74 Dossier | Launchpoint EPS HPS400 hybrid powertrain LaunchPoint’s ironless stator design is currently produced by initially handwinding the copper into the desired form (a key step towards customising and optimising copper fill) and then over-moulding it with a thermally conductive epoxy resin, which, once cured, is fastened to an outer, metallic aluminium ring. Unlike conventionally hand-wound stators, in which production engineers must pack wire into slotted structures in complex 3D geometries, LaunchPoint’s process largely just involves laying wire out into the desired form factor on a flat work surface. Hence, the winding processes will be easily automated via a robot with just two degrees of freedom (once production volumes hit about 1000 units per year and justify doing so). “We use a full-pitch wave winding with finely stranded litz wire, in which the individual strands are insulated to break up eddy currents by preventing them from circulating strand-to-strand. It is quite known in the art of ironless designs now that you really need to opt for litz wire in your windings,” Ricci says. While much of the specifics of the process are proprietary, LaunchPoint nods towards New England Wire Technology as its supplier of litz wire and a key source of consultation on the winding optimisation. “Beyond that, the proprietary resin about the copper is key to both heat removal and structural strength,” Ricci says. “Motor torque is directly linked to two factors – the current in the winding and the magnetic field from the rotor. The Halbach array maximises magnetic field, and the winding design maximises the stator current density. “Operating at maximum torque and maximum stator current density inevitably drives up heat in the copper, to the point that the stator runs up to 200 C. Finding a resin that could operate at such temperatures was a big challenge, but we’ve found and settled on a material that is rated to 220 C peak operating temperatures. “The resin is applied using a vacuumbased process similar to compression moulding. Once cured, it exhibits enough stiffness for surviving the vibration and torque pulses from the engine, and high thermal conductivity for the forced-air cooling to dissipate the heat extracted from the copper bundles.” SiC-powered motor control As the MOSFETS or IGBTs in a motor controller switch on and off, the highvoltage energy to the motor is being turned on and off to alternate the current up and down in a sinusoidal waveform. The gradient of the waveform relates to the motor inductance: when the transistors switch on, energy is stored in the inductance, and when they switch off, energy comes out of that inductance. In essence, motor inductance functions like piston engine flywheels, smoothing the electrical power pulses from the motor controller’s toggling switches and enabling smooth torque from the motor. “With our very low-inductance motor comes the issue of a tiny ‘flywheel’. With high energy or power output there’s a risk of creating huge current ripples. To mitigate that, we need transistors that turn on and off much faster than in the average e-motor to get smaller pulses,” Ricci explains. “A conventional motor drive turns on and off 20,000 times per second, but SiC MOSFETS can do 60,000-80,000 Hz. That makes our AC pulses three to four times smaller than conventional. It is like going from a V8 engine design to a V12 or V14 engine, just in terms of our power converter and MOSFETs.” The active system PMU contains SiC MOSFETS arranged to invert the DC input to an AC output or vice versa. A plethora of current and voltage sensors enable close feedback at high frequencies (though sensorless operation is available on request), and gate drivers with built-in desaturation protection are selected to help keep the system safe against shorting. A high-end DSP from Texas Instruments (TI) used in the PMU embeds the motor control logic; the selection of a microprocessor capable of keeping up with the SiC’s switching speeds having been vital to LaunchPoint. “We have really optimised our code to run our control loops at 50 kHz, which is a lot faster than most motorgenerator controllers, but gives us fine management of the sine waves. With our high pole counts, we need those high switching frequencies and highbandwidth, control-loop execution, June/July 2024 | Uncrewed Systems Technology The stators are first hand-wound to precision before being over-moulded in a thermally conductive epoxy, resulting in an entirely ironless component
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