I’m not sure the rotor is magnetised, as there is a coil in the centre of the alternator which is fed power from the regulator. As the voltage increases at the output the amount of current in the centre coil decreases and lowers the output to keep it at the correct voltage.
I believe the rotor just breaks the magnetic field to induce the current flow in the main windings. It is not a permanent magnet as far as I can tell.
Of course any and all information in this post could be totally wrong as it’s based on my own guesswork.
