Greybeard

Here’s a 3-minute on the idiot light:
Regardless of the design of an alternator or generator, the windings need some kind of external current to get the "flux line" in motion so that electrons can begin to flow.
In about 25% of all alternator designs, a small permanent magnet generator provides the excitation to energize the field windings so that they can develop sufficient electrical current to charge a battery.
The other 75% of designs, including most Nipondenso, sees field excitation current flowing directly through the idiot light when the ignition switch is selected to the "On" position, then into the alternator field winding (rotor) and regulator circuit to ground.
This small current in the field produces a very weak magnetic field, just enough to get the alternator to start generating when the engine is spooling up. As long as the alternator isn’t producing a charge, the regulator’s “ground plane” is closer charge-wise to the negative terminal than to the positive terminal, so the light comes on. This excitation current is also "regulating" the regulator to keep it from going directly into max conduction. Otherwise, with battery voltage lowest right after start, the alt would try to go into max charge as soon as it started cranking over, and the alternator would burn up before long.
As the generator spins, that weak magnetic field from the bulb current produces the initial output from the alternator. As the rotor speed increases, so does the excitation current that is being fed back to the field. Now, up to a point, the alternator is producing slightly less than battery volts, and the regulator is continuously seeing more internally produced voltage and less "excitation" voltage During this time the bulb continues to get dimmer and dimmer until alternator output exactly matches the battery voltage, at which time the bulb goes out completely.
As long as the alternator's regulated output remains above battery voltage, the light will stay off. With no accessories on, and the battery now recharged after start, the regulator voltage falls, and only rises to match the demand on the battery from the ignition system before dropping back to only slightly above battery voltage. During these low-voltage points, the bulb is actually feeling some current, but not enough to see with the naked eye.
Depending on what’s happening inside the regulator, the bulb can be “on” while the alternator is putting out correct voltage, or “out” while the battery is getting no charge at all, yet come on with the key turned to “ignition” before start. For example, a cracked diode or dirty brush contacts can result in a low output with dim light and a slowly dying battery.
And, in a correctly charging system with clean connections, a good battery, and properly working alternator with (clean brushes and slip rings with no leaking diodes) alternator speeds above about 500 rpm should have little or no bearing on the output, and blipping the throttle should hardly hange the intensity of the headlamps or fan speed.