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The d.c generator

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.

Student investigation
The foIIowing experiment is designed to investigate the variation of output voltage with rotation speed for a d.c. generator.

Set up a d.c. generator which may be rotated from rest by a motor. Use a voltmeter or data logger to record the instantaneous values of the e.m.f. generated as the speed of rotation is slowly increased from zero. Explain your results.

Characteristics of the series- and shunt-wound d.c. generators

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.

For the series-wound generator the voltage between the terminals rises with the current whereas for the shunt-wound machine the current variation is much smaller, a slow decrease being observed.
Can you explain these effects?
© Keith Gibbs 2011