Simplified design criteria for drivetrains in direct-drive wind turbines

Nominal power of wind turbines constantly increases. This goes along with an increase of the generator size, usually accompanied with an increase in Mass. To limit at the same time the tower top mass, the mass increase of the generator needs to be restricted. Changing the design concept of the generator to reduce mass, influences its modal behavior and thereby possibly the dynamics of the whole drivetrain. This work identifies simplified criteria for the parameter space restrictions of drivetrain designs, while taking into account the interaction of the generator electromagnetic and structural characteristics. The drivetrain is analysed as an integrated system using a simplified two MSS model to evaluate the dependency of system parameters and system stability. For two degrees of freedom an analytical solution is determined. Increasing the number of degrees of freedom to four, the parameter study is carried out using a full factorial analysis and a numerical solution approach. The results point out the necessity to take into account magnetic stiffness when designing the drivetrain to ensure system stability.