Modal testing of a rotor bearing system is performed by using an electro-magnetic exciter. The exciter, consisting of four electro-magnets equally spaced along the periphery, enables the rotating harmonic excitation of arbitrary frequency to the rotor system. The modal testing scheme utilizing the specially designed exciter features that the forward modes of an isotropic rotor system can be completely decoupled from the backward modes in frequency domain analysis. The transfer functions obtained by this scheme suggest that the conventional modal testing scheme may mislead the results when applied to rotating machinery. Analytical and experimental works are carried out to confirm the difference between the proposed and the conventional modal testing schemes.
