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Stator failure 6kV synchronous motor

Stator failure 6kV synchronous motor

Stator failure 6kV synchronous motor

(OP)
Recently we have had a ground fault in the stator of a 11MW synchronous motor. A condition measurement(PI,PD, surge) 1 month before the event showed no indication of deterioration. The motor is directly powered by a VF drive.Cable length is too short for voltage doubling. Measured rise times do not exceed 5000V/us. Motor is only 3 years in operation and winding temperatures never exceeded more than about 50 degrees C. The failure occured while in "idle" mode, taking about 1% load. So far we have no indication of a cause.

RE: Stator failure 6kV synchronous motor

Was the idle mode a common mode of operation or was it associated with something unusual such as an area shut down?

Have you had a chance to give a visual inspection of the stator to rule out the obvious causes such as contamination/moisture?

If you do not look for electrical faults in your VA analysis it may be helpful to have someone review your VA readings for anything unusual, but not bearing related.

Please keep us updated if you discover anything.

RE: Stator failure 6kV synchronous motor

henkv:

A few questions....

1. How far away from the neutral point did the fault occur?

2. A machine this size should be equipped with a 'Rotating Machine Surge Protection package'. Is this one protected? If yes, is the package located at the motor or far from it?

3. You said that the machine was in idle mode, do you normally shut it down from idle mode? If yes, what was the power factor at that time? the current? and what type of breaker is it that opens the entire circuit? Is there a breaker between the drive and the motor?

GH

RE: Stator failure 6kV synchronous motor

henkv:

What kind of load move this motor?
Do you know if not have problems with the peaks of the drive?
Any problems with the drive before the failure?

Alfredo Corona
Mexico

RE: Stator failure 6kV synchronous motor

All good questions.  A few more:

Since you say motor was tested (PI, surge) one month prior, I assume this was a maintenance outage. How long after machine restart did failure occur?  Was some form of space heating used to keep the machine dry?  

I would also go back and take another look at the surge test results to make sure there was no slight indication of failure detected/caused during that potentially-destructive test.

RE: Stator failure 6kV synchronous motor

Hi henkv

Stator shorted to ground after some tests were performed shortly before the failure.
You mention that PI,PD and Surge tests were performed ,but you don't mention at what voltage levels were these tests
performed at.
The PI (polarization Index) test is usually performed at much lower levels than the operating voltages.It measures the dialectric value of the insulation but does not stress motor to its operating levels.The same could be true for the surge tests.As for the PD (partial discharge) test,it too, may not stress the insulation to its operating levels.
Regardless of what tests one performs ,unless you apply
opearting voltages that stress windings (such as in  HI -Pot tests)Phase to phase or phase to ground insulation cannot be guaranteed (99%at least).
Understanding that TEST voltage levels for used motors would be slightly lower than FOR new motors (2Xnominal Voltage +1000 )is the rule for new motors.
The other suggestions regarding possible low ins.values due to Hi-humidity or others should not be discounted without further investigation.

GusD

RE: Stator failure 6kV synchronous motor

(OP)
Thanks for responses all.
-Motor is part of a steel rolling facility with a load cycle of say 3 mins full load, 6 mins idle mode(15% speed, 5% current, power factor ??)It is a 12 pole motor running at 170 rpm nominal speed.
-Inspection after failure showed no moisture. Some contamination was found, analysed in 2 labs, no cause for failure. There were also no traces of corona, loose wedges or dust from vibration problems.
-We have crowds looking at the measuring results, maybe one lead to an early indication coming up.
-Fault occurred in 1st coil, 1st turn, from motor terminals.
-No surge protection package installed, measured rise time within expectation from drive side and motor design spec, there is no breaker between motor and drive.
-We have regular bi-weekly shutdowns that we can use for condition measurements. Space heaters are in use. Motor had been in operation for a month, after the measurement before the fault occurred. We have an on-site motor maintenance team, which is experienced in large motors.
- We have had no problems with the motor/drive before the fault.
- Because of motor confuration - coils in parallel and in series, surge tests will give little information.
- Hi-potting will not be done as a maintenance test, other measurements are done at nominal voltage

RE: Stator failure 6kV synchronous motor

1 turn first coil is a sign of start up surge in a motor with the many poles. Is it a vandergraf roller?If they rewind them the should use high feq wire. The class of insulation is much higher than normal wire it would had the start up better. I agree with the person who wrote that P.I. tests use low voltage and may not show moisture. A baker surge tester would do the job better than most test I have seen it can stress the insulation properly. At a steel plant I did P.M. the oils and humidity was high. You may not be able to see it but it is there. Try the wire change if possible if it requires rewind.

RE: Stator failure 6kV synchronous motor

1st coil first turn is a sign of possible surge, although I'm not sure why would be limited to those associated with startup. Also 1st coil has highest voltage stress to ground.

Is there sign of turn or ground failure?  If it was a turn insulation failure, there is normally a lot of copper melting.  Is the location of failure on end turn or slot?

You say there are difficulties with surge testing on this particular motor, but surge testing was conduted.  Is it possible the failure/degradation begun during surge test but was not noticed at the time.

RE: Stator failure 6kV synchronous motor

henkv
You say that the PD test was done on this motor. I think that this test was done in offline situation and it's results didn't show any problem.if this is true it must be recommended that offline PD test doesn't able to detect all of the insulation defects. because the failure was occurred at 1st coil of the motor winding, I think that if there is any void or tree channel in insulation it would be recognized ( if suitable PDA instrument ws used).therefore insulation delamination may be occurred in your motor's winding.
Insulation delamination occur when thermal cycling applied on groundwall insulation because of different thermal expansion coefficients between copper and insulation or even different layers of insulation.
In stable thermal condition, e.g. in trips periods, gaps which is made by delaminaton dissapeared or very decreased and therefore insuation resistance and dissipation factor were affected notsensible                            

RE: Stator failure 6kV synchronous motor

(OP)
Induction, could you please explain what a vandergraf roller is? I don't think ours is. This motor drives a so-called 4-high mill through a 1:2 gearbox. We do use the Baker surge tester. Will look into the high freq wire, but I know that the wire for this motor has been insulated for surges according to IEEE 522. I am not quite sure what kind of startup you are referring to, as the motor is drive controlled, there are no large currents when we start running. The last time it was shut down was 3 weeks before the failure.

Electricpete, We assume it started with a turn short in the slot, turning into a ground fault. We have found melted copper/iron over about 40 cm length in one slot, but no large holes were seen. The motor was operating at a low current. We did not have difficulties measuring, but interpreting the surge test. On a motor with many parallel and series circuits, the response on the surge pulse does not really tell you a lot.

Mirbaghri, We have been told that an online PD measurement for HV PWM machines is not available yet. If you know a manufacturer with a working system, I would be very interested. If the motor ran hot, thermal cycling would certainly be considered as a cause, but as it is running very cool, this seems unlikely. Insulation resistance and dissipation factor had not changed in the last measurement before failure.

RE: Stator failure 6kV synchronous motor

Lots of possibilities.  Once again the point regarding the surge test was that it seems possible that a turn insulation failure occurred during the surge test (this is a potentially destructive test), but was not noticed at the time.

The fact that it was a turn insulation failure at the first coil is consistent with this theory (as well as surges from other sources).

Assuming that you plan on rewinding the motor, during the teardown you may have opportunity to isolate the coils and hi-pot and/or surge test other coils to failure?  That may give you a rough feeling for general strength of your ground and turn insulation (but of course may not detect  problems localized to one coil).

Regarding mirbaghri's suggestion of thermal cycling.... it seems it may not limited to motors which run abnormally hot.  Long length stators with a lot of up/down cycling may be susceptible.

RE: Stator failure 6kV synchronous motor

(OP)
We do have a plan in place where we will test the rest of the stator both electrically (surge, PI, PD) and then mechanically to see if we can find root cause. So we will have a comparison with other "first" coils to see if they have aged more than average. But we are interested to find out if other combinations of 6kV motors and VFD's have had similar problems. We do have a very frequent load/no load cycling. With the mechanical check of the coils we will find out if that cycling has caused serious delaminations. But then ...  delaminations should not result in turn to turn faults.

RE: Stator failure 6kV synchronous motor

Good points. Sorry I don't have any experience with similar failures.  

Out of curiosity, how did you measure your surge environment. That seems like a challenging measurement to make.  Did you characterize the peaks as well as the rise times?

Also I assume your guys are looking at the existing motor insulation system design. Clearly dedicated turn insulation would be required for this application (I have seen several  large motors without dedicated turn insulation... just used the strand insulation as turn insulation).

Presumably the machine has passed many surge tests including high levels when new.  But also isn't the theory that surges can degrade insulation over time. I have also heard that mylar tape insulation can degrade over time under voltage stress.



RE: Stator failure 6kV synchronous motor

Suggestion: It appears that it may have been a voltage spike since:
1. First turn is involved
2. Test have been performed recently
3. Turn to ground fault
4. Idle mode of operation implying moderate temperature, no interturn stresses due to high current, etc.
5. Light load conditions, implying higher voltages
6. The highest chance for the weakest insulation spot, i.e. the lowest insulation and surface resistances to ground
etc.
The more thorough analysis of the fault and consequent damages is needed.

RE: Stator failure 6kV synchronous motor

vandergraf roller is a self contianed unit, the motor runs in oil with the acctual gears within the roller itself. Ok it is not a vandergraph. That changes it a little. Are the heaters on the winding or in the endbell. Ok in such a slow speed motor such as that one the are many recurculating current cuase by the number of poles. Oh re-read your last statement directed at the connection. Big difference, if has blow before was it in the same spot?
More so than than high speed motors low speed motors suffer from high core loss. I have seen where in a case such as yours that the motor had been repared and not core loss tested and had iron loss. It may not look like much for the size of the core but the rating are much lower for slow speed motors. That siad, ask them to test the core and and see if there is a hot spot there. That speed in my experience calls for either the coil itself to be wraped every coil with nomex as phase paper. Pure nomex as slot cell. I test a motor in your instance "time frame" use a megger or P.I. tester for the general tests only surge test maybe once a month at most it really should not hurt it but not worth the chance. Really wish I could see it could give you a better diag. Could you email me a picture?

RE: Stator failure 6kV synchronous motor

(OP)
Coils of this motor have both wire insulation and 2 layers of mica tape turn insulation.
After long discussion, decision was made to replace whole stator and not rewind. So we eliminated the risk of finding damages to the iron package. When inspecting with a boroscope through the ventilation ducts, we did see some damage to the iron package, but could not quite assess the extend of the damage.

Electricpete, not quite sure what you mean with measuring the surge environment. Our approach is to take "fingerprint" measurements when the new motor arrives and then compare further measurements to this fingerprint. Only if a clear difference is noted will we start to analyse the cause for this difference. So far for these motors we did not find significcant differences. Surgetest was done 2 times after the fingerprint with about a year in between measurements.

Jbartos, we are certainly considering the spike as a possible cause, but cannot find any evidence for an external event that would be able to damage a well insulated motor and not the sensitive pc's/drives/I/O alongside it. I tend to disagree with your point 5 as the motor was running at low speed at the time of the fault, so the VFD would only have given it a 15%rms voltage. Yes we are planning a thorough root cause analysis, but still, the fault may have destroyed the evidence.

Induction, I am not sure how I gave an impression of a recurring fault, but this fault is the first and only we have had on these machines. If you post your email address here I can send you a picture, but that will not reveal much more then a simple description here: a 4 meter by 4 meter stator housing with a 2 meter inside diameter for the rotor. And on the insde of the stator a minute black spot of 10 cm wide, 40 cm long.

Oh yes, I have to come back on what I said on space heaters. We do have them, in the end bells, but do not use them. This is because there is a water to air cooler on top of the motor that is always on, with water that is always 27 degrees C. So there is a constant flow of warm air circulating through the airducts, keeping the motor nice and dry at 27 C (well above dewpoint).

RE: Stator failure 6kV synchronous motor

It sounds like you've looked at the space heaters closely but I don't undestand because I'm not too familiar with water-cooled motors. Does it have forced arir even with thte motor secured?  You cool it with hot water?  Also 27C is 80F, not necessarily that much higher than ambient unless in an air-conditioned space (is it?). Also the entire winding must be at that temperature... have you verified winding temp above ambient temp during shutdown using winding RTD?

By surge environment I was not referring to the motor surge withstand, but the power system surges... specifically your statement "Measured rise times do not exceed 5000V/us"  How was this measured? (what instrument, what location?).

RE: Stator failure 6kV synchronous motor

Suggestion/clarification to henkv (Electrical) May 1, 2003 marked ///\\\
Jbartos, we are certainly considering the spike as a possible cause, but cannot find any evidence for an external event that would be able to damage a well insulated motor and not the sensitive pc's/drives/I/O alongside it.
///Spikes do materialize alone or with surges in form of a surge peak.\\\
 I tend to disagree with your point 5 as the motor was running at low speed at the time of the fault, so the VFD would only have given it a 15%rms voltage.
///Agree. I meant the high input voltage to VFD. Many power electronic devices have input voltage range/window.\\\
 Yes we are planning a thorough root cause analysis, but still, the fault may have destroyed the evidence.
///Yes, however, certain traces of the fault remain, e.g. location, magnitude of the fault, some conditions under which developed, etc.\\\

RE: Stator failure 6kV synchronous motor

(OP)
Electricpete: Yes the air is forced through the cooler into the motor. Unfortunately the weather here in Holland is not that warm so 27 C is always a couple of degees above dewpoint. And yes we have checked this on the RTD's. The water is used for cooling lots of installations and is designed to be around 27 C. We once had a 2 day outage in winter (not that cold here) and at the end the water was still 27C!
Surge environment was a 200MHz scope with a fast HV probe measuring right at the motor terminals during rolling on the twin motor (different VFD)of the failed one. Later this was repeated on the VFD that drives the failed motor, when we had a replacement in place.

Jbartos: on your following point:///Agree. I meant the high input voltage to VFD. Many power electronic devices have input voltage range/window.\\\We have designed the supply side to be within range for both full load and no load conditions and we may have like a 10% increase on no load. I agree that this will have an effect in a worst case scenario.

I do appreciate everyone's input. We have 5 more motors like this and all are very critical for production. So we would love to come to a conclusive root cause.

Henkv

RE: Stator failure 6kV synchronous motor

One qeustion for you is this coolent water or chemical? Acctually two qeustions the cooler located above the motor and does the air get blown in or is it from rotor air flow. If you could give a outside picture or the motor also so I can see where the air vents are. One picture of the rotor also. What I mean by water is two things is this water reused or is really a chemical base I have seen both. the heaters in the endbells, I would say during times of shut down run them if possible. You may not hit the dew point but humidity can climb high during rainy days. I have seen completly disassembled motor have a high micro amp leak just from sitting on the cement.  

RE: Stator failure 6kV synchronous motor

Hi HenkV

After you received a lot of good info,I don't know if you have any better idea as to the cause of failure.
You still have another 5 motors being used .If the problem is a system problem (VFDs)rather than a motor pronblem, it could just as easy happened on any of the other motors.
When one hears of 1st coil,1st turn fault automaticaly we assume a Voltage spike from inverter use.
I just like to mention something that  is a characteristic of most windings (Form or Random).When coils enter or leave slots, they can be stressed with a downword pressure that weakens the Millar slot insulation.At times this insulation is actually cracked under the coils.It is true that it happens much more on Random windings than on formed coil.I have seen formed coil windings that have shown the tell tale marks.When this happens,motors may pass most tests,with age,this weak spot will be the weak link.This may not be the case, and it is easy to look for this condition.
If your rootcause does not show a possible power problem,how do you plan to test the other motors to make sure they are sound ? Henkv, If you don't mind, give us an idea how do you plan to check the other drives.If you are going to test'em.  

Thanks

GusD

RE: Stator failure 6kV synchronous motor

(OP)
Induction, You are right in saying to not only to depend on the water. Actually we are having an outage soon where the water system will be down and then we will use the space heaters. The motors are TEWAC with the coolers sitting on top of the motors and we use chemically treated water to prevent corrosion.

Gusd, we have inspected the places where the coils come out of the slot, but did not see any damage. The fault occurred some 40 cms from the side of the slot. If the VFD is ageing the insulation, we hope to get proof of that from the 1st coils of the other phases. If we don't find any cause .... we may have to get some more spares! We are doing some tests with a system from Delft university that may provide more insight from a PD test. When that proves to be helpful I'll let you know.

Henkv

RE: Stator failure 6kV synchronous motor

Suggestion to the previous posting: If the VFD is aging the insulation, then there is higher harmonic content on its output. The harmonic content can be mitigated by various means on the VFD output, e.g. reactors, filters, power line conditioners, improvements in DC-AC conversion of VFD, etc.

RE: Stator failure 6kV synchronous motor

"The fault occurred some 40 cms from the side of the slot"

Do you mean 40cm from the end of the slot? Inside the slot section or outside the slot section (in end-turn area)?

Just clarifying, (I thought you said before it was in the slot sectin?)

RE: Stator failure 6kV synchronous motor

Gents,
just to build on a comment made previously:  many years ago  we had two 300 HP Laurence-Scott wound rotor NS-type motors (3.3 kV 50 Hz stators) driving pumpsets at the bottom of a well.  The motors were air-cooled and well-ventilated except the air was basically being recirculated within the well.  Temperatures always remained well within acceptable but we experienced several consecutive motor failures, one very soon after a after complete re-wind.  It was then we discovered the atmosphere was mildly corrosive, I can't remember with what, but the introduction of fresh air eliminated motor failures for many years after.  The chemical in the air had been attacking the insulation.

RE: Stator failure 6kV synchronous motor

At forty cm from the slot there is possible a iron ring used to handly the surge. When large motors start under any load the windings do move out-ward because the energy developed to rotor from the stator core to the rotor core is at the point of saturation in the rotor. Once the rotor rotates the energy being developed into the bar of the rotor is being used it is a transformer action. Soft starting is the way to avoid damage during this time frame wye, delta. I witness 300 h.p. 2 poles move a 1.5" while being tested at have voltage what we call green no varnish or epoxy resin to hole it in place. The higher the speed the more movement at start-up, in a large water treatment plant the large pumps go through this many time during there life. Some jobs need to be cleaned internaly during this time I would V.P.I them agian filling any cracks may in the insulation and change the leads using a silicon glas braided lead wire which has a very long life. Henkv they would not let me put up my email.It is very possible that this is a normal failure depending on the use and the inviorment. It is a weak spot out side the iron the first turn move the most during start-up it is on the top or the botom and when the winding move it takes the most of the stress it pulls the insulation apart when it bends. No load, soft-starts or half voltage starts cuts the movement down lengthing the live of the insulation.

RE: Stator failure 6kV synchronous motor

(OP)
Location was 40 cm into the slot, from the side of the iron, sorry for the confusion. So firmly fixed.
Startup is current limited and no load, so we don't see these high forces in the slot or outside the slot.
On inspection after the failure, we checked for both moisture and corrosion, but none was found.

RE: Stator failure 6kV synchronous motor

Hi Purplepete.
Your aanswer to Henkv thread, brought back some bad memories of mistery motor burnouts.We had a Fire Water pumping station with 3 -50 hp vertical pumps.This building was somewhat isolated from the main buildings in this Plant and just like clock work,we were loosing a stator every 3 months or so.These motor burnouts went on for some time on equipment that ran very rarely.One day,we happened to be in the building when the Fire Crew came in to test their Extinguishers(Dry Chemical)
That kind of gave us the clue.
They too use to test many Extinguishers at 3 months intervals,and we lost motors nearlly every 3 months.
After they stopped this practice inside  the building the motor burnouts stopped as well.   

GusD

RE: Stator failure 6kV synchronous motor

Henkv sometimes the insulation is damaged when the coils are inserted it probable had no cell liner (just geussing). If you have the opportunity look in the slot before it opposite rotation see if one of the lamanations moved. It only takes one to put pressure on the next slot. It may have just been a weak coil wrap.

RE: Stator failure 6kV synchronous motor

Suggestion: Reading last a few postings, I tend to agree with the previous one, where there is addressed a weak coil wrap. If the failure is due to some material defect or lack of a certain quality, it is always difficult to identify the cause. Only, the second, the third, etc. faults can lead to more positive identification. There was one news category that I followed. Namely, the commuter planes were crashing. It took three crashes to have the FAA system to look into the plane design.

RE: Stator failure 6kV synchronous motor

(OP)
Induction,jbartos,  weak coil wraps were certainly checked for when we found the motor had failed. This was also a concern we had. But on inspection we could not find any bad wraps.

Henkv

RE: Stator failure 6kV synchronous motor

Suggestion: A bad wrap may also imply a defect in the wrap material or a mechanical injury of the wrap that may or may not easily be detected.

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