Principle of Operation of a Three-phase Induction Motor:
While the stator windings of a three-phase induction motor are linked to a three-phase balanced supply, the three-phase balanced currents build up a rotating flux wave. The rotating flux wave cuts the rotor conductors and induces voltage in the rotor conductors. As the rotor conductors are short- circuited these induced EMFs build up current in the rotor. These currents and the flux wave interact to generate torque in the rotor. As per the lenz's law, the developed torque makes the rotor to rotate in the direction of the flux wave so as to reduce the relative speed among the flux wave and the rotor conductors and thus reducing the cutting of flux lines by rotor conductors.
The rotating flux wave generated by the stator rotates at a speed of Ns (RPM) with respect to stator, where synchronous speed Ns is given by
Ns = 120 f /P
where f = Stator voltage frequency, Hz, and
P = Number of poles in the stator windings.
The rotor speed Nr of an induction motor is always somewhat less than the synchronous speed Ns.
The relative speed among the rotating flux and the rotor is represented usually as a fraction of the synchronous speed which is called slip.
Slip S = (Ns - Nr ) /Ns
The speed of induction motor rotor is
Nr = N s (1 - S )
The rotor induced emf shall have the frequency
f2 = S f
Since the rotor frequency is directly proportional to the slip, f2 is frequently called the slip frequency.