This paper studies the effects of voltage sags (dips), both symmetrical and unsymmetrical, on the three-phase synchronous machine (SM). The vast majority of the electrical power generation systems in the world is consist of synchronous generators coupled to the electrical network though a transformer. Voltage sags on SM cause speed variations, current and torque peaks and hence may cause tripping and equipment damage. The consequences of voltage sags in the machine behaviour depends on different factors such as its magnitude (or depth), duration and initial point-on-wave, and also on the parameters of the machine. In this study, threemachines of different nominal power have been used for simulate on MATLAB the effects of thevoltage sags on the machine under specific conditions.