Performance characteristics of synchronous motor drives with surface magnets and field weakening

Permanent-magnet synchronous motors offer high efficiency under normal operating conditions but when field-weakening is used to obtain a wide constant-power speed range a number of compromises must be made. Achievement of a constant-power speed range is dependent upon the product of stator current and stator circuit inductance and, with a surface-magnet rotor, added inductance is necessary. This causes reduction in performance at base speed. Performance is further degraded over the whole speed range owing to the effect of stator resistance. Efficiency at increased speeds is inevitably less than mould be obtained with constant voltage/frequency operation, owing to the necessary magnitude of stator current to obtain field weakening. An analysis of these performance characteristics is illustrated by application to an experimental disc-type machine