Spark image

The a.c. generator or alternator

A simplified form of the a.c. generator is shown in Figure 1(a).

A coil (the rotor) is rotated between the poles of a d.c. electromagnet (energised by the field coils), except in the case of a bicycle dynamo where a permanent magnet is used, and the e.m.f. generated is taken from the ends of the coil. These are connected to sliding contacts known as slip rings on the axle, and contact is made with these by two pieces of carbon (the brushes) which press against the slip rings. As the coil rotates it cuts through the lines of magnetic flux producing an induced e.m.f., the variation of which with time is shown by Figure 1(a). A much smoother output is obtained by having a number of coils wound on an iron core which is laminated to reduce eddy currents. The output of such a generator is shown in Figure 1(b).

In generators where the output current may be very large, as in a power station, it is the magnet that rotates while the coil remains at rest. A simplified version of this is shown in Figure 1(c). The advantage of this is that the slip rings and brushes have to carry only the small current needed to magnetise the rotating electromagnet while the current produced the static field coils may be many hundreds of amps. In modern alternators installed in a power station the e.m.f. generated will be some 25 kV and the current produced over 1000 A!

Effect of changing the speed of rotation of the coil on the induced emf

If the speed of rotation of the coil is changed two things happen (See Figures 2(a) and 2(b):
(a) since the rate of cutting of magnetic flux is increased the output emf will be incresead also in line with Faraday’s law
(b) the frequency of the output emf will be increased as well since the coil makes a revolution in a shorter time.

© Keith Gibbs 2011