Influence of Rotor Eccentricity on Electromagnetic Performance of Rotational Symmetrical Slotless PM Brushless Motor

This paper systematically investigates the influence of different rotor eccentricity types on the electromagnetic performance of rotational symmetrical slotless permanent magnet (PM) brushless motors, including static eccentricity, dynamic eccentricity, mixed eccentricity and axial eccentricity. Firstly, the analytical model of the slotless motor is deduced. Then, the influence of eccentricity on flux density, back-electromotive force (back-EMF) and electromagnetic torque is analyzed qualitatively by analytical model and finite element method (FEM). The results show that the influence of eccentricity on the rotational symmetrical slotless motor is similar to that of the rotational asymmetrical slotted motor. The static eccentricity leads to the imbalance of the three phase back-EMFs. The dynamic eccentricity not lead to three phase imbalance, but would introduce new order back-EMF harmonics. Mixed eccentricity not only leads to back-EMFs imbalance, but also introduce new harmonics. The back-EMFs distortion of above three types do not affect the average torque but generate additional torque harmonics. However, the axial eccentricity has almost no effect on the back-EMF and torque. Finally, a 6s/4p slotless PM motor is tested to validate the theoretical analyses and simulations.