DC excitation used in a synchronous alternator instead of AC excitation

Excitation in an alternator is the current applied to the rotor to make it a magnet with fixed polarity.  As the rotor turns, the magnetic field is applied successively to the stator windings, causing a positive, then zero, then negative then zero etc. magnetic flux in the stator windings.  The changing flux produces the generated voltage in the stator windings and the flux reversals are due only to the rotation of the rotor.  If we apply AC excitation voltage to the rotor, then its magnetic field will alternate due to the excitation and the stator windings will receive alternating flux varying both with rotation and AC excitation.  The resulting generated voltage is hard to predict but it won't be the nice three-phase AC that you want.  The rotor must be excited with DC.