A Method for Accurate Measurement of Anisotropic Thermal Conductivity of Impregnated Electrical Windings

This paper presents an experimental methodology developed to accurately measure the thermal conductivity of composite materials like impregnated electrical windings. The proposed approach utilises a custom-built heat flow metering system to analyse cuboidal material samples, which allow for the material thermal anisotropy to be derived. Validation and accuracy assessment are conducted by comparing the proposed method with the commercially available transient plane source method. The paper comprehensively discusses both the theoretical foundations and experimental results obtained from tests on several material samples, highlighting the efficiency of the proposed technique. Furthermore, the work includes a comparative analysis of various resins and conductor types, suggesting that the rectangular profile conductors with Epoxylite resin impregnation exhibits superior thermal conductivity both in transverse and longitudinal directions. The experimental findings have shown that the proposed method allows for more accurate measurements. For materials with high thermal conductivities (e.g., 180 W/m.K), a measurement deviation as low as 1.81% is achieved, while materials with low thermal conductivity (e.g., 0.2 W/m.K) are more susceptible to an increased method error, as theoretically shown through numerical simulations. Additionally, the study illustrates an impact of accuracy of the thermal conductivity data on the temperature predictions within an electrical machine demonstrator. The case study theoretical results revealed an increased thermal resistance between the winding and housing of up to 17%. The effect is particularly prominent when considering more conventional electrical insulation systems with poorer thermal properties.