What kind of alternator?
An automotive alternator has an "alternator control unit" (ACU) which senses the car's battery voltage(input), and it modulates the average current (~1A) to the field winding (rotor) of the alternator. The alternator's output current (typically 20-30A) is proportional to the field current and rpm (its like a current source, so its output voltage is whatever it needs to be). The feedback stabilizes the alternator output such that the battery voltage is 14.5V (the correct charging voltage), and the alternator output current exactly matches that being consumed by the car's electrical loads (ECU, instruments, AC, lights, radio, etc). It matters not what the frequency is because the stator winding (three phase, multiple poles) output current is full-wave rectified to DC with very low ripple.