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!
The induction motor relies on the phenomenon of electromagnetic induction for its operation and a simple investigation will demonstrate this (Remember that to obtain an induced e.m.f. either the conductor or the magnetic field may move.)