Analysis of Negative-Saliency Permanent-Magnet Machines

In this paper, a
negative-saliency
permanent-magnet (PM) synchronous machine analysis is presented. This particular saliency feature is achieved by replacing a portion of the magnet material by a soft iron piece over the rotor pole. In this manner, the
d-axis
inductance is increased, whereas the
q-axis
inductance is almost not affected, leading to the condition that
Ld
is higher than
Lq
(negative saliency) corresponding to the inverse condition of typical PM machines. An expression for the optimum pole configuration is derived. It is shown that, with appropriate control of the stator current based on the machine's saliency, the unfavorable effects of magnet reduction on torque production can be compensated. It is also shown that the machine saliency affects the location of the operating points when it operates under vector control. Finally, the theoretical analysis is validated with experimental results obtained from a prototype axial-flux PM machine that exhibits negative saliency.