Calculation of radial forces in surface PM motors with asymmetric stator windings

The magnetic forces acting on the rotor of Permanent Magnet Synchronous Motors are often neglected because of the symmetry of the winding. But for a fractional slot number per pole and per phase, the winding design is not symmetric. This leads to an asymmetry in the flux density distribution which is the source of the unbalanced magnetic forces. This paper presents an analytical model that leads to determine the magnetic forces acting on the rotor based on the spectral analysis of the air-gap flux density. A special care is taken to the harmonic contents of the stator equivalent current sheet that strongly depends on the winding arrangement. The model is validated by Finite Element computation and applied to the prediction of radial forces in different PM motor topologies. This study is performed on a 9 slots 8 poles teeth wounded motor and on two channel 14 poles fault tolerant PM motors under faulty operation designed for avionic applications.