This paper contributes to the modeling of vibration of magnetic origin of electrical machines. It is generally difficult to calculate the damping matrix of actual mechanical structure of electrical machines. Mechanical model that we present here is known as the modal superposition method. It allows us to take into account the damping coefficients of the mechanical structure which can be inferred from experience. Associated with a numerical code for the computation of magnetic force it can be of a great help in the predetermination of the vibration of an electrical machine.