Influence of End-Effect in Switched Flux Machines

Switched flux machines have all excitation sources in the stator and the salient rotor is simple and robust, which is suitable for the high-speed operation. The two-dimensional (2D) finite element (FE) is widely applied to the analysis and design of the switched flux machine, in which the end-effect is not accounted. Since the axial distribution of the flux density is not identical, it can be challenging for the accurate prediction of the switched flux machine performance. This paper aims to comparatively investigate the influence of the end-effect in both electrical excitation and permanent magnet switched flux machines, including switched flux permanent magnet (SFPM) machines, hybrid excitation switched flux (HESF) machines, wound field switched flux (WFSF) machines and the variable flux reluctance machines (VFRM). Firstly, the axial distribution of the flux density in the switched flux machine topologies is analysed, and it is found that the SFPM machine suffers from high end-region flux-leakage due to the shorter flux-leakage path. Secondly, the influence of the end-effect on the switched flux machine performances at different length/diameter (L/D) ratios, slot pole number combinations and current density excitation are analysed and compared under both 2D-FE and 3D-FE predictions. It shows that the end-effect has a significant impact on the average torque of the SFPM and HESF machines than the WFSF machine and the VFRM. In terms of the DC winding induced voltage, the end-effect will make the DC winding suffer from higher magnitude of the voltage pulsation. The underlying causes of the end-effect can be explained by the lower magnetic permeability difference between the conductor and insulator than the electric conductivity, and a higher magnetic permeability can ease end-effect.