A new design concept for permanent magnet vernier machine: positive-mutual-coupling for improved power factor and higher field-weakening capability

Abstract

This article investigates a permanent magnet vernier machine (PMVM) equipped with positive-mutual-coupling (PMC) winding. The conventional winding layout typically presents negative-mutual-coupling (NMC) among the three-phase windings due to the 120° spatial phase shift. It is illustrated that in the conventional NMC winding, the negative phase-mutual inductance leads to increasing q-Axis inductance and amplifying armature reaction. On the contrary, the investigated PMC winding exhibits positive phase-mutual inductance, cancellation effect on the q-Axis flux linkage, and brings the characteristic current closer to the current limit. This contributes to suppressing armature reaction, improving the power factor, reducing the terminal voltage, and enhancing the field-weakening performance. The generic design methodology to obtain PMC winding is presented and exemplified on a 24-slot, 5-Armature pole pair, and 19-rotor pole pair PMVM. Finite element analysis shows that the proposed PMC PMVM could improve the rated power factor and widen the constant torque region. With reduced terminal voltage and more voltage margins, the PMC PMVM can employ a higher q-Axis current to generate torque, thus significantly enhancing the output torque and power factor during field-weakening operations. As a result, the output power capability and constant power speed range (CPSR) are improved dramatically. Finally, a PMC PMVM prototype is manufactured to validate the efficacy of PMC winding in improving field-weakening and power factor.