When a 3 phase supply is given to the STATOR winding it sets up a rotating magnetic field in space. This rotating magnetic field has a speed which is known as the synchronous speed.This rotating magnetic field induces the voltage in rotor bars and hence short-circuit currents start flowing in the rotor bars. These rotor currents generate their self-magnetic field which will interact with the field of the stator. Now the rotor field will try to oppose its cause, and hence rotor starts following the rotating magnetic field.The MOMENT rotor catches the rotating magnetic field the rotor current drops to zero as there is no more relative motion between the rotating magnetic field and rotor. Hence, at that moment the rotor experiences zero tangential force hence the rotor decelerates for the moment.Squirrel Cage Induction MotorAfter DECELERATION of the rotor, the relative motion between the rotor and the rotating magnetic field reestablishes hence rotor current again being induced. So again, the tangential force for rotation of the rotor is restored, and THEREFORE again the rotor starts following rotating magnetic field, and in this way, the rotor maintains a constant speed which is just LESS than the speed of rotating magnetic field or synchronous speed.Slip is a measure of the difference between the speed of the rotating magnetic field and rotor speed. The frequency of the rotor current = slip × supply frequency