Motor Speed Control

Motor speed control is the regulation of how fast an electric motor turns by adjusting the electrical power supplied to it. The method used depends on the type of motor. The speed of a direct-current motor is broadly proportional to the voltage applied to its armature, so controlling that voltage controls the speed. The speed of an alternating-current induction motor depends mainly on the frequency of its supply, so adjusting frequency is the usual route to control.

Modern controllers rarely vary speed by simply wasting power in a resistor, because that is inefficient. Instead they switch the supply on and off rapidly using power semiconductor devices. Pulse-width modulation (PWM) is the dominant technique: the supply is switched at a fixed high frequency and the proportion of time it is on, the duty cycle, sets the average power delivered to the motor. Because the switching device is either fully on or fully off, it dissipates little heat, making PWM control efficient and well suited to battery-powered equipment.

Different switching devices suit different jobs. A triac can control the power delivered to small AC motors, such as those in fans and power tools, by switching on at an adjustable point in each mains half-cycle, a method known as phase control. For larger or more demanding loads, the insulated-gate bipolar transistor (IGBT) combines easy gate drive with the ability to handle high voltages and currents, and it is the standard switch in variable-frequency drives that control industrial AC motors. These drives synthesise an adjustable-frequency output to set the motor speed precisely.

Frequently asked questions

How does PWM control motor speed?

It switches the supply on and off rapidly and varies the fraction of time it is on, which changes the average power and therefore the speed, while keeping switching losses low.

What is a triac used for in motor control?

A triac switches AC power on at an adjustable point in each half-cycle to control the average power to small AC motors, a method called phase control.

Why are IGBTs used in industrial motor drives?

IGBTs handle high voltage and current while being easy to switch from a low-power gate signal, making them well suited to variable-frequency drives for large AC motors.