Although a far greater percentage of the electrical machines in service are a.c.
machines, the d.c. machines are of considerable industrial importance. The
principal advantage of the d.c. machine, particularly the d.c. motor, is that it
provides a fine control of speed. Such an advantage is not claimed by any a.c.
motor. However, d.c. generators are not as common as they used to be, because
direct current, when required, is mainly obtained from an a.c. supply by the use
of rectifiers. Nevertheless, an understanding of d.c. generator is important
because it represents a logical introduction to the behaviour of d.c. motors.
Indeed many d.c. motors in industry actually operate as d.c. generators for a
brief period. In this chapter, we shall deal with various aspects of d.c.
1.1 Generator Principle
An electric generator is a machine that converts mechanical energy into
electrical energy. An electric generator is based on the principle that whenever
flux is cut by a conductor, an e.m.f. is induced which will cause a current to flow
if the conductor circuit is closed. The direction of induced e.m.f. (and hence
current) is given by Fleming’s right hand rule. Therefore, the essential
components of a generator are:
(a) a magnetic field
(b) conductor or a group of conductors
(c) motion of conductor w.r.t. magnetic field.
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