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.
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Stator failure 6kV synchronous motor 1
- Thread starter henkv
- Start date
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.
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
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
- Thread starter
- #23
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
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
jbartos
Electrical
- Jan 15, 2000
- 6,440
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.
electricpete
Electrical
"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?)
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?)
purplepete
Electrical
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.
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.
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.
- Thread starter
- #28
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.
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.
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
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
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.
jbartos
Electrical
- Jan 15, 2000
- 6,440
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.
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