Optimal split ratio in small high speed PM machines considering both stator and rotor loss limitations

The split ratio which is defined as the ratio between rotor and stator outer diameters is critical for permanent magnet (PM) machines. In this paper, the split ratio in small high speed PM machines is optimized analytically by considering both stator and rotor loss limitations. By this way, the thermal constrains in both stator and rotor parts can be taken into account. It shows that both losses can significantly affect the optimal split ratio and the machine performance. By considering the influence of rotor losses, especially the rotor eddy current loss, the optimal split ratio is increased significantly in certain scenarios. The output torque is calculated analytically and the influences of several key design parameters on the optimal split ratio, i.e. slot/pole combinations and magnet materials, are also investigated in details. Finally, finite element (FE) analyses are carried out for validating of the analytical model.