Torque Density Improvement of Double-Stator HTS-Excitation Field-Modulation Machine by Using YBCO Compound Modulation Rotor

Electromagnetic torque $(T_{\mathrm{e}})$ and torque density are important indexes of offshore wind turbines. This paper analyzes the influence of the rotor thickness on $T_{\mathrm{e}}$ and torque density in a field modulation double-stator high-temperature-superconducting-excitation machine (DS-HTSM), which is a promising solution for offshore wind turbines. And the influence mechanism is also investigated by finite elements analysis (FEA). Furthermore, a YBCO compound modulation rotor composed of ferromagnetic blocks and YBCO blocks is proposed to improve the $T_{\mathrm{e}}$ and torque density of the DS-HTSM by Meissner effect. And its effectiveness is verified by FEA in a T-A formula model. The results show that the $T_{\mathrm{e}}$ and the torque density can be improved by 13.9% and 21.9% respectively after replacing the original rotor with the proposed rotor.