Phase Locked Loop Technique

  • 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.