Polyphase Induction Motors General Knowledge Quiz | Polyphase Induction Motors General Knowledge Short Questions & Answers

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Questions
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

Answer: windage losses
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
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43 A 3-phase slip ring motor has
A double cage rotor
B wound rotor
C short-circuited rotor
D any of the above

Answer: wound rotor
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.

Answer: 730 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

Answer: Ball bearings
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

Answer: D.C. shunt motor
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

Answer: smaller
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

Answer: crawling
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

Answer: 0.2 lagging

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