76 voltages from 200 to 800 V, given that the standard voltage for an aviation battery is 540 V. However, NEX Power remains aware that lower voltages are used among UAVs, but the proliferation of EVs and demands for power density are driving development of more 800 V systems. Despite integration of an inverter between the alternator and battery, NEX has still sought to closely match the latter two for the DC link voltage between them, for the sake of efficiency and expediency. Whichever battery is paired with the GenSet, the alternator must be able to charge it smoothly in a sustained manner, and the battery must also be able to start or restart the alternator without lags. Close voltage matching ensures minimal additional switching or stepping on top of converting the DC to AC, or vice versa. “When we worked with a South African customer, their battery was a maximum 290 V device, so having an alternator that could cope with that was very helpful,” Najafi adds. The alternator is provided by SycoTec, and is a radial-flux machine with permanent magnets of samarium-cobalt, being superior to most neodymium permanent magnets at performing in high temperatures. It also features an aluminium cooling case. In addition to voltage matching, additional key specifications included ensuring the rotor sleeve was designed with sufficient internal diameter to run on the GenSet’s shaft, as well as engineering it to output up to 15 kW and to handle speeds of up to 120,000 rpm. Thermal management was also a key concern for the alternator integration, with some custom work going into optimising it for the internal air cooling enabled through NEX’s integration of it upstream from the impeller. The inverter is COTS and supplied by BorgWarner, chosen primarily because its voltage was flexible to a similar degree as that of the alternator, and its compactness made it suitable for aviation integration (rather than bolting it onto the potentially hot engine housing, NEX Power keeps the inverter separate from the GenSet, allowing it to be installed wherever the UAV integrator has free space, or where they keep their battery). Naturally, the system being a bidirectional inverter was also key to ensuring that it could enable both battery charging and engine starting. Engine management As mentioned, the 15 kW GenSet is managed via an ECU, for which NEX Power is performing some final modifications. The company can also provide a FADEC system, although FADECs must be defined and based on the end-user’s platform and operations. “Our ECU approach uses a minimal sensor count because the type of inverter we’re using already reduces the number of sensors across the powertrain through sensor-less motor-generator control,” Najafi says. “For example, we don’t integrate any kind of actual, physical sensor on the shaft, but from reading the magnetic field of the alternator, we can estimate the shaft temperature and permanent magnet temperatures. That’s not a novelty of NEX Power, but of the SycoTec permanent magnet motor and the BorgWarner inverter technology, which BorgWarner refers to as its multi-level, high-speed, sensor-less control. “That, along with the bidirectional power and control capability, was another big reason we chose them. It keeps our control architecture simple.” That said, the GenSet does integrate temperature sensors on the alternator and in the gas microturbine’s exhaust manifold, enabling fine analysis of thermal deltas across the powertrain. Temperature and pressure sensors are also integrated on the diffuser, to closely gauge pressure levels between the compressor and the combustor. Certain sensors are also integrated on the current working prototypes, purely for testing and development guidance, such as vibration sensors to evaluate their vibration output. In operation, however, close integration of the inverter into the engine management enables close governance and tuning of speeds (and thus of fuel metering) to achieve specific levels of electric power generation. “Through extensive testing and calculation, we know exactly how much fuel we need to inject and how much August/September 2025 | Uncrewed Systems Technology Overall, the internals of the gas microturbine are kept simple, with the air bearing, impeller and turbine all mounting on a single common shaft
RkJQdWJsaXNoZXIy MjI2Mzk4