Novel Criteria for Reluctance Torque Utility of Permanent Magnet Motors

Abstract

This paper presents novel criteria for the reluctance torque utility of permanent magnet (PM) motors. The criteria can be quantified by the PM flux linkage Ψ_p of stator phase windings, and current I, as well as the direct- and quadrature-axis inductance L_d and L_q, respectively. Through both theoretical formulae and experiments, it is proved that a motor with large Ψ_p can utilize little reluctance torque even if the salient-pole ratio L_q/L_d is over 2.0. In such a case, the large salient-pole ratio only leads to an increase in iron loss, and hence a decrease in motor efficiency. The proposed criteria help one to understand the properties inherent in each motor, and also indicate that, in the above case, selecting the rotor geometry with less saliency can improve the efficiency. Results are given for two kinds of PM motors; one is a 4-pole Nd-Fe-B magnet motor with P_N =0.6kW, n_N =3600min^-1, Y-connection; the other is a 6-pole ferrite-magnet motor with P_N =0.2kW, n_N =1080min^-1, Y-connection.