ELECTRICAL MACHINE DESIGN 7

61. If the design of a 3-phase induction motor is done with a higher value of current density then
a) weight of copper will be more
b) temperature rise would be less
c) copper losses would be more
d) winding resistance would be less
ans. c

62. The value of operating power factor of a 3-phase induction motor is high say when
a) closed slots are used both on stator and rotor.
b) semiclosed slots are used both on stator and rotor.
c) open slots are used both on stator and rotor.
d) open and closed slots are used on stator and rotor respectively
ans. b

63. A 5hp ,400 V,4-pole , 50 Hz induction motor having 36 slots in the stator and 40 slots in the rotor may crawl synchronously at
a) 50 rpm
b) 150 rpm
c) 200 rpm
d) 750 rpm
ans. b

64. If the number of slots selected for a 3-phase, 6-pole induction motor is 42 , then it is possible to get
a) balanced fractional slot winding
b) full pitch integral slot winding
c) short pitch integral slot winding
d) none of the above
ans. a

65. If the stator of a 4-pole , 3-phase induction motor has 24 slots then the order of tooth ripples produced are
a) 5 and 7
b) 6 and 8
c) 9 and 11
d) 11 and 13
ans. d

66. Synchronous cusps in torque-speed curve of a 3-phase induction motor is due to
a) even harmonics
b) odd harmonics
c) both odd and even harmonics
d) none of the above
ans. b

67. For a 20 kW, 3-phase,400 V induction motor , the no load current is about
a) 100 A
b) 40 A
c) 10 A
d) 2 A
ans. c

68. At s=0 , the torque of an induction motor is
a) 0
b) equal to full-load torques
c) very high
d) very low
ans. a

69. Under full-load condition, the power factor of the rotor circuit of 3-phase induction motor is
a) about 0.2 lagging
b) about 0.5 lagging
c) almost unity
d) about 0.8 lagging
ans. c

ELECTRICAL MACHINE DESIGN 6

51. Damper winding is used to
a) reduce air gap harmonics flux
b) reduce oscillations
c) increase stability limits
d) resist the moisture
ans. b

52. When a synchronous generator is designed with lower SCR it
a) will give higher stability limit
b) will have lower SC current
c) will give better voltage regulation
d) will have high synchronous power
ans.b

53. An alternator with higher value of SCR has
a) poor voltage regulation and lower stability limit
b) better voltage regulation and higher stability limit
c) poor voltage regulation and higher stability limit
d) better voltage regulation and lower stability limit
ans. b

54. Under short circuit conditions, the power factor of a synchronous machine is
a) 1
b) about 0.8 lagging
c) almost zero lagging
d) 0.5
ans.c

55. If the stator winding of a 3-phase induction motor is delta connected the rotor winding
a) should be delta connected
b) should be star connected
c) should not be delta connected
d) may be delta or star connected
ans. d

56. An increase in number of poles of an induction motor results in
a) decrease in maximum pf
b) increase in maximum pf
c) no change in maximum pf
d) cannot be predicted
ans. a

57. The effect of leakage flux in case of 3-phase induction motor is to
a) reduce the torque produced
b) increase the torque produced
c) increase the operating power factor
d) none of the above
ans.a

58. For a 3-phase 50 Hz 4 pole induction motor having rotor diameter of 35 cm, the peripheral speed is
a) 20.1 m/s
b) 24.2 m/s
c) 26.9 m/s
d) 32.3 m/s
ans. c

59. Air gap of a polyphase induction motor is kept small to a
a) reduce the possibility of crawling
b) reduce the noise
c) reduce magnetizing current
d) obtain high starting torque .
ans. c

60. If an induction motor is designed with lesser air gap, then the motor will have
a) better cooling
b) better overload capacity
c) better power factor
d) lower power factor
ans. c

ELECTRICAL MACHINE DESIGN 5

41. Suitable value of flux density for design of large transformer is
a) 1.1 T
b) 1.4 T
c) 1.6 T
d) 1.9 T
ans. c

42. An iron cored transformer is working at a maximum flux density of 0.8 wb/m-sq.Its core is replaced by silicon steel core, working at a maximum flux density of 1.2 wb/m-sq. If total flux is to remain the same , what is the reduction in volume expressed as a percentage of the original volume? The frequency and voltage per turn are the same in both sides
a) 33 %
b) 9 %
c) 22 %
d) 11 %
ans. a

43. Tappings of a transformer are provided
a) at the phase end of lv side
b) at the phase end of hv side
c) at the neutral end of hv side
d) at the middle of the hv side.
ans. d

44. Under short-circuit conditions the windings of a transformer having 5% impedance will experience a radial force
a) 5 times of full load value
b) 20 times of full load value
c) 25 times of full load value
d) 400 times the full load value
ans. d

45. Addition of tubes to the transformer tank improves heat dissipation capacity because of
a) additional cooling surface
b) additional dissipation by radiation only
c) additional dissipation by convection only
d) additional dissipation by radiation and convection both.
ans. d

46. For a pulse transformer , the material used for its core and the possible turns ratio from primary to secondary are respectively as
a) ferrite; 20 : 1
b) ferrite; 1:1
c) laminated iron ; 1:1
d) powered iron; 1:1
ans.b

47. Heat dissipation by radiation in oil immersed transformers with cooling tubes is
a) about 0.6 Watt/metre-sq/degree centigrade
b) about 2 Watt/metre-sq/degree centigrade
c) about 10 Watt/metre-sq/degree centigrade
d) about 50 Watt/metre-sq/degree centigrade
ans. a

48. The slot leakage flux is more in case of
a) open type of slots
b) semi-closed slots
c) closed slots
d) open slots with aluminium conductors
ans. c

49. An ac winding has 2 slots per pole per phase. The slot harmonics will be
a) 5th and 7th
b) 11th and 13 th
c) 17th and 19th
d) none of the above.
ans. b

50. If m is the slots per pole/phase and A is any integer, then what order harmonics will have the same distribution factor as the fundamental
a) 6 Am+or- 1
b) 6 A+ or -m
c) 6 m+or-A
d) none of the above
ans. d

ELECTRICAL MACHINE DESIGN 4

31. The inductive reactance of a distribution core type transformer with concentric cylindrical coils of equal length can be reduced by
a) increasing the window height
b)  increasing the window  width
c) decreasing the yoke height.
d) increasing the operating flux density in the yoke.
ans. a

32. If all the dimensions of a transformer is doubled its iron loss will be
a) half
b) double
c) four times
d) 8 times
ans. d

33. The primary and secondary winding's are interlaced for
a) easiness of coil making
b) reduced leakage reactance
c) reduced cost.
d) uniform heating.
ans. b

34. It is necessary that magnetic coupling in a transformer is close. This is done to ensure
a) high efficiency
b) good regulation
c) good regulation and high efficiency
d) good regulation and high power factor.
ans. d

35. What is the arrangement of winding's in a core type single phase transformer?
a) Half lv inside and half hv outside on each core limb
b) LV on one code limb and hv on the other.
c) Sandwiched lv and hv discs on each core limb.
d) Half hv inside and half lv outside on each core limb.
ans. a

36. With increase in voltage , the window space factor of a transformer
a) decreases
b) increases
c) remains constant
d) decreases or increases depending upon whether it is a distribution or power transformer.
ans. a

37. No-load current in a large sized transformer is about
a) 10% of full-load current
b) 7% of full-load current
c) 4% of full-load current
d) 1% of full-load current
ans.d

38. A 200/400 ratio transformer has a secondary winding resistance of 0.5 ohms. The total resistance as referred to primary is about
a) 0.125 ohms
b) 0.5 ohms
c) 1 ohm
d) 2 ohms
ans. a

39. Typical value of per unit impedance for a transformer is
a) 0.2
b) 0.1
c) 0.05
d) 0.01
ans. c

40. If the iron loss in a transformer at full load is 600 W, the total  iron loss at 25% of overload will be
a) 600 W
b) 750 W
c) 1000 W
d) 400 W
ans. a

ELECTRICAL MACHINE DESIGN 3

21. Though the cost of single layer winding is less than that of double layer winding, single layer winding is not generally used for dc machines because it
a) leads to high reactance voltage and sparking commutation.
b) increases the effect of armature reaction under pole.
c) reduces the efficiency of operation.
d) leads to less number of segments.
ans. a

22. In a dc generator, emf commutation can be achieved by
a) keeping the brushes in the gap.
b) shifting the brushes ahead of gap.
c) shifting the brushes forward.
d) shifting the brushes backward.
ans. c

23. The mmf of an interpole is proportional to
a) armature current
b) field current
c) product of armature and field currents.
d) ratio of armature and field currents.
ans. a

24. The terminal voltage of a 20hp,230 V dc series motor under rated speed and no load condition is
a) 0 V
b) 230 V
c) about 5 to 20 V.
d) 250 V
ans. b

25. Modern starters used in dc shunt motors
a) are manual
b) use time delay relays.
c) use back emf sensing relays.
d) use time delay relays or back emf sensing relays.
ans. c

26. Magnetic material used for large transformers is
a) cast steel
b) hot rolled silicon steel
c) cold rolled grain oriented steel.
d) either b or c
ans. c

27. Thickness of core laminations of tranformers is
a) 18 cm
b) 25 cm
c) 20 cm
d) 15 cm
ans. c

28. In order to reduce hysteresis loss
a) core many be laminated.
b) silicon steel may be used as the core material
c) core may be constructed with any permanent magnet material such as alnico.
d) core may be impregnated with varnish.
ans. b

29. Multi-step core is used in a transformer to
a) increase the output.
b) decrease the cost of core material
c) decrease the cost of copper.
d) increase the efficiency.
ans. c

30. Gross window area in a transformer is 750 cm-sq. The approximate height of window is
a) 50 cm
b) 75 cm
c) 30 cm
d) 25 cm
ans. b

ELECTRICAL MACHINE DESIGN 2

11. As the number of poles in a dc machine increases the chances of flash over increases because
a) the voltage between the segments increases
b) the voltage between segments decreases
c) the length of the commutator decreases
d) effect of armature reaction increases
ans. a

12. Suitable value of number of poles for a 250 kw , 600 rpm dc motor is
a) 2
b) 4
c) 6
d) 8
ans. d

13. Air gap at the pole tips of a dc machine is kept more than that at the centre of the pole mainly to reduce
a) reactance voltage
b) effect of armature reaction
c) losses in armature core
d) noise of machine
ans. a

14. Number of parallel paths in a dc machine with duplex wave winding will be
a) 2
b) 4
c) 8
d) 16
ans. b

15. The number of parallel paths or a 4-pole duplex lap winding will be
a) 2
b) 4
c) 6
d) 8
ans. d

16. In a dc machine if the number of slot in the armature are more
a) cooling is likely to be poor
b) commutation will be poor
c) cost will be more
d) the flux pulsation will be more
ans. c

17. The number of brush arms is made sometimes equal to the number of poles in case of simplex dc wave winding to
a) avoid sparking commutation
b) reduce armature reaction
c) reduce the length of commutator and to equalise the number of coils in each parallel path.
d) none of the above
ans. a

18. Along with the rate of change of current, reactance voltage in dc machines also varies with
a) number of turns on the coil.
b) square of the number of turns on the coil.
c) square root of the number of turns of the coil.
d) none of the above.
ans. b

19. Width of carbon brush should be equal to
a) less than the width of one commutator segment.
b) the width of 1 to 2 commutator segments.
c) the width of 2 to 3 commutator segments.
d) the width of more than 3 commutator segments.
ans. c

20. The brushes are staggered to
a) avoid eating away of copper due to arcing.
b) avoid ridges and to have even commutator surface.
c) reduce brush contact loss
d) reduce armature reaction.
ans. b

ELECTRICAL MACHINE DESIGN

1. For a soft magnetic material there will be
a) steeply rising magnetization curve.
b) relatively narrow and small hysteresis loop.
c) less energy loss per cycle of magnetization.
d) all the above.


2. Silicon content in iron lamination is kept within 5% as it
a) increases hysteresis loss.
b) increases cost.
c) makes the material brittle.
d) reduces the curie point.
ans: c

3. For under frequency operation the eddy current loss
a) decreases.
b) increases.
c) remains unchanged.
d) cannot be predicted.
ans: a.

4. What is the suitable shape of plunger magnet to give almost flat force-stroke characteristics?
a) Flat faced .
b) Conical.
c) Stepped.
d) Tapered.
ans: d.

5. In a dc machine " contraction coefficient" is used to take into account the reduction of
a) air-gap area due to armature slots.
b) iron losses in the teeth due to lower tooth density.
c) armature mmf due to armature slots.
d) torque due to ventilating duets.
ans: a

6. Carter's coefficient is applicable for estimating
a) requirements of air-gap mmf.
b) flux distribution in air gap.
c) length of air gap.
d) no-load loss.
ans: a

7. A lap wound dc machine has 400 conductors and 8 poles. The voltage induced per conductor is 2 volts. The machine generates a voltage of
a) 100 V
b) 200 V
c) 400 V
d) 800 V
ans: a

8. The yoke of a dc machine
a) must be made of magnetic material.
b) should preferably be made of magnetic material but can be of non-magnetic material.
c) must be made of non-magnetic material.
d) is partially made of magnetic material and partially made of non-magnetic material.
ans: a.

9. The pole shoes of a dc machines
a) are always laminated.
b) are never laminated.
c) are sometimes laminated.
d) are partially laminated.
ans: a

10. Poles of a dc machines are often laminated to
a) reduce pulsation loss.
b) reduce armature reaction.
c) reduce iron weight.
d) dissipate more heat.
ans: a.