Thermal Conductivity Analysis of Electrical Insulation Systems in Hairpin Technology With Taped Profiled Litz Wire and Flat Enamelled Wire

Electric traction motors face new challenges for innovative concepts to reduce energy consumption of battery electric vehicles. A trend of high-speed motors for increased power density comes with disadvantage of AC losses for solid conductors. One pioneer technology to improve AC resistance at higher speeds in electric motors (EM) is HF litz wire. Concepts of profiled and taped litz wire as substitutes for flat enamelled wire in hairpin technologies like continuous wave winding are present in research and face challenges of different properties in stator production. Litz wire is a complex construct of enamelled wire and insulation materials with infinite variety. Thus, its properties and behaviour for winding technologies vary from solid wire, especially the thermal behaviour. Designers of EM are missing thermal conductivity of litz wire constructs to simulate and compare litz wire motor concepts. For electrical stator insulation systems, compatibilities of different litz wire insulations to secondary insulation systems are unknown. This paper analyses thermal behaviour of litz wire versus solid wire and the dependency of design parameters like single wire diameter, bundle design and taping material. Furthermore, it evaluates innovative impregnation systems in terms of transition resistances in-between primary and secondary insulation. Therefore, it uses a developed test setup with tracing of thermal flux during heating of specimen by electric current to carry out a field study of different insulation systems. In parallel, the paper studies and compares thermal conductivities of different conductor types with a calorimeter.