Evaluation of the electrodynamic forces in high-speed permanent magnet machines with rotor eccentricity

Abstract

In permanent magnet motors, the presence of rotor eccentricities can alter the airgap field distribution. This results in parasitic radial detent forces that can be reduced by connecting the stator phases in parallel. As a consequence, currents are passively induced in the windings when the rotor spins in an off-centered position, yielding balancing electrodynamic forces. Specific models were developed to predict these forces, but their complexity can be prohibitive. Therefore, this paper proposes to study the effect of the rotor off-centering in permanent magnet motors using a simpler model developed for electrodynamic bearings. This model consists in a linear differential equation with only four parameters that depend neither on the spin speed nor on the rotor position. As an illustration, the paper applies this model to the study of a high-speed, slotted permanent magnet motor. To support this, the main hypotheses of the model are validated in this particular case. Then, the centering electrodynamic forces in a staticeccentricity configuration are predicted using the model and compared to finite element simulation results. Finally, a preliminary study showing the impact of the width and permeability of the stator teeth on the centering force is performed.