Rapid contact wear on 100HP motor starter.

[MetalworkerMike]

I just finished cleaning the contacts on a 180A motor starter. It's running a 100HP 575V 3ph motor on a granulator (grinds up plastic... sporadic very heavy loading, then load reduces until it gets slammed again). The starter is a pretty good one - an Allen Bradley bulletin 100 B180 - replacement cost $1700. A new set of contacts for it is $400, so I just filed them down and re-used them. There was still enough silver left on them.
The question is what might cause the contacts to be heavily pitted in just 2 or 3 years. I've got contactors on machines, here, that are 40+ years old and still working fine. I've got one particular 37+ year old AB contactor on an air compressor that I _know_ has cycled at least 200,000 times, and it still works great. It looks like a reject prop from a low-budget Frankenstein movie, but it still works. This newish contactor on the granulator is rated for 180A (resistive) and it's only running 90A (inductive) so I'm not sure why it's getting hit so hard. The tech sheets specify up to 150HP at 575V, and the motor it's starting is only 100HP. Is there anything I can do about it? The contactor can't be taken apart... at least, not by any non-destructive method that I was able to find. You can remove the contacts and the coil, but the springs and the rest of it are all glued/snapped together, so I can't do much about alignment etc.
Any thoughts?
One person suggested vibration could be the culprit, and certainly there is plenty of vibration in the area. You can feel the shock-wave from the grinder hit your chest from 12 feet away when a roll of scrap is dropped in.

Mike

 

[waross]

I have found that the best life may be obtained from silver alloy contacts by ignoring them as long as possible. The black on silver contacts is more likely to be silver oxide than carbon. I understand that silver oxide is also a good conductor. I have seen silver contacts burned black and covered with "grapes" that were giving good service starting large motors DOL. Try ignoring the contacts for awhile and see if you get longer life than you do by cleaning the contacts.
Cleaning silver contacts often just wastes expensive silver.
respectfully

 

[electricpete]

The 180A "resistive" caught my attention because as we all know, a motor is not purely resistive.

On the other hand, it looks like it's a motor starter here:

http://www.ab.com/en/epub/catalogs/12768/229240/229254/229461/229477/tab5.html

Anyone have any idea why it's labeled with a "resistive" rating?

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[Marke]

Hello MetalworkerMike

The contacts are typically damaged in normal operation, by heat. There is a thermal rating, often referred to as the resistive rating, or the AC1 rating, and this is the maximum continuous current that the contact can control assuming that there is no overload.
If you control an inductive or capacitve load, you will have an overload when the contactor is closed. This high overload current will result in a high power dissipation in the contacts. The dissipation is proportional to the square of the current. A 20% overload results in 44% increase in power dissipation. In order to cope with high overload currents, such as that associated with starting induction motors, the contactor manufacturers provide derated values for different duties. In Europe, these are identified as AC2, AC3 AC4 etc.
What is not always obvious is just how much overload, for how long and how frequently. It is interesting to look at the criteria used by different manufacturers when they are determining the maximum rating for motor starting applications.
If we look at induction motors under full voltage starting, the start current can be anywhere from 500% to 900% rated motor current, and the starting time can be anything from cycles to tens of seconds. If you are starting a modern high efficiency motor with a locked rotor current of 750%, you are dissipating a higher current in the contacts during start and of you do not further derate the contacts, they will fail earlier than with motors with a Locked Rotor Current of 600%. Additionally, if you are starting high inertia loads with a start time of say 20 seconds, the temperature of the contacts will be very much higher than starting the same motor on a pump. Similarly, starting frequency will also influence the operating temperature of the contacts. In particular, the temperature is affected by the OFF (cool down) time before a restart.

If the contactor loading results in excessive contact temperatures, then the life of the contacts is going to be shortened.

Another factor to look for, is the voltage on the coil of the contactor, especially during start. If the coil voltage is reduced, the contact pressure can also be reduced. If the contact pressure is reduced, the contact resistance will increase and this will increase the contact temperature.

Blackened silver contacts is not a problem. I would not file them down just because they are black or pitted. You may be creating a problem by doing so.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[ScottyUK]

Grinder motors and shockwaves you feel 12 feet away - could this be an unusual motor design rather than the typical design the contactor is rated for? I'm not that familiar with US motor specs but if the contactor rating is based on the 'typical' motor (NEMA 'B'?) could the problems be because this motor isn't 'typical' but has a much more aggressive starting requirement?


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[MetalworkerMike]

The reason I opened the contactor and noticed the contacts were pitted was because the granulator had been stalled by too many rolls of scrap put in too quickly, without giving the motor a chance to come back up to speed, and it had kicked out the overload. When I went to re-start it it started 'oddly', without the kind of snappy torque that I would expect, and it seemed to run rough and just strangely. I tried starting it with the electrical cabinet open so I could watch the contactor, and I could see significant arcing during startup and operation. When I opened the contactor there was significant soot buildup present, though no melting - the soot all wiped off cleanly.
I filed the contacts and it started cleanly after that.

As to the efficiency of the motor, it is a Hyundai unit and I recall that it claims a power factor of 85% along with the expected service factor of 1.15, but I don't recall anything on the nameplate specifying lock rotor amperage. The 85% PF, though, struck me as being pretty reasonable, so I _think_ it's a reasonably efficient motor.

Coil voltage - electripete suggested that as a possibility, and at first I thought it was unlikely because it seemed the voltage would have to drop quite a bit to get below the holding voltage, but I have since realized that coupled with the vibration issue and others, it could certainly be a real contributor to the problem. I will test-start the unit with a meter on it that will measure the minimum voltage to the coil.

Mike

 

[oldfieldguy]

Short start-stop cycles and tripping under overload will both wreak havoc with contacts.

I've seen that start-stop thing happen as an unintended consequence of PLC programming.

Realize that the contacts have a thermal limit, too. Overheated contacts hit with the arc associated with tripping under an overloaded condition can be expected to melt and displace more metal than you'd want.

I've done some remediation on damaged contacts, limited to removing little beads of metal and jagged edges from arc craters. You have to be careful in this to maintain the original contour of the surface. Naturally, the manufacturer instructions say to replace damaged contacts instead of trying to clean them up, but the manufacturer isn't trying to get a unit back on line at two in the morning.

old field guy

 

[MetalworkerMike]

This granulator doesn't trip out that often... it takes half a day to get it going again, so believe me that if it happened often I'd be doing something about it.
I would guess that it may have tripped out 6 to 8 times in its lifetime. Still far more than I'd like, but not all operators are particularly interested in waiting for the machine to do its job.
As for remediation of contacts, my method is to file each one until it's reasonable, then I reassemble the starter and run a single file through all of the contacts on one side (load or line) at once while pressing the contacts together so that they all become parallel with one another. I haven't had any trouble yet, but that doesn't mean what I'm doing is intelligent... it just means that Murphy hasn't caught me at it yet.

Mike

 

[jraef]

#1 rule of contactor use:

NEVER file contacts as a permanent solution. When contacts are used properly, the "pitting" you see is actually HELPING them to conduct. The pits on one side mesh with the peaks on the other side, increasing your overall surface area contact for conduction and reducing the contact resistance. When you file them, you do two things: you drastically reduce the surface area, partly because you eliminate those nice peaks and valleys but also because nobody can file a perfectly mating surface by hand; and secondly you reduce the mass of the contact material which plays into the heat dissipation Marke discussed. Bottom line, the first time you file those contacts, you have ruined them and hasten their further deterioration, then you exacerbate the problem ever time you do it again until they end up welding, which then can smoke your motor and or severely damage your machinery. Size the contactor properly (as it appears it may be in this case) and leave it alone. If you had to file them to get running again, look at that as a temporary measure until you can replace the contacts completely.

PS, you may want to consider replacing that contactor with a NEMA design version for that application. They are built for extreme duty like that and can take a lot more thermal abuse than IEC contactors without oversizing. I would do that, along with using a solid state overload relay that provides jam protection (fixed over current trip) so that you don't have to wait for the thermal overload protection to take it off line, plus restart lockout to prevent someone from restarting too soon after a jam or overload. These overload relays are more expensive than cheap bimetal ones that typically come with the starters, but they're a lot less expensive than downtime! Waiting for a thermal overload to protect your motor from jams is hastening the demise of your motor windings as well.

 

[MetalworkerMike]

I wasn't expecting filing the contacts to be a permanent solution. If I had thought that it was a permanent solution then I probably wouldn't be here searching for a solution.
As I mentioned when I described my filing method, I use one file through all contacts on one side at once. Since the file is parallel it makes the contacts parallel, so I am not particularly worried about having reduced the contact area due to poor filing. As for the mass of the silver, it is pretty small compared to the mass of the copper, so I'm not sure how much of an effect removing half a gram of silver from each contact is going to have, but I will stand by the previous statement that the contactor did not work before filing, but works fine now. It connects properly with no visible arcing. That says to me that the filing helped.
I have had a grand total of two contactors weld on me, and both were small units with non-removable, non-accessible contacts which had, needless to say, never been filed.

As for leaving the contactor alone, once it stops working I _can't_ leave it alone. I have to fix it. I have no choice. It doesn't matter if it's the most lauded contactor ever devised by the mind of man, if it doesn't work then it needs to be dealt with.

Re-start lockout shouldn't be a problem, since it takes several hours to open the machine and clear out the material from it before you can start it up again, but that is assuming that the operator has a brain, and believe me when I tell you that I have given up on that assumption. Yes, it is possible that the operator might try to pound his way through the obstruction by repeatedly cycling the motor for one second per cycle or something like that. It seems to me, though, that no matter how much hardware you hang on a device to try to foolproof it, there are always bigger fools out there. The solution should be to educate the operators. Should be. Isn't, but should be.

Mike

 

[ScottyUK]

If this application turns out to be really troublesome or if reliability is proving difficult to obtain I really like the big old-fashioned bar type contactors. They're expensive so I usually reserve them for DC applications or the occasional severe service AC application. ABB's R-line range is one that I am familiar with. Telemecanique and Siemens also make them. I've never seen a contactor of this design fail without massive external influences (animals, molten metal, etc).

http://www.abb.co.uk/Product/seitp3...8e0040fe25.aspx?productLanguage=us&country=GB

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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[MetalworkerMike]

I have high hopes for isolating the contactor from vibration. I think that will be my first attempt at a solution (though I am still going to check the coil voltage on start-up to make sure it's not dropping). I'll keep checking it every few months to make sure it's behaving itself.
I got a lot of useful information in this thread, and it's appreciated.

Mike