Analysis and Comparative Study of a Novel Stator-Permanent-Magnet Axial-Flux Counter-Rotating Hybrid-Excitation Machine

This paper proposes a novel stator-permanent-magnet axial-flux counter-rotating hybrid-excitation machine (SPM-AFCRHEM) for unbalanced-load operation of counter-rotating propeller (CRP) systems, and investigates three topologies with different stator configurations. By utilizing U-shaped permanent magnets (UPMs) and field-regulation windings to achieve magnetic decoupling and field-regulation capability. Firstly, the field-regulation principle of SPM-AFCRHEMs are analyzed. The working harmonics and their respective torque contributions are calculated and evaluated by establishing the magnetomotive force (MMF)-permeance model. Subsequently, the torque performance of SPM-AFCRHEMs with different pole-slot combinations is compared under unified winding layout principles, and the risk of negative-sequence back-electromotive force (EMF) caused by stator yoke magnetic saturation is discussed. Finally, the machines are comprehensively evaluated through comparative studies of no-load characteristics, load performance, and field-regulation time constants. A prototype of the top-performing configuration is fabricated and experimentally validated, demonstrating that the proposed SPM-AFCRHEM represents a competitive solution for direct-drive CRP systems.