Investigation of Unbalanced Magnetic Force

Fractional-slot permanent magnet (PM) machines with an odd number of stator slots are attractive candidates for industrial applications due to their high torque density and low torque ripple. However, the unbalanced magnetic force (UMF) is detrimental to the bearing and yields noise and vibration. This presentation determines the existence criterion of UMF on both open-circuit and on-load conditions by analysis of air gap field and magnetic force waves, together with FE validation. In addition, five machines with a given odd slot number of 9 but different pole numbers, i.e. 2, 4, 6, 8 and 10 are compared in terms of UMF so that the influence of slot/pole combination on UMF can be determined. The additive and cancelling effects between the radial and tangential stresses are finally revealed.