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Old 05-16-2019, 05:05 PM
tac tac is offline
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Alternators

If you would like to know how alternators work, possible places that discuss this include:

https://alternatorparts.com/understa...ternators.html

The best, and most understandable however, is Nigel Calder’s book Boatowner’s Mechanical and Electrical Manual, well worth $20:

https://www.amazon.com/Boatowners-Me.../dp/0071790330

In short, an alternator’s output is controlled by regulating (changing) the current to the field windings. This is done two ways:

1. A “regulator” is included as part of the alternator. This is what you have. The regulator senses the battery voltage and uses a small(ish) current (0-5A or so) to the alternator’s field windings. The lower the field current, the lower the output current from the alternator (stator current). The alternator’s output, the stator current, is 3 phase AC, and is then converted to DC inside the alternator, generally using a bridge rectifier (6 or more diodes - “rectifier” is merely a 4 syllable word for the 2 syllable “diode”). In your case, the regulator is behind the finned retangular metal piece in your first photo. The two #16 (or so) wires coming out of that finned box (one is/was white, the other is now copper) are connected to the alternator’s field winding via carbon brushes (P/N 14) to a set of internal slip rings (P/N 18), at that nut.

2. The alternator has no built in regulator. You buy a regulator and connect its output to the alternator’s field winding terminals. The external regulator senses battery voltage and adjusts the field current, 0-5A, to control the alternator output.

In your case, the Motorola alternator output is a #8 (red) wire to the battery(s) + terminals through the ammeter and something called a “Volt Div”, which I guess is someone’s attempt at describing an Isolator, and a battery selector switch. A #8 wire is rated, for an engine space, at 68 amps. This is sufficient for the 37A alternator.

What Al was describing was eliminating any alternator internal regulator, and controlling the alternator output directly from the battery using a switch. In the control system world this is called bang-bang control, since the alternator output is switched on and off, rather than smoothly controlled over the full field 0-5 amp range. This used to be the standard way on cars, before about 1970, to regulate alternator output, using a voltage sensitive relay (regulator) on the firewall which contained a set of contact points that would switch the field current on and off. The John Luckless regulators on English cars were notorious for these contacts burning out every few thousand miles.

If you want to convert an internally regulated alternator to an externally regulated one, it ain’t difficult - see:

https://marinehowto.com/external-alt...on-conversion/

Finally, to answer your specific questions....

Switch ratings are NOT linearly scaleable. A switch rated at 20A @ 12VDC is NOT therefore rated at 200A at 120VDC. If the manufacturer does not specify, the rating for a 20A 12VDC switch would be 2A at 120VDC. It’s related to the power the switch can handle (volts x amps). To complicate it even more, a switch rated at 12VDC is often rated at a lower current at 12VAC. Likewise, your switch rated at 20 amps at 125 volts (probably AC) is highly unlikely to be rated at 200A DC. I’d be surprised if it were even rated as high as 20ADC. In short, always check the manufacturer’s data sheet, or ask them.

For an easy tutorial on switches, see:

https://www.digikey.com/en/articles/...witch-tutorial

Your plan is to switch the alternator’s output off with a manual switch on the #8 red wire. That will certainly disconnect the alternator from the battery, but when you switch the alternator’s output off with the engine running, the output at the alternator stator momentarily goes VERY high, generally high enough to destroy one or more diodes in the bridge rectifier, which reduces the alternator’s output to an .....ummm......unusable level. If you are correct that the “isolator” is defunk-it, then if it internally failed open circuit, it probably caused the alternator’s rectifiers to fail (go BOOM!).

Your concern about having battery voltage at the alternator output when the engine is off is no problem. That’s how the great majority of cars and boats are wired. The diodes in the bridge rectifier (if good) will only see battery voltage, less than 14VDC, or so. These diodes usually have a reverse voltage rating of over 100 volts, and so isolate the battery from the alternator, until the alternator’s output exceeds the battery voltage (i.e. the engine is running). Moverover, many alternators include an “isolation” or “blocking” diode right after the bridge rectifier. It’s purpose is to further isolate the alternator from the battery. If I ‘member right, the Motorola has this diode (P/N 11).

Last edited by tac; 05-16-2019 at 05:12 PM.
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