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The Lineman’s and
Cableman’s Handbook
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About the Editors
Thomas M. Shoemaker (deceased) was a Life Senior
Member of the Institute of Electrical and Electronics
Engineers, a consulting engineer, and the Manager of
the Distribution Department at the Iowa-Illinois Gas &
Electric Company.
James E. Mack is employed at MidAmerican Energy
Company. He is a Senior Member of the Institute of
Electrical and Electronics Engineers, a Member of the
National Society of Professional Engineers, a Member
of the Illinois Society of Professional Engineers, and an
Informational Member of ASTM International.
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The Lineman’s and
Cableman’s Handbook
Thomas M. Shoemaker, P.E., B.S.E.E.
Formerly: Consulting Engineer; Manager, Distribution Department,
Iowa-Illinois Gas & Electric Company; Member, Transmission and
Distribution Committee, Edison Electric Institute;
Captain, Signal Corps, U.S. Army; Life Senior Member,
Institute of Electrical and Electronics Engineers
James E. Mack, P.E., B.S.E.E., M.B.A.
MidAmerican Energy Company; Senior Member, Institute of Electrical and
Electronics Engineers; Member, National Society of Professional Engineers
Thirteenth Edition
New York Chicago San Francisco
Athens London Madrid
Mexico City Milan New Delhi
Singapore Sydney Toronto
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In Memoriam
Sadly, Thomas M. Shoemaker died on April 6, 2014.
Tom was a mentor, editor and author, and close friend.
In 1970, he joined Professor Edwin B. Kurtz (University
of Iowa) as co-author of The Lineman’s and Cableman’s
Handbook. Following Dr. Kurtz’ retirement, Tom was the
editor and author for the 5th through the 8th editions of
this work. He transferred the principal duties of editing
and authorship to me for the 9th and subsequent editions of this work. Tom was inducted into the International Lineman’s Museum Hall of Fame, Shelby, North
Carolina, in 2008.
jem
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Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ix
xi
xiii
1
Elementary Electrical Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2
Electric System
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
3
Substations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
4
Transmission Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
5
Distribution Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
109
6
Construction Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
121
7
Wood-Pole Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
131
8
Aluminum, Concrete, Fiberglass, and Steel Structures and Poles . . . .
153
9
Distribution Automation and the Smart Grid . . . . . . . . . . . . . . . . . . . .
175
10
Emergency System Restoration
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
189
11
Unloading, Hauling, Erecting, and Setting Poles . . . . . . . . . . . . . . . . .
203
12
Guying Poles
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
245
13
Insulators, Crossarms, and Conductor Supports . . . . . . . . . . . . . . . . .
279
14
Line Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
309
15
Distribution Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
351
16
Lightning and Surge Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
379
17
Fuses and Fuse Coordination
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
399
18
Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
429
19
Tap-Changing Transformers and Voltage Regulators
. . . . . . . . . . . . .
443
20
Transmission Tower Erection
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
453
21
Stringing Line Conductors
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
465
22
Sagging Line Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
475
23
Joining Line Conductors
497
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vii
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viii
Contents
24
Live-Line Maintenance with Hot-Line Tools . . . . . . . . . . . . . . . . . . . . .
519
25
Live-Line Maintenance from Insulated Aerial Platforms . . . . . . . . . .
551
26
Grounding
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
561
27
Protective Grounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
577
28
Street Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
591
29
Underground System
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
613
30
Laying Conduit
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
643
31
Manhole Construction
32
Pulling Cable
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
661
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
675
33
Splicing and Terminating Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
689
34
Underground Distribution
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
709
35
Vegetation Management: Tree Trimming . . . . . . . . . . . . . . . . . . . . . . . .
759
36
Distribution Transformer Installation, Selection, and Utilization . . . .
779
37
Electrical Drawing Symbols
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
813
38
Single-Line Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
817
39
Schematic Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
823
40
Voltage Regulation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
835
41Units of Measurement, Electrical Definitions,
Electrical Formulas, and Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . .
851
42
Maintenance of Transmission and Distribution Lines
. . . . . . . . . . . .
873
43
Rope, Knots, Splices, and Gear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
899
44
Climbing Wood Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
933
45
Protective Line and Personal Protective Equipment
. . . . . . . . . . . . . .
957
46
OSHA 1910.269 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
977
47
Resuscitation
48
Heart-Lung Resuscitation
49
Pole-Top and Bucket Truck Rescue
50
Self-Testing Questions and Exercises
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1091
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1103
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115
. . . . . . . . . . . . . . . . . . . . . . . . . . . 1125
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1133
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Preface
T
his Handbook is written for the apprentice, the lineman, the cableman, the foreman, the supervisor, and other employees of electric line construction contractors
and transmission and distribution departments of electric utility companies. It is
primarily intended to be used as an apprenticeship textbook and a home-study book to
supplement daily work experiences. This Handbook has 50 chapters; 11 chapters are
devoted to a general understanding of electricity, electrical terms, and electric-power
systems; 31 chapters are devoted to actual construction of overhead and underground
distribution and transmission lines and to maintenance procedures; and 7 chapters are
expressly devoted to safety guidelines. The final chapter can be used by the reader as a
self-examination of the information presented.
All chapters in this Thirteenth Edition have been revised where necessary to be
consistent with the newest equipment, techniques, and procedures. A special effort was
made to present all discussions clearly and in simple language. As in previous editions,
a large number of illustrations showing the construction and maintenance processes are
provided to assist the reader in a better understanding of the text. The illustrations
clarify many details that would require additional words to express. Many of the photographs were taken specifically for use in this edition. They portray the practices in use
by some of the foremost electric utility and contracting companies in the United States.
Methods of transmission-, distribution-, and rural-line construction have become
quite standardized since the First Edition of the Handbook was published in 1928. The
construction procedures described and illustrated are in most instances representative of
general practice. While each operating company has its own standards of construction to
which its linemen and cablemen must adhere, the procedures described explain why
things are done in a given way. Such basic knowledge will be helpful to the lineman or
cableman who is interested in learning the whys and wherefores of doing things one
way or another.
Safety is emphasized throughout this book. Of course, understanding the principles
involved in any operation and knowing the reasons for doing things a given way are the
best aids to safety. The opinion has become quite firmly established that a person is not
a good lineman unless he does his work in accordance with established safety procedures and without injury to himself or others. It is necessary for those engaged in electrical work to know the safety rules and the precautions applicable to their trades, as
specified in the National Electrical Safety Code, Occupational Safety and Health Act
(OSHA) standards, and their employers’ safety manuals and standards, and to make the
observance of safety rules and procedures an inseparable part of their working habits.
ix
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x
Preface
This Handbook places emphasis on the National Electrical Safety Code, OSHA standards, American National Standards Institute (ANSI) standards, and ASTM International (formerly known as the American Society for Testing and Materials) standards.
Important requirements of all of these are discussed, and all of these should be studied
for detailed work procedures. Many applicable codes and standards are referenced
throughout the text to assist the reader.
The lineman and the cableman must become acquainted with the minimum construction requirements and maintenance and operating procedures in the various
codes and standards to ensure the safety of the public and all workers. It is necessary
that all linemen and cablemen know the information in the National Electrical Safety
Code (ANSI C2) and that they adhere to the rules and procedures while performing
their work assignments.
The National Electrical Code details the rules and regulations for electrical installations, except those under the control of an electric utility. It excludes any indoor facility
used and controlled exclusively by a utility for all phases from generation through
distribution of electricity and for communication and metering, as well as outdoor
facilities on a utility’s own or a leased site or on public or private (by established rights)
property.
Reference material for the interested reader includes Standard Handbook for Electrical
Engineers, edited by Donald G. Fink and H. Wayne Beaty and published by McGraw-Hill;
National Electrical Safety Code (NESC) Handbook, by David J. Marne, P.E., published by
McGraw-Hill; IEEE Standard Dictionary of Electrical and Electronic Terms; Powerlineman
Magazine; and the Electric Power Research Institute’s (EPRI) publications.
The editor is well aware that one cannot become a competent lineman or cableman
from a study of the pages of this book alone. However, diligent study along with daily
practical experience and observation should give the apprentice an understanding of
construction and maintenance procedures—and a regard for safety—that should make
his progress and promotion rapid.
In conclusion, the lineman occupation has been properly recognized and chronicled in the film documentary series Storm Soldiers I and Storm Soldiers II. In 2006, the
International Lineman’s Museum, Shelby, North Carolina, established the Lineman’s
Hall of Fame.
James E. Mack
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Acknowledgments
I
wish to express my sincere appreciation to the many companies and their representatives who kindly cooperated in supplying illustrations, data, permissions to
publish, and valuable suggestions. I am especially grateful to David J. Marne of
Marne and Associates, Inc.; Dan Snyder of the American Iron and Steel Institute; Brian
Schaaf of the Cordage Institute; Austin Henry, Ryan Norlin, Andrew McNamara,
Derek Wahlheim, and Dave Litterst of MidAmerican Energy Company; Connie Hartline
of the American Public Works Association; Andy Price and Murray Walker of the
International Lineman’s Museum & Hall of Fame; Dave Bezesky of Merchant Job
Training and Safety; and numerous unnamed others.
James E. Mack
xi
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Lineman working from insulated bucket. The energized primary line conductor is covered with line
hose. The lineman has positioned himself for convenient access to the work being completed.
(Courtesy Pepco Technologies.)
The terms lineman and cableman, long-established and still current in the industry, are
beginning to be replaced by non-gender-specific titles in official documents and government publications. Both men and women are employed in these capacities in the
military and in the industry. To avoid awkwardness, this Handbook uses the masculine
pronoun, but it in no way implies that the jobs involved are held only by men.
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Introduction
L
inemen and cablemen construct and maintain the electric transmission and distribution facilities that deliver electrical energy to our homes, factories, and commercial establishments. They provide important skilled services to the electrical
industry—important because the health and welfare of the public are dependent on
reliable electric service.
When emergencies develop as a result of lightning, wind, or ice storms, linemen
and cablemen respond to restore electric service at any time of the day or night.
An understanding of electrical principles and their application in electrical construction and maintenance work is essential to completing the work safely, efficiently,
and reliably. New equipment and the public’s increased dependence on continuous
electric service require that all linemen and cablemen be highly skilled.
The time and effort spent studying these pages will increase the reader’s knowledge of electric distribution and transmission facilities and improve his skills. Every
A hot crew with their hot wagon. This photograph is from the 1930s when live line work was in its
infancy and was specialized at many companies. Lineman working on these crews developed
many effective methods and tools that are still used today.
xiii
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xiv
Introduction
lineman, cableman, and groundman should develop the highest possible level of skills
so that he will be able to meet the challenges of the work and be qualified for promotion
when the opportunity becomes available.
As a testament to the significance of the role that the lineman has had in the
United States, the following summary was submitted by Alan Drew, Northwest Lineman College, author of The American Lineman.
Evolution of the Lineman
In the 1840s, the use of the telegraph as a means of communication started in the
United States. To obtain the benefits of this enhanced method of communication, lines
would have to be constructed and maintained. It was found that telegraph lines could
be strung on trees if they were available but that wood poles provided the best method
of supporting the lines. The expansion of the telegraph system required men who
could set poles and then climb them to string the wire. The term “lineman” quickly
evolved as a title for those who worked on telegraph lines.
In the late 1870s the telephone was invented, and it began to replace the telegraph
as a means of communication. The telephone also needed lines to be constructed, which
were similar to the telegraph lines except they utilized more wires for the needed
circuits. The term “lineman” was well established and carried on into the telephone era.
In the late 1890s electric power started to become a useful form of energy, and
power plants and lines were built. This new form of energy immediately proved to be
considerably more hazardous than the telegraph and telephone systems. A new breed
of the “lineman” was now needed to work on these lines. These linemen took, and
were expected to take, many risks in working on power lines and equipment. A high
level of injuries occurred because of the limited training, and lack of proper equipment, construction standards, and safety rules. Although records are sketchy, in some
areas it has been said that one of every three linemen was killed on the job, and mostly
from electrocution.
As the power system evolved across the country there were many large line building projects, which resulted in linemen “booming around” from project to project.
These early linemen quickly established a reputation as individuals who worked hard,
took many risks, played hard, and took pride in their work.
In the late 1930s, the complexity of the lineman’s job was recognized as a good fit
for apprenticeship training. This resulted in the establishment of the apprentice lineman, and soon programs started to evolve across the country. This was a significant
step in the establishment of more formalized training to develop competent linemen.
The use of electricity brought with it a higher quality of life at home and the ability
to be more productive for businesses. When power outages occurred, the impact on
customers started to become significant. Linemen would quickly respond to these outages and make Herculean efforts in all types of weather to restore power. They soon
established a reputation as “heroes” in the eyes of many customers.
The early linemen established their reputation while working mainly on wood pole
lines. In the late 1950s, underground started to evolve as a popular and reliable way to
deliver power, and it soon became part of the lineman’s work. This added a new dimension and more complexity to the lineman’s job.
Today’s linemen face challenges similar to those of the past; however, today’s
customers rely heavily on the continuous delivery of power and are less tolerant of
outages. This results in considerably more work on energized lines than in the past.
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Introduction
xv
The lineman of today has a wide array of enhanced vehicles, tools, equipment, and
training to meet these challenges. In addition, OSHA rules, standards, and procedures
have greatly improved.
Linemen of today take pride in their work and remain heroes in the eyes of many
customers when their power is restored after a long outage. As we look back at the vast
amount of experience and knowledge that has been gained from the efforts of the pioneering linemen, it is appropriate to commit to maintaining their legacy.
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The Lineman’s and
Cableman’s Handbook
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CHAPTER
1
Elementary Electrical
Principles
Electron Theory
The basis of our understanding of electricity is the electron theory. This theory states
that all matter, that is, everything that occupies space and has weight, is composed of
tiny invisible units called atoms. Atoms in turn are subdivided into still smaller particles called protons, neutrons, and electrons. The protons and neutrons make up the
central core, or nucleus, of the atom, while the electrons spin around this central core in
orbits as illustrated in Fig. 1.1.
The protons and electrons are charged with small amounts of electricity. The proton
always has a positive charge of electricity on it, while the electron has a small negative
charge of electricity on it. The magnitude of the total positive charge is equal in amount
to the sum of all the negative charges on all the electrons. The neutron has no charge on
it, either positive or negative, and is therefore neutral and hence called neutron.
Atoms differ from one another in the number of electrons encircling the nucleus.
Some atoms have as many as 100 electrons spinning around the nucleus in different
orbits. The atom of hydrogen gas has only 1 electron. The atom of lead has 82 electrons.
Positive and negative charges of electricity attract each other; that is, protons attract
electrons. But the atom does not collapse because of this attraction. The spinning of the
electron around the nucleus causes a centrifugal force that just balances the force of
attraction and thus keeps them apart.
Electric Current
The electrons in the outermost orbit of an atom are usually not securely bound to the
nucleus and therefore may fly off the atom (Fig. 1.2) and move into an outer orbit of
another atom. These relatively free electrons normally move at random in all directions.
However, when an electrical pressure (voltage) is applied across a length of wire, the
free electrons in the wire give up their random motion and move or flow in one general
direction. This flow of free electrons in one general direction, shown in Fig. 1.3, is called
an electric current or simply current.1
By convention, however, the flow of current in a circuit is taken to be opposite in direction to that of
the electrons.
1
1
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2
Chapter One
Figure 1.1 An atom consisting of the
nucleus and revolving electrons. The nucleus
is composed of protons and neutrons. The
protons carry a positive electrical charge, the
electrons carry a negative electrical charge,
and the neutrons are neutral; that is, they
carry neither a positive nor a negative
charge.
Figure 1.2 Atom showing electron in outer
orbit leaving atom. The atom then has more
positive charge than negative charge. The
nucleus will therefore attract some other free
electron that moves into its vicinity.
Figure 1.3 Flow of free electrons in a conductor. Only electrons in the outer orbit are free to
move from one unbalanced atom to another unbalanced atom. This flow or drift of free electrons
is called an electric current.
Conductors and Insulators
Materials having many free electrons, therefore, make good conductors of electricity,
while materials having few free electrons make poor conductors. In fact, materials
that have hardly any free electrons can be used to insulate electricity and are called
insulators. Samples of good conductors are copper and aluminum. Samples of good
insulators are glass, porcelain, rubber, paper, polyethylene, and fiberglass.
Electric Circuit Compared with Water Circuit
An electric circuit is the path in which the electric current flows. The flow of electricity
in a wire is actually the simultaneous motion of countless free electrons in one direction.
It is often compared with the flow of a liquid like water. Electricity can then be said to
flow in a wire as water flows in a pipe. A simple water circuit, like the one shown in
Fig. 1.4, has a resemblance to a typical electric circuit, shown in Fig. 1.5. The similarity
between the water circuit and the electric circuit can give one an understanding of the
flow of electric currents.
Figure 1.4 illustrates water that is flowing in the pipe circuit in the direction shown
by the arrows. It is evident that this current of water flows because of a pressure exerted
on it. This pressure is produced by the rotary pump, often called the centrifugal pump,
which is driven by an engine. A water motor is connected on the end of the pipeline,
and therefore, all the water that flows around the circuit must pass through the motor.
It is plain that it will cause the motor to revolve and, therefore, deliver power to the
shaft and the rotating equipment connected by the shaft. Similarly, when an electric
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Elementary Electrical Principles
3
Water pipe
Rotary pump
powered by
gasoline engine
Water
motor
Rotary
pump
Water pipe
Generation
Transmission
Distribution
Figure 1.4 The water system.
Generation
Transmission
Distribution
Motor M
G Generator
Electric wire
Figure 1.5 The electric system.
current flows in a wire, it flows because an electric pressure causes it to flow. Thus, the
current in Fig. 1.5 is made to flow because of the electric pressure produced by the
dynamo, or electric generator, which is driven by an engine. As the electric current
flows along the wire, it will be forced to flow through the electric motor. This motor will
begin to revolve as the electricity begins to flow through it and will deliver power to the
shaft and the rotating equipment connected to the shaft.
Series Circuit
An electric circuit can be arranged in several ways, as long as the path for the electric
current is closed. The simplest arrangement is the so-called series circuit. The series
circuit has all the elements of the circuit connected onto each other, end to end, as
illustrated in Fig. 1.6. The same current from the battery flows through all the resistors.
Figure 1.6 A series circuit. The same
current flows through all the resistors.
R1
R2
+
120 V
–
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4
Chapter One
Parallel Circuit
Another arrangement is the so-called parallel or multiple connection. Each resistor is
individually connected across the battery, as shown in Fig. 1.7, instead of all the resistors
being connected onto each other, end to end, and then onto the battery. The resistors are
now said to be in parallel with each other.
Series-Parallel Circuit
A third arrangement is the combination of the series and the parallel circuits. Part of the
circuit is in series and part in parallel, as shown in Fig. 1.8. The same current flows
through each element of the series portion of the circuit. The current in the parallel part
of the circuit divides, and only a portion of the current flows through each of the parallel paths.
Electric System
The circuits in Figs. 1.4 and 1.5 can be seen to consist essentially of three main divisions. The section where the electricity is produced is called the generator section. That
part of the circuit which furnishes the path for the current from the place where it is
generated to where it is ultimately used is the transmission section, and the section
where the electricity is distributed and used is the distribution division. In the distribution division, electricity is used to light, heat, cool, and power. An actual electric
circuit has three parts: a generating station, a transmission line, and a distribution
system. An electric circuit with the different components is called an electric system.
The wires of the system serve to carry the electricity just as highways carry automobiles and railroad tracks carry trains. The reason one does not see the electricity moving
along the wire is because it is invisible. The wires and transformers appear lifeless;
however, they are very much alive and ready to do almost any work for us.
One should look on the generation, transmission, and distribution of electrical
energy as one does on the manufacture, shipment, and delivery of goods. Electricity is
different from a manufactured product, like shoes. The manufacturer of shoes can
Figure 1.7 A parallel circuit. Each
resistor is independent of the other
resistors and draws its own current.
Figure 1.8
+
120 V
–
R1
A series-parallel circuit.
R3
R2
R2
R4
+
120 V
–
01_Mack_Ch01_p001-030.indd 4
R1
R3
R5
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Elementary Electrical Principles
5
estimate the demand for shoes and then manufacture them in advance and put them in
a warehouse. Electricity has to be manufactured or generated at the very instant when
it is wanted. The customer flips a light switch to the on position or turns on the electric
range and an order is flashed back through the distribution system (the retail outlet),
the substation (the warehouse), the transmission line (bulk transportation), to the generator (the factory), and delivery of electricity must be made immediately.
It is important to observe that the current path or the transmission line must have a
return wire just as water must have a return pipe. The water passes out along the pipe
in one side of the circuit, through the water motor, where it does its work, and then
returns to the rotary pump in the other pipe (Fig. 1.4). The electricity passes out along
one wire to the motor, does its work, and then returns to the generator in the other wire
(Fig. 1.5), a path similar to the water circuit.
Electric Current
It has been pointed out that the flow of electricity in a wire is similar to the flow of water
in a pipe. When water flows in a pipe, one speaks of a current of water or a water current. Similarly, when electricity flows in a wire, it is called an electric current.
Ampere (Amp)
Generally, we want to know how much water is flowing in the pipe, and we answer by
saying “10 gal/sec.” In the same way, we can express how much electricity is flowing
in a wire by saying “25 amperes.” The ampere (amp) is the unit of electric current. One
can learn how much an ampere is by watching what it can do. An ordinary 60-watt,
120-volt incandescent lamp will require ½ amp. This means that ½ amp is flowing
through it all the time that it is glowing.
Ammeter
If we wish to measure the current of water flowing through a pipe, we place a meter
right in the pipeline. A meter for measuring the flow of water in a pipe is called a flowmeter. When such a meter is placed in the line, water flows through it, and the meter
indicates the number of gallons per second which pass through it. It is clear that the
meter must be inserted in the pipe so that the water flows through it. The number of
amperes of electric current flowing in a circuit can be measured by connecting a current
meter or an ampere meter in the circuit, as shown in Fig. 1.9. Since such a meter is to
read amperes, it is called an ampere meter or an ammeter. It should be noted that the
ammeter is inserted in the line in order that all the current taken by the motor may pass
through the meter.
Figure 1.9
circuit.
Ammeter in an electric
A
R1
+
120 V
–
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6
Chapter One
Electric Pressure
We know that a pressure causes a current of water to flow in a pipe. Likewise, in order
for current to flow in a circuit, an electric pressure must be present. Voltage is the electric
force or pressure that is required. Voltage is determined by the potential difference
between any two points in a circuit. Generators and batteries are devices that supply
voltage to circuits.
A hand valve in the water circuit will stop the flow of water if it is closed. The water
pressure would still be there, but water would not flow through the water motor or the
pipe. This can be demonstrated by turning off the water faucet in the kitchen sink. The
water flow ceases, but the pressure is still there. A switch placed in the electric circuit
will likewise prevent the flow of current if it is in the open position. The electric pressure will still be there with the switch open if the generator is being driven by the
engine. Thus, there can be pressure and no current.
Volt
One must be able to talk of its strength to learn something about electrical pressure.
This requires a unit with which to measure it; that unit is the volt. One volt will cause
1 amp to flow when impressed across a 1-ohm resistor. We can learn how much a volt
is by observing what it can do. We can note, for example, how much pressure or how
many volts are required, in general, to force a current through a doorbell, an iron, a
washing-machine motor, an electric range, a small factory motor, and a large factory
motor. The most common values are as follows:
An electric doorbell requires 2 volts to 5 volts.
An electric iron requires 120 volts.
A washing-machine motor requires 120 volts.
An electric range requires 240 volts.
A small factory motor requires 208 volts, 240 volts, or 480 volts.
A large factory motor requires 2,400 volts to 7,200 volts.
These values simply mean that so many volts are required to push or force the working
current through the devices or machines.
One can gain an idea of the strength of electric pressure by observing how much
of a shock is received when one puts one’s hands across the two wires of a circuit.
Under dry conditions, a person cannot detect or feel as little as 5 volts of electrical
pressure but can feel 50 volts. At 120 volts, nearly everyone will get a very unpleasant shock, even when a very light and brief contact is made with the wires. It may
prove fatal if a firm contact is made with the wires energized at 120 volts. If the
hands should be moist or wet and if a firm grasp is made across 120 volts, death is
likely to result. All voltages should be considered as dangerous and handled with
great care. All higher voltages should be well guarded, and no one except an authorized, fully trained, and qualified person has any business getting near them.
System voltages of 600 volts or less are classified as low voltages or utilization
voltages in the American National Standards Institute (ANSI) standard C84.1 for
Electric Power Systems and Equipment—Voltage Ratings (60 Hz). System voltages
of over 600 volts to 69,000 volts are classified as medium voltages, distribution
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Elementary Electrical Principles
7
voltages, or subtransmission voltages. System voltages of over 69,000 volts to
230,000 volts are classified as high voltages or transmission voltages. System voltages of over 230,000 volts to 1 million volts are classified as extra-high voltages
(EHV) or transmission voltages. Voltages of 1 million volts or more are classified as
ultra-high voltages (UHV) or experimental voltages.
Voltmeter
If we wish to measure the pressure in a water circuit, all we do is tap a pressure gauge
onto the pipeline. Everyone is familiar with such a gauge. The few points to be noted
are that the gauge is simply tapped on the pipeline at the point at which the pressure
is wanted so that the pressure at that place can get up into the gauge and make it
indicate. It is also evident that flow of water in the pipe is not disturbed by insertion
of the gauge.
In the same manner, we can measure electric pressure. Electric pressure or voltage
is measured utilizing the voltmeter. The voltmeter measures in volts the potential
difference between two points in an electric circuit. We simply connect the two leads
from a voltmeter across the line, as shown in Fig. 1.10. The current through the
voltmeter will then vary directly with the voltage, and the meter can be made to read
volts. It is to be noted that the current which flows to the motor does not flow through
the voltmeter. This is because the voltmeter is not a part of the circuit as the ammeter
is. Figure 1.11 shows the two meters, the ammeter and voltmeter, properly connected.
The ammeter reads the flow of current, and the voltmeter reads the pressure which
causes the current to flow.
Water Power
We have likened an electric current to a current of water. When a current of water
flows in a pipe in a simple circuit, as shown in Fig. 1.4, power is delivered to the
water motor. We know this because the water motor revolves and can do work. The
power delivered depends on the amount of water flowing and the pressure under
Figure 1.10
circuit.
Voltmeter in an electric
R1
+
120 V
–
V
Figure 1.11 Ammeter and voltmeter
correctly connected.
A
R1
V
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8
Chapter One
which it flows. This is self-evident, for more power will be developed if 50 gal/sec
flows through the water motor than if only 25 gal/sec flows through it. Furthermore, more power will be developed with 100 lb of pressure than with only 50 lb of
pressure. The power delivered in the pipeline to the motor thus depends on the
amount of water flowing and on the pressure.
Electric Power
The amount of power delivered by an electric circuit to an electric motor depends on
the number of amperes flowing and the number of volts of pressure in exactly the
same way as in water power. The greater the current, the larger the number of
amperes, the greater will be the amount of power developed by it; and the greater the
pressure, the more effect the current will have. The actual value of power in a directcurrent circuit (not true for an alternating-current circuit) is equal to the product of
volts times amperes; thus,
Power = volts × amperes
Watt
The unit of power in an electric circuit is the watt. An ordinary incandescent electric
lamp, when connected to an electric circuit, as in Fig. 1.12, will draw about 150 watts
from the circuit. An ordinary iron, when connected to a circuit, will draw about
550 watts of power from it. The motor shown schematically in Fig. 1.13 will draw
about 5,600 watts of power. It is plain that when we come to large machines, the
number of watts runs up quickly. Thus, the utilization of decimal system units to
quantify amounts is preferred. The decimal system is based on units and powers of
10. The unit for 1,000 is called kilo; therefore, 1,000 watts is called 1 kilowatt. This unit
Figure 1.12 Electric lamp taking
1.25 amps and 150 watts from 120-volt
circuit.
Figure 1.13 Motor drawing 23.4 amps
of current and 5,600 watts of power.
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Elementary Electrical Principles
9
is abbreviated kW. A kilowatt is equal to about 1-1/3 hp. The motor in Fig. 1.13 thus
draws 5.6 kW of power, or
5.6 kW × 1-1/3 hp/kW = 7½ hp.
Wattmeter
We have to measure electric power to know how much power any device or apparatus
is drawing from the line. A wattmeter registers watts or kilowatts, and by reading it one
can tell how much power any piece of apparatus is consuming. The amount of electric
power delivered by a circuit depends on the amperes flowing and the volts of pressure.
The meter must be connected so that the entire load current flows through it and the
voltage pressure is across it. A wattmeter is essentially a combination of two instruments, an ammeter and a voltmeter. It has an ammeter coil of low resistance, which is
connected into the circuit, and a voltmeter coil of high resistance, which is connected
across the circuit. A wattmeter will have four terminals or binding posts, two for the
current-coil leads and two for the voltage-coil leads.
Electric Energy
Electricity must act for a period of time in order to do useful work. The power expressed
in watts tells how much electricity is working, and the hours express the time during
which it acts. The product of these two factors gives the amount of work done.
Power × time = energy
or
Watts × hours = watthours
Likewise, 1,000 watthours equals 1 kilowatthour which is abbreviated 1 kWh.
A kilowatthour can thus be thought of as
1,000 watts acting for 1 hour = 1 kWh
Any other combination of volts, amperes, and hours whose product is 1,000 would
give 1 kWh of energy.
Watthour Meter
The total amount of electrical energy consumed over a period of time, such as a day
or month or year, is indicated by a watthour meter. We are all familiar with the common electric house meter. This device shows how much energy the lights, the television, the stereo, the iron, the toaster, the washing machine, etc., consume in the
course of a month. Solid-state watthour meters measure more than just consumption
of the kilowatthour. Main printed circuit boards are installed in each meter and can
be programmed to provide integrations and calculations that measure the peak and
average kilowatt demands as well as recording voltage swells, voltage surges, and
power interruptions.
Conductors
The pipes merely furnish the path for the flowing water in the water circuit. We would
say that the hole in the pipe was the path of the water if we were to be more exact.
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10
Chapter One
We know the water does not flow through the iron shell of the pipe. It actually passes
through the opening in the pipe inside the iron shell. The hole or opening in the pipe
may then be directly compared to the electric wire. The electricity, however, flows right
through the hard wire. It does not need an opening. It passes right through the metal.
But we find it can travel with less difficulty through some metals than through others.
It travels easily through copper; therefore, we often use copper wire. Copper is therefore called a good conductor. Any metal substance that offers little resistance to the flow
of electricity is called a good conductor.
Electricity passes still more easily through silver, but silver is too expensive to be
made into wires. Aluminum has been used extensively on long transmission lines
because of its rather good conduction and its light weight, making long spans between
poles or towers possible. It is in general use in distribution lines as well. Iron is not a
good conductor but is used as a supporting wire in parallel with or coated by copper or
aluminum.
Insulators
It was pointed out earlier that the hole in the pipes serves as the path for water. This
does not mean to imply that the shell of the pipe is useless and unnecessary. It is very
necessary. The shell serves to hold the water in its path. A pipe without a shell would
never transmit any water. There would not be any pipe, and the water would flow
everywhere. It is important to remember that the shell keeps the current of water in
its path.
In an electric circuit, there must be something to keep the current from leaving the
wire. The metal of the wire is its path, but there must be something to keep it from leaving the metal. For overhead installations, bare conductors are separated sufficiently
from each other based upon system voltages as defined in the clearance tables of the
National Electrical Safety Code (NESC). The air serves as a nonconductor of electricity
(a shell, or an insulator, to be more precise). Air has fewer free electrons than the wire
conductor and for this reason does not normally conduct electricity. For underground
and some overhead applications when the overhead conductors are desired to be placed
in closer proximity to one another due to space limitations than the NESC allows, a
shell is put around the wire just as in the case of the water pipe. This shell, however, is
not iron but is usually a rubber or polyethylene compound material. The layer of rubber
or polyethylene compound material is called insulation, and a wire so covered is called
an insulated wire.
Nonmetallic-sheathed cable—which is an assembly of two or more insulated conductors having an outer sheath of moisture-resistant, flame-retardant, nonmetallic
material—is usually used for housing wiring where the voltage is 120 or 240 volts.
Higher voltages are insulated with rubber or polyethylene.
Wires commonly lay on glass, porcelain, or polymer insulators installed on wooden
or porcelain crossarms. The crossarms are attached to poles. Glass, porcelain, and polymers are good insulators of electricity. Bare wires are carefully mounted on glass,
“petticoat” (named because of their resemblance to a skirt) porcelain insulators, or
polymer insulators to keep the wires from touching each other or the ground, thus
preventing the electricity from leaving the wires.
High-voltage transmission lines may be constructed with polymer suspension insulators. The suspension insulators are high strength, light weight, and durable. The
greater the length of the insulator, the higher the voltage it can withstand.
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Elementary Electrical Principles
11
Resistance
One can draw up an ice cream soda faster and with less exertion through a short
and large straw than through a long and small straw. The water circuit of Fig. 1.4
makes it easy to understand that less water will flow if the pipe is long and has a
small opening than if it is short and has a large opening, provided the pressure is
the same in both cases. This is to be expected, for in the long pipe the friction is
greater because of the greater length, and if the pipe is small in diameter, the friction
will be still greater because of the smaller space, through which the water must be
forced. It may be said that a long, narrow pipe offers more resistance to the flow of
water than a short, wide pipe. Likewise, an electric current flows along the path of
least resistance.
An electric conductor offers a resistance to the flow of current determined by its
length and its thickness or diameter. More friction must be overcome if the wire is very
long than if it is short; if it is of small cross section, it will take still more effort to crowd
the current through the wire.
Ohm
The unit of resistance for wire is the ohm (Ω). We can picture how much an ohm is by
noting how many feet of wire a given size takes to make an ohm of resistance. Wires
used for electric purposes are supplied in regular sizes of specified diameter. The
no. 10 copper wire has a diameter of about 1/10 inch and has 1 ohm of resistance for
each 1,000 ft of length. Thus 5,000 ft of this wire would have 5 ohms resistance. A wire
whose cross-section is one-half as large would have a resistance of 2 ohms for each
1,000 ft of length.
Ohm’s Law
Resistance reduces the amount of current that will flow in a circuit. The number of
amperes that will flow in a circuit will not be determined completely by the voltage or
pressure which causes the current to flow, but by the amount of friction or resistance in
the wires. The greater the resistance of a circuit, the smaller the current will be that flows;
and the smaller the resistance, the greater the current will be in a circuit with a constant
voltage. This general relation between voltage, current, and resistance is commonly
called Ohm’s law. It is a law because it has been found to hold true in every case. It is
written thus:
Current =
voltage
resistance
or
Amperes =
voltage
current
or
Ohms =
Resistance =
volts
ohms
volts
amperes
Voltage (V) = current (I) × resistance (R) or V = IR or Volts = amperes × ohms
The law as stated above applies only to direct-current circuits, circuits in which the
current continuously flows in one direction in the wire. The law is quite obvious
because it agrees with the common principle with which all of us are familiar, namely,
that the result produced varies directly in amount with the magnitude of the effort or
force and inversely with the resistance or opposition encountered.
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12
Chapter One
Rheostats
The current in a circuit may have to be reduced or increased. The resistance in the
circuit can be increased or decreased with a variable resistance that can be changed as
desired. More or less resistance is put into the circuit by turning the handle of the handoperated resistor, and the current accordingly decreases or increases. Variable resistors
are commonly called rheostats and are used to control or regulate the current flowing
in a circuit.
Direct Current
A water circuit has been used in which the flow of water was always in the same
direction around the circuit. It is evident from Fig. 1.4 that if the engine continues to
rotate in the same direction, the rotary pump will rotate in the same direction, and the
current of water will continue to flow in the same direction. The electrical equivalent
of this current is called a direct current. Direct current flows in the same direction in
a circuit. Direct current serves a very minor role in electric distribution. The principle
use of direct current at the distribution level is for elevators, electric furnaces, electroplating, etc.
Alternating Current
A rotary engine with a reciprocating pump, like the one shown in Fig. 1.14, will produce
a current of water which will first flow in one direction and then, as the piston moves in
the opposite direction, flow in the opposite direction. It can be seen in Fig. 1.14 that
when the piston is moving upward, the flow of water will be as shown by the arrows in
the pipe circuit, and when the piston moves downward, the direction of flow of water
will have to reverse. It is noted that in such an arrangement, the engine still keeps on
revolving in one direction as before. The pump, however, being of a different type,
causes the water current to flow in one direction for a very short time and then reverses
it and makes it flow in the other direction for a very short time. This changing of direction continues all the time. The electrical equivalent of the current that flows first in one
direction and then in the other direction is said to alternate in direction and is called an
alternating current. An alternating current is an electric current which periodically
passes through a regular succession of changing values, both positive and negative.
Figure 1.14
pump.
01_Mack_Ch01_p001-030.indd 12
Alternating-current water
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Elementary Electrical Principles
13
Cycle
The water pump illustrated in Fig. 1.14 causes the current to change its direction two
times for each revolution of the engine. When the piston moves up, the water has a
clockwise direction around the circuit, and when the piston moves down, it moves in a
counter clockwise direction, opposite to the hands of the clock. When the piston comes
up again, the water will then move in a clockwise direction. The water current reverses
its direction two times for each complete stroke of the pump. The complete stroke of the
pump with its two reversals of water flow is called a cycle. A cycle is, therefore, something which repeats itself.
Frequency
Frequency is the number of complete cycles made per second. If the pump made
60 complete strokes per second, the pump could be said to have a frequency of
60 cycles/sec. Frequency of electricity is expressed in hertz (Hz), which is the unit
of frequency equal to 1 cycle/sec. Almost all the electricity generated in the world
is of the alternating-current type. Most of the electricity generated and consumed
flows first in one direction in the wires and then in the other direction, just as water
does in Fig. 1.14, and most of the alternating current is 60-Hz (60-cycle) alternating
current. The currents in the house lamp, the toaster, the kitchen range, and the fan
motors all flow for a very short time in one direction around the circuit and then
quickly change and flow in the other direction. This continues all the time and at a
very high rate. These reversals generally take place at the rate of 120/sec or 7,200/
min, making the frequency 60/sec or 3,600/min. This is so fast that one cannot notice
any flicker in the light of a house lamp. These electric currents are known as 60-Hz
(60-cycle) alternating currents.
The prevailing electric frequency is 50 Hz outside North America. There is no
particular advantage associated with the lower 50-Hz frequency.
Alternating-Current Generator
The discussion of electric pressure or voltage stated that a generator or dynamo
produces electric pressure which causes current to flow. A description of the type of
generator that produces an alternating pressure will be provided. An electric pressure
which causes an alternating current to flow must alternate.
Elementary-Type Alternating-Current Generator
The simplest arrangement for generating an alternating pressure is shown in Fig. 1.15.
It consists of a steel horseshoe magnet and a bar of copper to the ends of which is
attached a wire, forming the circuit shown. When this bar is moved down between the
poles of the magnet so that it passes through the magnetism at the ends of the magnet’s poles, an electric pressure or voltage is generated in the wire. A magnet has two
poles: north and south. The north pole of a magnet is the magnet end, when freely
suspended, that aligns itself toward the earth’s north magnetic pole. If the bar is now
raised or moved upward so that it passes through the magnetism in the other direction, a voltage will again be generated in the wire, but this time the voltage will be in
the opposite direction. Simply stated: electricity can be produced by rotating a wire
loop in the presence of a magnetic field. All that is needed to obtain an alternating
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14
Chapter One
Figure 1.15 Elements of an
alternating-current generator.
voltage is a machine in which a wire moves through magnetism first in one direction
and then in the other and arrangements for connecting this moving wire to the outside
circuit. Such a device is shown in Fig. 1.16 and is called an alternating-current generator. The elementary electric alternating-current generator has a wire bent into the form
of a loop or coil so that it can be rotated continuously in one direction instead of having
to move up and down. The two sides of the coil will alternately pass through the magnetism at the north pole and through a magnetism at the south pole. The side of the
loop marked AB will generate a voltage in a direction from B to A (if this loop is
revolved in the direction of the arrow) and the other side of the loop marked CD will
generate a voltage in a direction from D to C at the instant shown. A current will flow
if the loop is connected to a closed circuit through rings at the ends of the loop by
means of sliding-contact brushes. When the loop is in the vertical position one-fourth
Figure 1.16 Elementary alternatingcurrent generator.
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Elementary Electrical Principles
15
Figure 1.17 Alternating-voltage wave. This chart gives the positive and negative values of
voltage for each position of the loop. Zero corresponds to vertical position of loop.
of a revolution later, the sides of the loop are not passing through any magnetism, and
during this instant there will be no voltage generated and no current flowing. When
the loop has advanced one-half of a revolution from the first position, the wires are
again passing through magnetism, causing a voltage to be generated and a current to
flow if the circuit is closed. This time, however, the current will flow in the opposite
direction, because the wires are passing different poles. An alternating current can
thus be taken from the rings. The curve shown in Fig. 1.17 results if the values of generated voltage are plotted against the corresponding positions of the coil. This is the
familiar sinusoidal waveform of an alternating voltage. Brushes sliding on the collector rings connect the loop to the electric circuit. The machine illustrated in Fig. 1.16 is
a generator of alternating currents.
The voltage in the coil will reverse its direction twice for every revolution of the coil,
just as the water changed its direction twice for every stroke of the pump. The frequency
in cycles per second will then be the same as the revolutions of the coil per second. If the
loop revolves 60 times/sec, the frequency of the voltage will be 60 Hz. The time required
for each revolution, and for each cycle, will be 1/60 sec.
Single-Phase Alternating-Current Generator
This generator has only two slip rings which connect to a simple circuit of two wires. A
generator with only two slip rings is known as a single-phase generator, single phase
meaning that the circuit has only two wires.
Two-Phase Alternating-Current Generator
A generator with another coil or loop placed on the rotating shaft so that the wires
of this loop lie halfway between those of the loop already described and with the
ends of the second loop brought out to another pair of slip rings will generate two
voltages, one in each loop. One loop would be passing through magnetism when the
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16
Chapter One
Figure 1.18 Voltage waves of two-phase generator.
other would be midway between the poles, and the voltage in one would be a maximum while the other was zero. The voltage in one loop would be ahead of the other
by the time necessary for the shaft to turn through the space separating the coils or,
in this case, one-fourth of a revolution (Fig. 1.18). A machine generating two voltages
is called a two-phase generator. The machine would have four wires leading from it
to make up the two circuits.
Three-Phase Alternating-Current Generator
Picture three coils or loops equally spaced on the shaft of the machine like the two coils
of the two-phase generator. A machine built with three coils and three pairs of collector
rings would obtain three alternating voltages in the three circuits, but each of these
voltages would be a little ahead of the others (Fig. 1.19). The machine would be called a
three-phase alternating-current generator. The cycles per second would again be equal
to the revolutions per second if the machine were of the simple type with only two
poles. The machine would have six slip rings and six wires leading from it, making up
the three circuits.
Figure 1.19 Voltage waves of three-phase generator. The corresponding zero values are
one-third revolution apart.
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Elementary Electrical Principles
17
It has been found that the sum of the currents at any instant in three of the six wires
is zero. Some of the currents are flowing away from the generator, others are flowing
toward the generator at the same instant, and the net sum of the currents in three wires
is zero. This fact makes it possible to do away with three of the six wires, thus making
three-phase circuit a line with only three wires. The generator will have only three
collector rings, and only three wires are connected to the machine.
Revolving-Field Alternating-Current Generator
The alternators described have stationary field poles with the conductors arranged to
move past the poles. It is better construction to have the armature conductors stationary and the poles revolving, called revolving-field alternators instead of revolvingarmature alternators. The principal advantages of the revolving field construction are
(1) the armature conductors can be more securely fastened and better insulated and
(2) the amount of material in the rotating part is considerably reduced.
Three-Phase Connections for Alternating-Current Generator
The three coils of the three-phase alternator can be connected in two ways, one to form
a Y connection and the other to form a delta (Δ) connection. The Y connection has one
set of corresponding ends of each of the three coils of the alternator connected together,
as shown in Fig. 1.20. The name “Y connection” is taken from the appearance of the
connection when shown as a diagram. The three free ends of the coils are connected to
the three-phase line.
The delta connection has the three coils connected in series as shown in Fig. 1.21.
The name is taken from the appearance of the connection when shown as a diagram.
The line wires are connected to the junction points or the corners of the delta.
Transmission Lines
Most of the electricity used in the world is three-phase alternating current. Longdistance alternating-current transmission-line towers or poles have three-phase wires
and a ground or static wire for lightning protection. In many cases, electric power companies provide lines with two sets of three wires each. Double circuits increase the
amount of power transmitted over the line, make more effective use of the right-of-way,
and improve the probability of ensuring continuous service.
Figure 1.20 Y connection of
three-phase alternating-current
generator.
Co
i
l
i
Co
l
Three-phase
line wires
Coil
Common junction
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18
Chapter One
il
Co
Co
il
Figure 1.21 Delta connection of
three-phase alternating-current
generator.
Three-phase
line wires
Coil
Power Factor
When an alternating voltage and the current which it causes to flow rise and fall in
value together and reverse direction at the same instant, the two are said to be “in
phase” and the power factor is unity. This condition is illustrated in Fig. 1.22. The same
formula for power as was discussed for direct current holds for alternating current
when the power factor is unity:
Power in watts = volts × amperes
The current and voltage waves are not in phase in most cases. They do not rise and
fall in value together, nor do they change direction at the same instant, but instead, the
current usually lags behind the voltage. Figure 1.23 illustrates the usual condition in
transmission and distribution circuits. The current and voltage are now said to be “out
of phase.” The current drawn by idle running induction motors, transformers, or underexcited synchronous motors lags even more than the current shown in the figure.
Occasionally, the current leads the voltage. An unloaded transmission line or an
overexcited synchronous motor or a static condenser takes leading current from the
line. When the current leads or lags the voltage, the power in the circuit is no longer
equal to volts times amperes but is calculated from the expression:
Watts = volts × amperes × power factor
Figure 1.22 Voltage and current waves in phase. The power factor under these conditions is
unity.
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Elementary Electrical Principles
Power factor =
19
watts
volts × amperes
Figure 1.23 Current wave lagging behind the voltage wave. This is the usual condition in
transmission and distribution circuits.
From this,
Power factor =
watts
volts × amperes
The power factor can thus be defined as the ratio of the actual power to the product
of volts times amperes. The latter product is generally called volt-amperes, or apparent
power. The value of the power factor depends on the amount the current leads or lags
behind its voltage. When the lead or lag is large, the power factor is small, and when the
lead or lag is zero, as when the current and voltage are in phase, the power factor is
unity. This is the largest value that the power factor can have. The power factor is usually
between 0.70 and 1.00 lagging. An average value often taken in making calculations is
0.80 lagging. Figure 1.24 is an example of an electric circuit with resistive and inductive
load and a lagging power factor.
Low Power Factor
The cause of low power factor is an excessive amount of inductive effect in the electric
consuming device, be it motor, transformer, lifting magnet, etc. Induction motors, when
lightly loaded, exhibit a pronounced inductive effect causing the current to lag the
voltage. Idle transformers likewise have a strong tendency to lower the power factor.
Figure 1.24 Electric circuit consisting
of a resistive and inductive load with
lagging power factor.
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20
Chapter One
Inductance
As the value of current changes in a circuit, properties in the conductor oppose the
change in the current and cause a time delay prior to the current reaching a steady state
value. This property is known as inductance. Inductance causes the current to lag the
voltage. Inductance is measured in units defined as henrys.
Capacitance
Capacitance is the direct opposite of inductance, just as heat is the opposite of cold,
sweet the opposite of sour, and day the opposite of night. Capacitance is the property
of a condenser, and a condenser is a combination of metal plates or foil separated from
each other by an insulator such as air, paper, or rubber. The capacitance, or the capacity
of the condenser to hold an electric charge, is proportional to the size of the plates and
increases as the distance between the plates decreases. Capacitance causes the current
to lead the voltage. Capacitance is expressed in units defined as farads.
Power-Factor Correction
One method of raising the low power factor is to add capacitors to the circuit, since
capacitance is the opposite of inductance and since too much inductance is the cause
of low power factor. Figure 1.25 is an example of an electric circuit in which the inductive load is offset by the capacitive load and the power factor has been corrected to
unity or 1.00. Capacitors are installed in the electric substation or in the electric distribution system on poles. The capacitors can be directly connected to the circuit or
switched on and off as needed.
Direct-Current Generator
We have carefully studied how an alternating current of water could be produced by
the pump shown in Fig. 1.14. We observed that the current of water circulated around
the pipe circuit first in one direction and then in the opposite direction. We observed
that the water current reversed its direction twice for each complete stroke of the pump.
All this we have carefully compared with the action of a real electric generator and
found the processes are quite alike. This much is fundamental in the generation of electricity, whether it is alternating-current or direct-current electricity. To obtain alternating current, we simply fastened slip rings to the ends of the loop and connected those
rings to the circuit by means of sliding brushes.
Direct-Current Water Pump
A direct current of water, which is a current that flows continually in one direction, can
be obtained by adding a set of valves to our water pump, as shown in Fig. 1.26. The
water will not be able to flow backward after it is once forced through the valve, and
Figure 1.25 Electric circuit consisting
of a capacitive, a resistive, and an
inductive load. The power factor has
been corrected to zero.
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Elementary Electrical Principles
Figure 1.26
21
Direct-current water pump.
therefore, the direction of the water flow will always be the same. The piston moves up,
forcing the water through the top valve, and as the piston drops, the top valve closes
and the lower valve rises, allowing the water to fill the cylinder. When the piston moves
upward again, the cylinder is full of water and more water will be forced into the circuit
through the top valve as before.
Elementary-Type Direct-Current Generator
A direct current of electricity can be obtained from the machine shown in Fig. 1.16 by
replacing the slip rings with a commutator. The commutator is the device which changes
an alternating voltage to a direct voltage. It corresponds to the valves in the hydraulic
direct-current generator. The process is called commutation. The commutator consists
of two bars of copper connected to the two ends of the loop or coil in the simple case.
These two copper bars revolve with the shaft, as do the slip rings. These bars have
brushes resting on them, which connect the machine to the electric circuit. The general
arrangement is shown in Fig. 1.27.
Commutation Direct-Current Generator
From Fig. 1.27, it is clear that the commutator will first connect one side of the loop
to one side of the circuit, and then as the shaft revolves, it will connect the other end
of the loop to the same side of the circuit. If the brushes are correctly set, this change
will take place when both sides of the loop are not generating any voltage, that is,
when the coil is midway between the poles. This is the point at which the voltage in
the loop changes direction. When A side of the loop is passing the south pole, the
voltage is in a given direction and feeds into brush B1, and D side of the loop feeds
into brush B2. But when the loop has revolved to its midway position, the direction
of the generated voltage in the loop is about to change, and just at this instant the
ends of the loop are also changed to the opposite sides of the circuit so that when the
loop revolves still further, the voltage into the circuit will remain the same. The voltage that alternates in the loop is made to produce a pressure in the same direction in
the outside circuit. Such a machine with its commutator is called a direct-current
generator.
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Chapter One
Figure 1.27 Elementary directcurrent generator.
Commercial Direct-Current Generator
The principal parts of a commercial direct-current generator are the frame, the poles,
the armature, the commutator, and the brushes. The purpose of the frame is to support
the north and south poles. The armature supports the conductors which constitute the
armature winding. The commutator, a split ring device made up of many copper segments, is also supported by the armature. The commutator keeps the torque on a DC
motor from reversing every time the coil moves through the plane perpendicular to the
magnetic field. A special brush rigging holds the brush holders and brushes.
Direct-Current Circuits
Direct-current (dc) circuits, as we have seen, require only two wires, one for the outgoing
current and the other for the return current. Circuits carrying direct current will,
normally, consist of only two wires. The use of direct current is generally confined to
special applications, such as electroplating, battery charging, and elevator operation, or
where fine speed control of industrial motors is required.
High-voltage dc circuits are used to transmit large amounts of power from remote
alternating-current (ac) generating stations, often located adjacent to generating stations, large coal mines or water reservoirs, to populated load areas. High-voltage dc
circuits are also used to transmit large quantities of electrical energy by insulated cable
circuits. Alternating-current systems that are not synchronized are being interconnected with high-voltage dc circuits. High-voltage dc transmission lines originate and
terminate in converter stations that change alternating current to direct current or
direct current to alternating current. The converter stations use solid-state electronic
devices to convert the energy from alternating current to direct current or direct current
to alternating current.
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Elementary Electrical Principles
23
Transformers
The voltages necessary to obtain economical transmission are higher than those which
can be directly generated by an alternator. The voltage at which electric power is used
in motors and lamps is less than that required for distribution. It is therefore necessary
to raise the voltage at the generating station to the value required for transmission
and to lower it at the point of consumption to the values required by the motors and
lamps on the system. The voltage must be stepped up, in the first case, and it must be
stepped down in the latter. The transformer is the apparatus used to make these
changes in ac voltage.
Hydraulic Transformer
The water circuit shown in Fig. 1.28 will make it clear how ac voltages are stepped up
and down. The water flows to and fro or back and forth in the circuit, since this is an ac
circuit. As it does this, it causes the piston in B to move up and down. Piston B is connected to piston C by a rod pivoted at P so that when B moves up and down, C moves
down and up. The pressure in B is very high and the current of water is small, just like
the voltage and current in a high-voltage transmission line. The pressure in cylinder C
will be low and the current of water large, because the size of the piston is so much
larger. A mechanism such as this could be used to transform, or step down, a water
circuit having a high pressure to one having a low pressure. The current on the lowpressure side would be much larger than the current on the high-pressure side.
The frequency of the current has not been changed. The only changes are the
decrease in pressure and the increase in current. It should be noted that the total power
delivered has not been changed, for under the discussion of power it was pointed out
that the power is dependent on the product of voltage and current. The same power can
be obtained from a circuit of high voltage and small current as from a circuit that has a
low voltage and a large current.
Electric Transformer
An electric transformer operates in the same manner as the hydraulic transformer.
The primary, or high-voltage side, of the transformer has a high electric pressure
and the current is small. The secondary, or low-voltage side, of the transformer has
a low pressure and the current is large. One can usually tell the high-voltage from
the low-voltage side of the transformer by observing the size of the insulators or
Figure 1.28 The hydraulic
transformer.
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Chapter One
Figure 1.29 Electric voltage
transformer.
bushings on the top of the transformer case. The high-voltage side must be better
insulated than the low-voltage side. The high- and low-voltage leads are brought
out on top of the tank.
Figure 1.29 illustrates the elementary parts of an ac electric voltage transformer.
Cold-rolled silicon steel strips are so arranged as to form a closed magnetic circuit.
These strips have placed on them two coils of insulated wire, one of which has many
turns of small wire and the other of which has a few turns of heavy, coarse wire. The
coil with the many turns is the high-voltage coil and is called the primary winding in
a step-down transformer. The other coil with the few turns is the low-voltage coil and
is called the secondary winding. Thus the winding into which current is brought is the
primary, and the winding from which current is taken is the secondary. The coils correspond to the two pistons in the water transformer, and the magnetic core acts as the
coupler between the two pistons.
The action in the transformer is somewhat as follows: The voltage applied to the
primary causes a current to pass through the primary coil. This current creates a magnetic flux in the core. The flux in the core cuts both the primary and the secondary
coils. This cutting of the primary coil creates a counter voltage in the primary coil
which very nearly equals the primary voltage applied. The current in the primary at
no load is only great enough to magnetize the core at no load. The secondary coil
being cut by the flux will have voltage at no load, but there is no current. Now, let us
apply load on the secondary. The load current in the secondary will create a counter
flux in the core which reduces the magnetic flux in the core. The reduction in magnetic flux in the core reduces the primary counter voltage. The reduction in primary
counter voltage increases the difference between the applied voltage and the counter
voltage. More current will therefore flow into the primary, thereby increasing the
magnetic flux to its former no-load value. This again raises the secondary induced
voltage to its original value. All these adjustments within the transformer take place
automatically and instantaneously.
It should be noted that in a modern transformer the primary and secondary windings are not placed on separate legs, but instead, each winding is generally divided into
two parts, and one-half of each winding is placed on each leg. This gives a more constant voltage with changes in load.
Transformer-Core Construction
The core of a transformer can be built in various shapes. The following types are in
general use:
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Elementary Electrical Principles
25
Core Type
The core is in the shape of a rectangle. Coils are placed on two legs of the core.
Shell Type
The core is rectangular in shape with a central leg in addition to the core first described.
Coils are placed on the central leg.
Cruciform Type
The cruciform core is a modification of the shell type. It is the same as if two shell-type
cores were set at right angles, using a common central core for the windings.
Wound-Core Type
The core is made of a ribbon of sheet steel wound in a spiral. After the core is wound
and annealed, the coils are wound on the core. This type of core is superseding other
core types for small distribution transformers.
Transformer-Core Material
An important consideration for a transformer is the core material because the core of an
energized transformer contributes to electric system losses. Transformer losses are classified as either no-load losses or full-load losses. The no-load losses on a lightly loaded
transformer are a result of the core material. The energy-efficient cores are constructed
of low-loss cold-rolled silicon strips or amorphous steel alloy strips.
Transformer Cooling
When the voltages of the primary and secondary windings are both low and the transformer has a small capacity, the transformer, that is, the steel core with its two windings,
is placed in a metal case merely to keep out dirt and moisture. This type of transformer
is designated as an air-cooled dry-type transformer. The core and coils of a high-voltage, large-capacity transformer are usually placed in a tank filled with oil. The oil is a
good insulator and helps to cool the windings. This type of transformer is designated
as oil-cooled. It will perhaps be remembered that whenever current flows in a wire,
there is a friction loss in heat. An illustration of this is found in the toaster or the iron.
The same is true in a transformer; whenever currents are flowing through the primary
and secondary windings, heat is given off which must be carried away so that the transformer may not become too hot and char or burn the insulation on the wire of the coils.
If it becomes too hot, the transformer will be ruined, just as a person will die from running a high temperature for a long time.
More power can be safely drawn from a transformer in cold weather than in hot
weather because of the lower ambient temperature. More power can be taken from a
transformer for short periods of time than can be taken off continuously. A transformer
which is overheated because of an overload should be replaced with a larger-capacity
rated transformer before it becomes damaged.
Methods of Cooling Transformers
The oil in the tank is the principal cooling agent. It keeps the windings cool by carrying
the heat from the coils to the surface of the tank. Many times the transformer case itself
is built so that it has much more surface for cooling than if it were smooth. The metal
sides of the tank are corrugated, or pipes are connected to the top and bottom of the
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26
Chapter One
tank so that whatever oil is in the pipes is exposed on all sides. The object in either case
is to increase the surface from which the heat can radiate. This is the same principle as
is used in the automobile radiator.
Very large transformers may have coils of pipes placed in the top of the tank through
which cold water is circulated by a pump. The cold water in the pipes cools the oil in
the tank. The water pipes are placed in the top of the tank because the hottest oil is
always at the top.
Transformer Temperature Limits
In general, the maximum safe temperature for such insulating materials as cotton, silk,
cambric, tape, fabric, etc., is 105°C, which corresponds to 221°F. It is impossible, however, to measure the actual temperature in the transformer because the exact location of
the hottest spot is not known, nor is it accessible. The temperature of the oil plus a 10°
to 15° correction adder is usually taken as an indication of the temperature of the windings. The temperature of the oil in large power transformers is usually obtained by
some form of thermometer.
Distribution transformers may have devices which indicate the maximum load on
the transformer. The rapid growth of loads on distribution systems makes it necessary,
in many cases, to make a check of the loads on transformers in order to determine
whether the transformer is still large enough to carry the load. An overload indicator
lamp makes use of a bimetallic thermostat to give an indication of the winding temperature. It turns on a red light which is visible from the ground if the transformer is
overloaded. A reset device makes it possible to turn the light off.
Three-Phase Transformer
A three-phase transformer is really three single-phase transformers in one case using a
single combined core. The core has much the same shape as the shell-type single-phase
transformer. The three-phase transformer, however, has single-phase primary and secondary windings on each leg of the core. The common core acts to supply flux for all
three phases.
A three-phase transformer weighs about two-thirds as much as the same capacity in
single-phase transformers. Its biggest disadvantage lies in the fact that if one phase
fails, the entire transformer must be taken out of service.
Large substation transformers employ forced-air and forced-oil cooling in order to
increase the output rating. By forced circulation of the oil through the external radiators and by blowing air against the radiators, the heat is more rapidly removed from
the hot oil.
Solid-State Circuit Components
Solid-state devices are used extensively to control electric power circuits. The
devices are manufactured from semiconductor materials. Semiconductor materials
individually have a high resistance to the flow of electric currents and thus have
been identified as semiconductors. Silicon and germanium are semiconductor
materials that are commonly used to manufacture solid-state devices. A large, pure
crystal of silicon has a high resistance to the flow of electric current. Silicon can be
made conductive by adding other materials (referred to as doping) that either add
or subtract electrons from the combination material. Materials used to dope the
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Elementary Electrical Principles
27
semiconductor materials are boron, phosphorus, gallium, arsenic, and others. A
silicon crystal that has been doped with phosphorus, which will donate free electrons to the material, is called an n-type semiconductor. An n-type semiconductor
will have an excess number of electrons. Silicon doped with boron, which takes
electrons away, is called a p-type semiconductor. A p-type semiconductor will have
a deficient number of electrons. The p-type material with a deficient number of
electrons is said to contain holes.
Diode Semiconductors
A semiconductor formed by modifying the silicon material on each end with a junction
in the middle separating the p-type and the n-type materials is called a diode (Fig. 1.30).
A diode semiconductor is used in a circuit as a current rectifier. As illustrated in
Fig. 1.30, the terminal of the diode on the p-type material is called the anode and the
terminal of the diode on the n-type material is called the cathode. When voltage is
placed across the diode semiconductor with the positive potential connected to the
anode (A) and negative potential connected to the cathode (K), the holes in the p-type
material and the electrons in the n-type material will both move toward the junction
between the n- and p-type materials. When the impressed voltage is great enough to
overcome the resistance of the barrier at the junction, the electrons will combine with
the holes and flow externally from the diode, leaving the anode and returning through
the cathode (Fig. 1.31).
The pulsating of the output voltage can be eliminated by adding a capacitor in
the circuit connected across the output terminals of the circuit x and y. Full wave
rectification can be obtained by adding a transformer on the input to the circuit with
a diode connected in series with each of the transformer end terminals and a resistor
Figure 1.30
Diode semiconductor.
Figure 1.31 Diode semiconductor connected to a 60-Hz ac supply voltage with a resistor in
series with it will produce a pulsating dc current through the resistor and a pulsating voltage
across the resistor that can be measured from terminals x and y of the circuit.
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28
Chapter One
Figure 1.32 Zener diode symbol and
operating-characteristics curve.
connected to the cathode of each diode and the neutral terminal on the transformer.
Four diodes can be used in a bridge circuit to increase the capacity of the rectified
output of the circuit.2
Zener Diode
A diode semiconductor that can be used as a voltage reference in an electric circuit is
called a Zener diode. The Zener diode is manufactured by doping the silicon crystal so
that when the diode is reversed bias at a voltage greater than the junction breakdown
value, a large current will flow through the diode. As the current increases through the
diode in the circuit, the voltage across the Zener diode within its rating will remain
practically constant (Fig. 1.32).
The operating curve for the Zener diode illustrates that the voltage across the diode,
when it is used in a circuit within its rating, varies from +0.7 volt when it is forward
biased to −5 volts when it is negatively biased. This characteristic permits its use as a
voltage reference when the proper resistance is connected in series with the diode in a
circuit with other components. When the voltage applied to the diode increases beyond
the breakdown value of the junction, the high current that flows in the circuit can be
used to initiate operation of auxiliary components in the circuit which will correct the
voltage variation and provide voltage regulation to the circuit.
Transistor Semiconductor
A semiconductor manufactured with three elements is called a transistor (Fig. 1.33).
Transistor semiconductors are constructed with two n-type (excess number of electrons)
and one p-type (deficient number of electrons) or two p-type and one n-type materials.
The current flowing from the emitter (E) to the collector (C) is controlled by the potential applied to the base (B). The base material is very thin.
A transistor can be used in a circuit to amplify a signal. An npn transistor connected
in a circuit with a collector (C) reversed biased with respect to the base (B) will have a
Additional information on semiconductors and basic solid-state conductor circuits can be obtained
from Basic Electronics: Theory and Experimentation by Fredrick W. Hughes (Prentice-Hall, 1984).
2
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Elementary Electrical Principles
Figure 1.33
Transistor semiconductors.
Figure 1.34
Schematic diagram of a circuit using a transistor to amplify a voltage.
29
small current flow from the emitter (E) to the base and a large current flow from the
emitter to the collector (Fig. 1.34).
When a small ac voltage is connected between the input terminal of the circuit and
ground, the bias voltage between the base (B) and the emitter (E) will be increased and
reduced accordingly. This will vary the current flow from the emitter (E) to the collector (C),
which will vary the voltage above and below some positive value between the output
terminal and ground. The output voltage will be in proportion to the input voltage
and amplified.
Thyristor Semiconductor
A semiconductor consisting of a layer of p-type, a layer of n-type, a layer of p-type, and
a layer of n-type materials with an anode (A) connection to one p-type material, and a
layer of n-type materials, a gate (G) connection to one p-type material, and a cathode (K)
connection to one n-type material is called a thyristor semiconductor (Fig. 1.35).
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30
Chapter One
Figure 1.35 Thyristor semiconductor
constructed with two p-type and two
n-type materials.
The thyristor semiconductor is used as a controlled rectifying device. The thyristor center np junction is reverse biased by the operating circuit. A current applied to
the gate (G) terminal will reduce the voltage necessary to overcome the reverse bias
of the center junction. The operation of the thyristor in a circuit designed to rectify an
ac voltage can be controlled by the signal applied to the gate (G) terminal. Controlling
the firing or operation of the thyristor is important when changing alternating current
(ac) to direct current (dc) and reversing the operation. Thyristors are used in converter stations to convert alternating current to direct current and to convert direct
current to alternating current. Thyristors are connected in series in circuits with other
circuit components to operate at the high voltages necessary to carry large quantities
of electric power over dc transmission lines.
Definitions
Definitions of electrical terms used in the text are given as they appear. American
National Standard definitions are listed in the IEEE Standard Dictionary of Electrical and
Electronic Terms.3
3
IEEE Standard Dictionary of Electrical and Electronic Terms. Wiley-Interscience, New York.
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