Development of a Radial-Flux Machine With Multi-Shaped Magnet Rotor and Non-Ferromagnetic Yoke for Low Torque Ripple and Rotor Mass

Torque ripple and rotor mass of electric machines are very significant for aerospace applications, as they may influence the vibration level and system dynamic performance greatly. However, it is challenging to reduce torque ripple and rotor mass while maintaining high output torque. In this paper, a multi-shaped magnet rotor with non-ferromagnetic yoke is proposed to achieve these three aspects simultaneously. The fundamental air-gap flux density and the average torque can be enhanced by the obliquely magnetized quasi-trapezoidal magnets with large flux emitting cross-section. Besides, the high-order magnetic harmonics and torque ripple can be effectively suppressed through the combination of the multi-shaped magnets with the variable magnetization directions. In addition, the triangular-shaped magnet in the multi-shaped magnet rotor helps to strengthen magnetic self-shielding effect of the rotor, which in turn helps to reduce the rotor mass by removing the yoke and replacing it with lower-density material. Studies have been carried out on the multi-shaped magnet rotor to validate its advantages. Firstly, the structure and operating principle of the multi-shaped magnet rotor are presented. And the magnetic field and average torque of the MMR design are formulated mathematically through field superposition method based on Laplace's and Poisson's equations. Then, the major structural parameters of the multi-shaped magnet rotor are determined. Following that, the electromagnetic performance of the multi-shaped magnet rotor is compared with those of conventional designs. Finally, one research prototype and test rigs are developed for experiments to verify the design concept and analytical models. The results shows that the average torque of the multi-shaped magnet rotor is increases by 8.4% and the torque ripple reduced by 46.9% compared with three-stage segmented Halbach array.