Paper Title
Hybrid Two-Stage Direct Drive Generator using Axialand Radialflux Permanent Magnet Machines for Multi-Megawatt Class Wind Turbine

Abstract
Direct drive generators for offshore multi-megawatt class wind turbine have evolved fromsynchronous machines permanent magnet excited. They are attracting more interest from the wind turbinemanufacturers due totheir higher energy density in comparison with gear train type generators. The directdrive generators in the ways of magnetic flux direction are divided into AFPM(Axial Flux Permanent Magnet)- and RFPM(Radial Flux Permanent Magnet)-type machines. The AFPM-type machine with the topology of one single-rotor and double-stators arrangement iscommonly used, where double iron-cored stator discs are installed and the rotor disc mounted by permanent magnets on its both sides is placed between the two iron-cored stator discs.The multi-stage AFPM-type machine could be assembled fromsingle modules and stator coils arrangedcircumferentially to give one stage. Additional stages wouldthen be stacked back to back along the machine. In this case, an increase of electric power production through multi-stage in axial direction is easily achieved due to its structural feature such as narrow width.On the other hand, the RFPM-type machine can be applied to a geometric feature of wide width.In this paper, the authors describe theevolution of a hybrid directdrive generator for 6.2MW with a multi-stageaxial flux topology,in which the two-stage AFPM-type machine for 5MW was composed of each topology model for 2.5MW with hollow shape of the rotor with annular disc, the stator and the main shaft mounted on coupled slew bearings. In addition, the RFPM-type machine with a capacity of 1.2MW was inserted to the hollow shape of the rotor. Keywords - AFPM-type machine,Hybrid directdrive generator, Offshore Wind turbine,Multi-stageaxial flux topology, RFPM-type machine.