Saliency analysis of PM machines with flux weakening capability

In PM machines, the fixed excitation provided by the magnets limits its suitability for high-speed applications. To correct this deficiency, a variety of designs capable of flux weakening have been proposed. To achieve this feature, a lower amount of magnet is considered as an approach to reduce the flux weakening current requirement. This paper investigates the application of this technique to a ldquonegative saliencyrdquo PM machine with X_d > X_q. It is shown that, with an appropriate control of the stator current based on machine's saliency, the unfavorable effects of magnet reduction on torque production can be compensated. The torque and flux control capability are quantified and an expression for the optimum pole configuration is derived. Finally, the effects of magnetic saturation on capability curves under optimum current control are studied.