Study of Multiphase Superconducting Wind Generators With Fractional-Slot Concentrated Windings

Superconducting (SC) generator offers a promising candidate for offshore direct-drive wind generators with power of 10 MW or higher. Since copper losses commonly dominate in the SC direct-drive wind generators with SC field windings and copper armature windings, an important way to increase the generator efficiency is to improve the armature winding design. Compared to traditional overlapping windings, fractional-slot concentrated windings (FSCW) have shorter end connection and higher slot fill factor, which has a great contribution to copper loss reduction. However, high content of magnetic motive force (MMF) harmonics is a big challenge for applying FSCW in SC generators. The asynchronous MMF harmonics may cause high eddy-current losses in the rotor assembly, resulting in efficiency reduction and more refrigeration burden. A novel multiphase FSCW with fewer MMF harmonics for SC generators is presented in this paper, followed by an MMF harmonic analysis through winding function method. Comparison with three-phase and traditional dual three-phase FSCW is performed. Characteristics of the generators are compared, in terms of back electromotive-force, electromagnetic torque, and rotor eddy-current losses by using finite element analysis. The advantages of the proposed FSCW are verified. Further, it is observed that the alternating components of the flux density at SC coil area is attenuated significantly by the proposed topology, indicating that the AC losses of the SC coils may be substantially reduced. The feasibility of applying FSCW topology in the design of SC wind generator is enhanced through the study.