ISSUE 011 Autumn 2021 Candela C-7 hydrofoil speedboat dossier l In conversation: Robert Hoevers l Battery recycling focus l Vehicle dynamics insight l ZeroAvia hydrogen-electric aircraft digest l Motor materials

The materials for the fixtures to hold the magnets in the rotor together are also changing. Rather than using steel, which is susceptible to eddy currents and heating, a thinner carbon fibre sleeve can provide the strength required with less weight. Rotors The design of the laminations in the rotor can generate different profiles of magnetic flux to mimic an IPM or SPM motor with different levels of performance. “We are using different designs of windings and different designs of architecture of the steel for the flux from the outside,” says Price. “We are looking to replace copper, which would mean you wouldn’t have to manufacture wire or hairpins, so you can cut the number of processes. “In the future, we will use a different winding system and a different process for applying those windings. These are cast windings, which are easy to manufacture, but you need optimisation in the inverter to manage that. That needs to be implemented in e-machines in the future, and it has to be designed at a system level for cooling to dissipate the heat so that you can have an air-cooled motor rather than a liquid-cooled one and still get the performance.” Adhesives and coatings The adhesives and coatings used in e-mobility motors are also critical to the performance of the motor, particularly for the thermal characteristics and the need for thin but rugged insulation and protective coatings for reliability. “In the future, stator and rotor designs will see a rise in voltage to 800-1200 V to reduce the charging time of battery packs, but the problem is having that voltage and a high current in a small space,” says Materials for motors in electric aircraft Rare earths are key to the performance and e iciency levels needed for electric aircraft. This means attention has to be paid to the supply chain for them. “We are trying to establish more rare earth suppliers with a UK and EU supply chain,” says Aaron Williams, director of strategic technology and business development at magnet supplier Arnold Magnetics. One way to do this is by recycling existing magnet materials to extract the rare earths. The motor also has to be as light as possible though, while cost is less of an issue. This leads to the use of carbon ibre rather than steel for the sleeves that hold the magnets together, cutting the weight by half. It also avoids the losses and heating that come from eddy currents in the steel sleeve. Using a lightweight, strong sleeve also allows di erent magnet topologies to be used, such as the Halbach array. This was developed in 1970, and uses a complex con iguration of magnets to ensure that all the magnetic lux is directed into the centre of the motor. This is achieved by aligning the magnets so that the lux is cancelled out on one side and therefore boosted on the other side. “This gives 3 to 8 percentage points higher e iciency,” says Williams. “Our Recoma material uses samarium and cobalt to be stable at the high temperatures in aerospace: when a neodymium magnet gets hotter it gets weaker, and it has a maximum temperature of 200 C. A samarium-cobalt magnet by contrast can operate up to 350 C, which is higher than any motor can operate as the wires and varnish would melt at that temperature.” The magnet can also be laminated to reduce the losses from eddy currents. The magnet layers are 1 2 mm thick, and a proprietary epoxy designed to bond to the coating on the magnet is used to hold the layers together. The operating temperature is also an important design factor, as there are other ways of generating electric power in aircraft. “Where we need to get to is small and single-aisle aircraft, and they will need an auxiliary power unit burning jet fuel to drive a 2 or 3 MW generator to produce power for the electric motors,” says Williams. “Our focus now is to make the generator and motor as e icient as possible.” The topology for a permanent magnet generator is di erent from that of a motor, as it runs at one optimum speed with less need for control. “Then it’s about making the rotor and stator as e icient as possible, by reducing the losses from the bearings and the heat generated in the eddy currents, so we try to optimise the slot ill on the stator,” he says. The slot ill factor is the amount of copper wire in the slot in the stator, and one way to increase this is to have even thinner laminations in the stator than there would be on other e-mobility platforms. These laminations are produced using a cold-forming process from a base of M19-grade steel. Although there is less focus on cost for an aircraft motor, using a thinner steel is more cost-e ective than a high-cobalt steel, Williams says. There is another approach, called Litz wire (named after the German word for braiding), that can also improve the ill factor. Rather than using a single copper wire, a Litz wire consists of many thin wire strands, individually insulated and twisted or woven together in a carefully designed pattern. For AC at frequencies above 1 MHz, that means most of the current is carried on the surface of the wires, minimising eddy currents and reducing the resistance and heating e ects in the wire. The particular design of the weave of the wires can be optimised for the shape and arrangement of the slots on the stator to further improve a motor’s e iciency. This laminated samarium-cobalt magnet is intended for higher temperature applications than those possible using neodymium (Courtesy of Arnold Magnetics) 60 Autumn 2021 | E-Mobility Engineering