Pulse Width Modulation (PWM) techniques for chopper or TRC
Q. Explain time ratio control and current limit control chopper method for speed control of d.c. series motor. State any four advantages of chopper based D.C. drive.
Q. List the control strategies used for chopper and explain TRC.
Q. List the various control techniques used in chopper and explain any one technique in detail.
Q. State working principle of chopper. Describe the constant frequency system of control technique of chopper.
Q. Explain with waveforms the control techniques used for chopper control.
Q. Describe constant frequency system of control technique of chopper.
Q. Define duty cycle of a chopper. Explain various control techniques used in chopper.
Working Principle of PWM Chopper
- In this strategy the switches are turned on at a constant chopping frequency. Figure shows the waveforms for PWM control.
- The ON time of the chopper is varied to vary the average output voltage.
- The total period T of one cycle of output waveform is constant. Therefore the chopping frequency f =1/T is also constant. The average output voltage is directly proportional to the ON time of chopper.
- In this technique, we change the ratio of on time of the chopper to the total time. Hence it is also called Time Ratio Control (TRC) technique.
Duty Cycle (D) of PWM Chopper
- The ratio of ON time to total time is defined as duty cycle “D”.
Duty cycle D = (TON / T) x 100%
But 1/T = f
∴ D = f TON
- The duty cycle D can be varied between 0 and 1 or 0 to 100%.
- The effect of change in the on time and duty cycle of the chopper is shown in Figure, which shows that increase in on time (or increase in duty cycle) will increase the average load voltage VLDC.
