This paper studies the dynamics of a 150 MW hydropower rotor system, in whichan elastic structure supports the generator guide bearing. A non-linear large-displacement force-deflection relationship for the bearing support is derived withinthe elastic range under quasi-static conditions, which is incorporated into theequations of motion for a rotor dynamic model. The proposed model includestwo tilting pad journal bearings, along with nonlinear magnetic interaction andunbalance forcing acting on the generator. The equation of motion is solved bytime-integrating procedures to study the system’s dynamics. A drastic softeningeffect is observed for the bracket within the elastic range, in which the nominalvalue represents merely 10% of the bearing stiffness. It is concluded that theexcessive support flexibility significantly reduces the lowest pair of frequencies,diminishes the damping ratios and alters the orbit characteristics during operation.Furthermore, the amplified eccentricity increases the risk of instability due to theradial magnetic pull.