E-Mobility Engineering | November/December 2023 65 Axial flux motors | Focus Systems thinking Electric and hybrid aerospace applications present their own set of considerations regarding the choice between AF and RF motor topologies, which overlap with those affecting other applications but in an intensified manner. “Due to the number of parameters and variables involved, not only in the motor but in the whole propulsion system, engineers are faced with a non-trivial challenge when assessing the optimal motor to deliver the whole system’s requirements,” an aviation motor expert says. “Oversights in assessing which motor to select for their given application can have significant downstream effects – affecting cost, time to market, even airframe design and whether the mission profile is achievable.” This expert also focuses on yokeless and segmented armature AF machines, and stresses that removing the motor’s yoke saves between 60 and 80% of the iron mass in the stator over a comparable RF machine. The large number of variables means that a whole-system approach is essential to making the optimum choice for the application. The aviation motor developer takes all the system’s parameters and uses its genetic algorithm toolkit to program the aircraft’s duty cycles into its simulation system; the parameters include propeller speeds, flight durations and other elements related to performance. “Then we run hundreds of simulations using our algorithm to help engineers identify the optimum motor solution in a whole-system context,” the expert says. “What used to take our customers a year to develop and learn can now be done within a few weeks.” In the design of any high-performance motor, optimising key components to reduce losses and increase efficiency, torque density and power density are a crucial part of the process. interior permanent magnet motors. As an aside, saliency refers to the amount of change in reluctance with rotor position, reluctance being the magnetic analogue of electrical resistance. A motor with low saliency will exhibit little change in reluctance with rotor position. For some applications, saliency is engineered into motors to improve control, efficiency and torque. It could also be important to consider whether peak power or continuous power is more important for an application, as this can have an impact on the choice of motor architecture. In this respect, AF motors support high peak power density but often face limitations in continuous power density because of thermal challenges. This is particularly true of yokeless AF motors with a central stator that has a limited, primarily radial heat rejection path. The topology of an AF motor, in which the large diameter of the rotor puts a large proportion of the magnetic material near the perimeter of the rotor, allows an increase in torque. It is generally accepted that the AF topology produces 30-40% more torque than a similarly powerful radial machine. However, an AF motor with a yokeless and segmented armature configuration will typically double that torque advantage. “When combined with the reduced weight, this means figures approaching four times the gravimetric torque density of current mainstream RF machines can be achieved,” one expert says. “Usually when speccing an AF motor, you would end up with something that has a much shorter axial length but is larger in diameter,” he adds. “Thanks to some thermal and magnetic tricks, yokeless and segmented armature machines have a similar radius to an RF example of equivalent power but maintain that much higher torque. “As AF motors typically also run at lower speeds, this can influence the requirements of the transmission type and, again, on packaging.” YASA’s yokeless and segmented armature AF motors are used in the Mercedes Vision One-Eleven EV, a homage to its C111 concept car of the 1970s (Courtesy of Mercedes-Benz)
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