This paper focuses on the effects of voltage sags, both symmetrical and unsymmetrical, on thethree-phase synchronous machine (SM). Voltage sags on SM cause speed variations, current and torque peaks and hence may cause tripping and equipment damage. The consequences ofvoltage sags on the machine behaviour depend on different factors such as sag's magnitude (or depth), duration, initial point-on-wave and the parameters of the electrical machine. In this study,three SMs of different rated power have been considered in order to simulate the voltage sageffects for specific conditions and analyze the machine stability.