# Help with most-mortem on motor



## frenchelectrican (Mar 15, 2007)

Mike.,

Is any chance any of those affected motours were run by VSD ?? if so you may have to dig little deeper on the issue.

I will say single phasing will useally take them a toll depending on how well it loaded if it was single phasing it may happend when the motours still running.

But once they stop and try to run again it will useally speed up the burn out progess on it.

Grease commation maybe so if someone did overpack the grease fitting or the bearing seal went bad.

Merci,
Marc


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## varmit (Apr 19, 2009)

From your ohmmeter readings, I would say that one, of the six, motor windings is open in the failed motors. Are these new motors? If so "Out of the box" failures are not that uncommon these days, but I would think that a 40% failure is unlikely.

Things to check: 
Check voltage, under load, on each phase to ground. It is possible that one phase, in the distribution to these motors is loose or broken somewhere. 

Are the motor overloads set and wired correctly?

Is there excessive voltage drop?

Are the affected units modified mechanically to move more air causing a higher motor load?

Are the affected motors started excessively?

Is there any mechanical binding hindering motor rotation?

Any severe lightning lately?

Check contact resistance on the starters.

Good luck. Finding "why" is always difficult.


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## Jlarson (Jun 28, 2009)

I'd say single phasing is a good possible cause. 

What type of starters, specifically what type of OLs are involved?


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## Mike in Canada (Jun 27, 2010)

Motors are of unknown vintage. No records kept. Maybe a year old, maybe 10 years old.

They're on regular magnetic starters.

The two in question blow fuses immediately so checking for voltage under load isn't in the cards.

I'm just trying to figure out what could cause one of the windings to lower in resistance. Internal short... what could cause it? A little voice keeps telling me it could be improper grease contamination.


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## Jlarson (Jun 28, 2009)

Does it look like someone's been greasing them? Any clues someone has been up there "maintaining" them.


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## JRaef (Mar 23, 2009)

I don't see how greasing, proper or not, is going to affect the winding resistance. But I don't know how you can "megger out VERY well" and have a resistance issue. Did you only use the megger for the resistance to ground test and then yous a VOM for the winding resistance? Or did you mean to say the megger test for leakage to ground was fine but the megger test of the winding was not?

If you used a VOM for the winding test, I wouldn't trust that. Small differences in probe connections can account for that much of a difference, that's why a megger uses much higher voltages. But if you used a megger and those are the results, that indicates a turn short, in other words you have conductors within the same winding that are shorting together, making that winding effectively shorter than the others. 

If possible, see if you can look at the ends of the windings and compare them to this set of photos, it will give you an idea of what to look for.


But turn shorts rarely cause fuses to blow instantly unless they are very catastrophic, which is not what you are seeing in those winding resistance numbers even if they did come from a VOM. A minor change in resistance would cause an overload eventually. But blowing the fuses instantly means it's a short circuit, not an overload. Something is probably going to ground somewhere, I would first suspect a bad connection or a bad motor lead. Did you megger the entire circuit at the stater panel or just the motor itself at the peckerhead?


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## electricalperson (Jan 11, 2008)

I dont know if this is a stupid question or not, but do you think the motor is jammed somehow? belt too tight? is it blowing the fuse as soon as they start or does it take a few seconds?


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## frenchelectrican (Mar 15, 2007)

Some case someone may installed wrong type of fuse that can act up like that as well.

If you have real Megger unit just unhook the motour et starter and check the conductors as well then do the motour check with Megger to see what it come up and only apply 2X of what the voltage the motour is running.

So if you have 240 volts then use 500 volt scale to read it ditto with 600 volters bring it up to 1000 to 1200 volts but keep it very breif { tyically a minuite max when you get that high of voltage level. }

JRaef did got this detail covered pretty good.

And Just suggest a idea as well get the recording voltmeter to read for X number of hours or jours { days } to see where the voltage surge show up.

Merci,
Marc


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