High-Speed Synchronous Reluctance Machines: Materials Selection and Performance Boundaries

This article presents a comprehensive comparative design exercise of synchronous reluctance (SyR) machines considering different soft magnetic materials and a wide range of speeds. First, a general design methodology able to consider all the consequences of selecting different materials is presented. In fact, magnetic nonlinearities, rotor structural limitations, and the rise of both stator and rotor iron losses are all considered. The adopted design approach allows achieving optimal stator and rotor geometries balancing all these competitive multiphysics aspects and keeping constant the cooling system capability. Both silicon-iron (SiFe) and cobalt-iron (CoFe) alloys with optimized magnetic and mechanical performance are examined to assess the maximum capabilities achievable with an SyR machine technology. The adoption of CoFe alloys leads to machines that outperform the SiFe counterparts up to a certain speed, above which, machines with SiFe provide better performance. Indeed, in the lower speed range, the effect of the higher saturation flux density of the CoFe materials is dominant, while for higher design speeds, their higher iron losses and lower yield strength, with respect to the SiFe ones, make the latter more convenient. All the design considerations are finally validated by comparing the predicted performance with the experimental test results on a 6.5-kW, 80-kr/min SyR machine prototype.