How to Vary the Speed on an AC Electric Motor

AC motors are constant speed devices but their speed can vary if you change the input voltage or frequency or the windings that make the motor rotate. The most common and efficient way of changing the speed is to vary the frequency by using an inverter as the power supply. This method has become popular with the development and decrease in cost of power electronics. Methods involving reducing the voltage to the motor windings with resistors, transformers or motor winding taps are still in use and are the low cost and preferred methods for certain clearly defined applications.

Use an inverter to power the AC motor. Select an inverter that can supply the motor voltage and current including the starting current. Use the range of speed variation required to select the range of frequency that the inverter must be able to supply. The inverter controls will vary the frequency supplied to the motor and the motor speed will vary accordingly.

Add a variable resistance in the motor circuit to reduce the voltage across the main winding if an inverter is too costly and accurate speed control is not required. The motor "slip," normally close to zero, will increase as reduced power is supplied to the motor, and the motor poles don't receive enough power to produce the force necessary to keep up their normal speed. The motor must be designed for high slip, and the resistance and circuit design is specific to the particular application.

Use a variable voltage transformer to change the voltage supplied to the main winding if you desire a high slip, reduced voltage speed control, which is more efficient. The variable voltage transformer has low losses compared with the variable resistor. The transformer may have a series of taps that yolu can change manually to control the motor speed or it may have a motorized tap changer. In either case, the motor speed changes in discrete increments and the detailed design is again specific to the installation where it is used.

Use an AC motor with tapped windings to change the speed if available. Such a motor has a certain number of taps on its main winding, which allow it to run with different voltages applied to its magnetic field. The number of taps and the number of speeds available for a particular motor is usually not more than four. This method is common in fans that have low, medium and high speeds with a switch selector. The exact speed in these devices is not important, and the speed control method is low cost.