Analysis and design of a low-speed, spoke-type coreless axial-flux permanent-magnet motor

Axial-flux permanent-magnet (AFPM) machines that implement flux focusing have been studied in the past with the aim of replacing rare earth magnets with ferrite permanent magnets (PMs). The use of NdFeB PMs, combined with similar topologies, has also been explored to increase torque density in the literature with respect to conventional surface-mounted PMs machines, achieving good results. This work investigates the flux-focusing technique in coreless structures with NdFeB PMs. The machine’s operating principle is described, and expected output performances are compared to surface-mounted PM machines with the aid of a mathematical model. Extensive 2D and 3D finite element analyses are reported, showing the effect of flux fringing on the output performance. Different rotor shapings that allow for mitigating the flux fringing are proposed. Furthermore, a low-speed direct-drive motor prototype is designed and built to achieve a high specific torque. A full set of open-circuit tests is reported, providing insight into loss mechanisms.