The influence of nonlinear magnetic pull on hydropower generator rotors

In large electrical machines the electromagnetic forces can in some situations have a strong influence on the rotor dynamics. One such case is when the rotor is eccentrically displaced in the generator bore. A strong unbalanced magnetic pull will then appear in the direction of the smallest air-gap. In this paper, the influence of nonlinear magnetic pull is studied for a hydropower generator where the generator spider hub does not coincide with the centre of the generator rim.The generator model consists of a four-degree-of-freedom rigid body, which is connected to an elastic shaft supported by isotropic bearings. The influence of magnetic pull is calculated for the case when the generator spider hub deviates from the centre of the generator rim. A nonlinear model of the magnetic pull is introduced to the model by radial forces and transverse moments.In the numerical analysis input parameters typical for a 70 MW hydropower generator are used. Results are presented in stability and response diagrams. The results show that this type of rotor configuration can in some cases become unstable. Therefore, it is important to consider the distance between the centreline of generator spider hub and the centreline of generator rim.