MCQ on synchronous motor 61-80

61. Slip rings are usually made of
(a) carbon or graphite
(b) brass or steel✔
(c) silver or gold
(d) copper or aluminium

62. An over excited synchronous motor is used for
(a) fluctuating loads
(b) variable speed loads
(c) low torque loads
(d) power factor corrections✔

63. When the voltage applied to a synchronous motor is increased, which of the following will reduce ?
(a) Stator flux
(b) Pull in torque
(c) Both (a) and (b)
(d) None of the above✔

64. The efficiency of a properly designed synchronous motor will usually fall in range
(a) 60 to 70%
(6) 75 to 80%
(c) 85 to 95%✔
(d) 99 to 99.5%

65. To limit the operating temperature an electrical machine should have proper
(a) voltage rating
(b) current rating✔
(c) power factor
(d) speed

66. Slip-rings in a synchronous motor carry
(a) direct current✔
(b) alternating current
(c) no current
(d) all of the above

67. A synchronous machine with large air gap has
(a) a higher value of stability limit
(6) a small value of inherent regulation
(c) a higher synchronizing power which makes the machine less sensitive to load variations
(d) all of the above✔

68. The armature current of the synchronous motor has higher values for
(a) high excitation only
(b) low excitation only
(c) both (a) and (b)✔
(d) none of the above

69. In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately
(a) one-third
(b) twice
(c) thrice✔
(d) six times

70. The angle between the rotating stator flux and rotor poles is called _____ angle.
(a) torque✔
(b) obtuse
(c) synchronizing
(d) power factor

71. Which of the following methods is used to start a synchronous motor ?
(a) Damper winding
(b) Star-delta starter
(c) Damper winding in conjunction with star-delta starter✔
(d) Resistance starter in the armature circuit

72. When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop
(a) inductor motor torque
(b) induction generator torque✔
(c) synchronous motor torque
(d) d.c. motor toque

73. An important advantage of a synchronous motor over wound round induction motor is that
(a) its power factor may be varied at will✔
(b) its speed is independent of supply frequency
(c) its speed may be controlled more easily
(d) none of the above

74. The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of
(a) zero degree
(b) two degrees
(c) five degrees✔
(d) ten degrees

75. Power factor of a synchronous motor is unity when
(a) the armature current is maximum
(b) the armature current is minimum✔
(c) the armature current is zero
(d) none of the above

76. Change of D.C. excitation of a synchronous motor changes
(a) applied voltage of the motor
(b) motor speed
(c) power factor of power drawn by the motor✔
(d) any of the above

77. While starting a synchronous motor by induction motor action, field winding is usually
(a) connected to D.C. supply
(b) short-circuited by low resistance✔
(c) kept open-circuited
(d) none of the above

78. A synchronous motor running with normal excitation adjusts to load increases essentially by increase in
(a) back e.m.f.
(b) armature current✔
(c) power factor
(d) torque angle

79. A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because
(a) synchronous motor has no slip
(b) stator supply is not required to produce magnetic field✔
(c) mechanical load on the rotor remains constant
(d) synchronous motor has large airgap

80. A synchronous motor working at leading power factor can be used as
(a) voltage booster
(b) phase advancer✔
(c) noise generator
(d) mechanical synchronizer