Computational model for calculating the dynamical behaviour of generators caused by unbalanced magnetic pull and experimental validation

This paper presents the modelling of the unbalanced magnetic pull (UMP) in generators and its experimental validation in a real machine. UMP is one of most relevant effects of electromechanical interactions in rotating machinery and manifests itself when rotor eccentricity causes an imbalance of the electromagnetic forces acting upon rotor and stator surfaces, so that a net radial force is developed. In small and stiff machines, like electrical motors, this phenomenon can be avoided by means of carefully assembling and manufacturing. On the contrary, big generators of power plants, supported by journal bearings, operate above their first critical speed behaving like flexible shafts, therefore the eccentricity of the active part of the rotor with respect to the stator is unavoidable. In the first part of the paper, an accurate model for calculating the UMP force is presented. This model is more general than those available in literature that are limited to circular orbits and is based on the actual position of the rotor inside the stator, therefore on the actual air gap distribution, regardless of the orbit type. The closed form of the non linear UMP force components is presented. In the second part, the experimental validation of the proposed model is presented, by considering a steam turbo generator of a power plant and by comparing the simulated dynamic behaviour in the time domain of the machine to the experimental one, without and with the excitation of the magnetic field in the generator.