A Review of High Torque Density Permanent Magnet Machines for Distributed Electric Propulsion

This paper presents a comprehensive review of high-torque-density permanent magnet (PM) machines for distributed electric propulsion (DEP) systems. First, the torque expressions of PM machines are derived and unified into a general analytical framework to identify the key influencing parameters that guide targeted design optimization. Critical design aspects, including material selection, winding topologies, and magnet configurations, are systematically analyzed. Emerging magnetic materials, such as soft magnetic composites (SMCs), dual-phase alloy materials, and amorphous materials, are all evaluated for their potential to enhance torque density and overall PM machine performance. In addition, advanced winding arrangements and innovative magnet configurations are discussed to achieve a superior output torque. Besides, torque enhancement strategies through multidisciplinary optimization and main material innovation are also summarized. Finally, fault-tolerant design approaches for high-torque-density PM machines in DEP systems, i.e., electric aircraft, vehicles, and ships, are critically reviewed. Overall, this paper aims to provide a unified perspective and highlight future research directions in the development of advanced high-torque-density PM machines.