What is Electrical Transformer - Definition, Construction, Working Principle , Circuit Diagram, Type and Parts

What is Transformer Principle - Definition and Working Concept with theory diagram explanation

Here we will discuss Electrical Transformer Notes. Electrical Transformer can be Define as a Device in which l electrical energy transfer from one circuit to another without any direct electrical connection by setting up magnetic medium with the help of mutual induction between two windings without changing its frequency. without changing its frequency it transfer energy.

How Transformer work-  Basic Working Concept of Transformer

The working principle of transformer is  depends upon Faraday's law of
electromagnetic induction
. Main function of working of transformer is mutual
. The mutual Induction is set up between two winding is
main reason for transformation of energy in electrical transformer.

What is Faraday's Laws of Electromagnetic Induction -

Faraday's laws says that , "the Rate of change of flux linkage with respect to time is directly proportional to the induced EMF in a conductor or coil".

Basic Theory of Transformer Working -

Alternating electrical Current source is applied to one winding called Sources windings or primary winding. The alternating current flow
through the winding produces a
Alternating flux because applied source is alternating. Second winding which is near to first or primary winding, will have linkage of alternating flux. As this flux is
Alternating its amplitude and direction,
there must be a change in flux linkage in the second winding or coil. So now  According to Faraday's law of electromagnetic induction , there must be an EMF induced in the second windings due to flux linkage. If the circuit is complete i.e. close than there must be a flowing current will set up. This is basic principles of working of Electrical Transformer.

Electrical Transformer Notes
Transformer Circuit Diagram 

Best Concept theory of Transformer Working Principle - Basic Understanding

Whenever we apply alternating current to an
electric coil, there will be an alternating flux
surrounding that coil. Now if we bring another coil
near the first one, there will be an alternating flux
linkage with that second coil. As the flux is
alternating, there will be obviously a rate of change in flux linkage with respect to time in the second coil. Naturally emf will be induced in it as per Faraday's law of electromagnetic induction.

Transformer Constructional Parts Definition -

There are three main parts of a transformer,
1. Primary/Source Winding of Transformer- where alternating current is applied and produces changing magnetic flux.

2. Magnetic Core of Transformer-
The magnetic flux is produced by the primary winding and it will flow through iron core which have some  reluctance value. Reluctance is similar to Resistance in Electrical Circuit. The reluctance path linked with secondary winding. It create a closed magnetic circuit. It link the primary circuit to secondary circuit though magnetic flux.

Click to Read Transformer Objective type Questions with Answer 

3. Secondary/load Winding of Transformer- the windings which is connected to load is secondary winding. Flux passes through the core and link with the secondary winding.

Dimensions of Transformer effect on transformer KVA rating, no load current, losses of transformer and rated voltage and current -

Here we have discussed very important topic of transformer i.e. Effect of transformer in various parameters of transformer i.e current, voltages, losses etc. Effect of Dimensions of core of transformer on No load current voltage, losses and KVA rating .  As we know that induced voltage of transformer on secondary side is given by 

E = 2π ƒ Ν Φ
ƒ -  is the flux frequency in Hertz, = ω/2π
Ν -  is the number of coil windings.
Φ -  is the flux density in webers.

Primary side = connected of source side.
Secondary side- connected to load side.

How voltage induces in secondary side of transformer -

Primary and secondary side connected by core made up of silicon iron. The flux is generated by primary source which flow in transformer core and is linked to secondary side which induced voltage in secondary side.

Electrical Transformer Notes

So core has fixed dimensions made at the time of transformer manufacturing. Here we will discuss what will happen if core of transformer increase by some factor (let x).

Effect of increasing the dimensions of transformer core -

When length, width, and height of transformer is increased by factor "X" than effect on
1) no load current of transformer - no load current will increase by factor "X" .

2) Rated voltage and Rated current - will increase by factor "X² " .

Electrical Transformer Notes

3) transformer losses - there are two type of transformer loss i.e. core loss and copper loss. Core loss is also known as iron loss or magnetising loss.
Iron loss is constant but copper loss is variable.
When core dimensions increased than Copper loss will increase by factor "X^3".

4) KVA rating of transformer - kva rating will be proportional to X is tp power 4 i.e. if transformer core dimensions is getting double than its KVA rating will be 2^4= 16 time.

Transformer Objective type - Questions Answer asked in Previous PSU exam

Today we have uploaded most important question from Electrical Core Subject i.e.Electrical Transformer. Transformer subject is one of those subjects for every electrical exam which has large weighed in any electrical competitive exam. 

Transformer basic Interview Question - 

1.In three pin plug 6 Amp. 220v AC rating. why  earth pin diameter is higher than ohter two pin?  what its purpose ?
Because Current flow in the conductor is inversely proportional to the conductor diameter. So if any short circuits occur in the system first high currents bypassed in the Earthling terminal.( R=Pl/a area of the conductor increases resistance value decreases)

2. Why should be the frequency is 50 hz only why not others like 45, 95 56 or anything , why 
should we maintain the frequency constant if so why it is only 50 ?
We can have the frequency at anyfrequency you like, but than you must also make your own motors, transformers or any other equipment you want to use.We maintain the frequency at 50hz or 60hz cos the world maintains a standard at 50 /60 hz and the equipment  are made to operate at these frequency.

3.What is excitation?
Excitation is applying an external voltage  to DC shunt coil in DC motors.

4.Different between megger and  contact resistance meter?
Megger used to measure cable resistance, conductor continuity, phase identification where as contact resistance meter used to measure low resistance like relays ,contactors.in an electric equipment in order to protect he equipment in occurance of fault in the system.

5.Why transformer rating in KVA instead of  KW?
The rating of any device depend on losses of the  machine. In a transformer the losses are iron losses  which depends on voltage and copper losses which  depends on current so there are no losses which  depend on power factor hence transformer is rated  in KVA.

There are 2 losses in transformer. One is copper  loss which depends on current and the other is iron loss which depends on voltage. These two factors are not affected by the power factor. This is why transformers are rated in KVA and not KW.

Single phase KVA = Amps x Volts/1000.
Single phase KW = Amps x Volts x pf/1000.
3 phase KVA = Amps x Volts x 1.73/1000.
3 phase KW = Amps x Volts x 1.73 x pf/1000.
yes there are two type of losses in transformer is that ohmic losses and core losses .
ohmic losses due to transformer current and core losses due to transformer voltage. these losses are due to transformer voltage and current. and these losses not depends on load ie kw that's the way its rating in kva..

Last year Questions Bank of Electrical Transformer 

1. In a transformer, the leakage flux of each winding is proportional to the current in that winding because
(a) Ohm’s law applies to magnetic circuits
(b) leakage paths do not saturate
(c) the two windings are electrically isolated
(d) mutual flux is confined to the core.

2. Which of the following is not a basic element of a transformer ?
(a) core
(b) primary winding
(c) secondary winding
(d) mutual flux.

3. In an ideal transformer,
(a) windings have no resistance
(b) core has no losses
(c) core has infinite permeability
(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
(c) eliminate magnetic hysteresis
(d) decrease reluctance of the common magnetic circuit.

5. Transformer cores are laminated in order to
(a) simplify its construction
(b) minimise eddy current loss
(c) reduce cost
(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
(c) 400
(d) 1250.

7. The primary and secondary induced e.m.fs. E 1 and E 2 in
a two-winding transformer are always
(a) equal in magnitude
(b) antiphase with each other
(c) in-phase with each other
(d) determined by load on transformer secondary.

8. A step-up transformer increases
(a) voltage
(b) current
(c) power
(d) frequency.

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
(c) a common magnetic circuit
(d) separate magnetic circuits.

10. A transformer transforms
(a) frequency
(b) voltage
(c) current
(d) voltage and current.

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
(c) equal to
(d) greater than.

12. In relation to a transformer, the ratio 20 : 1 indicates
(a) there are 20 turns on primary one turn on secondary
(b) secondary voltage is 1/20th of primary voltage
(c) primary current is 20 times greater than the secondary
(d) for every 20 turns on primary, there is one turn on

13. In performing the short circuit test of a transformer
(a) high voltage side is usually short circuited
(b) low voltage side is usually short circuited
(c) any side is short circuited with preference
(d) none of the above.

14. The equivalent resistance of the primary of a
transformer having K = 5 and R1 = 0.1 ohm
when referred to secondary becomes.......ohm.
(a) 0.5
(b) 0.02
(c) 0.004
(d) 2.5

15. A transformer has negative voltage regulation when its
load power factor is
(a) zero
(b) unity
(c) leading
(d) lagging.

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
(b) requires least voltage to perform the test
(c) needs minimum power input
(d) involves less core loss.

17. No-load test on a transformer is carried out to
(a) copper loss
(b) magnetising current
(c) magnetising current and no-load loss
(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
(c) total 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 normal flux
(c) iron core becomes fully saturated
(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
(c) 1.25
(d) 100.

21. In operating a 400 Hz transformer at 50 Hz
(a) only voltage is reduced in the same proportion as the
(b) only kVA rating is reduced in the same proportion as the
(c) both voltage and kVA rating are reduced in the same
proportion as the frequency
(d) none of the above.

22. The voltage applied to the h.v. side of a transformer
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
(c) 0.25
(d) 0.04.

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.
(c) total transformer loss depends on voltampere
(d) it has become customary.

24. When a 400-Hz transformer is operated at 50 Hz its
kVA rating is
(a) reduced to 1/8
(b) increased 8 times
(c) unaffected
(d) increased 64 times.

25. At relatively light loads, transformer efficiency is low
(a) secondary output is low
(b) transformer losses are high
(c) fixed loss is high in proportion to the output
(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 corresponding to
maximum efficiency is ....... kVA.
(a) 100
(b) 141.4
(c) 50
(d) 200.

27. If Cu loss of a transformer at 7/8th full load is 4900 W,
then its full-load Cu loss would be
(a) 5600
(b) 6400
(c) 375
(d) 429.

28. The ordinary efficiency of a given transformer is
maximum when
(a) it runs at half full-load
(b) it runs at full-load
(c) its Cu loss equals iron loss
(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
(c) 5
(d) 400.

30. The maximum efficiency of a 100-kVA transformer
having iron loss of 900 kW and F.L. Cu loss of 1600 W
occurs at ....... kVA.
(a) 56.3
(b) 133.3
(c) 75
(d) 177.7

31. The all-day efficiency of a transformer depends
primarily on
(a) its copper loss
(b) the amount of load
(c) the duration 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
(c) increase in transformer efficiency
(d) establishment of conductive link between primary and

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
(c) energy is transferred both inductively and 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
(c) magnetic quality of core material
(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
(c) 1.5
(d) 0.27

36. The essential condition for parallel operation of two 1-φ
transformers is that they should have the same
(a) polarity
(b) kVA rating
(c) voltage ratio
(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
(c) have a circulatory secondary current even when
(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
(c) 33
(d) 25

Answers of Objective Questions
1. b 2. d 3. d 4. d 5. b 6. a 7. c 8. a 9. c 10. d 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

Soon we will upload Multiple Choice Questions MCQ of other subjects like machine and power systems in PDF format with Answer. Though most of Question from Transformer asked from theory portion but we will also provide some Numerical Questions from Transformer for GATE IES and many PSUs practice. 

If you have any doubt on theoretical or numerical questions than please comment below.

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