Comparative analysis of unbalanced magnetic force in fractional-slot permanent magnet machines having external rotor topologies

This paper comparatively investigates the unbalanced magnetic force (UMF) of external rotor machines having pole and slot numbers differed by one but no rotor eccentricity. The 8-pole/9-slot and 10-pole/9-slot machines are firstly optimised for maximum torque density by nonlinear FE analysis. Then, the UMF of both machines are analysed by both analytical model and linear FE model. It shows that the optimised 10-pole/9-slot machine has larger average torque than the 8-pole/9-slot machine for the same copper loss. However, the 10-pole/9-slot external rotor machine has significantly larger UMF than the 8-pole counterpart, which is opposite to the case of internal rotor machines.