Yes indeed, but the output power will be pretty considerable as well.
keep in mind the eddy currents set up in the iron oppose the field coil's amp turns. so the volts per hz will decrease as rpm rises..
Yes, and that is very important. Its what sets the output current and keeps it constant as rpm rises.
If its a 65 amp alternator or an 85 amp alternator, whatever it is, the designer worked out the volts per hz and inductance of the windings to set a safe maximum current. We are not changing any of that. What we are doing is weakening the field at 48 volts not 12 volts to control the output voltage.
If it can deliver 65 amps at 14v (910 watts) it should be able to deliver 65 amps at 56v (3,640 watts).
It will really be spinning pretty fast, but the dc output current is limited only by the copper wire gauge in the star output winding, so that does not change. The diodes too will be quite happy at four times the output power.
One further thought on all this.
The original field winding needs around 14v at probably about 4 amps to generate a strong enough magnetic field, just safely below magnetic saturation, for full maximum rated output.
All the previous suggestions above ^^^ assumed we keep this original field winding.
It should not be too difficult to rewind the rotor, as its just a plain simple bobbin winding.
So what would be needed would be to rewind the rotor with wire of half the original diameter (one quarter the cross sectional area) but with four times as many turns.
So instead of having X turns and four amps, we have 4X turns with one amp.
Same ampere turns, same volume of copper, and sixteen times the ohmic resistance.
But we can now operate our home made voltage regulator at 48 volts output instead of 12 volts output.
Much more efficient., especially as its now only one amp required
And as oZ has already shown us, the rotor halves come apart very easily for a possible rewind.
If you don't have a humongous hydraulic press, your local garage or servo is certain to have one.
A six pack of beer on a hot day should have it apart in seconds.