Are you looking at Field on the stator or rotor field (automotive style)?
As pointed out if you store the resultant rectified AC as DC, you will need an inverter to convert back to AC to drive the alternator as a motor.
Also If you have a rotor field (DC two brushes), when used as a motor you have the choice to make it run as an induction motor (no DC field) or by connecting DC to the rotor field when close to synchronism will make it a synchronous motor.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.