Abstract This work presents a nonlinear adaptive output feedback excitation control, designed for a synchronous generator modelled by a standard third‐order model on the basis of the physically available measurements of relative angular speed, active and reactive electric power and terminal voltage. The power angle, which is a crucial variable for the excitation control, as well as mechanical power and the impedance of the transmission line connecting the generator to an infinity bus, are not assumed to be available for feedback. The feedback control achieves transient stabilization and voltage regulation when faults occur to the turbines or the transmission lines, such that parameters (mechanical power and line impedance) may permanently take any (unknown) value. The controller recovers by adaptation the unknown values and simultaneously generates trajectories to be followed by the states, that converge to the new equilibrium point. Copyright © 2004 John Wiley & Sons, Ltd.