The rotor of an induction motor cannot run with synchronous speed because

• In an induction motor when Voltage is applied to the stator of the induction motor, the current will flow through the winding, resulting in a magnetic field flux around the free space across the stator.
• The rotor is placed in such a way that this magnetic field induces a current in the rotor. This induced current will flow through the rotor winding causing another flux in the rotor.
• This flux in the rotor will lag stator flux.
• Due to this difference in flux, the rotor will experience a torque and will start to rotate at a speed less than synchronous speed due to lagging.

∴ Theoretically, the Induction motor can never run at synchronous speed. However, if by some external force, or system fault such as voltage surge, somehow speed of induction motor becomes equal to the synchronous speed, then there will be no more lagging between both the fluxes and no more current will be induced in the rotor winding. This will result in no torque on the rotor, and it will stop moving due to this.