When the stator of the induction motor is fed three phase supply, the synchronous rotating field is generated. The flux generated in the stator gets linked to the rotor conductor. The voltage is induced in the rotor if the relative speed difference exist between the synchronous speed(Ns= 120f/P) and the actual rotor speed. The difference between synchronous and actual speed of the rotor is called slip.
slip(s) = Ns -Nr
% slip = (Ns-Nr)/Ns x 100
At start,
Nr=0
Slip(s)= (Ns-0)/Ns = 1
At start, slip of the motor is unity.
When the motor starts accelerating, the slip decreases and at the rated KW the slip of the motor is about 3–5 % of the synchronous speed of IM.
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What is synchronous speed?Relation between speed and frequency
