A phase-locked loop control system for a synchronous motor

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.