Re: Field Circuit Resistance on an H

Posted by Steve - IN on March 19, 2003 at 02:54:21 from (12.222.31.186):

In Reply to: Field Circuit Resistance on an H posted by Hollerback on March 18, 2003 at 19:21:37:

Hollerback,
I'm guessing you have a 4 position light switch, and that the system originally had a cut out instead of a regulator (had a two wire box, now has a 3 or 4 wire box) - and that you're not talking about the headlight dimmer resistor - and are using a 12 volt battery. If so, the purpose of the big field resistor between the switch and the field or F terminal on the generator is to keep the generator from overcharging the battery -- effectively making you the regulator when you saw the ammeter in high charge mode for too long -- you switched in the resistor, or switched it out when you needed more charge.

Now that you have a regulator installed, and assuming it can control the old generator, the regulator will take over the job that the driver was once expected to do. The F terminal of the regulator goes to the F terminal on the generator. Instead of going to a cutout, the A or armature terminal of the generator now goes back to the Gen terminal of the regulator (the odd man out, as the other two or three are on the other side of the box). The Batt terminal on the regulator runs through the ammeter to the battery, and if you have a L or load terminal, it runs through a fuse and to the lights via the light switch. If your system is hooked up that way, and the regulator is polarized and doing its job, you should be OK. No field resistor needed. The two pole box, the cutout, was there to keep the the battery from draining into the generator at low speed. The regulator basically adds a second relay to the cutout setup and keeps the generator from under or over charging the battery -- as you've said.

If you've converted to a 12 volt battery and still have a 6 volt coil in a coil/distributor setup (not a magneto) you'll need about a 2 ohm resistor rated to handle about 50 watts to drop the ~4.5 amp draw of the coil down to around six volts. If you have a magneto, no worries, it's independent.

If you're talking about the light dimmer resistor - that curly thing - 3 lights will draw about ~4 amps each, ~12 amps total, or 144 watts at 12 volts. If we want to drop down to "half speed", ohms law says: Current= square root of power / resistance which says 12 amps drops to 6.0 amps with a 4 ohm resistor - which is different than the ~1-2 ohm resistor that was there for the 6 volt setup.

If none of my guesses are right - and you have basically a cutout so you still need the field resistor -- you can figure the resistance needed by Ohm's law above. Let's guess that your generator puts out around 20 amps at 12 or so volts, for around 240 watts of power. If you have a coil/distributor, you're going to have a draw of 4 to 5 amps to run the engine. If you have a magneto, no draw, it makes its own power. So if we want to be able to take the 20 amps down to about a nice, gentle 3 to 5 amp trickle charge with no lights on a magneto tractor, or around 7 amps if we have the draw of the coil ignition system. This is to keep the battery from frying when you run for a long time, and that ammeter has been reading +10 or +20 for way too long. So back to Ohm's law again -- taking 12 volts at 20 amps down to 7 amps requires just a shade under 5 ohms. With a magneto making no draw you need about 20 ohms for a 3.46 amp charge rate. To charge at ~5 amps, you need 9 ohms which works out to 20 amps in, 5.16 amps out. If you measure the output of your generator at working RPM's and read up on the recommended charge rates for the battery you're using -- you can select the resistor value to taste using Ohm's law and your knowledge of how you'll be using the tractor. There's no hard and fast, "must have" number for the value of the resitor.

Hope that covers all the possible bases, and answers your question as well.

Steve

Follow Ups:

• Re: Re: Field Circuit Resistance on an H Hollerback 12:45:13 03/19/03 (2)
  • Re: Re: Re: Field Circuit Resistance on an H Steve - IN 13:22:55 03/19/03 (1)
    • Re: Re: Re: Re: Field Circuit Resistance on an H Hollerback 17:12:38 03/19/03 (0)

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