Design and analysis of a kind of rotor ironless high torque density machine using Alnico magnet

Abstract

Conventional non-rare-earth permanent magnet (PM) electrical machine can hardly achieve comparable performance with the neodymium iron boron (NdFeB) machine due to the low coercivity and remanence. To solve this problem, a new kind of electrical machine using only Alnico magnet is proposed in this study. The novel rotor topology eliminates rotor iron core, and thus effectively limits armature reaction and improves torque density. A kind of rapid assessment method for the demagnetisation effect of q current in different slot–pole combinations through magnetomotive force and winding factor is presented and verified by 2D finite-element analysis. The optimisation of critical dimensions is demonstrated, and the trade-off between high PM utilisation and low demagnetisation effect is emphasised. After that, the good torque–current linearity and robust to demagnetisation characteristics of the proposed topology are proved through a comparison with two rare-earth-free machines. Finally, the performance comparison with a manufactured conventional NdFeB machine under the same volume constraints is provided. The result suggests that the proposed machine has a higher power factor, lower eddy-current loss, and rare-earth PM machine comparable torque density.