Novel Criterion for Reluctance Torque Utility of Permanent Magnet Motors

A novel criterion for the utility of reluctance torque in permanent magnet (PM) motors is developed. The criterion, reluctance torque ratio α , can be quantified by using the PM flux linkage Ψ p of the stator phase windings, the current I , and the direct- and quadrature-axis inductances Ld and Lq , respectively. It has been shown that the conventional salient-pole ratio is not helpful in clearly judging the utility, while the ratio α can be used to make such judgements across various types of motors with different applications and outputs. Through both theoretical consideration and experiments, it has been proved that a motor with large PM flux linkage utilizes little reluctance torque even if the salient-pole ratio Lq / Ld is over 2.0 and that a large Lq / Ld increases iron loss, thus decreasing motor efficiency. We have also shown that the above-mentioned loss increase is caused by the significant superposition of q -axis flux due to the large Lq and have concluded that, in the above-mentioned case, selecting a rotor geometry with less saliency can improve the efficiency. Results are given for two types of four-pole prototype motors with an output power of 0.6 kW, a rated rotation number of 3600 min−1, and Y-connection.