80. In case of 3-phase slip-ring induction motor, as the rotor resistance is increased, the starting torque
(c) increases upto a certain value of resistance and then remains constant.
(d) increases upto a certain value of resistance and then decreases. [A.M.I.E. Sec B. Winter 1995]
81. A 3-phase slip-ring induction motor having negligible stator impedance drives a constant torque load. If an additional resistance is included in the rotor circuit, what does the motor experience?
(a) Increase in both the stator current and the slip.
(b) No change in the stator current and increase in the slip.
(c) Increase in the stator current and no change in the slip.
(d) Decrease in the stator current and increase in the slip. [I.E.S. E.E.-II, 2008]
82. An induction motor can run at synchronous speed if it is operated
(a) on no-load.
(b) on full load.
(c) with emf injected in the rotor circuit.
(d) on voltage higher than the rated voltage.
83. The speed regulation of a 3-phase induction motor at full load is about
84. The speed of an induction motor depends on
(a) number of stator poles.
(b) stator supply frequency.
(c) input voltage to stator.
(d) all of the above.
85. Smooth speed control can be achieved by
(a) rotor resistance control only.
(b) rotor slip power control only.
(c) variation of supply frequency only.
(d) both rotor slip power control and variation of supply frequency.
86. The speed of a slip-ring three phase induction motor can be controlled from the rotor side by
(a) changing the supply voltage.
(b) changing the supply frequency.
(c) rheostatic control.
(d) changing the number of poles. [IES. E.E.II, 2008]
87. Which of the following methods are suitable for the speed control of squirrel cage induction motors?
- Voltage control.
- Rotor resistance control.
- Frequency control.
- Pole changing method.
Select the correct answer using the codes given below:
(a) 2, 3 and 4.
(b) 1, 3 and 4.
(c) 1, 2 and 3.
(d) 2 and 4. [I.E.S. E.E.-II, 2005]
88. A 3-phase induction motor fed from a 3-phase voltage regulator is suitable for driving loads whose torque is
(a) constant irrespective of speed.
(b) inversely proportional to the square of the speed.
(c) directly proportional to the square of the speed.
(d) inversely proportional to speed.
89. Synchronous speed of an induction motor can be increased by
(a) reducing mechanical friction.
(b) increasing supply voltage.
(c) increasing number of poles.
(d) increasing supply frequency. [A.M.I.E. Winter 1997]
90. In a variable speed induction motor drive V/f, is kept constant over a wide range of its frequency variation. The motor operates in the following mode :
(a) constant power mode.
(b) constant torque mode.
(c) variable power and variable torque mode.
(d) constant slip mode. [A.M.I.E. Sec B. Summer 1993]
91. In ac motor control V/f is kept constant to
(a) make maximum use of the magnetic circuit.
(b) make minimum use of the magnetic circuit.
(c) maximize the current drawn.
(d) make the power constant. [A.M.I.E. Sec B. Summer 1994]
92. While operating on variable frequency supplies, the ac motor requires variable voltage as well in order to
(a) protect the insulation.
(b) avoid the effect of saturation.
(c) improve the capabilities of the inverter.
(d) protect the thyristor from dv/dt . [E.E.-II, 1993]
93. A variable frequency induction motor is operated at constant V/f ratio. It has the following features over wide speed range regarding motor magnetising current (Im.) and maximum torque (Tm) respectively:
(a) Variable Im, constant Tm.
(b) Constant Im, constant Tm.
(c) Constant Im, variable Tm.
(d) Variable Im, variable Tm. [A.M.I.E. Sec B. Winter 1998]
94. In a 3-phase induction motor, if supply voltage and frequency are reduced by the same ratio, then slip at which maximum torque occurs is
(a) more and maximum torque Tem remains constant.
(b) more and Tem decreases.
(c) less and Tem decreases.
(d) less and Tem increases. [A.M.I.E. Sec B. Summer 2000]
95. A 3-phase squirrel cage induction motor has a full-load efficiency of 0.8 and a maximum efficiency of 0.9. It is operated at a slip of 0.6 by applying a reduced voltage. The efficiency of the motor at this operating point is
(a) less than 0.4.
(b) greater than 0.6.
(c) in the range of 0.8 ± 0.1.
(d) none of the above. [GATE E.E. 1998]
96. During plugging of an induction motor
(a) phase sequence is reversed.
(b) a dc source is connected to stator.
(c) one phase is open circuited.
(d) power is fed back to mains. [A.M.I.E. Sec B. 2002; 2003]
97. In dynamic braking of 3-phase induction motors
(a) the supply terminals of any two stator phases are interchanged.
(b) any two stator terminals are earthed.
(c) the stator terminals are switched over to a dc source from the ac supply.
(d) a dc voltage is injected in the rotor circuit.
98. In a 3-phase induction motor, the regenerative braking occur when
(a) the load is lowered by a hoisting machine.
(b) the load is raised by a hoisting machine.
(c) the number of poles is reduced in a pole-changing motor.
(d) the motor speed falls due to overload.
99. The stator of a 2/4 pole changing cage motor is initially wound for 2-poles. The reconnection of the stator winding to 4-poles through a change over switch, while the motor is running would result in
(a) constant torque drive.
(b) constant hp drive.
(c) plugging to standstill.
(d) regenerative braking to half the original speed. [U.P.S.C. I.E.S. 1992]
100. If a squirrel cage induction motor runs slow, the probable cause could be
(a) low voltage.
(c) shorted stator coils.
(d) one phase open.
(e) open stator coils.
(f) any of these.
101. If a squirrel cage induction motor run hot, the probable cause could be
(a) uneven air gap.
(c) low supply frequency.
(d) any of these.
102. If an induction motor hums during starting up, the possible cause could be
(a) open circuit.
(b) unequal phase resistance.
(c) inter-turn short circuit on rotor.
(d) any of the above.
103. Three phase induction motor should preferably be mounted on
(a) wooden structure.
(b) FVC platform.
(c) solid concrete foundation.
(d) rigid steel structure.
(e) either (c) and (d).
104. The large sized induction motors are protected against overload by
(a) high-voltage fuses.
(b) automatic cutouts.
(c) thermal or magnetic relays.
(d) rewirable fuses.
105. The insulating materials employed for motor windings are classified according to
(a) motor kW output rating.
(b) level of temperature rise.
(c) controller size.
(d) overload protection available.
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