In the d.c. generator the output from the rotor assembly is fed to a commutator where the brushes press against a split ring of copper. This means that a varying but unidirectional e.m.f will be produced. A d.c generator and its output is shown in simplified form in Figure 1.
As with the a.c. generator, the d.c. machine usually uses rotating field coils, a series of them being wound round in the core; the rotating coils and the core are known as the armature. The output is then much steadier, a ripple effect being obtained. The d.c. generator may be made 'self-exciting' by putting the field coils and armature in series or parallel, the current required for the field coils being produced by the generator itself. There is nearly always some residual magnetism in the core of the armature to aid the starting of such a generator.
The two graphs in Figure 2(a and b) show
how the voltage generated varies with the load current drawn from it for both a series-wound and a
shunt-wound machine.
