Comparative Conductor Design Study of Processing and Insulation Characteristics for Continuous Hairpin Stators Using Profiled High Frequency Litz Wires

Flat enamelled or extruded wire-based winding technologies – commonly known as hairpin winding technologies – for electric motors have become widely established as a standard for automotive applications. Trends of higher power densities and efficiencies in electric motors, paired with modern semiconductor technologies, result in innovative motor designs with higher switching frequencies. Past research has shown that, especially at high frequencies, rectangular enamelled wires can be disadvantageous compared to rectangular high frequency (HF) litz wires due to the alternating current (AC) losses exceeding the direct current (DC) losses. Electric motor types and designated drive cycles in a vehicle define the distribution of frequencies and thereby the share of losses. A variety of configurations exist for profiled HF litz wires based on the dimensions of the profiled litz wire, diameter of individual strands, length of lay, aspect ratio, chemical resistivity, ability for winding, bonding strength, or reinforced insulation material. Thus, HF litz wire design-rules for stator production are about the electromagnetics, insulation- and mechanical aspects. In previous research conducted, a process chain for processing of profiled high frequency litz wire in continuous hairpin winding technology was elaborated. Practical experience for processing profiled litz wires in the stator production process has been lacking. In this paper, the processability of different litz wire configurations regarding their electrical properties after the forming process steps is investigated to determine the material, process, and design limits for stators with stranded wire winding.