Evaluation of Winding Symmetry and Circulating Currents of Hairpin Windings

Hairpin windings are widely used in automotive traction motors due to their high slot fill factor and their highly automated manufacturing process. The hairpins are arranged in layers in the stator slots, which offer additional degrees of freedom compared to conventional stranded wire windings in the design process. However, a circulating current within the parallel branches can occur, if the electromagnetic force or the effective branch resistance of each parallel branch differs. Consequently, several design rules have been formulated in the past. In this research, designs of short-pitched and full-pitched hairpin windings are presented and their excitation of circulating currents is evaluated. Time-transient finite element analyses of the current displacement effects in hairpin conductors in a single slot and in an exemplary machine are performed. It is shown that commonly applied design guidelines are not sufficient to fully prevent circulating currents. As a consequence, the definitions of weak and strong symmetry are suggested, the latter of which ensuring that circulating currents are not excited.