ISSUE 011 Autumn 2021 Candela C-7 hydrofoil speedboat dossier l In conversation: Robert Hoevers l Battery recycling focus l Vehicle dynamics insight l ZeroAvia hydrogen-electric aircraft digest l Motor materials

Christoph Lomoschitz, global product manager for energy solutions at coatings supplier Axalta. “We now see effects from high-voltage generators happening in the powertrain motors, such as increased heat dissipation and partial discharge effects, and this impacts on the materials that have to withstand high temperatures for long periods of time,” he says. “The intrinsic resistance of copper windings rises with temperature, so there is a demand to keep the temperature low, but the more current that flows through the wires, the higher the temperature,” Lomoschitz says. “We can overcome that by lowering the thermal resistance, reducing the thermal flux barrier through the stator core into the cooling loop to keep distances low or integrate the cooling loop into the stator core. We also need a lamination coating for a compact seal of the core so that the cooling liquid does not leak.” Axalta’s insulating materials are based on a polymer resin with a ceramic filler. This is to increase the thermal capability as well as find a way for the material to flow evenly without creating voids or air pockets. The filler has a density of more than 50% of ceramic particles to ensure that the particles connect to transfer the heat. “We impregnate motor windings with a resin that has 2-3x higher thermal conductivity by doping the material with inorganic fillers,” Lomoschitz says. “That creates viscosity abnormalities or flow problems, and you can end up with air gaps. “This is the worst thing, as air is an ideal heat insulator. Our material is designed to be applied as a standard resin, so it flows well and doesn’t need special equipment or specific application parameters.” The choice of the viscosity of materials such as Axalta’s Voltertex 4224 depends on the motor design and the gap width between the wires or the wire to stator. The thinner the gap, the lower the viscosity of the material should be, but this also depends on the impregnating process. Some processes apply certain forces to move the resin. For example, a trickling process requires a low viscosity material, as gravity and capillary action is used; with a dip and bake, where there is air pressure, or with vacuum impregnation, the viscosity is less important. The thermal setting time of the resin is also a factor, as it determines how much time is available to get the material into the gaps. To reduce the cost of the assembly, this has to be fast, implying a lower viscosity and lower thermal performance. The challenge is to develop a material with high thermal conductivity that fills small gaps quickly and cures in 30 minutes rather than 90. The coatings and adhesives used in the stator are usually waterborne and heat-cured, and are there to insulate the sheets to reduce eddy currents. Using an adhesive avoids the need for interlocks or welding rods to hold the stator lamination layers together. The coating eliminates the air gap in the laminations for smoother distribution of heat, and can seal the stator core and give it a certain resistivity and protect against leakage. This approach allows holes to be drilled into the stator that can act as cooling channels. The coatings are applied to the stator steel in a steel mill, and brought to a dry but active state. The stator steel is shipped to a company that punches out the laminations and builds the stator stack, bonding the laminations with an oven cure or induction cure. The key factor is to keep the adhesion at a continuous high temperature, of between 150 and 180 C, with a peak temperature of 230-250 C. Wire enamel is applied to magnet wires to avoid damage from partial discharges. The higher voltages with shorter rise times lead to higher peak-to-peak voltages and more partial discharges. A standard enamel coating would last approximately 30 minutes before the discharges broke through to the copper underneath and damaged the rotor. The enamel has to last 1000 hours at the same coating thickness of 100 microns as used by the standard material. To a large extent, the partial discharges are generated by air voids in the slots of the rotor, so an effective process of impregnating the rotor E-motor stators are built using laminations of a specialist steel (Courtesy of Carpenter Technology) Autumn 2021 | E-Mobility Engineering 61 Deep insight | E-motor materials