Motor speed control circuits

Motor speed control circuits regulate how fast an electric motor turns, and often the direction in which it runs. A motor connected directly to a supply simply runs at full speed, so to vary its behavior a control circuit must adjust the power delivered to it. This is fundamental to robotics, model vehicles, fans, tools and many automated systems, where a motor must start gently, run at a chosen speed, reverse, or respond to a command from a controller or sensor.

A common and efficient way to control the speed of a direct-current motor is pulse-width modulation, in which the supply is switched on and off rapidly and the proportion of time it is on sets the average power and therefore the speed. Because the switching transistors spend little time in a partly conducting state, this method wastes little energy compared with simply dropping voltage across a resistor. To reverse a motor, the circuit must be able to swap the polarity applied to it, which is typically done with an arrangement of switches called an H-bridge.

Practical motor controllers add driver stages capable of handling the relatively large currents motors draw, along with protection against overload and the voltage spikes motors produce. In radio-controlled and robotic applications, the speed command often comes from a microcontroller or a servo signal, which the controller translates into the appropriate switching. Collections of motor control circuits include speed controllers, reversing drivers and servo and stepper circuits, illustrating how electronic switching governs mechanical motion.

Frequently asked questions

How is the speed of a DC motor controlled efficiently?

By pulse-width modulation, switching the supply on and off rapidly so the average power, and thus the speed, is set by the on-time.

How does a circuit reverse a motor?

By swapping the polarity applied to it, usually with a switching arrangement called an H-bridge.

Why do motor controllers need driver stages?

Motors draw large currents and produce voltage spikes, so robust driver and protection stages are needed to handle them safely.