Monday, October 22, 2012

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

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