Fast-response excitation systems are capable of influencing turbogenerator performance at subsynchronous resonance frequencies, and this paper shows how suitable excitation control can improve the damping of generator subsynchronous shaft oscillations. A multivariate frequency response method is used to investigate control capabilities and to identify the most beneficial control signals and stabilization requirements. An excitation control scheme is designed, employing a differential shaft speed signal, which stabilizes an initially unstable resonance condition and provides rapid damping of subsynchronous shaft oscillations.