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Top 100 MCQ Transformer questions and answers

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(A)       Choose the Correct Answer :

1. Which of the following does not change in a transformer? (ISRO, ORDNANCE , DMRC , DRDO Repeated )



3.  Frequency

4.All of the above

2. In a transformer the energy is conveyed from primary to secondary ?      

1.though cooling coil        

2.through air                                     

3.by the flux

4.none of the above


3.  Why are the transformer core are lamination ? (ISRO, ORDNANCE , DMRC , DRDO Repeated )                                                1.reduce hysteresis loss

2.reduce eddy current losses    

3.reduce copper losses 

4.reduce all the above losses

4. Which loss is not common between a transformer and rotating machines?

1.Eddy current loss

2.Copper loss

3.Windage loss

4.Hysteresis loss.

5. The degree of mechanical vibrations produced by the laminations of a transformer depends on

 1.tightness of clamping

2.gauge of laminations

3.size of laminations

4.all of the above.

6. The no-load current drawn by transformer is usually what percent of the full-load current? (ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.0.2 to 0.5 per cent

2.   2 to 5 per cent

3.12 to 15 per cent

4.20 to 30 per cent

7. In case there are burrs on the edges of the laminations of the transformer, it is likely to result in…                    



3.   higher eddy current loss

4.higher hysteresis loss

8. The path of a magnetic flux in a transformer should have….

1.high resistance

2.high reluctance

3.low resistance

4.   low reluctance.

9. No-load test on a transformer is carried out to determine (ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.copper loss

2.magnetizing current

3.magnetiing current and lose 

4.efficiency of the transformer 


10. The dielectric strength of transformer oil is expected to be… (ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.1 kV

2.33 kV 

3.100 kV                                                                                                                                      

 4.330 kV

11.Sumpner’s test is conducted on transformers to determine


2.stray losses

3.all-day efficiency

4.none of the above

12. The permissible flux density in case of cold rolled grain oriental steel is around

1.1.7 Wb/m2

2.   2.7 Wb/m2

3.  3.7 Wb/m2

4.  4.7 Wb/m2

13. During the short-circuit test on a small transformer the frequency is increased from 50 Hz to 200 Hz. The copper losses will increase by a factor of…

1.  16



4.¼ .

14. The efficiency of a transformer will be maximum when……..(ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.copper losses = hysteresis losses

2.hysteresis losses = eddy current losses

3.eddy current losses = copper losses

4.copper losses = iron losses

15. No-load current in a transformer is … (ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.  lags behind the voltage by about 750

2.leads the voltage by about 750

3.lags behind the voltage by about 150

4.leads the voltage by about 150

16. The purpose of providing the iron core in a transformer is to

1.provide support to windings

2.reduce hysteresis loss

3.decrease the reluctance of the magnetic path

4.reduce eddy current losses.

17. Which of the following is not a part of transformer installation? ( ORDNANCE Repeated )



3.Buchholz relay

4.  Exciter

18. Which conducting short-circuit test on a transformer the following side is short-circuited…..(ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.high voltage side

2.low voltage side

3.primary side

4.secondary side

19. In a transformer the toppings are generally provided on…… ( ORDNANCE  Repeated )

1.primary side

2.secondary side

3.low voltage side

4.high voltage side


 20. The use of higher flux density in the transformer design…..(ISRO, ORDNANCE , DMRC , DRDO Repeated )                   

1.  reduces weight per kVA

2.reduces iron losses

3.reduces copper losses

4.increases part load efficiency

21. The chemical used in breather for transformer should have the quality of…...(ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.ionizing air

2.absorbing moisture

3.cleansing the transformer oil

4.cooling the transformer oil

22. The chemical used in breather is…... (ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.asbestos fiber

2.silica sand

3.sodium chloride

4.   silica gel

23.A good transformer oil should be free from ….



3. moisture

4.   all the above


24. An ideal transformer has infinite values of primary and secondary inductances. The statement is


2.  false

25. An ideal transformer ratings are usually expressed in terms of




4.  kVA

26. The noise resulting from vibrations of laminations set by magnetic force, is termed as


2. boo

3.   hum

4. zoom

27. Hysteresis loss in a transformer varies as (Bmax= maximum flux density)


2.  Bmax1.6



28. Material used for construction of transformer core is usually




4.  silicon steel


29. The thickness of laminations used in a transformer is usually… (ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.  0.4 mm to 0.5 mm

2.4 mm to 5 mm

3.14 mm to 15 mm

4.25 mm to 40 mm

30. The function of conservator in a transformer is


1.to protect against internal fault

2.to reduce copper as well as core losses

3.to cool the transformer oil

4.to take case of the expansion and contraction of transformer oil due to variation of temperature of surroundings


31. The higher voltage for transmitting electrical power in India is….(ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.33 kV

2.66 kV

3.132 kV

4.400 kV

32. In a transformer the resistance between its primary and secondary is.. [ Very very important ] 


2.1 Ω

3.1000 Ω


33. A transformer oil must be free from….


2.   alkaline 


4. moisture

34. A Buchholz relay can be installed on..


2.air-cooled transformers

3.welding transformers

4.oil-cooled transformers


35. Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds..





36.The main reason for generation of harmonics in a transformer could be…

1.fluctuating load

2.poor insulation

3.mechanical vibrations

4.   saturation of core

37.Distribution transformers are generally designed for maximum efficiency around.(ISRO, ORDNANCE , DMRC , DRDO Repeated )

1.90% load

2.zero load

3.25% load

450% load

38. Which of the following properties is not necessarily desirable in the material for transformer core?

1.Mechanical strength

2.Low hysteresis loss

3.High thermal conductivity

4.High permeability

39. Helical coils can be used on…  [ Very very important ] 

1.low voltage side of high kVA transformers

2.high frequency transformers

3.high voltage side of small capacity transformers

4.high voltage side of high kVA rating transformers


40. High frequency transformers sometimes make use of ferrite cores because it has

1.high specific gravity

2.   high resistance

3.high hysteresis

4.low permeability

41. Cross over windings are used on…

1.low voltage side of high kVA rating transformers

2.current transformers

3.high voltage side of high kVA rating transformers

4.high voltage side of low kVA rating transformers


42. While conducting short-circuit test on a transformer the following side is short-circuited

1.   H.V .side

2.    L.V. side

3.primary side

4.secondary side

44.Harmonics in transformer result in…

1.Increase core losses

2.Increase I2R losses

3.Magnetic interference with communication circuits

4.   all of the above

45. Tertiary winding is provided in transformers having……

1.mesh/star winding

2.mesh/mesh winding

3.star/star winding

4.any of the above

46. The core used in high frequency transformer is usually…

1.copper core

2.cost iron core

3.air core

4.mild steel core

47. The full-load copper loss of a transformer is 1600 W. At half load the copper loss will be…. (ISRO, ORDNANCE , DMRC , DRDO Repeated )

Formula is (50
%)of square × copper loss

6400 W

1600 W

800 W

400 W

48. The value of flux involved in the e.m.f. equation of a transformer is…

1. average value

2.r.m.s. value

3.maximum value

4.instantaneous value


49.Silicon steel used in laminations mainly reduces…

1.   hysteresis loss

2.eddy current losses

3.copper losses

4.all the the above

50. Which winding of the transformer has less cross-sectional area?

1.Primary winding

2.Secondary winding

3.Low voltage winding

4.High voltage winding

51. Power transformers are generally designed to have maximum efficiency around…



3.near full-load

4.10% overload

52. Which of the following is the main advantage of an auto-transformer over a two winding transformer?

1.Hysteresis losses are reduced

2.Saving in winding material

3.Copper losses are negligible

4.Eddy losses are totally eliminated


53. During short-circuit test iron losses are negligible because….

1.the current on secondary side is negligible

2.the voltage on secondary side does not vary

3.  the voltage applied on primary side is low

4.full-load current is not supplied to the transformer

54. Two transformers are connected in parallel. These transformers do not have equal percentage impedance. This is likely to result in.

1.short-circuiting of the secondary

2.power factor of one of the transformers is leading while that of the other lagging.

3.transformers having higher copper losses will have negligible core losses

4.   loading of the transformers not in proportion to their kVA ratings

55. The changes in volume of transformers cooling oil due to variation of atmospheric temperature during day and night is taken care of by which part of transformer?

1.  Conservator



4.Buchholz relay


56.The transformer laminations are insulated from each other by..

1.mica strip

2.thin coat of varnish

3.   paper

4.any of the above

57.Which type of winding is used in 3-phase shell-type transformer?

1.Circular type

2.Sandwich type

3.Cylindrical type

4.Rectangular type

58. During open circuit test of a transformer…

1.primary is supplied rated voltage

2.primary is supplied full-load current

3.primary is supplied current at reduced voltage

4primary is supplied rated kVA

 59.Open circuit test on transformers is conducted to determine…

1.hysteresis losses

2.copper losses

3.  core losses

4.eddy current losses


60. Short circuit test on transformers is conducted to determine…

1.hysteresis losses

2.  copper losses

3.core losses

4.eddy current losses

61. For the parallel operation of single-phase transformers it is necessary that they should have…

1.same efficiency

2.same polarity

3.same kVA rating

4.same number of turns on the secondary side.


62.The transformer oil should have ……. voltatility and …. viscosity

1.   low ….. low

2.high ….. high

3.low ….. high

4.high ….. low

63.The function of breather in a transformer is…

1.to provide oxygen inside the tank

2.to cool the coils during reduced load

3.to filter the transformer cooling oil

4. to arrest flow of moisture when outside air enters the transformer.

64. The secondary winding of which of the following transformers is always kept closed?

1.Step-up transformer

2.Sept-down transformer

3.Potential transformer

4.Current transformer

65. The size of a transformer core will depend on….


2.area of the core

3.flux density of the core material

4.  (a) and (b) both

66. Natural air cooling is generally restricted for transformers up to…

1.  1.5 MVA

2.5 MVA

3.15 MVA

4.50 MVA

67. A shell-type transformer has….

1.high eddy current losses

2.  reduced magnetic leakage


3.negligible hysteresis losses

4.none of the above.

68. A transformer can have regulation  closer to zero…

1.on full-load

2.on overload

3.   on leading power factor

4.on zero power factor


69. A transformer transforms.. (ISRO, ORDNANCE , DMRC , DRDO Repeated )



3.current and voltage

4. power

70. Which of the following is not the standard voltage for power supply in India….

1.11 kV

2.33 kV

3.66 kV

4. 122 kV

71.A good voltage regulation of a transformer means..

(a) output voltage
fluctuation from no load to full load is least

(b) output voltage
fluctuation with power factor is least

(c) difference
between primary and secondary voltage is least

(d) difference
between primary and secondary voltage is maximum

Ans: a

72.Theno. of magnetic paths in a berry type transformer…

     a.  is one

     b are two

     c.are three
d. are numerous.


           73In a transformer, the linkage of magnetic flux takes place from….

                 a.  primary to secondary

                 b. secondary to primary

                c.  primary to secondary and from secondary to primary

               d. none of these.

74. Since, the cores of a transformer are made of silicon steel, therefore…

a. copper-loss remains low

b. eddy-current-loss remains low

d. hysteresis-loss remains low

75. For equal capacity, a shell type transformer as compared to a core type transformer will have ….

a. low iron-loss

b.  low copper and iron losses

c    high copper and iron losses.

d. low copper-loss


76.lf the supply frequency of a transformer is reduced then its iron-loss will be

a.  low  
b.  high
c. the same

d. very high

78. In a transformer the energy is conveyed from primary to secondary

a.through cooling coil

b.through air

c.  by the flux

d. none of the above

79.  The degree of mechanical vibrations produced by the laminations of a transformer depends on

(a)tightness of clamping

(b) gauge of laminations

(c) size of laminations

(d) all of the above

 80. The path of a magnetic flux in a transformer should have

a. high resistance

b. high reluctance

c. low resistance

d. low reluctance



81. Auto-transformer makes effective saving on copper and copper losses, when its transformation ratio is
(a) approximately equal to one
(b) less than one
(c) great than one
(d) none of the above
Ans: a
82. Minimum voltage regulation occurs when the power factor of the load is
(a) unity
(b) lagging
(c) leading
(d) zero
Ans: c


83. In a step-down transformer, there is a change of 15 A in the load
current. This results in change of supply current of

(a) less than 15 A
(b) more than 15 A
(c) 15 A
(d) none of the above
Ans: a
84. The efficiencies of transformers compared with that of electric motors of the same power are
(a) about the same
(b) much smaller
(c) much higher
(d) somewhat smaller
Ans: c

85. The noise produced by a transformer is termed as

(a) zoom

(b) hum

(c) ringing

(d) buzz

Ans: b


86. Which of the following loss in a transformer is zero even at full load ?

(a) Core loss

(b) Friction loss

(c) Eddy current loss

(d) Hysteresis loss

Ans: b

87. Which of the following is the most likely source of harmonics in a transformer ?

(a) poor insulation

(b) Overload

(c) loose connections

(d) Core saturation

Ans: d

88. If a transformer is continuously operated the maximum temperature rise will occur in…

(a) core

(b) windings

(c) tank

(d) any of the above

Ans: b

89. The hum in a transformer is mainly attributed to

(a) load changes

(b) oil in the transformer

(c) magnetostriction

d. mechanical vibrations 

Ans: c

90. The maximum load that a power transformer can carry is limited by its

(a) temperature rise

(b) dielectric strength of oil

(c) voltage ratio

(d) copper loss

Ans: c

91. The efficiency of a transformer, under heavy loads, is comparatively low because

(a) copper loss becomes high in proportion to the output

(b) iron loss is increased considerably

(c) voltage drop both in primary and secondary becomes large

(d) secondary output is much less as compared to primary input

Ans: a

92. An open-circuit test on a transformer is conducted primarily to measure

(a) insulation resistance

(b) copper loss

(c) core loss

(d) total loss


Ans: c

93. A no-load test is performed on a transformer to determine

(a) core loss

 (b) efficiency

(c) magnetising current

(d) magnetising current and loss

Ans: d

94. The voltage transformation ratio of a transformer is equal to the ratio of

(a) primary turns to secondary turns

(b) secondary current to primary current

(c) secondary induced e.m.f. to primary induced e.m.f.

(d) secondary terminal voltage to primary applied voltage

Ans: c

95. Part of the transformer which is most subject to damage from overheating is

(a) iron core

(b) copper winding

(c) winding insulation

(d) frame or case

Ans: c

96.For a transformer, operating at constant load current, maximum efficiency will occur at …

(a) 0.8 leading power factor

(b) 0.8 lagging power factor

(c) zero power factor

(d) unity power factor

Ans: d

97. Which of the following protection is normally not provided on small distribution transformers?

(a) Over fluxing protection

(b) Buchholz relay

(c) Overcurrent protection

(d) All of the above

Ans: b

98.Which of the following acts as a protection against high voltage surges due to lightning and switching?

(a) Horn gaps

(b) Thermal overload relays

(c) Breather

(d) Conservator

Ans: a

99.The efficiency of two identical transformers under load conditions can be determined by

(a) short-circuit test

(b) back-to-back test

(c) open circuit test

(d) any of the above

Ans: b

100.Which of the following insulating materials can withstand the highest temperature safely?

(a) Cellulose

(b) Asbestos

(c) Mica

(d) Glass fibre

Ans: c


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