Transformer MCQs

Transformers are the device which is capable of receiving AC power at one voltage and delivering it at another voltage. In this way  transformer help in achieve better transmission efficiency, while transferring the power over longer distances with high voltages,In this article include transformer MCQs which cover the basic concept of transformer(Correct Answer is in Bold)

Transformer MCQs

MCQs on Transformer

1-Magnetic flux path in a transformer should be  

  • high resistance
  • high reluctance
  • low resistance
  • low reluctance

02-Due to the vibration of laminations the sound produced is termed as

  • magnetostrication
  • boo
  • hum
  • zoom

03-Hysteresis loss in a transformer varies  with flux density as

  • Bmax
  • Bmax1-6
  • Bmax1-83
  • B max

04-To construct the transformer core the Material used  is usually

  • Wood
  • silicon steel
  • copper
  • aluminium

05-The lamination  thickness  used in a transformer is usually

  • 4 mm to 5 mm
  • 14 mm to 15 mm
  • 25 mm to 40 mm
  • 0.4 mm to 0.5 mm

06-In a transformer th function of conservator is

  • to project against ‘internal fault
  • to reduce copper as well as core losses
  • to cool the transformer oil
  • to take care of the expansion and contraction of transformer oil due to variation of temperature of surroundings

07-The highest voltage  which is used to transmit electrical power in Pakistan

(a) 33 kV.

(6) 66 kV

  • 132 kV
  • 660 kV

08-No-load on a transformer is carried out to determine

  • copper loss
  • magnetising current and loss
  • magnetising current
  • efficiency of the transformer

09-Transformer coil dielectric strength  should be

  • 12 kV
  • 33 kV
  • 100 kV
  • 330 kV

10-Sumpner’s test is conducted on transformers to determine

  • stray losses
  • all-day efficiency
  • temperature
  • none of the above

11-The permissible flux density in case of cold rolled grain oriented steel is around

  • 2.7 Wb/m2
  • 3.7 Wb/m2
  • 4.7 Wb/m2
  •  1.7 Wb/m2

12-The efficiency of a transformer will be maximum when

  • copper losses = hysteresis losses
  • hysteresis losses = eddy current losses
  • eddy current losses = copper losses
  • copper losses = iron losses Ans: d
  1. No-load current in a transformer
  2. leads the voltage by about 75°
  3. lags behind the voltage by about 15°
  4. lags behind the voltage by about 75°
  5. leads the voltage by about 15°

13-The main reason of providing an iron core in a transformer is to

  • provide support to windings
  • decrease the reluctance of the magnetic path
  • reduce hysteresis loss
  • reduce eddy current losses

14-Out of the following which is not a part of transformer ,,

  • Conservator
  • Exciter
  • Breather
  • Buchholz relay

15-During the short circuit test on a transformer which site of transformer should be short circuited

  • High voltage side
  • Low voltage side
  • Primary side
  • Secondary side

16-In the transformer which winding will have cross-sectional area

  • Low voltage winding
  • Primary winding
  • Secondary winding
  • High voltage winding

17-A transformer transforms

  • Voltage
  • power
  • current
  • frequency

18-A transformer increase and decrease the voltage of a D.C. supply because

  • Faraday’s laws of electromagnetic induction are not valid since the rate of change of flux is zero
  • there is no need to change the D.C. voltage
  • a D.C. circuit has more losses
  • none of the above

19-Transformer Primary winding is

  • is always a low voltage winding
  • is always a high voltage winding
  • could either be a low voltage or high voltage winding
  • none of the above

20-In a transformer which winding has more number of turns ?

  • Low voltage winding
  • Primary winding
  • Secondary winding
  • High voltage winding

21-The power transformer efficiency is of the order of

  • 100 per cent
  • 50 per cent
  • 25 per cent
  •  98 per cent

22-In a transformer the losses which remain constant irrespective of load changes are

  • hysteresis and eddy current losses
  • friction and windage losses
  • copper losses
  • none of the above

23-The  common method of power transformer cooling is

  • natural air cooling
  • air blast cooling
  • oil cooling
  • any of the above

24-No load current lags applied voltage in a transformer by an angle

  • 180°
  •  75°
  •  120″
  •  90°

25-The efficiency of a transformer depends upon

  • supply frequency
  • power factor of load
  • load current
  • both (b) and (c)

26-The function of conservator in a transformer is

  • provide fresh air for cooling the transformer
  • supply cooling oil to transformer in time of need
  • protect the transformer from damage when oil expends due to heating
  • none of the above

27-Ina transformer natural oil cooling is used upto a rating of

  • 3000 kVA
  • 1000 kVA
  • 500 kVA
  • 250 kVA

28-Power transformers are designed to have maximum efficiency at

  • 70% full load
  • 50% full load
  • nearly full load
  • no load

29-The maximum efficiency of a distribution transformer is

  • at no load
  • at 80% full load
  • at full load
  • at 50% full load

30-Transformer breaths in when

  • load on it increases
  • load remains constant
  • load on it decreases
  • none of the above

31-No-load current of a transformer has

  • has small magnitude and low power factor
  • has high magnitude and low power factor
  • has high magnitude and high power factor
  • has small magnitude and high power factor

32-Spacers are provided between adjacent coils

  • to insulate the coils from each other
  • to provide free passage to the cooling oil
  • both (a) and (b)
  • none of the above

33-If the secondary leakage flux is greater then

  • less will be the primary induced e.m.f.
  • less will be the primary terminal voltage
  • less will be the secondary induced e.m.f.
  • none of the above

34-The main reason of providing iron core in a step-up transformer is

  • to decrease the magnitude of magnetizing current
  • to provide coupling between primary and secondary
  • to increase the magnitude of mutual flux
  • to provide all above features

35-The power transformer is a constant

  • main flux device
  • voltage device
  • current device
  • power device

36-when two transformer operating in parallel will share the load which depend upon

  • leakage reactance
  • efficiencies
  • ratings
  • per unit impedance

37-when the transformer is operating continuously ,the maximum temperature is on which part of transformer

  • windings
  • core
  • tank
  • any of the above

38-The hum in a transformer is mainly attributed to

  • magnetostriction
  • load changes
  • oil in the transformer
  • mechanical vibrations

39-The maximum load of a power transformer  is limited by its

  • voltage ratio
  • temperature rise
  • dielectric strength of oil
  • copper loss

40-The efficiency of a transformer during the  heavy loads is low because

  • iron loss is increased considerably
  • voltage drop both in primary and secondary becomes large
  • secondary output is much less as compared to primary input
  • copper loss becomes high in proportion to the output

41-On a transformer open circuit test is conducted to measure.

  • core loss
  • insulation resistance
  • copper loss
  • total loss
  • efficiency

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

  • magnetising current and loss
  • core loss
  • copper loss
  • efficiency
  • magnetising current

43-The voltage transformation ratio of a transformer is equal to the ratio of

  • secondary induced e.m.f. to primary induced e.m.f.
  • primary turns to secondary turns
  • secondary current to primary current
  • secondary terminal voltage to primary applied voltage

44-which Part of the transformer will damage more during overheating

  • winding insulation
  • iron core
  • copper winding
  • frame or case
  • transformer tank

45-If a transformer is switched on to a voltage more than the rated voltage

  • its power factor will increase
  • its power factor will remain unaffected
  • its power factor will be zero
  •  its power factor will deteriorate

46-When the transformation ratio of auto transformer is ___________ it make effective saving on copper and copper losses

  • less than one
  • great than one
  • approximately equal to one
  • none of the above

47-Minimum voltage regulation occurs when the power factor of the load is

  • leading
  • unity
  • lagging
  • zero

48-In a step-down transformer, there is a change of 20 A in the load current. This results in change of supply current of

  • less than 20 A
  • more than 20 A
  • 10 A
  • none of the above

49–The efficiencies of transformers  as compared to the efficiency of same rating electric motor are

  • about the same
  • much higher
  • somewhat smaller
  • none of the above

50-What will happen if the transformers which is  working in parallel  not connected with same polarity ?

  • The power factor of the two trans-formers will be different from the power factor of common load
  • The transformers will not share load in proportion to their kVA ratings
  • Incorrect polarity will result in dead short circuit

51-when the two transformer working in parallel have different percentage impedance then

  • transformers will be overheated
  • 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
  • power factors of both the trans-formers will be same
  • parallel operation will be not possible

52-Tapping are generally provide in a transformer on ?

  • primary side
  • secondary side
  • low voltage side
  • high voltage side

53-The use of higher flux density in the transformer design

  • reduces iron losses
  • reduces weight per kVA
  • reduces copper losses
  • increases part load efficiency

54-The chemical which is used in breather for transformer should have the quality of

  • ionizing air
  • cleansing the transformer oil
  • cooling the transformer oil.
  • absorbing moisture

55-The chemical used in breather is

  • silica gel
  • asbestos fiber
  • silica sand
  • sodium chloride

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

  • true
  • false

57-The transformer ratings are usually expressed in terms of

  • volts
  • amperes
  • kW
  • kVA

58-Which of the following does not change in a transformer ?

  • Frequency
  • Current
  • Voltage
  • All of the above

59-In a transformer the energy is conveyed from primary to secondary

  • by the flux
  • through cooling coil
  • through air
  • none of the above

60-A transformer core is laminated to

  • reduce hysteresis loss
  • reduce eddy current losses
  • reduce copper losses
  • reduce all above losses

61-The degree of mechanical vibrations produced by the laminations of a transformer depends on

  • size of laminations
  • tightness of clamping
  • gauge of laminations
  • all of the above

62-The no-load current drawn by transformer is usually what percent of the full-load current ?

  • 0.3 to 0.5 per cent
  • 2 to 5 per cent
  • 15 to 20 per cent
  • 30 to 40 per cent

63-In a transformer the resistance between its primary and secondary is

  • zero
  • 1 ohm
  • 1000 ohms
  • infinite

64-A transformer oil must be free from

  • moisture
  • sludge
  • odour
  • gases

65-A Buchholz relay can be installed on

  • oil cooled transformers
  • auto-transformers
  • air-cooled transformers
  • welding transformers

66-Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds

  • 150°C
  • 40°C
  • 70°C
  •  100°C

67-The main reason for generation of harmonics in a transformer could be

  • fluctuating load
  • saturation of core
  • poor insulation
  • mechanical vibrations

68-Distribution transformers are generally designed for maximum efficiency around

  • 90% load
  • zero load
  • 50% load
  • 25% load

69-Which of the following property is not required in the material for transformer core ?

  • High thermal conductivity
  • Mechanical strength
  • Low hysteresis loss
  • High permeability

70-Star/star transformers work satisfactorily when

  • load is unbalanced only
  • on balanced as well as unbalanced loads
  • load is balanced only
  • none of the above

71-Delta/star transformer works satisfactorily when

  • load is balanced only
  • load is unbalanced only
  • on balanced as well as unbalanced loads
  • none of the above

72-Buchholz’s relay gives warning and protection against

  • electrical fault inside the transformer itself
  • electrical fault outside the transformer in outgoing feeder
  • for both outside and inside faults
  • none of the above

73-The magnetizing current of a transformer is usually small because it has

  • large leakage flux
  • laminated silicon steel core
  • fewer rotating parts
  •  small air gap

74-Which of the following does not change in an ordinary transformer ?

  • Frequency
  • Voltage
  • Current
  • Any of the above

75-The leakage flux in a transformer depends upon

  • load current
  • load current and voltage
  • load current, voltage and frequency
  • load current, voltage, frequency and power factor

76-The path of the magnetic flux in transformer should have

  • high reluctance
  • low reactance
  • high resistance
  • low resistance

77-Noise level test in a transformer is a

  • special test
  • routine test
  • type test
  • none of the above

78-Which of the following is not a routine test on transformers ?

  • Core insulation voltage test
  • Impedance test
  • Radio interference test
  • Polarity test

79-A transformer can have zero voltage regulation at

  • leading power factor
  • lagging power factor
  • unity power factor
  • zero power factor

80-Helical coils can be used on

  • high frequency transformers
  • high voltage side of small capacity transformers
  • low voltage side of high kVA transformers
  • high voltage side of high kVA rating transformers

81-Harmonics in transformer result in

  • increased core losses
  • increased  I2R losses
  • magnetic interference with communication circuits
  • all of the above

82-The core used in high frequency transformer is usually

  • copper core
  • cost iron core
  • mild steel core
  •  air core

83-The full-load copper loss of a transformer is 1600 W. At half-load, the copper loss will be

(a) 6000 W

(b) 1600 W

  • 800 W
  • 400 W

84- The value of flux involved in the e.m.f. equation of a transformer is

  • average value
  • r.m.s. value
  • maximum value
  • instantaneous value

85-Silicon steel used in laminations mainly reduces

  • hysteresis loss
  • eddy current losses
  • copper losses
  • all of the above

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

  • High voltage winding
  • Primary winding
  • Secondary winding
  • Low voltage winding

87-Power transformers are generally designed to have maximum efficiency around

  • no-load
  • half-load
  • near full-load
  • 15 % overload

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

  • Hysteresis losses are reduced
  • Copper losses are negligible
  • Eddy losses are totally eliminated
  • Saving in winding material


89-During short circuit test iron losses are negligible because

  • the current on secondary side is negligible
  • the voltage on secondary side does not vary
  • the voltage applied on primary side is low
  • full-load current is not supplied to the transformer

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

  • short-circuiting of the secondary’s
  • power factor of one of the transformers is leading while that of the other lagging
  • transformers having higher copper losses will have negligible core losses
  • loading of the transformers not in proportion to their kVA ratings

91-The changes in volume of transformer cooling oil due to variation of atmospheric temperature during day and night is taken care of by which part of transformer

  • Conservator
  • Breather
  • Bushings
  • Buchholz relay

92-An ideal transformer is one which has

  • no losses and magnetic leakage
  • interleaved primary and secondary windings
  • a common core for its primary and secondary windings
  • core of stainless steel and winding of pure copper metal
  • none of the above

93-When a given transformer is run at its rated voltage but reduced frequency, its

  • flux density remains unaffected
  • iron losses are reduced
  • core flux density is reduced
  • core flux density is increased

94-In an actual transformer the iron loss remains practically constant from no load to full load because

  • value of transformation ratio remains constant
  • permeability of transformer core remains constant
  • core flux remains practically constant
  • primary voltage remains constant
  •   secondary voltage remains constant

95-An ideal transformer will have maximum efficiency at a load such that

  • copper loss < iron loss
  • copper loss > iron loss
  • copper loss = iron loss
  • none of the above

96-If the supply frequency to the transformer is increased the iron loss will

  • not change
  • decrease
  • increase
  • any of the above

97-Negative voltage regulation is indicate that the load is

  • capacitive only
  • inductive only
  • inductive or resistive
  • none of the above

98-Iron loss of a transformer can be measured by

  • low power factor wattmeter
  • unity power factor wattmeter
  • frequency meter
  • any type of wattmeter

100-When secondary of a current transformer is open-circuited its iron core will be

  • hot because of heavy iron losses taking place in it due to high flux density
  • hot because primary will carry heavy current
  • cool as there is no secondary current
  • none of above will happen

101-The transformer laminations are insulated from each other by

  • mica strip
  • thin coat of varnish
  • paper
  • any of the above

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

  • Circular type
  • Sandwich type
  • Cylindrical type
  • Rectangular type

103-During open circuit test of a transformer

  • primary is supplied rated voltage
  • primary is supplied full-load current
  • primary is supplied current at reduced voltage
  • primary is supplied rated kVA

104-Open circuit test on transformers is conducted to determine

  • hysteresis losses
  • copper losses
  • core losses
  • eddy current losses

104-Short circuit test on transformers is conducted to determine

  • hysteresis losses
  • copper losses
  • core losses
  • eddy current losses

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

  • same efficiency
  • same polarity
  • same kVA rating
  • same number of turns on the secondary side.

106-The transformer oil should have              volatility and           viscosity.

  • low,low
  • high,high
  • low,high
  • high,low

107-The function of breather in a transformer is

  • to provide oxygen inside the tank
  • to cool the coils during reduced load
  • to cool the transformer oil
  • to arrest flow of moisture when outside air enters the transformer

108-The secondary winding of which of the following transformers is always kept closed ?

  • Step-up transformer
  • Step-down transformer
  • Potential transformer
  • Current transformer

109-The size of a transformer core will depend on

  • frequency
  • area of the core
  • flux density of the core material
  • (a) and (b) both

110-Natural air cooling is generally restricted for transformers up to

  • 1.5 MVA
  • 5 MVA
  • 15 MVA
  • 50 MVA

111-A shell-type transformer has

  • high eddy current losses
  • reduced magnetic leakage
  • negligibly hysteresis losses
  • none of the above

112-A transformer can have regulation closer to zero

  • on full-load
  • on overload
  • on leading power factor
  • on zero power factor

113-A transformer transforms

  • voltage
  • current
  • current and voltage
  • power

114-Reduction in core losses and increase in permeability are obtained with transformer employing

  • core built-up of laminations of cold rolled grain oriented steel
  • core built-up of laminations of hot rolled sheet
  • either of the above
  • none of the above

115-In a power or distribution transformer about 10 percent end turns are heavily insulated

  • to withstand the high voltage drop due to line surge produced by the shunting capacitance of the end turns
  • to absorb the line surge voltage and save the winding of transformer from damage
  • to reflect the line surge and save the winding of a transformer from damage
  • none of the above

116-For given applied voltage, with the increase in frequency of the applied voltage

  • eddy current loss will decrease
  • eddy current loss will increase
  • eddy current loss will remain unchanged
  • none of the above

117-Losses which occur in rotating electric machines and do not occur in trans formers are

  • magnetic losses
  • hysteresis and eddy current losses
  • copper losses
  • friction and windage losses

118-In a given transformer for a given applied voltage, losses which remain constant irrespective of load changes are

  • hysteresis and eddy current losses
  • friction and windage losses
  • copper losses
  • none of the above

119-  The secondary of a current transformer is always short-circuited under operating conditions because it

  • avoids core saturation and high voltage induction
  • is safe to human beings
  • protects the primary circuit
  • none of the above

120-In a transformer the resistance between its primary and secondary should be

  • zero
  • 10 Q

(c) 1000 Q

(d)infinity

121-A good voltage regulation of a transformer means

  • output voltage fluctuation from no load to full load is least
  • output voltage fluctuation with power factor is least
  • difference between primary and secondary voltage is least
  • difference between primary and secondary voltage is maximum

122-For a transformer, operating at constant load current, maximum efficiency will occur at

  • 0.8 leading power factor
  • 0.8 lagging power factor
  • zero power factor
  • unity power factor

123-Which of the following protection is normally not provided on small distribution transformers ?

  • Overfluxing protection
  • Buchholz relay
  • Overcurrent protection
  • All of the above

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

  • Horn gaps
  • Thermal overload relays
  • Breather
  • Conservator

125-The efficiency of two identical transformers under load conditions can be determined by

  • short-circuit test
  • back-to-back test
  • open circuit test
  • any of the above

126-Which of the following insulating materials can withstand the highest temperature safely ?

  • Cellulose
  • Asbestos
  • Mica
  • Glass fibre

127-Which of the following parts of a transformer is visible from outside ?

  • Bushings
  • Core
  • Primary winding
  • Secondary winding

128-The noise produced by a transformer is termed as

  • zoom
  • hum
  • ringing
  • buzz

129-Which of the following loss in a transformer is zero even at full load ?

  • Core loss
  • Friction loss
  • Eddy current loss
  • Hysteresis loss

130-Which of the following is the most likely source of harmonics in a transformer ?

  • poor insulation
  • Overload
  • loose connections
  • Core saturation

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  1. January 11, 2021

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