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Old 27-06-2013, 08:23   #1
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Manual Alternator/Voltage Regulator Jury Rig

(Thought there was an existing thread on this subject, but could not find it.)

Anyway, my Next Step 2, which has been doing a fine job of keeping my batteries happy for years, started developing problems recently so I put together a quick and dirty jury rig voltage regulator using a potentiometer scavenged from the stereo system. One side goes to battery positive the other to alternator field. Working fine, just have to keep a close eye on it of course so I don't cook my batteries or alternator.

For next season, I plan to build/buy a better version of this as a backup. An excellent device to have in the spares kit aboard any cruising boat I think.
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Old 27-06-2013, 11:40   #2
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Re: Manual Alternator/Voltage Regulator Jury Rig

Just out of curiosity, how much field current produces how much amps? And what kind of alternator do you have?

Seems like about 10 amps is 100% full field current on my Balmar 100 amp alternator, which produces 80-90 amps (Balmar alternators never seem to put out rated current) into 75% full batteries.

David
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Old 27-06-2013, 11:51   #3
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Re: Manual Alternator/Voltage Regulator Jury Rig

What size of pot did you use?
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Old 28-06-2013, 02:44   #4
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Re: Manual Alternator/Voltage Regulator Jury Rig

probably one that's way too small. you'd probably need something in the 40watt + range.
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Old 28-06-2013, 02:59   #5
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Re: Manual Alternator/Voltage Regulator Jury Rig

Thanks or sharing.

I would have thought the pot would not have a sufficient power rating for the job, but obviously its working fine.
I have seen people do a a similar thing with large wire wound fixed resistors on a multi position switch. This only allows a limited range of choices rather than the infinitely variable option you have, but the fixed resistors have a rating of 10 or 20w and multiple resistors can be used to get acceptable power dissipation.

Does anyone know what typical range of resistor values are needed?
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Old 28-06-2013, 03:55   #6
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Re: Manual Alternator/Voltage Regulator Jury Rig

At various times I have used a 12v DC speed controller, a number of 12v lights in series and even once an SS bike spoke as a heavy duty resister. If the regulator on the alternator has a voltage sensing terminal you can encourage it to defer it's current fold back by putting small diodes in series with it as these will depress the voltage being sensed by about 0.6v per diode.
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Old 28-06-2013, 06:40   #7
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Re: Manual Alternator/Voltage Regulator Jury Rig

I was wondering how many ohms the pot was, 1K?
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Old 28-06-2013, 06:54   #8
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Re: Manual Alternator/Voltage Regulator Jury Rig

The 12v doctors handbook is great little free book in PDF format. It can be downloaded here:

http://www.ankan.nl/download/12%20Vo...20Handbook.pdf

Page 160 discusses this in detail. It recommends about 10-20 ohm, but download the whole book and read the chapter youself.
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Old 28-06-2013, 09:04   #9
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Re: Manual Alternator/Voltage Regulator Jury Rig

Quote:
Originally Posted by djmarchand View Post
Just out of curiosity, how much field current produces how much amps? And what kind of alternator do you have?

Seems like about 10 amps is 100% full field current on my Balmar 100 amp alternator, which produces 80-90 amps (Balmar alternators never seem to put out rated current) into 75% full batteries.

David
My configuration is twin engines, each with a standard 65A alternator modified for external regulation. I have not tracked the field current to output in detail yet (the NS2 has always taken care of that for me), but it appears that less than 3.0A is very close to full output.

The NS2 settles in at about 1.5A to 2.2A field current for most of the bulk charge phase (higher initially for a short time but I have not captured that yet).

The POT is almost certainly under rated for the job (gets quite hot when I step it down). It was regulating speakers on a 50W per channel stereo so I assume it is rated for at least 50W, but I have not found specs on it yet. Assuming max charge voltage of 14.2V and max field amps of 3.0A I would need at least 42.6W. I think going bigger like 100W would be better for heat dissipation (maybe add a heat sink also). However, I am currently anchored in the San Blas....not exactly a lot of Radio Shacks around here.

I added a 12V 10W bulb into the circuit today...this lets me bleed off a lot of field current, reduce heat at the POT and still maintain tach signal.
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Old 28-06-2013, 09:26   #10
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Re: Manual Alternator/Voltage Regulator Jury Rig

NAPA auto parts sells a little rheostat with clamp and knob for heater fan speed adjustment. At least they did just a few years back. Heavy duty ceramic little thing and cheap. These manual regulators are handy in an emergency, or if you only have 15 mins to charge your batteries before everyone in the anchorage is taking the bus to town for the day!
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Old 28-06-2013, 18:45   #11
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Re: Manual Alternator/Voltage Regulator Jury Rig

Yes, I think a little off-season project will be to build a simple manual regulator (like the one in 12V Doctor) in a nice neat little enclosure with a few handy features (test probe points, 5A on/off breaker, maybe a field amps meter...) to stow in my electrical spares bin...just in case.

I have a new NS2 on the way, and plan to have the old one repaired as a spare (just a cooked trace to the terminal block I think), but sure like the idea of having a basic manual spare for when the fancy smart stuff packs it in.
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Old 29-06-2013, 06:42   #12
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Re: Manual Alternator/Voltage Regulator Jury Rig

Hey, thanks for the link to the 12v manual.
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Old 29-06-2013, 15:32   #13
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Re: Manual Alternator/Voltage Regulator Jury Rig

a wire jumper to straight 12v would get you home too. would just need to remove and rehook back up as needed and keep a good eye on it. with lower charge batteries it's going be at solid 12v for a while anyways.
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Old 29-06-2013, 18:15   #14
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Re: Manual Alternator/Voltage Regulator Jury Rig

Probably not prudent to feed straight 12v into the alternator excitation. It appears that without some type of current control mechanism you would be creating a positive feedback mechanism in that the alternator would generate high output voltage which would then create higher exciter current until something burnt out, possibly quiet spectacularly.

Years ago I discovered that stainless steel makes a pretty good resister material; probably due to it's metallurgical similarity to the nickel and chrome bearing material they make wire wound resisters from. There also tends to be a fair bit of it of one sort or another about the modern boat. In a previous post I stated I had used a stainless steel bike spoke as a resister (I may have used a 6V source as there are plenty of them available from my house batter banks) I have made temporary high Wattage resisters by winding nichrome resistance wire around a rod and mounting it between two nails driven into a piece of wood and a single strand of wire taken from a stainless wire rope would serve as well. If you use an alligator clip on one of the connections to the resister it can be moved back and forth to emulate a variable resister. In the bike spoke instance I just wound the connecting wires around the spoke and moved it back and forth with a screwdriver. Don't do it with your fingers as they could get burnt.

The overall aim is to source a controllable source of DC current to the rotor coil. If you can do this and monitor the alternators output voltage to the battery and keep it adjusted to below 14.4 (wet cell max charging voltage) you can generally get yourself out of trouble without destroying the battery or your electrics or electronics.
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Old 29-06-2013, 18:19   #15
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Re: Manual Alternator/Voltage Regulator Jury Rig

You can calculate the maximum field current by measuring the resistance of your alternator windings (or looking it up in the manual). For instance, Balmar uses 2.4 Ohm windings in nearly all their 12-Volt alternators, giving a maximum field current of 14.4/2.4 = 6A (all at nominal conditions).

In the bad old days Marinetics (RIP) used to sell a marine electrical panel with a built-in 10 or 20 Ohm, 50W rheostat. With a 10 Ohm you can drop the field current to just over 1A [14.4/(10+2.4) = 1.16A], with a 20 Ohm you can get down to 0.6A. The Marinetics panels had an inline toggle switch to allow you to shut off the field current entirely when you couldn't drop the alternator output any lower.

The maximum power dissipation in the rheostat occurs when the rheostat resistance = alternator winding resistance. At that point you will be at half maximum field current, and half the voltage drop will occur in the rheostat. For a 6A maximum field at 14.4 volts the power dissipation in the rheostat is 7.2V * 3A = 21.6W. At higher resistance settings the current will drop faster than the voltage drop across the rheostat, at lower settings the voltage drop (and the bulk of the power dissipation) will occur in the alternator.

We've had the Marinetics version on Anduril since 1975. It's been re-built several times because the rheostats wear out, but at $30 a pop that's not too bad. We usually carry a couple of spares and they always end up getting used in some far distant anchorage when somebody's regulator fails.

Just remember to either use a toggle switch or get your field current source from the ignition system - the field current will flow regardless of whether or not the alternator is spinning, so once you shut down the engine if you don't turn off the field current you'll just end up draining the batteries you just filled and keeping the engine room warm with the alternator/heater.
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