Practical alternating currents and power transmission, a practical treatise on the principles and applications of alternating currents to central station and power-house plants, including the elements of power transmission. Enriched by one hundred and seventy-two illustrations

Front matter

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Preface

The correct treatment of a subject like alternating currents has been regarded, with much truth, as being consistently mathematical on all occasions. The fact of the matter is, that however mathematical the treatment of the theory may seem to be, the practice is within the scope of any earnest reader's intelligence. In other words, opinions are apt to differ as to what is the correct treatment of a subject. To the student of academic forms, mathematics in this case would be the only reason for the treatment being given any consideration at all. To the practical man, interested in the most explicit and, to him, the least mathematical presentation possible, academic forms are looked upon askance. In consequence, the line of division is strongly drawn between the reading matter palatable to the mental taste of one and the other. Calculus is naturally a useful and time-saving tool in making an analysis, but how many regard it as a readily comprehended form of expression in connection with ordinary phraseology and practical, everyday work? There must be, instead of this, a departure if possible, one which is made only to suit those for whom such a departure is absolutely essential under the circumstances. Therefore this little volume, entitled " Practical Alternating Currents," was prepared in the belief that it represented what was wanted and what would

865696

6 PREFACE

prove comprehensible and useful. There is, and will always continue to be, much more to a subject than a volume on it will cover ; but then the purpose of the volume must be considered. It is here that light is best thrown upon the situation and misunderstanding avoided.

A book must stand upon its own merits, with its purpose evident to the reader's eye. Such a one as this has the purpose in view of discussing alternating currents in as unmathematical a form as possible. It is intended to cover practical conditions with reference to the principles upon which they rest, with arithmetic as the basis of all such calculations, comparisons, and results found therein. The subject has been entered into from a power-transmission-and-distribution standpoint. For this reason, some of the preliminary matter found in other books has been left out. Yet the author believes that enough ground has been covered to give a good working knowledge to the reader without such a preamble.

As far as possible the practice has been given first place, and all the author can honestly claim is a manner of explanation that, it is hoped, will prove to some an easier means of reaching the truth than has been their experience heretofore.

It must be understood, however, before this Preface is closed, that a non-producing class exist, whose main work consists in the assumption of superior attitudes toward the earnest efforts of others, particularly when crystallized in book form. They are found in the field of science, as everywhere else, and have their say. It

PREFACE 7

may be unnecessary to state that this volume is not intended for them, though in all likelihood it will not escape their attention.

If, instead of this class, the pages of this book are read with care by those for whom it is intended — ^the real workers — the author expects but one response. Imperfections are found in the productions of all hands, skilled or unskilled, and for that reason they will be found here as well, in all probability. The imperfections, however, are not the things to be judged, but the bulk of the subject-matter as it stands. On this basis the author is prepared to offer his work to those constituting themselves his readers, with complete contentment, for the outlines of the book are based upon natural law, and this has always seemed to him sufficiently stable to authoritatively support a treatise devoted to useful purposes.

Newton Harrison.

September, 1906.

Chapter I Pace

Introduction — Conditions of Electric Lighting — Character of Current Employed — Comparison of Circuits for Equal Power — Pressures of Standard Generators — Capacity of Machines — Selection of Lamps — Returns from Generators — Cost of High Efficiency — Elements of Power — Continuous Current Calculations — Sources of Power — Power from Waves 15

Chapter II

Power from the Sun — Vacuum-tube Lighting— Amount of Light per Inch of Tube — Efficiency — Light and Lamp Efficiency — Short Life with High Efficiency — Another Feature of Higher Lamp Efficiency — Cost of Power — Cost of Power in Various Localities 36

Chapter III

Methods of Distributing Electricity — Distinguishing Differences — Outline of Alternating- and Direct-current Systems — Position of Central Station — The Central Station in Practice — General Use of Polyphase Currents — Distribution by Wire— A Two- and Three-wire System Contrasted — ^The Power-factor— Angle of Lag— Self-induction — Lines of Force Surrounding a Circuit — Meaning of Angle of Lag 56 lo CONTENTS

Chapter IV Page

Self-induction and Resistance — Self-induction of a Coil — Capacity Reactance — Impedance — Light and Power Problems — Resonance in Circuits — The Transformer— Single-phase Current Power — Power-curves — When the Power-factor is Zero — Examples of Lag .... 76

Chapter V

The Transformer for Light and Power — Mutual Induction — Losses of Power — Ratio of Pressures — Without Load. — With Load — Relationship of Primary and Secondary Current — Calculating the e. m. f. of the Coils — Iron Losses — Mutual- and Self-induction — The Transformer Iron — The Permeability of the Iron — Calculation of Hysterisis of Iron — Permeability and Coercive Force — Formula for Hysteresis — Choice of Iron — Curve of Hysterisis Loss — Principles of Design .... 98

Chapter VI

Weight of Transformer Iron and Losses — Lag of Magnetism Behind the Magnetizing Force — Influence of Magnetism upon the Secondary Coils — The Two Causes of Hysteresis — Transformer Efficiencies — Construction of Modern Transformers for Light and Power — The Aging of Transformer Iron — Cooling of the Transformer — Eddy-current Losses — Varying the Thickness of the Iron — Rating of Transformers as to Temperature — Effect of Hysteresis and Eddy Currents During Overload — Effects of Increased Size in Transformers — Self-cooling Transformers — Advantages and Disadvantages of Oil . .123

CONTENTS II

Chapter VII Page

Regulation of Transformer for Lighting— Constant Current and Constant Potential — Constant-current Transformers — Constant Current from a Direct-current Generator — Comparison of Constant -current, Constant-potential, Direct- and Alternating-current Systems — High-tension Switches — Sparking Distances — Requirements of Switches — Point of Breaking the Circuit — Switches for I, ICO and 2,200 Volts — Transformer Fuse Blocks — Single- and Double-pole Fuse Boxes .... 147

Chapter VIII

Kinds of Alternators — Belt-driven Alternators of 16,000 Cycles — Frame and Bearing Construction — Armatures of Alternators — Armature Coils — The Fields of Alternators — Field Winding — Compensating Winding — Direct-connected or Engine-type Alternators — Further Subdivisions — Rotating-armature Type Alternators — Armature Windings — Generation of Parasitical Currents in Armature Conductors — Iron-clad Armatures — Division of the Frame — Field Winding — Rotating-field Engine-type Alternators — The Rotating Field Construction — Armatures of Rotating-field Alternators — Temperature of Armature 171

Chapter IX

Alternators in Synchronism — Mechanism for Obtaining Synchronism — Synchronous Cross-connections — Synchronism with High-tension Alternators — Synchronism for Two- and Three-phase Alternators — High-tension

12 CONTENTS

PAGE

Polyphase Alternators in Synchronism — Alternating-current Potential Regulators — Single-phase Motors — Self-starting Single-phase Motors — Making Two Phases from One Phase — Single-phase, Series-wound, Self-starting Motor — Defects of the Single-phase, Series-wound Motor — Practical Connections of a Series and Synchronous Motor — Advantages of the Synchronous Motor — Rotary Converters and Synchronous Motors . . 194

Chapter X

Purpose of Two-phase Generation of Power — ^The Two-phase Alternator — Angular Separation of a Two-phase Current — Winding of a Two-phase Armature — Weights of Wires with Different Phases — A Two-phase Threeand Four-wire System — Tracing a Rotary Two-phase Field — Coils Receiving the First Phase — Coils Receiving the Second Phase — Coils Receiving the First-phase Reversed — Coils Receiving the Second-phase Reversed — Tracing a Rotary Three-phase Field — Action of the First Phase — Action of the Second Phase — Action of the Third Phase — The Reversing Phases . . . .219

Chapter XI

Examination of the Three Sine Waves— The First Phase Forward — The First Phase Reversed — The Second Phase Reversed — The Third Phase Reversed — Tracing the Phases in the Circuits — More Current in One Main than Another — Generating a Three-phase Current — ^The Three-phase Generator — The Three Armature Windings — A Six-wire Three-phase System — A Four-wire Three-phase System — Two Phase with Three Mains . . 245

CONTENTS 13

Chapter XII Page

Classification of Alternator Armature Windings — ^Armature Slots per Phase — Three-phase Armature Coils Equally Loaded — ^Amount of Current and Pressure in each Phase in the Receiving Circuits — Amperes and Volts in a Three-phase Y-connected Receiving Circuit — Amperes and Volts in a Three-phase Delta-connected Receiving Circuit — Difference in Power Received by Delta and Y Circuits — Power in the Mains of a Three-phase System — Power the Same from a Delta- or Y-wound Armature — Three-phase Y System for Power Transmission — Practical Comparison of the Y and Delta Systems — Currents in the Two Sets of Conductors — Review of the Facts — Star and Mesh Two Phase — The Star-connected Four-phase System 270

Chapter XIII

Voltages Existing in a Star-connected Two-phase System — Voltages Existing in a Mesh Connected According to the Two-phase System — Amperages in a Star- and Mesh-connected Two- or Four-phase System — Comparison of the Copper Required for Different Systems — Two-phase Four- wire System with Transformers — Two Phase Four Wire to Two Phase Three Wire — Three-phase Three-wire Delta— Three Phase Three Wire Y or Star Connected- Combination Y and Delta — Changing Two Phase into Three Phase — A Complete Phase- and Pressure-transforming System — Changing Two Phase into Three Phase and Back into Two Phase — Features of Transformer Design — Causes Operating to Prevent the Development of a Constant Potential at the Terminals of the Secondary 292

14 CONTENTS

Chapter XIV Page

Features of Transformer Testing — Load of Transformers — Voltages of Transformers — Lighting and Power Transformers — Transformer Connections — Lightning Discharges — Primary Winding — Secondary Winding — Handling the Transformer — Sizes, Weights, and Oil Required — The Single-phase Inductor Alternator — The Rotor of the Inductor Alternator— Frequency of the Alternations 317

CHAPTER* XV

Development of Electromotive Force in Alternators — Waves with a High Peak — Sine Waves with a Low Peak — Waves with Distinct Irregularities — Station Machinery — Elements of Power and Their Efficiencies — Light, Heat, and Power Data — Definition of Energy — Defini tion of Force — Definition of Work — Definition of Power — Difference Between Electrical and Mechanical Horse-power — Horse-power of an Engine — The Steam Tur bine— A Watt— The Kilowatt Hour— Formula for Watts — Formula for Pressure — Formula for Current — Formula for Resistance — Formula for Kilowatts— Formula for Horse-power — Recapitulation 339

Chapter I ^- ' ' '• '

INTRODUCTION. CONDITIONS OF ELECTRIC LIGHTING. —

CHARACTER OF CURRENT EMPLOYED. — COMPARISON

OF CIRCUITS FOR EQUAL POWER. PRESSURES OF

STANDARD GENERATORS. — CAPACITY OF MACHINES. — SELECTION OF LAMPS. RETURNS FROM GENERA- TORS. COST OF HIGH EFFICIENCY. — ELEMENTS OF

POWER. CONTINUOUS CURRENT CALCULATIONS. —

SOURCES OF POWER. — POWER FROM WAVES.

Introduction. — The field of engineering has broadened sufficiently to include more than the term itself implies. Engineering, as now construed, is not limited entirely to the consideration of those problems hitherto regarded as belonging entirely to its peculiar domain. On the contrary, it embraces the most important problems of the hour, as far as material comforts are concerned. It enters into the field of illumination, with the purpose of pointing out, as well as crystallizing, the principles upon which that which is called illumination and wiring are based. It enters into the field of heating and cooking, regarding these domestic propositions as part of the scheme as now outlined. Hygienically, it not only offers to be the means by which sight is preserved by the development of a bright and uniform light in all homes and public places, but it offers to supply, by the use of , electricity, means by which the tem- *•:{*; peraatpiii.^ibjiildings, whatever their purpose may . . . . ^J^ej^i.s.^sustaipei .without the use of steam or heated '•\: ;•*: ^^i^*** ^i ik?C>wi«^*of light, electricity is a non-oxygenconsuming form of illumination, and as such is justifiably the superior of any other source of light suggested or utilized. As a source of heat it withdraws nothing from the surrounding atmosphere, leaving it as before with its full quota of oxygen gas. In these two respects the applications of electricity have such pronounced advantages that the only situation left to consider is that relating to the cost of production in either case. In that of light, the public estimation of its value for street and private lighting leaves little to be said. As regards its use for heating, the question of cost appears as the only element requiring a better solution. Although generally regarded as the only and ultimate means of light and heat in the future, electricity is not entirely within the scope of the general public as an ordinary commodity. When its introduction begins in the average home, then lighting will fill its expected niche in the category of household wants. Both light and heat as produced by electricity represent distinct departments of electrical engineering, requiring separate treatment and consideration. They, in common with power produced by the same means, are broad enough in their scope to cover a field of such great dimensions that, with the exception of the principles and methods employed in the transmission of intelligence, or isolated instances of the application of electricity in medical treatment or research, electrical engineering as thus defined enters into the operation of city and suburban life more than any other force to which reference could be made.

Conditions of Electric Lighting. — Without a demand for an electric-light equipment no community can feel justified in encouraging the erection of such a plant. The conditions of electric lighting are those which primarily arise through the necessity for a better form of illumination, more than through the fact that either a power or fuel source is conveniently near, or an enterprise of some sort is required to give that touch to town or city life best secured through the use of electricity for lighting. Yet whether these circumstances appear or not, the conditions of electric lighting are those which enter into a consideration of the following :

1. Whether the community to be supplied with a plant is compact and represented by stores and private dwellings, as well as needing light in its principal thoroughfares.

2. Whether the community to be supplied with a plant is of a scattered nature, though represented by stores and private dwellings, as well as needing light in its principal thoroughfares.

3. Whether the character of the community is sufficiently permanent to lead to the conclusion that further extensions of the system will be imperative to meet the demand for light.

4. Whether the growth of the community is sufficiently independent of large factories to possess the inherent power of expansion without regard to the success of said factories.

5. Whether the community is large enough in size to require more than one center of distribution in the lighting scheme, such as sub-stations, etc.

6. Whether a single station will be sufficient to supply all requirements when the expected contracts . for light are closed with the municipality and the private consumers desiring electricity.

7. Whether water-power or coal will be utilized for the purpose of operating the plant, or both ; in which case the temperature variations during the year will have to be taken into account to determine during what part of the year the water will be serviceable as a source of power.

8. Whether a contract can be closed with the village, town, or city, for public lighting, to partly guarantee the success of the project.

9. Whether the transportation of machinery is within the scope of the facilities afforded, or whether heavy additional expense for machinery would have to be added through inadequate railroad connections.

10. Whether skilled help can be secured with ease, or with that difficulty which would add unduly to the running expenses of the plant.

Along these lines the conditions of electric lighting may receive the attention necessary to secure an intelligent appreciation of all requirements. Other items, in addition to those enumerated, may be added, as fresh circumstances arise, when the final consideration of essential details will bring them to view. But although it is customary to regard this particular department of applied electricity as a purely technical proposition, it is clearly evident that as a technical business proposition it emphatically holds first place. Instances of where gas-lighting plants were installed long before the electric-light plant, have given rise to the question whether such competition, when keen, can lead to any form of success in a contracted community.

In other words, the best conditions for the healthy development of the electric-light industry are those vvhich first mean a popular demand for illumination of this character; second, the possibility of expansion on the part of the town ; and, third, a price for water-powef or fuel which permits a reasonable profit in its transformation from mechanical to electrical energy.

Character of the Current Elmployed. — The expense of installing an electric-light plant is only a part of the total expense invited by the enterprise. The erection of pole-lines or the interment of the conductors underground is well recognized as a feature of the construction which is as necessary as it is subsequently costly to keep in repair. In this respect the setting of the poles, the choice of the insulators, and the size of the copper conductors are bound to have a future effect upon the operation of the plant.

On the other hand, the town may be several miles in length (Fig. i),with its business center, important streets or avenues, and suburban homes stretching along over a considerable distance. A long pole-line would thus become a necessity for the distribution of the current to the various points of tap. If the town is grouped around a given center, with the population comparatively thick in numbers and then thin-

TOWN CENTER

• e ft

<9

& <0

Fig. I. — A Town with Lighting Greatly Distributed.

ning rapidly (Fig. 2), the problem of choosing current becomes much simplified. The currents to be generated belong to either of two kinds, as follows :

First — Alternating current of high pressure, used in conjunction with transformers for converting it to a lower pressure whenever necessary.

Second — Direct or continuous current of comparatively low pressure, for use just as generated in stores, streets, or homes along the line, or within easy access of it.

The use of alternating and direct current in small communities is governed by the geographical position of them, as already stated. The cost of an alternating- and direct-current plant for equal amounts of lighting displays this feature : That under propitious cir<:umstances it is better to use direct than single-phase alternating current, for the reason that motors cannot be readily run by the latter except when specially constructed. Whereas the direct is in some respects an advantage, it is also a more expensive form of energy to conduct ; because, if transmitted at a high pressure, it is not readily transformed down, and for equal powers in comparison with the alternating current, takes very much more copper.

But the proposition which practice has laid down is this, that when the towns are distributed over a large area, the alternating is by far the best current to employ. On the other hand, if a particular case a m

TOWN CE^f^ER 1

Q ^

Q

Q

Fig. 2. — A Town with Lighting Concentrated.

presents itself, of a village or town closely grouped, then the cost of a direct-current plant, with its convenience, would become readily apparent, if the use of power for small motors is considered. The conclusions reached are, therefore, as follows :

First — A straggling town covering considerable area and distance is a case where alternating current is the only form of electricity to employ.

Second — A small, compactly settled town is a case where the use of direct current is worthy of first consideration.

Comparison of the Circuits for Equal Power. — The only means of comparing the sizes of copper wire required in the case of a given amount of electrical

ONE MILE or CONDUCTOR

15,840 CIRCULAR MILS

ONE MILE OF CONDUCTOR

263,440 CIRCULAR MILS

Fig. 3. — Comparison of the Two Sizes of Conductors Carrying Equal Power.

energy transmitted, as high-pressure alternating or low-pressure direct, is by estimating the relative cross-sections of copper required for equal lengths of circuit. Taking a given case of 100 kilowatts of power, transmitted as alternating current a distance of one-half mile, at a pressure of 2,000 volts, the total watts would equal 100 X 1,000= 100,000. At 2,000 volts pressure the amperes would be equal to 100,000 -^ 2,000 = 50, without paying any attention to the losses experienced through inductive efforts, etc. At present the size of wire required with a single-phase system would be, with an allowance of 10 per cent, drop in the line, not much less than r M — ^^^8^.^ ^^ ^^^^ i n ft. X amperes X 12 volts drop or about a No. 8 wire B. & S. gauge. On the other hand, if the same method and the same line conditions are supplied in the case of the direct-current system, the figures present themselves as follows, with a 10 per cent, drop and a pressure of 500 volts : C. M. = circular mils. Amperes = ioo,opo watts -7-- 500 = 200.

or a wire which could only be strung as a stranded cable (Fig. 3), as its size would be more than No. 0000 B. & S. gauge, or about 24 No. 10 wires in circular mils. Comparing this cross-section of copper with that of the first, means 253,440-4- 15,840= 16 times. This result shows that, for a half-mile run, it is not practical to use 16 times as much copper in one case as in another. As two conductors are employed in each circuit, the total weight of copper would be really that of one mile of each of the given sizes.

Pressures of Standard Generators. — The outside demand has largely contributed to the adoption of certain standard pressures. These run as follows with direct- and alternating-current generators :

Direct current.

Alternating current

Generators of

Generators of

125 250 volts.

550 600

10,000 " 15,000 "

1,000 2,000

20,000 " 30,000" "

2,500 3,000

40,000 " 60,000 "

In this table with the last four quotations of 1,000, 2,000, 2,500, and 3,000 volts direct-current machines for high-tension arc lighting are kept in view. The last calculation, showing the difference in sizes of conductors for 500 direct and 2,000 volts alternating, would necessarily become the same if a high-tension direct-current system were introduced. But although this system would serve and be less expensive than the alternating for street lighting, it could not be employed for hall or residential lighting on account of the great danger involved.

Capacity of Machines. — The use of either alternating- or direct-current generators calls for a knowledge of what is to be expected of them when under heavier strain than usual. The overload factor, in other words, is a point of first importance in their purchase. According to the recommendations of the American Institute of Electrical Engineers, in its

Standardization Report, the ability of a generator to be overloaded 25 per cent, without its windings rising in temperature more than 27 (fegrees F. over the normal temperature of full load, or 50 per cent, for a short period of time without destructive sparking, is a sign of the value of the generator to meet the conditions of service. The questions relating to the best conditions under which electric lighting can be begun are not more important than those which re-

BLUE OLAS8 GREAT WASTE

RE0QLA88 QREAT WASTE

YELLOW GLASS LESS WASTE

OPAL GLASS MEDIUM WASTE

LESS WASTE

Fig. 4.— Light Wasted with Diflferent Screens.

late to the qualifications of a machine to perform its functions in this field of engineering.

Selection of Lamps. — The distinction to be drawn between one form of lighting and another is largely determined by the character of the illumination in view. From a broad standpoint, the difference is that of interior from exterior lighting. The lighting of office-buildings, theaters, large halls, and private dwellings differs in the nature of the lamp required when compared with the street. In the first case, many small centers of illumination become the means by which a comparatively uniform degree of lighting is secured. In the second case, more intense centers of illumination are the foundation for an outside system, by which the streets of a city are lit up over a series of contiguous areas. In the majority of cases where lighting is done the light is so developed that it falls directly upon the areas to be brightened. The light is not, as a rule, cast upon a given surface and then reflected downward or sideways. It is the object of lamp manufacturers to avoid, as far as possible, the dispersion of light, or its undue absorption through an inefficient and unscientific method of distribution. Although globes of all shades and colors are brought into commission, their use is only necessary because of the disagreeable intensity of the light and the desirability of shading or toning it down. Different colors are effective, from an aesthetic standpoint, in the production of pleasing hues; but any method which affects the color (Fig. 4) also diminishes the light. Ground glass, as well as opalescent shades or globes, is very wasteful in this respect. The resulting condition of affairs in the lighting field is somewhat illogical, for the reason that knowledge and skill are brought into play to develop a highly efficient generating, transmitting and distributing system with regard to the electrical energy involved; and this, when transformed as far as possible into light, is permitted to pass through globes and screens by which from 10 to 40 per cent, or over is wasted. Meter measurements, indicating excellent judgment in the method of generating electricity when contrasted, in some instances, with the returns obtained from candle-power measurements.

seem to show light losses of a serious nature. A given amount of power is productive, with modern lamps, of a given amount of light. In other words, a certain weight of kerosene oil, from which a horse-power could be obtained if exploded in a gas-engine ; a certain number of cubic feet of illuminating gas similarly utilized; a given weight of coal burned in a steam-boiler, or oil used as fuel for the same purpose ; a horse-power of mechanical energy obtained from a waterfall — all of these are sources of power which, with respect to lighting, produce a certain amount of candle-power. Power therefore represents light when so transformed ; and it merely becomes a question of the means or mechanism employed as to whether the transformation results in more or less light. Using screens or shades, however, which absorb a heavy percentage of the light is equivalent to using defective machinery in the way of inefficient boilers, engines, dynamos, or conductors, causing too heavy a line loss.

Returns from Generators. — Generators of different horse-power vary with respect to the returns they give when set into operation. A i-hp. dynamo is necessarily less efficient than a i,ooo-hp. machine. The percentages of efficiency increasing with the capacity of the generator is to be expected, as shown by the following table :

Power of generator

Percentage of in kilowatts.

efficiency.

I

70 " 80 lO

80 " 90

IS ■

85 "90

SO

■ 80

ISO

93 5

94-5

1,000

96 " 97

Cost of High Efficiency. — One of the best rules followed by conservative engineers is that of not reaching out for a too expensive efficiency. By this is meant the efforts made to overtop the market commercial efficiencies a few per cent., at a cost all out of proportion to the benefits gained. As ari example of this false economy, suppose a generator costs $1,000 and possesses an efficiency of 90 per cent. If it costs the manufacturer an extra $200 to gain a few per cent, more, then whatever economic advantages are supposed to arise from the few per cent, additional efficiency, are lost through the following: First, the additional investment and the interest it represents; second, the additional difficulty of making sales through the higher price; third, the necessity for special attention to preserve the advantages paid for ; and, fourth, the inability to control losses in light through its faulty manipulation when developed. For these, and perhaps additional minor reasons, the cost of high efficiency is sufficiently prohibitive, when obtained by too heavy an additional investment, to be a practical commercial proposition. The durability and amount of care required to give uniformly good returns are the governing influences in the selection of dynamos and motors. For equal investments, the machine of the most solid construction is by far the better choice.