The rotor-armature structure of an exciter rotor used in a steam turbine-generator unit is investigated in this work. The objective is to create a base verified linear dynamic model of the exciter rotor for fault diagnosis or nonlinear interactions analysis. Natural frequencies for the rotor-armature structure are obtained from modal testing in free-free conditions. An analytical model for the structure is then constructed using Timoshenko beam theory, in which the linear effect of contact interface between rotor and armature is included. The unknown parameters of the rotor-armature model are identified by comparing analytical and experimental results. The verified dynamic model serves as the base for fault diagnosis and nonlinear response analysis due to the rotor and armature interactions.