DC GENERATOR
1. principle
of DC generator
There are two types of generators, one is ac
generator and other is DC generator. Whatever may be the types
of generators, it always converts mechanical power to electrical
power.
we will come to the topic of principle of DC generator. Now the loop is opened and connected
it with a split ring as shown in the figure below. Split ring are made out of a
conducting cylinder which cuts into two halves or segments insulated from each
other. The external load terminals are connected with two carbon brushes which
are rest on these split slip ring segments.
- Working
Principle of DC Generator
It is seen that in the first half of the revolution current
flows always along ABLMCD i.e. brush no 1 in contact with segment a. In the
next half revolution, in the figure the direction of the induced current in the
coil is reversed. But at the same time the position of the segments a and b are
also reversed which results that brush no 1 comes in touch with the segment b.
Hence, the current in the load resistance again flows from L to M. The wave from of
the current through the load circuit is as shown in the figure. This current is
unidirectional.
This is basic working
principle of DC generator, explained by single loop generator model. The
position of the brushes of DC generator is so arranged that the change over of the
segments a and b from one brush to other takes place when the plane of rotating
coil is at right angle to the plane of the lines of force. It is so become in
that position, the induced emf in the coil is zero.
2. construction of DC Generator
- Yoke of DC Generator
Yoke or the outer frame of DC generator serves two
purposes,
·
It holds the magnetic pole cores of the generator and
acts as cover of the generator.
- Pole Cores and Pole Shoes of DC Generator
There are mainly two types of construction
available.
One: Solid pole core, where it is made of a solid
single piece of cast iron or cast steel.
Two: Laminated pole core, where it made of numbers of thin,
limitations of annealed steel which are riveted together.
- Armature Core of DC Generator
The purpose of armature core is to hold the armature winding and provide low reluctance path for
the flux through the armature from N pole to S pole.
Although a DC generator provides direct current but induced current in the armature is alternating in
nature.
- Armature Winding of DC Generator
Armature winding are generally formed wound. These are
first wound in the form of flat rectangular coils and are then pulled into their
proper shape in a coil puller. Various conductors of the coils are insulated
from each other. The conductors are placed in the armature slots,
which are lined with tough insulating material.
- Commutator of DC Generator
The commutator plays a vital role in DC
generator. It collects current from armature and sends it to the load as direct
current. It actually takes alternating current from armature and converts it to
direct current and then send it to external load.
- Brushes of DC Generator
The brushes are
made of carbon. These are rectangular block shaped. The only function of these
carbon brushes of DC generator is to collect current from commutator segments.
The brushes are housed in the rectangular box shaped brush holder or brush box.
As shown in figure, the brush face is placed on the commutator segment which is
attached to the brush holder.
- Bearing of DC Generator
For small machine, ball bearing is used and
for heavy duty DC generator, roller bearing is used. The bearing must always be
lubricated properly for smooth operation and long life of generator.
The curve which gives the relation between field current (If) and the
3.
Characteristic of Separately Excited DC Generator
- Magnetic or Open
Circuit Characteristic of Separately Excited DC Generator
The curve which gives the relation between field current (If) and the
generated voltage (E0) in the armature on no load is called magnetic or open circuit characteristic of a DC generator. The plot of this curve is practically same for
all types of generators, whether
they are separately excited or self-excited. This curve is also known as no load saturation characteristic curve of DC generator.
Here in this figure below we can see
the variation of generated emf on no load with field current for different
fixed speeds of the armature. For higher value of constant speed, the steepness
of the curve is more. When the field current is zero, for the effect residual
magnetism in the poles, there will be a small initial emf (OA) as show in
figure.
Let us consider a separately excited DC generator giving its no load voltage E0 for a
constant field current. If there is no armature reaction and armature voltage
drop in the machine then the voltage will remain constant. Therefore, if we
plot the rated voltage on the Y axis and load current on the X axis then the
curve will be a straight line and parallel to X-axis as shown in figure below.
Here, AB line indicating the no load voltage (E0).
When the generator is loaded then the voltage
drops due to two main reasons-
1. Due
to armature reaction,
2. Due
to ohmic drop ( IaRa ).
- Internal or Total Characteristic of Separately Excited DC Generator
The internal characteristic of the separately excited DC generator is obtained by subtracting the drops due to
armature reaction from no load voltage. This curve of actually generated
voltage ( Eg ) will
be slightly dropping. Here, AC line in the diagram indicating the actually
generated voltage (Eg ) with respect to load current. This curve is
also called total characteristic of separately excited DC generator.
- External Characteristic
of Separately Excited DC Generator
The external characteristic of the separately excited DC generator is obtained by subtracting the drops due to ohmic
loss ( Ia Ra ) in the
armature from generated voltage ( Eg ).
Terminal voltage(V) = Eg - Ia Ra.
This curve gives the relation between the terminal voltage (V) and load
current. The external characteristic curve lies below the internal
characteristic curve. Here, AD line in the diagram below is indicating
the change in terminal voltage(V) with increasing load current. It can be seen
from figure that when load current increases then the terminal voltage
decreases slightly. This decrease in terminal voltage can be maintained easily
by increasing the field current and thus increasing the generated voltage.
Therefore, we can get constant terminal voltage.
Separately excited DC generators have many advantages
over self-excited DC generators. It can operate in stable condition with any
field excitation and gives wide range of output voltage. The main disadvantage
of these kinds of generators is that it is very expensive of providing a
separate excitation source.
4. EMF
Equation of DC Generator
The derivation of EMF equation for DC generator
has two parts:
- Induced
EMF of one conductor
- Induced
EMF of the generator
One Armature Conductor
For one revolution
of the conductor, Let, Φ = Flux produced by each pole in weber (Wb) and P = number of poles in the DC generator. therefore, Total flux produced by aptionall the poles
 | |
And, Time taken to complete one revolution
Induced emf of one
conductor is
Derivation for Induced EMF for DC Generator
Let us suppose
there are Z total numbers of conductor in a generator, and arranged in such a
manner that all parallel paths are always in series. Here, Z = total numbers of
conductor A = number of parallel paths Then, Z/A = number of conductors
connected in series We know that induced emf in each path is same across the
line Therefore, Induced emf of DC generator E = emf of one conductor × number
of conductor connected in series.
Induced emf of DC generator is
Simple wave wound generator Numbers of parallel paths are
only 2 = A Therefore, Induced emf for wave type of winding generator is Simple lap-wound generator Here,
number of parallel paths is equal to number of conductors in one path i.e. P =
A Therefore,
Induced
emf for lap-wound generator is
5. Self Excited DC Generators
What are Self Excited DC Generators
The small amount of magnetism is present in the rotor
iron. This residual magnetic field of the main poles, induced electromotive
force in the stator coils, which produces initial current in the field
windings. Due to flow of small current in the coil, an increase in magnetic
field occurs. As a result, voltage output increases, in turn, increases the
field current. This process continues as long as the electromotive force in the
armature is more than the voltage drop in the field winding. But, after at
certain level, field poles get saturated, and at that point electric
equilibrium is reached, and no further increase in armature emf and increase in
current. The resistance of the field winding has certain fixed value, at which
self-excitation can be achieved. This resistance value may vary according to electric
parameters of the generator.
Types of DC Generators
DC Generators are
classified on the basis of the position of the field coils and armature in the
circuit and on the way of excitation.
- Series
Wound Generators
In
series wound generators, field winding and the armature winding are connected in series so that
current that passes through external circuit and through field windings, passes
from armature as shown in the figure below. The field coil of series wound
generator has low resistance, consist of a few turns of thick wire. If the load
resistance decreases, then current flow increases. As a result magnetic field
and output voltage increases in the circuit. In such type generator, output
voltage varies directly with respect to load current which is not required in
most of the application. Due to this reason, such types of generators are rarely used.
- Shunt
Wound DC Generators
In this type of
generator, the field winding is wired parallel to the armature winding so that
voltage is same across the circuit. Here, field winding has many numbers of
turns for the desired high resistance so that fewer armatures current can pass
through field winding and the reaming passes through load. So, IA =
IW + IL In shunt wound generator, as they are connected
parallel, current in the parallel branches are independent of each other.
Hence, the output voltage almost constant and if it varies then it varies
inversely with respect to load current. This is because of the voltage drop as
armature
Combination
Wound Generator
Compound wound
generator is advanced version of series wound generator and shunt wound
generator. the working principle of the generator is the combination of two
types so that it prevail over the disadvantages of both. It has both types of
winding; series field and shunt field winding. On the basis of their
connection, compound wound generators are of two types- short shunt compound
generator and long shunt compound generator.
Long
Shunt Compound Generator
Here the shunt
field winding is connected parallel to armature only as shown in figure. Series
winding is then connected in series.
- Short
Shunt Compound Generator
Here the shunt field winding is connected parallel to
armature only as shown in figure. Series winding is then connected in series
6. DC
Generators Performance Curves
Performance Curve of Compound Wound DC Generator
At no load, the
performance curve of this type of DC generator is same as that of shunt field
generators because at no load, there is no current in the series field winding.
When the load increases, then the terminal voltage drops due to the shunt DC
generator,but the voltage rise in the series DC generator compensates the
voltage drop. For these reason the terminal >voltage remains constant. The
terminal voltage can also make higher or lower by controlling the amp-turns of
the series field winding. In the diagram below, the curve FG is showing this
characteristic.
7. Applications
of DC Generators
There are various types of DC generators
available for several types of services. The applications of these DC
generators based on their characteristic are discussed below:
- Applications of Separately Excited DC Generators
This type of DC generators are generally more
expensive than self-excited DC generators
because of their requirement of separate excitation source. Because of that
their applications are restricted. They are generally used where the use of
self-excited generators are unsatisfactory.
1.
Because of their ability of giving wide range of voltage
output, they are generally used for testing purpose in the laboratories.
2.
Separately excited generators operate in a stable
condition with any variation in field excitation. Because of this property they
are used as supply source of DC motors,
whose speeds are to be controlled for various applications. Example- Ward
Leonard Systems of speed control.
- Applications of Shunt Wound DC Generators
The application of shunt generators is very much
restricted for its dropping voltage characteristic. They are used to supply
power to the apparatus situated very close to its position. These type of DC
generators generally give constant terminal voltage for small distance
operation with the help of field regulators from no load to full load.
1.
They are used for general lighting.
4.
They are also used for small power supply.
Applications of Series Wound DC Generators
These types of generators are restricted for the use
of power supply because of their increasing terminal oltage characteristic with
the increase in load current from no load to full load. We can clearly see this
characteristic from the characteristic curve of series
wound generator. They give constant current in the dropping
portion of the characteristic curve. For this property they can be used as
constant current source and employed for
various applications.
1.
They are used for supplying field excitation current
in DC locomotives for regenerative breaking.
2.
This types of generators are used as boosters to
compensate the voltage drop in the feeder in various types of distribution
systems such as railway service.
3.
In series arc lightening this type of generators are
mainly used.
- Applications of Compound Wound DC Generators
Among various types
of DC generators, the compound
wound DC generators are most widely used because of
its compensating property. Depending upon number of series field turns, the
cumulatively compounded generators may be over compounded, flat compounded and
under compounded. We can get desired terminal voltage by compensating the drop
due to armature reaction and ohmic drop in the in the line. Such generators
have various applications.
1.
Cumulative compound wound generators are generally
used for lighting, power supply purpose and for heavy power services because of
their constant voltage property. They are mainly made over compounded.
2.
Cumulative compound wound generators are also used for
driving a motor.
3.
For small distance operation, such as power supply for
hotels, offices, homes and lodges, the flat compounded generators are generally
used.
4.
The differential compound wound generators, because of
their large demagnetization armature reaction, are used for arc
welding where huge voltage drop and constant current is required.
At present time the applications of DC generators
become very limited because of technical and economic reasons. Now a days the electric
power is mainly
generated in the form of alternating current with the help of various power
electronics devices.
Sources:
- https://www.electrical4u.com/principle-of-dc-generator/
- https://www.electrical4u.com/self-excited-generators/
- https://www.electrical4u.com/emf-equation-of-dc-generator/
- https://www.electrical4u.com/construction-of-dc-generator-yoke-pole-armature-brushes-of-dc-generator/
