Something to be aware of is that only special high output alternators are made to operate at full output for more than a couple minutes. Alternators will not put out more than their rated output, even into a short, with 12 volts excitation. That is because although the output is DC, internally the windings operate on AC and their leakage inductance limits the output - as the speed goes up the induced voltage goes up, but so does the inductive reactance. In ordinary alternators the practical continuous output is limited by cooling - cooling means extra drag, even when the alternator is not putting out much current, so a compromise is made.
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