Development of rare earth free permanent magnet generator using Halbach cylinder rotor design

Direct-drive permanent magnet generators (DDPMGs) offer increased reliability and efficiency for wind turbine application over the more commonly used geared doubly-fed induction generators. However, deployment of DDPMGs is limited in the U.S. wind industry due to reliance on NdFeB permanent magnets, which contain critical rare earth elements Nd and Dy. To allow for the use of lower energy density, rare earth free permanent magnets, Halbach cylinders are employed as the rotor in a 3.5 kW PMG to concentrate magnetic flux over the rotor surface and increase magnetic loading. By varying the slot-to-pole ratio in Halbach PMGs (HPMGs), designs are developed, which allow for the use of ceramic, or hard ferrite, strontium iron oxide permanent magnets. At the 3.5 kW scale, the ceramic HPMGs are able to achieve rated performance at reduced average efficiency of between 82 and 87%, which is due to the difference in permanent magnet material properties. For scaling of the ceramic HPMGs to 3 MW, rated performance and high efficiency were achieved on average at rated speed, demonstrating the potential for a rare earth free PMG in commercial scale wind turbines.