Review of High Torque Density Permanent Magnet Machines for Electric Propulsion System

This paper presents a comprehensive review of high-torque density permanent magnet (PM) machines for electric propulsion systems. Firstly, the torque expressions for various electric motors are derived and unified into a general framework to highlight key influencing parameters, providing a foundation for targeted design considerations. Each influencing factor is then analyzed in detail. Secondly, critical design parameters, e.g., materials, winding topology, and magnet configurations, etc. are explored and analyzed. Emerging materials, e.g., soft magnetic composites (SMCs), dual-phase materials, and amorphous alloys, are introduced and evaluated for their potential benefits. Besides, additive manufacturing techniques, including 3D printing, are also discussed for application in producing iron cores, magnets, and windings. Besides, special attention is given to winding arrangements and magnet designs, with several advanced configurations highlighted. Afterwards, the article explores methods to further enhance torque density through material selection, design optimization, and cooling method. In the final section, the representative applications of high-torque density PM machines are discussed: in-wheel drive, electric aircraft, and electric ship. Overall, the paper concludes with remarks on future trends and prospects for the development of the high-torque density PM machines in modern electric propulsion applications.