Analytical methods for estimating equivalent thermal conductivity in impregnated electrical windings formed using Litz wire

This paper discusses two analytical methods used in estimating the equivalent thermal conductivity of impregnated electrical windings constructed with Litz wire. Both methods are based on a double-homogenisation approach consecutively employing the individual winding conductors and wire bundles. The first method is suitable for Litz wire with round-profiled enamel-coated conductors and round-shaped bundles; whereas the second method is tailored for compacted Litz wires with conductors and/or bundles having square or rectangular profiles. The work conducted herein expands upon established methods for cylindrical conductor forms [1], and develops an equivalent lumped-parameter thermal network for rectangular forms. This network derives analytical formulae which represents the winding's equivalent thermal conductivity and directly accounts for any thermal anisotropy. The estimates of equivalent thermal conductivity from theoretical, analytical and finite element (FE) methods have been supplemented with experimental data using impregnated winding samples and are shown to have good correlation.