What is the principle of 3 phase induction motor?

When three phase supply is given to the three phase stator winding of the induction motor,a rotating magnetic field is developed around the stator which rotates at synchronous speed.This rotating magnetic field passes through the air gap and cuts the rotor conductors which were stationary.Due to the relative speed between the stationary rotor conductors and the rotating magnetic field,an emf is induced in the rotor conductors.As the rotor conductors are short circuited, current starts flowing through it.And as these current carrying rotor conductors are placed in the magnetic field produced by the stator, they experiences a mechanical force i.e. torque which moves the rotor in the same direction as that of the rotating magnetic field.

the induction motor can't run at the synchronous speed because at synchronous speed the induction motor can not develop any torque to move the rotor from its stationary position.