Why armature is a rotor in dc machine and stator in ac machine?
The armature carries the majority of the current in the DC machine which means the commutator and brushes carry this current. The stator has the field windings on it and they carry only a small current. This arrangement makes the DC machines variable in speed but not as rugged as AC machines. The stator in an AC machine is driven by 3 phase AC creating a strong rotating armature field, and the rotor only needs to carry a small DC current to create a magnetic field that is driven to rotate by the strong armature field. The rotor uses slip rings and brushes which do not carry much current. AC machines are locked into a specific RPM.
