Investigation of Demagnetization in Air-Cored Halbach Machines With Different Number of Magnet Segments

Even under open-circuit, Halbach permanent magnet (PM) machines may partially suffer from an irreversible demagnetization issue. In this paper, the relationship between PM segment number per pole and electromagnetic performance reduction caused by demagnetization under both open-circuit and on-load is investigated in air-cored Halbach PM machines. The PM segment number has influence on effective PM thickness, which is related to on-load demagnetization withstand capability. The higher segment number, the larger effective PM thickness, and the better on-load demagnetization withstand capability. However, when segment number >3, the further improvement in effective PM thickness becomes less significant, and hence, the further improvement in on-load demagnetization withstand capability becomes less significant. Meanwhile, the influence of geometric demagnetization factors, including airgap length, PM thickness, and PM angular width, is investigated. It is found that Halbach PM machines with larger airgap length, and thicker PM have higher demagnetization withstand capability. Compared with the Halbach machines with 100% PM width ratio, a gap between adjacent PMs can mitigate the influence of demagnetization. An air-cored Halbach machine with PM 2-segments per pole has been fabricated and tested to verify the finite element results, such as cogging torque, back EMF, static torque, and average torque.