The transient stability of a turboalternator, connected to an infinite busbar through a reactive tieline, is investigated through Liapunov's direct method. The machine is fitted with an automatic voltage regulator (AVR) sensing the machine terminal voltage and its first time derivative. The AVR time lag is ignored and it feeds the machine directly. The governor is actuated by the load-angle deviation and its first two derivatives, and is represented by a differential equation involving one or two time lags only. The variation in the rotor direct, and quadrature-axis flux linkages has been included in the system model. Under different approximations, mathematical models for the AVR are suggested. Liapunov functions are proposed and the effect of variation in parameters of the AVR and governor on the critical fault clearing time, for a three-phase fault, is studied.