an electrical machine that functions as (1) a generator used to produce alternating current, or (2) a motor that converts electrical energy into mechanical work, or (3) a converter that changes the voltage or frequency of an electric current. AC machines are classified as synchronous or asynchronous.
Synchronous AC machines are devices in which the basic magnetic field is produced by direct current or by a permanent magnet and the rate of rotation of the rotor is directly related to the AC frequency by the expression n = 60f/p, where n is the rate of rotation of the rotor in rpm, f is the AC frequency in hertz, and ρ is the number of pole pairs in the magnetic system. Synchronous machines are used chiefly as AC generators and as motors in electric drives, and occasionally as DC-to-AC converters. They are also used as synchronous compensators for the phase shift between current and voltage in electrical networks, automatic measuring instruments, and automatic equipment that requires synchronization of the rate of rotation of control and actuating devices.
In asynchronous AC machines, the basic magnetic field is produced by an alternating current, and the frequency of rotation of the rotor is not strictly related to the frequency of the current in the stator winding and varies with the load. The most common type is the asynchronous machine without a commutator, which is used chiefly as an electrical motor. Asynchronous commutator motors are seldom used because they are more costly and less reliable than machines without commutators.
AC machines are components in dynamoelectric stages and are used as microelectrical machines. Synchronous and asynchronous AC machines have the property of reversibility; that is, they can operate as generators or motors.