Practically the power stations are divided into three-phase alternative current. A huge part of this power is used in the form of alternating current in industry, for lighting and domestic needs. With the help of converters, A.C. is converted into D.C. which provides advantage to the industries.There is a wide use of such installations.
As chief sources of power, D.C. generators are primarily used in self-contained plants such as automobiles and air planes, for electric arc welding, train car lighting, ill sub- marines, etc.
As per the method of excitation, D.C. generators are categorized as follows:
3.1.1. Separately excited generators
These are the type of generators whose field magnets are energized from a self-governing external source of D.C. current. The following figure represents such a generator:-
3.1.2. Self-excited generators
The field magnets of these generators are energized by the current they produce. Due to outstanding magnetism, there is always present some flux in the poles. When the armature is rotated, some e.m.f. and thus some induced current produced which is partly or fully passed through the field coils thereby strengthening the pole flux.
Further, self-excited generators can be divided, in accordance with how the field winding is connected into generators, as follows:
(i) Shunt wound generators
(ii) Series wound generators
(iii) Compound wound generators:
Important relations: The following figure refers to these important relations-
(iv) Power developed = EgIa
(v) Power delivered = VI
Where,Ish = shunt field current;
I (or Il) = load current,
Ia = armature current,
Ra = armature resistance,
Eg = generated e.m.f.
Rsh = shunt field resistance, and
V = terminal voltage.