Basic review: A look inside an ac induction motor

May 9, 2000
The basic principles of the induction motor have remained fundamentally unchanged for nearly a century. During that time, however, the materials and processes used to build these workhorse products have been improved continuously. As a result, today’s electric motor is very economical.

It has been some time since the Motor Doctor discussed the basics, so in this article we will review the anatomy of the ac induction motor. The first component we will look at is the stator winding.

The stator is the part of the motor that changes electrical energy into magnetic force. It consists of thin slices of die-cut metal (typically steel), called laminations, that are capable of being magnetized. These slices are held together by riveting, clamping, or welding, depending upon the size and use of the motor.

A long strand of insulated wire called a winding is wound repeatedly through the die-cut slots on the inside of the laminations. It carries the electrical current that will magnetize the laminations.

This winding is arranged into symmetrical pairs of bundles around the circumference of the laminations. These bundles produce distinct magnetic poles when they are energized.

During the manufacturing process, the bundles often are laced or sewn to hold them in position, and the entire stator assembly usually is dipped into electrical varnish and cured.

Opposing forces

The second major component found in all motors, the rotor assembly, is the moving part of the motor. The rotor also consists of steel laminations that are held together by a cast-aluminum frame.

The rotor core is immersed in the magnetic force supplied by the stator winding and creates an opposing magnetic force within itself (a process called induction). This opposing magnetic force causes rotational movement.

The term ac induction motor derives from the fact that the opposing magnetic fields are said to be induced from stator to rotor.

The other component found in the rotor is the shaft, a precisely sized rod of steel that is inserted into the middle of the rotor. The rotational movement turns the shaft, creating work. The end of the shaft may be keyed, slotted, or shaped to couple with another device.

Most fractional-horsepower motors are enclosed by a painted metal housing (or frame) designed to protect the stator and rotor core from moisture and contaminants.

End frames (also called end shields, end bells, or end brackets) are stamped steel, cast iron, or aluminum components. They hold the motor’s bearing system and are used to cap, shield, or complete the enclosure of the motor.

Often, the end frames come equipped with vent holes to improve airflow through the motor.

Every motor comes with a bearing system; its job is to prevent metal-to-metal contact by providing a constant film of lubrication between the shaft and the stationary bearing in which the rotor shaft rides.

Most electric motors are manufactured with one of two types of bearing systems, either sleeve bearings (rigid or self-aligning) or ball bearings.

Sleeve bearings come in a variety of surfaces including sintered iron, sintered bronze, babbitt, and others. Your best choice will depend upon the load and noise considerations.

A single-phase motor may have a capacitor and starting switch in addition to the components just described. The reason a single-phase motor needs these additional parts is that at least two phases are required to cause the magnetic fields produced by the stator and rotor to induce rotation in the rotor.

When the input current to the motor is only a single phase, the switch and capacitor (along with a second complete winding in the stator) are used in various combinations to create that second phase.

There is one other critical component found on ac induction motors, and that’s the nameplate. This is the one place where you will find information on the electrical, mechanical, and performance properties of that motor. If you are ever in doubt, refer first to the nameplate.

As you can see, there is not a lot of mystery to an ac induction electric motor. The components are made to a high degree of precision, providing you with a product that offers durable, reliable performance.