Steady-state asynchronous characteristics of salient-pole motors with rectifiers in the field circuit

The starting of brushless synchronous motors, in which a rectifier system is connected across the field circuit, presents a practical problem owing to the nonsinusoidal nature of the field-circuit current. In the case where the field winding is directly short-circuited by a rectifier, the d.c. component of field-circuit current produces high generating torques at low speeds and the machine will not start. The paper presents an analysis of the steady-state performance of this system and of the practical modifications of it, including shunt and series field resistors, which are adopted to achieve satisfactory starting performance. Computed results are shown to be in good agreement with test results on two laboratory machines. Particular attention is given, in both the theoretical treatment and in the experimental investigation, to establishing the true nature of the field-current waveshape, since this has an important influence on the torque performance.