Three-phase AC motors

By 2 August 2014

Three-phase AC motors are known as the ‘workhorses of industry’ because of their wide use and acceptance. They are popular because they are low in cost, compact in size, require less maintenance, withstand harsh industrial environments, etc.

Three-phase AC motors are a class of motors that convert the three-phase electric power supplied at the input terminals, to mechanical power at the rotating shaft, through the action of a rotating magnetic field, produced by a distributed winding on the stator.

Three-phase AC motors are broadly classified as:
1. Induction motor
2. Synchronous motor
3. Wound rotor induction motor.
Each motor operation is detailed briefly.

1. Induction motor
As the name implies, no voltage is applied to the rotor. The voltage is applied to the stator winding and when the current flows in the stator winding, a current is induced in the rotor by transformer action. The resulting rotor magnetic field will interact with the stator magnetic field, causing torque to exert on the rotor.

2. Synchronous motor
As the name suggests, rotor speed remains in synchronism with that of the stator magnetic field. The motor runs at the same speed. Unlike induction motors, synchronous motors are not self-starting. They have to be brought up to synchronous speed. Once they are locked then the rotor will continuously rotate.

3. Wound rotor induction motor
This motor has a ‘wire wound rotor’ from which three leads are brought out to the slip rings. It is possible to vary the rotor resistance. Introducing different resistances in the rotor circuit through the slip rings does this. The speed and the starting torque will now be variable.