This paper examines the coupled rotor-tower modes of a wind turbine over a wide rotor speed range. Since modes of the individual rotor blades vary with their azimuth positions, a multi-blade coordinate transformation is used to transform the blade modes to azimuth-independent rotor modes. The resulting rotor modes are physically meaningful and show interesting couplings with the tower modes; these couplings vary significantly with rotor speed due to a complex interplay of centrifugal, gyroscopic, and elastic forces. The coupled modes are extracted using a modal processor developed at NREL that post-processes the response data generated by a turbine simulation code. The processor offers a novel feature of isolating modes that become locked at a single frequency. Results indicate that over a specific rotor speed range, the rotor regressive in-plane mode and the tower lateral mode coalesce, resulting in a self-excited instability.
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