George, with all due respect, I SAY YES
Wellllllllll lets put our electrical thinking caps on.....
FIRST LETS GET THE TERMINOLOGY CORRECT: In an Alternator, the rotating component through which current is passsed via the slip rings to produce a rotating magnetic field IS CALLED THE ROTOR........The stationary coils of wire into which voltage is induced via a passing magnetic field IS CALLED THE STATOR. The Stators output is AC so its necessary to rectify it in order to produce DC charging voltage.
If you want to increase the output of an Alternator you can 1) Spin it faster or 2) Increase the Rotors magnetic field strength by pumping more current through it (controlling that rotor current is what the regulator does)
So long as the rotor windings will handle the extra current and the I Squared R heat can be sufficiently dissipated, enough current can pass through the rotor to likely yield a rectified loaded outout voltage of 48 VDC.
Next is the Stators ability to withstand the voltage (12 versus 48) and I think that wouldnt be any problem.
Next is the Stators ability to withstand the current youre going to pull out of the 48 volt alternator. At the higher 48 volts youre likely NOT going to pull the current (like 60 amps) that you were at 12 volts and its my best "guess" the Stator at 48 volts can supply the necessary charging current (but that depends on the load so we cant really say for sure) so the I Squared R heat dissipation wont be much if any more then when operated at 12 volts (Provided of course youre running at lower amps at 48 volts then when used at 12).
SO ITS MY OPINION A 12 VOLT ALTERNATOR YES CAN BE MODIFIED FOR 48 VOLT OPERATION. HOWEVER the heat dissipation has to be dealt with, so I doubt you can pump the same current as it does at 12 volts as if youre configured for 48 volts.
12 Volts at 60 amps means the Stator must dissipate 720 watts so at 48 volts thats 15 amps for 720 watts. The heat generated in the Stator is I Squared R and regardless if youre running at 12 or 24 or 48 volts I doubt it can dissipate a lot more heat, so at 48 volts I figure you cant still deliver 60 amps like it does at 12 volts (maybe 15 amps though)
Bob M (Mechanical Engineer understands heat dissipation better then me) and Janicholson am I wrong????