Review of Coreless Axial Flux Permanent Magnet Machines with Applications Including Electric Aircraft Propulsion

Axial flux permanent magnet (AFPM) machines with entirely or stator coreless configurations offer significant advantages, including low rotational losses, the absence of cogging torque, minimal torque ripple, and the potential for high specific power density. Advances in permanent magnet (PM) technology and cooling techniques have enabled fully coreless AFPM machines to achieve high magnetic and current loadings without core saturation, making them suitable for high-performance applications like electric aircraft propulsion. This paper reviews recent advancements in coreless AFPM machines, tracing their historical and chronological development. Various topologies, including single- and double-sided and multi-disk designs, are examined to identify the advantages and emerging trends in each configuration. Different winding configurations are compared, and their performance characteristics are discussed. Recent innovations in winding materials, such as printed circuit boards (PCBs), are highlighted. The rotor configurations are reviewed, with a focus on the development of flux density equations and specific attention to Halbach array PM rotor due to its relevance for electric aircraft propulsion. The paper also discusses electromagnetic modeling techniques, including proposed analytical and 2D approaches that reduce the computational burden of traditional 3D calculations. Sources of power loss are identified, along with mitigation strategies based on existing literature. A preliminary design procedure and sizing evaluation are presented, utilizing analytical torque and back electromotive force (B-EMF) calculations through an equivalent 2D linear model. Finally, recent advancements in electric machines designed for aircraft propulsion are reviewed, showcasing their electromagnetic performance and highlighting the potential benefits of coreless machines in this field.