Consideration to the operation limit of a field-weakening speed control system for a self-controlled synchronous motor

The operation limit as well as the dynamic performance of a field weakening speed control system for a self-controlled synchronous motor with constant margin angle of commutation is discussed in this paper. The steady-state characteristics of the motor are first shown for both field weakening control and DC input voltage control regions. It is shown that the experimental value of margin angle in the field weakening region was reduced as motor speed increased. A simple and effective method to keep the margin angle constant based on correcting the subtransient inductance of the motor in accordance with the air-gap flux is then proposed. This method is useful not only in the steady state, but also in the transient state when the speed reference is changed. Finally the dynamic responses and the operation limit of the field weakening speed control system when the speed reference is stepped up greatly are clarified for a motor having average constants. It is shown that a large operation range with field weakening is obtained for a large value of margin angle, and that a large value of short-circuit ratio of the motor widens the operation limit