Cryogenic Thermal System for Coreless Axial Flux PM Machine with Litz Wire Winding for Electric Aircraft Propulsion

Electric machines for aviation demand ultraefficient, compact designs with high specific power density, which can be achieved through advancements in both electromagnetic design and thermal management. This paper proposes and investigates an integrated thermal management approach for a coreless stator axial flux permanent magnet (AFPM) machine featuring a double-sided Halbach array PM rotor, two stators, and an aluminum nitride (AlN) cold plate positioned between the stators for direct cooling. The cold plate incorporates internal serpentine channels for liquid hydrogen circulation. The electromagnetic performance and efficiency of the machine at cryogenic temperatures are assessed using temperature-dependent data from the literature, indicating that as the temperature drops to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-140^{\circ} \mathrm{C}$</tex>, the remanence of the permanent magnets increases, and the resistivity of the Litz wire decreases continuously. These effects collectively enable the machine to achieve up to 99% efficiency at cryogenic conditions. To evaluate the feasibility of achieving this performance, thermal analysis is conducted using an analytically derived lumped thermal resistance network and computational fluid dynamics (CFD) simulations. The results show that the required thermal environment can be realized through precise cold plate design and stator winding manufacturing with careful potting using high thermal conductivity epoxy.