Unusual Motor Contactor Problem

It was noticed a few days ago that a contactor controlling a 75 HP motor load was - for lack of a better word - "clunking". While energized, the coil
of the Square D contactor would momentarily de-energize and then re-energize. It would do this randomly once every 15 or 20 seconds about 60 seconds after the load is started. The de-energization is so short that the load contacts don't open. But the clunking is quite loud and we can see movement (though not breaking) of the auxiliary contacts through the transparent covering. There is no noise, interruption or distortion of the 120 VAC coil supply at the coil terminals, as determined with an oscilloscope. The phenomenon also occurs with the load disconnected and the coil forced on.
We opened the contactor and examined the coil and the crimped winding interconnections within the contactor housing. Everything seemed solid there. There was no evidence of arcing or charring nor any burnt smell. I didn't want to unravel the tape binding the coil on the bobbin until a spare was available.
The 75 HP starting control uses an electronic soft start that's shunted by the contactor after the load accelerates to rated speed. For now, while we await a replacement, we're leaving the soft start energized after the contactor pulls in just in case the contactor's load contacts do open during a "clunk".
I suspect a broken wire in the coil winding that is vibrating and losing connection briefly and randomly after the coil warms up. I've never seen a coil fault quite like this. Usually, a broken coil winding will de-energize the coil long enough to drop out the load contacts. Anyone have experience with this type of fault?
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during
I would suspect the "soft start" stuff that's in parallel with the contactor is acting up. In s "soft start" system, the coil should be controled by the "soft start" electronics. Either through a error in the controller for the nature of the load the electronics may decide that the current is excessive and will act to reduce the voltage. But before the contactor opens and permits the solid state stuff to do it's voltage reducing job and current drops again and the controller activated the contactor again.
You might consider putting a old style ANALOG currrent probe on the load near where the controller senses load current. See if there is a current "glitch" when the contactor makes the noise. It could be something lilke a bearing that's has had better days that "drags" or even lets moving parts touch non moving parts.

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Well, if you don't mind getting your hands a little dirty, you disconnect the main power and the coil and run whatever voltate the coil wants to the coil and see whether is drops out. I really don't think it will but the "experiment/test" may ease your mind.

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Thanks. I've discounted the soft start as the cause of the problem. Without a load connected and the coil forced on, the fault still occurs. We do monitor and record analog current to the load and have not found an interruption of load current nor a current spike during a clunk. A faulty bearing or mechanical interference increasing friction enough so as to measurably affect load current would be noisy and would be detected during morning maintenance inspection.
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bargepole wrote:

Put a current probe on the contactor coil and watch it (and the coil voltage) with a transient recorder or properly triggered scope. You should be able to see if the coil os going open (or shorting some turns) intermittently.
--
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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Shrewd advice. I say that because that is exactly what we did today. As it happened, it seems that the supposed fault welded itself closed today since we did not observe the fault all day long.
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Put a Fluke 87 on the coil and monitor min voltage and max voltage. You might need two meters for this to catch the fault. If the coil chatters and your voltage is rock solid then replace the coil and retest. If it continues look at the controls/safeties and or the softstart.
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In my experience, this is a quite common failure mode for a contactor. If allowed to continue, it will eventually arc the contacts. I do not know the root cause of the phenomenon.
--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
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I'm surprised to read that you have found this to be a common problem. Our facility uses hundreds of contactors and I've never actually seen these particular fault characteristics in 19 years. It could be that the same supposed flaw in a coil of a smaller contactor with less armature mass actually allows the load contacts to open, making the fault more apparent. I don't doubt the load contacts will eventually arc and I hope to have the device replaced very soon.
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Does the contactor coil run through the thermals? I've seen the contacts on the "overloads" go bad many times.
Put a little neon pilot lamp on the coil itself and sit back and watch.
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Thanks. Yes, the coil contacts do run through the thermal overloads. That does seem to be the problem, as the thermal overloads test ok (continuity and test break) and as I've mentioned before, we're not seeing a voltage across the coil during a "clunk" as displayed on an oscilloscope.
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seem
Excuse my drunkenness. Of course, what I meant to say was "That does NOT seem to be the problem..." and "...we're not seeing a voltage DROP across the coil during a "clunk"as displayed on an oscilloscope".
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I am not sure of the environment your contactor is in. Is it clean, dirty, or damp, etc. I have seen contactors that are used in the Coal Mines and what happens is that the magnetic pole peice that actually pulls in the contacts get dirty or rusty. When this happens the thing chatters like mad. Maybe this is what is happening. I usally used a brillo pad to shine the pole pieces and that seemed to elimante a lot if not all of the chatter. Mind you, these are 3ph contactors like Westinghouse or Joy would make, so I don't know if this would apply to yours.
I don't know if this is what is happening with your particular contactor, but it may help to look into this.
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Good questions that underscore the incompleteness of my problem description. The environment is clean, dry and at constant temperature so I don't think that's an issue, but I'll remember that your attention to environmental conditions is an important aspect of problem definition.
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| I suspect a broken wire in the coil winding that is vibrating and losing | connection briefly and randomly after the coil warms up. I've never seen a | coil fault quite like this. Usually, a broken coil winding will de-energize | the coil long enough to drop out the load contacts. Anyone have experience | with this type of fault?
I have seen a relay that just barely tried to open (actually arced the main contacts) ... as opposed to fully opening ... when the cause was a short (happened to be in the wires leading to the winding in the case I observed). A coil winding break would likely last longer as you suspect, but a short that is just arcing occaisionally could do things on a shorter (pun not intended) time scale ... not enough to lift the contacts.
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On Thu, 05 Aug 2004 01:41:50 GMT, "bargepole"

You likely have a broken shading coil in the armature (stationary part) laminations. The shading coil is a loop of wire imbedded in the steel laminations which creates a shift in the magnetic field so that the plunger stays in with less energy than it took to pull it in. If it is broken it can interfere with the holding power of the armature. As current passes thru the contacts, it makes a loop-back that exerts a small amount of mechanical force opposing the coil magnetic holding force. If the shading coil is broken, that force can occasionally become strong enough to begin forcing the armature poles appart. If they start to spread, the resistance accross the contacts increases and the current drops off, lowering the magnetic force they exert, so the armature closes tight again.
You can relace the armature of some contactors, particularly NEMA designs, but most of the IEC style designs were not made for that so I suspect that you will need to replace the entire contactor.
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