Performance Comparisons Among Radial-Flux, Multistage Axial-Flux, and Three-Phase Transverse-Flux PM Machines for Downhole Applications

The aim of this paper is to provide performance comparisons among conventional radial-flux, multistage axial-flux, and three-phase transverse-flux permanent-magnet machines for downhole applications where the outer diameters are limited by well sizes, but the axial lengths can be relatively long. The comparison procedure is based on a high ambient temperature of 150°C, a small outer diameter of 100 mm, a current density of 4 A/mm² , an electrical loading of 20 kA/m, and a constant speed of 1000 r/min, with their output torques being from several newton meters to 105 N·m and power up to 18 kVA. Three machine prototypes are chosen and optimized individually in terms of maximum torque density on the basis of some common constraints. The comparisons are focused on the torque density, machine efficiency, and power factor with respect to different pole numbers and axial lengths. For a specific downhole application without an external cooling system, the obtained results provide an indication of machines best suited with respect to performance and size.