Simplified design method for litz wire

Abstract

A simplified approach to choosing number and diameter of strands in litz wire is presented. Compared to previous analyses, the method is easier to use. The parameters needed are only the skin depth at the frequency of operation, the number of turns, the breadth of the core window, and a constant from a table provided in the paper. In addition, guidance is provided on litz wire construction-how many strands or sub-bundles to combine at each twisting operation. The maximum number of strands to combine in the first twisting operation is given by a simple formula requiring only the skin depth and strand diameter. Different constructions are compared experimentally.

Litz wire losses: Effects of twisting imperfections

“…This ideal construction is difficult to realize with a large number of strands (more than six). and is commonly found in commercially available HF litz wires [2], [6], [12]. These twisting schemes ("PT" and "BW") feature correct twisting in azimuthal direction since imperfect azimuthal twisting is uncommon for HF litz wires.…”

“…Many different twisting schemes exist for HF litz wires [6], [14], [18], featuring different bundle structures, radial and/or azimuthal permutations, etc. However, most litz wires are constructed with the following twisting schemes:…”

“…The most common methods are listed below: and the losses due to proximity effects (internal and external) are modeled with a low-frequency asymptotic approximation. The packing pattern is only modeled with the fill factor [2], [6], [10].…”

“…Over the years, different calculation methods have been proposed for the calculation of the losses, which rely on analytical and/or numerical calculations [6]- [11]. However, it is found that the measured losses are often larger than the values computed with the aforementioned methods [1], [2], [12].…”

“…1. Each single strand is subject to skin and proximity effects and thus, the single strand diameter is selected with regard to acceptable losses [6]. The strands are twisted together in both radial and azimuthal directions: • The permutation in radial direction prevents eddy currents in case of an azimuthal magnetic field.…”