How Do Three-Phase Motors Work?

Three-phase motors are rotating electric machines powered from a three-phase source of alternating current. The motors have two main components: the stator and the rotor. A rotating magnetic field produced in the stator induces electromagnetic flux in the rotor. In turn, current in the rotor flows in the reverse direction of the rotating magnetic field, causing rotor motion. A three-phase induction motor does not need a starting device to operate.

The stator has several overlapping windings equipoised by an electric angle of 120 degrees. When the component is connected to a three-phase alternating source, it creates a rotating magnetic field. The phase sequence of supply lines and the manner in which the lines connect to the stator determine the direction in which the motor rotates. As such, switching the order of connections of any two primary terminals to the source reverses the direction of rotation.

The rotor is a cylindrical core with conductor-carrying parallel slots. A conductor is a dense copper or aluminum bar that fits in its slot in the rotor and has an end ring that short-circuits it. The slots are tilted to minimize magnetic humming noise and enhance smooth operation of the motor. The “slip” is the name for the difference between stator speed and rotor speed in a three-phase motor.