Measurement and Characterization of High Frequency Losses in Nonideal Litz Wires

Commercially available litz wires are fabricated in several ways, depending on the number of strands and on the cost frame, either as twisted strands, as twisted bundles of twisted strands or as braided strands. The bundles have a finite pitch length, and the permutation of strands within this length is not perfect. This paper illustrates the effects of this nonideal construction on skin and proximity losses in the winding and indicates some applications where this may become important. Test setups have been developed to measure skin losses and proximity losses separately, thus offering the possibility of quantifying the quality of real litz wires by comparing the test results with the ideal behavior.