Here is what I believe I know, so far. Bob M can help me, I know. When I get this sorted out, I may be able to pose as an "expert". The third brush in some generators served to limit the maximum amperage output of the unit. Without some control, the amperage output would rise to the point that heat would destroy the armature. Voltage output is directly proportional to the speed of the generator. The early units depended on the battery and the limited speed to "control" the voltage. A voltage regulator was added to the cutout in the next evolutionary step. The third component added to the regulator was the current (amperage) control. This eliminated the third brush in the generator. My question arises because it looks as if some 2 brush generators use the regulator with only the cutout and the voltage regulator coils. Limiting voltage output also serves to limit current output. But how is the current (amperage) controlled with a 2 coil regulator, 2 brush generator, if the tractor battery is very low and the tractor is jump started? Amperage output would be very high for a time while battery voltage is restored.

Hey Bus Driver - here's the deal: The two unit (two relay) voltage regulators are arranged so the "voltage regulator" relay also senses and limits output current. It does this with a low impedance (a few turns of heavy wire) coil wound outside the high impedance (many turns of fine wire) voltage sensing coil. The current coil is wired in series with the generator armature and sees all the generator output current. Now when output current exceeds the regulator, it pulls the regulator points open, dumping the field, thus reducing generator output voltage and hence current. In this photo of a 2 element regulator (from an SH), the regulator relay is the one closest to the camera. The current winding is the heavy copper coil wound outside the voltage sensing coil.

The two relay setup is a cheap, but imperfect solution. Adjusting the voltage setting affects the maximum current setting and vice-versa - ie. they are very difficult to adjust properly. Thus the more common 3 element regulators with separate relays for voltage, current and cutout control - while more costly, they do a more precise job of generator control.

Hope this makes senses! ...Bob M

Good explanation. Easily understood. I have been waiting over 40 years to find this explanation. Thanks. One other question has occurred to me. If this unit is used on a three brush generator, as you suggested in an earlier post, the third brush obviously continues to limit the current. Does this unit, in this application, do a satisfactory job of controlling the voltage? I once paid a repair shop to adjust the regulator on a 40 JD. They never got it to work properly.

Good question! Don't know the answer for sure, but here's what I think: The 3rd brush still acts as the primary generator current limiter. However the combination voltage/current regulator serves as a secondary current limiter - in the event of a short in the output wiring or something. Or maybe the roles are reversed. But one current limiting function backs up the other. That's my theory anyway...

Something else I've noticed: The generators provided with 2-element regulators are arranged with the 3rd brush set near the max current position, and the 3rd brush position is non-adjustable. At least that's how my SH an SM came from the factory. And that kinda supports my theory above. If anyone else knows different please advise!