motor current sidebands 45db down and stable - what shop checks to do?

No responses? I guess I made the question too difficult with too much irrelevant background information.

Let's forget about having a short availability of the motor.

The question is: what would be a reasonable shop tests to evaluate the condition of a rotor which tests marginal (45db) during on-line tests?

I'm thinking:
Single-phase test
rotor core loop test with infrared
visual inspect.


Does that sound like a good plan or would you add any other tests? Thanks.

I have often detected the defective bars using a growler as well as by current injection + infrared. However, sometimes a growler shows more than half the bars to be defective, so cannot always rely on it.

What is the point of the single-phase test, as the issue is to identify the defect location, isn't it?

With atleast two cases (these were 735 KW motors), all of the above & ultrasonic & dye penetrant did not indicate any flaw. We then had the repair shop de-braze the bars from the short-circuiting rings. It was then seen that many bars had just been inserted in the rings with some superfluous brazing on the outside. However, there was no material contact inside and the bars were effectively open.

Baker has a version of a surge tester (RT-1)that is designed for rotor bar testing. I found it too expensive to buy but some repair shops might have it over there.

Regards,

Aditya


P. S. What is a green paper test?

Thanks for your response.

You're right. The single phase test probably is not logical in this situation considering the motor would be disassembled anyway. Better to take advantage and get what data we can while disassembled.

So then I have
rotor core loop test with infrared
visual inspect

Maybe I should add growler? Any other comments? (if not I guess I'll ask the shop for their recommendations/preferences)

Baker off-line rotor bar tester... I have never heard of that. I don't see it here:
http://www.bakerinst.com/BakerWeb/Products/Products_Offline_Online.html


Green paper is flux paper. You can put it over portions of the core during the loop test (if not too hot) to show the flux pattern. Like this:
http://www.kjmagnetics.com/products.asp?cat=154

You can see the Baker instrument at http://www.baker-instrument.de/rotore.htm.

Reliance Motors, USA has a standard practice of injecting DC current between the two short-circuiting rings & viewng with an IR camera, better than a rotor flux loop.

Does this green paper work? They are mentioning it for permanent magnets only.

Regards,

Aditya

Thanks for the info on the Baker tester. Looks interesting.

On page 2 below you will see application of green magnetic flux paper to detecting faults in an induction motor.
http://www.lexseco.com/Files/rotorarticle.pdf

Our repair shop sent us a similar image once for a cast aluminum rotor.

quote:

"Reliance Motors, USA has a standard practice of injecting DC current between the two short-circuiting rings & viewng with an IR camera, better than a rotor flux loop."

Good point. I have seen that test done also: Instead of injecting current in wires looped around the core, you inject current into clamps placed on the end rings.

I hadn't given it much thought before, but now that you mention it, it makes sense that this test would be more sensitive to rotor bar problems than the loop-type core test. The reason is that it seems you will get a lot of bar current with direct injection. With the loop-type core test, I think the majority of flux is deep below the bars and doesn't induce much current in the bars.

I see this direct injection of current into endrings is the test they describe in the Lexseco link:

quote:

Lexseco:
"Squirrel cage rotor testing is a two part process. First we'll excite the cage winding by directly applying voltage through the shaft
clamps. If all is well each path or circuit should be equally excited. We use a magnetic paper to make this determination. Many
of you use iron filings, which is fine. The magnetic paper is just a little neater. Placing the paper on top of the rotor produces
a clear representation of the rotor circuit while excited by the Core Loss Tester's voltage. Our results with this method have been
excellent. Failed cage windings have consistently evidenced a void, gap, in the particle alignment of the paper.
Sometimes, the second phase of this qualification also helps. We'll warm the rotor by increasing the applied voltage. This tends
to demonstrate hot spots indicative of problems. Remember, since you're not attempting to determine the condition of the laminations, you need to look for uneven heating of the bars. We find the majority of problems occur at the connection of the bar and the endring. Depending on the specific nature of the problem, the fault area may also spark as the voltage attempts to cross the gap"

EP,
A picture is worth a thousand words. I am still a firm believer that the data we can collect while the motor is more than 75% loaded is the best to evaluate the rotor, but while apart, you can't beat current induction and infrared( see attached pic!)

Pole Pass sidebands in the range of 42-48 db indicate rotor bar crack may be developing or possible problems with high resistance joints. Best is to monitor and see if the value is going below than 44 db you see now. We use PdMA MCEmax, besides looking at the pole pass sideband we also check for swirl effect in the current spectrum at 300Hz, or 5th harmonic. The presence of swirl effect is a further indication of broken rotor bars. As you are planning a shutdown on this motor in March, I would suggest having someone with PdMA MCEmax to carry out Rotor Influence Check (RIC), which will provide a graphical representation of the magnetic coupling between the rotor/stator. Analysis of the waveform would help in confirming if there are broken rotor bars.

Download Article of Rotor Bar problems:

http://www.pdma.com/RotorFaultZone.pdf

http://www.pdma.com/PDF/CS0402.pdf

Thanks for the comments Aditya, Ron and UETS.

I decided to go with growler, visual inspecton, and induced current test with thermography cold and hot.

We should have some results in April.

[QUOTE] I would suggest having someone with PdMA MCEmax to carry out Rotor Influence Check (RIC)
[QUOTE]
It was interesting to see the results of the RIC test on a motor with 22 bad bars out of 51 from the PdMA site. As the PdMA states, there is some kind of anomaly on the RIC test.
I would like to ask you your opinion: In case that you see the results of the RIC test only on a motor similar to the one from the PdMA site, would you make a call?
I can see 3 waves that are virtually exact sine waves with total harmonic distortion probably lower than 5%. It seems to me to be a pretty poor resolution considering that 43% bars are broken or cracked. One has to think what would the RIC test look like if only 2 or 3 bars were broken?
Maybe I am missing some important feature of the RIC test because I do not use it. But if the departure from the perfect sine wave variation of the inductance reflects the gravity of the rotor bar problem, I would not bother to perform the RIC test at all.
jank

Jank, you seem to have a lot of negative to say about the PDMA test equipment (different threads), especially for someone who self-admittedly, has little to no experience with it. I personally have used this system with great success... as have many maintenance dept's of some of the biggest names in Industry... maybe they're on to something

As for EP's original question... your finial decisions were great choices. Too often rotor defects are missed at the repair shop level only because it was never inspected. Many people in industry have the opinion the "Rotor's Never Fail" or "In my 25 yrs I've never seen one fail"... because of this many people have become lax in inspection.

The Growler or green sheet (same principal), should find any breaks. The IR will give a great second opinion. However, If there is any amount of damage present, the shop personnel will probably see it during the visual inspection.

As for UETS's suggestion to do a RIC test... also a great test, but in my opinion if it is in the shop and dismantled anyway, better to go with the growler, etc. The RIC however earns its merit in that it is able to be preformed in the field, WITHOUT DISASSEMBLY. Meaning that with you would have confirmation of a possible rotor issue using 2 separate technologies (C.S.A & the RIC)... This would give you the confidence in making your decision to pull the motor for tear-down/inspection.

Vibration data would also help as it would give 3rd opinion.

[QUOTE]Originally posted by Druncle:
Jank, you seem to have a lot of negative to say about the PDMA test equipment (different threads), especially for someone who self-admittedly, has little to no experience with it. I personally have used this system with great success... as have many maintenance dept's QUOTE]

I have actually worked with PdMA for few months and I have done the RIC test. I must have tested at least 100 motors including some DC ones. I have been interested in this technology since something like 1996. So when I said that I do not use RIC test, it does not automatically mean that I don't know what I am talking about.
When you go back to those threads where I am critical of PdMA, I have never criticized their equipment (as you have suggested), but always talk about a very specific aspect of their testing.
But let's rather go to the technical aspect of the question: In case of the RIC test the explanations have been modified over time (no problem with that, we all change our minds). But one does not to have to have a ton of experience to know, that the residual magnetism does not induce anything into the stator at the standstill. Read in the PdMA Fault Zone Analysis "AIR GAP": "Evaluate the influence of the rotor's residual magnetic field on the stator's phase-to-phase inductance as the rotor is positioned".
It is statements like the one above that raise the suspicion about the rest. The RIC test is a very plain single phase test. There is no need to talk about residual magnetism.
jank

quote:

Origionally posted by Jank: Maybe I am missing some important feature of the RIC test because I do not use it. But if the departure from the perfect sine wave variation of the inductance reflects the gravity of the rotor bar problem, I would not bother to perform the RIC test at all.

Jank, not to turn this into a pissing match, but if you do have access to an MCEmax, I would suggest refering to pages 10-20 of the "Data Interpretation Guide." This is the section which outlines and expalins the RIC test (Theory behind the test, results etc).

Your assumption that anything aside from a perfect sinewave means an issue with the rotor is clearly debunked ... In fact in manufactured, copper bar rotors (as found in most larger/higher quality motors) are not likely to exhibit any sort of sinewave when the 3ph inductance vaules (measured in 18 increments over 1 pole face) are plotted. Most copper bar rotors are infact "Low Influence," meaning that the position of the rotor has little or no influence on the stators inductance.

The following table is from pg 13:

condition waveform


Normal: Smooth 3-phase sinusoidal waveforms or non-sinusoidal waveforms in Low-Influence Rotors

Rotor Defect: Erratic inductance throughout the peaks of the waveform or the development of sinusoidal activity on a Low Influence Rotor

Air Gap Eccentricity: Inconsistent variations in the amplitude of the waveforms. Static eccentricity sometimes causes a consistent separation in the three sinewaves, coupled with a low inductive imbalance.


I have easily tested >>>thousand motors using the PDMA system (not to mention other Brands) and have seen time and time again just how successful the RIC test can be. I will however, be the 1st to admit that it has some short comings:

1) Most effective when a baseline was taken (as with most technologies)
2)Difficult to preform in a plant setting when coupled to the load as the shaft needs to be turned. However, as this test is intended mostly as a follow-up/2nd opinion to other tests that the MCEmax offers, which may question the rotor/air gap/stator condition (high inductance imbalance reading in the Standard test or high pole-pass sidebands), I personnaly feel that decoupling is much easier then sending a motor to a shop for teardown.
3) In Cast rotors, casting voids may affect the shape (smoothness) on the sinewave... however, if you have baseline data as stated in #1, side-by-side comparions will allow you to see any changes from the healthy condition

If anyone has more interest, I can provide case studies of just how effective the RIC can be.

Druncle,

This is getting to sound way too much like a PdMA pitch. I am sure you have tons of data with MCEMax.

I too have data on a few thousand motors with Framatome & BJM products (& Baker & Tettex & others) and can make a pitch such that any of them sound like the best thing since sliced bread. Plenty of great case studies.

The more important point though is that none of them can be trusted blindly. With every instrument that I've come across, there have been many false results till I figured out the pitfalls. And no manuals give those. I have met many PdMA users whose instruments are gathering dust right now.

So, RIC may be good but nothing out of the world for sure.

Regards,

Aditya

Aditya,

I totally agree with you that none can be trusted blindly and as i have said in other posts, I have used many other instruments (Baker, BJM etc) and I like many of the features of each. These are all great products and the choice of one in perticular usually come down to what exact goals the end user has for his testing.

Anyone who swears that any of these Brands are the "Be-all, end-all" is sadly mistaken.

However, what I don't agree with is someone making negative posts about a perticular test or product without first doing a proper investagation... which, according to the staements of ______, they have not. That would be compareable to me saying, "You should never do surge testing b/c it can be destructive," when a surge test, when used properly is invaluable.... its just meant for new or reconditioned (clean) motors.

I have read many of your posts and can easily see that you have a personnal preference, however, these same posts show you have a great understanding of the topics... I'd be willing to bet that someone such as yourself could have many success stories with either of these testers (even your least favorite).

We all need to remember that Running down anything based on a personnel bias is not productive to anyone ....

Druncle,

I have no personal issues with you & apologize if I came across that way.

Jank has taken apart many manufacturers on this board, I don't think he is personally against any one. He does make us question ourselves & not accept things at face value and his contributions on this board are most valued. Guess I went off the handle a bit to see him being run down.

Regards,

Aditya

I agree with Aditya. Certainly anyone should be encouraged to express their opinions and the reasons behind them. Then we can all question and judge for ourselves which opinions to believe and which to not. All of jan's comments on this particular thread made good sense to me. The case study appeared to show RIC missing a call, and the presence of residual magnetism seems irrelevant to the NORMAL functioning of the RIC test to detect defective rotor bars (residual magnetism is only relevant as a possible source of interference in the test, as far as I know).

Also I was interested to hear Druncle's comments. I would be interested to see your case histories on RIC.

Aditya, no apoligies needed... no offence was taken. I certainly enjoy reading your postings (as well as those of Electricpete & Jank). I should apoligize if my comments seemed to be attacking. We have all seen previously how quickly comments get out of hand when we become unprofessional.

I will galdly post a couple case studies shortly,(away on assignment for a few more days)

In the meantime, the following link is to an article on the PDMA website which gives a better explaination of the theory behind their RIC test(http://www.pdma.com/Rotortest.html)

Not that we should take any manufacturer at face value without a question, but I believe some earlier comments regarding the RIC test("Somehow it seems to me, that performing a test that is not based on sound science is not an efficient use of time." in a different thread ) may be based on a misunderstanding of the test itself.

Not only should we not accept without question, but we should also remember to not reject an idea without giving it a chance.

Electricpete, I'm Looking forward to seeing your finial results... please keep us posted.

I would like to look into little more detail to explain my skepticism for the RIC test. Enclosed is a curve of reactance of the motor measured on all 3 phases of a 60 Hp motor, 480 V, aluminum cast rotor, closed rotor slots. The reactance was measured with variable voltage and I measured from as low as 0.118 Volts (phase to phase) to almost 30 Volts. Note that the curve for each phase is quite different. If measured for example with 5 Volts, there would be a huge inductive imbalance. Note the growth of the impedance as the voltage is lowered, then there is a maximum and the impedance goes down again.
The increase of the impedance is a very well known phenomenon and is pointed out in the standard IEEE 112. The growth appears on motors with closed slots on the rotor. As the voltage is lowered the flux that crosses the airgap doesn't bother to go around the bars of the rotor, but simply flows over the surface of the rotor. There is enough iron above the closed slots to support this flux. However if the voltage is large, particularly close to the nameplate voltage, the narrow bridges over the bars will saturate and the impedance will go down. You can see the decrease as the voltage approaches 30 Volts. Note also, that the 3 lines for each phase became one. In other words, the inductive imbalance disappears.
The decrease of the impedance, as the voltage approaches zero, is caused by decreasing the permeability from the maximum to the initial permeability.

Measuring the impedance with, let's say, 24 Volt AC RMS at 600 Hz is an equivalent of measuring with 2.4 Volts @ 60 Hz, from the flux densities point of view. The results are nowhere near the impedances the motor "sees" in normal operation. Large inductive "imbalances" may appear.
Let's now look at the LIR (low influence rotor), just an abbreviation that explains nothing. In my view, the majority of the LIR motors are LIR, because they have open rotor slots (mostly manufactured cages). When testing those motors with low voltages, the flux does not have a chance to travel over the surface of the rotor iron; it has to go around the bars. If a rotor is "LIR" it obviously has no inductive unbalance [unless there is a broken bar(s)].
Then there are motors with closed rotor slots. I have been testing rotors long enough to know that the iron on the surface of those rotors is notoriously non-symmetrical. On some slower motors some slots are even open (above the spider) while some are closed. The amount of iron over the rotor bars varies widely. It introduces asymmetry that can be seen as a variable inductance on the RIC test. Yet those asymmetries are totally irrelevant during the normal 60 Hz operation. (This paragraph may need some revisiting, and I will get back to it if somebody wants me to).

The attachment shows the impact of the low voltage on impedance measurement. Everybody can repeat such a test for himself. It requires only a variable transformer, voltmeter and ammeter and the will to do it. But there is another proof. You can go back to your MCE data. From the measurements of the inductive unbalance, take the average inductance and from that you can calculate the ratio of the locked rotor current to the nameplate current. The ratio should be something like 5 to 6 to 7 for majority of the motors. But from the MCE inductances you will find numbers much lower (2x, 3x). The explanation is, that the test does not actually see the rotor bars, it just sees the surface of the rotor and its irregularities. The difference between the open slots and the closed slots is striking. I have done that years ago on about 50 motors, unfortunately do not have the raw data any more.
One more thing: The results of the RIC test from the PdMA site
www.pdma.com/PDF/CS0402.pdf
The data from the link for the motor are: 3500hp, 4160 Volts, 3590 rpm, FLA= 425 Amps. We can also read the average inductance as measured by PdMA: 17.441mH. Assuming that the inductance was measured single-phase, line-to-line, the reactance of the motor per phase at 60 Hz is:
X= ½ w*L =1/2 *2*pi*f = ½*2 *pi* 60* 17.441/1000= 3.285 Ohm per phase. (The factor ½ reflects the fact that the measurement was made single-phase). So we can calculate the locked rotor current I:
I = (V/1.73)/ X= 2400/3.286= 730 Amps.
So the locked rotor current for this motor is 100*730/425= 171% of the full load current. NOT! Obviously something is wrong. The locked rotor current should be 500 or 600 %! The Reliance catalogue gives me data for 3500 hp, 2-pole, 4000 Volts, FLA= 429 Amps, Locked rotor current = 2516 Amps.
The answer is in the shape of the attached curves. The flux density during the inductance measurement was so miniscule (note that the frequency was 1200 Hz), that the measured impedance was totally wrong. The RIC test did not have a clue that there are bars (broken or not) on the rotor. The tiny magnetic flux bypassed the bars completely, flowing over the surface of the closed slots of the rotor. However it obediently created the RIC pattern (with some irrelevant "anomalies"). Considering that the waveforms are created from only 18 points while spinning the 3500 hp rotor in babbit bearings by hand (!), the shape is almost perfect.
It is quite obvious that a proper single-phase test with reasonable current would show high variations - 22 out of 51 bars were broken! I don't think that 425 Amps would be necessary; 200 Amps would be plenty good. A test like that would require much more than battery-powered instrument, but at least it would see the bars.
I am glad that the article http://www.pdma.com/Rotortest.html was brought to attention. Read for example on page 4 in the paragraph Testing with the Motor Disassembled on Growler Testing:"....If a rotor bar is broken, the alternating voltage at the location of the break will cause the thin piece of metal to vibrate. ..."
This article must have been on the net for good 12 years. Nobody seems to have noticed that it is the exact opposite. There are other pearls in that article that rival the: "influence of the rotor's residual magnetism on the stator's phase-to-phase inductance..." as pointed out before.
Another controversial topic, but this time with a big difference. The interested parties want to keep discussing.
The support I am receiving is encouraging.
jank

Originally posted by jank:

quote:

It was interesting to see the results of the RIC test on a motor with 22 bad bars out of 51 from the PdMA site. As the PdMA states, there is some kind of anomaly on the RIC test.
In case that you see the results of the RIC test only on a motor similar to the one from the PdMA site, would you make a call?
I can see 3 waves that are virtually exact sine waves with total harmonic distortion probably lower than 5%. It seems to me to be a pretty poor resolution considering that 43% bars are broken or cracked. One has to think what would the RIC test look like if only 2 or 3 bars were broken?

I have to admit, I have little experience with the RIC test but the above question adresses the core issue: How sensitive is this test?

I have read the paper and could not find a good reason for making a call the author made based on the slightly distorted sinwaves.

David

Good points.

On figure 3 the current signature shows pole pass sidebands only 36 db below the line frequency - an easy call from current signature.

But I think everyone agrees the on-line current test under load is more sensitive than the off-line tests.

The important comparison is RIC vs single-phase test (both off-line tests)... which would you have more faith in?

I have been studying Jan's graphs for awhile trying to figure out what they mean. I'll put it in my own words, and you tell me if I'm wrong.

What we see is impedance vary with voltage, and at low voltage it's unbalanced and as voltage increases it becomes balanced. (The low voltage power frequency region where the impedance is unbalanced is comparable to the RIC test on a voltz/hz basis.)

In a linear system, impedance doesn't change with voltage magnitude.

For a magnetic system, the impedance changes at the low end and the high end and is constant (linear) in the middle. So these test voltages (and presumably the pdma operating at similar volts/hz) must be in the low end where the exciting amp-turns are not much greater than the coercive amp turns (Hc) and the resulting flux density is not much greater than the residual flux density (Br). In these ranges the results are heavily influenced by the non-linear effects of Br and Hc. (Also we might expect the excitation current to be non-sinusoidal in this range.)

I think this is a similar conclusion to the fact that residual magnetism is known to intefere with the RIC test?

UPDATE ON THE ORIGINAL MOTOR
Yesterday, I observed the disassembly/inspection and test of this motor.

(As you remember, it had 45 dB pole pass sidebands stable over time)

Single phase test showed no significant variation in current as rotor was rotated.

2000 amps was injected from end to end of the shaft. We had about 6 hotspots. The worst was at the bottom of the motor 140C at 10 minutes. Several others 60C-100C. rotor core average temp ~ 30C. Green flux paper indicated there was normal current flowing in all these bars.

We pulled off the fans and steel endplate from the rotor for better inspection (slides 1 shows rotor before disassembly, 2 and 3 after disassembly).

At the location of the 140C hottest spot (slide 4), we could see a crack which ran along side of bar (between bar and slot in end-ring).

We saw several other cracks. The worst crack is shown in slide 5 (there was not hotspot at this location).

It seems like those cracks could result in an open circuit of the end-ring. I'm not sure how much the current injection tests current flowing in this pat (as opposed to core test). We couldn't do core test since the holes in the spider weren't big enough to insert the test cables.

I don't have the thermographic images yet. Should have those on Monday.

Any thoughts? The shop recommends that we melt out all the old solder and re-solder. I think that's just a few thousand dolars and a few days work. But based on time and to lesser extent budget considerations, the plant would rather get the motor back and reinstalled, save the money this year, and schedule to pull the motor again in 3 years to do that repair. Since it has been stable, that seems reasonable to me.

Any thoughts at this point on the severity? Has anyone seen anything like this?

Here is a repeat of the earlier powerpoint with some thermography shots added.

Slides 4,5,6 are the hottest spot, "location 1" at bottom of the rotor (reached 140C at 10 minutes). The hotspot shows up better from the side of the rotor (through gap between end of core and end ring) than from end of rotor. We did verify normaly current flow in this one with green paper.

Slides 7 and 8 are the worst crack by visual inspection, "location 2" at the top of the rotor. It didn't show as much of a hotspot from the side view. But from the end it's a strange pattern that I can't make sense of. Any thoughts on that? (slide 8?) We didn't use green paper at this location (I wish we would have).

We are going to do the logical thing and repair it now (melt out the solder and resolder), rather than putting it back in service and pulling it out again later for repair.

There are several good reasons not to take the thermography on the rotor too seriously.
Passing the current through the shaft means creating magnetic flux circulating in the back iron of the rotor around the shaft, similarly to the flux in the current transformer. If there is a conductive path around this flux, the current will circulate through this path.
If the cage is not insulated, the conductive path always exists. The path consist of: 1. Shaft, 2. Lamination on one end of the rotor, 3. Rotor bars, 4. Lamination on the other side of the rotor, 5. Shaft. The circuit is complete. In other words the path around the flux is quite complicated. Also, this path has never been designed to carry any current at all! So the current will find few "good" spots to transfer from the lamination to the bars or the rings. But the spots are not good enough; there is enough resistance to cause the spots to heat up. The thermography picks those spots as the "trouble" spots. But in fact those spots have nothing to do with the "week" spots of the cage.
Let's examine the posted example. If I measured correctly, the length of the iron is about 20", and the diameter about 30". The number of bars is about 66. From the data in the original posting one can calculate the current in the bar to be about 477 Amps(at full load). If the injected current through the shaft was 2000 Amps, then the current per bar could not have been higher than 33 Amps (2000/66). Yet according to the thermography it caused a noticeable heating.
Obviously, at the full load the watts in the "fault" would have to be considerably higher: (477/33)^2 = 209 times higher. And during the start-up (5x nameplate current) the situation would be even more serious. The watts in the fault would be (477 *5/33)^2=5223 times higher. If one calculates that the watts during the thermography reaches 100 W, then during the startup it would be 500 kW! The bad spot would probably evaporate.
I have tried the method on a rotor with aluminum cast bars. The end rings were touching the lamination. At about 4000 Amps through the shaft, the lamination on either end of the rotor were glowing red-hot. The extremely poor conductivity of the silicon steel lamination, transferring the huge current to the end rings resulted in this enormous heat build up.
I do not use this method any more.
jank

Those are great comments. I have been wondering about the path for current during the injection test and also during a rotor core loop test.. I'm still thinking about that.

The thermography did take us straight to the location of several cracks in this case like the one on the bottom at location 1. The crack is located between the bar and the endring and it extends the entire length of interface between bar and endring around the corner from the side view to the end view. Do you think from the visual images that the crack appears to be a problem?

Looking at the thermography at location 2, it looks like current might not flowing in the copper bars, although I'm not sure. But again it is different at this location than at the others, so it gives a clue of places for a closer visual inspection. But visual inspection is also of somewhat limited value.

It seems like we could get a better thermography test more directly testing the endring to bar joints if there were some means to clamp directly to the end-rings, but they are not very accessible for a clamp. How about if we had an ajdustable copper strap that you could cinch up tight all the way around the end ring to apply current?

Any other suggestions?

To know the rotor bar defects by injecting DC current between the two short-circuiting rings,
LOOKS TO BE A INTERESTING METHOD. How much dc voltage we can apply with respect to rated (ac)voltage. And what should be the duration. can we get the details of this test.?

It is good to see Aditya on the board.

DKSONI

In order to figure out what would such a test require, let’s start with the necessary current. Suppose you want to reach current in each bar equivalent to the full load current. Here is a formula to estimate the current per bar:
Ib=1.35 I V p/(Q f D L B)
Ib …..current in the bar
I…… stator current
V……Voltage phase to neutral
p……number of pole-pairs
Q…..number of rotor bars
f……frequency
D……Rotor diameter
L…….Iron length
B…….flux density in the airgap
For a small motor 7.5 kW, 50 Hz, 1500 rpm, 380 V line to line, 44 bars, L=0.124 m, D=0.153 m, p=2, B=0.62 Tesla, the current per bar is 355 Amp. Hence if you want to load each bar to nameplate current, you need:
355 *44=15620 Amp. Not an easy task. And it is just 7.5 kW motor.
jank

Here is another method to approximate rotor bar current. It is from an article in the August 2006 EASA Currents, by Chuck Yung and titled Rotor Bar Current: How To Determine. He is calculating the rotor as the secondary of a transformer with the primary being the stator and doing a turns ratio comparison:

“To calculate the approximate rotor bar current, use the following equation:

(.96 x stator slots x turns/coil x stator amps) / [(rotor bars / 2) x stator circuits x k]

where k = 1.0 for a wye connection
= 1.732 for a delta connection

For example, consider a 100 hp, 4-pole motor with 48 slots, 11 turns/coil, and connected 2-delta @ 460
volts and rated 120 amps. The rotor has 39 bars.

(.96 x 48 x 11 x 120) / [(39/2) x 2 x 1.732] = 900 amps”

Thanks

I realized I never updated this thread with results. Here are results (db difference changed from around 45 before repair to around 50 after repair):

11/19/98 45.4
12/15/04 44.6
1/11/07 45.3
4/xx/07 - Solder repair of bar/end ring joint
11/13/08 49.7

EP,
Excellent case study. I wanted to ask you if you ever captured any high res, low freq vibration data on this motor? I have always contended that the MCA, Vibration sideband modulation, and the good old analog amp meter are all showing the same thing. Importantly, the online current test quantifies it, as your study indicates.
The question now is, why didn't this motors readings not increase to the 60dB range?
Ron

This motor has 5 sisters. The historical db on those is 57 to 68. I don’t know why we didn’t get there on this motor.

The repair was described as melting out the solder and resoldering all bars on both ends.