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MULTIPLE-CHOICE QUESTIONS AND ANSWERS

1. In a transformer (a) All turns are equally insulated (b) The end turns are more strongly insulated (c) The

end turns are closely w ound (d) The end turns are w idely separated

2. Laminated insulations coated w ith varnish are normally used in the transformer (a) To reduce

reluctance of magnetic path (b) To reduce the effect of eddy current (c) To increase the reluctance of

magnetic path (d) To reduce the hysteresis effect

3. The required thickness of lamination in a transformer decreases w hen (a) The applied frequency

increases (b) The applied frequency decreases (c) The applied voltage increases (d) The applied

voltage decreases

4. Oil in transformer is used to (a) Transfer electrical energy (b) Insulate the w indings (c) Cool the

w indings (d) None of the above

5. The follow ing arrangement w ill reduce the reluctance of magnetic path of the transformer.

6. The w indings of a transformer are divided into several coils because (a) It is diff icult to w ind as one coil

(b) It reduces voltage per coil (c) It requires less insulation (d) None of the above

7. The size and construction of bushings in a transformer depend upon the (a) Size of w inding (b) Size of

tank (c) Current f low ing (d)Voltage supplied

8. Transformer humming sound is reduced by the (a) Proper bracing of transformers assemblies (b)

Proper insulation (c) Proper design (d) Proper design of w inding

9. Sludge in transformer oil is due to (a) Decomposition of oil (b) Decomposition of insulation (c) Moisture

content in oil (d) None of the above

10. A transformer used only for electrical isolation betw een tw o circuits has turns ratio w hich is (a) More

than unity (b) Less than unity (c) Equal to unity (d) More than 0.5

11. If 90 per cent of normal voltage and 90 per cent of normal frequency are applied to a transformer, the

per cent charge in hysteresis losses w ill be (a) 20% (b) 4.7% (c) 19% (d) 21%

12. If 110 per cent of normal voltage and 110 per cent of normal frequency is applied to a transformer, the

percentage change of eddy current losses w ill be (a) 10% (b) 20% (c) 25% (d) 21%

13. A transformer has tw o 2,400 V primary coils and tw o 240 V coils. By proper connection of the

w indings, the transformation ratio that can be obtained is (a) 10 (b) 5 (c) 20 (d) 9

14. A single-phase, 2,200/200 V transformer takes 1 A at the side or no load at a pow er factor of 0.385

lagging. The iron losses are (a) 167 W (b) 77 W (c) 88 W (d) 98 W

15. Neglecting resistance, at constant f lux density, the pow er required per kilogram to magnetize the iron

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core of a transformer is 0.8 W at 25 Hz and 2.04 W at 60 Hz. The pow er required per kilogram for 100

Hz is (a) 3.8 W (b) 3.63 W (c) 3.4 W (d) 5.2 W

16. Select the correct statement: (a) emf per turn of both the w indings are equal (b) emf per turn in HV

w inding is more than the emf per turn in LV w inding (c) emf per turn in HV w inding is less than the emf

per turn in LV w inding

17. The f lux involved in the emf equation of a transformer has (a) rms value (b) Average value (c) Total

value (d) Maximum value

18. The no-load current in a transformer lags the applied voltage by (a) 90° – 95° (b) About 80° – 85° (c) 0°

– 15° (d) About 110°

19. High leakage impedance transformers are used for applications such as (a) Pow er distribution (b)

Electrical toys (c) Arc w elding (d) Fluorescent lamps

20. A transformer 2,000 kVA, 250 Hz is operated at 50 Hz. Its kVA rating should be revised to (a) 400 kVA

(b) 10,000 kVA (c) 2,000 kVA (d) Cannot be revised

21. The transformer w hich w ill have the largest size is

(a) 100 kVA, 25 Hz (b) 100 kVA, 100 Hz (c) 100 kVA, 50 Hz (d) 100 kVA, 60 Hz

22. The magnetic f lux of a transformer follow s (a) High reluctance path (b) Low reluctance path (c) Low

conductivity path (d) High conductivity path

23. The eff iciency of a transformer is usually in the range of (a) 50–60% (b) 65–75% (c) 70–90% (d) 90–

98%

24. Transformer is laminated to reduce (a) Hystersis loss (b) Eddy current loss (c) Partly (a) and partly (b)

(d) Increases exciting current

25. Transformer changes the value of (a) Pow er (b) Frequency (c) Voltage (d) Current

26. Transfer of electrical pow er from primary to secondary in a transformer takes place (a) Electrically (b)

Electromagnetically (c) Magnetically (d) None of these

27. The thickness of a 50 Hz transformer lamination is (a) 0.35 cm (b) 0.35 mm (c) 0.33 m (d) 0.30 cm

28. A pow er transformer is a constant (a) Current device (b) Voltage device (c) Pow er device (d) Main

flux device

29. The value of f lux in the emf equation of a transformer is (a) rms (b) Average (c) Maximum (d) Integral

w ave cycle

30. The leakage f lux in a transformer depends upon the value of (a) Load current (b) Supply frequency (c)

Mutual f lux (d) None of these

31. The voltage regulation of a transformer at full-load 0.8 pow er factor lag is 6 per cent. Its voltage

regulation at full-load 0.8 pow er factor lead w ill be (a) Negative (b) 54% (c) Positive (d) Zero

32. The full-load eff iciency of a transformer at 0.85 p.f. lag is 97 per cent. Its eff iciency at full load 0.85

bpow er factor lead w ill be (a) 99% (b) 96% (c) 97% (d) 98%

33. If a load on secondary side of a transformer increases, the current on the primary side (a) Remains

constant (b) Increases (c) Decreases (d) None of these

34. If a sinusoidal exciting current is applied to a transformer, the mutual f lux produced is

(a) Negative (b) Sinusoidal (c) Zero (d) Flat top

35. To get sinusoidal f lux in a transformer the mutual f lux produced is (a) Non-sinusoidal (b) Sinusoidal (c)

Direct current (d) Zero

36. A 220V, 150V bulb is connected in series w ith the primary of a 220/1,100 V, 50 Hz transformer. If the

load on the secondary side is disconnected, the brightness of the bulb w ill (a) Decrease (b) Increase

(c) Be unaffected (d) Decrease to a very low value

37. The open circuit test of a transformer gives information about (a) Core losses of the transformer (b) Cu

losses of the transformer (c) Exciting current (d) None of these

38. The short circuit test of a transformer gives the information of (a) Cu losses of the transformer (b) Core

losses of the transformer (c) Winding circuit impedance (d) None of these

39. During testing of a transformer (a) Both SC and OC tests are performed at rated current (b) Both SC

and OC tests are performed at rated voltage (c) OC test is performed at rated voltage (d) SC test is

performed at rated current

40. All-day eff iciency of a transformer is (a) Equal to its pow er eff iciency (b) Less than its pow er

eff iciency (c) More than its pow er eff iciency (d) None of these

41. Routine eff iciency of a transformer depends upon the value of (a) Load current (b) Supply frequency

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(c) Pow er factor of load (d) Both (a) and (b)

42. A ferrite core has less eddy current loss than an iron core because ferrites have (a) Low er

permeability (b) High hysteresis (c) Alnico as the main constituent (d) High resistance

43. The maximum regulation of a transformer occurs at a pow er factor of (a) Unity (b) (c) (d)

44. The zero of regulation of a transformer occurs at a pow er factor of (a) Zero (b) (c) (d)

45. Circular coil sections are used because they (a) Reduce iron material (b) Reduce copper material (c)

Have the strongest mechanical shape (d) All of these

46. A transformer is connected to a constant voltage source. If the supply frequency decreases, the

magnetic f lux in the core w ill (a) Increase tow ards saturation (b) Decrease (c) Remain unchanged (d)

None of these

47. The pow er in autotransformer is transferred through (a) Inductive process (b) Convection process (c)

Conduction process (d) All of these

48. The cross-sectional area of the common portion of an autotransformer is (a) Kept constant (b)

Proportionally decreased (c) Proportionally increased (d) None of these

49. The eff iciency of an autotransformer for the same output compared to a tw o-w inding transformer is

(a) Greater (b) Lesser (c) Poor (d) None of these

50. The condition for successful parallel operation of transformer is (a) Correct polarity (b) Per unit

impedance based on their rating should be equal (c) Identical voltage and frequency rating (d) Equal

ratio of equivalent resistance to reactance (e) All of these

51. During parallel operation of transformers incorrect polarity w ill result in (a) Open circuit (b) Dead short

circuit (c) Regeneration of pow er (d) Pow er factor of transformer w ill be different from that of the

connected load

52. In an autotransformer of voltage ratio and 1 > 2, the fraction of pow er transferred inductively is (a)

(b) (c) (d)

53. A 10 kVA, 400/200 V sinlge-phase transformer w ith 10 per cent impedance draw s a steady short

circuit line current of (a) 50 A (b) 150 A (c) 250 A (d) 350 A

54. A 400/100 V, 10 kVA tw o-w inding transformer is reconnected as an autotransformer across a suitable

voltage source. The maximum kVA is (a) 50 kVA (b) 15 kVA (c) 12.5 kVA (d) 8.75 kVA

55. An autotransformer having a transformation ratio of 0.8 supplies a load of 10 kW. The pow er

transferred inductively from the primary to the secondary is (a) 10 kW (b) 8 kW (c) 2 kW (d) Zero

56. A single-phase induction regulator is a constant input transformer to obtain smooth variation of the

output voltage by varying the (a) Ratio of turns betw een primary and secondary w inding (b) Frequency

(c) Flux-density in the core (d) Angle betw een the magnetic forces of the primary and secondary

w indings

57. In operating a 400 Hz transformer at 50 Hz (a) Only voltage is reduced in the same proportion as the

frequency (b) Only kVA is reduced in the same proportion as the frequency (c) Both voltage and kVA

rating are reduced in the same proportion as the frequency (d) None of the above

58. In performing the short circuit test of a transformer (a) High-voltage side is usually short-circuited (b)

Low -voltage side is usually short-circuited (c) Any side is short-circuited w ith preference (d) None of

these

59. In performing open circuit test of a transformer (a) High voltage is usually kept open (b) Low voltage is

usually kept open (c) Any side can be kept open (d) None of the above

60. Transformer core usually uses grain-oriented laminated sheets. The grain orientation reduces (a)

Copper loss (b) Eddy current loss (c) Hysteresis loss (d) None of the above

61. In the circuit model of a transformer, the core loss is represented as (a) Series resistance (b) Series

inductance (c) Shunt resistance (d) Shunt inductance

62. While performing short circuit test on a transformer, the impressed voltage magnitude is kept constant

but the frequency is increased. The short circuit current w ill (a) Increase1 (b) Decrease (c) Remain the

same (d) None of the above

63. A transformer has a hysteresis loss of 30 W at 240 V, 60 Hz. The hysteresis loss at 200 V, 50 Hz w ill

be

(a) 20.8 W (b) 25 W (c) 30 W (d) 36 W

64. In a transformer, the exciting current w ill be in phase quadrature w ith the impressed voltage provided

(a) Only the leakage impedance drop is ignored (b) Only the core loss is ignored (c) Both the leakage

and impedance drop and the core loss are ignored (d) Only no-load copper loss is ignored

V V

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65. Magnetizing impedance of a transformer is determined by (a) Short circuit test (b) Open circuit test (c)

Both open circuit and short circuit tests (d) None of these

66. A transformer is to be tested at full-load conditions consuming only losses from the mains. We do (a)

Load test (b) Open circuit and short circuit tests (c) Back-to-back test (d) None of these

67. In a transformer operating at constant voltage if the input frequency increases the core loss w ill (a)

Increase (b) Decrease (c) Remain constant (d) Increase as the square of the frequency

68. Grain-oriented laminated sheet steel in a transformer reduces (a) Copper loss (b) Eddy current loss (c)

Hysteresis loss (d) None of above

69. 1 is the resistance of the primary w inding of a transformer. The turns ratio in terms of primary to

secondary is . The equivalent resistance of the primary referred to as secondary is (a) 1/ (b) 2 1

(c) 1/ 2 (d) None of these

70. Choose the correct statement: (a) emf per turn in high-voltage w inding is more than the emf per turn in

low -voltage w inding (b) emf per turn in both the f inding are equal (c) emf per turn in both the w indings

are not equal (d) None of these

71. If 2 and 2 be the induced emf in secondary w inding and 2 be the terminal voltage during load, the

regulation of the transformer is given by (a) (b) (c) (d)

72. One transformer has leakage impedance of 1 + 4 Ω and 3 + 4 Ω for its primary and secondary

w indings, respectively. The transformer has (a) Low voltage primary (b) High voltage primary (c)

Medium voltage primary (d) Insuff icient data

73. A single-phase transformer has its maximum eff iciency at 60 per cent of full load. At full load copper

loss w ill be (a) Equal to core loss (b) Less than core loss (c) More than core loss (d) Zero

74. The f lux in a magnetic core is alternating sinusoidally at a frequency of 600 Hz. The maximum flux

density is 2 T and eddy current loss is 15 W. If the frequency is raised to 800 Hz and maximum flux

density reduced to 1.5 T, the eddy current loss w ill be (a) Same (b) Reduced to half (c) Doubled (d)

Increased by 50%

75. The core f lux in a transformer depends mainly on (a) Supply voltage (b) Supply voltage and frequency

(c) Supply voltage, frequency and load (d) Supply voltage and load but independent of frequency

Answers

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