DynoMech undertook the analysis of a direct-drive 33MW turbine-generator (TG) unit of 3000 rpm, where, upon replacing the generator, vibrations of the order of 10 mm/sec and above were observed. High vibration levels were observed at Bearings #3 (LP Rotor Front End), # 4 (LP Rotor Rear End) & 5 (Generator Rotor Front End). These vibrations caused the TG set to run at less than its maximum output.
The 33 MW TG Unit was a direct drive system and hence called for a detailed rotor-dynamic analysis. Safety margins in terms of resonance frequencies, amplification factors, damping ratios, logarithmic decay and stability established for each Individual rotor would not be sufficient to ascertain a safe vibration limit of the direct drive TG Set since the three rotors (HP, LP & Generator) were coupled directly and would give rise to coupled vibrations which would often lead to lesser safety margins or resonance criteria.