What are AC motors? Technical Summary for Motion Engineers

Motors can be classified as either ac (alternating current) or dc (direct current).

An ac motor, like any electric motor, converts electrical energy to mechanical energy. AC motors take as their input an ac current, but differ from dc motors in that there is no commutation involved, and can be single or multi-phase.

Because ac motors have no commutators or brushes, they require less maintenance than brushed dc motors.

With dc motors, control is done by varying voltage and current, while on ac motors the voltage and frequency (along with the number of magnetic poles) are used to control the motor.

There is one common way to break down ac motors that is based on the magnetic principle that produces rotation. So there are two fundamental types of ac motors; induction motors and synchronous motors.

In induction motors, the key idea is the rotating magnetic field. The most common source of this in ac motors is the squirrel cage configuration. This is essentially two rings, one at each end of the motor, with bars of aluminum or copper connecting the two ends.

Induction motors have properties that make them especially well suited to a number of industrial as well as home appliance applications. For starters, they are simple and rugged motors that are easy to maintain. They also run at constant speed across a wide range of load settings, from zero to full-load. The only drawback is that induction motors are generally not amenable to speed control, although the availability of sophisticated variable-frequency drives means that even induction motors, usually three-phase induction motors, can now be speed controlled as well.

The other type of ac motor is a synchronous motor. Synchronous motors are so named because they run synchronously with whatever the frequency of the source is. The motor speed is fixed and doesn’t change with changes to the load or voltage. These motors are primarily used where the requirement is precise and constant speed. Most synchronous motors are used in heavy industrial applications, with horsepower ratings ranging from the low hundreds up to thousands of hp.

Synchronous motors can be used in motion control applications, but there are some down sides to using these motors. Because of the rotor size, the motor’s response in incrementing applications is typically not good. Also, because acceleration of inertial loads may not be as high as other motor types, these motors may operate at irregular speeds and produce undesirable noise. And generally, synchronous motors are larger and more costly than other motors with the same horsepower rating.