Comparison of Axial Flux PM Synchronous Machines With Different Rotor Back Cores

Rotor losses of axial flux permanent magnet (PM) synchronous machines with fractional windings depend on stator magneto-motive force (MMF) harmonics. To reduce rotor losses, a soft magnetic composite rotor can be used. This paper focuses on the comparison between two machines with the same soft magnetic composite stator and different rotor cores: one of solid steel and another of soft magnetic composite. The study evidenced the composite rotor improved the power density and the efficiency. Moreover, it highlighted a nonlinear salient-pole operation of the composite rotor machine, mainly connected to spatial harmonics.