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

I just threw out a Spa Creek unit including the instructions. There was more to it than just a rheostat.
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Old 20-02-2013, 16:56   #17
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Re: Typical alternator field current is ??

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Originally Posted by SoonerSailor View Post
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
Why not just cary a spare Ford reg. It uses the same plug as the Balmar, is easy to swap out and costs about $10.00....

To answer the original question it really depends upon the alt. I have seen some that reach full output at 4.7A and 12.5V +/- on the field and some that will draw over 6A and 13V+/- on the field....

This is easy to measure just put a clamp meter around the blue wire in bulk mode... I check field voltage, field current and alternator output with each install & set up and the field current can vary by the alt.......
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Old 20-02-2013, 17:16   #18
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My 220A alternators take 7A field current. I have two of those in parallel on an Ample Power regulator that can supply 15A field current.

My backup is a direct 12V wire.
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Old 21-02-2013, 06:06   #19
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Re: Typical alternator field current is ??

We were starting an offshore passage from Norfolk, Virginia to Rockland, Maine and were four or five hours out when the alternator stopped working. We did some diagnostics while underway and found that when we put 12 volts to the field circuit of the alternator we got 100 amps. Our diagnosis was a dead voltage regulator and then we had to decide whether to turn around and go back to Norfolk and waste at least a couple of days, alter course to enter another port and try to find a new regulator or to continue on the voyage and rig a temporary regulator.

I vaguely remembered that Nigel Calder had written about making a voltage regulator in his book "Boatowner's Mechanical and Electrical Manual" 2nd edition page 71. As Nigel says "A test wire connected to a 12- or 15-watt DC lamp will feed approximately 1 amp to the field winding (12 volt system)... The higher the lamp wattage, the more amperage it will pass. Divide the wattage by the system voltage to find out how many amps."

I, of course did not have the book on board so my hazy memory was all we had to go on as we wired a 12 light socket into a circuit to feed the field wire on the alternator. We tried it and it worked so we tried different bulbs until we found one that we liked the output and we then wired the bulb in series with a switch so we could turn on and off our single output level alternator. Since this was a five to six day offshore passage we could not have this light turned on during the night to kill our night vision so we mounted the light socket inside of a cupboard in the galley. We would monitor the voltage level of the batteries and turn on our alternator when we needed to charge. A nice side effect was that we had light inside of the pots and pans cupboard!

I highly recommend Nigel's books. I suggest that you get them and read them from cover to cover, even the stuff that you think that you will never use. You never know when a slight memory may solve a big problem for someone.
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Old 23-02-2013, 15:02   #20
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Re: Typical alternator field current is ??

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Originally Posted by Maine Sail View Post
Why not just cary a spare Ford reg. It uses the same plug as the Balmar, is easy to swap out and costs about $10.00....
Careful with the plug. The black wire in the Balmar plug is the ground and should be connected not to the Ford regulator's "I" terminal but rather to the case of the regulator.

See Saunter's 19-01-2013 post to the Balmar Regulator Failure thread.
http://www.cruisersforum.com/forums/...eply&p=1163856

'The 12v Doctor's Practical Handbook' has instructions for making a Spa Creek like alternator controller on page 161. If your library or bookstore does not have a copy, Google the title plus pdf.

As was said, the Ford alternator regulator is both the cheaper and better option. With it you get the first two stages of three stage alternator control in automatic without any intervention by you.
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Old 23-02-2013, 15:12   #21
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Re: Typical alternator field current is ??

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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.

AUTOMAC. John, I might have one laying around. We moved a few years ago and I don't know exactly where it may be anymore. Like a lot of things, too many...
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Old 23-02-2013, 16:04   #22
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Re: Typical alternator field current is ??

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AUTOMAC. John, I might have one laying around. We moved a few years ago and I don't know exactly where it may be anymore. Like a lot of things, too many.
That's it! Didn't Catalina supply them as standard for awhile? I had one many years ago, before most of these so-called smart regulators were available.
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Old 26-02-2013, 20:11   #23
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Re: Typical alternator field current is ??

Or in a pinch an old school light bulb in series with field.
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Old 09-10-2015, 14:22   #24
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Re: Typical alternator field current is ??

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So... 6 amps at about 14 volts = 84..................... you need about a 100watt rheostat I guess...
Probably better with a PWM controller, but add a freewheel diode to prevent flyback voltages being generated by the disconnecting field coil. 6A would only be required briefly, unless charging a very flat battery. If you can't reconfigure the alternator to accept high-side control, put a FET on the low side, with the PWM driving the gate.
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Old 09-10-2015, 14:29   #25
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Re: Typical alternator field current is ??

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Originally Posted by SoonerSailor View Post
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
I realise that this is an old thread but :
Current carrying capacity will remain prett much the same, regardless of setting. On big wirewound ones, current capacity will go up slightly at lower resistance setting, as overall heat dissipation will be the limiting factor rather than localised element heating.
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Old 10-10-2015, 01:40   #26
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Re: Typical alternator field current is ??

On my Yanmar I used a 12V motor speed controller permanently wired in parallel with the regulator to allow fast charging. I have not bothered with that for a long time now but often use a SS pushbike spoke and jumpers with crocodile clips as a heavy duty rheostat, usually when I want to connect the house batteries to the cranking battery alternator. The metallurgy of SS is not too far different from some of the resistance wire they use to wind rheostats.

Whilst on the subject of the resistance of SS I have had two recent experiences where this was detrimental.

The first was with new cranking batteries which came with little packets of nuts and washers for the terminals. After connecting up the batteries I found that they would not crank the engine properly. Since the terminal bolts were also SS I realized that by installing the battery cable terminals on the SS studs between two SS washers the current path was completely through SS. Threw the washers away and the problem solved itself.

The second was on an anchor winch motor where the common field terminal for the two sets of field coil kept melting the solder even with the use of high temperature solder. When re-assembling it after the latest incidence of melted solder I realized that the manufacturer had used SS nuts on the brass terminals and that the high currents passing through the high resistance SS nuts was causing the heating and the high thermal conductivity of the brass stud was allowing it the heat to melt the solder. Replaced the SS nut with brass nuts and hopefully this will solve the problem.

Relatively high electrical resistance of SS a two edged sword; in one instance it allows a bike spoke to be used as an emergency rheostat and in the second causes serious problems with high ampage electrical circuits.
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Old 10-10-2015, 01:46   #27
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Re: Typical alternator field current is ??

Further to my previous post on alternate uses and abuses of SS bike spokes and wire.

If you want to slice styrofoam into sheets a piece of SS wire stretched between two nails driven into a plank at the right distance wired to a large battery does the job very nicely. Bit off topic but handy to know.
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