AC Generator:

The  stationary  coils would have AC induced into them if DC field excitation is connected to  the  rotor that was show in the above figure. That arrangement  is known as a rotating field and stationary armature AC generator.

The rotating field, stationary armature typeAC  generator  is used when large power generation is included. Within this  category of generator a DC source is supplied to the rotating field coils that produces a magnetic field around the rotating element. As the rotor is turned through the prime mover, a field will cut the conductors of the stationary armature and an EMF will be induced into the armature windings.

Figure: Simple AC Generator - Rotating Field, StationaryArmature

This categories of AC generator has various advantages over the stationary field, rotating armature AC generator:  (1) a load could be linked to the armature without moving contacts within the circuit; (2) it is much simpler to insulate stator fields than rotating fields; and (3) much currents and higher voltages could be generated.