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.