Launchpoint EPS HPS400 hybrid powertrain | Dossier narrow enough, and they’re made with iron and steel that drives up the inductance and prevent use of high pole counts,” Ricci explains. “With six-phase, you get twice as many current pulses in your sinusoidal waveform output, so the valleys between are smaller, and you get much smoother, cleaner power output and better voltage regulation. In legacy aircraft with three-phase buses, electric engineers would install a big, heavy transformer to artificially shift three of the phases to make a six-phase output, with really large, powerful, 12-pulse rectifiers called ‘transformer rectifier units’ [TRUs] to handle the voltage and current ripple. “We get the advantage of 12-pulse rectification, without needing a transformer, largely thanks to our unique, ironless design. Six-phase does entail twice as many measurements as threephase, and current and voltage sensors don’t come for free, but once you’ve designed for it in your electric machine, it is not any harder to engineer or manufacture than our three-phase configurations.” Dual Halbach array rotor A radial-flux Halbach motor with an outrunner (internal stator, external rotor) design inherently self-confines all of its magnetic flux within the bore, making it an increasingly popular motor configuration. Given the axial-flux nature of LaunchPoint’s motor design, however, combined with the absence of iron in its stator, a dual-rotor design was necessary to confine the flux within the space occupied by the stator (and a single rotor design would have meant that the stator windings only received around half the magnetic field from the permanent magnets). Neodymium iron boron permanent magnets are selected to maximise the magnetic strength and hence power efficiency of the rotors, including grades such as ND48, ND45 and ND52. LaunchPoint can also work with hightemperature neodymium magnets or even samarium-cobalt magnets for applications where an extremely hot environment will tax the motor’s self-cooling, and DoDapproved (DFARS compliant) magnets if the mission requires it and the ensuing price can be covered. “Whichever magnet we’re using, the manufacturer cuts them to our specified shapes; then we bond them together in the aforementioned groups of magnets for the Halbach array, and then we epoxy them into our rotor plates,” Ricci says. “Those plates are machine-cut from titanium, including the pockets that the magnets insert into, but we don’t machine all the way through the plate. We leave a ‘floor’, which faces the air gap, and means the magnets can’t come unbonded, fly into the gap and jam the motor. That’s a huge reliability aid: even if the epoxy somehow dissolved, some of the magnets might slide away from the air gap a little, meaning a tiny loss of power, but the motor would keep working, with no critical loss of power mid-flight.” In some applications, LaunchPoint will also cut thin, titanium discs and weld them onto the outer face of each rotor, hermetically sealing the magnets against coming loose or into contact with anything that could harm them or the epoxy. Titanium has been chosen for high strength-to-weight and fatigue properties. “When a cylinder fires in a piston engine there is an incredible momentary torque pulse,” Ricci says. “The peak torque can be 15 times the average torque in a single cylinder engine – so if you design an electric motorgenerator for 10 Nm of average engine torque, you need it to withstand peaks of 150 Nm when attached to a piston engine. An aluminium rotor structure would fatigue and break off after a while; titanium wins out there.” Ironless stator Historically, ironless stators have been costly to manufacture, and challenging to produce and customise with copper fill factors above 20%, but their advantages, such as high weight efficiency, minimised thermal and eddy current losses, and smoother and quieter power output with no core iron-derived cogging have driven consistent interest in ironless designs, particularly for highspeed applications. 73 Uncrewed Systems Technology | June/July 2024 The rotor’s magnets (typically NdFeB) are slotted into machine-cut titanium ‘pockets’ that prevent them coming loose in the air gap
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