Unbalanced magnetic pull modelling in a two poles three phase generator

In rotating electrical machines with eccentric rotor, an imbalance of the electromagnetic forces acting upon rotor and stator surfaces occurs, so that a net radial force, known as Unbalanced Magnetic Pull (UMP), is developed. In case of large size machines, such as turbo-generators supported by oil-film bearings, the approximation of circular orbit normally used in literature for the geometric centre of the rotor is not acceptable. Nevertheless, the authors who have dealt with UMP in big size generators have disregarded that these rotor filtered orbits are elliptical and generally the orbit centre is not concentric with the stator. A more realistic model is introduced in this work and the actual distribution of the air-gap length during the rotation is determined in analytical terms, by taking into account the effects produced by the actual rotor orbit. The actual UMP is calculated by using the air-gap permeance approach and the simulation of the dynamical behaviour of a 320 MW generator is presented, showing the harmonic content of the UMP and the presence of non-linearities.