The reason why asynchronous motors are "asynchronous" is that the rotor speed is always lower than the speed of the rotating magnetic field generated by the stator, and the two cannot complete the electromechanical energy conversion process in a synchronized state.

The core working principle of an asynchronous motor is based on the interaction between an induced magnetic field and a rotating magnetic field. The stator windings of a three-phase asynchronous motor are spatially distributed with a 120° electrical angle difference. When a three-phase symmetrical current with a 120° time phase difference is applied, the three-phase pulsating magnetic fields combine to form a single magnetic field rotating at a fixed speed. This speed is called the synchronous speed—its magnitude is determined by the power supply frequency and the number of pole pairs in the motor's stator windings, calculated using the formula n₁ = 60f/p (where n₁ is the synchronous speed, f is the power supply frequency, and p is the number of pole pairs). This is the foundation of AC motor operation. Subsequently, this rotating magnetic field continuously cuts the closed conductors of the rotor, generating an induced current. The magnetic field generated by this induced current is synchronized with the rotating magnetic field, producing the electromagnetic torque that drives the motor. There must be relative motion between the motor speed and the rotating magnetic field, i.e., a speed difference, to satisfy the fundamental condition of electromagnetic induction: "a conductor cutting magnetic lines of force."

Therefore, the speed difference is the core prerequisite for the operation of an asynchronous motor: when the rotor rotates under the action of electromagnetic force, its speed will always be lower than the synchronous speed. If the rotor speed unexpectedly equals the synchronous speed, the relative motion between the rotor and the rotating magnetic field will be lost, the magnetic lines of force will no longer be cut, the induced current in the rotor circuit will disappear instantly, and the electromagnetic force on the current-carrying rotor conductor in the magnetic field will also return to zero. The electromagnetic torque that drives the rotor to rotate will naturally disappear, and the motor will be unable to continue operating or may even stop. Therefore, in the normal operation of an asynchronous motor, the rotor speed is always behind the synchronous speed. This "asynchronous" operating state is both an inevitable result of its working principle and the direct origin of the name "asynchronous" .