Theoretical analysis of shaft vibrations in two-pole induction machines considering static rotor eccentricity

The paper shows a theoretical analysis of the shaft vibrations in two-pole induction machines, caused by a static rotor eccentricity. By neglecting field damping effects and considering maximum homopolar flux, a maximum radial magnetic force is calculated - as a worst case - acting on the rotor in the direction of the smallest air gap and oscillating with double supply frequency. This magnetic force is implemented into a simplified analytical rotor dynamic model, considering the oil film stiffness and the damping of the sleeve bearings. Based on this model the equations of motion of the shaft centre and the shaft journals are derived and the orbit movements are mathematical described. Additionally the paper shows that elliptical orbit movements of the rotor occur and that the amplitudes of the shaft vibrations are not only influenced by the magnitude of the magnetic force but also by the direction of the magnetic force.