Asymmetry of the magnetic field in a synchronous generator will induce a magnetic unbalance pull. In this paper, a hydropower rotor system is modelled and the influence of electro-mechanical forces due to different amounts of real power is analysed. The electromagnetic field is solved with the finite element method and coupled with mechanical system as a magnetic stiffness. The mechanical model of the generator consists of a four degree of freedom rigid disc connected to an elastic shaft supported by two bearings with linear properties. It has been found that the unbalance magnetic pull increases for a lover amount of real power leading to a decrease of natural frequencies and an increase of unbalance response. Since many hydropower units are used for both part and full load it becomes important to model the electro-mechanical interaction when developing new generators, in order to improve the mechanical design.