68 November/December 2023 | E-Mobility Engineering returns, after which you are just adding weight and volume. “So in optimisation, there’s a lot of analysis to find that point. Also, if you drive the motor in a non-optimised way it will be very lossy, so we also consider how to drive the motor with the controller and the software.” Structural considerations The architecture of AF motors places a number of mechanical demands on the magnets. One particular demand is on securing magnets to the rotor backiron plate to withstand the radial forces generated by high rotational speeds. The composition of the magnets is not dissimilar to that used in RF, but their form and the way in which they are used are clearly different. A lot of development attention is therefore paid to cope with the tip forces generated by high rotational speeds of a larger rotor spinning alongside a stator, another expert notes. Ferromagnetic material cannot be used circumferentially between the magnets to prevent rotational acceleration from shearing them out of register on the rotor(s), because that would short-circuit the magnets and reduce their useful magnetic flux, another AF motor developer explains. Its expert says the magnets need to be robustly bonded to a back-iron in the case of yokeless dual-rotor, singlestator AF motors, where the backiron is needed to close the flux path. Alternatively, in dual-stator, single-rotor AF motors, where the flux passes straight through the rotor, the magnets tend to be housed in complicated composite cage structures. “For most other AF motors – particularly yokeless designs with concentrated windings – the flux harmonics experienced by the magnets leads to very high eddy current losses within them unless they are highly segmented, which means cutting them into small pieces that are then bonded together with an electrically insulating layer.” “What we are aiming for is to push our magnetic machine hard, which then gets the magnets very hot,” the aviation motor developer says. “We then dump and dissipate the heat from those magnets effectively so that they do not become demagnetised, which would cause permanent damage to the motor.” Staying cool The flat, thin basic shape of an AF motor provides a high surface area-to-volume ratio that is beneficial for cooling, but other aspects of the general architecture and different AF motor topologies complicate the picture. The AF topology maximises heat transfer from the magnets and into the surrounding structure, aided by other innovations around direct cooling to dissipate heat quickly. “This is the key aim of any magnetic machine, and axial flux motors can do it better than radial motors as the magnets are not cocooned inside other materials, so they can dissipate the heat quickly and efficiently,” one expert says. “In a radial flux motor the heat is dissipated by conduction through multiple layers before reaching an outer cooling jacket, which is simply not as efficient. “The AF machine also has other thermal management advantages. The heat generated in the coils can be removed efficiently by pumping oil directly over the coils and then transferring the oil to a radiator outside the motor.” Implementing liquid cooling for stators faces challenges in ensuring good flow around the stator cores and to ensure leak tightness, the academic expert notes. Despite some inherent advantages of the general architecture, innovative approaches to thermal management of AF motors have to be taken. For example, the yokeless and segmented armature topology allows fully sealed oil cooling of the stator elements, a solution that is not currently possible with RF motors, the second AF motor developer points out. Further, yokeless segmented armature designs featuring dual rotors place the outer face of each rotor in the open air, where it is easy to reject heat, whereas an AF topology with dual stators and a single rotor has that rotor permanently sandwiched in a hot environment, he says. Counter-intuitively, however, AF motors that place stators on either side of the rotor – known as the inner inrunner topology – have advantages in terms of losses when oil spray cooling is used, the electric, hydraulic and mechanical powertrain provider says. The aerospace motor developer notes, “In our AF motor there is only one thermal interface between the coils and the outside of the motor. The ability of our topology to efficiently cool the magnets in the rotor and the coils Focus | Axial flux motors Section through a YASA motor used in the Mercedes Vision One-Eleven showing the direct oil-cooled stator sandwiched by two magnet-bearing rotors (Courtesy of Mercedes-Benz)
RkJQdWJsaXNoZXIy MjI2Mzk4