Electrical frequencies, what do they mean? 1

[Pavel2]

I am doing vibration analysis, using the CSI 2120 analyzer, and have noticed that all the 3000 rpm motors (50Hz) in one of my departments suddenly showed an increase at 40 to 52 times running speed with 2 times line frequency sidebands. In acceleration it went up from 0.07 to 0.55 g's. I then remembered that the transformer feeding this section blew a coil and was replaced with another one. I assume that something in the transformer is reacting to what is probably the rotorbar frequencies in the different motors. Is this serious, what does it tell me about the condition of the transformer, and why the increase?
Regards from South Africa

 

[electricpete]

Most commonly, rotor bar pass frequency with 2XLF sidebands is an indication of eccentricity (due to soft foot or misalignment). If you go to the reliability-magazine.com and follow links to Alignment/Vibration forum you'll see a recent discussion entitled "stator slot pass frequency" which touches on rotor bar pass frequency patterns as well. Also use their search feature and look for RBPF or rotor AND bar for lots of discussion on this topic.

This does not match your scenario of course, where all 2-pole motors increased at once (unless there has been a drastic change in temperature which might affect the alignment of all of them... or maintenance shutdown where all where tampered with?)

The power supply would seem to be the only common link among different motors. The only remotely-possible contributors associated with the power supply in order of decreasing likelihood would seem to be:
1 - Increased voltage.
2 - Imbalanced voltage
3 - voltage harmonics? (not too likely).
4 - frequency - I seriously doubt you had a change in frequency.

If you are convinced there is a problem with the power supply I'd recommend checking all three phase-to-phase and phase-to-ground voltage magnitudes.

 

[ronfrend]

I have used the 2120 a lot and I have found similar results. Almost always the problem has been an imbalanced load as ElectricPete suggests. Again this ties in with the change of transformer. I would check your supply and also check any power factor correction circuits you might have. Chances are that this is not a problem - just different.