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

This article 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 (SE), dynamic eccentricity (DE), mixed eccentricity (ME), and axial eccentricity. First, 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 SE leads to the imbalance of the three-phase back EMFs. The DE not only leads to three-phase imbalance, but also introduces new order back EMF harmonics. ME not only leads to back EMF imbalance, but also introduces new harmonics. The back EMF distortion of the above three types does not affect the average torque but generates 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.