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The Construction of AC Motors

Oct 13, 2017

If you read our previous article, AC Motors: Basic Terminology, then you now understand:

  • The purpose of AC motors
  • NEMA standards for electrical equipment, and
  • The basic concept of torque, speed, and horsepower.

We’ll now discuss the construction of AC motors so you will be able to identify key differences of motor parts and how each product works for any given application.

Construction of AC Motors

Industrial applications frequently use AC induction motors. We’ll discuss 3-phase, 460V AC, asynchronous induction motors. When the speed of the rotating magnetic field is more than the speed of the rotor, you know it’s an asynchronous motor (see illustration). The AC motor consists of three basic parts: the stator, rotor, and enclosure.

AC Motor Construction

Construction of the Stator

The stator and the rotor are actually electrical circuits that work like electromagnets. The stator is the electrical part of the motor that doesn’t move. For a NEMA motor, the stator core is constructed with several hundred fine laminations.

Stator Construction

Stator Windings

Stator laminations are arranged to form a hollow cylinder. Each slot of the stator core receives a coil of insulated wire. An electromagnet is formed by each group of coils and the surrounding steel core. Motors operate thanks to electromagnetism. The power source is directly connected to the stator windings.

Stator Windings 2
Stator Windings

Construction of the Rotor

Within the electromagnetic circuit, the rotor is the part that rotates. The squirrel cage rotor is the most common type of rotor. Think of rotating exercise wheels used in pet hamster cages and you’ll understand the construction of the squirrel cage rotor. The rotor is made up of a stack of steel laminations that include uniformly spaced conductor bars surrounding the perimeter.
The rotor core is formed by the stacked laminations. Aluminum is die cast in the slots of the rotor core to form a series of conductors around the circumference of the rotor. The conductor bars are electrically and mechanically fused with end rings. Finally, the core of the rotor is mounted on a steel shaft.

Rotor Construction 3
Rotor Construction 2
Rotor Construction 1

The Motor Enclosure

The enclosure is made of a frame, or yoke, and two end brackets, also called bearing housings. The stator is installed inside the frame. The rotor sits inside the stator but separated by a slight air gap so that they don’t have any direct physical contact with each other. The purpose of the enclosure is to protect the operating and electrical parts of the motor from the harsh environment in which they operate. The bearings that are mounted on the shaft support the rotor, which enables it to turn. A fan is installed on the shaft for when the motor needs to be cooled down.

Motor Enclosure 1
Motor Enclosure 2

You should now be able to describe the construction of a squirrel cage AC motor. This knowledge should help you when you need to determine the right motor for your specific application. If you have any questions, we are happy to help. Contact us online and we will help you understand your options for AC motors and decide which is the best fit for your application.