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98 placed with their magnetic fields pointing in a left-up right-down pattern, which augments the magnetic field very strongly on one side of the array while cancelling it to almost zero on the other. Halbach arrays do not necessarily improve flux density in an e-motor constructed largely of iron – in fact, empirical data suggests that flux density is the same whether the rotor and stator are iron or a non-ferromagnetic material. That means manufacturers of motors that forgo iron cores can use Halbach array rotors to increase torque density while keeping weight low. Indeed, some e-motors now feature rotors made from aluminium or carbon composite as a result of continued advances away from iron or steel stator and rotor architectures. Whatever the rotor configuration, by and large the most popular electric motors still use permanent magnets based on rare earth elements, specifically neodymium and samarium-cobalt (with mixtures of other elements such as terbium and dysprosium to achieve differing qualities across a range of compound grades). However, rocketing demand for rare earth magnets, combined with concerns over supply chain resilience, has prompted many vehicle OEMs and e-motor producers to look towards alternatives. Obvious choices include greater use of ferrite magnets (compounds of iron oxide and various other elements) and alnico magnets (typically from aluminium, nickel and cobalt among other metals), both of which are being experimented with by many manufacturers to see how more power and torque density can be derived from them. Also, some are casting their gaze wider for supplies of rare earths, with potential sources in Africa, Japan, Australia and the US starting to yield results that could improve magnet supplies and prices in the years ahead. And in very recent years, a new technique aimed at getting more value from limited neodymium supplies has been used in electric motor/ generators for UAVs as well as EVs. This uses permanent magnets made from powdered neodymium and other magnetic materials that have been bonded and hardened within epoxy resin. As the magnetic materials are suspended in resin, they can be injection moulded into their rotors as desired. As well as bonding them very effectively onto or into their rotors, this enables a motor manufacturer to look beyond the conventional bar- or slot-shaped magnet, as well as the circular magnet of circumferential flux motors. Instead, some motors designed using this technique feature curved layers of magnetic material, simulated in CAD software and proven with real-world results to concentrate their magnetic flux in a way that gets more reluctance torque out of their rotors. An e-axle powered by a motor constructed in this way has achieved speeds of 34,000 rpm at high power, making it possibly the fastest in the world. This high speed and power raises the possibility that combining downsized versions of these motors with a gearbox could achieve the power and torque values of much larger conventional motors, while also being much lighter, less costly to produce, and less at risk from February/March 2022 | Unmanned Systems Technology Powdered magnetic material suspended in epoxy can be injection-moulded into rotors, resulting in extremely fast and recyclable motors (Courtesy of Aichi Steel) One interesting new innovation uses air scoops designed into the motor housing to pull more cooling air into the stator (Courtesy of ePropelled)