Electrical Engineering Practice MCQ : Synchronous Motor (part 2)

For previous part, check previous post.








51. 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%

Ans: c

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

(a) voltage rating

(b) current rating

(c) power factor

(d) speed

Ans: b

53. Slip-rings in a synchronous motor carry

(a) direct current

(b) alternating current

(c) no current

(d) all of the above

Ans: a

54. 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

Ans: d

55. 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

Ans: c

56. 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

(e) nine timesAns: c

57. The angle between the rotating stator flux and rotor poles is called _____ angle.

(a) torque

(b) obtuse

(c) synchronizing

(d) power factorAns: a

58. 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 Ans: c

59. 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

(e) none of the above

Ans: b

60. 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

Ans: a

61. 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

Ans: c

62. 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

Ans: b

63. 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

(e) all of the above

Ans: c

64. 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 Ans: b

65. 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

Ans: d

66. 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) become more

(b) become less

(c) remain constant

(d) none of the above

Ans: b

67. 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) remain same

(b) go down

(c) improve

(d) none of the above

Ans: c

68. 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

(a) induction motor torque in field winding

(b) induction motor torque in damper winding

(c) eddy current and hysteresis torque in pole faces

(d) reHetance motor torque due to saliency of the rotor

(e) all of the above methods

Ans: e

69. Armature of a synchronous machine is

(a) of reducing number of slip rings on the rotor

(b) armature is associated with large power as compared to the field circuits

(c) of difficulty of providing high voltage insulation on rotor

(d) all of the above reasons

Ans: d

70. 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 angle

(b) increase in back e.m.f. but decrease in armature current

(c) increase in both armature current and power factor which is lagging (d) increase in torque angle but decrease in back e.m.f. Ans: a

71. 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) augments it directly

(b) directly opposes it

(c) cross-magnetises it

(d) none of the above

Ans: b

72. Stability of a synchronous machine

(a) decreases with increase in its excitation

(b) increases with increase in its excitation

(c) remains unaffected with increase in excitation

(d) any of the above

Ans: b

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

(a) stator supply is relieved of responsibility of producing magnetic field

(b) mechanical load on the motor can be adjusted

(c) synchronous motor runs at synchronous speed

(d) synchronous motor has large air gap Ans: a

74. 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 become more

(b) will become less

(c) will remain unchanged (d) none of the above.

Ans: b

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

(a) speed of the motor is reduced

(b) power factor is decreased

(c) Eb (back e.m.f.) becomes less than V (applied voltage)

(d) Er (net resultant voltage) in armature is increased

(e) none of the above

Ans: d

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

(a) will refuse to start

(b) will overheat in spots

(c) will not come upto speed

(d) will fail to pull into step

Ans: a

77. If the field circuit of an unloaded salientpole synchronous motor gets suddenly opencircuited, then

(a) it runs at a slower speed

(b) the motor stops

(c) it continues to run at the same speed

(d) it runs at a very high speed Ans: b

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

(a) D.C. motor

(b) Reluctance motor

(c) Universal motor

(d) Synchronous motor

(e) Induction motor

Ans: d

79. The speed of a synchronous motor

(a) increases as the load increases

(b) decreases as the load decreases

(c) always remains constant

(d) none of the above

Ans: c

80. A rotory converter can also be run as a

(a) d.c. shunt motor

(b) d.c. series motor

(c) d.c. compound motor

(d) induction motor

(e) synchronous motor

Ans: e

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

(a) 10 per cent

(b) 6 per cent

(c) 4 per cent

(d) 2. per cent

(e) zero

Ans: e

82. Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor ? (a) Phase to phase winding resistance

(b) Stator winding to earthed frame

(c) Rotor winding to earthed shaft

(d) All of the above

Ans: d

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

(a) High field current

(b) Low short circuit ratio

(c) High core losses

(d) Low field current

Ans: d

84. In a synchronous motor, the maximum power developed depends on all of the following except (a) rotor excitation

(b) maximum value of coupling angle

(c) direction of rotation

(d) supply voltage

Ans: c

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

(a) supplied with unbalanced voltage

(b) under-loaded

(c) over-loaded

(d) none of the above

Ans: a

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

(a) stator frame

(b) rotor shaft

(c) pole faces

(d) none of the above

Ans: c

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

(a) more than the supply voltage

(b) less than the supply voltage

(c) equal to the supply voltage

Ans: a

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

(a) decrease both armature current and power factor

(6) decrease armature current but increase power factor

(c) increase armature current but decrease power factor

(d) increase both its armature current and power factor Ans: c

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

(a) vector sum of Eb and V

(b) arithmetic sum of Eb and V

(c) arithmetic difference of Eb and V

(d) vector difference of Eh and V

Ans: d

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

(a) zero

(b) one

(c) two

(d) infinity

Ans: a

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

(a) load on the motor

(b) d.c. excitation only

(c) both the speed and rotor flux

(d) none of the above

Ans: b

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

(a) the motor will stop

(b) the motor continue to run in the same direction

(c) the winding of the motor will burn

(d) the motor will run in the reverse direction

(e) none of the above

Ans: b

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

(a) remains constant at all loads

(b) varies with speed

(c) varies with the load

(d) varies with power factor

Ans: a

94. 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) the rotating stator flux and rotor poles

(d) none of the above

Ans: c

95. Hunting in a synchronous motor cannot be due to

(a) windage friction

(b) variable load

(c) variable frequency

(d) variable supply voltage

Ans: a

96. By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ? (a) By changing the supply frequency

(b) By interchanging any two phases

(c) By changing the applied voltage

(d) By changing the load.

Ans: a

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

(a) armature current and field current

(b) power factor and speed

(c) field current and speed

(d) field current and power factor Ans: a

98. 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) 3000 r.p.m.

(b) 1500 r.p.m.

(c) 750 r.p.m.

(d) none of the above

Ans: b

99. A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power fac

tor. On changing the excitation only, the armature current will have

(a) no change of power factor

(b) lagging power factor with over-excitation

(c) leading power factor with under-excitation

(d) leading power factor with over-excitation Ans: d

100. Which of the following motors is non-self starting ?

(a) D.C. series motor

(b) synchronous motor

(c) Squirrel cage induction motor

(d) Wound round induction motor Ans: b

101. In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then

(a) the motor is said to be fully loaded

(b) the torque generated is maximum

(c) the excitation is said to be zero per cent

(d) the excitation is said to be hundred per cent Ans: d

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

(a) prevent hunting and provide the starting torque

(b) reduce the eddy currents

(c) provide starting torque only

(d) reduce noise level

(e) none of the above

Ans: a

103. If the field of a synchronous motor is underexcited, the power factor will be

(a) zero

(b) unity

(c) lagging

(d) leading

Ans: c

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

(a) number of poles

(b) flux density

(c) rotor speed

(d) rotor excitation

(e) none of the above

Ans: d

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

(a) slip torque

(b) pull-out torque

(c) breaking torque

(d) synchronising torque

Ans: d

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

(a) high excitation only

(b) low excitation only

(c) both high and low excitation

(d) none of the above Ans: c

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

(a) Windage loss

(b) Copper losses

(c) Any of the above

(d) None of the above

Ans: a

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

(a) flux density

(b) horse power rating

(c) speed

(d) all of the above

Ans: a

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

(a) Eddy current losses in the conductors

(b) Iron losses in the stator

(c) Copper losses in the slot portion of the conductors

(d) Windage losses

(e) None of the above

Ans: d

110. The duration of sudden snort-circuit test on a synchronous motor is usually about

(a) one hour

(b) one minute

(c) one second

(d) none of the above

Ans: c

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-up torque

(b) pull-in torque

(c) pull-out torque

(d) none of the above

Ans: b

112. A synchronous machine with low value of short-circuit ratio has

(a) lower stability limit

(6) high stability limit

(c) good speed regulation

(d) good voltage regulation

(e) none of the above

Ans: a

113. The construction of a synchronous motor resembles (a) a series motor

(b) an induction motor

(c) an alternator

(d) a rotary converter

Ans: c

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

(a) stop

(b) run as induction motor

(c) function as static condenser

(d) burn with dense smoke

Ans: a

115 For power factor correction, synchronous motors operate at

(a) no-load and greatly over-excited fields

(b) no-load and under-excited fields

(c) normal load with minimum excitation

(d) normal load with zero excitation Ans: a

116. 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) locked-rotor torque

(b) synchronous torque

(c) pull up torque

(d) reluctance torque

Ans: a

117. Exciters of synchronous machines are

(a) d.c. shunt machines

(b) d.c. series machines

(c) d.c. compound machines

(d) any of the above

Ans: a

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

(a) rotor and stator teeth

(b) rotor and the stator poles of opposite polarity (c) rotor and the stator poles of the same polarity

(d) none of the above

Ans: b

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

(a) the stator current will be zero

(b) the stator current will be very small

(c) the stator current will be very high

(d) the back e.m.f. will be more than the supply voltage

(e) none of the above

Ans: a

120 The armature current of the synchronous motor

(a) has large values for low excitation i niy

(b) has large values for high excitation only

(c) has large values for low and high excitation

(d) any of the above

Ans: c

121. The maximum power developed in a synchronous motor will depend on

(a) the rotor excitation only

(b) the supply voltage only

(c) the rotor excitation and supply volt-age both

(d) the rotor excitation, supply voltage and maximum value of coupling angle (90°)

(e) none of the above

Ans: d

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

(a) static condenser

(b) condenser

(c) synchronous condenser

(d) none of the above

Ans: c

123. A synchronous motor develops maximum power when load angle is

(a) 45°

(b) 60°

(c) 90°

(d) 120°

Ans: c

124. In a synchronous motor, the breakdown torque is

(a) directly proportional to applied voltage

(b) directly proportional to the square of the applied voltage

(c) inversely proportional to applied voltage

(d) none of the above
Ans: a