You seem to be confusing a dynamo with an alternator. The dynamo has fixed field windings generating a magnetic field, and a rotating armature with a commutator and brushes to produce a direct current suitable for charging the battery.
An alternator has 3 (fixed) stator windings 120 degrees apart which produce 3 phase alternating current. This is converted into DC by a number (normally 6) of diodes linked together and called a rectifier. The magnetic field is produced by the rotor which has a DC voltage supplied to it by slip rings and brushes. This circuit is controlled by the regulator in response to the battery voltage and the demand from the aircraft systems. The following diagram is from a typical car installation which is very similar, the main difference is that it uses 3 extra diodes to provide the supply for the field circuit. This allows it to be completely self contained, and cars don't have any provision to isolate the alternator or battery when running:
http://www.familycar.com/Classroom/I...torCircuit.gif
The principal advantage of the alternator is that slip rings only carry a few amps of current and don't suffer the wear that dynamo brushes do. This allows an alternator to be run much faster and so give a better output low engine speeds.
Hope this helps.