Understanding Three Phase AC Motors Part 1: Overview

Three Phase AC Motor

The three phase AC motor is the workhorse of industrial and commercial facilities. It drives anything from conveyors, to fans, pumps, mixers and other heavy machines. It is not uncommon for large facilities to have hundreds of three phase AC motors.

The typical three phase AC motor has three main components: the stator, rotor, and enclosures. The enclosure houses the stator and rotor, while the rotor rotates within the stator. The rotor is the component which provides the rotating motion of the motor.


An Example of a Three Phase AC Motor

As its name implies, the stator is stationary. The stator is composed of many small metal sheets (also known as laminations), which are stacked together to form the overall core. Coils of wire run throughout the stator core. In regular operation, the coils are energized, which causes the rotor to begin rotation, through electromagnetic principles.

Again, as the name implies, the rotor is the rotating part of the motor. The most common type of rotor is the squirrel cage rotor. The rotor is similar to the stator in that it is typically constructed with many small metal sheets (laminations). However in the rotor, conductor bars are contained within the laminations. The rotor laminations enclose the motor shaft. As previously mentioned, when the stator coils are energized, the rotor begins to rotate. This is because of electromagnetic induction between the stator coils and the rotor conductor bars. Note that there is no direct physical contact between the rotor and stator.

Motor Dissection

Typical Motor Components

The enclosure protects the internal components of the motor from external elements, such as water or ice. Different enclosure types exist to protect against various types of external conditions.

Bearings are mounted on the shaft and allow it to turn.

Synchronous Motor

A synchronous motor is different from a three phase induction motor. There are some construction similarities between the two, and both operate similarly (with an energized stator coil), but their similarities end there.

While both rotate upon energization of the stator coil, the rotor coils of a synchronous motor is supplied DC power once the motor has accelerated. Hence, a strong magnetic field is then produced in the rotor, which locks the rotor in step with the rotating magnetic field of the stator. Unlike the typical three phase AC induction motor, the rotor of the synchronous motor rotates at the same speed as the rotating field, at synchronous speed.

Motor Slip

Electromagnetic principles govern the operation of a typical three phase AC motor. More specifically, when the stator coils of the motor are energized, a rotating magnetic field is generated in the coils, which then induces the rotor to begin rotating.

The speed at which the magnetic field rotates is always greater than the rotor rotation. This is because the magnetic field dictates the rotation speed, and due to efficiency limitations, some energy losses occur as the rotor experiences the induced magnetic field. This speed difference is known as slip, which is required to produce torque. Slip is load dependent. Sensibly, if the rotor were driving a load (such as a fan) , it would rotate slower than the rotating magnetic field due to torque resistance from the load. A larger load slows down the rotor, and a smaller load speeds up the rotor.

Slip can be expressed with the following formula




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