- The speed control of DC motor may be open loop or closed loop.
- The accurate speed control of DC motor is done by analog servo feedback closed loop system.
- The speed of DC motor is measured by tachometer.
- The error signal is generated by comparing speed of AC techno generator and reference signal.
- The speed of DC motor may be not accurate when it is measured by analog device and its speed regulation may be greater than 0.2%.
- This type of speed control is not accurate in the industrial drive.
- As the analog device measurement is not accurate, the phase locked loop control system is used for DC motor speed control.
- The speed of motor is converted into digital pulse train and it is compared with input reference signal.
Phase
locked loop system
- The block diagram of DC motor speed control by phase locked loop system is shown in the figure A.
- The speed of motor is converted into digital pulse train and it is compared with input reference signal in this method.
- The phase detector compares two signals and generates output accordingly.
- The output of the phase detector feeds at low pass filter.
- The high frequency components are filtered partly or completely by low pass filter and it generates continuous DC signal.
- The output of the low pass filter is given to the converter input.
- When the reference speed and speed of DC motor are equal, both frequencies are locked or synchronize.
- The phase detector generates constant voltage at output and it is directly proportional to phase difference between two quantities.
- Therefore the speed of motor is constant with reference of load.
- If there is change in speed due to any disturbance, the output of the AC techo generator changes accordingly.
- This will change the output of phase detector but the reference signal and feedback signals are always locked.
- The response of phase locked loop is very fast, ideally it is nil but actually it is 0.002%.
- The application of phase locked loop is to control DC motor drive and synchronize communication system.
