The current through the exciter coil is varied by the Regulator for two reasons: 1. The output voltage of the alternator would increase far beyond 14V with the increase of the alternator's rpm if the rotor's magnetic field is not controlled. The Regulator senses the alternator's output voltage and, if it starts going above 14V, it reduces the current through the exciter coil which reduces the magnetic field. This "counter action" prevents the alternator from going above 14V (or whatever is the pre-set voltage by the Regulator design). 2. When the load on the alternator increased, it would cause voltage drop of the alternator's output if the magnetic field is not controlled. Again, the Regulator senses the alternator's output voltage and, to counter the voltage drop, the Regulator increases the current through the exciter coil (=stronger magnetic field) and maintains 14V. Of course, there are limits as to how well the alternator can keep the voltage at 14V under load. With rather high loads, and as the alternator heats-up, the voltage may not be held at steady 14V but may drop to ~13.6 which is still fine. The frequent failures of your Regulators could be due to a partial short in the rotor (exciter coil) windings. See the Denso manual I have attached, I think you will find info on what the resistance of the exciter coil should be.
Update: after the last (new) regulator burnt out again in 30 minutes, the alternator shop suggested to replace the battery. For now, the car is running with rebuilt (again) alternator and a brand new Interstate battery. It idles with 14.3 - 14.5V shown at the starter + terminal. If the regulator burns out again, wiring issues are the only remaining items and those get expensive quick.
Hi Mitchell I don’t think anyone answered you but my battery light came on only once voltage dropped way down. The alternator likely failed about 15 to 30 minutes earlier. I think the voltage was about 10.5 or 11 can not remember but was not a great setup because it did not tell you the alternator was failing but rather that it had failed for a while. Hence I periodically check it through the cigarette lighter when on long drives like my road trip to Vancouver 1200km I suspect(guessing) that you have about 0.75hrs from failure before my car shut off and about 15 minutes from when the light came on Just roughly what I experienced
I gave up on older style alternators. A few years back i installed an modern ASCOT 130D. It is able to generate the required voltage at lower RPM. I made an aluminum alternator pulley 7% bigger (yet still half the weight of the original). I get 12.7 Volts at Idle and 14.4 at 1500RPM.
