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Free download in PDF Polyphase Induction Motors General Knowledge Short Questions & 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.
41
Short-circuit test on an induction motor cannot be used to determine
[A]
windage losses
[B]
copper losses
[C]
transformation ratio
[D]
power scale of circle diagram
42
The starting torque of a 3-phase squirrel cage induction motor is
[A]
twice the full load torque
[B]
1.5 times the full load torque
[C]
equal to full load torque
[D]
None
Answer: 1.5 times the full load torque
43
A 3-phase slip ring motor has
[A]
double cage rotor
[B]
wound rotor
[C]
short-circuited rotor
[D]
any of the above
44
5 H.P., 50-Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the
costliest ?
[A]
730 r.p.m.
[B]
960 r.p.m.
[C]
1440 r.p.m.
[D]
2880 r.p.m.
45
A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will
eventually happen ? It will
[A]
stall after sometime
[B]
stall immediately
[C]
continue to run at lower speed without damage
[D]
get heated and subsequently get damaged
Answer: get heated and subsequently get damaged
46
Which type of bearing is provided in small induction motors to support the rotor shaft ?
[A]
Ball bearings
[B]
Cast iron bearings
[C]
Bush bearings
[D]
None of the above
47
The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the
following machines
[A]
D.C. series motor
[B]
D.C. shunt motor
[C]
universal motor
[D]
none of the above
48
It is advisable to avoid line starting of induction motor and use starter because
[A]
it will run in reverse direction
[B]
it will pick up very high speed and may go out of step
[C]
motor takes five to seven times its full load current
[D]
starting torque is very high
Answer: motor takes five to seven times its full load current
49
If a 3-phase supply is given to the stator and rotor is short circuited rotor will move
[A]
in the opposite direction as the direction of the rotating field
[B]
in the same direction as the direction of the field
[C]
in any direction depending upon phase squence of supply
[D]
None
Answer: in the same direction as the direction of the field
50
As load on an induction motor goes on increasing
[A]
its power factor goes on decreasing
[B]
its power factor remains constant
[C]
its power factor goes on increasing even after full load
[D]
its power factor goes on increasing up to full load and then it falls again
Answer: its power factor goes on increasing up to full load and then it falls again
51
Slip ring motor is recommended where
[A]
speed control is required
[B]
frequent starting, stopping and reversing is required
[C]
high starting torque is needed
[D]
all above features are required
Answer: all above features are required
52
In an induction motor the relationship between stator slots and rotor slots is that
[A]
stator slots are equal to rotor slots
[B]
stator slots are exact multiple of rotor slots
[C]
stator slots are not exact multiple of rotor slots
[D]
none of the above
Answer: stator slots are not exact multiple of rotor slots
53
Low voltage at motor terminals is due to
[A]
inadequate motor wiring
[B]
poorely regulated power supply
[C]
any one of the above
[D]
none of the above
Answer: any one of the above
54
A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the
motor
[A]
will continue running burning its one phase
[B]
will continue running burning its two phase
[C]
will stop and carry heavy current causing permanent damage to its winding
[D]
will continue running without any harm to the winding
Answer: will stop and carry heavy current causing permanent damage to its winding
55
A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor
[A]
will continue running burning its one phase
[B]
will continue running burning its two phases
[C]
will stop and carry heavy current causing permanent damage to its winding
[D]
will continue running without any harm to the winding
Answer: will continue running burning its one phase
56
Size of a high speed motor as compared to low speed motor for the same H.P. will be
[A]
bigger
[B]
smaller
[C]
same
[D]
any of the above
57
Slip of an induction motor is negative when
[A]
magnetic field and rotor rotate in opposite direction
[B]
rotor speed is less than the synchronous speed of the field and are in the same direction
[C]
rotor speed is more than the synchronous speed of the field and are in the same direction
[D]
none of the above
Answer: rotor speed is more than the synchronous speed of the field and are in the same direction
58
If an induction motor with certain ratio of rotor to stator slots, runs at 1/7 of the normal speed, the phenomenon will be termed as
[A]
humming
[B]
hunting
[C]
crawling
[D]
cogging
59
The ‘cogging’ of an induction motor can be avoided by
[A]
proper ventilation
[B]
using DOL starter
[C]
auto-transformer starter
[D]
having number of rotor slots more or less than the number of stator slots (not equal)
Answer: having number of rotor slots more or less than the number of stator slots (not equal)
60
The power factor of an induction motor under no-load conditions will be closer to
[A]
0.2 lagging
[B]
0.2 leading
[C]
0.5 leading
[D]
unity
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