What Are The Main Types Of AC Motors?

The two most common types of AC motors are induction motors and synchronous motors.

 

If you have an induction motor (or asynchronous motor), it always relies on a small difference in speed between the stator rotating magnetic field and the rotor shaft speed (called slip) to induce rotor current in the rotor AC winding. As a result, your induction motor cannot produce torque near synchronous speed where induction (or slip) is irrelevant or ceases to exist.

 

In contrast, a synchronous motor at your plant or facility does not rely on slip-induction for operation and uses either permanent magnets, salient poles (having projecting magnetic poles), or an independently excited rotor winding. The synchronous motor produces its rated torque at exactly synchronous speed.

 

Other types of motors include eddy current motors, and AC and DC mechanically commutated machines in which speed is dependent on voltage and winding connection.

 

How is the Motor's Speed Determined?

The speed of your AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding. This speed is derived from a common formula involving variables that include synchronous speed (in RPM), AC power frequency, and the number of poles per phase winding.

 

It turns out that the actual RPM for your induction motor will be less than its calculated synchronous speed by an amount known as slip (referenced above), that increases with the torque produced. With no load, the speed of your motor will be very close to synchronous. When loaded, a standard motor might have between 2–3% slip, and special motors may have up to 7% slip.