Transformer MCQ Questions and Answers

61. If per unit impedances of two transformers connected in parallel are not equal, then which one of the following statements is correct?

  1. The power factor of the two transformers will be different from that of the connected load.
  2. Transformers will get overloaded
  3. Dead short circuit occurs.
  4. The transformer with higher per unit impedance will share more load.
Answer. b

62. A 1 kVA, 200/100 V, 50 Hz, single-phase transformer gave the following test results on 50 Hz:

OC (LV side): 100 V, 20 watts

SC (HV side): 5 A, 25 watts

It is assumed that no-load loss components are equally divided. The above tests were then conducted on the same transformer at 40 Hz.

Tests results were

OC (HV): 160 V, W1 watts

SC (LV): 10 A, W2 watts

Neglecting skin effect, W1 and W2 will be

  1. W1 = 16 watts, W2 = 25 watts
  2. W1 = 25 watts, W2 = 31.25 watts
  3. W1 = 20 watts, W2 = 20 watts
  4. W1 = 14.4 watts, W2 = 25 watts
Answer. d

63. Two transformers operating in parallel will share the load depending upon their

  1. Leakage reactance
  2. Ratings
  3. Per unit impedance
  4. Efficiency
Answer. c

64. Consider the following statements relating to the constructional features of a large power transformer

  1. The conservator is used to maintain the level of oil in the transformer tank.
  2. The bushing is used to protect transformer insulation against lightning over-voltages.
  3. The Buchholz relay is an over current relay.
  4. Silica gel is used to absorb moisture

Which of these statements are correct?

  1. ii and iii
  2. i, ii, iii and iv
  3. i, ii and iv
  4. i and iv
Answer. d

65. The function of oil in a transformer is to provide

  1. Insulation and cooling
  2. Protection against lightning
  3. Protection against short circuit
  4. Lubrication
Answer. a

67. The use of higher flux density in the transformer design

  1. Reduces the weight per kVA
  2. Increases the weight per kVA
  3. Has no relation with the weight of transformer
  4. Increases the weight per kW
Answer. a

68. Two transformers, with equal voltage ratio and negligible excitation current, connected in parallel share load in the ratio of their kVA rating only, if their p.u. impedances (based on their own kVA) are

  1. equal
  2. in the inverse ratio of their ratings
  3. in the direct ratio of their ratings
  4. purely reactive
Answer. a

69. Two 10 kV/440 V, 1-phase transformers of 600 kVA and 350 kVA are connected in parallel to share a load of 800 kVA. The reactances of the transformers referred to the secondary side are 0.0198 Ω and 0.0304 Ω respectively (resistances negligible). The load shared by the two transformers will be, respectively

  1. 484.5 kVA and 315.5 kVA
  2. 315.5 kVA and 484.5 kVA
  3. 533 kVA and 267 kVA
  4. 267 kVA and 533 KVA
Answer. a

70. A 4 kVA, 400/200 V single-phase transformer has resistance of 0.02 p.u. and reactance of 0.06 p.u. Its actual resistance and reactance referred to h.v. side, are, respectively

  1. 0.2 ohm and 0.6 ohm
  2. 0.8 ohm and 2.4 ohm
  3. 0.08 ohm and 0.24 ohm
  4. 2 ohm and 6 ohm
Answer. b

71. In a 100 kVA, 1100/220 V, 50 Hz single-phase transformer with 2000 turns on the high-voltage side, the open-circuit test result gives 220 V, 91 A, 5 kW on loW-voltage side. The core-loss component of current is, approximately

  1. 9.1 A
  2. 22.7 A
  3. 45.0 A
  4. 91 A
Answer. b

72. The full-load copper-loss and iron-loss of a transformer are 6400 Wand 5000 W respectively. The copper-loss and iron-loss at half load will be, respectively

  1. 3200 W and 2500 W
  2. 3200 W and 5200 W
  3. 1600 W and 1250 W
  4. 1600 W and 5000 W
Answer. d

73. A 20 kVA, 2000/200V, 1-phase transformer has name-plate leakage impedance of 8%. Voltage required to be applied on the high-voltage side to circulate full-load current with the low-voltage winding short-circuited will be

  1. 16V
  2. 56.56V
  3. 160 V
  4. 568.68 V
Answer. c

74. Generally, the no-load losses of an electrical machine is represented in its equivalent circuit by a

  1. parallel resistance with a low value
  2. series resistance with a low value
  3. parallel resistance with a high value
  4. series resistance with a high value
Answer. c

75. A single-phase transformer when supplied from 220 V, 50 Hz has eddy current loss of 50 W. If the transformer is connected to a voltage of 330 V, 50 Hz, the eddy current loss will be

  1. 168.75 W
  2. 112.5 W
  3. 75 W
  4. 50 W
Answer. b

76. While performing the short-circuit test on the transformer the _________ side is short-circuited.

  1. High voltage side
  2. Low voltage side
  3. Primary side
  4. Secondary side

Answer. b

77. Which of the following is not a part of the transformer installation?

  1. Conservator
  2. Breather
  3. Buchholz relay
  4. Exciter

Answer. d

78. The purpose of providing an 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

Answer. c

79. The no-load current in a transformer

  1. lags behind the voltage by about 75°
  2. leads the voltage by about 75°
  3. lags behind the voltage by about 15°
  4. leads the voltage by about 15°

Answer. a

80. The efficiency of a transformer will be maximum when

  1. copper losses = iron losses
  2. copper losses = hysteresis losses
  3. eddy current losses = copper losses
  4. hysteresis losses = eddy current losses

Answer. a

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