Electric Flytrain EFT-Hybrid-1x | Dossier and oil system, with an additional stator-mounted temperature sensor for the alternator and two battery temperature sensors. The engine also integrates sensors for lambda, fuel flow and fuel level. Alternator The alternator is supplied by Acutronic Power Systems, and provides both the electric current for charging the battery and a starter functionality for the engine. It is a derivative of the company’s S676-800U-02. As Kevin Peryea, chief engineer at Acutronic, explains, “The S676-800U-02 is a 203 mm-diameter starter/alternator, it’s designed with 18 slots in the stator, 22 magnets in the rotor, and uses a lot of the materials used elsewhere in our product line. However, we have improved on the typical silicon steel laminations by moving to a higher performance, higher quality and thinner silicon steel lamination, as EFT are running it at high continuous revs to achieve the Hybrid1x’s high output level.” He adds that the permanent magnets used in the Hybrid-1x’s alternator have remained the same as in the standard version: a hightemperature neodymium, specifically the N45SH(T) grade. An aircraft-grade aluminium, 7075-T6, is also used in the construction of the rotor – again, standard for this alternator and across most of Acutronic’s alternators. “When building the alternator we use a commercial, high-temperature electrical insulation system rated for heat levels up to 200 oC, which makes it safe for the electric machine to run up against the IC engine and also keeps its own heat from spreading to other systems,” Peryea says. “We design our alternators to be as efficient as possible but there are always some heat losses. “Because we pull upwards of 8 kW out of the alternator continuously at about 50 V, the amperage is really high, so we hand-wind the copper wires in our stators to maximise the fill ratio. We worked with EFT to adjust the winding scheme to match their targeted operating performance, and of course everything else about the alternator and PMU must also be matched to the engine’s performance, or else we risk overloading the engine during start-up or failing to generate electricity efficiently.” The hand-winding enables small modifications to parameters such as the voltage constant, with three iterations of the alternator having been produced over the course of r&d. That is influenced by the data EFT accumulated from its motor test benches, not only on the threephase output from the alternator but also on the DC output from the PMU. Such testing investigated the alternator’s torque at low rpm to optimise starting behaviour, measured its electrical output at peak shaft horsepower to ensure the choice of alternator was neither too small nor too big and heavy, and checked that the kV rating – the correlation between rpm and three-phase output voltage – was paired with the operating voltage for the intended vehicle platforms. “And through CAD, we were able to play with things like the alternator’s diameter, whether it should be an inrunner or outrunner, and whether the magnets were surface-mounted or interior-slotted to the rotor, all of which were critical to laying the foundations of how our powertrain would operate,” Kahnert comments. “Ours eventually became a system where the stator is in the centre and the rotor wraps around it, as that makes for better torque than the alternative. The windings were key to controlling the relationship between voltage outputs and rpm levels, both in starting mode and electricity generation mode, as that really comes down to the thickness and number of turns of the stator wire. You can have thicker wire and fewer turns to get lower voltage and higher current, or use thinner wire with more turns to increase voltage and reduce current.” With each iteration of the alternator delivered by Acutronic as per EFT’s requests, EFT also worked with independent testing facilities to gain detailed evaluations of power curves and efficiency ratings at different voltages, in order to inform the next iteration. 87 Uncrewed Systems Technology | June/July 2023 An open view of the cooling housing that encases the alternator and cylinder heads
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