6v generator to 12v generator

1 - A 6 volt generator will run hotter when operated on 12 volts. This is because running on 12 volts doubles the current thru the generator field coils. And since heating of the coils varies as the SQUARE of the current (P = I^2 * R for you engineers...), doubling the current causes 4 times as much heat to be developed in the fields. Probably OK if the tractor is used only itnermittantly in cool weather. But if it's run hard in 100 deg ambients, you will increase the risk of frying the generator.

2 - You can tweak the regulator to make 12 volts by increasing the spring tension on the regulator relay. This is the relay whose contacts are help CLOSED by spring tension. (The other relay is the cutout - it's contacts are help OPEN by a spring). It's kinda fussy to set just right, but it can be done. However on a 2-element regulator like the Farmalls use, the voltage regulator relay serves also as a current regulator. So when you mess with the voltage setting you increase the current limit. If your battery shorts a cell or a fault develops in the wiring, the regulator won't limit current to a safe value for the generator, and poof!.....your generator is again fried.

Bottom line is you can make your 6 volt generator give 12 volts - just be aware of the risks!

First, what is the 'weak link' in the generator? I know (now) that my 6-volt gen is running double current in the field windings, but I am also running only half current in my armature (this was the reason I considered it safe and went ahead and did it). Shouldn't this help me out? I guess it would be best to run both sets of winding below their design amperage, but I was wondering how sensitive the field actually is to over-current, especially when my armature is only making one-quarter the heat than it would if run at 6 volts. Also, the exterior field windings seem more likely to shed the extra heat. I agree this is not optimal but was also wondering how different the 12-volt field windings are from the 6-volt (wire guage, number of turns, etc).

Also, do you know what the typical rated field current is for these type generators? I guess I could go out and measure the ohms of the field and divide it into 6 (or 12), but that would not tell me what my field is rated to take. I guess I should be somewhat cautious since I think my generator is oiginal and the insulation isn't the greatest. But I was just wondering how cautious I should be. For the wndings, I would take it that the field would be the 'safer' of the two to run over-current (no brushes, etc.) I was just curious if you knew how much.

Thanks for your insightful, technical remarks. I learned something!

Dave

For the field winding, running over the design current causes higher temperatures in the winding as previously discussed. High temperatures - especially deep in the windings - result in rapid deterioration of the insulation. When the insulation deteriorates enough that a short develops between windings or to a pole shoe, you've got failure. Also higher temperature may cause the solder in soldered connections to slowly creep - especially with the vibration experienced by a generator humg on the side of an engine. When enough solder creeps out or weakens in a connection, the winding goes open and again you've got failure.

The armature on the other hand usually fails when a solder joint at the commutator lets go. These joints are under considerable mechanical stress due to centrifugal force when in operation. So vibration and metal fatigue also may play a role. However armature winding insulation failure can also occur, same as in the field windings.

Field resistance is determined by the wire gage and by the length of wire comprising the winding. Typical rated field current for a 6 volt generator? Depends on the factors above, also on the coil temperature (higher temperature -> increased resistance -> reduced current). However 2-4 amps is what I've typically observed on a 6 volt system.

Regarding relative heating between the field and armature after switching to 12 volts, analysis is a bit more complex for the armature. Granted, the reduced armature current results in reduced heating (P=I^2*R). However armature current is not cut in exactly in half like you'd expect on 12 volts. This due to the increased field magnetic strength caused by the increased field current draw. End result is heating is somewhat reduced, but NOT by a factor of 50%. Further, because the current in each winding reverses polarity with each 1/2 revolution, there's hysteresis and eddy current heating going in the iron parts of the armature. How this affects the total heating in the armature is beyond my analytic capability however (I'm an ME by training, not an EE!)

Bottom line is I guess I'll agree with you: You are probably less likely to experience problems running over current through the field vs the armature. But that's just my guess....

jhgold@stic.net

Thanks for the help.

JG