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?

 

[dominar]

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.

 

[dominar]

Picture attached.

 

[ScottyUK]

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 I_inst 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.


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

 

[jraef]

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

 

[dominar]

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.

 

[waross]

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

 

[timithye]

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.

 

[jraef]

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

 

[busbar]

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.