1. Synchronous motors are generally not self-starting because
Correct : B. the direction of instantaneous torque reverses after half cycle
2. In case one phase of a three-phase synchronous motor is short-circuited the motor will
Correct : A. not start
3. A pony motor is basically a
Correct : A. small induction motor
4. A synchronous motor can develop synchronous torque
Correct : C. only at synchronous speed
5. A synchronous motor can be started by
Correct : D. any of the above
6. A three-phase synchronous motor will have
Correct : C. two slip-rings
7. Under which of the following conditions hunting of synchronous motor is likely to occur ?
Correct : A. Periodic variation of load
8. When the excitation of an unloaded salient pole synchronous motor suddenly gets
disconnected
Correct : A. the motor stops
9. When V is the applied voltage, then the breakdown torque of a synchronous motor
varies as
Correct : A. V
10. The power developed by a synchronous motor will be maximum when the load angle
is
Correct : C. 90°
11. A synchronous motor can be used as a synchronous capacitor when it is
Correct : D. over-excited
12. A synchronous motor is running on a load with normal excitation. Now if the load on
the motor is increased
Correct : D. power factor will decrease and armature current will increase
13. Mostly, synchronous motors are of
Correct : C. salient pole type machines
14. The synchronous motor is not inherently self-starting because
Correct : A. the force required to accelerate the rotor to the synchronous speed in an instant is absent
15. As the load is applied to a synchronous motor, the motor takes more armature current
because
Correct : B. the rotor by shifting its phase backward causes motor to take more current
16. Synchronous motor always runs at
Correct : A. the synchronous speed
17. An over-excited synchronous motor takes
Correct : A. leading current
18. The working of a synchronous motor is similar to
Correct : B. transmission of mechancial power by shaft
19. The minimum armature current of the synchronous motor corresponds to operation at
Correct : B. unity power factor
20. In a synchronous motor, the magnitude of stator back e.m.f. £& depends on
Correct : A. d.c. excitation only
21. If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in
mechanical degrees is
Correct : B. 3
22. For V-curves for a synchronous motor the graph is drawn between
Correct : A. field current and armature current
23. The back e.m.f. of a synchronous motor depends on
Correct : C. load angle
24. A synchronous motor can operate at
Correct : D. lagging, leading and unity power factors
25. In a synchronous motor which loss varies with load ?
Correct : C. Copper loss
26. A synchronous motor can be made self starting by providing
Correct : D. none of the above
27. The oscillations in a synchronous motor can be damped out by
Correct : C. providing damper bars in the rotor pole faces
28. The shaft of synchronous motor is made of
Correct : A. mild steel
29. When the field of a synchronous motor is under-excited, the power factor will be
Correct : B. lagging
30. The speed regulation of a synchronous motor is always
Correct : D. zero
31. The percentage slip in case of a synchronous motor is
Correct : D. zero
32. The operating speed of a synchronous motor can be changed to new fixed value by
Correct : C. changing frequency
33. A synchronous motor will always stop when
Correct : C. excitation winding gets disconnected
34. riunting in a synchronous motor takes place
Correct : B. when load varies
35. When load on an over-excited or under excited synchronous*motor is increased, rate
of change of its armature current as compared with that of power factor is
Correct : B. less
36. The rotor copper losses, in a synchronous motor, are met by
Correct : A. d.c. source
37. The maximum power developed in a synchronous motor occurs at a coupling angle of
Correct : C. 90°
38. When the stator windings are connected in such a fashion that the number of poles
are made half, the speed of the rotor of a synchronous motor
Correct : D. increases to two times the original value
39. In which of the following motors the stator and rotor magnetic field rotate at the same speed ?
Correct : B. Synchronous motor
40. Synchronsizingpower of a synchronous machine is
Correct : B. inversely proportional to the synchronous reactance
41. Synchronous motors are
Correct : A. not-self starting
42. The standard full-load power factor ratings for synchronous motors are
Correct : C. unity or 0.8 leading
43. A synchronous motor running with normal excitation adjusts to load increases
essentially by increase in
Correct : B. armature current
44. A synchronous motor has better power factor as compared to that of an equivalent
induction motor. This is mainly because
Correct : B. stator supply is not required to produce magnetic field
45. A synchronous motor working at leading power factor can be used as
Correct : B. phase advancer
46. Slip rings are usually made of
Correct : B. brass or steel
47. An over excited synchronous motor is used for
Correct : D. power factor corrections
48. When the voltage applied to a synchronous motor is increased, which of the following
will reduce ?
Correct : D. None of the above
49. The efficiency of a properly designed synchronous motor will usually fall in range
Correct : C. 85 to 95%
50. To limit the operating temperature an electrical machine should have proper
Correct : B. current rating
51. Slip-rings in a synchronous motor carry
Correct : A. direct current
52. A synchronous machine with large air gap has
Correct : D. all of the above
53. The armature current of the synchronous motor has higher values for
Correct : C. both (a) and (b)
54. In a synchronous motor running with fixed excitation, when the load is increased
three times, its torque angle becomes approximately
Correct : C. thrice
55. The angle between the rotating stator flux and rotor poles is called _____ angle.
Correct : A. torque
56. Which of the following methods is used to start a synchronous motor ?
Correct : C. Damper winding in conjunction with star-delta starter
57. When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop
Correct : B. induction generator torque
58. An important advantage of a synchronous motor over wound round induction motor
is that
Correct : A. its power factor may be varied at will
59. 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
Correct : C. five degrees
60. Power factor of a synchronous motor is unity when
Correct : B. the armature current is minimum
61. Change of D.C. excitation of a synchronous motor changes
Correct : C. power factor of power drawn by the motor
62. While starting a synchronous motor by induction motor action, field winding is
usually
Correct : B. short-circuited by low resistance
63. Which of the following motors will be used in electric clocks ?
Correct : D. A.C. synchronous motor
64. If in a synchronous motor, driving mechanical load and drawing current at lagging
power factor from constant voltage supply, its field excitation is increased, then its power factor
Correct : B. become less
65. A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will
Correct : C. improve
66. While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by
Correct : D. all of the above methods
67. Armature of a synchronous machine is
Correct : D. all of the above reasons
68. If excitation of a synchronous motor running with a constant load is decreased from
its normal value, ignoring effects of armature reaction, it leads to
Correct : A. increase in both armature current and power factor angle
69. When a 3-phase synchronous generator is supplying a zero power factor lagging load,
the armature field affects the main field in the following way
Correct : B. directly opposes it
70. Stability of a synchronous machine
Correct : B. increases with increase in its excitation
71. The power factor of a synchronous motor is better than that of induction motor because
Correct : A. stator supply is relieved of responsibility of producing magnetic field
72. If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor
Correct : B. will become less
73. A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because
Correct : D. Er (net resultant voltage) in armature is increased
74. If one-phase of a 3-phase synchronous motor is short-circuited, motor
Correct : A. will refuse to start
75. If the field circuit of an unloaded salientpole synchronous motor gets suddenly open- circuited, then
Correct : B. the motor stops
76. In which of the following motors the stator and rotor fields rotate simultaneously ?
Correct : D. Synchronous motor
77. The speed of a synchronous motor
Correct : C. always remains constant
78. A rotory converter can also be run as a
Correct : D. synchronous motor
79. The maximum speed variation in a 3-phase synchronous motor is
Correct : D. zero
80. Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor ?
Correct : D. All of the above
81. Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current ?
Correct : D. Low field current
82. In a synchronous motor, the maximum power developed depends on all of the following except
Correct : C. direction of rotation
83. In a 3-phase synchronous motor, the negative phase sequence exists when the motor is
Correct : A. supplied with unbalanced voltage
84. In a synchronous motor, damper windings are provided on
Correct : C. pole faces
85. The induced e.m.f. in a synchronous motor working on leading power factor will be
Correct : A. more than the supply voltage
86. The effect of increasing the load on a synchronous motor running with normal
excitation is to
Correct : C. increase armature current but decrease power factor
87. The net armature voltage of a synchronous motor is equal to the
Correct : D. vector difference of Eh and V
88. The ratio of starting torque to running torque in a synchronous motor is
Correct : A. zero
89. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
Correct : B. d.c. excitation only
90. A 3-phase synchronous motor is running clockwise. If the direction of its field current
is reversed
Correct : B. the motor continue to run in the same direction
91. The magnitude of field flux in a 3-phase synchronous motor
Correct : A. remains constant at all loads
92. The torque angle, in a synchronous motor, is the angle between
Correct : C. the rotating stator flux and rotor poles
93. Hunting in a synchronous motor cannot be due to
Correct : A. windage friction
94. By which of the following methods the constant speed of a synchronous motor can be
changed to new fixed value ?
Correct : A. By changing the supply frequency
95. In a synchronous motor, V-curves represent relation between
Correct : A. armature current and field current
96. In a 3-phase, 4-pole, 50 Hz synchronous motor, the frequency, pole number and load
torque all are halved. The motor speed will be
Correct : B. 1500 r.p.m.
97. A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have
Correct : D. leading power factor with over-excitation
98. Which of the following motors is non-self starting ?
Correct : B. synchronous motor
99. In a synchronous motor it the back e.m.f. generated in the armature at noload is
approximately equal to the applied voltage, then
Correct : D. the excitation is said to be hundred per cent
100. In a synchronous motor, the damping winding is generally used to
Correct : A. prevent hunting and provide the starting torque