Design of Dual Rotor Axial Flux Permanent Magnet Generators with Ferrite and Rare-Earth Magnets

This paper discusses dual rotor axial flux machines with surface mounted and spoke type ferrite permanent magnets (PMs) with concentrated windings; they are introduced as alternatives to a generator with surface mounted Nd-Fe-B magnets which is analyzed in [10]. The output power, voltage, speed and air gap clearance for all the generators are identical. The machine designs are optimized for minimum mass using a population-based algorithm, assuming the same efficiency as the Nd-Fe-B machine. A finite element analysis (FEA) is applied to predict the performance, electromotive force, developed torque, cogging torque, no load losses, leakage flux and efficiency of both ferrite generators and that of the Nd-Fe-B generator. To minimize cogging torque, different rotor pole topologies and different pole arc to pole pitch ratios are investigated by means of 3D FEA. It was found that the surface mounted Ferrite generator topology is unable to develop the nominal electromagnetic torque, and has higher torque ripple and is heavier than the spoke type machine. Furthermore, it was shown that the spoke type Ferrite PM generator has favorable performance and could be an alternative to rare-earth PM generators, particularly in wind energy applications. Finally, the analytical and numerical results are verified using experimental results.