Impact of Conductive Airgap Sleeve on Parasitic Capacitances and Bearing Current Risk in a High-Specific-Power Hairpin-Winding Traction Machine

Abstract

The pursuit of higher specific power in electric vehicle traction machines drives the adoption of advanced design solutions, including direct liquid cooling via hollow hairpin conductors and elevated operational speeds. One emerging concept involves placing a thin sleeve in the airgap, for example, to arrange direct oil cooling of the stator winding. Often this sleeve is manufactured from carbon fibre composite having non-zero, yet moderate, electric conductivity. However, the electrostatic implications of such a conductive sleeve – specifically its influence on parasitic capacitances and the associated risk of inverter-induced non-circulating (EDM) bearing currents – remain unexplored. This paper investigates the impact of an airgap sleeve on the parasitic capacitance distribution and bearing voltage ratio (BVR) in a high-specific-power permanent magnet synchronous machine (PMSM) equipped with a direct-oil-cooled hollow hairpin winding. Using 2D finite element analysis (FEA), the key parasitic capacitances are computed for three configurations: the machine with no sleeve, with a non-conductive sleeve (having relative permittivity $\varepsilon_{\mathrm{r}}=3$), and with a floating electrically conductive sleeve inserted in the airgap. The bearing voltage ratio, which quantifies the fraction of common-mode voltage mirrored across the bearings, is calculated for all cases. The results reveal how the presence of the sleeve alters the electrostatic field distribution and modifies the machine’s capacitive voltage divider parasitic structure, thereby affecting the EDM bearing current risk.