Electrical Power Engineering Multiple Choice Questions and Answers (MCQs) Part 2:Â
52. The pair used for frequency converters is
(a) squirrel cage induction motor and synchronous motor.
(b) slip-ring induction motor and synchronous motor.
(c) slip-ring induction motor and squirrel cage induction motor.
(d) all of above.
Answer : (a) squirrel cage induction motor and synchronous motor.
53. For the same rating, the size of the single-phase induction motor is about ………. that of the corresponding three-phase induction motor.
(a) three times
(b) the same as
(c) 1.5 times
(d) one-third
Answer : (c) 1.5 times
54. A fluctuating voltage supply is detrimental to a refrigerator motor but not to a ceiling fan, although both are single-phase induction motors because, the refrigerator motor
(a) is made more robust than the fan motor.
(b) is subjected to short-duty cycle but the fan motor is subjected to continuous duty.
(c) is enclosed in a sealed unit while the fan motor is open to the environment.
(d) load is constant, but the fan motor load is voltage dependent.
Answer : (d) load is constant, but the fan motor load is voltage dependent.
55. Ward-Leonard controlled dc drives are usually used for ……. duty excavators.
(a) light
(b) medium
(c) heavy
Answer : (c) heavy
56. Speed control by variation of field flux results in
(a) constant power drive.
(b) variable power drive.
(c) constant torque drive.
(d) variable torque drive.
Answer : (a) constant power drive.
57. In variable speed dc motor drive the armature voltage control leads to
(a) constant power operation.
(b) constant torque operation.
(c) variable torque operation.
(d) randomly varying power operation.
Answer : (b) constant torque operation.
58. Speed of an induction motor is controlled by injecting voltage into the rotor winding. Frequency of the injected voltage should be
(a) main frequency.
(b) frequency corresponding to required speed.
(c) frequency corresponding to rotor speed.
(d) slip frequency.
Answer : (d) slip frequency.
59. The slip power recovery scheme is used in induction motor for speed control in the range
(a) above synchronous speed.
(b) below synchronous speed.
(c) both above and below synchronous speed.
(d) none of the above.
Answer : (c) both above and below synchronous speed.
60. The electric braking system commonly employed in rolling mills, elevators and printing presses is
(a) plugging.
(b) rheostatic.
(c) dynamic.
(d) regenerative.
Answer : (a) plugging.
61. For rheostatic braking of two series motors connected in parallel
(a) cross-connection is better.
(b) equaliser connection is better.
(c) both are equally good.
(d) none of the two is required.
Answer : (a) cross-connection is better.
62. Dynamic braking is employed to brake
(a) non-reversing drive.
(b) reversing drive.
(c) in both (a) and (b).
Answer : (c) in both (a) and (b).
63. The condition of regenerative braking can be achieved by
(a) speed higher than no-load speed of overhauling load.
(b) increasing the excitation while supply voltage remains constant.
(c) increasing the armature current.
(d) either (a) or (b).
Answer : (d) either (a) or (b).
64. In a dc shunt motor, regenerative braking is limited by
(a) saturation of flux at high speed.
(b) saturation of flux at low speed.
(c) armature current at high speed.
(d) either (a) or (b).
Answer : (b) saturation of flux at low speed.
65. For regenerative braking with dc series motors, its field windings are ……… excited.
(a) separately
(b) series
(c) shunt
(d) either (b) or (c)
Answer : (a) separately
66. The most economical method of electric braking is
(a) plugging
(b) dynamic braking with separate excitation.
(c) dynamic braking with self excitation.
(d) regenerative braking.
Answer : (d) regenerative braking.
67. The plugging provides ……… braking torque in comparison to rheostatic and regenerative braking systems.
(a) negligible
(b) small
(c) highest
Answer : (c) highest
68. Rheostatic braking may be applied to an induction motor provided
(a) it is a squirrel cage type.
(b) it is a wound type.
(c) separate dc source for field excitation is available.
(d) variable external resistance is available.
Answer : (c) separate dc source for field excitation is available.
69. The motor enclosure used for collieries, chemical plants is
(a) flame proof type.
(b) splash proof type.
(c) totally enclosed type.
(d) pipe ventilated type.
Answer : (a) flame proof type.
70. The motor enclosure used in woodworking industry is
(a) protected type.
(b) totally enclosed fan cooled type.
(c) flame-proof type.
(d) splash-proof type.
Answer : (b) totally enclosed fan cooled type.
71. The motor enclosure used for industrial purposes is
(a) protected type.
(b) drip proof type.
(c) totally enclosed type.
(d) open type.
Answer : (a) protected type.
72. Rotor of a motor is usually supported on
(a) bush bearings.
(b) thrust bearing.
(c) ball or roller bearings.
(d) any of the above.
Answer : (c) ball or roller bearings.
73. Ball-bearings are
(a) used up to 75 kW motors.
(b) of long life and low friction loss.
(c) costlier and noisy particularly at high motor speed.
(d) all of the above.
Answer : (d) all of the above.
74. Sleeve bearings
(a) are normally of bronze.
(b) have self-lubricating properties.
(c) are used where noise is to be avoided.
(d) all of the above.
Answer : (d) all of the above.
75. Hot bearings of a dc Motor may be caused by
(a) poor ventilation.
(b) incorrect voltage.
(c) loose coupling.
(d) lack of or dirty lubricant.
Answer : (d) lack of or dirty lubricant.
76. Excessive motor vibration is caused by
(a) worn bearings.
(b) open armature coil.
(c) bent shaft.
(d) excessive brush tension.
Answer : (a) worn bearings.
77. Excessive sparking at the brushes may be caused due to
(a) loose coupling.
(b) worn bearings.
(c) dirt on commutator.
(d) open armature coil.
Answer : (c) dirt on commutator.
78. Intermittent sparking at the brushes of a dc motor may be caused due to
(a) intermittent load.
(b) open armature coil.
(c) loose coupling.
(d) incorrect voltage.
Answer : (b) open armature coil.
79. The least expensive drive is
(a) belt drive.
(b) rope drive.
(c) chain drive.
(d) gear drive.
Answer : (a) belt drive.
80. For very high speed ratio the indispensable drive is
(a) direct drive.
(b) rope drive.
(c) chain drive.
(d) any of the above.
Answer : (b) rope drive.
81. In rotating electrical machines, the insulation temperature limit for class B type is
(a) 105°C
(b) 130°C
(c) 150°C
(d) 180°C
Answer : (b) 130°C
82. For a particular motor, the cooling time constant is usually
(a) smaller than the heating time constant.
(b) greater than the heating time constant.
(c) equal to the heating time constant.
Answer : (b) greater than the heating time constant.
83. For a particular type of motor, the heating time constant
(a) increases with the increase in size.
(b) decreases with the increase in size.
(c) same for all sizes.
Answer : (a) increases with the increase in size.
84. For estimation of power rating of an electric drive, its losses can be considered to be proportional to
(a) power.
(b) (power)2.
(c) (power)3.
(d) torque.
Answer : (b) (power)2.
85. The duty cycle of a motor is
Load        Duration
20 kWÂ Â Â Â Â Â Â Â Â Â Â Â 120 s
10 kWÂ Â Â Â Â Â Â Â Â Â Â Â 120 s
stopped     280 s
The continuous rating of motor suitable for above application would be
(a) 20 kW
(b) 16.4 kW
(c) 14.14 kW
(d) 9 kW
Answer : (c) 14.14 kW
86. The heating time constant of a totally enclosed motor is relatively
(a) lower.
(b) higher.
(c) independent of type of enclosure.
Answer : (b) higher.
87. For a certain industrial application, an overrated ac motor was selected. It will lead to operation with
(a) higher efficiency and better pf.
(b) lower efficiency and poorer pf.
(c) higher efficiency and poorer pf.
(d) lower efficiency and better pf.
Answer : (b) lower efficiency and poorer pf.
88. A flywheel is generally used in
(a) a cement mill drive.
(b) a paper mill drive.
(c) a rolling mill drive.
(d) a sugar centrifuge drive.
Answer : (c) a rolling mill drive.
89. In overhead travelling cranes
(a) continuous rating motors are used.
(b) continuous minimum rating motors are used.
(c) short-time rating motors are used.
(d) any of the above.
Answer : (c) short-time rating motors are used.
90. 15 minute rated motors are suitable for
(a) light duty cranes.
(b) medium duty cranes.
(c) heavy duty cranes.
(d) all of the above.
Answer : (a) light duty cranes.
91. For medium duty cranes the short-time rating motor used is
(a) 10 minutes.
(b) 15 minutes.
(c) 30 minutes.
(d) one hour.
Answer : (c) 30 minutes.
92. Load equalization is desirable in the case of
(a) very large refrigeration and air-conditioning plants.
(b) rolling mills, electric hammers, presses, reciprocating pumps.
(c) lathes, woodworking machines, paper making machines, shapers and slotters.
(d) travelling cranes, lifts.
Answer : (b) rolling mills, electric hammers, presses, reciprocating pumps.
93. The phase controlled rectifiers used in speed control of dc motors converts fixed ac supply voltage into ………. output voltage.
(a) variable dc
(b) variable ac
(c) variable frequency ac
(d) full rectified ac
Answer : (a) variable dc
94. A four quadrant operation requires
(a) two full converters in series.
(b) two full converters connected back to back.
(c) two full converters connected in parallel.
(d) two semi converters connected back to back.
Answer : (b) two full converters connected back to back.
95. The following converters can feed power in any of the four quadrants
(a) semi-converter.
(b) full converter.
(c) dual converter.
(d) combination of a semi- and full-converters.
Answer : (c) dual converter.
96. For speed control of dc motors using controlled rectifiers, armaÂture voltage control gives speed
(a) above rated speed.
(b) below rated speed.
(c) above as well as below rated speed.
Answer : (b) below rated speed.
97. Armature of a dc motor is fed from a phase controlled rectifier where as its field is supplied from a constant dc source. To reduce the speed of the motor regeneratively
(a) the firing angle of the converter should be increased.
(b) the polarity of the dc voltage should be reversed.
(c) the polarity of the armature induced voltage should be reversed.
(d) the firing angle should be varied in the range 90° to 180° simultaneously reversing the armature connections.
Answer : (a) the firing angle of the converter should be increased.
98. For the speed control of ac drive, the preferred method using thyristors is
(a) phase control.
(b) integral cycle control.
(c) chopper control.
(d) all are equally good.
Answer : (c) chopper control.
99. The semiconductor device used for speed control of single phase induction motors in every day domestic use is
(a) SCR.
(b) power transistor.
(c) Triac.
(d) power MOS.
Answer : (c) Triac.
100. Which of the following motor is suitable for driving cranes, hoists, centrifugal pumps, conveyor belt etc. ?
(a) DC series motor.
(b) DC shunt motor.
(c) DC compound wound motor.
(d) Any of the above.
Answer : (a) DC series motor.
101. Heavy-duty steel-works cranes having wide load variations are equipped with
(a) dc series motors.
(b) plain squirrel cage induction motors.
(c) wound rotor induction motors.
(d) synchronous motors.
Answer : (a) dc series motors.