Effect of magnet properties on power density and flux-weakening performance of high-speed interior permanent magnet synchronous machines

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Abstract
This paper investigates the impact of varying magnet properties on the performance characteristics of interior permanent magnet (IPM) machines for traction applications with a special emphasis on the power density and flux-weakening performance. Several IPM machines utilizing four different grades of magnet materials are optimized to meet a set of traction machine specifications with a minimum total active mass. The designs are then compared based on total active mass, efficiency, flux weakening, high-speed operating characteristics, and a number of other metrics. Results show that magnets with a higher energy-product generally achieve higher torque density in the final machine. However, the relationship between torque density and magnet energy product is nonlinear and requires consideration of other factors including coercivity. These results lead to a better understanding of the engineering tradeoffs encountered by designers who are evaluating use of Dysprosium(Dy)-free Neodymium-Iron-Boron (NdFeB) or ferrite PM material as lower-cost alternatives to conventional rare-earth magnets.

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