Why does the rotor rotate of induction motor ?

 Induction motor has a stationary part known as stator containing status winding and a rotor having bar in squirrel gauge induction motor and three phase winding in slipping induction motor.

When three phase winding of stator fed by three phase supply then a rotating magnetic field  of constant magnitude 1.5 times  of maximum flux due to any phases is generated. Rotating at synchronous speed NS = 120 f/p. 

This flux passage through the air gap and cut the rotor bar as that they are still stationery.

Due to the relative velocity between the rotating flux and the stationary rotar conductor, EMF is induced in the rotor conductor according to faraday's law of electromagnetic induction.

The the frequency of the induced EMF is same as supply frequency, magnitude is proportional to the relative velocity between the two rotating flux and rotor conductor. And direction is given by Fleming's right hand rule.

since the rotor conductors are short circuit by end ring so current start flowing whose direction is given by lenz's law. It opposes the cause producing it. Here the cause is relative speed so to reduce the relative speed rotor starts rotating in the same direction of rotating flux. so it starts rotating.