Intermittent AC Motor Short?

[rdub]

I have a 50 HP 3-phase, 460VAC motor circuit that experienced a fault. The motor is operated by an across-the-line starter. (I didn't realized motor starters had so much smoke compacted into them!). A megger test did not show any difference in the windings or wiring to the motor so the motor was put back into service and appears to work fine. Could there be some problem with the motor that will raise its ugly head again?

 

[itsmoked]

Well stop and think about it... Holding a starter in your hands..feel how heavy it is? That translates to a LOT of compressed smoke!

Starters do fail on their own. They have a lot of abuse. The coil can short out inside and as it burns it shorts thru large chunks of its windings which drops the resistance dramatically causing it to draw ever larger amounts of power releasing ever more of the contained smoke. Finally the contactor starts to g r a d u a l l y open which can cause nasty arcing of the contacts and further released smoke.

You seem to have done your due diligence on the motor. Unless violent driven load motion was noticed or some other anomaly not fitting with the above description was noted, you probably won't have any further problems.

 

[rdub]

From looking at the contacts,(which were now laying on the bottom of the enclosure)looks like they were welded for a short time before they blew off. So I'm thinking the contactor itself was OK.

 

[jraef]

You may have had a coil, armature or wiring problem. Chattering of the coil causes extremely rapid heating of the contacts, and they can go into melt down in a matter of seconds depending on how they were sized. Low voltage on the coil usually will not do that because responsible contactor designers know enough to have a threshold voltage at which the coil either holds or releases, but it can still happen under the right conditions. I have seen it happen on systems supplied by portable generators when the generator runs out of fuel and there is no undervoltage release. The voltage AND FREQUENCY drop at the same time and the coils can chatter. Sometimes it is just a chattering of another control device in the coil circuit such as a fluttering pressure or float switch without a dead band. The result is the same.

I also see this a lot when people try to push a contactor closed by hand. On large contactors, your hand cannot exert enough force to overcome the magnetic forces that are trying to drive the contacts apart, and they chatter wildly, smoking the contacts. I have been challenged on that concept by electricians, but then I counter challenge them to remove the contact assemblies and place their fingers between the movable and stationary parts of the armature as I energize the coil and try to prevent it from smashing their fingers. None have taken me up on that.

A piece of debris in the pole face of the armature will cause problems as well, especially something like wire insulation material or drill shavings that get stuck on it. It doesn't let the magnet pole faces touch, so the field fluctuates wildly, causing chatter as well. I once had my suspicion about insects doing the same thing, but that was proven false once when I saw a large spider splatter mark on the face of an armature that was working fine.

A line or load lead that heated and cooled off enough to loosen it's connection into the lug will eventually overheat as the resistance across that connection increases, then it fails all at once catastrophically.


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[ccjersey]

The industrious little ant (in their thousands) can do a number on a contactor.