Dear Becar, motor rated rpm is 995. We noticed from the display of control room where values of Lf and vibration values from installed vibration sensors are coming that fluctuation of Lf (which is almost between 49.8 and 50.2 Hz) causes change of vibration from 2mm/s to 4mm/s or even more. When Lf is 50.2Hz, vibration will be 4 or 5 mm/s and when Lf drops below 50hz, vibration values decreases to around 2mm/s. Maximum fluctuation in Lf is about 0.5Hz.
Speed(RPM) = 120 f(Hz) / P(Poles)
So Fluctuation in speed will be 120*0.5/6=10RPM
So we can say that maximum fluctuation in RPM due to fluctuation in Lf is 10rpm causes change in vibration from 2mm/s to 4mm/s.
Thus phenomenon is quite strange for us and it only in load condition. This fluctuation is not observed in no load condition.