Controlling DC motor

Controlling the sense of rotation with a H-bridge

The circuit below shows the power unit to control the direction of rotation of a DC motor. In this so-called H-bridge transistors serve as electronic switches. In power electronics, MOSFETs or IGBTs are often used to switch large currents.

H-Bridge for DC motor - function

H-Bridge for DC motor - function, how a H-bridge works

Which transistors have to be switched on for motor clockwise (UA> 0) and motor counterclockwise (UA <0) rotation?
What tasks have the diodes have in this H-bridge?

Controlling the speed of rotation

The pulse diagram of the applied armature voltage illustrates the principle of pulse width modulation PWM. The course of the motor or armature voltage is recorded by an oscilloscope. The following picture of the armature voltage shows how low and high speed can be achieved:

Voltage curve of a typical PWM signal

Voltage curve of a typical PWM signal

Parameters which describe this rectangular signal:

  • f = 1 / T   f frequency, T period duration
  • Duty cycle = ti / T

Combining the principle of pulse width modulation with the H-bridge, speed and sense of rotation can be controlled continuously:


MOS-FET = Metal–Oxide–Semiconductor Field-Effect Transistor

IGBT = Insulated Gate Bipolar Transistor

PWM `= Pulse Width Modulation