72 November/December 2023 | E-Mobility Engineering in AF motors and, in its simplest form, operates in a similar way to noisecancelling headphones. “NVH is always a system-related issue,” another expert points out. “The motor, inverter and transmission have to be tuned together to achieve the best NVH properties.” Reliability and longevity As with any relatively new technology, a vital aspect of the engineering effort that goes into RF motors is ensuring their reliability and durability, particularly in harsh environments and over high mileages and thousands of operating hours. Careful design of the sealing for liquid-cooling channels and of the internal motor cavity to protect it from the external environment is vital, for example. Other key issues include bearing loads, bearing alignment and axial pre-loads. Durability testing that exposes motors to thermal shock and vibration is also crucial. The importance of such testing is magnified in aerospace because of the safety-critical nature of the powerplant. The aircraft AF motor expert emphasises the importance of specific durability testing and highly accelerated life testing. “Throughout this, we subject the motor to repeated cycles that escalate in severity of vibration, temperature and performance.” A deep understanding of how the machines are designed and how they fail is also crucial. “This starts with a fault tree analysis, where we look at every component and analyse how they could fail and then remove or protect against those causes through design.” While regulatory standards and safety requirements don’t take motor architecture into account, AF architecture and related design details do present some safety benefits, a couple of the developers say. All the high-voltage elements of the motor are buried deep within the machine and isolated in a dielectric oil, reducing the risk of short-circuits to the chassis. The aerospace motor developer points to the separation between coils in its AF machines, saying, “They are separated not only through physical clearance but also through immersion in a dielectric oil. No coil has any overlapping connection touching another coil, which is really difficult to do on a radial machine.” This makes them much more robust to electrical failures. Future directions AF motor developers continue to improve their core products in terms of power density, efficiency and integration into drivelines. One developer highlights improved stator encapsulation and better manufacturing techniques, plus the integration of a motor control unit that shares the housing and coolant with its motor, eliminating phase cables. The company has also developed a single-speed transmission with multiple gear ratio options to provide a ‘costoptimised’ e-drive. Another developer points to a multiplication in power density, from a few kW/kg a decade ago to around 15 kW/kg in its production motors. It plans to double that as well. The aviation specialist reports power density increasing by a third over the past 18 months. Meanwhile, it has also been working on what its expert refers to as dual electric architectures that enable them to fit two AF motors side by side in the same size package as a single motor. “This has major benefits for the safety profile of any fully electric aircraft,” he says. The electric, hydraulic and mechanical powertrain specialist points to breakthroughs in manufacturing processes and motor design that have enabled higher cycle rates in production. It has also designed an oil-cooled AF motor with a very high continuous power that is intended for commercial vehicles. Future improvements in the core technology are likely to focus on higher rotational speeds leading to higher power density, this being enabled by lighter rotors that are better balanced and assembled with finer control of air gap tolerances. Another r&d goal is further reduction of eddy current losses in magnets through lower space harmonics (variations in the distribution of magnetic field strength) in the windings’ magneto-motive force, along with shielding against high-frequency flux harmonics. A third direction is increased inverter switching frequency, multi-level inverters and/or more advanced modulation schemes to reduce current harmonics and improve voltage utilisation. Finally, increased rotor saliency and flux focusing could take some pressure off the supply of rare earth magnets by easing requirements on the grade and volume of magnetic material needed. Acknowledgements The author would like to thank Dr Chris Lines at Saietta, Dr Tim Woolmer and Simon Odling at YASA, Mukesh Patel at Evolito, Martin Bauer at Wafios, Dave Kemper at Omni Powertrain Technologies, and Henrik Born at RWTH Aachen University for their help with researching this article. Focus | Axial flux motors Evolito’s 285 kW D1500 yokeless and segmented armature motor in 2 x 3-phase form offers extra redundancy for safety in aircraft applications (Courtesy of Evolito)
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