The Rotax alternators are permanent magnet type. Thus they do not have field windings.
I can not believe that, "These regulators reject any unused electrical power
as heat." Van's must have been misinformed about this. What Rotax
document is this statement based on? Suppose that 21 amps are available but
not used by the aircraft. If those 21 amps were turned into 300 watts of heat,
how is that heat dissipated? Where are the cooling fins?