AC induction motors are one of the most common types of motors used in a variety of applications due to the fact that they run off of AC voltage which is readily available at every outlet. They also run quietly and have a long life.

All AC motors have the same basic components, a stator and rotor. The stator is the stationary coils in the motor where the current is produced to create the magnetic field. This magnetic field induces a current in the rotor bars causing the rotor to rotate. (For more information on how AC motors rotate see: “How Does a Single Phase Motor Work?”.)

Key Characteristic: Running Speed

Two important characteristics of AC motors that need to be considered for any application are running speed and starting torque. Running speed is dependent on the power supply frequency, the number of motor poles and the amount of slip. The frequency and number of poles define what the synchronous speed of the motor will be. Our blog post on Synchronous and Induction motors will further explain the differences between these motor types. (See Figure 1 to determine synchronous speed.)

The amount of speed below the synchronous speed the motor runs at (expressed as a percent), is defined as the slip. AC motors require slip to induce current in the rotor, and the amount of slip changes as the load on the motor changes. In order to change the speed of an AC motor, the frequency must be changed. This is accomplished by a motor control, and the most common is a Variable Frequency Drive (VFD). Without a control, the motor speed is fixed by the equation in Figure 1.

Key Characteristic:  Starting Torque

Another key characteristic is the starting torque of the motor. In comparison to other motor types, starting torque is the chief limitation of an AC motor. A single phase motor will not start on and must have help. Single phase motors are defined by the methods they use to start. Some common types of single phase motors are the shaded pole motor, the split phase motor, the permanent split capacitor motor (also called the single value capacitor motor), and the two value capacitor motor. All these motor types either use an out of phase secondary coil or a capacitor to create a secondary phase to start the motor. Remember, if your application requires the motor to start with a load on it, consult your motor manufacturer to ensure the motor has enough torque to start at load and to ensure the correct motor type is specified for your application.

In comparison to single phase motors, three phase motors have a higher power density, a higher starting torque, and are more efficient than single phase motors. They start on their own, eliminating the need for a starting winding or capacitor. The same speed calculation applies to three phase motors as single phase, so a VFD is required to change the motor speed. Also, when a three phase power source is unavailable, controls are capable of converting single phase power into three phase power, making the three phase motor more versatile.

AC motors work great for a number of different applications such as pumps, conveyors, and commercial products. Remember the key characteristics when considering an AC motor, and consult your motor manufacturer with your specific application specifications to ensure you are getting the correct motor for your application.