90 Focus | Motor controllers capacitors are still the leading cause of failures on any motor controller, most often due to imperfect installations or selections. The latter can especially occur when a distinct lack of quality control or validation testing has been carried out by the supplier, to affirm key characteristics such as capacitor ripple current (which most capacitor manufacturers don’t do). It can also happen when the capacitor has not been installed cleanly enough on the board. When these are not done, inevitably the ESCs are asked to perform beyond the capacitors’ operating window, potentially causing them to break after some tens of hours, or explode within seconds. Modern ESC architectures designed for reliability will avoid this problem by first testing their supplied capacitors where necessary, and then integrating enough capacitors inside or on the PCB such that the vehicle manufacturer does not need to hand-solder their own capacitor banks onto the boards. This helps scalability on the OEM’s end, skipping the need for laborious and potentially inconsistent or unsafe board mounting processes. Such is the inconsistency that can happen when capacitor installation is left to the uncrewed vehicle manufacturer, that safe input lead lengths are limited to 12 cm as a result. Naturally, this can be heavily restricting for any OEMs who may want a larger UAV than 12 cm cables permit, especially if using VTOL drive modules whose dimensions far exceed 12 cm (a common sight for heavy-lift applications). Controlling capacitor installation inhouse, by contrast, enables input leads of 3 m lengths, and saves UAV engineers and technicians from needing to solder anything. The resulting ESC may be slightly larger than those which ship without their capacitors integrated, but saving the endintegrator from soldering, verifying solder joints, or redesigning their UAVs’ drive architectures to accommodate the ESC and its cabling is invaluable. And though most ESC manufacturers continue to get good reliability and lifespan from using electrolytic capacitors and ceramic capacitors for their bulk capacitance, some ESCs now come with hybrid polymer capacitors, which may provide extra vibration and shock resilience over the other types. However, integrating high numbers of small ceramic capacitors greatly reduces the individual inertia (and hence shock and vibration concerns) of each, as well as spreading electrical and thermal loads broadly among them, making for an efficient design strategy. Electrolytic capacitors, by contrast, tend to remain disadvantageous in these respects even when installed and arranged creatively, and may still need to be replaced on the customer’s end, unlike their ceramic and hybrid polymer counterparts. SiC Across the EV world, silicon carbide (SiC) power transistors are becoming widespread, not only for their great efficiency in high voltage, high speed switching, but also for how much more available they have become over the past five years. Notably, however, SiC has not quite proliferated among UAV ESCs, with some manufacturers having moved away from them due to a number of reasons. For one, despite the higher quantities supplied, SiC MOSFETs remain pricey: often 10-20 times the price of high-end silicon FETs. Additionally, UAVs today rarely operate at the 800 V levels that bring out the real efficiency advantages of SiC; for 60-120 V UAVs, SiC adds a lot of cost, to not very much benefit. But the keyword there is “rarely”. Some very large motor controllers have been recently engineered and made available to suit both very large, very heavy-lifting UAVs as well as urban air taxi-type eVTOL aircraft (UAM). For these, 800 V architectures are necessitated by the hundreds of kilograms and hence hefty amounts of current (up to or exceeding 200 A per ESC) to be pulled in such applications. Motor controllers in this category are being engineered for compliance with DO-178 and DO-254 standards (which respectively detail software and hardware airworthiness requirements), a vital aid in UAV and UAM certifiability. One can also find SiC transistors within their architectures. While silicon IGBTs remain the most common choice among 800 V inverter and motor controller devices, the proliferation and improvement of SiC MOSFETs (as well as peripherals for enabling SiC-based bridge circuits) has furthered SiC’s efficiency advantages. Rising commercial availability has also made SiC less expensive to acquire, reducing the cost barrier of switching from IGBTs to SiC. Size optimisation of 800 V motor controllers typically requires multiple boards inside such devices – at the very least, one board for hosting the power management components and another for the control and intelligence components – which can be stacked atop one another to prevent a cumbersomely wide solution. However, between SiC MOSFETs and IGBTs of equivalent power throughput, one often finds the SiC to be the smaller of the two. That, combined with SiC’s much greater thermal efficiency (reducing the size and weight of any cooling mediums they may need) means that SiC inherently makes for smaller and lighter motor controllers. One can expect to see a significant re-growth of SiC-based devices April/May 2025 | Uncrewed Systems Technology SiC MOSFETs make particular sense as 800 V powertrains proliferate among heavy-lifting UAVs and autonomous air taxis (Image courtesy of Embention)
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