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post #1 of 24 Old 08-03-2008 Thread Starter
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Alternator regulator operation

Today during sail (motoring, really) I realized that my alternator is not charging - it was barely putting out 12.8V.

I have an external regulator from Xantrex, so I was able to adjust the ACCEPT voltage (this probably regulates "field" whatever it is). A few turns and it was charging again at 14.1V as the manual says it should.

Here is the thing though - it used to charge just fine with old settings (I didn't follow voltage closely but ACR always turned on when alternator was on, so it was definitely above ACR closing voltage which is 13.1V).

So, now I am wondering - what would cause this? Could there be something wrong with an alternator (which I just masked by adjusting the regulator?)?
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post #2 of 24 Old 08-03-2008
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I don't think your alternator is the problem. Typically, they either work, or don't work.

I would put a volt meter on your battery terminals and then the hot terminal on your alternator to independently confirm/verify that your other volt meter is giving you accurate information. It could be that everything is fine but for some reason your volt meter has a bad connection. If your voltmeter is accurate, I'd suspect a voltage regulator problem but continue to monitor the situation since your fix.

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I don't think your alternator is the problem. Typically, they either work, or don't work.

I would put a volt meter on your battery terminals and then the hot terminal on your alternator to independently confirm/verify that your other volt meter is giving you accurate information. It could be that everything is fine but for some reason your volt meter has a bad connection. If your voltmeter is accurate, I'd suspect a voltage regulator problem but continue to monitor the situation since your fix.

I used a main panel voltmeter and also a separate digital meter - so I am fairly certain the readings are correct. I guess regulators go bad too sometimes, this one is from PO (last electric device on the boat that I did not replace ) so it could be pretty old. Guess I'll keep watching this thing.
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post #4 of 24 Old 08-04-2008
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It is pretty strange that something changed since the last time the engine ran. Could the adjusting screw you used to change the acceptance voltage have gradually moved over time?

Ray
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It is pretty strange that something changed since the last time the engine ran. Could the adjusting screw you used to change the acceptance voltage have gradually moved over time?

I thought about it but it looks extremely unlikely. It is one of those little adjustable parts (most likely a potentiometer of some sort) and I really don't see it moving on its own. Not impossible, but not my first explanation.
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post #6 of 24 Old 08-04-2008
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Could a dirty connection somewhere be adding resistance to the circuit?
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post #7 of 24 Old 08-04-2008
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You can jumper full voltage (like from he output post) straight to the field connection on the alternator. This basically bypasses the controller. The alternator should then crank out max output. That will tell you if the problem is the controller or the alternator. Obviously you don't want to leave it like this for a long time!

This is also a good trick to know if you regulator goes out while at sea. However, you do have manually control your charging so you don't ruin your batteries. You just full field the alternator until your voltage comes up to 14.4 or so then remove the field jumper for a while and repeat.
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This problem kept me up last night. I first thought it was probably a connector between the regulator and the field terminal that had resistance in it. I thought the regulator would be sending out the right voltage to the field terminal but that it just wasn't getting there. But when Brak was able to overcome this with a manual adjustment, I start doubting that theory. So anyway, last night I considered that if continues to be "flaky" and give intermitent outputs, I would start to suspect that it's the slip ring brushes inside the alternator that are dirty or worn out. At any rate, I'll be interested to see what the real problem turns out to be.

Ray
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So dirty brushes would increase resistance? How can I check that - do I need to take the alternator off and open it up?

I am fairly sure it is not an external connector (as hard as it is to understand my engine's wiring harness - it was done by PO's mechanics when repowering and its one part of electric system I just don't get - lots of wires back and forth, all in one duck-tape-bound tube). Still, I am fairly sure I found the field connector and it looks just fine.

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This problem kept me up last night. I first thought it was probably a connector between the regulator and the field terminal that had resistance in it. I thought the regulator would be sending out the right voltage to the field terminal but that it just wasn't getting there. But when Brak was able to overcome this with a manual adjustment, I start doubting that theory. So anyway, last night I considered that if continues to be "flaky" and give intermitent outputs, I would start to suspect that it's the slip ring brushes inside the alternator that are dirty or worn out. At any rate, I'll be interested to see what the real problem turns out to be.
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IIRC, the voltage regulator controls the amount of current going to the spinning electro-magnet in the middle of the alternator. A stronger magnetic field induces more current to come out of the wire windings around the spinning protion. The electricity from the windings alternates back and forth from positive to negative as the north/south poles of the spinning electro-magnet spin pass them. The three wire windings pass through diodes. They act as an on/off switch and only allow the current to pass through only one way, either positive to negative, giving us D.C. current for our D.C. system.

If the brushes that make contact with the slip ring terminals to the electro-magnet are worn/dirty/weak springs, the regulator may be telling the electro magnet to produce a strong current, but it can't because it's not getting the voltage it needs. I recollect that you can check with this an ohm meter, but I don't remember how to do it off the top of my head. I had some alternator problems two years ago and found some detailed testing procedures online for checking the diodes. Without that, it would require splitting the case open and taking a look at the brushes. New brushes are typically .75 to .50 inches long. If they're getting down to .25 inches, I would suspect they're worn out.

Ray
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1983 Fraser 41
La Conner, WA


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