Folks, Can you please help me to identify what's happening in the cooling tower fan according to the data attached herewith?
VFD motor gearbox driven fan, a sudden increase in vibration level and noise level caused by approximately @2x vibration with fan run speed sidebands. Modulation is evident at the time waveform. Motor was running @16.5Hz at the time of the survey, fan run speed @2.011Hz, No. of blades 7 making the blade passs vibration @14.072Hz, GMF1 @484Hz, GMF2 @154Hz.
No apparent signs of looseness were found during the visual inspection (inspection was limited though). Last oil sample in August result was normal. No significant increase in gearmesh vibration levels. Oil level was checked at normal level.
Is there a possibility of resonance being excited somehow?