Comparative electrical design of radial- and axial-flux permanent magnet synchronous machines under space limitation

Comparative electrical designs of radial- and axial-flux permanent magnet machines (AFPMs) under space limitation are described in this paper. The target machines are conventional radial-flux machines and axial-flux machines with a single stator between two rotors. An analytical equation for designing axial-flux machines is derived considering given dimensions. The torque densities are calculated under dimensional limitations. From calculated results, the torque density of axial-flux machines depends on axial lengths, though that of radial-flux machines is independent of axial lengths. The back yoke thickness and the stator slot height of axial-flux machines are included in the axial length. Axial-flux machines have the possibility of having higher torque density than radial-flux machines if the windings are filled in the slot at a rate proportional to the increase of the slot space. However, the optimal axial length of axial-flux machines depends on the electrical loading because of the magnetic saturation of the stator teeth.