Explicit criteria for reluctance torque utility of permanent magnet motors

This paper presents an explicit criteria for reluctance torque utility of permanent magnet (PM) motors. The criteria can be quantified by PM flux linkage Ψ_p of stator phase windings, and current I, as well as direct- and quadrature-axis inductance L_d and L_q. Through both theoretical formula and experiments, it is proved that the 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 invented criteria helps one to understand the property inherent in each motor, and also indicates that in the above case selecting the rotor geometry with less saliency can improve the efficiency. Results are given for two types of 4-pole prototype motors with P_N = 0.6 kW, n_N = 3600 min^-1, Y-connection.