Synchronous Motors MCQ – Multiple Choice Questions

In this article, we shared the synchronous motor multiple choice questions and answers.

Synchronous Motors MCQ Questions

cut way synchronous motor

The below list provides all the multiple-choice questions related to the synchronous motors.

Answers are available at the bottom of the article.

1. In case one phase of a three-phase synchronous motor has short-circuited the motor will

A)run with excessive vibrations
B)run at 2/3 of synchronous speed
C)take less than the rated load
D)not start

2. A synchronous motor that works on a leading power factor and does not drive a mechanical load is called as

A)none of these answers
B)static condenser
C)synchronous condenser

3. 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

A)will remain unchanged
B)will become less
C)will become more
D)none of these answers

4. Armature of a synchronous machine is kept fixed because

A)of difficulty of providing high voltage insulation on rotor
B)of reducing number of slip rings on the rotor
C)armature is associated with large power as compared to the field circuits
D)all of these answers

5. A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because

A)power factor is decreased
B)none of these answers
C)Er (net resultant voltage) in armature is increased
D)Eb (back e.m.f.) becomes less than V (applied voltage)
E)speed of the motor is reduced

6. As the load is applied to a synchronous motor, the motor takes more armature current because

A)the rotor by shifting its phase backward causes motor to take more current
B)the back e.m.f. decreases causing ah increase in motor current
C)the rotor strengthens the rotating field causing more motor current
D)the increased load has to take more current

7. A synchronous motor working at leading power factor can be used as

A)noise generator
B)phase advancer
C)voltage booster
D)mechanical synchronizer

8. Slip rings are usually made of

A)copper or aluminium
B)silver or gold
C)brass or steel
D)carbon or graphite

9. The armature current of the synchronous motor has higher values for

A)low excitation only
B)both high excitation only and low excitation only
C)none of these answers
D)high excitation only

10. An important advantage of a synchronous motor over wound round induction motor is that

A)none of these answers
B)its speed is independent of supply frequency
C)its power factor may be varied at will
D)its speed may be controlled more easily

11. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

A)both the speed and rotor flux
B)d.c. excitation only
C)load on the motor
D)speed of the motor

12. In a synchronous motor, the maximum power developed depends on all of the following except

A)rotor excitation
B)direction of rotation
C)supply voltage
D)maximum value of coupling angle

13. If the field winding of an unloaded salient pole synchronous motor is open circuited, the motor will

A)run as induction motor
C)function as static condenser
D)burn with dense smoke

14. The rotor copper losses, in a synchronous motor, are met by

A)supply lines
B)d.c. source
C)motor input
D)armature input

15. The shaft of synchronous motor is made of

B)chrome steel
C)mild steel
D)stainless steel

16. 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

A)directly opposes it
B)augments it directly
C)none of these answers
D)cross-magnetises it

17. If the field of a synchronous motor is under-excited, the power factor will be


18. While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerating torque is produced by

A)induction motor torque in damper winding
B)reluctance motor torque due to saliency of the rotor
C)eddy current and hysteresis’ torque in pole faces
D)induction motor torque in field winding
E)all of these answers

19. A synchronous motor will always stop when

A)supply voltage fluctuates
B)excitation winding gets disconnected
C)supply voltage frequency changes
D)load in motor varies

20. Synchronous motor always runs at

A)more than synchronous speed
B)less than synchronous speed
C)none of these answers
D)the synchronous speed

21. The construction of a synchronous motor resembles

A)a series motor
B)an induction motor
C)an alternator
D)a rotary converter

22. Hunting in a synchronous motor takes place

A)when supply voltage fluctuates
B)when load varies
C)motor is under loaded
D)when power factor is unity

23. A synchronous motor can operate at

A)unity power factor only
B)lagging power factor only
C)leading power factor only
D)lagging, leading and unity power factors

24. Which of the following motors will be used in electric clocks?

A)D.C. shunt motor
B)D.C. series motor
C)A.C. induction motor
D)A.C. synchronous motor

25. In a synchronous motor, the damping winding is generally used to

A)none of these answers
B)reduce the eddy currents
C)provide starting torque only
D)prevent hunting and provide the starting torque
E)reduce noise level

26. The size of a synchronous motor decreases with the increase in

A)all of these answers
B)flux density
D)horse power rating

27. The synchronous motor is not inherently self-starting because

A)a rotating magnetic field does not have enough poles
B)the rotating magnetic field is produced by only 50 Hz frequency currents
C)the force required to accelerate the rotor to the synchronous speed in an instant is absent
D)the starting device to accelerate the rotor to near synchronous speed is absent

28. A synchronous motor develops maximum power when the load angle is


29. Which of the following methods is used to start a synchronous motor ?

A)Damper winding
B)Resistance starter in the armature circuit
C)Damper winding in conjunction with star-delta starter
D)Star-delta starter

30. When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop

A)synchronous motor torque
B)none of these answers
C)inductor motor torque
D)d.c. motor toque
E)induction generator torque

31. For V-curves for a synchronous motor the graph is drawn between

A)power factor and field current
B)armature current and power factor
C)field current and armature current
D)terminal voltage and load factor

32. 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

A)none of these answers
B)become less
C)remain constant
D)become more

33. A synchronous motor can be used as a synchronous capacitor when it is


34. The percentage slip in case of a synchronous motor is


35. The operating speed of a synchronous motor can be changed to new fixed value by

A)changing the supply voltage
B)using brakes
C)changing the load
D)changing frequency

36. Stability of a synchronous machine

A)decreases with increase in its excitation
B)remains unaffected with increase in excitation
C)increases with increase in its excitation
D)any of these answers

37. The speed regulation of a synchronous motor is always


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

A)increases to two times the original value
B)decreases to half the original value
C)remains same as the original value
D)tends to becomes zero

39. In a 3-phase synchronous motor, the negative phase sequence exists when the motor is

A)none of these answers
D)supplied with unbalanced voltage

40. The magnitude of field flux in a 3-phase synchronous motor

A)varies with the load
B)varies with power factor
C)varies with speed
D)remains constant at all loads

41. Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current ?

A)Low short circuit ratio
B)High core losses
C)High field current
D)Low field current

42. In a synchronous motor it the back e.m.f. generated in the armature at no load is approximately equal to the applied voltage, then

A)the torque generated is maximum
B)the excitation is said to be zero percent
C)the motor is said to be fully loaded
D)the excitation is said to be hundred percent

43. The induced e.m.f. in a synchronous motor working on leading power factor will be

A)less than the supply voltage
B)none of these answers
C)more than the supply voltage
D)equal to the supply voltage

44. Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor?

A)Phase to phase winding resistance
B)Rotor winding to earthed shaft
C)All of these answers
D)Stator winding to earthed frame

45. A three-phase synchronous motor will have

A)one slip-ring
B)three slip-rings
C)two slip-rings
D)no slip-rings

46. A synchronous motor can be started by

A)pony motor
B)any of these answers
C)providing damper winding
D)D.C. compound motor

47. The oscillations in a synchronous motor can be damped out by

A)oscillations cannot be damped
B)providing damper bars in the rotor pole faces
C)maintaining constant excitation
D)running the motor on leading power factors

48. An over-excited synchronous motor is used for

A)power factor corrections
B)low torque loads
C)fluctuating loads
D)variable speed loads

49. When V is the supply voltage and R is the rotor resistance per phase, the mechanical power developed by the synchronous motor will be proportional to


50. When load on an over-excited or underexcited synchronous motor is increased, rate of change of its armature current as compared with that of power factor is


51. The coupling angle or load angle of synchronous motor is defined as the angle between the

A)rotor and the stator poles of the same polarity
B)none of these answers
C)rotor and the stator poles of opposite polarity
D)rotor and stator teeth

52. Which of the following losses, in a synchronous motor, does not vary with load?

A)Windage loss
B)Copper losses
C)Any of these answers
D)None of these answers

53. In which of the following motors the stator and rotor magnetic field rotate at the same speed?

A)Reluctance motor
B)Universal motor
C)Induction motor
D)Synchronous motor

54. 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

A)increase in both armature current and power factor which is lagging
B)increase in both armature current and power factor angle
C)increase in torque angle but decrease in back e.m.f.
D)increase in back e.m.f. but decrease in armature current

55. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

A)both the speed and rotor flux
B)none of these answers
C)d.c. excitation only
D)load on the motor

56. In a synchronous motor, the armature current has large values for

A)low excitation only
B)both high and low excitation
C)high excitation only
D)none of these answers

57. At no-load, the armature current in a synchronous motor

A)leads the applied voltage by 90°
B)lags the applied voltage by 90°
C)none of these answers
D)is zero
E)is in phase with the applied voltage

58. 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

A)none of these answers
B)1500 r.p.m.
C)3000 r.p.m.
D)750 r.p.m.

59. The efficiency of a properly designed synchronous motor will usually fall in range

A)75 to 80%
B)99 to 99.5%
C)85 to 95%
D)60 to 70%

60. The maximum value of torque that a synchronous motor can develop without losing its synchronism is known as

A)synchronising torque
B)slip torque
C)breaking torque
D)pull-out torque

61. A synchronous machine with large air gap has

A)a higher synchronizing power which makes the Machine less sensitive to load variations
B)a higher value of stability limit
C)a small value of inherent regulation
D)all of these answers

62. To limit the operating temperature an electrical machine should have proper

B)power factor
C)current rating
D)voltage rating

63. A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed

A)the winding of the motor will burn
B)the motor will run in the reverse direction
C)the motor continue to run in the same direction
D)the motor will stop

64. If one-phase of a 3-phase synchronous motor is short-circuited, motor

A)will not come upto speed
B)will refuse to start
C)will fail to pull into step
D)will overheat in spots

65. Under which of the following conditions is hunting of synchronous motor is likely to occur?

A)Small and constant load
B)Over-loading for long periods
C)Periodic variation of load

66. A synchronous motor running with normal excitation adjusts to load increases essentially by increase in

A)armature current
B)power factor
C)torque angle
D)back e.m.f.

67. The speed of the synchronous motor

A)increases with increase in load
B)always remains constant
C)decreases as load increases
D)varies with power factor

68. The torque angle, in a synchronous motor, is the angle between

A)the supply voltage and the back e.m.f.
B)magnetising current and back e.m.f.
C)none of these answers
D)the rotating stator flux and rotor poles

69. Exciters of synchronous machines are

A)any of these answers
B)d.c. shunt machines
C)d.c. compound machines
D)d.c. series machines

70. Slip-rings in a synchronous motor carry

A)alternating current
B)direct current
C)no current
D)all of these answers

71. The working of a synchronous motor is similar to

A)none of these answers
C)gear train arrangement
D)transmission of mechancial power by shaft
E)distribution transformer

72. 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

A)go down
C)none of these answers
D)remain same

73. The net armature voltage of a synchronous motor is equal to the

A)vector difference of Eb and V
B)arithmetic sum of Eb and V
C)vector sum of Eb and V
D)arithmetic difference of Eb and V

74. A synchronous motor has a better power factor as compared to that of an equivalent induction motor. This is mainly because

A)synchronous motor has large airgap
B)synchronous motor has no slip
C)stator supply is not required to produce magnetic field
D)mechanical load on the rotor remains constant

75. In a synchronous motor which loss varies with load?

A)Windage loss
B)Bearing friction loss
C)Core loss
D)Copper loss

76. If the load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is


77. The standard full-load power factor ratings for synchronous motors are

A)unity or 0.8 leading
B)unity or zero
C)unity or 0.8 lagging
D)zero or 0.8 leading

78. Synchronsizing power of a synchronous machine is

A)none of these answers
B)inversely proportional to the synchronous reactance
C)direcly proportional to the synchronous reactance
D)equal to the synchronous reactance

79. Power factor of a synchronous motor is unity when

A)the armature current is zero
B)none of these answers
C)the armature current is minimum
D)the armature current is maximum

80. Which of the following losses is not dissipated by tile stator core surface in a synchronous motor?

A)Iron losses in the stator
B)None of these answers
C)Windage losses
D)Copper losses in the slot portion of the conductors
E)Eddy current losses in the conductors

81. Higher the applied ____voltage will be the stator flux and____ will be the pull-in torque

A)lower, greater
B)greater, lower
C)lower, lower
D)greater, greater

82. The angle between the rotating stator flux and rotor poles is called____ angle

C)power factor

83. The power factor of a synchronous motor is better than that of an induction motor because

A)stator supply is relieved of responsibility of producing magnetic field
B)synchronous motor runs at synchronous speed
C)mechanical load on the motor can be adjusted
D)synchronous motor has large air gap

84. The minimum armature current of the synchronous motor corresponds to operation at

A)zero power factor leading
B)0.707 power factor lagging
C)0.707 power factor leading
D)unity power factor

85. Which of the following motors is nonself starting?

A)synchronous motor
B)D.C. series motor
C)Wound round induction motor
D)Squirrel cage induction motor

86. Change of D.C. excitation of a synchronous motor changes

A)all of these answers
B)applied voltage of the motor
C)any of these answers
D)power factor of power drawn by the motor
E)motor speed

87. The mechanical displacement of the rotor with respect to the stator, in the polyphase multipolar synchronous motor running at full load, is of the order of

A)five degrees
B)ten degrees
C)two degrees
D)zero degree

88. Dust and dirt accumulation on motor winding will result in

A)increased core losses
B)shorting of coils
C)eddy current flow
D)a rise in winding temperature

89. The maximum power developed in a synchronous motor occurs at a coupling angle of


90. In a synchronous motor, damper windings are provided on

A)none of these answers
B)pole faces
C)stator frame
D)rotor shaft

91. A synchronous motor can develop synchronous torque

A)when under loaded
B)while over-excited
C)only at synchronous speed
D)below or above synchronous speed

92. If the field circuit of an unloaded salient pole synchronous motor gets suddenly open-circuited, then

A)it continues to run at the same speed
B)it runs at a slower speed
C)the motor stops
D)it runs at a very high speed

93. The power developed by a synchronous motor will be maximum when the load angle is


94. For power factor correction, synchronous motors operate at

A)no-load and under-excited fields
B)no-load and greatly over-excited fields
C)normal load with minimum excitation
D)normal load with zero excitation

95. If the synchronous motor, properly synchronized to the supply is running on no load and is having negligible loss then

A)the stator current will be very small
B)none of these answers
C)the stator current will be very high
D)the back e.m.f. will be more than the supply voltage
E)the stator current will be zero

96. The duration of a sudden short-circuit test on a synchronous motor is usually about

A)one minute
B)none of these answers
C)one second
D)one hour

97. In which of the following motors the stator and rotor fields rotate simultaneously?

A)Induction motor
B)Synchronous motor
C)Reluctance motor
D)D.C. motor
E)Universal motor

98. The speed of a synchronous motor

A)decreases as the load decreases
B)increases as the load increases
C)always remains constant
D)none of these answers

99. Synchronous motors are

A)essentially self-starting
B)none of these answers
C)not-self starting

100. The angle between the rotor poles and stator poles, in a synchronous motor, is known as

A)power factor angle
B)synchronizing angle
C)angle of retardation
D)torque angle

101. Synchronous motors are generally not self-starting because

A)the direction of instantaneous torque reverses after half cycle
B)startes cannot be used on these machines
C)starting winding is not provided on the machines
D)the direction of rotation is not fixed

102. A synchronous motor can be made self starting by providing

A)damper winding on rotor poles
B)damper winding on stator
C)damper winding on stator as well as rotor poles
D)none of these answers

103. By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value?

A)By changing the load.
B)By changing the applied voltage
C)By changing the supply frequency
D)By interchanging any two phases

104. The maximum speed variation in a 3- phase synchronous motor is

A)2 percent
B)4 percent
D)10 percent
E)6 percent

105. The back e.m.f. of a synchronous motor depends on

C)load angle
D)all of these answers

106. The armature current of the synchronous motor

A)has large values for low excitation only
B)has large values for low and high excitation
C)any of these answers
D)has large values for high excitation only

107. Hunting in a synchronous motor cannot be due to

A)variable supply voltage
B)windage friction
C)variable load
D)variable frequency

108. While starting a synchronous motor by induction motor action, the field winding is usually

A)connected to D.C. supply
B)none of these answers
C)short-circuited by low resistance
D)kept open-circuited

109. The ratio of starting torque to running torque in a synchronous motor is


110. The back e.m.f. in the stator of a synchronous motor depends on

A)none of these answers
B)rotor speed
C)flux density
D)number of poles
E)rotor excitation

111. The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as

A)pull-out torque
B)pull-up torque
C)pull-in torque
D)none of these answers

112. 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

A)no change of power factor
B)leading power factor with under-excitiltion
C)lagging power factor with over-excitation
D)leading power factor with over-excitation

113. When V is the applied voltage, then the breakdown torque of a synchronous motor varies as


114. A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased

A)power factor will decrease and armature current will increase
B)power factor will increase but armature current will decrease
C)power factor as well as armature current will decrease
D)power factor as well as armature current will increase

115. A rotory converter can also be run as a

A)d.c. series motor
B)synchronous motor
C)d.c. compound motor
D)d.c. shunt motor
E)induction motor

116. In a synchronous motor, the forced vibrations will be maximum when driving

A)a centrifugal blower
B)a reciprocating air compressor
C)none of these answers
D)any of these answers
E)a centrifugal pump

117. When the voltage applied to a synchronous motor is increased, which of the following will reduce?

A)Pull in torque
B)None of these answers
C)Both Stator flux and Pull in torque
D)Stator flux

118. When the field of a synchronous motor is under-excited, the power factor will be


119. Mostly, synchronous motors are of

A)smooth cylindrical type machines
B)induction type machines
C)salient pole type machines
D)alternator type machines

120. In a synchronous motor, V-curves represent the relation between

A)field current and power factor
B)power factor and speed
C)field current and speed
D)armature current and field current

121. The maximum torque which a synchronous motor will develop at rest for any angular position of the rotor, at rated stator supply voltage and frequency, is known as

A)synchronous torque
B)reluctance torque
C)locked-rotor torque
D)pull up torque

122. In a synchronous motor, the breakdown torque is

A)none of these answers
B)inversely proportional to applied voltage
C)directly proportional to applied voltage
D)directly proportional to the square of the applied voltage

123. A pony motor is basically a

A)small induction motor
B)D.C. shunt motor
C)D.C. series motor
D)double winding A.C./D.C. motor

124. The effect of increasing the load on a synchronous motor running with normal excitation is to

A)decrease both armature current and power factor
B)increase both its armature current and power factor
C)decrease armature current but increase power factor
D)increase armature current but decrease power factor

Click Here for Answers

If you liked this article, then please subscribe to our YouTube Channel for Electrical, Electronics, Instrumentation, PLC, and SCADA video tutorials.

You can also follow us on Facebook and Twitter to receive daily updates.

Next Quiz:

Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address

Leave a Comment