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Old 02-21-2016, 01:13 PM   #1
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How can I damage my alternators?

I understand that an alternator will be damaged (diodes?) if it is disconnected while the engine is running. Is the damage done by disconnecting the field wire or the heavy charge wire?
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Old 02-21-2016, 01:19 PM   #2
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Heavy charge wire. You can disconnect the field and it just quits making output, won't harm anything.
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Old 02-21-2016, 01:23 PM   #3
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"Is the damage done by disconnecting the field wire or the heavy charge wire? "

THE output "heavy charge wire" or simply shutting off a dumb rotary switch causes the damage.

Turning off the field current is fine , then you could disconnect the battery wire.

A better grade of rotary switch will have field disconnects built in.

They DO have to be wired in , just having them on the switch does nothing.

Internally these are "break before break " switches.

The field is cut first then the battery is cut.

This is a simple cheap setup that should be on every boat with a rotary switch.
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Old 02-21-2016, 02:43 PM   #4
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The switches that FF describes above have an automatic field disconnect feature which has a parallel field switch that turns off the alternator before the switch disconnects power.


But much more common are one, two, all, off switches that make before break. That means that when you switch from one to two or from two to all, the switch makes contact with the new position first before it breaks contact with the old one. This assures that there is always a load connected, presuming that a load is connected to 1 and 2.


But there is no protection with these switches if you switch to off. It will blow the diodes then.


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Old 02-22-2016, 07:23 AM   #5
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Thanks everyone - exactly what I needed to know.

David: master switches like you describe make a lot of sense. I will have to check the specs on mine
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Old 02-22-2016, 10:54 AM   #6
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I understand that an alternator will be damaged (diodes?) if it is disconnected while the engine is running. Is the damage done by disconnecting the field wire or the heavy charge wire?
Charge wire as has been mentioned before. I recently changed how my own boat was wired. Keep in mind that I have a small single diesel and a single battery bank. As is typical from the factory, my alternator was wired to the Common post of a 1/2/Both/off switch (make before break type). The problem of course is that if someone were to inadvertently pass through the off position while the engine was running, it could damage the alternator.

I changed this so that the alternator output goes directly to the battery bank, bypassing the 1/2/Both/Off switch. I made some other changes as well, but now the alternator is always charging the batteries when the engine is running. This not only eliminates the concern of damaging the alternator but reduces the number of electrical connections between the alternator and the batteries. FWIW
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Old 02-23-2016, 07:34 AM   #7
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Add an $18 buck RV solenoid on the house side to charge and you will have a seamless charge setup that requires Nothing to have the house charge with the engine on , automatically.

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Old 02-23-2016, 08:02 AM   #8
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Dave: That is the setup I have - except that the start and house batt are fed via a battery isolator. Fairly safe providing the isolator itself doesn't malfunction.


Fred: I like KISS. Can you please elaborate a bit on your suggestion.
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Old 02-23-2016, 09:13 AM   #9
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Many boats I see rewired, some from factory, have the alternator charge wire led to the positive post on the starter.
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Old 02-23-2016, 10:27 AM   #10
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Many boats I see rewired, some from factory, have the alternator charge wire led to the positive post on the starter.
I have seen this on a lot of sailboats. It has the same effect of wiring the alternator to the common post on a 1/2/Both/Off switch. It is easy to do as the alternator and starter are typically very close to each other.

My plan was to add a dedicated start battery for the engine. I was going to use an echo charger to charge the start battery as both my charger and my alternator are wired directly to the positive post on the house bank. Very simple system.
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Old 02-23-2016, 01:04 PM   #11
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THE output "heavy charge wire" or simply shutting off a dumb rotary switch causes the damage.
Turning off the field current is fine , then you could disconnect the battery wire.
A better grade of rotary switch will have field disconnects built in.
They DO have to be wired in , just having them on the switch does nothing.
Internally these are "break before break " switches.
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Old 02-23-2016, 02:50 PM   #12
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Shoalwaters,
If you have discharged or dead batts recharging w an alternator may overwork it and cause damage. Motorcycles are prone to that especially. Better to charge w charger and then use your boat batt.
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Old 02-23-2016, 03:15 PM   #13
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Shoalwaters,
If you have discharged or dead batts recharging w an alternator may overwork it and cause damage. Motorcycles are prone to that especially. Better to charge w charger and then use your boat batt.
Eric: Your alternator should be fine with a 3 stage external voltage regulator with a battery temperature sensor. That's a pretty standard set up these days. With the exception of one stator, our alternator failures have all been bearings. When we replace the bearings, I do the brushes at the same time.
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Old 02-24-2016, 07:20 AM   #14
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" our alternator failures have all been bearings"

Loose belt or fan used with wrong direction fins might be cause.

Overheating the alt can cause the lubricant to depart....
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Old 02-24-2016, 09:16 AM   #15
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" our alternator failures have all been bearings"

Loose belt or fan used with wrong direction fins might be cause.

Overheating the alt can cause the lubricant to depart....
FF...
You caught my attention... trying to understand your point #1...seems counter intuitive so trying to understand the rationale

loose belt = bearing failure?

I have been under the (incorrect?) impression that loose belt would affect output but belt too tight would relate to bearing failure?

Wouldn't a loose belt = slippage = lower output and LESS heat than if at full output?
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Old 02-26-2016, 07:33 AM   #16
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AS a loose belt slips, the FRICTION of the belt sliding past the pulley heats it to a grand temperature.

Enough to melt the grease in the end bearing.

Try looking with an IR gun.
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Old 02-26-2016, 12:33 PM   #17
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AS a loose belt slips, the FRICTION of the belt sliding past the pulley heats it to a grand temperature.
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FF

Duh... I forgot about friction.
Like gravity we sometimes overlook the obvious

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Old 02-27-2016, 05:39 PM   #18
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Chuckle

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I understand that an alternator will be damaged (diodes?) if it is disconnected while the engine is running. Is the damage done by disconnecting the field wire or the heavy charge wire?
The title of your post gave me a chuckle. My first thought was "why would you want to?" Of course I understood what you really wanted to know. Sorry but couldn't resist.
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Old 02-27-2016, 08:21 PM   #19
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I ran the charge wire directly to the battery post of the on-off-1+2 battery switch, with an 80 amp red blue sea fuse holder inline. At least that way moving switches wont ever inadvertently disconnect the alternator.
So with 2 engines and 2 banks and 2 switches, each alternator charges it's own bank. And I could move switches to combine them etc...
One bank is for starting, one bank for house.

It works for me so far.
Only negative might be if all switches are off, the charge wire is still hot. But not been a problem. Say you worked on engine and somehow short the charge wire, it would spark and blow the fuse. I almost never turn off those big switches.
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Old 02-27-2016, 08:51 PM   #20
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FF, in my experience it makes no difference which way the fan turns. Its still centrifugal. Probly better if it goes the correct direction but I've not seen it make a diff.
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