Download ELECTROTECHNOLOGY N3

ELECTROTECHNOLOGY N3
PJA Bakker
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Publishers
Preface
This book was written at the request of the publishers and colleagues in vocational training in the Department
of Education and Culture and the Department of Education and Training.
The contents of this book are of such a nature that it should be completed within eight to nine weeks. This
was the express purpose and as a result you may find some sections too concise. I am, however, well aware
that the subject is studied by both electrical and mechanical students who have further study in mind,
specifically Electrotechnics N4.
My intention was to provide basic, essential knowledge for students, especially those who are novices in the
field of electricity and electronics. I suggest that lecturers make constant and adequate use of practical
demonstration as prescribed in the syllabus.
Any textbook has its fair share of errors and shortcomings and colleagues may experience the same in this
work. Therefore I shall appreciate constructive criticism to enable maximum utilisation of its potential by
students and lecturers alike.
The use of supplementary works by lecturers is essential to broaden students' knowledge and for their
thorough comprehension of the subject. As mentioned before, time is a limiting factor; nevertheless, I remain
confident that the contents of this book cover the syllabus to the extent that examination papers should be
attempted with confidence.
In conclusion, my sincere thanks to all those who provided much appreciated support in this task,
especially my wife who typed the manuscript with so much patience.
THE AUTHOR
Foreword
This textbook was written with the express purpose of serving as a guide to the tuition of Electrotechnology
N3 offered by the Department of Education and Culture and the Department of Education and Training. In
effect a long-felt need has been satisfied as this very well illustrated and thorough textbook covers the syllabus
for Electrotechnology N3 and links up with the Engineers' Certificate of Competency.
Mr Bakker is an experienced senior lecturer who regularly obtains very good results in the subject. At present
he is also examiner of the subject and I am convinced that the book will be a boon to lecturers and students
alike. I thank him for a task well done.
A. G. CAWOOD
PRINCIPAL
SASOLBURG TECHNICAL COLLEGE
Contents
1. Direct current machines
1.1
1.2
1.3
Construction of a direct current
machine.
Armature reaction
Commutation.
I
3
4
2. Direct current generators
2.1
2.2
2.3
Operation of a dc generator .
The elementary dc generator ..
Methods of excitation.
7
8
9
Operation of a dc motor
Back emf..
Emf equation of a dc machine.
The speed of a dc motor ...
The torque of an electric motor ..
Types of direct current motors.
Speed control of dc motors.
Direct current motor starters.
Overload protective devices ...
Reversing the direction of rotation
of dc motors
.................
14
15
16
17
18
19
22
24
25
26
Losses in dc machines .....
Efficiency of dc machines .
Determination of efficiency
30
31
32
5. Alternating current theory
5.1
5.2
5.3
5.4
5.5
5.6
5.7
Definitions of alternating current
terms ................................
Generation of an alternating
current ............................
Value of the induced emf .............
Instantaneous value of an alternating
quantity ...........................
Maximum, rms and average
values ..............................
The mid-ordinate rule ..........
Alternating current circuits ...
55
59
60
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
Construction and principle of
operation ...
Double-wound transformer ..
The transformer ratio
Three-phase transformers.
Single-phase transformers connected
in delta and star ...............
Power in three-phase circuits
The auto transformer ...
Losses in transformers
Cooling of transformers.
65
66
66
67
68
69
70
70
70
7. Electronics
4. Efficiency of direct current machines
4.1
4.2
4.3
47
51
52
53
54
6. Transformers
3. Direct current motors
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
Series circuits .
Power and power factor
Active and reactive components .
Resonance in a series circuit.
More examples on series circuits.
Parallel alternating current
circuits ..
5.14 Resonance in parallel circuits
5.15 Three-phase alternating current
circuits ..
5.8
5.9
5.10
5.11
5.12
5.13
37
38
38
39
39
40
44
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
Semiconductor devices ..............
The p-n junction diode ........
Ac to dc conversion (rectification)
Transistors .........................
Transistor configurations ......
The silicon-controlled rectifier
(SCR) .................................
The cathode-ray oscilloscope.
Principles of digital logic ......
Number systems ..... . . . . . . . . . . . .
72
73
74
77
78
79
82
83
89
8. Measuring instruments
8.1
8.2
8.3
8.4
Basic mechanisms ... ...................
Moving-iron instruments ..............
Moving-coil instrument ................
The dynamometer instrument ........
92
93
95
96
8.5
8.6
8.7
8.8
Connection of instruments in
single-phase circuits........... .... . .. ... 98
Connections of instruments in
three-phase circuits. . . . . . . . . . . . . . . . . . . . . . 98
Instrument shunts and series
resistances. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Instrument transformers........... . . 100
Appendices
A
B
C
D
E
---
Syllabus
Definitions
SI system......................................
IEC symbols.....
Exam papers..................................
103
103
104
109
III
1. Direct current machines
The operation of direct current motors and generators is not only the same in principle, but the practical design does not differ much either. A direct
current motor can be used as a generator and vice
versa. However, if we consider the purpose of each
machine and where it is used, there are a few differences in the construction. Generators are normally
installed in buildings where they operate under ideal
atmospheric conditions. They may therefore be of
an open-type design. A motor, on the other hand,
may be installed where it is subject to abnormal
weather conditions, temperature changes, gases, etc.
The result is that often the motor must be constructed
in such a way that it is totally enclosed, and this
requires some way of cooling the motor.
which are either moulded or screwed onto the ends
of the poles, hold the field coils in position. They
also increase the efficiency of the magnetic path.
1.1.2 The armature core
The armature core consists of plate steel or silicon
steel laminated plates which are insulated from each
other with varnish. It is mounted on a shaft and
clamped down with the aid of end plates. Laminated
plates are used in order to reduce eddy currents.
Slots are stamped on the periphery of the core to
accommodate the armature windings (see also Fig.
1.1 ).
1.1.3 The commutator
1.1 Construction of a direct current machine
1.1.1 The stator and field poles
Fig. 1.1 shows the general arrangement of a fourpole dc machine. The yoke P is that part of the
machine which forms the outer casing of the machine
and supports the main field system. It is made of
wrought iron or steel and forms the magnetic circuit
of the main poles. The main poles R are either
moulded with or bolted to the yoke. The pole shoes,
A commutator is an integral part of any dc machine
but its purpose differs as far as generators and
motors are concerned.
A generator must supply a voltage which remains
constant in direction and magnitude. It is therefore
necessary to use a commutator to ensure that a
steady or direct voltage will be obtained from the
alternating emf generated in the rotating conductors.
In the case of a direct current (dc) motor the purpose
of the commutator is to provide a difference in
R
....
stator
FIG. 1.1 General arrangement of a dc machine
_----
polarity between the armature and the field and in
this way produce motion.
A commutator consists of a number of wedge-shaped
segments of hard-drawn copper. The segments are
built up in a cylindrical form, and fixed by V-rings.
They are insulated from one another by thin layers
of mica. The segments are wedge-shaped so that
they will be held in position and not be moved by
the centrifugal force of the armature. The mica between the segments is undercut to ensure the free
movement of the brushes between the segments.
The armature conductors are soldered to each segment.
The types of brushes normally used are the following:
• The electrographite brush: It consists of graphitised carbon. Its coefficient of friction is low and it
has a high current-carrying capacity.
• Graphite brushes: They are made of graphite, are
mechanically weak and therefore not used in large
motors.
• Copper-graphite brushes: They are manufactured
of a compressed mixture of copper and graphite,
and can take a high current density. The degree of
hardness is determined by the amount of copper
that is added.
• Carbon brushes: Carbon brushes are cheap and
are used in motors with a low current density.
1.1.5 Armature windings
~-------F==t -
-
Two methods are used to wind the armatures of
direct current machines:
• lap winding
• wave winding.
The choice of the type of winding is determined by
factors such as the size and function of the machine.
The windings are placed in the slots on the circumference of the armature. In most cases they are
wound beforehand and are then fixed into the slots
with the necessary insulation.
Two very important factors concerning the design
of an armature, and thus also the choice of the
windings, are:
• the pole pitch, i.e. the distance between the centre
of the poles
• the coil pitch, i.e. the distance between the coil
sides.
When machines are designed, the pole and coil
pitches are normally made equal so that the two coil
sides move simultaneously under two different poles.
B
FIG. 1.2 Construction of a commutator
In the above figures A represents the copper segments, B the mica insulation, C the point of connection on the segment, D the mica insulation between
the V-rings and the copper segments, and E the
armature shaft.
1.1.4 Brushes and brushgear
Lap winding
Brushes for use in dc machines mainly consist of a
carbon and graphite mixture. The brushes are obtainable in different grades of hardness which are determined by the percentage of graphite in the mixture.
Care must be taken that brushes of the correct
shape, size and grade are used for a specific machine.
The brushes are normally placed in a brush holder
and kept in position on the commutator by means
of a spring. The correct pressure on the brush is of
great importance, because excessive pressure will
damage the brush and shorten its lifetime. In normal
machines the pressure on the brushes varies_ between
0,7 and 1,1 N/cm 2•
Brushes are "embedded" on the commutator, i.e.
they are ground with emery paper so that they take
on the arc of the commutator. Consequently the
total contact area of a brush is used and this ensures
maximum current flow.
Lap winding is also known as an overlap or parallel
winding. The two ends of each coil are connected to
adjacent commutator segments. The shape of the
lap winding is shown in Fig. 1.3.
The number of parallel circuits formed by a lap
winding is the same as the number of poles. The
total current in the machine is divided equally among
the parallel circuits and therefore this type of winding
is especially suitable for high currents.
The number of brushes and poles in a lap-wound
machine is the same because brushes with the same
polarity are connected.
Most large generators and motors rotating at normal
speed are fitted with lap-wound armatures because
of their ability to carry high currents. Welding
generators are a good example. A lap winding is
normally called a high-current, low-voltage winding.
2
This effect is called armature reaction and it can be
defined as follows:
Armature reaction is the distortion of the main
magnetic field as a result of the current flowing in
the armature conductors, or it is the effect of the
armature ampere-turns upon the value and the distribution of the magnetic flux entering and leaving
the armature core.
FIG. 1.3 The lap winding
Wave winding
In contrast with a lap winding, the two ends of each
coil of a wave winding are not connected to adjacent
commutator segments, but to segments a certain
distance apart (approximately two pole pitches).
The windings are such that only two parallel circuits
are formed and thus only two brushes are used,
irrespective of the number of poles. Each circuit
consists of a number of conductors in series and it
forms half of the total number of conductors on the
armature.
1.2.1 Armature reaction in a dc generator
Let us consider the principle of a simple de generator.
Fig. 1.5 (a) shows the distribution of the field due to
the main poles only. In this case there is obviously
no armature current and the flux is distributed uniformly (N to S).
In Fig. 1.5 (b) an armature is shown in position. The
magnetic field shown is due to the armature current
only and it flows in the direction in which it will
actually flow if the machine is used as a generator.
The direction of this flux is at right angles to the
centres of the pole shoes and the armature core;
therefore the flux caused by the armature current is
called cross flux.
Fig. 1.5 (c) shows the resultant distribution of the
main field and the armature field when the armature
rotates in an anti-clockwise direction. The cross flux
opposes the main flux over the leading halves of the
pole faces and reduces the flux density, while the
flux density is strengthened over the trailing halves
because the cross- and the main flux support each
other. The total flux remains unaltered although the
fluxes are distributed unevenly. As shown in Fig. 1.5
(c), the resultant flux is distorted in the direction of
rotation. This only happens in the case of a direct
current generator.
FIG. 1.4 The wave winding
The emf generated in a wave winding is equal to the
emf induced in one half of the total number of
conductors (due to the two parallel circuits). It can
be deduced that a wave winding is a high-voltage,
low-current winding.
Wave-wound machines are mainly used where low
and medium currents are required.
Owing to this distorted field the brushes must be
shifted in the direction of rotation from the geometric
neutral axis through a certain angle to a position
called the magnetic neutral axis (see Fig. 1.5 (c)).
The purpose is to reduce or eliminate sparking on
the brushes because a voltage will be induced in the
coils if the brushes remain in their original positions,
i.e. at 90 0 with the main field (see Fig. 1.5 (a)).
1.2 Armature reaction
When the armature of any direct current generator
or motor rotates, then the two fields of the machine
are acting upon each other, i.e. the one field has an
influence on the movement and distribution of the
other.
3
()
-
magnetic neutral axis
N
-- -..- -
N
-~
s
(a)
brushes
- geometric neutral axis
(8 = lagging)
FIG. 1.6 Armature reaction in a dc motor
s
N
1.3 Commutation
The emf generated in the conductors of a dc armature is an alternating emf. The current flows in one
direction when the conductor is moving under the N
pole and in the reverse direction when it is moving
under the S pole. This reversal of current in a coil
has to take place while the two commutator segments, to which the coil is connected, are being
short-circuited by a brush. This process is termed
commutation.
As previously mentioned, the purpose of the commutator differs in dc motors and generators. In the
case of the generator the commutator must change
the alternating emf to a direct current. In a motor it
must provide a difference in polarity between the
armature and the field in order to produce motion.
(b)
....
\
s
(c)
geometric neutral axis
1.3.1 Commutation and sparking on the
brushes
«() = leading)
To obtain satisfactory collection of current from a
rotating commutator by means of brushes, it is
essential that the brushes should make good contact
with the commutator. Even with good contact,
sparking may take place. When a coil passes from
one side of the brush to the other it is transferred
from, say, the right to the left of the brush. For a
brief period the commutator segments joined to the
coil are short-circuited by the brush, and it is during
this very short period that the current in the coil has
to be reversed.
In Fig. 1.7 (a) coil A is about to leave the circuit
comprising the coils under the N pole; in Fig. 1.7 (b)
coil A is short-circuited by the brush, and in Fig. 1.7
(c) the coil has been completely transferred to the
circuit of the coils under the S pole. The current has
been completely reversed, compared to the one in
Fig. 1.7 (a).
Fig. 1.5 Armature reaction in a de generator
Distribution of flux due to (a) poles alone;
(b) armature current; (c) poles and armature
current
1.2.2 Armature reaction in a dc motor
Since current flows through the armature conductors
of a motor, a magnetic field will be set up around
the conductors. This magnetic field will also distort
the main field, just as in the case of a generator. It is
therefore obvious that the dc motor is also subject to
the effect of armature reaction.
The direction of distortion of the flux in a motor is
opposite to that in a generator. To eliminate sparking, the brushes of a motor must also be shifted to
the magnetic neutral axis, but in this case against the
direction of rotation (Fig. 1.6).
4
~
This means that the current density has become very
high and an arc is easily formed when the segment
leaves the brush.
By using carbon brushes the contact resistance is
considerably increased and commutation greatly
improved. Various grades of carbon brushes possessing different contact resistances are manufactured;
the most suitable grade for a particular machine is
usually determined experimentally.
I
(a)
~---~
(b) Shifting the brushes: When the brushes of a
generator are moved forward to the magnetic neutral
zone, the short-circuited coils will not be generating
any emf. Reversal of the current in a short-circuited
coil will then be determined by the relative resistance
of that coil and by the areas of contact between
brushes and segments.
It is preferable to move the brushes a little further
than the magnetic neutral zone to allow the shortcircuited coil to cut the fringing flux of the next
main pole.
II.'
\\
(b)
~--I_N
The disadvantage of this method is that, in order to
achieve optimal commutation, the brushes have to
be moved each time the current changes.
To improve commutation in a generator the brushes
must be moved forward in the direction of rotation.
In a motor they have to be moved backwards against
the direction of rotation. The brushes are then placed
on the magnetic neutral plane.
(c)
FIG. 1.7 (a), (b), (c), Commutation
Fig. 1.7 (b) illustrates the position of coil A in the
middle of the commutation period. If, at this instant,
the current is reduced to exactly zero, the commutator segments 2 and 3 will share the current uniformly,
as indicated.
The current in the coils must be reversed at exactly
the correct rate. The armature coils possess considerable inductance, and thus the change in the current
will be retarded automatically. It is therefore essential to support the reversal of the current by arranging that the coils actually cut through a magnetic
field and that they are such that a reversing emf is
established, opposing the emf of self-induction.
(c) Interpoles: It is customary to fit interpoles in all
dc motors and generators, other than the very
smallest types. These are smaller pole pieces placed
between each pair of main field poles, and their
purpose is to ensure sparkless commutation at any
load within the capacity of the machine.
When an armature is operating under conditions of
load, conductors under the main poles tend to establish a flux with opposite polarity to that required for
satisfactory commutation. Winding sufficient turns
on the interpoles neutralises the effect of these
"armature ampere-turns" or armature field.
Since the armature field varies with the load, the
strength of the interpole fields also varies, since they
are excited by the main current.
The correct sequence of poles in a motor is as
follows: the main poles are followed by interpoles of
the same polarity in the direction of rotation (Fig.
1.8 (a». In the case of a geneator with interpoles the
sequence is reversed, i.e. main poles are followed by
interpoles of opposite polarity in the direction of
rotation (Fig 1.8 (b».
1.3.2 Methods of improving commutation
It is obvious that the commutation process is subject
to various factors which may have a negative influence on it. Four methods are normally used to
limit these factors.
(a) Increasing the brush contact resistance: If the
brush contact resistance is made very low a large
current still flows from the segment to the brush.
5
compensating winding
'\
~
motion of armature
FIG. 1.9 Compensating windings
FIG. 1.8 (a) Interpoles in a motor
Exercise 1
1. Draw a neat, fully labelled sketch of a fourpole dc machine.
2. Briefly discuss the following components of a
dc machine
(a) stator
(b) field poles
(c) armature core.
3. What is the function of the brushes in a dc
machine?
4. Discuss four types of brushes normally used in
dc machines.
5. Give a full description of the brushgear, installation and maintenance of brushes of a dc
machine. (Also refer to other sources.)
6. Draw simple sketches of (a) a lap-winding and
(b) a wave winding and discuss each winding
fully. Indicate the pole and coil pitches on the
wave winding.
7. What is meant by armature reaction?
8. Discuss armature reaction fully with the aid of
sketches.
9. Explain what is meant by commutation in (a) a
dc motor and (b) a dc generator.
10. Explain the different methods used to improve
commutation.
FIG. 1.8 (b) Interpoles in a generator
(d) Compensating windings: Direct current motors
which have to operate with a wide variety of speeds
and on which excessive overloads may occur, are
generally equipped with armature-compensating
windings. With a wide range of speed variations it is
essential to run the machine with a considerably
reduced field strength at the top speed, and the
action of the armature ampere-turns would distort
the field were not some special steps taken to prevent this.
The method used consists of making slots on the
faces of the main poles. The windings are placed in
the slots and connected in such a way that the
ampere-turns are equal and opposite to the ampereturns of the armature conductors opposite the pole
(Fig. 1.9).
Motors with armature-compensating windings are
far more expensive than the ordinary, standard
types, and normally compensating windings are only
used in very large machines.
6
2. Direct current generators
The use of dc generators is very limited because
energy is normally generated as alternating current.
In general dc generators are used to provide the
generating supply of ac generators or to change ac
to dc for industrial applications.
through the magnetic flux induces an emf in the
conductor and this can be measured on the voltmeter V.
If the conductor moves downwards, the current
flows in the direction indicated by the arrows. Upward
movement of the conductor results in current flow
in the opposite direction. This observation shows
that the direction of the flow of current depends on
the direction of movement of the conductor. In the
same way a change in the direction of the magnetic
flux is responsible for a change in direction of the
ind uced current.
The magnitude of the induced emf in a conductor
therefore depends on:
• the strength of the magnetic field;
• the rate at which the magnetic flux is cut by the
moving conductor;
• the number of active conductors connected In
series;
• the number of pairs of poles used.
We have mentioned that the direction of the induced
2.1 Operation of a de generator
The principle of electromagnetic induction was discovered by Michael Faraday. After several experiments he defined the concept "electromagnetic induction" as follows:
When a cond uctor cuts a magnetic flux or is cut by
a magnetic flux, an emf is generated in the conductor.
The magnitude of this generated emf is directly
proportional to the rate at which the conductor cuts
the magnetic flux or is cut by the magnetic flux.
In Fig. 2.1 the principle of generation is explained in
an elementary way. The movement of the conductor
v
FIG. 2.1 Principle of generation
7
emf depends on various factors. There are two ways
to deduce the direction of the induced or generated
emf:
• Fleming's right-hand rule
• Lenz's law.
The former is empirical, but the latter is fundamental
in that it is based upon electrical principles.
• Fleming's right-hand rule: If the index finger of
the right hand points in the direction of the magnetic
flux and the thumb is pointed in the direction of
motion of the conductor relative to the magnetic
field, then the middle finger, held at right angles to
both the thumb and the index finger, indicates the
direction of the induced emf (Fig. 2.2).
A
I
/
I
/
./
I
/'
FIG. 2.3 Elementary dc generator
I
B ...- - - - - - - - ( '
I
I
I
I
I
I
I
I
I
I
)
I
I
I
1
+
/
/
--
c
....
o
g
A = index finger (flux)
-go
c
r-------~..------___;.,--­
u
B = thumb (motion)
§
= middle finger (induced emf)
FIG. 2.2 Fleming's right-hand rule
FIG. 2.4 Waveform of induced emf
• Lenz's law: The direction of an induced emf is
always such that it tends to set up current opposing
the motion or the change of flux responsible for
inducing that emf.
This law is used in all electric machines where the
concept of induction applies, e.g. motors and transforme~.
+
2.2 The elementary de generator
'~time
5~-----~!....-_------''''---
Fig. 2.3 shows an armature coil which is connected
to a commutator with two segments. When the
armature rotates the flux is cut by the conductors
and an emf is induced. This is an alternating emf
and the commutator changes the ac to a direct
current. The wave of the induced emf is sinusoidal
(Fig. 2.4) and the output waveform of the dc generator is as shown in Fig. 2.5.
FIG. 2.5 Output wave of a dc generator
8
2.3 Methods of excitation
Excitation of a machine as such refers to the supply
of current to the field winding in order to provide a
magnetic flux for the generation of an emf.
2.3.1 Separate excitation
I~
In a separately excited generator the exciting current
required by the field coils is obtained from an external dc source - usually from another dc generator
or even a battery. Separate excitation is seldom used
with direct current generators but is normal practice
in the case of alternating current generators.
Fig. 2.6 shows a diagram of a separately excited
generator deriving its magnetising current from a
battery. A rheostat is connected in series with the
field winding F of the machine and its purpose is to
control the current through the winding. This results
in the flux also being controlled. The ammeter A I
indicates the current through the winding. This controlled field induces an emf in the armature circuit
which in turn is connected to the load.
With switch S open, the open circuit voltage can be
read on the voltmeter V, and with S closed the
terminal voltage can be read. Ammeter A 2 indicates
the load current.
A small change in field current will result in a large
change in the load current and therefore the current
flow must be controlled very carefully. The field
winding cannot be excited to an unlimited extent
because the rate of increase in flux, and thus also the
emf, decreases as the magnetising current increases.
This is due to the gradual saturation of the iron
parts of the magnetic circuits.
Fig. 2.7 shows the load characteristic of a separately
excited generator.
I
load current
FIG. 2.7 Load characteristic
The terminal voltage gradually decreases with an
increase in load current. This decrease in terminal
voltage is due to armature reaction and the voltage
drop in the armature circuit.
Application: This generator is often used in automatic motor control systems. In these systems the
field current is controlled by an amplifier and the
output is used to drive a motor.
2.3.2 Self-excited generators
In contrast with the separately excited generator,
this type supplies its own generating current.
In the case of the separately excited generator, the
generated emf quickly reaches its maximum value,
while in the case of the self-excited generator it may
take a few seconds before maximum value is reached.
As the field winding of this type of generator is
connected to its own armature (see Fig. 2.8) it is
dependent upon the residual magnetism in the iron.
s
,....----{ A I
-
full load current
I
I
)----.
+
v
F
rheostat
load (changing)
FIG. 2.6 Separately excited generator
9
It can be deduced that the output voltage of a series
generator is dependent on the applied load and thus
the voltage will increase when the load increases.
rheostat
field windings
armature
field windings
load
FIG. 2.8 Self-excited generator
Residual magnetism is the magnetism which remains
in the iron even after the current has been switched
off. As soon as the generator starts turning, the
armature conductors cut through this weak field
and a weak emf is generated. This small emf forces a
small current through the field winding, thus increasing the flux. In this way the flux and emf are built
up to a maximum.
These generators are classified according to the type
of field winding used and may be subdivided in:
• series generators
• shunt generators
• series-parallel or compound generators.
FIG. 2.9 Series generator
A
b
(a) The series generator
In a series generator the field coils, the armature and
the external circuit are all connected in series. This
means that the same current that flows through the
external circuit (load) also flows through the field
coils and the armature. The field current, which is
also the load current, is relatively large and therefore
the required magnetic flux density is obtained from
a small number of field windings. These windings
are normally manufactured of thick wire.
Fig. 2.9 shows a schematic diagram and Fig. 2.10
illustrates the load characteristic of the series generator.
Under no-load conditions there is no current flow
and therefore only a very small emf will be induced
in the armature. This emf is due to the residual
magnetism and is indicated by ab on the curve.
If a load is connected there will be a flow of current
and the field strength and terminal voltage will increase. A further increase in current will increase the
field strength and a high voltage will be generated in
the armature winding. At point A any further increase in load current will not result in a higher
voltage, because the magnetic field has reached saturation point.
a
load current
FIG. 2.10 Load characteristic of a series generator
Application: The series generator is seldom used but
its normal application is as a booster on dc transmission lines. The fact that its supply voltage is
proportional to the armature current makes it suitable for this type of application.
(b) The shunt generator
The field coils and armature windings of a shuntwound generator are connected in parallel (Fig. 2.11).
Owing to this parallel connection the current through
the field coils is determined by the supply voltage
and the field resistance (l
= ~).
The field wind-
ings consist of a large number of turns and a relatively small current is required to prod uce the necessary flux density for the generator.
The basic operation of a shunt generator is as follows:
• The armature of the generator is driven up to the
required speed.
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• The lines of force due to the residual magnetism
in the main poles are cut by the annature conductors.
• The initial cutting of flux ind uces an emf in the
armature conductors.
• This induced emf is applied across the field since
the field is connected in parallel with the armature.
• A current flows in the field coils causing a field
which strengthens the residual field.
• The armature conductors are now cutting a
stronger field and the induced emf increases until
maximum voltage is obtained.
• At this stage the magnitude of the terminal voltage
can be controlled by the rheostat.
field windings
armature
B
C
j
(1)
en
I
~
"0;;>
c;
I
full load
~
c
"§
/
/
~
/
V
I
I
I
/'
A
load current
FIG. 2.13 Load characteristic of a shunt generator
Application: The shunt machine is the type of de
generator most frequently used. However, the load
current must be limited to a value well below the
maximum value, thereby avoiding excessive variation of the terminal voltage. It is therefore used
where a constant voltage is required, e.g. battery
charging and excitation for ac generators.
load
(c) Series-parallel or compound generators
The series-parallel generator is a compound series
and shunt generator and possesses the properties of
both of these machines. There are two sets of field
coils one in series and the other in parallel with the
armature -- and one coil of each are mounted on the
same pole piece (Fig. 2.14).
The circuit diagram is shown in Fig. 2.] 5.
FIG. 2.11 Shunt generator
field current
FIG. 2.12 Open circuit characteristic of a shunt
generator
Load characteristic q( a shunt generator: When a
shunt generator is loaded the maximum terminal
voltage is obtained in a very short period of time.
This is shown by A and B in Fig. 2. 13. When the
load is increased there is a decrease in terminal voltage which is partly due to the increased luRu voltage
drop in the armature winding and partly to armature
reaction. The shunt current is also decreased and
therefore the flux and the generated emf are reduced,
thereby causing a further reduction in terminal
voltage (B to C).
If the load current is increased beyond C there will
be a sharp drop in terminal voltage and therefore C
is the ultimate point for generation.
+
FIG. 2.14 Compound generator - field coils
series field coils
load
FIG. 2.15 Compound generator - circuit
11
overCOll pounded
The shunt coils are connected in parallel with the
armature and consist of many turns of a thin type of
wire. The resistance is high and the current is small
when compared with the armature current. These
coils supply the main magnetic flux as required by
the machine.
The series coils must be able to carry the high
armature current and thus consist of a few turns of
wire with a large cross-sectional area. The resistance
of these coils is low and it does not have a marked
influence on the emf.
level-eompounded
, undercompounded
differential series-parallel
load current
FIG. 2.17 Characteristics of compound-wound
generators
Cumulative compound generator: In this case the
series and the shunt field coils are connected in such
a way that the fields support each other. The two
coils carry currents in the same direction so that the
total magneto-motive force is the sum of the mmPs
in the two coils (Fig. 2.16).
Fig. 2.18 show~ the shunt circuit connected across
the main terminals. This method is called a "longshunt" connection. Alternatively, the shunt winding
may be connected across the armature terminals
and then it is said to be a "short-shunt" connection
(Fig. 2.19).
It is not really important which method is used,
because it has little effect on the ampere-turns of the
machine. The type of connection is determined by
the application of the machine.
...----_ _Gi_cr=-::J
l
shunt field coils
R
series field coils
+
FIG. 2.16 Cumulative compound generator
load . - -
FIG. 2.18 Compound generator - long shunt
The machine reacts to a light load in exactly the
same way as a shunt generator under no-load conditions. When the load increases, the effect of the
series coils automatically increases the excitation.
By suitable regulating it can be arranged that the
series windings provide only the additional excitation necessary to maintain a constant voltage. The
machine is then called "level-compounded".
By having relatively powerful series coils the flux
may be made to increase appreciably as the load
increases. The machine is then regarded as being
"overcompounded". In this case the terminal voltage
is also larger than the no-load voltage.
If the full-load terminal voltage for some reason or
another is less than the no-load terminal voltage the
generator is said to be "under-compounded".
shunt field coils
R
series field coils
IOadl
FIG. 2.19 Compound generator - short shunt
12
Diflerentiaf compound Kenerator: In this type of
generator the direction of the shunt and the series
fields is such that they oppose each other (Fig. 2.20).
Therefore the resulting field becomes weaker and
the terminal voltage falls quickly with an increase in
load current. This is shown in Fig. 2.21.
Application: Compound generators are designed to
eliminate the drop in terminal voltage when the load
is increased. This voltage drop is undesired in feeder
and lighting systems, and over-compounded generators are normally used to compensate for this problem.
Exercise 2
I. Define Faraday's law.
2. Name four factors that determine the magnitude
of the induced emf in a conductor.
3. Explain Fleming's right-hand rule.
4. Define Lenz's law.
5. Explain fully what is meant by (a) separate
excitation and (b) self-excitation.
6. Draw and describe the load characteristic of a
separately excited generator.
7. Draw and explain the load characteristic of a
series generator.
8. What is meant by residual magnetism?
9. Explain the basic operation of a shunt generator.
ID. Give as many applications as possible of (a) the
series generator, (b) the shunt generator and (c)
the compound generator.
II. Explain fully the load characteristic of a shunt
generator.
/2. What is the difference between (a) cumulative
and (b) differential compound generators?
13. Draw neat schematic diagrams of (a) long-shunt
and (b) short-shunt compound generators.
14. Draw and describe the characteristics of an
over- and undercompounded generator.
15. What is meant by "level-compounded"?
+
FIG. 2.20 Differential compound generator
load current
FIG. 2.21 Load characteristic of a differential
generator
13