The excitation system is the principal element that influences the dynamic behavior of alternators. This paper presents a new excitation structure based on a wound field synchronous machine (exciter machine), a diode bridge rectifier and a dc chopper. The proposed structure is associated to a H_∞ voltage controller. The dynamic behavior of the synchronous generator is evaluated by experimental tests in the presence of unknown variations of the the power system operating conditions. Experimental results are presented and discussed.