Performance of multi-stage axial-flux machines with MMEs

This study covers the investigation into the reduction of rare-earth magnets in a permanent magnet (PM) machine via the use of multiple rotors to enable a PM generator to be comprised of mixed magnetic elements, hereafter referred to as MME, by substituting ferrite materials into some rotors. The particular machine covered in this paper featured four rotors, with two on the external sides and two inside, along with a trio of stators interlaced between the rotors. The efficiency, power, and cost of this machine are compared herein to explore available benefits of such a configuration as described. It was shown from the finite element analysis results that there was a clear benefit to using cheap ferrite materials in some rotors but not all, allowing for a huge cost decrease while only a small decrease in efficiency. The machine's purpose was for use in a low speed, direct-drive, wind turbine. Therefore, the rated operating speed was required to be very low and was chosen to be 180 RPM which meant 40 poles were needed on the rotors to achieve 10 kVA at 60 Hz.