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Old 09-09-2017, 09:08   #1
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AFD

Has anyone installed an independent Alternator Field Disconnect, on their sailboat? Our boat is 37 feet long with 28 horsepower so there are times when the full power is required. I am considering the power loss to the charging system to be equivalent to a larger engine at times of need.
If you have done this how has it affected your external regulator and power to the shaft?
BobV
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Old 09-09-2017, 09:15   #2
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Re: AFD

Yes, I've done it on couple boats. I just used a common switch at the instrument panel. Mine were field activated alternators with external regulators though. Nothing too difficult about it. Frankly though, if your engine runs for 20 mins or so before you need any extra power, the charging circuit has tapered off very much by that time and the alternator isn't pulling much power. It would be unusual to start the engine and immediately head into a strong adverse situation likely. Although I did have that one time....
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Old 09-09-2017, 12:34   #3
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Re: AFD

Thank you Cheechako
I am just switching the charging system over to external regulation now and was not sure if the dropping current load would be enough to free up the power to the shaft. I was also wondering how the external regulators would react to the dead field, so it sounds like a regulator with a "Force to Float" function would resolve any issue of urgent max-power requirements. I am of course speaking of anchor ditching urgency, not an out-of-beer, experience.
BobV
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Old 09-09-2017, 13:52   #4
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Re: AFD

I've simply put a SPST switch in the supply lead to the external regulator in the past. Instantly removes all electrical load from the alternator, no harm done. Also useful to reset the regulator to its startup condition, meaning that you can force it to return to bulk charging after a time out.

But I agree that in a 30+ hp engine (guessed at by your boat size) there will be little noticeable difference in available power.

Jim
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Old 09-09-2017, 15:10   #5
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Re: AFD

Jim thank you for your input.
The engine is rated at 28 horse power at 2500 rpm. The regulator I am looking at does not have a ramp-up function so start up is at full load. I was wondering if the field is off, then turned on when the engine is running, how that would affect the regulator. By your experience it should not have any effect other than to boot it to it's bulk charge program.
I note that you no longer have a switch in place has this affected your charging system or your start-up loading
BobV
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Old 09-09-2017, 15:17   #6
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Re: AFD

Quote:
Originally Posted by nautitrix View Post
Jim thank you for your input.
The engine is rated at 28 horse power at 2500 rpm. The regulator I am looking at does not have a ramp-up function so start up is at full load. I was wondering if the field is off, then turned on when the engine is running, how that would affect the regulator. By your experience it should not have any effect other than to boot it to it's bulk charge program.
I note that you no longer have a switch in place has this affected your charging system or your start-up loading
BobV
Bob, that setup was on our previous boat, and startup was never a problem per se. That was a 150 Amp alternator on a single belt and it would sometimes slip on startup, but not when re-starting the cycle because by then the batteries were full enough that their acceptance was reduced.

Have not put one on this boat as yet (only owned her 14 years... I'll get around to it soon...) but would not hesitate to do so if the need cropped up!

Jim
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Old 09-09-2017, 19:03   #7
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AFD

Depending on your regulator, you could also trick it by the alternator temp sensor connection.

The force to float switch will also cut it way back, but that regulator I removed as it went to float way prematurely, you could return it to normal by cycling the force to float switch, but give it fifteen minutes or so and its back to float, and you have to cycle the switch again.
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Old 10-09-2017, 02:30   #8
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Re: AFD

The balmar s have a small engine mode. A switch cuts charging lower but still charges abit. That it's exact purpose.

To cut the whole thing. I would think you'd want t to kill the ignition to the reg. not the field to the alt
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Old 10-09-2017, 03:15   #9
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Re: AFD

I've had a field disconnect on every alternator we've had for the last 40 years. Depending on the boat/engine/system we used them a lot or almost never. Easiest and fastest way to remove alternator load entirely, and no risk of damage to the alternator. Imagine you are in a maneuvering situation and you suddenly hear belt squeal - it happens no matter how good your maintenance program - shut down the alternator and take pretty much all the load off the belt until you have leisure to address the problem.

For (manual - subject to human fallibility) soft start you can put a rheostat in series with the AFD switch. Check out this old thread:

Typical alternator field current is ??

The bottom line - something like a 10-25 Ohm/50-100W rheostat can be had for a small sum. When shutting down the engine you can dial it up to maximum resistance, then on startup dial it back to zero slowly and let the regulator take over. This also gives you a manual regulator of sorts if your fancy, expensive one packs it in someplace inconvenient.
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