Structural mass minimization of large direct-drive wind generators using a buoyant rotor structure

The aim of this paper is to minimize the structural mass of large direct-drive wind generators using a ring-shaped buoyant rotor structure. A surface mounted permanent-magnet machine configuration is chosen for the proposed generator. The active mass of the generator is estimated by an electromagnetic design. Structural deformation equations for the proposed generator are derived, and the structural dimensions of the generator are initially determined from the equations. Using a three-dimensional finite element analysis, the structural dimensions are modified and the structural mass is estimated. The total mass of the proposed generator estimated by the design and the analysis is 89, 195 and 430 tonnes for 5, 10 and 20 MW wind turbines. The results indicate that the proposed generator is lighter than conventional direct-drive generators, and becomes lighter than the geared generator in scaling up larger than 9 MW.