Study of Demagnetization Risk in PM Machines

Nowadays, many applications are demanding more and more compact, cost effective, and robust electric drives. By using rare earth magnets, it is possible to achieve electrical machines with rather high torque volume ratio, accomplishing this way the compactness requirement. Nevertheless, rare earth permanent magnets are very sensitive to the temperature, so that the risk of being partially demagnetized arises as their working temperature increases. In addition, as it has been seen during the last years, the market of rare earth metals is quite volatile. The price of magnets could rise suddenly, leading to a significant impact on the cost, and so in the competitiveness of permanent magnet machines. Due to this fact, it is essential to reduce the dependency on rare earth market in order to accomplish cost-effective drive solutions. Some key points to face this problem can be to reduce the magnet volume, or to use magnets with worse thermal properties (magnets with less dysprosium). Nevertheless, in both cases, the adopted solution would lead to an increase in the risk of demagnetization, affecting to the robustness of the machine. In this paper, a methodology based on finite element analysis and experimental test is proposed to evaluate the demagnetization risk in permanent magnet machines. By applying this methodology, it is possible to accomplish competitive machine designs, achieving a good tradeoff between cost and robustness. The proposed methodology has been applied in a practical case, for the optimization of ten industrial motors, achieving good results.