Influence of rotor back-iron on optimal Halbach PM machines

Abstract

Due to transition permanent magnets (PMs) with high tangential magnetization component and located between main PMs, i.e., N and S poles, Halbach machines are featured for self-shielding effect. Consequently, rotor back-iron is not essential. In this paper, Halbach machines with optimal PM widths and optimal magnetization angles for the maximum torque are investigated. It is found that regardless of PM segment number per pole, in optimal air-cored Halbach machines, PM widths are almost equal and magnetization angle shifts between adjacent PMs are almost equal too. Therefore, the equal air-cored machines can be regarded as the optimal machines. In optimal Halbach machines with rotor back-iron, main PM widths are much larger than transition PM widths and magnetization angle shifts between main and transition PMs are lower than corresponding equal magnetization angle shifts. Therefore, the airgap field in optimal iron-cored Halbach machines exhibits a significant 3rd harmonic field component and higher fundamental airgap field, and consequently the larger torque. Meanwhile, as the PM becomes thicker and the split ratio becomes lower, PM widths and magnetization angle shifts between adjacent PMs in optimal iron-cored machines both change from corresponding unequal values to almost equal. A prototype of optimal air-cored Halbach machine is fabricated and tested to verify the finite element analysis results.