Three Phase Induction Motors(Engineering > Electrical Engineering ) Questions and Answers

Question 1. In an induction motor, the ratio of rotor cu loss and rotor input is given by
  1.    S
  2.    1-S
  3.    1/S
  4.    S/1-S
Explanation:-
Answer: Option A. -> S
Question 2. The frequency of rotor current in a 6-pole , 50 Hz, 3-phase induction motor running at 950 rpm is
  1.    2.5 Hz
  2.    1.5 Hz
  3.    5 Hz
  4.    0.05 Hz
Explanation:-
Answer: Option A. -> 2.5 Hz
Question 3. The no load speed of an induction motor depends on
  1.    The number of its poles
  2.    The maximum flux/poles
  3.    The supply frequency
  4.    Both supply frequency and number of poles
Explanation:-
Answer: Option D. -> Both supply frequency and number of poles
Question 4. The rotor of induction motor
  1.    None of the listed here
  2.    Always revolves in the direction of stator flux
  3.    Insufficient data
  4.    Always revolves in the direction opposite to stator flux
Explanation:-
Answer: Option B. -> Always revolves in the direction of stator flux
Question 5. The induction motor has lagging power factor during
  1.    Both starting and operation
  2.    Starting only
  3.    None of the listed here
  4.    Operation only
Explanation:-
Answer: Option A. -> Both starting and operation
Question 6. An induction motor is so called because its operation depends on
  1.    Self induction
  2.    Hysteresis
  3.    Eddy currents
  4.    Mutual induction
Explanation:-
Answer: Option D. -> Mutual induction
Question 7. The magnetising current drawn by a 3 phase induction motor is about _______ of full load stator current.
  1.    10 to 15%
  2.    30 to 50%
  3.    15 to 20 %
  4.    5%
Explanation:-
Answer: Option B. -> 30 to 50%
Question 8. The starting torque of a 3-phase induction motor is _______ supply voltage.
  1.    Directly proportional to sqaure of
  2.    Directly proportional to
  3.    None of the listed here
  4.    Independent of
Explanation:-
Answer: Option A. -> Directly proportional to sqaure of
Question 9. A high starting torque can be obtained in a 3phase induction motor by _______
  1.    None of the listed here
  2.    Decreasing rotor resistance
  3.    Increasing rotor resistance
  4.    Increasing rotor reactance
Explanation:-
Answer: Option C. -> Increasing rotor resistance
Question 10. If the supply voltage of a 3-phase induction motor is increased two times, then torque is _______
  1.    Decreased two times
  2.    Increased two times
  3.    Increased four times
  4.    Decreased four times
Explanation:-
Answer: Option C. -> Increased four times
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