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92 Focus | Electric motors deserts or rapid shore-to-ship logistics, which also demand higher ingress protection such as IP67, which is not possible with outrunners. In addition to better agility, robustness and heat dissipation, inrunners also typically offer smoother torque output and longer lifetimes. And while that has historically come with higher weights and prices, manufacturers have worked hard to overcome these drawbacks. For instance, designing inrunner stators with a higher-than-traditional pole count intrinsically raises their torque density (ratio of torque to volume). It also reduces the amount of iron needed relative to copper, and by extension their overall weight. Also, experimentation (followed by validation testing) with different metals and designs has led to some manufacturers producing their motors with higher percentages of gold and aluminium in their plugs, or making shafts hollow or from titanium, in the name of weight reduction as well as improved heat dissipation. Furthermore, while some inrunners have needed a gearbox to translate their higher speeds into higher torques for propulsion, increasing pole counts and hence torque density enables their speed to be lower for a given power output, eliminating the need for gearboxes in most applications. Another important manufacturing consideration concerns whether the windings in electric motors are better wound by hand (with the copper wires tied around the stator teeth manually by human workers) or by using automated winding machines. While it is easy to assume the future is automated, many e-motor manufacturers in the industry continue to hand-wind, despite having the wherewithal to invest in automated production. Hand-winding grants production flexibility, allowing engineers to adjust motor designs to the specific needs of a client or application on an ad hoc basis. The tactile nature of hand-winding inherently lends itself to different copper packing densities, targets for which can be adjusted at a moment’s notice rather than having to go through a computer interface and wait for machines to spool up or down. Copper densities are also typically higher in hand-wound motors, given that human hands and eyes can more easily sense how best to force more lengths of wire into slots of teeth. This is critical for the short- and long- term effectiveness of e-motors. The more copper in a motor, the more current can be run through it without mounting thermal losses, and output torque is generally proportional to current. Hand- winding also typically results in shorter winding heads (the crossover sections at the smaller sides of the windings), which reduces inner electrical resistivities and improves heat transfer. Conversely, radial flux motors continue to be rather more commonly used than axial flux motors, but not dramatically so. But in addition to these two geometries, some newer approaches to configuring magnetic flux are rapidly growing in uptake and commercial availability. The most prominent of these is the circumferential flux motor, which features copper windings that are wrapped in a ‘C’ shape around circular permanent magnets. The coils encase about 300 º of the magnets’ circumference; by doing so, they cover a denser region of magnetic flux than conventional geometries, and hence produce more force per volume. As such, these motors do not need the tiny air gaps of radial or axial flux motors – rather than 0.1 to 0.5 mm typical of those designs, an air gap of 1 to 3 mm can be sufficient. That means circumferential flux motors can tolerate the ingress of particles such as sand and dust more easily than conventional arrangements, as the particles are less likely to cause a jam between stator and rotor. It also means they can be manufactured to coarser tolerances, potentially lowering production costs per unit quite significantly, and they can be made from plastics or composites in place of iron laminations, reducing weight as well as inertia, which could also improve response efficiency. As a result, tests of these motors suggest they could be up to 17% February/March 2022 | Unmanned Systems Technology Experiments with different kinds of wires, voltages, currents and more are revealing new ways to improve torque and power efficiency (Courtesy of ePropelled)