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Old 18-12-2021, 08:10   #1
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alternator fires

Does anyone have any anecdotes about alternators literally burning up?

I have heard of one recently and was unaware of the potential danger, particularly with high output alternators.

Also, I have thought of alternator faults as most likely self limiting, but a short to ground in a running alternator I guess is basically an arc welder capable of starting a hard to put out Metal Fire:

https://www.wtamu.edu/~cbaird/sq/mob...t-metals-burn/
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Old 18-12-2021, 08:18   #2
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Re: alternator fires

main battery shut off switch and shut the engine off, hit it with a dry chem. Better yet get a externally regulated alternator with temp control on the battery and alternator and fuse it properly while using proper wire, connections and routing.
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Old 18-12-2021, 08:38   #3
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Re: alternator fires

all good advice, but in the recent incident, they shut down the engine, disconnected power, and emptied all their fire extinguishers …but the metal continued to burn until cooled with sea water poured over it. funny thing a prominent marine alternator company’s tech support told me metal is not combustible.
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Old 18-12-2021, 08:46   #4
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Re: alternator fires

In all likelihood the energy was from the batteries back feeding into a short. A fuse at the battery end of the alternator conductor is a simple fix.


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Old 18-12-2021, 09:06   #5
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Re: alternator fires

Usually what happens is that a bearing fails and overheats, people notice, engine is shut down. If that isn't done eventually the rotor will contact the windings which is hot and loud. A short in the windings can cause overheating and lead to a thermal cascade. Aftermarket regulators with alternator temperature sensors should shut down the field when this starts to happen.

Usually the fires self-extinguish when the engine is stopped. I suppose in some cases power from the battery could contribute but that would have to be rare as there are diodes to prevent it, and diodes pretty much always fail open rather than failing short.

Never heard of magnesium or other combustible metals used to make an alternator. Usually there's a steel rotor, copper windings on steel laminations, and an aluminum housing. I suppose somewhere someone has made a magnesium alloy housing to save weight. Takes high temperature to get it lit but very hard to put out
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Old 18-12-2021, 09:19   #6
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Re: alternator fires

I agree with the casualty scenario by Jammer in comment #5. In my opinion, the metal was not burning, the insulation on the windings, which is essentially varnish, burned.
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Old 18-12-2021, 09:28   #7
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Re: alternator fires

aluminum housing sounds combustible (per my attached link).

and I wonder, even though its un-informed speculation, if the powder coating of aluminum alternator cases is such a good idea if the coating were to bind oxygen in
close proximity to the heated aluminum.

also, alternator temperature sensors can go bad, so I think I will be testing them as a regular maintenance item in the future.

we have twin 150A/12V alts connected to really high charge acceptance rate batteries.

and a canvas bucket on a rope will be moved to an easy access location as well.
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Old 18-12-2021, 09:30   #8
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Re: alternator fires

i would think the varnish coating would be a pretty quick burn? unlikely to sustain a hot fire for long?
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Old 18-12-2021, 09:54   #9
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Re: alternator fires

Quote:
Originally Posted by Journeyman View Post
aluminum housing sounds combustible (per my attached link)...
Doesn’t sound like it to me, reading the same article:

Quote:
First, if you have a solid chunk of metal, it is hard to get oxygen atoms close enough to the majority of the metal atoms to react. In order to burn the metal, each metal atom has to get close enough to an oxygen atom to bond to it. For large chunks of metal; like spoons, pots, and chairs; most of the atoms are simply too deeply buried to have any access to oxygen molecules. Furthermore, metals don't vaporize easily. When you burn a chunk of wood or a wax candle, the fuel particles readily vaporize, meaning that with just a little heat, they shoot out into the air where they have better access to oxygen atoms. In contrast, solid metals tend to have their atoms very tightly bound together, meaning that it is much harder to use heat to vaporize the metal. Also, organic materials like wood or cloth contain a lot of their own oxygen, whereas raw metals don't. This is one reason why it is much harder to burn a metal spoon than a wooden spoon, even though they both consist of large chunks of material.

With this fact in mind, all we have to do is manually break apart the metal atoms in order to get them to burn better. In practice, this means grinding the metal down to a fine powder.....
The article is interesting, but all we have is anecdotal evidence that metal actually burned in a hard to extinguish fire. Any photos of the alternator after the fire? Is the case gone? Substantially melted? Without that kind of information “metal burning” is just speculation.
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Old 18-12-2021, 10:01   #10
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Re: alternator fires

if it was mag, putting water on it would have made the situation much worse before it got better and I doubt there would be enough, continuous water to be applied to it via bucket to overcome the effect that takes place when doing so to get it cool enough to stop the reaction (splitting H20 into H and o2).
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Old 18-12-2021, 11:09   #11
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Re: alternator fires

Quote:
Originally Posted by HeywoodJ View Post
Doesn’t sound like it to me, reading the same article:



The article is interesting, but all we have is anecdotal evidence that metal actually burned in a hard to extinguish fire. Any photos of the alternator after the fire? Is the case gone? Substantially melted? Without that kind of information “metal burning” is just speculation.
unfortunately the alt is not available and i am just going on the skipper’s description of it as a hard to extinguish “metal fire” with no adjacent material having ignited. so yes, this is speculation …hence my interest in whether others have any similar anecdotes
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Old 18-12-2021, 11:51   #12
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Re: alternator fires

All of my cars have alternator fuses, but none of my boats. Something amiss in the Rules? I do use isolators from Alt to Batt, a bit of a help, but not a fuse. When the engine is not spinning, the B+ term is zero.
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Old 20-12-2021, 07:26   #13
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Re: alternator fires

I have had an alternator fail on a van that I owned. It had a short in the windings and overheated and lit on fire - presumably the winding coatings. It was in winter in Canada and all I had was snow to try to extinguish it. It had so much internal heat that cooling down the exterior with snow would put the fire out, but then a few seconds later it would restart. Probably took me about three minutes to fully extinguish it using snow. I didn’t have a fire extinguisher with me nor any water along so can’t offer any experience in that regard.

Overall though, this sort of an experience would be a very disconcerting prospect on my boat.
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Old 20-12-2021, 08:41   #14
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Re: alternator fires

Quote:
Originally Posted by team karst View Post
All of my cars have alternator fuses, but none of my boats. Something amiss in the Rules? I do use isolators from Alt to Batt, a bit of a help, but not a fuse. When the engine is not spinning, the B+ term is zero.
While the concept of battery separation has merit, be aware that the use of battery isolators can screw up the sensing of voltage by the regulator, due to the 0.7 volt drop across the isolator diodes. I had trouble with charging and in the end I removed the isolator, and charging was fine after that.

Could someone advise what size and type of fuse should be used? I assume it is on the main output cable? My alt is the legendary Delco Remy CS-144 rated at 100 amps continuous, though typically only needs to output half that at most.


Cheers, RR.
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Old 20-12-2021, 09:30   #15
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alternator fires

I use remote sensing. It removes the diode V drop from affecting the charge voltage
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