A DC generator operate on the principle of Dynamic induced emf explain by fleming right hand rule
Fleming's right-hand rule : (for
generators) shows the direction of induced current when a conductor
moves in a magnetic field. It can be used to determine the direction of
current in a generator's windings.
Induced emf in generator : e=B*l*v*sinθ, Here the induced emf is in
form of Sine, so generated emf in DC generator is AC instead of DC. To convert in unidirectional DC signal use rectifying as Commutator.