Reduction of torque and radial force fluctuation in permanent magnet synchronous motors by means of multi-objective optimization

This work relates to external rotor type permanent magnet synchronous machines having surface mounted permanent magnets and two-layer non-overlapping windings. A short review of approaches to reduce fluctuation of forces presented in literature is given. As the reduction is associated in most cases with a negative influence on other important design goals, multi-objective optimizations are applied in conjunction with the finite element analysis yielding PARETO-optimal solutions with different preferences. PARETO-optimal solutions regarding the permanent magnet magnetization distribution and the current profiles minimizing the torque fluctuation of a 12-slot 8-pole machine and the higher frequency radial force harmonics in a 12-slot 10-pole machine are presented and discussed. A discrete PARETO-optimal magnetization distribution has been identified which promises a higher fundamental PM flux linkage amplitude compared to radial magnetization, but at a significantly lower cogging torque and torque ripple utilizing the balancing of open circuit voltage harmonics. Furthermore, it is shown that individual current harmonics affect individual radial force harmonics dominantly. A test setup is proposed and the numerically obtained instantaneous torque and the influence of current harmonics on the radial force amplitudes are verified with good agreement.

I. Introduction

THE permanent magnet synchronous machines (PMSM) have been advantageously applied as direct drives for fans for many years. Here, magnetically induced vibration and noise are an important concern. Resonant vibrations cannot be avoided across the operating range of this variable-speed drive. A significant rise of natural frequencies or an increase of mechanical damping is associated with higher costs. There-fore, not only is it necessary to coordinate structural dynamics and force excitation, but excitation amplitudes must also be reduced. A short overview of approaches to reduce fluctuation of forces presented in literature will be given below.