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96 Focus | Electric motors more conventional methods for series production. Another manufacturer meanwhile is operating a proprietary automated system that weaves thick ‘mats’ of copper wire, which can then be impregnated with epoxy and cured into the stator’s cylindrical shape. The epoxy holds the coils in place as securely (if not more so) as metallic teeth, and the system can be swiftly customised to achieve a wide array of thicknesses, pole counts and patterns of copper, thereby setting the voltage constants, power densities and other performance specifications of the resultant motor in the factory. That means they can be tailored to exact application parameters without needing masses of production machinery or recalibrating them for customisations. Also, previous approaches to making iron-less stators have been poor when it came to inserting large volumes of copper. They have generally achieved little more than 20-30% copper density, which has made them unsuitable for high-torque applications such as propulsion and mobility. However, this new proprietary approach can achieve copper densities of 50-60%, effectively doubling the torque-to-mass ratios of past attempts and resulting in fill factors suitable for a range of unmanned vehicles. As mentioned, some manufacturers have emphasised that, regardless of core type, only hand-wound stators can achieve higher copper volumes, despite that needing more man-hours per unit and hence higher cost. An automated means of achieving high copper fill factors, with an iron-less stator to boot, could therefore be a game-changer for unmanned vehicle OEMs in terms of cost, agility, payload capacity and reliability. Encasing the coils in some kind of resin is similar to the potting often carried out in EV motors. Potting provides very high protection of the windings against damage but has historically been rare in UAV-grade products owing to the extra weight the material adds to stators. A tangible shift is now occurring though, with some e-motor companies in the unmanned space having found through tests that the longevity and reliability benefits gained from potting (and some other anti-corrosion techniques) can outweigh the increased mass, especially now that UAVs are expected to start running daily logistics and inspection tasks over potentially populated areas. Potting can even increase overall thrust efficiency (shaft output power or payload-carrying capacity relative to total electrical input power) at higher speeds if the windings are impregnated with a thermally conductive resin to draw heat out to a broader and inherently more ‘coolable’ surface area than wire insulation. It is therefore crucial to keep in mind that, nowadays, weight does not in fact tell the whole story when it comes to carrying more with less. Rotors Among unmanned vehicle markets and suppliers, there is still something of a split between rotors designed with surface-mounted permanent magnets (SPMs) and those with interior permanent magnets (IPMs), although the pros and cons of each are largely agreed upon. SPM rotors tend to be lighter and less prone to magnetic short-circuits, but the adhesives used to bond them to a rotor’s circumference must be strong enough to withstand the centrifugal forces imparted on the magnets during high-speed rotation. In general though, that limits their maximum safe mechanical speed relative to IPMs. Also, applying the bonding medium and the magnets atop them in a uniform way is complex, whether performed manually or by automated machinery, compared with IPM rotors. An IPM rotor must be machined with very fine tolerances in order for its magnets to fit in its slots without vibrations, but once the magnets are installed, they are extremely secure and well-protected against any kind of damage or failure. If the machining can be automated, IPM rotors also lend themselves very well to large batch production – far more so than SPM rotors, although the latter can be a more sensible design choice for prototyping or short production runs. In other developments concerning magnet arrangements, more manufacturers are turning their efforts towards producing Halbach array rotors. These have their permanent magnets February/March 2022 | Unmanned Systems Technology The modern emphasis on supply chain resilience is motivating e-motor companies to seek new sources of – or alternatives to – rare earth magnets (Courtesy of ePropelled)