Intermittent AC Motor Short?
Intermittent AC Motor Short?
(OP)
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?





RE: Intermittent AC Motor Short?
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.
RE: Intermittent AC Motor Short?
RE: Intermittent AC Motor Short?
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.
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more." Nikola Tesla
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The industrious little ant (in their thousands) can do a number on a contactor.
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RE: Intermittent AC Motor Short?