The rotor in an alternator is at the very center of the magnetic field creation. The rotor creates the electromagnetic field that inductively creates amperage flow in the stator windings. This is opposite of a DC generator where a field current is created in the stationary external portion of the generator and induces voltage into the rotating armature.
The alternator rotor is constructed of many coils of insulated with varnish copper wire wound directionally around a solid iron core. The core is then
centered and balanced on a steel shaft for high speed rotation and affixed by some design of bearings on both ends.
At both ends of the metal core and copper winding assembly are heavy gauge metal plates bent over the windings with triangular shaped metal fingers called poles. These pole fingers are spaced alternately around the
outer circumference of the rotor so that they are as close as possible at top and bottom yet not touching from north to south.
As a current is introduced into the copper wire the metal triangular pieces at either end of the rotor assembly become electromagnets. Because of the direction the copper wire is turned in relation to the position of the metal fingers: each triangle finger becomes alternately a north and south pole. When the electromagnetic field is in proximity of the stator winding assembly, a current is induced into the stator.
In order to spin the rotor at high revolutions while maintaining a current in the rotor field, a copper slip ring is attached to the shaft and the field wire is attached. Then a second copper ring with the other end of the field wire attached or soldered to it. Stationary copper/ carbon mix brushes are then mounted to a frame with voltage fed through them. In this way the rotor can be used with variable currents at variable RPM’s