An alternator, preferably for an electric power plant (1) comprises a stator (5), a rotor (6), provided with a plurality of electrical windings, and a collector assembly (7), which is configured to supply current to the electrical windings of the rotor (6) and is provided with a chamber (18) and at least one sliding contact (27) housed inside the chamber (18); the alternator (3) comprises a monitoring device (8) for monitoring the conditions of at least one sliding contact (27), which is provided with at least one first temperature sensor (36); of at least one first ozone concentration sensor (37a, 37b); of at least one first photo detector (38), configured to detect the intensity and the frequency of the light events occurring inside the chamber (18); of a control device (34) configured to determine at least one operating state of at least one sliding contact (27) selected between one correct operating state and at least one first incorrect operating state on the basis of the data detected by the first temperature sensor (36), by the first ozone concentration sensor (37a, 37b) and the first photo detector (38).