While taking trail of a new motor some abnormal sound(humming sound) observed in Motor solo run.
Initially motor trail taken after alignment on 06/06/18.The vibrations were normal until 750 RPM and once it reached 750 RPM abnormal sound and increase in vibration levels observed and max vibration of 6.1 mm/sec observed at Motor DE Horizontal with harmonics of 80 Hz.After 850 RPM the vibration levels decreased to 1.2 mm/sec.Please find the spectrum of the same.
The motor was not taken into operation due to abnormal sound.
The same motor was taken trail again on 08/08/19 on floor.The vibration levels gradually increased to max vib of 2.3 mm/sec with abnormal sound.Spectrum indicates frequency of 66.25 Hz at 990 RPM.The frequency decreases as RPM decreases.Please find the spectrum of the same.
Both Drive end and non drive end bearings inspected and no noticeable defects observed.
Motor Details as follows
NDE bearing:NU 226M(insul)
DE bearing:NU 6230M + 6230 C3
The abnormal sound disappeared once the supply is switched off.
What could be the reason for abnormal sound and high vibration ??