Analytical Design and Experimental Testing of a Self-Cooled, Toroidally Wound Ring Motor With Integrated Propeller for Electric Rotorcraft

This article presents a ring motor that is wrapped around the outer diameter of a propeller for electric rotorcraft. The approach is motivated by the greater aerodynamic efficiency afforded by larger diameter propellers with lower rotational speeds, ultimately minimizing battery energy requirements in electric rotorcraft. Lower rotational blade speeds are mismatched with electric machines that derive power density from high-speed operation, thus a machine design with very high torque density is proposed to allow lower speed operation without the aid of gearing. By wrapping the machine around the outer diameter of the blades, the innate lever arm drastically increases the torque density of the machine without the need for liquid cooling. An analytical design approach is used to characterize the machine design space and design outcomes are verified with finite-element analysis. The final prototype design targets the “heavy-lift” drone space and was tested to verify performance. Measurements of back electromotive force and max torque per amp, along with unloaded and loaded (propeller) testing, benchmark the design and highlight the relative impacts among the various loss mechanisms. The measured specific torque density of the air-cooled design is 6.25 N·m/kg - total mass, 11.1 N·m/kg - active mass only. Key areas for improvement, such as bearing drag reduction and winding fault tolerance, are identified.