Multi-Objective Design Optimization and Comparison of 2-layer versus 3-layer PM-Assisted Synchronous Reluctance Machines using a Blend of Rare-Earth and Rare-Earth-Free Magnets

Abstract

Permanent Magnet Assisted Synchronous Reluctance Machines (PMASynRM) provide a low-cost alternative to the Surface PM Machines due to the use of relatively lower grades of rare-earth (RE) or RE-free magnets, as the performance degradation due to weaker magnets is compensated by the presence of reluctance torque. However, the weaker magnets suffer from high risk of demagnetization leading to unreliable motor operation. Using a blend of RE and RE-free magnets has the potential to overcome this issue. This paper presents four combinations of RE and RE-free magnets as well as two RE-free magnet combinations for their potential utilization in PMASynRM. Two different PMASynRM rotor topologies having 2-layers and 3-layers of rotor magnets arranged in U-shape and enabled with the six different magnet combinations are subjected to a multi-objective design optimization. The designs are optimized to minimize the volume of RE magnets, the demagnetization risk of RE-free magnets as well as torque ripple while achieving performance comparable to that of the Chevy-Bolt motor, taken here as the baseline design. Additionally, the minimization of the mechanical stress developed in the rotor laminations are also considered. The evaluation of various performance parameters along with approximate cost assessment based on magnet mass for the best designs of each magnet combinations are obtained for both rotor topologies. A quantitative comparison is also presented to highlight the various tradeoffs and suggest the best topology for further investigation.