Polyphase Induction Motors General Knowledge Quiz | Polyphase Induction Motors General Knowledge Multiple Choice Questions(MCQs) with Answers

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Questions
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

Answer: Air 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
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63 Lubricant used for ball bearing is usually
A graphite
B grease
C mineral oil
D molasses

Answer: grease
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

Answer: 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

Answer: efficiency
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

Answer: 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

Answer: 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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