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Nema 6 Contactor contacts pitting Vendor says it is normal

Nema 6 Contactor contacts pitting Vendor says it is normal

Nema 6 Contactor contacts pitting Vendor says it is normal

(OP)
We have a motor control center (MCC) that has 8 - 480V Nema size 6 starters (FVNR).  These starters control 8 - 400 HP motors with a KVA code of 'G' and is high efficiency rated.  The motors are started Across-the-line (480V).  The Full Load Current (FLA) for a motor is 418 amps and has a locked rotor amp (LRA) rating of 2900 amps. The motors are connected to natural gas compressors.  High efficiency motors can be as high as 13 times FLA.  The vendor says that they sized the Nema 6 starter per the KVA code 'G' that is on the motor's name plate.

The start up current was measured by the vendor to be 4603 amps for about 17ms, then it drops down below 2900 amps within 4 cycles about 80ms.  The voltage dropped from 489 volts to 441 volts with a frequecny of 59.8 Hz.  With the motor un-coupled from the compressors, the start up current was 3221 amps which is within Nema 12.36 that allows 1.8 - 2.8 times the 2900 LRA.

From time to time the contactors will weld together and has to be pulled apart and there is pitting of the contactors.  The vendor says that this is normal, but I do not think so.  The life cycle of these contactors will be significantly reduced.  The vendor sites Nema IC 2-2000 (Performance Requirements and Tests section) that says the contactors meet the 10 times the FLA of the motor (code 'G') on the name plate for 0.1 seconds.

We found after testing that 1 set of 350MCM was run instead of 2 runs of 350MCM cable.  Since the addition of the 2nd run of 350MCM, we have not seen any welding of contact, but we still get the high inrush currents.

The vendor does not want to address this issue and continues to skirt this issue.  I know that soft starts or oversizing the contactors to Nema 7 are solutions, however, they are cost prohibitive right now.  

Would anyone on this forum have any input or other solutions?  

 

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

The welding sounds like it is a result of the prolonged start caused by the undersized cable to the motor. The prolonged start resulting from the low terminal voltage causes a higher I2t energy loss in the poles of the contactor and is melting the faces.

Regarding the currents, you are seeing inrush for the first few ms until the rotating magnetic field establishes itself, then locked rotor current until the motor picks up speed. Sounds fairly normal. Inrush is much larger than LRC, although the source impedance normally calms things down a little.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

How old are the contactors and how many starts have they seen?

A properly applied NEMA contactor should be able to handle a lot of starts.

Contact welding does not sound normal, unless recommended maintenance inspection intervals have been exceeded.  

Contacts will wear out eventually, but they shouldn't weld shut if properly sized and maintained.  

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

Who's the vendor?

Given the current economic downturn vendors should be kissing lot of frogs to keep customers happy. If the factory tech support is not helpful, try a distributor/rep for that factory. You may be able to get some help that way.

http://www.clrwtr.com/ABB-Controls-Contactors.htm

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

(OP)
We are doing a start up and we have had no more than 20 starts.  The Vendor is Eaton Cutler Hammer.

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

Pitting of contacts in normal, in fact there is a school of thought that some pitting actually helps because the pits on one side correspond to craters on the other, so they act to increase the surface area. In addition, a NEMA size 6 contactor is the appropriate device for this task (although I disagree with Across-the-line starting of 400HP motors on principal). NEMA contactor design guidelines take the LRC and even magnetization inrush current values into consideration.

But welding is not acceptable. Having to pull them apart is not only abmormal, but extremely dangerous. Even assuming you were opening a disconnect before attempting this (please tell me you did), what would happen if an overload occurred? That vendor is nuts to suggest this is "normal", they should be severely punished!
hammer

But I agree with ScottyUK, the welding is indicative of an abnormal starting condition, i.e. a voltage drop and corresponding current increase caused by the severely under sized conductors. 350kcmil is not even close to what you would run for a motor that size, even if the controller was sitting right on top of the motor and you used 90C rated conductors. The way I see it, to go with single conductors the minimum size would have been 1000kcmil (not accounting for voltage drop if there is any distance involved) or for parallel, 2 x 300kcmil if you have no more than 3 in a raceway. I think your solution was correct, assuming you did your homework on the VD.

If the vendor sized and supplied the conductors, I would hold them accountable. If not, I'm affraid you are on your own, or you can pursue the installing contractor if there was one (which from the looks of it is doubtful).


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: Nema 6 Contactor contacts pitting Vendor says it is normal

Welding may also occur if the control voltage dips so low that the contactor is not being tightly pulled in during starting.  Contactor chatter could probably lead to welding of main contacts.  

Anything below about 85% voltage at the starter can be a problem for some control circuits.  

jraef is correct, pitting is unavoidable for contacts. But welding is another story and a significant safety concern.  Especially on a starter this large.  

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

Good point about the control voltage drop dpc. Chatter can definitely cause welding as well.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

(OP)
Since we added the second run of 350 MCM, we have not experienced any welding.  

The disconnect was opened before the contacts were pulled apart.

The vendor only supplied the Motor starters and contactors.  The one run of 350 MCM was due to a distribution error when the drawings where sent to the contractor.  The contractors boss did not send  out the correct drawings. The installing contractor never received the updated drawings from his boss that showed 2 runs of 350 MCM.  The contractors boss is paying for the removal of the original one run of 350 MCM due to overheating and found that it did not pass megger tests.  The contractor pulled in 2 new runs of 350 MCM.  

We did look at the possibility that the contactors via the coils were causing chatter.  The vendor set up a varistor to control voltage and tested that the coils on the contactors were within design limits.   That eliminated the contactor chatter possibility.  

I am in agreement with ScottyUK on this issue.  

This was my first posting on ENG-Tips and you guys were great in providing feedback and answers.  

I appreciate it very much.  Thank you.

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

Nice to hear that the contractor stood up for his work, but it's troubling to see that he installed it in the first place without questioning the application. Must have let the apprentices do it unsupervised.

Glad it worked out though, thanks for the follow through. Often times we never know the final outcome of things in here. Welcome to the forum!


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

hopefully the electrician questioned this. I have had many cases of the drawings being dead wrong. unfourtunately sometimes the engineers dont want to hear that they are wrong and will fight any changes. if its a case of the control scheme not being right or its something they missed altogether i would just pull the extra wire and leave it in a j-box. when the problem showed up at start up i was able to fix it fast because i already pulled the wires. when it came down to safety and the engineers were being stubborn i would flat out refuse to do it the wrong way. the electrical contractor should have done the same here. you could have had a nasty flashover

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

The contact surfaces look normal, but those B and C phase arc chutes look as though they have had some serious thermal abuse, likely confirming that there was a fairly severe current situation going on there. Does everything move freely still, i.e. no binding from being warped? If so, I would at least clean as much of that carbon off of there as you can before putting it back into service. That can provide a flashover path if it builds up. Don't use sanding cloth or anything that will leave material behind or fall into the works, but at least spray some contact cleaner in there and wipe the sides of those arc chutes really forcefully with a cloth.

If there is any binding, you may want to consider seeing if CH sells replacement arc chutes. Most mfrs do on the big stuff like that.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

(OP)
There is no binding.  

We will clean the arc chutes.  The client thinks that the contactor appearance looks abnormal.  A Siemens tech was on site today and mentioned that the contactors looked pretty bad.  I believe the motors that were run the most when we had only one run of 350 MCM would show the most wear.

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

Not too bad, but looks like it has broken a fairly heavy current a few times. Did you have any problems with the motor stalling and tripping the thermal O/L before the cable problem was corrected?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

(OP)
We did have breaker trips.  The vendor set the magnetic trip on its highest setting which is 6300 amps which is out of the 13 x FLA range early on in the startup.  I having the electrician reduce the trip setting to 5400 amps which is the 2nd highest setting and will be within the 13 x FLA range.

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

(OP)
I have attached another picture of the contactors.  These are the same contactors used when we had undersized cable to the contactor and motor.  The metal seems to be still melting when we have the 2 runs of 350 MCM run to the motor.  Is this wear typical of contactors of this size or do you think the previous damage is spreading or running over the contacts?  Thanks.

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

I wouldn't be too worried about the pole faces, the damage looks fairly typical for a contactor which has been in service.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

I seriously doubt you have any worrysome damage there. The pitting looks well within normal to me.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

(OP)
Thank you.

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

(OP)
Attached is a picture of welded contacts with phase C on the right burnt up on West 480V LVMCC.

Prior to finding this, the West LVMCC main breaker tripped and the MCC lights went out (lights are on the MCC with these contacts).  The East 480V LVMCC main breaker tripped as a result of instantaneous event.  The two LVMCC are separated by a tie breaker which was open.

The operator turned the main breaker back on and the motor on these contacts started back up and smoke was comming out of these contact when they were found as shown in the attached picture.

It was found that phase C on the 400 HP, 480V motor to be shorted.

These contactors are Nema size 6 contactors rated for 400 HP motors.

We continue to see accelerated wear on other contactors and the is a concern that the other contactors will weld and do more damage.

These motors are started Across-the-Line.  So we see about 11 X FLA (418A) for about 17 ms and it falls below the LRA (2900A) as the motor accelerates in about 3-4 seconds.

There is a concern that there is some other electrical issues that may have caused the motor short according to the customer.  I am not so sure that a short in the motor was caused by the motor starters and electrical system.

Any ideas or words of wisdom?

Thank you.

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

What was phase C shorted to? From the look of the damage it was a phase-earth fault otherwise it seems odd that phase C pole is totalled while A and B are pretty much unmarked. What are the system parameters - fault level, clearance time, etc.

Are you assuming this is short-circuit damage which would have been cleared by the breaker and not a severe overload which would have been cleared by the O/L relay? I'm not convinced that this damage is solely due to a through-fault cleared elsewhere unless the bus at the MCC has an outrageously high fault level or the fault existed for rather more than a few cycles. I suspect that the contactor tried to open the fault and and the damage is the result of arcing in the contactor as it opened. Don't forget that a fault at the motor will be significantly lower than at the bus, epsecially if the cable is of any length: would the breaker mag trip have operated for a fault at the remote end of the cable? If the breaker Iinst is higher than the remote end fault level then the chances are the contactor was left to clear a fault considerably larger than its breaking capability and you are looking at the result.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

ScottyUK has given you a nicely done "guesstimation", but keep in mind that is all any of us can do from afar. You had a serious serious fault here on a very large (and likely expensive)piece of equipment, one that warrants a detailed investigation by a qualified professional. While I would agree with Scotty here, I also think you are beyond what an internet site can do for you. I would say that you need not only a forensic analysis of what happened, but also a detailed look at your protection coordination as well. Something like that should not have happened, the energy in that fault should have been interrupted at the level of a device rated and capable of interrupting it.

I strongly recommend hiring a PE with experience in large motor control and power distribution systems.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

(OP)
There was a short on phase A to B on one set of parrallel motor leads and and short on B to C on the 2nd set of motor leads.  We are sending the motor to a repair shop to investigate what exactly happened in the motor.

This is what was recorded in the field:

First set of parallel wires.

 

A to B (No Good)

A to C (Good)

B to C (Good)

B to A (No Good)

C to B (Good)

C to A (Good)

 

A to Ground (Good)

B to Ground (Good)

C to Ground (Good)

  

Second set of parallel wires.

 

A to B (Good)

A to C (Good)

B to C (Good)

B to C (No Good)

C to A (Good)

C to B (No Good)

 

A to Ground (Good)

B to Ground (Good)

C to Ground (Good)

 
No good meaning zero (0).


The Breaker feeding these contacts are Eaton 600A HCMP breakers and it did not open.  The Main breaker in the LVMCC are Eaton 520 Digitrips with no gfi did open.  The mag trip on the HMCP is set to 5434 amps with full load of motor at 418 amps.

Thanks.

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

A couple of comments, subject to the previous disclaimers regarding advice from the internet.
A short circuit in a motor or motor feeders may lead to secondary damage.
Seeing two contacts in good condition and one destroyed suggests to me that for some reason the third contact did not close completely. I have seen double break contactors where one side welded strongly and firmly (as a result of a dead short) and the other side was open slightly. In my instance the welded contact prevented the other contacts from closing. However it is possible that in your instance the contact may have welded so as to allow the good contacts to close, but on the third contact the weld held open a small air gap which lit up an arc when energized.

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

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

Where I work, we use vacuum starters for all NEMA size 4 and 5 starters. We also have a general guideline where anything above 200hp is medium voltage (2300V.)

Starters in general will arc and pit when opening/closing, not much you can do about it. The vacuum starters will keep the splatter from leaving the contact path (in a vacuum, the arc is extinguished) and arc chutes (there are none), they sound a whole lot less violent when compared to air-break and the footprint is smaller.

Price can be an issue since vacuum starters usually a little more expensive.

RE: Nema 6 Contactor contacts pitting Vendor says it is normal

Usually a LOT more expensive, and there is another issue: Vacuum contactors are not considered fully isolating, because they can "leak" when in the Off condition. After a while, even in the vacuum there is a vaporization of the contact material and inside of that sealed chamber, it eventually coats the inside surface of the vacuum bottle, providing a high impedance conduction path.  


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Nema 6 Contactor contacts pitting Vendor says it is normal


Posting a little late here, but I don't see any mention of doing an in-service AC millivolt-drop test on each contact set.  Might give some indication of relative contact condition.

 

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