Alternators and generator heads produce electricity via electromagnetic induction. AC current is output from the alternator, but it can be rectified into DC current.
Some alternators and generator heads utilize a rotating armature in a stationary magnetic field that has been produced through permanent magnets or electromagnetics. To invert the electrical load a commutator is used, which consists of a segmented ring and contact brushes. As the ring rotates with the armature, the brushes collect current along the ring at intervals. The output is always going in one direction—direct current.
Locating the fielding windings on the rotor, with the armature windings remaining stationary, is also an option. This eliminates the commutator, but another current inverter technique must be employed if DC output is required. This can be accomplished by creating the magnetic field with permanent magnets placed on the rotor, or with a second smaller generator called an exiter that produces an electromagnetic current. This type of alternator is known as "brushless."