Modal analysis of a teetered-rotor wind turbine using the Floquet approach

This paper examines the operating modes of a twobladed teetered wind turbine modeled with seven degrees of freedom: nacelle yaw, hub teeter, flapping of each blade, tower fore-aft bending, tower lateral bending, and tower twist. Because of the gyroscopic asymmetry of its rotor, this turbine’s dynamics can be quite distinct from that of a turbine with three or more blades. This asymmetry leads to system equations with periodic coefficients that must be solved by the Floquet approach to extract the correct modal parameters. Results illustrate how the turbine modes become more dominated by the centrifugal and gyroscopic effects as the rotor speed increases. Under certain design conditions, the gyroscopic coupling may cause phase locking or frequency coalescence of the yaw and teeter modes resulting in self-excited dynamic instabilities.