Definition:
“An electrical generator is a device which converts
mechanical energy into electrical energy. DC generator produces direct
current”.
PRINCIPLE:
In a DC generator an emf is induced whenever
magnetic flux is cut by a conductor.
WORKING:
Shown in the above figure is a rectangular copper
conductor loop ABCD rotating in a clockwise direction about its own axis in a
uniform magnetic field provided by permenant magnets or electromagnets.
Two ends of coil are connected to two slip rings R1
and R2 which are insulated from eachother and from the central shaft.
Two collecting brushes B1 and B2 made of carbon or
copper are pressed against the slip rings. Their function is to collect the
current induced in the coil.
Electric current produced whenever coil rotates
between the magnetic poles. The deflection of galvanometer needle shows the
current is induced.
Important
characteristics of a DC generator:
i.
Load
saturation characteristic (E0/If):
It is also called open-circuit characteristic or No magnetic. It gives
the relation between the no-load generated emf in armature (E0) and the
field or exciting current (If). This characteristic is same
for the separately excited and the selfexcited generators.
ii.
Internal
characteristic (Eg/Ia):
It practically gives the relation between the emf (Eg)
actually induced in the armature conductors (after considering the
demagnetizing effect of armature reaction) and armature current Ia.
It is also known as total characteristic.
iii.
External
characteristic (V/Ia ): It shows
the relation between the terminal voltage
iv.
dc generator is mechanical energy
into convert electrical energy which used in machine part of stator in a out
part & rioter is inner part and voltage induced armature winding and
current pass is Field winding it is magnetic flux generate.
DC
generators are classified based on their method of excitation. So on this basis
there are two types of DC generators:-
1.
Separately excited DC generator
2. Self
excited DC generator
Self excited DC generator
can again be classified as 1) DC Series generator 2) DC Shunt
generator and 3) DC Compound generator.
Let’s take a brief
look at how all these differ.
1. Separately excited DC generator
As you can guess
from the name itself , this dc generator has a field magnet winding which is
excited using a separate voltage source (like battery). You can see the
representation in the below image. The output voltage depends on the speed of
rotation of armature and field current. The higher the speed of rotation and
current – the higher the output e.m.f
2. Self Excited DC Generator
These are
generators in which the field winding is excited by the output of the generator
itself. As described before – there are three types of self excited dc
generators – they are 1) Series 2) Shunt and 3) Compound.
A series
DC generator is shown below in
fig (a) – in which the armature winding is connected in series with the field
winding so that the field current flows through the load as well as the
field winding.Field winding is a low resistance,thick wire of few turns. Series
generators are also rarely used!
A shunt DC
generator is shown in figure
(b), in which the field winding is wired parallel to armature winding so that
the voltage across both are same. The field winding has high resistance and
more number of turns so that only a part of armature current passes through
field winding and the rest passes through load.
A compound
generator is shown in figure
below. It has two field findings namely Rsh and Rse. They are basically shunt
winding (Rsh) and series winding (Rse). Compound generator is of two types – 1)
Short shunt and 2) Long shunt
Short
shunt:- Here the shunt field
winding is wired parallel to armature and series field winding is connected in
series to the load. It is shown in fig (1)
Long
shunt:- Here the shunt field
winding is parallel to both armature and series field winding (Rse is wired in
series to the armature). It is shown in figure (2)
So you have got a
basic idea about the types of DC generators! Now you may know that these
generators are used only for special industrial purposes where there is huge
demand for DC production. Otherwise electrical energy is produced by AC
generators and is transmitted from one place to other as AC itself. When a DC
power is required, we usually convert AC to DC using rectifiers.
