Hi guys,
I think this was already discussed several times in some other posts, but still i was not satisfied when the problem comes to me..

The bad actor is an Ac Induction Motor, VEM make, 1793RPM, 2100KW, 4000V, 337A.This motor was overhauled 2 times in our workshop due to rotor bar problem. Last time when we removed it from the plant, the level was 10Gs. After overhauling we tested it in our workshop at no-load condition(with half coupling) and found that there was still a distinct peak at RBPF...So we decided to overhaul it again in an outside worshop.Today i tested the Motor again and found the same signature..What may be the cause?I think the levels will come up when we load the Motor,ryt?

I'm attaching the spectrum here..The Non-Drive End reading was taken directly from the Motor body because the bearing housing was not accessible..

Have a nice day!!

DBK-2003.doc (47 Kb, 50 downloads)

I'll repeat some of my opinions FWIW:
* High RBPF pattern vib can be inherent in the design of a perfectly healthy motor.
* 2 - pole motors definitely are more prone.
* High RBPF pattern vib can be increased by soft foot problem on 2-pole motor as reported in two case studies in P/PDM magazine.
* In theory, rbpf pattern vib can also be increased by eccentricity.
* rbpf pattern magnitude can increase with load.

Have you seen the Siemens article?

Recognizing that RBPF +2LF pattern can be normal on a healthy motor, I would think it should never be cause to call a motor bad unless there is some other factor like pole pass sidebands.

TIOMOAICBTWA. Interested to hear other opinions or experiences.

Jenish,

10 Gs is a fairly high amplitude but I am inclined to agree with Pete, "Slotpass" and/or "Rotorbar Pass" does not necessarily make a good motor bad.

Now a 10G amplitude might say "something is wrong", but looking at the spectrums you have posted here, the motor looks fairly good.

In these "repairs" which were done on this motor, especially the one after the 10G peak was seen, what was found on inspection? What work was actually done?

Remember, this is only my opinion and I could be totally wrong.

This message has been edited. Last edited by: Ralph Stewart,

Pete & Ralph,
Thanks a lot!!

I couldn't find any pole pass frequency around the 1x. But i think something went wrong here!! We checked this Motor in our elecrical worshop with half coupling as i said before. The name plate data shows 1793RPM. But when i checked the RPM with my analyzer, it was showing 1800.6. Can an AC Induction Motor run above itz synchronous speed? If possible what are the factors affecting this? How can i determine a pole pass frequency in this case?Ha ha ha...

About your questions Ralph, i don't have a clear answer now coz i was involved in this case now only. Anyway i will try to find what they did before!!

Have a nice day!!

quote:

But when i checked the RPM with my analyzer, it was showing 1800.6. Can an AC Induction Motor run above itz synchronous speed?

Checking for an "exact" speed with an analyzer in a spectrum, if that is what you did, is not the best way. A strobe light (good one) might be the better.

I am no electrical engineer, but as far as the speed being less than 1 rpm over 1800 rpm, I would say it is quite possible to "capture" such a speed with an averaged spectrum (or averaged tach light signal) since the rotor is free to more or less do what it wants with no load. A strobe light might shine some light on this question.

I do not think I would "worry" myself with 0.6 rpm error from a spectrum, of course that is only my opinion and I could be totally wrong.

Remember too that spectra have limited resolution. SAay for example that you have a 400 line spectrum with a 400 Hz Fmax. Each bin is 1 Hz wide so a peak might be at 59.8 Hz but you'll read it out as 60 Hz with the cursor. FFTs make pretty bad tachs.

There are methods to read the exact frequency of a peak in a spectrum by comparing the height of lines in adjacent bins. Another trick used by experienced analysts is to look at higher-order frequencies, such as vane rate, gear mesh and even rotorbar frequencies and dividing by the number of elements to determine the rotation rate.

This is a good point to make a comment about zoom. Yes, you might be able to zoom with 16,000 lines over a 5 Hz bandwitdh as the manufacturer claims. Just remember the time to acquire a single sample is 1/(bin width). In this example, the bin width would be 5/16,000 = 0.0003 Hz, so 1 sample will take 3200 seconds or 53 minutes! EVERYBODY is stuck using the same physics, so don't believe anyone who tells you otherwise.

Jon
Spintelligent Labs

I agree with the others the speed doesn't appear unusual.

You'd expect 1793 +/- a few RPM at full load and 1800 +/- a few RPM at no-load.

The +/- a few may include measurement error and the way the peaks are extracted from the spectrum, and more importantly may include tiny deviations from 60hz supply frequency normal in the US. If your speed was reading low (not the case), you would also consider variations from differences in voltage (low or unbalanced voltage can give higher slip) and also differences in rotor temperature.

Hi guys,

Thanks for your valid infromation...

Let me tell you about some of my observations.. I agree with you guys that for precisely measuring the speed, our analyzers are not good enough!!

I took a reading with 50Hz, 3200 lines..
So the measurement accuracy will be 50/3200 = 0.015625Hz.
Say, we are looking for a speed of 29.97Hz
Then 29.97/0.015625 will be 1918.08.. That means the closest no: of data points will be 1918..
1918*0.015625 = 29.96875Hz.
So theoretically we have to expect only that much difference in the spectrum.ryt?

My RBPF is at 122420cpm and i have 68 bars. So there also it will come as 1800.294..RPM. Anyway i agree with Pete that at no-load, the motor may run at +/-1800. May be there is some fluctuation in the supply frequency.

But still the problem remains!!How can you identify a pole pass frequency around the 1X in this situation? Or is it not possible to identify any rotor bar problems during no-load testing?

Have a nice day!!

IF pole pass modulation were present during uncoupled run, then likely the only way to detect it would be to monitor for over a period of time. Pole pass modulation would show up as very slow change in magnitude over time (maybe a 5 or 10 minute oscillation period).

BUT, we should also recognize that rotor bar problems might not show up at all until load is placed on the machine. One rule for current signature analysis is to be above 75% load.