MULTIPLE-CHOICE QUESTIONS AND ANSWERS
1. In a transform er
(a) All turns are equally insulated
(b) The end turns are more strongly insulated
(c) The end turns are closely wound (d) The end turns are widely separated
2. Lam inated insulations coated with varnish are norm ally used in the transform er
(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 lam ination in a transform er decreases when
(a) The applied frequency increases (b) The applied frequency decreases
(c) The applied voltage increases
(d) The applied voltage decreases
4. Oil in transform er is used to
(a) Transfer electrical energy (b) Insulate the windings
(c) Cool the windings
(d) None of the above
5. The following arrangem ent will reduce the reluctance of m agnetic path of the transform er.
6. The windings of a transform er are divided into several coils because
(a) It is difficult to wind 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 transform er depend upon the
(a) Size of winding (b) Size of tank
(c) Current flowing (d)Voltage supplied
8. Transform er hum m ing sound is reduced by the
(a) Proper bracing of transformers assemblies
(b) Proper insulation
(c) Proper design
(d) Proper design of winding
9. Sludge in transform er oil is due to
(a) Decomposition of oil
(b) Decomposition of insulation
(c) Moisture content in oil (d) None of the above
(a) More than unity (b) Less than unity (c) Equal to unity (d) More than 0.5
11. If 90 per cent of norm al voltage and 90 per cent of norm al frequency are applied to a transform er, the per cent charge in hysteresis losses will be
(a) 20% (b) 4.7% (c) 19% (d) 21%
12. If 110 per cent of norm al voltage and 110 per cent of norm al frequency is applied to a transform er, the percentage change of eddy current losses will be
(a) 10% (b) 20% (c) 25% (d) 21%
13. A transform er has two 2,400 V prim ary coils and two 240 V coils. By proper connection of the windings, the transform ation ratio that can be obtained is
(a) 10 (b) 5 (c) 20 (d) 9
14. A single-phase, 2,200/200 V transform er takes 1 A at the HT side or no load at a power 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 flux density, the power required per kilogram to m agnetize the iron core of a transform er is 0.8 W at 25 Hz and 2.04 W at 60 Hz. The power 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 statem ent:
(a) emf per turn of both the windings are equal
(b) emf per turn in HV winding is more than the emf per turn in LV winding
(c) emf per turn in HV winding is less than the emf per turn in LV winding
17. The flux involved in the em f equation of a transform er has
(a) rms value (b) Average value (c) Total value (d) Maximum value
18. The no-load current in a transform er lags the applied voltage by
(a) 90° – 95° (b) About 80° – 85° (c) 0° – 15° (d) About 110°
19. High leakage im pedance transform ers are used for applications such as
(a) Power distribution (b) Electrical toys
(c) Arc welding
(d) Fluorescent lamps
20. A transform er 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 transform er which will 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 m agnetic flux of a transform er follows
(a) High reluctance path
(b) Low reluctance path
(c) Low conductivity path (d) High conductivity path
23. The efficiency of a transform er is usually in the range of
(a) 50–60% (b) 65–75%
(c) 70–90% (d) 90–98%
(a) Hystersis loss
(b) Eddy current loss
(c) Partly (a) and partly (b) (d) Increases exciting current
25. Transform er changes the value of
(a) Power
(b) Frequency
(c) Voltage (d) Current
26. Transfer of electrical power from prim ary to secondary in a transform er takes place
(a) Electrically
(b) Electromagnetically
(c) Magnetically (d) None of these
27. The thickness of a 50 Hz transform er lam ination is
(a) 0.35 cm (b) 0.35 mm
(c) 0.33 m
(d) 0.30 cm
28. A power transform er is a constant
(a) Current device (b) Voltage device
(c) Power device
(d) Main flux device
29. The value of flux in the em f equation of a transform er is
(a) rms
(b) Average
(c) Maximum (d) Integral wave cycle
30. The leakage flux in a transform er depends upon the value of
(a) Load current (b) Supply frequency
(c) Mutual flux
(d) None of these
31. The voltage regulation of a transform er at full-load 0.8 power factor lag is 6 per cent. Its voltage regulation at full-load 0.8 power factor lead will be
(a) Negative (b) 54% (c) Positive (d) Zero
32. The full-load efficiency of a transform er at 0.85 p.f. lag is 97 per cent. Its efficiency at full load 0.85 bpower factor lead will be
(a) 99% (b) 96% (c) 97% (d) 98%
33. If a load on secondary side of a transform er increases, the current on the prim ary side
(a) Remains constant (b) Increases (c) Decreases (d) None of these
34. If a sinusoidal exciting current is applied to a transform er, the m utual flux produced is
(a) Negative (b) Sinusoidal (c) Zero (d) Flat top
35. To get sinusoidal flux in a transform er the m utual flux produced is
(a) Non-sinusoidal (b) Sinusoidal (c) Direct current (d) Zero
36. A 220V, 150V bulb is connected in series with the prim ary of a 220/1,100 V, 50 Hz transform er. If the load on the secondary side is disconnected, the brightness of the bulb will
(a) Decrease
(b) Increase
(c) Be unaffected (d) Decrease to a very low value
37. The open circuit test of a transform er gives inform ation 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 transform er gives the inform ation 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 transform er
(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 efficiency of a transform er is
(a) Equal to its power efficiency
(b) Less than its power efficiency
(c) More than its power efficiency (d) None of these
41. Routine efficiency of a transform er depends upon the value of
(a) Load current
(b) Supply frequency
(c) Power 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) Lower permeability
(b) High hysteresis
(c) Alnico as the main constituent (d) High resistance
43. The m axim um regulation of a transform er occurs at a power factor of
(a) Unity (b)
(c)
(d)
44. The zero of regulation of a transform er occurs at a power 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 transform er is connected to a constant voltage source. If the supply frequency decreases, the m agnetic flux in the core will
(a) Increase towards saturation (b) Decrease
(c) Remain unchanged
(d) None of these
47. The power in autotransform er is transferred through
(a) Inductive process
(b) Convection process
(c) Conduction process (d) All of these
48. The cross-sectional area of the com m on portion of an autotransform er is
(a) Kept constant
(b) Proportionally decreased
(c) Proportionally increased (d) None of these
49. The efficiency of an autotransform er for the sam e output com pared to a two-winding transform er is
(a) Greater (b) Lesser (c) Poor (d) None of these
50. The condition for successful parallel operation of transform er 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 transform ers incorrect polarity will result in
(a) Open circuit
(b) Dead short circuit
(c) Regeneration of power
(d) Power factor of transformer will be different from that of the connected load
52. In an autotransform er of voltage ratio and V1 > V2, the fraction of power transferred inductively is
(a)
(b)
(c)
(d)
53. A 10 kVA, 400/200 V sinlge-phase transform er with 10 per cent im pedance draws 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 two-winding transform er is reconnected as an autotransform er across a suitable voltage source. The m axim um kVA is
(a) 50 kVA (b) 15 kVA (c) 12.5 kVA (d) 8.75 kVA
55. An autotransform er having a transform ation ratio of 0.8 supplies a load of 10 kW. The power transferred inductively from the prim ary 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 transform er to obtain sm ooth variation of the output voltage by varying the
(a) Ratio of turns between primary and secondary winding
(b) Frequency
(c) Flux-density in the core
(d) Angle between the magnetic forces of the primary and secondary windings
57. In operating a 400 Hz transform er 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 perform ing the short circuit test of a transform er
(a) High-voltage side is usually short-circuited
(b) Low-voltage side is usually short-circuited
(c) Any side is short-circuited with preference
(d) None of these
59. In perform ing open circuit test of a transform er
(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. Transform er core usually uses grain-oriented lam inated sheets. The grain orientation reduces
(a) Copper loss
(b) Eddy current loss
(c) Hysteresis loss (d) None of the above
61. In the circuit m odel of a transform er, the core loss is represented as
(a) Series resistance (b) Series inductance
(c) Shunt resistance (d) Shunt inductance
62. While perform ing short circuit test on a transform er, the im pressed voltage m agnitude is kept constant but the frequency is increased. The short circuit current will
(a) Increase1
(b) Decrease
(c) Remain the same (d) None of the above
63. A transform er has a hysteresis loss of 30 W at 240 V, 60 Hz. The hysteresis loss at 200 V, 50 Hz will be
(a) 20.8 W (b) 25 W (c) 30 W (d) 36 W
64. In a transform er, the exciting current will be in phase quadrature with the im pressed 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
65. Magnetizing im pedance of a transform er is determ ined by
(a) Short circuit test
(b) Open circuit test
(c) Both open circuit and short circuit tests
(d) None of these
66. A transform er is to be tested at full-load conditions consum ing only losses from the m ains. We do
(a) Load test
(b) Open circuit and short circuit tests
(c) Back-to-back test (d) None of these
67. In a transform er operating at constant voltage if the input frequency increases the core loss will
(a) Increase
(b) Decrease
(c) Remain constant (d) Increase as the square of the frequency
(a) Copper loss
(b) Eddy current loss
(c) Hysteresis loss (d) None of above
69. R1 is the resistance of the prim ary winding of a transform er. The turns ratio in term s of prim ary to secondary is a. The equivalent resistance of the prim ary referred to as secondary is
(a) R
1
/a (b) a
2
R
1
(c) R
1
/a
2
(d) None of these
70. Choose the correct statem ent:
(a) emf per turn in high-voltage winding is more than the emf per turn in low-voltage
winding
(b) emf per turn in both the finding are equal
(c) emf per turn in both the windings are not equal
(d) None of these
71. If E2 and V2 be the induced em f in secondary winding and V2 be the term inal voltage during load, the regulation of the transform er is given by
(a)
(b)
(c)
(d)
72. One transform er has leakage im pedance of 1 + j 4 Ω and 3 + j 4 Ω for its prim ary and secondary windings, respectively. The transform er has
(a) Low voltage primary
(b) High voltage primary
(c) Medium voltage primary (d) Insufficient data
73. A single-phase transform er has its m axim um efficiency at 60 per cent of full load. At full load copper loss will be
(a) Equal to core loss
(b) Less than core loss
(c) More than core loss (d) Zero
74. The flux in a m agnetic core is alternating sinusoidally at a frequency of 600 Hz. The m axim um flux density is 2 T and eddy current loss is 15 W. If the frequency is raised to 800 Hz and m axim um flux density reduced to 1.5 T, the eddy current loss will be
(a) Same
(b) Reduced to half
(c) Doubled (d) Increased by 50%
75. The core flux in a transform er depends m ainly on
