January/February 2024 | E-Mobility Engineering 28 Housing matters The HPDM-250 is built with an enclosure consisting of an inner and outer section (whereas in the HPDM-30, the inner section forms the entirety of the housing). The inner section is an approximately cylindrical part, additively printed from aluminium alloy for low weight and high heat transference. It features a circular bore for housing the stator and rotor, as well as a plethora of liquid coolant channels. In the HPDM-250, a series of flat surfaces are arranged radially around the outermost part of the inner housing, upon which the inverter power electronics are mounted. In the HPDM-30, the inverter-power electronics are arranged axially off the back of the inner section. “In the -30, the inverter gets enclosed within the printed housing, but in the -250 a second, outer housing is necessary to protect the inverter boards, as they sit around the outside of the cooling channels and the structural inner housing, so the outer housing is constructed from bent sheet metal and then sealed, and it functions as both EMI [electromagnetic interference] containment and general environmental protection for the electronics inside,” says Liben. “We use direct metal laser sintering [DMLS] for the inner housing structure. Additive manufacturing has come a long way in the last few years, such that we can print multiple parts per printer simultaneously, and it isn’t overly cost or time prohibitive anymore, but we are evaluating a switch to casting the inner section when we get to large enough volumes.” While other additive manufacturing techniques were available, DMLS was deemed the most technologically mature, particularly for aluminium alloys, and the batch production of components was enabled by the larger format of these printers. Coolant designs For thermal management, water-glycol liquid coolant typically flows through channels that run helically in several turns inside the radius of the inner housing between the stator and the power-converter modules. Cold liquid enters one side of the powertrain, increases in temperature as it collects heat from the stator and inverter, and then leads through a port on the opposite side. In the HPDM-250, the coolant then flows through the gearbox housing, where it extracts heat from the gearbox oil. For a hybrid application, fuel could also be pumped through the channels to be used as coolant before being burned in an engine, which removes the requirement for (and mass of) a dedicated coolant pump and radiator. “The coolant-channel design is actually deceivingly complex, since you need to balance the convective heat transfer coefficient with the pressure drop, and also with the housing stiffness – that last one is particularly important because our power electronics are closely connected to the electric motor and its vibrations, so the geometry of the cooling channel influences the behaviour of the motor under highvibratory conditions,” Liben notes. “We could have gone with a system of several parallel axial channels from front to back with manifolds at either end, and that might have increased our total volumetric flow rate and reduced the required pumping power, thanks to the less restricted path taken by the coolant. But it would have decreased the flow velocity per channel quite significantly, and so the coolant wouldn’t be turbulent enough for a good convective heattransfer coefficient,” he points out. Lastly, the housing holds the rotor shaft in high-temperature bearings. These are hybrid bearings with metal raceways and ceramic rolling elements, which are used for their long lifespan, high speed tolerance and low temperature rise. Additionally, while some e-motors can suffer arcing through their metal ball bearings between the inner and outer bearing races (due to the build-up of common mode voltages on the rotor, which are drawn towards the chassis ground), this is prevented by H3X’s use of ceramic rolling elements, combined with the design of a grounded connection to the rotating shaft. Inverter challenges H3X’s inverters use SiC devices and have a power-conversion efficiency exceeding 99%. SiC MOSFETs are an increasingly common sight across e-mobility for their fast switching speeds, high power density and high efficiency, but using SiC in aerospace poses a bigger challenge than in road H3X’s PCB layouts are carefully designed to minimise inductance and interference between circuits
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