There are two main types of AC induction motors commonly used in industry: single-phase motor and polyphase motor.

In the polyphase motor category, the most commonly used is the three phase induction motor.

In single-phase electric motor unit, five different subgroups are usually used, which are split phase, capacitor start (CS is a variant of split-phase motor), shaded pole, permanent split phase capacitor (PSC) and capacitor start running capacitor (CSCR is a variant of the PSC motor).

Usually speaking, split phase, CS and CSCR are ruled out in the case of variable speed control because they both have a starting winding or a contact point that needs to reach 75% of the motor's full load speed before disconnecting the starting winding. However, usually less than 75% when controlling the speed. If the starting switch is not disconnected, the contacts or starting windings will quickly burn out and the motor will overheat and stop.

When deciding which kind of induction motors to use, these motors can be mechanically made to reach their maximum speed. And they can theoretically be used if they are never slow enough to reengage the starting contact during normal operation.

As for the rest of the motors, shaded poles, PSC and polyphase are the best choice for choosing a motor that will control the speed. Due to its inherent design, the shaded pole motor is the most economical of the three, but it is only available at less than 1/4 horsepower. Due to limitations, it also has a low starting and running torque and is inefficient. Therefore, for single-phase operations with power greater than 1/4 and/or high operating torque, permanent split capacitor motors are usually used. This type of motor is one of the more expensive single-phase motors, but it is very reliable and has good running and starting capability, and allows for speed control by electronic devices. The function of a permanent split capacitor motor most closely approximates that of a three-phase motor.

Three-phase motors are available with partial and total horsepower options, but generally the power savings will not exceed the cost until they reach the total horsepower range. It works essentially the same way as a single-phase motor, having three instead of one hot wire, controlling speeds that need to be different from single-phase controllers. And the functions of single phase electric motors are slightly different from three-phase motors.

There are generally four ways to control the speed of a motor. The simplest is a variable speed motor. The other three groups are passive speed controls, solid-state controls, and mechanical devices.

Regardless of the application, it is common to check with the motor manufacturer what the speed control standards are, as some motors are not available for speed control. Also, note that all safety issues require thermal overload protection as part of the speed control system. There is often a large amount of heat generated by the motor. When using the speed controller to prevent accidents or damage, it is recommended that the overload protector can be part of the motor or the controller itself.

After years of the development of industrial motors, the ways of controlling speed are also in development. To choose an appropriate way to control speed, we should take into account the type of motors that it would be applied in, the cost and the efficiency limit involved. Some controller may cost low when not as user-friendly as other controllers, but it can still get the job done.