The magnetizing current comes from an external source but once you are creating the power you can use a small part of that current to maintain the magnet. As stated you can control the field strength to vary the output from either the rotor or the stator. You can pull the produced energy as DC of the rotor if a commutator is installed or as AC if slip rings are used. I think most big generators take the output off the stator as the amount of current you can pull off a rotor is affected by user maintainable parts like brushes. I is often easier to use the rotor as the exciter and control and input the magnetic field there. I assume you change to dc and then to AC to control the frequency. I assume the reason the magnetizing current must come off the grid is to ensure you don't put power on to a grid that has tripped or been shut off for any reason like maintenance.
