Contactor major overload

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PMichael944

Member
Location
PA
Occupation
Electrician
Reaching out for as much input as possible. Working for an adhesives production company. Company makes and packages adhesives and such but not really important to question beyond that. The initial processing begins with mixing, for which we have multiple industrial sized mixers. The one in question, is a 500G, 3 phase, 480v 250A 60HZ mixer.

From the main electrical panel power is run to electrical cabinet in mech room, then to mixer which has its own control panel on machine also. (I’m not the best at explaining things but gonna try really hard to give mental picture) So the electrical cabinet, power comes in to 250A disconnect and from there directly out to two main contractors, each having its own thermal overload. The contractors basically control two seperate function of the mixer. Dispersion and planetary. All wiring through is properly sized. Here’s where fun begins.
When I first started, they had been having issues with dispersion side overheating and tripping out overload. Process of determining why, I eventually replaced leads from d/c to contactor making them exact length and torquing connections. Finally seemed to solve issue.
Friday, the planetary contactor side overheated (b and c phases temp when shot it were 210-250 degrees and A phase read at 417 degrees). Wires had begun to disintegrate . Contactor became welded closed. Niether the d/c nor overload tripped.

The machine was manually shut down immediately at that point.
I am just basically looking for any ideas from anybody as to what thoughts are on what would cause this. I just do not want to overlook anything or miss anything that could allow to happen again.
Thank You
 
It is very hard to give you advice without being on site but I can tell you that if the wiring is overheated it is likely because there is a LOT more current than the wires are rated for. Ten or twenty or fifty percent extra is probably not going to be your problem.

If the over current protection is not functioning to protect the wiring, you need to figure out why.

I would start with measuring the actual current in the wires and making sure they are sized properly.

It's possible the motors are dragging down due to mechanical binding and someone decided the solution was to disable the overloads. Maybe with a jumper. Maybe the overload just failed.

If you are experiencing contact welding it may be time to replace the contactor.

You may have multiple issues as well that are making for a confusing mess of symptoms.

One possibility is some kind of ground fault that is producing enough current to overload the wires but not trip the OCPD. Maybe a cover that is clamped over a wire.

Could be all kinds of things.
 
If we assume that the system is designed properly, my first guess would be a motor failure that resulted in a single phase event. Old, NEMA style overloads will not react in time to prevent damage in such a case.
 
You have one motor and two contactors? Is it a 2 speed motor then? Or is out a Part Winding starter and someone THOUGHT it was two speed? (Happens more than it should). Can you post pictures of the motor nameplate and the control cabinet?
 
I see the welding of the contacts as a particular problem when the motor has not faulted and opened the OCPD. But ... you did not mention what the OCPD is rated for, just that the disconnect (unfused?) is rated at 250A. Anyway, If a motor contactor is subject to being opened and closed rapidly (poor control wire connections, or other problems in the control circuit), the heat generated at the contacts can be enough to weld them. I have also seen them weld due to a motor fault, with the contacts welding when they try to open a circuit with more amps than they are rated to interrupt.

Certainly you need to replace the contactor and any damaged wire. You should also check the motor and leads for opens, shorts, and grounds with a megger, check the settings of the OCPD and the overload, and, when starting back up, the actual amps the motor is drawing.

Not all motor faults can be identified with a megger, and there may be something else going on that we aren't thinking of. Be handy if you have one of those nifty bluetooth amp clamps so you can have the bucket closed and stand back when you take amp readings.
 
Jraef said:
You have one motor and two contactors? Is it a 2 speed motor then? Or is out a Part Winding starter and someone THOUGHT it was two speed? (Happens more than it should). Can you post pictures of the motor nameplate and the control cabinet?
Click to expand...
No two motors. One for dispersion and one for planetary.
 
Thank you for replies. I will try to post pictures tomorrow of what resulted from incident. (Has been shut down until parts - hopefully - arrive tomorrow.
Will actually try to include pics of whole set up. I’ve been reading these forums for long time, but first time posting
Thanks again
 
ptonsparky said:
Overheated wires at the contactor or starter screams poor connections. Not just the lugs but the contacts of the starter. Things do wear out. I agree with replace the contactor.
Click to expand...
and cut out or replace any length of conductor that has been damaged by the heating effects or even anything else in that area like the overload assembly, conductor links between overload and the contactor, etc. that may have experienced the overheating.
 
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