Design and Analysis of a PMSM with Direct Oil-Cooled Hollow Hairpin Windings

Abstract

The demand for high specific power and material efficiency in electrified transportation requires electric machines capable of sustaining high continuous current densities. However, conventional thermal management techniques are severely bottlenecked by the thermal resistance of winding insulation and long heat transfer paths, limiting the extraction of DC and high-frequency AC losses generated in standard solid rectangular hairpin conductors. This paper presents comprehensive design and multiphysics analysis of a novel Permanent Magnet Synchronous Machine (PMSM) utilizing direct oil-cooled hollow hairpin windings. By extruding a continuous hollow channel directly through the copper conductors, dielectric cooling fluid can be circulated within the active heat source, virtually eliminating thermal barriers. While the hollow channel reduces the effective copper area and increases localized losses, this design trade-off unlocks massive thermal extraction capabilities. Electromagnetic and thermal Finite Element Analysis (FEA) validate the proposed 300 kW traction motor design. The results demonstrate that the machine can operate continuously at a high nominal current density of $18 \mathrm{~A} / \mathrm{mm}^{2}$. Despite high winding losses, the direct in-conductor cooling system stabilizes the maximum stator temperature at 172°C. While this remains within Class 180 (H) thermal limits, the narrow margin necessitates rigorous thermal monitoring to prevent accelerated insulation aging.