A phase-locked loop control system for a synchronous motor

Abstract

The viewpoint that a hysteresis synchronous motor operates similar to a phase-locked loop of the type common in communications applications is discussed. This viewpoint leads to a simple electrical equivalent circuit model for the motor. The model adequately describes the motor's motion with respect to synchronism and, specifically, the well-known hunting characteristic phenomenon. The motor's hunting characteristic cannot be tolerated in systems which require very accurate speed control. It is then necessary to employ external phase-lock control to eliminate the motor's hunting. A two-loop approach is suggested which effectively dampens the motor's response to a torque disturbance. A classical linear analysis of the motor embedded within the control system is given. The results of the analysis establish the overall response as being virtually independent of the motor's hunting characteristic, i.e., in the absence of transport lag. It is further shown that transport lag within the loop eventually limits the system's performance.