Sunday, March 14, 2021

AC MACHINES 8

Ac machines mcq, gate eee, electrical engineering , btech eee questions, previous eee gate papers











AC MACHINES

91. The following starting method for an induction motor is inferior from the point of view of poor starting torque per ampere of the line current drawn
a) Direct-on-line starting
b) Auto transformer method of starting
c) series inductor method of starting
d) star-delta method of starting
ans. c

92. A 3-phase induction motor is started by direct-on-line switching at the rated voltage. If the starting current drawn is 6 times the full-load current, and the full -load slip is 4%, then the ratio of the starting developed torque to the full load torque is approximately equal to
a) 0.24
b) 1.44
c) 2.40
d) 6.00
ans. b

A three-phase squirrel cage induction motor has a starting current of seven times the full load current and a full load slip of 5%
93. If an autotransformer is used to reduced voltage starting to provide 1.5 per unit starting torque, the auto-transformer ratio (%) should be
a) 57.77 %
b) 72.56%
c) 78.25%
d) 81.33%
ans. c
94. If a star-delta starter is used to start this induction motor, the per-unit starting torque will be
a) 0.607
b) 0.816
c)1.225
d) 1.616
ans. b
95. If a starting torque of 0.5 per unit is required then the per unit starting current should be
a) 4.65
b) 3.75
c) 3.16
d) 2.13
ans. c

96. An ac induction motor is used for a speed control application. It is driven from an inverter with a constant v/f control. The motor nameplate details
V: 415 V
Ph: 3
f: 50 Hz
N: 2850 rpm
The motor is run with the inverter output frequency set at 40Hz, and with half the rated slip. The running speed of the motor is
a) 2400 rpm
b) 2280 rpm
c) 2340 rpm
d) 2790 rpm
ans.

97. The speed of a 4-pole induction motor is controlled by varying the supply frequency while maintaining the ratio of supply voltage to supply frequency (V/F) constant. At a rated frequency of 50 Hz and a rated voltage of 400 V, its speed is 1440 rpm. find the speed at  30 Hz, if the load torque is constant.
a) 882 rpm
b) 864 rpm
c) 840 rpm
d) 828 rpm
ans. c

98. A 3-phase induction motor is driving a constant torque load at rated voltage and frequency. If both voltage and frequency are halved, the following statements relate to new conditions if stator resistance, leakage reactance, and core loss are ignored.
P. The difference between synchronous speed and actual speed remains the same
Q. The air-gap flux remains the same
R. The stator current remains the same
S. The PU slip remains the same
among the above, correct statements are
a) all
b) P ,Q,R
c) Q, R, S
d) P, S
ans. b

99. A 3-phase squirrel cage induction motor has full-load efficiency of 0.8 and 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
ans. c

100. A 3-phase slip-ring induction motor is fed from the rotor side with the stator winding short-circuited. The frequency of the currents in the short-circuited stator is
a) slip frequency
b) supply frequency
c) frequency corresponding to rotor speed
d) zero
ans. a

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