A recently completed study by researchers from the National Renewable Energy Laboratory (NREL) and Oak Ridge National Laboratory (ORNL) of the anisotropic—i.e., directionally dependent—thermal conductivity of packed copper wire for electric-drive vehicle (EDV) motor applications is providing a baseline for the assessment of new materials and winding structures. The findings of the study are published in the ASME Journal of Thermal Science and Engineering Applications.
Improved thermal management of windings in electric motors makes it possible to maximize operational efficiency and longevity and reduce component footprint, allowing manufacturers to meet consumer demands for high-performance, reliable, and long-lasting EDVs.
Anisotropy is the quality of exhibiting properties with different values when measured along axes in different directions. As a relevant example, in 2015, a team led by a group of researchers at Berkeley Lab experimentally confirmed strong in-plane anisotropy in thermal conductivity, up to a factor of two, along the zigzag and armchair directions of single-crystal black phosphorous nanoribbons. (Earlier post.)
When the NREL/ORNL researchers evaluated packed copper wire windings used in vehicle applications, they found anisotropic properties, with distinctive differences between their 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%.
Researchers examined 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 collective results from all three test methods indicated that the κ_app of the packed copper wire was significantly higher in the direction parallel to the wires than in the perpendicular direction.
The low κ_app values in the perpendicular direction indicated the copper wires were isolated and did not significantly affect heat conduction in this direction. On the other hand, heat conduction parallel to the copper wires showed the wire had a significant impact.
This supports 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.
These test results provide a valuable baseline for comparing new materials, and for highlighting methods for examining the thermal impact of new materials for winding structures relevant to motor applications. This ongoing study will help manufacturers and researchers design high-performance, long-lasting motors for EDVs.
Wereszczak AA, Emily Cousineau JJ, Bennion K, et al. (2017) “Anisotropic Thermal Response of Packed Copper Wire.” ASME. J. Thermal Sci. Eng. Appl. 9(4):041006-041006-9. doi: 10.1115/1.4035972