Electrical engineering - Christie
ELECTRICAL ENGINEERING
The theory and characteristics of electrical circuits and machinery.
BY CLARENCE CHRISTIE
McGRAW-HILL BOOK COMPANY, NEW YORK, 1917
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PREFACE TO THE SECOND EDITION
The second edition contains all the material in the original text but much of it has been rewritten and a great deal of new material added.
The more important additions include sections on complex alternating waves and wave analysis on polyphase alternating current circuits, on the construction of the characteristic curves of direct-current generators and motors, on the design of direct and alternating-current machinery, on the Blondel diagram for the synchronous motor, on the symbolic method of analysis of the induction motor, on alternating-current commutator motors, and finally a chapter on electrical measuring instruments.
The chapter on direct-current machinery has been entirely rewritten and much enlarged, and to make it complete a short chapter outlining the design of a direct-current generator has been added.
This chapter and the other sections dealing with design are not intended to cover the work required in a course on design but only to give the student some idea of the formula and constants involved.
The chapter on measuring instruments has been placed at the end of the book because some of the principles involved cannot be well understood by the student until he has mastered the theory of the more important electrical machines.
PREFACE TO THE FIRST EDITION
This book has been compiled as a foundation for lecture courses for junior and senior students in Electrical Engineering.
The theory and characteristics of electrical machines are developed from the fundamental principles of electrostatics and electromagnetics. Only the more standard types have been discussed since familiarity with the principles of their operations will guide the student to a complete understanding of other machines which differ only in minor respects. This general groundwork may be extended to suit the requirements of particular classes.
CONTENTS
Electrostatics
Magnetism and Electromagnetics
Electric Circuits
Electric Circuits
Complex Alternating-current Waves
Polyphase Alternating-current Circuits
Direct-current Machinery
Design of a Direct-current Generator
Synchronous Machinery
Transformers
Converters
Induction Motor
Alternating-current Commutator Motors
Transmission Systems
Electrical Instruments
CHAPTER I - ELECTROSTATICS
1. Electrification. - Bodies which are charged with electricity are said to be electrified. Charges are of two kinds called positive and negative. Bodies which have a positive charge are acted upon by forces tending to make them give up their charge; bodies which have a negative charge are acted upon by forces tending to convey a positive charge to them equal to their negative charge. These forces are exerted through the medium separating the charges and the medium is in a state of stress.
The body with the positive charge is at a higher potential than the body with the negative charge and the difference of potential between the two is a measure of the tendency for electricity to pass from one to the other.
2. Electrical Conductors and Insulators. - If two metallic bodies charged to different potentials are joined by a metal wire electricity will flow from one to the other until the potential of both is the same and the transfer of electricity will take place almost instantaneously. The metal wire is therefore a good conductor of electricity; or, it offers a low resistance to the passage of electricity through it.
If the two charged bodies had been joined by a glass rod, there would have been no transfer of electricity between them, or, it would have taken place so slowly that it could only be detected by the most delicate instruments. Glass is therefore a very bad conductor; or, it offers a very high resistance to the passage of electricity. It is called a non-conductor or insulator.
As all materials conduct to a certain extent, it is not possible to divide them absolutely into conductors and insulators, but, since the resistance of a good insulator is many million times that of a good conductor, they may be so divided for practical purposes.
In the first class are silver, which is the best conductor, copper and other metals, graphite, impure water and solutions of salts. In the second class are air, which when dry is an almost perfect insulator, glass, paraffin, ebonite, porcelain, rubber, shellac, oils and the numerous insulating compounds used in electrical engineering.
3. Electrostatics and Electromagnetics. - Electrostatics comprises phenomena related to electric charges at rest and to the stresses produced in the fields surrounding them. These phenomena become of great importance where very large differences of potential must be provided for, as for example in the design and operation of all high voltage apparatus and systems.
Electromagnetics comprises phenomena related to electricity in motion, that is, to currents of electricity and the magnetic fields produced by them. Almost all the problems to be solved by the electrical engineer come under this head.
4. Laws of Electrostatics. - First Law. - Like charges of electricity repel one another; unlike charges attract one another.
Second Law. - The force exerted between two charges of electricity is proportional to the product of their strengths and is inversely proportional to the square of the distance between them; it also depends on the nature of the medium separating them.
5. Coulomb. - One electrostatic unit of quantity is that quantity which, when placed at a distance of 1 cm. in air from a similar quantity, repels it with a force of one dyne.
6. Electrostatic Field. - Any space in which electrostatic forces act is called an electrostatic field. The direction of the force at any point in the field is the direction in which a unit positive charge placed at the point tends to move and its intensity is the force in dynes exerted on the unit charge.
The electrostatic field is conveniently represented by lines of electrostatic induction or dielectric flux drawn in the direction of the force. In air the number of lines per square centimeter is equal to the force in dynes at the point and in a medium of dielectric constant k the number of lines per square centimeter is equal to k times the force. This may be stated in another way: Unit electrostatic force produces one Une of dielectric flux per square centimeter in air and k lines per square centimeter in a medium of dielectric constant k.
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The body with the positive charge is at a higher potential than the body with the negative charge and the difference of potential between the two is a measure of the tendency for electricity to pass from one to the other.
2. Electrical Conductors and Insulators. - If two metallic bodies charged to different potentials are joined by a metal wire electricity will flow from one to the other until the potential of both is the same and the transfer of electricity will take place almost instantaneously. The metal wire is therefore a good conductor of electricity; or, it offers a low resistance to the passage of electricity through it.
If the two charged bodies had been joined by a glass rod, there would have been no transfer of electricity between them, or, it would have taken place so slowly that it could only be detected by the most delicate instruments. Glass is therefore a very bad conductor; or, it offers a very high resistance to the passage of electricity. It is called a non-conductor or insulator.
As all materials conduct to a certain extent, it is not possible to divide them absolutely into conductors and insulators, but, since the resistance of a good insulator is many million times that of a good conductor, they may be so divided for practical purposes.
In the first class are silver, which is the best conductor, copper and other metals, graphite, impure water and solutions of salts. In the second class are air, which when dry is an almost perfect insulator, glass, paraffin, ebonite, porcelain, rubber, shellac, oils and the numerous insulating compounds used in electrical engineering.
3. Electrostatics and Electromagnetics. - Electrostatics comprises phenomena related to electric charges at rest and to the stresses produced in the fields surrounding them. These phenomena become of great importance where very large differences of potential must be provided for, as for example in the design and operation of all high voltage apparatus and systems.
Electromagnetics comprises phenomena related to electricity in motion, that is, to currents of electricity and the magnetic fields produced by them. Almost all the problems to be solved by the electrical engineer come under this head.
4. Laws of Electrostatics. - First Law. - Like charges of electricity repel one another; unlike charges attract one another.
Second Law. - The force exerted between two charges of electricity is proportional to the product of their strengths and is inversely proportional to the square of the distance between them; it also depends on the nature of the medium separating them.
5. Coulomb. - One electrostatic unit of quantity is that quantity which, when placed at a distance of 1 cm. in air from a similar quantity, repels it with a force of one dyne.
6. Electrostatic Field. - Any space in which electrostatic forces act is called an electrostatic field. The direction of the force at any point in the field is the direction in which a unit positive charge placed at the point tends to move and its intensity is the force in dynes exerted on the unit charge.
The electrostatic field is conveniently represented by lines of electrostatic induction or dielectric flux drawn in the direction of the force. In air the number of lines per square centimeter is equal to the force in dynes at the point and in a medium of dielectric constant k the number of lines per square centimeter is equal to k times the force. This may be stated in another way: Unit electrostatic force produces one Une of dielectric flux per square centimeter in air and k lines per square centimeter in a medium of dielectric constant k.
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