The endurance of electrically powered drones as aerial vehicles is very important and can be increased by, among other things, more efficient electric motors, lower mass as well as measures to reduce drag and improve rotor blade aerodynamics. The aim of the "AluMotor" project is to increase efficiency and power density of electric drone powertrains by introducing the concept of cast aluminum coils. To facilitate this, an innovative tool concept for the investment casting process is devised which results in a significantly higher slot filling compared to conventional round wire coils in the stator and contributes to increased efficiency via reduction of losses. Furthermore, the use of aluminum instead of copper combines reductions in coil weight, resource usage  and raw material costs with improved heat dissipation at identical performance1, since the electrical conductivity disadvantage of aluminum is compensated by the higher slot filling factor. The coils are plugged onto the teeth of an innovatively designed electric motor and tested against a reference drone motor. Aluminum cast coils are realized by means of a cuvette-based investment casting process. Process development includes the conceptualization of the coils for the injection molding process, the design of a casting cluster, the production of the coils in cuvette investment casting as well as their post-processing, including the application of an insulating coating. The complexity of the coil geometry and the resulting extended flow paths at low wall thicknesses of 1-2 mm pose a major challenge. The CAD data of the coil is optimized so that lost wax models for investment casting can be produced in an undercut-free injection mold without the need for cores or slides. A plasma coating on the mold surface eliminates the need for release agents. An additional challenge is the need for processing of pure aluminum dictate by the application’s electrical requirements. Its high oxygen affinity significantly complicates melting. For this reason, casting is done within an evacuated casting chamber, while melting occurs in an inert gas atmosphere. At the end of the filling process, the cuvette containing the solidifying metal is pressurized to counteract the formation of cavities, thus ensuring very good casting results². The subsequent post-processing steps include separating the castings from the gating system and deburring. The latter is important to ensure a homogeneous thickness of the insulation layer. Complexity of the geometry and the low yield strength of pure aluminum impede this step, which must however be carried out reliably to ensure full functionality. The reworked coils are coated by means of a powder coating process to provide electrical insulation. The use of the new coil and motor design plus the innovative housing and bearing concept result in a mass reduction in excess of 30 % compared to the reference system.

References
[1] M. Gröninger, D. Schmidt, et.al. IEEE, 1st International Electric Drives Production Conference (EDPC), 2011
[2] M. Gröninger, D. Schmidt, et.al., Patent EP 3 637 590 A3, Gießtechnisch hergestellte elektrische Spule, 2013