Design and control of brushless permanent magnet motors for constant power range with minimum inverter rating

Flux-weakening of permanent magnet motors is commonly introduced above a base speed beyond which constant power is required. It is shown that lower inverter rating is achieved by also flux-weakening at the base speed. The motor should operate with equal currents and equal power factors at the extremes of the constant power range, with the current lagging the voltage at the base speed and leading the voltage at the maximum speed. Though arbitrarily high inverter utilisation is possible with high currents, it is suggested that a good compromise is to set the current advance angle at the base speed equal to the power angle there. For the broad constant power range limit, the resulting inverter rating is reduced by 23% compared to flux-weakening only above the base speed.