Comparative Study of PM Demagnetization in Air- and Iron-Cored Halbach Machines

In Halbach machines, the potential demagnetization of permanent magnets (PMs) should be considered to avoid irreversible demagnetization (ID). In this paper, the Halbach machines with air- and iron-cored rotors are optimized for maximum average torque and minimum torque ripple, and their electromagnetic performances with consideration of PM demagnetization are compared. It is found that due to low and high permeabilities of air- and iron-cored rotors, the air-cored machine suffers from the larger ID area while the iron-cored machine suffers from more severe ID level in the ID area. When the influence of ID on torque reduction is considered, it is found that due to the field interaction between PMs in air-cored machines, ID influence is more severe at low current and high PM temperature. However, due to significantly strong armature field in iron-cored machines, ID influence is more severe at high current and high PM temperature, which even results in lower torque than that of the air-cored machine. Besides, a notched iron-cored rotor is proposed to improve the ID withstand capability in the iron-cored machine. Prototypes with air- and iron-cored rotors are fabricated and tested to verify the finite element analysis.