Nema 6 Contactor contacts pitting Vendor says it is normal 1

[dominar]

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?

 

[ScottyUK]

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.


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[dpc]

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

 

[damiens]

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

 

[dominar]

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

 

[jraef]

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!


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).


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[dpc]

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

 

[jraef]

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


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[dominar]

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.

 

[jraef]

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!


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[krbernier]

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

 

[dominar]

I have attached a picture of the contacts. Do these look normal?

 

[jraef]

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.


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[dominar]

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.

 

[ScottyUK]

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?


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

 

[dominar]

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.

 

[dominar]

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.

 

[ScottyUK]

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


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

 

[jraef]

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


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[dominar]

Thank you.