How to Control the Speed of an AC Electric Motor

An AC motor is an electric motor driven by an alternating current (AC) and consists of two basic parts: an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. Being able to control the speed of the motor has numerous advantages, and this guide will examine multiple ways to control a motor's speed.

As noted above, controlling the speed of an AC electric motor has a variety of advantages, including reducing the audible noise, power efficiency, and enhanced control over the motor application. Although they are constant speed devices, AC motor speeds can vary if the frequency, input voltage, or the windings that make the motor rotate are changed.

A common and efficient means of changing a motor's speed is to vary the frequency by use of an inverter as the power source. With the technological advancements and decreased cost of power inverters, this a frequently used, popular option. Methods involving reducing the voltage to the motor windings with transformers, resistors, or motor winding taps are still in use as well.

If you plan to use an inverter to power the AC electric motor, it is important to choose one that can supply not only the motor voltage and running current but the starting current as well. Use the range of speed variation required to select the range of frequency that the inverter must be able to supply. The controls of the inverter can be used to vary the frequency supplied to the motor, and the motor speed will then vary accordingly.

If completely accurate speed control isn't critical for the motor application, adding a variable resistance in the motor circuit to reduce the voltage across the main winding can also be done. The motor "slip"—the difference between the synchronous speed of the electric motor magnetic field and the shaft rotating speed—which is usually close to zero, will increase since reduced power is supplied to the motor. Additionally, the motor poles don't receive adequate power to produce the necessary force to maintain their normal speed, and in this method the motor must be designed for high slip.

A more efficient option is to use a variable voltage transformer. This method will change the voltage supplied to the main winding, resulting in a high slip and reduced voltage speed control. The variable voltage transformer has low losses compared with the variable resistor. The use of a transformer may have a series of taps, which changes the voltage ratio, to control the motor speed. These taps can be changed manually, or the transformer might  have a motorized tap changer. Regardless, the motor speed is changed in discrete increments and the specific design is particular to the installation where the transformer is used.

Other Methods for AC Motor Speed Control

Another method for AC motor speed control is to use an AC motor that has tapped windings to change the speed. This is a method most commonly found in house fans that have high-, medium-, and low-speed switches. These motors feature a preset number of taps on the main winding, allowing it to operate with different voltages applied to its magnetic field. The number of taps and speeds available for a motor is usually not more than four. An exact speed in these types of applications is not critical, and controlling the speed by this option is very economical.

A Variable Frequency Drive (VFD) is another option and is an AC motor controller that drives the motor by varying the frequency and voltage supplied to it. Frequency (or hertz) is directly related to the motor’s speed (RPMs), so the faster the frequency, the faster the RPMs. If the motor's application doesn't require it to run at full speed, a VFD can be utilized to decrease the frequency and voltage to meet the motor’s load requirements. When the application’s motor speed requirements vary, a VFD will effectively turn down or up the motor speed, meeting the speed requirement. The use of a VFD can provide for reduced energy consumption and costs, increased production via tighter process control, and extend equipment life while reducing maintenance requirements.

Finally, regulating the amount of voltage across the terminals of a motor through Pulse Width Modulation (PWM) can also control the speed of the motor. As the term implies, PWM speed control works by operating the motor with a rapid series of “ON" and "OFF” pulses and varying the duty cycle. The power applied to the motor is controlled by varying the width of these applied pulses, which in turn varies the average voltage applied to the terminals of the motor. By modulating or changing the timing of these pulses, the speed of the motor can be controlled. So, the longer the pulse is “ON,” the motor will rotate faster, while conversely, the shorter the time the pulse is “ON,” the slower the motor will rotate.

With a few simple adjustments or changes, controlling the speed of an AC electric motor is possible. Check out Zoro.com for a wide range of AC speed controls from leading suppliers.

The product statements contained in this guide are intended for general informational purposes only. Such product statements do not constitute a product recommendation or representation as to the appropriateness, accuracy, completeness, correctness or currentness of the information provided. Information provided in this guide does not replace the use by you of any manufacturer instructions, technical product manual, or other professional resource or adviser available to you. Always read, understand and follow all manufacturer instructions.