Analytical Design of an Easily Manufacturable, Air-Cooled, Toroidally Wound Permanent Magnet Ring Motor with Integrated Propeller for Electric Rotorcraft

Abstract

This paper presents an analytical design method to develop a ring motor that is wrapped around the outer diameter of a propeller. This 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 of the outer diameter of the blades, the innate lever arm drastically increases torque density of the machine without the need of liquid cooling. An analytical design approach is used to characterize the machine design space and design outcomes are verified with FEA. The final proposed design targets the heavy lift drone space. The design yields a 400mm diameter propellor that spins at 1650rpm producing 225N thrust. The proposed machine delivers 6 kW of mechanical power with 91% efficiency and 12 Nm/kg continuous specific torque.