Electricals 9

62. The magnetising current of a transformer is usually small because it has
(a) small air gap
(b) large leakage flux
(c) laminated silicon steel core
(d) fewer rotating parts
Ans: a

63. Which of the following does not change in an ordinary transformer ?
(a)     Frequency
(b)     Voltage
(c) Current
(d) Any of the above
Ans: a

64. Which of the following properties is not necessarily desirable for the material for transformer core ?
(a) Low hysteresis loss
(b) High permeability
(c) High thermal conductivity
(d) Adequate mechanical strength
Ans: c

65. The leakage flux in a transformer depends upon
(a)     load current
(b)     load current and voltage
(c) load current, voltage and frequency
(d) load current, voltage, frequency and power factor
Ans: a

66. The path of the magnetic flux in transformer should have
(a) high reluctance
(b) low reactance
(c) high resistance
(d) low resistance
Ans: b

67. Noise level test in a transformer is a
(a)     special test
(b)     routine test
(c) type test
(d) none of the above
Ans: c

68. Which of the following is not a routine test on transformers ?
(a) Core insulation voltage test
(b) Impedance test
(c) Radio interference test
(d)     Polarity test
Ans: c

69. A transformer can have zero voltage regulation at
(a) leading power factor
(b) lagging power factor
(c) unity power factor
(d) zero power factor
Ans: a

70. Helical coils can be used on
(a)     low voltage side of high kVA transformers
(b)     high frequency transformers
(c) high voltage side of small capacity transformers
(d) high voltage side of high kVA rating transformers
Ans: a

71. Harmonics in transformer result in
(a) increased core losses
(b) increased I2R losses
(c) magnetic interference with communication circuits
(d) all of the above
Ans: d

72. The core used in high frequency transformer is usually
(a) copper core     
(b) cost iron core
(c) air core
(d) mild steel core
Ans: c

73. The full-load copper loss of a trans¬former is 1600 W. At half-load, the copper loss will be
(a) 6400 W
(b) 1600 W
(c) 800 W
(d) 400 W
Ans: d

1.74.   The value of flux involved m the e.m.f. equation of a transformer is
(a)     average value
(b)     r.m.s. value
(c) maximum value
(d) instantaneous value
Ans: c

L.75. Silicon steel used in laminations mainly reduces
(a) hysteresis loss
(b) eddy current losses
(c) copper losses
(d) all of the above
Ans: a

76. Which winding of the transformer has less cross-sectional area ?
(a) Primary winding
(b) Secondary winding
(c) Low voltage winding
(d) High voltage winding
Ans: d

77. Power transformers are generally designed to have maximum efficiency around
(a) no-load
(b) half-load
(c) near full-load  
(d) 10% overload
Ans: c

.78. Which of the following is the main advantage of an auto-transformer over a two winding transformer ?
(a) Hysteresis losses are reduced
(b) Saving in winding material
(c) Copper losses are negligible
(d) Eddy losses are totally eliminated
Ans: b

79. During short circuit test iron losses are negligible because
(a) the current on secondary side is negligible
(b) the voltage on secondary side does not vary
(c) the voltage applied on primary side is low
(d) full-load current is not supplied to the transformer
Ans: c

80. Two transformers are connected in parallel. These transformers do not have equal percentage impedance. This is likely to result in
(a)     short-circuiting of the secondaries
(b)     power factor of one of the transformers is leading while that of the other lagging
(c) transformers having higher copper losses will have negligible core losses
(d) loading of the transformers not in proportion to their kVA ratings
Ans: d

22. In a given transformer for given applied voltage, losses which remain constant irrespective of load changes are
(a) friction and windage losses
(b) copper losses
(c) hysteresis and eddy current losses
(d)     none of the above
Ans: c

23. A common method of cooling a power transformer is
(a) natural air cooling
(b) air blast cooling
(c) oil cooling
(d)     any of the above
Ans: c

24. The no load current in a transformer lags behind the applied voltage by an angle of about
(a) 180°
(b) 120"
(c) 90°
(d) 75°
Ans: d

25. In a transformer routine efficiency depends upon
(a) supply frequency
(b) load current
(c) power factor of load
(d) both (b) and (c)
Ans: d

27. Natural oil cooling is used for transformers up to a rating of
(a) 3000 kVA
(b) 1000 kVA
(c) 500 kVA
(d) 250 kVA
Ans: a

28. Power transformers are designed to have maximum efficiency at
(a) nearly full load
(b) 70% full load
(c) 50% full load  
(d) no load
Ans: a

29. The maximum efficiency of a distribution transformer is
(a)     at no load
(b)     at 50% full load
(c) at 80% full load
(d) at full load
Ans: b

30. Transformer breaths in when
(a) load on it increases
(b) load on it decreases
(c) load remains constant
(d) none of the above
Ans: b

31. No-load current of a transformer has
(a) has high magnitude and low power factor
(b) has high magnitude and high power factor
(c) has small magnitude and high power factor
(d) has small magnitude and low power factor
Ans: d

32. Spacers are provided between adjacent coils
(a)     to provide free passage to the cooling oil
(b)     to insulate the coils from each other
(c) both (a) and (b)
(d) none of the above
Ans: a

33. Greater the secondary leakage flux
(a) less will be the secondary induced e.m.f.
(b) less will be the primary induced e.m.f.
(c) less will be the primary terminal voltage
(d) none of the above
Ans: a

34. The purpose of providing iron core in a step-up transformer is
(a)     to provide coupling between primary and secondary
(b) to increase the magnitude of mutual flux
(c) to decrease the magnitude of mag-netizing current
(d) to provide all above features
Ans: c

35. The power transformer is a constant
(a) voltage device
(b) current device
(c) power device   
(d) main flux device
Ans: d

36. Two transformers operating in parallel will share the load depending upon their
(a) leakage reactance
(b) per unit impedance
(c) efficiencies
(d)     ratings
Ans: b

37. If R2 is the resistance of secondary winding of the transformer and K is the transformation ratio then the equivalent secondary resistance referred to primary will be
(a) R2/VK
(b) R2IK2
(c) R22!K2
(d) R22/K
Ans: b

38. What will happen if the transformers working in parallel are not connected with regard to polarity ?
(a) The power factor of the two trans-formers will be different from the power factor of common load
(b) Incorrect polarity will result in dead short circuit
(c) The transformers will not share load in proportion to their kVA ratings
(d)     none of the above
Ans: b

39. If the percentage impedances of the two transformers working in parallel are different, then
(a) transformers will be overheated
(b) power factors of both the transformers will be same
(c) parallel operation will be not possible
(d)     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
Ans: d

40. In a transformer the tappings are generally provided on
(a)     primary side
(b)     secondary side
(c) low voltage side
(d) high voltage side
Ans: c

1 TO 20 Transformer MCQ

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