Consider the simplified view of a 3 phase induction motor shown below.
The stator hosts a three phase winding distributed symmetrically on its inner periphery. This stator winding is energised from a three phase supply.
The rotor also hosts a 3 phase winding on its periphery. But, the rotor winding is not energised from any source and is short- circuited on itself.
Three phase Induction motor working principle:
(1) When the 3 phase stator winding is energised from a 3 phase supply, a rotating magnetic field is produced which rotates around the stator at synchronous speed.
(2) The rotating magnetic field cuts the rotor conductors, which as yet, are stationary. Due to this flux cutting, emfs are induced in the rotor conductors. As rotor circuit is short circuited, therefore, currents start flowing in it.
(3) Now, as per Lenz's law , "the direction of induced current will be such that it opposes the very cause that produced it " .
(4) Here, the cause of emf induction is the relative motion between the rotating field and the stationary rotor conductors. Hence, to reduce this relative motion, the rotor starts rotating in the same direction as that of the stator field and tries to catch it but, can never catch it due to friction and windage and therefore emf induction continues and motor keeps rotating.
Thus, principle of 3 phase induction motor also explains why rotor rotates in same direction as the rotating field and why induction motor is self-starting.
When rotor winding is short-circuited with no resistance in series, it is called a squirrel cage induction motor and when rotor winding is shorted through a resistance in series, it is called slip ring induction motor.
