Unbalanced magnetic pull due to asymmetry and low-level static rotor eccentricity in fractional-slot brushless permanent-magnet motors with surface-magnet and consequent-pole rotors

We report on an investigation into the unbalanced magnetic pull (UMP) in ferrite-magnet fractional-slot brushless permanent-magnet motors due to either magnetic asymmetry or static rotor eccentricity. We justify the work in terms of the establishment of total indicated runout (TIR) tolerance and use 10% eccentricity as the nominal tolerance. The UMP will generate force and vibration, which causes wear. We investigated several machines with different pole numbers, slot numbers, and winding arrangements. Some of the windings contain sub-harmonics, and we found these to be more susceptible to UMP when there is rotor eccentricity and also to produce vibrating UMP. We found that consequent rotor poles produce high UMP under centered and rotor eccentricity conditions. We conducted detailed finite-element studies to calculate the UMP.