The formula to determine HP is HP= Torque X RPM/5252.
5252 is a constant. Search google and you'll get the story of where it comes from.
If 5252 is a constant and the torque required to turn the alt to get the 50amps is also constant (same electrical load, just different RPM, right?) then it goes without saying that increasing speed will increase the HP demand.
Lets try this then; let's imagine it takes 5ft/lbs to turn the alternator to generate 50 amps. Hypothetical number, just for shits and giggles. Then at 1000RPM we get:
5ft/lbs X 1000RPM/5252= 0.95HP
And at 10000RPM we get:
5ft/lbs X 10000RPM/5252= 9.5HP.
It takes 10X more HP to spin the same alternator pushing the same 50 amps it would seem... Very simplified way to see it but you get the picture. It probably takes more than 10X because of drag but no need to make this anymore complicated.
If I made a mistake in the math I encourage ANYONE to cerrect me as it's been a while since I last played with this
.