Dear All,
I want to check for rotor bar problem of a 3.3KV SCIM but our current probe's (AEMC Instrument, Model MR411) maximum operating voltage is only 600V.
Is it OK to take data for MCSA in the CT?
I already tried it using SKF CMVA65 data collector. However, data is fluctuating considerably. Is this fluctuation expected?
Please share me your experience.
Thanks.

MRTA

For medium voltage motors, you would always take data at the CT secondary.

What do you mean by fluctuation? If you are referring to the modulation at slip frequency, this is standard in case of severe rotor bar damage.

Just take care to verify the data. I have seen cases where significant hunting was seen in the measurement CT, but none when re-checked at the protection CT. Those turned out to be loose connections or other CT problems, nothing in the motor.

Regards,

Aditya

I second the discussion by Aditya.

We take all our data at the CT secondary for our motors that have CT's (which is our medium voltage motors and only a very few of our 480v motors). And we do it using our Entek datapak data collector.

Pole pass sidebands around LF in the spectrum are equivalent to modulation of the TWF at pole pass frequency. So as was mentioned, the magnitude oscillation you are seeing may well be associated with those pole pass sidebands you are checking for.

The test should be performed at high load - at least above 50% and preferably above 75%. Also make sure you have enough resolution to see separate those pole pass sidebands from the main frequency

Hello All!
If I am wrong, please, tell me about it, I want simply to add some thoughts to yet existent discussion.

quote
For medium voltage motors, you would always take data at the CT secondary.
quote

quote
We take all our data at the CT secondary for our motors that have CT's
(which is our medium voltage motors and only a very few of our 480v motors).
quote

Is the harmonic content of REAL and MEASURED currents are the same? or what is the distortion due to CT usage?
What current and potential transformer (CT and PT) are used for MV motors (existent or special) -in other words, I say about frequency response of those CT's and PT's-is it practically constant for all interesting frequency range?

Point is that we often see Power Quality measurements on MV side. On question "What do you measure?" answer often was "Mainly Harmonics on MV side". On our question about frequency response of CT often there was no any IN ESSENCE reply...They simple use existent and yet installed CT.

Another point is that now in Russia there is no any Standard on frequency response on CT, only for first harmonic of line frequency. It is enough for voltage sag, swell, outages measurement, but not enough for harmonic measurement and especially for on-line MV motors diagnostic.

(As you can note, hardware for power quality measurements and on-line motor diagnostic is the absolutely same (PT and CT -ADC - PC Notebook with software for data registration); naturally, software for those tasks is different, but common part exist in those two tasks-Harmonics...)

Best regards,
Victor


(Technical director
A&Alpha Consulting

Victor Petoukhov
Ph.D, IEEE member e-mail: a.and.alpha<at>gmail.com
http://www.motor-diag.com )

I used to ask this question but no clear-cut answer was ever provided by the clients. Practically, you have no choice but to use the existing CT.

As long as the faults are being correctly picked up & the spectral data seems to be valid, no issues. Secondly, the idea of opening an energized medium voltage panel & placing current clamps around the cables is not exciting at all. Finally, the voltage has to be taken from the PT only, cannot connect it directly to 6600 V.

I had taken data on some large low voltage motors (200 kW, 415 V) directly from the power cable as well as from the CT. The direct measurement does seem to be more sensitive (see attached data), however I wonder if the difference is sufficient to be worth the risks involved.

Regards,

Aditya

Data_comparison.doc (220 Kb, 24 downloads)

Dear Aditya, thank you very much for reply.
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Practically, you have no choice but to use the existing CT.
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Why? Why not to deenergize medium voltage panel and place "right" CT with known freqs. response characteristic? Yes, it is more complicated, but (in my humble opinion) it is more correct.


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however I wonder if the difference is sufficient to be worth the risks involved.
quote

Yes, you are quite right, but only for this motor.
More correct to look at freqs. response characteristic of used CT.

Especially because
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The direct measurement does seem to be more sensitive
quote


Victor

Hello, everyone!
I ‘m familiar with similar instrument ( CMVA60) and like Aditia, in the past I had taken data of our low voltage motor ( 110 – 200 kW , voltage in Serbia is “only” 380 V) directly as well as from the CT. I have same experience that direct measurements gives more sensitive values but I can ‘t save and share data with you because ,unfortunately, my SKF Machine analyst can not open MCSA data but always when I had bad result in direct measurement it was also bad from CT's measurement.
I agree that the difference is not worth the risks in measuring on LV motors and thinking about direct measurement on 3,3 kV is pure lunacy!!
MRTA,
I share my set-up for MCSA with Gurusamy on this board for CMVA 60 about two months ago
http://maintenanceforums.com/eve/forums/a/tpc/f/7161085912/m/9201014453 and it can be useful for you. In the past I had problem when in setup for option Motor nameplate data instead OFF I tried Specify motor with rated full load RPM and Rated full and No load Amps. This option did not work good because then data, in my case, shows values for speed from nameplate and I had to manually find the highest pick in the spectra on the left side of 50Hz pick and right speed of the motor. If I select option OFF, instrument automaticly find sidebands and right value of motor speed!
You have very good instrument and If you , in measuring from CT ,found motor with result below 40 dB, verified with Enveloped AC motor current and vibration analysis (haystacks at 1-6 frequency of running speed) don't hesitate, overhaul it and putt away into repair shop
Have nice day and good hunting

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

Dear All,

Apologies for my late response..
I thank you so much for the time you've given in responding to my inquiry and for sharing your knowledge and experiences.

The fluctuation that I meant is the instrument manipulated value in dB. As what PANTA mentioned, using SKF instrument, we can obtain the dB reading and there is also automatic diagnosis of rotor bar condition depending on the dB value. But during data gathering, we experience fluctuation of the dB value, say, within 30~50dB range. I'd like to know if this is normal. Of course there are other considerations or factors in rotor bar condition diagnosis and we also evaluate. It's just that the instrument is new to us.

We'll cosider your inputs in our next measurement.
I'll give feedback when we have reults. So that I can also share it to others..

MRTA

Hello, MRTA!
I didn’t have experience with that big fluctuation (30 - 50dB). The difference in value during gata gathering is below 1 dB in my readings.

First, you should look in your setup in field for Number of average (I usually choose 4) and in field for Average Overlap (My favourite number is 50%).
If your choice is None for Average overlap then 'Each spectrum ensemble is new data added to the sum'. And you could obtain huge difference in each average.
If you choose 50% Average overlap instrument use 50% of exiting data and 50% of new data to calculate new FFT and then readings between each average are similar.

Second, bear in mind Aditya’s post and his experiences with significant differences in readings from protective and measurement CT. I didn’t have similar problem but you could check out ,connect current clamp on another CT ,shoose right sensitivity for ratio of CT and check the value of motor current on Microlog screen for every average.

Good hunting!

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

MRTA,

Please clarify regarding the dB fluctuation:

1. Is it happening between consecutive measurements?

2. Is the difference between the upper & lower sidebands?

3. Are you seeing data in the 'Live' mode & observing fluctuations?

Victor,

I guess there will be some advantages with known CT frequency response. Do you follow this procedure with your clients? Do you use a split-core for this purpose & is its accuracy acceptable?

Regards,

Aditya

Hello Everyone!

Regarding the fluctuations, I am seeing it in the 'Live' mode before data is being saved.

But now, i have to try again with reference to the set-up given by PANTA. I see lapses in my set-up which I think the significant cause of the seemingly "abnormal" fluctuations.

To PANTA and ADITYA, you've taken one's hat off in my person!

Aditya,

I remember story which was almost appr. 7 years ago. The first question we was asked by grey-headed, expert and wise Chief power engineering specialist of one large enterprise was about frequency response of used CT. We say then "We do not exactly know, but it is not very important". "When what we are talking about? and how I can believe your measurements (here can be artifacts), because you deal with harmonics, and measured signal contain motor current harmonics,INFLUENCED by CT. May be influence of CT is minimal in your measurement frequency range. But, please, prove it. " -he said.

So, for us it is a principal item of measurement (as atialiasing, subsequent DSP of measured signals, etc).
Simply we well remember those words and do not want to hear them again.

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Do you follow this procedure with your clients?
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Yes, and firmly. Either clients use recommended CT for MV motors, or they are not our clients NOW. (But they became our clients later, we have such examples- as we all well know, "everything flows, everything changes").

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Do you use a split-core for this purpose & is its accuracy acceptable?
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We use different types of CT for MV motors diagnostic.
Please, visit www.lem.com and contact their local representatives.

Regards,

Victor

I haven't given much thought to frequency response. Good example data posted by Aditya.

Looking for pole pass sidebands around 1*LF, I don't think the frequency response will be important because the frequencies of interest are very very close to each other and we are looking at a ratio.

If one were looking for power quality, it seems the frequency response would be important.

As far as those swirl frequency pole-pass sidebands below 5*LF mentioned in the other post... I am not clear what kind of comparison would be made (do we compare them to 5*LF or to 1*LF?). If one were looking for a dB down from the 5*LF peak, I don't think it would be important (as long as there is sufficient dynamic range) since the frequencies are close and the ratio would be unaffected. If we tried to ratio those sidebands to the fundamental, then the frequency characteristics would be important.

EPete & Jank,

Do you have an idea about whether the presence of these sidebands around the line frequency harmonics is more likely to be eccentricity or bar damage? As mentioned earlier, GE & ABB seem to contradict each other on this point.

Regards,

Aditya

Aditya - It's a good question that I'm trying to understand also. To help keep the threads focused, I replied in the other thread:

http://maintenanceforums.com/eve/forums?a=tpc&s=9171034...191033863#9191033863

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