Reason for rotation of rotor in induction motor

         When 3 phase stator winding is fed with 3 phase supply a magnetic flux of constant magnitude but rotating at synchronous speed is set up.This flux passes through the air gap and cut the stationary rotor conductors .Due to the relative speed between rotating flux and stationary conductors an e.m.f is induced in the rotor conductors according to Faraday's laws of electromagnetic induction.The frequency of induced e.m.f is same as that of supply frequency.
The magnitude of induced e.m.f is propotional to the relative velocity between flux and conductors.Its direction is given by Fleming's right hand rule.
     Since rotor bars or conductors form a closed circuit,  rotor current is rotating in such a way that it always opposes the very cause which produce it In this case the cause which produce the rotor current is relative velocity between flux and conductor .Hence to reduce the relative speed the rotor starts running as in same direction that of flux.