Why is slip necessary in an Induction motor?
The magnetic field of the stator, rotating faster than the physical speed of the rotor passes through the rotor conductors. That induces current in the rotor. Current in the rotor generates a magnetic field in the rotor that rotates at the same speed as the stator magnetic field, but lagging in position. That creates rotor torque. At synchronous speed, the rotor would be turning at the same speed as the rotor magnetic field and no torque would be produced. With no load attached to the motor shaft, very little torque is required to keep the rotor moving, so the speed is nearly synchronous. Any increase in load reduces the rotor speed and increases the rotor current and torque.