Pulse Width Modulation(PWM) is a modulation technique by which the width of a pulse is changed while keeping the frequency constant. PWM is one of the powerful techniques used in control systems today. It is used in wide range of application which includes: speed control, power control, measurement and communication.
The speed of a DC motor is dependent on the load given to it, voltage and current supplied to its input. For a fixed load we can control the power supplied to the DC motor by changing the width of the pulse applied to the DC motor. When power changes the speed of the motor also changes accordingly.
As shown in the above figure a pulse has on-time and off-time. On-Time is the duration of time signal is high. Off-Time is the duration of time signal is low. The longer the on-time of the pulse the more the voltage provided by the pulse. Period is represented as the sum of on-time and off-time of PWM signal. Duty cycle is represented as percentage of time signal remains on during the period of the PWM signal. As shown in the figure, Ton denotes the on-time and Toff denotes the off time of signal. Period is the sum of both on and off times and is calculated as shown in the equation below:
Ttotal = Ton + Toff
Duty Cycle is calculated as on-time percentage of the time period. Using the period calculated above, duty cycle is calculated as:
D = Ton/(Ton + Toff)
PWM signal when used at different duty cycles gives a varying voltage at the output.