An AC motor is a mechanical device used to convert mechanical energy into AC electrical energy. Due to the tremendous development of the AC power system, AC motors have become the most commonly used motor. Compared with DC motors, AC motors have a simple structure, convenient manufacturing, and are relatively sturdy due to the lack of a commutator (see commutator for DC motors), making them easy to make into motors with high speed, high voltage, high current, and large capacity. The coverage range of AC motor power is large, from a few watts to hundreds of thousands of kilowatts, or even millions of kilowatts. In the early 1980s, the largest steam turbine generator had reached 1.5 million kilowatts.

According to statistics, AC motors can be classified in two ways:

AC motor by function

AC motors are usually divided into several categories based on their functions: AC generators, AC motors, and synchronous condensers. Due to the reversibility of the working state of the motor, the same motor can be used as both a generator and an electric motor.

It is not very accurate to divide motors into generators and electric motors, but some motors mainly operate as generators and others as electric motors.

AC motors by type

AC motors are divided into two categories according to their varieties: synchronous motors and asynchronous motors. The speed ns of the synchronous motor rotor is the same as the speed of the rotating magnetic field, which is called synchronous speed. There is a strict relationship between ns and the frequency of the connected AC (f), as well as the number of magnetic pole pairs (P) of the motor.

Ns=60f/P

In China, the power frequency is 50 Hz, so the synchronous speed of one pair of pole motors in three-phase AC motors is 3000 rpm, and the synchronous speed of two pair of pole motors in three-phase AC motors is 1500 rpm, and so on. The speed of the rotor of an asynchronous motor is always lower or higher than the speed of its rotating magnetic field, hence the name asynchronous. The difference between the rotor speed of an asynchronous motor and the rotational magnetic field speed (referred to as the slip) is usually within 10%.