Axial flux motors | Focus E-Mobility Engineering | November/December 2023 71 The powertrain developer argues that AF and RF machines can be engineered to provide very similar torque characteristics, emphasising that the company can provide motors with almost any reasonable torque and speed characteristics, and there is an analogous flexibility in terms of packaging, its expert says. “While the shapes of AF and RF motors are different, with an integrated motor, inverter, and transmission systems you can end up with about the same package but with higher performance per unit volume and weight for AF motor solutions,” he says. Observing the evolution of applications at one removed from the motor developers themselves, the wire bending specialist notes that AF motors allow a lot of design freedom for wheel hub drives, e-axles and even inwheel applications. However, the need for a motor at each wheel adds more components such as inverters to the system and thereby increases costs. Control questions Controlling AF motors efficiently, particularly under dynamic driving conditions, naturally means taking their inherent electromagnetic characteristics into account. One developer explains that AF motors typically don’t exhibit any significant saliency. “That means all the torque is produced from the interaction with the permanent magnet flux, there being no reluctance torque component,” its expert explains. “However, this simplifies the control of the machine, as torque is simply proportional to the q-axis current, and negative d-axis current is used for field weakening as required. Maps are therefore not required for effective maximum torque-per-ampere control.” He notes that one challenge stems from the fact that AF motors usually have relatively low winding inductance. “This means that either a high switching frequency or a multi-level inverter topology is necessary to keep the current ripple low,” he says. “Otherwise, high current ripple leads to additional loss in the machine and increased NVH. “On a positive note, the lower inductance means the torque control of the machine is very responsive, exhibiting high-bandwidth control of the current.” NVH finesse NVH is a major topic in vehicle design as a whole, and must also be considered in AF motor design and development. Key aspects of design include nominal air gap size and expected variations in it resulting from factors such as rotors not being perfectly parallel, differences between air gaps, tolerance stacks and so on. Also important are rotor vibration modes and their natural frequencies, which are affected by stiffness and mass, and rotor balance. From an electromagnetic point of view, an expert says, NVH performance is affected by the amplitude and frequency of cogging torque, which itself is affected by factors such as slot opening, skewing, slot/pole combination, and concentrated versus distributed winding configurations. Mechanically, large rotor diameters and high rotation speeds bring a risk of vibration. The academic expert suggests that an additional air gap bearing on the outer diameter of the rotor could provide a solution, but acknowledges that it would entail some loss of efficiency. One AF motor developer points to the application of current harmonic injection, explaining that it is particularly good for addressing NVH issues Some suppliers of axial flux motors Australia Evans Electric – www.evans-electric.com.au Austria Miba +43 7613 2541 0 www.miba.com France Whylot +33 5 65 10 12 00 www.whylot.com Germany Brand Group +49 2947 889 0 www.brandgroupinternational.com Thyssenkrupp Steel +49 203 52 0 www.thyssenkrupp-steel.com Wafios +49 7121 146 0 www.wafios.com Italy Lucchi R +39 0541 739211 www.lucchirimini.com Slovakia Emrax +386 82053850 www.emrax.com Switzerland Phi-Power +41 44 555 89 67 www.phi-power.com UK Evolito +44 1865 967700 www.evolito.aero Horizon Technologies +44 2036 089996 www.horizontechnologies.eu Saietta Electric Drive +44 1869 233121 www.saietta.com Oxford Propulsion +44 1865 802480 www.oxfordpropulsion.co.uk YASA +44 1865 952 100 www.yasa.com USA Arnold Magnetics +1 585 385 9010 www.arnoldmagnetics.com Beyond Motors +1 781 428 3653 www.beyondmotors.io Omni Powertrain Technologies +1 713 635 6331 www.omnipowertrain.com Turntide Technologies +1 877 776 8470 www.turntide.com
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