Comparison of Simulation Methods for Angular Dependency when Modeling Demagnetization

Rare earth element free magnets, such as ferrites, are weaker than their more common alternatives and therefore more prone to demagnetization. Additionally, in permanent magnet electrical machines they are commonly tangentially magnetized and placed in a spoke-type geometry to enhance the magnetic field. In this geometry, the magnets are subject to inclined fields, which are normally not accounted for when evaluating demagnetization risk. In this paper, different methods for demagnetization of permanent magnets are applied to two motors under two distinct loading conditions and the results are compared. It is concluded that it is more important to consider inclined fields when studying ferrite magnets than rare earth element magnets. The common, simplified method of omitting inclined fields is shown to be the most conservative method for evaluating demagnetization. However, more precise results can be reached with methods taking inclined fields into account, especially if the models are adapted for the specific magnet type. Two motors with similar performance have been designed, one with rare earth element magnets and one with ferrites. The motor with ferrites is more sensitive to demagnetization and the bulk of the magnet reaches the demagnetization limit at a lower current than for the motor with rare earth element-based magnets. Finally, demagnetization of magnets is compared for two different rotor positions, emphasizing the importance of considering rotor position when evaluating demagnetization risk.