PHASE LOCKED LOOP D.C. MOTOR SPEED CONTROL SYSTEM

Abstract

This Thesis deals with the development and testing of an analog type phase locked loop d.c. motor speed control system, To start with, a new fast measuring phase detector capable of giving a d.c. output voltage is developed. This output voltage is proportional to the phase angle difference between the reference and feedback signals. Using this phase detector, a cosine wave crossing firing angle control circuit and a half-controlled thyristor bridge, a PLL speed control system for a £ h.p. separately excited d.c. motor is fabricat ed. An a.c. tachogenerator is used to obtain a feedback signal. Stability analysis of the basic scheme is carried out. A first derivative of the output voltage of the phase detector is used to stabilise the drive. The effect of various parameters on the stability of the system is also determined. The steady state performance of the drive has been studied. Certain performance characteristics, such as supply power factor, .fundamental power factor and harmonic content are obtained. The transient response curves of the system for step changes in load torque and reference frequency have been determined-. The effect of adding a second derivative of the phase detector output voltage for control purposes is discussed. Integral Square of Error (IS!) minimisation criterion is used to obtain the optimum values of the

differentiating circuit parameters. The transient response of the drive to a step change in load is plotted for the optimum values of the parameters. The effect of smoothing reactor on the transient response of the drive is discussed, Experimental oscillograms are also included. The lock range and capture range of the drive are determined. The causes for speed hunting phenomenon and the precautionary steps taken to prevent it are discussed.