

=0.6 − j 2.94 + 44.8 − j 33.6

= 45.4 − j 2.94 + 44.8 − j 33.6

= 45.4 − j 36.54 = 58.3 ∠38.86º

Thus I lags behind the supply voltage by an angle of 38.86°.

1128

Electrical Technology

Tutorial Problems 32.1

1. The number of turns on the primary and secondary windings of a 1−φ transformer are 350 and 35

respectively. If the primary is connected to a 2.2 kV, 50-Hz supply, determine the secondary voltage

on no-load. [220 V] (Elect. Engg.-II, Kerala Univ. 1980)

2. A 3000/200-V, 50-Hz, 1-phase transformer is built on a core having an effective cross-sectional area

of 150 cm

and has 80 turns in the low-voltage winding. Calculate

(a) the value of the maximum flux density in the core

(b) the number of turns in the high-voltage winding. [(a) 0.75 Wb/m

(b) 1200]

3. A 3,300/230-V, 50-Hz, 1-phase transformer is to be worked at a maximum flux density of 1.2 Wb/m

in the core. The effective cross-sectional area of the transformer core is 150 cm

. Calculate suitable

values of primary and secondary turns. [830; 58]

4. A 40-kVA, 3,300/240-V, 50 Hz, 1-phase transformer has 660 turns on the primary. Determine

(a) the number of turns on the secondary

(b) the maximum value of flux in the core

Internal drops in the windings are to be ignored. [(a) 48 (b) 22.5 mWb (c) 12.1 A; 166.7 A]

5. A double-wound, 1-phase transformer is required to step down from 1900 V to 240 V, 50-Hz. It is

to have 1.5 V per turn. Calculate the required number of turns on the primary and secondary windings

respectively.

The peak value of flux density is required to be not more than 1.2 Wb/m

. Calculate the required

cross-sectional area of the steel core. If the output is 10 kVA, calculate the secondary current.

[1,267; 160; 56.4 cm

; 41.75 A]

6. The no-load voltage ratio in a 1-phase, 50-Hz, core-type transformer is 1,200/440. Find the number

of turns in each winding if the maximum flux is to be 0.075 Wb. [24 and 74 turns]

7. A 1-phase transformer has 500 primary and 1200 secondary turns. The net cross-sectional area of

the core is 75 cm

. If the primary winding be connected to a 400-V, 50 Hz supply, calculate.

(i) the peak value of flux density in the core and (ii) voltage induced in the secondary winding.

[0.48 Wb/m

; 60 V]

8. A 10-kVA, 1-phase transformer has a turn ratio of 300/23. The primary is connected to a 1500-V,

60 Hz supply. Find the secondary volts on open-circuit and the approximate values of the currents

in the two windings on full-load. Find also the maximum value of the flux. [115 V; 6.67 A; 87 A;

11.75 mWb]

9. A 100-kVA, 3300/400-V, 50 Hz, 1 phase transformer has 110 turns on the secondary. Calculate the

approximate values of the primary and secondary full-load currents, the maximum value of flux in the

core and the number of primary turns.

How does the core flux vary with load ? [30.3 A; 250 A; 16.4 mWb; 907]

10. The no-load current of a transformer is 5.0 A at 0.3 power factor when supplied at 230-V, 50-Hz. The

number of turns on the primary winding is 200. Calculate (i) the maximum value of flux in the core (ii)

the core loss (iii) the magnetising current. [5.18 mWb; 345 W; 4.77 A]

11. The no-load current of a transformer is 15 at a power factor of 0.2 when connected to a 460-V, 50-Hz

supply. If the primary winding has 550 turns, calculate

(a) the magnetising component of no-load current

(b) the iron loss

12. The no-load current of a transformer is 4.0 A at 0.25 p.f. when supplied at 250-V, 50 Hz. The number

of turns on the primary winding is 200. Calculate

(i) the r.m.s. value of the flux in the core (assume sinusoidal flux)

(ii) the core loss

(iii) the magnetising current. [(i) 3.96 mWb (ii) 250 W (iii) 3.87 A]

Transformer

1129

13. The following data apply to a single- phase transformer:

output : 100 kVA, secondary voltage; 400 V; Primary turns: 200; secondary turns: 40; Neglecting the

losses, calculate: (i) the primary applied voltage (ii) the normal primary and secondary currents (iii)

the secondary current, when the load is 25 kW at 0.8 power factor.

(Rajiv Gandhi Technical University, Bhopal 2000) [(i) 2000 V, (ii) 50 amp, (iii) 78.125 amp]

32.10. Transformer on Load

When the secondary is loaded, the secondary current I

is set up. The

magnitude and phase of I

with respect to V

is determined by the charac-

teristics of the load. Current I

is in phase with V

if load is non-inductive, it

lags if load is inductive and it leads if load is capacitive.

The secondary current sets up its own m.m.f. (=N

) and hence its

own flux Φ

which is in opposition to the main primary flux Φ which is

due to I

. The secondary ampere-turns N

are known as

demagnetising amp-turns. The opposing secondary flux Φ

weakens

the primary flux Φ momentarily, hence primary back e.m.f. E

tends to be

reduced. For a moment V

gains the upper hand over E

and hence

causes more current to flow in primary.

Let the additional primary current be I

′. It is known as load

component of primary current. This current is antiphase with I

′. The

additional primary m.m.f. N

sets up its own flux Φ

′ which is in

opposition to Φ

(but is in the same direction as Φ) and is equal to it in

magnitude. Hence, the two cancel each other out. So, we find that the

magnetic effects of secondary current I

are immediately neutralized by

the additional primary current I

′ which is brought into existence exactly

at the same instant as I

. The whole process is illustrated in Fig. 32.17.

Hence, whatever the load conditions, the net flux passing through

the core is approximately the same as at no-load. An important deduction is that due to the constancy

of core flux at all loads, the core loss is also practically the same under all load conditions.

As Φ

= Φ

′ ∴ N

= N

′∴I

′ =

IKI

×=

Hence, when transformer is on load, the primary winding has two currents in it; one is I

and the other

is I

′ which is anti-phase with I

and K times in magnitude. The total primary current is the vector sum

of I

and I

′′

′.



0

I

k=1

()a

()b





k=1

()c

Negligible





k=1

Fig. 32.18

Fig. 32.17





Load





1130

Electrical Technology

In Fig. 32.18 are shown the vector diagrams for a load transformer when load is non-inductive and

when it is inductive (a similar diagram could be drawn for capacitive load). Voltage transformation

ratio of unity is assumed so that primary vectors are equal to the secondary vectors. With reference

to Fig. 32.18 (a), I

is secondary current in phase with E

(strictly speaking it should be V

). It causes

primary current I

′ which is anti-phase with it and equal to it in magnitude ( K = 1). Total primary

current I

is the vector sum of I

and I

′ and lags behind V

by an angle φ

In Fig. 32.18 (b) vectors are drawn for an inductive load. Here I

lags E

(actually V

) by φ

Current I

′ is again antiphase with I

and equal to it in magnitude. As before, I

is the vector sum of I

′

and I

and lags behind V

by φ

It will be observed that φ

is slightly greater than φ

. But if we neglect I

as compared to I

′ as in

Fig. 32.18 (c), then φ

= φ

. Moreover, under this assumption

′ = N

= N

∴

21 2

22 1

IIN

′

== =

It shows that under full-load conditions, the ratio of primary and secondary currents is constant.

This important relationship is made the basis of current transformer–a transformer which is used with

a low-range ammeter for measuring currents in circuits where the direct connection of the ammeter is

impracticable.

Example 32.12. A single-phase transformer with a ratio of 440/110-V takes a no-load current

of 5A at 0.2 power factor lagging. If the secondary supplies a current of 120 A at a p.f. of 0.8 lagging,

estimate the current taken by the primary. (Elect. Engg. Punjab Univ. 1991)

Solution. cos φ

=0.8, φ

= cos

−1

(0.8) = 36°54′

cos φ

=0.2 ∴φ

= cos

−1

(0.2) = 78° 30′

Now K = V

= 110/440 = 1/4

∴ I

′ = KI

= 120 × 1/4 = 30 A

=5 A.

Angle between I

and I

′

= 78° 30′ − 36° 54′ = 41° 36′

Using parallelogram law of vectors (Fig. 32.19) we get

(5 30 2 5 30 cos 41 36 )

++××× °′

= 34.45 A

The resultant current could also have been found by resolving

′ and I

into their X and Y -components.

Example 32.13. A transformer has a primary winding of

800 turns and a secondary winding of 200 turns. When the load

current on the secondary is 80 A at 0.8 power factor lagging,

the primary current is 25 A at 0.707 power factor lagging.

Determine graphically or otherwise the no-load current of the

transformer and its phase with respect to the voltage.

Solution. Here K = 200/800 = 1/4; I

′ = 80 × 1/4 = 20 A

= cos

−1

(0.8) = 36.9°; φ

= cos

− 1

(0.707) = 45°

As seen from Fig. 32.20, I

is the vector sum of I

and I

′. Let

lag behind V

by an angle φ

cos φ

+ 20 cos 36.9° = 25 cos 45°

Fig. 32.19

36 54'

78 30'



30A

120A

Fig. 32.20

36.9



80A

20A



Transformer

1131

∴ I

cos φ

= 25 × 0.707 − 20 × 0.8

= 1.675 A

sin φ

+ 20 sin 36.9° = 25 sin 45°

∴ I

sin φ

= 25 × 0.707 − 20 × 0.6

= 5.675 A

∴ tan φ

= 5.675/1.675 = 3.388

∴φ

= 73.3°

Now, I

sin φ

= 5.675

∴ I

= 5.675/sin 73.3° = 5.93 A

Example 32.14. A single phase transformer takes 10 A on no load at p.f. of 0.2 lagging. The

turns ratio is 4 : 1 (step down). If the load on the secondary is 200 A at a p.f. of 0.85 lagging. Find

the primary current and power factor.

Neglect the voltage-drop in the winding. (Nagpur University November 1999)

Solution. Secondary load of 200 A, 0.85 lag is reflected as 50 A, 0.85 lag in terms of the primary

equivalent current.

= 10 ∠−φ

, where φ

= cos

−1

0.20 = 78.5° lagging

=2 − j 9.8 amp

’= 50 ∠−φ

where φ

= cos

−1

0.85 = 31.8°, lagging

’ = 42.5 − j 26.35

Hence primary current I

= I

+ I

′

=2 − j 9.8 + 42.5 − j 26.35

= 44.5 − j 36.15

| = 57.333 amp, φ = 0.776 Lag.

φ =

44.5

cos 39.10 lagging

57.333

−

=°

The phasor diagram is shown in Fig. 32.21.

Tutorial Problems 32.2

1. The primary of a certain transformer takes 1 A at a power factor of 0.4 when it is connected across

a 200-V, 50-Hz supply and the secondary is on open circuit. The number of turns on the primary is

twice that on the secondary. A load taking 50 A at a lagging power factor of 0.8 is now connected

across the secondary. What is now the value of primary current ? [25.9 A]

2. The number of turns on the primary and secondary windings of a single-phase transformer are 350

and 38 respectively. If the primary winding is connected to a 2.2 kV, 50-Hz supply, determine

(a) the secondary voltage on no-load,

(b) the primary current when the secondary current is 200 A at 0.8 p.f. lagging, if the no-load current

is 5 A at 0.2 p.f. lagging,

3. A 400/200-V, 1-phase transformer is supplying a load of 25 A at a p.f. of 0.866 lagging. On no-load

the current and power factor are 2 A and 0.208 respectively. Calculate the current taken from the

supply. [13.9 A lagging V1 by 36.1°]

4. A transformer takes 10 A on no-load at a power factor of 0.1. The turn ratio is 4 : 1 (step down). If

I



(Ref)

Fig. 32.21

1132

Electrical Technology

a load is supplied by the secondary at 200 A and p.f. of 0.8, find the primary current and power factor

(internal voltage drops in transformer are to be ignored). [57.2 A; 0.717 lagging]

5. A 1-phase transformer is supplied at 1,600 V on the h.v. side and has a turn ratio of 8 : 1. The

transformer supplies a load of 20 kW at a power factor of 0.8 lag and takes a magnetising current of

2.0 A at a power factor of 0.2. Calculate the magnitude and phase of the current taken from the h.v.

supply. [17.15 A ; 0.753 lag] (Elect. Engg. Calcutta Univ. 1980)

6. A 2,200/200-V, transformer takes 1 A at the H.T. side on no-load at a p.f. of 0.385 lagging. Calculate

the iron losses.

If a load of 50 A at a power of 0.8 lagging is taken from the secondary of the transformer, calculate the

actual primary current and its power factor. [847 W; 5.44 A; 0.74 lag]

7. A 400/200-V, I-phase transformer is supplying a load of 50 A at a power factor of 0.866 lagging. The

no-load current is 2 A at 0.208 p.f. lagging. Calculate the primary current and primary power factor.

[26.4 A; 0.838 lag] (Elect. Machines-I, Indore Univ. 1980)

32.11. Transformer with Winding Resistance but No Magnetic Leakage

An ideal transformer was supposed to possess no resistance, but in an actual transformer, there

is always present some resistance of the primary and secondary windings. Due to this resistance,

there is some voltage drop in the two windings. The result is that :

(i) The secondary terminal voltage V

is vectorially less than the secondary induced e.m.f. E

by an amount I

where R

is the resistance of the secondary winding. Hence, V

is equal to the

vector difference of E

and resistive voltage drop I

∴ V

= E

− I

...vector difference

(ii) Similarly, primary induced e.m.f. E

is equal to the vector difference of V

and I

where R

is the resistance of the primary winding.

= V

− I

...vector difference

I' KI

 E

()a



()c





I' KI

()b







Fig. 32.22

The vector diagrams for non-inductive, inductive and capacitive loads are shown in Fig. 32.22 (a),

(b) and (c) respectively.

32.12. Equivalent Resistance

In Fig. 32.23 a transformer is shown whose primary and secondary windings have resistances of

and R

respectively. The resistances have been shown external to the windings.

Transformer

1133

It would now be shown that the resistances of the

two windings can be transferred to any one of the two

windings. The advantage of concentrating both the

resistances in one winding is that it makes calculations

very simple and easy because one has then to work in

one winding only. It will be proved that a resistance of

in secondary is equivalent to R

in primary. The

value R

will be denoted by R

′− the equivalent

secondary resistance as referred to primary.

The copper loss in secondary is I

. This loss is supplied by primary which takes a current of I

Hence if R

′ is the equivalent resistance in primary which would have caused the same loss as R

secondary, then

′ = I

or R

′ = (I

)

Now, if we neglect no-load current I

, then I

= I/K

. Hence, R

′ = R

Similarly, equivalent primary resistance as referred to secondary is R

′ = K

In Fig. 32.24, secondary resistance has been transferred to primary side leaving secondary circuit

resistanceless. The resistance R

+ R

′ = R

+ R

is known as the equivalent or effective resistance

of the transformer as referred to primary and may be designated as R

∴ R

= R

+ R

′ = R

+ R

/ K

Similarly, the equivalent resistance of the transformer as referred to secondary is

= R

+ R

′ = R

+ K

This fact is shown in Fig. 32.25 where all the resistances of the transformer has been concentrated

in the secondary winding.

Fig. 32.24 Fig. 32.25

It is to be noted that

1. a resistance of R

in primary is equivalent to K

in secondary. Hence, it is called equivalent

resistance as referred to secondary i.e. R

2. a resistance of R

in secondary is equivalent to R

in primary. Hence, it is called the

equivalent secondary resistance as referred to primary i.e. R

′.

3. Total or effective resistance of the transformer as referred to primary is

= primary resistance + equivalent secondary resistance as referred to primary

= R

+ R

′

= R

+ R

4. Similarly, total transformer resistance as referred to secondary is,

= secondary resistance + equivalent primary resistance as referred to secondary

= R

+ R

′

= R

+ K

Fig. 32.23

* Actually I

# 2/I

′ = I/K and not I

# 2/I

. However, if I

is neglected, then I

′ = I

1134

Electrical Technology

Note : It is important to remember that

(a) when shifting any primary resistance to the secondary, multiply it by K

i.e. (transformation ratio)

(b) when shifting secondary resistance to the primary, divide it by K

32.13. Magnetic Leakage

In the preceding discussion, it has been assumed that

all the flux linked with primary winding also links the

secondary winding. But, in practice, it is impossible to

realize this condition. It is found, however, that all the

flux linked with primary does not link the secondary but

part of it i.e. Φ

completes its magnetic circuit by passing

through air rather than around the core, as shown in Fig.

32.26. This leakage flux is produced when the m.m.f. due

to primary ampere-turns existing between points a and

b, acts along the leakage paths. Hence, this flux is known as primary leakage flux and is proportional

to the primary ampere-turns alone because the secondary turns do not link the magnetic circuit of Φ

The flux Φ

is in time phase with I

. It induces an e.m.f. e

in primary but not in secondary.

Similarly, secondary ampere-turns (or m.m.f.) acting across points c and d set up leakage flux Φ

which is linked with secondary winding alone (and not with primary turns). This flux Φ

is in time

phase with I

and produces a self-induced e.m.f. e

in secondary (but not in primary).

At no load and light loads, the primary and secondary ampere-turns are small, hence leakage fluxes

are negligible. But when load is increased, both primary and secondary windings carry huge currents.

Hence, large m.m.f.s are set up which, while acting on leakage paths, increase the leakage flux.

As said earlier, the leakage flux linking with each winding, produces a self-induced e.m.f. in that

winding. Hence, in effect, it is equivalent to a small

choker or inductive coil in series with each winding

such that voltage drop in each series coil is equal to

that produced by leakage flux. In other words, a

transformer with magnetic leakage is equivalent to

an ideal transformer with inductive coils connected

in both primary and secondary circuits as shown in

Fig. 32.27 such that the internal e.m.f. in each inductive

coil is equal to that due to the corresponding leakage

flux in the actual transformer.

= e

and X

= e

The terms X

and X

are known as primary and secondary leakage reactances respectively.

Following few points should be kept in mind :

1. The leakage flux links one or the other winding but not both, hence it in no way contributes

to the transfer of energy from the primary to the secondary winding.

2. The primary voltage V

will have to supply reactive drop I

in addition to I

. Similarly E

will have to supply I

and I

3. In an actual transformer, the primary and secondary windings are not placed on separate legs

or limbs as shown in Fig. 32.27 because due to their being widely separated, large primary and

secondary leakage fluxes would result. These leakage fluxes are minimised by sectionalizing and

interleaving the primary and secondary windings as in Fig. 32.6 or Fig. 32.8.

Fig. 32.26

Load



Fig. 32.27

Transformer

1135

32.14. Transformer with Resistance and Leakage Reactance

In Fig. 32.28 the primary and secondary windings of a transformer with reactances taken out of

the windings are shown. The primary impedance is given by

()

Similarly, secondary impedance is

given by

()

RX+

The resistance and leakage reactance of

each winding is responsible for some voltage

drop in each winding. In primary, the leakage

reactance drop is I

(usually 1 or 2% of V

Hence

= E

+ I

+ jX

) = E

+ I

Similarly, there are I

and I

drops in secondary which combine with V

to give E

= V

+ I

+ jX

) = V

+ I

The vector diagram for such a transformer for different kinds of loads is shown in Fig. 32.29. In

these diagrams, vectors for resistive drops are drawn parallel to current vectors whereas reactive

drops are perpendicular to the current vectors. The angle φ

between V

and I

gives the power factor

angle of the transformer.

It may be noted that leakage reactances can also be transferred from one winding to the other in

the same way as resistance.

∴ X

′ = X

and X

′ = K

and X

= X

+ X

′ = X

+ X

and X

= X

+ X

′ = X

+ K



















Fig. 32.29

Fig. 32.28

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Electrical Technology

R+R'

X+X'

R+R'

X+X'

Fig. 32.30 (a) Fig. 32.30 (b)

It is obvious that total impedance of the transformer as referred to primary is given by

01 01

()

...Fig. 32.30 (a)

and Z

02 02

()

...Fig. 32.30 (b)

Example 32.15. A 30 kVA, 2400/120-V, 50-Hz transformer has a high voltage winding resistance

of 0.1 Ω and a leakage reactance of 0.22Ω. The low voltage winding resistance is 0.035 Ω and the

leakage reactance is 0.012 Ω. Find the equivalent winding resistance, reactance and impedance

referred to the (i) high voltage side and (ii) the low-voltage side.

(Electrical Machines-I, Bangalore Univ. 1987)

Solution. K = 120/2400 = 1/20; R

= 0.1 Ω, X

= 0.22 Ω

= 0.035 Ω and X

= 0.012 Ω

(i) Here, high-voltage side is, obviously, the primary side. Hence, values as referred to primary side

are

= R

+ R

′ = R

+ R

= 0.1 + 0.035/(1/20)

= 14.1

ΩΩ

Ω

= X

+ X

′ = X

+ X

= 0.22 + 0.12/(1/20)

= 5.02

ΩΩ

Ω

22 2 2

01 01

14.1 5.02

+= + =

ΩΩ

Ω

(ii) R

= R

+ R

′

= R

+ K

= 0.035 + (1/20)

× 0.1 = 0.03525

ΩΩ

Ω

= X

+ X

′ = X

+ K

= 0.012 + (1/20)

× 0.22 = 0.01255

ΩΩ

Ω

22 2 2

02 02

0.0325 0.01255

+= +

0.0374

ΩΩ

Ω

(or Z

= K

= (1/20)

× 15 = 0.0375 Ω)

Example 32.16. A 50-kVA, 4,400/220-V transformer has R

= 3.45 Ω, R

= 0.009 Ω. The values

of reactances are X

= 5.2 Ω and X

= 0.015 Ω. Calculate for the transformer (i) equivalent resis-

tance as referred to primary (ii) equivalent resistance as referred to secondary (iii) equivalent reac-

tance as referred to both primary and secondary (iv) equivalent impedance as referred to both pri-

mary and secondary (v) total Cu loss, first using individual resistances of the two windings and

secondly, using equivalent resistances as referred to each side.

(Elect. Engg.-I, Nagpur Univ. 1993)

Solution. Full-load I

= 50,000/4,400 = 11.36 A (assuming 100% efficiency)

Full-load I

= 50,000/2220 = 227 A; K = 220/4,400 = 1/20

(i) R

0.009

3.45 3.45 3.6

(1 / 2 0)

+= + = +=

7.05

ΩΩ

Ω

(ii) R

= R

+ K

= 0.009 + (1/20)

× 3.45 = 0.009 + 0.0086 = 0.0176

ΩΩ

Ω

Also, R

= K

= (1/20)

× 7.05 = 0.0176 Ω (check)

Transformer

1137

(iii) X

= X

+ X

′ = X

+ X

= 5.2 + 0.015/(1/20)

= 11.2

ΩΩ

Ω

= X

+ X

′ = X

+ K

= 0.015 + 5.2/20

= 0.028

ΩΩ

Ω

AlsoX

= K

= 11.2/400 = 0.028 Ω (check)

(iv) Z

22 2 2

01 01

( ) (7.05 11.2)

+= + =

13.23

ΩΩ

Ω

22 2 2

02 02

( ) (0.0176 0.028)

+= + =

0.03311

ΩΩ

Ω

AlsoZ

= K

= 13.23/400 = 0.0331 Ω (check)

(v) Cu loss = I

+ I

= 11.36

× 3.45 + 227

× 0.009 = 910 W

AlsoCu loss = I

= 11.36

× 7.05 = 910 W

= I

= 227

× 0.0176 = 910 W

Example 32.17. A transformer with a 10 : 1 ratio and rated at 50-kVA, 2400/240-V, 50-Hz is

used to step down the voltage of a distribution system. The low tension voltage is to be kept

constant at 240 V.

(a) What load impedance connected to low-tension size will be loading the transformer fully at

0.8 power factor (lag) ?

(b) What is the value of this impedance referred to high tension side ?

(Elect. Engineering-I, Bombay Univ. 1987)

Solution. (a) F. L. I

= 50,000/240 = 625/3 A; Z

240

(625/ 3)

1.142

ΩΩ

Ω

(b) K = 240/2400 = 1/10

The secondary impedance referred to primary side is

′ = Z

= 1.142/(1/10)

= 114.2

ΩΩ

Ω

′ = KI

= (1/10) × 625/3 = 20.83 A

Example 32.18. The full-load copper loss on the h.v. side of a 100-kVA, 11000/317-V, 1-phase

transformer is 0.62 kW and on the L.V. side is 0.48 kW.

(i) Calculate R

, R

and R

in ohms (ii) the total reactance is 4 per cent, find X

, X

and X

ohms if the reactance is divided in the same proportion as resistance.

(Elect. Machines A.M.I.E,. Sec. B, 1991)

Solution. (i) F.L. I

= 100 × 10

/11000 = 9.1 A. F.L. I

= 100 × 10

/317 = 315.5 A

Now, I

= 0.62 kW or R

= 620/9.1

= 7.5 Ω

= 0.48 kW, R

= 480/315.5

= 0.00482 Ω

′ = R

= 0.00482 × (11,000/317)

= 5.8 Ω

% reactance =

101 01

9.1

100 or 4 100, 48.4

11000

IX X

××

×= ×=Ω

+ X

′ = 48.4 Ω. Given R

′ = X

′

or (R

+ R

′)/R

′ =(X

+ X

′)/X

′ (7.5 + 5.8)/5.8 = 48.4/X

′ ∴ X

′ = 21.1 Ω

∴ X

= 48.4 − 21.1 = 27.3 Ω, X

= 21.1 × (317/11000)

= 0.175

ΩΩ

Ω

Example 32.19. The following data refer to a I-phase transformer :

Turn ratio 19.5 : 1 ; R

= 25 Ω ; X

= 100 Ω ; R

= 0.06 Ω ; X

= 0.25 Ω. No-load current =

1.25 A leading the flux by 30°.

The secondary delivers 200 A at a terminal voltage of 500 V and p.f. of 0.8 lagging. Determine

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Electrical Technology

by the aid of a vector diagram, the primary applied voltage, the primary p.f. and the efficiency.

(Elect. Machinery-I, Madras Univ. 1989)

Solution. The vector diagram is similar to Fig. 30.28 which has been redrawn as Fig. 32.31. Let us

take V

as the reference vector.

∴ V

= 500 ∠0° = 500 + j0

= 200 (0.8 − j 0.6) = 160 − j 120

= (0.06 + j 0.25)

= V

+ I

= (500 + j 0) + (160 − j 120) (0.06 + j 0.25)

= 500 + (39.6 + j 32.8) = 539.6 + j 32.8 = 541 ∠3.5°

Obviously, β =3.5°

= E

/K = 19.5 E

= 19.5 (539.6 + j 32.8)

= 10,520 + j 640

∴ − E

= − 10,520 − j 640 = 10,540 ∠ 183.5°

′′

′ = − I

K = (− 160 + j 120)/19.5

= − 8.21 + j 6.16

As seen from Fig. 32.31, I

leads V

by an angle

= 3.5 ° + 90° + 30° = 123.5°

∴ I

= 1.25 ∠ 123.5°

= 1.25 (cos 123.5° + j sin 123.5°)

= 1.25 (− cos 56.5° + j sin 56.5°)

= − 0.69 + j 1.04

= I

′′

′

+ I

= (− 8.21 + j 6.16) + (− 0.69 + j 1.04)

= − 8.9 + j 7.2 = 11.45 ∠141°

− −

− E

+ I

= − 10,520 − j 640 + (− 8.9 + j 7.5) (25 + j 100)

= − 10,520 − j 640 − 942 − j 710

= − 11,462 − j 1350

= 11,540 ∠186.7°

Phase angle between V

and I

is = 186.7° − 141° = 45.7°

∴ primary p.f. = cos 45.7° = 0.698 (lag)

No-load primary input power = V

sin φ

= 11,540 × 1.25 × cos 60° = 7,210 W

= R

+ K

= 0.06 + 25/19.5

= 0.1257 Ω

Total Cu loss as referred to secondary = I

= 200

× 0.1257 = 5,030 W

Output = V

cos φ

= 500 × 200 × 0.8 = 80,000 W

Total losses = 5030 + 7210 = 12,240 W

Input = 80,000 + 12,240 = 92,240 W

η = 80,000/92,240 = 0.8674 or 86.74%

Example 32.20. A 100 kVA, 1100/220 V, 50 Hz, single-phase transformer has a leakage impedance

of (0.1 + 0/40) ohm for the H.V. winding and (0.006 + 0.015) ohm for the L.V. winding. Find the

equivalent winding resistance, reactance and impedance referred to the H.V. and L.V. sides.

(Bharathiar Univ. Nov. 1997)





 E

Fig. 32.31

Transformer

1139

Solution. Turns ratio = (N

) = (V

/ V

) = 1100/220 = 5

(i) Referred to H.V. side :

Resistance = r

+ r

= 0.1 + (25 × 0.006) = 0.25 ohm

Reactance = x

+ x

′ = 0.4 + (25 × 0.015) = 0.775 ohm

Impedance = (0.25

+ 0.775

)

0.5

= 0.8143 ohm

(ii) Referred to L.V. side :

Resistance = 0.25/25 = 0.01

(or resistance = 0.006 + (0.1/25) = 0.01 ohm)

Reactance = 0.775/25 = 0.031 ohm

Impedance = 0.8143/25 = 0.0326 ohm

32.15. Simplified Diagram

The vector diagram of Fig. 32.29 may be

considerably simplified if the no-load current

is neglected. Since I

is 1 to 3 per cent of

full-load primary current I

, it may be neglected

without serious error. Fig. 32.32 shows the

diagram of Fig. 32.29 with I

omitted altogether.

In Fig. 32.32, V

, V

, φ

are known, hence

can be found by adding vectorially I

and I

to V

. Similarly, V

is given by the

vector addition of I

and I

to E

. All the

voltages on the primary side can be transferred

to the secondary side as shown in figure, where

the upper part of the diagram has been rotated

through 180°. However, it should be noted

that each voltage or voltage drop should be

multiplied by transformation ratio K.

The lower side of the diagram has been

shown separately in Fig. 32.34 laid horizontally

where vector for V

has been taken along

X-axis.

It is a simple matter to find transformer regulation as shown in Fig. 32.34 or Fig. 32.35.

It may be noted that V

= KV

− I

+ jX

) = KV

− I



KI X

KI R



I(X +

22 1

I(R +

22 1

220

Fig. 32.33 Fig. 32.34

Fig. 32.32

* Also, V

= (V

+ I

) #3/K



KI X

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Electrical Technology

32.16. Total Approximate Voltage Drop

in a Transformer

When the transformer is on no-load, then V

is ap-

proximately equal to E

. Hence E

= KE

= KV

. Also,

where

is secondary terminal voltage on no-

load, hence no-load secondary terminal voltage is KV

The secondary voltage on load is V

. The difference be-

tween the two is I

as shown in Fig. 32.35. The ap-

proximate voltage drop of the transformer as referred to

secondary is found thus :

With O as the centre and radius OC draw an arc cutting OA produced at M. The total voltage drop I

= AC = AM which is approximately equal to AN. From B draw BD perpendicular on OA produced.

Draw CN perpendicular to OM and draw BL parallel to OM.

Approximate voltage drop

= AN = AD +DN

= I

cos φ + I

sin φ

where φ

= φ

= φ (approx).

This is the value of approximate voltage drop for a lagging power factor.

The different figures for unity and leading power factors are shown in Fig. 32.36 (a) and (b)

respectively.

202

102

KV = V

102

()a





202

102

()b

Fig. 32.36

The approximate voltage drop for leading power factor becomes

cos φ ± I

sin φ)

In general, approximate voltage drop is (I

cos φ ± I

sin φ)

It may be noted that approximate voltage drop as referred to primary is

cos φ ± I

sin φ)

% voltage drop in secondary is=

202 2 02

cos sin

100

IR IX

φ± φ

202 2 02

02 02

100 100

cos sin

IR IX

φ± φ

= v

cos φ ± v

sin φ

where v

202 1 01

02 1

100 100

percentage resistive drop =

IR I R

202 1 01

02 1

100 100

percentage reactive drop =

IX I X

Fig. 32.35





202

102

Transformer

1141

32.17. Exact Voltage Drop

With reference to Fig. 32.35, it is to be noted that exact voltage drop is AM and not AN. If we add the

quantity NM to AN, we will get the exact value of the voltage drop.

Considering the right-angled triangle OCN, we get

= OC

− ON

= (OC + ON) (OC − ON) = (OC + ON) (OM − ON) = 2 OC × NM

∴ NM = NC

/2.OC Now, NC = LC − LN = LC − BD

∴ NC = I

cos φ − I

sin φ ∴ NM =

202 202

(cos sin)

IX IR

φ− φ

∴ For a lagging power factor, exact voltage drop is

= AN + NM = (I

cos φ + I

sin φ) +

202 202

(cos sin)

IX IR

φ− φ

For a leading power factor, the expression becomes

=(I

cos φ − I

sin φ) +

202 202

(cos sin)

IX IR

φ+ φ

In general, the voltage drop is

=(I

cos φ ± I

sin φ) +

202 202

(cos sin)

IX IR

φ± φ

Percentage drop is

202202 202202

( cos sin ) 100 ( cos sin ) 100

φ± φ × φ φ ×

IR IX IX IR

= (v

cos φ ± v

sin φ) + (1/200) (v

cos φ

sin φ)

The upper signs are to be used for a lagging power factor and the lower ones for a leading power

factor.

Example 32.21. A 230/460-V transformer has a primary resistance of 0.2 Ω and reactance

of 0.5 Ω and the corresponding values for the secondary are 0.75 Ω and 1.8 Ω respectively. Find

the secondary terminal voltage when supplying 10 A at 0.8 p.f. lagging.

(Electric. Machines-II, Bangalore Univ. 1991)

Solution. K = 460/230 = 2; R

= R

+ K

= 0.75 + 2

× 0.2 = 1.55 Ω

= X

+ K

= 1.8 + 2

× 0.5 = 3.8 Ω

Voltage drop = I

cos φ + X

sin φ) = 10 (1.55 × 0.8 + 3.8 × 0.6) = 35.2V

∴ Secondary terminal voltage = 460 − 35.2 = 424.8 V

Example 32.22. Calculate the regulation of a transformer in which the percentage resistance

drops is 1.0% and percentage reactance drop is 5.0% when the power factor is (a) 0.8 lagging

(b) unity and (c) 0.8 leading. (Electrical Engineering, Banaras Hindu Univ. 1988)

Soluion. We will use the approximate expression of Art 30.16.

(a) p.f. = cos φ = 0.8 lag µ=v

cos φ + v

sin φ = 1 × 0.8 + 5 × 0.6 = 3.8%

(b) p.f. = cos φ = 1 µ=1 × 1 + 5 × 0 = 1%

− −

− 2.2%

Example 32.23. A transformer has a reactance drop of 5% and a resistance drop of 2.5%. Find

the lagging power factor at which the voltage regulation is maximum and the value of this

regulation. (Elect. Engg. Punjab Univ. 1991)

Solution. The percentage voltage regulation (µ) is given by

1142

Electrical Technology

µ=v

cos φ + v

sin φ

where v

is the percentage resistive drop and v

is the percentage reactive drop.

Differentiating the above equation, we get

= − v

sin φ + v

cos φ

For regulation to be maximum, dµ/dφ = 0 ∴ − v

sin φ + v

cos φ = 0

or tan φ = v

= 5/2.5 = 2 ∴ φ = tan

−1

(2) = 63.5° Now, cos φ = 0.45 and sin φ = 0.892

Maximum percentage regulation = (2.5 × 0.45) + (5 × 0.892) = 5.585

Maximum percentage regulation is 5.585 and occurs at a power factor of 0.45 (lag).

Example 32.24. Calculate the percentage voltage drop for a transformer with a percentage

resistance of 2.5% and a percentage reactance of 5% of rating 500 kVA when it is delivering

400 kVA at 0.8 p.f. lagging. (Elect. Machinery-I, Indore Univ. 1987)

Solution. % drop =

(% ) cos (% ) sin

RI XI

φφ

where I

is the full-load current and I the actual current.

∴ % drop =

(% )

rating rating

XkVAR

RkW

kVA kVA

In the present case, kW = 400 × 0.8 = 320 and kVAR = 400 × 0.6 = 240

∴ % drop =

2.5 320 5 240

500 500

××

32.18. Equivalent Circuit

The transformer shown diagrammatically in Fig. 32.37 (a) can be resolved into an equivalent

circuit in which the resistance and leakage reactance of the transformer are imagined to be external to

the winding whose only function then is to transform the voltage (Fig. 32.37 (b)). The no-load





()a

()b

Ideal

Transform

Fig. 32.37

current I

is simulated by pure inductance X

taking the magnetising component I

and a non-inductive

resistance R

taking the working component I

connected in parallel across the primary circuit.

The value of E

is obtained by subtracting vectorially I

fromV

. The value of X

= E

and of

= E

. It is clear that E

and E

are related to each other by expression

= N

= K.

To make transformer calculations simpler, it is preferable to transfer voltage, current and impedance

Transformer

1143

either to the primary or to the secondary. In that case, we would have to work in one winding only which is

more convenient.

The primary equivalent of the secondary induced voltage is E

′ = E

/K = E

Similarly, primary equivalent of secondary terminal or output voltage is V

′ = V

/K.

Primary equivalent of the secondary current is I

′ = KI

For transferring secondary impedance to primary K

is used.

′ = R

, X

′ = X

, Z

′ = Z

The same relationship is used for shifting an external load impedance to the primary.

The secondary circuit is shown in Fig. 32.38 (a) and its equivalent primary values are shown in Fig.

32.38 (b).

()a

Load

E' =E

V' =V /K

R' =R /K

X' =X /K

Z' =Z /K

()b

Fig. 32.38

The total equivalent circuit of the transformer is obtained by adding in the primary impedance as shown

in Fig. 32.39. This is known as the exact equivalent circuit but it presents a somewhat harder circuit

problem to solve. A simplification can be made by transferring the exciting circuit across the terminals as in

Fig. 32.40 or in Fig. 32.41 (a). It should be noted that in this case X

= V



V

I

R

X

Z

Load



I

R

Z

Load

Fig. 32.39 Fig. 32.40

Further simplification may be achieved by omitting I

altogether as shown in Fig. 32.41(b).

From Fig. 32.39 it is found that total impedance between the input terminal is

Z = Z

+ Z

| | (Z

′ + Z

′) =

()

′+ ′





+′+′



ZZ Z

ZZZ

where Z

′ = R

′ + jX

′ and Z

= impedance of the exciting circuit.

This is so because there are two parallel circuits, one having an impedance of Z

and the other

having Z

′ and Z

′ in series with each other.

∴ V

()

′+ ′





+′+′



ZZ Z

ZZZ

1144

Electrical Technology

V

I

Z

Load

I=I



I=I



Z

Load

Fig. 32.41 (a) Fig. 32.41 (b)

Example 32.25. The parametres of a 2300/230 V, 50-Hz transfomer are given below :

= 0.286 Ω R

′ = 0.319 Ω R

= 250 Ω

= 0.73 Ω X

′ = 0.73 Ω X

= 1250 Ω

The secondary load impedance Z

= 0.387 + j 0.29. Solve the exact equivalent circuit with

normal voltage across the primary.

Solution. K = 230/2300 = 1/10; Z

= 0.387 + j 0.29

′ = Z

= 100 (0.387 + j 0.29) = 38.7 + j 29 = 48.4 ∠36.8°

∴ Z

′ + Z

′ = (38.7 + 0.319) + j(29 + 0.73) = 39.02 + j29.73 = 49.0 ∠ 37.3°

= (0.004 − j 0.0008); Z

= 1/Y

= 240 + j48 = 245 ∠11.3°

+ (Z

′ + Z

′) = (240 + j48) + (39 + j29.7) = 290 ∠15.6°

∴ I

2300 0

()

0.286 0.73 41.4 33

()

∠°



′+ ′

++∠°



′+ ′

ZZ Z

2300 0

54.8 33.7

42 33.7

∠°

=∠−°

∠°

Now I

′ =

245 11.3

54.8 33.7

( ) 290 15.6

∠°

×=∠−°×

′+ ′ + ∠ °

ZZ Z

= 54.8 ∠ − 33.7° × 0.845 ∠− 4.3° = 46.2 ∠ − 38°

()

49 37.3

54.8 33.7

( ) 290 15.6

′+ ′

∠°

×=∠−°×

+′+′ ∠°

ZZZ

=54.8 ∠−33.7° × 0.169 ∠ 21.7° = 9.26 ∠−12°

Input power factor = cos 33.7° = 0.832 lagging

Power input = V

cos φ

= 2300 × 54.8 × 0.832 = 105 kW

Power output = 46.2

× 38.7 = 82.7 kW

Primary Cu loss = 54.8

× 0.286 = 860 W

Secondary Cu loss = 46.2

× 0.319 = 680 W; Core loss = 9.26

× 240 = 20.6 kW

η = (82.7/105) × 100 = 78.8%; V

′ = I

′ Z

′ = 46.2 × 48.4 = 2,240V

∴ Regulation =

2300 2240

100

2240

−

×=

2.7%

Example 32.26. A transformer has a primary winding with a voltage-rating of 600 V. Its

secondary-voltage rating is 1080 V with an additional tap at 720 V. An 8 kW resistive load is

connected across 1080-V output terminals. A purely inductive load of 10kVA is connected across

the tapping point and common secondary terminal so as to get 720 V. Calculate the primary current

and its power-factor. Correlate it with the existing secondary loads. Neglect losses and magnetizing

current. (Nagpur University, Winter 1999)

Transformer

1145

Solution. Loads are connected as shown in Fig. 32.42.

8000

7.41 at unity p.f.

1080

= 10000/720 = 13.89 at zero lagging p.f.

These are reflected on to the primary sides with appropriate ratios of turns, with corresponding power-

factors. If the corresponding transformed currents are represented by the above symbols modified by

dashed superscripts,

I′

= 7.41 × 1080/600 = 13.34 A at unity p.f.

I′

= 13.89 × 720/600 = 16.67 A at zero lag. p.f.

Hence, I

=[I

′

+ I

′

]

0.5

= 21.35 A, at 0.625 lag p.f.

Fig. 32.42

Correlation : Since losses and magnetizing current are ignored, the calculations for primary current

and its power-factor can also be made with data pertaining to the two Loads (in kW/kVAR), as supplied by

the 600 V source.

S = Load to be supplied : 8 kW at unity p.f. and 10 kVAR lagging

Thus, S = P + jQ = 8 − j 10 kVA

S =(8

+ 10

)

0.5

= 12.8 kVA

Power − factor = cos φ = 8/12.8 = 0.625 lag

Primary current = 12.8 × 1000/600 = 21.33 A

32.19. Transformer Tests

As shown in Ex 32.25, the performance of a transformer can be calculated on the basis of its equivalent

circuit which contains (Fig. 32.41) four main parameters, the

equivalent resistance R

as referred to primary (or secondary

), the equivalent leakage reactance X

as referred to

primary (or secondary X

), the core-loss conductance G

(or resistance R

) and the magnetising susceptance B

(or

reactance X

). These constants or parameters can be easily

determined by two tests (i) open-circuit test and (ii) short-

circuit test. These tests are very economical and convenient,

because they furnish the required information without actually

loading the transformer. In fact, the testing of very large a.c.

machinery consists of running two tests similar to the open

and short-circuit tests of a transformer.

Small transformer

1146

Electrical Technology

32.20. Open-circuit or No-load Test

The purpose of this test is to determine

no-load loss or core loss and no-load I

which

is helpful in finding X

and R

One winding of the transformer –

whichever is convenient but usually high

voltage winding – is left open and the other

is connected to its supply of normal voltage

and frequency. A wattmeter W, voltmeter V

and an ammeter A are connected in the low-

voltage winding i.e. primary winding in the

present case. With normal voltage applied to the primary, normal flux will be set up in the core, hence

normal iron losses will occur which are recorded by the wattmeter. As the primary no-load current I

(as

measured by ammeter) is small (usually 2 to 10% of rated load current), Cu loss is negligibly small in primary

and nil in secondary (it being open). Hence, the wattmeter reading represents practically the core loss under

no-load condition (and which is the same for all loads as pointed out in Art. 32.9).

It should be noted that since I

is itself very small, the pressure coils of the wattmeter and the

voltmeter are connected such that the current in them does not pass through the current coil of the

wattmeter.

Sometimes, a high-resistance voltmeter is connected across the secondary. The reading of the voltmeter

gives the induced e.m.f. in the secondary winding. This helps to find transformation ratio K.

The no-load vector diagram is shown in Fig. 32.16. If W is the wattmeter reading (in Fig. 32.43),

then

W = V

cos φ

∴ cos φ

= W/V

∴ I

= I

sin φ

, I

= I

cos φ

∴ X

= V

and R

= V

Or since the current is practically all-exciting current when a transformer is on no-load (i.e. I

≅ I

) and

as the voltage drop in primary leakage impedance is small*, hence the exciting admittance Y

of the trans-

former is given by I

= V

or Y

= I

The exciting conductance G

is given by W = V

or G

= W/V

The exciting susceptance B

()

−YG

Example. 32.27. In no-load test of single-phase transformer, the following test data were

obtained :

Primary voltage : 220 V ; Secondary voltage : 110 V ;

Primary current : 0.5 A ; Power input : 30 W.

Find the following :

(i) The turns ratio (ii) the magnetising component of no-load current (iii) its working (or loss)

component (iv) the iron loss.

Resistance of the primary winding = 0.6 ohm.

Draw the no-load phasor diagram to scale. (Elect. Machine A.M.I.E. 1990)

Solution. (i) Turn ratio, N

= 220/110 = 2

(ii) W = V

cos φ

;cos φ

= 30/220 × 0.5 = 0.273 ; sin φ

= 0.962

= I

sin φ

= 0.5 × 0.962 = 0.48 A

* If it is not negligibly small, then I

= E

i.e. instead of V

we will have to use E

E=V

Low

High

Fig. 32.43

Transformer

1147

(iii) I

= I

cos φ

= 0.5 × 0.273 = 0.1365 A

(iv) Primary Cu loss = I

= 0.5

× 0.6 = 0.15 W

∴ Iron loss = 30 − 0.15 = 29.85 W

Example 32.28. A 5 kVA 200/1000 V, 50 Hz, single-phase transformer gave the following test

results :

O.C. Test (L.V. Side) : 2000 V, 1.2 A, 90 W

S.C. Test (H.V. Side) : 50 V, 5A, 110 W

(i) Calculate the parameters of the equivalent circuit referred to the L.V. side.

(ii) Calculate the output secondary voltage when delivering 3 kW at 0.8 p.f. lagging, the input

primary voltage being 200 V. Find the percentage regulation also.

(Nagpur University, November 1998)

Solution. (i) Shunt branch parameters from O.C. test (L.V. side) :

= V

= 200

/90 = 444 ohms, I

= 200/444 = 0.45 amp

= (1.2

− 0.45

)

0.5

= 1.11 amp, X

= 200/1.11 = 180.2 ohms

All these are referred to L.V. side.

(ii) Series-branch Parameters from S.C test (H.V side) :

Since the S.C. test has been conducted from H.V. side, the parameters will refer to H.V. side.

They should be converted to the parameters referred to L.V. side by transforming them suitably.

From S.C. Test readings, Z = 50/5 = 10 ohms

R = 110/25 = 4.40 ohms, X = (10

− 4.4

)

0.5

= 8.9 ohms

These are referred to H.V. side.

For referring these to L.V. side, transform these using the ratio of turns, as follows :

= 4.40 × (200/1000)

= 0.176 ohm

= 8.98 × (200/1000)

= 0.36 ohm

Equivalent circuit can be drawn with R

and X

calculated above and r

and x

as above.

L.V. Current at rated load= 5000/200 = 25 A

L.V. Current at 3 kW at 0.8 lagging p.f. = (3000/0.80)/200 = 18.75 A

Regulation at this load = 18.75 (r

cos φ + x

sin φ)

= 18.75 (0.176 × 0.80 + 0.36 × 0.6)

= + 6.69 Volts = + (6.69/200) × 100% = + 3.345%

This is referred to L.V. side, and positive sign means voltage drop.

Regulation in volts ref. to H.V. side = 6.69 × 1000/200 = 33.45 V

With 200 V across primary (i.e. L.V. side), the secondary (i.e. H.V. side)

terminal voltage = 1000 − 33.45 = 966.55 V

Note : Since approximate formula for voltage regulation has been used, the procedure is simpler, and faster.

32.21. Separation of Core Losses

The core loss of a transformer depends upon the frequency and the maximum flux density when

the volume and the thickness of the core laminations are given. The core loss is made up of two parts

(i) hysteresis loss W

= PB

1.6

max

f as given by Steinmetz’s empirical relation and (ii) eddy current loss W

= QB

max

where Q is a constant. The total core-loss is given by

= W

+ W

= PB

1.6

max

+ QB

max

If we carry out two experiments using two different frequencies but the same maximum flux

density, we should be able to find the constants P and Q and hence calculate hysteresis and eddy

current losses separately.

1148

Electrical Technology

Example 32.29. In a transformer, the core loss is found to be 52 W at 40 Hz and 90 W at 60 Hz

measured at same peak flux density. Compute the hysteresis and eddy current losses at 50 Hz.

(Elect. Machines, Nagpur Univ. 1993)

Solution. Since the flux density is the same in both cases, we can use the relation

Total core loss W

= Af + Bf

or W

/f = A + Bf

∴ 52/40 = A + 40B and 90/60 = A + 60B ; ∴ A = 0.9 and B = 0.01

At 50 Hz, the two losses are

= A

= 0.9 × 50 = 45 W ; W

= Bf

= 0.01 × 50

= 25 W

Example 32.30. In a power loss test on a 10 kg specimen of sheet steel laminations, the maximum

flux density and waveform factor are maintained constant and the following results were obtained:

Frequency (Hz) 25 40 50 60 80

Total loss (watt) 18.5365066104

Calculate the eddy current loss per kg at a frequency of 50 Hz.

(Elect. Measur. A.M.I.E. Sec B, 1991)

Solution. When flux density and wave form factor remain constant, the expression for iron loss

can be written as

= Af + Bf

or W

/f = A + Bf

The values of W

/ f for different frequencies are as under :

f 25 40 50 60 80

/ f 0.74 0.9 1.0 1.1 1.3

The graph between f and W

/f has been plotted in Fig. 32.44. As seen from it, A = 0.5 and

B = 0.01

∴ Eddy current loss at 50 Hz = Bf

= 0.01 × 50

= 25 W

∴ Eddy current loss/kg = 25/10 = 2.5 W

Example 32.31. In a test for the determination of the losses of a

440-V, 50-Hz transformer, the total iron losses were found to be 2500 W

at normal voltage and frequency. When the applied voltage and

frequency were 220 V and 25 Hz, the iron losses were found to be 850 W.

Calculate the eddy-current loss at normal voltage and frequency.

(Elect. Inst. and Meas. Punjab Univ. 1991)

Solution. The flux density in both cases is the same because in

second case voltage as well as frequency are halved. Flux density remaining the same, the eddy current loss

is proportional to f

and hysteresis loss ∝ f.

Hysteresis loss ∝ f = Af and eddy current loss ∝ f

= Bf

where A and B are constants.

Total iron loss W

= Af + Bf

∴

= A + Bf ...(i)

Now, when f = 50 Hz : W

= 2500 W

and when f = 25 Hz ; W

= 850 W

Using these values in (i) above, we get, from Fig. 32.44

2,500/50 = A + 50 B and 850/25 = A + 25 B ∴ B = 16/25 = 0.64

Hence, at normal p.d. and frequency

eddy current loss = Bf

= 0.64 × 50

= 1600 W

Hystersis loss = 2500 − 1600 = 900 W

Fig. 32.44

Transformer

1149

Example 32.32. When a transformer is connected to a 1000-V, 50-Hz supply the core loss is

1000 W, of which 650 is hysteresis and 350 is eddy current loss. If the applied voltage is raised to

2,000 V and the frequency to 100 Hz, find the new core losses.

Solution. Hysteresis loss W

∝

1.6 1.6

max max

BfPBf

Eddy current loss W

∝ B

max

= QB

max

From the relation E = 4.44 fNB

max

A volt, we get B

max

∝ E/f

Putting this value of B

max

in the above equations, we have

1.6 0.6

PfPEf

−







and W







QfQE

In the first case, E = 1000 V, f = 50 Hz, W

= 650 W, W

= 350 W

∴ 650 = P × 1000

1.6

× 50

− 0.6

∴ P = 650 × 1000

−1.6

× 50

0.6

Similarly, 350 = Q × 1000

∴ Q = 350 × 1000

−2

Hence, constants P and Q are known.

Using them in the second case, we get

= (650 × 1000

−1.6

× 50

0.6

) × 2000

1.6

× 100

−0.6

= 650 × 2 = 1,300 W

= (350 × 1000

−2

) × 2,000

= 350 × 4 = 1,400 W

∴ Core loss under new condition is = 1,300 + 1,400 = 2700 W

Alternative Solution

Here, both voltage and frequency are doubled, leaving the flux density unchanged.

With 1000 V at 50 Hz

= Af or 650 = 50 A ; A = 13

= Bf

or 350 = B × 50

; B = 7/50

With 2000 V at 100 Hz

= Af = 13 × 100 = 1300 W and

= Bf

= (7/50) × 100

= 1400 W

∴ New core loss = 1300 + 1400 = 2700 W

Example 32.33. A transformer with normal voltage impressed has a flux density of 1.4 Wb/m

and a core loss comprising of 1000 W eddy current loss and 3000 W hysteresis loss. What do these

losses become under the following conditions ?

(a) increasing the applied voltage by 10% at rated frequency.

(b) reducing the frequency by 10% with normal voltage impressed.

(Electrical Machinery-I, Madras Univ. 1985)

Solution. As seen from Ex. 32.32

= PE

1.6

−0.6

and W

= QE

From the given data, we have 3000 = PE

1.6

−0.6

...(i)

and 1000 = QE

...(ii)

where E and f are the normal values of primary voltage and frequency.

(a) Here voltage becomes = E + 10% E = 1.1 E

The new hysteresis loss is W

= P (1.1 E)

1.6

−0.6

...(iii)

Dividing Eq. (iii) by (i), we get

3000

=1.1

1.6

; W

= 3000 × 1.165 = 3495 W

1150

Electrical Technology

The new eddy-current loss is

= Q (1.1. E)

∴

1000

= 1.1

∴ W

= 1000 × 1.21 = 1210 W

(b) As seen from Eq. (i) above eddy-current loss would not be effected. The new hysteresis loss is

= PE

1.6

(0.9 f)

−0.6

...(iv)

From (i) and (iv), we get

3000

= 0.9

−0.6

, W

= 3000 × 1.065 = 3,196 W

= P (1.1 E)

1.6

(1.1 f)

−0.6

∴

3000

=1.1

1.6

× 1.1

−0.6

= 1.165 × 0.944

∴ W

= 3000 × 1.165 × 0.944 = 3,299 W

As W

is unaffected by changes in f, its value is the same as found in (a) above i.e. 1210 W

Example 32.34. A transformer is connected to 2200 V, 40 Hz supply. The core-loss is 800 watts

out of which 600 watts are due to hysteresis and the remaining, eddy current losses. Determine the

core-loss if the supply voltage and frequency are 3300 V and 60 Hz respectively.

(Bharathiar Univ. Nov. 1997)

Solution. For constant flux density (i.e. constant V/f ratio), which is fulfilled by 2200/40 or

3300/60 figures in two cases,

Core-loss = A f + B f

First term on the right-hand side represents hysteresis-loss and the second term represents the

eddy-current loss.

At 40 Hz, 800 = 600 + eddy current loss.

Thus, A f = 600, or A = 15

B f

= 200, or B = 200/1600 = 0.125

At 60 Hz, core-loss = 15 × 60 + 0.125 × 60

= 900 + 450

= 1350 watts

32.22 Short-Circuit or Impedance Test

This is an economical method for determining the following :

(i) Equivalent impedance (Z

or Z

leakage reactance (X

or X

) and

total resistance (R

or R

) of the transformer

as referred to the winding in which the mea-

suring instruments are placed.

(ii) Cu loss at full load (and at any de-

sired load). This loss is used in calculating

the efficiency of the transformer.

(iii) Knowing Z

or Z

, the total

voltage drop in the transformer as referred

to primary or secondary can be calculated and hence regulation of the transformer determined.

In this test, one winding, usually the low-voltage winding, is solidly short-circuited by a thick conductor

(or through an ammeter which may serve the additional purpose of indicating rated load current) as shown

in Fig. 32.45.

Fig. 32.45

L.V.

Supply

Low

High

Short Circuit

Transformer

1151

V' =0

E=E'

I=I



V' =0

I=I'

Short Circuit

Fig. 32.46

A low voltage (usually 5 to 10% of normal primary voltage) at correct frequency (though for Cu losses

it is not essential) is applied to the primary and is cautiously increased till full-load currents are flowing both

in primary and secondary (as indicated by the respective ammeters).

Since, in this test, the applied voltage is a small percentage of the normal voltage, the mutual flux Φ

produced is also a small percentage of its normal value (Art. 32.6). Hence, core losses are very small with

the result that the wattmeter reading represent the full-load Cu loss or I

R loss for the whole transformer i.e.

both primary Cu loss and secondary Cu loss. The equivalent circuit of the transformer under short-circuit

condition is shown in Fig. 32.46. If V

is the voltage required to circulate rated load currents, then Z

Also W = I

∴ R

= W/I

∴ X

()

01 01

−

In Fig. 32.47 (a) the equivalent circuit

vector diagram for the short-circuit test is

shown. This diagram is the same as shown

in Fig. 32.34 except that all the quantities

are referred to the primary side. It is obvious

that the entire voltage V

is consumed in

the impedance drop of the two windings.

If R

can be measured, then knowing

, we can find R

′ = R

− R

. The impedance triangle can then be divided into the appropriate

equivalent triangles for primary and secondary as shown in Fig. 32.47 (b).

32.23. Why Transformer Rating in kVA ?

As seen, Cu loss of a transformer depends on current and iron loss on voltage. Hence, total

transformer loss depends on volt-ampere (VA) and not on phase angle between voltage and current

i.e. it is independent of load power factor. That is why rating of transformers is in kVA and not in kW.

Example 32.35. The primary and secondary windings of a 30 kVA 76000/230, V, 1-phase

transformer have resistance of 10 ohm and 0.016 ohm respectively. The reactance of the transformer

referred to the primary is 34 ohm. Calculate the primary voltage required to circulate full-load

current when the secondary is short-circuited. What is the power factor on short circuit ?

(Elect. Machines AMIE Sec. B 1991)

Solution. K = 230/6000 = 23/600, X

= 34 Ω,

= R

+ R

= 10 + 0.016 (600/23)

= 20.9 Ω

SC 1 01



()a

R

X

Z

()b

Fig. 32.47

1152

Electrical Technology

22 22

01 01

20.9 34 40

+= +=Ω

F.L., I

= 30,000/6000 = 5 A ; V

= I

= 5 × 40 = 200 V

Short circuit p.f. = R

= 20.9/40 = 0.52

Example 32.36. Obtain the equivalent circuit of a 200/400-V, 50-Hz, 1-phase transformer from

the following test data :

O.C test : 200 V, 0.7 A, 70 W – on L.V. side

S.C. test : 15 V, 10 A, 85 W – on H.V. side

Calculate the secondary voltage when delivering 5 kW at 0.8 p.f. lagging, the primary voltage

being 200V. (Electrical Machinery-I, Madras Univ. 1987)

Solution. From O.C. Test

cos φ

= W

∴ 200 × 0.7 × cos φ

=70

cos φ

= 0.5 and sin φ

= 0.866

= I

cos φ

= 0.7 × 0.5 = 0.35 A

= I

sin φ

= 0.7 × 0.866 = 0.606 A

= V

= 200/0.35 = 571.4 Ω

= V

= 200/0.606 = 330 Ω

As shown in Fig. 32.48, these values refer to primary

i.e. low-voltage side.

From S.C. Test

It may be noted that in this test, instruments have been placed in the secondary i.e. high-voltage

winding whereas the low-voltage winding i.e. primary has been short-circuited.

Now, as shown in Art. 32.32

= V

= 15/10 = 1.5 Ω ; K = 400/200 = 2

= Z

= 1.5/4 = 0.375 Ω

Also I

= W ; R

= 85/100 = 0.85 Ω

= R

= 0.85/4 = 0.21 Ω

22 2 2

01 01

0.375 0.21 0.31

−= − = Ω

Output kVA = 5/0.8 ; Output current I

= 5000/0.8 × 400 = 15.6 A

This value of I

is approximate because V

(which is to be calculated as yet) has been taken equal

to 400 V (which, in fact, is equal to E

Now, Z

=1.5 Ω, R

= 0.85 Ω∴X

1.50.851.24

−=Ω

Total transformer drop as referred to secondary

= I

cos φ

+ X

sin φ

) = 15.6 (0.85 × 0.8 + 1.24 × 0.6) = 22.2 V

∴ V

= 400 − 22.2 = 377.8 V

Example 32.37. Starting from the ideal transformer, obtain the approximate equivalent circuit

of a commercial transformer in which all the constants are lumped and represented on one side.

A 1-phase transformer has a turn ratio of 6. The resistance and reactance of primary winding

are 0.9 Ω and 5 Ω respecitvely and those of the secondary are 0.03 Ω and 0.13 Ω respectively. If

330 -V at 50-Hz be applied to the high voltage winding with the low-voltage winding short-

circuited, find the current in the low-voltage winding and its power factor. Neglect magnetising

current.

Fig. 32.48

Transformer

1153

Solution. Here K = 1/6 ; R

= R

+ R′

= 0.9 + (0.03 × 36) = 1.98 Ω

= X

+ X

′ = 5 + (0.13 × 36) = 9.68 Ω

∴ Z

()

9.68 1.98

= 9.9 Ω; V

= 330 V

∴ Full-load primary current I

= V

= 330/0.9 = 100/3 A

As I

is negligible, hence I

= I′

= 100/3 A. Now, I′

= KI

F.L. secondary current I

= I′

K = (100/3) × 6 = 200 A

Now, Power input on short-circuit = V

cos φ

= Cu loss = I

∴ (100/3)

× 1.98 = 330 × (100/3) × cos φ

; cos φ

= 0.2

Example 32.38. A 1-phase, 10-kVA, 500/250-V, 50-Hz transformer has the following constants:

Reactance : primary 0.2 Ω; secondary 0.5 Ω

Resistance : primary 0.4 Ω; secondary 0.1 Ω

Resistance of equivalent exciting circuit referred to primary, R

= 1500 Ω

Reactance of equivalent exciting circuit referred to primary, X

= 750 Ω

What would be the reading of the instruments when the transformer is connected for the open-

circuit and short-circuit tests ?

Solution. While solving this question, reference may please be made to Art. 30.20 and 30.22.

O.C. Test

= V

= 500/750 = 2/3 A ; I

= V

= 500/1500 = 1/3 A

∴ I

1/3) (2/3)





= 0.745 A

No-load primary input = V

= 500 × 1/3 = 167 W

Instruments used in primary circuit are : voltmeter, ammeter and wattmeter, their readings being

500 V, 0.745 A and 167 W respectively.

S.C. Test

Suppose S.C. test is performed by short-circuiting the l.v. winding i.e. the secondary so that all

instruments are in primary.

= R

+ R′

= R

+ R

; Here K = 1/2 ∴ R

= 0.2 + (4 × 0.5) = 2.2 Ω

Similarly, X

= X

+ X′

= 0.4 + (4 × 0.1) = 0.8 Ω

(2.2 0.8 )

= 2.341 Ω

Full-load primary current

= 10,000/500 = 20 A ∴ V

= I

= 20 × 2.341 = 46.8 V

Power absorbed = I

= 20

× 2.2 = 880 W

Primary instruments will read : 46.8 V, 20 A, 880 W.

Example 32.39. The efficiency of a 1000-kVA, 110/220 V, 50-Hz, single-phase transformer, is

98.5 % at half full-load at 0.8 p.f. leading and 98.8 % at full-load unity p.f. Determine (i) iron loss

(ii) full-load copper loss and (iii) maximum efficiency at unity p.f.

(Elect. Engg. AMIETE Sec. A Dec. 1991)

Solution. Output at F.L. unity p.f. = 1000 × 1 = 1000 kW

F.L. input = 1000/0.988 = 1012.146 kW

F.L. lossses = 1012.146 − 1000 = 12.146 kW

If F.L. Cu and iron losses are x and y respectively then

x + y = 12.146 kW ...(i)

1154

Electrical Technology

Input at half F.L. 0.8 p.f. = 500 × 0.8/0.985 = 406.091 kW

Total losses at half F.L. = 406.091 − 400 = 6.091 kW

Cu loss at half-load = x (1/2)

= x/4

∴ x/4 + y = 6.091 ...(ii)

From Eqn. (i) and (ii), we get (i) x = 8.073 kW and (ii) y = 4.073 kW

(iii) kVA for η

max

= 1000 ×

4.073/8.073

= 710.3 kVA

Output at u.p.f. = 710.3 × 1 = 710.3 kW

Cu loss = iron loss = 4.037 kW ; Total loss = 2 × 4.037 = 8.074 kW

∴η

max

= 710.3/(710.3 + 8.074) = 0.989 or 98.9 %

Example 32.40. The equivalent circuit for a 200/400-V step-up transformer has the following

parameters referred to the low-voltage side.

Equivalent resistance = 0.15 Ω ; Equivalent reactance = 0.37 Ω

Core-loss component resistance = 600 Ω ; Magnetising reactance = 300 Ω

When the transformer is supplying a load at 10 A at a power factor of 0.8 lag, calculate (i) the

primary current (ii) secondary terminal voltage. (Electrical Machinery-I, Bangalore Univ. 1989)

Solution. We are given the following :

= 0.15 Ω, X

= 0.37 Ω ; R

= 600 Ω, X

= 300 Ω

Using the approximate equivalent circuit of Fig. 32.41, we have,

= V

= 200/300 = (2/3) A

= V

= 200/600 = (1/3) A

22 2 2

(2/ 3) (1/3)

+= +

= 0.745 A

As seen from Fig. 32.49

tan θ =

1/3 1

2/3 2

; θ = 26.6º

∴φ

= 90º − 26.6º = 63.4º ; Angle between I

and

I′

= 63.4º − 36.9º = 26.5º; K = 400/200 = 2

I′

= KI

= 2 × 10 = 20 A

(i) I

= (0.745

+ 20

+ 2 × 0.745 × 20 × cos 26.5º)

1/2

= 20.67 A

(ii) R

= K

= 2

× 0.15 = 0.6 Ω

= 22 × 0.37 = 1.48 Ω

Approximate voltage drop

= I

cos φ + X

sin φ)

= 10 (0.6 × 0.8 + 1.48 × 0.6) = 13.7 V

∴ Secondary terminal voltage = 400 − 13.7

= 386.3 V

Example 32.41. The low voltage winding of a 300-kVA, 11,000/2500-V, 50-Hz transformer has

190 turns and a resistance of 0.06. The high-voltage winding has 910 turns and a resistance of

1.6

Ω

. When the l.v. winding is short-circuited, the full-load current is obtained with 550-V applied

to the h.v. winding. Calculate (i) the equivalent resistance and leakage reactance as referred to h.v.

side and (ii) the leakage reactance of each winding.

Solution. Assuming a full-load efficiency of 0.985, the full-load primary current is

36.9

63.4

36.9







10A



Fig. 32.49

Transformer

1155

= 300,000/0.985 × 11,000 = 27.7A

∴ Z

= 550/27.7 = 19.8 Ω ; R′

= R

= 0.06 (910/190)

= 1.38 Ω

∴ R

= R

+ R′

= 1.6 + 1.38 = 2.98 Ω

()

22 2 2

01 01

(19.8 2.98 )

−= −

= 19.5 Ω

Let us make another assumption that for each winding the ratio (reactance/resistance) is the same, then

= 19.5 × 1.6/2.98 = 10.5 Ω

X ′

= 19.5 × 1.38/2.98 = 9.0 Ω ; X

= 9(190/910)

= 0.39 Ω

(a) R

= 2.98

ΩΩ

Ω ; X

= 19.5

ΩΩ

Ω (b) X

= 10.5

ΩΩ

Ω : X

= 0.39

ΩΩ

Ω

Example 32.42. A 230/115 volts, single phase transformer is supplying a load of 5 Amps, at

power factor 0.866 lagging. The no-load current is 0.2 Amps at power factor 0.208 lagging. Calcu-

late the primary current and primary power factor.

(Nagpur University Summer 2000)

Solution. L.V. current of 5 amp is referred to as a

2.5 amp current on the primary (= H.V.) side, at 0.866

lagging p.f. To this, the no load current should be added,

as per the phasor diagram in Fig. 32.50. The phase

angle of the load-current is 30º lagging. The no load

current has a phase angle of 80º lagging. Resultant of

these two currents has to be worked out. Along the

reference, active components are added.

Active components of currents =

2.5 × 0.866 + 0.2 × 0.208

= 2.165 + 0.0416

= 2.2066 amp

Along the perpendicular direction, the reactive components get added up.

Reactive component = 2.5 × 0.5 + 0.2 × 0.9848

= 1.25 + 0.197 = 1.447 amp.

= 2.2066 − j 1.447

φ =tan

−1

1.447

2.2066

= 33.25º ; as shown

Tutorial Problems 32.3

1. The S.C. test on a 1-phase transformer, with the primary winding short-circuited and 30 V applied to

the secondary gave a wattmeter reading of 60 W and secondary current of 10 A. If the normal applied

primary voltage is 200, the transformation ratio 1 :2 and the full-load secondary current 10 A, calculate

the secondary terminal p.d. at full-load current for (a) unity power factor (b) power factor 0.8 lagging.

If any approximations are made, they must be explained. [394 V, 377.6 V]

2. A single-phase transformer has a turn ratio of 6, the resistances of the primary and secondary windings

are 0.9 Ω and 0.025 Ω respectively and the leakage reactances of these windings are 5.4 Ω and 0.15

Ω respectively. Determine the voltage to be applied to the low-voltage winding to obtain a current of

100 A in the short-circuited high voltage winding. Ignore the magnetising current. [82 V]

3. Draw the equivalent circuit for a 3000/400-V, I-phase transformer on which the following test results

were obtained. Input to high voltage winding when 1.v. winding is open-circuited : 3000 V, 0.5 A, 500

Fig. 32.50. Phasor diagram for Currents

30°



Ref

2.5 A

0.2 A

80°

1156

Electrical Technology

W. Input to 1.v. winding when h.v. winding is short-circuited : 11 V, 100 A, 500 W. Insert the

appropriate values of resistance and reactance.

= 18,000

ΩΩ

Ω, X

= 6,360

ΩΩ

Ω, R

= 2.81

ΩΩ

Ω, X

= 5.51

ΩΩ

Ω] (I.E.E. London)

4. The iron loss in a transformer core at normal flux density was measured at frequencies of 30 and 50

Hz, the results being 30 W and 54 W respectively. Calculate (a) the hysteresis loss and (b) the eddy

current loss at 50 Hz. [44 W, 10 W]

5. An iron core was magnetised by passing an alternating current through a winding on it. The power

required for a certain value of maximum flux density was measured at a number of different frequencies.

Neglecting the effect of resistance of the winding, the power required per kg of iron was 0.8 W at 25

Hz and 2.04 W at 60 Hz. Estimate the power needed per kg when the iron is subject to the same

maximum flux density but the frequency is 100 Hz. [3.63 W]

6. The ratio of turns of a 1-phase transformer is 8, the resistances of the primary and secondary

windings are 0.85 Ω and 0.012 Ω respectively and leakage reactances of these windings are 4.8 Ω and

0.07 Ω respectively. Determine the voltage to be applied to the primary to obtain a current of 150 A

in the secondary circuit when the secondary terminals are short-circuited. Ignore the magnetising

current. [176.4 W]

7. A transformer has no-load losses of 55 W with a primary voltage of 250 V at 50 Hz and 41 W with

a primary voltage of 200 V at 40 Hz. Compute the hysteresis and eddy current losses at a primary

voltage of 300 volts at 60 Hz of the above transformer. Neglect small amount of copper loss at no-

load. [43.5 W ; 27 W] (Elect. Machines AMIE Sec. B. (E-3) Summer 1992)

8. A 20 kVA, 2500/250 V, 50 Hz, 1-phase transformer has the following test results :

O.C. Test (1.v. side) : 250 V, 1.4 A, 105 W

S.C. Test (h.v. side) : 104 V, 8 A, 320 W

Compute the parameters of the approximate equivalent circuit referred to the low voltage side and

draw the circuit. (R

= 592.5

ΩΩ

Ω ; X

= 187.2

ΩΩ

Ω ; R

= 1.25

ΩΩ

Ω ; X

= 3

ΩΩ

Ω)

(Elect. Machines A.M.I.E. Sec. B Summer 1990)

9. A 10-kVA, 2000/400-V, single-phase transformer has resistances and leakage reactances as follows :

= 5.2 Ω, X

= 12.5 Ω, R

= 0.2 Ω, X

= 0.5 Ω

Determine the value of secondary terminal voltage when the transformer is operating with rated

primary voltage with the secondary current at its rated value with power factor 0.8 lag. The no-load

current can be neglected. Draw the phasor diagram. [376.8 V] (Elect. Machines, A.M.I.E. Sec B, 1989)

10. A 1000-V, 50-Hz supply to a transformer results in 650 W hysteresis loss and 400 W eddy current

loss. If both the applied voltage and frequency are doubled, find the new core losses.

= 1300 W ; W

= 1600 W] (Elect. Machine, A.M.I.E. Sec. B, 1993)

11. A 50 kVA, 2200/110 V transformer when tested gave the following results :

O.C. test (L.V. side) : 400 W, 10 A, 110 V.

S.C. test (H.V. side) : 808 W, 20.5 A, 90 V.

Compute all the parameters of the equivalent ckt. referred to the H.V. side and draw the resultant ckt.

(Rajiv Gandhi Technical University, Bhopal 2000)

[Shunt branch : R

= 12.1 k-ohms, X

= 4.724 k-ohms Series branch : r = 1.923 ohms, x = 4.39 ohms]

32.24. Regulation of a Transformer

1. When a transformer is loaded with a constant primary voltage, the secondary voltage

decreases* because of its internal resistance and leakage reactance.

Let

= secondary terminal voltage at no-load.

* Assuming lagging power factor. It will increase if power factor is leading.

Transformer

1157

= E

= EK

= KV

because at no-load the impedance drop is negligible.

= secondary terminal voltage on full-load.

The change in secondary terminal voltage from no-load to full-load is =

− V

. This change divided

is known as regulation ‘down’. If this change is divided by V

, i.e., full-load secondary terminal

voltage, then it is called regulation ‘up’.

∴ % regn ‘down’ =

02 2

100

−

and % regn ‘up’ =

02 2

100

−

In further treatment, unless stated otherwise, regulation is to be taken as regulation ‘down’.

We have already seen in Art. 32.16 (Fig. 32.35) that the change in secondary terminal voltage from no-

load to full-load, expressed as a percentage of no-load secondary voltage is,

= ν

cos φ ± ν

sin φ (approximately)

Or more accurately

=(ν

cos φ ± ν

sin φ) +

200

(ν

cos φ " ν

sin φ)

∴ % regn = ν

cos φ ± ν

sin φ ...approximately.

The lesser this value, the better the transformer, because a good transformer should keep its

secondary terminal voltage as constant as possible under all conditions of load.

(2) The regulation may also be explained in terms of primary values.

In Fig. 32.51 (a) the approximate equivalent circuit of a transformer is shown and in Fig. 32.51 (b), (c)

and (d) the vector diagrams corresponding to different power factors are shown.

The secondary no-load terminal voltage as referred to primary is E′

= E

/K = E

= V

and if the

secondary full-load voltage as referred to primary is V ′

(= V

/K) then

% regn =

100

′

−

Fig. 32.51

From the vector diagram, it is clear that if angle between V

and V ′

is neglected, then the value of

numerical difference V

− V ′

is given by (I

cos φ + I

sin φ) for lagging p.f.

∴ % regn =

101 1 01

cos sin

100

IR IX

φ+ φ

= ν

cos φ + ν

sin φ

where

101

100

= ν

and

101

100

= ν

...Art. 32.16

As before, if angle between V

and V

′ is not negligible, then

% regn = (ν

cos φ ± ν

sin φ) +

200

(ν

cos φ " ν

sin φ)

(3) In the above definitions of regulation, primary voltage was supposed to be kept constant and

the changes in secondary terminal voltage were considered.

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Electrical Technology

As the transformer is loaded, the secondary terminal voltage falls (for a lagging p.f.). Hence, to keep

the output voltage constant, the primary voltage must be increased. The rise in primary voltage required to

maintain rated output voltage from no-load to full-load at a given power factor expressed as percentage of

rated primary voltage gives the regulation of the transformer.

Suppose primary voltage has to be raised from its rated value V

to V

′, then

% regn. =

100

′

−

Example 32.43. A-100 kVA transformer has 400 turns on the primary and 80 turns on the

secondary. The primary and secondary resistances are 0.3

Ω

and 0.01

Ω

respectively and the

corresponding leakage reactances are 1.1 and 0.035

Ω

respectively. The supply voltage is 2200 V.

Calculate (i) equivalent impedance referred to primary and (ii) the voltage regulation and the sec-

ondary terminal voltage for full load having a power factor of 0.8 leading.

(Elect. Machines, A.M.I.E. Sec. B, 1989)

Solution. K = 80/400 = 1/5, R

= 0.3 Ω, R

= R

+ R

= 0.3 + 0.01/(1/5)

= 0.55 Ω

= X

+ X

= 1.1 + 0.035/(1/5)

= 1.975 Ω

(i) Z

= 0.55 + j 1.975 = 2.05 ∠74.44º

(ii) Z

= K

= (1/5)

(0.55 + j 1.975) = (0.022 + j 0.079)

No-load secondary voltage = KV

= (1/5) × 2200 = 440 V, I

= 10 × 10

/440 = 227.3 A

Full-load voltage drop as referred to secondary

= I

cos φ − X

sin φ)

= 227.3 (0.022 × 0.8 − 0.079 × 0.6 ) = − 6.77 V

% regn. = − 6.77 × 100/440 = − 1.54

Secondary terminal voltage on load = 440 − (− 6.77) = 446.77 V

Example 32.44. The corrected instrument readings obtained from open and short-circuit tests

on 10-kVA, 450/120-V, 50-Hz transformer are :

O.C. test : V

= 120 V; I

= 4.2 A; W

= 80 W; V

, W

and I

were read on the low-voltage side.

S.C. test : V

= 9.65 V; I

= 22.2 A ; W

= 120 W − with low-voltage winding short-circuited

Compute :

(i) the equivalent circuit (approximate) constants,

(ii) efficiency and voltage regulation for an 80% lagging p.f. load,

(iii) the efficiency at half full-load and 80% lagging p.f. load.

(Electrical Engineering-I, Bombay Univ. 1988)

Soluion. It is seen from the O.C. test, that with primary open, the secondary draws a no-load current

of 4.2 A. Since K = 120/450 = 4/15, the corresponding no-load primary current I

= 4.2 × 4/15 = 1.12 A.

(i) Now, V

cos φ

=80 ∴ cos φ

= 80/450 × 1.12 = 0.159

∴φ

= cos

−1

(0.159) = 80.9°; sin φ

= 0.987

= I

cos φ

= 1.12 × 0.159 = 0.178A and I

= 1.12 × 0.987 = 1.1 A

∴ R

= 450/0.178 = 2530

Ω Ω

Ω and X

= 450/1.1 = 409

ΩΩ

Ω

During S.C. test, instruments have been placed in primary.

∴ Z

= 9.65/22.2 = 0.435 Ω

= 120/22.2

= 0.243 Ω

0.435 0.243 0.361

−=Ω

The equivalent circuit is shown in Fig. 32.52.

(ii) Total approximate voltage drop as referred to primary is I

cos φ + X

sin φ).

* Assuming φ

= φ

= cos

−1

(0.8).

Transformer

1159

Fig. 32.52

Now, full-load I

= 10,000/450 = 22.2 A

∴ Drop = 22.2 (0.243 × 0.8 + 0.361 × 0.6) = 9.2 V

Regulation = 9.2 × 100/450 = 2.04%

F.L. losses = 80 + 120 = 200 W;

F.L. output = 10,000 × 0.8 = 8000 W

η = 8000/8200 = 0.9757 or 97.57%

(iii) Half-load

Iron loss = 80 W; Cu loss = (1/2)

× 120 = 30 W

Total losses = 110 W; Output = 5000 × 0.8 = 4000 W

∴η= 4000/4110 = 0.9734 or 97.34%

Example 32.45. Consider a 20 kVA, 2200/220 V, 50 Hz transformer. The O.C./S.C. test results

are as follows :

O.C. test : 220 V, 4.2 A, 148 W (1.v. side)

S.C. test : 86 V, 10.5 A, 360 W (h.v. side)

Determine the regulation at 0.8 p.f. lagging and at full load. What is the p.f. on short-circuit ?

(Elect. Machines Nagpur Univ. 1993)

Solution. It may be noted that O.C. data is not required in this question for finding the regulation.

Since during S.C. test instruments have been placed on the h.v. side i.e. primary side.

∴ Z

= 86/10.5 = 8.19 Ω; R

= 360/10.5

= 3.26 Ω

8.19 3.26 7.5

−=Ω

F.L. primary current, I

= 20,000/2200 = 9.09 A

Total voltage drop as referred to primary = I

cos φ + X

sin φ)

Drop = 9.09 (3.26 × 0.8 + 7.5 × 0.6) = 64.6 V

% age regn. = 64.6 × 100/2200 = 2.9%, p.f. on short-circuit = R

= 3.26/8.19 = 0.4 lag

Example 32.46. A short-circuit test when performed on the h.v. side of a 10 kVA, 2000/400 V

single phase transformer, gave the following data ; 60 V, 4 A, 100 W.

If the 1.v. side is delivering full load current at 0.8 p.f. lag and at 400 V, find the voltage applied

to h.v. side. (Elect. Machines-I, Nagpur Univ. 1993)

Solution. Here, the test has been performed on the h.v. side i.e. primary side.

= 60/4 = 15 Ω ; R

= 100/4

= 6.25 Ω ; X

15 6.25

−

= 13.63 Ω

F. L . I

= 10,000/2000 = 5A

Total transformer voltage drop as referred to primary is

cos φ + X

sin φ) = 5 (6.25 × 0.8 + 13.63 × 0.6) = 67 V

Hence, primary voltage has to be raised from 2000 V to 2067 V in order to compensate for the total

voltage drop in the transformer. In that case secondary voltage on load would remain the same as on

no-load.

Example 32.47. A 250/500-V transformer gave the following test results :

Short-circuit test : with low-voltage winding short-circuited :

20 V; 12 A, 100 W

Open-circuit test : 250 V, 1 A, 80 W on low-voltage side.

Determine the circuit constants, insert these on the equivalent circuit diagram and calculate

applied voltage and efficiency when the output is 10 A at 500 volt and 0.8 power factor lagging.

(Elect. Machines, Nagpur Univ. 1993)

1160

Electrical Technology

Solution. Open-circuit Test :

cos φ

=80 ∴ cos φ

= 80/250 × I = 0.32

= I

cos φ

= I × 0.32 = 0.32 A, I

(1 0 . 32 ) 0 . 95

−=

= V

= 250/0.32 = 781.3 Ω, X

= V

/ I

= 250/0.95 = 263.8 Ω

The circuit is shown in Fig. 32.53 (a).

Short-circuit Test :

As the primary is short-circuited, all values refer to secondary winding.

Fig. 32.53 (a) Fig. 32.53 (b)

∴ R

short-circuit power

100

F.L. secondary current

= 0.694 Ω

= 20/12 = 1.667 Ω ; X

(1.667 0.694 )

−

= 1.518 Ω

As R

and X

refer to primary, hence we will transfer these values to primary with the help of

transformation ratio.

K = 500/250 = 2 ∴ R

= R

= 0.694/4 = 0.174 Ω

= X

= 1.518/4 = 0.38 Ω ; Z

= Z

= 1.667/4 = 0.417 Ω

The equivalent circuit is shown in Fig. 32.53 (a).

Efficiency

Total Cu loss = I

= 100 × 0.694 = 69.4 W ; Iron loss = 80 W

Total loss = 69.4 + 80 = 149.4 W ∴η =

5000 0.8 100

4000 149.4

××

= 96.42%

The applied voltage V

′ is the vector sum of V

and I

as shown in Fig. 32.53 (b).

= 20 A ; I

= 20 × 0.174 = 3.84 V ; I

= 20 × 0.38 = 7.6 V

Neglecting the angle between V

and V

′, we have

′

= OC

= ON

+ NC

= (OM + MN)

+ (NB + BC)

= (250 × 0.8 + 3.48)

+ (250 × 0.6 + 7.6)

′

= 203.5

+ 157.6

∴ V

′ = 257.4 V

Example 32.48. A 230/230 V, 3 kVA transformer gave the following results :

O.C. Test : 230 V, 2 amp, 100 W

S.C. Test : 15 V, 13 amp, 120 W

Determine the regulation and efficiency at full load 0.80 p.f. lagging.

(Sambalpur University, 1998)

Solution. This is the case of a transformer with turns ratio as 1 : 1. Such a transformer is mainly

required for isolation.

Transformer

1161

Rated Current =

3000

230

= 13 amp

Cu-losses at rated load = 120 watts, from S.C. test

Core losses = 100 watts, from O.C. test

At full load, VA output = 3000

At 0.8 lag p.f., Power output = 3000 × 0.8 = 2400 watts

Required efficiency =

2400

100%

2400 220

= 91.6%

From S.C. test, Z =

= 1.154 ohms

R =

120

15 15

= 0.53 ohm, X =

1.154 0.53

−

= 1.0251 ohm

Approximate voltage regulation

= IR cos φ + IX sin φ = 13[0.53 × 0.8 + 1.0251 × 0.6]

= 13[0.424 + 0.615] = 13.51 volts

In terms of %, the voltage regulation =

13.51

100%

230

= 5.874%

Example 32.49. A 10 kVA, 500/250 V, single-phase transformer has its maximum efficiency of

94% when delivering 90% of its rated output at unity p.f. Estimate its efficiency when delivering its

full-load output at p.f. of 0.8 lagging. (Nagpur University, November 1998)

Solution. Rated output at unity p.f. = 10000 W. Hence, 90% of rated output = 9,000 W

Input with 94% efficiency = 9000/0.94 W

Losses = 9000((1/0.94) − 1) = 574 W

At maximum efficiency, variable copper-loss = constant = Core loss = 574/2 = 287 W

At rated current, Let the copper-loss = P

watts

At 90% load with unity p.f., the copper-loss is expressed as 0.90

× P

Hence, P

= 287/0.81 = 354 W

(b) Output at full-load, 0.8 lag p.f. = 10,000 × 0.80 = 8000 W

At the corresponding load, Full Load copper-loss = 354 W

Hence, efficiency = 8000/(8000 + 354 + 287) = 0.926 = 92.6%

Example 32.50. Resistances and Leakage reactance of 10 kVA, 50 Hz, 2300/230 V single phase

distribution transformer are r

= 3.96 ohms, r

= 0.0396 ohms, x

= 15.8 ohms, x

= 0.158 ohm.

Subscript 1 refers to HV and 2 to LV winding (a) transformer delivers rated kVA at 0.8 p.f. Lagging

to a load on the L.V. side. Find the H.V. side voltage necessary to maintain 230 V across Load-

terminals. Also find percentage voltage regulation. (b) Find the power-factor of the rated load-

current at which the voltage regulation will be zero, hence find the H.V. side voltage.

(Nagpur University, November 1997)

Solution. (a) Rated current on L.V. side = 10,000/230 = 43.5 A. Let the total resistance and total

leakage reactance be referred to L.V. side. Finally, the required H.V. side voltage can be worked out

after transformation.

Total resistance, r = r

′ + r

= 3.96 × (230/2300)

+ 0.0396

= 0.0792 ohms

Total leakage-reactance, x = x

′ + x

= 15.8 × (230/2300)

+ 0.158

= 0.316 ohm

1162

Electrical Technology

For purpose of calculation of voltage-magnitudes, approximate formula for voltage regulation can be

used. For the present case of 0.8 lagging p.f.

′ = V

+ I [r cos φ + x sin φ]

= 230 + 43.5 [(0.0792 × 0.8) + (0.316 × 0.6)]

= 230 + 43.5 [0.0634 + 0.1896] = 230 + 11 = 241 volts

Hence, V

= 241 × (2300/230) = 2410 volts.

It means that H.V. side terminal voltage must be 2410 for keeping 230 V at the specified load.

(b) Approximate formula for voltage regulation is : V

′ − V

= I [r cos φ ± x sin φ]

With Lagging p.f., +ve sign is retained. With leading power-factor, the −ve sign is applicable. For the

voltage-regulation to be zero, only leading P.f. condition can prevail.

Thus, r cos φ − x sin φ =0

or tan φ = r/x = 0.0792/0.316 = 0.25

or φ = 14º, cos φ = 0.97 leading

Corresponding sin φ =sin 14º = 0.243

H.V. terminal voltage required is 2300 V to maintain 230 V at Load, since Zero regulation condition is

under discussion.

Example 32.51. A 5 kVA, 2200/220 V, single-phase transformer has the following parameters.

H.V. side : r

= 3.4 ohms, x

= 7.2 ohms

L.V. side : r

= 0.028 ohms, x

= 0.060 ohms

Transformer is made to deliver rated current at 0.8 lagging P.f. to a load connected on the L.V.

side. If the load voltage is 220 V, calculate the terminal voltage on H.V. side

(Neglect the exciting current). (Rajiv Gandhi Technical University, Bhopal, Summer 2001)

Solution. Calculations may be done referring all the parameters the L.V. side first. Finally, the

voltage required on H.V. side can be obtained after transformation.

Rated current ref. to L.V. side = 5000/220 = 22.73 A

Total winding resistance ref. to L.V. side = r

′ + r

= (220/2200)

× 3.4 + 0.028

Total winding-leakage-reactance ref. to L.V. side = x

′ + x

= (220/2200)

× 7.2 + 0.060 = 0.132 ohm





Fig. 32.53(c)

In the phasor diagram of Fig. 32.53 (c).

QA = V

= 220 volts, I = 22.73 A at lagging phase angle of 36.87º

AB = Ir, AD = Ir cos φ = 22.73 × 0.062 × 0.80 = 1.127 V

DC = Ix sin φ = 22.73 × 0.132 × 0.60 = 1.80 V

Transformer

1163

OC = 220 + 1.127 + 1.80 = 222.93 volts

BD = Ir sin φ = 0.85 V

B′F = x cos φ = 2.40 V

CF = 2.40 − 0.85 = 1.55 V

′ = OF = (222.93

+ 1.55

)

0.50

= 222.935 volts

Required terminal voltage of H.V. side = V

= 222.935 × (2200/220) = 2229.35 volts

[Note. In approximate and fast calculations, CF is often ignored for calculation of magnitude of V

′. The

concerned expression is : V

′ = V

+ Ir cos φ + Ix sin φ, for lagging P.f.]

Example 32.52. A 4-kVA, 200/400 V, single-phase transformer takes 0.7 amp and 65 W on Open-

circuit. When the low-voltage winding is short-circuited and 15 V is applied to the high-voltage

terminals, the current and power are 10 A and 75 W respectively. Calculate the full-load efficiency

at unity power factor and full-load regulation at 0.80 power-factor lagging.

(Nagpur University April 1999)

Solution. At a load of 4 kVA, the rated currents are :

L.V. side : 4000/200 = 20 amp

And H.V. side : 4000/400 = 10 amp

From the test data, full-load copper-loss = 75 W

And constant core-loss = 65 W

From S.C. test, Z = 15/10 = 1.5 ohms

R = 75/100 = 0.75 ohm

Hence x =

1.5 0.75

−

= 1.30 ohms

All these series-parameters are referred to the H.V. side, since the S.C. test has been conducted

from H.V. side.

Full-load efficiency at unity p.f. = 4000 / (4000 + 65 + 75)

= 0.966 = 96.6%

Full load voltage regulation at 0.80 lagging p.f.

= Ir cos φ + I x sin φ

= 10 (0.75 × 0.80 + 1.30 × 0.60) = 16.14 Volts

Thus, due to loading, H.V. side voltage will drop by 16.14 volts (i.e. terminal voltage for the load

will be 383.86 volts), when L.V. side is energized by 200-V source.

32.25. Percentage Resistance, Reactance and Impedance

These quantities are usually measured by the voltage drop at full-load current expressed as a

percentage of the normal voltage of the winding on which calculations are made.

(i) Percentage resistance at full-load

% R =

101 1 01

111

100 100

IR I R

VVI

×= ×

202

100

= % Cu loss at full-load

% R = % Cu loss = ν

...Art.32.16

(ii) Percentage reactance at full-load

% X =

101 202

100 100

IX IX

×= ×

= ν

1164

Electrical Technology

(iii) Percentage impedance at full-load

% Z =

101 202

100 100

IZ IZ

×= ×

(iv) %Z =

(% % )

It should be noted from above that the reactances and resistances in ohm can be obtained thus :

%%Cu loss

100 100

RV V

××

; Similarly R

%%Cu loss

100 100

RV V

××

100 100

XV V

×ν×

××

; Similarly X

100 100

ν×

××

It may be noted that percentage resistance, reactance and impedance have the same value whether

referred to primary or secondary.

Example 32.53. A 3300/230 V, 50-kVA, transformer is found to have impedance of 4% and a Cu

loss of 1.8% at full-load. Find its percentage reactance and also the ohmic values of resistance,

reactance and impedance as referred to primary. What would be the value of primary short-circuit

current if primary voltage is assumed constant ?

Solution. % X =

22 22

(% % ) (4 1.8 ) 3.57%

ZR−=−=

( Cu loss = % R)

Full load I

= 50,000/3300 = 15.2 A (assuming 100% efficiency). Considering primary winding, we

have

% R =

01 1

100

1.8 3300

1.8

100 15.2

=∴= =

3.91

ΩΩ

Ω

Similarly % X =

01 1

100

3.57 3300

3.57

100 15.2

=∴= =

7.76

ΩΩ

Ω

Similarly Z

43300

100 15.2

8.7

ΩΩ

Ω

Now

Short-circuit current

Full load current

100

S.C. current = 15.2 25 =

∴×

380 A

Example 32.54. A 20-kVA, 2200/220-V, 50-Hz distribution transformer is tested for efficiency

and regulation as follows :

O.C. test : 220 V 4.2 A, 148 W –l.v side

S.C. test : 86 V 10.5 A, 360 W – l.v. side

Determine (a) core loss (b) equivalent resistance referred to primary (c) equivalent resistance

referred to secondary (d) equivalent reactance referred to primary (e) equivalent reactance referred

to secondary (f) regulation of transformer at 0.8 p.f. lagging current (g) efficiency at full-load and

half the full-load at 0.8 p.f. lagging current.

Solution. (a) As shown in Art 32.9, no-load primary input is practically equal to the core loss.

Hence, core loss as found from no-load test, is 148 W.

(b) From S.C. test, R

= 360/10.5

= 3.26

ΩΩ

Ω

= K

= (220/2200)

× 3.26 = 0.0326 Ω

* Short circuit I

Now, Z

100

11 1

100 100

100

%% %

VI I

VZ Z I Z

×× ×

=∴=

Transformer

1165

(d) Z

8.19

10.5

==Ω

(8.19 3.26 )

−=

7.51

ΩΩ

Ω

(e) X

= K

= (220/2200)

× 7.51 = 0.0751 Ω

(f ) We will use the definition of regulation as given in Art. 32.24 (3).

We will find the rise in primary voltage necessary to maintain the output terminal voltage constant from

no-load to full-load.

Rated primary current = 20,000/2200 = 9.1 A

′ =

[2200 0.8 9.1 3.26) (2200 0.6 9.1 7.51) ]

×+× + ×+× =

2265 V

∴ % regn =

2265 2200

100

2200

−

×=

2.95%

We would get the same result by working in the secondary. Rated secondary current = 91 A.

(220 0.8 91 0.0326) (220 0.6 91 0.0751) ] 226.5

V×+× + ×+× =

∴ % regns. =

226.5 220

100

220

−

×=

2.95%

(g) Core loss = 1.48 W. It will be the same for all loads.

Cu loss at full load = I

= 9.1

× 3.26 = 270 W

Cu loss at half full-load = 4.55

× 3.26 = 67.5 W (or F.L. Cu loss/4)

∴η at full-load =

22,000 0.8 100

20,000 0.8 148 270

××

×+ +

97.4%

∴η at half-load =

10,000 0.8 100

10,000 0.8 148 67.5

××

×+ +

97.3%

Example 32.55. Calculate the regulation of a transformer in which the ohmic loss is 1% of the

output and the reactance drop is 5% of the voltage, when the power factor is (i) 0.80 Lag (ii) unity

(iii) 0.80 Leading. (Madras University, 1997)

Solution. When 1% of output is the ohmic loss, p.u. resistance of the transformer, ε

= 0.01

When 5% is the reactance drop, p.u. reactance of the transformer ε

= 0.05

(i) Per Unit regulation of the transformer at full-load, 0.8 Lagging p.f.

= 0.01 × cos φ + 0.05 × sin φ = 0.01 × 0.8 + 0.05 × 0.06 = 0.038 or 3.8%

(ii) Per Unit regulation at unity p.f. = 0.01 × 1 = 0.01 or 1%

(iii) Per Unit regulation at 0.08 Leading p.f. = 0.01 × 0.8 − 0.05 × 0.6 = − 0.022 or − 2.2%

Example 32.56. The maximum efficiency of a 500 kVA, 3300/500 V, 50 Hz, single phase

transformer is 97% and occurs at 3/4

full-load u.p.f. If the impedance is 10% calculate the

regulation at fullload, 0.8 p.f. Lag. (Madurai Kamraj University, November 1997)

Solution. At unity p.f. with 3/4

full load, the output of the transformer

= 500 × 0.75 × 1 kW = 375 kW

0.97 =

375

375 2

where P

= core loss in kW, at rated voltage.

At maximum efficiency, x

= P

(0.75)

= P

where x = 0.75, i.e. 3/4

which is the fractional loading of the transformer

1166

Electrical Technology

= copper losses in kW, at rated current

()

{}

11 3

(375) 1 ½ 375 5.8 kW

20.97 97

×−=××=

= 5.8/(0.75)

= 10.3 kW

Full load current in primary (H.V.) winding =

500 1000

3300

151.5 amp

Total winding resistance ref. to primary

10.3 1000

0.44876 ohm

(151.5)

= % resistance =

151.5 0.44876

100 %

3300

×=

2.06%

= % Impedance = 10%

= % reactance =

100 4.244

−=

9.7855%

By Approximate formula at 0.8 p.f. lag

% regulation = ε

cos φ + ε

sin φ

= 2.06 × 0.8 + 9.7855 × 0.6

= 1.648 + 5.87 = 7.52 %

Example 32.57. A transformer has copper-loss of 1.5% and reactance-drop of 3.5% when tested

at full-load. Calculate its full-load regulation at (i) u.p.f. (ii) 0.8 p.f. Lagging and (iii) 0.8 p.f.

Leading. (Bharathithasan Univ. April 1997)

Solution. The test-data at full-load gives following parameters :

p.u. resistance = 0.015, p.u. reactance = 0.035

(i) Approximate Voltage − Regulation at unity p.f. full load

= 0.015 cos φ + 0.035 sin φ

= 0.015 per unit = 1.5%

(ii) Approximate Voltage − Regulation at 0.80 Lagging p.f.

= (0.015 × 0.8) + (0.035 × 0.6) = 0.033 per unit = 3.3%

(iii) Approximate Voltage Regulation at 0.8 leading p.f.

= I

cos φ − I

sin φ

= (0.015 × 0.8) − (0.035 × 0.6) = − 0.009 per unit =

− −

− 0.9%

32.26. Kapp Regulation Diagram

It has been shown that secondary terminal voltage falls as the load on the transformer is increased

when p.f. is lagging and it increases when the power factor is leading. In other words, secondary terminal

voltage not only depends on the load but on power factor also (Art. 32.16). For finding the voltage

drop (or rise) which is further used in determining the regulation of the transformer, a graphical

construction is employed which was proposed by late Dr. Kapp.

For drawing Kapp regulation diagram, it is necessary to know the equivalent resistance and

reactance as referred to secondary i.e. R

and X

. If I

is the secondary load current, then secondary

terminal voltage on load V

, is obtained by subtracting I

and I

voltage drops vectorially from

secondary no-load voltage

Now,

is constant, hence it can be represented by a circle of constant radius OA as in Fig. 32.54.

This circle is known as no-load or open-circuit e.m.f. circle. For a given load, OI

represents the load

current and is taken as the reference vector, CB represents I

and is parallel to OI

, AB represents I

and is drawn at right angles to CB. Vector OC obviously represents I

and is drawn at right

Transformer

1167

angles to CB. Vector OC obviously rep-

resents secondary terminal voltage V

Since I

is constant, the drop triangle

ABC remains constant in size. It is seen

that end point C of V

lies on another

circle whose centre is O′. This point O′

lies at a distance of I

vertically be-

low the point O and a distance of I

to its left as shown in Fig. 32.54.

Suppose it is required to find the

voltage drop on full-load at a lagging

power factor of cos φ, then a radius OLP

is drawn inclined at an angle of φ with

OX. LM = I

and is drawn horizon-

tal MN = I

and is drawn perpen-

dicular to LM. Obviously, ON is no-

load voltage

. Now, ON = OP =

. Similarly, OL is V

. The voltage

drop = OP − OL = LP.

Hence, percentage regulation ‘down’ is =

100 100

OP OL

OP OP

−

×=×

It is seen that for finding voltage drop, triangle LMN need not be drawn, but simply the radius OLP.

The diagram shows clearly how the secondary terminal voltage falls as the angle of lag

increases. Conversely, for a leading power factor, the fall in secondary terminal voltage decreases till for an

angle of φ

leading, the fall becomes zero; hence V

. For angles greater than φ

, the secondary

terminal voltage V

becomes greater than

The Kapp diagram is very helpful in determining the variation of regulation with power factor but it has

the disadvantage that since the lengths of the sides of the impedance triangle are very small as compared to

the radii of the circles, the diagram has to be drawn on a very large scale, if sufficiently accurate results are

desired.

32.27. Sumpner of Back-to-Back Test

This test provides data for finding the

regulation, efficiency and heating under load

conditions and is employed only when two

similar transformers are available. One

transformer is loaded on the other and both

are connected to supply. The power taken

from the supply is that necessary for supplying

the losses of both transformers and the

negligibly small loss in the control circuit.

As shown in Fig. 32.55, primaries of the

two transformers are connected in parallel

across the same a.c. supply. With switch S

open, the wattmeter W

reads the core loss

for the two transformers.

Fig. 32.55

Fig. 32.54

Supply

1168

Electrical Technology

The secondaries are so connected that their potentials are in opposition to each other. This would

so if V

= V

and A is joined to C whilst B is joined to D. In that case, there would be no secondary

current flowing around the loop formed by the two secondaries. T is an auxiliary low-voltage trans-

former which can be adjusted to give a variable voltage and hence current in the secondary loop

circuit. By proper adjustment of T, full-load secondary current I

can be made to flow as shown. It is

seen, that I

flows from D to C and then from A to B. Flow of I

is confined to the loop FEJLGHMF

and it does not pass through W

. Hence, W

continues to read the core loss and W

measures full-load

Cu loss (or at any other load current value I

). Obviously, the power taken in is twice the losses of a

single transformer.

Example 32.58. Two similar 250-kVA, single-phase transformers gave the following results

when tested by back-to-back method :

Mains wattmeter, W

= 5.0 kW

Primary series circuit wattmeter, W

= 7.5 kW (at full-load current).

Find out the individual transformer efficiencies at 75% full-load and 0.8 p.f. lead.

(Electrical Machines-III, Gujarat Univ. 1986)

Solution. Total losses for both transformers = 5 + 7.5 = 12.5 kW

F.L. loss for each transformer = 12.5/2 = 6.25 kW

Copper-loss at 75% load =

()

37.5

kW 2.11 kW

×=

Output of each transformer at 75% F.L. and 0.8 p.f. = (250 × 0.75) × 0.8 = 150 kW

η =

150

150 2.5 2.11

97%

32.28. Losses in a Transformer

In a static

transformer, there

are no friction or

windage losses.

Hence, the only

losses occuring

are :

(i) Core

or Iron Loss : It

includes both

hysteresis loss

and eddy cur-

rent loss. Be-

cause the core

flux in a trans-

former remains

practically con-

stant for all loads

(its variation being

1 to 3% from no-load to full-load). The core loss is practically the same at all loads.

Hysteresis loss W

= η B

1.6

max

f V

watt; eddy current loss W

= PB

max

watt

These losses are minimized by using steel of high silicon content for the core and by using very thin

Typical 75kVA Transformer Losses vs. Load

3,000

2,500

2,000

1,500

1,000

500

Total Loses (Watts)

Load (%)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

150

Rise

110

Rise

TP-1 150

C Rise

Transformer

1169

laminations. Iron or core loss is found from the O.C. test. The input of the transformer when on no-

load measures the core loss.

(ii) Copper loss. This loss is due to the ohmic resistance of the transformer windings. Total Cu loss

= I

+ I

= I

+ I

. It is clear that Cu loss is proportional to (current)

or kVA

. In other

words, Cu loss at half the full-load is one-fourth of that at full-load.

The value of Cu loss is found from the short-circuit test (Art. 32.22).

32.29. Efficiency of a Transformer

As is the case with other types of electrical machines, the efficiency of a transformer at a particular load

and power factor is defined as the output divided by the input–the two being measured in the same units

(either watts or kilowatts).

Efficiency =

Output

Input

But a transformer being a highly efficient piece of equipment, has very small loss, hence it is impractical

to try to measure transformer, efficiency by measuring input and output. These quantities are nearly of the

same size. A better method is to determine the losses and then to calculate the efficiency from ;

Efficiency =

Output Output

Output + losses Output + Cu loss + iron loss

or η =

Input Losses

losses

Input Input

−

=−

It may be noted here that efficiency is based on power output in watts and not in volt-amperes, al-

though losses are proportional to VA. Hence, at any volt-ampere load, the efficiency depends on power

factor, being maximum at a power factor of unity.

Efficiency can be computed by determining core loss from no-load or open-circuit test and Cu loss

from the short-circuit test.

32.30. Condition for Maximum Efficiency

Cu loss = I

or I

= W

Iron loss = Hysteresis loss + Eddy current loss = W

+ W

= W

Considering primary side,

Primary input = V

cos φ

η =

11 1 1 01

11 1

11 1 11 1

coscos losses

cos cos

VI I R WVI

VI VI

φ− −φ−

φφ

=1 −

101

11111

cos cos

IR W

VVI

−

φφ

Differentiating both sides with respect to I

, we get

11 1

cos

−+

For η to be maximum,

= 0. Hence, the above equation becomes

cos

11 1

cos

or W

= I

or I

or Cu loss = Iron loss

1170

Electrical Technology

Fig. 32.56

The output current corresponding to maximum efficiency is I

(/ )

It is this value of the output current which will make the Cu loss equal to the iron loss. By proper

design, it is possible to make the maximum efficiency occur at any desired load.

Note. (i) If we are given iron loss and full-

load Cu loss, then the load at which two losses

would be equal (i.e. corresponding to maximum

efficiency) is given by

= Full load ×

()

Iron loss

F.L. Cu loss

In Fig. 32.56, Cu losses are plotted as a

percentage of power input and the efficiency

curve as deduced from these is also shown. It is

obvious that the point of intersection of the Cu

and iron loss curves gives the point of maximum

efficiency. It would be seen that the efficiency

is high and is practically constant from 15%

full-load to 25% overload.

(ii) The efficiency at any load is given by

η =

full-load kVA p.f.

100

( full-load kVA p.f.) +

xWW

××

××+

where x = ratio of actual to full-load kVA

= iron loss in kW ; W

= Cu loss in kW.

Example 32.59. In a 25-kVA, 2000/200 V, single-phase transformer, the iron and full-load cop-

per losses are 350 and 400 W respectively. Calculate the efficiency at unity power factor on

(i) full load (ii) half full-load. (Elect. Engg. & Electronic, Bangalore Univ. 1990 and

Similar example in U.P. Technical University 2001)

Solution. (i) Full-load Unity p.f.

Total loss = 350 + 400 = 750 W

F.L. output at u.p.f. = 25 × 1 = 25 kW ; η = 25/25.75 = 0.97 or 97%

(ii) Half F.L. Unity p.f.

Cu loss = 400 × (1/2)

= 100 W. Iron loss remains constant at 350 W, Total loss = 100 + 350

= 450 W.

Half-load output at u.p.f. = 12.5 kW

∴η= 12.5/(12.5 + 0.45) = 96.52%

Example 32.60. If P

and P

be the iron and copper losses of a transformer on full-load, find the

ratio of P

and P

such that maximum efficiency occurs at 75% full-load.

(Elect. Machines AMIE Sec. B, Summer 1992)

Solution. If P

is the Cu loss at full-load, its value at 75% of full-load is = P

× (0.75)

= 9P

/16. At

maximum efficiency, it equals the iron loss P

which remains constant throughout. Hence, at maximum

efficiency.

=9P

/16 or P

= 9/16.

Example 32.61. A 11000/230 V, 150-kVA, 1-phase, 50-Hz transformer has core loss of 1.4 kW

and F.L. Cu loss of 1.6 kW. Determine

(i) the kVA load for max. efficiency and value of max. efficiency at unity p.f.

(ii) the efficiency at half F.L. 0.8 p.f. leading (Basic Elect. Machine, Nagpur Univ. 1993)

Transformer

1171

Solution. (i) Load kVA corresponding to maximum efficiency is

= F.L. kVA ×

Iron loss 1.6

250

F.L. Cu loss 1.4

=×

= 160 kVA

Since Cu loss equals iron loss at maximum efficiency, total loss = 1.4 + 1.4 = 2.8 kW ;

output = 160 × 1 = 160 kW

max

= 160/162.8 = 0.982 or 98.2%

(ii) Cu loss at half full-load = 1.6 × (1/2)

= 0.4 kW ; Total loss = 1.4 + 0.4 = 1.8 kW

Half F.L. output at 0.8 p.f.= (150/2) × 0.8 = 60 kW

∴ Efficiency = 60/(60 + 1.8) = 0.97 or 97%

Example 32.62. A 5-kVA, 2,300/230-V, 50-Hz transformer was tested for the iron losses with

normal excitation and Cu losses at full-load and these were found to be 40 W and 112 W respectively.

Calculate the efficiencies of the transformer at 0.8 power factor for the following kVA outputs :

1.25 2.5 3.75 5.0 6.25 7.5

Plot efficiency vs kVA output curve. (Elect. Engg. -I, Bombay Univ. 1987)

Solution. F.L. Cu loss = 112 W ; Iron loss = 40 W

(i) Cu loss at 1.25 kVA = 112 × (1.25/5)

= 7 W

Total loss = 40 + 7 = 47 W Output = 1.25 × 0.8 = 1 kW = 1,000 W

η = 100 × 1,000/1,047 = 95.51 %

(ii) Cu loss at 2.5 kVA = 112 × (2.5/5)

= 28 W

Total loss = 40 + 28 = 68 W

Output = 2.5 × 0.8 = 2 kW

η = 2,000 × 100/2,068 = 96.71%

(iii) Cu loss at 3.75 kVA

= 112 × (3.75/5)

= 63 W

Total loss = 40 + 63 = 103 W

η = 3,000 × 100/3,103 = 96.68 %

(iv) Cu loss at 5 kVA

= 112 W

Total loss = 152 W = 0.152 kW

Output = 5 × 0.8 = 4 kW

η =4 × 100/4.142 = 96.34 %

(v) Cu loss at 6.25 kVA

= 112 × (6.25/5)

= 175 W

Total loss = 125 W = 0.125 kW ; Output = 6.25 × 0.8 = 5 kW

η =5 × 100/5.215 = 95.88 %

(vi) Cu loss at 7.5 kVA = 112 × (7.5/5)

= 252 W

Total loss = 292 W = 0.292 kW ; Output = 7.5 × 0.8 = 6 kW

η =6 × 100/6.292 = 95.36 %

The curve is shown in Fig. 32.57.

Example 32.63. A 200-kVA transformer has an efficiency of 98% at full load. If the max.

efficiency occurs at three quarters of full-load, calculate the efficiency at half load. Assume negli-

gible magnetizing current and p.f. 0.8 at all loads. (Elect. Technology Punjab Univ. Jan. 1991)

Fig. 32.57

1.25 2.5 3.75 5 6.25 7.5

Efficiency

Load, kVA

1172

Electrical Technology

Solution. As given, the transformer has a F.L. efficiency of 98 % at 0.8 p.f.

F.L. output = 200 × 0.8 = 160 kW ; F.L. input = 160/0.98 = 163.265 kW

F.L. losses = 163.265 − 160 = 3.265 kW

This loss consists of F.L. Cu loss x and iron loss y.

∴ x + y = 3.265 kW ...(i)

It is also given that η

max

occurs at three quarters of full-load when Cu loss becomes equal to iron

loss.

∴ Cu loss at 75 % of F.L. = x (3/4)

=9 x/16

Since y remains constant, hence 9 x/16 = y ...(ii)

Substituting the value of y in Eqn. (i), we get x + 9 x/16 = 3265 or x = 2090 W; y = 1175 W

Half-load Unity p.f.

Cu loss = 2090 × (1/2)

= 522 W ; total loss = 522 + 1175 = 1697 W

Output = 100 × 0.8 = 80 kW ; η = 80/81.697 = 0.979 or 97.9 %

Example 32.64. A 25-kVA, 1-phase transformer, 2,200 volts to 220 volts, has a primary resistance

of 1.0

Ω

and a secondary resistance of 0.01 Ω . Find the equivalent secondary resistance and the

full-load efficiency at 0.8 p.f. if the iron loss of the transformer is 80% of the full-load Cu loss.

(Elect. Technology, Utkal Univ. 1998)

Solution. K = 220/2,200 = 1/10 ; R

= R

+ K

= 0.01 + 1/100 = 0.02

ΩΩ

Ω

Full-load I

= 25,000/220 = 113.6 A ; F.L. Cu loss = I

= 113.6

× 0.02 = 258 W.

Iron loss = 80% of 258 = 206.4 W ; Total loss = 258 + 206.4 = 464.4 W

F.L. output = 25 × 0.8 = 20 kW = 20,000 W

Full-load η = 20,000 × 100/(20,000 + 464.4) = 97.7 %

Example 32.65. A 4-kVA, 200/400-V, 1-phase transformer has equivalent resistance and reactance

referred to low-voltage side equal to 0.5

Ω

and 1.5

Ω

respectively. Find the terminal voltage on the

high-voltage side when it supplies 3/4th full-load at power factor of 0.8, the supply voltage being 220

V. Hence, find the output of the transformer and its efficiency if the core losses are 100 W.

(Electrical Engineering ; Bombay Univ. 1985)

Solution. Obviously, primary is the low-voltage side and the secondary, the high voltage side.

Here, R

= 0.5 Ω and X

= 1.5 Ω. These can be transferred to the secondary side with the help of

the transformation ratio.

K = 400/200 = 2 ; R

= K

= 2

× 0.5 = 2 Ω ; X

= K

= 4 × 1.5 = 6 Ω

Secondary current when load is 3/4 the, full-load is = (1,000 × 4 × 3/4)/400 = 7.5 A

Total drop as referred to transformer secondary is

= I

cos φ + X

sin φ)* = 7.5 (2 × 0.8 + 6 × 0.6) = 39 V

∴ Terminal voltage on high-voltage side under given load condition is

= 400 − 39 = 361 V

Cu loss = I

= 7.5

× 2 = 112.5 W Iron loss = 100 W

Total loss = 212.5 W output = (4 × 3/4) × 0.8 = 2.4 kW

Input = 2,400 + 212.5 = 2,612.5 W η = 2,400 × 100/2,612.5 = 91.87 %

Example 32.66. A 20-kVA, 440/220 V, I-φ, 50 Hz transformer has iron loss of 324 W. The Cu

loss is found to be 100 W when delivering half full-load current. Determine (i) efficiency when

* Assuming a lagging power factor

Transformer

1173

delivering full-load current at 0.8 lagging p.f. and (ii) the percent of full-load when the efficiency will

be maximum. (Electrotechnique-II, M.S. Univ., Baroda 1987)

Solution. F.L. Cu loss = 2

× 100 = 400 W ; Iron loss = 324 W

(i) F.L. efficiency at 0.8 p.f. =

20 0.8

100

(20 0.8) 0.724

×+

= 95.67 %

(ii)

kVA for maximum

F.L. kVA

Iron loss 324

F.L. Cu loss 400

= 0.9

Hence, efficiency would be maximum at 90 % of F.L.

Example 32.67. Consider a 4-kVA, 200/400 V single-phase transformer supplying full-load

current at 0.8 lagging power factor. The O.C./S.C. test results are as follows :

O.C. test : 200 V, 0.8 A, 70 W (I.V. side)

S.C. test : 20 V, 10 A, 60 (H.V. side)

Calculate efficiency, secondary voltage and current into primary at the above load.

Calculate the load at unity power factor corresponding to maximum efficiency.

(Elect. Machines Nagpur Univ. 1993)

Solution. Full-load, I

= 4000/400 = 10 A

It means that S.C. test has been carried out with full secondary flowing. Hence, 60 W represents

full-load Cu loss of the transformer.

Total F.L. losses = 60 + 70 = 130 W ; F.L. output = 4 × 0.8 = 3.2 kW

F.L. η = 3.2/3.33 = 0.96 or 96 %

S.C. Test

= 20/10 = 2 Ω ; I

= 60 or R

= 60/10

= 0.6 Ω ; X

20.6

−

= 1.9 Ω

Transformer voltage drop as referred to secondary

= I

cos φ + X

sin φ) = 10 (0.6 × 0.8 + 1.9 × 0.6) = 16.2 V

∴ V

= 400 − 16.2 = 383.8 V

Primary current = 4000/200 = 20 A

kVA corresponding to η

max

= 4 ×

70/ 60

= 4.32 kVA

∴ Load at u.p.f. corresponding to η

max

= 4.32 × 1 = 4.32 kW

Example 32.68. A 600 kVA, 1-phase transformer has an efficiency of 92 % both at full-load and

half-load at unity power factor. Determine its efficiency at 60 % of full-load at 0.8 power factor lag.

(Elect. Machines, A.M.I.E. Sec. B, 1992)

Solution.

η =

cos

100

()cos

iCu

xkVA

xkVA W xW

××φ

××φ++

where x represents percentage of full-load

is iron loss and W

is full-load Cu loss.

At F.L. u.p.f. Here x =1

∴ 92 =

16001

WIW

××

××++

× 100, W

+ W

= 52.174 kW ...(i)

At half F.L. UPF. Here x = 1/2

92 =

1/2 600 1

(1 / 2) 6 00 1 (1 / 2)

iCu

××

××++

× 100 ;

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Electrical Technology

∴ W

+ 0.25 W

= 26.087 kW ...(ii)

From (i) and (ii), we get, W

= 17.39 kW, W

= 34.78 kW

60 % F.L. 0.8 p.f. (lag) Here, x = 0.6

η =

0.6 600 0.8 100

(0.6 600 0.8) 17.39 (0.6) 34.78

×××

××+ +

= 85.9 %

Example 32.69. A 600-kVA, 1-ph transformer when working at u.p.f. has an efficiency of 92 %

at full-load and also at half-load. Determine its efficiency when it operates at unity p.f. and 60 % of

full-load. (Electric. Machines, Kerala Univ. 1987)

Solution. The fact that efficiency is the same i.e. 92 % at both full-load and half-load will help us

to find the iron and copper losses.

At full-load

Output = 600 kW ; Input = 600/0.92 = 652.2 kW ; Total loss = 652.2 − 600 = 52.2 kW

Let x = Iron loss − It remains constant at all loads.

y = F.L. Cu loss − It is ∝ (kVA)

. ∴ x + y = 52.2 ...(i)

At half-load

Output = 300 kW ; Input = 300/0.92 ∴ Losses = (300/0.92 − 300) = 26.1 kW

Since Cu loss becomes one-fourth of its F.L. value, hence

x + y/4 = 26.1

Solving for x and y, we get x = 17.4 kW ; y = 34.8 kW ...(ii)

At 60 % full-load

Cu loss = 0.62 × 34.8 = 12.53 kW ; Total loss = 17.4 + 12.53 = 29.93 kW

Output = 600 × 0.6 = 360 kW ∴η = 360/389.93 = 0.965 or 96.5 %

Example 32.70. The maximum efficiency of a 100-kVA, single phase transformer is 98% and

occurs at 80% of full load at 8 p.f. If the leakage impedance of the transformer is 5 %, find the

voltage regulation at rated load of 0.8 power factor lagging.

(Elect. Machines-I, Nagpur Univ. 1993)

Solution. Since maximum efficiency occurs at 80 percent of full-load at 0.8 p.f.,

Output at η

max

= (100 × 0.8) × 0.8 = 64 kW ; Input = 64/0.98 = 65.3 kW

∴ Total loss = 65.3 − 64 = 1.3 kW. This loss is divided equally between Cu and iron.

∴ Cu loss at 80 % of full-load = 1.3/2 = 0.65 kW

Cu loss at full-load = 0.65/0.8

= 1 kW

% R =

Cu loss 100

100 1

100

×=×

= 1 % = v

; v

= 5 %

∴ % age regn. = (1 × 0.8 + 5 × 0.6) +

200

(5 × 0.8 − 1 × 0.6)

= 0.166 %

Example 32.71. A 10 kVA, 5000/440-V, 25-Hz single phase transformer has copper, eddy current

and hysteresis losses of 1.5, 0.5 and 0.6 per cent of output on full load. What will be the percentage

losses if the transformer is used on a 10-kV, 50-Hz system keeping the full-load current constant ?

Assume unity power factor operation. Compare the full load efficiencies for the two cases.

(Elect. Machines, A.M.I.E., Sec. B, 1991)

Solution. We know that E

= 4,44 f N

A.. When both excitation voltage and frequency are

doubled, flux remains unchanged.

F.L. output at upf = 10 kVA × 1 = 10 kW

Transformer

1175

F.L. Cu loss = 1.5 × 10/100 = 0.15 kW ; Eddy current loss

=0.5 × 10/100 = 0.05 kW ; Hysteresis loss = 0.6 × 10/100 = 0.06 kW

Now, full-load current is kept constant but voltage is increased from 5000 V to 10,000 V. Hence,

output will be doubled to 20 kW. Due to constant current, Cu loss would also remain constant.

New Cu loss = 0.15 kW, % Cu loss = (0.15/20) × 100 = 0.75 %

Now, eddy current loss ∝ f

and hysteresis loss ∝ f.

New eddy current loss = 0.05 (50/25)

= 0.2 kW, % eddy current loss = (0.2/20) × 100 = 1 %

Now, W

= 0.06 × (50/25) = 0.12 kW, % W

= (0.12/20) × 100 = 0.6 %

100

10 0.15 0.05 0.06

+++

= 87.4 %

100

20 0.15 0.2 0.12

+++

= 97.7 %

Example 32.72. A 300-kVA, single-phase transformer is designed to have a resistance of 1.5 %

and maximum efficiency occurs at a load of 173.2 kVA. Find its efficiency when supplying full-load

at 0.8 p.f. lagging at normal voltage and frequency. (Electrical Machines-I, Gujarat Univ. 1985)

Solution. % R =

F.L. Cu loss

100

Full-load

; 1.5 =

F.L. Cu loss

100

300 1000

∴ F.L. Cu loss = 1.5 × 300 × 1000/100 = 4500 W

Also, 173.2 = 300

Iron Loss

4500

; Iron loss = 1500 W

Total F.L. loss = 4500 + 1500 = 6 kW

F.L. η at 0.8 p.f. =

300 0.8

100

(300 0.8) 6

×+

= 97.6 %

Example 32.73. A single phase transformer is rated at 100-kVA, 2300/230-V, 50 Hz. The

maximum flux density in the core is 1.2 Wb/m

and the net cross-sectional area of the core is 0.04 m

Determine

(a) The number of primary and secondary turns needed.

(b) If the mean length of the magnetic circuit is 2.5 m and the relative permeability is 1200,

determine the magnetising current. Neglect the current drawn for the core loss.

circuit with rated voltage, the power input was 400 W. Determine the efficiency of the transformer

at 75 % of full-load with 0.8 p.f. lag.

(d) If the same transformer is connected to a supply of similar voltage but double the frequency

(i.e., 100 Hz). What is the effect on its efficiency ? (Elect. Engg., Bombay Univ. 1988)

Solution. (a) Applying e.m.f. equation of the transformer to the primary, we have

2300 = 4.44 × 50 × N

× (1.2 × 0.0.4) ∴ N

= 216

K = 230/2300 = 1/10 N

= KN

= 216/10 = 21.6 or 22

(b) AT = H × l =

1.2 2.5

1989

4 10 1200

−

×= =

µµ

π× ×

∴ I =

1989

216

= 9.21 A

Cu loss at 75 % of F.L. = (0.75)

× 1200 = 675 W

Total loss = 400 + 675 = 1075 kW

Output = 100 × (3/4) × 0.8 = 60 kW ; η = (60/61.075) × 100 = 98.26 %

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Electrical Technology

(d) When frequency is doubled, iron loss is increased because

(i) hysteresis loss is doubled − W

∝ f

(ii) eddy current loss is quadrupled − W

∝ f

Hence, efficiency will be decreased.

Example 32.74. A transformer has a resistance of 1.8 % and a reactance of 5.4 %. (a) At full

load, what is the power-factor at which the regulation will be : (i) Zero, (ii) positive-maximum ?

(b) If its maximum efficiency occurs at full-load (at unity p.f.), what will be the efficiency under these

conditions ?

Solution : Approximate percentage regulation is given, in this case, by the relationship

1.8 cos φ ± 5.4 sin φ.

(a) Regulation :

(i) If regulation is zero, negative sign must be applicable. This happens at leadings p.f.

Corresponding p.f. = tan φ = 1.8/5.4 = 0.333 leading

φ = 18.44

leading

(ii) For maximum positive regulation, lagging p.f. is a must. From phasor diagram, the result can

be obtained.

Corresponding tan φ = 5.4/1.8 = 3, φ = 71.56 lagging

% Voltage regulation = 1.8 cos φ + 5.4 sin φ = 5.7 %

(b) Efficiency : Maximum efficiency occurs at such a load when

Iron losses = Copper losses

This means Iron-losses are 1.8 %.

Efficiency = 100/(100 + 1.8 + 1.8) = 96.52 %

Example 32.75. A 10 kVA, 1 phase, 50 Hz, 500/250 V transformer gave following test results :

OC test (LV) side : 250 V, 3.0 A, 200 W

SC test (LV) side : 15 V, 30 A, 300 W.

Calculate efficiency and regulation at full load, 0.8 p.f. lagging.

(Nagpur University, Summer 2000)

Solution. For efficiency calculations, full load current should be calculated, on the L.V. side in

this case,

F.L. Current =

10,000

250

= 40 amp

Short-circuit test data have been given at 30 A current on the L.V. side.

r losses at 40 A L.V. side =

()

300

Watts = 533.3 Watts

At rated voltage, iron losses (from O.C. test) = 200 Watts

F.L. Output at 0.8 P.F. = 10,000 × 0.8 = 8000 Watts

Hence, η =

8000

100 %

8000 4733.3

= 91.6 %

For regulation, series resistance and reactance parameters of the equivalent circuit have to be

evaluated, from the S.C. test.

Series Impedance, Z =

= 0.5 ohm

Series resistance, r =

300

30 30

= 0.333 ohm

Transformer

1177

Series reactance, x =

0.5 0.333

−

= 0.373 ohm

By Approximate formula,

p.u. regulation at full load, 0.8 p.f. lagging

250

[0.333 × 0.8 − 0.373 × 0.6] = 6.82 × 10

−3

p.u.

When converted into volts, this is 6.82 × 10

−3

× 250 = 1.70 volt

Example 32.76. A 40 kVA, 1-ph, transformer has an iron loss of 400 W, and full copper loss of

800 W. Find the load at which maximum efficiency is achieved at unity power factor.

(Amravati University, Winter 1999)

Solution. If x = fraction of rated load at which the efficiency is maximum.

= Iron−loss = 400 W

= F.L. copper−loss = 800 W

Then x

= P

On substitution of numerical values of P

and P

, we get

x = 0.707

Hence, the efficiency is maximum, at unity p.f. and at 70.7 % of the rated Load. At this load,

copper−loss = Iron−loss = 0.40 kW

Corresponding output = 40 × 0.707 × 1

= 28.28 kW

Corresponding efficiency =

28.28

28.28 0.4 0.4

= 97.25 %

Extension to Question : (a) At what load (s) at unity p.f. the efficiency will be 96.8 % ?

40 70 125

96.5

96.8

97.25

97.5

0.90

P.f.

Unity

P.f.

Load

Fig. 32.58. Efficiency variation with load

Solution. Let x = Fractional load at which the concerned efficiency occurs, at unity p.f.

40 0.8 0.40

xx++

= 0.968

This gives the following values of x : x

= 1.25 x

= 0.40

1178

Electrical Technology

Thus, at 40 % and at 125 % of the rated load, the efficiency will be 96.8 % as marked on the graph,

in Fig. 32.58.

(b) How will maximum-efficiency condition be affected if the power factor is 0.90 lagging ?

Solution.

The condition for efficiency-variation-statement is that the power factor remains constant. Thus, for

0.90 lagging p.f., another curve (Lower curve in Fig. 32.58) will be drawn for which the maximum efficiency

will occur at the same value of x (= 0.707), but

Maximum efficiency =

40 cos

40 cos 0.80 0.40

φ+ +

28.28 0.90

(28.28 0.90) 0.80

×+

= 97 %

Example 32.77. A 10 kVA, 500/250 V, single-phase transformer gave the following test results:

S.C. Test (H.V. side) : 60 V, 20 A, 150 W

The maximum efficiency occurs at unity power factor and at 1.20 times full-load current. De-

termine full-load efficiency at 0.80 p.f. Also calculate the maximum efficiency.

(Rajiv Gandhi Technical University, Bhopal, Summer 2001)

Solution. Full-load current on H.V. side = 10,000/500 = 20 Amp

S.C. test has been conducted from H.V. side only. Hence, full-load copper-loss, at unity

p.f. = 150 watts

(a) Maximum efficiency occurs at 1.2 times full-load current, at unity p.f. corresponding copper-

loss = (1.2)

× 150 = 216 watts

At maximum efficiency, copper-loss = core-loss = 216 watts

Corresponding Power-output = 1.2 × 10,000 × 1.0 = 12 kW

Hence, maximum efficiency at unity P.f. = (12)/(12 + 0.216 + 0.2160) = 0.9653 = 96.53 %

(b) Full-load efficiency at 0.80 P.f.

Output Power at full-load, 0.80 P.f. = 10,000 × 0.8 = 8000 W, constant core-loss = 216 W

Corresponding copper-loss = 150 W

Total losses = 366 W

Hence, efficiency = (8000/8366) × 100 % = 95.63 %.

32.31. Variation of Efficiency with Power Factor

The efficiency of a transformer is given by

η =

Input - Losses

Output

Input Input

=1 −

Losses Losses

Input ( cos losses)

=−

φ+

Let, losses/V

= x

∴η=1 −

losses /

cos (losses / )

φ+

=1 −

/cos

(cos ) 1 ( / cos )

=−

φ+ + φ

The variations of efficiency with power factor at

different loadings on a typical transformer are shown in

Fig. 32.59.

Fig. 32.59

0.99

0.98

0.97

0.96

00.51.0 1.5

Efficiency

Full Load

Transformer

1179

Tutorial Problems 32.4

1. A 200-kVA transformer has an efficiency of 98 % at full-load. If the maximum efficiency occurs at

three-quarters of full-load, calculate (a) iron loss at F.L. (b) Cu loss at F.L. (c) efficiency at half-load.

Ignore magnetising current and assume a p.f. of 0.8 at all loads.

[(a) 1.777 kW (b) 2.09 kW (c) 97.92%]

2. A 600 kVA, 1-ph transformer has an efficiency of 92 % both at full-load and half-load at unity power

factor. Determine its efficiency at 60 % of full load at 0.8 power factor lag.

[90.59%] (Elect. Machines, A.M.I.E. Sec. B, 1992)

3. Find the efficiency of a 150 kVA transformer at 25 % full load at 0.8 p.f. lag if the copper loss at full

load is 1600 W and the iron loss is 1400 W. Ignore the effects of temperature rise and magnetising

current. [96.15%] (Elect. Machines, A.M.I.E. Sec. B, 1991)

4. The F.L. Cu loss and iron loss of a transformer are 920 W and 430 W respectively. (i) Calculate the

loading of the transformer at which efficiency is maximum (ii) what would be the losses for giving

maximum efficiency at 0.85 of full-load if total full-load losses are to remain unchanged ?

[(a) 68.4% of F.L. (ii) W

= 565 W ; W

= 785 W]

5. At full-load, the Cu and iron losses in a 100-kVA transformer are each equal to 2.5 kW. Find the

efficiency at a load of 65 kVA, power factor 0.8. [93.58%] (City & Guilds London)

6. A transformer, when tested on full-load, is found to have Cu loss 1.8% and reactance drop 3.8%.

Calculate its full-load regulation (i) at unity p.f. (ii) 0.8 p.f. lagging (iii) 0.8 p.f. leading.

[(i) 1.80% (ii) 3.7 % (iii)

−−

−0.88%]

7. With the help of a vector diagram, explain the significance of the following quantities in the open-

circuit and short-circuit tests of a transformer (a) power consumed (b) input voltage (c) input current.

When a 100-kVA single-phase transformer was tested in this way, the following data were obtained

: On open circuit, the power consumed was 1300 W and on short-circuit the power consumed was

1200 W. Calculate the efficiency of the transformer on (a) full-load (b) half-load when working at

unity power factor. [(a) 97.6% (b) 96.9%] (London Univ.)

8. An 11,000/230-V, 150-kVA, 50-Hz, 1-phase transformer has a core loss of 1.4 kW and full-load Cu

loss of 1.6 kW. Determine (a) the kVA load for maximum efficiency and the minmum efficiency (b)

the efficiency at half full-load at 0.8 power factor lagging. [140.33 kVA, 97.6% ; 97%]

9. A single-phase transformer, working at unity power factor has an efficiency of 90 % at both half-load

and a full-load of 500 kW. Determine the efficiency at 75 % of full-load. [90.5%] (I.E.E. London)

10. A 10-kVA, 500/250-V, single-phase transformer has its maximum efficiency of 94 % when delivering

90 % of its rated output at unity power factor. Estimate its efficiency when delivering its full-load

output at p.f. of 0.8 lagging. [92.6%] (Elect. Machinery, Mysore Univ, 1979)

11. A single-phase transformer has a voltage ratio on open-circuit of 3300/660-V. The primary and

secondary resistances are 0.8 Ω and 0.03 Ω respectively, the corresponding leakage reactance being

4 Ω and 0.12 Ω. The load is equivalent to a coil of resistance 4.8 Ω and inductive reactance 3.6 Ω.

Determine the terminal voltage of the transformer and the output in kW. [636 V, 54 kW]

12. A 100-kVA, single-phase transformer has an iron loss of 600 W and a copper loss of 1.5 kW at full-

load current. Calculate the efficiency at (a) 100 kVA output at 0.8 p.f. lagging (b) 50 kVA output at

unity power factor. [(a) 97.44% (b) 98.09%]

13. A 10-kVA, 440/3300-V, 1-phase transformer, when tested on open circuit, gave the following figures

on the primary side : 440V ; 1.3 A ; 115 W.

When tested on short-circuit with full-load current flowing, the power input was 140 W. Calculate

the efficiency of the transformer at (a) full-load unity p.f. (b) one quarter full-load 0.8 p.f.

[(a) 97.51% (b) 94.18%] (Elect. Engg-I, Sd. Patel Univ. June 1977)

14. A 150-kVA single-phase transformer has a core loss of 1.5 kW and a full-load Cu loss of 2 kW.

Calculate the efficiency of the transformer (a) at full-load, 0.8 p.f. lagging (b) at one-half full-load

1180

Electrical Technology

unity p.f. Determine also the secondary current at which the efficiency is maximum if the secondary

voltage is maintained at its rated value of 240 V. [(a) 97.17% (b) 97.4% ; 541 A]

15. A 200-kVA, 1-phase, 3300/400-V transformer gave the following results in the short-circuit test. With

200 V applied to the primary and the secondary short-circuited, the primary current was the full-load

value and the input power 1650 W. Calculate the secondary p.d. and percentage regulation when the

secondary load is passing 300 A at 0.707 p.f. lagging with normal primary voltage.

[380 V ; 480%]

16. The primary and secondary windings of a 40-kVA, 6600/250-V, single-phase transformer have

resistances of 10 Ω and 0.02 Ω respectively. The leakage reactance of the transformer referred to the

primary is 35 Ω. Calculate

(a) the primary voltage required to circulate full-load current when the secondary is short-circuited.

(b) the full-load regulations at (i) unity (ii) 0.8 lagging p.f. Neglect the no-load current.

[(a) 256 V (b) (i) 2.2% (ii) 3.7%] (Elect. Technology, Kerala Univ. 1979)

17. Calculate :

(a) F.L. efficiency at unity p.f.

(b) The secondary terminal voltage when supplying full-load secondary current at p.f. (i) 0.8 lag

(ii) 0.8 lead for the 4-kVA, 200/400 V, 50 Hz, 1-phase transformer of which the following are the

test figures :

Open circuit with 200 V supplied to the primary winding-power 60 W. Short-circuit with 16 V

applied to the h.v. winding-current 8 A, power 40 W. [0.97 ; 383 V ; 406 V]

18. A 100-kVA, 6600/250-V, 50-Hz transformer gave the following results :

O.C. test : 900 W, normal voltage.

S.C. test (data on h.v. side) : 12 A, 290 V, 860 W

Calculate

(a) the efficiency and percentage regulation at full-load at 0.8 p.f. lagging.

(b) the load at which maximum efficiency occurs and the value of this efficiency at p.f. of unity, 0.8

lag and 0.8 lead. [(a) 97.3%, 4.32% (b) 81 kVA, 97.8%, 97.3% ; 97.3%]

19. The primary resistance of a 440/110-V transformer is 0.5 Ω and the secondary resistance is 0.04 Ω.

When 440 V is applied to the primary and secondary is left open-circuited, 200 W is drawn from the

supply. Find the secondary current which will give maximum efficiency and calculate this efficiency

for a load having unity power factor.

[53 A ; 93.58%] (Basic Electricity & Electronics. Bombay Univ. 1981)

20. Two tests were performed on a 40-kVA transformer to predetermine its efficiency. The results were:

Open circuit : 250 V at 500 W

Short circuit : 40 V at F.L. current, 750 W both tests from primary side.

Calculate the efficiency at rated kVA and 1/2 rated kVA at (i) unity p.f. (ii) 0.8 p.f.

[96.97% ; 96.68% ; 96.24% ; 95.87%]

21. The following figures were obtained from tests on a 30-kVA, 3000/110-V transformer :

O.C. test : 3000 V 0.5 A 350 W ; S.C. test ; 150 V 10 A 500 W

Calculate the efficiency of the transformer at

(a) full-load, 0.8 p.f. (b) half-load, unity p.f.

Also, calculate the kVA output at which the efficiency is maximum. [96.56% ; 97% ; 25.1 kVA]

22. The efficiency of a 400 kVA, 1-phase transformer is 98.77 % when delivering full load at 0.8 power

factor, and 99.13 % at half load and unity power factor. Calculate (a) the iron loss, (b) the full load

copper loss. [(a) 1012 W (b) 2973 W] (Rajiv Gandhi Technical University, 2000)

Transformer

1181

32.32. All-day Efficiency

The ordinary or commercial efficiency of

a transformer is given by the ratio

Output in watts

Input in watts

But there are certain types of transformers

whose performance cannot be judged by this

efficiency. Transformers used for supplying

lighting and general network i.e., distribution

transformers have their primaries energised all

the twenty-four hours, although their

secondaries supply little or no-load much of

the time during the day except during the house

lighting period. It means that whereas core

loss occurs throughout the day, the Cu loss

occurs only when the transformers are loaded. Hence, it is considered a good practice to design such

transformers so that core losses are very low. The Cu losses are relatively less important, because they

depend on the load. The performance of such is compared on the basis of energy consumed during a

certain time period, usually a day of 24 hours.

∴η

all-day

Output in kWh

Input in kWh

(For 24 hours)

This efficiency is always less than the commercial efficiency of a transformer.

To find this all-day efficiency or (as it is also called) energy efficiency, we have to know the load cycle

on the transformer i.e., how much and how long the transformer is loaded during 24 hours. Practical

calculations are facilitated by making use of a load factor.

Example 32.78. Find the all-day efficiency of 500-kVA distribution transformer whose copper

loss and iron loss at full load are 4.5 kW and 3.5 kW respectively. During a day of 24 hours, it is

loaded as under :

No. of hours Loading in kW Power factor

6 400 0.8

10 300 0.75

4 100 0.8

40–

(Elect. Machines, Nagpur Univ. 1993)

Solution. It should be noted that a load of 400 kW at 0.8 p.f. is equal to 400/0.8 = 500 kVA.

Similarly, 300 kW at 0.75 p.f. means 300/0.75 = 400 kVA and 100 kW at 0.8 p.f. means 100/0.8 = 125 kVA

i.e., one-fourth of the full-load.

Cu loss at F.L. of 500 kVA = 4.5 kW

Cu loss at 400 kVA = 4.5 × (400/500)

= 2.88 kW

Cu loss at 125 kVA = 4.5 × (125/500)

= 0.281 kW

Total Cu loss in 24 hrs = (6 × 4.5) + (10 × 2.88) + (4 × 0.281) + (4 × 0)

= 56.924 kWh

The iron loss takes place throughout the day irrespective of the load on the transformer because

its primary is energized all the 24 hours.

The world first 5,000 KVA amorphous transformer

commissioned in August 2001 in Japan

1182

Electrical Technology

∴ Iron loss in 24 hours = 24 × 3.5 = 84 kWh

Total transformer loss = 56.924 + 84 = 140.924 kWh

Transformer output is 24 hrs = (6 × 400) + (10 × 300) + (4 × 100) = 5800 kWh

∴η

all-day

output

5800

output + losses 5800 140.924

= 0.976 or 97.6 %

Example 32.79. A 100-kVA lighting transformer has a full-load loss of 3 kW, the losses being

equally divided between iron and copper. During a day, the transformer operates on full-load for 3

hours, one half-load for 4 hours, the output being negligible for the remainder of the day. Calculate

the all-day efficiency. (Elect. Engg. Punjab Univ. 1990)

Solution. It should be noted that lighting transformers are taken to have a load p.f. of unity.

Iron loss for 24 hour = 1.5 × 24 = 36 kWh ; F.L. Cu loss = 1.5 kW

∴ Cu loss for 3 hours on F.L. = 1.5 × 3 = 4.5 kWh

Cu loss at half full-load = 1.5/4 kW

Cu loss for 4 hours at half the load = (1.5/4) × 4 = 1.5 kWh

Total losses = 36 + 4.5 + 1.5 = 42 kWh

Total output = (100 × 3) + (50 × 4) = 500 kWh

∴η

all-day

= 500 × 100/542 = 92.26 %

Incidentally, ordinary or commercial efficiency of the transformer is

= 100/(100 + 3) = 0.971 or 97.1 %

Example 32.80. Two 100-kW transformers each has a maximum efficiency of 98 % but in one the

maximum efficiency occurs at full-load while in the other, it occurs at half-load. Each transformer

is on full-load for 4 hours, on half-load for 6 hours and on one-tenth load for 14 hours per day.

Determine the all-day efficiency of each transformer. (Elect. Machines-I, Vikram Univ. 1988)

Solution. Let x be the iron loss and y the full-load Cu loss. If the ordinary efficiency

is a maximum at 1/m of full-load, then x = y/m

Now, output = 100 kW ; Input = 100/0.98

∴ Total losses = 100/0.98 − 100 = 2.04 kW

∴ y + x/m

=2.04

Ist Transformer

Here m = 1 ; y + y = 2.04 ; y = 1.02 kW and x = 1.02 kW

Iron loss for 24 hours = 1.02 × 24 = 24.48 kWh

Cu loss for 24 hours = 4 × 1.02 + 6 × (1.02/4) + 14 (1.02/10

) = 5.73 kWh

Total loss = 24.48 + 5.73 = 30.21 kWh

=4 × 100 + 6 × 50 + 14 × 10 = 840 kWh

∴η

all-day

= 840/870.21 = 0.965 or 96.5 %

2nd Transformer

Here 1/m =1/2 or m = 2 ∴ y + y/4 = 2.04

or y = 1.63 kW ; x = 0.14 kW

Output = 840 kWh ...as above

Iron loss for 24 hours = 0.41 × 24 = 9.84 kWh

Cu loss for 24 hours = 4 × 1.63 + 6 (1.63/4) + 14 (1.63/10

) = 9.19 kWh

Total loss = 9.84 + 9.19 = 19.03 kWh

∴η

all-day

= 840/859.03 = 0.978 or 97.8 %

Transformer

1183

Example 32.81. A 5-kVA distribution transformer has a full-load efficiency at unity p.f. of 95

%, the copper and iron losses then being equal. Calculate its all-day efficiency if it is loaded throughout

the 24 hours as follows :

No load for 10 hours Quarter load for 7 hours

Half load for 5 hours Full load for 2 hours

Assume load p.f. of unity. (Power Apparatus-I, Delhi Univ. 1987)

Solution. Let us first find out the losses from the given commercial efficiency of the transformer.

Output = 5 × 1 = 5 kW ; Input = 5/0.95 = 5.264 kW

Losses = (5.264 − 5.000) = 0.264 kW = 264W

Since efficiency is maximum, the losses are divided equally between Cu and iron.

∴ Cu loss at F.L. of 5 kVA = 264/2 = 132 W ; Iron loss = 132 W

Cu loss at one-fourth F.L. = (1/4)

× 132 = 8.2 W

Cu loss at one-half F.L. = (1/2)

× 132 = 33 W

Quarter load Cu loss for 7 hours = 7 × 8.2 = 57.4 Wh

Half-load Cu loss for 5 hours = 5 × 33 = 165 Wh

F.L. Cu loss for 2 hours = 2 × 132 = 264 Wh

Total Cu loss during one day = 57.4 + 165 + 264 = 486.4 Wh = 0.486 kWh

Iron loss in 24 hours = 24 × 132 = 3168 Wh = 3.168 kWh

Total losses in 24 hours = 3.168 + 0.486 = 3.654 kWh

Since load p.f. is to be assumed as unity.

F.L. output = 5 × 1 = 5 kW ; Half F.L. output = (5/2) × 1 = 2.5 kW

Quarter load output = (5/4) × 1 = 1.25 kW

Transformer output in a day of 24 hours = (7 × 1.25) + (5 × 2.5) + (2 × 5) = 31.25 kWh

all-day

31.25

100

(31.25 3.654)

= 89.53 %

Example 32.82. Find “all day” efficiency of a transformer having maximum efficiency of 98 %

at 15 kVA at unity power factor and loaded as follows :

12 hours – 2 kW at 0.5 p.f. lag

6 hours – 12 kW at 0.8 p.f. lag

6 hours – at no load. (Elect. Machines-I, Nagpur Univ. 1993)

Solution. Output = 15 × 1 = 15 kW, input = 15/0.98

Losses = (15/0.98 − 15) = 0.306 kW = 306 W

Since efficiency is maximum, the losses are divided equally between Cu and iron.

∴ Cu loss at 15 kVA = 306/2 = 153 W, Iron loss = 153 W

2 kW at 0.5 p.f. = 2/0.5 = 4 kVA, 12 kW at 0.8 p.f. = 12/0.8 = 15 kVA

Cu loss at 4 kVA = 153 (4/15)

= 10.9 W ; Cu loss at 15 kVA = 153 W.

Cu loss in 12 hrs = 12 × 10.9 = 131 Wh ; Cu loss in 6 hr = 6 × 153 = 918 Wh.

Total Cu loss for 24 hr = 131 + 918 = 1050 Wh = 1.05 kWh

Iron loss for 24 hrs = 24 × 153 = 3,672 Wh = 3.672 kWh

Output in 24 hrs = (2 × 12) + (6 × 12) = 96 kWh

Input in 24 hrs = 96 + 1.05 + 3.672 = 100.72 kWh

∴η

all-day

= 96 × 100/100.72 = 95.3 %

1184

Electrical Technology

Example 32.83. A 150-kVA transformer is loaded as follows :

Load increases from zero to 100 kVA in 3 hours from 7 a.m. to 10.00 a.m., stays at 100 kVA from

10 a.m. to 6 p.m. and then the transformer is disconnected till next day. Assuming the load to be

resistive and core-loss equal to full-load copper loss of 1 kW, determine the all-day efficiency and

the ordinary efficiency of the transformer. (Electrical Machines-II, Indore Univ. 1990)

Solution. Since load is resistive, its p.f. is unity.

Average load from 7 a.m. to 10 a.m. = (0 + 100)/2 = 50 kVA i.e., one-third F.L.

Load from 10 a.m. to 6 p.m. = 100 kVA i.e., 2/3 of F.L.

Ordinary Efficiency

In this case, load variations are not relevant.

Output = 150 × 1 = 150 kW ; Iron loss = Cu loss = 1 kW ; Total loss = 2 kW.

∴ Ordinary η = 150/(150 + 2) = 0.9868 or 98.68 %

All-day Efficiency

Cu loss from 7-10 a.m. = 3 × (1/3)

× 1 = 0.333 kWh

Cu loss from 10 a.m. to 6.p.m. = 8 × (2/3)

× 1 = 3.555 kWh

Total Cu loss for 24 hrs = 0.333 + 3.555 = 3.888 kWh

Total iron loss for 24 hrs = 24 × 1 = 24 kWh

Losses for a day of 24 hrs = 27.888 kWh

Output for 24 hrs = 3 × (50 × 1) + 8 (100 × 1) = 950 kWh

∴η

all-day

950 100

(950 27.888)

= 97.15 %

Example 32.84. Find the all-day efficiency of a 50 kVA distribution transformer having full load

efficiency of 94 % and full-load copper losses are equal to the constant iron losses. The loading of

the transformer is as follows, the power factor being 1.0.

(i) No load for 10 hours (ii) Half load for 5 hours

(iii)25 % load for 6 hours (iv) Full load for 3 hours. (Sambalpur University, 1998)

Solution. At full load unity p.f.

efficiency = 94 % =

50,000

50,000 2

50,000

0.94



−





,orP

1 1 0.94

50,000

20.94

−







= 25,000 ×

0.06

0.94

= 1596 Watts

Hence, full load Cu-losses = 1596 Watts

(a) Energy required in overcoming Cu-losses, during 24 hours

(i) No load for 10 hours : zero

(ii) At half load, Cu-losses = (0.5)

× 1596 Watts = 399

Energy in 5 hours =

399 5

1000

kWh = 1.995 kWh

(iii) At 25 % load, Cu-loss = (0.25)

× 1596 = 99.75 Watts

Energy in 6 hours =

6 99.75

1000

= 0.5985 kWh

(iv) Energy lost during 3 hours of full load =

1596 3

1000

= 4.788 kWh

(b) Energy lost in constant core-losses for 2 hours =

1596

1000

= 38.304 kWh

Transformer

1185

All-day efficiency =

350

100

350 38.304 7.3815

= 88.454 %

Example 32.85. A 10 kVA, 1-ph transformer has a core-loss of 40 W and full load ohmic loss of

100 W. The daily variation of load on the transformer is as follows :

6 a.m. to 1 p.m. 3 kW at 0.60 p.f.

1 p.m. to 5 p.m.

−

8kW at 0.8 p.f.

5 p.m. to 1 a.m. full load at u.p.f.

1 a.m. to 6 a.m. no load

Determine all day efficiency of the transformer (Amravati University, 1999)

Solution. Fractional loading (= x) and the output kWh corresponding to load variations can be

worked out in tabular form, as below :

S.N. Number x = x

in kW Output in Copper Loss

of hours kWh in kWh

3/0.6

= 0.5 0.50

× 0.10 = 0.025 3 × 7 = 21 0.025 × 7 = 0.175

8/0.8

= 1.0 0.10 8 × 4 = 32 0.1 × 4 = 0.40

10/1

= 1.0 0.10 10 × 8 = 80 0.1 × 8 = 0.8

4 5 Zero Zero Zero Zero

Output in kWh 21 + 32 + 80

= 133

Ohmic Loss, in kWh 0.175 + 0.40 + 0.80

= 1.375

Core loss during 24 Hrs =

400

1000

× 24 = 0.96 kWh

Hence, Energy efficiency (= All day Efficiency) =

133

133 +1.375 + 0.96

× 100 = 98.3 %

Example 32.86. A transformer has its maximum efficiency of 0.98 at 15 kVA at unity p.f.

During a day, it is loaded as follows :

12 hours : 2 kW at 0.8 p.f.

6 hours : 12 kW at 0.8 p.f.

6 hours : 18 kW at 0.9 p.f.

Find the all day efficiency. (Manomaniam Sundaranar Univ. April 1998)

Solution. Let 15 kVA be treated as full load.

Output at maximum efficiency = 15000 × 1 watts

Input = 15000/0.98 watts

Losses = Input − Output = 15000 (1/0.98 − 1) = 15000 × 2/98 = 306 Watts

At maximum efficiency, since the variable copper-loss and constant core-loss are equal.

Full load copper-loss = Constant core-loss = 306/2 = 153 Watts

Let the term x represents the ratio of required Load/Full load.

Output = 15 × cos φ

load kVA

Xmer Rating

1186

Electrical Technology

Following tabular entries simplify the calculations for all-day efficiency.

S.N. x Hrs x

in kW Energy in Copper Output during the

-loss in kWh period in kWh

2/0.5

12 0.01088 0.131 24

12/0.8

1.0

6 0.153 0.918 72

18/0.9

4/3

6 0.272 1.632 108

Total output during the day = 204 kWh

Total copper-loss during the day = 2.681 kWh

Total core-loss during the day = 0.153 × 24 = 3.672

All day efficiency = (204/210.353) × 100 = 96.98 %

Tutorial Problems 32.5

1. A 100-kVA distribution transformer has a maximum efficiency of 98 % at 50 % full-load and unity

power factor. Determine its iron losses and full-load copper losses.

The transformer undergoes a daily load cycle as follows :

Load Power factor Load duration

100 kVA 1.0 8 hrs

50 kVA 0.8 6 hrs

No load 10 hrs

Determine its all-day efficiency. (Electrical Engineering, MS Univ. Baroda 1979)

2. What is meant by energy efficiency of a transformer ?

A 20-kVA transformer has a maximum efficiency of 98 percent when delivering three-fourth full-load

at u.p.f. If during the day, the transformer is loaded as follows :

12 hours No load

6 hours 12 kWh, 0.8 p.f.

6 hours 20 kW, u.p.f.

Calculate the energy efficiency of the transformer.

(Electrical Technology-III, Gwalior Univ., 1980)

32.33. Auto-transformer

It is a transformer with one winding only, part of this being common to both primary and

secondary. Obviously, in this transformer the primary and secondary are not electrically isolated from

each other as is the case with a 2-winding transformer. But its theory and operation are similar to those

of a two-winding transformer. Because of one winding, it uses less copper and hence is cheaper. It is

used where transformation ratio differs little from unity. Fig. 32.60 shows both step down and step-up

auto-transformers.

As shown in Fig. 32.60 (a), AB, is primary winding having N

turns and BC is secondary winding

having N

turns. Neglecting iron losses and no-load current.

221

112

VN I

VNI

Transformer

1187

The current in section CB is vector

difference* of I

and I

. But as the two

currents are practically in phase oppo-

sition, the resultant current is (I

− I

)

where I

is greater than I

As compared to an ordinary 2-

winding transformer of same output, an

auto-transformer has higher efficiency

but smaller size. Moreover, its voltage

regulation is also superior.

Saving of Cu

Volume and hence weight of Cu, is

proportional to the length and area of

the cross-section of the conductors.

Now, length of conductors is proportional to the number of turns and cross-section depends on current.

Hence, weight is proportional to the product of the current and number of turns.

With reference to Fig. 32.60,

Wt. of Cu in section AC is ∝ (N

− N

) I

; Wt. of Cu in section BC is ∝ N

− I

∴ Total Wt. of Cu in auto-transformer ∝ (N

− N

) I

+ N

− I

)

If a two-winding transformer were to perform the same duty, then

Wt. of Cu on its primary ∝ N

; Wt. of Cu on secondary ∝ N

Total Wt. of Cu ∝N

+ N

∴

Wt. of Cu in auto-transformer

Wt. of Cu in ordinary transformer

121221

11 2 2

() ()

NNINII

NI N I

−+ −

= I −

112

11 ;

NNI

NIK



=− =− = =





+×

Wt. of Cu in auto-transformer

) = (1 − K) × (Wt. of Cu in ordinary

transformer W

)

∴ Saving = W

− W

= W

− (1 − K) W

= KW

∴ Saving = K × (Wt. of Cu in ordi-

nary transformer)

Hence, saving will increase as K ap-

proaches unity.

It can be proved that power trans-

formed inductively is input (1 − K).

The rest of the power = (K × input) is

conducted directly from the source to the

load i.e., it is transferred conductively to

the load.

Fig. 32.60

I=(I I)

1c2

Load

I=(I I)

2c1

Load

Step up auto-transformer

* In fact, current flowing in the common winding of the auto-transformer is always equal to the difference

between the primary and secondary currents of an ordinary transformer.

(a) (b)

1188

Electrical Technology

Uses

As said earlier, auto-transformers are used when K is nearly equal to unity and where there is no

objection to electrical connection between primary and secondary. Hence, such transformers are

used :

1. to give small boost to a distribution cable to correct the voltage drop.

2. as auto-starter transformers to give upto 50 to 60 % of full voltage to an induction motor during

starting.

3. as furnace transformers for getting a convenient supply to suit the furnace winding from a 230-V

supply

4. as interconnecting transformers in 132 kV/330 kV system.

5. in control equipment for 1-phase and 3-phase electrical locomotives.

Example 32.87. An auto-transformer supplies a load of 3 kW at 115 volts at a unity power

factor. If the applied primary voltage is 230 volts, calculate the power transferred to the load

(a) inductively and (b) conductively. (Basic Elect. Machines, Nagpur Univ, 1991)

Solution. As seen from Art 32.33

Power transferred inductively = Input (1 − K)

Power transferred conductively = Input × K

Now, K = 115/230 = 1/2, input ≅ output = 3 kW

∴ Inductively transferred power = 3 (1 − 1/2) = 1.5 kW

Conductivley transferred power = (1/2) × 3 = 1.5 kW

Example 32.88. The primary and secondary voltages

of an auto-transformer are 500 V and 400 V respectively.

Show with the aid of diagram, the current distribution in the

winding when the secondary current is 100 A and calculate

the economy of Cu in this particular case.

Solution. The circuit is shown in Fig, 30.61.

K = V

= 400/500 = 0.8

∴ I

= KI

= 0.8 × 100 = 80 A

The current distribution is shown in Fig. 32.61.

Saving = KW

= 0.8 W

- Art 32.33

∴ Percentage saving = 0.8 × 100 = 80

Example 32.89. Determine the core area, the number of turns and the position of the tapping

point for a 500-kVA, 50-Hz, single-phase, 6,600/5,000-V auto-transformer, assuming the following

approximate values : e.m.f. per turn 8 V. Maximum flux density 1.3 Wb/m

Solution. E = 4.44 f Φ

N volt

4.44 4.44 50

= 0.03604 Wb

Core area = 0.03604/1.3 = 0.0277 m

= 277 cm

Turns of h.v. side = 6600/8 = 825 ; Turns of L.V. side = 5000/8 = 625

Hence, tapping should be 200 turns from high voltage end or 625 turns from the common end.

32.34. Conversion of 2-Winding Transformer into Auto-transformer

Any two-winding transformer can be converted into an auto-transformer either step-down or

step-up. Fig. 32.62 (a) shows such a transformer with its polarity markings. Suppose it is a 20-kVA,

Fig. 32.61

500 V

100

20 A

100 A

400 V

80 A

Transformer

1189

2400/240 V transformer. If we employ additive polarity between the high-voltage and low-voltage sides,

we get a step-up auto-transformer. If, however, we use the subtractive polarity, we get a step-down auto-

transformer.

Fig. 32.62

(a) Additive Polarity

Connections for such a polarity are shown in Fig. 32.62 (b). The circuit is re-drawn in Fig. 32.62 (c)

showing common terminal of the transformer at the top whereas Fig. 32.62 (d) shows the same circuit

with common terminal at the bottom. Because of additive polarity, V

= 2400 + 240 = 2640 V and V

2400 V. There is a marked increase in the kVA of the auto-transformer (Ex. 32.90). As shown in

Fig. 32.62 (d), common current flows towards the common terminal. The transformer acts as a step-up

transformer.

(b) Subtractive Polarity

Such a connection is shown in Fig. 32.63 (a). The circuit has been re-drawn with common polarity

at top in Fig. 32.63 (b) and at bottom in Fig. 32.63 (c). In this case, the transformer acts as a step-down

auto-transformer.

Fig. 32.63

The common current flows away from the common terminal. As will be shown in Example 32.91, in

this case also, there is a very large increase in kVA rating of the auto-transformer though not as marked as

in the previous case. Here, V

= 2400 − 240 = 2160 V.

Example 32.90. For the 20-kVA, 2400/240-V two-winding step-down transformer shown in

Fig. 32.63 (a) connected as an auto-transformer with additive polarity as shown in Fig. 30.61 (d),

compute

1190

Electrical Technology

(i) original current capacity of HV-windings.

(ii) original current capacity of LV-windings.

(iii) k VA rating of auto-transformer using current capacity of current LV winding as calculated

in (ii) above.

(iv) per cent increase in kVA capacity of auto-transformer as compared to original two-winding

transformer.

(v) values of I

and I

in Fig. 30.61 (d) from value of I

used in (iii) above.

(vi) per cent overload of 2400-V winding when used as an auto-transformer.

(vii) comment on the results obtained.

Solution. (i) I

= 20 × 10

/2400 = 8.33 A (ii) I

= I

/K = 8.33 × 10 = 83.3 A

(iii) kVA rating of auto-transformer V

= 2640 × 83.3 × 10

−3

= 220 kVA

(iv) Per cent increase in kVA rating =

220

100

×=

1100%

(v) I

= 220 × 10

/2400 = 91.7A, I

= I

− I

= 91.7 − 83.3 = 8.4 A

(vi) Per cent overload of 2400 V winding = 8.4 × 100/8.33 = 100.8%

(vii) As an auto-transformer, the kVA has increased tremendously to 1100% of its original value with

LV coil at its rated current capacity and HV coil at negligible overload i.e. 1.008 × rated load.

Example 32.91. Repeat Example 30.64 for subtractive polarity as shown in Fig. 32.62 (c).

Solution. (i) I

= 8.33 A (ii) I

= 83.3 A

(iii) New kVA rating of auto-transformer is 2160 × 83.3 × 10

−3

= 180 kVA

(iv) Per cent increase in kVA rating =

180

100

×=

900%

(v) I

= 180 × 10

/2400 = 75 A, I

= I

− I

= 83.3 − 75 = 8.3 A

(vi) Per cent overload of 2400V winding = 8.3 × 100/8.33 = 100%

(vii) In this case, kVA has increased to 900% of its original value as a two-winding transformer with

both low-voltage and high-voltage windings carrying their rated currents.

The above phenomenal increase in kVA capacity is due to the fact that in an auto-transformer energy

transfer from primary to secondary is by both conduction as well as induction whereas in a

2-winding transformer it is by induction only. This extra conductive link is mainly responsible for the

increase in kVA capacity.

Example 32.92. A 5-kVA, 110/110-V, single-phase, 50-Hz transformer has full-load efficiency of

95% and an iron loss of 50 W. The transformer is now connected as an auto-transformer to a

220-V supply. If it delivers a 5-kW load at unity power factor to a 110-V circuit, calculate the

efficiency of the operation and the current drawn by the high-voltage side.

(Electric Machinery-II, Banglore Univ. 1991)

Solution. Fig. 32.64 (a)

shows the normal connection

for a 2-winding transformer. In

Fig. 32.64 (b) the same unit

has been connected as an auto

transformer. Since the two

windings are connected in se-

ries, voltage across each is 110

The iron loss would

Fig. 32.64

110

Prim

ary

Secondary

110

(

)

2-W

inding

Transform

220

110

(

)

Auto-Transform

Load

Transformer

1191

remain the same in both connections. Since the auto-transformer windings will each carry but half the

current as compared to the conventional two-winding transformer, the copper loss will be one-fourth of the

previous value.

Two-winding Transformer

η = 0.95 ∴ 0.95 =

output

5,000

output + losses 5,000 50 Cu loss

∴ Cu loss = 212 W

Auto-transformer

Cu loss = 212/4 = 53 W; Iron loss = 50 W ∴ η =

5,000

0.9797 or . %

5,000 53 50

97 97

Current of the h.v. side = 5103/220 = 23.2 A

Example 32.93. A transformer has a primary voltage rating of 11500 volts and secondary volt-

age rating of 2300 volts. Two windings are connected in series and the primary is connected to a

supply of 11500 volts, to act as a step-up auto transformer. Determine the voltage output of the

transformer.

Question extended : If the two winding transformer is rated at 115 kVA, what will be the kVA

raitng of the auto-transformer ? (Madras University, 1997)

Solution. As in Fig. 32.65 (a), 115 kVA, 11500/2300 V, transformer has the current ratings of 10 A

and 50 A.

Referring to Fig. 32.65 (b), the step-up connections have been shown. Winding currents have to

be at the same rated values. As in Fig. 32.65 (b), the voltage obtainable at B

− B

is 13800 V, and from

a load-current of 50 A can be supplied.

kVA rating = 13800 × 50 × 10

−3

= 690

(b) 2 winding transformer Rating : 115kVA (b) Auto transformer

Fig. 32.65

Example 32.94. An 11500/2300 V transformer is rated at 100 kVA as a 2 winding transformer.

If the two windings are connected in series to form an auto-transformer, what will be the possible

voltage ratios ? (Manonmaniam Sundaranar Univ. April 1998)

Solution. Fig. 32.66 (a) shows this 2-winding transformer with

rated winding currents marked .

Rated current of 11.5 kV winding = 100 × 100/11500 = 8.7

Amp

Rated current of 2300 V winding = 43.5 Amp

Fig. 32.66 (b) and Fig. 32.66 (c) show autotransformer

Fig. 32.66 (

2 winding transformer

10A

11500/2300V

PRI SEC 50

1192

Electrical Technology

connections. On H.V. side, they have a rating of 13.8 kV. On L.V. side, with connection as in Fig. 32.66

(b), the rating is 2300 V. On L.V. side of Fig. 32.66 (c), the output is at 11.5 kV.

Thus, possible voltage ratios are : 13800/2300 V and 13800/11500 V.

With both the connections, step-up or step-down versions are possible.

Extension of Question : Calculate kVA ratings in the two cases.

Fig. 32.66

Windings will carry the rated currents, while working out kVA outputs.

In Fig. 32.66 (b), Input current (into terminal A

of windings A

-A

) can be 8.7 Amp with H.V.-side-

voltage ratings as 13.8 kV. Transformation ratio = 13800/2300 = 6

Hence, kVA rating = 13.8 × 8.7 = 120

Output current = 120 × 1000/2300 = 52.2 Amp

Current in the winding B

-B

= Difference of Output current and Input current

= 52.2 − 8.7 = 43.5 A, which is the rated current of the winding B

-B

In Fig. 32.66 (c). Similarly, transformation ratio = 13800/11500 = 1.2

kVA rating = 13800 × 43.5 × 10

−`3

= 600

Output current = 600 × 1000/11500 = 52.2 Amp

Current carried by common winding = 52.2 − 43.5 = 8.7 A, which is rated current for the winding

-A

. Thus, with the same two windings give, a transformation ratio closer to unity gives higher kVA

rating as an auto transformer.

Thus, a 100 kVA two winding transformer is reconnected as an autotransformer of 120 kVA with

transformation ratio as 6, and becomes a 600 kVA autotransformer with transformation ratio as 1.2.

Example 32.95. A two-winding transformer is

rated at 2400/240 V, 50-kVA. It is re-connected as a

step-up auto-transformer, with 2400 V input. Calcu-

late the rating of the auto-transformer and the induc-

tively and conductively transferred powers while deliv-

ering the rated output at unity power-factor.

(Nagpur University, Winter 1999)

Solution. With 50 kVA as the rating, the rated currents

on the two sides are 20.8 A (2400-V side) and 208 A

(240 − V side). With the required re-connection, the 2400-

V winding will work as a common winding. As shown in

Fig. 32.67, the winding common to input and output can

carry 20.8 A, the output current can be 208 A with a

voltage of 2640 V, which means that the output of this auto

Fig. 32.67

Transformer

1193

transformer is (2640 × 208) = 550 kVA.

The corresponding input-current is

(208 × 2640/2400) = 229 A.

The ratio of turns in this case is given by

k = 2640/2400 = 1.1

With the step-up job, and k =1.1

Rating of Auto-transformer

Rating as two-winding transformer

1.1

~1 0.1

This gives the rating as auto-transformer of 550 kVA.

At unity power-factor, the rated load = 550 kW.

Out of this, the “inductively” transferred power

= Power handled by the common winding

= (2400 V) × (20.8 A) × 10

− 3

= 50 kW.

= Rated output as a two-winding transformer.

Remaining Power = 550 kW − 50 kW = 500 kW.

This power of 500 kW is “conductively” transferred as is clear from the division of currents at the

input node. Out of the total current of 229 A from the source, 208 A goes straight to the output. The

remaining current of 20.8 A is through the “common” and “inductive” path, as marked in the

Fig. 32.67.

32.35. Parallel Operation of Single-phase Transformers

For supplying a load in excess of the rating of

an existing transformer, a second transformer may

be connected in parallel with it as shown in Fig.

32.68. It is seen that primary windings are

connected to the supply bus bars and secondary

windings are connected to the load bus-bars. In

connecting two or more than two transformers in

parallel, it is essential that their terminals of similar

polarities are joined to the same bus-bars as in Fig.

32.68. If this is not done, the two e.m.fs. induced

in the secondaries which are paralleled with incorrect

polarities, will act together in the local secondary

circuit even when supplying no load and will hence

produce the equivalent of a dead short-circuit as

shown in Fig. 32.69.

There are certain definite conditions which

must be satisfied in order to avoid any local circulating currents and to ensure that the transformers share the

common load in proportion to their kVA ratings. The conditions are :

1. Primary windings of the transformers should be suitable for the supply system voltage and fre-

quency.

2. The transformers should be properly connected with regard to polarity.

3. The voltage ratings of both primaries and secondaries should be identical. In other words, the

transformers should have the same turn ratio i.e. transformation ratio.

4. The percentage impedances should be equal in magnitude and have the same X/R ratio in order to

avoid circulating currents and operation at different power factors.

Fig. 32.68

1194

Electrical Technology

5. With transformers having different kVA ratings, the equivalent impedances should be

inversely proportional to the individual kVA rating if circulating currents are to be avoided.

Of these conditions, (1) is easily comprehended ; condition (2) is absolutely essential (otherwise

paralleling with incorrect polarities will result in dead short-circuit). There is some lattitude possible

with conditions (3) and (4). If condition (3) is not exactly satisfied i.e. the two transformers have

slightly different transformation or voltage ratios, even then parallel operation is possible. But due to

inequality of induced e.m.fs. in secondaries, there will be even on no-load, some circulating current between

them (and therefore between the primary windings also) when secondary terminals are connected in parallel.

When secondaries are loaded, this localized circulating current will tend to produce unequal loading condition.

Hence, it may be impossible to take full kVA output

from the parallel connected group without one of the

transformers becoming over-heated.

If condition (4) is not exactly satisfied i.e.

impedance triangles are not identical in shape and

size, parallel operation will still be possible, but the

power factors at which the two transformers operate

will be different from the power factor of the common

load. Therefore, in this case, the two transformers

will not share the load in proportion to their kVA

ratings.

It should be noted that the impedances of two

transformers may differ in magnitude and in quality (i.e. ratio of equivalent resistance to reactance). It is

worthwhile to distinguish between the percentage and numerical value of an impedance. For example,

consider two transformers having ratings in the ratio 1 : 2. It is obvious that to carry double the current, the

latter must have half the impedance of the former for the same regulation. For parallel operation, the

regulation must be the same, this condition being enforced by the very fact of their being connected in

parallel. It means that the currents carried by the two transformers are proportional to their ratings provided

their numerical impedances are inversely proportional to these ratings and their percentage impedances

are identical.

If the quality of the two percentage impedances is different (i.e. ratio of percentage resistance to

reactance is different), then this will result in divergence of phase angle of the two currents, with the

result that one transformer will be operating with a higher and the other with a lower power factor than

that of the combined load.

(a) Case 1. Ideal Case

We will first consider the ideal case of two transformers having the same voltage ratio and having

impedance voltage triangles identical in size and shape.

Let E be the no-load secondary voltage of each transformer and V

the terminal voltage ; I

and

the currents supplied by them and I-the total current, lagging behind V

by an angle φ (Fig. 32.70(a)



Load

Fig. 32.70 (a) Fig. 32.70 (b)

Fig. 32.69

Load Busbars

Primary Busbars

Transformer

1195

In Fig. 32.70 (b) a single triangle ABC represents the identical impedance voltage triangles of both the

transformers. The currents I

and I

of the individual transformers are in phase with the load current I and

are inversely proportional to the respective impedances. Following relations are obvious.

I = I

+ I

; V

= E − I

= E − IZ

Also I

= I

or I

= Z

∴ I

= IZ

/(Z

+ Z

) and I

= IZ

/(Z

+ Z

)

(b) Case 2. Equal Voltage Ratios

Let us assume that no-load voltages of both secondaries is the same i.e. E

= E

= E, and that the two

voltages are coincident i.e. there is no phase difference between E

and E

, which would be true if the

magnetising currents of the two

transformers are not much different

from each other. Under these

conditions, both primaries and

secondaries of the two transformers

can be connected in parallel and there

will circulate no current between them

on on-load.

However, if we neglect the

magnetising admittances, the two transformers can be connected as shown by their equivalent circuits in Fig.

32.71. The vector diagram is shown in Fig. 32.72.

From Fig. 32.71 (a) or (b) it is seen that it represents two impedances in parallel. Considering all

values consistently with reference to secondaries, let

, Z

= impedances of the transformers

, I

= their respective currents

= common terminal voltage

I = combined current

It is seen that I

= I

= IZ

...(i)

where Z

is the combined impedance of Z

and Z

in parallel.

1/Z

=1/Z

+ 1/Z

Hence Z

= Z

/(Z

+ Z

) ...(ii)

From equation (i), we get

= IZ

/(Z

+ Z

) and I

= IZ

/ (Z

+ Z

)

Multiplying both sides by common terminal voltage V

, we have

; Similarly V

Let V

I ×10

−3

= S-the combined load kVA. Then, the kVA carried by each transformer is

1+ /

and

AB BA

+1+/

SS S

ZZ ZZ

...(iii)

Hence, S

and S

are obtained in magnitude as well as in phase from the above vectorial

expressions.

The above problem may be solved graphically, although somewhat more laboriously. As shown in Fig.

32.72, drawn I

and I

with an angular difference of (φ

−φ

) and magnitude (according to some suitable

scale) inversely proportional to the respective impedances. Vector sum of I

and I

gives total combined

current I. The phase angle and magnitude of I will be known from the conditions of loading, so that angle φ

Fig. 32.72



IZ =

AA BB

Fig. 32.71

()a ()b

Load

1196

Electrical Technology

between V

and I will be known. Inserting this, the transformer currents I

and I

become known in

magnitude and phase with respect to V

Note. (a) In equation (iii) above, it is not necessary to use the ohmic values of resistances and reactances,

because only impedance ratios are required.

(b) The two percentage impedances must be adjusted to the same kVA in the case of transformers of

different rating as in Ex. 32.101.

transformation ratio are to share the load equally, then their impedances should be equal i.e. equal resistances and

reactances and not numerical equality of impedances. In general, for transformers of different ratings but same

transformation ratio, their equivalent impedances must be inversely proportional to their ratings if each transformer

is to assume a load in proportion to its rating. For example, as said earlier, a transformer operating in parallel with

another of twice the rating, must have an impedance twice that of the large transformer in order that the load may

be properly shared between them.

Example 32.96. Two 1-phase transformers with equal turns have impedances of (0.5 + j3) ohm

and (0.6 + j10) ohm with respect to the secondary. If they operate in parallel, determine how they

will share a total load of 100 kW at p.f. 0.8 lagging ? (Electrical Technology, Madras Univ. 1987)

Solution. Z

= 0.5 + j 3 = 3.04 ∠80.6° Z

= 0.6 + j 10 = 10.02 ∠86.6°

+ Z

=1.1 + j13 = 13.05∠85.2°

Now, a load of 100 kW at 0.8 p.f. means a kVA of 100/0.8 = 125. Hence,

S = 125 ∠ − 36.9°

125 36.9 10.02 86.6

13.05 85.2

∠− °× ∠ °

∠°

= 96 ∠ − 35.5°

= a load of 96 × cos 35.5° = 78.2 kW

125 36.9 3.04 80.6

13.05 85.2

∠− °× ∠ °

∠°

= 29.1 ∠− 41.5°

= a load of 29.1 × cos 41.5° = 21.8 kW

Note. Obviously, transformer A is carrying more than its due share of the common load.

Example 32.97. Two single-phase transformers A and B are connected in parallel. They have

same kVA ratings but their resistances are respectively 0.005 and 0.01 per unit and their leakage

reactances 0.05 and 0.04 per unit. If A is operated on full-load at a p.f. of 0.8 lagging, what will be

the load and p.f. of B ? (A.C. Machines-I, Jadavpur Univ. 1985)

Solution. In general, S

and S

where S is the total kVA supplied and Z

and Z

are the percentage impedances of these transformers.

Now, Z

= 0.005 + j 0.05 per unit ; % Z

= 0.5 + j 5 and %Z

= 1 + j 4

Let S

∠ − 36.87°

where S

represents the rating of transformer A (and also of B).

= S

∠− 36.87° ×

(0.5 5)

(1 4)

= S

∠− 36.87°×

20.7

∠8.3° = 1.22 S

∠− 28.57°

Transformer

1197

It is obvious that transformer B is working 22% over-load and its power factor is

cos 28.57° = 0.878 (lag)

Example 32.98. Two 1-phase transformers A and B rated at 250 kVA each are operated in

parallel on both sides. Percentage impedances for A and B are (1 + j 6) and (1.2 + j 4.8) respectively.

Compute the load shared by each when the total load is 500 kVA at 0.8 p.f. lagging.

(Electrical Machines-II, Indore Univ. 1989)

Solution.

2.2 10.8

=0.55∠2.1° ;

1.2 4.8

+ 2.2 10.8

= 0.45∠− 2.5°

= 500∠− 36.9° × 0.45∠− 2.5° = 225

∠−∠−

∠−39.4°

=500∠− 36.9° × 0.55∠2.1° = 275

∠−∠−

∠− 34.8°

Obviously, transformer B is overloaded to the extent of (275 − 250) × 100/250 = 10%. It carries

(275/500) × 100 = 55% of the total load.

Example 32.99. Two 100-kW, single-phase transformers are connected in parallel both on the

primary and secondary. One transformer has an ohmic drop of 0.5% at full-load and an inductive

drop of 8% at full-load current. The other has an ohmic drop of 0.75% and inductive drop of 2%.

Show how will they share a load of 180 kW at 0.9 power factor.

(Elect. Machines-I, Calcutta Univ. 1988)

Solution. A load of 180 kW at 0.9 p.f. means a kVA of 180/0.9 = 200

∴ Load S = 200 − 25.8°

(0.5 8) (0.5 8)(1.25 12)

(1 .2 5 1 2)

1.25 12

jjj

++−

96.63 4

96.65 2.4

145.6 145.6

∠°

= 0.664∠2.4°

(0.75 4)(1.25 12)

145.6

jj+−

48.94 4 49.1 5

145.6 145.6

j−∠−°

= 0.337∠−5°

= 200∠− 25.8° × 0.337 ∠−5° = 67.4∠−30.8°

∴ kW

= 67.4 × cos 30.8° = 67.4 × 0.859 = 57.9 kW

= 200 ∠−25.8° × 0.664 ∠ 2.4° = 132.8 ∠−23.4

= 132.8 × cos 23.4° = 132.8 × 0.915 = 121.5 kW

Note. Second transformer is working 21.5% over-load. Also, it shares 65.7% of the total load.

Example 32.100. A load of 200 kW at 0.85 power factor lagging is to be shared by two transform-

ers A and B having the same ratings and the same transformation ratio. For transformer A, the full-

load resistive drop is 1% and reactance drop 5% of the normal terminal voltage. For transformer

B the corresponding values are : 2% and 6%. Calculate the load kVA supplied by each transformer.

Solution. Z

=1 + j 5; Z

= Z + j 6

1198

Electrical Technology

=0.8 + j 0.1 ;

= 1.23 − j 0.514

Load kVA = kW/p.f. = 200/0.85 = 235

kVA = S = 235(0.85 − j 0.527)

= 200 − j 123.8 ( cos φ = 0.85; sin φ = 0.527)

AB AB

S=S

Z+Z 1+(Z/Z)

200 123.8 200 123.8

1 (0.8 0.1) 1.8 0.1

−−

++ +

= 107.3 − j 74.9

(107.3 74.9 )

= 131; cos φ

= 107.3/131 = 0.82 (lag)

Similarly S

Z+Z

1+(Z /Z )

200 123.8 200 123.8

1 (1.23 0.514) 2.23 0.154

−−

+− −

= 93 − j 49

∴ S

(93 49 )

= 105; cos φ

= 90/105 = 0.888 (lag)

(As a check, S = S

+ S

= 131 + 105 = 236. The small error is due to approximations made in

calculations.)

Example 32.101. Two 2,200/110-V, transformers are operated in parallel to share a load of

125 kVA at 0.8 power factor lagging. Transformers are rated as below :

A : 100 kVA ; 0.9% resistance and 10% reactance

B : 50 kVA ; 1.0% resistance and 5% reactance

Find the load carried by each transformer. (Elect. Technology, Utkal Univ. 1989)

Solution. It should be noted that the percentages given above refer to different ratings. As

pointed out in Art. 32.35, these should be adjusted to the same basic kVA, say, 100 kVA.

% Z

= 0.9 + j 10 ; % Z

= (100/50)(1 + j 5) = (2 + j 10)

Z+Z

0.9 10 (0.9 10) (2.9 20)

(2.9 20)

2.9 20

jjj

++−

(202.6 11)

202.9

408.4 08.4

∠3.1°

= 0.4968 ∠ 3.1°

Z+Z

(2 10)(2.9 20) 206 11

408.4 08.4

jj j+− −

206.1

408.4

∠ − 3.1°

= 0.504 ∠− 3.1°

Also cos φ = 0.8, φ = cos

−1

(0.8) = 36.9°

Z+Z

= 125 ∠− 36.9° × 0.504 ∠− 3.1° = 63 ∠ − 40°

Z+Z

= 125 ∠− 36.9° × 0.4968 ∠ 3.1° = 62.1 ∠ − 33.8°

Transformer

1199

Example 32.102. A 500-kVA transformer with 1% resistance and 5% reactance is connected in

parallel with a 250-kVA transformer with 1.5% resistance and 4% reactance. The secondary

voltage of each transformer is 400 V on no-load. Find how they share a load of 750-kVA at a p.f. of

0.8 lagging. (Electrical Machinery-I, Madras Univ. 1987)

Solution. It may be noted that percentage drops given above refer to different ratings. These should

be adjusted to the same basic kVA i.e. 500 kVA.

% Z

= 1 + j 5 = 5.1 ∠ 78.7° ; % Z

()

500

250

(1.5 + j 4) = 3 + j 8 = 8.55 ∠ 69.4°

%(Z

) =4 + j 13 = 13.6 ∠ 72.9° ; S = 750 ∠− 36.9°

Z+Z

750 36.9 8.55 69.4

13.6 72.9

∠− °× ∠ °

∠°

= 470 ∠ − 40.4°

= 470 kVA at p.f. of 0.762 lagging

Z+Z

750 36.9 5.1 78.7

13.6 72.9

∠− °× ∠ °

∠°

= 280 ∠ − 31.1°

= 280 kVA at p.f. 0.856 lagging

Note. The above solution has been attempted vectorially, but in practice, the angle between I

and I

is so

small that if instead of using vectorial expressions, arithmetic expressions were used, the answer would not be

much different. In most cases, calculations by both vectorial and arithmetical methods generally yield results that

do not differ sufficiently to warrant the more involved procedure by the vector solution. The above example will

now be attempted arithmetically :

= 5.1 Ω, Z

= 8.55 Ω ; I

= Z

= 8.55/5.1 = 1.677 ∴ I

= 1.677 I

Total current = 750,000/400 = 1875 A

I = I

+ I

; 1875 = 1.677I

+ I

= 2.677 I

∴ I

= 1875/2.677

∴ S

= 400 × 1875/2.677 × 100 = 280 kVA

= 1.677 × 1875/2.677

= 400 × 1.677 × 1875/2.677 × 1000 = 470 kVA

Example 32.103. Two single-phase transformers A and B of equal voltage ratio are running in

parallel and supply a load of 1000 A at 0.8 p.f. lag. The equivalent impedances of the two transformers

are (2 + j3) and (2.5 + j5) ohms respectively. Calculate the current supplied by each transformer

and the ratio of the kW output of the two transformers.

(Electrical Machines-I, Bombay Univ. 1986)

Solution. Z

= (2 + j 3), Z

= (2.5 + j 5)

Now, =

2.5 5

= (1.54+ j 0.2) ; I

= I

(1.54 + j 0.2)

Taking secondary terminal voltage as reference vector, we get

I = 1000(0.8 − j0.6) = 800 − j 600 = 200 (4 − j 3)

Also, I = I

+ I

= I

(1.54 + j 0.2) + I

= I

(2.54 + j 0.2)

∴ 200(4 − j 3) = I

(2.54 + j 0.2) ; I

= 294.6 − j 259.5 = 392.6

∠−∠−

∠− 41.37°

= I

(1.54 + j 0.2) = (294.6 − j 259.5) (1.54 + j 0.2) = 505.6 − j 340.7

= 609.7 ∠− 33.95°

The ratio of the kW output is given by the ratio of the in-phase components of the two currents.

output of

505.6 1.7

294.6 1

Note. Arithmetic solution mentioned above could also be attempted.

1200

Electrical Technology

Example 32.104. Two transformers A and B, both of no-load ratio 1,000/500-V are connected

in parallel and supplied at 1,000 V. A is rated at 100 kVA, its total resistance and reactance being

1% and 5% respectively, B is rated at 250 kVA, with 2% resistance and 2% reactance. Determine

the load on each transformer and the secondary voltage when a total load of 300 kVA at 0.8 power

factor lagging is supplied.

Solution. Let the percentage impedances be adjusted to the common basic kVA of 100.

Then %Z

=(1+ j 5) ; %Z

= (100/250)(2 + j 2) = (0.8 + j 0.8)

Z+Z

(1 .8 5 .8)

= 0.839 ∠ 5.9

Z+Z

0.8 0.8

(1 .8 5 .8)

= 0.1865 ∠ − 27.6°

Now, S = 300 ∠ − 36.9° = 240 − j 180

Z+Z

= 300 ∠− 36.9° × 0.1865 ∠ − 27.6° = 55.95 ∠ − 64.5°

Z+Z

= 300 ∠ − 36.9° × 0.839 ∠ 5.9° = 251.7 ∠ − 31°

Since Z

and Z

are in parallel, their combined impedance on 100 kVA basis is

Z+Z

(1 5)(0.8 0.8)

(1 .8 5 .8)

= 0.6 + j 0.738

Percentage drop over Z

is = (0.6 × 240/100) + (0.738 × 180/100) = 1.44 + 1.328 = 2.768%

∴ V

500 2.768

500

100



−





= 486.12 V

Example 32.105. Two 1-φ transformers are connected in parallel at no-load. One has a turn

ratio of 5,000/440 and a rating of 200 kVA, the other has a ratio of 5,000/480 and a rating of

350 kVA. The leakage reactance of each is 3.5%.

What is the no-load circulation current expressed as a percentage of the nominal current of the

200 kVA transformer.

Solution. The normal currents are

200 × 10

/440 = 455 A and 350 × 10

/480 = 730 A

Reactances seen from the secondary side are

3.5 440

100 455

= 0.034 Ω,

3.5 480

100 730

= 0.023 Ω

The difference of induced voltage is 40 V. The circulating current is

= 40/0.057 = 704 A = 1.55 times the normal current of 200 kVA unit.

Tutorial Problems 32.6

1. Two single-phase transformers A and B of equal voltage ratio are running in parallel and supplying a

load requiring 500 A at 0.8 power factor lagging at a terminal voltage of 400 V. The equivalent

impedances of the transformers, as referred to secondary windings, are (2 + j 3) and (2.5 + j 5) ohm.

Calculate the current supplied by each transformer.

(Note. The student is advised to try by arithmetic method also). [I

= 304 A ; I

= 197 A]

2. Two single-phase transformers A and B are operating in parallel and supplying a common load of

1000 kVA at 0.8 p.f. lagging. The data regarding the transformers is as follows :

Transformer

1201

Transformer Rating %Resistance % Reactance

A 750 kVA 3 5

B 500 kVA 2 4

Determine the loading of each transformer. [S

= 535

∠− ∠−

∠− 34.7° ; S

= 465

∠−∠−

∠− 39.3°]

3. Two transformers A and B give the following test results. With the low-tension side short-circuited,

A takes a current of 10 A at 200 V, the power input being 1000 W. Similarly, B takes 30 A at 200 V

; the power input being 1,500 W. On open circuit, both transformers give a secondary voltage of 2200

when 11,000 volts are applied to the primary terminals. These transformers are connected in parallel

on both high tension and low tension sides Calculate the current and power in each transformer when

supplying a load of 200 A at 0.8 power factor lagging. The no-load currents may be neglected.

(Hint : Calculate Z

and Z

from S.C. test data)

= 50.5 A, P

= 100 kW ; I

= 151 A, P

= 252 kW] (London University)

4. Two 6600/250-V transformers have the following short-circuit characteristics : Applied voltage

200 V, current 30 A, power input 1,200 W for one of the transformers ; the corresponding data for the

other transformer being 120 V, 20 A and 1,500 W. All values are measured on the H.V. side with the

L.V. terminals short circuited. Find the approximate current and the power factor of each transformer

when working in parallel with each other on the high and low voltage sides and taking a total load of

150 kW at a p.f. of 0.8 lagging from the high voltage bus-bars.

= 13.8 A, cos

φφ

φA = 0.63 ; I

= 15.35 A , cos

φφ

φB = 0.91] (Electrical Engg-IV, Baroda Univ. 1978)

5. Two 11,000/2,200-V, 1-phase transformers are connected in parallel to supply a total load of 200 at 0.8

p.f. lagging at 2,200 V. One transformer has an equivalent resistance of 0.4 Ω and equivalent reac-

tance of 0.8 Ω referred to the low-voltage side. The other has equivalent resistance of 0.1 Ω and a

reactance of 0.3 Ω. Determine the current and power supplied by each transformer.

[52 A ; 148 A ; 99 A ; 252 kW]

6. A 2,000-kVA transformer (A) is connected in parallel with a 4,000-kVA transformer (B) to supply a

3-phase load of 5,000 kVA at 0.8 p.f. lagging. Determine the kVA supplied by each transformer

assuming equal no-load voltages. The percentage volt drops in the windings at the rated loads are as

follows :

Transformer A : resistance 2% ; reactance 8%

Transformer B : resistance 1.6% ; reactance 3%

[A : 860 kVA, 0.661 lag ; B : 4170 kVA, 0.824 lag] (A.C. Machines-I, Jadavpur Univ. 1979)

7. Two single-phase transformers work in parallel on a load of 750 A at 0.8 p.f. lagging. Determine

secondary voltage and the output and power factor of each transformer. Test data are :

Open circuit : 11,00/13,300 V for each transformer

Short circuit : with h.v. winding short-circuit

Transformer A : secondary input 200 V, 400 A, 15 kW

Transformer B : secondary input 100 V, 400 A, 20 kW

[3,190 VA : 80 kVA, 0.65 lag ; B : 1,615 kVA ; 0.86 lag]

In this case, the voltage ratios (or transformation

ratios) of the two transformers are different. It means

that their no-load secondary voltages are unequal. Such

cases can be more easily handled by phasor algebra

than graphically.

Let E

, E

= no-load secondary e.m.f.s of the

two transformers.

= load impedance across the secondary.

The equivalent circuit and vector diagram are also

shown in Fig. 32.73 and 32.74.

Fig. 32.73

I=I +I

Load

1202

Electrical Technology

It is seen that even when secondaries are on no-load, there will be some cross-current in them because

of inequality in their induced e.m.fs. This circulating current I

is given by

= (E

−−

− E

)/(Z

+ Z

) ...(i)

As the induced e.m.fs. of the two transformers are equal to the total drops in their respective circuits.

∴ Z

+ V

; E

= I

+ V

Now, V

=IZ

= (I

+ I

) Z

where Z

= load impedance

= I

+ (I

+ I

) Z

...(ii)

= I

+ (I

+ I

) Z

...(iii)

∴∴

∴ E

−−

− E

−−

− I

...(iv)

= [(E

−−

− E

) + I

]/Z

Substituting this value of I

in equation (iii),

we get

+ [{(E

−−

− E

) + I

}/Z

]/Z

=[E

−−

− (E

−−

− E

]/[Z

+ Z

.(Z

+ Z

)] ...(v)

From the symmetry of the expression, we get

= [E

+ (E

−−

− E

+ Z

)] ...(vi)

Also, I = I

+ I

Á Â Â Á

Á Â L Á Â

Å Æ + Å Æ

Æ Æ + Æ (Æ + Æ )

By multiplying the numerator and denominator of this equation by 1/Z

and the result by Z

we get

= IZ

Á A Â B

Á B L

ÅÆ+ Å Æ

Æ + 1/Æ + 1/ Æ

The two equations (v) and (vi) then give the values of secondary currents. The primary currents may

be obtained by the division of transformation ratio i.e. K and by addition (if not negligible) of the no-load

current. Usually, E

and E

have the same phase (as assumed above) but there may be some phase

difference between the two due to some difference of internal connection in parallel of a star/star and a star/

delta 3-phase transformers.

If Z

and Z

are small as compared to Z

i.e. when the transformers are not operated near short-

circuit conditions, then equations for I

and I

can be put in a simpler and more easily under-standable

form. Neglecting Z

in comparison with the expression Z

+ Z

), we have

)

−

Á B A B

LA B A B

Å Æ Å E

Æ (Æ + Æ Z + Z

...(vii)

)

−

BA A B

LA B A B

Å Æ Å E

Æ (Æ + Æ Z + Z

...(viii)

The physical interpretation of the second term in equations (vii) and (viii) is that it represents the

cross-current between the secondaries. The first term shows how the actual load current divides

between the loads. The value of current circulating in transformer secondaries (even when there is no-

load) is given by* I

= (E

−−

− E

)/(Z

+ Z

) assuming that E

> E

. It lags behind E

by an angle α

given by tan α = (X

+ X

)/(R

+ R

). If E

+ E

the ratios of the currents are inversely as the

impedances (numerical values).

* Under load conditions, the circulating current is

ABABL

ZZZZZ

−

If Z

= ∞ i.e. on open-circuit, the expression reduces to that given above.

Fig. 32.74



Transformer

1203

If in Eq. (iv) we substitute I

= I

−−

− I

and simplify, we get

−

BAB

AB AB

IZ E E

Z+Z Z+Z

...(ix)

Similarly, if we substitute I

= I

−−

− I

and simplify, then

−

AAB

AB AB

IZ E E

Z+Z Z+Z

...(x)

In a similar manner, value of terminal voltage V

is given by

= IZ

+−

AB BA AB

EZ EZ IZZ

Z+Z

...(xi)

These expressions give the values of transformer currents and terminal voltage in terms of the load

current. The value of V

may also be found as under :

As seen from Fig. 30.70.

=(E

−−

− V

)/Z

= (E

−−

− V

= (E

−−

− V

I=V

= I

+ I

or V

= (E

−−

− V

+ (E

−−

− V

∴∴

∴ V

+ Y

)=E

+ E

or V

AA BB

LAB

EY EY

Y+Y Y

...(xii)

Eq. (xi) gives V

in terms of load current. But if only load kVA is given, the problem becomes more

complicated and involves the solution of a quadratic equation in V

Now, S = V

I. When we substitute this value of I in Eq. (xi), we get

()

+−

AB BA AB 2

EZ EZ SZZ/V

or V

+ Z

)

−−

− V(E

) + SZ

= 0

When V

becomes known, then I

and I

may be directly found from

= E

−−

− I

and V

= E

−−

− I

Note. In the case considered above, it is found more convenient to work with numerical values of imped-

ances instead of % values.

Example 32.106. Two transformers A and B are joined in parallel to the same load. Determine

the current delivered by each transformer having given : open-circuit e.m.f. 6600 V for A and 6,400

V for B. Equivalent leakage impedance in terms of the secondary = 0.3 + j3 for A and 0.2 + j1 for B.

The load impedance is 8 + j6. (Elect. Machines-I, Indore Univ. 1987)

Solution. I

()

+−

AB A B L

AB L A B

EZ E E Z

ZZ Z(Z +Z)

Here E

= 6,600 V ; E

= 6,400 V ; Z

= 8 + j 6; Z

= 0.3 + j 3 ; Z

= 0.2 + j 1

6600(0.2 1) (6600 6400) (8 6)

(0.3 3)(0.2 1) (8 6) (0.3 3 0.2 1)

++ − +

++++ +++

jj j j j

117 − j 156 = 195A in magnitude

Similarly, I

()

−−

BA A B L

AB L A B

EZ E E Z

ZZ Z(Z +Z)

6400 (0.3 3) (6600 6400) (8 6)

(0. 3 3) (0. 2 1) (8 6 ) (0.5 4)

jjjj

+− − +

+++ +++

= 349 − j 231 = 421 A (in magnitude)

1204

Electrical Technology

Example 32.107. Two 1-φ transformers, one of 100 kVA and the other of 50 kVA are connected

in parallel to the same bus-bars on the primary side, their no-load secondary voltages being 1000 V

and 950 V respectively. Their resistances are 2.0 and 2.5 per cent respectively and their reactances 8

and 6 percent respectively. Calculate no-load circulating current in the secondaries.

(Adv. Elect Machines, A.M.I.E. Sec. B, 1991)

Solution. The circuit connections are shown in Fig. 32.75.

Ist transformer

Normal secondary current = 100,000/1000 = 100 A

1000 2.0

100 100

= 0.2 Ω ; X

1000 8

100 100

= 0.8 Ω

2nd Transformer

Normal secondary current = 50,000/950 = 52.63 A

= 950 × 2.5/100 × 52.63 = 0.45 Ω

= 850 × 6/100 × 52.63 = 1.08 Ω

Combined impedance of the two secondaries is

()( )

AB A B

RR X X

+++

0.65 1.88 1.99

+=Ω

∴ I

= (1000 − 950)/1.99 = 25.1 A ; α = tan

−1

(1.88/

0.65) = 71°

Example 32.108. Two single-phase transformers, one of 1000-kVA and the other of 500-kVA

are connected in parallel to the same bus-bars on the primary side ; their no-load secondary voltages

being 500 V and 510 V respectively. The impedance voltage of the first transformer is 3% and that

of the second 5%. Assuming that ratio of resistance to reactance is the same and equal to 0.4 in

each. What will be the cross current when the secondaries are connected in parallel ?

(Electrical Machines-I, Madras Univ. 1985)

Solution. Let us first determine the ohmic value of the two impedances. Also, let the secondary

voltage be 480 V*.

Full-load I

= 1000 × 1000/480 = 2083 A ; F.L. I

= 500 × 1000/480 = 1042 A

9500

100 100 2083

××

= 0.0072 Ω

5510

100 1042

= 0.0245 Ω

510 500

(0.0072 0.0245)

−

= 315.4 A

Note. Since the value of X/R is the same for the two transformers, there is no phase difference between E

and E

Example 32.109. Two transformers A and B of ratings 500 kVA and 250 kVA are supplying a

load kVA of 750 at 0.8 power factor lagging. Their open-circuit voltages are 405V and 415 V

respectively. Transformer A has 1% resistance and 5% reactance and transformer B has 1.5% resis-

tance and 4% reactance. Find (a) cross-current in the secondaries on no-load and (b) the load shared

by each transformer.

* Though it is chosen arbitrarily, its value must be less than either of the two no-load e.m.fs.

Fig. 32.75

Load Bus Bars

Supply Bus Bars

Transformer

1205

Solution. As said earlier, it is more convenient to work with ohmic impedances and for that purpose,

we will convert percentage value into numerical values by assuming 400 volt as the terminal voltage (this

value is arbitrary but this assumption will not introduce appreciable error).

Now I

= 1% of 400 ∴ R

1400

100 1250

= 0.0032 Ω

where I

= 500,000/400 = 1250 A

= 5% of 400 ; X

5 400

100 1250

= 0.016 Ω (i.e. X

= 5R

)

In a similar way, we can find R

and X

; R

= 0.0096 Ω; X

= 0.0256 Ω

∴ Z

= 0.0032 + j 0.016 = 0.0163 ∠ 78.5° ; Z

= 0.0096 + j 0.0256 = 0.0275 ∠69.4°

+ Z

= 0.0128 + j 0.0416 = 0.0436 ∠ 72.9°

Next step is to calculate load impedance. Let Z

be the load impedance and V

the terminal voltage

which has been assumed as 400 V.

∴ (V

) = 750 ∠− 36.9°

∴ Z

= 400

× 10

−3

/750 ∠− 36.9° = 0.214 ∠ 36.9° = (0.171 + j 0.128) Ω

(a)I

−

Z+Z

(405 415)

0.0436 72.9

−

∠°

= − 230 ∠− 72.9°

(b)I

405 0.0275 69.4 (405 415) 0.214 36.9

0.0163 78.5 0.0275 69.4 0.214 36.9 0.0436 72.9

×∠°+−×∠°

∠°×∠°+∠°×∠°

= 970 ∠ − 35°

Similarly, I

415 0.0163 78.5 (405 415) 0.214 36.9

0.0163 78.5 0.0275 69.4 214 369 0.0436 72.9

×∠°−−×∠°

∠°× ∠°+ ∠°× ∠°

∴ S

= 400 × 970 × 10

−3

∠− 35° = 388 ∠− 35° kVA ; cos φ

= cos 35° = 0.82 (lag)

= 400 × 875 × 10

−3

∠42.6° = 350 ∠− 42.6° kVA

cos φ

= cos 42.6° = 0.736 (lag)

Example 32.110. Two transformers A and B are connected in parallel to a load of (2 + j 1.5)

ohms. Their impedances in secondary terms are Z

= (0.15 + j 0.5) ohm and Z

= (0.1 + j 0.6) ohm.

Their no-load terminal voltages are E

= 207 ∠ 0° volt and E

=205 ∠0° volt. Find the power

output and power factor of each transformer. (Elect. Machines-I, Punjab Univ. 1991)

Solution. Using the equations derived in Art. 30.34 (c), we have

()

)

+−

AB A B L

AB L A B

EZ E E Z

ZZ +Z(Z +Z

= (0.15 + j 0.5) Ω ; Z

= (0.1 +j 0.6) Ω ; Z

= (2 + j 1.5) = 2.5 ∠36.9°

∴ I

207(0.1 0.6) (207 205) (2 1.5)

(0.15 0.5)(0.1 0.6) (2 1.5)(0.25 1.1)

jj j j

++− +

++++ +

24.7 127.2 129.7 79

1.435 2.715 3.07 117.9

+∠°

−+ ∠°

= 42.26 ∠− 38.9°A = (32.89 − j 26.55)A

()

)

−−

EZ E E Z

ZZ +Z(Z +Z

BA A B L

AB L A B

205(0.15 0.5) 2(2 1.5) 103 75

1.435 2.715 3.07 117.9

+−+ ∠°

−+ ∠°

= 33.56 ∠ − 42.9° = (24.58 − j 22.84) A

Now V

′ = IZ

= (I

+ I

) Z

= (57.47 − j 49.39) (2 + j 1.5) = 189 − j 12.58 = 189.4 ∠− 3.9°

1206

Electrical Technology

p.f. angle of transformer A = − 3.9°− (−38.9°) = 35°

∴ p.f. of A = cos 35° = 0.818 (lag) ; p.f. of B = cos [−3.9° − (−42.9°)] = 0.776 (lag)

Power output of transformer A is P

= 189.4 × 42.26 × 0.818 = 6,548 W

Similarly, P

= 189.4 × 33.56 × 0.776 = 4,900 W

Example 32.111. Two transformers have the following particulars :

Transformer A Transformer B

Rated current 200 A 600 A

Per unit resistance 0.02 0.025

Per unit reactance 0.05 0.06

No-load e.m.f. 245 V 240 V

Calculate the terminal voltage when they are connected in parallel and supply a load impedance

of (0.25 + j 0.1) Ω.(Elect. Machines-I, Sd. Patel Univ. 1981)

Solution. Impedance in ohms = Z

× N.L. e.m.f./full-load current

= (245/200)(0.02 + j 0.05) = 0.0245 + j 0.0613 Ω = 0.066 ∠68.2°

= (240/600)(0.025 + j 0.06) = 0.01 + j 0.024 Ω = 0.026 ∠67.3°

Z = (0.25 + j 0.1) = 0.269 ∠ 21.8° ; Z

+ Z

= 0.0345 + j 0.0853 = 0.092 ∠68°

+ Z

) = 0.269 × 0.092 ∠89.8° = 0.0247 ∠89.8° = (0 + j 0.0247)

= 0.066 × 0.026 ∠ 135.5° = (−0.001225 + j 0.001201)

∴ Z

+ Z

) = (

−−

− 0.00125 + j 0.259) = 0.0259 ∠ 92.7°

Let us take E

as reference quantity.

Also E

is in phase with E

because transformers are in parallel on both sides.

= 245(0.01 + j 0.0245) = 2.45 + j 5.87

= 240(0.0245 + j 0.0613) = 5.88 + j 14.7

+ E

= 8.33 + j 20.57 = 22.15 ∠ 67.9°

Now, I =

22.15 67.9

0.0259 92.7

∠°

AB BA

AB L A B

EZ +EZ

Z Z +Z (Z +Z )

= 855 ∠ − 24.8°

∴ V

= IZ

= 885 ∠ − 24.8° × 0.269 ∠21.8° = 230 − 3°

Tutorial Problems 29.1

1. A 1000-kVA and a 500-kVA, 1-phase transformers are connected to the same bus-bars on the primary

side. The secondary e.m.fs. at no-load are 500 and 510 V respectively. The impedance voltage of the

first transformer is 3.4% and of the second 5%. What cross-current will pass between them when the

secondaries are connected together in parallel ? Assuming that the ratio of resistance to reactance is

the same in each, what currents will flow in the windings of the two transformers when supplying a

total load of 1200 kVA.

[(i) 290 A (ii) 1577 and 900 A] (City & Guilds, London)

2. Two transformers A and B are connected in parallel to supply a load having an impedance of

(2 + j 1.5Ω). The equivalent impedances referred to the secondary windings are 0.15 + j 0.5 Ω and

0.1 + j 0.6 Ω respectively. The open-circuit e.m.f. of A is 207 V and of B is 205 V. Calculate (i) the

voltage at the load (ii) the power supplied to the load (iii) the power output of each transformer and

(iv) the kVA input to each transformer.

[(i) 189 ∠− 3.8°V (ii) 11.5 kW (iii) 6.5 kW, 4.95 kW (iv) 8.7 kVA, 6.87 kVA]

Transformer

1207

QUESTIONS AND ANSWERS ON TRANSFORMERS

Q.1. How is magnetic leakage reduced to a minimum in commerical transformers ?

Ans. By interleaving the primary and secondary windings.

Q.2. Mention the factors on which hysteresis loss depends ?

Ans. (i) Quality and amount of iron in the core (ii) Flux density and (iii) Frequency.

Q.3. How can eddy current loss be minimised ?

Ans. By laminating the core.

Q.4. In practice, what determines the thickness of the laminae or stampings ?

Ans. Frequency.

Q.5. Does the transformer draw any current when its secondary is open ?

Ans. Yes, no-load primary current.

Q.6. Why ?

Ans. For supplying no-load iron and copper losses in primary.

Q.7. Is Cu loss affected by power factor ?

Ans. Yes, Cu loss varies inversely with power factor.

Q.8. Why ?

Ans. Cu loss depends on current in the primary and secondary windings. It is well-known that current

required is higher when power factor is lower.

Q.9. What effects are produced by change in voltage ?

Ans. 1. Iron loss.........varies approximately as V

2. Cu loss.........it also varies as V

but decreases with an increase in voltage if constant kVA

output is assumed.

3. Efficiency.........for distribution transformers, efficiency at fractional loads decreases with in-

crease in voltage while at full load or overload it increases with increase in voltage and vice-

versa.

4. Regulation.........it varies as V

but decreases with increase in voltage if constant kVA output

is assumed.

5. Heating.........for constant kVA output, iron temperatures increase whereas Cu temperatures

decrease with increase in voltages and vice-versa.

Q. 10. How does change in frequency affect the operation of a given transformer ?

Ans. 1. Iron loss .........increases with a decrease in frequency. A 60-Hz transformer will have nearly

11% higher losses when worked on 50Hz instead of 60 Hz. However, when a 25-Hz trans-

former is worked on 60 Hz, iron losses are reduced by 25%.

2. Cu loss.........in distribution transformers, it is independent of frequecy.

3. Efficiency.........since Cu loss is unaffected by change in frequency, a given transformer effi-

ciency is less at a lower frequency than at a higher one.

4. Regulation.........regulation at unity power factor is not affected because IR drop is independent

of frequency. Since reactive drop is affected, regulation at low power factors decreases with

a decrease in frequency and vice-versa. For example, the regulation of a 25-Hz transformer

when operated at 50-Hz and low power factor is much poorer.

5. Heating.........since total loss is greater at a lower frequency, the temperature is increased with

decrease in frequency.

1208

Electrical Technology

OBJECTIVE TESTS – 32

1. A transformer transforms

(a) frequency

(b) voltage

(d) voltage and current.

2. Which of the following is not a basic element of

a transformer ?

(a) core

(b) primary winding

(d) mutual flux.

3. In an ideal transformer,

(a) windings have no resistance

(b) core has no losses

(d) all of the above.

4. The main purpose of using core in a transformer

is to

(a) decrease iron losses

(b) prevent eddy current loss

(d) decrease reluctance of the common mag-

netic circuit.

5. Transformer cores are laminated in order to

(a) simplify its construction

(b) minimise eddy current loss

(d) reduce hysteresis loss.

6. A transformer having 1000 primary turns is

connected to a 250-V a.c. supply. For a

secondary voltage of 400 V, the number of

secondary turns should be

(a) 1600 (b) 250

7. The primary and secondary induced e.m.fs. E

and E

in a two-winding transformer are always

(a) equal in magnitude

(b) antiphase with each other

(d) determined by load on transformer second-

ary.

8. A step-up transformer increases

(a) voltage (b) current

9. The primary and secondary windings of an

ordinary 2-winding transformer always have

(a) different number of turns

(b) same size of copper wire

(d) separate magnetic circuits.

10. In a transformer, the leakage flux of each wind-

ing is proportional to the current in that winding

because

(a) Ohm’s law applies to magnetic circuits

(b) leakage paths do not saturate

(d) mutual flux is confined to the core.

11. In a two-winding transformer, the e.m.f. per turn

in secondary winding is always.......the induced

e.m.f. power turn in primary.

(a) equal to K times

(b) equal to 1/K times

(d) greater than.

12. In relation to a transformer, the ratio 20 : 1 indi-

cates that

(a) there are 20 turns on primary one turn on

secondary

(b) secondary voltage is 1/20th of primary

voltage

secondary current.

(d) for every 20 turns on primary, there is one

turn on secondary.

13. In performing the short circuit test of a trans-

former

(a) high voltage side is usually short circuited

(b) low voltage side is usually short circuited

(d) none of the above.

(Elect. Machines, A.M.I.E. Sec. B, 1993)

14. The equivalent resistance of the primary of a

transformer having K = 5 and R

= 0.1 ohm

when referred to secondary becomes.......ohm.

(a) 0.5

(b)0.02

(d) 2.5

15. A transformer has negative voltage regulation

when its load power factor is

(a) zero

(b) unity

(d) lagging.

Transformer

1209

16. The primary reason why open-circuit test is

performed on the low-voltage winding of the

transformer is that it

(a) draws sufficiently large on-load current for

convenient reading

(b) requires least voltage to perform the test

(d) involves less core loss.

17. No-load test on a transformer is carried out to

determine

(a) copper loss

(b) magnetising current

(d) efficiency of the transformer.

18. The main purpose of performing open-circuit

test on a transformer is to measure its

(a) Cu loss

(b) core loss

(d) insulation resistance.

19. During short-circuit test, the iron loss of a

transformer is negligible because

(a) the entire input is just sufficient to meet Cu

losses only

(b) flux produced is a small fraction of the nor-

mal flux

(d) supply frequency is held constant.

20. The iron loss of a transformer at 400 Hz is 10

W. Assuming that eddy current and hysteresis

losses vary as the square of flux density, the

iron loss of the transformer at rated voltage but

at 50 Hz would be....... watt.

(a)80 (b) 640

21. In operating a 400 Hz transformer at 50 Hz

(a) only voltage is reduced in the same propor-

tion as the frequency

(b) only kVA rating is reduced in the same pro-

portion as the frequency

the same proportion as the frequency

(d) none of the above.

22. The voltage applied to the h.v. side of a trans-

former during short-circuit test is 2% of its rated

voltage. The core loss will be.......percent of

the rated core loss.

(a)4 (b) 0.4

23. Transformers are rated in kVA instead of kW

because

(a) load power factor is often not known

(b) kVA is fixed whereas kW depends on load

p.f.

ampere

(d) it has become customary.

24. When a 400-Hz transformer is operated at 50

Hz its kVA rating is

(a) raduced to 1/8

(b) increased 8 times

(d) increased 64 times.

25. At relatively light loads, transformer efficiency

is low because

(a) secondary output is low

(b) transformer losses are high

(d) Cu loss is small.

26. A 200 kVA transformer has an iron loss of 1 kW

and full-load Cu loss of 2kW. Its load kVA cor-

responding to maximum efficiency is ....... kVA.

(a) 100 (b) 141.4

27. If Cu loss of a transformer at 7/8th full load is

4900 W, then its full-load Cu loss would be

.......watt.

(a) 5600 (b) 6400

28. The ordinary efficiency of a given transformer

is maximum when

(a) it runs at half full-load

(b) it runs at full-load

(d) it runs slightly overload.

29. The output current corresponding to maximum

efficiency for a transformer having core loss of

100 W and equivalent resistance referred to

secondary of 0.25 Ω is ....... ampere.

(a)20

(b)25

(d) 400

30. The maximum efficiency of a 100-kVA trans-

former having iron loss of 900 kW and F.L. Cu

loss of 1600 W occurs at ....... kVA.

(a) 56.3 (b) 133.3

1210

Electrical Technology

ANSWERS

1. d 2. d 3. d 4. d 5. b 6. a 7. c 8. a 9. c 10. b 11. c 12. d 13. b 14. d

15. c 16. a 17. c 18. b 19. b 20. b 21. b 22. d 23. c 24. a 25. c 26. b 27. b 28. c 29. a 30. c

31. d 32. d 33. c 34. a 35. b 36. a 37. d 38. c

31. The all-day efficiency of a transformer depends

primarily on

(a) its copper loss

(b) the amount of load

(d) both (b) and (c).

32. The marked increase in kVA capacity produced

by connecting a 2 winding transformer as an

autotransfomer is due to

(a) increase in turn ratio

(b) increase in secondary voltage

(d) establishment of conductive link between

primary and secondary.

33. The kVA rating of an ordinary 2-winding

transformer is increased when connected as an

autotransformer because

(a) transformation ratio is increased

(b) secondary voltage is increased

conductivity

(d) secondary current is increased.

34. The saving in Cu achieved by converting a

2-winding transformer into an autotransformer

is determined by

(a) voltage transformation ratio

(b) load on the secondary

(d) size of the transformer core.

35. An autotransformer having a transformation ratio

of 0.8 supplies a load of 3 kW. The power

transferred conductively from primary to

secondary is.......kW.

(a) 0.6 (b) 2.4

36. The essential condition for parallel opearation of

two 1-φ transformers is that they should have

the same

(a) polarity

(b) kVA rating

(d) percentage impedance.

37. If the impedance triangles of two transformers

operating in parallel are not identical in shape and

size, the two transformers will

(a) share the load unequally

(b) get heated unequally

when unloaded

(d) run with different power factors.

38. Two transformers A and B having equal outputs

and voltage ratios but unequal percentage

impedances of 4 and 2 are operating in parallel.

Transformer A will be running over-load by .......

percent.

(a)50 (b)66

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