Sub-synchronous low-frequency vibration causing rotor dynamic instability is a major vibration fault in rotating machinery. General experience suggests that the low-frequency vibrations of sub-synchronously unstable rotors are mainly caused by bearing instability(film whirl and oil film oscillation)and airflow excitation. This paper conducts a modeling analysis of the low-frequency vibration fault occurring in large synchronous condensers due to sub-synchronous instability.The Riccati transfer matrix method is applied to calculate the natural frequencies and stability of the condenser rotor.Subsequently

numerical simulations based on Hertz′s contact theory are performed to analyze the vibration response waveforms and spectra of the condenser rotor under contact conditions.This study investigates the mechanism and fault characteristics of rotor subsynchronous low-frequency vibration faults induced by contact and non-contact interactions

eliminating the commonly held belief regarding the instability fault of the condenser bearings. Additionally

it corrects the traditional understanding that contact mainly leads to rotor operating frequency vibrations

confirming that contact interactions are the primary cause of lowfrequency vibrations in condensers.The findings guide on-site repairs to eliminate contact faults

allowing the condenser to return to normal operation.This deepens the understanding of sub-synchronous low-frequency vibration faults in condensers and provides useful references for vibration management in equipment.