Three types of multibody models are presented for the investigation of the internal dynamics of a drive train in a wind turbine. The first approach is limited to the analysis of torsional vibrations only. Then, a rigid multibody model is presented with special focus on the representation of the bearings and the gears in the drive train. The generic model implementation can be used for parallel as well as for planetary gear stages, both with helical or with spur gears. Thirdly, the extension to a flexible multibody model is presented as a method to include directly the drive train components' flexibilities. Finally, a comparison of the different modelling techniques is discussed for a wind turbine's drive train with a helical parallel high speed stage, two planetary stages and a flexible rotor. The focus is on the calculation of eigenfrequencies and corresponding mode shapes. Furthermore, the influence of the helix angle and the bearing stiffnesses on the results of these calculations are discussed and the eigenmodes of the planetary stages are classified as rotational, translational or out-of-plane modes.
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