Development of a Production Process for Formed Litz Wire Stator Windings

In the field of automotive electric traction machines, the hairpin stator technology has become one of the established standards. This uses rigid, solid rectangular insulated copper wires instead of flexible round enameled wires. This technology is associated with higher efficiencies on the product side due to high slot fill factors and replaces rather stochastic winding processes on the process side with deterministic automated manufacturing and assembly processes. At higher-frequency operating points of electrical machines, however, rectangular copper wires are at a disadvantage compared with high frequency litz wires, because the AC losses are exceeding the DC losses. Against this background, the formed litz wire technology represents one of the next innovations in electric motor production. The starting material is a high frequency litz wire, which is profiled, compressed and afterwards formed to replace rectangular copper wires in hairpin stator production. From the changed starting material, challenges arise from the necessary adaptation of the individual production processes. This paper describes a general process chain of formed litz wire winding technology and the deriving challenges in production.