Design of Rare Earth Free Permanent Magnet Generators

Abstract: Low speed permanent magnet (PM) synchronous generators (SGs) are commonly used in renewable energy. Rare earth (RE) PMs such as neodymium-iron-boron are a popular choice due to their high performance. In 2011 supply and cost issues were added to the previously existing environmental concerns regarding REPM raw materials as the world's major producer China imposed export restrictions. This thesis aims to investigate and propose design solutions for PMSGs that do not use REPMs. Two approaches are used: to design generators using the cheaper and more abundant ferrite PM materials, and to investigate how properties of new PM materials influence SG design.A ferrite PM rotor is designed to replace a REPM rotor in an experimental 12 kW wind power generator. The new design employs a flux concentrating spoke type rotor to achieve performance similar to the old REPM rotor while using ferrite PMs. The ferrite PM rotor design is built. The air gap length, magnetic flux density in the air gap, PM remanence, and voltage at both load and no load are measured. The generator has lower no load voltage than expected, which is mainly explained by lower than specified remanence of the ferrite PMs in the prototype. With the measured remanence inserted into the calculations some discrepancy remains. It is found that the discrepancy can be explained by the magnetic leakage flux in the end regions of the spoke type rotor, which is not modeled in the two dimensional simulations used for the design calculations.To investigate the influence of PM material properties three different PM rotor topologies are optimized for torque production using PM materials described by their remanence, recoil permeability, and demagnetization resistance. Demagnetization is considered using currents determined by a novel, winding design independent short circuit model. It is found that the spoke type rotor gives the highest torque of the three rotor topologies for low remanence materials as long as the PMs have sufficient demagnetization resistance. For high remanence materials the surface mounted PM rotor can give higher torque if the demagnetization resistance is high, but otherwise a capped PM rotor gives higher torque.