Performance comparisons among radial flux, multi-stage 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 (RF), multi-stage axial flux (AF) and three-phase transverse flux (AF) permanent magnet (PM) 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 150degC, 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 rpm. 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 and the efficiency of the machines 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 the performance and size.