Besides load, a DC motor's speed depends chiefly on the applied armature voltage. Pulse-width modulation has become the standard way to vary that voltage with low losses.

Basic principle: voltage and speed

For a given load, a DC motor's speed is approximately proportional to supply voltage - this is the basis of every speed control scheme. Depending on motor design, however, speed droop under load and the usable control range differ.

How PWM works

Pulse-width modulation (PWM) switches the supply voltage on and off rapidly instead of reducing it continuously. The armature winding's inductance and the motor's mechanical inertia smooth this pulse, so the motor behaves approximately as if a continuous DC voltage were applied - whose effective level depends on the duty cycle (the ratio of on-time to period). Compared with linearly reducing the voltage, this generates significantly less heat loss in the drive electronics.

H-bridge and direction of rotation

An H-bridge of four switches allows the motor to be driven via PWM while also reversing its direction of rotation, without changing the wiring. It is the standard basic circuit for electronic control of DC motors.

Note: PWM drive with steep rectangular voltage edges can radiate electromagnetic interference (EMI). At a low PWM frequency in the 1-2 kHz range, an audible whine can also occur at the motor - both should be considered when selecting the motor and drive electronics.

Open-loop or closed-loop control?

A simple PWM drive applies a fixed duty cycle and does not react to load changes. If speed must stay constant regardless of load, it has to be measured and fed back to adjust the supply voltage - only this closed control loop turns open-loop drive into closed-loop control. How different duty types affect thermal loading under frequent starting and braking is explained in our Duty Types S1-S3 article.

Which information helps in selecting the drive?

  • Required speed range and control accuracy
  • Whether direction reversal is required
  • Permissible EMI emission for the application
  • Available supply voltage and power electronics
  • Feedback: is speed or current measurement available?

Frequently asked questions

How does DC motor speed depend on voltage?

For a given load, a DC motor's speed is approximately proportional to the applied armature voltage - a lower voltage gives a lower speed.

How does PWM speed control work?

Pulse-width modulation switches the supply voltage on and off rapidly. Winding inductance and the motor's mechanical inertia smooth this pulse, so the motor behaves approximately as if a continuous DC voltage were applied, whose effective level depends on the duty cycle.

What is an H-bridge used for?

An H-bridge allows speed to be controlled via PWM while also reversing the motor's direction of rotation, without changing the wiring.

When is open-loop control not enough and closed-loop control needed?

When speed must stay constant regardless of load, it has to be measured and fed back to adjust the supply voltage - open-loop control without feedback cannot do this.

Sources (selection): olliw.eu (PWM basics), IEC 60034-1 (duty types).

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