Concentric-Coil Distributed Winding Design for Large Direct-Drive Fully Superconducting Machines

Abstract

Fully superconducting generators (FSCGs) use superconducting wires for both the field and armature windings to reach a high torque density for wind turbine applications. The AC armature winding of an FSCG design mostly employed racetrack tooth-coils to form fractional-slot concentrated windings. This winding type has high leakage flux and excessive harmonics, especially when used in superconducting machines. Other attempts like applying full-pitch racetrack coils can only be with one slot per pole per phase which also has significant slotting harmonics. This paper proposes an innovative concept of double-layer concentric coils to enable distributed windings with a larger integral number of slots per pole per phase. The concept successfully breaks the limitation that racetrack coils cannot be built with such integral slots. This paper then assesses the performance of the proposed winding concept within a context 20 MW FSCG design. The assessment finds out that the proposed concept has balanced three phases but reduces inductance compared to single-layer windings. The force on each concentric coil can significantly be different, thereby complicating the mechanical design of these superconducting coils.