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Old 22-08-2011, 10:27   #31
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Re: Damage to Alternator ?

A late Welcome Aboard Cruisers Forum Moonfire
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Old 22-08-2011, 10:50   #32
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Re: Damage to Alternator ?

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I dont know this alternator, so bear with me if I'm all wet, but arent these 6 diodes the big ones on the back of the case.. possible pressed in..?
Yes, those are the ones. They must dissipate a lot of heat so they are pressed into a metal heatsink.

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Old 22-08-2011, 11:08   #33
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Re: Damage to Alternator ?

Bill-
"A MUCH BETTER solution is to wire the alternator's output directly to the house batteries,"
That would seem to be a better idea, but it also means that when (not if, but when) the alternator has a shorted diode failure, the alternator will drain the house bank completely. So there is a weakness in that approach as well.

Some alternators these days have different types of internal protection, so they can be run without damage during switching or disconnecting. Some don't. Some also have spike protetion built into them, to clamp any spikes form getting in or out during routine startup and shutdown. From what I've heard in the automotive world, the units with spike protection also are a mixed blessing, since all spike protection devices eventually burn out--at which point the alternator fails, often faster than an unprotected one

Damned if you do, damned if you don't.

What was the line from Ghostbusters? "Don't touch that switch!" (On the ectoplasmic containment unit.)
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Old 23-08-2011, 22:31   #34
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Re: Damage to Alternator ?

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A battery (bank) is an excellent buffer.
Does anybody really understand how a battery can buffer a spike produced by a collapsing electromagnetic field? I can see how a battery can absorb a good portion of a positive-going load dump transient, but what does it do with a negative-going spike? How does an electrolytic capacitor (i.e., a battery) absorb any energy when it's negatively biased?
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Old 23-08-2011, 22:39   #35
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Re: Damage to Alternator ?

While I have an automatic horn blower for reduced-visibility conditions, the horn is so loud I could only sleep if I was "dead tired."

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Old 24-08-2011, 11:10   #36
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Re: Damage to Alternator ?

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Does anybody really understand how a battery can buffer a spike produced by a collapsing electromagnetic field? I can see how a battery can absorb a good portion of a positive-going load dump transient, but what does it do with a negative-going spike? How does an electrolytic capacitor (i.e., a battery) absorb any energy when it's negatively biased?
Im sorry, I think you want the "Brain Surgeons and Physics Doctorate" forum.
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Old 24-08-2011, 11:26   #37
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Re: Damage to Alternator ?

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Originally Posted by hellosailor View Post
Bill-
"A MUCH BETTER solution is to wire the alternator's output directly to the house batteries,"
That would seem to be a better idea, but it also means that when (not if, but when) the alternator has a shorted diode failure, the alternator will drain the house bank completely. So there is a weakness in that approach as well.

Some alternators these days have different types of internal protection, so they can be run without damage during switching or disconnecting. Some don't. Some also have spike protetion built into them, to clamp any spikes form getting in or out during routine startup and shutdown. From what I've heard in the automotive world, the units with spike protection also are a mixed blessing, since all spike protection devices eventually burn out--at which point the alternator fails, often faster than an unprotected one

Damned if you do, damned if you don't.

What was the line from Ghostbusters? "Don't touch that switch!" (On the ectoplasmic containment unit.)
Hellosailor,

Yeah, that could happen but I don't think it's anywhere near as frequent an occurrence as you might imagine. It's also the case with the way alternators are often wired directly to the starter solenoid and on to the battery...without a fuse. And, it's possible in virtually all automobiles, but how often have you heard of it happening in autos?

Diodes generally fail in one of two ways: short or open. If one of the six diodes on the high side fails open, then the alternator continues to work OK but at reduced output. A short could cause a great deal of current to drain, but if the alternator circuit is protected by a fuse as it should be then there's a good chance the fuse will blow.

As you say, damned if you do and damned if you don't :-)

Bill
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Old 24-08-2011, 11:52   #38
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Re: Damage to Alternator ?

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Im sorry, I think you want the "Brain Surgeons and Physics Doctorate" forum.
Oh... thousand pardons... please carry on...
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Old 24-08-2011, 12:03   #39
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Re: Damage to Alternator ?

Bill-
"but how often have you heard of it happening in autos?"
An alternator diode going bad and taking out the battery? If I had to guess, maybe one in a thousand or one in five hundred among the car owners I know. Probably more like one in one or two hundred, if you cut that down to the long-term car owners, since the folks who replace 'em every year or three don't have time to wear out an alternator or battery.
Now one in a thousand, or even five thousand, would seem like "don't worry about it" but change that to a boat in some location where it would have really been nice to have a battery in the morning...<G>...
I've known a number of diesels that came with manual crank handles and tried with the best to get them going on two or three ocassions over the years. Never seen it happen though. It was easier to get ashore, get a battery back out to the boat, and do it that way.
I try (try!) to follow that school of engineering which is always trying to make things idiot proof. Because on occasion, I switch teams and join the idiots.<G>
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