ESWAR COLLEGE OF ENGINEERING
Topics : Principle of operation of DC Machines- EMF equation – Types of
generators, Magnetization and load characteristics of DC generators
Although a far greater percentage of the electrical machines in service
are AC machines, the DC machines are of considerable industrial importance.
The principal advantage of the DC machine, particularly the DC motor, is that it
provides a fine control of speed. Such an advantage is not claimed by any AC
However, DC generators are not as common as they used to be, because
direct current, when required, is mainly obtained from an AC supply by the use
of rectifiers. Nevertheless, an understanding of DC generator is important
because it represents a logical introduction to the behaviour of DC motors.
Indeed many DC motors in industry actually operate as DC generators for a brief
period. In this chapter, we shall deal with various aspects of DC generators.
Principle of Operation of a DC Generator
An electric generator is a machine that converts mechanical energy into
All the generators work on a principle of dynamically induced e.m.f. This
principle is nothing but the Faraday's law of electromagnetic induction. It states
that, 'whenever the number of magnetic lines of force i.e. flux linking with a
conductor or a coil changes, an electromotive force is set up in that conductor or
coil.' The change in flux associated with the conductor can exist only when there
exists a relative motion between a conductor and the flux. The relative motion
can be achieved by rotating conductor with respect to flux or by rotating flux with
respect to a conductor. So a voltage gets generated in a conductor, as long as
there exists a relative motion between conductor and the flux.
Such an induced e.m.f. which is due to physical movement of coil or
conductor with respect to flux or movement of flux with respect to coil or
conductor is called dynamically induced e.m.f.
So a generating action requires following basic components to exist,
i) The conductor or a coil ii) The flux iii) The relative motion between conductor
In a practical generator, the conductors are rotated to cut the magnetic
flux, keeping flux stationary. To have a large voltage as the output, the numbers of
conductors are connected together in a specific manner, to form a winding. This
winding is called armature winding of a DC machine.