In this thesis, a rotor dynamical model is made for a hydropower generator. The difference in this model compared to a number of others, is that both radial and tangential electromagnetic forces are taken into account. A number of authors have presented general theories concerning the electromagnetic forces in electrical machines, such as electrical motors. In a couple of these theories the electromagnetic forces is presented like two components. One radial, acting in the direction of the smallest airgap between the rotor and the stator, and one tangential force component. Most of these theories are concerning electrical motors and not electrical generators. For the case of motors the tangential force component is in one way or another expressed like a function of the difference of frequency for the motor and the AC line voltage. This difference is called slip. In the generator treated in this thesis, slip does not occur. But, recent numerical simulations have been performed which indicates that there exists a tangential electromagnetic force component. The rotor model is simplified to four degrees of freedom in the equation of motion. Both the rotor and the stator have been considered stiff. To solve these equations the state space approach is used and the simulations are based on an explicit Runge-Kutta formula, the Dormand-Prince pair. The electromagnetic force used is linearized for up to 10 percent relative eccentricity. Above that limit the electromagnetic force grows exponentially. The tangential force component has been assumed to be a function proportional the radial force component. Different magnitude and direction of the tangential component have been studied. This thesis shows that the electromagnetic force and especially the tangential force component have a significant effect on the dynamics of the generator. For small positive values the dynamics close to critical speed are affected in a similar way as increased damping while negative values behaves like decreased damping. However, when the magnitude of the tangential force grows above a certain value, both negative and positive, the system becomes unstable. Even if the model is far too simple one can still conclude that the tangential force have an important influence on the generator. Therefore the author will continue with research to find out the nature and the rotor dynamical consequences of this tangential force.