An electric generator works on the Lenz’s law. If the flux linking a coil changes a voltage is produced in the coil. The voltage is proportional to rate of change of the flux, and polarity is such that it would try to produce (if path is available), a current whose flux would compensate the change of original flux. Thus, to make a generator, we essentially need to have a coil in which we cyclically change the flux somehow.
In most generators this coil is called the armature. It is stationary and is called the stator. A second coil called the field (or exciter) coil carrying dc current is the rotor. It is made to rotate with respect to the armature so that the armature receives a cyclically varying magnetic flux. This induces into the armature an alternating voltage which can drive a load connected to the armature. The power required for the load current comes from the mechanical ‘prime mover’ which rotates the field coil.
The source of DC supplying the rotor is not moving but the coil is moving. Hence, a pair of slipping carbon brushes is required to make slipping contacts on a segmented rotor. These brushes will occasionally cause sparking, (and hence, electromagnetic interference) and will also need replacement due to wear out.
Permanent Magnet Generator
But a permanent magnet with sufficient magnetic strength and rotating at the same frequency can produce the same effect. So we can replace the rotor field coil with a permanent magnet, and we have a permanent magnet generator (PMG). A permanent magnet generator eliminates the need for a rotor coil and the slip brushes. This makes a simpler and more robust construction.
But the PMG has a disadvantage also. The magnetic strength may change with time. This will change the output voltage even if all other parameters are kept same. Output voltage also changes when the load current changes. In a generator with current excitation these changes can be easily compensated by controlling the field excitation current. With a flux produced by a permanent magnet there is no way to control the flux strength and hence, to compensate for the output voltage variations. However, there is no difficulty in using PMG’s where a subsequent stage of voltage regulation is present.