Researchers from the National Renewable Energy Laboratory and Oak Ridge National Laboratory investigated the anisotropic—or directionally dependent—thermal conductivity of packed copper wire for electric-drive vehicle (EDV) motor applications.

Improved thermal management of windings in electric motors leads to enhanced operational efficiency and reduced component footprint, allowing manufacturers to be better equipped to meet consumer demands for high-performance, reliable and long-lasting EDVs.

Evaluation of packed copper wire windings used in vehicle applications revealed anisotropic properties, with Analysis of the anisotropic thermal conductivity of packed copper used in electric motors is helping to improve the performance and reliability of electric-drive vehicles. Image credit: Kevin Bennion, NRELAnalysis of the anisotropic thermal conductivity of packed copper used in electric motors is helping to improve the performance and reliability of electric-drive vehicles. Image credit: Kevin Bennion, NRELdistinctive differences between parallel and perpendicular measurements. The thermal conductivity proved to be over two orders of magnitude higher in the direction parallel to the wires than in the perpendicular direction, for a wire packing efficiency of approximately 50 percent.

The researchers then tested 670- and 925-μm-diameter varnish-impregnated copper wire specimens with an insulation coating thickness of 37 μm. The interstices were filled with a conventional varnish material and also contained some remnant porosity. The apparent thermal conductivity perpendicular to the wire axis was about 0.5–1 W/mK, whereas it was more than 200 W/mK in the parallel direction.

The thermal conductivity of the wire was measured using laser flash, transient plane source and transmittance characterization methods both parallel and perpendicular to the axis. A measurement of apparent thermal conductivity (κ_app) was used to factor in not just the bulk thermal conductivity (κ) but also the interfacial thermal resistances, which can lower the apparent thermal conductivity

The test methods confirmed that the k_app of the packed copper wire was significantly higher in the direction parallel to the wires than in the perpendicular direction. Low values in the perpendicular direction indicated the copper wires were isolated and did not significantly affect heat conduction in this direction. However, heat conduction parallel to the copper wires showed the wire had a significant impact, supporting the expectation that increasing the thermal conductivity of both the wire-insulating coating material and the material in the interstices can significantly increase the apparent thermal conductivity perpendicular to the wire orientation in packs of aligned copper wire.