Analysis of coupling between two sub-machines in co-axis dual-mechanical-port flux-switching PM machine for fuel-based extended range electric vehicles

The permanent magnet (PM) field coupling between inner and outer machines of co-axis dual-mechanical-port flux-switching PM (CADMP-FSPM) machines is investigated. Firstly, the relationships between the inner and outer stator teeth are analytically evaluated, with three key stator teeth types defined, i.e. series, parallel, and independent teeth. Secondly, the negative effects of PM field coupling, including high even-order electromotive force (EMF) harmonics, three-phase EMFs asymmetry and DC bias component in flux-linkages, are investigated and verified by two CADMP-FSPM machines, namely, 5/6-12/22, and 5/6-18/42 structures. It is found that for avoiding the negative effects of PM field coupling, all inner and outer stator teeth types should be the same, thus, a 10/12-12/22 structure CADMP-FSPM machine is introduced for analysis. Thirdly, the performance of the 10/12-12/22, 5/6-12/22, and 5/6-18/42 structures, featured by PM field distributions, d-axis flux-linkage ripples, cogging torques, electromagnetic torques, losses and efficiencies, are comparatively analysed by finite element (FE) analysis. The results indicate that the 10/12-12/22 structure exhibits the lowest PM field coupling level and the best performance. Moreover, the 10/12-12/22 structure can avoid all the negative effects of PM field couplings. A prototyped 10/12-12/22 CADMP-FSPM machine is built and tested to verify the FE predicted results.