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Old 20-02-2013, 10:45   #1
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Typical alternator field current is ??

I want to have a rheostat on board to replace the external regulator in the event of failure.

What is a typical range of field currents for a 100 amp alternator?

Trying to decide what wattage and resistance range for the rheostat.

Thanks
Chip
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Old 20-02-2013, 11:09   #2
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Re: Typical alternator field current is ??

Back in the day on a different boat, I had a QuadCycle regulator connected to a 100 amp Balmar alternator.
The field lead was fused with a 6 amp.
Never had a problem with it.
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Old 20-02-2013, 11:12   #3
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Re: Typical alternator field current is ??

Hmmm not sure. The manual regulator that CEI used to make was a pretty big rheostat... probably 2" diameter. You could just get an old school prestolite/ford truck regulator to use in a pinch....
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Old 20-02-2013, 11:15   #4
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Re: Typical alternator field current is ??

Why not just carry a spare regulator?

Maintenance it your best option. The things that can take out regulators are:
Arcing (short) brushes.
Lose wires to the battery or alternator.
Old dead batteries (continuos high amp charging).
Badly corroded wiring/cables (resistance).
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Old 20-02-2013, 12:41   #5
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I have a Balmar alt with int regulator. The external 3 stage regulator was a Balmar MC-612, but it died - probably from overheating due to its location in the engine compartment.

I like the idea of having a dirt simple backup to the regulator, hence the desire for a rheostat.
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Old 20-02-2013, 12:56   #6
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Re: Typical alternator field current is ??

Quote:
BALMARŪ alternators are available in a wide range of sizes, mounting configurations and amperage outputs to replace or upgrade a
wide range of marine alternators. All BALMARŪ alternators are “P” type (positive on the field wire), with max field current demand 6 amps
and 2.4 ohm rotors in 12/14 volt models.
The only regulator I have had for years is a rheostat, unfortunately it's on the boat and I can't remember the size (wattage or resistance). Somewhere around 50W by eyeball, and probably 25 or 50 Ohm.
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Old 20-02-2013, 12:56   #7
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Re: Typical alternator field current is ??

With a rheostat you will need to monitor the output and adjust or fry your batteries or alternator. I have tested alternator systems by simply running a 12v wire to the field with no rheostat. Thinking about that... it seems the rheostat wouldnt have to be that heavy..... can you measure how many amps your field wire draws when you first start your engine now...?
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Old 20-02-2013, 13:04   #8
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Re: Typical alternator field current is ??

So... 6 amps at about 14 volts = 84..................... you need about a 100watt rheostat I guess...
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Old 20-02-2013, 15:03   #9
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Re: Typical alternator field current is ??

Yes, the field current is typically less than 6 amps max. May be some higher output alternators that draw more, and when not making max output at low revs, the current is likely somewhat less. Thus the above suggested 100 watt pot is quite safe.

Back in the days before good regulators were available I used such a system. It does require monitoring, but it isn't all that hard and I never fried any components.

Cheers,

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Old 20-02-2013, 15:37   #10
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A 10 ohm 100 watt rheostat can be had for $18. I'll give that a shot. At 10 ohms should cut alt output down into the 20's at least, and i already have a switch to cut the field current completely.

Thanks for everyone's input.

Chip
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Old 20-02-2013, 15:43   #11
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Re: Typical alternator field current is ??

Can't figure out a good way to post a data table here, sorry

At 14.4 Volts and an alternator resistance of 2.4 Ohms (per Balmar):

_Field________Rheostat______Rheostat
Current_____Resistance____Power Dissipation
_(A)_________(Ohms)________(Watts)
__6____________0_____________0
__5___________0.48___________12
__4___________1.2___________19.2
__3___________2.4___________21.6
__2___________4.8___________19.2
__1___________12____________12
0.001________14398___________0

So, a 15 or 20 Ohm rheostat should give pretty much all the turn-down you can get, it would take an entirely different unit to work the field current range below 1A.

The maximum power dissipated by the rheostat is ~22W, however, that is done at a resistance value of 2.4 Ohms, and the power dissipation is almost as high in the 1.2 Ohm case. So, there is a very short section of the rheostat that is in use and actually dissipating the heat, thus a larger wattage rheostat is required in order to dissipate the heat using only the short section of the resistance winding. You'd have to check the specs carefully to see what the specific power capability is and how far you would have to oversize.

I think the one we use is:

RJS12RE Ohmite | Mouser

Although several other options here:

Rheostats | Mouser

They're not cheap anymore, a spare basic regulator might be the same price
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Old 20-02-2013, 15:52   #12
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Re: Typical alternator field current is ??

I see we cross-posted while I was trying to get that table formatted

10Ohm/100W seems a good compromise, at about 1/4 resistance you will be dissipating about 1/4 the rated power.
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Old 20-02-2013, 15:59   #13
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Re: Typical alternator field current is ??

Anybody remember the old Spa Creek alternator control? It was just a rheostat mounted with a switch and a voltage meter I think. If I ever find one at a yard sale cheap I would like to pick it up for a back up.
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Old 20-02-2013, 16:34   #14
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The 10 ohm rheostat I'm looking at (ebay 130697152024) is rated at 3.2 A. To put that through 10 ohms would take a 32 volt drop and dissipate abt 100 watts, hence its power rating.

I wonder if the 3.2 A limit holds at lower resistance settings or if current carrying capacity increases as long as the 100 watt limit is respected? I imagine the answer is somewhere in between.

Chip
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Old 20-02-2013, 16:50   #15
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Re: Typical alternator field current is ??

A manual field control is not a bad idea. But watch the alternator temperature. Few can survive full rated power for very long. Plenty of Balmars have turned to charcoal inside as proof. 50% rated power seems to work for long term.
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