Governing of motor speed is taken as an example for the purpose of study. Details have been shown in the figure.

With the help of a keypad, information regarding the speed to be maintained is fed to the controller.

Now the controller starts the motor. Any digital signal from the controller is converted to an analog signal (firing pulse) by a D/A converter.

This firing pulse is given to the inverter, which controls the frequency of the input supply.

Now the motor speed is changed according to the output frequency of the inverter.

A tachometer is used to provide feedback control. The tachometer converts the motor speed into an electrical analog signal.

The analog signal is amplified and converted to a digital signal using a A/D converter.

Now the controller compares the feedback signal and the required speed and generates an error signal.

This error signal is fed to the D/A converter again.

The process briefed till now is continued until the required motor speed is achieved. Thus the variation of speed is governed.