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Free download in PDF Polyphase Induction Motors General Knowledge Multiple Choice Questions(MCQs) with Answers for competitive exams. These short objective type questions with answers are very important for Board exams as well as competitive exams. These short solved questions or quizzes are provided by Gkseries.
61
Which motor is preferred for use in mines where explosive gases exist ?
[A]
Air motor
[B]
Induction motor
[C]
D.C. shunt motor
[D]
Synchronous motor
62
An induction motor can run at synchronous speed when
[A]
it is run on load
[B]
it is run in reverse direction
[C]
it is run on voltage higher than the rated voltage
[D]
e.m.f. is injected in the rotor circuit
Answer: e.m.f. is injected in the rotor circuit
63
Lubricant used for ball bearing is usually
[A]
graphite
[B]
grease
[C]
mineral oil
[D]
molasses
64
Blocked rotor test on induction motors is used to find out
[A]
leakage reactance
[B]
power factor on short circuit
[C]
short-circuit current under rated voltage
[D]
all of the above
65
The circle diagram for an induction motor cannot be used to determine
[A]
efficiency
[B]
power factor
[C]
frequency
[D]
output
66
What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ?
[A]
The slip of the motor will be 5%
[B]
The rotor will not run
[C]
The rotor will run at very high speed
[D]
The torque produced will be very large
Answer: The rotor will not run
67
The torque of a rotor in an induction motor under running condition is maximum
[A]
at the unit value of slip
[B]
at the zero value of slip
[C]
at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
[D]
at the value of the slip which makes the rotor reactance half of the rotor
Answer: at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
68
In the squirrel-cage induction motor the rotor slots are usually given slight skew
[A]
to reduce the magnetic hum and locking tendency of the rotor
[B]
to increase the tensile strength of the rotor bars
[C]
to ensure easy fabrication
[D]
none of the above
Answer: to reduce the magnetic hum and locking tendency of the rotor
69
The complete circle diagram of induetion motor can be drawn with the help of data found from
[A]
noload test
[B]
blocked rotor test
[C]
stator resistance test
[D]
all of the above
70
A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6 poles. The slip of the machine will be
[A]
0.01
[B]
0.02
[C]
0.03
[D]
0.04
71
The stating torque of the slip ring induction motor can be increased by adding
[A]
external inductance to the rotor
[B]
external resistance to the rotor
[C]
external capacitance to the rotor
[D]
both resistance and inductance to rotor
Answer: external resistance to the rotor
72
A change of 4% of supply voltage to an induction motor will produce a change of appromimately
[A]
4% in the rotor torque
[B]
8% in the rotor torque
[C]
12% in the rotor torque
[D]
16% in the rotor torque
Answer: 16% in the rotor torque
73
The shape of the torque/slip curve of induction motor is
[A]
parabola
[B]
hyperbola
[C]
rectangular parabola
[D]
straigth line
Answer: rectangular parabola
74
The power scale of circle diagram of an induction motor can be found from
[A]
stator resistance test
[B]
no-load test only
[C]
short-circuit test only
[D]
noue of the above
Answer: short-circuit test only
75
The vafcie of transformation ratio of an induction motor can be found by
[A]
open-circuit test only
[B]
short-circuit test only
[C]
stator resistance test
[D]
none of the above
Answer: short-circuit test only
76
In a three-phase induction motor
[A]
power factor at starting is high as compared to that while running
[B]
power factor at starting is low as compared to that while running
[C]
power factor at starting in the same as that while running
[D]
None
Answer: power factor at starting is low as compared to that while running
77
In three-phase induction motors sometimes copper bars are placed deep in the rotor to
[A]
improve starting torque
[B]
reduce copper losses
[C]
improve efficiency
[D]
improve power factor
Answer: improve starting torque
78
Which is of the following data is required to draw the circle diagram for an induction motor ?
[A]
Block rotor test only
[B]
No load test only
[C]
Block rotor test and no-load test
[D]
Block rotor test, no-load test and stator resistance test
Answer: Block rotor test, no-load test and stator resistance test
79
In case of 3-phase induction motors, plugging means
[A]
pulling the motor directly on line without a starter
[B]
locking of rotor due to harmonics
[C]
starting the motor on load which is more than the rated load
[D]
interchanging two supply phases for quick stopping
Answer: interchanging two supply phases for quick stopping
80
In a three-phase induction motor
[A]
iron losses in stator will be negligible as compared to that in rotor
[B]
iron losses in motor will be neg¬ligible as compared to that in rotor
[C]
iron losses in stator will be less than that in rotor
[D]
iron losses in stator will be more than that in rotor
Answer: iron losses in stator will be more than that in rotor
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