Litz wire losses: Effects of twisting imperfections

High-Frequency (HF) litz wires are extensively used for the windings of Medium-Frequency (MF) magnetic components in order to reduce the impact of eddy current losses that originate from skin and proximity effects. Literature documents different methods to calculate eddy current losses in HF litz wires, however, most of the computation methods rely on perfect twisting of the strands, which is often not present in practice. This paper analyzes the implications of imperfect twisting on the current distribution among the different strands of HF litz wires and the corresponding losses by means of a fast 2.5D PEEC (Partial Element Equivalent Circuit) method. The effects of different types of twisting imperfections (at the bundle-, sub-bundle-, or strand-level) are examined. It is found that imperfect twisting can lead to increased losses (more than 100 %). However, perfect twisting of the strands, which is difficult to achieve, is often not required, i.e. suboptimal twisting is sufficient. Analytical expressions are given for distinguishing between critical and uncritical imperfections. The experimental results, conducted with a 7.5kHz/65kW transformer, reveal a reduction of the error on the predicted losses from 52 % (ideal HF litz wire model) to 8 % (presented model) and, thus, confirm the accuracy improvement achieved with the proposed approach.