larger systems for marine, industrial and defence markets, with the long-term focus being the hybrid and fully electric aircraft market as it grows.” The HPDM-1500 and larger multisector motors do not use a gearbox, because shaft speed drops inversely, proportional to the number of sectors. Thus, while the HPDM-250 has a shaft speed of 20,000 rpm, the HPDM-1500 has 2500 rpm. This is because the rotor-tip speed is maintained across platforms, so as the rotor diameter increases, rpm decreases. The torque increases with the square of the number of sectors; hence the gearbox becomes unnecessary for the biggest motors, enabling direct drive of the propellers. “Using similar pole and winding designs across all our motors, [plus] similar inverter topologies, similar thermal management – it’s all key to how we’ve created this modular and scalable core technology platform, which forms the building blocks of increasingly large, but no less powerdense electric powertrains,” Liben says. Stator winding insulation Of the various subsystem-level innovations that are common among the motors, the first one that H3X chose to discuss with us was its HV stator winding insulation – a critical solution to the challenge of maximising continuous power output against high operating temperatures, which the company believes is arguably the biggest bottleneck in electric powertrain performance. H3X’s solution, which it invented, is a proprietary composite material, combining multiple existing ceramic- and polymer-based insulating materials – the exact formula is closely guarded. Traditional e-motor manufacturers often sidestep the thermal bottleneck by running an active and direct cooling medium in contact with the stator windings. This involves either flooding the stator with oil or designing in-slot cooling channels that enable waterglycol to run immediately adjacent to the conductors. Each strategy mandates dedicating copious space in the stator to components that are not conducting current. “Just pumping coolant at higher flow rates won’t solve it, because the temperature rise from the coolant to the winding hotspot, due to the HV insulation, is a much larger fraction of the total temperature rise than anything attributable to the convective heat transfer between the cooling liquid and the housing,” Liben says. “We’re talking maybe a 10 C rise from the liquid to the housing, whereas you can develop hundreds of degrees celsius temperature difference due to the winding insulation.” This is why H3X developed the most thermally conductive stator winding it could, minimising the thermal bottleneck and creating more room in the stator for the copper conductor. The result is a bulk-winding thermal conductivity of 8.7 W/m-K and a thermally continuous current density of 47 A/mm2, with additional improvements in the pipeline. For reference, H3X comparatively baselines current state-of-the-art high-performance motors at 1.5 W/m-K and 25 A/mm2. Thermally conductive “Ceramics are exceptionally thermally conductive for non-metallic materials, while still being very electrically insulative. They don’t quite have the electrical insulation performance of polymers, but they do take the edge in thermal conductivity,” Liben says. “We’ve incorporated several different classes of ceramics and polymers into our insulation system in many different form factors. Although we’ve not used anything radical, it’s still a very different insulation approach to anything else we’ve seen out there.” In addition, the stator is fully encapsulated to prevent partial discharge. This refers to the occurrence of minor arcing across small voids in winding insulation, which contributes to winding degradation over time. This was an important quality tracked by H3X while developing its insulation as it is a safety critical consideration for flight. Overall, the high thermal conductivity January/February 2024 | E-Mobility Engineering The HPDM-1500 consists of eight independent sectors of rotor and stator; one may think of it as eight HPDM-250s unrolled and joined into one circle Up to six HPDM-1500s can be mounted on a common shaft, which would result in a 9 MW electric motor 24
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