Influence of Wire Tolerances on Hairpin Shaping Processes

Abstract

Due to the increasing sales of electric vehicles, new production technologies must be developed for meeting the growing demand for high productivity and quality in electric drives manufacturing. In comparison to conventional winding technologies, the hairpin technology provides significant advantages regarding the ability for automation, the productivity as well as the attainable filling factors, but also exhibits technological weaknesses concerning the process reliability. Since the shaping of hairpin coils represents the initial core process of the winding production by hairpin technology, geometric tolerances of the hairpin contour after shaping have a major impact on the downstream manufacturing processes. Especially the insertion of the coils into the stator slots as well as the twisting and contacting of the open coil sides are highly affected by variations in the positioning and orientation of the hairpin legs. Besides the process-based deviations caused by vibrations and tool displacements, the tolerances of hairpin contours are frequently induced by fluctuations of the geometric and material properties of the wire - like the dimensions and radii as well as the Young's modulus and flow curve. In this paper, a holistic analysis of geometric and material tolerances of flat wires and their effect on the accuracy of hairpin shaping processes is shown. For this purpose, essential material properties of flat winding wires are characterized by means of tensile tests at first and subsequently used as boundary conditions for a numerical sensitivity analysis of a tool-bound hairpin shaping process. The FE-based analyses are carried out in Abaqus FEA using a fully parametrized simulation model that is validated by means of CT measurements. The derived knowledge about interdependencies between fluctuations of wire properties and the reliability of hairpin shaping processes enables the cost-effective definition of wire tolerances in compliance with quality specifications.