Contactors

Yes. Or none at all.

What do you mean?

If one of the contactors on three phase system is severely damaged, would that cause abnormal voltage?

That's what he said.

You can check the contact point with an ir thermometer, if it is making contact and supplying voltage on all three legs, a poor contact will generally heat up....but why would you be using a damaged contactor...? Replace it....

We are not.

I'm investigating damage compressor on a large chiller. One thing I notice that the contactors were cleaned and filed down, there was a note from the mechanical service contractor asking the owner to change the contactors, but I don't think they did. They just tried to clean and file them. So I know that the contactors were not at the same thickness or size.

You failed to define "abnormal voltage", but can we ASSume you meant LOW voltage? If so, then yes that is possible, but it would be the result of high contact resistance, which generally reveals itself rather quickly as severely over heated contacts in the contactor.

"Filing" of contacts is something that should be done ONLY in an emergency as one awaits replacement contacts of a new contactor. Contacts often appear to people to be "burned" because they are pitted, but that is actually the NORMAL appearance. The pitting actually HELPS reduce contact resistance, because a pit on one side generally corresponds to a peak on the other side, so the overall surface area making contact is increased. Filing them down removes those matching surfaces, lowers the surface area, increases the resistance and then HASTENS the failure.

Those guys were cheap.

Looking at the three sets you can see that by filling the contactors, the edges were smaller, which in turn made the area of contact smaller. So each contactor had different surface areas. That led to different voltage at each leg," I'm not an electrical engineer", but I think that led to the motor damage.

Depending on the type of contactor there are some I would have no hesitation to dress the contacts. I've had a file in my box that has never been used for anything else for 40+ years.
But with modern rubbish, contact replacement is often the only way. Very large "pips" I would just knock off if the contact still had useful life in it.

Unfortunately as with much of the new stuff it's a case of "chuck the whole lot away"

I'm afraid this is one that comes down to experience.

If the points were filed, and the silver oxide surface removed, it's quite possible one or more of the contact points could have welded itself in the closed position, or formed a poor contact surface, resulting in any number of scenarios involving the voltage supply....I would look for discoloration of the contact mounting plate...

Knackered. Replace them.

I've that hundreds of times of times. its a common industry practice

Please download the document that i gave. You NEVER File contacts. They are made from silver alloy, and you are throwing away the remaining life when you file them....

I live in America. we save time and throw junk away. the tech on thid job did the right thing. he got the machine online and noted a proper repair. perhaps in India they have some labor intensive practice to preserve silver, thats too costly here, we're far more concerned with keeping equipment online. file them and order a new set of contacts for larger starters, toss the little ones in the trash

The maintenance practice is from ABB Europe, equally applicable to America. Not just India. i worked for Cutler Hammer before, and the practice was the same ... no filing. Silver oxide, which is a good conductor, may be dark and look like something asking to be cleaned, but is a good conductor. No need to file it off. Just some cleaning by wet rags is enough. Replacement of all contacts, of course, is the best, provided arc chutes and other parts are reasonably carbon-free. One has to weigh the cost of the contactor vis-a-vis the cost of equipment being controlled.

What hasn't helped is the OP not saying the make of the contactors at the outset.

If you worked for Cutler Hammer you will know of BHI contactors. No sintered silver on the contacts just plain and simple hard copper copper.
We used other manufacturers such as MC&S (rebadged Laurence Scott) and Allen West. Rated at up to 1200A.

No way would I throw contacts away that could be dressed and made good. Even sintered silver can be re-profiled in the machine shop.

ABB are but one manufacturer, they don't write the rules for others.

Quite.

However, the point that ABB makes is that there is really no need to dress the contacts at all, since the sooty appearance really indicates that the contacts are "run-in" after a few thousand operations, and arcing is actually less than when the contacts were new. And silver-oxide is a good conductor anyway. Cutler-Hammer also said this when i worked at Milwaukee, and at Brookhirst Igranic, Bedford, a long time ago....

An extract from a CH paper......

I would never dress a contact because of pitting. One of our problems was much of the gear was obsolete. A set of contacts for an Allen West 1200A contactor was five years ago £2200 to replace. You did what was necessary to keep them going.

I did solve the problem, we kept on stock 480V 315A 3P Telemecanique contactors. I used one contactor per phase. They were operating at 1750V but they only had to make on load the load being broken at 3.3KV.
I kept a close eye on them, happy to say they showed no sign of contact wear. It was a bit unconventional I know but it turned out to be the cheap option.
I didn't do it without consulting Telemecanique first and carrying tests on the contactors first. Telemecanique agreed it was feasible but wouldn't give their blessing, but they wanted to know the outcome.

My apologies if i came across as condescending. i fully agee with your views, now that i have understood. i have had some similar experiences, though not as broad as yours since we were severely constrained by foreign currency restrictions,, so we had to make do with local clones of those magnificent products

Sorry again...

First of all, I would like to thank you for helping me with this issue; your opinion is very valid to me.

The question I have that if the contactors were filed and the surface area for each contactor is different, you can see from the profile picture that the contactors were concaved and it was the not the same on each side of the contactor, by doing so wouldn't that cause a major problem.

As each moving contact is independent, with its own spring and guideway, chances are high that the fixed and moving contact tips will match well and handle the steady-state conduction of the current. However, the actual situation may be different, a severe short-circuit may have passed through that contact, to the extent that base material is exposed. In such a case, replacement of all six contacts is better.

You haven'r responded to the aspect of better motor protection. Can you tell us something about the relay that you have?

phase imbalance leading quickly to phase loss - see post 1

if bad enough you can have voltage drop on the load side,

Please see the picture where you can see the evidence of the filling

If poor contact had been made the bar with the upper contacts would be discolored, orange and black...those look normal to me from here....

Now these are burnt....

G.A. Personally I've never, ever measured any "abnormal" voltages on severely worn contactors. Phase loss or shorted phases, yes many times.

Given the exchanged so far, I'm curious to know what qualifies you to evaluate the failure?

Also, I am curious to know why you have not engaged a local, knowledgeable person to advise you?

Thats easy "I'm investigating damage compressor on a large chiller."

Blame the contactor for the chiller damage and there is no warranty for the chiller.

Standard pass the buck.

No we are actually on the side of the contactor, I'm a mechanical engineer and I have over 30 years of experience, but I have never seen this, where the motor is burnt and the mechanical parts are clean except the screen. I have contacted electrical engineers in my office, but I want to gather as much experience as possible. I don't feel that the mechanical contractor was at fault for this chiller, they did a proper job working in a bad situation. The owner was told that the contactors were going, and they could weld, and they offered to get some and change them, the cheap owner elected to work on that part themselves only to burn the compressor motor in two days and going after the contractor for the loss. By the way for the one who want to know, I took the whole compressor a part and the mechanical section is excellent.

Assuming you did not single phase, I would be more concerned with the overall protectection scheme, and even the time delay on restarting (short cycle), and the compressor off loading before the restart.

I have seen significant damage to AC compressor motors on soft lines that droop excessively on the inrush. Fix that problem and the compressor starts cleanly.

So many issues, and so few answers. I would be looking far beyond just the contacts - like the whole power system, and the motor protection scheme, that obviously did not protect the motor.

The system was way too big for the process.

They were using 200 ton chiller with two 100 ton compressors, when I checked the system all they needed was 20 ton chiller, they were told that the system is too big but they kept ignoring the mechanical contractor recommendations. The chiller was short cycling due to the lack of load, screw compressors could only go down to about 40% of the total load, which in this case 40 ton, it will run at 40% but you will be asking for trouble. Those guys were running at half that. In my opinion, and I'm not an electrical engineer, the compressor was recycling due to the lack of load which in turn caused motor jitters, that damaged the contactors and the coil insulations. Damaged contactors lead to voltage drop on one of the legs, this was 3 Ph, that caused the compressor motor to burn out and the compressor to be useless now.

I am guessing (due to an almost complete lack of vital information from the OP!) that a phase was either lost completely or dropped the voltage severely.

The results being probably that a 3 phase motor overheated and/or stalled, but the HRC fuses were not blown and the motor was therefore severely damaged....

Good modern equipment can do many things to restrict damage (this list is not complete):-

1) Phase voltage monitoring

2) Temperature monitoring of motor and other equipment.

3) In the event of a problem being noted, the motor could be stopped and/or some form of alarm started.....

It therefore appears that equipment and personnel safety was of secondary concern and now the OP is trying to blame someone else for the problems.........

As many have said, better protection is required for the motor. The most inexpensive is an IEC type Thermal Overload Relay, with built-in single-phasing sensitivity. Something like this differential-slider mechanism.

This is from Eaton, USA. They also have much more sophisticated electronic relays which protect the motor against many kinds of faults. You may benefit by checking them out.

However, the failure you have seems most likely to be from single-phasing.

Good luck.

It will depend on the type of contractor you are using, there are various reasons of contractor failure, the one reason is the contractor does not close, it will open and flashes over. you cant lead the damage contractors to abnormal voltage.

I have some damaged contactors that I will send you then you can replace all healthy contactors with damaged ones.......