Hi guys,
What may be the problem of this Motor?

AC Induction Motor
Make : - GAMAK; GM-315-L-4-b
1780RPM; 200KW

The spectrum is attached. The sidebands are at 7246 CPM. The Rotor bar/Stator Slot details were unknown.

Have a nice day!!

Doc2.doc (214 Kb, 78 downloads)

Jenish,

The SBs are likely spaced at 7200 CPM rather then 7246 CPM which can be verified if data is taken with better resolution. The only thing that might be a matter of concern is that SBs are larger in amplitude then RBPF itself ( could be resonance related), and it appears that 48x is the RBPF.

How does the spectrum look like in the lower frequency range ( in velocity units)?

David

Hi Jenish,

Line frequency 50 or 60 HZ?. If it is 60 HZ, can it be 2XLine frequency?. If yes, then possibility of Loose Rotor bars.

Hi Jenish
I looked at this post a few days ago and from memory, and a rough calculation it seems you have a 4 pole motor and a 2 X line frequency thing happening, possibly a softfoot condition?

Hi guys..

Thanks for your valuable comments!!

By the way, the spectrum posted was a high resolution spectrum with 6400 lines. The zoomed spectrum is attached here.. The sidebands are at 7247CPM...

The line frequencies here in Saudi is 60Hz.. But there may be some fluctuations in the frequencies due to the problem in our Power Plant!! I will check about that later..

There is no pole pass frequency sidebands around 1X..

Is it softfoot or any other electrical problem?

Why the sidebands are quite bigger than the primary RBPF?

Have a nice day!!

Doc1.doc (140 Kb, 19 downloads)

If we consider only the forces (no resonance amplification), the electromagnetic forces at one of the sideband frequencies (either upper or lower) is often expected to be higher than at RBPF frequency. This result may seem non-intuitive, but pops out directly from fourier analaysis of the magnetic forces when we consider the number of poles and dslots.

Personally, I tend not to get excited about RBPF pattern. Especially if there are no pole pass sidebands present (and if there were pole pass sidebands, I would look at current signature).

The key thing imo is to confirm that this is a RBPF pattern, and not something more sinister like a bearing defect pattern. It certainly looks like RBPF pattern to to me based on spectrum and TWF (although a little zoom-in on TWF would help... no sign of impact/ringdown would be a good thing). If you want to investigate furhter you line frequency, take a look in your spectrum at the peak near 7200cpm and use the peak label feature to estimate that peak as exactly as you can. The closer it matches your sideband spacing, the more confidence you can have that you are looking at a RBPF pattern.

If it is a 2-pole motor, there is a small chance that you might be able to lower RBPF pattern by removing soft foot... but I wouldnt bet on it. Some motors just have that pattern show up strongly as a result of their design.

This message has been edited. Last edited by: electricpete,