Torque ripple reduction in fractional-slot Interior PM machines optimizing the flux-barrier geometries

As far as the permanent magnet (PM) machine design is concerned, there are two recent trends. The first deals with the adoption of fractional-slot (FS) non-overlapped coil windings for the stator, which minimizes the length of the end- windings with a consequent benefit on the reduction of copper cost and losses. The second regards the choice of rotor structures with Interior PM (IPM rotors), which exhibits a reluctance torque components and allows a low-cost PM to be used. However, the adoption of FS winding (characterized by a high MMF harmonic content) together with an IPM rotor (which is an anisotropic structure) is not devoid of drawbacks. Certainly the biggest one is the high ripple in the torque behaviour. An analytical procedure to analyze and minimize such a torque ripple is presented in this paper. Some solutions are compared, taking the PM heights and the flux-barrier end angles as design variables. Some finite element results are also presented to integrate the analysis, including the effect of stator saturation and the presence of open slots.