Analytical Calculation and Experimental Validation of Litz Wires Axial Thermal Conductivity

Litz wires are widely used in electrical machines with reduced ac losses, compared to traditional random winding. The reduced ac losses can improve both electrical machines' electromagnetic performance, such as efficiency and thermal performance. Thermal aspect is identified as a key enabler for next-generation high power density electrical machines, where thermal modeling plays a critical role. Equivalent slot thermal conductivity is one of the most challenging parameters to be determined in the process of thermal model development, due to various components, such as copper, insulation, and impregnation resin. There are extensive literature where the litz wire thermal conductivity in the radial direction is reported. However, the thermal conductivity in the axial direction is not well studied, which is critical to determine the heat transfer, such as for electrical machines with end-winding spray-cooling system. In this article, the axial thermal conductivity is investigated with analytical calculations and experimental validation. First, nine litz wire samples are selected with various types, such as rectangular and circular shapes, with varnish process and without. Two different analytical methods are then introduced and presented to calculate the litz wire thermal conductivity in axial direction, based on equivalent length theory and equivalent medium theory, respectively. Finally, experimental tests are conducted, and results are compared to those obtained from the proposed analytical methods. Guidelines are also provided to predict the litz wire axial thermal conductivity in this field.