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three wire alternator help

I'm trying to understand the difference between one and three wire alternators but I'm still confused. I understand that the three wire in addition to the high current output uses a remote sense wire to eliminate the error due to the IR drop to a distribution point ( versus regulating at the alternator) What I don't get is the last wire and what its purpose is .I've read that its used to possibly run a light on the dashboard but if you go to the trouble of remote sense the + voltage why not remotely sense the ground and compensate for that IR drop as well. This is for a marine application and I'd expect similar drops there because the ground is carried through wires ( versus a chassis). I've found lot of information via Google but nothing that would allow me to draw a connection diagram for my friend Thanks in Advance Bob

Reply to
BoborAnn
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The third wire connects to the circuit node in the regulator where it gets current to power itself. The regulator has an internal diode trio that will derive operating current from the alternator windings, but this only happens once the alternator is up to speed. A common problem with 3-wire alternators hooked up as a single wire is that they don't start charging until you rev the engine up, usually to a couple thousand RPM. You can buy "low turn-on" regulator packs that come on at lower speed, but do not eliminate the problem entirely. The third wire connects to the ignition switch, optionally through a light bulb, so that the regulator is powered at all times when the key is on. The bulb will light until the alternator is supplying enough voltage to power itself.

I've been down this road researching an alternator conversion for an old tractor. The best tech information I found was at:

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Also useful was:
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Of course this site tends to try to sell you their products.

As for the idea of remotely sensing battery ground; While this is technically feasible, the ground cable is usually much heavier than the charging wire, since it occaisionally carries hundreds of amperes of starting current, so under normal running/ charging conditions it is a pretty insignificant voltage drop.

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Ol' Duffer

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BoborAnn

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