Comparison of electromagnetic performance of 10MW superconducting generators with different topologies for offshore direct-drive wind turbines

This paper compares the electromagnetic performance of 10MW superconducting (SC) generators with three different topologies, i.e., iron-cored stator and rotor (ISIRT), iron-cored stator and air-cored rotor (ISART), and air-cored stator and rotor (ASART). The objective is to provide a powerful insight into the advantages and disadvantages of the different topologies, and to establish some design guidelines for selecting an appropriate direct drive SC generator for offshore wind turbine applications. Firstly, the structures of the three SC generator topologies are introduced. Then, the influence of the SC coil cross sectional area on torque capability is compared. After that, three SC generators with different topologies are optimized respectively for further comparison, including the active material cost, weight, harmonics in the electromotive force (EMF), torque ripple, field harmonics in the SC coil, and forces on the rotor and stator components, etc. It is found that, with the same SC quantity, the torque capability of the iron-cored stator and rotor topology is much better than that of the other two topologies. However, the advantage becomes less significant when a larger area of the SC coil is employed. The air gap flux density waveform of the ASART is much smoother than those of the ISIRT and ISART. The torque ripples of the ISIRT and the ISART are much higher than that of the ASART. The field harmonics (both amplitude and frequency) in the SC coil of the ASART are the lowest. For the ISIRT, most of the force on the rotor is acting on the rotor iron, and thus, the SC coil is more likely to be safe from a mechanical performance point of view and the design of the corresponding supporting structure is simple. However, for the air-cored rotor topologies, nearly all the force is acting on the SC coil. For the air-cored stator, the force mainly acts on the armature winding, while for the iron-cored stator, it is mainly on the stator teeth. Due to the excellent mechanical performance of iron, the iron-cored stator is therefore more robust.