Optimal Designs of Wound Field Switched Flux Machines with Different DC Windings Configurations

Wound field switched flux (WFSF) machines exhibits characteristics of the simple robust rotor, flexible flux-adjustable capability, and no risk of demagnetization. However, they suffer from a poor torque density compared with permanent magnet machines due to the saturation. Therefore, in this paper, two WFSF machines with single- and double-layer DC windings, respectively, are optimized for the maximum torque. The end-winding (EW) lengths differ in these two machines, which can affect the optimal design. Design parameters including the DC to armature winding copper loss ratio, slot area ratio and split ratio are optimized when two machines have the same copper loss and overall sizes. In addition, the influence of the flux density ratio, total copper loss, air-gap length and aspect ratio on the optimal split ratio is investigated using the finite element method and results are explained through the analytical model accounting for the saturation. It is discovered that the EWs have no effect on the optimal copper loss ratio, which is unity. In terms of the slot area ratio, the machine with single-layer DC windings prefers smaller DC slot areas than armature slot areas. In the WFSF machine with longer EWs, the optimal split ratio becomes smaller. Moreover, compared with other parameters, the flux density ratio can significantly affect the optimal split ratio.