Comparative Study of Dual-Rotor Slotless Axial-Flux Permanent Magnet Machines with Equidirectional Toroidal and Conventional Concentrated Windings

Axial-flux permanent magnet (AFPM) machines are gaining popularity in low speed and high torque applications due to their high torque density, high efficiency and compact structure. In this paper, a dual-rotor slotless (DRS) AFPM machine equipped with equidirectional toroidal winding (EDTW) is proposed and compared with that equipped with conventional single layer and double layer concentrated windings (SL and DLCWs). Firstly, the differences between the DRSAFPM machine with EDTW and that with conventional CWs in coil layout, electromotive force (EMF) and winding magnetomotive force (MMF) are revealed. Considering the special coil layout of the EDTW, the coil factor and size equations of the DRSAFPM machines with EDTW and conventional CWs are presented. Secondly, the electromagnetic performance including air-gap field, back-EMF, torque and efficiency of the DRSAFPM machines with EDTW and conventional CWs are analyzed based on the three-dimensional finite element method (3D-FEM). The simulation results validate the correctness of the coil factor and size equations and indicate the EDTW-DRSAFPM machine has superiority in back-EMF amplitude and torque density. Finally, a prototype of the EDTW-DRSAFPM machine is manufactured and tested, which validates the feasibility of the EDTW-DRSAFPM machine and the correctness of the analysis.