Transformer MCQ Questions and Answers

41. For the parallel operation of transformers, which of the following conditions must be satisfied?

  1. Same voltage ratios
  2. Must be connected in proper polarities
  3. Re/Xe Ratio should be same
  4. Same kVA rating
Answer. b

42. What is the core loss in a high frequency ferrite core transformer used in SMPS power supply?

  1. 10% of rated power
  2. 5% of rated power
  3. 2% of rated power
  4. 1% of rated power
Answer. d

43. Consider the following tests:

  1. Load test
  2. Short circuit test
  3. OC test
  4. Retardation test

Which of the above tests are to be conducted for the determination of voltage regulation of a transformer?

  1. ii only
  2. i only
  3. iii and iv
  4. ii and iii
Answer. a

44. When are eddy-current losses in a transformer reduced?

  1. If laminations are thick.
  2. If the number of turns in primary winding is reduced.
  3. If the number of turns in secondary winding is reduced.
  4. If laminations are thin.
Answer. d

45. The full-load copper loss and iron loss of a transformer are 6400 W and 5000 W, respectively. What are the above copper loss and iron loss, respectively at half-load?

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

46. A transformer has a percentage resistance of 2% and percentage reactance of 4%. What are its regulations at power factor 0.8 lagging and 0.8 leading, respectively?

  1. 3.2% and -1.6%
  2. 4% and -0.8%
  3. 1.6% and 3.2%
  4. 4.8% and 0.6%
Answer. b

47. Cores of large power transformers are made from which one of the following?

  1. Hot-rolled steel
  2. Cold-rolled non-grain oriented steel
  3. Cold-rolled grain-oriented steel
  4. Ferrite
Answer. c

48. What is the load at which maximum efficiency occurs in case of a 100 kVA transformer with iron loss of 1 kW and full-load copper loss of 2 kW?

  1. 100 kVA
  2. 70.7 kVA
  3. 50.5 kVA
  4. 25.2 kVA
Answer. b

49. A single-phase transformer rated for 220/440 V, 50 Hz operates at no load at 220 V, 40 Hz. This frequency operation at rated voltage results in which one of the following?

  1. Increase of both eddy-current and hysteresis losses
  2. Reduction of both eddy-current and hysteresis losses
  3. Reduction of hysteresis loss and increase in eddy-current loss
  4. Increase of hysteresis loss and no change in the eddy-current loss
Answer. d

50. Sludge formation in transformer oil is due to which one of the following?

  1. Ingress of dust particles and moisture in the oil.
  2. Appearance of small fragments of paper, varnish, cotton, and other organic materials in the oil.
  3. Chemical reaction of transformer oil with the insulating materials.
  4. Oxidation of transformer oil.
Answer. d

51. Two transformers when operating in parallel will share the load depending upon which of the following?

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

52. Consider the following statements regarding transformers:

  1. The function of the magnetizing component of no load current is to sustain the alternating flux in the core.
  2. Short circuit test is performed to find core losses only.
  3. The function of the breather in transformer is to arrest flow of moisture when outside air enters the transformer.

Which of these statements are correct?

  1. i and ii
  2. i and iii
  3. i, ii and iii
  4. ii and iii
Answer. b

53. On which of the following factors does hysteresis loss depend?

  1. Flux density
  2. Frequency
  3. Thickness of lamination
  4. Time

Select the correct answer using the code given below:

  1. ii and iii
  2. i and ii
  3. iii and iv
  4. i and iv
Answer. b

54. A 500 kVA transformer has constant losses of 500 W and copper losses at full load are 2000 W. Then at what load, is the efficiency maximum?

  1. 500 kVA
  2. 250 kVA
  3. 125 kVA
  4. 1000 kVA
Answer. b

55. In a large power transformer, a conservator drum is provided above the completely oil filled transformer tank and connected to it by a short pipe. The conservator drum is linked to external air through a breather. What is the purpose of providing the conservator?

  1. To store reserve oil to make up oil losses due to leakage
  2. To prevent explosion due to rise in oil pressure inside the tank during a fault.
  3. To accommodate change in oil-level during the “load-cycle” of the transformer load.
  4. To exert additional pressure by the conservator-oil on the oil inside the main tank to prevent disintegration of oil at high temperature.
Answer. c

56. If the voltage applied to a transformer primary is increased by keeping the V/f ratio fixed, then the magnetizing current and the core loss will, respectively,

  1. decreases and remain the same
  2. increases and decreases
  3. remain the same and remain the same
  4. remain the same and increases
Answer. d

57. If the iron core of a transformer is replaced by an air core, then the hysteresis losses in the transformer will

  1. increase
  2. decrease
  3. remain unchanged
  4. become zero
Answer. d

58. If P1 and P2 be the iron and copper losses of a transformer at full load, and the maximum efficiency of the transformer is at 75% of the full load, then what is the ratio of P1 and P2?

  1. 9/16
  2. 10/16
  3. 3/16
  4. 3/4
Answer. a

59. Percentage resistance and percentage reactance of a transformer are 1% and 4%, respectively. What is voltage regulation at power factor 0.8 lagging and 0.8 leading?

  1. 2.4% and 0.8%, respectively
  2. 3.2% and -1.6%, respectively
  3. 3.2% and -3.2%, respectively
  4. 4.8% and 1.6%, respectively
Answer. b

60. Two single-phase transformers A and B with equal turn’s ratio have reactances of j3 Ω and j9 Ω referred to secondary. When operated in parallel, the load-sharing of 100 kW at 0.8 pf lag between A and B transformers would respectively be

  1. 75 kW and 25 kW
  2. 60 kW and 40 kW
  3. 20 kW and 80 kW
  4. 25 kW and 75 kW
Answer. a
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