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94 more power-efficient than conventional e-motors of roughly equivalent size and current levels, not to mention much lighter, making them ideal for heavy-lift UAV applications for example. Regardless of all these arrangements though, one final aspect of electric motors that remains unchanged is that the bearings in them are still widely reported as their most common cause of failure. With bearing technology being quite mature, high-end manufacturers still largely opt for the same kinds of radial ball bearings as 10 years ago, sometimes with a 2Z non-contact seal or a 2RS for harsh environments. Although stainless steel or chrome- steel bearings are still the most widespread, some manufacturers are also investigating ceramic bearings in their r&d, as they could extend motor lifespans. Advancing stators In principle, the stator of an electric motor is built from layers of laminated iron or steel, which provide mechanical strength and improved torque from magnetic fields. They are also relatively easy to machine- cut to exact geometries and tolerances for a very tight air gap with the rotor. When stacked together, the layers form a circle of teeth, which are wound or otherwise filled with insulated copper windings. The windings form the motor’s electromagnetic coils, and companies have been researching ways to increase copper fill factors without damaging the insulation on the wires, given their aforementioned importance to motor torque. For instance, some r&d is looking into the benefits of using thicker versus thinner wires, while related efforts are investigating how different wires behave at higher frequencies. Higher frequencies in the alternating current can cause eddy currents and hence the ‘skin effect’ in the wires, driving up their resistance and by extension copper losses. A solution to many of the issues surrounding copper windings is to reduce the required current for electromotive force (and hence the losses associated with higher currents) by increasing the voltage. As a result, more and more electric motors are moving from 48 V and 60 V architectures towards 100-150 V, in the same way that EV manufacturers are going from 300- 400 V in their battery inputs and network architectures to 600-800 V. The laminations on the metal core layers meanwhile are critical for reducing eddy currents and hence the associated magnetic losses they cause. The constant changes in the magnetic field (which is key to the ‘push’ and ‘pull’ the stator coils exert on the rotor magnets) is resisted in a solid stator core, which causes the eddy currents. Reducing the core’s contiguous cross- sectional area by using stacked, thin layers coated with impermeable lacquers, rather than a single solid chunk of metal, February/March 2022 | Unmanned Systems Technology Circumferential flux motors feature iron-less stators, high magnetic flux density and wider air gaps than radial or axial flux motors (Courtesy of Motion Robotics) A new proprietary technique for automated weaving enables very copper-dense iron-less stators with high torque as well as reduced mass, cost and lead times (Courtesy of Alva)