INTRODUCTION
A machine, which converts mechanical energy
into electrical energy, is called a Generator.
This energy conversion is based on the dynamically induced emf.
According to the Faraday's law of electromagnetic Induction, an induced emf is
produced in the conductor which cuts the magnetic flux. This emf causes a
current to flow in the conductor if its circuit is closed.
Hence the basic essentials for an
electrical generator are: -
(i) Magnetic field;
(ii) Conductor or conductors and;
(iii) Relative motion between magnetic field
and conductors.
CONSTRUCTIONAL DETAILS OF A DC
GENERATOR
Here we are dealing with a DC generator.
But there is a good similarity with a DC motor also as far as construction is
concerned.
The following are the main parts of a DC
generator:
·
Yoke
·
Pole core or pole shoes
·
Field coils
·
Armature core
·
Armature winding
·
Commutator
·
Brushes and bearings
YOKE OR MAGNET FRAME
This is the outer part of the DC generator.
It provides the mechanical supports for the poles and acts as a protecting cover
for the whole machine. It carries the magnetic flux produced by the poles.
Yokes are made out of cast iron or cast steel. The modern process of forming
the yoke consists of rolling a steel slab round a cylindrical mandrel and then
welding it at the bottom. The feet and the terminal box etc. are welded to the
frame afterwards. Such yokes possess sufficient mechanical strength and have
high permeability.
POLE CORE OR POLE SHOES
The field magnet consists of pole cores and
pole shoes. The pole shoes serve two purposes.
(i) They spread out the flux in the air gap
and also being the larger cross section reduced the reluctance of the magnetic
path.
(ii) They support the exciting coils.
FIELD COILS
The field coils or pole coils, which consist
of copper wire, are former-wound for the current dimension. Then the former is
removed and the wound coil is put into place over the core.
THE ARMATURE CORE
It houses the armature conductors or coils
and causes them to rotate and hence cut the magnetic flux of the field magnets.
Its most important function is to provide a path of very low reluctance to the
flux through the armature from North Pole to South Pole. It is laminated to
reduce the loss due to eddy currents. Thinner the lamination, greater will be
resistance offered to the induced emf and hence smaller the current. And thus
the loss is also small.
ARMATURE WINDING
It is generally former wound. These are
first wound in the form of flat rectangular coils and then are pulled into their
proper shape in a coil puller.
Two types of winding are mainly used -
namely lap winding & Wave winding.
Lap winding
In lap winding finish end of one coil is
connected to a commutator segment and to the start end of the adjacent coil
situated under the same pole and similarly all coils are connected. Since the
successive coils overlap each other and hence the name (Ref. fig.1).
Wave winding
It is also called as series winding. In
this winding, the coil side is not connected back but progresses forward to
another coil sides. In this way the winding progresses, passing successively
every N pole and S pole till it returns to coil side from where it was started.
As the winding shape is wavy, the winding is, therefore, called wave winding
COMMUTATOR
The commutator, whose function is to
facilitate the collection of current from the armature, is cylindrical in
structure, built up of segments of high conductivity, hard drawn copper
insulated from one another by mica sheets. It also converts alternating current
into unidirectional current (DC).
BRUSHES&BEARINGS
The function of brushes is to collect
current from the commutator. These are rectangular in shape, made of carbon
normally. These brushes are housed in brush holder usually of the box type
variety.
Generally ball bearings are employed due to
their reliability but for heavy duty, roller bearings are also used. The balls
and rollers are generally packed in hard oil for quieter operation. Sleeve
bearings are also used where low wear is required.
E.M.F. EQUATION OF A
GENERATOR
Generated E.M.F, E = φZNP/60A Volts
Where, P = No. of poles.
φ = Flux per pole in (Weber)
Z =Total nos. of conductors.
N = r.p.m.
A = nos. of parallel paths in Armature.
(A = 2 for wave winding & A = P for lap winding)
PRINCIPLE OF OPERATION
The principle of electromagnetic
induction, discovered by Faraday, states that when a conductor is moved across
a magnetic field so as to cut the lines of force, electromotive force or
E.M.F. measured in volts, is generated across the conductor. Thus, if an open
loop or wire is made to rotate between the poles of a permanent magnet, as
shown in fig.3 and fig.4, there will be a tendency for electricity to flow
through the wire. The magnitude of this EMF or voltage, depends on the speed of
rotation, and on the strength of the magnet, i.e. "the magnetic
flux".
The direction of voltage generated in a
conductor depends on the direction of the motion of the conductor across a
magnetic field and the direction of the field itself. Since the magnet has two
poles, two conductors can be connected together in series to form a loop and
their voltages will be additive. Several loops can be joined together to form a
coil having a number of turns, all the voltages being added together. For each
half revolution, embracing one complete pole, the voltage will start from zero,
rise to a maximum and fall to zero again. For the remaining half revolution a
similar series of events will occur, but the direction of the voltage is
reversed. This very simple form of alternating current (A.C.) generator is
shown in fig.3.
To change this primitive machine into a
direct current (D.C. generator) fig.4, it is necessary to introduce a
commutator. In order to attain constancy of direction, the ends of the loop,
instead of being connected to slip rings, are connected to a split metal ring,
the two halves being insulated from each other. By placing the collecting
brushes (C & D) on the commutator in such a position that the voltage
induced in the loop is zero when the brushes change from one segment to the
other voltage at the brushes will be uniform in direction, although it will
still be alternating, commutator simply alters the connection of the loop to
the external circuit at the instant when the induced electromotive force
changes in direction.
If the loop of wire is closed by connecting
the brushes (C&D) to an external resistance(R), which represents the 'load'
imposed on the machine, electric current will flow through the loop and the
resistance (R). In practice, the amount of current, which flows, is measured in
amperes (amps). The magnitude of the current, which will flow through the
circuit, depends on the voltage generated and on the value of the joint
resistance of the loop of wire and the external resistance. Voltage, which the
machine is capable of developing at the certain speed and with a magnet of the
given strength, the current flow, measured in volts, divided by the total
resistance of the circuit, measured in ohms.
If the loop of wire be rotated in one
direction, the current will flow in the wire under the south pole (S), in the
direction of the arrow, that is, away from the brush (C), and then in the wire
under the north pole (N) towards the brush (D). From the brush (D), it will go
to the external circuit and then back to the brush (C); thus completing the
electric circuit. After rotating such that the position of the segments is
reversed, it will be noticed that the picture remains identical and therefore
the current flow will be in the same direction.
Although the primitive direct current
generator, so far described, produces a uni-directional current flow, it is
obvious that for each revolution of the coil the induced current will start
from zero value, rise to a maximum value, fall to zero then rise to a maximum
value again and finish at the zero point from which it started. However, by
increasing the number of coils and spreading them out evenly, the flow of
current can be made very nearly constant. This also means that there would be
an increased number of segments in the commutator in proportion to the
increased number of coils. In practice, a direct current generator has many
coils consisting of insulated copper wire or strip, and in order to concentrate
the magnetic flux where it is required, they are embedded in slots in a
soft-iron laminated cylinder. This assembly is called the armature.
The permanent magnet of the original
example is replaced by an electro-magnet having many poles wound with insulated
copper wire; these are field coils and are referred to as the field system. The
field strength, or excitation, depends upon the number of turns of wire on each
pole and on the magnitude of current flowing through the wire.
From this it can be seen that there are two
ready means of regulating the output of the generator; one by varying the speed
of rotation of the armature and the other by altering the magnetic strength of
the field system. The variation of speed of rotation is readily obtained by
varying the governor setting on the diesel engine, which drives the armature,
and by inserting variable resistance in the field system, the amount of current
flowing through the coils of the Electro-magnets can be varied.
In a diesel locomotive, the driver of the
locomotive makes these adjustments, as required, by moving his control handle,
thereby simultaneously affecting engine speed and generator excitation. The
main generator frame is coupled directly to the diesel engine flywheel casing.
The armature is of the single bearing type, that is to say, one end of the
shaft is coupled to the engine flywheel, and the other end is supported in a
roller bearing, housed in an end plate bolted to the generator frame. The main
generator is self ventilated, having its own fan which draws air through the
machine so as to cool the winding's and maintain them at a safe working
temperature.
COMMUTATION
We have seen that current induced in the
armature conductors of a DC generator is alternating and to make it
unidirectional in the external circuit we use commutator. Also the flow of
direction of current in the conductor envisages as the conductor's position
changes from one pole to another i.e. as conductors pass out of the influence
of a `N' pole and other that of a `S' pole the current in them is reversed.
This reversal of current takes place along the Magnetic Neutral Axis (MNA).
Thus, commutation is a group of phenomena
related to current reversal in the conductors of an armature winding when they
place through the M.N.A. where they are short-circuited by the brushes placed
on the commutator.
Commutation is said to be good if there is
no sparking between the brushes and commutator when current reversal in the coil
section takes place. Contrary to that, it is said to be poor if there is
sparking at the brushes during current reversal in the coil section.
TYPES OF GENERATORS
In accordance with the method of excitation
D.C. generators are divided into two categories -
1.
Separately excited Generator
2.
Self excited generators.
Since the separately excited generators
have limited application we look forward for self-excited generators.
Generators with self-excitation can be
divided according to the way of the field winding connection into following
categories-
1.
Shunt-excited generators
2.
Series excited generators and,
3.
Compound-wound generators
CHARACTERISTICS OF GENERATOR
There are 3 important characteristics of a
DC generator.
1. Open circuit characteristic (O.C.C.)
2. External characteristic or load
characteristics
3. Internal characteristic or total
characteristic
O.C.C. or NO LOAD CHARACTERISTIC
It represents the relation between
generated E.M.F. and field current. If it is practically the same for all types
of generator whether they are self-excited or separately excited.
EXTERNAL OR LOAD CHARACTERISTIC
It is a curve representing the relation
between the terminal voltage V and the load current IL.
INTERNAL OR TOTAL CHARACTERISTIC
It is a curve, which represents the
relation between the generated EMF.(Eg.) and armature current Ia.
CRITICAL RESISTANCE
The value of that resistance due to which
field resistance line becomes tangent to the O.C.C. curve is called critical
resistance.
SUMMARY
Necessary information regarding operating
principle, constructions, characteristic of DC generators have been given in
this unit. These information will help in maintaining the machines to ensure
reliability and their trouble free functioning. Some information have been
given about commutation of DC machines, which would prove to be important to
understand behavior of DC machines. Sketches and diagrams have been included in
this unit to understand the block with more practical and systematic approach.
SELF-ASSESSMENT EXERCISES
1.
Name different components of a
dc generator and describe their functions.
2.
State the EMF equation of a
generator and mention detail names of different symbols.
3.
Explain the commutation process
of a dc machine with necessary diagrams.
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