Why Induction Motors Are Asynchronous

The Core Factor Behind the “Asynchronous” Nature of Induction Motors

An induction motor is called “asynchronous” because the rotor speed is always lower than the speed of the rotating magnetic field generated by the stator. The two never achieve complete synchronization during the electromechanical energy conversion process. The fundamental operating principle of an induction motor is based on the interaction between the induced magnetic field and the rotating magnetic field.

In a three-phase induction motor, the stator windings are spatially distributed with a 120° electrical angle difference. When a three-phase symmetrical current with a 120° phase shift is applied, the three-phase pulsating magnetic fields combine to form a single rotating magnetic field at a constant speed. This speed is known as the synchronous speed, which is determined by the power supply frequency and the number of pole pairs of the stator winding. It can be calculated using the formula：n₁=60f/p

where n1 is the synchronous speed (in RPM), f is the supply frequency (Hz), and ppp is the number of pole pairs. This synchronous speed forms the fundamental basis for AC motor operation.

The rotating magnetic field continuously cuts through the rotor conductors, generating an induced current. The magnetic field produced by this induced current interacts with the rotating magnetic field, creating the electromagnetic torque that drives the motor. For electromagnetic induction to occur, there must be relative motion between the rotor speed and the rotating magnetic field—that is, a speed difference.

This speed difference is the core prerequisite for an induction motor to operate: while the rotor rotates under electromagnetic force, its speed is always slightly less than the synchronous speed. If, unexpectedly, the rotor were to reach synchronous speed, the relative motion between the rotor and the rotating magnetic field would vanish. As a result, the magnetic flux would no longer be cut, the induced current in the rotor circuit would instantly disappear, and the electromagnetic force acting on the current-carrying rotor conductors would drop to zero. Consequently, the electromagnetic torque driving the rotor would vanish, and the motor would be unable to continue running, potentially coming to a stop.

Therefore, during normal operation, the rotor speed of an induction motor always lags behind the synchronous speed. This “out-of-sync” operation is both a natural outcome of its working principle and the direct reason why it is called an “asynchronous” motor.