In a rotating field synch generator, the rotor provides the flux and you can design to stator to produce any voltage (within practicality) for that flux. So, you can run the 24 V stator without doing anything to the rotor.

Muthu
www.edison.co.in

Tugboat Rotor flux is - as usual - proportional to ampere-turns (amps x turns). The 12v alternator requires 4 A into a 4 ohm rotor winding circuit: let's say 100 turns of some wire size. The winding "space" is the area under the pole head that will restrain the wire from moving into the airgap - which means there's a limit on the number of turns/space available for a given diameter and pole count. You indicate your 24v design uses 2.5 A into a 10 ohm circuit resistance. All else being equal, the 24v rotor uses (4/2.5) x (number of 12v turns), with a correspondingly smaller cross-section ... which ends up being a higher resistance circuit.

If you just push 2.5A thru the 12v rotor winding, you'll only develop 2.5/4 x the flux necessary, which means your alternator will only produce about (2.5/4) x 24 volt = 15 v output. Adding an external resistor to the circuit isn't going to help raise that output voltage. On the other hand, you could regulate on voltage and "push" 4 A thru the rotor winding to develop sufficient flux ... since the winding is already designed for that ... but you'd have some more losses somewhere in the system.

Converting energy to motion for more than half a century

It sounds like the best option is to use the 12 volt regulator (4 amp capacity) with a voltage divider to half the sensed voltage.