Analysis and Reduction of Unipolar End Leakage Flux in Consequent-Pole PM Machines

Abstract

This paper firstly investigates unipolar end leakage flux of consequent pole (CP) permanent magnet machines (PMMs) with different slot-pole number combinations and winding configurations. It shows that unipolar end leakage flux, mainly due to CP rotor, is affected by slot-pole number combinations, while the armature reaction has negligible effect on the amplitude of end leakage. The unipolar end leakage flux decreases with the increase of rotor pole number for the analyzed fractional slot concentrated winding CP PMMs but increases when the rotor pole number further increases for overlapping winding vernier machines. Secondly, the influence of critical design parameters on end leakage and average torque of CP PMMs is investigated, and the results show that the end leakage exhibits significant sensitivity to shaft diameter and PM pole arc. Thirdly, a simple and effective non-magnetic rotor structure, either a non-magnetic shaft or a non-magnetic shaft together with a non-magnetic ring inside the rotor core, is proposed for the first time to reduce the unipolar end leakage flux without sacrificing the torque capability. The non-magnetic shaft alone can reduce approximately 90% of end leakage flux, while the non-magnetic ring alone can reduce approximately 40%. Additionally, 95% of end leakage flux can be reduced when combining a non-magnetic ring with a non-magnetic shaft (which could be integrated into a single shaft) compared with the conventional rotor with a magnetic shaft. Considering the manufacturing complexity, a non-magnetic shaft alone is highly recommended. The analyses have been validated by experiments on prototyping CP machines with different slot-pole number combinations, magnetic/non-magnetic shafts, and conventional/composite rotors.