A: The rotor speed of induction motor is less than the synchronous speed of rotating magnetic field. R: At synchronous speed, motor torque is zero.
electrical machines
Note: Each of the following questions consists of two statements, one labelled Assertion (A) and the other labelled the Reason (R). Examine the two statements carefully and decide if the Assertion (A) and Reason (R) are individually true and if so whether the Reason (R) is correct explanation of the Assertion (A). Select your answers to these questions using the codes given below:
A. Both A and R are true and R is a correct explanation of A.
B. Both A and R are true but R is not a correct explanation of A.
C. A is true but R is false.
D. A is false but R is true.
1. A: The rotor speed of induction motor is less than the synchronous speed of rotating magnetic field.
R: At synchronous speed, motor torque is zero.
2. A: The leakage reactance of a 3-phase induction motor should be small.
R: A small value of leakage reactance will increase the maximum power output of motor.
3. A: The rotor resistance of a deep bar type cage induction motor is higher than the running resistance.
R: The extra resistance present in the rotor circuit at starting is cut out by a centrifugal switch during
normal running.
4. A: In a single-phase induction motor, while the motor is connected to single phase ac supply and is
rotating, the two fields (stator and rotor field) combine to form a resultant field revolving at a
synchronous speed.
R: The rotor field created by rotor currents acts at right angle to the stator field and also lags the stator
field by 90 electrical degrees in the time phase.
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