We are facing an issue on our cement plant. It's raw mill main drive motor of 4300kw. On no load condition, vibration is normal but as the feed come, vibration reaches to 4-6 mm/sec (fluctuating between 4 and 6) and whole vibration is on 85Hz (which is natural frequency of end shield bearing housing as told by manufacturer). According to manufacturer, this frequency is being excited by some forcing frequency which is unknown.
Second thing which is observed is vibration amplitude is greatly influenced by line frequency. Vibration exceed 4mm/s as line frequency exceeds 50 hz, and it reduces to 2mm/s as line frequency drops below 50 hz i.e 49.9 hz.
Thirdly, at no load, dominant frequency is 2Lf (0.3mm/s) whereas at load conditions 2Lf is there (same magnitude i.e. 0.3mm/s) but dominant frequency is 85Hz as discussed above.
Fourth observation is, 2Lf and 85hz disappears as the motor is switched OFF.
Spectrums are attached for reference. Please guide.
PS: An year ago, insulation resistance of this motor was dropped below allowed value and then it was sent to vendor where is rotor was cleaned and painted/polished.