Need 120A current limiter for 200A source
It sounds like you have a dedicated motor to drive the alternator, which I presume you have some sort of speed control over it.
A good alternator may provide 50% of rated current at an idle of 800 RPM and 100% at 1500 RPM or more. So you simply limit the current by the motor speed and an ammeter for measuring current.
e.g.
200 AMP Alternator for Nissan Sentra, 2002 - 2006 1.8L L4
Idle Output:    100+ AMPS
Full Output:    200+ AMPS @ 1200 Engine RPM !
There are also field current limiting methods but this may be simpler.