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Motor problem - loss of torque, slow acceleration with open shaft, noisy

Motor problem - loss of torque, slow acceleration with open shaft, noisy

Motor problem - loss of torque, slow acceleration with open shaft, noisy

(OP)
I got asked to give a second opinion on a motor which we have at our overhaul contractor's works. The machine is a form-wound 3300V 250kW type on a 355 frame. It suffered a failed DE bearing which cooked itself, hence it was removed from site and sent for overhaul. The stator was untouched during the rebuild and we don't believe there has ever been a rewind, so we are reasonably confident that the stator connections are correct. The physical construction means that inadvertently swapping polarity of a winding could only occur during a rewind. The stator passed a Baker test.

Following rebuild the motor is noisy with a continuous low-frequency noise, not a growl or buzz, more of a rumble. The motor accelerates very slowly on an open shaft test where there is only windage and bearing friction to overcome. The rotor is mechanically free and coasts for a long period once moving. On this basis we think there is a gross lack of torque during acceleration rather than any mechanical drag. If the bearings were dragging to the point of stalling a 250kW machine they would be torn to bits, and this isn't happening. During a start attempt the motor crawled up to speed over about a minute or so, and the peak current captured by the clamp meter was just over running current, i.e. abnormally low for a DOL start.

The motor load is a centrifugal pump which currently presents very little load due to process conditions requiring very low flow, so it is entirely possible that it has been running in this state for a while without causing any apparent problem at the load..

My opinion from the symnptoms is that there is something wrong with the rotor rather than the stator. My current theory is that there is a fracture in the squirrel cage end-ring itself, and that some current is passing into a relatively high resistance steel retaining ring located inside the copper end-ring. As a result the currents in the squirrel cage are much smaller than expected, causing loss of torque and keeping the line current low. There a few electrical burns which indicate passage of current and perhaps light arcing. The brazes where the bars meet the ring, and the ring itself, are obscured by a heavy film of varnish and overspray. There are no visible cracks, but I'm getting it cleaned down to bare metal in prep for dye penentrant and/or ultrasonic NDT on the end ring and the brazing.

Has anyone seen a similar set of symptoms before? Am I on the right track, or am I totally off into the weeds? Any thoughts on other possible sources of the problem?

I am not certain whether the copper ring was cast and machined, or hewn out of solid plate: can anyone with experience of manufacturing this type of machine offer an opinion which is most likely? It may be of significance to the NDT guys, plus it will influence whether repair is viable or not.

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Did you try the single-phase low voltage test for broken rotor bars? If current remains fairly constant as you turn the rotor, you can rule out rotor bar problems. But, otoh, if there's no contact at all to one of the shorting rings, you may see a low but even current.

Next thing is to find a motor with similar data and do the same test, if current is significantly higher in the known-good motor - then you have a rotor problem.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

(OP)
Can we discuss that test a bit further Gunnar?

I know the motor shop did some testing prior to us being summoned, and they were confident that the bars themselves were sound. They are an old experienced shop so I give their opinion credibility, but I'm quite happy to ask them to undertake additional tests and I think they'll be happy to work with us.

Stator is star connected, 3.3kV with FLC of about 90A. We can certainly muster 240V single phase at a fairly high current. I think 240V will probably keep the line current within acceptable limits. I'm reasonably confident in understanding how a broken bar will manifest itself in a reduction in line current as the broken bar passes a pole, but I've not seen this test done for a cracked ring and don't really know what results it is likely to produce.

I don't have many similar motors to do a comparison with, so direct comparison is out of the question right now - the only similar machine is the duty pump while this one is in bits.



RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Scotty

I get the impression that youv'e done that test before. But we take it from the start, nonetheless.

3300 V delta would mean around 1900* V from star center to motor terminal. At standstill, the motor will draw LRA at 1900 V. Since very many things are nicely linear, it is safe to assume that LRA will be proportional to voltage.

If LRA is six times rated current, or 540 A, that would mean that 240 V should result in around 540*240/1900=68 A. That may be a problem in most shops, so I recommend a lower voltage, say 24 V from a transformer. That should result in just below 7 A.

If the shorting ring has separated completely, then you will not get that current. It will be considerably lower.

If the shorting ring is connected to a few bars, you will notice that current varies when the rotor is turned. Increasing when the bars are in the field under the active pole.

If the shorting ring is connected to most bars, except one or two, there will be expected current except for when the broken connections are under the active pole.

If you get expected LRA (for the used voltage) and there's no significant variation variation when the rotor is turned 360 degrees, you can assume that the rotor is OK. I doubt that you will get that result.


*Of course, you can use two twerminals and 3300 V in your calculations. I don't know, really, why I thought it would be better to use just one winding. Use what you find more convenient. If the star point isn't available it is obvious what the more convenient choice is smile

Check to see if you can find LRA for that particular motor and use that number instead of the assumed 6 times.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

OK, remember now. If you use the star point, you don't need to worry about the to winding's voltage and phase relation.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

(OP)
Thanks Gunnar. I'd calculated for the supply across two lines and had estimated a current of about 40A or so which seems to match your calculation. It's been a while since I was on this type of work and wanted to check I wasn't about to get it embarrassingly wrong: I've been on switchgear replacement for a few years but a reshuffle now has me back in Maintenance. The diagnostic notes you provided are useful. smile

The shop has an 11kV supply of moderate capacity so it can do an unloaded full voltage run on most motors. 40A at 240V certainly isn't a problem. The star point is awkward to access, so I'll be going line-line.

My gut feeling is that there's a crack in the ring near a notch for a rotor bar, but I'm speculating. Will post some photos if I can get some.


DesertFox - thanks. This link is probably closer to what I was looking for, but your link has some good information.

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

The EASA link is a good one for rotor testing.

You might be tempted at first glance to think there is no danger of overheating during that single phase test as long as you keep the current below full load current. A little further thought reveals of course that is not true because there is not rotor rotation for cooling. So give some thought to how long you will apply current and whether you will monitor stator temperature.

=====================================
(2B)+(2B)' ?

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Quote:

Any thoughts on other possible sources of the problem?
Only very simple stuff which I'm sure you've already covered...

Just to clarify ... there are only stator three leads available? If so that rules out swapping the polarity of 1/3 phases as you know.

Were the stator currents verified balanced during the run? That would rule out gross supply imbalance.

What type of supply? (It might suggest some very remote scenarios).

It's hard to think of anything else other than a rotor problem. But with such an apparently severe rotor problem, I was a little surprised to hear no report of twice-slip frequency pulsating in the current, noise or vibration at some point during the start. But you never know. Good luck.

=====================================
(2B)+(2B)' ?

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

(OP)
Thanks ePete, sound advice as normal.

The star point isn't brought out to the terminal box, and the way the leads are configured there's no practical way that two ends could be transposed except during a rewind which we don't think has taken place. We could get at the star point with additional effort, but until we've eliminated the more likely problems I'm not going to incur the extra cost of pulling the stator pack out of the casing.

The three line currents were balanced during the run, and the stator resistances are balanced on a ducter test.

Supply is 3300V resistance-earthed star annd is well balanced.

Very good point about limiting the duration of the test because of the lack of cooling, or perhaps using a lower voltage to keep I²R losses down as Gunnar suggested earlier.

I haven't seen the vibration signatures, but there wasn't audibly a very low frequency modulation of the sound it was producing. I wish I had my old low frequency analyser from the power plant, it was great for jobs like this.

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Hi, Scotty,
Try to perform the test by connecting the stator to the three phase reduced voltage (approx. 10 % of nominal) instead of single phase.

http://winding.wix.com/design

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

(OP)
Hi zlatkodo,

What outcome would you expect? On a good motor with no load it should rotate. On this one with a known problem, how would this test help localise the fault?

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

LOL ScottyUK,
I was [b]AT[/b] that EASA convention in San Francisco and attended that lecture in your link! A few years later I was involved in a discussion about finding broken rotor bars in large motors and mentioned using a "growler" as depicted in that paper, because I remembered seeing it discussed. The people I was engaged with at the time brushed me off, saying that growlers were only useful for DC armatures, not AC rotors. I tried in vain at that time to find that info but eventually gave up (my Google-Fu skills were still weak then). Now, when I no longer need it of course, you post a link to the EXACT paper I was looking for at that time. Weird.

As I read your post, I immediately went to broker rotor connections somewhere in that motor, I have seen several very similar applications and that was what it turned out to be. When I was more heavily in the Soft Starter business, I noticed that a side benefit to that technology is in its uncanny ability to "find" rotor bar problems. My working theory is that the soft starters don't really create them as some people assume, but many motors already have rotor bar problems and the users never realize it because if accelerating Across-the-Line, the motor passes through the obvious effects too fast. When slowed down with a soft starter, it becomes much much more evident.

In fact come to think of it, you might try hooking up a small LV soft starter to your motor as a low power source to find it. Always seemed to work for me!

"Will work for (the memory of) salami"

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Hi, Scotty,
Damage of the rotor can be detected using a method of three A-meters ( one in each stator phase) and stator connected to reduced voltage ( 10 - 25 % of nominal ).
You need to start with a very low voltage and then slowly increase and select the approx. maximum voltage while the rotor is yet stationary.
Turn the rotor slowly by hand and determine maximum and minimum values of each phase current in all positions from 0 to 360 degrees. One rotor turn is sufficient.
At serviceable rotor the currents in the stator phases in all positions of the rotor will be the same, and if there is a broken bar or bad contact between the bar and end ring, the currents will vary depending on the position of the rotor.

http://winding.wix.com/design

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

I was thinking in-situ growler also Jeff. I would try low voltage across A-B and measure voltage across A-C while turning the rotor.
zlatkodo's suggestion would be the current version rather than the voltage version of the test.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

I really canot see the benefit of using two or three ammeters. I use one, as described earlier. Also, with three phases connected, it may be difficult to hold the rotor of larger machines and if you let them go they soon accelerate to rated speed. A heavy motor that hasn't been bolted down is not something you want to see running with more than a few RPM.

Finding three identical analog ammeters can also be tricky. So, unless the three ammeter method offers something extaordinary, I cannot see any reason to use more than one ammeter and a low voltage single phase supply. That, one an easily find anywhere. Three identical analogmeters and a reduced three-phase supply is not something you find in the field. Hardly ever in the electrical departmant of any large industry. I have a couple of large winder shops nearby (Imtech is one of them) and none of them have the necesary three identical analog ammeters. Using a three channel recorder is one way out, but that doesn't bring more than a single channel recorder.

Is this a cultural thing? "I say tomaato, you say tomayto"?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Hello ScottyUK

Sympthons are evidencing a possible broken bar or broken ring in the squirell cage, the suggested test are fine to find this kind of problems, I.M.O. a single phase test could show the problem, if not, more accuracy testing most be performed, you could try with one core tester, is possible that you can test the rotor with high current,during the test the squirrel cage is covered with magnetic paper and you could see if some bar is broken.

An analysis of vibration records coul also help.

Regards

Carlos

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

A growler test and a single phase rotor test are, in principle and practice, the same test. In the case of the growler, the motor is disassembled and the coil that is used to measure the varying inductance of the defective rotor is a separate device (external to the motor). This test requires a fixture upon which the rotor can be mounted and rotated while positioned above the growler.

In the case of the single phase rotor test, one phase of the stator winding is used to detect the rotor fault and the motor frame, brackets, and bearings are used to position the rotor and allow rotation.

If you have a rotor fault of the magnitude that would cause the starting fault that you describe, either test will detect it.

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Hello everyone,
In our motor shop we have induction motor with wound rotor for voltage regulation and three ammeter *one per phase* on the control cabinet .This gives us the conditions for reliable finding problems on the rotor on the way as suggested Zlatkodo . Sometimes we must warm up motor and then damage of the rotor is larger and more visible changes in current .
Good luck

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

I can't say I've heard of 3-phase off-line method before.

The single-phase off-line test is more standard. I can give three EASA refs, 3 EPRI refs and a textbook reference for the single-phase offline test. I haven't come across any references for the three-phase off-line test.

Preference for one or the other method may depend on equipment available.
Personal safety would require some more consideration during 3-phase test when people are manually rotating the rotor.

Is there supposed to be any sensitivity advantage of 3-phase test over single-phase test?

=====================================
(2B)+(2B)' ?

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

I should have also mention that single phase test is used by our local shops in my part of the USA.

=====================================
(2B)+(2B)' ?

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Epete,
I am from Eastern Europe and I guess that's zlatkodo is from the same area .The principle to give on the motor small percent of the voltage and gently turning is very easy to perform .You should not have a special preparation for this test because it might be the start of open shaft testing after reparation of electric motor. I don*t have some references for this test in English but we suggest that you should try to perform . Mainly worker who hold the shaft gives an indication to colleague who raises the voltage until he can control the shaft.There are not safety consideration because the torque of motors is very small and it is easily control shaft of the motor . Sometimes the start electric motors with controlled voltage when passing cut down torque is moment when can be seen variation of current on ammeters and for experienced people in our repair shop can be a sign of damage to the rotor .
Good luck

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

With the motor running open shaft at full speed, you will see the current swinging very noticably at a low frequency (slipfrequency) if you have a damaged rotor bar.
I have observed currents swinging on a standard moving iron meter with the current swinging at 40+ %.

Otherwise, I favour the single phase approach with a low voltage applied and the rotor slowly turned. Easy and safe to do.
Best regards,
Mark

Mark Empson
Advanced Motor Control Ltd

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Quote:

With the motor running open shaft at full speed, you will see the current swinging very noticably at a low frequency (slipfrequency) if you have a damaged rotor bar.
I have observed currents swinging on a standard moving iron meter with the current swinging at 40+ %.
Hmm. Well at no load, the vast majority of the stator current is magnetizing current, which is unaffected by the rotor bar defect. There is very little current in the rotor (only enough to produce torque to overcome losses) which is affected by the rotor bar defect.

With a large magnetizing component unaffected by rotor bar defect and a tiny load component affected by rotor bar defect, I don't think that looking for no-load current variation will be a very sensitive test.

=====================================
(2B)+(2B)' ?

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

And sleep frequency is almost zero.Motor must be loaded and at pump aggregates where the load is constant variation of current is obvious on old iron ammeter . I worked for several years Motor current Signature analysis and found over 50 problems on the rotor at low voltage and high voltage motors .Lastly I could on the basis of the ammeter to the cubicles and current variation to predict that the motor has a problem with the rotor bares.
Good luck .

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

A word of caution.

I have been working with electric motors and drives for more than 40 years now. And I have quite often been asked to go and diagnose motors with suspect rotors. I must have been checking at least 500 motors - probably more.

I have only found four (that I can remember) actual rotor bar or shorting ring failures. All other cases were false alarms with fluctuating load and unstable drive being the most common cause of ammeter swinging. The second most common cause is interference between motor frequency and grid frequency when the DC link capacitors have a high ESR or have failed.

BTW, it is SLIP, not SLEEP. And BARS not BARES. Sorry to be such a brute, but I think that you are better off knowing the right words.

I agree that the motor must be loaded for the rotor bar damage to show up significantly on an ammeter. Low voltage and turning the rotor while observing current is a simple and safe way to diagnose broken rotor bar and end ring problems.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Hi Pete

While I agree with your theory, I was asked to look at a machine some years ago, on a star/delta starter (wye delta) that would not start in star, but did start in delta albeit at much reduced current. When the motor was running, the current was observed to swing significantly at around 2 - 4 second cycle time.
The motor was dismantled and there were a number of broken rotor bars that had been arcing.
When these bars were repaired, the motor performed correctly.

Gunnar, I agree, very few problems are actually due to rotor bars, but I did see one large motor that had frequent ring problems.
This motor had been poorly manufactured with the rings breaking away from the bars after a relatively short time.
The repairer decided that the problem was related to too much overhang of the bars, so shortened them considerably before re welding the rings back on. There was now little room for expansion and cracked welds became a frequent problem. This motor was also on a star delta starter so subject to high transients during start.

Quote:

Low voltage and turning the rotor while observing current is a simple and safe way to diagnose broken rotor bar and end ring problems.
Works for me!!

Mark Empson
Advanced Motor Control Ltd

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Thanks Mark.

My comments (hm, well) expressed surprise because I was surprised to hear what you said.

Now thinking some more and seeing your explanation - if I had to guess I'd say that was a very severe rotor defect, based on
1 - reduced current noticeable during start. Looking at equivalent circuit, the effect of increasing R2 is to decrease both current and torque during the start.
2 - 2-4 second pole pass frequency even at open-shaft/no-load conditions. That implies the motor is really struggling just to make up for normal losses.

So maybe the friction losses were enough loading to create a significant load component in your case. (would be interesting to know how the average measured no-load current compared to fla and to expected no-load current).

At any rate I'll stick with my comment that it's not "sensitive". Meaning it is not a good way to ferret out a hard-to-find problem. But it's interesting to know maybe no-load current oscillation will show up for a really severe problem.

My comments (like a lot recently) are a little off-topic to Scotty's problem, but I still think it's interesting.

Going back to Scotty's op

Quote (Scotty)

During a start attempt the motor crawled up to speed over about a minute or so, and the peak current captured by the clamp meter was just over running current, i.e. abnormally low for a DOL start.
Abnormally low current. Seems consistent with what you (Mark) reported.

=====================================
(2B)+(2B)' ?

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Clarification:

Quote ((electricpete, ORIGINAL))


So maybe the friction losses were enough loading to create a significant load component in your case.
should have been

Quote ((electricpete, CORRECTED))


So maybe the friction losses were enough loading to create a significant load torque component which was significant compared to the drastically reduced torque from severe defect in your case.

=====================================
(2B)+(2B)' ?

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

maybe both were right. The losses created a:
- load torque significant compared to reduced torque capability
- load current component significant compared to magnetizing current

=====================================
(2B)+(2B)' ?

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

Hi Pete

Yes, if you consider the rotor with essentially all bars open circuit, or close to it and just one relatively high impedance loop still in circuit (2 bars) then the lowest impedance could appear as two poles on the rotor and as that rotates at just below the synch speed, that low impedance will slowly rotate through the phases.

My suggestion was in response to Scotty's description which was very low start current and of course that reflects in an even lower start torque, If the bar to ring bonds are all badly cracked, then it is possible to have essentially a high impedance path primarily through two bars only.
It will be interesting to hear what they find.

Mark Empson
Advanced Motor Control Ltd

RE: Motor problem - loss of torque, slow acceleration with open shaft, noisy

That's an extreme rotor failure. Never come across anything like it.

But, when you think about it, it all sounds very plausible.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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