Hairpin Winding Technology for Electric Traction Motors: Design, Prototyping, and Connection Rules

Compared to traditional windings, hairpin windings feature higher fill factors and increased robustness to mechanical stresses. However, due to the high cross-sectional area of the rectangular conductors, the current density is not uniform when the motor operates at high speeds. To mitigate the skin effect, parallel paths are often required. In this context, an appropriate design analysis and the formulation of suitable connection rules are mandatory. In this paper, the main design issues and the resulting construction aspects of hairpin windings are analyzed. By modeling Joule losses in the hairpin windings as function of the frequency and highlighting the main differences with traditional windings, a set of rules is presented to obtain a correct configuration. Both symmetric and asymmetric bar layouts are considered to point out the optimal configurations that mitigate the skin-effect-driven increase of current density and expose balanced impedances to the parallel paths of phase currents.