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

magnetic field using an excitation coil in the rotor, so the production of the motor is independent of the raw materials markets. The power to the inside of the motor is wireless and so is wear-free. The motor operates with 96% efficiency, and we can scale it easily from small test cars to commercial vehicles. “To get the energy to the rotor, it uses an AC field that is converted to DC for the magnet coils that induce a magnetic field into the air gap,” he adds. “The magnitude of this field can be controlled easily.” To obtain this change in material, Mahle simulated various motor designs, adjusting different parameters until an optimum was found. This approach is much faster and cheaper than conventional physical prototyping. Stator materials A stator is built up of very thin layers of specialist steel, with a balance being struck between the thickness of the metal and its structural rigidity. The layers are made as thin as possible to reduce the eddy currents that are induced by the magnetic field and cause heating. This heat reduces the efficiency of the motor and needs to be removed from the rotor, often by a liquid-cooling system. However, other materials such as insulating layers between the lamination layers can also be used to take heat away from the stator. The steel used for the rotors and stators in motors for electric aircraft and high-performance vehicles is different from mainstream designs, the aim being to provide the highest magnetic performance and strength in order to achieve the lowest weight. “Most alloys are silicon steel, but we use a cobalt-based alloy in stators and rotors,” says Nir Vaks, global director of magnetic materials developer Carpenter Technology. “Using a lot of cobalt means you can push more magnetic flux density through the alloy core, and the more magnetic flux you can push through the alloy, the more power you can generate.” The alloy in each layer of the lamination in the stator can be as thin as 0.002 in (a thickness grade of 002) up to 0.014 in for a high-performance motor. There are three tiers of processing for the cobalt alloy. Carpenter’s proprietary chemistry produces the mechanical strength in four grades, from HC 27 for e-mobility motors, 50 and 50A for aerospace, to 50HS for higher strength but a lower magnetic capability. Then there is a hot and cold rolling process followed by heat treating the alloy with temperature cycles and quenching. Carpenter uses a hydrogen- rich environment that allows more control of the mechanical, thermal and electromagnetic properties. The HC 27 grade contains 27% cobalt that supports a flux of 1.8 T, rising to 49% for the 50 grade, while the 50A has additional niobium to support a magnetic flux of up to 2.3 T and 50HS has vanadium, giving higher strength but a flux of 2.2 T. There are also other trace elements that are important for the structure of the grains in the metal. As Jaydip Das, senior manager, Applications Engineering, at Carpenter explains, “We add vanadium and sometimes manganese to refine the grains, and it is the post-process heat treatment that defines the combinations for 50A and 50HS. Larger grain gives higher magnetic permeability and lower losses but less strength. “On top of that we can tweak the grain boundaries and size to control the distribution of the grains. For example, if you want a high-strength area you make sure it has smaller grains.” The laminations are combined in a stack, but there is a trade-off between the bonding materials and methodologies and the fixture ring to maximise the mechanical strength and the magnetic performance, says Vaks. The metal in the moving rotor for example has to be stronger than the stationary stator. Problems with the supply of rare earths has prompted some manufacturers to develop e-motors that are magnet-free (Courtesy of Mahle) 58 Autumn 2021 | E-Mobility Engineering Deep insight | E-motor materials