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PLATE I.
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ELECTEIC POWEE
TEANSMIS8I0N
A PRACTICAL TREATISE
FOR PRACTICAL MEN
LOUIS BELL, Ph. D.
MEMBEB AMERICAN INSTITUTE OF ELECTRICAL ENOINEEBS
FOURTH EDITION
REVISED AND ENLARGED
NEW YORK
McGRAW PUBLISHING CO.
1006
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GOPYBIOHT, 1906, BY
McGRAW PUBLISHING CO.
New York
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94186
MA: r 1906
^u-iw^h
•3
PREFACE TO FIRST EDITION.
This volume is designed to set forth in the simplest possible
manner, the fundamental facts concerning present practice in
electrical power transmission.
Busy men have little time txy spend in discussing theories of
which the practical results are known, or in following the
derivation of formula) which no one disputes. The author has
therefore endeavored, in introducing such theoretical consid-
erations as are necessary, to explain them in the most direct
way practicable; using proximate methods of proof when pre-
cise and general ones would lead to mathematical complica-
tions without altering the conclusion for the purpose in hand,
and stating only the results of investigation when the processes
are undesfrably complicated.
In writing of a many-sided and rapidly changing art, it is
impossible in a finite compass to cover all the phases of the
subject or to prophesy the modifications that time will bring
forth; hence, the epoch of this work is the present and the
point of view chosen is that of the man, engineer or not, who
desires to know what can be accomplished by electrical power
transmission, and by what processes the work is planned and
carried out. This treatment is not without value to the stu-
dent who wishes to couple his investigations of electrical
theory with its application in the hands of engineers, and puts
the facts regarding a very great and important development of
applied electricity in the possession of the general reader.
Such apparatus as is described is intended to be typical of
the methods used, rather than representative of any particular
scheme of manufacture or fashion in design. These last
change almost from month to month, while the general con-
ditions remain fairly stable, and the underlying principles are
of permanent value.
Janiuiry, 1897,
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PREFACE TO FOURTH EDITION.
In the three years which have elapsed since the third edition
of this work went to press, there have been very few sensational
changes in electric power transmission. Plants have multiplied
and higher voltages have become common without any radical
innovations in general practice. There has been very con-
siderable improvement in station accessories and in details of
construction, especially of the line. The resources of the art
with respect to distribution of energy for various purposes are
steadily growing richer, much apparatus special a few years
since has become standardized, and altogether the aggregate
of minor changes has made a new edition imperative. It has
been needful to devote some special attention to the important
accessory apparatus of which modern stations are full, and to
make use of considerable new material of a more general sort,
as well as to eliminate some descriptive matter which had to
do with things which are obsolete and without historical im-
portance. It has now become a hopeless task to keep track
of 10,000 volt plants which are at present utterly commonplace,
so that in retaining merely as a matter of general interest, a list
of high voltage transmission plants, nothing under 20,000 volts
is included. This fact expresses better than words the trend
of recent advances in the art.
September J 1905.
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CONTENTS.
Craptbr. pAoa.
I. Elementary Principles 1
II. General Conditions op Power Transmission ... 23
III. Power Transmission by Continuous Currents ... 77
IV. Some Properties op Alternating Circuits .... 125
V. Power Transmission by Alternating Currents . . 158
VI. Alternating Current Motors 217
VII. Current Reorganizers 280
VIII. Engines and Boilers 309
IX. Water-Wheels 349
X. Hydraulic Development 387
XI. The Organization op a Power Station 418
XII. Auxiliary and Switchboard Apparatus 455
XIII. The Line 474
XIV. Line Construction 536
XV. Methods op Distribution 581
XVI. The Commercial Problem 639
XVII. The Measurement of Electrical Energy 660
XVIII. High Voltage Transmission 687
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LIST OF TABLES.
Efficdsnct of Wire Rope Drives 39
Wire Ropes and Pullets Therefor 42
Loss OF Head in Hydraulic Pipes 47
Loss OF Air Pressure in Pipes 52
Efficiencies of Electric Motors 62
Efficiencies of Electric and Other Transmissions .... 74
Performance of Small Polyphase Motors 270
&rEAM Consumption of Enoines 318
EvAPORATiyB Power of Fuels 329
Evaporative Tests of Boilers * . . . 330
Coal Consumption of Engines 332
Table for Weirs 391
Properties of Steel Hydraulic Pipe 408
Properties of Copper and Other Wires 486
Size, Resistance, and Weiohts of Copper Wires 509
Natural Tangents, Sines, and Cosines 522
Size, Weight, and Tensile Strength of Line Wires .... 539
Sizes and Weights of Wooden Poles 551
Tensile Strength of Woods 554
Properties of Direct and Alternating Current Arcs . . . 592
Cost of Power with Various Engines 640
Cost of Intermittent Power with Various Engines .... 643
Cost of Electric Motors 644
Typical List of Discoxtntb 657
List of American Transmission at or above 20,000 Voi/rs . 702
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ELECTRIC TRANSMISSION OF POWER.
CHAPTER I.
ELEMENTARY PRINCIPLES.
It has long been the fashion to speak of what we are
pleased to call electricity as a mysterious "force," and to
attribute to everything connected with it occult characteristics
better suited to mediaeval wizardry than to modem science.
This unhappy condition of affairs has, in the main, come about
through the indistinctness of some of our fundamental ideas
and inexactitude in expressing them.
To speak specifically, there has been, even hi the minds
and writings of some who ought to know better, a tendency
toward confusing the extremely hazy individuality of *' elec-
tricity" with the sharply defined properties of electrical
energy. We have been so overrun by theories of electricity,
two-fluid, one-fluid, and non-fluid — by electrically ** charged"
atoms and duplex ethers, that we have well-nigh forgotten the
very great uncertainty as to the concrete existence of elec-
tricity itself. Even admitting it to be an entity, it most
assuredly is not a force, mysterious or otherwise. Electrical
force there is, and electrical energy there is, and with them
we can freely experiment, but for most practical purposes
"electricity" is merely the numerical factor connecting the
two. It is related to electrical energy much as that other
hypothetical fluid ''caloric" was supposed to be related to
heat energy. The analogy is not absolutely exact, but it
nevertheless summarizes the real facts in the case.
The day has passed wherein we were at liberty to think of
"electricity" as flowing through a material tube or as plas-
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2 ELECTRIC TRANSMISSION OF POWER.
tered upon bodies like a coat of paint. The things with ^vhich
we have now to deal are the various factors of electrical
energy.
It is the purpose of this chapter to treat of that form of
energy which we denominate electrical, to discuss its relation
to other forms of energy and some of the transformations
which they may reciprocally imdergo.
Speaking broadly, energy is power of doing work. The
energy of a body at any moment represents its inherent capacity
for doing work of some sort on other bodies. This, however,
must not be understood as implying that the aforesaid energy
is limited by our power of utilizing it. We may or may not be
able to employ it to advantage or xmder possible conditions.
As an example, take the massive weight of a pile driver.
Raised to its full height it possesses a certain amount of gravi-
tational energy — a possibility of doing useful work. This
energy is temporarily unemployed and appears only as a stress
on the supporting rope and frame-work. Under these cir-
cumstances, wherein the energy exists in static form, it is
generally known as potential energy.
Now let the weight fall and with swiftly gathering velocity
it strikes the pile and does work upon it, settling it deep into
the mud. The energy due to the blow of the moiring weighty
energy of motion in other words, is called kinetic. But at the
bottom of its fall the weight still has potential energy with
reference to points below it, and we realize this as the pile
settles lower and each successive blow becomes more forceful.
At some point we are unable further to utilize the fall, and
have then reached the limit of the available energy in this par-
ticular case.
We must not forget, however, that each time the weight
was lifted, work had to be done against gravitation to give the
weight its point of vantage with respect to available energy.
This work was probably done by utilizing the energy of
expanding steam — in other words, the energy of the steam
was transformed through doing work on the piston into kinetic
energy of the latter, which, through doing work against gravi-
tation, has been enabled again to reappear as the energy of
a falling body, and to do work on the driven pile. And back
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ELEMENTARY PRINCIPLES, 8
of the steam energy is the heat energy, by which work is
done on the water in the boiler, and yet back of this the chemi-
cal energy of the coal, transformed into heat energy and doing
work on the minute particles of iron in the boiler, for we know
that heat is a species of kinetic energy.
Even the work done on our pile is not permitted to go un-
transformed into energy. Part is transformed into heat energy
through friction and compression of the pile, part through fric-
tion of the water, and part raises ripples that may lift against
gravity chips and pebbles on a neighboring shore. Other frac-
tions go into the vibrational energy of sound ; into heating the
weight so that it gives out warmth — radiant energy — to the
hand when held near it and to the surrounding air; and into
electrical work done on the weight and neighboring objects, for
the weight unquestionably receives a minute amount of elec-
trical energy at each blow. Thus, a comparatively simple
mechanical process involves a long series of transformations
of energy.
No energy is ever created or destroyed, it merely is changed
in form to reappear elsewhere, and work done is the link
between one form of energy and another. And we may lay
down another law of almost as serious import: No form of
energy is ever transformed completely into any other.
On the contrary, the general rule is that with each transfor-
mation several kinds of energy appear in varying amounts, and
among them we may always reckon heat. The object of any
transformation is usually a single form of energy, hence practi-
cally no such thing as perfectly efficient transformation can be
obtained. The energy by-products for the most part cannot
be utilized and are frittered away in useless work or in storing
up kinds of potential energy that cannot be employed.
The greatest loss is in heat, which is dissipated in various
ways and cannot be recovered. The presence of unutilized
heat always denotes waste of energy.
From what has gone before, we can readily appreciate that
when we do work with the object of rendering available a
particular kind of energy, the method must be intelligently
selected, else there will result useless by-products of energy
which will seriously lower the efficiency of the operation.
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4 ELECTRIC TRANSMISSION OF POWER,
Whenever possible we utilize potential energy already exist-
ing in securing a transformation. Thus if heat is wanted, the
easiest way of getting it is to bum coal, and to allow its energy
to become kinetic as heat. If we want mechanical work done,
we set heat energy to work in the most efficient way practi-
cable. If electrical energy is desired, we set the energy of
st^am to revolving the armature of a dynamo. If the right
method of transformation is not chosen, much of the energy
will turn up in forms that we do not want or cannot utilize.
Burning coal is a very bad way of getting sound, just as play-
ing a comet is but a poor means of getting heat, although a
fire does produce a trifling amount of soimd, and a comet by
continual vibration must be warmed to a minute degree.
These seem, and perhaps are, extreme instances, but when
we realize that, somewhat to the discredit of human ingenuity,
less than one-twentieth of the electrical energy supplied to an
incandescent lamp appears in the form of light, the comparison
becomes grimly suggestive.
Understanding now that in order to obtain energy in any
given form (such as electrical), particular methods of transfor-
mation must be used in order to secure anything like efficiency,
we may look a little more closely at various types of energy to
discover the characteristics that may indicate efficient methods
of transformation, particularly as regards electrical energy.
Speaking broadly, one may divide energy into three classes:
1st. Those forms of energy which have to do with move-
ments of, or strains in, masses of matter. In this class may
be included the ordinary forms of kinetic energy of moving
bodies and the like.
2d. Those which are concerned with movements of, or
strains in, the molecxiles and atoms of which material bodies
are composed. In this class we may reckon heat, latent and
specific heats, energy of gases, and perhaps chemical energy.
3d. All forms of energy which have to do with strains which
can exist outside of ordinary matter, i.e., every kind of radiant
energy and presumably electrical energy.
These classes are not absolutely distinct; for example, we
do not know the relation of chemical energy to the third class,
nor of gravitational energy to any class, but such a division
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ELEMENTARY PRINCIPLES, 5
serves to keep clearly in our minds the kind of actions to
which our attention is to be directed.
It is only within the past few years that we have been able
with any certainty to classify electrical energy, and even now
much remains to be learned. P'or a very long while it has
been known that light, i.e., luminous energy, must be propa-
gated through a medium quite distinct from ordinary matter
and possessing certain remarkable properties. It was well
known that luminous energy is transferred through this
medium by vibratory or wave motion. Even the period of
the vibrations and the lengths of the waves were accurately
measured, and from these and similar measurements it has
been possible to classify the mechanical properties of this
medium, universally called "the ether," until we really know
more about them than about the properties of many kinds of
ordinary matter — a number of the rare metals, for example.
The next important step was the discovery, verified in the
most thorough manner, that what had been known as radiant
heat, such as we get from the sun or any very hot body, is
really energy of the same kind as light. That is, it was found
to be energy of wave motion of precisely the same character
and in the same medium, differing only in frequency and wave
length. It also has turned but in similar fashion that what
had been called "actinic" rays, that are active in attacking a
l)hotographic plate and producing some other kinds of chemi-
cal action, are only light rays of shorter wave length than
usual, and so ordinarily invisible to the eye.
So much having been ascertained, it became clear that
instead of three kinds of energy — "heat, light, and actinism,"
we were really dealing with only one — radiant energy, \nbrat-
ing energy in the ether, varying in effect as it varies in fre-
quency. Speaking in an approximate way, such wave energy
has a frequency of six hundred thousand billion vibrations per
second and a velocity of propagation of about a hundred and
eighty-five thousand miles per second, so that each wave is
not far from one fifty-thousandth of an inch long. These
dimensions are true of light waves; chemical action can be
produced by waves of half the length, while so-called heat
rays may be composed of waves two or three times as long as
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6 ELECTRIC TRANSMISSION OF POWER.
those of light. Such figures are startling, but they can be
verified with an accuracy greater than that of ordinary
mechanical measurements.
We see that this radiant energy is capable of producing
various disturbances perceptible to our senses, such as chemi-
cal action, light, and heat, and that these different effects
simply correspond to waves of energy having different fre-
quencies and wave lengths. This being so, it is not umiatural
to suppose that at still different frequencies other effects
might be noted. This idea gains further probability from the
experimental fact that waves of very different frequency
traverse the ether with precisely the same velocity, showing no
signs of slowing down or dying out, so that there seems to be
no natural limit to their length.
During the past half dozen years it has been clearly shown
that "radiant energy" is capable of producing profound
electrical disturbances, such as violent oscillations of electrical
energy in conducting bodies, and that these effects exist what-
ever the frequency of the ether waves concerned. This ver3'
important fact was clearly foreseen by Maxwell more than
twenty years ago, regarding light, and his prediction has been
thoroughly verified through the persistent researches of the
late Professor Hertz and others.
This discovery is often expressed by saying that radiant
energy is an electro-magnetic disturbance, or that light is one
kind of electrical action. It is more strictly accurate to say
that radiant energy, just as it produces chemical disturbances
on the photographic plate, affects the eye as light, and material
bodies as heat, is also capable of producing electrical effects
when transferred to the proper media. Most of our experi-
ments on its electrical effects have been performed with waves
many thousand times longer than those of light, but their gen-
eral character has proved to be exactly the same.
A given substance may be differently related to waves of
radiant energy of different lengths, but the phenomena are
still essentially the same. For instance, a plate of hard
rubber is thoroughly opaque to waves of a length correspond-
ing to light, but is quite transparent to those of considerably
greater length, such as can produce thermal or electrical
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ELEMENTARY PRINCIPLES. 7
effects. A plate of alum will let through light waves and very
long waves, but will stop most of those which are efficient in
producing heat. A thick sheet of metal is quite opaque to all
known waves of radiant energy. Hence the fact noted long
ago by Maxwell, that all good conductors are opaque to light,
although the converse is not true.
The substance of all this is, that the same sort of disturb-
ance in the ether which produces light is also competent to set
up electrical actions in material bodies, and conversely, such
actions may and do produce corresponding disturbances in the
ether, which are thus transferred to other bodies. Such a
transference corresponds to all that we know concerning the
velocity with which electrical and electro-magnetic disturb-
ances pass from body to body. It is equally certain that this
velocity totally transcends anything we could hope to obtain
from bodies having the dynamical properties of ordinary
matter, while it does fit exactly the dynamical properties of
the ether.
We are thus forced to the conclusion that when an electrical
current, as we say, "passes along" a wire, whatever a "cur-
rent" may be, it is not simply transferred from molecule to
molecule in the wire as soimd or heat would be, but that there
is an immensely rapid transfer of energy in the neighboring
ether that reaches all points of the wire almost simultaneously.
It takes a measurable time for the electrical energy to reach
and utilize the centre of the wire, although its progress along
the surface, thanks to the free ether outside, is enormously
rapid.
Thus takes place what is generally called a "flow of elec-
tricity" along the wire. Looking at the process more closely,
the nearest approach to flow is the transfer of energy along
the wire by means of stresses in the ether which in turn set up
strains in the matter along their course.
Whenever we cause in matter the particular stress which we
call electromotive force for lack of a more exact name, the
resulting strain is electrification, and if the stress be applied
at one point of a conducting body, the strain is immediately
transferred to other points by the stresses and strains in the
surrounding ether. Wherever this transference of strain exists
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8 ELECTRIC TRANSMISSION OF POWER.
we have an electrical current, although this name is generally
reserved for those cases in which there exists a perceptible
transference of energy by the means aforesaid. If the condi-
tions are such that energy must be steadily sxipplied to keep
up the electromotive stress, we have such a state of things as
we find in a closed circuit containing a battery.
To cause such a flow of energy we must first find means of
setting up electromotive stress capable of being propagated
through the ether. Now atoms and molecules are the only
handles by which we can get hold of the ether. Only in so
far as we can work through them can we do work on the
ether.
As a matter of fact, we cannot do work of any kind on the
molecules of a body without setting up electrical stresses of
some sort. In most cases of mechanical work, which in the
main produces stress on the molecules only by strains in
the mass, the energy appears mainly as heat, and is only inci-
dentally electrical, as for instance the energy wasted in a
heated journal.
When, however, by any device we do work more directly on
the molecules of a body, or on the atoms which compose the
molecules, we are more than likely to transform much of this
work into electrical energy. As a rough example of the two
kinds of action just mentioned, pounding a body heats it with-
out causing any considerable electrification, while on the other
hand rubbing it rather gently, sets up a considerable electrifica-
tion without heating it noticeably.
In fact, for many centuries, friction was the only known
method of causing electrification. Later, as is well known, it
was discovered that certain sorts of chemical action, which has
to do directly with interchanges of energy between molecxiles,
were very potent in electrical eflPects. With this discovery
came the ability to deal with steady transfers of electrical
energy in considerable amount (electric currents), instead of
the relatively slight and transitory effects previously known
(electrification, "frictional" electricity).
To clear up the real nature of this difTerence it is well to
consider what we mean by saying that a body is electrified, or
has an electrical charge. In other words, what is electrifica-
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ELEMENTARY PRINCIPLES. 9
tion? Not very many years ago this qxiestion would have
been answered by saying that a quantity of a substance, posi-
tive electricity (or negative as the case might be), had been
communicated to the body in question; that this remarkable
substance could reside only at the surface of the body and
was able to produce in surrounding bodies exactly an equal
quantity of negative electricity; that this '* charge" of elec-
tricity would repel another ^'charge" of the same substance
placed near it, or attract a charge of its opposite, the other
substance called negative electricity; and mxich more to the
same effect. All this was a very convenient hypothesis — it
explained, after a fashion, the common facts and enabled
investigators to discover many important electrical relations
and laws. But it expressed much more than there was any
reason to know. From the standpoint of our modem doc-
trines of energy, electrification is a very different thing.
Let an electromotive stress (from whatever source) be
applied to a body, a metallic sphere for example, long enough
to transfer to it a finite amount of energy. This energy
appears as stresses and strains in the ether everywhere about
the body under consideration and thence extends to the mole-
cules and atoms of neighboring bodies, causing "induced
charges." It is as if one were to fill a box with jelly, and then
pull or push or twist a rod embedded in its centre. The
result would be strains in the rod, the jelly, and the box, and in
a general way the total stress on the box woxild equal that on
the rod. By proper means we could detect the strain all
through the substance of the jelly, but most easily by its varia-
tions from place to place.
We do not know exactly what sort of a strain in oxir ether
jelly is prodxiced l>y electromotive stress, but we do know that
it possesses the quality of endedness, so that the strains in
the matter concerned, i.e., in the ball and surrounding bodies,
are equal and opposite.
In fact, the two "charges" are in effect the two ends of the
same strain in the ether. They appear to us to be real attri-
butes of the two opposed surfaces, because at these surfaces
the dynamical constants, such as density, elasticity, etc., of
the medium through which the strain is propagated, change
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10 ELECTRIC TRANSMISSION OF POWER.
in value, and differences in state of strain there become physi-
cally manifest.
In electric currents we have a very different state of things.
The energy supplied by the electromotive stress, instead of
becoming potential as electrostatic strain, and producing
"charge," does work and is transformed into other kinds of
energy, thermal or chemical, mechanical or luminous.
When a stress of whatever kind is applied to a body, only a
limited amount of energy can be transferred by it so long as
the energy remains potential. Thus, in our box of jelly before
referred to, a twist of given intensity applied to the stick, as
for instance by a string wound around it and pxilled by a given
weight, can only transfer energy until the stresses produced in
the jelly come to an equilibrium with it. On the other hand, if
the box were filled with water and the stick were the axle of a
sort of paddle wheel, the very same intensity of twist could go
on communicating energy to the water as long as one chose
to apply the necessary work.
This roughly expresses the difference between electric
charge and electric current, viewed from the standpoint of
energy. An electromotive stress applied to a wire charges it
and then the transfer of energy ceases. If the same stress be
applied under conditions that allow work to be done by it,
energy will be transferred so long as the stress is kept up. In
an open electric circuit we have a charge as the result of elec-
tromotive stress. When the circxiit is closed, i.e., when a
continuous medium is furnished on which work can be done,
we have an electric current. The amount of this work and
the flow of electrical energy that produces it depend on the
nature of the circuit. Certain substances, especially the
metals, and of metals notably copper and silver, permit a
ready continuous transfer of energy in and about them. Such
substances are called good conductors. The real transfer
of energy takes place ultimately via the ether, but its amount
is limited by the amount and character of the matter through
which work can be done.
Whenever the strains in the ether, such as we recognize in
connection with electrical charge, shift through space as when
a current is flowing, other strains bearing a certain relation
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ELEMENTARY PRINCIPLES. 11
to the direction of flow are made manifest. Where there
is a rapid and intense flow of energy, these strains are very
great and important compared with any electrostatic strains
that exist outside the conducting circuit. In other cases they
may be quite insignificant. These strains are electro-magnetic,
and with them we have to do almost exclusively in practical
electrical engineering. They appear wherever there is a moving
electrical strain, whether produced by moving a charged body
or causing the charge upon a body to move.
Both kinds of strains exist in radiant energy, as in other
cases of flowing energy. The stresses in electro-magnetic
energy are at right angles both to the electrostatic stresses
and to the direction of their motion or flow. If, for example,
Fi«}. 1. Fid. 2.
we have a flow of electrical energy in a straight wire (Fig. 1),
the electro-magnetic stresses are in circles about it.
If A be a wire in which the flow of energy is straight down
into the paper, the electro-magnetic stresses are in circles in
the direction shown by the arrow heads. If the wire be bent
into a ring (Fig. 2), with the current flowing in the direction of
the arrows, then the electro-magnetic stresses will be (follow-
ing Fig. 1) in such direction as to pass downward through the
paper inside the ring.
These electro-magnetic stresses constitute what we call a
magnetic field outside the wire. The intensity of this field can
be increased by increasing the flow of energy in the desired
region in the systematic way suggested by Fig. 2. If, for
example, we join a number of rings like Fig. 2 into a spiral
coil shown in section in Fig. 3, in which the current flows
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12
ELECTRIC TRANSMISSION OF POWER.
downward into the paper in the lower edge of the spiral,
there will be pro<luced a ma^etic field in which the stresses
have the direction shown by the arrows. Such a spiral consti-
tutes a genuine magnet, and if suspended so as to be free
to move would take up a north and south position with it.s
right-hand end toward the north. In and about the spiral
there exists a magnetic "field of force," which is merely another
way of saying that the ether there is xmder electro-magnetic
stress. Its condition of strain is closely analogous to that
about an electrified body, and, as in that case, there is no
\ V,X \.^J I
Fio. 3.
work done on the ether after the strains are once established,
since the energy then becomes potential. While this is being
accomplished, work is done just as when a body is charged.
If, now, setting up such an electro-magnetic field requires
energy to be spent by causing a current to flow in the spiral,
we shoxild naturally expect that if the same field could be set
up by extraneoas meahs, energy would momentarily be spent
on the spiral in producing stresses and strains similar to those
that set up the original field. This is found to be so, the
process working backward as well as forward.
If, for example, we have two rings (Fig. 4), and by sending
a current around one, transfer energy to the medium outside
it, this energy will set up an electromotive stress in the other
ring. The direction of this stress is not at once obvious, but
we can get a very clear idea of it by considering the work
done. If current is started in A (Fig. 4), in the direction
shown, electro-magnetic stresses are produced in the direction
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ELEMENTARY PRINCIPLES, 13
of the arrow C If these are to do work on B, the electro-
motive stress in the latter cannot have such a direction as to
set up on its own account a magnetic field that would assist
that of A, otherwise we could increase the field indefinitely
without added expenditure of energy. Therefore, the electro-
motive stress in B, and hence the current, must be in a direc-
tion opposing the original current in -4, as shown in the figure.
In like manner if the current in A be stopped and the field
due to it therefore changes, there are changes in the electro-
magnetic stresses about By that again set up an electromotive
stress in it. If, however, this change of stress is to do work,
the electromotive stress in B must be of such direction as to
PlO. 4.
opf)ose by its field the change in the field of A — i,e,, it must
change its direction and will now give us a current in the
same direction as the original one in A. All this follows the
general law, that if work is to be done by any stress it must
be against some other stress. There can be no work without
resistance.
In Fig. 4 we have the fundamental facts of current induction
on which depend most of our modem methods of generating
and working with electrical energy. Summed up they amount
to saying that whenever there is a change in the electro-mag-
netic stresses about a conductor, work is done upon it, depend-
ing in direction and magnitude on the direction and magnitude
of the change in the stresses.
This is equally true whether the stresses change in absolute
value or whether the conductor changes its relation to them.
Thus, in Fig. 4, if A carries an electrical current the result on
B is the same whether the field of A changes through cessation
of the current, or whether the same change in the stresses
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14 ELECTRIC TRANSMISSION OF POWER.
about B is produced by suddenly pulling B away from A. The
rate at which work is done depends on the rate at which the
stresses are caused to change, as might be expected. So long
as the stresses are constant with reference to the conductor in
which current is to be induced, no work can be done xipon it.
These principles form the foundation of the dynamo, motor,
alternating current transformer, and many other sorts of elec-
trical apparatus. Their details may differ very widely, but we
can get all the fundamental ideas from a consideration of Figs.
3 and 4. To define somewhat the specific idea of the djmamo,
consider what happens when a conducting wire is thrust into
a magnetic field such as is produced by a coil, as in Fig. 5.
As in Fig. 3, let the current in the coil be flowing down-
ward into the paper in the lower half of the figure. -4 is a
wire perpendicular to the plane of the paper in front of the
coil, its ends being united at any distant point that is con-
ri(^ 5.
venient. Knowing that moving the wire into the field will set
up electromotive stresses in it, we can as before determine
their direction by remembering that work must be done.
That is (see Fig. 1), the induced current will flow through A
downward into the paper. In passing out of the field, the cur-
rent would be xipward.
We have so far neglected the rest of the circuit. To be
exact, we should consider it as in Fig. 6. Following the same
line of reasoning as in Fig. 5, we see that while the ring A is
entering the magnetic field the current induced in it must be
opposite to that in the inducing coil (see Fig. 4). When the
coil is leaving the field, however, this direction will be reversed.
Considering the coil A as a whole, we see that so long as the
total field tending to set up stresses in it is increasing, a cur-
rent will be induced opposed to that in the inducing coil.
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ELEMENTARY PRINCIPLES.
15
While the total field is diminishing, the induced current will
be in the other direction. The work that is spent in moving
the coil A will for the most part reappear as electrical energy
in that coil. Arrange the parts of Fig. 6 so that the motion
of A can be accomplished uniformly and continuously, and
i-tl-*-++rfi-«-t++-l--
*■^■Ti-^-^t-♦■t^-t+4^-
H-rf-f-f-r+-tf"^-H-H
Ht-i-4-t-t-i-4-»-|-ff-»-f
ht -•-+■»- t-T'l-fH-f^t
d t) o o d o D^c^':C
I
FlO. 6.
we should have a true, though rudimentary, d)aiamo. Such
a structure could be made by fixing A to the end of an arm
pivoted at the other end and then revolving the arm so that
at each revolution the coil A would sweep through the field
of the magnetizing coil (see Fig. 7). The result of this, as
we have seen, would be on entering the field, a current in
one direction, and on leaving, a current in the other. There
would thus be an alternating current developed in the ring A,
Fig, 7.
If it were cut at some point, and wires led down the arm and
to two metal rings on the axis B, we could obtain, by pressing
brushes on these rings, an alternating current in any outside
circuit. To make more of the revolution of the arm useful,
we could arrange inducing coils in a circle about B. There
would then be an alternation as A passed each coil.
All these devices, however, would produce comparatively
weak effects, because it is difficult to produce powerful mag-
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16 ELECTRIC TRANSMISSION OF POWER,
netic stresses in so simple a way. There are very few materials
in which magnetic stresses are easily set up or propagated.
Chief among these is iron, which bears somewhat the same rela-
tion to magnetic actions that copper does to electrical ones.
By giving to the coil in Fig. 7 a core of soft iron, the electro-
magnetic effects obtained from it would be greatly enhanced.
They are comparatively fee])le in air, and the more iron we put
in their path the better. Developing this idea, we have in
Fig. 8 a much better device for setting up electric currents.
Here the coil of Fig. 7 is wound around an iron core, the ends
of which are brought near together. The arm of Fig. 7 is
also of iron with enlarged ends, and the ring A is replaced by
a coil of several turns.
The magnetic stresses brought to bear on the coil A are thus
made comparatively powerful. Following out on Fig. 8 the
reasoning applied to Fig. 7, we see that considerable electro-
motive stresses would be set up by the revolution of X, alter-
nating in direction at each half revolution. In fact, A is the
armature of a simple alternating dynamo, having two poles
N and ;S', so called from their magnetic relations (see Fig. 3).
We have not thus far considered the source of the electro-
magnetic field involved. It may be obtained as shown by
utilizing the electro-magnetic stresses set up by a wire convey-
ing electrical energy, or on a small scale from permanent mag-
nets. The essential fact, however, is that by forcing a wire
through a region of electro-magnetic stress, electromotive
stresses are set up in that wire, the action in every case being
in such direction as to compel us to do work on the wire.
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ELEMENTARY PRINCIPLES. 17
This work appears as electrical energy in the circuit including
the moving wire.
Now return to Fig. 5 and consider the effect if the wire A is
carrying a steady flow of electrical energy. It will set up
electro-magnetic stresses about it as already described. If
the current be downward into the paper in A, these stresses
will be opposed to the stresses in the field. Inasmuch as we
have seen that in setting up such a current, work had to be
done in forcing the wire into the field, it follows that given
such a current, there must be between its field and that of the
coil a repulsive force which had to be overcome by doing the
work aforesaid. In other words, there must have been a
tendency to throw A out of the field of the coil. Just as
work had to be spent to produce electrical energy in ^4, so
electrical energy will be spent in keeping up the stresses
around A that tend to drive it out of the magnetic field. If
the current in A were in the other direction, the stresses in its
field and that of the coil would be concurrent instead of
oppose<l, and their resultant would tend to draw wire and coil
together, i.e., work would have to be spent to keep them
apart. This is the broad principle of the electric motor. It
is sometimes referred to as simply a reversal of the dynamo,
but it really makes no difference whether the structure in
which the action jiLst described takes place is well fitted to
generate current or not. (iiven a magnetic field and a wire
carrying electrical energy, and there will be a force between
them depending in direction on the directions of the electro-
magnetic stresses belonging to the two. If either element is
arranged so as to move and still keep up a similar relation
of these stresses we have an electric motor. Whether so
arranged as to fulfil this condition with alternating currents,
or in such manner as to require currents in one direction only,
the principle is the same.
So far as unidirectional or "continuous" currents are con-
cerned they are usually obtained from dynamo electric machines
similar in principle to Fig. 8. This machine, if the ends of
the winding on the armature be connected to two metal rings
insulated from each other, serves as a source of alternating
currents which can be taken off the two rings by brushes
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18 ELECTRIC TRANSMISSION OF POWER,
pressed against them. If it is necessary to obtain currents in
one direction only, this can be readily done by reversing the
connection of the outside circuit to the windings at the same
moment that the current reverses in them. The simplest way
of doing this is by a "two part commutator," such as is
shown in diagram in Fig. 9. Here A is the shaft surrounded
by an insulating bushing. On this are fitted two half rings, C
and C\ of metal (the commutator segments). On these bear
brushes B and B\ If the ends of the winding are connected
to C and C, and the brushes are so placed that they pass from
one segment to the other at the moment when the current in
the winding changes its direction, the direction of the current
with respect to the brushes and the outside circuit with which
they are connected obviously remains constant.
In the actual practice of dynamo building very many refine-
ments have to be introduced to serve various purposes, but the
underlying principle remains the same, i,e., to set up in a con-
ductor electromotive stresses by dragging it into and out of
the strained region of ether under an electro-magnetic stress.
According as the dynamo is intended for producing con-
tinuous or alternating currents, its structure is somewhat
modified with its particular use in view. These modifications
extend not only to the general arrangement but to the details
of the winding. Alternating dynamos usually have a more com-
plicated magnetic structure than continuous current macliines,
and are almost invariably separately excited, i.e., have their
magnetizing current supplied from a generator specialized for
producing continuous current. The magnetic complication is
really only apparent, as it consists merely of an increased nuni-
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ELEMENTARY PRINCIPLES. 19
ber of magnet poles, due to the desirability of obtaining toler-
ably rapid alternations of current.
D)mamos designed for producing continuous current are
modified with the armature as a starting point. The winding
is very generally much more complicated than that of an alter-
nator, and the commutator that serves to reverse the rela-
tion of the windings to the brushes at the proper moment is
correspondingly elaborate. The magnetic structure is usually
comparatively simple. The whole design is necessarily sub-
ordinated to securing proper comnmtation. Continuous cur-
rent dynamos are almost universally self-excited, that is, the
current which magnetizes the field is derived from the brushes
of the machine itself. Whatever the character of the machine
the electromotive force generated in it increases with the inten-
sity of the magnetic field (that is, with the magnitude of the
electro-magnetic strains which affect the armature conductors),
with the speed (that is, with the rate of change of electro-
magnetic stress about these moving conductors), and with
the number of turns of wire of which the electromotive forces
are added. The capacity of the machine for furnishing elec-
trical energy varies directly with the electromotive force and
with the capacity of the armature conductors for transmitting
the energy without becoming overheated. Practically all the
energy lost in a dynamo appears in the form of heat, which
must be limited to an amount which will not cause an undue
rise of temperature.
It is not the purpose of this chapter to deal with the prac-
tical details of dynamo design and construction. For these,
the reader should consult special treatises on the subject,
which consider it with a fulness which would here be quite
out of place. Special machines, however, will be briefly dis-
cussed in their proper places and in relation to the work they
have to do.
Having now considered the principles which underlie the
transformation of mechanical into electrical energy, we may
profitably take up the fundamental facts in regard to the
measurement of that form of energy and the units in which it
and its most important factors are reckoned.
All electrical quantities are measured directly or indirectly
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20 ELECTRIC TRANSMISSION OF POWER.
in terms of the dynamical units foimded upon the imits of
length, mass, and time. These derived dynamical units can
serve alike for the measurement of all forms of energy, so
that all have a common ground on which to stand. As the
electrical units are derived directly from the same units that
serve to measure ordinary mechanical effects, electrical and me-
chanical energies are nuitually related in a perfectly definite way.
A natural starting point in the derivation of a working
system of electrical units may be found in electro-magnetic
stress, such as is developed about an electrical circuit or a
permanent magnet. To begin with, the mechanical units that
may serve to measure any form of energy are derived from
those of length, mass, and time. These latter are almost uni-
versally taken as the centimetre, gramme, and second, the
'* C. Ci. S. " system. Starting from these the unit of force is that
which acting for one second on a mass of one gramme can
change its velocity by one centimetre per second. This unit is
called the dyne, and as a magnetic stress it is equivalent to a
push of about ?4 5*<jiyxF ^f a pound's weight on a similar "imit
pole" one centimetre distant. This unit is inconveniently small
for practical use, and before long some nuiltiple of it is likely
to be given a special name and used for practical reference. In
fact, one megadyne (i.e., 1,()()(),(K)() dynes) is very nearly equiva-
lent to the weight of a kilogramme. Magnetic measiu'e-
ments may thus be made by direct reference to the dyne and
centimetre, since the unit ]>()le is that which repels a similar
pole 1 centimetre distant, with a force of 1 dyne.
Referring now to what has been said about the causes which
vary the electromotive force produced in a dynamo, we fall at
once into the definition of the unit electromotive force, which
is that produced when field, velocity, and length of wire imder
inducticm are all of unit value. The unit electromotive force
is, then, that which is generated in one centimetre of wire
moving one centimetre per second, perpendicular to its own
length, straight across unit field, which is that existing one
centimetre from unit pole as indicated above. This unit, too,
is inconveniently small, so that one hundred miUion times this
(|uantity is taken for the practical unit of electromotive force
and called the volt.
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ELEMENTARY PRIXCIPLES. 21
The unit electrical current is that which flowing through one
centimetre length of wire will create unit field at any point
equidistant from all parts of the wire (as when the wire is
bent to a curve of 1 centimetre radius). One-tenth of this cur-
rent is taken as the working unit and called the ampere.
The unit electrical resistance (one ohm) is that through
which an electromotive force of one volt will force a current of
one ampere.
The C. G. S. unit of work is that due to unit force acting
through unit distance; that is, one dyne acting through one
centimetre. As this is too small to be generally convenient,
ten million times this amoimt is taken as the working unit
(called the joule). This is a little less than three-quarters of a
foot-pound (exactly .7373). The unit rate of doing work is
one joule per second. This unit rate is called the watt, and
translating this into English measure, one watt ecjuals tA
horse- power.
Although the watt is often spoken of as an electrical unit, it
belongs no more to electrical than to any other form of energy.
It only remains to show the relation of the watt to the more
strictly electrical units just mentioned. Recurring to our
definition of the volt, let us suppose that the resistance of the
circuit of which the moving wire is a part is such that unit
electromotive force produces unit current in it. The stress
between the field of the moving wire and the other unit field
in which it moves is then one dyne at unit distance. In main-
taining this for one second at the given rate of moving (1 cm.
per second) the work done is, as above, one C. G. S. unit. At
this rate, if the E. M. F. were 1 volt and the current 1 ampere,
the work would be one joule and the rate of doing work one
watt. If either E. M. F. or current were changed, the work
would be proportionally changed. So, the number of volts
multiplied by the number of amperes is numerically equal to
the watts, i.e., we have obtained the dynamical equivalent
of the two factors that make up electrical energy as ordinarily
reckoned. So the output of any dynamo in watts is deter-
mined by the volt-amperes produced, and we see the reason
of the ordinary statement that 746 volt-amperes make one
horse-power. This is always true whether the output is steady
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22 ELECTRIC TRANSMISSION OF POWER.
or variable, so long as we give to the product of volts and
amperes their true concurrent values.
What few other electrical units appear in practical work will
be referred to in their proper places.
It has been the purpose of this chapter, not so much to set
forth the ordinary elements of electrical study, as to present
these elements as viewed from the standpoint of energy. The
author has purposely avoided the conception of electricity as
a material something, to lay the greater emphasis on the
paramount importance of electrical energy. The present
recrudescence of a material theory of electrical charge in no
way affects the validity of the principles here laid down, since
it deals merely with a possible mechanism behind the stresses
and strains which are experimentally apparent.
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CHAPTER II.
GENERAL CONDITIONS OF POWER TRANSMISSION.
The growth of human industry depends on nothing more
than upon the possession of cheap and convenient power.
Labor is by far the largest factor in the cost of many manu-
factured articles, and in so far as motive power is cheap and
easy of application it tends to displace the strength of human
hands in all manufacturing processes, and so to reduce the
labor cost and to set free that labor for other and less purely
machine-like purposes.
Therefore industrial operations have steadily gravitated
toward regions where power is easily procured, often at the
sacrifice of certain other advantages. This is in no wise
better shown than by the growth of cities around easily avail-
able water-powers, even in regions where both raw material
and finished product became subject to considerable cost of
transportation. With the introduction of the steam engine
came a corresponding tendency to gather factories about
regions of cheap fuel. These localities, like those in which
water-power is plentiful, seldom coincide with centres of cheap
material and transportation, so that it has generally been
desirable to strike an average condition of maximum economy
by transporting the necessary power, stored in the form of
fuel, to some advantageous point.
Experience has shown, however, that, while the hauling of
coal is a simple and comparatively cheap expedient, fuel
utilized for running heat engines is in very many cases so
much more expensive than hydraulic power as to be quite out
of competition in cases where the latter can be transmitted,
with a reasonable degree of economy, to places that are favor-
able for its utilization. And in general it is found that there
is a wide field for the transmission of power obtained from a
given source, in competition with power from some other
source utilized in situ,
23
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24 ELECTRIC TRANSMISSION OF POWER.
The sources of energy on which we may draw for mechanical
power to be employed on the spot or transmitted elsewhere
are very diversified, although few of them are to-day utilized
in any considerable amount. Taking them in the order of
their present importance we arrive at something like the fol-
lowing classification:
I. Fuel,
II. Water-power.
III. Wind.
IV. Solar radiation.
V. Tidal and wave energy.
VI. Internal energy of the earth.
Of these only the first two play any important part in our
industrial economy. The third is employed in a very small
and spasmodic way, the fourth and fifth although enormous in
amount are almost untouched, while the last is not at present
used at all, owing to inherent difficulties.
I. The world's supply of fuel is almost too great for intel-
ligible description. Aside from a widely distributed and
steadily renewed supply of wood, the extent and capacity of
available coalfields give promise that for a very long time to
come fuel will be the chief source of energy. Coal is found
in nearly every country, and in most quite plentifully, while
exploration both in old fields and in new, is constantly bringing
to light fresh supplies. Many computations concerning the
probable duration of the coal supply have been made, but they
are generally unreliable owing to the great probability that
only a very small proportion of the available coal is as yet
known to mankind. Certaui it is that there is unlikely to be
a marked scarcity of fuel for several centuries to come, even at
the present rate of increase in it-s consumption. Still, it is
altogether probable that it may become considerably clearer
than at present within perhaps the present century, owing to
the increased difficulty of working the older mines and the
comparative inaccessibihty of new ones.
Besides coal we have petroleum and natural gas in unknown
but surely very great quantities, since the distribution of both
is far wider than has generally been supposed. At present the
cost of these as fuel does not differ widely from that of coal,
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GENERAL COXDiriOXS OF POWER TRAXSMISSIOX, 25
but appearances indicate that they are likely to be sooner
exhausted.
Every improvement that is made in the generation of power
by st^am and its subsequent distribution, helps to economize
the fuel supply and stave off the already distant day when fuel
shall be scarce. The work of the past half century has by direct
improvement in steam practice, nearly if not quite doubled
the energy available per ton of fuel. Beyond this much has
been done along collateral lines. Particularly, explosive vapor
engines have been developed to a point at which they are for
small powers decidedly more economical than steam engines.
Gas engines of moderate size, 5 to 25 HP, are readily ob-
tained of such excellence as to give a brake-horse-power hour
on an expenditure of little, if any, more than 20 cu. ft. of ordi-
nary gas, reducing the cost per HPH to below that of power
from a steam engine of similar size. Engines using an explo-
sive mixture of air and petroleum vapor are at least equally
economical, in fact more so unless the comparison be made
with very cheap gas.
These explosive engines have nearly double the net efficiency
of steam engines as converters of thermal energy into mechani-
cal power, and are capable of giving under favorable circum-
stances 1 HPH on the thermal e(iuivalent of less than 1 pound
of coal.
IL Water-power derived from streams is not distributed with
the same lavish impartiality as fuel, but nevertheless exists in
many regions in sufficient amount to be of the greatest impor-
tance in industrial operations. Available streams exist around
almost every mountain range and are capable of furnishing an
amount of power that is seldom realized. In the United States
the total horse-power of the improved water-power is approxi-
mately 1,500,000. New England is especially rich in this re-
spect, as is, too, the entire region bordering on the Appalachian
range. The Rocky Mountains are less favored, the available
water being rather small in amount, on account of the smaller
rainfall and the severe cold of the winters.
The Pacific slope is rather better off, and the high price of coal
operates to hasten the development of every practicable power.
All over the country are scattered small water-powers, and one
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26 ELECTRIC TRANSMISSION OF POWER,
of the interesting results of the growth of electrical power
transmission has been to bring to light half forgotten falls, even
in familiar streams. Abroad, Switzerland is rich in powers of
moderate size, as is the entire Alpine region, while a few years
of experience in electrical transmission will probably cause the
discovery or utilization of many water-powers that have hardly
been considered, even in highly developed countries. Of the
world's total water-power supply we know little more than
of its coal supply, but it is quite certain, now that transmission
of power over very considerable distances is practicable, that
the employment of the one will every year lessen the relative
inroads upon the other. And this is in spite of the fact that
water is by no means always cheaper than steam as a motive
agent.
III. Wind as a prime mover has been employed on a rather
small scale from the very earliest times. Were it not for the
extreme irregularity of the power supplied by it in most places,
the windmill would be t()-<lay a very important factor in the
problem of cheap power. Unhappily, winds in the same place
vary most erratically, from the merest breeze to a hurricane
sweeping along at the rate of 5() to 75 miles an hour. As all
strengths of wind within very wide limits must be utilized by the
same apparatus running at all sorts of speeds, it is no easy
matter to employ it for most sorts of work. It seems especially
unfitted for electrical work, and yet several small private plants
have obtained good results from windmills used in connection
with storage batteries.
In ordinary winds the great size of the wheel necessary for a
moderate power militates against any very extensive use. For
example, with a good breeze of 10 miles per hour a wheel about
twenty-five feet in diameter is needed to produce steadily a
single effective horse-power, and the rate of rotation, about 30
revolutions per minute, is so low as to be inconvenient for many
purposes. Hence windmills are generally used for very small
work which can be done at variable speed, such as pumping,
grinding, and the like, for which they are unexcelled in cheap-
ness and convenience. For large work we can hardly count
much on wind-power, in spite of ingenious speculations to the
contrary, and as a source of power for general distribution it
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GENERAL CONDITIONS OF POWER TRANSMISSION. 27
is out of the question, for such as it is we have it already dis-
tributed. It must ratlier be regarded as a local competitor of
distributed power, and even so only in a small and limited field.
IV. Aside from being in a general way the ultimate source
of nearly all terrestrial energy, the sun steadily furnishes an
amount of radiant energy, which if converted into mechanical
power would more than supply all possible human needs. Its
full value is the equivalent of no less than ten thousand horse-
power per acre of surface exposed to the perpendicular rays of
the sun.
This prodigious amount is reduced by perhaps one-third
through atmospheric absorption before it reaches the sea level,
and in cloudy weather by a very much larger amount. Never-
theless, with clear sunlight the amount of energy practically
available, after making all allowances for increased absorption
when th6 sun is low, and for the hours of darkness in any given
place, is very great. If we suppose the radiant energy to be
received cm concave mirrors kept turned toward the sun and
arranged so as to utilize the heat in the boiler of a steam or
vapor engine, the average result after making all allowances for
losses would be one mechanical horse-power for each 100 square
feet of mirror-aperture, available about ten hours per day.
Very important pioneer work was done on solar engines
by John Ericsson and by M. Mouchot more than a quarter
century ago, but it is only within the past few years that the
solar engine has approached really commercial form. At the
present time solar heating apparatus is being regularly pro-
duced although on a rather small scale, and gives good economic
results. The solar motor is essentially a steam engine supplied
with steam by a boiler placed in the focus of a concave mirror.
This is shaped like an open umbrella with its handle pointed
toward the sun. The umbrella is carried on a polar axis at
right angles to the handle and pointing toward the celestial
pole. The actual mirror, is segmental, built upon a steel
frame, of rectangles of plane thin glass silvered on the back.
Each segment is about six inches wdde and two feet long, sup-
ported by cushioned clamps at the comers, and the whole are
arranged to focus the sun's rays on a cylindrically disposed
blackened boiler formed of copper tube. The structure is
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28 ELECTRIC TRANSMISSION OF POWER.
supported on a polar axis about which it is moved automatically
by steps every few minutes, remaining locked in the intervals
to avoid needless strain on the clockwork. There is also a
motion in declination to take care of the apparent motion of
the sun, adjusted by hand every day or two as becomes neces-
sary. The engine is generally a rather highly organized one,
worked condensing and with superheated steam at a pressure
of 200 lbs. per square inch or more. The net result is one brake
HP for each 100 square feet of mirror surface. The mirror
structure becomes rather unwieldy when of dimensions great
enough to supply an engine of more than 15 HP, so that for
greater powers several mirrors with their boilers should be
coupled together. The initial cost of each equipment is high,
say $250 per horse-power, but the fuel cost is nil and the
attendance required very little, so that even now there are
localities where its use is economical. The full power is
available about eight hours per day, and there is upon the earth's
surface a vast, irregular equatorial belt in which such solar
engines can be successfully used for irrigation and other pur-
poses. The power is steady, and reliable during the hours of
sunshine, and gives constant speed like any other steam engine.
It is worth mentioning that general heating and cooking appa-
ratus on the same plan is entirely practicable in regions of
scant fuel and high sun, and has been tried successfully.
V. Of tidal energy but little use has yet been made. Here
and there, both here and abroad, are small tidemjUs, feebly sug-
gesting the enormous store of tidal power as yet unutilized.
The intermittent character of tidal currents and the small
extent of the rise and fall generally available, make the practical
part of the problem somewhat difficult. The easiest way of
harnessing the tides is to let the rising water store itself in
artificial reservoirs, or natural ones artificially improved, and
then during the ebb to use it with water-wheels. But usually
the head is so small that for any considerable power stored the
area of reservoir must be very large, and the wheels must be
of great size in order to make the stored water do its work
before the rising tide checks further operations. The average
tide is seldom more than 10 to 12 feet along our coast, and of
this hardly more than half could be utilized to give even a few
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GENERAL CONDITIONS OF POWER TRANSMISSION. 29
hours of daily service. At 6 feet available head about 100
cubic feet of water must be stored for each horse-power-minute,
even with the best modem turbines. Hence for say 1,000
HP available for 5 hours there must be impounded 30,000,000
cubic feet of water, making a pond 6 feet deep and almost
120 acres in extent.
Tidal operations are therefore likely to be restricted to a
few favored localities where through special configuration of
the ground natural reservoirs can be found, and where the rise
of the tide is several times the figure nameil. In rare cases, by
the use of more than one reservoir and outlet, work may be
made nearly or quite continuous. Still, with all these difficul-
ties the possibilities of tidal power are enormous in certain
cases. Take for example the Bay of Fundy with its 40 feet of
normal tidal rise. If half this head can be used in practice 30
cubic feet will be required per horse-power-minute, and a single
square mile of reservoir capacity gained by damming an estu-
ary or cutting into a favorable location on shore will yield
62,000 horse-power ten hours per day in two five-hour intervals.
Generally speaking, economic conditions are not favorable for
such an employment of the tides, but in some localities a
peculiarly fortunate contour of the shore coupled with high
local cost of fuel may render it easy and profitable to press the
tides into service. The author has had cxH^asion to investigate
a few cases of this kind in which the commercial outlook was
good. The main difficulties in utilizing the tides are two: first,
the very variable head; and, second, the short daily periods in
which the outflow can be advantageously used. Moreover,
these periods shift just as the times of high tide shift, by a
little less than an hour per day, so that if the power were used
directly it would often be available only at very inconvenient
times.
To work the tides on a really commercial scale, therefore,
some system of storing power is absolutely necessary. And
since one would have to deal with very large amounts of
power, much of the time the entire output of the plant, the
storage must be fairly cheap and efficient. For work on the
scale contemplated, it is probable that the storage battery is
the most available method. Used in very large units in
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30 ELECTRIC TRANSMISSION OF POWER,
a colossal plant, most of the serious objections to the
storage battery are in great measure obviated, since attend-
ance and repairs can be part of the duties of a regular main-
tenance department, inspecting, testing, and repairing damaged
cells, casting and filling new plates, and keeping the plant in
first-class working condition all the time.
The cost of battery tvould be, of course, a serious matter, but
not prohibitive, and its efficiency could probably be kept as
high as 80 per cent. The best idea of the economic side of
the case can be gained by investigating a hypothetical case of
tidal storage, based, for convenience, on the square mile
of reservoir just mentioned. To simplify the case we will
assume use of the power locally, so as not to complicate the
situation by the details of a long-distance transmission. We
will take the generators, which can be worked at steady full
load, at 94 per cent efficiency. Then the efficiency to the dis-
tributing lines would be
.94 X .80 = .752.
At this rate, the 62,000 HP available would give substan-
tially 35,0(K) KW; i.e., 350,000 KW-hours daily. Storage
capacity would have to be provided for this whole amount
in a gigantic battery, weighing about 18,0(K) tons and cost-
ing in the neighborhood of three million dollars. To this,
of course, the cost of the electrical and hydraulic machinery
must be added, and beyond this must be reckoned the really
very uncertain cost of the reservoir and hydraulic work. In
spite of all this, an assured market for the output would lead
to economic success under conditions quite possible to be
realized. If extensive transmission had to accompany the
enterprise there would be still further loss of efficiency, so
that the final figure would not exceed 60 per cent, which
would reduce the salable power to about 27,(X)0 KW. Evi-
dently this would have to command a very good price, to carry
the burden of the heavy investment, which would probably
rise to between $10,000,000 and $15,000,000. The cost of
such an enterprise is so formidable that it is practically out of
the question, unless it can reach a market for j)ower in which
a ver^' high price is admissible. When fuel begins to get
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GENERAL CONDITIONS OF POWER TRANSMISSION, 31
scarce it will be profitable to utilize the tides on a large scale;
until then, their use will be confined to isolated cases in which
local causes lead to high cost of other power and tidal storage
is unusually cheap.
All these considerations apply with similar force to wave
motors, which have been often suggested, and now and then
used, as sources of power. The energy of the waves is very
great, as the havoc wrought by storms bears witnass; but it is
most irregular in amount, and requires very large apparatus
for its utilization. What is worse, the power is intermittent,
so that to be of any material advantage it must be brought to
a steady output by means of storage of energy in some form.
The periodicity of wave motion is so low, roughly about 6 to
10 crests per minute, that flywheels and the like are of little use,
and storage is practically reduced to a question of compressing
air or pumping water. Even if some such wasteful intermedi-
ary were not necessary, and one could work directly by means
of floats or their equivalent, a float would have to have a dis-
placement of at least one ton per horse-power, even if work-
ing in a pretty heavy sea, and under ordinary circumstances
several times that amount of displacement. At best, wave
motors are cumbersome, and give small promise of economic
development while other sources of energy are available.
VI. Of the earth's internal heat energy there is little to be
said. It is quite unused save as an occasional source of hot
water, and except in a very few cases could not be employed at
all, much less to any advantage. Immense as is its aggregate
araoimt, it is, save at isolated points, so far separated from the
earth's surface as to be very difficult to get at. Hot springs,
very deep artesian wells, and some volcanic regions, furnish the
only feasible sources of terrestrial heat energy, so that the
whole matter is only of theoretical interest.
We see that at present only two sources of energy, viz.,
fuel and water-power, are worthy of serious consideration in
connection with the general problem of the transmission and
distribution of power. The other sources enumerated are
either very irregular, uncertain in amount, or so difficult of
utilization as to remove them at once from the sphere of prac-
tical work.
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32 ELECTRIC TRANSMISSION OF POWER.
Granted, then, that fuel and water-power are and are likely
long to remain the dominant sources of energy, let us look more
closely into their possibilities. From each energy can be
readily transmitted and distributed by any suitable means;
each, in fact, can be transferred bodily to a distant scene of
action without any transformation from its own proper form.
In fact, for certain purposes and under certain conditions such
is the very best method. Fuel for ordinary heating and water
for such uses as hydraulic mining can be taken as cases in
point. In a more general way, both fuel and water for the
development of mechanical power may often profitably be
transferred from place to place.
The conditions of economy in the transmission of fuel as
such are comparatively easy to examine and define. Coal
may be produced at the mine for a certain quite definite
cost per ton. It can be transported over railroads and
waterways for an easily ascertainable price. vSuch a trans-
mission may be said to have a definite efficiency, as for example
90 per cent, when the total transportation charges against a ton
of coal amount to 10 per cent of its final value. From this
standpoint it is cjuite possible to transmit power at this very
high efficiency even to the distance of hundreds of miles. If
the final object be the distribution of power on a large scale, as
from a great central station, this transmission by transporta-
tion of fuel is often at once the most reliable and the cheapest
method.
Transformation of the fuel energy at its source into some
other form for the purpose of transmission is generally only
justifiable, first, when by so doing fuel not available for trans-
portation at a high efficiency can be rendered valuable by trans-
formation of its energy, or second, when it is to be utilized
at some distant point in a manner which compels a loss of
efficiency greater than that encountered in transmission. As
an example of the first condition , fully one-third of the coal as
ordinarily mined is unfitted, through its finely divided condition
or poor quality, for transportation over considerable distances.
Its commercial value is so small i)er ton that it could not be
carried far without incurring charges for carriage amounthig
to a large part of its value. Hence, every coal mine accumu-
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GENERAL CONDITIONS OF POWER TRANSMISSION. 33
lates a mountainous culm pile that is at present not only
valueless but cumbers the ground. This waste product could
sometimes be very profitably employed in generating power
which could be transmitted at a relatively very high efficiency
and sold at a good price.
A specimen of the second kind may be found in the somewhat
rare case of power which must be used in small units scattered
over a considerable territory, so that they could be replaced
with a great gain in efficiency by a single large generating
station. Such a state of affairs might be found in certain
mining regions where coal and iron mines are interspersed.
This must not be confounded with the very ordinary case of
distributing energy from a central station to various scattered
points, for we are here considering only the original source of
the fuel.
When an extensive distribution of energy from a power
station is contemplated, electrical or similar transmission of
power to that station is generally economical only on the condi-
tion above expressed, of using fuel otherwise valueless, since the
facilities for transportation to points at which power distribu-
tion on a large scale would be profitable, are generally good
and fairly cheap. All this applies to piping gas or petroleum
as well as to hauling coal, with the difference that neither gas
nor petroleum has any waste corresponding to culm, and hence
the transportation of each of them becomes a process entirely
comparable with the transmission of energy and directly com-
peting therewith. It has even been proposed to pipe coal dust
by pneumatic power for fuel purposes.
Water-power is by no means always cheaper than fuel, but as
a general rule it is, and by such an amount that it can be
transformed into electrical energy and transmitted to at least
a moderate distance without losing its economic advantage.
It therefore is usually the cheapest source from which to derive
power for general distribution on a large scale.
It is very difficult to give a clear idea of the relative cost of
steam and water-power, for while the one can be predicted for
any given place with fair accuracy, the other is subject to
immense variations. Once established, a water-power plant
can be operated very cheaply, but the cost of developing the
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34 ELECTRIC TRANSMISSION OF PO\^ER,
water-power may be almost anything, and each ease must be
figured by itself. It is easy to obtain estimates of the cost of
developing a given stream and to form a close estimate of
both the interest charges to be incurred and the additional
expense of repairs and of operation. The cost of steam-
power for the same conditions can be accurately estimated.
The details of such estimates we will discuss later. In general,
one can only safely say that the costs of steam and water-
power overlap, as it were, so that while the more easily
developed water-powers are cheaper sources of energy than
fuel at any ordinary price, there are many cases in which the
great cost of development of difficult water-powers prohibits
competition with steam except where fuel is very dear. Much
depends on the topography of the country, the amount and
reliability of the available head of water, the price at which
water rights can be obtained and various other local conditions.
To utilize the normal minimum power of a stream is gener-
ally comparatively easy, while so to take account of high water
as to obtain nearly the full continuous working power of the
stream often means great added expense for storage capacity
and works to control and regulate the flow.
In addition we have to consider two distinct phases of the
comparative cost — first, the cost of steam and water as prime
movers for a source of power to be distributed, and second,
the relation between these costs and that of steam-power at
the points where the distribution takes place.
(liven a proper source of energy, there is vast variety in the
character of the work of transmission and distribution that is
to be undertaken. In the first place, the point of utilization
may be distant anywhere from a few hundred feet to many
miles, and at that point the object may be the delivery of
mechanical power in a single unit, in one or several groups of
allied units, in one or several widely scattered groups, or
finally for transformation into some other form of energy in
the most direct way possible.
There is no single method of power transmission which
meets in the best possible manner all these widely varying con-
ditions. Although electrical transmission is the most general
solution of the difficult problem in hand, there are cases in
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GENERAL CONDITIONS OF' POWER TRANSMISSION. 86
which other methods are preferable and should be adopted.
Those besides electric transmission which have come into con-
siderable use are the following:
I. Wire Rope Transmission.
II. Hydraulic Transmission.
III. Compressed Air Transmission.
IV. Gas Transmission.
It will be well to look into the distinguishing characteristics
of these and their relation to electrical transmission, with the
purpose of finding the advantages and limitations of each, so
that the proper economic sphere of each may be determined,
before taking up the electrical work which forms the main
subject of this volume. Each method will be found to have
its own legitimate place.
I. The transmission of power by wire ropes is merely a ver}'
useful extension of the ordinary process of belting. Belts are
made of material which will not stand exposure to the weather,
and which being of low tensile strength is heavy and bulky in
proportion to the power transmitted. The advantage of wire
rope over belting lies in its high tensile strength and freedom
from deterioration when used out of doors. To gain the
fullest benefit from these properties it is necessary to use
light ropes driven at high speed.
It should be borne in mind that the power transmitted by
anything of the nature of belting depends directly on the
speed and the amount of ])ull exercised. If the force of the
pull is 100 pounds weiglit and the speed of belt or rope is
4,000 feet per minute, the amount of power transmitted is
400,000 foot-pounds per minute or (since 1 horse-power is 33,000
foot-pounds per minute) about 12 IIP. The greater the speed,
the more power transmitted with the same pull, or the less
the pull for the same power. Wire ro})e can be safely run
at a considerably higher sj)eed than belting and is much stronger
in proportion to its size and weight. It does not often replace
belting for ordinary work, for the reason that owing to its
small size it does not grip ordinary pulleys anywhere nearly
in proportion to its strength. Hence, to best take advantage
of its ability to transmit large powers, the rope speed nuist be
high and the pulleys unusually large in diameter to give sufE-
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36 ELECTRIC TRANSMISSION OF POWER.
cient surface of contact. Such large wheels are inconvenient
in most situations, and as the alternative is a number of ropes
which are troublesome to care for, rope driving save for outdoor
work is rather uncommon.
A typical rope transmission is shown diagrammatically in
Fig. 10. Here A and B are two wheels, usually of cast iron,
generally from 5 to 15 feet in diameter and with deeply grooved
Fio. 10.
rims. They are connected by a wire rope perhaps from ^ inch
to li inch in diameter, which serves to transmit the power as
the wheels revolve. The rope speed is usually from 3,000 to
5,000 feet per minute, sometimes as high as 6,000. The distance
between the centres of A and B may be anything required by
the conditions up to four or even five hundred feet. Greater
distances are seldom attempted in a single span, as, if the rope
is not to be overstrained by its own weight, it must be allowed
to sag considerably, compelling the pulleys to be raised to
keep it clear of the ground, and subjecting it to danger from
swaying seriously by reason of wind pressure or other acci-
dental causes.
The rope employed is of special character. The material is
the best charcoal iron or low steel, and the strands are usually
Fig. 11.
laid around a hemp core to give added flexibility. The rope
generally employed in this country is of six strands with seven
wires per strand, and is shown in cross-section in Fig. 11.
Even with the hemp core there is still in an iron rope sufficient
resistance to bending to make the use of pulleys of large
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GENERAL CONDITIONS OF POWER TRANSMISSION. 37
diameter necessary. Sometimes each separate strand is made
A\ith a hemp core, or is composed of nineteen small wires
instead of seven larger ones, to increase the flexibility and to
make it possible to use smaller sheaves and drums, as in hoist-
ing machinery.
Steel rope is slightly more costly than iron, but gives greater
durability. The wheels on which these ropes run are fur-
nished with a deep groove around the circumference, pro-
vided with a relatively soft packing at the bottom on which
the rope rests, and which serves to increase the grip of the
rope and to decrease the wear upon it. Fig. 12 shows a section
of the rim of such a wheel. The bushing at the bottom of the
Fig. 12.
groove, upon which the rope directly bears, has been made of
various materials, but at present, leather and especially prepared
rubber are in most general use. The small pieces of which
the bushing is composed are cut to shape and driven into the
dovetailed recess at the bottom of the groove. The bushings
have to be replaced at frequent intervals, and the ca5Ies them-
selves have an average life of not nmch over a year.
When a straightaway transmission of a few hundred feet is
necessary, when the power concerned is not great, and the size
of the pulleys is not a serious inconvenience, this transmission
by wire rope is both very cheap and enormously efficient.
No other known method can compete with it within these
somewhat narrow limitations. For a span of ordinary length
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38 ELECTRIC TRANSMISSION OF POWER.
and the usual rope speeds, the efficiency has been shown by
experiment to be between 96 and 97 per cent. At a dis-
tance of four to five hundred feet the weight and sag of the
rope becomes a very serious inconvenience, and the arrange-
ment has to be modified. Perhaps the most obvious plan is
Fio. 13.
to introduce a sheave to support the slack of the cable, as
shown in Fig. 13.
On longer spans several sheaves become necessary, and both
the slack and the tight portions of the cable need such sui)port.
In cable railway work, the most familiar instance of power
transmission by wire ro[)es, numerous sheaves have to be
employed to keep the cable in it« working position in the
somewhat contracted conduit. These reduce the efficiency of
the system considerably, so that the power taken to run the
cable light is often greater than the net power transmitted.
In aerial cable lines multiple sheaves are seldom used, and the
more usual procedure is to subdivide the transmission into
several independent spans, thus lessening swaying and sagging
as well as the length of rope that nuist be discarded in case of a
serious break. This device is shown in Fig. 14. It employs
intermediate pulley stations at which are installed double
Fkj. 14.
grooved pulleys to accommodate the separate cables that
form the individual spans. Such a pulley is shown in section
in Fig. 15. The spans may be three or four hundred feet
long; as soon as the length gets troublesome another pulley
station is employed. There is necessarily a certain small loss of
energy at each such station. This is approximately proportional
to the number of times the rope passes over a pulley. From
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GEXERAL COXDITIOXS OF POWER TRAXSMISSIOX. 39
the best experimental data availalile the officioncy of a rope
transmission extended by separate spans is nearly as follows:
Number of spans . .
Per cent efficiency .
..123460789 10
. .96 .94 .93 .91 .89 .87 .8(5 .86 .84 .82
These figures are taken to the nearest per cent and are for
full load only. At half-load the loss in each case would be
doubled. For instance, a 10-span transmission at half-load
Fio. 16.
would give about 64 per cent efficiency. The pulley stations
consist of the double-grooved wheel before mentioned mounted
on a substantial and rather high pedestal or frame-work.
In this country a timber frame is generally used; abroad
Fig. 16.
a masonry pier is more common. A convenient fonii of
frame-work is shown in Pig. 16. An idea of its dimensions may
be gained from the fact that the wheel is likely to be 6 to 10
feet in diameter.
It is interesting to note that the efficiency just given for a 10-
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40
ELECTRIC TRANSMISSION OF POWER,
span transmission at full load is quite nearly the same as would
be obtained from an electrical-power transmission at moderate
voltage over the same distance, assuming a unit of say 50 HP
or upward. The first cost of the latter would be considerably
higher than that of the rope transmission, but the repairs
would certainly be much less than the replacements of cable,
bringing the cost per HP at full load to about the same fig-
ure by the two methods.
From actual tests of electrical apparatus we have the fol-
lowing efficiency for a transmission of 50 HP 5,000 feet, assum-
ing 2,000 volts and 2 per cent line loss, which would require
Fig. 17.
a wire less than one-fourth of an inch in diameter. Efficiency
at full load 81 per cent, at half -load 72. These values are
lower than those attainable with machinery of the most recent
type, which should give at least 86 per cent at full load and
80 per cent at half-load for the complete transmission, which
beats out rope transmission at a distance much less than
5,000 feet. Except at full load the electrical transmission has
a very material advantage. This advantage would be greatly
increased if the transmission were in anything but a straight
line. An electric line can be carried around any number of
comers without loss of efficiency, while a rope transmission
cannot. If it becomes needful to change the direction of a
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GENERAL CONDITIONS OF POWER TRANSMISSION. 44
rope drive, it is done at a station provided with a pair of rope
wheels connected by bevel gears set at any required angle.
Fig. 17 shows such a station in diagram. The loss of energy
in such a pair of bevel gears amounts to from 7 to 10 per
cent, more often the latter. The bevel gears may be avoided
by a sheave revolving in a horizontal plane, and carrying
the turn in the cable, but while this arrangement is tolerably
efficient, it greatly decreases the life of the rope.
From what has been said, it will be seen that while cable
transmission is for short distances in a straight line both cheap
and very efficient, at 2,000 to 3,000 feet it is equalled and sur-
passed in efficiency by electric transmission, with lesser main-
tenance although greater first cost. The steel rope for a 50
HP transmission of 5,000 feet would cost about $400, and
replacement brings a considerable charge against each HP
delivered. If the transmission is not straightaway, or if
branches have to be taken off en rotUCy the efficiency of the
s)rstem is considerably reduced by gear stations, while even
aside from these the efficiency is high only at or near full load.
But the general simplicity and cheapness of cable transmission
have made it a favorite method, and there have been many such
installations, some of them of a quite elaborate character.
Most of them are small, since the amount of power that can
be transmitted by a single rope is limited to 250 or 300 HP.
Ropes suited to a larger power are too heavy and inflexible;
1| inch is about the greatest practicable diameter of cable,
and even this requires pulleys between 15 and 20 feet in diam-
eter for its proper operation. Besides, even at moderate
distances the rope transmission suffers in wet or icy weather,
so that at anywhere nearly equal costs the electrical drive
is to be preferred save in the simplest cases.
I'nder all circumstances the need of replacing the cables
every year or so causes a high rate of maintenance. The fol-
lowing table, giving the sizes of iron-wire cables and pulleys
necessary for transmitting various amounts of power, will help
to give a clearer idea of the conditions of cable transmission
and aid in defining its limited but useful sphere. Speed is given
in revolutions per minute, and pulley diameter is the smallest
permissible. These figures are, as will readily be seen, for rope
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42
ELECTRIC TR.lSSMfSSlOX OF POWER.
Diameter of Rope.
Speed.
Diameter of Pulley.
HP.
V
150
6'
26
9 '/
1«
140
r
3o
r
140
8'
45
\\"
100
10'
85
80
12'
100
80
14'
140
w
80
14'
150
speeds of not far from 3,0()0 feet per minute. This ran fre-
quently be safely raised to 5,0()0 with somewhat larger pulleys
than those given and increased revolutions, while for steady
loads the tension can be slightly augmented without danger.
So while the figures given are those suitable for ordinary
running with a good margin of capacity, the HP given can be
nearly doubled when all conditions are favorable.
But from all these figures it is sufficiently evident that
rope transmission is very limited in its applicability and is
not at all suited to work of distribution in small units. For
a good many years, however, a wire-rope transmission, now
practically sup)erseded by electric driving, was operated at
SchafThausen on the falls of the Rhine. The power station
delivered more than 600 HP to a score of consumers over
distances of half a mile or so. There were two bevel-gear
stations, and on the average, five cable spans between the
power station and the consumer, so that the efficiency even
at full load wa« somewhere between 60 and 70 per cent and
ordinarily very much less. Nevertheless, in default of any
better means of transmission at the time of installation, some
twenty years since, the plant did fairly successful work, even
from a commercial standpoint. In this country the system
is very little used save for short straight runs between building
and building across streets, for instance.
II. Noting, then, that cable transmission does excellent work
in its proper place, but is unsuited for the distribution of power
or for transmissions of anything save the simplest sorts, we
may pass to the hydraulic method of transmitting and distrib-
uting power. This in its crude form of small water-motors
attached to ordinary city mains is very familiar, but nothing
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GENERAL COSDITfOXS OF POWER TRAXSMISSIOX. 43
more extensive has been attempted in this country. Al)road
there are a number of hydraulic power plants specially intended
for the distribution of power for general use, and the method is
one which has been fairly successful. There are two distinct
types of hydraulic plant, one utilizing such pressure as is
available naturally or by pumping to reservoirs, the other
employing very high artificial pressures, up to 75() pounds per
square inch, and used only for special purposes.
There are s(unewhat extensive works of the former kind at
Zurich, Geneva, and Genoa, the effective head of water being
in each case not far from 5()0 feet. In each case the power
business has been an outgrowth of the nuuiicipal water-supply
system. At Zurich and Cieneva elevated reservoirs are supplied
by pumping stations driven by water-power. At Genoa the
head is a natural one, 20 miles from the city, and nuich of the
fall is utilized IS miles from (ienoa in driving the fine constant
current electric ])lant described elsewhere in this volume.
At Zurich there is in addition to the ordinary low pressure
water system a special high service reservoir supplying power
to a large electric station and to small consumers. Water is
pumped 6,000 feet into this reservoir through an 18-inch main,
and the total power service from both systems is something
like 500 HP, reckoned on a ten-hour basis. The price charged
is from $37 to $80 per HP per year.
The Geneva plant is on a much larger scale, the total turbine
capacity being about 4,500 HP. Here, as at Zurich, there are
two sets of mains, one at nearly 200 feet head, the other at
about 450. Each supplies water for both ]>ower and general
purposes. The high pressure service reservoir is about 2^
miles from the city, and the working pressure is supplied
indifferently from this or from the pumps direct. There is an
electric light plant with 600 HP in turbines driven by the
pressure water, and a large number of smaller consumers.
Water is supplied to the electric light company for as low as
$15 per HP per year.
Both these installations are extensions of the city water
service, and have done excellent work. Operated in this way
the economic conditions are somewhat different from those to
be found in a hydraulic plant established by private enterprise
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44
ELECTRIC TRANSMISSION OF POWER.
for power only. An inquiry into the efficiency of such a
system may be fairly based on the facts given. At Zurich, for
example, the efficiency from turbine shaft to reservoir cannot
well exceed 75. The distributing mains must involve a loss
of not less than 10 per cent, while the motors cannot be
counted on for an efficiency of over .75. The total efficiency
from turbine shaft to motor shaft is then about .75 x .75 X .90
= 50.6 per cent. The character of the motors has an impor-
tant influence on the economy of the system, particularly at
low loads. The motors most used particularly for small powers
r~\
Km;, is.
are oscillating water engines of the type shown hi Fig. 18.
The form shown is made by Schmid of Zurich. It possesses, in
common with all others of similar construction, the undesirable
property of taking a uniform amount of water at uniform speed ,
quite irrespective of load. The mechanical efficiency falls off
like that of a steam engine, friction being nearly constant.
Better average results are secured with impulse turbines (see
Chapter IX) of which the efficiency varies but little as the load
falls off, or for high rotative speeds wdth impulse wheels like the
Pel ton, shown in Fig. 19, as adapted for motors of moderate
power. At half-load, i.e., half flow, the losses in distributing
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GENERAL CONDITIONS OF POWER TRANSMISSION. 46
mains would be reduced to about one-third, while the efficiency
of the engine motors would certainly not be lowered by less
than 5 per cent. The total half-load efficiency would then be
.75 X .97 X .70 = 50.9 per cent, actually a trifle higher than
at full load. This apparently remarkable prop)erty is shared
by all transmissions wherein the transmission loss proper is
fairly large.
The second type of hydraulic distribution of power is that
at very high pressures and employing a purely artificial head.
The pressures involved are usually 700 to 800 pounds per square
FlO. 19.
inch, and a small amount of storage capacity is gained by
employing .what are known as hydraulic accumulators, fed by
the pressure pumps. These accumulators are merely long
vertical cylinders adapted to withstand the working pressure,
which is kept up by a closely fitting and enormously heavy
piston. The distribution of power is by iron pipes leading to
the various water motors. This high pressure water system
is a device almost peculiar to England, and has been slow in
making headway elsewhere. Its especial advantage is in con-
nection with an exceedingly intermittent load, such as is
obtained from cranes, hoists, and the like. This is for the
reason that with a low average output a comparatively small
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46 ELECTRIC TRAXSMISSION OF POWER,
engine and pump working continuously at nearly uniform load
can keep the accumulators charged, while the rate of output
of the accumulators is enormous in case of a brief demand for
very great power.
Power plants on this hydraulic accumulator system are in
operation in the cities of London, Liverpool, Hull, and Bir-
mingham, p]ngland, and at Marseilles, France. The Lon-
don plant is the most important of those mentioned, con-,
sisting of three pumping and accumulator stations and about
60 miles of mains. The total number of motors operated was
in 1892 about 1,700. The charges are by meter, and are based
on intermittent work, being quite prohibitive for continuous
service — from $200 to $500 per effective HP per year of 3,000
hours. The largest accumulators have pistons 20 inches in
diameter and 23 feet stroke, giving a storage capacity of only
24 horse-power-hours each. While very convenient for the
supply of power for intermittent service only, this system, like
hydraulic supply at low pressure, is rather inefficient, the more
so as it has been found advisable to employ hydraulic motors
of the piston type, althougli special Pelton motors have been
used in some cases.
Any hydraulic system suffers severely from the inefficiency
of pump and motors and from loss of head in the pipes. The
amount of power that can be transmitted in the mains is (juite
limited, since the permissible velocity is not large. About
3 feet per second is customary — more than this involves
excessive friction and danger from hydraulic shock. At this
speed a pipe about 2 feet in diameter is necessary to transmit
5(K) HP under 500 feet head.* The power delivered increases
directly with the head, but as the pressure increases the largest
practicable size of pipe decreases, and on the high pressure
systems nothing larger than 12 inches has been attempted, and
even this requires the use of solid drawn steel
Whatever the size of pipe, the loss in head is quite nearly
inversely as the diameter and directly as the square of the
velocity. Even for high pressure systems this loss is by no
means negligible, since the pipes used are rather small.
The following table gives the loss of head in feet per 100 feet
* Cost per mile laid in average uiipaved ground about §15,000.
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GEXERAL CONDITIOSS OF POWER TRAXSMISSIOX. 47
of pipe and at a uniform velocity of 3 feet per second. This
applies to pipe in good average condition. When the pipe is
new and quite clean, the losses may he slightly less. If the
pipe is old and incrusted, the above losses may be nearly
doubled. Hends and branches still further reduce the working
pressure.
Diameter ....
1" 2"
1
3"
1
1 ''
5"
ir
7"
1
1 8"
1
w
1
|12"
Loss of Head . . .
1
4.89 1 2.44
1
1.62
1.22
.98
.81
1 ' 1
.70 , .61 .49 .41
_- : ^ - r _ -^-_
1
1 1 !
"" . ~ 1 ~
Diameter ....
14" ' 1«"
18"
20"
22"
24" 1 2«"
1
28"
40"
36"
Loss of Head . . .
.35 .32
.27
.25
.22
.20
.19
.17
.10
.13
We may now look into the efficiency of these high pressure
hydraulic systems. Of the mechanical horse-power applied to
the pump we cannot reasonably hope to get more than 75 per
cent as energy stored in the accunmlators. Tests on the
Marseilles plant have sho\^ai 70 to 80 per cent efficiency between
the indicated steam power and the accunmlators, the former
figure at the speeds corresponding to full working capacity. As
the pumps were direct acting the difference between brake and
indicated HP was presumably very small. The motors can be
counted on for about .75 efficiency, and the losses of head in
the pipes for any ordinary distribution cannot safely be taken
at less than 5 per cent. Hence the full load efficiency is about
.75 X .75 X .95 = .53. The efficiency at full load is thus not
far from that of the low pressure system, but at half-load it
suffei-s from the use of ])iston motors, generally necessary on
account of the too high speed of rotary motors at high pressure.
At even 500 pounds per s(iuare inch pressure the normal speed
of a Pelton wheel of say 20 HP would be over 4,000 r. p. m., and
could not be greatly reduced without seriously cutting down
the efficiency. At half-load the piston motors could not be
relied on for over .65 efficiency, reducing the total efficiency,
even allowing for greatly lessened pipe loss, to about 45 per
cent. On the whole, the hvdraulic accimuilator svstem must be
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48 ELECTRIC TRANSMISSION OF POWER.
regarded as a very ingenious and occasionally useful freak. It
may now and then be useful as an auxiliary in the storage of
energy from a very irregular power supply.
The strongest point of hydraulic transmission is its ready
adaptability in connection with water supply systems for gen-
eral purposes. Skilfully installed, as for instance at Geneva,
it furnishes convenient, reliable, and fairly cheap motive power.
As a distinct power enterprise the high first cost is against it,
and the efficiency is never really good. All this applies with
even greater force to the special high pressure systems, which
suffer from inability to cope with continuous work, thus seri-
ously limiting the possible market. Even for intermittent ser-
vice the charges are enormously high. ^
The methods of power transmission already mentioned are
then somewhat limited in their usefulness by rather well
defined conditions, which make their employment advisable in
some cases and definitely inadvisable in general.
III. We may now pass to the pneumatic method of transmit-
ting power, which is far more general in its convenient appli-
cability than either of the others, and which is the only system
other than electric which has been extensively applied in prac-
tice to the distribution of power in small units, although only
short distances have been involved in any of the plants
hitherto operated, and the possible performance at long dis-
tances is more a subject of speculation than of reasonable cer-
tainty. Transmission of power by compressed air involves
.essentially three elements: An air compressor delivering the
air under a tension of from 50 to 100 or more pounds per
square inch into a pipe system, which conveys the compressed
air to the varioiLs motors. These motors are substantially
steam engines in mechanical arrangements, and indeed almost
any steam engine can be readily adapted for use with com-
pressed air. The compressor itself is not unlike an ordinary
steam pump in general arrangement. Its appearance in the
smaller sizes is well sho^Mi in diagram in Fig. 20. The system
was originally introduced about fifty years ago for mining
purposes, and owed its early importance to its use in working
the drills in the construction of the Hoosac, Mont Cenis, and
St. Gothard tunnels. Since then it has come to be used on a
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GENERAL CONDITIONS OF POWER TRANSMISSION, 49
very extensive scale for drilling operations, and recently has
also been applied for the distribution of power for general pur-
poses, particularly in Paris, where the only really extensive
system of this kind is in operation. Its best field has been
and still is in mining operations where the escaping air is a
welcome addition to the means of ventilation and where, as a
rule, the distances are not great.
Transmission of power by piping compressed air has even
for general distribution certain very well marked advan-
tages. The subdivision of the power can be carried on to
Pig. 20.
almost any extent, and the motors are fairly efficient, simple,
and relatively cheap. In addition, the power furnished to
consumers can very easily be metered. The loss of energy
can be kept within moderate limits, and the mains themselves
are not liable to serioiLs breakdowns, although losses from
leakage are frequent and may be large. Finally, the system
is exceptionally safe. On the other hand, the efficiency of
the system, reckoned to the motor pulleys, is unpleasantly
low. The mains for a transmission of any considerable length
are very costly, and the compressed air has no considerable
use aside from motive power, instead of being applicable,
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60 ELECTRIC TRANSMISSION OF POWER.
like electric or even hydraulic transmission of power, to divers
profitable employments quite apart from the furnishing of
mechanical energy. To obtain a clearer idea of the nature
of these advantages and disadvantages, let us follow the process
of pneumatic transmission from the compressor to the motor,
looking into each stage of the operation with reference to
its efficiency and economic value.
The compressor is the starting point of the operation. Fig.
20 shows in section a typical direct acting steam compressor,
one of the best of its class. It consists essentially of the air
cylinder A and a steam cylinder B, arranged in line and having
a common piston rod. The steam end of the machine is
simply an ordinary engine fitted with an excellent high speed
valve gear worked l)y two eccentrics on the crank shaft of
the flywheels G, which serve merely to steady the action of
the mechanism. The air cylinder A is provided with a simple
piston driven by an extension of the steam piston rod.
At each end of the air cylinder are automatic poppet valves
E E, which serve to admit the air and to retain it during the
process of compression. F is the discharge pipe for the com-,
pressed air leaving the cylinder. In the compressor shown
there are two steam and two air cylinders connected with the
cranks 90° apart, thus giving steady rotation in spite of the
character of the work. In some machines the pistons and
piston rods are hollow and provided with means for maintaining
water circulation through them, to assist in cooling the air.
Round the air cylinder is a water jacket shown in the cut just
outside the cylinder wall. The purpose of this is to keep the
air, so far as pos.sible, cool during compression, and thus to
avoid putting upon the machine the work of compressing air
at a pressure enhanced by the heat that always is produced
when air is compressed. And just here is the first weak point
of the compressed air system. However efficient is the
mechanism of the compressor, all heat given to the air during
compression represents a loss of energy, since the air loses
this heat energy before it reaches the point of consump-
tion. The higher the final pressure which is to be reached,
the more useless heating of the air and the lower efficiency.
Hence the water jacket, which, by abstracting part of the heat
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GENERAL CONDITIONS OF POWER TRANSMISSION. 51
of compression, aids in averting needless work on the air dur-
ing compression. Even the most thorough jacketing leaves
much to be desired, generally leaving the air discharged at
from 200° to 300° F., more often the latter. A cold water
spray is often used in the compressing cylinder. This is some-
what more thorough than the jacket, but is still rather in-
effective. Both serve only to mitigate the evil, since they
cool the air by absorbing energy from it, and at best cool it
very imperfectly. A careful series of investigations by Riedler,
perhaps the best authority on the subject, gives for the efficiency
of the process of compression from .49 to .72. These figures,
derived from seven compressors of various sizes and types,
include only those losses which are due to heat, valve leakage,
clearance, and the like, taking no account of fricticmal losses
in the mechanism. These are ordinarily about the same as
in a steam engine, say 10 per cent, so that the total efficiency
of a simple compressor may be taken as .44 to .65, the latter
only in large machines under very favorable conditions. The
most considerable recent improvement in compressors is the
division of the compression into two or more stages, as the ex-
pansion is divided in compound and triple expansion engines.
This limits the range of heating that can take place in any
given cylinder, and greatly facilitates effective cooling of the
air. Riedler has obtained from two-stage machhies of his
own design a compressor efficiency of nearly .9. Allowing for
the somewhat greater friction in the mechanism, the total
efficiency was found to be about .76. In general, then, we may
take the total efficiency of the single stage compressors usually
employed in this country as .5 to .6, very rarely higher, while
the best two-stage compressors may give an efficiency slightly
in excess of .75. For steady working, .75 would be an excel-
lent result.
We may next look into the action of the compressed air in
the mains. As in the case of w^ater, the frictional resistance
and consequent loss of pressure vary directly with the square
of the velocity of the air and inversely with the diameter of
the pipe. Hy reducing the one and increasing the other, the
efficiency of the line may be increased at the cost of a con-
siderable increase in original outlay. Any attempt to force the
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ELECTRIC TRANSMISSION OF POWER,
output of the main rapidly increases the losses. At a working
gauge pressure of 60 pounds per square inch, which is in very
frequent use, the per cent of pressure lost per 1,000 feet of
pipe of various diameters is given in the following table — the
velocity being taken at 30 feet per second :
Diameter . . .
1"
2"
3"
4"
5"
4.37
6"
12"
1
18" 24"
30"
48"
Per cent loss . .
21.8
10.9
7.8
5.45
3.00
1.0
0.00
0.6
.33
.25
The friction in the pipes is proportionally greater in small
pipes than in large, and this table is taken as correct for the
medium sizes. No allowance is made for increase in velocity
through a long main, for leakage, nor for draining traps,
elbows, curves, and other extra resistances, so that as in prac-
tice the larger and longer mains suffer the more from these
various causes, the table will not be found widely in error for
ordinary cases. Very large straightaway mains will give
somewhat better results, and the five last columns of the table
are computed from Riedler's experiments on the Paris air
mains, 11 J inches in diameter and 10 miles long. All losses
are included. Ix)sses in the air mains can therefore be kept
within a reasonable amoimt in most cases. With large pipes
and low velocities, power can be transmitted with no more loss
than is customary in the conductors of an electrical system.
Small distributing pipes, however, entail a serious loss if they
are of any considerable length.
The motor is the last element of pneumatic transmission to
be considered. Generally it is almost identical with an ordi-
nary steam engine; in fact, steam engines have been often
utilized for air, and common rock drills may be used indif-
ferently for steam or air with sometimes slight changes in the
packing of the pistons and piston rods. Some special air
motors are in use with slight modifications from the usual
steam engine type. In most of these the air is used expan-
sively and at a fairly good efficiency. Tests l)y Riedler on the
Paris system show for the smaller air motors an efficiency of as
high as 85 per cent so far as the utilization of the available
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GENERAL CONDITIONS OF POWER TRANSMISSION. 63
energy in the air is concerned, or, taking into account the
mechanical losses, 70 to 75 per cent. Occasional results as
low as 50 to 60 per cent were obtained even when the air was
used expansively, while if used non-expansively the total effi-
ciency was uniformly below 40 per cent. Tests on an adapted
steam engine with Corliss valve gear gave a pneumatic effi-
ciency of .90, with a total efficiency of .81. These figures are
under more than usually favorable conditions.
One of the principal difficulties with air motors is freezing,
due to the sudden expansion of the compressed air, and the
congelation of any moisture carried with it. It is quite use-
ful, therefore, to supply to the motor artificially a certain
amount of heat, sufficient to keep the exhaust at the ordinary
temperature, especially if the air has been cooled by spray
during compression. This heating process is very frequently
extended so as not only to obviate all danger of freezing but to
add to the output of the air motor by giving to the compressed
air a very considerable amount of energy. The air is passed
through a simple reheating furnace and delivered to the motor
at a temperature of about 300° Fahrenheit. The energy de-
livered by the motor is composed of that actually transmitted
through the mains pliLS that locally furnished by the reheater.
The amount of fuel used is not great, usually from i to J of
a pound of coal per horse-power-hour, and the increase of
power obtained is about 25 per cent of that which would
otherwise be obtained from the motor. This means that the
heat is very effectively utilized. Reheating is not a method of
increasing the efficiency of the system, as is sometimes sup-
posed, but a convenient way of working a hot air engine in
conjunction with an initial pressure obtained from air mains.
It increases the operating expense by a very perceptible though
rather small amount, and gains a good return in power. In so
far it is desirable, but it no more increases the efficiency of the
pneumatic transmission than would power from any other
source added to the power actually transmitted.
We are now in a position to form a clear idea of the real
efficiency of transmission of power by compressed air. Taking
the compressor and motor efficiencies already given, and assum-
ing 10 per cent loss of energy in the mains, we have for the
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54 ELECTRIC TRANSMISSION OF POWER.
total eflicieiicy from indicated horse-power at the compressor
to brake-horse-power at the motor: .75 x .90 x .80 = .54
for large two-stage compressors and large motors; while with
ordinary apparatus it would be about .70 X .90 x .75 = .47.
At half-load these figures would be reduced to about .45 and
.35 respectively. In operating drills, which are motors in
which the air is used non-expansively and to which the air is
carried considerable distances through small pipes, the total
efficiency is almost ahvays below rather than above .30. The
efficiency of .54 given above cannot well be realized without
recourse to artificial heating to enable the air to be used expan-
sively without trouble from freezing.
Compressed air has been mainly used for mining operations,
where its entire safety and its ventilating effect are strong
point-s in it,s favor. More rarely it is employed for general
power purposes. Of such use the Popp compre^ssed air system
in Paris is the best and the only considerable example.
This great work started from a system of regulating clocks
by compressed air established a quarter-century ago. Nearly
a decade later the use of the compressed air for motors began,
and after several extensions of the old plant the present station
was built. It contains four 2,000 HP compound compressoi-s,
of which three are regularly used and the fourth held in r(»serve.
The steam cylinders are triple expansion, worked with a steam
pressure of 180 pounds. The air pressure is 7 atmospheres,
and the new mains are 20 inches in diameter, of wrought iron.
There are in all more than 30 miles of distributing main, most
of it of 12 inches and imder in diameter. A very large number
of motors of sizes from a fan motor to more than 100 HP arc
in use. Their total amount runs up to several thousand HP,
even though the majority of them are less than a single horse-
power. Except in very small motors, reheaters are used,
raising the temperature of the air generally to between 200°
and 300° F. The efficiency of the w^hole system from Pro-
fessor Kennedy's investigations is about 50 per cent under
very favorable conditions. The prices charged for powder
have not been generally known , but are understood to be some-
what in excess of $100 per horse-power per working year.
An interesting addition to the apparatus of pneumatic trans-
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GENERAL CONDITIONS OF POWER TRANSMISSION. 55
mission has recently appeared. It is a modification of the
ancient "trompe/' or water blast, used for centuries to feed
the forges of Catalonia, very simple in operation and cheap to
build. In its present improved form it is known as the Taylor
Hydraulic Air Compressor, and an initial plant of very respect-
able size has been in highly successful operation for nearly two
years past at Magog, P. Q., from which the data here given
have been obtained.
The compressing apparatus which is shown in Fig. 21 is in
principle an inverted siphon having near it*s upper end a series
of intake tubes for air, and at the bend a chamber to collect
the air which, entrained in the form of fine bubble??, is carried
down with the water column, which flowing up tlie short arm
of the sijihon escapes into the tail race. In Fig. 21, A Is the
penstock delivering water to the supply tank B. In this
tank Is the mouth of the down tube C, contracted by the
inverted cone C so as to lower the hydraulic pressure and
allow ready access of air from the surrounding apertures.
The air bubbles trapped in the water sweep down C, which
expands at the lower end, and finally enters the air tank 1).
Here the water column encounters the cone K, which flattens
into a plate at the base. Thus spread out and escaping into
the air chamber by the circuitous route shown by the arrows,
the air bubbles from the water accumulate in the top of the
air tank, while the water itself rises up the shaft E, and flows
into the tail race F, The air in D is evidently under a pressure
due to the height of the water column up to F, and quite
independent of the fall itself, which consequently may vary
greatly without aflfecting the pressure of the stored air, a very
valuable property in some cases, as in utilizing tidal falls.
F'rom D the compressed air is led up through a pipe, P, for
distribution to the motors. To get more pressure, it is only
necessary to burrow deeper with the air tank, not a diffi-
cult task where easy digging can be found. The fall and rate
of flow determine the rate at which the air is compressed,
and contrary to what might be supposed, the process of com-
pression is quite efficient. It is quite sensitive to variations
in the amount of flow, the efficiency changing rapidly with the
conditions of inlet; and since there certainly is a limit to the
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56
ELECTRIC TRANSMISSION OF POWER,
rio. 21,
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GENERAL CONDiriONS OF POWER TRANSMISSIOX. 57
amount of air that can be entrained in a given volume of
water, the process is likely to work most efficiently at moderate
heads and with large volumes of water. In the Magog com-
pressor about 4 cubic feet of water are required to entrain
1 cubic foot of air at atmospheric pressure, and it is open to
question as to how far this ratio could be improved. This
ratio, too, would be changed for the w^orse rapidly in attempt-
ing high compression, so that the Magog results probably rep-
resent, save for details, very good working conditions. The
dimensions of the Magog apparatus are given in the accom-
panying table, which is followed by the details of one of the
tests made by a very competent body of engineers.
The general dimensions of the compressor plant are:
Supply penstock 60 inches diameter
Supply tank at top 8 feet diameter by 10 feet high
Air inlets (feeding numerous small tubes) 34 2-inch pipes
Down tube 44 inches diameter
Down tube at lower end 60 inches diameter
Length of taper in down tube, changing from 44-iuch to
60-inch diameter 20 feet
Air chamber in lower end of shaft 16 feet diameter
Total depth of shaft below normal level of head water about 150 feet
Normal head and fall about 22 feet
Air discharge pipe 7 inches diameter
Flow of water, cubic feet, minute 4292.
Head and fall in feet 19.509
Gross water HP 158.1
Cubic feet compressed air per minute, reduced to atmos-
pheric pressure 1 148.
Pressure of compressed air, lbs 53.3
Pressure of atmosphere, lbs 14.41
Effective work done in compressing air, HP 111.7
Efficiency of the compressor, per cent 70.7
Temperature of external air, Fahr 65.2
Temperature of water and compressed air, Fahr 66.6
Moisture in air entering compressor, per cent of saturation 68.
Moisture in air after compression, per cent of saturation 36.
The efficiency given is certainly most satisfactory, being
quite as high as could be attained by a compound compressor
of the best constniction driven by a turbine, and for the head
in question at a very much lower cost. It is probable that
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58 ELECTRIC TRANSMISSION OF POWER.
the test given does not represent the best that can be done by
this method, and the indications are that within a certam,
probably somewhat limited, range of heads the hydraulic com-
pressor will give as compressed air a larger proportion of the
energy of the water than any other known apparatus. Just
what its limitations are, remains to be discovered, but several
plants are now under construction which will throw consider-
able light upon the subject. ^
In certain cases the power of getting compre.ssed air direct
from hydraulic power by means of a simple and, under favor-
able conditions, cheap form of apparatus, is very valuable,
and while it is unlikely to change radically the status of pneu-
matic transmission, it is an important addition to available
engineering methods. As in most pneumatic plants, the
Magog installation is worked in connection with reheaters.
A similar plant on a somewhat larger scale is in successful
operation near Norwich, Conn.
IV. In point of convenience and efficiency, compressed air
is nearer to electricity for the distribution of power over large
areas than any other method. The only other system that
approaches them is the transmission of gaseous fuel for use
in internal combustion engines. At equal pressures one can
send through a given pipe twenty times as much energy stored
in gas as in air. A good air motor requires about 450 cubic
feet of air at atmospheric pressure per indicated HP hour,
while a gas engine will give the same power on a little over 20
cubic feet of gas. But the cases wherein the distribution of
gas would be desirable in connection with a transmission over
a long line of pipe are comparatively few. Particularly this
system has no place in the development of water-powers, the
most important economic function of electrical transmission.
Nevertheless it must be admitted that for simple distribution
of power a well-designed fuel gas system is a formidable com-
petitor of any other method yet devised, particularly in the
moderate powers — say from 5 to 25 HP.
We are now in a position to review the divers sorts of power
transmission that have been discussed, and to compare them
with power transmission by electricity.
Without going deeply into details, which will be taken up in
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GENERAL CONDITIONS OF POWER TRANSMISSION. 69
due course, we may say that electrical niachiuory possesses one
advantage to an unique extent — high efficiency at moderate
loads. Machinery in which the principal losses are frictional
is subject to these in amount nearly independent of the load;
hence the efficiency drops rapidly at low loads. In dynamos,
motors, and transformers, however, the principal losses de-
crease rapidly with the load, so that within a wide range of
load the efficiency is fairly uniform. Fig. 22 gives the efficiency
curves for a modem dynamo, motor, and transformer. The
100
TRA
1
no
/
/^
>
,^
^^OVN^
hO
z
u
o
/
/
/I
^0^0^
SO
/
i
1
70
/
>
4
\
i
( FULL
LOAD
Fio. 22.
generator curve is from a 2(X) KW 5()0-volt direct-current
machine, the motor curve from a smaller machine of the same
type, and the transformer curve from a standard type of about
30 kilowatts capacity. Jn the generator curve the variation of
efficiency from half load to full load is less than 2 per cent,
in the motor only 2\ per cent, and in the transformer just
1^ per cent. In addition, the efficiency of all three at full
load is very high. Hence, not only is an electrical power
transmission of great efficiency if the loss in the line be moder-
ate, but this efficiency persists for a wide range of load. As
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60 ELECTRIC TRANSMISSION OF POWER.
in hydraulic and pneumatic transmission, the efficiency of the
line depends on its dimensions; so that by increasing the weight
of copper in the line, the loss of energy may be decreased
indefinitely. And since the loss of energy in the line dimin-
ishes as the scjuare of the current, the percentage of loss at
constant voltage diminishes directly with the load.
Hence, the total efficiency may be constant or even increase
from half load to full load, even with a quite moderate loss in
the line. In pneumatic and hydraulic transmission this con-
dition may occur, but only with large loss in the mains, since
the efficiencies of the generator and motor parts of such systems
decrease too rapidly to be compensated by the gain in the
main, unless its efficiency is low at full load. Hence, for
ordinary cases of distribution in which the average load is
considerably less than full load, often only J to ^ of full load,
electric transmission has a very material advantage over all
other methods. To appreciate this we need only to run over
the details of electrical power transmission and compare the
results with those which we have obtained for the other
methods described.
There are to be considered in electrical power transmission,
as in transmission of every sort, two somewhat distinct prob-
lems :
First, the transmission of energy over a considerable dis-
tance and its utilization in one or a few large units.
Second, the distribution of power to a large number of small
units at moderate distances from the centre of distribution.
This latter case may sometimes also involve the transmission
of power to a real or fictitious centre of distribution. This
second problem is the commoner, and, while not so sensational
as the transmission of power at high voltage over distances of
many miles, is of no less commercial importance.
We have all along been considering, in treating of transmis-
siori of power by ropes and by hydraulic and pneumatic engines,
the case first mentioned, excepting in so far as some special
distributions have been referred to. We have already the data
for figuring the efficiency of an electric power transmission
•with large units. In cases of this kind the distance between
the generator and motor is likely to be much greater than in
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GENERAL CONDITIONS OF POWER TRANSMISSION, 61
the case of distribution to small motors from some central
point, and the loss in the line, the only uncertain figure in the
transmission, would generally range from 5 to 10 per cent.
In case of distributing plants intended to furnish from a single
point small units of power over a moderate distance, it is
generally found that losses in the line of from 2 to 5 per cent
do not involve excessive cost of copper. In cases where
a. distribution is coupled with the transmission of power to the
central point, the loss from the distant generator to the motors
is in most cases from 10 to 15 per cent.
Taking up first the transmission of power from one or more
large generators to one or more large motors, we may take
safely the commercial efficiency of the generator as that given
by the curve. Fig. 22, and that of the motors as at least as
good as that given for a motor in the same figure. The effi-
ciency of the line for moderate distances may be taken as
95 per cent. It should be noted that the efficiencies of large
alternating generators and motors do not differ materially
from those shown; in fact, are quite certain to be above them.
We thus have for the efficiency in a transmission of this kind:
94 X 95 X 93 = 84 per cent. This is largely in excess of
that which could be obtained at distances of say a couple of
miles by any other method of transmission.
Even more extraordinary is the efficiency at half load in
this ca.se, which is 92 x 97.5 x 91 = 81.6 per cent. It
should be borne in mind that these efficiencies are taken from
experiments with ordinary machines, and the efficiencies are
those which can be bettered in practice. These results show
the great advantage to be derived from electrical transmission
when, as in most practical cases, full load is seldom reached.
It is most important for economical operation to employ a
system which will give high efficiency at low loads, and it
would be worth while so to do even if the efficiency at full
load were not particularly good. With electrical machinery,
however, there is no such disadvantage. Even at one-fourth
load the efficiency of the electrical system still remains good.
It is nearly 73 per cent on the a.ssumed data. The efficiencies
thus given are from the shaft of the generator to the pulley
of the motor inclusive.
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62
ELECTRIC TRANSMISSION OF POWER.
In the case of distributed motors supplied from a central
point not very distant from any of them, the efficiencies of
generator and line remain about as before, but the motor
efficiencies for the sizes most often employed are below that
just given. The average motor efficiency is largely dependent
on the skill with which the units are distributed. It has often
been proposed to drive separate machines by individual motors,
while in other cases comparatively long lines of shafting are
employed, grouping many machines into a dynamical unit
operated by a motor. To secure economy it is desirable on
the one hand to use fairly large motors well loaded, while on
the other hand the losses in shafting and belting must be kept
down.
The larger the motors, the better their efficiency at all
loads and the less the average cost per HP, but with small
motors the cost and inefficiency of shafts and belts may be
in large measure avoided. The most economical arrange-
ment depends entirely upon the nature of the load. Much
may be said in favor of individual motors for each machine,
but so far as total economy is concerned, this practice is best
limited to a few cases - - machines demanding several HP (say
5 or more) to operate them, machines so situated as to neces-
sitate much loss in transmitting power to them, and certain
classes of portable machines. In applying electric power
to workshops already in operation, the group system will
usuall}'^ give the best results, individual motors being used
only for such machines as might otherwise cause serious loss
of power. The following table gives the average full load
efficiencies that may safely be expected from motors of various
sizes, irrespective of the particular type employed.
HP of motor . .
1
72
8
7S
5
81
.1,.
sa j 80
15
86
20
87
25
88
40
90
50
90
Per cent efficiency
92
These are commercial efficiencies reckoned from the electri-
cal input to the mechanical output at the pulleys. Below 5 HP
the efficiencies fall off rapidly. At partial loads the efficiencies
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GENERAL COSDITIOXS OF POWER TRANSMISSIOX, 63
are somewhat uncertain, inasmuch as some motors are designed
so as to give their maximum efficiency at some point below
full load, while others work with greater and greater efficiency
as the load increases until heating or sparking limits the out-
put. The former sort are most desirable for ordinary work-
shop use, while the latter are well suited to intermittent work
at very heavy loads, as in hoisting. The difference in the two
types of machine is very material. It is easily possible to
procure motors that will not vary more than 5 per cent in
efficiency from full load to half load, and this even in machines
as small as 2 or 3 HP. We may now calculate the efficiency
of an electric distribution with motors of moderate size — such
a case as might come from the electrical equipment of large
factories. The generator efficiency may be taken as before
at .94 and that of the line at .95, while the motors nuist be
laken close account of in order to estimate their collective
efficiency. Assuming the sizes of motors in close accordance
with those in several existing installations of similar character,
we may sum them up about as follows:
5 3 HP
o 5 HP
10 10 HP
10 20 HP
5 26 HP
2 f)0 HP
In all 37 motors, aggregating 565 HP. The mean full load
efficiency of this group is very nearly .N7. The efficiency of
the system is then
.94 X .95 X .87 = 77.6.
This result requires full load throughout the plant, a some-
what unusual condition with any kind of distribution. From
the data already given, the half load efficiency should be about
.92 X .975 X .82 - .785.
Between the limits just computed should lie the commercial
efficiency of any well-designed motor distril)ution reckoned
from the dynamo pulley. In the case of steam-driven plants
it is often desirable to consider the indicated HP of the engine
as the starting point, and the (juestion immediately arises as
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64
ELECTRIC TRANSMISSION OF POWER,
to the commercial efficiency of the combination of d3mamo
and engine. In cases where high efficiency is the desideratum
direct coupling is usually employed, saving thereby the loss
of power, perhaps 5 per cent, produced by belting. The
losses in such direct-coupled units vary considerably with the
size and type of both machines. Fig. 23 shows the efficiency
of two such combinations at various loads. Curve A is from
an actual test of the combination; curve B from tests of an
engine and dynamo separately. Each unit was of several
xuo
90
m
:r^
:==
z
~*7D
y
</
^^""^'^^^
.0^^^
b.
bJ
60
/
^
/^
60
A
/
^
40
/
1
4
k
i
H
•4 FULL
LOAD
Fi«. 23.
hundred HP. The high result from curve A is mainly due
to very low friction.
These curves give handy data for computing the total effi-
ciency of a motor plant from the motor pulleys to the indicated
horse-power of the driving engine. Taking the combined
engine and dynamo efficiency from i4,and assuming the same
figures as before on motors and line, we have at full load,
.88 X .95 X .87 = .727.
And from the same data at half load,
.78 X .975 X .82 = .651.
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GENERAL CONDITIONS OP POWER TRANSMISSION. 65
For certain computations, as in case of figuring out a com-
plete installation, the above efficiencies are convenient. They
show that in very many instances the distribution of power by
electric motors is very much more economical of energy than
any other method employed. In ordinary manufacturing
operations power is generally transmitted to the working
machines through the medium of lines of shafting of greater
or less length. These are very rarely belted directly to the
machines, but transfer power to them through one or more
countershafts. Often the direction of shafts is changed by
gearing or quarter turn belts, and even when the power is
distributed through only a single large building there will be
found more often than not, intervening between the driving
engine and the driven machine, three belts and two lines of
shafting of consi derable length, and not infrequently still other
belts and shafts. It very often happens, too, that to keep in
operation one small machine in a distant part of the shop, it is
necessary to drive a long shaft the friction of which consumes
half a dozen times as much power as is actually needed at the
machine. The constant care required to keep long lines of
shafting in operative condition is an irritating and costly con-
comitant. The necessary result is a considerable loss of power,
which, being nearly constant in amount, is very severe at
partial loads.
Mowing 5 per cent loss of energy for each transference
of power by belting, a figure in accordance with facts, and 10
per cent loss for each long line shaft driven, it Is sufficiently
evident that from 20 to 25 per cent of the brake-horse-power
delivered by the engine must be consumed even under very
favorable circumstances by the belting and shafting at full
load. This means an efficiency at half-load of from 50 to 60
per cent only, and at le&ser loads a very low efficiency indeed.
The large number of careful experiments carried out on
shafting in different kinds of workshops, and under various
conditions, shows that only imder very exceptional circum-
stances LS the loss of power by shafting between the engine
and the driven machines as low as 25 per cent. Far more
often it is from 30 to 50 per cent, and sometimes as high as
75 or 80 per cent. The figures, which have been well estab-
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66 ELECTRIC TRANSMISSION OF POWER.
lished, regarding the efficiency of the transmission of power
by motors, show that at full load it is comparatively easy
to exceed 75 per cent efficiency; thus more than equalling the
very best results that can be obtained with shafting. At half-
load and below, the advantage of the electric transmission be-
comes enormous, even supposing shafting to be at its very
best.
Compared with ordinary transmission by shafting, the motor
system is incomparably superior at all loads, so that it may
easily happen that a given amount of work can be accomplished
through the medium of a motor plant with one-half the steam
power required for the delivery of the same power through
shafts and belts. Such results as this have actually been
obtained in practice. It is, therefore, safe to conclude that
the distribution of power by motors is, under any ordinary
commercial conditions, at least as efficient as the very best
distribution of power by shafting at full load, and much more
efficient at low loads. Under working conditions in almost all
sorts of manufacturing establishments, light loads are the rule
and full loads the rare exception; consequently the results of
displacing shafting by motor service have, as a rule, been
exceedingly satisfactory in point of efficiency, and the lessened
operating expense more than offsets the extra cost of installa-
tion.
In one early three-phase plant, that of Escher, Wyss & Co.
at Winterthur, Switzerland, 300 HP in 32 motors worked from
a 12-mile transmission line displaced far greater capacity in
steam engines, and similar results on a smaller scale are not
uncommon.
To add force to this comparison between the efficiency of
shafting and of motors, the following results from electrical
distribution plants already installed may be pertinent. A
typical example of the sort is from a plant installed some years
since in a fire-arms factory at Herstal, Belgium. There were
there installed 17 motors of an aggregate capacity of 305 HP,
driven by a 300-KW generator direct-coupled to a 500-HP
compound -condensing engine. The efficiency guaranteed from
the shaft of dynamo to the pulleys of the motors is 77 per cent.
Since its first installati(m, the plant has been increased by the
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GENERAL CONDITIONS OF POWER TRANSMISSION. 67
addition of a second direct-coupled dynamo and the total horse-
power of motors is 428. A second notable installation of
motors in the same vicinity is at the metallurgical works of
La Soci^t^ de la Vielle-Montagne, consisting of a 375-KW 500-
volt dynamo direct .driven at a speed of 80 revolutions per
minute by a 600-HF compound-condensing engine. The plant
consists of 37 motors with an aggregate HP of 329. The full
load efficiency of the plant from dynamo shaft to motor pulley
is 76 per cent. The loss in the lines, both in this case and
in the preceding, is very small, only 2 per cent. They are
both typical cases of transmission to motors driving groups
of machines, and in spite of rather low dynamo efficiencies
at full load, these being in each case 90 per cent, the results
obtained are in close accordance with those already stated as
appropriate to similar cases. As an example of work under
more favorable conditions, the early three-phase power plant
at Columbia, S. C, may be instanced.
The problem here undertaken was to drive a very large cot-
ton mill, utilizing for the purpose a water-power about 800 feet
distant. Two 500-KW dynamos direct-coupled at a speed of
108 turns per minute deliver current at 550 volts to an under-
ground line connecting the power station with the mills.
The motors are suspended from the ceiling, and each drives
several short countershafts. The motors are wound for the
generator voltage without transformers, and are of a uniform
size, 65 HP each. The commercial efficiency of this plant,
taken as a whole from the shaft of the dynamo to the pulleys
of the motors, is not less than 82 per cent at full load. This
good result is due to the use of large motors, and to the small
line loss of 2 per cent as in the preceding foreign examples.
These results are thoroughly typical, and can regularly be
repeated in practice. In general a net efficiency of 80 to 85
per cent can be counted upon in plants of the approximate
size of those here mentioned, assuming the apparatus now
commercially standard. Even smaller plants can be counted
on to give nearly or quite as good results, since the differ-
ence in efficiency, supposing motors of the same size to be
used, between a d3mamo of 100 KW and one of 400 or 500 KW
is hardly more than 1 |>er cent at full load, supposing machines
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68 ELECTRIC TRANSMISSION OF POWER.
of the same general design to be employed, nor is there any
substantial difference in efficiency between plants employing
direct current and those using polyphase apparatus, as may
be judged from the figures just given.
We are now in position intelligently to compare the trans-
mission and distribution of power by electric means with the
other methods which have sometimes been employed.
All comparisons between methods of transmitting power
have to be based in a measure on their relative efficiency.
Now, in every such method there are three essential factors:
1st, the generating mechanism, which receives power direct
from the prime mover and in conjunction with which it is
considered; 2d, the transmitting mechanism, which may be an
electric line, a pipe line, ropes, or belts; and 3d, the motor
part of the transmission, which receives power from the trans-
mitting mechanism and delivers it for use. For a given
capacity of the generating and receiving mechanisms, the
efficiency of each at all loads is determined within fairly close
limits. The transmitting mechanism, however, is not so
closely determined, save in the case of the rope drive.
Electric, pneumatic and hydraulic transmission lines are all
subject to the general principle that the loss in transmission
can be made indefinitely small by an indefinitely large expen-
diture of capital, enormous cross-section in the one case, or
huge pipe lines in the others. The efficiency of these methods
is, therefore, a fluctuating quantity depending on that loss in
the transmitting mechanism which may be desirable from an
engineering or economical standpoint. In making compar-
isons between these methods, there is a wide opportunity for
error unless some common basis of comparison is predeter-
mined. In the next case any such comparison must differ
widely in its results according to the character of the power
distribution which is to be attempted. We have already seen
that with the rope drive, distribution is very difficult, while
with electric and pneumatic systems it is comparatively
easy.
A general valuation of the commercial possibilities of these
divers matters is, therefore, hard to make except in a general
way. We can, however, by assuming a given transmission of
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GENERAL CONDITIONS OF POWER TRANSMISSION. 69
given magnitude and character, and further assuming such
loss in the transmitting mechanism as might reasonably be
expected in practice, arrive at a reasonably accurate conclu-
sion for the case considered. As a very simple example of
power transmission, let us take the delivery of power over a
distance of two miles, the delivery being in one unit or, at
most, two units. We will assume the same indicated HP fur-
nished at the generating end of the line in each case, of which
as much as possible is to be delivered at the receiving station,
the losses in transmissicm being taken as 10 per cent of the
power delivered to the line; this is to cover all losses of
energy by resistance and leakage on the electrical line or
loss of pressure and resulting expenditure of energy, leakage,
friction, and all other sources of loss in the other cases.
As the same indicated power is generated in each case, we
will suppose a modem plant with compound condensing en-
gines costing complete with buildings $50 per HP. We will
further assume that each indicated horse-power per working
year of 3,000 hours will cost $18; this covering all expenses
except those chargeable to interest and depreciation. For
this simple case we have the following costs of initial plant
and of operation per mechanical horse-power delivered from
the motor, full load only })eing considered. The four meth-
ods considered are rope driving, pneumatic, pneumatic with
reheating apparatus at the motors, and electrical. The prices
are from close estimates of the cost in each case. The dyna-
mos are supposed to. be direct-coupled. The compressors tr)
be direct-acting, two-stage compressors. The steam cylinders
Corliss compound-condensing type. The air-pressure assumed
is 60 lbs. above atmospheric pressure. The electric voltage
3,000. The rope speed about one mile per minute. Interest
and depreciation are taken at 10 per cent of the total cost of
the plant, save in the case of the rope drive, where an addi-
tional charge for renewal of cable is made on the supposition
that the cable will last somewhere from 18 months to 2 years,
which is fully as favorable a result as can fairly be expected.
The following are the comparative estimates:
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70 ELECTRIC TRANSMISSION OF POWER.
Rope, EFFiciENrY 67 per cent.
COST.
Steam plant $50,000
Pulley stations 26,000
Cables, steel 17,000
ToUlcost 192,000
OPERATING EXPENSE.
1,000 l.HP at ai8 118,000
Interest and depreciation on plant, at 10 per cent . . . 7,500
Depreciation of cable 8,000
133,500
Net HP produced, 672.
Cost per HP-year, $49.
Pneumatic, Efficiency 64 per cent.
COST.
Steam plant, excluding engines $35,000
Compressors . 17,000
Air mains laid, 12 inches 18,000
Air motors 12,000
Total cost $82,000
operating expense.
1,000 I.HP at $18 $18,000
Interest and depreciation, at 10 per cent 8,200
$26,000
Net HP delivered, 640.
Cost per HP-year, $48.
Air Reheated, Apparent Efficiency 65 per cent.
COST.
Steam plant, excluding engines $35,000
Compressors 17,000
Air mains laid 18,000
Air motors and rchealers with chimney, etc. .... 14,000
Total cost $84,000
operating expense.
1,000 I.HP at $18 . .• $18,000
Interest and depreciation, at 10 per cent 8,400
Coal and labor for reheating 1,500
$27,900
Net HP delivered, 650.
Cost per HP-year, $43.
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GENERAL CONDITIONS OF POWER TRANSMISSION. 71
Electric, Efficiency 73 per cent.
COST.
Steam plant 860,000
Dynamos 18,000
Line 3,000
Motors 13,000
Total cost $84,000
OPERATING EXPENSE.
l,000I.HPat$18 $18,000
Interest and depreciation, at 10 per cent 8,400
Electrician 1,500
$27,900
Net HP, 730.
Cost per HP-year, $38.
It appears at once that the rope drive is beyotul the range of
its efficient use. Its first cost is greater than that of either of
the other methods, and the expense is carried to a very high
figure by the item of depreciation on the cables, which cannot
be avoided; hence in spite of a high efficiency, the cost per
HP year delivered rises to $49. We may next consider the
schedule of cost for the pneumatic system. In this case the
most formidable item is the cost of the air-mains, which should
be at least 12 inches in diameter. Nevertheless, the total initial
cost is the lowest of the four. The operating expense is also
the lowest, but the very low efficiency of the pneumatic system
without reheating raises the cost per HP delivered to a very
considerable amount — almost as much as in the case of the
rope drive. Reheating would almost always be used in con-
nection with a plant of this size, and with reheating the result
is much more favorable. The initial expenditure is somewhat
increased by the addition of the reheaters, piping and chinniey.
The operating expense is also slightly increased by the coal
necessary for reheating, taken at J of a pound per HP per hour,
and the small amount of additional labor involved in caring
for the reheaters, disposing of the ashes, and looking after
the reheating plant generally. The apparent efficiency in
this Case is very excellent, 65 per cent being reasonably attain-
able, and the cost per HP year falls to $43, shov/ing conclu-
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72 ELECTRIC TRANSMISSION OF POWER.
sively enough the advantage of reheating; at least, where
the units are so large that the presence of a reheater is not
a practical nuisance.
Finally, we come to the electric power transmission. In
this case the most striking feature is the low cost of the line,
supposed here to be overhead. It may be noted, however,
that an underground line, consisting of cable laid in conduit,
still leaves the cost per HP year lower than that of any of
the other methods. Operating expense is fairly increased
by the addition of an electrician to the cost of the indicated
horse-power, interest, and depreciation. The total first cost
ife practically the same as that of air with reheater, as is also
the operating expense. Tlie added efficiency, however, brings
the cost per HP year to $38; decidedly the lowest of the four
cases considered. It may be thought that difference of loss
in transmission might possibly alter the relation of the electric
plant to the air-plant with reheaters, but an added efficiency
of line would in either case be accompanied by added expen-
diture of not very different amounts in the two cases, and the
efficiency of the electric plant would always be enough higher
than that of the air-plant to give it the advantage in net cost
per HP, however the two plants might be arranged. We thus
find that at a distance of two miles the electric transmission
has a material advantage, air with reheaters, air without
reheaters, and rope drive following it in the order named.
The pneumatic method would at the distance of one mile,
as may readily be computed, take about the same relative
position as before, since the efficiency maintains approxi-
mately the same relation to the others.
The pneumatic plant gains in first cost at this lesser dis-
tance, not enough, however, to alter the final result. At
half a mile distance, the rope drive will be found to be the
cheapest in fii-st cost, and also, through its enormous efficiency,
to be a little the cheapest per HP delivered, in spite of the
large depreciation in the cables, while the electric and pneu-
matic systems would be very close together, the electric,
however, still retaining a slight advantage due to its greater
efficiency. Neither can, in point of absolute cost of power
delivered, compete with the rope drive at this distance for
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GENERAL CONDITIONS OF POWER TRANSMISSION. 73
this simple transmission at full load, although both would
surpavss it were there any considerable distribution of the
powder. Figures that have heretofore been given on the
relative cost and efficiency of such transmissions have as a
rule been in error in two very essential particulars: first, the
efficiencies of the electrical system have been greatly under-
estimated owing to the poor machines with which the first
experiments were made; second, the commercial advantage of
reheating in the pneumatic transmission has not generally been
given its proper weight. It is, as has been already stated, not
a method of increasing the efficiency, but of increasing the
power delivered by addition of energy at the receiving end of
the line under very favorable conditions. The figures just
given are believed to be as nearly exact as roughly assumed
conditions permit.
One modification in the electric transmission should here
be noted. The recent introduction of the steam turbine has
rendered it possible to lower the cost of the generating plant
very materially, while retaining a cost of power at the prime
mover not in excess of that here given. The generating unit
also comes in for some reduction in the combined frictional
loss, so that the final cost per HP year would on the basis here
taken probably fall to $35 or $36, giving the electrical system
a still greater advantage. This statement does not mean
broadly that a turbo-generator can regularly deliver power
six or eight per cent cheaper than an ordinary generating
set, but merely that it would probably do so under the cir-
cumstances here assumed.
All these estimates are subject to change of prices from
year to year, but no changes are likely to be sufficient to alter
the relative position of the methods compared as regards
cost. It is not a difficult matter to construct a set of estimates
arranged to favor any given method. In the long run, what-
ever minor variations may appear in the items here given,
the totals will be found to scale up or down in about the same
ratios.
At less than full load and hence under variable loads, the elec-
tric system enjoys the unique advantage of having the losses
of energy in every part of the system decrease as the load
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74 ELECTRIC TRANSMISSION OF POWER.
decreases, while in rope driving all the losses are practically
constant, and in the hydraulic and pneumatic systems all
arc nearly constant save that in the pipe-line.
Hence, under low and varying loads, electric transmission has
a great additional advantage. Since in distributions of power
employing a considerable number of motors light load on the
motors is the invariable rule, as soon as we depart from the
very simple case discussed the electrical system gains in rela-
tive economy at every departure. These more general cases
have already been described, and gathering the results we
may construct the following table, showing the efficiency of
each svstem under full and half-loads:
System. Full l.osid. Half Load.
Wire rope . 67 46
Hydraulic high pressure 53 46
Hydraulic low pressure 6() 5()
Pneumatic ^ 60 40
Pneumatic reheated (virtual efficiency) . 66 60
Electric 73 66
The efficiencies in the electric system as here given are
lower than would be reached practically in large plants. The
present practice of using generators and motors woimd for
pressures up to 10,000 or 12,000 volts makes a most material
diiTerence in the matter of efficiency. For a well-designed
transmission of a few miles in units of say 500 KW and up-
wards, one may fairly expect to get at full load as much as 94
per cent from generator and motor, and i)erha})s 98 per cent
from the line, givhig a total efficiency of transmission of
.94 X .94 X .98 = .866
at full load and of nearly .85 at half-load or say 79 and 72 per
cent respectively when reckoning efficiency from the indicated
HP of the engine as in the foregoing comparisons. This means
far higher efficiency than can be obtained by any other method
at any but the shortest distances.
All the figures must be taken as approximate. They are
under conditions fairly comparable except in case of the low-
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GENERAL CONDITIONS OF POWER TRANSMISSION. 75
pressure hydraulic system, in which the large proix)rti()n of
loss due to pipe-friction operates to hold up the half-load effi-
ciency to an abnormal degree. With the ordinary proportion
of small motors this half-load efficiency would be nearer 40
than 50 per cent. The electric system is easily the most
efficient at any and all loads. Of the others, wire-rope trans-
mission, if the distributed units are fairly large, holds the
second place for short distances, and the pneumatic system
with energy added at the motors by reheating, at moderate
and long distances. Without reheating it occupies the last
place in order of efficiency, although even so, it is, next to
electricity, the most convenient method of distributing power.
In fact, electricity and compressed air are the only two
systems available for the general distribution of energy. The
latter is, save for a single system in Paris, used only on a small
scale, and in this country hardly at all save in mining. Of
course, the very largest power stations are those belonging to
electric railway systems in the largest American cities. Several
of these exceed 50,000 HP in generator capacity and frequently
in actual output, notably the systems in Boston, Brooklyn,
and Philadelphia. Recent advances in electrical engineering,
particularly the effective utilization of alternating currents,
have greatly cheapened the distribution of electrical energy,
and other systems are now seldom installed for ordinary pur-
poses. A few pneumatic and hydraulic plants will continue
to be used, owing to the large capital already invested in them,
but new work is, and in the nature of things must be, almost
exclusively electrical. As the transmission of power from
great distances becomes more common and the radii of dis-
tribution themselves increase, the electrical methods gain more
and more in relative value, and all others become more ineffi-
cient and impracticable.
We have now discussed in some detail the sources of natural
energy which are available for human use, and the most promi-
nent of the systems employed for their utilization. We have
found that for practical purposes steam-power and water-
power must at present be used to the virtual exclusion of all
others, the former perhaps less than the latter save for dis-
tribution of power over short distances.
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76 ELECTRIC TRANSMISSION OF POWER.
Of the methods of distribution we have found all save com-
pressed air and electricit}'- very limited in their application,
the hydraulic systems to special classes of work under favorable
topographical conditions, and rope transmission to extremely
short distances and small numbers of power units delivered.
Both are noticably inefficient. The pneumatic system is very
general in its applicability, but of very low intrinsic efficiency.
When used in connection with reheating apparatus it requires
additional care, and the motors, like steam-engines, are heavy
and inconvenient. The electric system on the other hand em-
ploys motors which are compact and far more efficient than
any other type of machine for delivering mechanical power,
run practically without attention, and can be placed in any sit-
uation or position that is convenient. Furthermore, in average
working efficiency the electric system is 10 to 15 per cent
higher than any other yet devised, so that it is more economical
in use at nearly all distances and under nearly all conditions.
Finally, it unites with power distribution the ability to furnish
light and heat, thus gaining an immense commercial advan-
tage. This advantage is shared only by gas transmission,
which up to the present time remains of doubtful value on
account of the cost of the motors, their imperfect regulation,
and their inefficiency at moderate loads. Gas transmission,
however, is likely to grow in importance, owing to the great
improvements in small gas-engines stimulated by the rapid
development of the automobile. Having now overlooked
the advantages of electrical power, it is proper to pass to the
details of the methods employed for its utilization, and thence
to the general problem of its economical generation, trans-
mission and distribution.
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CHAPTER ITT.
POWER TRANSMISSION BY CONTINXTOUS TFRRENTS.
Up to the present time the larger part of electrical power
transmission has been done by continuous currents. All the
earlier plants were of this type, and even now, when trans-
mission by alternating currents, polyphase and other, is gen-
erally used, the older type of apparatus is still being installed
on an extensive scale, and on account of the large number of
plants now in operation, even if for no other reason, wdll prob-
ably remain in use for a long time to come. New power
transmission plants, both here and abroad, are, save for rare
exceptions, for alternating currents, and in many cases this
practice is almost absolutely necessary, but there still remain
many cases wherein the conditions are as well met in the old-
fashioned way.
Chief among these may be mentioned electric railway work,
which in America alone certainly requires more than a full
million horse-power in generators and motors. Certain diffi-
cult work at variable speed and load, and many simple trans-
missions over short distances, are at present best handled by
continuous current machinery. As alternating practice ad-
vances, many, perhaps all, of these special cases will be elimi-
nated, but we are dealing with the art of power transmission as
it exists to-day, and hence continuous current working deserves
consideration.
The broad principle of the continuous current generator has
already been explained, but its modifications in actual work
are important and worthy of special investigation. Tn a gen-
eral way, continuous currents are almost always obtained by
commuting the current obtained from a machine which would
naturally deliver alternating currents. This process is, how-
ever, by no means a.s simple as I"ig. 9 would suggest. With a
two-part commutator the resulting current, although unidirec-
tional, would necessarily be very irregular, owing to the fact
77
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KLECTRIC TRANSMISSION OP POWER,
that the total current drops to zero at the moment of com-
mutation. Such a current is ill fitted for many purposes, and
the commutator would be rapidly destroyed by sparking if the
machine were of any practical size.
To avoid these difficulties, the number of coils on the arma-
ture is increased, and they are so interconnected that, while
each coil has its connection to the outside current reversed as
before, when its electromotive force is zero, the other coils in
which the E. M. F. still remains in the right direction continue
in circuit imchanged. In this way the E. M. F. at the brush
is the sum of the E. M. F.'s of a number of coils, each of which
is reversed at the proper moment. The number of commutator
segments is increased proportionally to the number of coils,
Fia. 24.
and the commutator thus becomes a comparatively complicated
structure. The result, however, is that the total E. M. F. of
the armature cannot vary by more than the variation due to a
single coil. The nature of this modification is shown in Fig.
24, which shows a four-part commutator connected to a four-
coil drum armature.
An eight-part winding of modern type is shown in Fig. 25.
Tracing out the currents in this will give a clear idea botli of
a typical winding and of the process of commutation.
In commercial machines the number of individual coils and
of commutator segments often exceeds 100, but the principle
of the winding is the same. Nearly all the early dynamos had
several turns of wire per coil, as in Fig. 24, but at present, in
most large machines, one turn constitutes a complete coil.
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TRANSMISSlOyi liY CONTINUOUS CURRENTS.
79
This subdivision is to avoid sparking at the commutator,
which becomes destructive if the current be large and the
E. M. F. per commutator segment more than a few volts.
If each coil generates a considerable voltage, there is even
under the best conditions of commutation a strong tendency
for sparks to follow the brush across the insulation between
segments, or even to jump across this insulation elsewhere.
As this goes from bad to worse, and rapidly ruins the commu-
tator, every precaution has to be taken against such a con-
tingency. The E. M. F. generated by each coil is kept low
by subdividing the winding, and in large machines it is the
FlQ. 26.
rule that the E. M. F. of a single loop is quite all that can
safely be allotted to a single commutator segment.
Present good practice indicates that, in generators for light-
ing, up to 100 or 150 volts the voltage between brushes should
be subdivided so that it shall not exceed 3 or 4 volts for
each segment between the brushes. For 500 or 600 volt
machines it should not ordinarily exceed 8 or 10 volts, while
for dynamos of moderate output and even higher voltage it
may rise to 20 volts or more.
The reason for these differont figures is that the destructive-
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ELECTRIC TRANSMISSION OF POWER.
ness of the spark depends on the amount of current which is
liable to be involved. On a low voltage commutator intended
for heavy currents, even very moderate sparking may gnaw
the segments seriously, while the spark of an arc machine, in
spite of its venomous appearance, may do very little harm, as
the maximum current in the whole bar will not exceed 8 or 10
amperes. Consequently the voltage per bar in such cases is
sometimes 50 or more, while in very large incandescent ma-
chines, and in those designed for electrolytic purposes, the
E. M. F. per bar is often less than 2 volts or even below 1 volt.
Windings like those of Figs. 24 and 25 are of the so-called
Flo. 2G.
dram type, in which each convolution extends around the whole
body of the armature, either diametrically or nearly so.
Another sort of armature winding frequently used, although
less now than formerly, is the Gramme, so called from its
inventor. Here the iron body of the armature is, instead
of being cylindrical, in the form of a massive ring of rect-
angular cross-section. The windings are looped through and
around this ring, fitting it firmly and closely. Fig. 26, which
shows in diagram a winding in ten sections, furnishes a good
example of the Gramme construction. There may be one or
several turns per coil, as in drum windings. These two gen-
eral types of windings are used with various modifications in
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TRANSMISSION BY CONTINUOUS CURRENTS. 81
nearly all continuous current dynamos. Each has its good
and bad features. The Gramme winding makes it very easy
to keep down the voltage per segment, inasmuch as for each
external armature wire there is a commutator bar, while in
the drum form there is but one bar for two wires. It is also
mechanically solid even when wound with small wire, and no
two adjacent wires can have a considerable voltage between
them, thus making it easy to build an armature for high
E. M. F. On the other hand, the drum winding gives a very
compact armature of easy construction, and the magnetism
induced in it is less Hkely to disturb that of the field.
In the small machines once usual, the Gramme type was pre-
ferred for high voltages on account of the ease with which it
could be repaired, while the drum was liked for its simplicity
of mechanical construction as a whole and excellent efficiency
as an inductor. In modern practice the differences between
these types have become much less marked. With large units,
particularly of the multipolar form now usual, the drum wind-
ing is as easily insulated as the Granune, for with the winding
now used in such cases there need be no considerable voltage
between adjacent wire^?, and repairs are of very infrequent
occurrence. In fact, the drum winding can be made quite as
accessible as the other, and is on the whole cheaper and
simpler. Almost tlie sole advantage of the Gramme (or ring)
winding is that of low voltage per commutator bar. Mechan-
ically, too, there is less difference than formerly, for the coils
are in both types generally bedded in slots in the iron of the
armature core.
It must be noted that the armature of the modem dynamo,
unless of small size or unusually high voltage, is seldom wound
with wire in the ordinary sense of the word. Instead, the
conductors are bars of copper, usually of sections rectangular
rather than round, and generally lacking any permanently
attached insulation. Whatever the winding, the conductors
on the armature face are inclosed in close fitting tubes of
mica and specially treated paper or the like, and then put
in place on the armature core or in more or less completely
closed channelis cut in it. If on the core surface, the bars are
often not insulated on the exterior surface at all. If the arma-
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ELECTRIC TRANSMISSION OF POWER.
ture core be slotted, the insulating material is preferably put
in position first and the bar put in afterward. As to the rest
of the winding, it is completed by connectors of copper strip or
rod soldered to the face conductors and insulated in a substan-
tial manner. Thus each convolution, whether of ring or drum
winding, is composed of from two to four pieces.
A typical modem ring winding is shown in Fig. 27. It well
exemplifies the construction above mentioned, and in this case
the uninsulated faces of the exterior conductors form the com-
mutator of the machine. Such a construction of course ex-
FlQ. 27.
eludes iron clad armatures, and is best fitted for a machine
having a field magnet inside the ring armature. A similar
arrangement which avoids the above limitations, uses the side
connectors of the ring as commutator segments. The general
principle, however, is the same, whether the commutator
forms part of the winding proper or is a separate structure.
An iron clad drum winding of typical character is shown in
Fig. 28. Here the exterior bars are fitted into thoroughly
insulated slots in the core, and wedged firmly into place by
insulating wedges. Sometimes the bars themselves are shaped
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TRANSMISSION BY CONTIXUOVS CVRREXTS.
83
so as to act as wedges. In either case the bars are held almost
as solidly as if they formed an integral part of the core. The
commutator in these windings must be a separate affair. Fig.
28 shows well the nature of the winding, with its slotted core,
ventilating spaces, and massive bars — in this example 4 per
slot. The end connectors lie in a pair of reverse spirals, one
outside the other, and separated by firm insulation. The
relation of these connectors to the rest of the winding is illus-
trated in Fig. 25.
Between the modem drum and ring armatures it is needless
to discriminate. Both have been successfully used in dynamos
of the largest size, but the iron-clad drum is in the more gen-
eral use, while the use of ring annatures is steadily declining.
Fig. 28.
It is very unusual to find a standard generator of recent build
of 100 KW or more output with a regular wire wound arma-
ture, and the most of them have some modification of the bar
windings just described.
We have briefly reviewed here the armature windings at
present in general use and may now pass to the various wind-
ings employed for the field magnets. These are, in continuous
current dynamos, almost always connected with, and supplied
with current from, the armature winding, thus making the
machines self-exciting. As the armature is turned the action
begins with the weak residual magnetism left in the field mag-
nets, and the current set up by the small E. M. F. thus produced
is passed around and gradually strengthens the magnets, build-
ing them up to full strength. If this residual magnetism is
very feeble, as may haj)pen when it is knocked out of the iron
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ELECTRIC TRANSMISSION OP POWER.
by rough handling or the continual jarring of a long journey,
it is sometimes quite difficult to get the machine into action.
The simplest form of field winding, and the one which was
most extensively used at first, is that in which the current from
one of the brushes passes around the field magnet coils on its
way to or from the external circuit of the machine, as sho^^^l
in Fig. 29. This series winding possesses more than one ad-
vantage. It consists of a comparatively small number of eon- .
volutions of rather large v^^re and so is cheap to wind, it is,
for this same reason, little liable to injury and easy to repair
when injured; and what is of particular importance, whenever
FlO. 29.
Kiu. 30.
the series dynamo is called upon for more current, the mag-
netizing power of the field is raised by the increase, thus
increasing the electromotive force. This property, once con-
sidered a disadvantage, becomes of great value in modem
windings adapted for the purpose. As the generation of
p]. M. F. at the start depends entirely on the residual mag-
netism, series wound machines do not *' build up" full voltage
very easily unless the resistance of the outside circuit is fairly
low, thus giving the current a chance.
The common shunt winding shown in Fig. 30 almost describes
itself. The brushes are, independently of the circuit, connected
to magnetizing coils of relatively fine wire. Although such a
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THANSMLSSION BY CONTINUOUS CURRENTS. 85
field wiiuling i.s slightly harder to construct and to maintain, it
produces a magnetic field that is relatively free from any actions
in the working circuit of the machine. So long as the E. M. F.
at the brushes is unaffected by changes of speed, the field will
be quite steady except as a very large current in the exterior
circuit may reduce the voltage available for the field by causing
a loss of voltage in the armature. If the armature resistance
be very small, there will be almost a constant E. M. F. at the
brushes except as the current flowing in the armature may
produce a magnetization opposed to the shunt field. For a
considerable time, then, the shunt winding was always used
when a constant E. M. F. was recjuired. At the same time, it
permits the E. M. F. to be varied, if desired, with a very small
loss of energy, by the simple expedient of putting a variable
resistance in circuit with the field magnets.
As the principles of dynamo construction became better
known, it was apparent that the above method of getting a
constant E. M. F. was rather expensive. To build an armature
that would carry a heavy current without noticeable loss of
voltage and to inclose it in fields so strong as to be disturbed
only in a minute degree by the magnetizing effects of such
current, was a task requiring much care and a great amount of
material. Even if this difficult problem were solved, the con-
stant voltage would be at the brushes of the machine and not
at the load, where it is needed.
An easy way out of these difficulties is found by considering
an important property of the series-wound machine just men-
tioned, i.e.y the rise of E. M. F. as the load on the external
circuit rises. If now one takes a good shunt- wound dynamo
and adds to the field magnets a few series turns wound in the
same direction as the shunt, the result is an follows: At no
load, the voltage at the brushes is that due to the shunt alone.
As the load comes on this voltage would naturally fall off by the
loss of voltage from armature resistance and reaction. The
series turns, however, at this juncture strengthen the field and
thus compensate for these losses. This is the compound wind-
ing^ now almost universally used. It is shown in diagram in
Fig. 31. Ordinarily the series tunis are more than would be
needed for merely compensating the losses due to armature
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ELECTRIC TRANSMISSION OF POWER.
resistance and reaction, so that the voltage at the brushes
under load will rise enough to make up for the increased loss
in the line due to carrying heavier current.
Machines thus over-compounded five or ten per cent are in
very common use.
The foregoing gives the rudiments of the machines used for
generating direct current. It now remains, before taking up
the question of power transmission proper, to consider briefly
the use of such machines as motors. The underlying principle
has been already discussed. The power of a motor to do
FlQ. 31.
work depends on the stress of the magnetic field on conductors
carrying curent in it and free to move. This stress is virtu-
ally the same as that which has to be overcome in using the
machine as a generator, and reaches a very considerable
amomit in machines of any size.
In motors with the field strengths often used, the actual
drag between the field and the armature wires may amoimt at a
rough approximation, to nearly an ounce pull on each foot of
conductor in the field for every ampere flowing through the
wire. With a 20 HP motor the actual twisting effort or torque
at the surface of the armature might easily be considerably
over a hundred pounds pull. Forces of this size emphasize
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TRANSMISSION BY CONTINUOUS CURRENTS. 87
the need of solid armature construction, with the conduc-
tors firmly locked in place, particularly since the magnetic
drag is not steady, but conies somewhat violently u[)on the
conductors as they enter the field. With the old smooth core
armatures wound with wire, the conthictors not infretiuently
worked loose and chafed each other, and even the entire wind-
ing has been known to slip on the core. In modern windings,
either iron-clad or modified smooth core, such accidents are
nearly impossible.
When the armature conductors of the motor cut through its
field as the armature revolves, an electromotive force is neces-
sarily generated in them as in every other case when the
magnetic forces on a conductor change. There is thus pro-
duced, as a necessary part of the action of every motor, a
counter electromotive force in the armature. This electro-
motive force plays a very important part in the internal
economy of the motor.
In the first place, the magnitude of the counter electromo-
tive force determines the amount of current that can flow
through the motor when supplied at a given voltage. The
resistance of the armature from brush to brush may be only
a few thousandths or even ten thousandths of an ohm,
while the applied voltage may be several hundred volts. The
resulting current, however, is not that which would flow
through the given resistance under the pressure applied, but
the flow is determined by the difference between the applied
electromotive force and the counter E. M. F. of the motor, so
that in starting a motor when the armature is at rest and there
is therefore no counter E. M. F., a resistance must be inserted
outside the armature to cut do^vn the initial riLsh of current.
In the second place, the counter electromotive force mea-
sures the output of the motor for any given current. It does
this because the very same things, i.e., strength of field,
amount of wire under induction, and speed, which determine
the output for a given current, also determine the magnitude
of the counter electromotive force.
Therefore, when the machine is running as a motor, while
the energy supplied to it is the product of the voltage by the
amperes which flow through the armature, the output of the
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88 ELECTRIC TRANSMISSION OF POWER.
motor is determined by the product of the counter electromo-
tive force into the selfsame current; hence, under given con-
ditions, the ratio between the impressed and counter electro-
motive forces of the motor determines the efficiency of the
motor. The difTerencc between these electromotive forces
determines the input of energy, since it determines the cur-
rent which may flow; therefore, as the counter electromotive
force increases, the efficiency of the motor increases, but the
output is limit<jd by the decreased input.
With a fixed electromotive force supplied to the armature,
the output of the motor per ampere of current will diminish as
the counter electromotive force diminishes, but the total
amperes flowing will increase because the difference between
the applied and counter E. M. F. has also increased. Thus
the total output increases, although at a lower efficiency,
when the counter E. M. F. decreases. Since the input (which
is determined by the difTerencc between counter and applied
E .M. F.'s) nmltiplied by the efficiency (which is determined
by the counter E. M. F.) equals the net output of the motor,
this output will be at a maximum when the counter E. M. F.
and the effective E. M. F. are equal to each other. This fol-
lows from the general law, that the product of two quantities,
the sum of which is fixed, will be a maximum when these
quantities are c<iual.
It must be distinctly understood, however, that at this point
of theoretical maxinunn output the motor is very inefficient,
and that mechanical considerations prevent the efficiency
being wholly determined by the counter E. M. F., while spark-
ing and heating generally j)revcnt working with the counter
E. M. F. equal to the effective E. M. F.
In actual practice motors are worked under very diverse
conditions, and some of these it is worth while to take up in
detail, following the preceding generalizations. The energy
may be supplied at constant current, at constant voltage, with
neither current nor voltage constant, at fixed or variable
speed, and subject to a wide variety of conditions; the motors
may be wound either series, shunt, compound, or with various
modifications of these windings, and may be either self regu-
lating with respect to various requirements, or regulated by
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TRANSMISSION BY CONriNUOUS CURRENTS. 89
extraneous means. In the ordinary i)roblenis dealt with in
power transmission, these conditions may be classified in a
fairly simple way as follows :
Case I. Series-wound motors at constant current.
Case II. Series-wound motors at constant voltage.
Case III. Series-wound motors with interdependent current
and voltage.
Case IV. Shunt-wound motore at constant voltage.
The first class is now rarely found in practice, and is of
real commercial importance only in a few cases. The second
class is very widely used in a particular case, to wit: electric
railway service, and consequently it is of great practical im-
portance. The third class of motors is used occasionally with
great success but not very extensively, while the fourth includes
the vast majority of all the continuous current machines
running for purposes other than electric railway service.
These latter cases, therefore, it is worth while to take up
somewhat thoroughly.
Case I. — Series-wound motors operated with a constant cur-
rent originally came into use in connection with arc lighting
circuits, which for some years formed the most generally
available source of current. Such lines are fed from dynamos
in which the current is kept constant by special regulation,
while the voltage rises and falls in accordance with the load,
consisting of lamps or motors in scries with each other. We
are therefore relieved of any concern about the current, since
it is kept constant (juite irrespective of what happens in the
motor.
Under these circumstances, in a series-wound motor, the
torque will be constant, since the field is constant, and the
output of the motor will vaiy directly with the speed. If it
be loaded beyond its capacity, it simply refuses to start the
load, inasmuch as its tor<|Uc is limited by the current. If it
starts w^ith a load within its limit of torque, its speed will
steadily increase until that limit is reached. This may be
comparatively soon if the load is a rapidly increasing one, or
the machine may race until its own friction of air and bearings,
magnetic resistances, and the induction of idle currents in the
core and frame serve to furnish resistance up to its limit of
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90 ELECTRIC TRANSMISSION OF POWER.
torque. When running at a given speed, any increase of load
causes the speed to fall off, while decrease of load produces
racing. Unless these tendencies are controlled, this type of
machine becomes almost useless for practical purposes, as
regularity of speed under change of load is generally highly
desirable. In fact, the tendency to run at constant torque is
generally inconvenient. To obviate this very serioius difficulty,
various devices have been tried with tolerable success. The
commonest is to vary the torque in accordance with the load
by changing the field strength, or by shifting the brushes so as
FlO. 32.
to throw the armature coils out of their normal relation to the
magnetic field.
Since the object of such changes is to vary the output at
constant current, and since this output is measured by the
counter E. M. F. of the motor, the real problem of such regu-
lation is to vary the counter E. M. F. in proportion to the
output desired. Therefore, the same general means that
serve to accomplish this end in an arc dynamo, keeping the
current constant and varying the E. M. F., will serve to regu-
late the corresponding motor.
As in this case the speed is the thing to be held constant,
the usual means taken for working the regulating devices is a
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TRANSMISSION BY CONTINUOUS CURRENTS. 91
centrifugal governor, which generally acts to shift the brushes
or to put in circuit more or less of the field winding, which for
this purpose is divided into sections. In still other arrange-
ments the governor acts to slide the armature partially into or
out of the field, or to work a rheostat which shunts the field
magnet, as in the Brush regulator for constant current. An
excellent example of a small constant current motor regulated
on the last mentioned principle is shown in Fig. 32.
As to the operation of these regulating devices, it is tolerably
good if everything is carefully looked after and kept in adjust-
ment. The efficiency of such motors is not generally as high
as that of other types at light loads, owing to the nearly con-
stant loss in the armature due to constant current working.
At and near full load the efficiency may be good.
In addition, the current is highly dangerous, coming as it
does from generators of very high voltage, and even the voltage
across the brushes is, in machines of any size, sufficient to
give a dangerous or even fatal shock. A 10 HP motor, for
example, on the customary 10-ampere circuit, would have a
difference of potential of about 800 volts between the brushes
at full load. As a few such motors would load even the largest
arc dynamos, besides being dangerous in themselves, opera-
tions have generally been confined to smaller units. On
account of the danger and the mechanical and other difficulties,
the arc motor has come to be looked upon as a last resort, is
seldom or never used when anything else is available, and, to
the credit of the various manufacturers be it said, is nearly
always sold and installed with a specific explanation of its
general character and the precautions that must be taken
with it.
In spite of all these objections, the constant current motor
often does good and steady work, and some such motors have
been used for years without accident or serious trouble of any
kind. They have been employed, however, only sparingly for
power transmission work of any kind, and when so used are
mostly on special circuits of 50 to 150 amperes.
Case II. — Series motors worked at constant potential are
very widely used for electric railway service and other cases,
such as hoisting, in which great variations of both speed and
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92 ELPJCTRIC TRANSMISSION OF POWER.
torque are desirable. When supplied at constant potential, the
speed of a series-wound motor varies widely with the load. In
any case the speed increases until the counter E. M. F. rises
high enough to cut the current down to the amount necessary
to give the torque sufficient for that load and speed.
If the field be strengthened, the motor will give a certain
output at a lower speed than before; if it be weakened, at a
higher speed; the torque behig in these cases correspondingly
increased or decreased.
The tonjue increases rapidly with the current, so that when
the counter E. M. F. is small, or zero, as in starting from rest,
the torque is very great, a property of immense value in start-
ing heavy loads. For m starting, not only is the current
through the armature large, but the field is at its maxinmm
strength. If the field strength varied directly as the current,
the torque would vary nearly as the square of the current.
As a rule, however, these, like most other motors, are worked
with a fairly intense magnetization of the fields, so that doub-
luig the magnetizing current b}- no means doubles the strength
of the field. In fact, most series motors for constant potential
circuits are of the type used for electric railways, and wound
so that the field magnets are nearly saturated even with very
moderate currents. Hence the torque in such cases increases
but a trifle faster than the current. This construction is
adopted in order to reduce the amount of iron necessary to
secure a given strength of field, and so to lighten and cheapen
the motor.
It is quite obvious that while series motors at constant
potential have the advantage of being able to give on occasion
very great tonjue, they suffer from the same disadvantage as
constant current motors, in that they are not self-regulatuig
for constant speed. Centrifugal governors could, of course,
be applied to them, but since it happens that most work
requiring great torque also requires variable speed, nothing
of the kind is usually necessary.
As previously explained the speed can be easily regulated to
a certain extent by changing the field strength, thus changing
the counter E. M. F., but owing to the peculiarity of design
just noted, this method is rather ineffective, requiring a
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TRANS^fISSION BY CONTINUOUS CURRENTS.
93
great change in the field winding for a moderate change in
speed.
In general, when a considerable range of speed is needed,
constant potential working is abandoned and the speed is
changed by varying the impressed E. M. F. by means of a
rheostat. If this E. M. F. be lowered, the current decreases
and the speed sags off imtil the new counter E. M. F. is low
enough to let pass just enough current to maintain the output
at the reduced speed. When the applied E. I\I. F. is increased
the reverse action takes place. Under these circumstances, for
Km. 33.
a fixed load the current is approximately the same, independent
of the speed; for with a uniform load the torque is constant,
while the output (i.e., rate of driving the load) varies. Many
railway motors are regulated in the manner just described,
although in addition the field strength is sometimes varied by
cutting out or recombining fields and by series parallel control.
Rheostatic control necessarily wastes energy, and the greatest
recent improvement in railway practice consists in reducing
the E. M. F. applied to the car motors by throwing them in
series. This secures a low speed economically, though the
rheostat still comes into play at intermediate speeds.
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94 ELECTRIC TRANSMISSION OF POWER,
Speaking broadly then, series-wound motors, while possess-
ing many valuable properties, are limited in their usefulness
by their tendency to vary 'widely in speed when the load
changes. Hence they are used chiefly in cases where the
speed is to be varied deliberatel3^ A typical early motor of
this class, used for hoists and the like, with rheostatic control,
is shown in Fig. 33.
In spite of the difficulty in regulation, the series motor pos-
sesses some considerable advantages: The field coils being of
coarse wire are easily and cheaply wound, even in motors for
very high voltage; the same quick response to changes in
current or load that makes it hard to obtain uniform speed is
also most important in many kinds of work; the powerful
initial torque Is coupled with the useful property of prompt
reversal. All these make the series motor preiMiunent for cer-
tain purposes, especially where severe work is to be coupled
with hard usage.
There is one case, too, in which the series-wound motor can
be made accurately self-regulating for constant speed — a case
somewhat peculiar and unusual, but yet worthy of special
attention.
Case III. — We have seen that when the load on a series
motor supplied at a certain voltage increases, the speed falls
ofT until the increasing current due to the lessened counter
E. M. F. raises the torque sufficiently to meet the new con-
ditions. Imagine now the impressed P]. M. F. to be so varied
that the slightest increase of current in the motor is met by
a rise in the E. M. F. applied to it. Evidently the speed
would not have to fall as before, for the greater applied voltage
would furnish ample current for all the needs of the load. If
the variation in voltage could be made to depend on change
of torque, not giving the speed time to change, the regulation
would be almost perfect. Such a method has been proposed,
but owing to mechanical difficulties has not been used to any
extent.
It is possible, however, so to combine a special motor and
generator that the former will be very closely uniform in speed,
quite independent of the load. In this connection we must
revert to the properties of the , series- wound dynamo. If such
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TRANSMISSION BY CONTINUOUS CURRENTS.
95
a machine be driven at constant speed its electromotive force
will increase with the current, since the strength of field,
here the only variable factor in the voltage, will increase with
the current. If the field magnets of the generator are
unsaturated, that is, not so strongly magnetized as to require
considerable current to produce a moderate increase of mag-
netization, they will respond very promptly to an increase of
load by raising the voltage. If such a generator be connected
to a series- wound motor of proper design, the pair will work
together almost as if connected by a belt instead of a long
line, and the motor will nm at a nearly uniform speed, since
Fia. 34.
the least diminution of speed, with its accompanying increase
of current, will be met by a rise in the voltage of the genera-
tor. Such an arrangement is shown in diagram in Fig. 34.
In this cut A is the generator supplying current to the
motor B. The machines should be of practically the same
capacity, for the generator cannot supply current except to the
one motor without disturbing the regulation. Whenever the
load on B changes, a very small reduction in speed suffices to
raise the voltage of A and thereby to hold up the speed of B.
To this end the field magnets of B must be more strongly
saturated than those of A, else the same increase of current
will raise the counter E. M. F. of the motor and defeat the
purp<»se of the c<)m])ination. If the fielcis of the two machines
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9& ELECTRIC TRANSMISSION OF POWER.
are properly designed, the generator will increase its voltage
under increasing load just enough to hold the motor at speed,
as a very slight change in current immediately reacts on the
generator.
It is even possible to make the motor rise in speed under
load if the generator is sufficiently sensitive to changes of cur-
rent. This is generally needless, but it is often useful so to
design A and B that the former will rise in voltage fast enough
not only to compensate for the added load on the motor but
for the added loss of energy in the line, entailed by the in-
crease of current, thus regulating the motor even at a long
distance. The difference of saturation between the generator
and motor fields new! not involve material difference of design,
since it may be effected by shunting the motor field. When
properly adjusted, the system is capable of holding the motor
speed constant within two per cent through the range of load
for which the machines are planned.
It should be noted in connection with Fig. 34 that, whereas
the current circulating in the armature of a generator tends to
disturb the magnetic field in one direction, in a motor the same
reaction is in the opposite direction. For the current in the
motor is driven through the armature against the coimter
E. M. F., i.e., in the direction opposite to that of the current
the machine would give if running as a generator. As the
effect of the reaction is to skew the direction of the magnetic
field that affects the armature conductors, and the conunuta-
tion must take place when the commuted coil is not imder a
varying induction, the armature reaction compels one to shift
the brushes slightly away from the position they would have if
the field were perfectly symmetrical. This shifting is in the
direction of armature rotation in a generator, but for the
reason above noted has the opposite direction in a motor. In
either case it need be only a few degrees.
Case IV. — Shunt-wound motors' are almost invariably
worked on constant potential circuits, to which they are partic-
ularly well suited. They form by far the largest class of motors
in general use and owe this advantage mainly to their beautiful
self-regulating properties.
The shunt motor is in construction practically the same as
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TRANSMISSIOX BY CONTiyUOUS CURREXTS. 97
a shunt-wound dynamo. Let us look into the action of such a
machine when supplied from a source of constant voltage. If
the design be reasonably efficient, the armature will have a
very low resistance and the shunt circuit, which includes the
field coils, a resistance several himdred times greater. When
such a machine is supplied with current of constant voltage at
its brushes and is running at any given speed and load, the
current through the armature is practically determined by the
counter E. M. F. developed, the armature resistance being
almost negligible. The shunt is of high resistance and takes
a certain small amount of current, determined by the voltage
across the brushes. Now let the load increase; the field is,
aside from loss of voltage on the line, practically constant, and
the first effect of the added load is, as in a series motor, to
reduce the speed. But this lowers the coimter E. M. F., and
consequently the armature current rises and the torque is
increased, thereby enabling the motor to operate under the
larger load. The torque necessary to enable the motor to
maintain an increased load varies directly as the load and is
also directly proportional to the current. But since the cur-
rent is closely proportional to the difference between the
impressed and counter E. M. F.'s, it is possible to design a
machine so as to run at almost exactly constant speed.
The constancy depends really on the armature resistance,
small as it is. For example, a motor is designed to run at 100
volts. Running light the comiter E. M. F. is 99.9 volts, and
with an armature resistance of 0.01 ohm the current will be 10
amperes. The work done is say 1 HP. Now let a full load,
say 20 HP, be thrown on. The torque will have to be in-
creased 20 times, requiring 200 amperes. But this will flow
through the armature under an effective pressure of 2 volts.
Hence the counter E. M. F. will only have to fall to 98 volts to
provide current enough to meet the new condition. As the
coxmter E. M. F. varies directly as the speed, a fall in speed of
less than 2 per cent will follow the increase of load. This
computation neglects all questions of armature reaction as
well as the effect of this minute fall in speed on the output,
but fairly represents a case that might actually be met with in
the best modem practice. In fact, shunt motors have been so
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98 ELECTRIC TRANSMISSION OF POWER,
designed as to vary no more than li per cent in speed from
no load to full load. A variation of 5 or 6 per cent is, how-
ever, more usual.
When supplied from an over compounded generator so that
the impressed voltage may increase with the load, a shunt
motor can be operated even more closely to constant speed
than indicated above, since there is no longer need for a fall
in speed to maintain the requisite difference between the
impressed and counter E. M. F.'s. In such case any tendency to
fall in speed is at once corrected by the rise in voltage on the
line. This scheme is seldom used, however, since it is ill fitted
for simultaneously operating a number of motors at varying
loads, and for single units has no particular advantages over
the series-wound pair previously noted, or a very simple
arrangement of alternating apparatus.
Not only can the shunt motor be worked at nearly constant
speed, but it also has the advantage of permitting a consid-
erable range of sp)eed variation without sacrificing much in the
matter of efficiency. We have already seen that a change in
field strength involves a change of speed, since it necessarily
alters the counter E. M. F., which in turn modifies the current.
In a shunt motor the immediate effect of a decrease of field
strength is to lower the counter E. M. F., letting more current
through the armature and increasing the torque. Hence, the
speed rises until the current and torque adjust themselves to
the requirements of the load. On the other hand, if the field
be strengthened, the current necessary to carry the load can-
not be obtained without a fall in speed. It is clear that the
changes of speed thus obtained may be quite considerable, for
in a motor such as that just described a variation of 10 per
cent in the field would produce an immense variation of cur-
rent, which would have to be compensated by a change in
speed as great as the change in the field. Inasmuch as these
field changes can be produced by varying the field current,
which is always small, through a rheostat in the circuit, this
change of field strength can be accomplished with but a tri-
fling waste of energy. If the field magnets are comparatively
unsaturated, it is not difficult to obtain perhaps 50 per cent
variation in speed. A motor designed for such work is, how-
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TRANSMISSION BY CONTINUOUS CURRENTS.
99
ever, bulky, as it must if necessary be possible to get torque
enough to handle the full load with a field much below its
normal strength.
It should be noted that even when running at a considerably
modified speed, the motor must still be nearly self-regulating
for changes in load, for the conditions that govern self-regu-
lation are within moderate limits unaffected by the particular
strength of field employed. Only when the armature reaction
has been greatly modified will the regulation be sensibly
disturbed.
Fig. 35.
A device sometimes used to improve the regulation of motors
essentially shunt wound is the so-called differential winding.
This consists of an additional field winding in series with the
armature, but around which the current flows in such a direc-
tion as to demagnetize the field. The total field strength is
then due to the difference between the magnetizing power of
the shunt and of this regulating coil. When the load on the
motor increases, the additional current due to a minute change
of speed will weaken the field, and thence cause the motor to
run faster until the counter E. M. F. adjusts the current to'the
new speed and output. Differential winding obviously requires
an extra expenditure of energy in the field, since the shunt and
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100
ELECTRIC TRANSMISSION OP POWER.
series turns act against each other. Fig. 35 shows the Sprague
motor wound on this differential plan, now only of historical
interest, but which through its good qualities did much to
popularize the electric motor in America. Plate II shows
in Fig. 1 a Westinghouse bi-polar shunt motor, and in Fig.
2 a G. E. six-pole shunt motor for slow speed.
Various modifications of shunt- and series-wound motors
have from time to time appeared, devised for particular adapta-
tion to special purposes or sometimes merely for the sake of
novelty. None of them, however, are of sufficiently general
importance to find a place here except a single ver}'^ beautiful
method of obtaining efficiently a very wide range of speed.
The principle of this method is to work the motor at normal
Fig. 36.
full excitation, but to deliver to the armature a current of
variable E. M. F. so that a given current and hence torque
may accompany very various values of the counter E. M. F.
Fig. 36 shows the connections employed to effect this result.
Here C is the working motor, B the special generator which
feeds its armature, A the motor used to drive this generator,
and D the rheostat and reversing switch in the generator field
circuit which allows the generator E. M. F. to be varied. In
the figure the motor // is shown as a synchronous alternating
machine with a commutator from which are fed the fields of
the three machines. In ordinary central station practice A is
a continuous current motor, and the fields are fed direct from
the distributing mains. The result of this arrangement is that
the motor field C remains at full strength, while the armature
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Fio. 1.
I'lG. 2.
1M.ATE II.
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TRANSMISSION BY CONTINUOUS CURRENTS. 101
current can be brought to any required strength at any desired
armature speed within a very wide range. Hence, C can give
full load torque or even more while the armature is merely
turning over a few times per minute, and the speed can be
brought up with the utmost delicacy and held at any desired
point. And at every speed the motor holds its speed fairly
well, irrespective of changes of load. For elevators, hoists,
and similar work this device is extremely useful. The only
objection to it is the cost of installing the two extra machines,
which is of course considerable. Nevertheless the regulation
attained is so beautiful and perfect that the cost often becomes
a minor consideration, and the device is very widely used in
cases where variable speed is essential.
POWER TRANSMISSION AT CONSTANT CURRENT.
In its general aspect this method must now be regarded as a
makeshift. It came into existence at a time when the only
circuits extensively installed were those for arc lighting, and
hence, if motors were to be used at all, they must needs be of
the constant current type. As incandescent lighting became
more common the arc motors were gradually replaced by shunt
motors worked at constant potential. A few constant current
plants especially for motor service have been installed both
here and abroad, but for the most part they have merely
dragged out a precarious existence, and in this coimtry have
been abandoned.
There is good reason for this. The motors usually regulate
indifferently, and there is serious objection to running high
voltage wires into buildings when it can be avoided.
The objections of the insurance companies alone are quite
sufficient to discourage the practice. The constant current
has often been advocated for long distance transmission of
power where high voltage is a necessity. For such service the
method has the great advantage that the motors do not need
extraordinary insulation except from the ground. A constant
potential service at 5,000 volts continuous current would be
utterly impracticable, if distribution of power in moderate
units were to be attempted, while with constant currents it
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102 ELECTRIC TRANSMISSION OF POWER,
is entirely feasible, although objectionable on the grounds
mentioned. In addition, unless a proposed transmission be
for power alone, the constant current method shares with con-
stant potential of high voltage the very grave difficulty that
an incandescent lamp service is out of the question, without
secondary transformation of the necessarily high line voltage
to a very moderate pressure. This is somewhat expensive with
continuous currents of any kind, and at once introduces the
Fio. :J7.
troublesome question of regulation at constant current into
the problem.
To reduce the energy sent over a high voltage continuous
current Hne to a pressure at which incandescent lamps can l)e
fed, two methods are possible. We may pass by the plan
of using many lamps in series as of very Hmited appli-
cability and forbidden by the fire underwriters. First, the
required power may be received by a motor of appropriate
size, which is belted or coupled to a low voltage generator.
This device does the work, but it involves installing three
times the capacity of the lamps desired in machinery of a
somewhat costly character, and losing in the motor and gen-
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TRANSiMISSION BY CONTINUOUS CURRENTS. 103
erator perhaps 15 or 20 per cent of the energy supplied from
the line. The other alternative is to employ a composite
machine combining the functions of motor and generator.
This piece of apparatus is variously known as a motor-genera-
tor, dynamotor, or continuous current converter. It is a
dynamo electric machine having a double-wound armature and
two commutators. One winding with its commutator receives
the high line voltage and operates as a motor. The other
winding and its commutator furnishes, as a dynamo, low
tension current. The field is common to both ^vindings. Fig.
37 shows a small machine of this kind, adapted to receive 5,000
volts from the line, and to deliver 110 volts, or vice versa.
This particular machine works at constant voltage on both
circuits. Either circuit, however, could be made to work at
constant current, provided the means of regulation for this
purpose were so chosen as to leave the field and speed un-
changed.
The cost of a motor generator, while less than that of two
separate machines, is still high, and although its efficiency is
somewhat greater than that of the pair mentioned above, it is
obtained at the cost of a rather complicated armature, which,
from a practical standpoint, is quite objectionable.
In spite of the difficulties incident to working incandescent
lamps from a high, voltage constant current circuit, the ease
with which such circuits can be worked, even if for power
alone, at voltages far above those available on the constant
potential system, encouraged their installation during the
period between the first efforts at long distance transmission
and the more recent date at which alternating current appa-
ratus has become thoroughly available. For some years it
was constant current or nothing, so far as long distance trans-
mission, coupled with distribution, was concerned.
As a result of the various adverse conditions mentioned,
transmission at constant current has never made really any
headway in American practice, and in fact the method has been
followed to a noticeable extent in only one locality — San Fran-
cisco. There, through the activity of local exploiters, constant
current power circuits were early established and remained
in fairly successful operation for several years.
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104 ELECTRIC TRANSMISSION OF POWER.
There were until recently three companies op)erating con-
stant current circuits in San Francisco for the distribution
of power. The currents empoyed were of 10, 15, and 20 am-
peres. Most of the motors were small, a very large propor-
tion of them being under one horse-power. The total num-
ber of motors in circuit on the various systems was between
six and seven hundred.
Except in San Francisco, what few constant current motors
have been in operation were operated on regular arc circuits.
Their use has been much discouraged by the operating com-
panies, and very few such motors are now manufactured or
sold ; in fact, constant current distribution in modem American
practice is almost non-existent. Abroad, the conditions are
somewhat different, and on the Continent constant current
distribution for long distance transmission work has been ex-
ploited to a very considerable extent, probably owing to the
early and successful establishment of a number of transmission
plants for single motors worked on the series system. There
are several successful plants operated at constant current, one
of the most considerable of them being that at Genoa, which
is an excellent example of the kind and as such is worth more
than a passing mention, even although the probability is that it
will seldom be duplicated, at least on this side of the Atlantic.
The Genoa transmission is derived from the River Gorzente,
which about twenty years ago was developed for hydraulic pur-
poses, artificial lakes being established and a tunnel about IJ
miles long being built for an outlet. Beyond the tunnel, an
aqueduct some fifteen miles in length conveyed the water to
Genoa, where a considerable amount of power is utilized
directly. In this development there was left at the mouth of
the tunnel an unused fall of nearly 1,200 feet aside from the
head employed in the aqueduct. This has been developed
electrically. It was divided into three partial falls of 338,357,
and 488 feet, respectively. At each of these was erected a
generating station with its own transmission line. These
stations were named after the three renowned electricians,
Galvani, Volta, and Pacinotti. The first mentioned station
was the first installed. It consists of two generators operated
in series. Each is of about 50 KW capacity, giving 47 amperes
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TRANSMISSION BY CONTINUOUS CURRENTS. 105
T-i
g^iJ
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106 ELECTRIC TRANSMISSION OF POWER,
with a maximum pressure of 1,100 volts. Current is kept con-
stant by regulating by hand the speed of the dynamos,
through the gate which controls the turbines. Each dynamo
is provided with an automatic switch, short- circuiting the ma-
chine in case of extreme rise in voltage. This Galvani station
was a preliminary or experimental station, and was followed up
by the establishment of two others which supply the power to
Genoa. One of these stations, which is thoroughly typical of
the system employed, is shown in Fig. 38. It consists of four
turbines, each driving a pair of dynamos of a little less than
50 KW output at 45 amperes and about 1,000 volts.
These dynamos are similar to those in the Galvani station,
but the regulation for constant current is obtained in a dif-
ferent manner. The dynamos are separately excited, the
fields being supplied in parallel from a small dynamo driven
by a separate waterwheel. The speed of this exciter is auto-
matically varied by controlling its turbine in response to
changes of current in the circuit. All the dynamos are oper-
ated in series, and like those in the Galvani station are direct
coupled in pairs. The machines are insulated with enormous
care, heavy layers of mica being placed between the magnets
and the bed plates, while the windings themselves are very
elaborately protected. Carbon brushes are employed, and
the commutators are reported to behave admirably. Each
dynamo is protected by most elaborate safety devices, as in the
Galvani station. The regulation is said to be excellent, even
under considerable changes of load.
The third station, Pacinotti, contains eight machines of the
same capacity as those in the preceding. They are governed
as in the Galvani station by controlling their speed. This,
however, is done by an electrical motor-governor controlled
by a relay on the main line and worknig in one direction or
the other as the occasion may recjuire. In all the stations are
carried out the same thorough precautions regarding insula-
tion, and each machine has around it an insulated floor sup-
ported on porcelain. The line voltage from each of the two
stations last mentioned is from 6,000 to 8,000, and two
circuits are carried into Genoa, the extreme distance of trans-
mission being about eighteen miles. The conductors, of wire
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TRANSMISSION BY CONTINUOUS CURRENTS. 107
9 mm. in diameter (nearly No. 00 B. & S.), are for the most
part bare, except when passing through villages, and are sup-
ported on oil insulators carried by wooden poles, save in some
few cases where iron poles have been used. The two circuits
running into Genoa transmit power for motors only. The loss
in the line at full load is 8 per cent.
At full load, nearly 1,000 HP is transmitted over the lines,
the motors being of all sizes between 10 and 120 horse-power.
They are of the ordinary series-wound type, and their speed is
automatically controlled by centrifugal governors, which act
by varying the field strength. Fig. 39 shows one of the Genoa
motors, fitted with an automatic governor acting upon a com-
FlO. 39.
mutated field winding. They are provided with carbon
brushes, and are reported to operate very successfully.
It is to be noted, however, that the motors are placed in
special rooms with insulated floors and walls, owing to the
enormous voltage which has to be taken into the buildings.
They are fitted with heavy fly-wheels to assist the governors,
and with automatic switches to short circuit around the motor
in case of excessive voltage. The motors are under the
special care of skilled assistants connected with the staff of
the generating station, who inspect the lines and go over the
motors at intervals of a few days. These extraordinary pre-
cautions both in the matter of insulation and skilled attendance
account in great measure for the success of what, under
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108 ELECTRIC TRANSMISSION OF POWER.
American conditions, would have almost infallibly resulted in
disastrous failure. The efficiency of the plant from turbine
shaft to motor pulleys is said to be a little over 70 per cent.
As may be judged from this description, the whole instal-
lation is of enormously complicated character, although per-
haps as simple and efficient as any alternating plant of the
same early date. The plan of the Volta station for the most
part explains itself. The switchboards for each machine with
their plugs for connecting the pair of dynamos coupled to it are
shown at -4, dynamos at fi, the exciters at C, exciter switch-
board and rheostat at D, and the solenoids, which control the
exciter turbines, at E. Lightning arresters are shown at F.
These consist of a spark gap, impedance coils in series with
the line and condenser shunted around them. Every motor is
provided with a similar lightning arrester. Taken altogether,
this Genoa plant is an excellent example of the constant cur-
rent system followed to its legitimate conclusion. A descrip-
tion of the system is a sufficiently condemnatory criticism
judged from our present point of view; at the same time, it
should be remembered that while this station was being built,
the method adopted was practically as good as any available
in the existing state of the art, and that the system has in
more recent installations been materially improved. En-
couraged by the favorable results obtained at Genoa, a similar
station was soon afterwards built delivering a maximum of
700 HP at Brescia at an extreme full-load pressure of about
15,000 volts over a 12-mile line. Since these stations nearly
a dozen others have been installed, aggregating about 17,000
HP, and their performance has been uniformly good. In
spite of a predilection for modem polyphase work one must
admit that a system which has been installed to such an
extent, and of late in competition with alternating methods,
is far from moribund. Two strong points it undoubtedly has;
freedom from all inductive disturbances, and the property of
carrying its extreme voltage only at full load, the importance
of which will be discussed later. It has shown itself capable
of doing steady and efficient work over long distances and
under cHmatic conditions by no means favorable. The Con-
tinental makers of this class of machinery have gone far be-
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I'LATE III.
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TRANSMISSION BY CONTINUOUS CURRENTS. 109
yond anything that has been attempted in American practice
and have turned out constant current dynamos of really
remarkable properties.
At present, machines of 50 to 60 amperes have been given
successfully E. M. F. as high as 3,500 volts, while those of
100 to 150 amperes have gone to 2,500 volts. As they are
usually coupled in pairs, a single unit may have a capacity of
about 700 KW, each component machine giving over 300^
Without pushing beyond present apparatus it then becomes
possible to arrange a plant of 1,000 to 1,500 KW having a
working E. M. F. at full load of 10,000 to 14,000 volts. Such
a plant is not especially complicated and is nearly as easy to
operate as an alternating plant. For a load of a few large
motors, it is capable of good work, without, however, present-
ing any advantages over a polyphase system save that the
line is simpler and the insulation requirements less severe.
An alternating power station of similar output would contain
practically as many generators, for sake of security. When
it comes to combined lighting and power service the constant
current system is hard pushed. In practice, recourse is had
to motor generators. Perhaps the best idea of the situation
may be given by a brief description of the Swiss transmission
from Combe-Garot to La Chaux-de-Fonds, a distance of 32
miles. At the former place are installed 8 generating units
each giving 150 amperes at 1,800 volts, giving a total capacity
of 2,160 KW at 14,400 volts. These generators are six pole
Thury machines with drum armatures, and are series wound.
Regulation is by automatic variation of the speed of the tur-
bines, the normal full load speed being 300 r. p. m. The line
is overhead, of cables having a cross section of about 300,000
cm., bare except in the towns where the power is delivered —
Lode, and La Chaux-de-Fonds at the end of the line. Motors
aggregating 2,400 HP are in circuit at these points, 2,000 HP
being used in the transforming stations. All motors above 20
HP are upon the high tension circuit. The substation at La
Chaux-de-Fonds is typical of the methods employed. It is
equipped with motor-generators of 200 HP working a three
wire system at 320 volts between the outside wires. One of
the motor-generator units is shown in Plate III. It is composed
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ELECTRIC TRANSMISSION OF POWER.
of two six pole machines with a fly-wheel between them. The
machine to the left is the motor and upon it is mounted the
automatic speed regulator. The principle upon which this
works is shown in F\g. 40. The regulating shimt around the
fields and the brush shifting mechanism are simultaneously
actuated by the dogs thrown into gear by the governor. This
form of governor is very generally used for. the motors upon the
system.
The efficiency of both generator and motor under test has
Fig. 40.
been shown to be 93.5 per cent, or 87 per cent for the com-
plete imit. Similar motor-generators in connection with a
storage battery furnish current at 550 volts for railway service.
Now the drop in the line at full load is 6 per cent, so that
w^e are in position to make a very close estimate of the effi-
ciency of the system from waterwheel to low tension mains.
It is obviously
93.5 X .94 X .87 = 76.5 per cent.
This is a very creditable figure for the total efficiency, and it is
worth while comparing it with the results ordinarily reached in
polyphase working. Taking the generator at 94 per cent, the
raising and reducing transformers at 97.5 per cent each, the
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TRAXSMISSION BY COXTINUOUS CURREXTS, 111
line at .94, and the distributing banks of transformers at 96.5,
we have
.94 X .94 X .975 x .975 X .965 = .81.
The difference is substantially that due to the difference in
efficiency between the static transformers and the motor gen-
erator. If the comparison be made w4th the railway part of
the propositiojt, assuming the use of a rotary converter, the
case would stand about as follows:
.94 X .94 X .975 x .975 x .94 = .79.
In the simple case of large motors the advantage lies rather
the other way, for the constant current plant would show
.935 X .94 X .935 = 82.1
against, for the alternating plant,
.94 X .94 X .975 X .975 x .94 = .79.
This merely indicates that after passing the voltages which
can be derived directly from the armature, more is lost in the
transformers than is gained in a low voltage winding. The
figures for the efficiency of the alternating plant are taken
from actual data on machines of about the capacity concerned.
To tell the unvarnished truth, a constant current transmission
coupled with a three wire distribution at 220 to 250 volts on
a side is capable of giving even the best alternating system a
pretty hard rub over moderate distances. In this coimtry
no constant current power transmission machinery is avail-
able, but where it is readily to be had, it is by no means out of
the game. A still larger constant current transmission plant
is now in operation between the falls of the Rhone at Saint-
Maurice, and Lausanne, a distance of 34.8 miles. The first
installation is of five pairs of generators aggregating 3,300
KW, giving at full load a combined voltage of 23,000. The
current in this case is 150 amperes, as in the Combe-Garot
plant just described.
POWER TRANSMISSION AT CONSTANT POTENTIAL.
The transmission of power to series-woimd motors at con-
stant potential is a branch of the art which as regards station-
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112 ELECTRIC TRANSMISSION OF POWER,
aiy motors has been developed only in special cases. It is,
however, the method universally employed for electric rail-
way work. Two or three sporadic efforts have been made to
operate electric railway systems at constant current, but with
such indifferent success that the method has been abandoned.
Coimting in electric railways, it is safe to say that at present
the majority of all electrical power transmission in the world
is done ^\'ith series motors worked at constant potential or as
nearly constant potential as may be practicable. As before
mentioned, regulation is generally obtained through the use of
rheostats in series with the motors, thereby cutting down the
applied voltage, or by throwing the motors either in parallel
or in series with each other, or in the third place by a combi-
nation of the above methods. Concerning the operation of
motors thus arranged, sufficient has been said to explain the
situation clearly. The general good properties of the method
are most prominently exhibited in the simplicity of the motor
windings and the very powerful effort which can be obtained
in starting the motors from rest. These properties are of
extreme value in railway service.
Aside from the operation of electric railways, series motors
at constant potential are frequently employed for hoists and
similar work where a powerful starting torque and considerable
range of speed at the will of the operator are desirable. In
spite of the large use of motors for such purposes, there are no
plants either here or abroad which may be said to be operated
exclusively after this method, for it is generally found desir-
able to combine in the same system series-wound motors for
severe work and shunt-wound motors for purposes where uni-
form speed is of prime importance. As a rule the power trans-
mission so accomplished is over a comparatively small distance
and really involves the problem of distribution more than
transmission alone. A very large number of electric hoists
designed by different makers are in use at various points
throughout this and other countries, doing service in mines,
operating elevators of one kind or another, working derricks, and
travelling cranes and employed for a large variety of similar
purposes. Many of the motors employed are of the ordinary
railway type.
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TRAXSAflSSIOX BY COXTIXUOUS CURREXTS. 113
The voltage utilized for this work, in America at least, is
usually either 200 to 250 volts or 500 to 600 volts, the former
being most generally used in mines, where difficulties of insu-
lation are considerable, or in operating motors supplied by
three- wire systems already installed. The latter voltage is gen-
erally selected for work above ground. None of the plants so
equipped are, however, sufficiently large or characteristic to be
worth a detailed description. The power installations and the
methods of distribution are in general, closely similar t« those
employed for electric railway work. Plants of higher vol-
tage than from 500 to 600 are so infrecjuent as to be hardly
worth considering in practical engineering. It is perfectly
possible to wind series motors for voltages considerably ex-
ceeding this figure, say for 1,000 or 1,200 volts, or. in rare
cases, more, provided the units arc of tolerable size, but inas-
much as most plants for the distribution of power require both
large and small motors, w^ound both series and shunt, the
general voltage is in nearly every case kept at a point at which
it will be easy to meet these varied reciuirements; therefore
500 volts, the American standard for railway practice, has
usually been selected.
The only noteworthy exception may be found in the use
of the Edison three-wire system for distribution of power
to railway and other motors. By this method it become}?
possible to transmit the power at the virtual voltage of 1,000
and to employ 1,000 volt motors, either series- or shunt- wound
for the larger units in order to help in preserving the general
balance, while at the same time using motors of all sizes with
any kind of direct current winding, connected between the
middle wire and either of the outside wires. The advantages
of such an arrangement are very evident, and if the number
of motors be considerable, so that it is possible to balance the
system with a fair degree of accuracy, we have at our disposal
a very convenient niethod for the distribution of continuous
currents. It is interesting to note that this scheme foimd its
first considerable development in electric railway service itself.
Of course, the use of both 110 and 220 volt motors on Edison
three-wire systems is very common, but the extension of the
plan to operating electric roads, and under conditions which
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114 ELECTRIC TRANSMISSION OF POWER.
as regards balance are somewhat trying, is a considerable
step toward an individual method.
The method of working electric railways on the three-wire
system is well shown in Fig. 41. Here the road is a double
track one, to which the method is generally best suited. The
ground, track, and supplementary wires serve as a neutral
wire, both tracks being placed in parallel for this purpose
and thoroughly bonded. On a double track road, the cars
running on each side of the system will be substantially the
same in number, and if the total number of cars be consider-
able, a very fair balance can be obtained, although never as
good as is customarily and necessarily used in an Edison three-
wire system for' lighting. In order to still further improve
the balance of the system and prevent its being disturbed as
Fig. 41.
might otherwise occur by a blockade on one track at some
point, it is better to make the trolley wire above each track
consist of sections of alternate polarity and of convenient
length, so that even in case of a bk)ckade, stopping a con-
siderable number of cars, the load would be removed almost
equally from both sides of the system.
Installed in this way, a railway system is operated at a virtual
voltage of 1,000, and the saving of copper over the ordinary
distribution at 500 volts is considerable, in spite of the inevitable
lack of balance and the loss of the track as part of the main
conducting system. In one large French plant electric loco-
motives are used, each utilizing both sides of the three- wire
system so as to preserve a complete balance. Nevertheless, the
three-wire distribution for trainways has proved generally
imsatisfactory on account of the complication of the overhead
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TRANSAflSSION BY COXTiyUOUS CURRENTS, 115
wiring and the difficulty of preserving balance, so that it has
entirely disappeared from American practice. A few years
ago it was tried in Bangor, Maine ; Portland, Oregon ; and
experimentally elsewhere, but was abandoned after no very
long experience as troublesome and unreliable. It has found
some use abroad, even in a few recent plants, and is reported
to be successful in spite of the difficulties here mentioned.
The increase in voltage obtained by its use is not sufficient
to answer the general purposes of interurban service, and the
ease with which power can be transmitted by altematuig
currents and utilized through rotary converters, or even directly
upon the cars, has obviated the necessity for so dubious a
method of obtaining higher economy of transmission.
INTERDEPENDENT DYNAMOS AND MOTORS.
Aside from the distribution of power for railway purposes,
by far the most interesting kind of power transmission by
continuous currents is that in which a special combination of
two series machines is employed, giving a self-regulating
system comprising a motor unit and a generator unit. This
plan has been successfully used abroad, but has never been
employed in American engineering practice except in an
experimental and tentative way, owing largely to the difficulties
that have been encountered in the production of large direct-
current generators for high voltage.
While it is not at all a difficult matter to build a machine
giving five or six thousand volts with a rather small current,
such as is used in arc lighting, the troubles at the commuta-
tor have proved forbidding when any attempt has been made
to use currents large enough to obtain units of any consider-
able size. Power transmission in this country took its first
stimulus from the development of polyphase apparatus and
methods, and consequently, so far as the art has now been
carried forward, it has been almost wholly in the line of alter-
nating current work. It is obvious, nevertheless, that a
system of power transmission such as we are considering,
possesses great convenience where single units are to be operated
over moderately long distances. In the first place, the induc-
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116 ELECTRIC TRANSMISSION OF POWER.
tive difficulties familair with alternating currents are avoided.
In the second place, the motors are self-starting imder load,
a condition that was not true of alternating machinery prior
to the introduction of the polyphase system. Through the
energy of several foreign engineers, notably Mr. C. E. L. Brown,
much was done in power transmission by this method long
before alternating current apparatus had been suitably devel-
oped. The same difficulties were encountered abroad as
here. It proved to be very difficult to build machines of suffi-
cient voltage and of any considerable output.
In this connection it is noteworthy that nearly all the plants
of this character on the Continent have been installed at rela-
tively low voltages, most of thein less than 1,000, correspond-
ing in general character to the American plants over similar
distances worked at constant potential. In the very few
instances where long distances have been attempted, the
usual method has been to employ generators and motors per-
manently connected together in series, on account of the
impracticability of getting sufficient power in one unit at a
very high voltage. This proceeding somewhat complicates
the system. In addition, the generators and motors have to
be especially designed for each other in order to secure regu-
lation; which, of itself, is a considerable disadvantage.
This last difficulty may be in part avoided by using a shunt
around the field coils of the generator, thereby changing its
regulation under variations of current. A similar device is
widely used in this country in connection with compoimd-
wound generators, where a shunt applied across the terminals
of the series coils is used to regulate the compounding. In
either case, the obvious result of such a shunt is to diminish
the change in the field produced by a given increase in cur-
rent. In this way the necessity for special* machines can be
partially obviated. The plants installed on this peculiar series
plan have been imiformly successful, and permit of the con-
venient transmission of moderate amounts of power over con-
siderable distances. Such plants have even been employed
in connection with motor-generators to supply a general
distribution system, though evidently at a high cost for appa-
ratus.
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TRANSMISSION BY CONTINUOUS CURRENTS. 117
In order to distribute low tension currents from such a
transmission system, it is necessary to employ either a motor,
coupled to a dynamo, or a composite machine with a double
winding, as in case of transmission at constant current. Either
alternative involves the loss of energy substantially equiva-
lent to that lost in two dynamo-electric machines of the
capacities concerned. These losses arc necessarily much more
serious than those in an alternating current transformer, as
has already been seen. They are likely to amoimt to from 12
to 15 per cent, so that quite aside from the efficiency of the
generating dynamo and of the line, the price paid for the privi-
lege of obtaining a low tension current is considerable.
For the dehvery of power alone, where motors in series
coupled to appro j)riate generators can be used, the method
is well fitted for use under certain circumstances and is closely
approximate in efficiency to that which would be obtained
by an alternating current transmission over the same dis-
tance. It is worth mentioning that one of the longest of the
early power transmissions was operated upon this now obsoles-
cent system.
The plant in question is that which was installed nearly
fifteen years ago for operating the Biberest Paper Mills, near
Soluere, Switzerland. The power is derived from the River
Suze, near Bienne, and the distance of transmission is a little
less than twenty miles. At the generating station the avail-
able head of water is about forty-five feet, and the quantity
is sufficient to generate about 400 HP. The power station
contains a 400-HP turbine running at 120 revs, per minute,
of which the vertical shaft is connected by means of beveled
gear to two 130-KW dynamos. They are six pole machines.
Gramme wound, and give at 275 revs, per minute about 40
amperes at 3,300 volts. The two machines are connected
in series, giving a working potential of 6,600 volts on the line.
It should be noted that great care is taken in insulating them,
the bed plates being carried on porcelain insulators. Carbon
brushes are employed. The line is a bare copper wire, 7
millimetres in diameter, about No. 1 B. & S. gauge. The
line runs through a mountainous country, and is Uberally
provided vrith lightning arresters at various points.
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ELECTRIC TRANSMISSION OF POWER.
The two motors at the mills are duplicates of the gener-
ators, the only modifications being those to insure their self-
regulation. They run at 200 revs, per minute on 6,000 volts
delivered, and give about 155 HP each. The commercial
efficiency of this interesting system is somewhat in excess of
75 per cent at full load. Fig. 42 shows one of the motors on
its foundation, and its coupling to its mate. This plant con-
stitutes the most striking example of long distance trans-
Fio. 42.
mission by series-wound interdependent generators and motors,
and probably exhibits the system at its best.
In this country the system has not been used in anything
more than an experimental way, owing principally to two
reasons: first, for short distances, involving not more than
1,000 volts, shunt-wound generators and motors working on
the two-wire or three-wire systems afford better opportmiity
for distribution, inasmuch as their use is not limited to single
mechanical units; second, no serious demand for long distance
transmission arose in America prior to the development of
the alternating system to the point at which alternating
motors became thoroughly practicable. It has been charac-
teristic of American electrical engineering that it has occupied
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TRANSMISSION BY CONTINUOUS CURRENTS. 119
«
itself with one thing at a time. The development of the
electric light was followed by a concentration of energy on the
electric railroad, and this has of late years been succeeded by
extensive power transmission enterprises, often in themselves
involving railway work. Such a mental habit is not conducive
to an even development, but probably accomplishes quite as
much as a more symmetrical advance.
CONSTANT POTENTIAL SYSTEMS.
Shunt- or compound-woimd generators used in connection
with shunt-wound motors have found very extensive use
in this country, working over inconsiderable distances. The
very obvious advantage of such a system is that it permits the
ready distribution of power as well as its easy transmission.
If it becomes necessary to transmit power from one point to
another, the chances are nuich more than even that at the
distributing end of the Hne it will be desirable to utilize the
power in a number of units of varying size. Such an arrange-
ment bars out transmission from series dynamos unless upon
the constant current system with its inherent difficulties of
regulation, whereas -with shunt- wound apparatus the problem
is easy. It often happens, as previously mentioned, that at
the receiving end both series and shunt motors are used, the
former for hoisting and similar work, the latter for operation
at constant speeil.
The growth of the electric railway has encouraged the estab-
lishment of such transmission plants, and their number is very
considerable, scattered over all parts of the Union, not a few
of them being in the mining regions of the Rocky Mountains
and on the Pacific coast, as well as in various isolated plants
through the rest of the country. In most of them, the dis-
tances being moderate, an initial voltage of from 500 to 600
has been employed; more rarely, voltages ranging from 1,000
to 1,800. Plants of these latter voltages have now generally
been replaced by polyphase machines. The efficiency of this
method of transmission is about the same as that of the series
method, just described, but ^ith the advantage that the
shunt motor supplied at constant potential can advantageously
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120 ELECT HIC TRAXSMhSSION OF POWER.
be (listri})uted wherever the work is to he done, while with inter-
dependent series units any distribution of power has to be ac-
compHshed by means of shafting and belting or its equivalent.
The net efficiency from generator to driven machine is likely
to be rather better with the transmission at constant potential
than in the case just discussed. The generators and motors
are of nearly the same efficiency; the line at ordinary dis-
tances is customarily worked at about the same pressure in'
both methods, but distribution by shafting is far less efficient
at any but short distances than distri})ution of electric power
by wire. The loss from the centre of distribution to individual
motors will very seldom exceed 5 per cent, w^hile the loss in
equivalent shafting will seldom be less than 10 per cent, and
more often 20 or more; in fact, it generally turns out upon
investigation that so far as efficiency is concerned there is
a noticeable saving in transmitting power electrically, even
within the limits of a mill or large factory, over the results
which can be obtained by the use of transmission by shafts
and belts. In a large building where the power is to be widely
distributed, it seldom happens that the loss in the shafting is
less than 25 per cent. Anything in excess of this figure repre-
sents remarkably good practice. With motors, 80 per cent
efficiency, if the units are of tolerable size, can be reached
without much difficulty, and there are comparatively few
cases where the efficiency need fall lower than 75. In such a
plant, installed some years since in a Belgian gun factory,
and described in the last chapter, the guaranteed efficiency
was 76.6 per cent. As the efficiency of the dynamo was
reckoned at but 90 per cent, the total efficiency would in
practice be raised without difficulty to 78 or 79 per cent at full
load.
As regards efficiency in general, aside from the disadvan-
tages previously mentioned in changing the voltage of direct
current circuits, the efficiency of transmission by such currents
is in itself as high as has ever been reached by other means.
There is no material difference between the efficiency of direct
and alternating current generators, nor between the efficiency
of direct current motors and the polyphase motors, at least,
p,mong alternating motors. In these particulars, the direct
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TRASSMISSIOy BY CONTINUOUS CURRENTS. 121
current is able to hold its own against all coniors, and in the
cost of motors it has at present a material advantage.
As regards transmission of power over considerable dis-
tances, a case has already been mentioned in which the result
is as good as can reMS()na])ly be expected. Direct current,
however, continually rujis into the limitati(m of available vol-
tage as sixm as distribution is to be attempted. Where single
or a few large motor units are to be used, consisting of either
single machines or groups operated as a unit, the efficiency of
the system is likely to be as high as that obtained from units
of similar magnitude on alternating current systems. The
only disadvantage of the direct current in point of efficiency
in this particular case is that if the amount of power to be
transmitted be large, it is necessary to use generators and
motors coupled in series, while if alternating currents were
used, one would have the advantage of employing a single
machine of equivalent capacity. The principal disadvantage
of direct current machinery is the commutator, which at high
voltages is likely to be sooner or later the source of consider-
able trouble. Careful mechanical and electrical construction
may materially reduce this difficulty, but it always remains to
be faced, and is liable at any time to become troublesome.
On long lines, the direct current has the advantage of pro-
ducing no inductance in the line, an advantage, however,
which does not apply to plants which can advantageously be
operated as single units. Such a single vmit system, arranged
for alternating currents, can have the inductance of the cir-
cuit completely nullified by the simple expeclient of strength-
ening the field of the motor.
It must be remembered, however, that in several particulars
continuous current has peculiar advantages. In the first
place, it is well known that a direct current is decidedly less
dangerous than an alternating current of the same nominal
voltage, so far as the question of life is concerned. The differ-
ence between the two is even greater than would be indicated
by the difference in maximum voltage.
An alternating current has a maximum voltage of approxi-
mately 1.4 times its mean effective voltage, and in addition to
this an alternating current is certainly intrinsically more
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122 ELECTRIC TRANSMISSION OF POWER.
dangerous by reason of the greater shock to the nervous
system producec^ by the alternations of E. M. F. The ease
of tranforming alternating current to a lower voltage partially
obviates this objection, but the fact remains. So far as
danger of fire is concerned, the continuous current has the
power of maintaining a much more formidable arc than an
alternating current of the same effective voltage; but, on the
other hand, the alternating current has somewhat greater
maximum voltage with which to start the arc, so that, practi-
cally, honors are even.
The increase of experience with resonance and kindred
phenomena, acquired on long lines and at high voltages, has
emphasized the fact that alternating transmission work is not
exactly a bed of roses for the engineer, and when it comes to
a question of transmission at 50,000 volts or so, difficulties
multiply. At and above this pressure, there can be little
doubt that insulation is a very difficult task, and there is
equally little doubt that of two lines, one constant current
and the other polyphase, transmitting the same energy at the
same effective voltage, the former would be in trouble much
less frequently than the latter. In the first case there are but
two wires involved, while in the second there are certain to be
three, and generally considerations of inductance would lead
to not less than six hi a plant of large size. And when the
point is reached where insulation is a costly matter, the extra
wires and precautions may easily outweigh any intrinsic saving
in copper. The constant current plant, too, always has the
advantage that it is only working at its maximum voltage
during the peak of the load and the rest of the time has a
very considerable factor of safety.
Whether the increased cost and complication of the gener-
ating station of a constant current system can ever be endured
for the sake of these advantages is a matter open to discussion ;
it certainly cannot be answered in the negative offhand, how-
ever. The continuity of service possible in an alternating
plant materially above 50,000 volts is an imknown quantity,
and in the absence of data upon this pohit one is not justified
in estimating the importance of an alternative method.
It is a mistake, however, to suppose that the considerably
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TRANSMISSION BY CONTINUOUS CURRENTS. 123
increased maximum voltage in an alternating current involves
much greater danger of leakage, or of breaking down insula-
tion under all circumstances. Under many conditions it is
highly probable that the electrolytic strain from continuous
current on insulating materials, particularly when damp, is
more destructive than the added electrostatic strain of an
alternating current. Within any voltages now regularly
employed, the total difference is probably immaterial. In the
matter of one of the great dangers to an overhead line and
apparatus, i.e., injury from lightning, direct current has a very
material advantage in that it is possible to use coils of con-
siderable self-induction in connection with such circuits, so
as to keep oscillatory discharges, Uke lightning, out of the
machines. This is well shown in the singular freedom of arc
lighting stations from serious damages to the machines by
lightning, as compared with stations containing other kinds of
electrical apparatus. In this case the magnets of the arc
machines themselves act as a powerful ijiductance, tending to
throw the lightning to earth. High voltage shunt-wound
dynamos and alternators are much more sensitive in this
respect.
Consequently, part of the price one has to pay for the privi-
lege of utilizing alternating currents is extra care with respect
to protective devices against lightning. In the present state
of the art, the best field for combined transmission and dis-
tribution of power by continuous currents is in cases involving
distribution over moderate distances, within, say, a couple of
miles from the centre of distribution, and even then in prob-
lems where lighting is not an essential part of the work. The
voltage of a lighting circuit is determined by the voltage of
the lamps which can be employed upon it, and this is so
limited that if lighting is to be done on the same circuit as
power distribution there are few cases where such a com-
bhied system can be successfully used with continuous cur-
rents. The field seems at present to be somewhat widened
by the advent of 3-wire systems at 220 to 250 volts on a side,
but their place in the art is hardly yet secure, although their
use is extending.
At all long distances continuous current is at a disadvantage
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124 ELECTRIC TRANSMISSION OF POWER.
in point of available voltage where distribution is to be done,
and has in most cases no very material advantages for single,
unit work. It will require considerable further advance in
dynamo building to render continuous current thoroughly
available for high voltages, and even then only in units of
moderate size, say 300 to 400 KW. In this lies its weakness.
Its strength is largely in its present firm foothold in electrical
practice, and in the fact that standard apparatus for continu-
ous currents is available everywhere, and is manufactured
cheaply in large quantities by numerous makers. It is, fur-
thermore, interchangeable to a degree which will never be true
of alternating-current machinery until there is far greater
miity in alternating-current practice than we are likely to have
for some years to come.
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CHAPTER IV.
SOME PROPERTIES OF ALTERNATING CIRCUITS.
We have already seen in Chapter I that the current nor-
mally produced by a dynamo-electric machine is an alternating
one, SO that a continuous current exists in the external circuit
only in virtue of the commutator. Until within the last
fifteen years the original alternating current was utilized to
but a trivial extent. Nevertheless, it possesses certain prop-
erties so valuable that their practical development has wrought
a revolution in applied electricity.
To describe these properties with any degree of complete-
ness would require several volumes the size of the present, and
would involve mathematical considerations so abstruse as to be
absolutely imintelligible to any save the professional reader.
We shall therefore at the very start drop the academic methods
of treatment and confine ourselves, so far as possible, to the
physical facts concerning those properties of alternating cur-
rents which have a direct bearing on the electrical transmission
of energy. This discussion will therefore be somewhat imcon-
ventional in form, although adhering rigidly to the results of
experiment and mathematical theory. The student who is
interested in the exact development of this theory will do
well to consult the excellent treatises of Fleming, Mascart
and Joubert, Bedell and Crehore, and Steinmetz, all of which
are full of valuable demonstrations.
The fundamental differences between the behavior of con-
tinuous and of alternating currents lie in the fact that in the
former case we deal mainly with the phenomena of a flow of
electrical energy already steadily established, while in the
latter case the phenomena of starting and stopping this flow
are of primary importance. These differences are akin to those
which exist between keeping a railway train in steady motion
over a uniform track, and bringing it up to speed from a
state of rest. In steady running the amount of, and the
125
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126
ELECTRIC TRANSMISSION OF POWER.
variations in the power needed, depend almost wholly on the
friction of the various parts, while in starting both the power
and its variations are profoundly affected by the inertia of the
mass, the elasticity of the parts, and other things that cut
little figure when the train is up to a uniform speed.
The characteristic properties of alternating currents are due
mainly to the starting and stopping conditions, and are only in-
cidentally affected by the circumstance that the flow of current
alternates in direction. As, however, this alternating type of
current is in general use and its uniform oscillations give the
best possible opportunity for observing the effect of repeated
stops and starts, we will look into the generation of alternat-
ing current, not forgetting that for certain purposes we shall
FlO. 43.
have to recur to the phenomena of a single stop or start in the
current.
Fig. 43 shows an idealized generator of alternating currents.
It is composed of a single loop of wire arranged to turn con-
tinuously in the space between the poles of a magnet. This
space is a region of intense electromagnetic stress directed
from pole to pole, as indicated by the dotted lines. The two
ends of the loop are connected to two insulated metallic rings
connected by brushes to the terminals A and B of the external
circuit. We have already seen that what we call electromotive
force appears whenever the electromagnetic stress about a
conductor changes in magnitude. Now, in turning the loop as
shown by the arrow, the electromagnetic stress through it
changes, and of course sets up an electromotive force. In
the initial position of the loop shown in Fig. 43, it includes
evidently the maximum area under stress; after it has turned
through an angle a, this area will be much lessened, and when
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PROPERTIES OF ALTERNATING CIRCUITS. 127
a = 90°, the loop will be parallel to the plane of the electro-
magnetic stress, and hence can include none of it at all.
But the resulting electromotive stress is, other things being
equal, proportional to the rate at which work is expended in
imiformly turning the coil, i.e., it is proportional to the rate^
of change in the electromagnetic stress included by the coil.
This rate is, during a single revolution, greatest when the
sides of the loop are moving directly across the lines of stress,
and least when moving nearly parallel to them. Hence, we see
from Fig. 43 that the electromotive force in our coil will be a
maximum when a = 90° or 270° and a minimum in the two
intermediate positions. For a simple loop it is easy to compute
exactly the way in which the electromotive force will vary as
the loop turns. The area of strain included by the coil in any
FlO. 44.
position is proportional to the cosine of the angle a, hence, for
uniform motion the rate of change in the area is proportional
to the sine of o. Therefore, the E. M. F. at every point of the
revolution is proportional to sine a.
If now we draw a horizontal line and measure along it equal
distances corresponding to degrees, and then, erecting at each
degree a line in length proportional to the sine of that par-
ticular angle, join -the ends of these perpendiculars, we shall
have an exact picture of the way in which the E. M. F. of
our loop rises and falls. Fig. 44 is such a curve of E. M. F. —
a so-called "sine wave," which is expressed by the equation,
E = E^ sin at.
This simple form of E. M. F. curve — the "sine wave" — is
assumed to exist in most mathematical discussions of alternat-
ing current to avoid the frightful complications which would
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128
ELECTRIC TRANSMISSION OF POWER.
result from assuming such E. M. F. curves as often are foimd in
practice. This assumption is somewhat rash, for a true sine
wave is never given by any practical generator, but the error
does not often invalidate any of the conclusions, for the exact
form of the wave only matters in discussing certain cases, where
it can often be taken into account without much difficulty.
Actual alternating generators give curves of E. M. F. greatly
influenced by the existence of an iron armature core which
collects the lines of force so that as the core turns the change
of stress through the armature coils is not directly propor-
tional to anything in particular. A glance at the rudimentary
dynamo of Fig. 8, Chapter I, will suggest the reason. It is evi-
Fig. 46.
dent enough that the armature could turn almost 30° from the
horizontal with scarcely any change in the magnetic relations
of the coils. The result would be a wave with a very flat and
depressed top, since the rate of change of induction would be
very moderate when it should be considerable. The practical
bearings of wave form on power transmission work will be taken
up in the next chapter. At present it will suffice to say that the
best standard alternators give a fairly close approximation to
the sine form. Fig. 45 shows the E. M. F. curve of one of the
great Niagara generators. This is an excellent example of
modem practice and shows a form slightly flatter than the sine
curve and with a mere trace of depression at the crest. Plenty
of machines are in operation, however, that give curves not
within hailing distance of being sinusoidal — e.g. Fig. 46, which
shows the E. M. F. curve of one of the earliest alternators
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PROPERTIES OF ALTERNATING CIRCUITS, 129
designed for electric welding. In this case there is a far sharper
wave than the sinusoidal, of a curious toothed form. Many of
the early alternators with ironclad armatures gave curves quite
far from the sine form, generally rather pointed, while the
tendency in recent machines has been rather in the opposite
direction, toward curves like Fig. 45, although seldom so nearly
sinusoidal. The general equation for their E. M. F. is,
£=J?iSina/ + ^8sin3a/+^5sin5a^ . . . E(2n-i)sin(2n—l)at.
FlO. 46.
In other words, the E. M. F. is built up of the fundamental
frequency and its odd harmonics.
Now as to the current produced by this oscillating electro-
motive force. In ordinary work with continuous currents, the
current corresponding to each successive value of the E. M. F.
would be very easy to determine by simple reference to Ohm's
E
law, C = - • If the dynamo of Fig. 43 gave one volt maximum
R
E. M. F. and were connected through a simple circuit of one
ohm resistance the maximum current would be one ampere,
and the current at all points would be directly proportional to
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130 ELECTRIC TRANSMISSION OF POWER.
the voltage. Hence if the E. M. F. varied as shoT\7i in Fig. 44,
the current would vary in precisely the same manner, and the
curve showing its variation would, if drawn to the same scale,
exactly coincide with the curve of Fig. 44. This would be
generally true if we had only resistance to consider, and the
treatment of alternating currents would then be very simple.
But the starting and stopping of current which takes place
periodically in alternating circuits produces great changes in
the electro-magnetic stresses about the conductors, and these
changes are in turn capable of very important reactions.
They give to the alternating current its most valuable proper-
ties, but also involve its action in very curious complications.
Turn back to Chapter I and examine Fig. 4. We see from
it that whenever the electro-magnetic stresses about a circuit
mm
« /
Fig. 47.
as A, change by the variation of the current flowing in it, an
E. M. F. is set up in the parallel circuit B^ opposing the
change of E. M. F. in A. This fact, as we shall see later, is
the root of the alternating-current transformer. Suppose now
that in the circuit of our alternator is a coil of wire woimd in
close loops as shown in Fig. 47. Here A and B are the dyna-
mo terminals, C the general circuit, and D the aforesaid coil.
Let an E. M. F. be started m the direction A C D. The result-
ing current flows through D, but the electro-magnetic stresses
set up by the current about, for instance, the loop e, produce
an E. M. F. in neighboring coils tending to drive current in
a direction opposite to that from the dynamo. In other
words, e acts toward / just as A acted toward B in Fig. 4.
Thus each turn tends to oppose the increase of current in
the others. When the current in C ceases to vaiy, of course
the reactive E.M. F.'s in e and / stop, for there is for the moment
no change of stress to produce them, but as the main current
begins to decrease, the reactive E. M. F.'s set in again.
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PROPERTIES OF ALTERNATING CIRCUITS. 131
The main or "impressed" E. M. F. is thus opposed in all
its changes by the reactive or "inductive" E. M. F. due to
the combined action of the loops at D. Hence tlie impressed
E. M. F. in driving current through the system has to over-
come, not only the resistance of the conductors, but opposing
electromotive forces. Therefore since a part of the impressed
E. M. F. is taken up with neutralizing the inductive E. M. F.,
only the remainder is effective against the true resistance of
the circuit. Ohm's law, then, cannot apply to alternating cir-
cuits in which there is inductive action, except in so far as we
deal with the "effective" E. M. F. The relation between the
E E
impressed E. M. F. and the current is not C = —f but C = —
R R
FlO. 48.
less a quantity depending on the amoimt of inductive E. M. F.
encountered.
This state of things leads to two very important results:
First, the current in an inductive circuit is less than the im-
pressed E. M. F. would indicate. Second, this current reaches
its maximum later than the impressed E. M. F. For the
current depends on the effective P]. M. F., and for each partic-
ular value of this the impressed E. M. F. must have had time
to rise enough to overcome the corresponding value of the
inductive E. M. F. The current is thus damped in amount
and caused to lag in "phase" as sho\^Ti in Fig. 48. The heavy
line here shows the variations of the impressed E. M. F., and
the light line the corresponding variations of current in a
circuit containing inductive reaction — inductance. The dis-
tance a b represents the "angle of lag," while 6 c is 180° as
shown in Fig. 44. Very similar relations are found in prac-
tice, although the lag is often greater than shown in the cut,
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132 ELECTRIC TRANSMISSION OF POWER.
particularly when alternating motors are in circuit. Fig.
49 shows the curves of E. M. F. and current from a very small
alternating motor at the moment of starting. The angle of
lag in this case is a trifle over 45*^, and the curves are much
closer to sine waves than is usual.
The inductive E. M. F., as has already been explained, is due
to the magnetic changes produced by the variation of the cur-
rent. Just as in the d3aiamo of Fig. 43, the actual amount of
E. M. F. is directly proportional to the rate of change in mag-
netic stress, which is in turn proportional to the change of cur-
rent. The inductive E. M. F. is therefore at every point
proportional to the rate of variation of the current. But the
PlO. 49.
current wave is, like the impressed E. M. F. wave, still approxi-
mately a sine curve, for it has been merely shifted back
through the angle of lag, and although damped, it has been
simply changed to a different scale. Being still essentially
a sine curve, its rate of variation is a cosine curve, or, what
is the same thing, a sine curve shifted backward a quarter
period, 90®. Indeed, this is at once evident, for, since the
current varies most slowly at its maximum, the inductive
E. M. F. must be a minimum at that point, i.e., it must be
90® behind the current in phase, while since E. M. F. and cur-
rent vary symmetrically, in general the forms of the two curves
will be similar. The effective E. M. F. which is actually
engaged in driving the current is a wave in phase with the
current it drives, and of similar shape, i.e., a sine curve.
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PROPERTIES OF ALTERNATING CIRCUITS.
183
We have, then, in an inductive circuit three E. M. F.'s to be
considered :
I. The impressed P]. M. F., acting on the circuit.
II. The inductive E. M. F., opposing I.
III. The effective E. M. F., the resultant of I and II.
Plotting the respective curves, they bear to each other the
relation shown in Fig. 50. Here a is the impressed E. M. F.,
b the effective E M. F. (or the current) lagguig behind a
through an angle usually denoted by <A, and c is the induc-
tive E. M. F. 90° behind b. Now since b is the resultant of
the interaction of a and c, and we know that b and c are 90°
apart in phase, it is comparatively easy to find the exact rela-
tion between the three.
For we can treat electromotive forces acting at known angles
with each other just as we would treat any other forces work-
Fio. 50.
Fig. 61.
ing conjointly. If for example we have a force A B, Fig. 51,
acting simultaneously with a force B C, at right angles to it,
the magnitudes of the forces being proportional to the lengths
of the lines, the result is the same as if a single force in magni-
tude and direction A C were working instead of the two com-
ponents. This is a familiar general theorem that proves par-
ticularly useful in the case in hand.
If we take A B equal to the effective E. M. F., and B C equal
to the inductive E. M. F., then ^1 C is the impressed E. M. F.
It at once appears that the angle between A B and A C is <l>j
the angle of lag. Then from elementarj*^ trigonometry it
appears that
AB ^ ACcos<l>
CB = ACsm<l> = AB tan <^, hence
CB
tan<A=.— .
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134 ELECTRIC TRANSMISSION OF POWER,
We are therefore in a position to determine the three E. M.
F.'s and the angle <^, knowing any two of the four quantities.
Thus for a given impressed E. M. F., £, such as is found on
any constant-potential alternating circuit, the effective E. M.
F. which determines the current is given by E cos <l>. As <l>
grows less and less through decrease of the inductive E. M. F.,
A Cf the impressed E. M. F. necessary for a given current
also decreases, and finally, when <l> becomes zero, A C = A B.
In other words, the impressed E. M. F. is then simply that
needed to overcome the ohmic resistance.
For any particular current, then, A B is directly propor-
tional to the resistance of the circuit, while C B is directly
proportional to the "inductance" of the circuit, that prop-
erty of the particular circuit which determines the inductive
E. M. F. Calling this / we may redraw Fig. 51 in a very
convenient form — Fig. 52. Here we see the relation between
R and / in determining the impressed E. M. F. necessary to
drive a certain current through an inductive circuit. The
magnitude of the E. M. F. evidently is Vfl^ + /' if the units
of measurement are chosen correctly, and it is always pro-
portional to this quantity, which is related to the impressed
E. M. F. as resistance is to the effective E. M. F.
Hence v i2' + P has sometimes been called "apparent
resistance." The more general name, however, is impedance,
which indicates the perfectly general relation between E. M. F.
and current. If / be zero, as in a continuous-current circuit,
then the impedance becomes the simple resistance. We can
now write out some of the general relations of current and
E. M. F. in alternating circuits as follows, calling E the im-
pressed E. M. F. as before:
E
C = ^ K" ■\- P
E ^ C '^ K" + P
and with respect to the angle of lag,
I ^
y R? + p- ^
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PROPERTIES OF ALTERNATING CIRCUITS.
135
/ = ie tan <^
tan <^ = - .
K
Hence, knowing the angle of lag and the resistance of a
circuit, the inductance can be found at once. The angle
of lag, depending on the ratio of / and iJ, must be the same
for all circuits in which this ratio is the same. Also in
any circuit of given inductance, increasing the resistance
diminishes the angle of lag, while of course also diminishing
the current for a given value of E, In fact, since / does not
represent work done, for the inductive E. M. F. represents
FlO. S3.
merely a certain amount subtracted from the impressed
E. M. F. by the reaction of the circuit, any process which for
a given value of E increases the energy actually spent in the
circuit is accompanied by a diminution of the angle of lag.
This freedom of the circuit from any energy losses due to
/ is a fact of the greatest importance. It is fully borne out
by experiment, and there is besides good physical reason for
it. For since current and E. M. F. are the two factors of
electrical energy, there can be no energy when the product of
these factors is zero. Note now Fig. 53, developed from Fig.
'50. Hence a is the line of zero E. M. F. and current, h the
current curve for a single alternation, and c the correspond-
ing curve of inductive E. M. F. 90° behind the current.
When 6 is a maximum, c is zero, and vice versa. And since
c is equally above and below the zero line during each alterna-
tion of current, the average E. M. F. is zero, and therefore
the average energy throughout the alternation is zero. The
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136 ELECTRIC TRANSMISSION OF POWER,
same conditions would evidently continue if instead of an
alternation we took a complete cycle (i.e., the whole curve
from the time the current starts in a given direction until
it starts in the same direction again) or any number of cycles.
Thus / must be entirely dropped out of consideration in dis-
cussing the question of work done in an alternating circuit.
And since E differs from £4, the effective E. M. F., only by
a fimction of /, the energy value of which is zero, the energy
in the circuit is exactly measured by E^ and the corresponding
current which, as we have seen, is in phase with it. But
El = E cos <l>.
Hence, multiplying both members of this equation by C to
reduce to energy.
Energy = C ^j = C £ cos <^.
That is, the energy in an alternating circuit is equal, not to
the impressed E. M. F. multiplied by the current, but to their
product multiplied by the cosine of the angle of lag. The
product C E is sometimes called the apparent energy to dis-
tinguish it from C E^y the actual energy. This apparent
energy is that obtained by measuring the amperes and the
impressed volts and taking their product. The real energy
is that which would be obtained by putting a wattmeter in
circuit. Hence
watts
volt-amperes
a convenient and common method of measuring the angle of
lag. If in addition the value of / is wanted, it can be obtained
at once from the expression for tangent <^ already given.
We thus see that the energy in an inductive circuit is not
directly proportional to the voltage as measured, but to the
effective voltage, which is less by an amount depending on
the inductance. This difference is sometimes referred to as
the ''inductive drop" in a circuit. The result is that to drive
a given current through an inductive circuit the generator
nuist give a voltage depending on the impedance of the cir-
cuit. On the other hand, if an inductive circuit be fed from
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PROPERTIES OF ALTERNATING CIRCUITS. 137
a given impressed E. M. F. the current required to represent
a given amount of energy exceeds that required in a non-induc-
tive circuit, in the ratio of 1 to the cosine of the angle of lag.
The net result, then, of inductance in an alternating circuit is
to increase the E. M. F. at the generator required to produce
a given E. M. F. at the load, and to increase the current re-
quired to deliver a given amoimt of energy.
The E. M. F. and current are here supposed to be meas-
ured in the ordinary way, by properly designed voltmeters
and ammeters. In power transmission work, inductance in the
circuit (line or load or both) means that the dynamo has to
give voltage enough to overcome the impedance of the system
and still to deliver the proper number of volts at the motor,
while the motor will take extra current enough to compensate
for the lag between the E. M. F. at its terminals and the re-
sulting current.
The dynamo thus has to be capable of giving a little extra
voltage, and the motor must be able to stand a little extra cur-
rent. In other words, both machines must have sufficient mar-
gin in capacity to take care of this matter of lagging current.
We have already seen the general relation between resist-
ance, inductance, and impedance. Let us now look into the
quantity last mentioned so as to see its numerical relation to
the others. If a circuit has a certain resistance in ohms and
a given inductance, what is its impedance, i.e., the ratio
between the measured voltage and the measured current?
The real question involved is the value of the inductive E.
M. F. This, like any other E. M. F., is proportional to the
rate of variation of the electro-magnetic stress which produces
it. Its total magnitude depends on the rate of variation of the
current and the ability of this current to set up stresses which
can affect neighboring conductors as in Fig. 43. This latter
property depends on the number of turns, their locality with
reference to each other, and other similar conditions which
depend simply on the physical nature of the circuit, and so for
any given circuit are settled once for all. These properties
are defined on the basis of their net effect, and the ratio of the
rate of variation of the current to the inductive E. M. F. pro-
duced by it in a given circuit is usually known as L, the
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188 ELECTRIC TRANSMISSION OF POWER.
"coefficient of self-induction" of that circuit. The total
inductive E. M. F. is then equal to L, multiplied by the actual
rate of current variation expressed in such units as will fit the
general system by which E, R and other quantities are con-
cord an tly measured.
Expressed in this way, the rate of current variation in an
alternating circuit is 2 «■ n, where n is the number of cycles per
second, and v has its ordinary meaning of 3.1416. Hence the
inductance of the circuit is numerically 2 v n L, the last factor
being dependent on the nature of the circuit and denoting
the inductance per unit rate of current variation. The 2 tt n
factor gives the actual rate of current variation, which may
change to any amount, while L remains fixed. L therefore
may at all times in a given circuit be expressed in terms of
any unit that is conveniently related to other electrical units.
Such a unit inductance is the henry, which is the inductance
corresponding to an inductive E. M. F. of 1 volt when the
inducing current varies at the rate of 1 ampere per second.
If, therefore, L for any circuit is known in henrys, the
total inductance / is 6.28 n L.
We are now ready to apply a numerical value to / in Fig.
52 and the resulting equations.
For example, let us suppose that a certain alternating circuit
has a resistance of 100 ohms and L = 0.1 henry. The im-
pressed E. M. F. is 1,000 volts. What will be the current and
its angle of lag? Lay off A B, Fig. 54, 100 units long. Then
at B, to the same scale erect a perpendicular B C, 2 ir n L in
height. If we are dealing with an alternating circuit of 60 ^
per second, such as is often used for power transmission, 2
IT n L will be 37.7 units high. Now join A C, and the result-
ing length on the same scale is the impedance in ohms. But
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PROPERTIES OF ALTERNATING CIRCUITS. 139
A C = Vi22 + p = Vl00» + 37.72 _ 106.9 nearly. And since
the current equals the impressed E. M. F. divided by the im-
pedance, the current in tliis case would be 9.36 amperes instead
of the 10 amperes due if there had been no inductance. And
since tan <^ = ■- , it is here .377, which corresponds to an angle
R .
of 20° 40'.
Also, since E =- C y^+P, we can readily find the im-
pressed E. M. F. required to produce in this circuit any given
current. For C = 10 amperes, E = 1,069 volts, and so on.
w^atts
We have seen that cos <^ = — so that in the
volt-amperes
case in hand where cos <^ = .936 the actual energy in the cir-
•cuit is 93.6 per cent of that indicated by the readings of volt-
meter and ammeter.
This factor, cos <^, connecting the apparent and the real
energy, is known as the "power factor" of the circuit.
As I — R tan <^, and in any given case n is known, L can
readily be obtained from a measurement of lag in a circuit of
known resistance. It must be remembered, however, that if
the inductance is due to a coil having an iron core, the value
of L will change when the magnetization of the iron changes,
so that results obtained with a certain current will not hold
exactly for other currents. The values of L found in practice
cover a very wide range, from a few thousandths of a henry in
a small bit of apparatus like an electric bell, to some hundreds
of henrys in the field magnets of a big dynamo. L in fact is
nearly as variable as R.
As a practical example in inductance effects we may consider
the effect of alternating current in a long straightaway circuit.
Suppose for example we have a circuit 50,000 ft. long composed
of No. 4 B. & S. copper wires. The wires are about 1 ft. apart
and about 20 ft. above the ground. What voltage will be
required to deliver 10 amperes through this circuit at 130
cycles per second, and what will be the angle of lag? The
resistance of this wire is 0.25 ohms per 1,000 ft. L, its co-
efficient of induction, is .0003 henry per 1,000 ft. The total
resistance of the circuit is then 25 ohms, and its total induc-
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140
ELECTRIC TRANSMISSION OF POWER,
tance, /, = 6.28 x 130 X.03 = 24.5. Plotting as before these
values, in Fig. 55 we have the impedance equal to v25M-24^
= 35 ohms. Hence E must be 350 volts instead of the 250
that would suffice in the case of continuous current. Tan
24.5
i> = = .98. The corresponding angle is 44° 25'. The
^o
ratio of impedance to resistance in this case is 1.4:1. This
ratio, often called the impedance factor, is a very convenient
way of treating the matter, and tables giving its value for
common cases will be given later. In case of apparatus being
connected to the circuit, the computation of its effect is easy.
If it has resistance R^ and inductance P then the total impe-
dance of the circuit will be V(/e -f R^y + (/ + ^Y and so
on for any number of resistances and inductances, the impe-
dance being always equal to the square root of the squared
sum of the resistances plus the squared sutn of the inductances.
Thus an inductance added anywhere in circuit changes the
total impedance and the angle of lag.
There are several ways of looking at inductance, according
as one wishes to deal more particularly with inductive E. M. F.,
the changes in electro-magnetic stress which produce it, or
the energy changes which accompany it. The first point of
view is the one here taken, in accordance with the definition
of the henry just given. Hence the henry may be called unit
inductance, in which case the quantity / which we have been
considering measures the inductive E. M. F., and since it is
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PROPERTIES OF ALTERNATING CIRCUITS. I4l
the product of the inductance for unit rate of current change
multiplied by 2 w n, it is sometimes referred to as inductance-
speed, now conventionally termed reactance.
In alternating-current working inductance may easily be-
come quite troublesome, through the '^inductive drop" in
the line and the necessity of sometimes delivering a current
quite out of proportion to the energy. Thus in alternating-
current lighting plants during the hours of daylight when the
actual load is small, the current may be of quite imposing size
from the lag produced by the inductance of the unloaded
transformers in circuit. The sort of thing which happens may
readily be figured out. Suppose we are dealing with a trans-
former or other inductive apparatus having a resistance of 5
ohms and L = 1 henry. The impedance at 60 <^ will then be
V 5^ + (6.28 X 60 X ly == V25 + 376.8^ = 377.8 ohms, sub-
stantially the same as the inductance alone, and under an
impressed E. M. F. of 1,000 volts the resulting current would
377 8
be 2.65 amperes. But tan <^ = — -— = 75.56. Hence <l> = 89°15'
5
and cos <l> = .013. Therefore while the apparent energy is
2.65 X 1,000 = 2,650 watts, the real energy is only 2,650 X.013
watts = 34 -I- : really the loss due to heating the conductor.
This is of course a very exaggerated case, as it takes no account
of the energy that would be required to reverse the magneti-
zation in whatever iron core the apparatus might have. It
does, however, show very clearly that the current flowing
depends practically on the inductance and very little on the
resistance, and that the angle of lag is so great that the dis-
crepancy between apparent and real energy may also be very
great. In practice cos <^ may fall as low as 0.1 on single
pieces of apparatus, and ranges up under varying conditions
of load to .95 or more.
These practical considerations naturally raise a question as
to the effect of impedances in parallel. The joint impedance
of two impedances in series must first be discussed.
The resistance of two resistances in parallel is of course
familiar. If i? = 2 ohms and i2^ = 4 ohms, then their joint
resistance ig the reciprocal of the sum of their reciprocals,
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142
ELECTRIC TRANSMISSION OF POWER.
thus,
(R + R') =
i + i
= 1 J ohms.
We have seen, however, that impedances cannot be added
in the ordinary manner. If we take two impedances made up
respectively of i? = 4, / = 3, and R^ = 6, /' = 3, we must pro-
ceed as in Fig. 56. The first impedance is 5, the second 6.70.
The true impedance of the two in series is given by the dotted
Unes, and is 11.66, not 11.70. That is, the impedances must be
added geometrically, since unless <A = <^i the arithmetical sum
of the impedances does not represent the facts in the case.
■r^7
FlO. 66.
Similarly, while it is perfectly true that the joint impedance
of two impedances in parallel is equal to the reciprocal of the
•sum of their reciprocals, the summation must be done as in
Fig. 56 to take account of the difference of phase which may
exist in the two branches. Taking the data just given, the
reciprocals of the two impedances are .20 and .149 respectively.
Drawing these on any convenient scale as in Fig. 57, preserv-
ing between them the angle due to the difference of phase as
given by <l> and <^4, we find the geometrical sum of the recipro-
cals to be .348, of which the reciprocal is 2.87. This is the
joint impedance of the two which we have thus geometrically
added.
This same process can be extended to any number of impe-
dances in parallel. In a precisely similar way any number of
directed quantities may be laid off and geometrically added,
the final sum being in direction and magnitude the line from
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PROPERTIES OF ALTERNATING CIRCUITS. 143
the starting point to the finish. It is important to note that
since the currents in such cases are generally not in phase
with each other, it usually happens that the sum of the currents
in the branches differs from the current in the main circuit,
as they are ordinarily measured. It is in fact a prominent
characteristic of alternating circuits that both currents and
voltages are liable to vary in a way at first sight very erratic.
Particularly is this the case when there is capacity in the cir-
cuit, a condition which we will now investigate.
By a circuit having capacity, we mean one so constituted
that E. M. F. applied to it stores up energy in the form of
electrostatic stress, which starts this energy back in the form
of current when the constraining E. M. F. is removed.
Such a condition exists whenever two conductors are
separated by an insulating medium, or dielectric, as in the
ordinary condenser of Fig. 58. Here A and B are two metal
plates separated by a layer, C, of some insulating material.
If now these plates are connected to the terminals of a dyna-
mo they become electrostatically charged. The electrostatic
stress tends to draw the plates together, and in addition sets
up intense strains in the dielectric C, rendering potential
thereby a certain amount of energy which flows into the
apparatus in the form of electric current. This energy is
returned as current if the original electromotive stress is
removed and A and B are connected together. The medium
behaves just as if it were a strained spring, and when it returns
its energy to the circuit it does so spring-fashion with rapid
oscillations, dying out the more slowly the less resistance they
encounter.
The capacity of such a condenser is the quantity of energy
which it can store up as electrostatic strains in C. It is pro-
portional to the area of the plates, to the E. M. F. produc-
ing the strains, and to the "dielectric constant" of C, that is,
the coefficient which for that particular substance measiu*es its
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144 ELECTRIC TRANSMISSION OF POWER.
power to take up electrostatic strains. Oddly enough the
capacity decreases as C grows thicker, indicating that the
intensity of the strain is the thing which counts rather than
the volume of dielectric. Without knowing the exact character
of electrostatic strain, it is difficult to get a clear mechanical
idea of the state of things which causes the energy stored to
increase as the thickness of C diminishes. A similar condition,
however, holds for a wire held tightly at one end and twisted
at the other; the shorter the \\ire, the more energy stored for
a given angle of twist.
As in the case of inductance, for practical purposes the unit
of capacity is taken in terms of unit pressure, t.e., one volt.
Unit capacity, then, in terms of energy, is the capacity of con-
denser in which one watt-second can be stored under an elec-
FIO. 58.
tromotive stress of one volt. This capacity is one farad, and
as it is many thousand times larger than anything found in
practice, of it (the microfarad) is more often used.
^ ' 1,000,000 ^
When a condenser is used with an alternating current, the
rate at which energj'^ is stored and delivered evidently increases
with the frequency, or) what is the same thing, for a given
alternating E. M. F. the greater the frequency the greater the
current received and delivered by the condenser.
Numerically the current in a condenser of capacity k farads,
supplied by an E. M. F. of e volts at n cycles per second, is
C = 2 wnek J
which is simply the current due to e volts and k farads mul-
tiplied by the frequency expressed in angular measure. Thus,
if we have a 2 microfarad condenser fed by an alternating
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PROPERTIES OP ALTERNATING CIRCUITS. 145
E. M. F. of 2,000 volts and 130 cycles per second, the current
flowing is
^ 2,000 X 6.28 X 130 X 2 ^
C = -—^- = 3.26 amperes.
In such an alternating circuit, then, there will be a substantial
current flowing in spite of the fact that there is a break in the
conductor at the condenser. In short, the circuit acts as if it
2,000
had a resistance of -f— — =613 ohms, which is the impedance of
3.26 ' ^
the circuit. More exactly the impedance is • It should
2 7r n k
be noted here that some writers refer to this fundamental
condenser function (2 w n) k sls capacity-speed. Capacity-im-
pedance really is a negative reactancej often termed conden^ance.
To see the relation which this capacity-impedance bears to
other impedances in the circuit, it is necessary to look into
the properties of the E. M. F. of the condenser. As energy is
stored in the condenser the opposing stresses in it increjise
until the applied E. M. F. can no longer force current into it
and the condenser is fully charged. At the moment, then,
when current ceases to flow, the E. M. F. of the condenser
tending to discharge it is at a maximum. Hence, since the
one has a maximum as the other is zero, the E. M. F. of the
condenser and the charging current are 90° apart iji phase.
But the inductive E. M. F. is also 90° from the current, and,
as we have seen, lagging. It has its maximum when the
current is rary^ing niost rapidly; and when the strejigth of cur-
rent in a given direction is increasing, the inductive E. M. F.
in the same direction is diminishing, as shown in Fig. 53. As
regards capacity, however, the moment of maximum condenser
E. M. F. in a given direction is that at which the current
thereby becomes zero, so that as the current changes sign it
has behind it the thrust of the full E. M. F. of the discharging
condenser, while at the same moment, as we have just seen,
the opposing inductive E. M. F. is at its maximum. Hence the
E. M. F. of the condenser has a maximum in one direction
when the inductive E. M. F. has its maximum in the other
direction. The two are thus 180° apart in phase, and each
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146
ELECTRIC TRANSMISSION OF POWER.
being 90° from the current, the condenser E. M. F. must be
regarded as 90° ahead of the current, just as the inductive
E. M. F. is 90° behind it.
The condition of affairs is sho^\'n in Fig. 59. Here a a is
the line of zero current and E. M. F. All quantities above
this line may be regarded as -I , and all below it as — ; 6 is a +
b
Fio. 69.
wave of current to which appertains c c the curve of inductive
E. M. F. lagging 90° behind the current, and d d the condenser
E. M. F., leading the current 90°.
It is evident that these two E. M. F.'s always are opposing
each other — when one is retarding the current the other is
accelerating it, and vice versa.
The condenser E. M. F. has no effect on the total energy
no. 60.
of the circuit for the same reason that held good in respect
to Fig. 53; it is obviously akin to a spring, alternately receiv-
ing and giving up energy, but absorbing next to none.
Capacity may be considered as negative inductance in many
of its properties. If, as in Fig. 59, it is in amount exactly
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PROPERTIES OF ALTERNATING CIRCUITS.
147
equivalent to the inductance, the total effect on the circuit is
as if neither capacity nor inductance were in the circuit. In
such case it is as if C /?, Fig. 51, should be reduced to zero.
The impressed E. M. F. then becomes equal to the effective
E. M. F., the angle of lag vanishes and the circuit behaves as
if it contained resistance only. If the condenser E. M. F. is
not quite large enough to annul the inductance it simply
reduces it.
Fig. 60 illustrates the effect of varying amounts of capa-
city. In the main triangle ABC, the sides have the same
signification as in Fig. 51. Since the capacity E. M, F. is
180° from, i.e., directly opposite to, the inductive E. M. F., the
effect of adding the capacity E. M. F. C D, is to reduce the
effective inductance to B D and give as an impressed E. M. F.
A D and an angle of lag <^i. Now increasing C D to equal C B,
the inductance is annulled, <^ becomes zero, and the impressed
and effective E. M. F.'s are the same. Then increase C D still
further so that it becomes C E, Now the inductance C B
not only is neutralized but is replaced by a negative inductance
B E. The angle of lag now becomes an angle of lead, <^2» the
necessary impressed E. M. F. rises to A E, and the circuit
behaves as regards the relations between current, E. M. F., and
energy, just as it did when affected by inductance. There is
the same discrepancy between real and apparent energy, the
same necessity for more current to represent the same energy.
But adding inductance now decreases the angle of lead. From
a practical standpoint capacity by itself is objectionable, but
capacity in a line containing inductance is sometimes a very
material advantage.
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148 ELECTRIC TRANSMISSION OF POWER.
The nature and reality of this curious phenomenon of
"leading" current in an alternating circuit may be appre-
ciated by an examination of Fig. 61. This shows the actual
curves of current and E. M. F. taken from a dynamo working
on a condenser in parallel with inductance. The maximum of
the current wave is very obviously in advance of the maximum
of the E. M. F. wave, though by a rather small amoimt (ac-
tually about 6°). The capacity in this case was between 2 and
3 microfarads.
Treating capacity as a negative inductance enables us to
compute its effects quite easily. We have already seen how
to reckon the impedance of a condenser; using the word im-
pedance here in its proper sense of apparent resistance by
whatever caused. This quantity we can add geometrically to
the ohmic resistance of a circuit and obtain the net impedance
just as in Fig. 54. We must bear in mind, however, that the
capacity E. M. F. is 180° from the inductance E. M. F., though
each is at right angles to the effective E. M. F. which is con-
cerned with the ohmic resistance.
Instead, then, of computing the total impedance as
Vr^ + P^ it becomes y r* + (^ r) j the second t^rm under
the radical being the square of the apparent resistance due to
the capacity, just as P expressed the square of the apparent
resistance due to inductance.
Suppose, for example, we have a resistance of 100 ohms in
series with a condenser of 4 microfarads capacity. The
impressed E. M. F. is 2,000 volts at 130 cycles per second.
What is the total impedance, the resulting current, and the
angle <^, in this case an angle of lead? Here
, 6.28 X 130 X 4 __^^
2vnk = = .003266
1,000,000
— ~ - = 306.
2vnk
Laying off the resistance A B in Fig. 62 as in Fig. 54, and
drawing to the same scale at right angles (downward to
2 vnk
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PROPERTIES OF ALTERNATING CIRCUITS.
149
emphasize its opposition to the inductance of Fig. 54), we
have for the length of the diagonal A C, which represents the
total impedance, VlO(p + 306^ = 322 ohms. The current
flowing is then, 6.21 amperes. The angle ^ is determined as
before by tan <^ "" Too "" ^'^^' whence <l> = 72°, cos <^ = .309,
so that we are dealing with a "power factor" like that pro-
duced by a heavy inductance, although the current leads the
E. M. F. instead of lagging behind it. If we consider an
inductance in series with this circuit, we should have to reckon
FlO. 82.
it upward in Fig. 62, thereby subtracting it from the former
length B C.
Suppose for example for the given inductance L = .3 heniy.
Then / = 2 Trn L = 245. If in Fig. 62 we draw 245 on the
scale already taken, upward from C, we shall reach the point
D, B D therefore is 61, and A Z), the resulting impedance.
The new
is Viocp ^ 6P = 117 ohms.
2 000
fore [^_ = 17.09, and as tan i>^
current is there-
61, <l>, = 31°.5, being still
117
an angle of lead.
It is easy to see that for a certain value of 7, the capacity
effect and inductance effect would exactly balance each other.
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150 ELECTRIC TRANSMISSION OF POWER.
This value is obviously 2 «• n L = > since then in Fig.
2irnk
62, JB C — C D = 0, and the impedance and resistance are
the same thing, while ^ becomes zero.
In actual circuits the capacity is seldom in series with the
inductance. It is usually made up of the aggregated capacity
of the line wires with air as the dielectric, the capacity of
any underground cables that may be in circuit, and finally
the capacity of the apparatus, transformers, motors, and the
like, that may be in circuit. Generally the major part of the
total inductance is in the apparatus rather than the line, and
hence in parallel with the capacity. In many cases nearly
all the inductance and capacity is due to the apparatus, and
the two may be regarded as in parallel substantially at the
S
X
_ ^
Fio. 63.
ends of the line. The inductance of generators and trans-
formers may amount to several henrys, while their capacity is
by no means small, though very variable, like the inductance.
For example, the capacity of a large high-voltage generator or
transformer may often amount to several tenths of a micro-
farad. Armored or sheathed cable has a capacity of from a
quarter to a half or more, microfarad per mile. Altogether
one may expect to find a capacity of several microfarads
frequently, and large fractions of a microfarad very often.
Suppose now we have in parallel a capacity A , Fig. 63, of 2
microfarads, and an inductance of .5 henrys, the resistance con-
nected with each being insignificant. Assuming as before 2,000
volts and 130 cycles, what is the total impedance of the com-
bination, and the resulting current? We have already seen
how impedances in parallel are to bo treated. In the case in
hand the impedance of A is — ^^ ^^^ =613 ohms,
* 6.28 X 130 X .000,002
and that of B Is 6.28 X 130 x .5 = 408 ohms. Now remem-
fel
o
A B°
a
O
C3
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PROPERTIES OF ALTERNATING CIRCUITS,
151
bering that in adding impedances their geometrical sum is to
be taken, and that joint impedance is the reciprocal of the
geometrical sum of the reciprocals of its components, we can
proceed as follows : The reciprocal of 613 is .00163. This we
will lay off to any convenient scale just as in Fig. 57. As it is
capacity-impedance, we will draw it downward for the sake of
uniformity, making A J5, Fig. 64. Now take the inductance.
The reciprocal of 408 is .00245. As the inductance and capa-
city E. M. F.'s are here as before at an angle of 180°, we must
draw this upward from B, giving us the distance B C. The
geometrical sum is then A C = .00082, of which the recip-
B
Fia. 64.
rocal gives the resultant impedance as 1,219 ohms. Hence
2 000
the net current in the line under 2,000 volts is ' =1.6
ampere. But under the same pressure the current in A would
obviously be 3.26 amperes and that in the inductance B would
be 4.90 amperes. We have then the curious phenomenon of
a total current in the line smaller than that through either of
the two impedances in circuit. It is as if A and B formed a
local circuit by themselves in which the condenser A served
as a species of generator. It is quite evident that the total
energy of the system, however, is that due to the current in
the line, so that the phases in A and B are greatly displaced.
If the resistances in the circuit were quite negligible, the net
current in the line would be indefinitely small when A = B,
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152 ELECTRIC TRANSMISSION OF POWER.
that is, when L = - • Of course, however, the true impedances
K
of both A and B are modified by the resistances, however small,
so that in Fig. 64 the impedances will always be at a small angle
with the E. M. F.'s instead of being coincident. Hence the net
current can never become zero, though when the impedances
of A and B are large compared with the resistances, the line
current will be very small when L = - •
K
This case is in sharp contrast to that in which condenser
and inductance are in series with each other. For then the
line current is increased as L approaches - instead of becom-
ing smaller relatively to the branch currents, although in each
case the same relation between capacity and inductance gives
the maximum "power factor" on the circuit, since whatever
the current, under this condition it depends most nearly on the
resistance alone. When the resistance is quite perceptible in
comparison with the impedances of A and B, we should form a
'resultant impedance with each, and then combine the two
somewhat as in Fig. 56.
If then we have an inductive load of any kind in circuit, a
condenser in parallel therewith will reduce the current on the
line and thereby increase the "power factor*' of the system.
It does this, too, without any material loss of energy and with-
out necessarily increasing the amount of current flowing through
the inductance under a given E. M. F. on the line. Were
condenser and inductance in series, the power factor could
likewise be improved up to a certain point, but trouble would
be encountered in that the condenser would necessarily have
to be large enough to let pass enough current to supply the
energy required in the inductance at full load.
In all practical cases the relations between resistance,
capacity, and inductance, which have just been set forth, are
somewhat modified by the existence of losses of energy in the
circuit quite apart from these due merely to overcoming of
resistance. Energy is required to reverse the magnetization
of the iron cores of inductance coils, and to reverse the electric
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PROPERTIES OF ALTERNATING CIRCUITS.
158
strains in the dielectric of condensers. It therefore happens
that with a condenser in circuit its apparent current is not
exactly 90° ahead of its impressed E. M. F., as shown in Fig.
59, but a trifle less, so that the current has a small component
in phase with its E. M. F., thus supplying the energy in ques-
tion. The deviation from 90° is generally but a small fraction
of a degree. The same sort of thing happens when an induc-
tance having an iron core is in circuit. However small the
resistance, the lag still misses 90° by enough to take accoimt
of the energy required for magnetic losses. The variation
from 90° in this case may amount to 30° or more. Hence
RESISTANCE
the failure to take accoimt of these energy losses in the exam-
ple given on page 141.
The result is that in adding inductance and capacity effects,
one sometimes seems not to get so simple results as in Fig. 64,
but something more like Fig. 65. Here it is clear that no com-
bination of capacity and inductance can leave the circuit free
from everything except resistance, for both the inductance and
the capacity demand energy in the circuit beyond that ex-
pended in the resistance. Evidently, however, ^ may be
reduced to zero if the relation between capacity and induc-
tance is just right. Thus while the lag may be reduced to
zero, no combination can dodge the energy losses. Whenever
all the energy losses are taken into account, the true induc-
tance and capacity E. M. F.'s will be found 180° apart and
90** from the energy, exactly where they belong.
Closely connected with this subject is the matter of reso-
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154
ELECTRIC TRANSMISSION OF POWER,
nance, which will be taken up in connection with the discus-
sion of the line. Briefly the phenomenon is this: We have
seen that the E. M. F. of a condenser is a maximum when the
current is zero, so that as the current changes sign the thrust
of the condenser E. M. F. is behind it. Now if the condenser
E. M. F. synchronizes with this current, the impressed E. M. F.
is added to it, imposes an added stress on the condenser dur-
ing the next alternation, catches therefrom an additional kick
as it passes through zero again, and so on. Thus the net
effective E. M. F. Is raised by the action of the condenser,
and would increase enormously but for its being frittered
away in overcoming resistance and supplying such energy
Fig. 66.
losses as we have just been considering. By avoiding these
losses as far as possible, one can actually raise the voltage on
an alternating circuit to twenty-five or thirty times its nominal
amoimt by employing a condenser of the proper capacity.
Even when the impressed E. M. F. and the current are not
quite in phase, one has always a component of the condenser
E. M. F. tending to act in a similar manner. Whether it
actually produces a sensible rise of voltage depends on its rela-
tions to the frequency and resistance with which it has to deal.
In fact, it is the addition of this same condenser E. M. F. to
the circuit that enables one to neutralize inductive E. M. F.
Whether or not the neutralization of inductance by capacity
produces a real resonant rise of voltage depends on the fre-
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PROPERTIES OF ALTERNATING CIRCUITS. 165
quency and whether the energy losses are small or large. If
they are small enough to let the sum of the impressed and the
condenser E. M. F. accumulate during several alternations
there will be a noticeable increase of voltage, otherwise not.
The dynamics of rcsonance may perhaps be best understood
by a very pretty mechanical analogue due to Dr. Pupin. The
apparatus on which it is based is shown in Fig. 66. It is a
torsional pendulum composed of a heavy bar A suspended by
a sHjf elastic wire B, from a light circular bearing plate C.
This plate rests in a recess a, with a frictional resistance which
can be regulated by the screw showTi in the cut. Such an
apparatus acts much like an electric circuit, having induc-
tance, capacity, and ohmic resistance. The moment of inertia
of the bar A corresponds to self-induction, the elasticity of
B to condenser capacity as we have just noted in connection
with Fig. 58, and the friction of C to the resistance. More-
over, if / is the moment of inertia of the bar A, and B the
reciprocal of the elastic capacity of the wire, then within cer-
tain values of the frictional resistance the oscillation period of
the pendulum thus formed is, in seconds,
7^ = 2^ VTb,
This correspcmds most beautifully to the time constant of an
electric circuit, which is, if the energy losses are within cer-
tain limits,
1,000
wherein L is in henrys, C the capacity in microfarads, and
the denominator comes from the units being thus chosen.
Now, if this pendulum be given a twist it will oscillate at
constant frequency imtil the friction gradually brings it to
rest with oscillations of steadily decreasing amplitude. If,
however, at the end of each complete swing it should receive a
slight push, its oscillations would continue and would increase
in amplitude up to a limit set by the frictional resistance.
The condition for such permanent increase of amplitude is
that the frequency of the pushes must coincide with the period
of the pendulum. In the electrical case, resonance thus
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156 ELECTRIC TRANSMISSION OF POWER,
occurs when the frequency and the time constant of the cir-
cuit are equal. Further, maintaining our auxiliary pushes at
their original frequency, suppose / to be decreased by taking
weight off A progressively. As the time constant of the
pendulum thus diminished a point would be found, and that
very soon, at which resonance would cease, and the same
result would follow increase of /, so that when the circuit
begins to get out of tune the resonance soon becomes rather
trivial. If, however, the pushes were supplemented by others
of 3, 5, 7, etc., times the frequency, corresponding to the
harmonics found in an ordinary alternating circuit, new points
of resonance would appear when the period of A assumed
corresponding values.
As to the magnitude of the resonant effect, in the torsional
pendulum case the amplitude evidently increases with the
strength of the pushes, their absolute frequency, which mea-
sures the energy supplied, and the moment of inertia of A^
which stores this energy. It decreases in virtue of the fric-
tional resistance. Corresponding reasoning holds in the elec-
trical case, and to a first approximation the E. M. F. in a
completely resonant circuit is
in which E is the impressed E. M. F. concerned, L the induc-
tance in henrys, R the resistance in ohms, and n the frequency.
In case of resonance with harmonics, n and E refer to the
frequency and magnitude of the harmonic implicated, and E'
becomes a resonant component of the E. M. F. wave. This
subject will be discussed more at length in Chapter XIII.
We have now glanced at the most striking characteristics
of alternating currents — those concerned with the phenomena
of inductance and capacity.
It rcmains to note very briefly some other physical proper-
ties that are of practical importance.
The most important single property of alternating current
is the ease with which it can be changed inductively from one
voltage to another. If a circuit carrying such a current is put
in inductive relation with another circuit as in Fig. 4, Chapter
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PROPERTIES OF ALTERNATING CIRCUITS. 157
I, the electro-magnetic stresses set up by the first circuit can be
utilized to produce alternating current of any desired voltage
in the second circuit. The details of the operation will be
taken up later; suffice it to say here that it is essentially the
transformation of the electro-magnetic energy due to one
circuit into electrical energy in another circuit.
Alternating currents can be regulated in amount by putting
inductance in the circuit without losing more than a very
trifling amount of energy. This very property, however, is
troublesome when an alternating current is used for magnetiz-
ing purposes. It is very difficult to get a large current to flow
around a magnet core because of the high inductance, and even
then the magnetic and other losses in the core are serious imless
great care is taken. These difficulties have stood in the way
of getting a good alternating motor until within the past few
years, and even now such motors have to be designed and con-
structed with the greatest care to avoid trouble from induc-
tance and iron losses. For some classes of work, such as teleg-
raphy and electrolytic operations, the alternating current is
ill suited save under special conditions and with special appar-
atus. For the general purposes of electrical power transmis-
sion it is singularly well fitted, from the great ease with which
transfonnations of voltage can be made, certain very valuable
properties of the modem alternating motor, and the great
simplicity and efficiency with which regulation can be efifected.
The only inconvenience attending transmission by alter-
nating current is that incurred when direct current must for
one reason, or another, be supplied. This is in a fair way to
be greatly reduced by both increasing use of alternating cur-
rent in distribution, and by improvement in apparatus for
obtaining direct current from an alternating source.
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CHAPTER V.
POWER TRANSMISSION BY ALTERNATING CURRENTS.
Broadly considered, we may say that all systems of trans-
mitting power by alternating currents are closely akin in
principles and characteristics. The growth of the art, how-
ever, has proceeded along several lines, and certain conven-
tional distinctions have come to be observed in considering
the methods employed for rendering the alternating current
applicable to the working conditions of power transmission.
Alternating systems are usually classified as either mono-
phase or polyphase. By the former term is generally imder-
stood a system generating, transmitting, and utilizing a simple
alternating current such as shown in diagram in Fig. 44. By
the latter is meant a system generating, transmitting, and
utilizing two or more such currents differing in phase and
combined in various ways. As regards the systems, this dis-
tinction is sufficiently sharp, but as regards individual parts of
such systems the line of demarcation is sometimes hazy, since
a monophase current may be the source of derived polyphase
currents, and on the other hand polyphase currents may be so
combined as to give a monophase resultant. Mixed systems
involving unsymmetrical phase relations may properly be
called heterophase.
As regards apparatus, any device that performs all its func-
tions in a normal manner when deriving all its energy from a
simple alternating current should be classified as monophase.
If its functions require the cooperation of energy received
from two or more alternating currents differing in phase,
the apparatus is essentially polyphase.
For certain purposes the one system is best adapted, for
certain other purposes the other is most advantageous, but
the underlying principles are the same, and the apparatus has
much the same general properties.
The material of alternating transmission work may be classi-
158
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TRAXS\fISSIOX BY ALTERXATIXG CURREXTS. 159
fied as follows, the transmission line itself being reserved for
discussion in a separate chapter in connection with other line
work:
I. Generators. III. Synchronous Motors.
II. Transformers. IV. Induction Motors.
In addition to these, there have been recently introduced
alternating series-wound motors with commutators which will
be discussed in their proper place.
After a tolerably careful examination of the practical prop-
erties of this apparatus in its various forms,we shall be. able
to appreciate its application to the electrical transmission of
power under various circumstances. Subsidiary apparatus of
all kinds will be referred to elsewhere, and the divers systems
that have been exploited can best be considered after we
have looked into the characteristics of their component
parts.
Alternating power transmission is now going through the
stage of development that is inseparable from the rise of a
comparatively new art — the planting time of "systems," if
one may be allowed the simile. It is sufficiently certain
alread}'' that the same sort of plant will not do equally well
under all circumstances.
The principles of the alternating current dynamo have
already been explained, but the constructional features of such
machines are sufficiently distinct from those of continuous
current dynamos to warrant examination in considerable
detail.
The modifications peculiar to alternators are in general due
to two causes ; first, the general use of a fairly high frequency,
and, second, the necessities of rather high voltage.
We have already seen that, while an ordinary continuous
current dynamo fitted with collecting rings will give alternat-
ing current, the frequency is rather low. To secure a higher
frequency it becomes necessary to increase the number of poles,
the speed, or both. Increasing the number of the poles is
the usual method employed, since continuous current dynamos
are generally for the sake of keeping up the output operated at
speeds as high as the conditions of economical use render
desirable. So we usually find that for equal outputs alternators
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160
ELECTRIC TRANSMISSION OF POWER.
have many more poles,
speed, and frequency is,
The general relation between poles,
w = - TT
2 60
where p is the number of poles, A" the revolutions per min-
ute, and n the complete cycles per second.
For example, belt-driven continuous current dynamos of 100
to 500 kilowatts usually run at speeds from 600 down to 300,
and have four or six poles, thus giving 15 to 20 cycles per sec-
Fio. 67.
ond, while modem alternators of similar size and speed have
from 12 to 24 poles, thus adapting them for a frequency of
30^ to 60 '^. Machines for the older frequencies of 120^ to
140 '^ were usually even more liberally provided with poles
unless driven at speeds considerably above those mentioned.
The general appearance and design of a typical belted alter-
nator is shown in outline in Fig. 67. This is a 150 KW gener-
ator running at 600 revolutions per minute, and shows admir-
ably the general characteristics of rather numerous poles, low
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fttAMMlSSION BY ALTERNATING CURRENTS, l6l
base, and massive bearings that nowadays belong in common
to machines by nearly all makers. Such alternators usually
have very powerful field magnets, and the projecting pole-
pieces are usually built up of iron plates Uke the armature, for
the same purpose of preventing eddy currents in the iron. The
ring of field magnets is split on the level of the centre of the
shaft, for convenience in removing the armature. The weight
Feo. 68.
of belt-driven generators of the output named is usually six
or seven tons.
This same general type is commonly adhered to whatever
the nature or voltage of the armature winding, save in the
case of special machines.
The winding of a modem alternator is nearly always widely
dififerent from continuous-current windings. In alternators the
voltage is generally from 1,000 volts up, seldom below 500 volts,
and to obtain this the windings corresponding to the numerous
poles are almost universally connected in series instead of in
parallel.
This necessitates connecting the numerous armature coils
in a very characteristic way. For when a given armature
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162 ELECTRIC TRANSMISSION OF POWER.
coil is approaching one of the north poles of the field magnet
and is generating current in a given direction, the next arma-
ture coil is necessarily approaching the neighboring south pole,
and if woimd in the same direction as the first coil would
generate a current flowing in the opposite direction. Hence
if all the armature coils are to be in series, they must be
wound alternately in opposite directions, as shown in Fig. 68.
This arrangement throws in series the E. M. F.'s generated
by all the armature coils. Sometimes for convenience the
halves of the armature are connected in parallel, thus giving
half the voltage and twice the current by a simple change in
connections. Fig. 69 shows in diagram such a winding for a
Fio. 69.
16-pole field, and its relation to the collecting rings. Note
that each half of the winding preserves the characteristics
shown in Fig. 68.
In practical machines as built to-day, the armature coils are
nearly always bedded in slots in the armature core. The
early American machines were generally built with smooth
armature cores, and upon these flat coils were laid and held in
place by an elaborate system of binding wires. This construc-
tion has been virtually abandoned by all the principal manu-
facturers in favor of the so-called "iron-clad" armature, which
has the double advantage of great mechanical solidity and of
permitting the armature coils to be wound in forms thoroughly
insulated, and then dropped into place in their slots and
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TRANSMISSION^ BY ALTERNATING CURRENTS. 163
firmly wedged in position. The winding is, therefore, very
little liable to damage and easily replaced if necessary.
The slotted armature cores are variously arranged in dif-
ferent macnhies, but always with the same object in view.
Fig. 70 shows one widely used arrangement of slots. Here
the coils are wound in forms and thoroughly insulated. They
are then pushed into place in the previously insulated slot,
each coil enclosing a single armature tooth. When firmly in
FiQ. 70.
place the insulating material is put into position above them
and a hard-wood wedge is driven into the dove-tailed upper
portion of the slot, holding the coils and their surrounding
insulation permanently in place. The coils here shown con-
sist of only four turns of heavy wire. Often there are many
more turns per coil, and frequently the round wire is replaced
by rectangular bars. In generators for use with raising trans-
FiQ. 71.
formers each coil sometimes consists of a single turn of bar
copper, but whatever the nature of the coil the slots are
. arranged much as here shown.
Another familiar form of slotted armature is shown in
Fig. 71. The coils are, as in the case just mentioned, woimd
in forms and solidly insulated. They are then sprung over
the armature teeth into place and tightly wedged. The
slots are carefully insulated also, and by the time the winding
is completely assembled it is so thoroughly insulated that
repairs are few and far between. The special peculiarity of
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164 ELECTRIC TRANSMISSION OF POWER.
this form of core is that the outer comers of the teeth are cut
away, so that the coils come more gradually into the field of
the pole-pieces than if the edges were sharp. The object of
this device is to obtain a curve of E. M. F. more nearly accord-
ing with the sine wave form, and experience shows that the
plan works successfully. Without such precautions the E. M. F.
curve is very likely to be quite irregular, and even with
them it is generally none too smooth. The pole-pieces of
alternators are very often similarly rounded off or chamfered
away for the same purpose.
Nearly all modem alternating windings are like those just
indicated, of the drum type. The Gramme winding is seldom
or never employed, as it is hard to wind and repair and has,
for alternators, no compensating advantages. Nor has the
flat coil \\inding without iron core found a permanent place in
American practice, although it is somewhat used abroad.
There is considerable likelihood of eddy currents in the arma-
ture conductors of such machines unless they are indi\'idually
very thin, and for this and obvious mechanical reasons Ameri-
can designers have adhered to the iron-clad armature, which
is admirable mechanically and magnetically, and have taken
other means to escape the difficulty of its high ijiductance.
As in other dynamos, the theoretical E. M. F. generated by
an alternator depends on the strength of the magnetic field,
the number of armature conductors under induction, and
the speed at which they are driven through the field. As an
altcmator receives load the p]. M. F. at its terminals is reduced
by three several causes.
First, there is a loss of voltage due to energy lost in the
armature conductors. This depends simply on the current
and resistance and is numerically equal to C R.
Second, there is self-induction in the armature windings, '
which, as we have already seen, involves an inductive E. M. F.,
lagging 90® behind the impressed E. M. F. The effect of
this is to partly neutralize the impressed E. ^f. F., as in all
cases of inductance. The amount of this disturbance depends
on the frequency and the magnetic relation of the armature
coils to each other and to the field magnets. This relation
of course varies according to the relative position of the arma-
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TRANSMISSION BY ALTERNATING CURRENTS. 166
ture teeth which carry the coils. In Fig. 72, purposely shown
with somewhat exaggerated teeth, the armature is in the
position of minimum inductance, for the magnetic field set
up by the armature coils is not here much strengthened by
the presence of the pole-pieces. If, however, the armature
were shifted forward or backward so that each tooth would
be just opposite a pole-piece, the field from the armature
coils would traverse an almost complete loop of iron and the
inductance of the armature would be a maximum. In this
position the armature teeth might be almost as good magnet
poles as the field poles themselves; at all events, consecutive
Fio. 72.
teeth would be united by an almost continuous iron core, and
the armature inductance would be very high.
One of the best ways of reducing this inductance and its
train of troubles is to make the magnetization due to the field
magnets as strong as is practicable. This not only utilizes the
iron of the field magnets and armature to the best advantage,
but, so to speak, preempts its power of receiving magnetiza-
tion so that the current about the armature teeth finds a poor
field for its inductive operations. In addition, this strengthen-
ing of the field enables the required E. M. F. to be obtained
with fewer turns per tooth. This of itself is a great advan-
tage, since increasing the number of turns in an iron-cored
coil runs up the inductance with appalling rapidity. A glance
at Fig. 73 will show the reason why. Suppose we have a
looped iron core wound with four turns of vire, a, 6, c, d. If
we pass a certain alternating current around two turns, a and
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166
ELECTRIC TRANSMISSION OF POWER.
hj we shall have a certain inductance due to the reaction of
the change in magnetism on these two coils. Now, pass the
same current around all four coils. The magnetization will
be approximately doubled and the number of turns on which
it acts will also be doubled. That is, each coil is acted upon
by double the force and there are twice as many total coils.
Hence, the total inductance will be about four times as great
as at first, and in general it will increase with the square of
the number of turns. If, however, as just suggested, the
core is nearly saturated already, adding the two extra turns,
c and d, will not anywhere nearly double the magnetization,
Fig. 73.
since iron already magnetized responds less and less to addi-
tional magnetizing force as this force increases.
Hence, diminishing the number of armature turns that can
act conjointly in producing effective magnetization lowers the
inductance very rapidly.
The third disturbing cause which tends to reduce the
effective E. M. F. of an alternator is the reaction of the
armature current, through the resulting magnetization, on
the field magnets. We have already seen that when a closed
coil is driven into and out of a magnetic field the induced
current is always in such direction as to cause work to be
done in driving the coil. But, since the current due to enter-
ing the field is equal and opposite to that produced in leaving
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TRANSMISSION BY ALTERNATING CURRENTS. 167
the field, the total mafi^etizations due to these currents are
equal and opposite, and if one opposes the field due to a pole-
piece the other will in an equal degree strengthen that field.
Hence, provided these two actions are applied alike; i.e.,
are symmetrical with respect to the field, the total effect of
armature current will be neither to weaken nor strengthen
the field.
In practice the effect of the armature reaction is two-fold.
If the current be nearly in phase with the E. M. F. the main
result of the magnetic field set up by the armature is to
Fig. 74.
distort that due to the field without greatly weakening it as
a whole. The result of this distortion is that the E. M. F.
does not increase and decrease steadily following a sine wave,
but becomes irregular. The working E. M. F., as measured
on a voltmeter, changes but a trifle, but the maximum
E. M. F. becomes subject to great variations. Fig. 74 shows
in a very striking manner the result of field distortion from
a purely non-inductive load. Here a is the E. M. F. curve
on open circuit and b is the curve as modified by the armature
reaction at nearly full load. The arrow shows the direction
of rotation of the armature. In this case the maximum
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168 ELECTRIC TRANSMISSION OF POWER.
voltage was jncreased about 30 per cent, while the measured
voltage was nearly constant. Bearing in mind that the
E. M. F. at any moment is due to the rate of change of the
magnetic induction through the armature, and not to the
absolute amount of that induction, it is tolerably obvious that
the effect of field distortion due to armature reaction may
vary widely according to the shape and position of both
the pole-pieces and the armature teeth. It may increase the
maximum voltage as above, or decrease it fully as much, but
if it is of any considerable magnitude it always deforms the
E. M. F. wave very materially.
If, however, through armature inductance or inductive load
the current lags behind the E. M. F., we have a very different
state of affairs. The current reaches its maximum after the
armature coil has passed beyond the position of maximum
E. M. F., and the net magnetization produced by it chokes
back the field, at the same time greatly distorting it.
If the only effect of armature reaction and inductance were
to cause a loss of voltage there would be little cause for alarm.
But as shown in Fig. 74, the E. M. F. wave-shape often un-
dergoes profound changes, which may greatly increase the
chance for serious resonance. As already noted, alternating
generators, monophase and polyphase alike, give in practice an
E. M. F. wave which is not sinusoidal, but contains the odd
harmonics of the fundamental frequency. These are a neces-
sary result of the variations in magnetic reluctance and arma-
ture reactance when the armature is in various angular
positions, as well as of subsidiary reactions in transformers
and other apparatus. The harmonics of even order do not
appear, since, unless a machine is deliberately made unsym-
metrical, all the variations in E. M. F. are complete within
each half period, the second half of the cycle merely showing
a reversal of sign. Hence, only those harmonics appear which
are themselves symmetrical with respect to a half period of
the fundamental, i.e., by construction all the harmonics are
of odd order. These harmonics have a very real existence,
and can readily be identified by testing electrically for reso-
nance, or even by hunting for them with a telephone in some
cases. By taking the wave form of the machine by the con-
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TRANSMISSION BY ALTERNATING CURRENTS. 169
tact method or photographically, the nature and magnitude of
the harmonics are at once made evident.
Fig. 75 shows the wave form of a machine that was carefully
studied by Steinmetz. It is from a three-phase generator hav-
ing but one armature tooth per phase per pole, and giving 150
KW at 2,000 volte and 60 -^. Curve A is the E. M. F. wave of
one coil to the common connection, at no load, B is the wave
as calculated from a summation of the harmonics up to the
fifteenth, and C shows the residual traces of still higher har-
monics. To reduce the vertical scale to primary volte, mul-
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FlQ. 76.
tiply by 10. Analysis of this wave showed that it corre-
sponded approximately to the following equation :
Sin a - .12 sin (3a - 2.3) - .23 sin (5a - 1.5)
+ .134 sin (7a - 6.2).
In other words the third harmonic has about 12 per cent, the
fifth about 23 per cent, and the seventh about 13 per cent of
the amplitude of the fundamental.
At full load the shape of this wave is changed in a most sin-
gular manner. The armature reaction shifte the magnitudes
and positions of the variations in the magnetic field and of the
harmonics due to them. Fig. 76 shows the wave form from
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170
ELECTRIC TRANSMISSION OF POWER.
this machine under load. The central depression of Fig. 75
is replaced by a slight hollow between a high peak and a shoul-
der, and the wave is conspicuously unsymmetrical, as might
readily be predicted from the general effect of the armature
reaction. The approximate equation to the wave of Fig. 76 is
Sin a - .176 sin (3a + 11.7) - .085 sin (5a - 33.8)
+ .01 sin (7a -f 26.6).
The effect of the armature reaction due to load has been
greatly to strengthen the third harmonic, greatly to weaken
the fifth, and nearly to suppress the seventh.
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Fia. 76.
Obviously changes of this sort may have a very great effect
in the matter of resonance. Suppose, for example, that the
conditions on the line at light load were such as to give
marked resonance with the seventh harmonic of the frequency.
Now, imder all ordinary working conditions this harmonic
would be practically absent; but if a large part of the load were
thrown off, resonance would suddenly appear, and with the
lessened armature reaction the general voltage would rise
sharply, so that serious results might follow. In case of a high
voltage generator, say for 10,000 volts, having the curves just
given, at load the seventh harmonic would only have an ampli-
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TRANSMISSION BY ALTERNATING CURRENTS. 171
tude of about 100 volts, while this amplitude would suddenly
rise to 1,340 volts, increased perhaps four or five times by
resonance, when the load was thrown off. Under other con-
ditions throwing on load might produce an equally unpleasant
effect.
Lest it should be suppi^sed that these wave distortions with
the presence of strong high harmonics are extraordinary and
Fio. 77.
of merely theoretical importance, examples of such action
from recent machines of first class make, obtained in commer-
cial service, are here given. Fig. 77 shows the E. M. F. curve
from a 750 KW, 5,500 volt engine-driven three-phaser, dis-
torted by the presence of a strong thirteenth harmonic. The
generator had a monodontal winding which is prone to give
lower harmonics, but these higher ones were mainly due to
Fio. 78.
the disturbing effect of a synchronous motor load. Fig. 78
shows E. M. F. and current waves from a 1,500 KW three-
phase turbo-generator, also on a synchronous motor load, and
displaying conspicuous harmonics of the twenty-third order,
in this case not traceable to the nature of the load, but structural
and merely aggravated by the running conditions. The gen-
erator had four slots per phase per pole in this case, but the
magnetic density in the teeth was rather low. Under ordinary
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172 ELECTRIC TRANSMISSION OF POWER.
circuiTistances these harmonics are probablj'^ quite harmless,
but their frequency is so great that they might easily cause
serious results with but a very moderate amount of capacity in
the system. The curves shown are from oscillograph curves,
reported by Dr. W. M. Thornton to the British Institute
Electrical P^ngineers, and are quite sufficient to prove the im-
portance of the subject.
Such eccentricities can be avoided by scrupulous care in
design, at least for the most part, and should be eliminated
from every machine used upon circuits where by reason of
unusual length, or the presence of cables, there is danger of
resonant effects. At the voltages now generally used for power
transmission, insulation is difficult enough without incurring
the risks that come from preventable dangers of this sort.
The magnetizing and demagnetizing effects of the arma-
ture current in case of inductive load no longer can balance
each other, for they are unsymmetrical with respect to the
poles. If the angle of lag is large the result will be a very
serious weakening of the field, and a correspondingly large
drop in the effective voltage. For example, a certain alter-
nator of 120 KW output has 40 turns of wire per armature
tooth, carrying a normal full load current of 60 amperes.
There is thus a possible demagnetizing force of 2,400 ampere-
turns at full load. The ampere-turns per pole-piece in the
same machine are 3,600, so that if the current should lag
enough to give the armature reaction full play, as might
happen from excessive armature inductance alone, the total
net magnetizing force would be reduced to a third of its nor-
mal amount and the resulting voltage to a half or less. It
is in fact common enough to find alternators that require
from 50 to 100 per cent increase in the exciting ampere-turns
to hold them at normal voltage under a full-load current
lagging even 15° or 20°.
Between inductance and armature reaction the effective
E. M. F. of alternators generally falls off rapidly imder load
unless special care be taken with the design. The loss from
ohmic resistance is usually trivial compared with those just
named. It is, in fact, perfectly practicable to build an alterna-
tor with inductance and armature reaction so exaggerated,
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TRANSMISSION BY ALTERNATING CURRENTS. 173
that a very slight increase in current will cut down the voltage
so rapidly as to keep the current virtually constant. This plan
was successfully carried out in the remarkable Stanley alter-
nating arc machine of a few years ago.
In this case the current varied only about 10 per cent, while
the voltage varied between a few volts and over 2,000. An
automatic short-circuiting switch was provided to avert dan-
gerous rise of voltage in case of an accidental open circuit.
In so-called constant potential alternators, as usually built,
the inherent regulation is by no means good. Fig. 79 gives
an excellent idea of the performance of some of the earlier
machines in this respect, and it is about what one would find
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Fig. 79.
in many alternators now in service, except for their compound
winding.
It has often been held that high inductance and large arma-
ture reaction are desirable in alternators in order to prevent
bum-outs in case of accidental short circuits. While it is per-
fectly true that sufficiently crude armature design does produce
this effect, by limiting the possible current, it is equally true
that a machine with sufficient inductance and reaction to serve
as a practical safeguard will regulate so atrociously as to be
imder many circumstances incapable of decent commercial
service under present conditions. When it was sufficient for
an alternator to give current that with sufficient hand regula-
tion could supply house to house transformers most of the
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174
ELECTRIC TRANSMISSION OF POWER.
time, high inductance machines, which are easy and cheap to
build, answered the purpose.
At present, when the importance of good regulation is gener-
ally understood, and most large alternating plants must look
forward to assuming a motor load, low inductance machines
with small armature reaction are essential for first-class service.
For power transmission plants with heavy mixed loads of
lights and motors, no other class of machine should be toler-
ated, or can be used without incessant annoyance.
Most even of the older alternaLors are compound-wound
to compensate for armature effects, and are thus enabled to
work successfully up to outputs at which the voltage begins to
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Fia. 80.
fall off too fast to be thus compensated. So long as the com-
pounding process actually gives good regulation, it is useful
and enables the generators to be worked at a high output. As
a matter of fact when used with generators of the older type,
even compounding left much to be desired. As alternating
practice has gradually improved, compound-wound alternators
have been more skillfully designed, and recent machines give
on non-inductive load a very fair approximation to constant
potential. Fig. 80 shows the E. M. F. of a modem over-com-
pounded alternator at varying load. If, however, the current
has even a moderate lag behind the E. M. F.. owing to induc-
tance in the machine or the load, the machine will no longer give
constant potential, and the voltage may fall off rapidly as the
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TRANSMISSION BY ALTERNATING CURRENTS. 175
load comes on, as shown in the cut. The reason for this we
have already found in the extra increase of field excitation
necessary to compensate for the demagnetizing effect of arma-
ture reaction. Incidentally if the current commuted to supply
the series field lags much, the process of commutation cannot
Fig. 81.
go on normally without adjusting the brushes to compensate
for the lag.
Therefore, for inductive load the compounding has to be
greatly increased, and even then is correct only for a particu-
lar inductance.
It must be understood that alternators are compounded on
the same general principles as continuous current machines,
except that instead of the current for the series winding being
derived from the general commutator of the dynamo, it is
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176
ELECTRIC TkANSMlJSSlON OF POWER.
generally obtained from a simple special commutator. A
shunt around this commutator diverts most of the main cur-
rent, while a portion is rectified and passed around the fields.
Fig. 81 shows in diagram a common compounding arrange-
ment. The two collecting rings A and B with the commutator
C are moimted on the armature shaft. Brushes on A and B
take off the alternating current. One of these rings, A, leads
directly to line. The current going to the other ring is divided,
part passing aroimd C through the resistance box D, and part
being rectified by the commutator for use in the series field.
This commutator has as many segments as there are pairs of
poles in the field, the alternate sections being electrically, united.
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Fig. 82.
By varying the resistance D, the amount of current diverted
into the field can be varied, and the compounding may thus be
arranged to keep the voltage constant at the terminals or at
any point on the line. A similar change in D may be made
to adjust the compounding for inductive load of any given
power factor.
For non-inductive loads, or for inductive loads of constant
power factor, this compounding gives good results, but for a
load of widely varying power factor it is nearly worthless
unless supplemented by hand regulation.
If compounding is to be successfully used for keeping con-
stant potential on a circuit of lights and motors subject to
considerable variations in the power factor, it must be applied
to a generator of very low inductance and armature reactioa.
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TRANSMISSION BY ALTERNATING CURR£!NTS. 177
Otherwise no adjustment of the compounding for any particu-
lar power factor will give approximately constant potential
when the power factor varies.
For example it would be hopeless to attempt to compound
in the ordinary way an alternator having a characteristic
like Fig. 79, so that it would be tolerable on a commercial cir-
cuit of lights and motors. On the other hand, a generator
having a voltage characteristic like Fig. 82 could readily be
so compounded. Here the fall in voltage at constant field
excitation, from no load to full load (non-inductive), is about
3J per cent. Under inductive load this fall would be in-
creased considerably, but from the usual ratio of inductive
drop to armature reaction found in the best modern gener-
ators, the variation for the power factors likely to be encoun-
Fio. 83.
tered with a mixed load would be somewhat smaller than the
original drop. The total variation . from no load to full in-
ductive load would then be between 6 and 7 per cent, and
with compounding adroitly adjusted for average conditions
the greatest variation from normal voltage could easily be
brought within 2 per cent. A little intelligent hand regu-
lation at certain times of the day would improve even this
good result.
These considerations apply to polyphase as well as to mono-
phase generators. The advent of polyphase work has done
much to improve all alternators, and especially with respect to
regulation.
The generation of polyphase alternating currents is a very
simple matter. The object in view is the production of two or
more similar currents differing in phase by some convenient
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178 ELECTRIC TRANSMISSION OF POWER.
amount, usually 60° or 90°. To obtain two currents 90° apart
in phase, it is only necessary to clamp together the shafts of
two common alternators, so that, for a construction like Fig.
70, the slots of one armature would be opposite the teeth of
the other armature. The armatures would then give currents
90° apart in phase. Sucl^ combination alternators were built
for the Columbian Exposition by the Westinghouse Company,
and were used for the principal lighting and power circuits.
These structures arc, however, expensive for the output obtained,
and the two windings arc nearly always put on a single arma-
ture core, and spaced as just described. Fig. 83 shows dia-
grammatically a winding of this nature. There are. four times
Fig. 8*.
as many armature slots as there are field poles. Each coil
spans two teeth. The coils shown by solid lines form one phase
winding, the dotted coils the other phase winding. Each set
of coils is connected as an ordinary monophase winding, and
the terminals are brought out to two pairs of collecting rings.
Such a winding gives two simple alternating currents related
in phase as shown in Fig. 84. The armature core is very fully
occupied by the two windings, rather more advantageously
than it could be by a single winding, so that the machine gives
a somewhat better output as a two-phaser than would be
possible with a simple alternator of the same dimensions.
And, what is of more importance, the regulation of the machine
as a two-phaser is much better than it would be as a single-
phaser. In the first place the armature inductance is greatly
reduced by the distribution of the windings and the reduction
of the ampere-turns per armature tooth. Second, the same
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TRANSMISSION BY ALTERNATING CURRENTS. 179
causes act to cut down the armature reaction in case of a lagg-
ing current. Anything that improves the intrinsic regu-
lation also means greater output for unimproved regulation.
Moreover, the increased number of armature teeth gives a
more imiform reluctance than in the case of fewer teeth, and
hence tends to give a better approximation to a sinusoidal
wave form.
So, aside from the value of polyphase currents for motor pur-
FlG. 8S.
poses, which we shall presently examine, polyphase winding is
valuable on its own accoiuit as increasing output and improv-
ing regulation. In fact, diphase wandings were devised for this
purpose before their importance in the operation of motors
became generally known.
The value of a subdivided winding in reducing inductance
and armature reaction was greatly emphasized by the intro-
duction of polyphase generators, and it was a short step from
monodonotal windings having one coil and virtually one tooth
per phase per pole, to windings in which each phase winding is
split up into several sets of coils in adjacent slots, thereby
still further decreasing the effective inductance and armature
reaction. Such windings may be called polyodontaly from their
several teeth per phase per pole, and are very generally used in
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180 ELECTRIC TRANSMISSION OF POWER.
the best recent machines. A fine example of this class of
winding is shown in Fig. 85. This is a quarter section of the
armature of one of the original 5,000 HP Niagara generators,
showing a portion of one coil belonging to a single phase.
The full winding is composed of two conductors per slot, half
the total slots, in alternate groups, belonging to each phase.
Such complete subdivision of the coils results in low induc-
tance and a very low armature reaction. A similar winding
could be used for a monophase generator, and will have to be
employed if monophase machines come to be used extensively
for power transmission purposes. The form of armature slot
used for polyodontal windings is shown in Fig. 86, a single
segment of one of the core plates of the armature of the Niagara
two-phaser. The appearance of one of these great machines
Fio. 86.
complete is admirably shown in the frontispiece, showing the
interior of the Niagara station. The field magnets are re-
volved instead of the armature, although they are exterior to
it. A very pow^erful fly-wheel effect is gained by this arrange-
ment, since the weight of the revolving structure, turning at
250 r. p. m., is about 75 tons, half of this being in the field
itself. This is about 12 feet in diameter, a single forged steel
ring with twelve massive pole-pieces secured to its inner face.
The normal voltage of the machine is about 2,250, and the
frequency is 25^. The stationary armature is provided with
six ample ventilating ducts, through which air is forced by
the revolving field. Fig. 87 shows a vertical section of the
whole apparatus with its shaft and upper bearings. A hun-
dred and forty feet below the generator is the turbine which sup-
ports by hydraulic pressure the weight of the revolving mass,
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TRANSMISSION BY ALTERNATING CURRENTS. 181
save a ton or two of residual weight, which may be either posi-
tive or negative, and which is taken care of by a thrust bearing.
The full load of this generator is 775 amperes on each of the
two circuits, and at this load the commercial efficiency is
nearly 97 per cent — a figure very close to the possible max-
imum. The exciting current for the fields is derived from a
rotary transfonner, and is led into the revolving magnets
through a pair of collecting rings shown in Fig. 87 at the
Fio. 87.
extreme top of the shaft. The armature current is of course
taken from stationary binding posts. Altogether this Niagara
machine was a fine specimen of polyphase construction.
When three-phase currents instead of two-phase are to be
generated, separate armatures are out of the question, and a
winding similar to that of Fig. 83 is frequently employed. To
obtain the three currents, however, three separate windings
are employed, arranged as in Fig. 88. The coils are connected
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182
ELECTRIC TRANSMISSION OF POWER.
SO that a, a, a, etc., form one phase winding, 6, 6, etc., a second,
and Cy c, etc., the third. The close similarity of this winding
to the two-phase shown in Fig. 83 is at once apparent.
It is worth noting that these three windings are spaced 60®
apart, instead of 90°, as in a winding for two phases. Natur-
ally, therefore, the currents generated would be different in
Fio. 88.
phase by only 60®, giving the arrangement of currents shown
in Fig. 89. This is homologous with the two-phase current
system of Fig. 84.
In practice it is necessary, however, to have the sym-
metrical arrangement of phases given by three similar cur-
Fio. 89.
rents 120° apart. This is very easily obtained in the external
circuit by winding one set of the armature coils in a direc-
tion reversed from the other two, or by merely reversing the
termmals in making connections. The result of this is a true
three-phase current, such as is shown in diagram in Fig. 90.
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TRANSMISSION BY ALTERNATING CURRENTS. 183
It has now the curious property that at all times the system is
simultaneously carrying currents substantially equal in both
directions, as will readily appear from inspection of the curves.
With such a current it is usual to combine the circuits cor-
responding to the several armature windings. Otherwise we
would be compelled to deal with circuits of six wires, and the
generator would have six collecting rings.
Moreover, the distribution circuits formed by combining
the circuits as just indicated have the advantage of economy
in copper, as we shall presently see. Hence, the three-phase
system has become the mainstay of electrical power transmis-
sion so far as the principal circuit is concerned. The genera-
Fio. 1)0.
tors may be two-phase and the distributing circuits two-phase
when convenience dictates, but the main line is, save in very
rare instances, worked three-phase. The change from two-
phase to three-phase, or the reverse, is accomplished in a beau-
tifully simple and efficient manner, to be described later.
Under certain circumstances the use of a two-phase generator
has at least the theoretical advantage that the currents in
the respective armature windings, being in quadrature, can
have little or no mutual reaction, so that the two phases are
more independent than the three phases of a three-phaser.
As might be expected, the subdivision of windings in a three-
phase armature results in small inductance and armature reac-
tion, smaller in fact than would be found in a similar two-phase
winding. Nevertheless, experience shows that if the annature
has only a single coil per phase per pole, the reaction is too
great for first-class regulation, and the curve of E. M. F. is
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184
ELECTRIC TRANSMISSION OF POWER,
rather too vAAq a departure from the sine wave. It is quite
usual, therefore, to adopt the polyodontal construction with
from two to four coils per phase per pole. A machine carefully
designed on these lines can be made to give excellent regulation,
with voltage not varying more than 3 or 4 per cent from no
load to full non-inductive load, and is capable of giving a very
close approximation to a true sinusoidal wave, a valuable
characteristic for longidistance transmission. Fig. 91 shows
the wave form giv^n by one of these polyodontal three-phasers.
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The full curve shows the actual E. M. F., the dotted line the
corresponduig sine curve, and the irregular line at the base of
the figure the difference between the two.
There are several methods of connecting a three-phase wind-
ing to its external circuit. The two chiefly used are generally
known as the "star'' and "mesh'' connections. In the former,
one end of each of the three windings is brought to a common
jimction, and the three remaining ends are connected to three
line wires. The three Unes then serve in turn as outgoing and
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TRANSMISSION BY ALTERNATING CURRENTS. 185
return circuits, the maximum current shifting in regiilar rota-
tion from one to the others in succession. The three E. M. F/s
in the three coils differ in phase by 120°, owing to the reversal
of which we have spoken. We may draw the star connection
diagrammatically in Fig. 92, drawing the three coils ah c 120®
apart to show the relation of the E. M. F.'s and currents,
A
Fig. 92.
although they lie on the armature as shown in Fig. 88. Three
of the terminals meet at the point o, the others are connected
respectively to the lines A, B^ C. As the three windings on the
armature are alike, the E. M. F.'s generated by the three coils
are equal. So if each winding a, 6, c, is designed for 1,000
volts, that will be the voltage between the point o and each of
the three lines A, B, C, Clearly, however, the voltage between
Fio. 93.
any two of these lines, as A and B, is a very different matter,
since it results from the addition of the voltages of a and 6,
which are, however, 120° apart in phase. They must then be
added geometrically. Now the chord of 120° is V3 times the
radius, so that the geometrical sum of the voltages a and 6,
120° apart, is 1.732 times either of them. The voltages then
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186 ELECTRIC TRANSMISSION OF POWER.
between A and B in the case in hand aggregate 1,732. The
same is evidently true of the other pairs of lines B, C, and C, A,
The other ordinary three-phase connection is the mesh, in
which the six terminals of the three coils are united two and
two, and the lines are connected to the three points of junc-
tion. This arrangement is shown diagrammatically in Fig. 93.
Here each coil must generate the full E. M. F. between any
two of the lines, but the current in any line, as B, is made up of
the geometrical sum of the currents in a and 6, differing in
phase, just as the E. M. F. between lines in Fig. 92 was made
up of the sum of two E. M. F.'s. The current inB being then
so constituted, is V3 times the current in a or 6, and so on for
the other lines. In the mesh connection we deal with resul-
tant currents just as in the star we find resultant E. M. F.'s.
An armature designed for a given working voltage, measured
in the ordinary way between lines, would, if planned for star
connection, have fewer turns of larger wire than if intended for
mesh connection. This is sometimes convenient, and is useful
in keeping the voltage between coils low. The mesh connec-
tion on the other hand has more turns of smaller wire, as the
current is diminished while the E. M. F. in each coil is the full
E. M. F. between lines. This property is useful under certain
conditions, as it makes the E. M. F. between any two lines
somewhat less dependent on the actions going on in the other
pairs of lines. The same windings can of course be connected
either star or mesh, according to the dictates of convenience.
Both these combination circuits have in common one immensely
valuable property. They require for the transmission of a
given amount of energy at a given percentage of loss, only
75 per cent of the weight of copper reqyired for the same trans-
mission at the same working voltage, by continuous current
or by any alternating system having two wires per phase.
That is, if 100 tons of copper are required for a given transmis-
sion by continuous current, single-phase alternating, two-
phase with two circuits, or three-phase with three circuits,
75 tons will suffice for the same transmission by the star or
mesh three-phase circuit without any increased loss of energy.
The proof of this saving is very simple. Assume a three-phase
circuit carrying a non-inductive load at V volts between lines,
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TRANSMISSIOX BY ALTERNATING CURRENTS. 187
the current in each line being / and the resistance r. Then for
a star connection, as we have already seen, the voltage in
each branch to the neutral point o (Fig. 92) is V — --r, the cur-
V3
rent in each branch is /, the power in each branch is— ^,- IV,
V3
and the total power is IV ^S.
The loss in each branch of the circuit is obviously Pr, and
the total loss for the above power 3/V. Now let the same
amount of power be transmitted by a single-phase circuit at
the same voltage V, The current will evidently have to be
/ v3. Let r' be the resistance of one of the two monophase
wires, such that the total loss shall be 3Pr as before. The
resistance of the complete circuit will be 2-/, and the total loss
6P/. But since
6Pr' = 3Pr,
2
That is, the resistance of each of the monophase wires must be
only one-half the resistance of a single three-phase wire. The
cross section of each monophase wire must then be double the
cross section of one three-phase wire. If the weight of the lat-
ter be w, the total weight of the three-phase copper will be
SWy while the weight of th^ two monophase leads of double
cross section will evidently be 4w for a circuit of the same
length. A mesh connected three-phase system leads to ex-
actly the same result, since tlie voltage in each branch is V
(see Fig. 93), the current is — — , the power per branch — r/F,
V3 V3
the total power IV V3, and the loss 3Pr, as before.
The result seems so singular that in the early days of the
three-phase system it was slow to be accepted by the public,
until checked experimentally wdth the greatest precision, and
by various experimenters. A similar saving can be effected
by the use of some other polyphase combination circuits, but
it happens that the three-phase combination is the one least
open to practical objections.
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188 ELECTRIC TRANSMISSION OF POWER.
In actual working the two-phase system is nearly always
installed with a complete circuit per j)hase as regards the dis-
tribution circuits, unless for short connections to apparatus;
the three-phase system is used with the star or mesh combina-
tion, except for occasional special work, and the more compli-
cated polyphase systems are practically not used at all, save
that in working rotary converters, the final connection is some-
times with four, six or even more phases to take better advan-
tage of the armature winding.
In speaking of the voltage of an alternating circuit, it must
be borne in mind that we do not mean the voltage correspond-
ing to the extreme crest of the E. M. F. wave, but that vol-
tage which, multiplied by the current in a non-inductive circuit,
equals the energy in that circuit. This effective working vol-
tage bears no fixed relation to the real maximum voltage, since
Fig. M.
their ratio evidently varies with the shape of the E. M. F. wave.
For a sine wave the ratio is 1.414, so that an alternating working
pressure of 1,000 volts means a -maximum voltage of 1,414.
As may be judged from Fig. 91, this ratio is very nearly true
for the best modem alternators.
Save in rare instances the work of power transmission is
done by two-phase or three-phase currents. Abroad some pure
single-phase plants are in operation with fairly good results,
but the difficulty of getting good smgle-phase motors has so far
rather checked development along this line.
In this comitr}', a decade since, the so called "monocyclic"
system, now obsolete, was introduced in a few plants where the
motor load was merely incidental to lighting.
In this system there was a main armature winding to which
the lighting circuit was connected as in ordinary single-phase
working, while a subsidiary armature winding furnished mag-
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TRANSMISSION BY ALTERNATING CURRENTS. 189
netizijig current for the motors. The general arrangement
of the armature coils is shown in Fig. 94. The winding in
the small intermediate slots was of the same size of wire as
the main coil, but had only one-fourth as many turns, and
consequently one-quarter the main E. M. F. This so-called
"teaser" E. M. F. was obviously 90^ in phase from the main
E. M. F. The relation of the two E. M. F.'s is better showTi
in Fig. 95, where A B C is the main E. M. F. and B D the teaser .
E. M, F. The generator had three collecting rings, of which
the middle one was connected to D. The outer rings had the
full E. M. F. between them, while between D and C the E. M. F.
was the geometrical sum oi B C and B D, approximately
.56 of the main E. M. F. For niotor service the resultant
E. M. F.^s differing in phase were variously combined, usually
into approximately three-phase relation, although in normal
JT =^B T:
Fia. 95.
rimning all the currents in the motor remained in very nearly
the same phase. The object of this system was to obtain for
lighting purposes a perfectly simple circuit, the voltage of
which should be quite undisturbed by actions going on in the
subsidiary motor circuit, which object was attained if the
generator was so arranged as to hold its voltage closely under
inductive load.
A similar device for simplifying the operation of lighting
circuits is a three-phase system arranged to supply the entire
lighting service from two of it.s lines, as A and B, Fig. 92.
The other two connections B C and A C would only be used
for motor service, and if desirable the coils h and c could take
up very little space on the armature. Still another of these
heterophase schemes employs regular single-phase alternators
for the lighting work, and a small adjunct machine in phase 90°
from the others, and connected with them to form a two-phase
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190
ELECTRIC TRANSMISSION OF POWER,
circuit with one common wire. This connection is used for
starting ordinary'' two-phase motors.
In general, the heterophase systems have no substantial
advantage over the ordinary polyphase systems, and are rarely
employed. In the chapter on centres of distribution, the
working properties of various altematii>g systems wdll be
taken up in more detail.
In general construction and arrangement of parts all alter-
nators are similar. Those specially intended for power trans-
Fio. %.
mission are sometimes, however, modified for convenience in
obtaining high voltage or for direct coupling to water wheels.
The vertical shaft arrangement as exemplified in the original
Niagara machines is now and then used both in this country and
abroad. Machines for 3,000 to 5,000 volts and upward are
best constructed with stationary armatures, to avoid mechan-
ical strains on the high voltage insulation. In following this
design the armature is usually exterior to the field magnets
as it is indeed in the later generators of the great Niagara plant.
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TRANSMISSION BY ALTERNATING CURRENTS. 191
It is very doubtful whether the fly-wheel effect gained by
revolving an exterior magnet compensates for the great inac-
cessibility of the high voltage armature.
A characteristic example of the revolving field alternator is
shown in section in Fig. 96.* It is a large polyphase generator
for direct connection to a water-wheel, and the cut gives a good
idea of its mechanical arrangement. The stationary armature
is assembled on the interior of a supporting circular box girder
cast in upper and lower halves. In the fourth quadrant of the
cut this is seen in section, bearing dovetailed projections for
supporting the laminae of armature iron. These are curved
segments as shown in the third quadrant, twelve segments to
the entire circle. In assembling the armature each layer
breaks joints with the next, and when the whole mass of
lamime is built up it is held firmly together by heavy end plates
which are secured by bolts passing through the space left
between the laminse and the supporting girder. This stage of
the construction is seen in the second quadrant. Finally after
the armature coils are in place they are protected by a seg-
mental ventilated shield as seen in the first quadrant. The
revolving field magnet is likewise built up of segmental laminae
dovetailed to supporting castings, which are in turn carried
by the two heavy steel plates which, bolted to the hub, form
the driving spider. As in most such constructions the pole
tips are of separate laminae dovetailed or interlocked with the
laminje of the polar projections. The field coils are held in
place by shoes and radial bolts to relieve the pole tips of the
centrifugal stress. For lower speed machines the poles are
often solid save for the dovetailed laminated tips, and are
simply held to the rim of the field spider by radial bolts.
The construction of such machines is very various, but the
main point is that the high voltage windings are stationary,
kept well clear of each other, and singularly accessible so that
damaged coils are very easily replaced. Current is led to the
field by two small slip rings. Even for low voltage machines
this construction is very generally preferred by reason of the
greater security of the windings, and the absence of the large
slip rings and their collecting devices.
• See Trans. A. I. E. E. Feb., 1004
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192
ELECTRIC TRANSMISSIOX OF POWER.
Still another form of alternator in which the armature,
and field windings as well, are stationary, is found in the ^* in-
ductor" dynamo. Of this the most familiar types are those
introduced by Mr. Mordey in England and by Mr. Stanley in
this country. In such machines the magnetic circuit through
the armature coils established by fixed field coils is periodi-
cally closed and opened by revolving pole pieces which them-
selves carry no wire. The principle is illustrated in Fig. 97, a
Fio. 97.
cross section of the Stanley inductor dynamo. Here the cir-
cular yoke C carries two rings of laminae each provided with
windings, B, arranged much as in the alternator just described.
Within these rings revolve two sets of laminated polar pro-
jections bonie on a massive spider which completes the mag-
netic circuit. The stationary field winding A surrounds the
spider as a whole without touching it. Evidently all the
poles at one end of the spider are north poles and those at the
other end south poles, and the armature coils are connected
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Fig. 1.
Fio. 2.
PLATE IV.
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TRANSMISSION BY ALTERNATING CURRENTS. 193
accordingly. As a rule inductor dynamos are not economical
of material owing to the nature of the magnetic circuit, and
their gain in security and simplicity over a revolving field
alternator of the ordinary sort is hardly enough to balance
the disadvantage, so that they are now less used than formerly,
although in themselves excellent machines.
Plate IV shows the field and the two halves of the armature
of a modern high voltage polyphase generator for direct con-
nection to the prime mover. In this case the diameter of the
armature frame is so great that it has been found desirable to
design it as a hollow circular truss in order to give it the ne-
cessary rigidity against distortion by its own weight and by
inequality of magnetic pull, if there were a trifling eccentricity
due to wear of the bearings. In some of the early machines
of large diameter, flexure from the weight alone was very
troublesome. Half the armature coils are shown in place and
wedged in, and a coil belonging in the second half is all ready
to put in place. Four shapes of coils are necessary to com-
plete this winding, but they can be kept well clear of each
other at the ends and are easy to put in and take out, so that
in case of damage a coil can be easily replaced, although it
may sometimes be necessary to move several others to get at
the damaged one. An injured coil, however, can readily be put
out of circuit. by cutting it loose at the ends, msulating them,
and connecting the adjacent coils of the same phase across the
dead one. A generator so temporarily repaired in a few min-
utes can be run imtil opportunity oiffers for permanent repairs,
and can even be worked in parallel with others without material
difficulty.
To facilitate repairs the armatures of large revolving pole
machines are often carried on a sliding bed, so that they can
be shifted by their o^^ti width along the shaft, exposing the
windings of both armature and field.
The field is really a compact, massive fly-wheel with the
poles bolted on its rim, the poles surfaces being shaped so as to
give as nearly as may be a sinusoidal wave. The pole-pieces
are generally laminated, at least near the tips, and are some-
times provided with ventilating spaces like those in the ar-
mature.
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19-4
ELECTRIC TRANSMISSION OF POWER.
The advantage of revolving field generators is so great in
point of easy insulation and ready collection of even very great
currents, that this type of machine has been rapidly displac-
ing the older form for high voltage work, and indeed for large
work of every kind. In such generators voltages of 10,000
and 12,000 are now quite common, and the limit has not been
reached.
Fig. 98 shows the efficiency curve of one of the huge modern
high voltage three-phasers. It is from a 5,000 KW, 11,000
volt directed connected generator for the Interborough Rapid
Transit Co., of New York City, and while it does not show the
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small frictional and air resistance losses, is very striking as
illustrating first, the very high efficiency reached by such
machines, and second, the remarkably uniform efficiency at
varying loads. The curve passes 94 per cent at a little below
quarter load, reaches a full load efficiency of 98 per cent, and
rises even slightly higher on a 25 per cent over load. The regu-
lation of this generator is also excellent, being upon non-induc-
tive load in the vicinity of 5 per cent. It is built with a revolv-
ing 40 pole field 32 feet in diameter and the armature winding
is distributed in four slots per phase per pole, each slot contain-
ing three bars.
In large polyphase generators the question of automatically
regulating the voltage in response to changes of load is a seri-
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TRANSMISSION^ BY ALTERNATING CURRENTS. 195
ous one, and no final solution of it has as yet been reached.
It is not economical to build generators with so small inherent
variation of voltage as is in itself desirable. In small poly-
phase machines compounding has been accomplished with an
arrangement of parts similar to that shown in Fig. 81, the
connections being so modified as not to take the commutated
current from a single phase. This is troublesome in machines
requiring considerable energy for the field excitation, and be-
sides it only compounds correctly for a particular value of the
power factor, which in many plants is constantly changing.
Several modem methods of compounding direct the com-
FlG. 99.
pounding at the exciter. A rotary converter is used as ex-
citer, and the voltage at its commutator, which depends on
the alternating voltage applied at the slip-rings, is modified in
various ways in response to changes in the magnitude and
phase of the working currents from the generator. A tj'-pical
plan of this kind, successfully applied by the author some ten
years ago, is shown diagrammatically in Fig. 99. Here the
generator fields A' A' are fed from the commutator end of a
rotary converter F F. Current from the main collecting rings
a is led to the collecting rings b of the exciter through the re-
active coils c c c on the cores M M il/, which are also wound
with series turns d d din the main leads of the generator. At
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196
ELECTRIC TRANSMISSION OF POWER.
light loads the voltage at b is cut clown by the reactance, while
as the main current increases or lags the series turns d d d raise
the voltage a h, and hence strengthen the generator field. By
properly proportioning the coils c c c, d d d, and theif cores
M M My the apparatus can be made to regulate the voltage
very closely for all loads of the generator, inductive or non-
inductive, or even may over-compound on inductive load so as
to compensate for the change in the inductance of the system.
Fig. 100 shows the working of this device when arranged to
show extreme over-compounding on inductive load. The gen-
erator chosen was one which uncompounded would drop its
voltage about 40 per cent on a heavy inductive load. Curve
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A shows the regulation of the secondary voltage on non-induc-
tive load, curs'^e B the over-compoimding produced by a load of
induction motors running light, having a power factor of not
over 0.25.
The same general principle has been lately applied in several
forms with very promising results. An interesting modifica-
tion is the compensated field alternator recently brought out
by the General Electric Company, and shown in Plate V.
Here the exciter armature is on the shaft of the main machine,
and is in a field having the same number of i)oles, so that it
revolves synchronously pole for pole with its generator. Ex-
citer and main fields are fed in shimt from the exciter commu-
tator, but the exciter armatiure also receives through its col-
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Fio. 2.
PLATE V.
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TRANSMISSIOJSf BY ALTERNATING CURRENTS. 197
lector rings an auxiliary current derived from series trans-
formers in the main leads of the generator. This device holds
the voltage with beautiful precision under ordinary changes of
load and lag, but the necessity of being in mechanical S3mchron-
ism is somewhat embarrassing, save in high speed machines.
Another very pretty method of regulation by compounding
the exciter, is that due to Prof. F. G. Baum* and shown dia-
grammatically in Fig. 101. In this device a little generator of
a few hundred watts capacity is mechanically driven in syn-
chronism with the main generator (?. Its fields A A' are
excited by a few turns of the main generator current. The
Fig. 101.
armature B has a very simple winding, one terminal of which
goes to a soUd ring connected by a brush with the lead 6, the
other goes to a pair of opposite segments about 90° wide and
thence to the lead V . When the fields are excited from the
main current and the armature is turning in synchronism, the
machine evidently gives a partially rectified current, more or
less of the waves of one polarity being bitten off short, accord-
ing to the position of the segments of the divided ring. The
oscillograph record of the resulting pulsatorj"- current is shown
in Fig. 102. Now if the current in the main line lags the
effect is precisely the same as if the segmental ring had been
turned forward a little, thus increasing the amplitudes of the
* Trans. A. I. E. E. May, 1902.
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198 ELECTRIC TRANSMISSION OF POWER.
peaks, and hence the effective E. M. F. of the pulsatory current.
A leading current produces precisely the opposite effect. This
automatically varied current excites the field of a second little
machine C which compoimds the exciter through the series
coil D, The rheostats R R enable the compounding to be
accurately adjusted. The effect of this apparatus is to regu-
late not only for varying current but for variations of power
factx)r, in a very satisfactory manner. It may also be applied
to synchronous motors and rotary converters.
Along such lines as these, good results are certainly attain-
able, and in addition there are several automatic devices for
working a rheostat in the generator field so as to hold the vol-
tage constant, irrespective of load or lag. These with other
regulating apparatus will be described in another chapter.
FlO. 102.
As a matter of fact, in much power transmission work com-
pound winding is not necessary, since the machines hold their
voltage closely without it if well designed, and in large plants
the variations of load are usually so gradual that the voltage at
the end of the transmission line can be easily kept constant
by hand regulation. Again, in many transmission plants sev-
eral lines are fed by one generator, so that no compounding
would suit all the lines; and whenever a substation is in-
stalled, the secondary voltage has to be kept constant by
special regulation in any event.
TRANSFORMERS.
The alternating current transformer is merely a glorification,
as it were, of the fundamental idea showxi in Fig. 4, page 12.
The loops A and B are expanded into massive coils and are
given a very perfect magnetic core of laminated iron, but the
principle is unchanged.
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TRANSMISSIOJSf BY ALTERNATING CURRENTS. 199
In Fig. 103, ^ is a core composed of soft iron plates perhaps
j^jj inch thick, stamped into the form shown, and then built
up together like the leaves of a book, 5 is a coil of insulated
wire wound in a spiral around one side of the core, and C is a
single loop of heavy insulated copper bar around the other side.
Now suppose an E. M. F. is suddenly applied to the terminals
of the coil By the loop C being left open. Current will flow
through B in amount determined by its resistance and induc-
tan,ce, setting up a magnetic field throughout the mass of A.
If the current is an alternating one an alternating magnetic
field "will be set up in Ay and the current in B will settle down
to that value which is determined by the resistance and induc-
Fio. 103.
tance of the coil. The energy represented by this current is
spent in heating the coil and in doing work by the reversal of
magnetism in the core A. The current thus engaged lags
behind its E. M. F. as in other cases of uiductive circuit, the
power factor at no load being in ordinary cases from .6 to .7.
Now close the loop C Current opposing the current in B
will be at once set up. The magnetizing effect of this reverse
current opposes the magnetization due to B, and hence tends
to cut down the inductance imposed on By which is, as we have
already seen, determined by the magnetic induction through
its core. To this action B simultaneously responds wdth an
increased current, so that any increase of the current in C and
its consequent demagnetizing action, is automatically compen-
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200 ELECTRIC TRANSMISSION OF POWER.
sated by an increased current in B, The increase of energy
represented by this compensates for the energy due to the
current in C. Energy is thus virtually transferred from the
primary circuit B to the secondary circuit C.
Now as to the voltage of these two circuits. The energy
in the two circuits is evidently equal save for losses in the iron
and copper, which amount ordinarily to only a few per cent.
For any given magnetization in A the inductive E. M. F.
in S is proportional to the total number of turns in the coils;
so also the induced E. M. F. in the secondary is proportional
to the number of turns in it. That is for a certain rate of
change of the magnetic induction in A, the induced E. M. F.
is the same per turn throughout A, whether that E. M. F.
appears as inductance in B or secondary E. M. F. in C.
Hence, the E. M. F.'s across the terminals of the primary and
secondary coils are proportional to the respective numbers
of turns in those coils. But the energy in the two is sub-
stantially equal, and hence the currents in primary and secon-
dary must be inversely proportional to the respective E.M. F.'s
In Fig. 103 are shown seven primary turns and one secondary
turn. Therefore, the secondary E. M. F. is one-seventh the
primary E. M. F., and the primary current is one-seventh the
secondary current. For the same density of current in am-
peres per square inch the secondary turn must have seven
times the cross-section of the primary conductor. By simply
changing the relative number of primary and secondary turns
— the ratio of transformation — electrical energy at any vol-
tage can be transformed to any other voltage with trifling loss
if the apparatus be properly designed.
The losses which exist arc of three kinds. First is the loss
due to the resistance of the copper. This at light loads is
very trifling, but increases with the square of the load, being
numerically equal in watts to C -R, as in all cases of loss
through resistance.
Second comes the loss through hysteresis — virtually mag-
netic friction — produced by the alternate reversals of mag-
netization in the iron core. This is nearly constant at all
loads and is kept as low as possible by securing the best pos-
sible iron, and working it at rather low magnetization, since
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TRANSMISSION BY ALTERNATING CURRENTS. 201
the hysteretic loss increases very rapidly as the iron is more and
more strongly magnetized.
Finally comes the loss from eddy currents in the core.
This is due to the fact that the core is a fairly good conductor,
and currents are induced in it for precisely the same reason
that they are induced in the secondary winding. These eddy
currents are largely reduced by carefully laminating the core
across the natural direction of flow of these currents, and
insulating the laminae with sheets of tissue paper or with
varnish. The loss from eddy currents is, generally speaking,
of about the same magnitude as the hysteretic loss, and in
transformer practice the two are usually lumped together and
denominated core loss.
By careful construction and design these losses can be kept
very small compared with the total output. The following
data from a test of a 7,500 watt transformer designed for a
frequency of 15,000 to 16,000 alternations per minute, about
125 to 135 ^, will give a clear idea of the results that can be
reached commercially even in small transformers.
Output 7.5 KW
Transformation ratio 20 : 1
Full load amperes (primary) 3.6
Full load amperes (secondary) 72.0
Resistance (primary) ohms 6.15
Resistance (secondary) ohms . . . . . . . .012
Total C» R loss (watts) 148.
Total core loss (watts) 78.
Primary cuiTent (no load) 063
Power factor (no load) .505
Total C R drop (per cent) 1.0
The efficiency curve of this transformer at various loads is
given in Fig. 104. The interesting feature of this curve is the
very uniform efficiency from half load to full load, with a maxi-
mum of 97.4 per cent at three-quarters load. This is the
result of a relatively very small core loss. Even at one-tenth
the normal load the efficiency is still good, over 90 per cent,
although the curve falls more rapidly below half load.
The larger transformers, such as are used for hea^'y power
transmission work, are even more efficient than the small one
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202
ELECTRIC TRANSMISSION OF POWER.
here described, although the room for increase is now very
limited. Within the last few years the improvement in com-
mercial transformers has been very great. In practice they
are seldom so simple in form as in Fig. 103, the core plates
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being universally built up of several pieces, so that the coils
may be wound in forms and slipped into their respective places
on the core. One of the forms which has been widely used
is shown removed from its case in Fig. 105. The hollow
rectangle A forms the main part of the core, while the bridge
piece, B, is built up separately as the core of the coils, together
B
Fig. 105.
with which it is forced into the position shown. The secon-
dary coil immediately surrounds the bridge, and outside of it
is the primary coil. Both coils are of course elaborately
insulated. Another familiar form of transformer is shown in
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TRANSMISSION BY ALTERNATING CURRENTS, 203
Figs. 106 and 107. Here the core is built up of straight rec-
tangular slips of iron into a hollow rectangle upon the longer
sides of which the coils are fitted, as in Fig. 106, separated
by heavy sheet insulation in the manner shown. The whole
assembled core and coils are shown in longitudinal section in
Fig. 107. This form of construction gives the coils a large
available cooling surface and simplifies their insulation some-
FlO. 106.
what, although magnetically the arrangement of Fig. 105 is
to be preferred.
As transformers are usually inclosed in tight iron boxes to
protect them from the weather, the heat generated in the coils
and core has a rather poor chance to escape, and the tempera-
ture may therefore rise higher than is safe for the insulation.
It is usual to take special precautions to prevent this over-
heating. One of the commonest and best devices for this
Fig. 107.
purpose is the subdivision of the core into bunches of laminse
separated by air spaces.
This arrangement is well shown in Fig. 108, in which the
core is provided with a dozen of these ventilating spaces.
The arrangement of the coils is somewhat like that of Fig. 105.
As an additional precaution against overheating, the trans-
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204 ELECTRIC TRANSMISSION OF POWER.
former case is often filled with heavy mineral oil after the
core is in place. This both provides additional insulation,
and facilitates the transfer of heat from the core and coils to
the iron case, whence it is radiated to the surroimding air. In
very large transformers the primary and secondary windings
are often built up of thin flat sections assembled with spaces
between them. '
For huge transformers such as are used for substation
work, means are generally provided for artificial cooling. Two
methods are at present in use for this purpose. One is the
use of a blast of air from a small blower streaming through
Fio. 108.
the interstices provided in core and coils, and rapidly carrying
away the heat generated. The other is appUed to oil-filled
transformers, and consists in cooHng the oil by a worm in
the transformer case through which cold water is allowed
to flow, or with a small pump circulating the oil itself slowly
through a worm cooled by water. Either plan is very effective,
and both are extensively used.
With properly designed tranformers there is no difficulty in
dealing with any voltage now in use, without the device of
connecting transformers in series, which was formerly often
employed for high voltage. Plate VI shows a type of the
latest transformer practice in an oiled-cooled 900 KW West-
inghouse transformer. It is designed for use at 25 -^ to give
60,000 volts upon the transmission lines. Its splendid efficiency
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PLATE VI.
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TRANSMISSION BY ALTERNATING CURRENTS. 206
curve, which speaks for itself, is shown iji Fig. 109. If trans-
formers are of similar size and design, they can be run in
parallel with the utmost facility, and may very often be thus
"banked" most advantageously, as with such comiection it
is easy to proportion the number of transformers in use to the
load, so that they can be worked nearly at full load, and con-
sequently at their best efficiency.
In general the larger the transformer the higher its effi-
ciency, though the improvement is very slow after the out-
put reaches 25 KW or thereabouts. The curve of Fig. 110
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PROPORTION OF FULL LOAD
Fio. 100.
shows the change in half -load efficiency with the size of trans-
former as found in ordinary American practice.
The data here given relate to transformers of the kind
employed for power transmission work, as now produced by
the best makers. The sizes above 50 KW are frequently
artificially cooled. The frequency is taken at 60 '^ to 70 ^,
and the figures do not apply to transformers originally
designed for higher frequencies. At lower frequencies the
efficiencies are Ukel)'^ to be a fraction of a per cent lower, but
at any frequency within the range of ordinarj" working a first-
class transformer of 50 KW capacity or upward can be
depended on for a full load efficiency of just about 98 per
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206
ELECTRIC TRANSMISSION OF POWER,
cent, and a half load efficiency about one per cent lower.
With care m planning a substation equipped with these large
transformers, the loss under normal conditions of working
should not exceed 2^ per cent.
For polyphase work it is the almost universal custom in this
country to employ simply groups of ordinary standard trans-
formers. Abroad, composite transformers, transforming two
or more phases in a single structure, are often used. The
intent of this arrangement is to utilize more fully the iron
core by making it common to the several phase windings.
Three laminated cores, with the laminae running vertically, are
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united at the ends by laminated yokes. Each core receives
the primary and secondary windings belonging to a single
phase, while the iron belongs to the three in common. The
arrangement is akin to the mesh connection of three-phase
circuits.
It is a question whether the common use of the core iron
is a sufficient offset to the loss incurred in operative flexibihty.
Separate transformers for each phase can be readily shifted
about or reconnected in case of accident, while if anything
happens to a polyphase transformer it is likely to put out of
action a considerably greater capacity than in the other case.
Nevertheless, three-phase transformers are considerably used
abroad and very recently they have come into current prac-
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TRANSMISSION BY ALTERNATING CURRENTS. 207
tice here. Fig. Ill shows, removed from its oil-filled ease, a
three-phase transformer of about 50 KW capacity. The
arrangement of the cores is akin to that of Fig. 107, but with
three wound cores instead of two. Similar transformers are
now being made of several thousand kilowatts capacity, but
whether they will have a permanent place in the art remains
to be seen. They are at present emphatically special, and it
is somewhat dubious whether they present sufficient advan-
tages to compensate for the extra capacity jeopardized in case
of trouble.
Fio. u
Several arrangements of transformers are employed in poly-
phase working corresponding to the various arrangements of
polyphase circuits. For example, in two-phase systems the
transformers are generally connected as shown in Fig. 112.
This is simply one transformer per phase connected in the ordi-
nary manner. The two phases are kept distinct both as regards
primary and secondary sides of the circuit. Fig. 113 shows
the composite circuit method of connection. Both primary
and secondary circuits have one wire common to both phases.
In this case there is between the outside wires of the system
a higher voltage than exists between either outside wire and
the common wire. This voltage is of course the geometrical
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208
ELECTRIC TRANSMISSION OF POWER.
sum of two separate phase-voltages. As these are 90®
apart the resultant voltage is V2 times either component.
Not infrequently the primary arrangement of Fig. 112 is com-
bined with the secondary circuit of Fig. 113. This is the ordi-
nary connection of two-phase motors, which are often built for
this three-wire circuit. As a rule all lighting connections and
all long circuits of any kind are made as shown in Fig. 112.
Transformers for three-phase circuits, are, like the circuits
themselves, very seldom worked with the phases separated,
but in nearly every case are combined in the star or mesh
Fig. 112.
connection. The former is useful in dealing with very high
voltages, since the individual transformers do not have to carry
the full voltage between lines. Fig. 114 shows a diagram of
the star connection and Fig. 115 the corresponding mesh. In
each a, 6, c, are the primary leads, and A, B^C the correspond-
ing secondary leads. Of the two connections the mesh is
rather the more in use except for high voltage work, and for
secondary distribution with a connection to the common
junction of the transformer system, which connection has for
certain purposes very great advantages.
Whether the star or the mesh connection is employed, one
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TRANSMISSION BY ALTERNATING CURRENTS. 209
transformer per phase is required, and this condition is some-
times inconvenient as rendering necessary the use of three
small transformers where a two-phase system would need but
two. To obviate this difficulty, what may be called the
A/WW\
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FlO. 113.
"resultant mesh" connection is extensively used, particularly
for motors. The principles on which this is based have already
been set forth.
Briefly, if one takes the geometrical sum of two E. M. F.'s
b <h
not in phase with each other, the resultant will be less than the
arithmetical sum of the components, and not in phase with
either. From the examples of geometrical summation already
discussed, it is evident that by varying the magnitudes of the
components and the angle between them, i.e., their phase
difference, the resultant may have any desired value and any
direction with reference to either component.
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210
ELECTRIC TRANSMISSION OF POWER.
The "resultant mesh" three-phase connection is shown
in Fig. 116. It is composed of two transformers instead of
three as in Fig. 115, the E. M. F. between the points A and C
being the resultant derived from the two existing secondaries.
Each of these secondaries contributes its part of the output
in the resultant phase, and the secondary circuit behaves
substantially as if it were derived from the ordinary mesh
connection. This arrangement is very convenient in motor
work, since it is very simple and allows the use of two trans-
formers when desirable for the required output. Sometimes
a motor is of a size that is fitted better by three standard
transformers than by two, or the reverse, and with the choice
FIO. 115.
of the two mesh connections it is often possible to avoid some
extra expense or to utilize transformers that are on hand.
A very beautiful application of this principle of resultant
E. M. F. is the change of a two-phase system into a three-
phase, or vice versa. The method of doing this is shown in
Fig. 117. Suppose we have two equal E. M. F.'s 90° apart, as
in the ordinary two-phase system, as the primary circuit. The
secondary E. M. F.'s will still be 90® apart, but can be of any
magnitude we please. Let one of these secondaries A C give
say 100 volts, and tap it in the middle so that the halves, A D
and D C will each be 50 volts; now wind the other secondary,
B Dy for 50 V3 volts, and connect one end of it to the middle
point of the first secondary. Taking now the geometrical
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TRANSAfIS;SlON BY ALTERNATINC CURRENTS. 211
sums of B D with the two halves of A C, the resultants are
equal to each other and to A C, and leads connected to i4, B,
and C will give three equal E. M. F.s 120® apart, forming a
three-phase mesh with two resultant E. M. F.'s instead of one
Pig. 116.
as in Fig. 116. The actual connection of a 1,000 volt two-phase
system to form a 100 volt three-phase secondary system is
shown in Fig. 118 . Reversing the operation by supplying
three-phase current to the three-phase side of the system
gives a resultant two-phase circuit.
This change-over process is valuable in that it allows a
three-phase transmission circuit to be used for the sa^'ing in
copper characteristic of it, in connection with two-phase
generating and distributing plants, and permits two-phase and
three-phase apparatus to be used interchangeably on the same
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212
ELECTRIC TRANSMISSION OF POWER,
circuit, wh\ch is sometimes advantageous. A somewhat
analogous arrangement permits the transformation of a
monocyclic primary circuit into a three-phase or two-phase
secondary form, as may be convenient, and in fact any sys-
tem with two or more phases may be transformed into any
other similar system in the general manner described.
It is worth noting that the three-phase-two-phase trans-
formation sho\\Ti in Fig. 118 can in an emergency be very
readily made' without special transformers if. one has avail-
able transformers of ratios 9: 1 and 10: 1, respectively, both
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these being obtainable commercially. For the latter tapped
from the middle of the secondary, as is common for three-
wire work, gives the left-hand half of Fig. 118, while the 9: 1
transformer is sufficiently near the required ratio to give the
rest of the combination. Such an extemporized arrangement
is very serviceable in operating three-phase induction motors
from two-phase mains or vice versa, and can be put together
very easily. In default of this it is easy enough in using
standard transformers of makes in which the secondary wind-
ings are fairly accessible, to tap the secondary winding so as
to leave about 12 per cent of it dead-ended, and this forms
the supplementary transformer required.
The electrician will do well to familiarize himself with the
handling of transformers in all sorts of connections, for in a
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TRANSMISSION BY ALTERNATING CURRENTS. 213
sudden emergency a little deftness in this respect will often
extricate him from an uncomfortable comer. For instance,
one can connect transformers backwards to get high voltage
for testing, or with the usual three-wire secondaries trans-
form twice and reach half the primary voltage, or put several
secondaries in series, with the corresponding primaries in mul-
tiple, or do many other things occasionally useful. The chief
things to be borne in mind are that the normal currents in
primaries and secondaries must not be exceeded, that the
polarities must be kept straight and great care must be exer-
cised not inadvertently to get any coils on short circuit.
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FlQ. 119.
One of the most useful temporary expedients is boosting the
primary voltage by means of a standard transformer to meet
excessive drop in a long feeder. The process is exceedingly
simple, being merely the connection of the secondary in series
with the line to be boosted, while the primary is put across the
mains as usual. The result is that the feeder voltage is raised
by nearly the amount of the secondary voltage. Fig. 119
shows a convenient way of arranging the connections, in which
one of the primary lines is so connected to a double throw
single-pole switch that while boosting goes on with the switch
in the position shown, on throwing the switch to the reverse
position the booster is cut out and the line receives its current
as usual. It must be remembered in such boosting that the
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214 ELECTRIC TRANSMISSION OF POWER.
strain on the transformer insulation is more severe than usual,
and in particular that the strain between secondary coils and
core is the full primary voltage, for which provision is seldom
made in insulating secondaries from cores. Hence, in rigging
a booster transformer one of the oil-insulated type should be
chosen, and it should be very carefully insulated from the
ground. For the same reason the boosting transformer
should be of ample capacity, so that it will not be likely to
overheat, and should in general be treated rather gingerly, like
any other piece of apparatus subject to unusual conditions.
Nevertheless, it is capable of most effective service if properly
operated.
All these systems which involve resultant E. M. F.'s are
open to certain practical objections which may or may not be
important according to circumstances.
In the first place, the resultant E. M. F. is less than the
sum of the E. M. F/s for which the transformers in the com-
ponent circuits are wound. For instance, in Figs. 116 and 118,
100 resultant volts are derived from transformers aggregating
respectively 200 and 186.7 volts, through the secondaries of
which the resultant current has to flow. In the former case
one-third and in the latter case two-thirds of the total current
is thus derived at a disadvantage, using up more transformer
capacity for a given amount of energy than if the transformers
were used in the normal manner. On a small scale the dis-
advantage is seldom felt, but in heavy transmission work with
large transformers it may be quite serious.
Second, the disturbance of any one component voltage from
drop or inductance, or any shifting of phase between the com-
ponents from unequal lag, disturbs all the resultant E. M. F.'s
This, again, may or may not be of importance, but it must
always be borne in mind, as ia every case of combined phases.
It is possible by combinations of transformer similar to
those described to obtain at some sacrifice in transformer
capacity a single-phase resultant E. M. F. from polyphase
components, or to split up a single-phase current, by the aid of
inductance and capacity, into polyphase currents. Neither
process is employed much commercially, since both encounter
in aggravated form the difficulties common to resultant phase
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TRANSMISSION BY ALTERNATING CURRENTS. 215
working mentioned above, and others due to the special form
of the combinations attemptcnl. Combining polyphase cur-
rents for a single-phase resultant is a process that would be
very seldom useful, but the reverse process if it were success-
fully carried out might l>e of very great importance in certain
distribution problems, and especially in electric railway prac-
tice, although in working on the large scale that offers the best
field for alternating motors the disadvantage of two trolleys is
at a minimum. One very ingenious method of splitting an al-
ternating current into three-phase components is the following,
due to Mr. C. S. Bradley, one of the pioneers in polyphase
VlG. 120.
work. His process is essentially twofold, first splitting the
original current into a pair of components in quadrature
and then combining these somewhat as in Fig. 118. The appa-
ratus is shown in diagram in Fig. 120. Here A is the gen-
erator, B the simple primary of one transformer elenient, D a
condenser, n and I the sections of the compound transformer
primary, and g, A, i, /, k the secondary transformer sections.
The condenser D is so proportioned that acting in conjunc-
tion with the compound primary n I the original current
is split into two components in quadrature, in B and n I respec-
tively. Then the secondaries are so intercoimected as to
produce three-phase resultant currents which are fed to the
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216 ELECTRIC TRANSMISSION OF POWER.
motor M. The coil i gives one phase, the resultant of g and k
another, and the resultant of h and / the third. The combi-
nation of these resultants gives a more uniform and stable
phase relation under varying loads than would be obtained
from two-phase secondaries fed by B and n I respectively.
The condenser is necessary in getting a correct two-phase re-
lation in the primaries to start with, and even so the E. M. F.'s
will not stay in quadrature imder a varying load on the sec-
ondaries unless the condenser capacity be varied, but the re-
combination in the secondaries partially obviates this difficulty.
A device brought out abroad by M. Korda for a similar purpose
omits the condenser and splits the monophase current into two
components 60° apart by variation of inductance alone, and
these are utilized to give three-phase resultants. The phase
relations thus obtained are, however, unstable, as must always
be the case in phase splitting by inductance alone. For
the energy supplied by a monophase current is essentially
discontinuous, while the energy of a polyphase circuit has no
periodic zero values, so that in passing from one to the other
there should be storage of energy during part of each cycle such
as is obtained by the condenser of Fig. 120.
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CHAPTER VI.
ALTERNATING CURRENT MOTORS.
The principles of the synchronous alternating motor are a
snare for the unwary student of alternating current working,
since they involve, when discussed in the usual way, rather
complicated mathematical considerations. And the worst of
it is that the generalized treatment of the subject often causes
one to lose sight of the fundamental ideas that are at the root of
alternating and continuous current motors alike. The sub-
ject is at best not very simple, and unless we are prepared to
attack the general theory with all its many considerations, it is-
desirable not to cut loose from the common basis of all motor
work.
Recurring to the rudimentary facts set forth in Chapter T,
we see that an electric motor consists essentially of two work-
ing parts — a magnetic field and a movable wire carrying an
electric current. The motive power — torque — is due to the
reaction between the magnetic stresses set up by the current
and those due to the field. The refinements of motor design are
concerned with the efficient production of these two sets of
stresses and their coordination in such wise that their reac-
tion shall produce a powerful torque in a uniform direction.
In continuous current motors, for example, the field mag-
nets are energized by a part or the whole of the working-
current, and this current is passed, before entering the arma-
ture, through a commutator like that of the generator, so
that in the armature the direction of the currents through the
working conductors shall be reversed at the proper time, so as
to react in a uniform direction with field poles which are con-
secutively of opposite polarity. Were it not for the commu-
tator the armature would, on turning on the current, stick fast
in one position, as may happen when there is a defect in the
winding.
Now, since the function of the commutator in the generator
m
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218 ELECTRIC TRANSMISSION OF POWER,
is to change a current normally alternating, so that it shall
flow continuously in one direction, and since the object of the
commutator in the motor is periodically to reverse this current
in the armature coils, thus getting back to the original current
again, one naturally asks the reason for going to all this trouble.
Why not let the generator armature do the reversing instead
of providing two commutators — the second to undo the work
of the first?
The reason is not far to seek. In a generator running at
uniform speed the reversals of current take place at certain
fixed times — whenever an armature coil passes from pole to
pole, quite irrespective of the needs of the motor. The com-
mutator on the other hand reverses the current in the motor
armature coils in certain fixed positions with respect to the
field poles so as to pn>duce a continuous pull, irrespective of
what the generator is doing.
If we abolish the commutators the motor will nm properly
only when the alternating impulses received from the gen-
erator catch the armature coils systematically in the same
positions in which reversal would be accomplished by the
coitimutator. Hence for a fixed speed of the generator the im-
pulses will be properly timed only when the motor armature
is turning at such a speed that each coil passes its proper
reversal point simultaneously with each reversal of the genera-
tor current. If generator and motor have the same number
of poles, this condition will be fulfilled only when they are
running at exactly the same number of revolutions per minute.
In any case they must run synchronously pole for pole, so that
if the motor has twice as many poles as the generator, it will
be in synchronism at half the speed in revolutions per minute,
and so on.
If we try to dispense with the commutators when starting
the motor from rest, the action will obviously be as follows:
The first impulse from the generator might be in either direc-
tion, according to the moment at wliich the switch was thrown.
The reaction between this current in the armature coils and
the field poles might tend to pull the armature in either direc-
tion, but long before the torque could overcome the inertia
of the armature a reverse impulse would come from the gen-
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ALTERNATING CURRENT MOTORS. 219
erator and undo the work of the first. Consequently the motor
would fail to start at all.
If the impulses from the generator came very slowly indeed,
so that the first could give the armature a start before the
second came, the armature would stand a chance of getting
somewhere near its proper reversal point before the arrival of
the reverse current, and thus might get a helping pull that
would improve matters at the next reversal, but the direction
of the first impulse would be quite fortuitous. Starting the
armature in either direction before the current is thrown on
gives it a better chance to go ahead if the first impulses in the
wrong direction are not strong enough to stop it altogether.
00000 '^tt%r 00000
Fio. 121.
We see, then, that an alternating current derived directly
from the generator does not give reversals in the motor coils
that are equivalent to the action of a commutator, save at
synchronous speed. Except at this speed the current from
the generator does not reverse in the motor armature coils
when the latter are in the proper position.
Fig. 121 will give a clear idea of the conditions of affairs in
the field and armature conductors of a continuous current
motor. Here S and A^ are the poles, and + and — mark the
positions of the positive and negative brushes with reference
to the armature winding. The solid black conductors carry
current flowing down into the plane of the paper. The white
conductors carry current upward. The armature turns in
the direction of the arrow, and as each conductor passes under
the brush the current in it is reversed. This distribution of
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220
ELECTRIC TRANSMISSION OF POWER.
current is necessary to the proper operation of the motor, and
if the brushes are moved the motor will run more and more
weakly, and then stop and begin to rim in the opposite direc-
tion, imtil when the brushes have moved 180° the motor will
be running at full power in the reverse direction. This final
position means that the currents in the two halves of the
armature have exchanged directions, so that the conductors
originally attracted toward N and repelled from S, are now
repelled from N and attracted toward S, If alternating
current from the generator is led into the windings, the dis-
tribution of current showTi in Fig. 121 must be preserved, and
since in abolishing the commutator the alternating current
leads are permanently connected to two opposite armature
coils through slip rings, the distribution of Fig. 121 can only
Fro. 122.
be preserved when these leads change places by making a half
revolution every time the current reverses its direction. Other-
wise the distribution of currents will be changed, and the
motor will fail to operate, since each reversal of current will
catch the armature in a wrong position, and may tend to
turn it in the wrong direction as much as in the right.
Hence such a motor must run in synchronism, or not at all,
and to operate properly it must either be brought to full syn-
chronous speed before the alternating current is turned on, or
nursed into action by running the generator very slowly, work-
ing the motor into synchronous running at very low speed,
and then gradually speeding up the generator, thus slowly
pulling the motor up to full speed. In practice the former
method is uniformly employed, and the machine used as a
synchronous motor is substantially a duplicate of the alternat-
ing generator as already described. In fact, it is an alternating
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ALTERNATING CURRENT MOTORS. 221
generator worked as a motor, just as a continuous current
motor is the same thing as the corresponding generator.
Fig. 122 gives a clear idea of the way in which synchnmous
alternating motors may be employed for power transmission.
Here G is the generator driven from the pulley P. S is a
switch connecting the generator to the line wires L L\ At the
motor end of the line is a second switch S\ which can connect
the line either with the synchronous motor 3/, or the starting
motor M\ This latter is usually some form of self-starting
alternating motor to which current is first applied, ii/' then
gradually brings M up to synchronous speed ; when the switch
S' is thrown over, the main current is turned on M , and then
the load is thrown on the driving pulley P' by a friction clutch
or some similar device.
Such a system has certain very interesting and valuable
properties. We can perhaps best comprehend them by
comparing them with the properties of continuous current
motor systems.
In the alternating system both generator and motor are
usually separately excited, which means really that the field
strengths are nearly constant; as constant in fact as those
in a well designed shimt-wound generator and motor for con-
tinuous current.
Now we have seen that this latter system is beautifully self-
regulating. Whatever the load on the motor, the speed is
nearly constant, and the current is closely proportional to the
load. If the load increases, the speed falls off just that minute
amoimt necessary to lower the count<?r E. M. F. enough to
let through sufficient current to handle the new load. The
effective E. M. F. is the difference between Ej the impressed
E. M. F. and E'y the counter E. M. F. The current produced
by this E. M. F. is determined by Ohm's law.
E - E'
C = (1)
r
where r is the armature resistance, and since we have seen that
the output of the motor is measured by the counter E. M. F.,
W^CE' (2)
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222
ELECTRIC TRANSMISSION OF POWER.
where W, in watts, includes frictional and other work. E',
neglecting armature reaction, is proportional to the speed of
the armature, which falls under load just enough to satisfy
equation (2) by letting through the necessary current.
Now we have seen that when we abandon the commutator
the motor has to run at true synchronous speed, or else lose
its grip entirely. How can it adjust itself to changing condi-
tions of load? If the load increases, more current is demanded
to keep up the output, but the field strength remains constant,
and the counter E. M. F. of the motor cannot fall by reduction
of speed. We must note that while in a continuous-current
motor the counter E. M. F. of the armature is constant at
uniform speed, in an alternating motor the counter E. M. F.
Fig. 123.
varies like that of the generator, following approximately a
sinusoidal curve, as the position of the armature with respect
to the field poles varies.
Hence at any given instant the counter E. M. F., the speed
and field strength remaining the same, depends on the position
of the motor armatui-e. In Fig. 123 we have a pair of alternat-
ing machines, generator A and motor B. In normal running
at light load, the two are nearly in opposite phase, since of
course the impressed and counter E. M. F.'s are virtually in
opposition.
Now, if there is an increase of load the motor armature sags
backward a little imder the strain, thereby lessening the com-
ponent of its counter E. M. F. that is in opposition to the
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ALTERNATING CURRENT MOTORS. 223
impressed E. M. F. The current increases, and with it the
torque, and the sagging process stops when the torque is great
enough to carry the new load as synchronous speed. The
change of phase in the counter E. M. F. thus takes the place
of change of absolute speed in the continuous current motor,
by the same general process of increasing the E. M. F. effec-
tive in forcing current through the circuit. This efTective
E. M. F. is generally by no means in phase with the impressed
E. M. F., and in general the current and the impressed E. M. F.
are not in phase in a synchronous motor. Here, as elsewhere,
the input of energy is
C E cos 4>,
while the output, which in the continuous current motor is
simply the product of the current and the counter E. M. F.,
in the synchronous motor depends evidently on such parts of
both as are in phase with each other, t. e,,
W = CE' cos <l>' (3),
in which <^ is the angle between current and counter E. M. F.
Likewise the current, which in the continuous current motor
depends on the effective E. M. F. and the resistance, now de-
pends on the counter E. M. F. and the impedance I. So that
^ E - E'
C = — (4).
In this equation the values of all the quantities depend on
their relative directions, and by combining geometrically the
factors of (4) we can form a clear idea of the singular relations
that may be found in synchronous motor practice.
The construction is similar to that found in Fig. 51, page 133.
In Fig. 124, we will start with an assumed impressed E. M. F.
of 1,000 volts, a counter E. M. F. of 800 volts and an impe-
dance composed of 5 ohms resistance and 10 ohms equivalent
inductance.
To begin with, we will lay off the impressed E. M. F. A B,
and then the counter E. M. F. B C, which as we have seen is
in partial opposition to A B. In this case ^1 C is the resultant
E. M. F., which, on the scale taken, is 300 volts. This, then, is
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224
ELECTRIC TRANSMISSION OF POWER,
the available E. M. F. taken up by the inductive and ohmic
drops in the armature. The next step is to find C (eq. 4)
from /, and the value of E — F/, just obtained. To obtain /,
we must combine resistance and inductance, as shown in Fig. 55.
Performing this operation, it appeai-s that / = 11.18. Hence
300
in the case in hand C = -•= 26.8 4- amperes. As to the
11.18 ^
direction of this current, we know that it is at right angles to the
inductive E. M. F., i. c, is in phase with the resistance in Fig.
125. Solving that triangle to obtain the angle between the
current and impedance, it turns out to be a little over 63°,
being the angle whose tangent is — • Laying off this angle a
5
from A C, the impedance in Fig. 124, we find the current to be
FlQ. 124.
in the direction A D. This current then is out of phase with
the impressed E. M. F. by the angle of lag DAB. It is also
out of phase with the counter E. M. F., though by chance very
slightly, and lags behind the resultant E. M. F. A C, by the
angle a. Being nearly in phase with the counter E. M. F., the
gross output of the motor is approximately 26.8 X 800 = 21.4
KW.
Now, what happens when the load increases? The motor
armature sags back a few degrees imder the added torque, and
the counter E. M. F. takes the new position B C. The new
resultant E. M. F. is A C, which on the scale taken equals 450
450
volts. The new value of the current is C = = 40.25
11.18
amperes, and its phase direction, 63° from A C, is A D', The
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ALTERNATING CURRENT MOTORS, 226
new angle of lag is then D' A B, showing that under the larger
load the power factor of the motor has improved. If C B
should lag still more, A C, together with the current, would
keep on increasing. Evidently, too, the angle of lag D' A B
will grow less and less until A C B becomes a right angle,
when in the case shown it will be very minute, and the power
factor will be almost unity. Beyond this point the angle
CAB will obviously begin to decrease, and D' A B will begin
to open out, again lowering the power factor at very heavy
loads.
Hence it appears that at a given excitation there is a par-
ticular load for which the power factor is a maximum, and it
is evident from the figure that in the example taken this maxi-
mum will be higher as the inductance of the system decreases,
INDUCT ANCE=10
Pig. 125.
and also will pertain to a smaller output. Let us now see
what happens when the excitation of the motor is varied. In
Fig. 126 the conditions are the same as before, except that we
assume counter E. M. F.'s of 500 volts corresponding to C and
1,100 volts corresponding to C. Examining the former, the
resultant E. M. F. is A C = 528 volts, the corresponding cur-
rent is 47 + amperes and the angle of lag D A 5 is much
greater than before. The power factor evidently would still
be rather bad under increased loads, and worse yet, when at
lighter loads the angle ABC decreases. Lessened inductance,
however, would help the power factor by decreasing the angle
CAD, and hence BAD, Now, consider the result of in-
creasing the motor excitation to 5 C = 1,100 volts. The
resultant E. M. F. now becomes A C, being shifted forward
nearly 90°, its value is 280 volts and the current is 25 +
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226 ELECTRIC TRAMSMTSSION OF POWER,
amperes. But this current is now in the direction A /)', a
being the same as before, and hence it no longer lags, but
leads the impressed E. M. F. by nearly 45°. The power fac-
tor is therefore still bad, but gets better instead of worse
imder loads greater than that showai. Inductance in the
system now improves the power factor, and combined with
heavy load might bring the current back into phase with the
impressed E. M. F.
The counter E. M. F.'s corresponding to C and C are
rather extreme cases for the assumed conditions, but it is easy
to find a value for the excitation which would annul the lag
exactly for a particular value of the load. Laying off in Fig.
126, C A B =^ C AD' we find the required counter E. M. F.,
which is very nearly 910 volts. At the particular output cor-
PlQ. 126.
responding to this condition, the power factor is unity, the
current and the impressed E. M. F. are in phase, and since
the current is therefore a minimum for the output in question,
the efficiency of the conducting system is a maximum. At
this point, too, the energy is correctly measured by the product
of volts and amperes, so that if wattmeters are not at hand
the input at a synchronous motor can be closely approximated
at any steady load by varying the field until the armature cur-
rent is a minimum, and reading volts and amperes.
Throughout this investigation it has been assumed that the
ratio of resistance and inductance has been constant. This is
not accurately true, but is approximately so when the induc-
tance is fairly low. The phenomenon of leading current in a
synchronous motor system does not indicate that the current,
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ALTERNATING CURRENT MOTORS, 227
in some mysterious way, has been forced ahead of the E. M. F.
which produces it, for the impressed E. M. F. is not responsible
for the current, which is determined solely by the resultant
E. M. F. behind which the current invariably lags.
The net practical result of all this is that a snychronous
alternating motor, under varying excitation, is capable of
increasing, diminishing, or annulling the inductance of the sys-
tem with which it is connected, or can even produce the same
result as a condenser in causing the current to lead the im-
pressed E. M. F. The maximum torque of the motor, which
determines the maximum output, is determined by the greatest
possible value of C E' cos <^' consistent with the given im-
pedance and electromotive forces. The stronger the motor
field, and the less the armature inductances and reactions of
both generator and motor, the greater the ultimate load that
can be reached without overburdening the motor and pulling
it out of step.
As regards the relation in phase between current and im-
pressed E. M. F., the three commonest cases are those for
which the currents were computed for Figs. 125 and 126. The
first, and commonly the most desirable, is that in which the
current lags slightly at small loads, gradually lags less and less,
comes into phase, or very nearly so, at about average load, and
lags slightly again at heavy loads. The maximum efficiency
of transmission, reached when the lag touches zero, is then at
about average load. The second and commoner case is when
the motor is rather under-excited, so that the lag merely
reaches a rather large minimum, never touching zero. The
third case is that in which the current leads at all moderate
loads, passes through zero lag, and then lags more and more.
The average power factor may be the same as in the first case,
but more energy is required for excitation, and no advantage
is gained except in carrying extreme loads, often undesirable on
account of overheating, or in modifying the general lag factor.
It is highly desirable for economy in transmission that the
product of current and E. M. F. should be a minimum for the
required load. This condition can be fulfilled for the motor
circuit at any load by changing the excitation until the current
for that load becomes a minimum. Further, the field of a
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228
ELECTRIC TRANSMISSION OF POWER,
uniformly loaded motor may in the same way be made to bring
the entire line current of the system to a minimum if the
motor be of sufficient capacity. Thus a synchronous motor
load can be made very useful in improving the general condi-
tions of transmission. By changing the motor excitation as
the load on the motor of the system varies, the power factor
can be kept at or near unity for all working loads.
Fig. 127 shows the power factor of a synchronous motor
somewhat under-excited, and that of a similar machine with a
field strong enough to produce lead at moderate loads. With
H H H
PROPORTION OF FULL LOAD
FlO. 127.
proper adjustment of its field, the effect of a synchronous
motor on the general conditions of distribution is very bene-
ficial. In curve A, Fig. 127, the indications are that the motor
had rather a high inductance and armature reaction, and the
excitation was decidedly too low for good results. Curve B
is from a 300 HP motor, ^ith its field adjusted for zero lag at
about I load. The inductance was low and the armature
reaction small. The result is somewhat startling. Even at i
load the power factor (current leading) is about .93. At half
load it has passed .99; touches unity, and then slowly diminishes
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ALTERNATING CURRENT MOTORS,
229
to very nearly .98 (lagging) at full load. In this case the gen-
erator was held accurately at voltage while the excitation of
the motor was uniform. Both were polyphase machines woimd
for 2,500 volts.
When a synchronous motor is used in this manner, it obvi-
ously will show, at the same load, values of the current varying
if the excitation be varied. For any load the minimum current
is given by that excitation which brings the current into phase
10 80
AMPERES IN FIELD
Fio. 128.
with the impressed E. M. F. This point is fairly well defined.
At less excitation the current lags, with more it leads.
Fig. 128 shows for a particular instance the relations between
the current and the excitation of the motor field, at full load
of the motor. It is evidently easy to adjust the excitation to
the proper point.
In the practical work of power transmission the synchronous
motor has several salient advantages to commend it. At con-
stant frequency it holds its speed absolutely, entirely indepen-
dent of both load and voltage until, from excessive load or
greatly diminished voltage, it falls out of phase and stops.
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230 ELECTRIC TRANSMISSION OF POWER.
It constitutes a load that is substantially non-inductive, so
that it causes no embarrassing inductive complications in the
system, and takes current almost exactly in proportion to its
work.
Finally it can be made to serve the same end as a condenser
of gigantic capacity in compensating for inductances else-
where in the system, and thus raising the general power factor
substantially to unity.
As compensating disadvantages, it must run at one fixed
uniform speed imder all conditions, it is not self-starting, and
it requires the constant use of a continuous-current exciter.
For many purposes the fixed speed is no objection, and in
most large work the exciter can be used without inconvenience.
Inability to start unaided, even when quite unloaded, is on the
other hand a very serious matter, and has driven engineei-s to
many ingenious subterfuges. The simplest of these is to pn)-^
vide a starting motor, which is supplied with power by any
convenient means, and serves to bring the main machine up to
synchronous speed. Then the main current is thrown on, the
motor falls into synchronism, and the load is taken up by
means of a clutch. The difficulty is to start the starting
motor. In transmissions of moderate length, continuous
current may be delivered over the main line from the exciter
of the generator to the exciter of the motor, which is there-
by driven as a motor, and brings the alternating motor up
to speed. As the energy required for this work is not great,
say 10 per cent of the whole power transmitted, it can often
be delivered quite easily. At long distances, however, the drop
becomes too great for the moderate voltages available with
continuous current, and other methods have to be used.
The best known of these is that indicated in Fig. 122 in
which the synchronous motor is brought up to speed by an
induction motor and thefn clutched to its load after which the
induction motor is thrown out of action.
Another method sometimes used is a special commutator to
rectify the current applied to the main motor armature, thus
directing the impulses so as to secure a small starting torque,
enough to bring the motor to speed. Then the commutator
is abandoned and the motor falls to running synchronously.
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ALTERNATING CURRENT MOTORS.
231
An ingenious modification of this plan is found in a type of
self-starting synchronous motor built by the Fort Wa)nae
Electric Corporation, shown in Fig. 129.
This machine has a double-wound armature. The main
winding is of the kind usual in alternators, wound in slots in
the armature core, and the leads belonging to it connect with
the collecting rings via the bnishes on the pulley end of the
shaft.
The other winding is a common continuous current drum-
PlO. 129.
winding, laid uniformly on the exterior of the armature.
It is provided with a regular commutator as shown in the
figure.
The field is of laminated iron, and the field coils are in dupli-
cate, there being a coarse wire winding which in starting is in
series with the commutated armature winding, and a fine wire
winding cut out in starting, but used alone when the motor is
at speed.
The motor in question is started by turning the alternating
current, reduced to a moderate voltage by transformation, into
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232 ELECTRIC TRANSMISSION OF POWER.
the series field and the commutated winding. The machine
then starts with a good torque, and when it has reached syn-
chronous speed, indicated by the pilot lamp on the top of the
motor being thrown into circuit by a small centrifugal gov-
ernor, a switch is thrown over, sending the main current
through the alternating winding and closing the fine wire field
circuit upon the commutator, at the same time cutting out the
series coils. The motor then runs synchronously, the excita-
tion being furnished by the fine wire winding. This construc-
tion is best suited for rather small machines, as the double-
winding is rather cumbersome for large motors.
At present the tendency in synchronous motor practice is
wholly toward the use of polyphase machines. These will
start, when properly designed, as induction motors, or may be
started by separate motors. When at speed the field excita-
tion is thrown on, and the machine thereafter runs in synchro-
nism. As such motors in starting, as induction motors, take
a very heavy current they are generally provided with starting
motors, although at a pinch they may be brought to speed
at reduced voltage independently, especially if it is practi-
cable to dix)p the frequency temporarily and thus to bring
generator and motor up to speed together. Synchronous
polyphase motors possess the same general properties as other
synchronous motors, and as most power transmission work
is now done by polyphase currents, they are \\idely used.
In general transmission work, synchronous motors find
their most useful place in rather heavy work, which can be
readily done at constant speed.
They have high power factors even when used for very
var}''ing loads, and are valuable in neutralizing inductance
in the line and the rest of the load. Even when not deliber-
ately used for this purpose, they raise the general power factor,
and thus have a steadying effect that is very useful. When
working imder steady load and excited correctly, they almost
eliminate the lagging current that sometimes becomes so
great a nuisance in alternating current working.
The polyphase synchronous motors will run steadily even
if one of the leads be broken, working then as monophase
machines, and by stiffening the excitation will generally carry
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ALTERNATING CURRENT MOTORS. 233
their full normal loads without falling out of synchronism ; but,
of course, with increased heating.
In one case that came to the author's notice, such aji accident
befell a three-phase synchronous motor, which went quietly on
dri>nng its load of 1,700 looms for four hours, imtil the mill shut
down at night.
For small motor work synchronous machines are somewhat
at a disadvantage, from the complication of the exciter and
inability to start under load. In sizes below 100 HP they have
been very generally superseded by the far simpler and more
convenient induction motor, the use of which is a most charac-
teristic feature of modem power transmission. In the use of
synchronous motors, both monophase and polyphase, there has
been often encountered an annoying and sometimes alarming
phenomenon known as "hunting," or where several machines
are involved, as " pumping. " In mild cases it appears merely as
a small periodic variation or pulsation of the current taken by
the motor, often sufficient to cause embarrassing periodic vari-
ations in the voltage of the system. The frequency is ordi-
narily one or two periods per second, varying irregularly in
different cases, but being nearly constant for the same machine.
The amplitude may vary from a few per cent of the normal
current upwards. Generally the amplitude remains nearly
constant after the phenomenon is fairly established, but some-
times it sets in with great violence and the amplitude rapidly
increases until the motor actually falls out of synchronism.
This is usually the result of pumping between two or more
motors, and seems to be especially serious in rotary convert-
ers, not only throwing them out of synchronism, but throwing
load off and on the generators ^^dth dangerous violence.
Fig. 130 shows a facsimile of a record from a recording volt-
meter showing the pulsation of the voltage on the system
produced by the hunting of a 300-HP synchronous motor. It
set in as the peak of the load came upon the system and per-
sisted until the peak subsided, when it was gotten imder con-
trol, only to break out again when the late evening load fell off. »
During the eariy evening it was so severe as to produce pain-
ful flickering in all the incandescents on the circuit.
In this case the dynamo tender was inexperienced and had
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ELECTRIC TRANSMISSION OF POWER,
not acquired the knack of so juggling the field current as to
suppress the hunting. A few months later the same system
was in regular operation without the least trouble from hunt-
ing, the operators by this time having been thoroughly broken
in. In the majority of cases adroit variation of the field
strength abolishes hunting, which almost always starts with a
Fig. 130.
sharp change in load or power factor. Just how to handle the
excitation to obtain the best results is a matter of experiment
in each particular case, but except in cases of unusually seri-
ous character the knack is soon acquired. A rather strong
field often steadies things, although if strong enough to pro-
duce leading current the trouble is sometimes aggravated.
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ALTERNATING CURRENT MOTORS. 236
But iji this case, as in operating alternators in parallel,
the best running conditions have to be learned by expe-
rience.
Much yet remains to be learned about the exact nature of
hunting, but its general character is about as follows: A
sudden change in current or phase causes the armature to seek
a new position of equilibrium. In so doing the sudden change
in the armature reaction momentarily changes the field
strength, which aggravates the instabilty already existing and
causes the armature imder the influence of its own inertia to
overreach and run beyond its normal position of equilibrium.
Then the field recovers and the armature swings back, once
more shifting the field and again overrimning, and so on ad
nauseam. The pulsation of the exciting current in cases of
hunting is generally very conspicuous, and the periodicity of
the himting seems to correspond in general with the time con-
stant of the field magnetization.
A fly-wheel on the motor or direct connection to a heavy
machine generally increases the trouble by increasing the
mechanical momentum of the armature, while belted and flex-
ibly cozmected motors suffer less. Heavy drop in the supply
lines, which makes the voltage at the motor sensitive to varia-
tions of current, and low reactance in the armature, which
favors large fluctuations of current, are conditions specially
favorable to violent hunting. Rotaiy converters in which the
armature current and its reactions are very heavy, compared
with that component of the current which is directly concerned
with the rotation of the machine as a synchronous motor, are
subject to peculiarly vicious hunting, which has often risen to
the point where it threw the rotary out of synchronism.
They are far less stable in this particular than ordinary syn-
chronous motoi-s, and cannot readily be controlled by varying
the excitation on account of the consequent variation of vol-
tage on the continuous current side.
Motors and rotaries having their pole pieces not laminated,
but solid, often show less tendency to hunt than machines with
laminated poles. If the poles are solid any violent swaying of
the armature current with reference to them is checked and
damped by the resulting eddy currents, so that the himting is
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ELECTRIC TRANSMISSION OF POWER.
pretty effectively choked. For the same reason alternators in
parallel are less likely to pump if they have solid poles, and most
foreign machines are built in such wise. Here, where laminated
poles are just now the rule, recourse is had to ** bridges" or
"shields." These are essentially heavy flanges of copper or
bronze attached to the edges of the poles, so that fluctuations of
armature reaction and of field are damped by heavy eddy cur-
rents whenever they arise, the bridges acting indeed like a
rudimentary induction motor winding. An example of such
practice is shown in Fig. 131, which shows a portion of the
FlO. 131.
revolving field of a large polypnase machine fitted with mas-
sive castings, bridging the spaces from pole piece to pole piece
and serving at once to hold the field coils rigidly wedged into
place and to check pumping. A similar device is used in con-
nection with many rotary converters with a very fair degree
of success. Occasionally pumping may be traced to some
definite cause like a defective engine governor having a periodic
vibration, but more often the phenomenon is purely electro-
magnetic. The use of shields or solid pole pieces constitutes
the best general remedy, for, while adjustment of the field
is often effective, it is often desirable to adjust the field
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ALTERNATING CURRENT MOTORS, 237
for other purposes, and the necessity of varying it to
suppress hunting is sometimes very embarrassing, if not
impossible.*
INDUCTION MOTORS.
An induction motor is a motor into which working current
is introduced by electromagnetic induction instead of by
brushes. It has therefore two distinct, although coordinated,
fimctions — transformer and motor. To understand its action
we must take care not to confuse these fimctions, and this is
best done by recurring to the fundamental principles that are
at the root of all motors of whatever kind.
An electric motor consists of these essential parts, viz.: A
magnetic field, a movable system of wires carrying electric
FlO. 132.
currents, and means for organizing these two elements so as
to produce continuous torque.
These parts are beautifully shown in their elementary
simplicity in Barlow's wheel, Fig. 132, invented some three-
quarters of a century ago.
In this machine A^ S is the permanent field-magnet, the
arms of the star-shaped wheel are the current-carrying con-
ductors, and a little trough placed between the magnet poles,
and partly filled with mercury, serves with the wheel as a com-
mutator. Its function is to shift the current from one con-
ductor to the next following one, when the first passes out of
an advantageous position. In other words it keeps the cur-
rent flowing so as to produce a continued torque, irrespective
of the movement of the conductors. Such is precisely the Timc-
* For the mathematical theory of the sabject see Steinmetz, Trans.
A. I. E. E. Hay, 1002.
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ELECTRIC TRANSMISSION OF POWER,
tion of the modem commutator, and it is interesting to note
that the device of making the armature conductors themselves
serve as the commutator is successfully used in some of the
best modem machines.
These same fundamental parts are found alike in motors
designed for continuous or for alternating currents. We have
already seen that a series-wound motor can serve for use with
both kinds of current, since the commutator distributes the
current alike for both, and since the direction of the torque is
determined by the relative direction of the main field and that
due to the moving conductors, alternations which affect both
symmetrically leave the torque unchanged.
Fio. 138.
We have seen also that if the distribution of currents given
by the commutator can be simulated by supplying the arma-
ture with alternating impulses timed as the commutator would
time them, we can dispense with the commutator, and sub-
stitute two slip rings. In this case, however, the motor will
run only when in synchronism, since then only will the alternat-
ing impulses from the generator be properly distributed in the
armature, as has already been explained. Besides, the current
has to be introduced into the armature through brushes bear-
ing on a pair of slip rings, and an exciter is required to supply
the field. If one could use an alternating field, and induce the
currents in the armature as one would in the secondary of a
common transformer, the machine would be of almost ideal
simplicity.
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ALTERNATING CURRENT MOTORS.
289
This is what is accomplished in the induction motor. The
field is supplied with alternating current, and the working cur-
rent is induced directly in the armature conductors.
To this end the brushes used in the previous examples may
be replaced by a pair of inducing poles, carrying the primary
windings, to which the armature windings play the r61e of
secondary. These armature windings are therefore closed
on themselves, instead of being brought out to slip rings.
For this short-circuited winding various forms are employed,
the simplest being shown in Fig. 133. It consists of a set of
copper bars thrust through holes near the periphery of the
Fio. 134.
laminated armature core, and all connected together at each
end by heavy copper rings.
The simplest arrangement of field and inducing poles is
shown in Fig. 134. Here each pair of opposite poles is provided
with a separate winding, so that the circuit A A supplies alter-
nating current to one pair and B B to the other pair. The
armature we will assume to be like Fig. 133. Now apply an
alternating current to A A, The windings of the armature
which enclose the varying electromagnetic stress will have set
up in them a powerful alternating current almost 180° behind
the primary current, i.e., in general opposed to it in direction, as
considerations of energy require. The armature will not turn,
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240 ELECTRIC TRANSMISSION OF POWER.
however, for two very good reasons: first, the current in it is
far out of phase with the magnetization of the poles; and
second, this current is quite symmetrical w^ith respect to
the poles, so that the only effect could be a straight push or
pull without the slighest tendency to attract or repel one
side of the armature more than the other.
To produce rotation as a motor, there must be not only a
current in the armature conductors, but there must be field
poles magnetized and disposed so as to produce a torque upon
these conductors.
Suppose, now, an alternating current to be sent around the
circuit B B. If it is applied simultaneously with the current
in A il, we shall be no better off than before, for since the two
pairs of poles act together and just alike, there is no magnetiza-
tion in phase with the armature current, and nothing to cause
the armature to turn either way.
To obtain rotation we must arrange the two sets of poles so
that one pair may furnish a magnetic field with which the cur-
rent induced by the other pair is able to react. The simplest
way of doing this is to supply B B with current 90^ in phase
behind the current in A A. Then when the current induced
hy A A rises, it finds the poles B B energized and ready to
attract it, for the magnetization in B B and the current are
less than 90^ apart in phase. The less the lag of the arma-
ture current behind its E. M. F., the more nearly will the
magnetization of these field poles be in phase with the armature
current, and the more powerful will be the torque produced.
The B B set of poles necessarily induce secondary currents in
the armature in their turn, toward which the A A poles serve
as field during the next alternation. The directions of both
armature current and field magnetization are now reversed,
so that, as in the commutating motor, the torque is im changed.
The next alternation begins the cycle over again, and so the
motor runs up to speed. Its direction of rotation depends
evidently upon the relative directions of magnetization in the
two sets of poles, for these determine the direction of the
armature current and the nature of the field poles that act
upon it. Reversing the current in A -4 or B B will therefore
reverse the motor, while reversing both will not.
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ALTERNATING CURRENT MOTORS. 241
The speed of the armature is determined in a rather inter-
esting manner. When the armature is in rotation the electro-
magnetic stresses which act upon a given set of armature
conductors are subject to variation from two causes. First is
the variation in magnetization, due to changes in the primary
current; second, the variation due to the armature coils mov-
ing as the armature turns, so as to include more or less of the
magnetic stress. The E. M. F. in the armature conductors is
due to the summed effect of these two variations. And since
the two are in opposition, if the armature were moving fast
enough to make a half revolution for each alternation of the
field, the E. M. F. produced would be zero, since the rates of
change in the field and in the area of stress included by the
armature coils would be equal.
This means that the armature must always run at less than
synchronous speed — enough less to produce a net armature
E. M. F. high enough to give sufficient armature current for
the torque needed.
Under varying loads, therefore, an induction motor behaves
much Uke a shimt-wound continuous current motor. In both,
the armature current is due to the net effect of an applied and
a coimter E. M. F., the former being delivered from the line
through brushes in the one case and by induction in the
other. In neither case can the speed rise high enough to
equalize these two E. M. F.'s. There is, however, a very curi-
ous and interesting form of induction motor which runs at
tnie synchronous speed until the load upon it reaches a certain
point, when it falls out of step like any other synchronous
motor, or under certain circumstances falls out of synchronism
and then operates like an ordinary asynchronous motor.
Its operation in synchronism seems a paradox at the first
glance; but the principle involved is really simple, although
the exact theory of the motor is a bit complicated. As has
already been noted, if the rates of change of magnetic induc-
tion due to the pulsation of the field and to the cutting of
the field by the armature coils are equal and opposite, there
will be no E. M. F. in these coils, and obviously no energy
can be transferred from field to armature. If, however, the
E. M. F. wave due to the change of magnetization in the field
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242 ELECTRIC TRANSMISSION OF POWER.
and that due to the motion of the armature coils through the
field are very diflferent in shape, there can still be a periodical
resultant E. M. F., generally of a very complicated description,
accompanied by a transfer of energy even at full synchronous
speed. A very irregular wave shape in the E. M. F. of supply,
or a distortion of it due to extraordinary armature reactions,
may produce this condition. Fig. 135 shows the primary
E. M. F. wave form as taken by the oscillograph across the
terminals of such a synchronous induction motor, and the cor-
responding current wave, which emphasizes the significance of
the facts just given. The condition is best reached in small
motors having sharply salient field poles. The writer has
never seen one which would start from rest unaided, the great
PlO. 135.
field distortion necessary being in the way, but once spun up
to or near synchronism they work admirably on a small scale.
The conditions of energy supply are obviously such as to be
highly unfavorable in motors of any size, but for laboratory or
other purposes where synchronous speed is wanted they are
very convenient for an output of i HP or so, and form a
very striking modification of the ordinary induction motor.
They have, up to the present, been made mostly by the Holt-
zer-Cabot Electric Co.
If the load on an induction motor increases, demanding an
increased torque, the armature slows down a trifle, until the
new armature E. M. F. and resulting current are just sufficient
to meet the new conditions. In the continuous current motor
this speed is determined by the resistance of the armature, to
which the current corresponding to a given decrease of speed
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ALTERNATING CURRENT MOTORS.
243
is necessarily proportional. In the induction motor the arma-
ture resistance plays a precisely similar r61e. Fig. 136 shows
the actual speed variation of a 100 HP induction mbtor in terms
of its output. The maximum fall in speed under full load is a
trifle less than 3 per cent, and even this result is sometimes
surpassed in induction motors for especial purposes, even a 1
per cent variation having been reached. A motor with higher
armature resistance w^ould fall more in speed, like a shunt
motor with a rather high armature resistance. We thus see
that the induction motor, as it should, behaves much like any
other motor; the torque is produced in the same way, and
obeys similar laws; the motor is similarly self-starting, and
works on the same general principles throughout. Obviously
the magnitude of the annature current in an induction motor is
s
'^A ■^' ^»^a
6ft ^
il
y
• HP. OUTPUT
Flo. 136.
determined, not by the armature resistance alone, but by its
impedance. As, however, the presence of reactance shifts the
phase of the current, and that component of current which is
effective in producing torque depends upon the resistance,
the relation just explained holds good. That current is deliv-
ered to the armature by induction is a striking feature, but
not one that implies any radical difference in principle.
It is not even necessary to use a polyphase circuit for work-
ing induction motors, for, imder certain conditions, the same
set of poles can perform the double duty of delivering current
and interacting with it to produce torque.
The principles of the induction motor, as here given, thus
become part of the general theory of the electric motor which
applies aHke to machines for continuous and alternating cur-
rent, quite independent of particular methods of construction
or operation.
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ELECTRIC TRANSMISSION OP POWER,
The great pioneers in induction motor work, Tesla, Ferraris,
and some others, preferred to view the matter from the special
rather than the general standpoint, and hold to the theory of
the rotary pole action of induction motors — very beautiful,
mathematically, but unfortunately hiding the Icinship of induc-
tion to other motors, and distracting attention from the trans-
former action, which is so prominent.
From this point of view the two pairs of poles in Fig. 134
FIG. 187.
Fia. 138.
Pig. 139.
co-act to produce an oblique resultant magnetization, which
shifts around the field^ producing a iijoving system of poles,
following the sequence of the current phases, and dragging
around the armature after them, by virtue of the currents in-
duced in it. Figs. 137, 138, 139 show the rudimentary prin-
ciples of the rotary pole. In Fig. 137 an annular field magnet
is wound with two circuits A A and B B, supplied with alter-
nating currents 90® apart in phase. The polarity of the
armature is represented diagrammatically by the rotating
magnet A^ S.
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ALTERNATING CURRENT MOTORS. 245
Now, when the current in il il is maximum (and that in J5 B
is consequently zero), the field has poles at P and P', which
exert a torque on the armature poles. As the current falls in
A A and rises in B B, the resultant poles move forward to P^
and P/ (Fig. 138), followed by the armature. When the cur-
rent BBisA maximum, and A A has become zero, the poles are
at P2 and P,' and so on. In order that the revolving poles
may induce, current in the armature, the latter must slip
behind so as to produce relative motion and change in electro-
magnetic stress.
This point of view is very interesting and instructive. It
deals, however, not directly with the two field magnetizations
— the functions of which have just been discussed — but with
a resultant rotary magnetic field, which may or may not have
a concrete existence, according to circumstances. It by no
means follows that because two equal energizing currents are
90® apart in phase, they must or do form a resultant rotary
magnetic field, or that, if they are so organized as to give a
ph3rsical resultant, their individual functions are superseded
and must be neglected.
The two views of the induction motor here set forth are not
in any way conflicting; they merely represent two methods of
treatment of the same phenomena. As it happens, the rotary
field point of view is from a mathematical standpoint the
easier, for it treats the resultant instead of its components,
and hence has been the oftener used, but in discussing certain
classes of induction motors, it is by no means convenient, and
is less general than the analytical method, which deals with the
separate components. In most commercial induction motors
there is imdoubtedly a resultant rotary field, but however con-
venient it may be to consider the motors in that light, it is not
well to lose sight of the general actions of which the rotary
field is a special case.
As a matter of fact, the several currents in a polyphase in-
duction motor may be so distributed that they cannot produce
a resultant rotary magnetization, and in certain heterophase
and monophase motors the "rotary field," in so far as one is
formed by the field, may revolve in one direction while the
armature starts and runs strongly in the other direction.
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ELECTRIC TRANSMISSION OF POWER,
Hence, the view here taken of the induction motor has been
generalized for the purpose of bringing out its relation to the
general theory of motors, and to take account of induction
motors, in explaining which the rotary pole theory would have
to be, as it were, dragged in by the ears.
Salient poles, like those of Fig. 134, are seldom used, and
the induction motor as generally constructed, consists of two
FlO. 140.
short concentric cylinders of laminated iron, slotted on their
opposed faces to receive the windings. Sometimes these slots
are open, and again they are simply holes close to the surface
of the iron.
The relation of the parts is well shown in Fig. 140, a 6 HP
two-phase motor by C. E. L. Brown.
In this case the exterior ring is the primary, and the revol-
ving ring the secondary element of the motor. The primary
^^dnding is of coils of Gne wire threaded through the core
holes, while the secondary member is wound, if one may use
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ALTERNATING CURRENT MOTORS.
247
the term, with solid copper rods imited at the ends by a broad
copper ring. The clearance between primary and secondary
is very small in all induction motors, almost always less than
I inch, sometimes less than ^ inch. The smaller the clear-
ance the better the machine as a transformer.
The primary of an induction motor is wound much as the
armature of a polyphase generator is wound, as described
already. Fig. 141 shows in diagram a two-phase winding for a
24 slot primary, and Fig. 142 a three-phase winding for the
same primary. In the former there are two sets of coils, A
Fig. 141.
and B, each forming a separate phase winding; in the latter the
three sets, A, B, d may be united to form either a "star" or
^'mesh" three-phase winding. In practice the primary winding
is nearly always polydontal, for the same general reasons that
hold for generator armatiu*es, but especially to keep down
inductance. For the same reason the secondary winding is
polydontal. As an example of the best usage in this respect,
Rg. 145 shows the number and relation of primary and
secondary slots in the motor shown in Fig. 140. There are no
less than 40 primary slots for a four-pole winding, t.6., 5 slots
per phase per pole, while the secondary has 37 slots, this od<J
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ELECTRIC TRANSMISSION OF POWER.
number being chosen to reduce the variation in the magnetic
relations of primary and secondary due to diflferent positions
of the armature.
Induction motors with fixed primary have the great advan-
tage of having no moving contacts, and no liigh voltage wind-
ings exposed to the strains due to revolution. On the other
hand a revolving primary makes it very easy to vary the resist-
ance in the secondary circuit, which is often desirable. Both
forms are used, the latter only rarely. Inasmuch as a large
proportion of the hysteretic loss occurs in the primary, since
FlO. 142.
in the secondary the variation of the magnetization is small,
a revolving primary, being of less dimensions than its secon-
dary, gives a slight advantage in efficiency. There is, however,
small reason to suppose that on the whole it is easier to build
one form than the other for a given efficiency with the same
care in designing.. In recent practice it is not imcommon to
wind the primaries of large induction motors for voltages up
to 10,000.
Motors with revolving primary are no longer regularly man-
ufactiu^d, the vastly superior simplicity of the other con-
struction being generally recognized. Plate ^"11 shows the
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Fig. 1.
FlO. X
PLATE Vn.
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ALTERNATING CURRENT MOTORS.
249
motors now in common use in this country. Fig. 1 is a stan-
dard Westinghouse "Type C C " motor of 10 HP. It is exceed-
ingly simple in construction, and efficient in operation. It has
a "squirrel-cage" armature similar to that of Fig. 133, but
the bars are in open slots and are of rectangular section, a
construction which gives a lower armature reactance than if
the iron were closed over the armature bars. These motors
start with a powerful torque, approximately two or three
times the torque at rated full load, when the full line voltage
is thro^sTi upon the primary, but of course take, under these
conditions, a very heavy current, so that in practice it is usual
to start them at reduced voltage, which gives all the torque
necessary without calling for excessive current. This is ac-
complished by means of a so-called auto-converter, of which
the essential connections are shown in Fig. 143. With the
switch in the starting position the applied voltage is only a
quarter or a half the normal voltage, the actual amount being
adjusted by means of the variable connections sho'WTi, and
when the motor has come up to its full speed under the starting
conditions the switch is suddenly thrown over, putting the full
working voltage in circuit. The actual appearance of the
latest form of auto-starter is shown in Fig. 144. The starting
voltage is applied in several steps and the whole device is
immersed in oil. It is necessary to let the motor reach its full
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ELECTRIC TRANSMISSION OF POWER.
speed with the lower voltage before making this change, else
there will be a needlessly severe current due to the sudden
acceleration under full voltage, and the change should be
made quickly, lest the armature speed should fall oflF during
the change and produce the same unpleasant result. When
intelligently handled the starting current can be kept within
very reasonable limits, but the auto-converter should be ad-
justed when set up to give at starting merely the voltage
needed to start under the required torque, an excess of voltage
meaning excess of cun-ent. Fig. 2 of Plate VII is a General
FlO. U4.
Electric **Type L" motor of 35 HP. The mechanical design
is very simple, giving a light and well ventilated structure.
The bearings can be shifted to compensate for wear. The
winding of the armature is a regular three-phase bar winding
furnished with starting resistances within the spider, which are
cut out gradually by means of a ring moved by the lever seen
just within the bearing spider. The starting resistances are
in many sections and can be short-circuited very gradually,
holding the primary current practically constant from start to
full speed, even when starting \mder a heavy torque. The
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ALTERNATING CURRENT MOTORS.
251
start is made with the lever pulled out to its fullest extent,
and it is gradually pushed home luitil full speed is reached.
Such motors are peculiarly well adapted for use on lighting
circuits, and in large sizes requiring heavy starting torque.
The start can be made with very moderate currents, and the
torque per ampere is considerably greater than in any motor
starting on reduced primary voltage, which is the compensa-
tion for the rather elaborate starting device. Neither of the
FIO. 146.
companies mentioned holds rigidly to the constructions here
shown, but the cuts show their best standard practice. There
is very little difference in the essential properties of the two
fonns, and both are very widely used.
These recent motors are nearly all made with extremely
small clearance between armature and field, from -Ar to A inch
or less, even in large motors. This practice renders it easy to
design for a good power factor, but may, and sometimes does,
cause trouble mechanically, as might be anticipated. It is not
difl&cult to make thoroughly good motors without resorting to
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ELECTRIC TRANSMISSION OF POWER,
such extreme measures, unless the designer is hampered by
troublesome specifications in other particulars. Demand for
slow speed motors at a periodicity of 60^, and insistence on a
uniformity of speed at various loads that would not for a mo-
ment be demanded in direct current motors, are responsible
for serious and needless impediments in induction motor .
design.
The "Type L'* motors just described have on the armature
Fia. 146.
a regTilar three-phase winding of rectangular Jjjars united by
end connectors. A simple fotu'-pole form of such a winding is
shown in Fig. 146. It obviously is more troublesome to con-
struct than a "squirrel cage" "s^inding, but it possesses certain
advantages. Conspicuously, it renders it possible to insert the
resistance in the secondary circuit at starting, which in the
"squirrel cage" would be a very difficult matter, although it
has been tried.
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ALTERNATING CURRENT MOTORS.
258
If the field is very uniform, with a thoroughly distributed
winding, there is very Httle difference in the actual perform-
ance of the two kinds of armatures (drum-wound and "squir-
rel cage") when at speed. In case of a motor with salient
poles or with few winding slots in the field, the drum armature
has a very considerable advantage, owing to the fact that the
currents in it are directed into definite paths which they must
follow at ail times, while in the ''squirrel cage" form the cur-
rents are only uniformly organized when there is a imiform
field. In the early motors, therefore, the drum- wound arma-
FlQ. 147.
ture had a great advantage, but as the art of designing has
advanced the two types have become closely approximated in
their properties.
In this country the windings of induction motors are gener-
ally placed in open or nearly open slots, as in the case of the
motors shown in Plate VII. Abroad the arrangement of wind-
ings in holes as shown in Fig. 145 is very common. Each
procedure has its advantages. The American practice renders
it very easy to place the windings, and to put a very large
amount of copper upon the armature, for open slots can be
made radially deep and filled true, while holes unless rather
large can only be trued by reaming, which implies a round
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ELECTRIC TRANSMISSION OF POWER,
hole, unfavorable if a great amount of copper is to be crowded
upon the armature. Hence, with open slots it is easier to sub-
divide the winding into many slots, thus reducing the armature
reactance. On the other hand, open slots are extremely unfa-
vorable as regards power factor, since the iron surfaces opposed
in armature and field are very greatly reduced, and hence the
tendency to use extremely small clearances in order to make
the best of a bad matter. The European practice is on the
whole better as regards power factor, but does not facilitate
the construction of motors of very low armature resistance,
and is considerably more difficult of proper execution. The
FlO. 148.
matter really hinges on the relative cost of labor here and
abroad. With cheap labor the manufacturer can afford to go
into little refinements if it is otherwise worth while, but at
American labor-rates handwork has to be minimized. On the
whole, the American motors are fully up to foreign standards
in general design, although the tendency here has been to
make a fetish of uniformity of speed, even at the expense of
more important characteristics.
In motors such as those just described, with distributed
windings and no sharply defined polar areas, the consecutive
exchange of motor and transformer functions among the wind-
ings is almost lost sight of in the presence of the very apparent
phenomenon of resultant revolving poles, but the appearance
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ALTERNATING CURRENT MOTORS. 265
of the latter is a necessary result of the persistence of the
former. These induction motors are generally operated from
the secondary circuits of transformers, although the large
sizes (50 HP and upward) are sometimes wound for use of
the full primary voltage up to 2,000 volts or more.
An early form of induction motor which possesses some
interestuig features is the Stanley machine, shown in Figs. 147,
148, 149. The field in Fig. 147 is composed of two separate
rings of laminated iron, each having eight polar projections.
These field rings are assembled side by side with the poles
** staggered," as shown in the cut. Each field is energized
separately, one from each branch of a two-phase circuit. The
armature, Fig. 148, is composed of two separate cores assem-
Fio. 160.
bled side by side. The secondary winding, Fig. 149, polyodon-
tal as usual, is common to the two cores. The transformer
and motor functions are here separated, for each half of the
machine acts alternately as transformer and motor, each set of
fields inducing current which serves for motor purposes in the
other half of the machine. There is no rotary field in the
ordinary sense of that term, since there is no physical resul-
tant of the two field magnetizations, nothing but the alterna-
tion of transformer and motor functions that is a characteristic
of all polyphase induction motors.
These motors have been generally used in connection with
condensers to improve the power factor, and to facihtate this
practice have been usually woimd for 500 volts.
A step further in the direction of simplicity, but generally
inferior to both polyphase and heterophase forms, are the true
monophase induction motors. The principle of these motors is
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ELECTRIC TRANSMISSION OF POWER,
shown in Fig. 150. Here there is but one set of poles energized
by the circuit A, while 6, c, d, are portions of the armature wind-
ing, which may be a simple squirrel cage, or a complex bar
winding similar to those used in polyphase motors.
If A be supplied with an alternating current, induced cur-
rents will be produced in the armature, out of phase with the
field magnetization and symmetrical with respect to it, so that
no torque is produced.
If, however, we spin the armature up to nearly synchronous
speed, the armature currents will lag, from self-induction,
Pio. 151.
behind the E. M. F. set up by the field, so that they have an
angular displacement with respect to the field at a time when
the latter is still active. There is, therefore, torque between
these two elements — in the direction of the initial rotation.
The motor will thus run, when once started, equally well in
either direction.
In every motor there must be not only a field magnetization
and current in a movable conductor substantially in phase
with each other, but there must be a stable angular displace-
ment between the two in order to ensure continuous torque.
In continuous current motors this displacement is secured by
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ALTERNATING CURRENT MOTORS. 257
the position of the brushes. In polyphase induction motors it
is obtained by the space relation of the sets of poles combined
with the time relation of the two or more currents.
In the monophase motor this angular displacement is due
to the displacement of the armature currents by inductance.
Hence, there is a particular value of the inductance correspond-
ing to the best condition of torque, more or less than this
being especially injurious in this type of motor.
In practice monophase induction motors are built in very
FlO. 162.
much the same form as polyphase motors, and for the same
reason, i.e., to make the structure good as a transformer. In
fact, the same motor structures are often used for both types.
Fig. 151 shows the manner of winding a six-pole monophase
primary, homologous with Figs. 141, 142. A monophase
induction motor of 120 HP by Brown, Boveri & Co., is shown
in Fig. 152. Monophase induction motors are not yet used to
any large extent in this countr}', and abroad their use is gen-
erally confined to motors much smaller than the example
shown.
A moment's reflection will show that while the supply of
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ELECTRIC TRANSMISSION OF POWER.
energy to a polyphase motor is substantially continuous, in
monophase motors it is essentially intermittent, so that the
latter give less output for the same structure, while the depen-
dence of the torque on the armature inductance generally
leads to low power factors.
Nevertheless, cases arise in which it is extremely convenient
to use single phase motors. There are still many small light-
FlO. 163.
ing plants equipped only with single phase generators, the
extreme simplicity of the circuits out-weighing the" advan-
tage of polyphase service in economy of copper and ready
availability of motor service. For the occasional motors de-
sirable on such systems some form of monophase machine is
important. There are also some large lighting plants that
serve their territory both by continuous current and in the
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ALTERNATING CURRENT MOTORS.
25d
remoter districts by alternating current for which such motors
are useful, and even on polyphase systems there are isolated
demands for motors which can best be filled by a circuit run
from a single phase.
Monophase motors are, too, rather more cheaply installed
on account of the simpler circuits and lower cost of trans-
formers, so that there is a genuine though at present rather
limited demand for them. As a result there have been deter-
mined and measurably successful efforts to produce practical
monophase motors capable of use at least in small sizes, with-
out the impairment of general regulation likely to come from
low power factor and large current at starting. Abroad the
ordinary monophase type just described is used, generally
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Started light and taking up its load with a clutch, but here a
self-starting motor is universally demanded.
One of the recent American contributions to the list of
monophase motors is somewhat out of the ordinary in that it
starts as an induction motor by the aid of a commutator.
This is the Wagner motor shown in Fig. 153. In its general
construction it is a pure monophase motor with an armature
winding the coils of which are at one end connected \ivith a
commutator. This has bearing on it a pair of brushes which
close upon themselves those armature coils which are in such
angular relation with the field magnetization as to give a
strong motor reaction with it. By thus keeping in action
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260
ELECTRIC TRANSMISSION OF POWER,
only coils giving an efficient torque in one direction, the
necessary directed torque at starting is secured, and when the
motor reaches a predetermined speed a compact little centri-
fugal governor throws over a short-circuiting ring, converting
the motor into an ordinary monophase induction motor. It
is possible to start under load with this device by drawing
rather heavily on the mains for current, but in any except the
smallest sizes it is better to start light.
Fig. 155 shows the characteristic curves of a recent 4 HP
Wagner motor, which gave a highly creditable performance for
a monophase motor of so small size. It comes much nearer
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representing real commercial conditions than the curves of
Fig. 154, and, as we shall presently see, does not make a bad
showing as compared with the polyphase motors ordinarily
found upon the market.
Although monophase motors as a class start at a great dis-
advantage compared with polyphase motors, they can be made
to give pretty good results at load by extraordinary care in
designing. Fig. 154 shows the curves obtained from a certain
Brown motor by Professor Amo. The motor was nominally
of 15 HP, but was evidently overrated at that load. Never-
theless, within a certain range of load the performance of this
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Fig. 2.
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ALTERNATING CURRENT MOTORS. 261
motor compares well with that of the best polyphase motors
of similar size. This motor had an extremely small air gap,
and shows doubtless a record performance in several respects,
but it proves that barring the matter of starting it woidd
be possible to turn out a pretty usefid machine of the mono-
phase type if anybody desired it, although it is certain that
at anything like equal cost of construction the polyphase
motor must retain the advantage. Fig. 1, Plate VIII, shows
a recent monophase motor of Westinghouse make. It is in
appearance hardly distinguishable from the polyphase motors,
and its operative qualities are said to be excellent. Speed
regidation, never any too easy in induction motors, is almost
out of the question in the monophase form. Still, within its
limitations it has its uses.
A very ingenious flank movement has recently been made
by the General Electric Co., upon the monophase problem in
a monophase induction motor with condensers. In polyphase
motors the usefulness of condensers had been shown by the
Stanley type previously mentioned, and the same device seems
specially usefid in overcoming the low power factor to which
the monophase form is especially prone. Plate VIII, Fig. 2,
shows the 2 HP monophase motor of this type, moimted upon
a base which contains the condenser. Its weight is 295 lbs.,
its nominal speed 1,800 r.p.m. at 60^^ and its slip at full load
2.75 per cent. Its fidl load eflSciency is 75 per cent, and
power factor 92 per cent. The condenser is hermetically sealed
in a tin case and is connected not as a shunt to the whole
field, but is closed upon an independent phase winding, so
that the motor belongs rather to the split-phase class than
to the strictly monophase. The armature likewise is given a
winding akin to that of the ordinary polyphase motors and is
provided with a starting resistance, in series with the armature
windings at starting and cut out automatically as the arma-
ture nears speed, by a centrifugal switch. An automatic
clutch pulley is also provided on these motors to further facil-
itate starting with moderate current.
As might be expected from these features the motor is singu-
larly free from starting and power factor difScidties, and, at
the cost of some complication to be sure, meets the end for
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262
ELECTRIC TRANSMISSION OF POWER.
which it was designed, of furnishing a motor suitable for con-
nection to single phase lighting circuits without material risk
of injiuing the regulation.
Fig. 156 shows the characteristic curves from a 5 HP motor
of this class. It will be observed that the full load power
factor is .95 and the real efficiency at the same point .80, which
is certainly an excellent showing. Obviously the power factor
is a matter of proportioning the condensers, and in motors
of this class of 10 HP, and upwards the power factor is raised to
unity, or the ciurent is even made to lead at certain loads.
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Fio. 156.
These various recent forms of monophase motor have come
into somewhat considerable, though scattered use, mostly in
sizes below 10 HP, although now and then motors of several
times this power have been installed. When judiciously in-
stalled they can undoubtedly be made to give good service.
There has also very recently been introduced a most inter-
esting type of single phase alternating ciurent motor derived
from and closely resembling in its properties the ordinary
direct current series motor. It is in fact a series motor spe-
cialized for alternating current working.
The direction of rotation of a series motor depends entirely
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ALTERNATING CURRENT MOTORS. 268
on the direction of the magnetizations produced by the field
and armature respectively. Consequently it does not change
if the direction of the current be reversed at the motor termi-
nals, but only if it be reversed as between field and armature.
Hence, such a motor would run even if supplied at its terminals
with alternating current, provided enough such current coidd
be forced through in spite of the high inductance of the ma-
chine. The first step toward this end would evidently be to
reduce the frequency, but evidently even at low frequency the
losses from eddy currents in the solid iron would be serious,
and the next obvious step is to construct the field as well as
the armature of iron laminated like a transformer core. In
fact it has been known for a long time that a series-wound
motor with a laminated field would operate after a fashion
when fed with low frequency alternating currents, say at 8 or
10 periods per second. The recent work has been in the direc-
tion of so specializing this machine as to keep down the in-
ductance and to reduce the sparking to reasonable limits when
operating at the lower commercial frequencies.
The chief electrical feature of the a.c. series motor is that its
total counter E. M. F. is the geometrical sum of the E. M. F.'s
induced by the motion of the armature conductors and those
due to reactance in the armature and field respectively. Now
the apparent watts supplied are measured by the product C E^y
where E^ is the impressed E. M. F. while the useful energy is
determined by E, the motor E. M. F. as in any other motor.
Hence, in order that an a.c. series motor should have a good
power factor and apparent efficiency, it is necessary to make
E large compared with the reactances of armature and field.
To do this the niunber and the speed of the armature wires
may be increased on the one hand and the reactances kept
down upon the other. The first condition points to a motor
having a relatively simple field and a very powerful high speed
armature, while the second condition calls for low frequency
and very careful designing against reactance.
To reduce the field reactance the turns on the field must
be kept low, since one cannot reduce the effective field magnet-
ization without reducing that in the armature also, and to
maintain the field with few turns, requires a small air gap of
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264 ELECTRIC TRANSMISSION OF POWER.
large area. Since one cannot reduce the air gap beyond a
certain point without mechanical difficulties, and cannot
increase its area much without increasing the general dimen-
sions of the motor, the saving of reactance in the field is neces-
sarily rather limited.
One can, however, considerably reduce the armature reac-
tance by winding a neutralizing coil so as to surrovmd the
revolving armature in a plane approximately perpendicular
to the line joining the brushes. This is known as a compen-
sating coil and the motor fitted with it as a compensated
series motor. Fig. 157 shows this arrangement in diagram.
The result is to very greatly diminish the net armature reactance
so that the power factor may be
carried to .90 or even more. Here
A is the armature, F the field, and
C the compensating coil.
The recently introduced single-
phase motors for electric traction
generally belong to this type, and
in certain cases these machines
may be useful for variable speed
work on commercial circuits, for
they behave imder supply at varied
voltage quite like d.c. series motors
and indeed can be worked on d.c. circuits. Fig. 158 shows a
Westinghouse single-phase railway motor with the armature
removed, showing the field coils and the compensating coils.
A modification of the same idea is shown in Fig. 159, where
the compensating coil is short circuited, the motor being other-
wise arranged as before. Still another commutating type of
a.c. motor is that shown diagrammatic ally in Fig. 160 in which
there are field and compensating coils in series, but the arma-
ture is short circuited upon itself. This is substantially like
Prof. Elihu Thomson's repulsion motor in the original form
of which, however, the coils F and C were replaced by a small
resultant coil in an intermediate position with' respect to the
brush line. This motor too has been developed for railway
work, and has much the same properties as the regular series
compensated type.
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ALTERNATING CURRENT MOTORS.
265
All these alternating motors have good power factors when
working near their normal speeds, often rising to .90 and
Fio. 168.
more, but their efficiency is generally materially less than that
of d.c. motors, or polyphase induction motors of similar out-
put. The losses from the more complex windings, from eddy
Fio. 169.
Fio. 160.
currents and from hysteresis are enough to cut down efficiency
generally 5 to 10 per cent, more often near the latter figure.
By careful design of the commutation sparking can be kept
within reasonable bounds, at least within a moderate range
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ELECTRIC TRANSMISSION OF POWER.
of speedy although the conditions for sparkless operations can
never be as favorable as in d.c. motors.
Several other modifications of the commutating a.c. motor
have been devised, but they all depend on principles similar
to those already mentioned, and may be expected to perform
in about the same way. None of them can reasonably be
expected to do materially better than the series compensated
aoaoaoio 6000 70 so soiooiiouo a.p]
Fio. 161.
type first described. They are one and all intended for low
frequency work generally 25 -^ as a maximum. The higher
the frequency the harder to build a good commutating motor.
Hence, whatever place such motors may find in traction, they
will probably be of rather limited applicability on ordinary
commercial circuits of the present usual frequency of 60^,
although they may be occasionally useful.
The practical properties of good modem induction motors
are strikingly similar to those of shunt-wound or separately
excited continuous current motora.
For the same output, the induction motor generally has the
advantage in weight, owing to the fine quality of iron which has
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ALTERNATING CURRENT MOTORS,
267
to be employed, but its laminated structure and rather com-
plicated primary winding make it fully as expensive to build, in
spite of the absence of a commutator.
In point of commercial eflficiency there is but little differ-
ence. It is not difficult to build an induction motor which is
fully up to the average efficiency of other motors of similar
output and speed. And what is of greater importance, the
question of sparking being eliminated, the point of maximum
efficiency can quite easily be brought somewhere near the aver-
OUTPUT-BRAKE H.P..
FlO. 162.
age load. It must be remembered that here, as elsewhere,
the last few per cent of efficiency are somewhat costly, and not
always found in the rank and file of commercial machines.
The weak point of commercial induction motors is apt to be
the power factor. Of course low power factor means demand
for current quite out of proportion to the output, and hence
greater loss in the lines and greater station capacity. In
addition, a heavy lagging current makes regulation of voltage
on the system anything but easy.
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ELECTRIC TRANSMISSION OF POWER.
Now, it is perfectly feasible to build induction motors with
power factors so high as to avoid these practical difficidties
almost entirely. But this result is somewhat expensive,
whether reached by finesse in design, or by the addition of con-
densers, and it is therefore not always attamed.
Slow speed induction motors, large and small, are subject
to bad power factors, and so in fact are all induction motors
having many poles. The best results, however, are very good
indeed. A power factor of .9 or thereabouts at normal load
t 10 li u i» — A Hb m k m
MECHANICAL HORSE POWER
FlO. 163.
is quite unobjectionable in practice, and this figure can be
reached or closely approximated by careful design.
In point of efficiency there is little difficulty in reaching
satisfactory figures. The actual properties of polyphase induc-
tion motors can be best appreciated by the examination of
their characteristic curves, showing the variations of efficiency,
power factor, and speed under varying loads. Fig. 161 shows
these curves for a 75 HP three-phase motor built by the General
Electric Company. It is a 60^^ motor, intended for severe ser-
vice, and hence is arranged to carry considerable overload at a
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ALTERNATING CURRENT MOTORS.
269
good efficiency. The fall in speed from no load to fidl load is
but 3 per cent, and the starting torque is 80 per cent greater
than full running torque, with an expenditure of current closely
proportional to the torque. The commercial efficiency reaches
91.1 per cent, and the power factor 84.3 per cent, which is not
bad for so large a motor intended for considerable overloads.
Fig. 162 shows the characteristics of a Westinghouse two-
phase induction motor of 50 HP for 25-^. Its properties, as
might be expected of a well-designed motor for so low a fre-
iooa ■KMtfuJbofr
FlO. 161
quency, are admirable, particidarly the great efficiency at
small loads.
Fig. 163 shows the properties of a polyphase motor of 20
HP at 130^, used with Stanley condensers to keep down the
results of the inductance encountered at so high a frequency.
The effect of this device, particularly at moderate loads, is
very striking indeed. Without condensers one could not
obtain such a power factor even at full load. While the con-
denser does not perfectly compensate for inductance, it does
so sufficiently well for all practical purposes. In other prop-
erties the motor is not so especially remarkable.
These curves are from the manufacturers' tests, and the
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ELECTRIC TRANSMISSION OF POWER.
author believes them to be entirely trustworthy, although they
probably represent good results. Better curves than these are
occasionally obtained, generally for some individual reason.
Now and then a "freak" motor is produced, with enormously
high efficiency or power factor, like a certain 5 HP three-phase
motor designed and tested by the author, which gave at full
load a power factor of .94.
On the other hand, it is unfortunately true that many com-
mercial induction motors are not as good in point of efficiency
and power factor as they ought to be. A series of tests of
induction motors under the direction of Professor D. C. Jack-
son was published a few years since, which gives data so instruc-
tive and impartial as to be well worth reproduction here. The
motors tested were, except for a 10 HP Westinghouse two-
phase, all of 5 HP nominal capacity, and by the following
makers: Westinghouse, Fort Wayne Electric Corporation (syn-
chronous self-starting monophase), Stanley, AUegemeine Elec-
tricitats Gesellschaft, General p]lectric Company. In addition,
results of tests on Oerlikon and Brown motors are inclu4ed in
the results. Fig. 164 shows the efficiency curves and regu-
lation of the several machines, and the table gives a general
view of their respective properties.
COMPARATIVE QUALITIES OF INDUCTION MOTORS.
1
2
3
4
6
610
7
8
1
L
I
p3
5
6
6.26
6
666
10
4.6
6
Torque, In Per
Cent of Full
Load.
140
100
104
186.5
171
147
163
131
46
90
136
138.6
142
232
^ -L
3
Efficiencies, Per
Cent.
77.8 65.2 70.6 77
83.8 54.5 72.5 80.6 83.8
Power Factors, Per
Cent.
9.7,79
7 79.6
3.7188
4.5182
3.7:79.161
4.683. 8'66.6
I I
171.9
62.2 77.2
75.2 85
67
78.4
78.fl|77
79.5I77.8
87.788
76.6 76.4 13
67.5 10. <
Si 174
82. 6,16. J
•8.5 6.1
37.6
5:25.5
81.6
5'44.7
8.27.6
81.9 81.li80 II6 40.4
73.8 78.4,79. 1,88.4 22.2 51 .3
62
80.283.882 89
62.3
67.3
3 83.7
5,73.7
659.8
480.3
6|81.7
71.6
64
84
80.3
70.3
73.4
85.2
87.2
Note. — 6, 7, and 8 were not run up to maximum load, on test.
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ALTERNATING CURRENT MOTORS. 271
Ix)oking over these results, Nos. 3, 5, and 8 are decidedly
the best of the lot. Of these, No. 3 is possessed of a fairly high
and very uniform power factor, but rather moderate efficiency.
It starts well, and ^ith a moderate current has sufficient
margin of capacity for all ordinary work, but its speed falls
considerably under load. No. 5 has extraordinary efficiency at
all loads, starts admirably, and can carry a tremendous over-
load — more than double its rated capacity. Moreover, it
regulates very closely. The power factor, however, is so bad
as to be a curiosity, having apparently been sacrificed to
obtain great maximum output, which is for many purposes use-
less. No. 8 is a far better all-round machine than any of the
others, has a good maximum efficiency at a little below full
load, and an excellent power factor. Professor Jackson notes
that since, at an output of 3J HP, No. 3 has an efficiency of
75.5, and a power factor of 83 J, while No. 5 shows respec-
tively 85 and 59, the station capacity for the latter must be con-
siderably greater than for the former. That is, the apparent
efficiency of Nq. 3, which determines the necessary station
capacity, is 64 per cent, while that of No. 5 is 50 per cent.
Hence, to supply one brake HP with No. 5 motors, there must
be a station capacity of 2 HP, while with No. 3 motors 1.56 HP
is sufficient. But with No. 8 the efficiency is about .83, and the
power factor about .80, giving an apparent efficiency of .66,
which is better than either No. 3 or No. 5. Motors like
No. 3 are excellent for the power station, but hard on the
customer, while No. 5 is admirable for the customer, but bad
for the station. No. 8 is fair to both parties.
Most of the motors shown start quite well enough for ordi-
nary purposes. Neither heavy starting torque nor ability to
carry large overloads is needed in ordinary motor work.
Large torque per ampere is, however, desirable. It is best
secured by using at starting a non-inductive resistance in the
secondary circuit as found in many existing motors. The
actual effect of this resistance is as follows: It reduces the
current drawn from the mains so that the motor will not
seriously disturb the voltage on the lines at starting; by dimin-
ishing the current flowing in the armature it limits the arma-
tiu-e reaction so that it may not beat back the field so as to
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272 ELECTRIC TRANSMISSION OF POWER.
interfere with proper starting, nor distort it so as to produce
dead points; and, finally, it largely increases the torque per
ampere, which greatly aids in starting under load.
The function first mentioned is very important where lights
and motors are to be operated, since if a motor is capable of
starting imder heavy load it is likely to take at starting a
pretty large current, which may pull down the voltage in the
neighborhood merely in virtue of ohmic drop. Besides, the
power factor of an induction motor at starting is only about
.7, so that the heavy current lags severely and still further
interferes with proper regulation.
The heavy lagging current set up in the armature is likely
to distort the field seriously, sometimes so much as to block
the starting of the motor, sometimes merely producing dead
points, i.e., points of no torque, or greatly weakening the
torque in certain positions of the armature. The introduc-
tion of resistance in the secondary circuit both diminishes the
current and its angle of lag, and thus keeps down the arma-
ture reaction. In some motors the reluctance of the magnetic
circuits is sensibly the same in all angular positions of the
armature, so that there are no points of noticeably weak
torque either with or without a starting resistance. But
some motors otherwise excellent have sufficient variations of
reluctance to produce bad dead points when the armature
reactance is severe, while these nearly or quite disappear by
adding resistance in the secondary circuits.
The use of resistance in the secondary at starting obviously
throws forward the phase of the secondary current so that it
is in better relation to the field magnetization, and hence
although the numerical value of the current is reduced, its
effective component is increased. The considerations which
affect the relations between torque and current in the arma-
tures of induction motors are in reality quite simple. The
absolute value of the current, other things being equal, is
determined by the armature impedance, and is the same for
the same impedance whatever the relation between the reac-
tance and resistance comiponents of that impedance. The
ratio between these components, however, determines the
phase angle of the armature current, so that for a given value
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ALTERNATING CURRENT MOTORS, 273
of the cun*ent the torque depends on the ratio between resis-
tance and reactance in the armature. x
By lessening either the resistance or reactance of the arma-
ture a motor is obtained in which a very large ciu'rent flows
at starting, but reducing the impedance by cutting down
reactance gives the resulting current a better phase angle
than that obtained by reducing resistance alone. For a given
motor the maximum torque is obtained when the ratio of
resistance and reactance is unity, i.e., when
7 = R.
Now, one can cut down the resistance by increasing the
allowance of armature copper, and can diminish the reactance
by subdividing the winding so that there shall be many slots
in the armature, and the minimum possible number of turns
per slotr Also the better the mutual induction between field
and armature the less the reactance of either member is likely
to be, so that by close attenticm to design it is possible greatly
to reduce the armature reactance. In commercial motors the
relation between resistance and reactance in the armature is
generally from
/ = 3 i2 to / = 10 fi.
Hence, when large torque per ampere is desired the simplest
thing to do is to insert non-inductive resistance in the secon-
dary, and when
/ = 22
the given motor ^vill be at its best with respect to starting
torque. With
7 = ft
the maximum torque will be obtained when both are as small as
possible. Hence, if very great starting torque is desired, the
motor should be designed witH very low armature resistance
and reactance.
The slip of the motor below synchronous speed depends
upon the armature resistance in induction motors, just as in
continuous current motors the slip below the speed at which
the armature would give the impressed E. M. F. is determined
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274 ELECTRIC TRANSMISSION OF POWER.
by armature resistance. In each case the slip measures the
percentage o#^ energy lost in the armature, so that if an induc-
tion motor, for example, runs loaded at 5 per cent slip the loss
of efficiency in the armature is 5 per cent.
Commercial induction motors vary widely in slip — from as
little as 1 per cent to 8 or 10 per cent, according to design.
It must not for a moment be supposed, however, that small
slip implies high efficiency of the motor. One can put, in
designing a motor, most of the loss into the armature or into
the field, as one pleases, and it is pretty safe to say that if there
is remarkably little in the armature there will be an unusual
amount in the field, unless cost is utterly disregarded. Prob-
ably the best all-around results can be obtained by dividing
the permissable loss nearly equally between armatiu^ and
field.
There is a very simple relation between the static and run-
ning torques of an induction motor, the static and running
currents, and the slip, as follows:
T ,,C»
— — is •
T. C.>
In this equation T, is the static torque, C, the static current,
Tg and C, torque and current of the slip S, and S that slip
expressed as a percentage. As an example of the application
of this formula, suppose the full load current of a certain
motor is 60 amperes per phase, the current with the armature
at rest 400 amperes, and the sUp at full load is 5 per cent.
Then
T ^^ 160,000 ^^^
— = .05 X =2.22,
T, 3600
I.e., the static torque will be 2.22 times the full load running
torque. Of course, if a motor is to have a powerful starting
torque it must take a pretty heavy current, but the extra
resistance at starting helps very materially in keeping the
current within bounds. An adjustable secondary resistance
makes it easy to bring to speed any load that the motor will
carry continuously, without demanding excessive current.
As to overload, an ability to carry 25 per cent more than
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ALTBRNATtNO CURRENT MOTORS.
276
the rated capacity is ample, save in rare cases, and greater
margin than this usually means some sacrifice in efficiency
or power factor at normal loads. For most work an effi-
ciency curve Uke that of No. 8 is preferable to one like that
of No. 5. When great margin of capacity is needed, it is best
to use a motor deliberately adjusted to such use, and not to
expect it of a motor properly designed for ordinary service.
The speed of induction motors is best regulated by inserting
19
-3^
A
11
yL
7^
10
tecH
/
r^
c
Q
i—
c=:
^
-
8
•0—
y
y"
1'
/
X
L
>
/
—
ye
J
y
/
4
'i
y
r
f
I
/
•r- —
.
/
f
1
, r
<
oT""
r""
1 <
%
9 <
EtpMd
iolO
Df.|>.
\.^'
o \
1 \
s r
» 1
\ il
Fio. 165.
a non-inductive resistance in the secondary circuit. Under
these circumstances the motor can be made to nm at con-
stant torque over a very wide range of speeds by varjdng the
resistance, just as one would regulate a street-car motor.
Fig. 165 shows the variation in speed, current, and power
factor in a 15 HP three-phase motor fitted with rheostatic
control. The speed was varied at constant torque from about
1,400 r. p. m. down to 150 r .p. m. Curve B shows the variation
of the power factor, in this case high at all speeds, and curve
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276 ELECTRIC TRANSMISSION OF POWER,
C shows the slight variation iji input. Operated in this way,
the motor behaved almost exactly like a series-w'ound direct
current motor with rheostatic control. Such a rheostat is
used in operating hoists and the like with induction motors.
Regulation by varying the primary voltage is highly imsatis-
factory, since the torque /alls off nearly in proportion to the
square of the voltage, so that at low speeds the output is
enormously reduced. Regulation by any method involving
resistance is of course inefficient, not materially more so, how-
ever, than in the case of continuous current motors. It should
be understood that all these remarks concerning torque,
regulation, and the like apply to polyphase induction motors
and do not hold true in general of monophase motors.
The w^eak point of induction motor practice is in the heavy
inductance likely to be encountered imless motors with first-
class power factors are used. It is depressing to find the
current capacity of your generator exhausted long before it
has reached its rated output in kilowatts, and if the motor
service is part of a general system, the effect of a bad power
factor on regulation is disastrous.
With generators of moderate inductance and good motors,
general distribution by polyphase currents gives admirable
results. The station manager should see to it that his motors
are not of excessive size for their work, and are good in the
matter of power factor. A few motors for very variable loads
can be handled readily enough, but no motor with a bad
power factor should be tolerated simply because it is cheap.
Power factors of at least .85 at full load, and .80 at two-thirds
load, are quite obtainable except in case of some special motors,
and should be insisted upon rigorously.
One polyphase station operating more than fifty induction
motors showed, when tested by the author, about .65 as aver-
age power factor when carrying all the motors. Rigorous
inspection of the motors installed would have raised this
figure to .75, although the existing power factor actually gave
no trouble, there being ample generator capacity.
So much for polyphase induction motors. Monophase
motors generally fail to give so uniformly good results. Oc-
casional extraordinary results have been reported from the
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ALTERNATING CURRENT MOTORS 277
latter, but in the author's opinion they concern motors which
belong to the "freak " class alluded to, and cannot be expected
in commercial practice. Monophase motors are usually weak
in power factor save at certain loads, and start badly, most
of those in use abroad being started without load. Even so,
the starting current is large, as may be safely concluded from
the discreet silence preserved on this topic in all descriptions
of monophase motor installations. In general power trans-
mission work the incandescent lamp and the induction motor
are the chief factors. Sjmchronous motors are valuable in
their proper place, and arc lighting and continuous current
work are sometimes relatively important. The alternating
current systems are now far enough developed to be entirely
workable and trustworthy for incandescents and motors.
The alternating arc lamp is, however, not quite in condition
to replace the continuous current arcs for all purposes and
under all circumstances, and for work specially suited to con-
tinuous cmrents reliance has at present to be placed in various
current reorganizing devices, which are, so far, of rather
indeterminate ultimate value. Whether they are to have a
large permanent place in the art, or whether their sphere will
gradually be much contracted, is uncertain. At all events
it is sufficiently clear that the main body of power transmission
will have to depend on alternating currents, at least for a long
while to come.
Even if continuous current should be obtained somewhat
directly from coal in the near or far future, the result would be
not to increase power transmission by continuous currents,
but to render the transportation of coal by far the cheapest
method of transmitting energy.
The relative importance of polyphase, heterophase, and
monophase systems is a question often raised. The present
indications are that the polyphase systems, in virtue of in-
creased output of generators, possible economy in copper and
general convenience, have come to stay. The monophase
motor problem has not yet been satisfactorily solved in any
general way, and until it has been solved, the monophase sys-
tem must remain subordinate, like the heterophase systems,
which are special rather than general in their appUcability.
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278 ELECTRIC TRANSMISSION OF POWER,
The much mooted question of frequency will be referred to in
its different bearings in connection with other topics. The
frequencies once common, 120^ to 135^, are rapidly passing
out of use for all important work. They are inconveniently
high for long lines by reason of inductance, are troublesome
for large units, lead to high inductance in the system, and
have for their only compensating advantage, lessened cost of
transformers. Both here and abroad lower frequencies have
come into use. In this country 60^ seems to be the favorite
frequency, except for work with rotary converters, when 25-w
to 35-^ is usual. Both these last are too low for general
practice, since the cost of transformers is greatly increased;
the former is unsuitable for incandescent service, miless with
extremely low voltage lamps, and both are unsuitable for
alternating arcs. It is now pretty generally recognized for
the above reason, that the adoption of so low a frequency as
25-w in the great Niagara plant was an error of judgment,
perhaps brought about by an overestimate of the importance
of rotary converters in general distribution. The only appa-
ratus which at present demands low frequency is the single-
phase commutating motor. Should it come into great use
plants of 25*^ or less may be necessary, but for general dis-
tribution it is always preferable to keep the frequency high
enough for incandescent lamps, which are the most profitable
kind of load.
On the other hand, abroad a compromise frequency of 40^
to 50^ is in general use. In the author's opinion there are
very few cases in which lower frequencies than these are
desirable, and none in which less than 30--^ should be toler-
ated for general distribution work; 50^ or 60^ meets general
requirements admirably, and only in rare cases is the use of
rotary converters of sufficiently commanding importance to
call for a lower frequency.
In connection with this topic we may consider a verbose
controversy which has raged of late, respecting the advantages
of certain irregular forms of alternating current waves vs, a
true sine wave. The facts in a nutshell are as follows: Cer-
tain complex current waves, whose irregularity is due to the
presence of harmonics of higher frequency, have been found
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ALTERNATING CURRENT MOTORS. 279
to give slightly better efficiency in transformers than sine
waves of the same nominal frequency. Such waves, however,
do not hold their form under varying conditions of load, and
by reason of their harmonics of higher frequency raise the
inductance of the line and apparatus, increase the probability
of resonance on the line, hamper all attempts to balance the
inductance of the system by condensers or synchronous motors,
and finally sometimes interfere with the proper performance of
induction motors. The use of such wave forms, then, is likely
to lead to very embarrassing complications in a power trans-
mission system, and their sole advantage is far better secured
by using a sine wave of slightly increased frequency, than by
interpolating a set of worse than useless harmonics.
It is needless to say that all cases of power transmission
cannot be treated alike — there is no system that will meet all
conditions in the best possible manner. The best results will
be obtained by treating, in the preliminary investigation,
each problem as an imique and independent case of power
transmission, and afterward boiling down the conclusions to
meet practical conditions. Avoid, when you can, apparatus
of peculiar sizes and speeds — remember that you are after
results, not electrical curios. See to it that what is done is
done thoroughly, and for general guiding principles keep your
voltage up and your inductance down, and watch the line.
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CHAPTER VII.
CURRENT REORQANIZERS.
Whatever method may be employed for the transmission
of power in any given case, it will often be found that the
current delivered at the receiving station is not of the charac-
ter needed. Sometimes in transmissions for special purposes
no difficulty ^vill be met, but frequently, especially in the
transmission of power for general distribution, both continu-
ous and alternating currents are needed, whereas only one is
at hand. For all electrolytic operations, for most railway
work at present, for telegraphy, and sometimes for arc light-
ing, continuous current is necessary, while alternating current
is necessary for convenient application to electric furnaces,
electric welding, electro-cautery and other minor purposes.
So whichever kind of current is transmitted the other must be
derived from it for certain uses.
All devices for thus changing alternating to direct currents,
or vice versa, with or without accompanying change of voltage,
may properly be called current reorganizers.
Three classes of such apparatus have come into considerable
use: 1. Commutators; 2. Motor dynamos; 3. Rotary con-
verters. These classes are quite distinct from each other;
each has advantages and faults peculiar to itself, and all three,
especially the last named, are in e very-day practical use to a
greater extent than would seem probable at first thought.
We have already looked into the matter of commutation in
Chapter I, and have seen how the naturally alternating cur-
rents in a continuous current dynamo are rectified and
smoothed. Given, then, an alternating current received from
a distant generator, and it would seem an easy matter to-
receive this ciurent upon a commutator and deliver it as con-
tinuous current. In point of fact there are very serious diffi-
culties in this apparently simple process.
The current received is a set of simple alternations shown
280
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CURRENT REORGANIZERS, 281
diagrammatically in Fig. 166. The figure shows three complete
periods. Now, if such a current be sent into a simple two-part
commutator, such as is shown in Fig. 9, Chapter I, revolving
at such a speed that the brushes will be just passing from one
segment to the other every time the current received changes
direction, the result will be a rectified current, shown in Fig.
167, unidirectional, it is true, but far from continuous. Vari-
FlG. 166.
ous modifications of this simple rectifying apparatus have been
and are in extensive use for supplying current to the field
magnets of alternating generators. As these machines are
generally multipolar, the two-part commutator has been modi-
fied so as to reverse the current at each alternation. Fig.
168 shows one of the simple forms of commutator arranged
for self-exciting alternators. It consists of a pair of metal
cylinders mounted on and insulated from the dynamo shaft.
Each cylinder is cut away into teeth, and the two are moimted
so that the teeth interlock with insulation between them.
Each pair of consecutive teeth acts like the ordinary two-part
commutator, and there are of course a pair of teeth for every
pair of poles, so that the commutator acts at each alternation.
The resulting rectified current is then led aromid the field
magnets of the generator, furnishing either the whole excita-
tion, or enough to compoimd the machine. Such a current,
Fio. 187.
however, is so fluctuating that it is by no means the equivalent
of an ordinary continuous current for magnetizing purposes,
hence in most modem machines the main exciting current is
furnished by a small exciting dynamo, driven from the alter-
nator shaft or by separate means, while the rectified current is
used only now and then for compoimding.
This simple current reorganizer is very successful for the
purpose described. But it must be remembered that the
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282 ELECTRIC TRANSMISSION OF POWER.
amount of energy concerned is trifling, only a very few kilo-
watts being required to compound even the largest alternators.
And despite this, there is often trouble from sparking, such
commutators being notoriously hard to keep in good order.
In applying the same process to rectifying currents on a larger
scale, the difficulties from sparking are very serious, in fact
generally prohibitive. And the worst of it is that they are
inherent. The root of the trouble is that the alternating
current on a line used for general purposes cannot be kept
accurately in step with the motion of the commutator. To
ensure sparkless commutation the conditions must be as shown
in Fig. 169.
The alternations of the current and E. M. F. are shown by the
Fio. 168.
solid line, while the brushes at the moment of passing from one
commutator segment to the next must take the position b 6,
with respect to the current. That is, they must pass from one
segment to the next at the moment when the current, jiLst
reversing, is practically zero. So long as the electromotive
force and the current are in phase with each other, as shown
in the solid line, the current will be rectified without trouble-
some sparking. But when the current lags behind the
E. M. F., as shown by the dotted line of Fig. 169, there is trouble
at once. The brushes, as can be seen from the dotted pro-
longations of b b, nmst break a considerable current, and there
is certain to be sparking. Nor can any point be found for the
brushes at which they will not have either to break this cur-
rent or to pass from one segment to the next while there is
considerable E. M. F. between segments. The case is bad
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CURRENT REORGANIZERS,
283
enough in a compounding commutator having a position fixed
with reference to the E. M. F. of the machine and dealing
with low voltage and moderate current. The inevitable result
is sparking that can be only mitigated by shifting the brushes,
and more or less demoralization of the compoimding. If the
current be received from a distant generator on a commuta-
tor driven by a synchronous motor, the condition of things
is much worse. When the current lags (or leads), not only
are the brushes generally thrown out of step with it, but if
there is a sudden change of phase the inertia of the commuta-
ting apparatus will put it at serious variance for the time with
both current and E. M. F. Add to this the disturbances of
phase produced by armature reaction in both generator and
motor, and one has a set of conditions that renders sparking
Fio. 169.
absolutely certain. The most that can be done to help matters
is to employ palliative measures to delay the destruction of the
commutator. Aside from this sparking, it is nearly out of the
question to hold the voltage of the rectified current steady if
the phase is shifting, as it often is likely to be.
Incidentally may be mentioned the fact that in working such
a commutating apparatus, just as in rotary converters, the
direction of the rectified current will be uncertain; the brush
which happens to be on a positive segment when the brush
circuit is closed, will stay positive, as can readily be seen by
tracing out the rectifying process in Fig. 168. In ordinary
compoimding commutators this imcertainty is absent, for with
the brushes in a fixed position the positive segments will
always be under the same brush, since the segments are fixed
with reference to the armature coils.
No small amoimt of time and money has been spent in try-
ing to work out a successful synchrcmizing commutator. The
main trouble is, of course, sparking, and the exasperating part
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284 ELECTRIC TRANSMISSION OF POWER,
of the problem is that while on a small scale, as in compound-
ing alternators, fair results can be obtained, the difficulties
increase enormously with the output, so that every attempt on
a scale really worthy of serious consideration has ended in
discouragement and the scrap heap.
The great usefulness of such apparatus if of reasonably good
qualities, has made this field of experimentation very interest-
ing, and a vast amount of ingenuity has been expended in
elaborately devised plans for reducing sparking and mini-
mizing the evil results of shifting phase. An example of such
work, of more than usual merit, was shown at the International
Congress of 1893 at Chicago. This was the current reorganizer
PlO. 170.
devised by C. Pollak, for use in connection with accumulator
installations. It was intended specifically for charging accum-
ulators, and is very ingeniously adapted to that use. Its
general appearance is shown by Fig. 170. The apparatus
consists of a small synchronous motor driving a commutator,
which has, m the example shown, eight segments coupled alter-
nately in parallel so as to produce the effect of Fig. 168. The
Pollak commutator is, however, peculiar in that the spaces
between segments are of nearly the same width as the segments
themselves, while the collecting brushes are set in pairs, so
that by setting one of each pair ahead of, or behind the other,
the ratio of segment width to space width can be changed. In
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CURRENT REORGANIZERS, 286
charging accumulators the E. M. F. of the charging current
must always, to prevent waste of energy, exceed the coimter
E. M. F. of the battery. Hence a current rectified as in Figs.
167 and 168 cannot successfully be used. The arrangement of
segments jiust described enables the brushes to be so set that
contact with a segment is made at the moment when the rising
E. M. F. of the alternating side is exactly equal to the counter
E. M. F. of the battery, and broken when the falling E. M. F.
reaches the same value. Only that part of the current wave
of which the E. M. F. exceeds the counter E. M. F. of the
battery is used, the charging circuit being open during the
remainder of the period. When well adjusted and used on
a circuit nearly non-inductive, the machine in question is
almost sparkless and very well adapted for the particular pur-
pose intended. It is also highly efficient, the only losses being
those in the motor, plus brush friction. The total amount of
these need be but trifling, probably less than 5 per cent of the
output.
But such apparatus cannot be considered as a general
solution of the problem, for while quite successful for an
output of 10 KW or so, it has not been tested in large sizes,
nor under the conditions of inductance ordinarily to be ex-
pected on a power transmission circuit. For the reasons
already adduced the chances for success are not good, particu-
larly since all questions of sparking become very grave when
large currents nmst be dealt with. This difficulty is well
known in dynamo working. For instance, in an arc machine
there may be frequent recurrence of the long, wicked-looking
blue sparks familar to every dynamo tender, without notice-
able damage to the commutator, while in a low voltage
generator sparking of much less formidable appearance may
put the machine out of business in a very short time.
Bearing all this in mind, it is but natural to expect that
another particular solution of the reorganizing problem might
be foimd for arc lighting. Here the irregularity of a "recti-
fied" current is of small consequence, while the small amount
of current cannot cause really destructive sparking if other con-
ditions are fairly favorable. So it is that we find commutating
apparatus in quite successful use for arc lighting in connection
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286
tJLECTRiC TRANSMISSION Of POWER.
with alteniating stations. The form of apparatus shown in
Fig. 171, designed by Ferranti, has been introduced in
several British stations with good results. The comniutating
mechanism is of course used in connection with a "constant
current" transformer, arranged so as automatically to hold
the current closely imiform under all variations of load.
Each conmmtating unit supplies two separate arc circuits of
moderate capacity — twelve lights in each. How well the same
device works at several times the E. M. F. necessary to supply
so small a series, is now being demonstrated. The present
tendency in central station practice is to employ very high
FiQ. m.
voltages for arc lighting — 50 to 100 or 125 lamps in series,
thus greatly simplifying both the station equipment and the
circuits. The rectifier should at least be able to replace the
smaller generators now in use, and such machines are now built
for as many as sixty lights. This is probably practical — in
fact there seems to be no good reason why the rectifier should
not be entirely available wherever it is desirable to work
series arc circuits in connection with a transmission plant.
Although not in use sufficiently long to enable one to pass a
final judgment, the machine is at least promising and worth
careful investigation. There seems to be some doubt as to the
successful working of these rectifiers at anything except rather
low. frequencies, 30 to 40^ or less, but such a difficulty would
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CURRENT REORGANIZERS. 287
appear to be constructional rather than inherent. It is possi-
ble that the alternating arc lamp will be developed far enough
to render continuous current arcs entirely unnecessary, but this
remains yet to be proved, although the inclosed alternating arc
now gives highly successful results, particularly in street
lighting.
All rectifjdng commutators now in practical service are of
very limited output — not much exceedhig 10 to 20 KW, an
amoimt merely trivial so far as large enterprises are concerned.
For railway work or incandescent lighting, these very interest-
ing machines cannot be considered in the race at present. The
general problem is as yet unsolved by such means, useful as
they may be for special purposes.
The current delivered by rectifiers is in a measure discon-
tinuous, and, hence, is not the full equivalent of an ordinary
continuous current. The PoUak machine, however, which is
intended to be used with a somewhat flat-topped alternating
current wave, has been successfully employed for working
motors as well as for charging accumulators. It is not impos-
sible that such apparatus may yet be constructed of sufficient
capacity to be of much practical service, although the difficul-
ties, as has already been pointed out, are very considerable,
and of a kind very hard to overcome. Of course, polyphase
currents can be rectified by following the same process as with
monophase current, and a successful apparatus would often
find some place in transmission plants.
The advantages of the rectifying commutator are simplicity,
efficiency, and cheapness, particularly the last. The working
parts are a small synchronous motor, made self-exciting (and
self-starting) by a commutator, and one or more rectifying
commutators driven by this motor. To obtain 100 KW out-
put, it is not necessary, as in other forms of current reorgan-
izers, to have a machine nearly as large and costly as a 100
KW dynamo. On the contrary, a one or two horse-power
motor would be amply powerful to drive the commutator,
and the whole affair could hardly cost a quarter as much as
a d)aiamo of the ^ same capacity, besides being of greater
efficiency, particularly at partial loads. But a hundred kilo-
watts is far beyond the output of any rectifier that has yet been
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288 ELECTRIC TRANSMISSION OF POWER,
put to commercial service, and even a hundred kilowatts is but a
fraction of the output that is often desirable in a single imit.
On the other hand, a rectifier must require at least the same
care as a dynamo, and must in every practical case be employed
in connection with reducing transformers to bring the alter-
nating current to the right voltage. The regulation too, is
somewhat dubious, since compound winding is out of the
question. And the current is at best disjointed, likely to
produce needless hysteresis, and of a character rather hard to
measure conveniently.
To sum up, the rectifying commutator, while quite good
enough for certain particular purposes, has so far given no
definite promise of general usefulness. All of the serious
attempts to develop it on a considerable scale have ended in
failure. It is not effectively reversible, so that the task of
converting continuous to alternating currents is quite beyond
it. While the cheapness, lightness, and efficiency of such
apparatus puts it in these particulars far ahead of any other
type of current reorganizer, the verdict of experience has so
far been adverse in spite of these advantages, and engineers
have been driven to other and more cumbersome devices.
The most obvious method of deriving continuous from alter-
nating currents, is to employ an alternating current motor in
driving a continuous current dynamo. The two machines
may be connected in any convenient way, by belting, clutching
the shafts together, or by putting them in even more intimate
connection by placing two armatures on the same shaft or two
windings on the same core.
The procedure first mentioned is not infrequent, particularly
when a transmission of power plant is installed in connection
with an existing lighting or power station. A synchronous
motor is installed in place of the previously used engines,
belted m any convenient way to the existing generators,
and the operation of the station goes on as before. Further
description is unnecessary, as the apparatus is in no way out
of the ordinary, and not at all specialized for the conversion of
alternating to continuous currents. As a rule such installa-
tions have temporary, and have been replaced later by special
apparatus worked directly from the transmission system.
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CURRENT REORGANIZERS. 289
A more interesting way of accomplishing the same result is
by the use of a twin machine comprising motor and generator
on the same bed plate, or even on the same shaft. In this way
the reorganizing apparatus is formed into a compact unit,
convenient to install and to operate, and possessing an effi-
*ciency higher than that of two belted machines, by the belt
losses and more or less of . the bearing friction. The total
increase of efficiency is perhaps 5 per cent, when the com-
parison is between a pair of coupled machines and a pair
directly belted, or more if the belting be indirect. Moreover,
PlO. 172.
the motor and dynamo parts of the machine can each be
designed so as to give the best efl&ciency and economy of
construction possible at the given mutual speed. A unit of
this class is shown in Fig. 172 — an early Siemens continuous
alternating transformer. The motor part is wound for 2,000
volts, monophase, and the dynamo part, of the well-known Sie-
mens internal pole-type, with overhung armatiu-e and brushes
directly on the windings, delivers continuous current at 150
volts. In this case the machine has three bearings, although
in many cases it would be quite possible to get along with
two. The main advantage of this duplex form of machine is
the complete independence of the two component parts in
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290 ELECTRIC TRANSMISSION OP POWER.
their electrical relations. The motor part can be designed
for any desired voltage or number of alternations. It can
often, except in very long transmissions, take the line voltage
directly without need for reducing transformers, while the
number of alternations can be chosen solely with reference to
general conditions and without considering the direct cmrent '
end of the machine at all. This, as will be seen when we have
considered some other types of current reorganizers, is a very
valuable property, since it gives the power of obtaining con-
tinuous current in a thoroughly practical way from alternating
currents of any frequency. Other reorganizers can be worked
to advantage only within a somewhat limited range of fre-
quency. Again, the motor dynamo can be compounded on
the continuous current side without in any way reacting upon
the alternating circuit, and the two circuits can be regulated
independently in any desired manner. All difficulties due to
lagging current can be eliminated, and the continuous ciurent
side can be kept at constant pressure irrespective of loss in
the main line or any variations of voltage or phase occur-
ring in it.
Finally, the apparatus can as readily give alternating current
from continuous, as the reverse, and with the same indepen-
dence in each case.
The compensating disadvantages are high first cost and
rather large loss of energy in the double transformation. As
to the former count, it may be said that the advantages gained
in possible range of frequency and flexibility in the matter of
voltage go far to offset the increase of cost. Often such
a motor dynamo is the only possible way of securing the
necessary current. For example, if one wished continuous
current for heavy motor service, such as hoists and the like,
where the only current available was monophase alternating
of 125^, or even of 60-^ for that matter, the motor dynamo
would be the only practical way of solving the problem.
As regards efficiency the motor dynamo should be, and is,
a little better than motor and dynamo separately, owing to
lessened friction of the bearings. Its efficiency should be as
great as 85 per cent at full load, and might easily be 2 or
3 per cent higher, in large machines. At half load it should
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Fig. 1,
FlQ. 2.
PLATE IX.
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CURRENT REORGANIZERS, 291
be say 82 to 85 per cent. Practice too often shows results
several per cent below those mentioned, but this is because
motor dynamos have usually been of very small size and
sometimes have been made up from any machines of the right
speed that were at hand.
The usual synchronous motor may In small motor genera-
tors be replaced to advantage by an induction motor, which
is simpler than the s3rnchronous form and requires no brushes.
Such a combination is shown in Fig. 173. This machine is
Fig. 173.
specially designed for furnishing charging current for auto-
mobile batteries. Through most residence districts only alter-
nating current is available, and the convenience of such an
apparatus is very great.
The motor is a monophase induction machine of the class
shown in Plate VIII, Fig. 2, suited to ordinary lighting circuits.
Of late such machines have assumed considerable importance,
and many large imits have been produced. Plate IX shows in
Fig. 1 a 500 KW quarter-phase set running at 400 r. p. m. It
consists of an 8 pole 500 KW railway generator coupled directly
to a 20 pole 2,200 volt synchronous motor, the two machines
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Fig. 1.
FiQ. 2.
PLATE IX.
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CURRENT REORGANIZERS.
291
be say 82 to 85 per cent. Practice too often shows results
several per cent below those mentioned, but this is because
motor dynamos have usually been of very small size and
sometimes have been made up from any machines of the right
speed that were at hand.
The usual synchronous motor may hi small motor genera-
tors be replaced to advantage by an induction motor, which
is simpler than the synchronous form and requires no brushes.
Such a combination is shown in Fig. 173. This machine is
X
I f /i
FlO. 178.
specially designed for furnishing charging current for auto-
mobile batteries. Through most residence districts only alter-
nating current is available, and the convenience of such an
apparatus is very great.
The motor is a monophase induction machine of the class
shown in Plate VIII, Fig. 2, suited to ordinary lighting circuits.
Of late such machines have assumed considerable importance,
and many large units have been produced. Plate IX shows in
Fig. 1 a 500 KW quarter-phase set running at 400 r. p. m. It
consists of an 8 pole 500 KW railway generator coupled directly
to a 20 p)ole 2,200 volt synchronous motor, the two machines
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292 ELECTRIC TRANSMISSION OP POWER,
having a common bearing between them. An interesting
feature of this set is the exciter mounted on the same shaft, an
8 KW multipolar generator, so that the whole outfit is self-
contained. The frequency in this case is 66-^, a periodicity at
which such motor generators have a material advantage over
other apparatus for a like purpose.
Fig. 2 is out of the ordinary in that the motor is of the
induction, type, instead of the ordinary synchronous machine.
The set shown is of 100 K W output, and comprises an ordinary
6 pole 600 volt railway generator coupled to a 12 pole three-
phase induction motor, running at 600 r. p. m., the periodicity
being 60^. Induction motors have recently come into consid-
erable use in this sort of work, in spite of somewhat lower effi-
ciency than the corresponding synchronous motors. It is safe to
say that the difference in efficiency is 2 or 3 per cent, and while
the S3mchronous motor may be overexcited so as to improve
the power factor of the system, the induction motor always
introduces lagging current. Yet a number of motor generators
with induction motors are now being built of capacity from 500
to nearly 1,000 KW. The real reason for the use of induction
motors on so large a scale is the trouble which has been experi-
enced at many times and places from hmiting. These troubles
do not get widely advertised outside the stations where they
occur, but it is a fact that in the use of rotary converters and
synchronous motors on a large scale very serious and formi-
dable developments of this phenomenon have occurred, so that
in spite of the use of shields it has under certain conditions,
especially when incandescent lighting circuits were to be fed, .
seemed wise to have recourse to induction motors. It is, how-
ever, probably best to regard this as a temporary expedient,
as synchronous motors, at least, can be practically freed from
hunting by proper design and construction, and possess very
considerable advantages. The demand for machines of extreme
multipolar construction, a demand based largely on fashion,
and the use of laminated pole pieces, are responsible for a
good share of the trouble. Rotary converters, as we shall pres-
ently see, present even more serious problems.
In these large motor dynamos it is possible to reach full load
efficiencies in the neighborhood of 90 per cent, and figures
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CURRENT REORGANIZERS.
293
fully up to that point have actually been obtained. As large
synchronous motors can readily be wound for 10,000 or 12,000
volts, under favorable conditions motor dynamos can be used
without reducing transformers, which averts a loss of 2.5 or 3
per cent, that would otherwise be incurred.
From the duplex machines just described it is but a short
step to the composite dynamotor, so called, of which the
armature is double wound. The primary or high voltage
winding may of course be either altematmg or continuous.
Fig. 174.
The secondary winding is likewise for either current, and may
well be fitted with both commutator and collecting rings.
A favorite arrangement of the windings is to place the
secondary coils in slots in the armature core, apply a sheath-
ing of insulation, and then to wind the primary coils on the
smooth surface thus formed. The commutators or rings
are placed one at each end of the armature, as in the con-
tinuous current transformer shown in Fig. 37, Chapter III.
A typical dynamotor of this sort is shown in Fig. 174. This
is specifically intended to derive a high voltage alternating cur-
rent for testing purposes from a low voltage continuous cur-
rent. The output is small, only a fraction of a kilowatt
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294
ELECTRIC TRANSMISSION OF POWER,
and the armature is in the ordinary bipolar field used for small
motors. The motor or primary winding is for 110 volts,
continuous, and the secondary for 5,000 volts, alternating. Of
course these voltages might be anything desirable, since in so
small a machine there are no difficulties in the way.
Another excellent specimen of the same type is Fig. 175, a
Lahmeyer **umformer'* of about 30 KW output. It is pri-
marily a continuous current transformer, with 675 volts primary
and 115 volts secondary. It is fitted, however, as shown in
FlO. 175.
the cut, with collector rings outside one of the bearings, from
which three-phase current at about 70 volts can be taken.
There are four field poles, and as the normal speed is 850
revolutions per minute, the three-phase current is at a fre-
quency of a little less than 30-w per second.
This was one of the machines exhibited at the Frankfort
Exposition of 1891, and fortunately an efficiency test of it is
available, dealing, however, only with continuous currents.
From the nature of the case the efficiency with a three-phase
secondary would not differ substantially from that found, so
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CURRENT REORGANIZERB.
295
that the curve, Fig. 176, gives a closely approximate idea of the
general efficiency of such apparatus in the smaller sizes. At
full load the commercial efficiency is very nearly 85 per cent,
while at half load it has dwindled to 77 per cent. This is not
bad for a small machine, and in a unit of 100 KW or more could
undoubtedly be raised several per cent. It should be at least as
high as can be obtained from a duplex motor d3aiamo, in fact
rather higher, since the bearing friction and core losses are
diminished. The composite machine is also cheaper, since but
one field is \ised, and it has a certdn advantage in that the arma-
ture reactance due to the motor and dynamo windings tend to
81
10
80K.MI
Fio. 176.
oppose each other, and hence to diminish possible sparking and
disturbance of the field. It has the same independence of pri-
mary and secondary voltage as the duplex motor dynamo.
On the other hand, by reason of a common field, the period-
icity of the currents in both windings must be the same. It
must be remembered that a continuous current armature has a
periodicity just as truly as an alternating armature. The cur-
rent as generated in each is alternating, but in the former it is
commuted before leaving the generator. Now, the frequency
of these alternations depends directly on the nmnber of poles
and the revolutions per minute, being in fact the numerical
product of the two. So if one of these composite d3aiamotors
be \ised with the continuous current winding as primary, the
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296 ELECTRIC TRANSMISSION OF POWER.
frequency of the alternating secondary is fixed, since the
speed of the machine cannot be changed without involving
both primary and secondary voltages. If the alternating cur-
rent side be used as the primary, the speed of the machine is
•fixed by the number of alternations, and whatever the voltage
of the secondary, the frequency must be the same as that of
the primary. Now it is a fact well known to dynamo designers,
that continuous current dynamos generating a high frequency
current prior to its commutation are troublesome and costly
to build. Most continuous current dynamos have an intrinsic
frequency of 15 to 25-^ per second. * To increase these figures
to 40^ involves some difficulty, particularly in large machines,
while 50 to 60^ are rather hard to reach, unless in sizes of
100 KW and below.
Hence, in spite of the good points of the composite dyna-
motor, it is of limited utility compared with the dujJex machine
previously described, particularly since there is a simpler way
of doing the same work with a higher efficiency.
This is found in the so-called rotary or synchronous converter,
now used on a very large scale.
This machine is nothing more than a continuous current
dynamo fitted with collecting rings in addition to ijie com-
mutator. These rings are connected to appropriate points of
the armature winding, and supplied with alternating currents
of the same frequency which would be generated by the arma-
ture if the machine were used as a dynamo. The brushes
being raised, the machine is nothing but a synchronous motor
running without load at its normal speed. Now, when, the
brushes are put down, the alternating current simply flows
through the armature much as if it were generated therein, is
commuted and passes out upon the line. This commutation
takes place under just the same general conditions as if the
machine were used as a generator. Meanwhile a portion of
the current supplied is passing as before, not through the
brushes but through the 'wdnding to the collecting rings, keep-
ing up the action as a motor. Of the total current then, a
small part forces its way against the E. M. F. set up in the
windings by the field, and supplies the motor function; a far
greater part, in amount determined by the resistance and
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CURRENT REOROANIZERS. 297
inductance of the armature, flows as if urged by this E. M. F.,
to the brushes, and supplies the generator function of the
machine. But a single armature winding serves to drive the
armature and to furnish a large output of commutated current.
And this current is not simply rectified, but is of exactly the
same character as if generated in the armature.
Inasmuch as the armature is revolving in a magnetic field,
the transfer of energy through the rotary converter is not in
the last analysis a case of pure conduction and commutation.
A part of the energy spent in the motor part of the armature
goes into dynamical increase of output in the part which
for the moment acts as generator. There is thus a motor
FlO. 177.
generator action in the same armature. Of the total energy
delivered from the d.c. side in a monophase converter like
Fig. 177, a little more than 40 per cent of the energy is dynam-
ically transferred, in the polyphase forms much less, say
12 to 24 per cent according to the number of armature taps.
The required motor activity in the converter is thus consider-
ably in excess of that required merely to spin the armature at
synchronous speed.
The character of the winding in a rotary converter is gen-
erally precisely the same as in a continuous current generator,
the only addition being two or more leads from symmetrically
placed iK)ints in the winding to the collecting rings. These leads
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298
ELECTRIC TRAXSML<SIOX OP POWER.
can l)c 8o arrangoil as lo :\tti: a :
natinj? current or, if desire i. a iii
lattcT forms are trenersilly pTviV
in^ HVnchrtMious ino^»r> :hty c
1 ho in(>noph».<o niaohir.e h:t> :
and by no nu\Hns s^i:v.]»'o r..t :hxi:
of the armature in a s::..yrie 1
phiiHo). Here the ov^nur.u.us
rlnj? in 16 ^vtions, F^'n: ;be
mwi may Ix* applicii or ^iibi
::• Tj-pbase sr-stem for the alter-
.»- « 'T ihr^ee-phase system. The
rr^i, fdi^ce like the correspond-
sii. t:»e n^ie self -starting, while
l»e r.r urhi to speed by special
N. Tji. 177 sbo-ws the character
•r.v ili^ r TATV converto" (mono-
rur!^i.i '•ii.iinfi: is a Gramme
rrusbfs B. B, continiiotis ctir-
r. i« bile ibe brushes on the col-
liM-hh^i vin»x (\ (\ <|v->-, - -Sf ^,^^ ,f .^.•f f. ,y ^^ altematinj;
rnuiMU S\io]\ ^'^ ) :i,\, c " -\ st^'-vT h \'h.Tii\y of puTposes as
h»ll.u\N \ ron;\^;j,Niv o..-^.- : /.y j. ». . 2, Ahemating ctiT-
ivMt d\ujnu,N. :^ v\v ,, ;v.;^i .-.—:-::■': 'T. 4, Synchronous
♦dhMuoHn^i t\)oi,N^ ,\ 0.': • ' ^ i> :/':*7:..s:lr.i: e:»r.verter. 6.
i^«p'»-^'^ tN>iM'\ .N.'v.-..-^ j.-Y ..^ :.'- y >, ppbei with four
'"^'"'<''^''' '»' ^^ ^^^- -.N a: :,^ :..-- 7-,- .".-•..,:<, oAch oi^e i<nn-
*"" ''*'^ ^^•M.i,^.v ... ,^,,. , ,, ,^ ., - ,. .~:.^ -< .,f ^y^^ armatuie.
'"1^'"^'^ tMnv,^, .V ^. ,,.. , v.v, ;:.->,^. c Gt^riiT^s rings.
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298
ELECTRIC TRANSMISSION OF POWER.
can be so arranged as to form a monophase system for the alter-
nating current or, if desired, a two- or three-phase system. The
latter forms are generally preferred, since like the correspond-
ing synchronous motors they can be made self-starting, while
the monophase machine has to be brought to speed by special
and by no means simple methods. Fig. 177 shows the character
of the armature in a simple bipolar rotary converter (mono-
phase). Here the continuous current^ wdnding is a Gramme
ring in 16 sections. From the brushes B, B, continuous cur-
rent may be applied or withdrawn, while the brushes on the col-
Fio. 178.
lecting rings C, C, perform the same office for the alternating
current. Such a machine may serve a variety of purposes as
follows: 1. Continuous current dynamo. 2. Alternating cur-
rent dynamo. 3. Continuous current motor. 4. Synchronous
alternating motor. 5. Continuous alternating converter. 6.
Alternating continuous converter.
Diphase rotary converters are usually supplied with four
collecting rings connected to form two circuits, each one join-
u\£r the windings in two opposite quadrants of the armature.
Triphase transformers generally have three collecting rings,
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Fio. 1.
Fig. 2.
PLATE X.
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CURRENT REORGANIZERS, 299
with their respective leads tapped into the windings 120°
apart. The connections vary somewhat for different kinds of
armature windings, but are the same in effect as those just
indicated. One of the early practical machines of this sort
exhibited at the Frankfort Exposition of 1891 is shown in Fig.
178. It is of the flat ring type usual to dynamos of Schuckert
make, and is fitted with four collecting rings outside the bear-
ing at the commutator end. The rings were arranged for
either monophase or diphase connection. The rotary converter
thus organized attracted great attention, and was successfully
operated in its manifold and diverse functions. It should
be noted that if driven as a dynamo, such a machine can furnish
continuous and alternating current simultaneously, a property
sometimes convenient, and now not infrequently utilized.
These rotary converters in the diphase and triphase forms
are playing a very important part in electric railway operations
involving considerable distances, and a large number of them
are in highly successful use. A good idea of the modem tyi>e
of rotaiy converter is shown in Fig. 2, Plate X. This is one of
the 400 KW machines installed in 1894 to operate the electric
railways in the city of Portland, Ore. It is designed to deliver
continuous current at nearly 600 volts, and receives its energy
from Oregon City, about fourteen miles away, where is in-
stalled a triphase transmission plant. The motive power is
derived from the great falls of the Willamette River. Current
is generated at 6,000 volts, with a frequency of 33-^ per second,
and is given to the rotary converters at about 400 volts,
from the secondaries of the reducing transformers. Fig. 1,
Plate X, shows a 250 KW Westhighouse diphase machine,
adapted for use on a 60-^ circuit and giving continuous current
at 250 volts. An interesting feature of this machine is the
diphase induction motor with its armature on an extension of
the main shaft. This serves to bring the machine to speed
without calluig for the excessive current that would be required
if the main lines were closed upon the converter armature
itself. The monophase form of this very interesting apparatus
has not yet come into much practical use, not through any in-
herent faults, but because most of the power transmission has
so far been accomplished with diphase and triphase currents.
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800 ELECTRIC TRANSMISSION OF POWER.
The efficiency of these machines is, as might be expected
from their character, practically the same as ordinary con-
tinuous current dynamos of the same output, or rather better
on account of the shorter average path for the current in the
armature. In fact, so far as general properties go, they are
dynamos. They furnish at present by far the most available
means of deriving continuous from alternating currents, for
they are simple, of great efficiency, and of about the same price
as other generators of the same capacity. In point of fact, a
well-designed polyphase rotary converter has rather better
output and efficiency than the corresponding generator, since
for the reason just noted the armature losses are diminished.
Bearing this in mind, it is apparent that increasing the number
of points at which the armature is tapped for the alternating
current supply, thus shortening the average path to the brushes,
will, other things being equal, lessen the armature loss. In
practice it is found that a three-phase converter with
three armature taps is considerably better than a monophase
converter with two; a quarter-phase converter with four is
somewhat better still, while a three-phase connection with
separate phases and six taps gives even a higher output and
efficiency. The net result is that while a monophase con-
verter is rather inferior to the corresponding dynamo the
two- and three-phase converters are considerably better than
the corresponding dynamos. Quarter-phase converters are
always connected for four collecting rings, and large three-
phase converters not infrequently have six, to gain the advan-
tage just mentioned.
Efficiencies as great as 96 per cent at full load have been
obtained from large rotary converters, with 93.6 per cent at
half load. These figures are from a three-phase, six collect-
ing ring converter of nearly 1,000 KW output.
As already indicated, there is a strong tendency toward the
use of low periodicity, 25^^ to 30>^ in rotary converters. This
is partially due to the complication of the commutator in high
frequency converters, partly to the current fashion for
extremely low rotative speeds, and partly to lack of finesse on
the part of the average designer. That converters for a fre-
quency as high as 60^ are entirely feasible even in capacities
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CURkENT REORGANIZBRS. 801
up to several hundred kilowatts admits of no discussion, as
the machine put in evidence in Plate X, of which a number
are in successful operation, plainly shows. It is imdoubtedly
easier and cheaper to build them for somewhat lower periodici-
ties, but there seems very little reason for going so low as is the
current custom, and it tends needlessly to multiply special
types of apparatus.
And yet the simplicity of the rotary converter is attained
at the cost of certain practical inconveniences that cannot
lightly be passed by. Their source is the employment of a
single field and armature winding for all the purposes of the
apparatus. The results are, first, complete interdependence
of the alternating and continuous voltages, and, second, con-
sequent difficulties of regulation that are occasionally very
troublesome.
The immediate result of a single winding is that there is an
approximately fixed ratio between the alternating and the con-
tinuous voltage. The former is always the lass, and, while
varied by changes in the number of phases determined by the
connections, is approximately the alternating voltage that
would be yielded by the machine driven as a generator. This
is, for monophase or diphase connections, about seven-tenths
of the continuous current voltage, and for three-phase connec-
tions about six-tenths. The proportions would approximate to
1 V3
/- and ^ respectively, if the alternating E. M. F.'s were
V2 2 V2
sine waves, which they never are when derived from an ordi-
nary continuous current armature. In service the real pro-
portions may, and generally do, vary by several per cent,
according to the excitation. In a particular two-phase case
the actual ratio was .68, and in a three-phase case .65. If,
therefore, a rotary converter be used for supplying continuous
current, the applied alternating current must be of lower pres-
sure than the derived continuous, in about the proportion
above noted. This compels the use of reducing transformers
in every case of power transmission involving this apparatus.
Further, any cause that affects the alternating pressure affects
the continuous as well. Line loss, inductance, resonance effects,
as well as changes at the generators, all influence the vol-
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302 ELECTRIC TRANSMISSION OF POWER.
tage at the continuous current end of the rotary transformer.
Nor can this voltage be freely altered by changing the field
strength, since, as we have already seen, this may profoundly
change the inductance of the alternating circuit, which is for
many reasons undesirable. The field windings of rotaries are
either shunt from the d.o. side or compound. The former
winding gives much the steadier power factor and, hence,
is rather desirable for close regulation of a steady load, while
the latter is advantageously used for railway loads and the like.
The best results are obtained by carefully adjusting the gen-
erator, line, and rotary converters to work together. Other-
vnse there is likely to be trouble in regulation.
For these reasons in cases where close regulation is neces-
sary, as for incandescent lighting, preference has frequently
been given to the motor generator with double field and arma-
ture, as in the large Budapest system installed by Schuckert &
Co., who were among the pioneers in developing the rotary
converter. In this case the transmission is at 2,000 volts
diphase, at which pressure current is delivered to the motor
end of the motor generators placed in substations at conve-
nient points. In such a plant the increased cost of the duplex
machines is not so great as might be supposed, for reducing
transformers are needless, and the output of both generators
and motors can be forced to the utmost limit of efficient oper-
ation, without fear of injuring the regulation, which is reduced
to the easy problem of accurately compounding a continuous
current generator. The net efficiency of the Budapest trans-
formation is said to be 85 per cent. Some recent experiments
on the relative efficiency and cost of motor generators and
rotary converters are as follows : The sets compared were of
200 KW capacity for changing triphase current from the
Niagara circuits at 11,000 volts, 25^ into continuous current
at 120 to 150 volts. The efficiencies given are net, including
the necessary provisions for obtaining a variation of 25 per
cent in the finally resulting voltage:
Motor-
Oenerator.
Transformers
and Rotaries.
Difference.
Full load
87.40
89.87
2.47%
f load
85.54
88.70
3.16%
}load
81.42
84.00
8.48%
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CURRENT REORGANIZERS. 808
The extra apparatus required with the rotaries brought the
two methods to substantially the same cost, but for lighting
work the motor generators gave the better results.
From the foregoing it is sufficiently evident that every case of
current reorganization cannot be successfully met by the same
apparatus. For certain small work the rotating commutator
seems to be fairly weU suited, and for occasional purposes it
is somewhat cheaper and more efficient than any of its rivals.
Next in point of efficiency and cheapness comes the rotary
converter, infinitely better for heavy work than any commutat-
ing device, and finding very extensive application to electric
railway work. Finally, for work requiring very close regulation,
the motor generator is specially well suited, closer to the rotary
transformer in cost and efficiency than would be supposed off-
hand, and unique in the complete independence of its working
circuits.
Practice in this line of operations has not yet settled into
fixed directions, and is not likely so to do just at present.
Each plant nmst therefore be considered by itself and treated
symtomatically.
American usage is at present tending strongly toward the
rotary converter, on account of its ready adaptation to railway,
service, but, in view of the work that has been done on alternat-
ing motors for such service, it is an open question how far
current reorganization will be generally necessary in the future,
although just now it is of very great practical importance.
As the price of copper rises, the use of current reorganizers
becomes more and more important in railway work, and for
this particular use the rotary converter is generally chosen.
There should be mentioned here some curious and valuable
devices for obtaining rectified alternating currents, based
upon the phenomena of polarization.
Obviously, if one could find a conductor which would let pass
cuiTents in one direction, and block those in the other, the
result of putting it in an alternating circuit would be that all
the current impulses in one direction would be suppressed, so
that the resulting current would be a series of separated half-
waves of the same polarity. It would be as if in Fig. 166 all
the half-waves above the base line were erased. Now such a
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804 ELECTRIC TRANSMISSION OF POWER,
conductor is actually obtainable in certain electrolytic cells in
which a counter electromotive force or severe polarization
resistance impedes current flowing in a particular direction.
Under favorable circumstances the selective action is quite
complete, so that the alternating current becomes unidirec-
tional. Fig. 179 shows the current curve for a complete
cycle as modified by electrolytic rectification. The positive
half of the wave is practically wiped out of existence. The
efficiency of these electrolytic devices as regards the energy
rectified is quite low, and most of the apparatus constructed
has been upon a very small scale, but there are certain purposes,
like energizing induction coils, for which it may occasionally
be of service. It is given place here more on account of its
Fro. 179.
general interest than for any practical value. It works best,
like most other rectifying devices, at low frequencies.
The latest and in some respects the most interesting device
for obtaining continuous currents from an alternating source,
is the vapor, or mercury arc converter. Its action depends
on the mechanism of current flow in the electric arc. As is
well kno^Ti, the current is carried across the space between the
terminals of an electric arc by a blast of vapor streaming from
the negative to the positive electrode. An arc cannot start
until this stream has been established, for which reason arcs
are generally started by touching the electrodes momentarily
together. For the same reason on a low frequency alternating
circuit, or generally unless a considerable mass of conducting
vapor lingers between the poles, the arc readily goes out, since,
granted that the arc is struck at all, the negative stream dies
with the pulse of current that produced it, and the following
alternation can only get through by starting a new stream
from the other electrode as negative.
Now, the arc formed about a mercury negative pole in vacuo
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CURRENT REORGANIZERS.
806
has this remarkable property, that, while once started the
stream can be maintained by a few volts, it takes many thou-
sand volts to initiate or to reestablish the stream over any
material gap. Hence, if the stream is once started it can be
kept in action continuously by a rather low voltage current,
but can be reversed only by an enormous E. M. F. in the
opposite direction.
If, however, the original negative stream can be kept going
Po«j(Sto£l«({tcod««
VttXLy^JSuMH rode
/WVVVWWsA/^
a
A»<XSapplf —
FlO. 180.
it will transmit freely current impulses in the original direc-
tion while reverse impulses will lack the potential required to
reverse the stream. Upon this property of the mercury arc
the vapor converter is based, and the essential feature of its
operation is the preservation of the negative stream by send-
ing overlapping impulses, so that once started the original
stream shall not die out. The extremely ingenious method
of doing this is shown in Fig. 180. Here A is an exhausted
bulb 8 or 10 inches in diameter, containing two positive elec-
trodes side by side, and a mercury negative electrode D.
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806
ELECTRIC TRANSMISSION OF POWER.
At B is a fairly stiff reactance. The two positives are connected
to the terminals of an auto-converter C and its middle point
is connected to B through the proposed d. c. circuit.
The apparatus is started by tipping the bulb until a supple-
mentary mercury positive touches the negative and as the
bulb is tipped back the negative stream starts.
Let us say that the current let through is via the right hand
electrode. Owing to the reactance B the current, lagging,
persists until the E. M. F. rising in the left hand connections
FIO. 181.
has had time to start via the same negative stream, a current
through the other positive electrode. Positive electrodes
virtually in the same negative vapor blast can thus exchange
work freely, provided the blast be not interrupted.
The two sides of the circuits thus keep up the interchange,
working alternately, but utilizing as \vill be seen from the
consecutive directions of flow, both sets of alternations. By
this same cause the effective E. M. F. of the rectified current
is something less than half the nominal a. c. voltage applied
to the apparatus as a whole. By a stroboscopic examination
the sequence of the operations can be very beautifully seen.
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CURRENT REORGANIZERS. 307
Two-phase or three-phase currents can be made operative in
a very similar manner so that the process is a general one.
It can be made operative at any commercial frequency.
Fig. 181 shows the constant current form of the same device.
The letters have the same significance although the electrode
tube is of different shape, and the coil C is here the secondary
of a constant current transformer. The resulting current from
the vapor converter is evidently not uniform but somewhat
pulsatory as if received from a dynamo having very few seg-
ments in the commutator.
Fig. 182 from an oscillograph record * of the current form
Fio. 182.
derived from the constant current converter like Fig. 181,
shows the facts in the case admirably.
The efficiency of such apparatus is high. There is a small
back E. M. F. of about 15 volts to overcome, the ohmic and
hysteretic loss in the transformer and reactance, and some
heating of the c<jnverter tube. The higher the voltage applied
to the tube the less current for a given energy and the better
the efficiency, and the voltage may be anything that will not
strike a reverse arc in the tube. At current of a few amperes
the working a. c. voltage may even be 25,000 volts. At mod-
erate voltages the back E. M. F. is more important and the
current rises for the same energy so as to sooner reach the
heat endurance of the tube.
The constant potential form is now commercially avail-
able in moderate capacities, say up to 25 or 30 KW at 115 to
120 volts, the efficiency being about 75 to 80 per cent. These
converters are designed for charging storage batteries and
* Steinmetz, tr. A. L E. E. June, 1906.
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308 ELECTRIC TRANSMISSION OF POWER,
similar light work. The constant current form is beginning
to be used for arc lights, giving d. c. arcs off an a. c. circuit,
using d. c. voltages up to 4,000 or 5,000 volts. The efficiency
of such sets is probably between 80 and 90 per cent, and the
power factor is reported to be .90 or better.
The apparatus is very beautiful in principle, and has thus
far developed no serious operative defects. Its life is somewhat
uncertain and a good deal of experimenting is still needed to
bring it into standard form, but it is altogether very promising.
Whether it is to be available for large powers remains to be
seen, but it is certain to find a wide commercial use so soon
as it has been far enough standardized to enable the price to be
brought down to a manufacturing basis. At present the
figures are too near those charged for motor generators to
encourage any widespread enthusiasm.
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CHAPTER VIII.
ENGINES AND BOILERS.
Mechanisms that constitute the link between natural sources
of energy and mechanical power are called prime movers. So
far as the electrical transmission of energy is concerned, but
two classes of prime movers, steam engines and water-wheels,
have to be seriously considered. All others sink into insigni-
ficance or are limited to special and rarely-occurring cases.
When power is transmitted electrically over considerable dis-
tances the prime mover is usually a water-wheel, since, as yet,
the transmission of power from coal fields has been hardly
more than begun, although when long electrical lines be-
come somewhat more familiar, coal may become a frequent
source of energy. Where the distribution of power from a
central point is to be accomplished, the prime mover is fre-
quently a steam engine.
The general principle of the steam engine may be fairly
supposed to be somewhat familiar to the reader, but the con-
ditions of economy are not always so clearly understood. The
source of power in an engine is the pressure of the steam,
which must be utilized as fully as possible to get anything like
efficient working. Since the pressure is in direct proportion
to the temperature in any gas, the proportion of the total pres-
sure which can be used depends on the original temperature
at which its use is begun, and the temperature at which one
ceases to use it and rejects it together with all the energy it
then possesses. These temperatures are not to be reckoned
from the ordinary zero of a thermometer, but from the so-
called absolute zero. This is that point from which, if the
temperature of a gas be reckoned, its pressure will be directly
proportional to the temperature. It is 461° below zero,
Fahrenheit, that is, 493° below the melting point of ice. It is
determined by the consideration that any gas at this melting
point loses ^i^ of its pressure for a change m temperature of
309
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310 ELECTRIC TRANSMISSION OF POWER.
one degree, hence, if it could be cooled down 493^, would lose
its pressure and would have given up all of its energy. Count-
ing from this absolute zero, then, one can utilize that part of
the whole energy of a gas which lies between the temperature
at which the gas begins to work and that at which it ceases to
do work. In other words the efficiency of any engine operated
by gaseous pressure is:
,
in which T^ is the absolute temperature of the gas when
it begins to do work in the engine, and 7\ the absolute temper-
ature at which its work ends. In practice, T^ is the tem-
perature of the steam when it enters the cylinder, and T^
the temperature of exhaust or condensation. Steam permits
the use of but a Umited range of temperature on account
of the temperature at which it liquefies, and bothers us by
condensing as it expands, even in the cylinder. It must be
remembered that while we are limited by our possible range of
temperature to a low total efficiency in any heat engine, of the
energy that can possibly be obtained within this limitation,
a very good proportion is recovered in the best modern engines
— from one-half to three-fourths. The remainder is lost in
various ways, largely through radiation of heat and cylinder
condensation. Besides these thermal losses a portion of the
energy utilized is wasted in friction of the mechanism.
From these considerations we may derive the following
general principles of engine efficiency:
I. The steam should be admitted at the highest pressure
feasible and exhausted at the lowest pressure possible.
This indicates that high boiler pressure should be used, and
that it is better to condense the steam than to expel it into the
air, as by condensing most of the atmospheric pressure can
be added to the working range of pressure in the engine. In
the next place it is evident that the steam should be sent into
the engine at full boiler pressure, and finally condensed after
expanding and yielding up its pressure as completely as
possible.
II. Waste of heat in the engine should be stopped as far
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ENGINES AND BOILERS. 311
BS possible. This means checking losses from the cylinder
by radiation and conduction, and internal loss from cylinder
condensation. The first principle laid down has for its ob-
ject the increase of the possible efficiency, while this second
principle bears on the securing of as large a proportion as
possible of this possible efficiency. It requires the preven-
tion of escape of heat externally by protecting the cylinder,
and incidentally shows the advantage of high pressure and high
piston speed in securing as much work as possible without in-
creasing the size of the working parts, and hence their chance
for radiation. On the other hand, it indicates the danger of
working with too great a range of temperature in the cylinder
thus producing cyUnder condensation.
III. The work of the engine should be the maximum practi-
cable for its dimensions and use. This secures high mechan-
ical efficiency as the previous principles secure high thermal
efficiency. To fulfill this condition high steam pressure and
high piston speed are necessary, and the latter usually means
also rather high rotative speed. The importance, too, of fine
workmanship in the hioving parts is evident.
It will be realized that some of the conditions just pointed
out are mutually incompatible to a certain extent. Every-
thing points, however, to the great desirability of a condens-
ing engine, worked with a high initial steam pressure and
great piston speed. The tendency of the best modem prac-
tice is all in this direction, and the efficiency of engines is con-
stantly improving. The greatest advances of the past decade
or two have been in the introduction of compound engines.
The principle here involved is the lessening of thermal
losses in the cylinder by avoiding extremes of temperature
between the initial and the final temperature of the steam ex-
panded into it. Compound engines simply divide the expan-
sion of the steam between two or more cylinders, so that the
temperature range hi each is limited, without hmiting the
total amount of expansion.
Following the same line of improvement, triple and quad-
ruple expansion engines are becoming rather common, although
the value of the last mentioned is somewhat problematical at
present.
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312 ELECTRIC TRANSMISSION OF POWER,
For practical purposes steam engines may be classified in
terms of their properties, somewhat as follows :
First, there is the broad distinction between condensing and
non-condensing engines. The former condense the exhausted
steam and gain thereby a large proportion of the atmospheric
pressure against which the latter class is obliged to do work in
exhausting the steam. Where economy of operation is se-
riously considered, the non-condensing engine has no place, if
water for condensation is obtainable.
Each of these classes falls naturally into subclasses, depend-
ing on the number of steps into which the expansion is
divided — simple, compound, triple expansion, etc. Of these
the first may now and then be desirable, where the size is small
and coal very cheap, but for the general distribution of energy
the last two are more generally useful. Furthermore, each of
the subclasses mentioned may be divided into two genera,^
depending on the nature of the valve motions that control the
admission and rejection of the steam. To follow out the first
principle of economy laid down, the steam must be admitted
at a uniform pressure as near that of the boiler as possible, the
admission should be stopped short after entrance of enough
steam for the work of the stroke, the steam allowed to expand
the required amount, and then rejected completely at the lowest
possible pressure. The admission valves should therefore
open wide and very rapidly, let in the steam for such part of
the stroke as is necessary, and then as promptly close. The
exhaust valves should open quickly and wide when the expan-
sion is complete, and stay open imtil nearly the end of the
stroke, closing just soon enough to cushi(m the piston at the
end of its stroke. In proportion to the completeness with
which these conditions are met, the use of the steam will be
economical or wasteful. The two genera of engines referred to
are those in which the motions of the admission and exhaust
valves are independent of each other or dependent. Fig. 183
shows in section the cylinder and valves of an mdependent
valve engine, Corliss type. The arrows show the flow of the
steam. The admission valve on the head end of the cylinder
has just been opened, as also has the exhaust valve on the
crank end.
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ENGINES AND BOILERS.
313
The essential point of the mechanism is that the admission
valves open and close at whatever time is determined by the
action of the governor without in the least affecting the work-
ing of the exhaust valves. In the Corliss valve gear the
steam valves are closed by gravity, or by a vacuum pot, and
are opened by catches moved by an eccentric rod, and released
at a point determined by the governor, which thus varies the
point of cut-off according to the load. Ordinarily the admis-
sion of steam is thus cut off in a simple engine at full load
after the piston has traversed from one-fifth to one-quarter of
its stroke, according to the pressure of the steam. If the cut-
off is too late in the stroke, there is not sufficient expansion of
Fio. 183.
the steam ; if too early the steam is partially condensed by too
great expansion. For every initial pressure of steam there is a
particular degree of expansion which gives the best results in a
given engine.
Fig. 184 shows the valve motion of one of the best of the
dependent valve genus. Steam is just being admitted at the
head end both around the shoulder of the hollow piston valve
and through the ports at the other end of the valve via the
interior space. At the crank end the exhaust port has just
been fully opened. It will be seen that any change in the
conditions of admission also involves a change in the condi-
tions of exhaust, and although some variation may take place
in the latter without serious result on the economy, simplicity
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314
ELECTRIC TRANSMISSION OF POWER.
in the valve gear has been gained at a certain sacrifice of
efficiency in using the st^am. Both independent and depen-
dent valve engines have many species differing widely in
mechanism, but retaining the same fundamental difference.
Of the two genera, the independent valve engine has the
material advantage in efficiency, and under similar conditions
of pressure, capacity, and piston speed consumes from 10 to 20
per cent less steam for the same effective power. It there-
FlO. 184.
fore is generally employed, in spite of somewhat greater first
cost, for all large work, often in the compound or triple ex-
pansion form. Except in small powers, or for exceptionally
high speed, the dependent valve engine has few advantages,
and in the generation of power on a large scale, such as for
the most part concerns us in electrical transmission work, it
hardly has an important place.
It must not be supposed that between the various sorts of
engines mentioned there are hard and fast lines. In the
economical use of steam a very large non-condensing engine
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ENGINES AND BOILERS. 815
may surpass a smaller condensing one, or a fast rimning
dependent valve engine, a very slow running one with inde-
pendent valves. Broadly, however, we may lay down the
following propositions concerning engines of similar capacity:
I. Condensing engines will always furnish power more
economically than non-condensing ones. This is particularly
true at less than fiJl load, since the loss of the atmospheric
pressure may be taken as a constant source of inefficiency,
which, like mechanical friction, is very serious at low loads.
For example, a triple expansion engine working at one-(juarter
load in indicated HP, will be likely to have its ccmsumption of
steam per IHP, increased from 15 to 25 per cent above the con-
sumption per IHP at full load; while worked non-condensing,
the increase would be from 50 to 100 per cent. Hence, for
electrical working where light loads are frequent, condensing
engines are an enormous advantage. With simple or compound
engines the same general rule holds good as for triple-expan-
sion engines, ^vith the additional point that light loads affect
their economy even more, when worked non-condensing. It
must be borne in mind that if any engine is to do its best under
varying loads, its valve gear and working pressure must be
arranged with this in mind, else the advantage of high expan-
sion and condensing may be thrown away. It is frecjuently
said that triple expansion engines do not give good results in
electric railway work. When this is the case there has been
improper adjustment of engine to load.
II. Among engines having the same class of valve gear,
compound engines give better economy than simple ones, and
triple expansion better than compound. This is true irre-
spective of the nature of the load, supposing each engine to be
suitably adjusted to the work it has to do. In rare cases, owing
to exceedingly cheap fuel and short working hours, it may hap-
pen that the advantage of a triple expansion engine over a
compound in economy of coal may be more than offset by
increased interest on investment, but at the present cost of
engines and boilers, this could not well occur unless in the
case of burning culm or poor coal obtained at a nominal
price.
III. As regards speed of engines, there is always advantage
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816 ELECTRIC TRANSMISSION OF POWER.
in high piston speed both as respects first cost and mechanical
efficiency. So far as the economical use of steam goes, speed
makes little difference save as it sometimes involves a change
in the valve gear. Most high-speed engines have valve gear
of the dependent sort, which puts them at a disadvantage
except in so far as lessened cylinder condensation and friction
may offset the losses due to less efficient distribution of the
steam. But the best dependent valve engine is uniformly less
economical than the best independent valve engine of the
same class and subclass. Even the lessened friction of the
small high-speed pistons does not offset this difference in
intrinsic economy.
As regards actual economy in the st^am consumption, the
size of engine has a powerful though somewhat indeterminate
influence. Even at full load, simple non-condensing dependent
valve engines of moderate size require from 30 to 40 lbs.
of steam per indicated horse-power hour. Only in very large
engines, such as locomotives, and specially fast running engines
such as the Willans» does the steam consumption of these
dependent valve engines fall below 30 lbs., and not very often
even in these cases. Worked condensing the same machines
use from 20 lbs., hi exceedingly favorable cases, to 25 or 30 lbs.
more commonly.
Independent valve engines, simple and non-condensing, will
give the indicated HPH on 25 to 30 lbs. of steam, occasion-
ally on as little as 22 to 23 lbs. With the advantage of con-
densation these figures may be reduced to say 18 to 25 lbs.,
the former figure being somewhat exceptional and probably
very rarely attained in practice.
Passing now to compoimd non-condensing engines, the effect
of compounding on efficiency is about the same as that of con-
densing. Ordinary dependent valve engines of compound
construction require from 20 to 25 or 30 lbs. of steam per
IHP hour. The former result is very exceptional, and seldom
or never reached in practice, while the last mentioned would
be considered rather high. Independent valve compound
engines are so seldom worked non-condensing, that the data of
their performance are rather meagre; 18 to 25 lbs. of steam
is about the usual amount, however.
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ENGINES AND BOILERS. 317
When condensation is employed, on the other hand, the
dependent valve engines are in rather infrequent use. When
ihe need for economy is so felt as to lead to the use of com-
pound engines, it also leads to the use of economical valve
gear. The steam consumption of dependent valve compound
condensing engines is quite well known, however, and is
usually from 16 lo 24 lbs. per IHP hour. The first mentioned
figure is rarely reached, and only in special types of engines.
Plenty of tests on compound condensing engines with inde-
pendent valves are available; 14 to 20 lbs. of steam covers
the majority of results. Occasional tests run down to and
even below 12 and as high as 22 lbs.
It is noticeable that in compoimd engines the difference
between dependent and independent valve gear is l6ss than
with simple engines. This is due to a variety of causes. The
larger range of expansion used in compound engines tends to
lessen the deleterious effects of moderate variations in the
distribution of the steam, and besides, the valve gear of com-
pound engines is not infrequently composite, the high-pres-
sure cylinder having independent valves and the low-pressure
cylinder dependent ones.
The same arrangement is often used in triple expansion
engines, so that, in conjunction with the condition before
mentioned, it is usually true that the economy of dependent
valve triple expansion engines is much nearer that of indepen-
dent valve ones than would be at first supposed. Without
condensing, a dependent valve triple expansion engine may be
expected to require from 19 to 27 lbs. of steam per IHP hour.
With condensation such engines perform much better, the steam
consumption being reduced to 14 to 20 lbs.
Nearly all triple expansion engines, however, are built with
independent valves, at least in part, the intention being to
secure the most economical performance possible. Under
favorable conditions their steam consumption runs as low as
12 lbs. per IHP hour, and seldom rises above 18 lbs. In a
few exceptional cases the record has been reduced below
12 lbs., but such results cannot often be expected. Any-
thing under 13 lbs. of steam per HP is good practice for
running conditions.
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ai8
ELECTRIC TRANSMISSION OF POWER.
All the figures given refer in the main to good sized engines
of at least 200 HP and over, operated at full load and at
favorable ratios of expansion. It must be clearly understood
that there is for each steam pressure a particular ratio of
expansion which will give the most economical result — less ex-
pansion than this rejects the steam at too high a temperature;
more, causes loss by condensation, etc. Compound and triple
expansion engines permit greater expansion of the steam with-
out loss of economy, hence allow higher steam pressure and a
greater temperature range — hence higher thermal efficiency.
Good practice indicates that for simple engines the boiler
pressure should be not less than 90 to 100 lbs. per square
inch, for compound engines not less than 120 to 150, and for
triple expansion engines not less than 140 to 150, and thence
up to 175 or 200 lbs.
We may gather the facts regarding steam consumption into
tabular form somewhat as follows:
Kind of Engine.
Steam per I HP.
General Range.
Steam per IHP.
Working Average.
Simple, non-condensing dep v
30-40
25-30
20-30
18-25
20-28
18-26
10-24
14-20
14-20
12-18
12-14
38
Simple, non-condensing indep. v
Simple, condensing dep. v
28
25
Simple, condensing indep. v
21
Compound, non-condensing, dep. v
Compound, non-condensing indep. v
Compound, condensing dep. v
24
22
20
(/Ompound, condensing indep. v
17
Triple, condensing dep. v
17
Triple, condensing indep. v
14
Triple large, condensing indep. v
13
The engines considered are supposed to be of good size —
say 200 to 500 IIP, ami to be worked steadily at or near full
load. The figures given as working average are such as may
be safely counted on with good engines, kept in the best
working condition, and operated at at least the boiler pres-
sures indicated. The steam is supposed to be practically dry
and the piping so protected as to lose little by condensation.
These results are such as may regularly be obtained in prac-
tice, and indeed it is not uncommon to find them excelled.
Compound condensing engines of large size not infrequently
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ENGINES AND BOILERS. 319
work down to 13 lbs. of steam, and triple expansion con-
densing engines down to 12 lbs., which result will be guar-
anteed by most responsible builders.
Unfortunately, engines employed for electrical work are com-
paratively seldom kept at uniform full load. Furthermore,
they are subject to all sorts of variations of load. In electric
railway service there arc sudden changes from light loads to
very heavy ones, while in electric lighting there is generally a
gradual increase to the maximum load, which continues an
hour or two, followed by a rather gradual decrease. Thase
variations affect the economy of the engines unfavorably — at
certain loads there is not enough expansion, at others decidedly
too much. The variations in economy are largely controlled
by the proportioning of the engine to its work. To say that
an engine is of 500 HP means little unless the statement be
coupled with a definite explanation of the circumstances. If
that output is obtained by admitting steam for half the stroke,
the engine will work at 500 HP very uneconomically, sup-
posing a simple engine to be under consideration. Its point
of maximum economy may be perhaps 300 HP. On the other
hand, 500 HP may be given when cutting off the steam at one-
fifth stroke. In this case the engine will be working near its
point of maximum economy, and at 300 HP will require much
more steam per IHP. It could give probably 600 to 700 HP
at a longer cut-off, and is really a much more powerful engine
than the first. For uniformity it is better to rate an engine at
the HP of maximum economy, whatever the real load may be.
The relation of load to economy is well shown in the curves of
Fig. 185.
Curves 1, 2, 4, and 5, are of engines so rated as to have
their maximum economy near full load. Curve 3, (m the
other hand, is from an engine inteiided to give its highest
economy at about three-cjuarters load. For very variable
output this is the preferable arrangement, while for large
central station work, when the number of units is large enough
to permit loading fully all that are running at any one time,
it is better to have each unit give its veiy best economy near
full load and to vary the number of units according to the re-
quirements of total load.
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ELECTRIC TRANSMISSION OF POWER.
For electric railway service under ordinary conditions, it is
best to employ an engine which at full load is worked to a
high capacity, and hence somewhat uneconomically, while at
lesser loads, which more nearly correspond with the average
conditions, its economy will be at a maximum. For electric
lighting service it is preferable to have the point of maximum
PER CENT LOAD l.H.P.
Fio. 185.
economy fall more nearly at full load. For power service,
which is on the one hand more uniform than railway service, and
less uniform than electric lighting work, it is probably best to
employ an engine having characteristics between those just
mentioned". In every case attention must be paid to the
character of the load as regards average amount and con-
stancy in the choice of an appropriate engine for the work.
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ENGINES AND BOILERS.
821
In cases where the variations of load are likely to be very
sudden, great mechanical strength of all the moving parts is
absolutely necessary, and an attempt should be made in plan-
ning the power station to arrange the engine for its best econ-
omy at average load as nearly as this can be predicted.
With care in planning an electric power station the engines
can be made to give an exceedingly good performance, much
0^ «.?£ 1.0
FRQPORTION TfiAT ACTUAL J.OAD BEXH3 TO RATED POWER
Fig. 186.
better than was considered possible a few years ago. Fig.
186 shows a set of curves from the experiments of Prof. R. C.
Carpenter giving the performance of engines of different kinds
over a wide range of loads, from mere friction load up to 50
per cent overload. The results are in pounds of water
evaporated per indicated HPH. The immense advantage to
be gained by using compound and triple expansion condensing
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822 ELECTRIC TRANSMISSION OF POWER.
engines appears plainly from the curves. Another conspicu-
ous fact is the great economy attained by such engines over a
wide range of load. It is a common fallacy to suppose that
while compound or triple expansion condensing engines are all
well enough at steady load, simple engines have the advantage
if the load varies over a wide range. The facts in the case as
shown in Fig. 186 are exactly the reverse: not only do the high
expansion engines have the advantage of the simple engines
at their rated loads, but at all loads, and particularly light ones.
And their advantage is so great that imder any ordinary cir-
cumstances the use of a simple or a non-condensing engine for
power generation is wilful waste of money. If the saving in
first cost were great the mistake might be excusable, but the
greater amount of steam required for nmning simple engines
means larger boiler capacity, which nearly offsets the lower
cost of engine. For example, a glance at Fig. 186 shows that
a triple expansion condensing engine requires only half the
boiler capacity demanded by a non-condensing automatic engine
for the same output. In other words, if the former requires
500 HP in boilers, the latter will need 1,000 HP in boilers for
exactly the same service. And the same holds true of the
capacity of the stack, feed-pumps, steam-piping, water-piping,
and, to a certain extent, even of the building, so that it is
almost always poor economy to buy a cheap type of engine.
The greatest improvement in economy made in recent years
has been the introduction of superheating which American
engineers have been somewhat slow in adopting. This is simply
the heating of the steam as such on its way to the engine.
The steam prior to use is passed through a special reheater,
frequently with an independent furnace, and given additional
heat energy, the working temperature being thus raised
sometimes to 600° or 700° F. This largely increases the
range of working temperature possible to the engine, and hence
the efficiency, at a relatively small expense for extra fuel.
The very high temperature of the steam compels extra
precautions in the lubrication, and for some years this lubri-
cation bug-a-boo stood in the way of substantial progress.
At present it is entirely practicable to lubricate the cylinders
successfully even up to the figures mentioned above, and
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ENGINES AND BOILERS.
323
it Ls being done abroad though the prejudice in this country
still persists. The results are startling, the steam consumption
under test havmg repeatedly run down near 'and even below
10 lbs. of steam per IHP hour in compound condensing engines.
The result of a recent test of a 21 x 36 X 36-inch mill engine
are given in Fig. 187 and represent the highest efficiency yet
attained. It will be noted that under the test conditions with
steam superheated to 720® - 750® F. the steam consumption
increased for the heavy loads just as it rises for overloads in
the curves of Fig. 186. The fact in each instance merely
implies that a certain amount of expansion corresponds to
9.5
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maximum economy, and less than this amount injures econ-
omy although it increases the possible output.
Fig. 187 is merely an extreme instance of the general principle,
due to starting the expansion at a relatively very high temper-
atin-e. There is no doubt whatever of the practicability of
reducing steam expenditure 20 to 30 per cent below that
found in the best current practice by an amount of super-
heating applicable without any considerable difficulty. Super-
heaters have already been introduced here as auxiliaries to
steam turbines with pretty good effect, but have not yet come
into more than occasional use for general purposes, and even
so are very rarely worked for what they are really worth
Large gas engines are beginning to come into use as prime
movers for electrical purposes, and one such plant of 12,000
KW capacity is just being installed in San Francisco. The
gas engine in large sizes shows very great thermal efficiency,
giving the brake HP hour on the thermal equivalent of 1 lb.
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S24 ELECTRIC TRANSMISSION OP POWER.
of coal or even less, and is to-day becoming a formidable com-
petitor of steam engines for many purposes. Working as it
does from a very high initial temperature, its theoretical claim
to efficiency is valid enough, and the difficulties of lubrication
at the temperature involved have proved less serious than
was first supposed. The main trouble is the fact that ordi-
narily only every fourth stroke is a working stroke, so that for
a given number of impulses per revolution of the fly-wheel the
gas engine becomes far more heavy and complex than the
steam engine. Nevertheless, the gain in fuel economy is so
valuable that the incentive to use gas engines is great. They
are usually worked in the large sizes with natural or "producer"
gas, sometimes with gas from the blast furnaces of the steel
industry, in other words with cheap gas unsuited for illumi-
nating purposes, and have the merit of being very quickly
brought into action when required. Difficulties of governing,
once serious, have now been in great measure eliminated.
Many blunders are made by being too hasty in buying
engines for electric service, and not sufficiently studying the
problem. For uniform loads the selection of the engines can
be made easily. For variable loads it requires great astute-
ness and experience, nor is it safe to argue from experience
based on other kinds of variable service. No engines can be
subject to greater variations of load than are met in marine
engines driving a ship in a high sea. If the screw rises from
the water the whole load is thrown off, and resumed again with
terrible violence when the screw is submerged. Nevertheless
an engine, which is so arranged as to perform well luider these
trying circumstances, might perform badly when put on
electric railway or power service, not because of its inability
to stand the far less severe changes of load, but for the reason
that the average load would be much further from its full
capacity than in the case of marine practice. For large rail-
way and power service it is best to use direct connected units,
for the sake of compactness and economy. If a station is of
sufficient magnitude to employ four or five 500 HP engines,
direct connecting is advisable in nearly every case.
It has been said that such a plant has a lack of flexibility
that is dangerous in case of sudden and great variations of
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ENGINES AND BOILERS. 825
load. This ia not true if the engines have been intelligently
proportioned for the work they have to do, although in some
cases there has been trouble due to the fact that the engines
were ill-fitted to operate successfully under the changes of
load to which they were subjected. As a matter of economy
both in engines and dynamos, it is desirable to work direct
coupled plants at a fairly high speed. There is no need of ex-
aggerating the size of both engine and dynamo for the sake of
rimning at 50 to 70 revolutions per minute, when equally good
engines and dynamos of smaller size and less weight can be
obtained by running at 90 to 120 revolutions or more. Much
of the unwieldiness charged against large direct coupled units
has been the result of yielding to the importunities of some
engine builder who wanted to sell a very large machine, and
putting in an engine and draamo working at absurdly and un-
necessarily low speed.
Electric power transmission, with a steam engine as the
prime mover, is most likely to be developed in the direction of
very large plants, to which these remarks apply most forcibly,
particularly as in order to make transmission of power from
a steam-operated station profitable, it is necessary to seek the
very highest efficiency. Apart from the cost and inconvenience
of very low speed luiits, it must be borne in mind that the
mechanical efficiency of large low speed engines with heavy
pistons and enormous fly-wheels, is lower than that of those
designed for more reasonable speeds, which gives added reason
for moderation in planning direct coupled units.
Throughout the design of a power station the probability of
light loads must be considered. Not only does this have an
important bearing on the economy of the engines, but it influ-
ences that of the boilers as well. The cost of operation de-
pends on the coal consumption, and this in turn not only on
the amount of steam that must be produced, but on the effi-
ciency of its production.
There is, however, no classification of boilers on which one
can safely rest in judging of their economy. There is much
more difference in economy between a carefully fired and a
badly fired boiler of the same kind, than there is between the
best and the worst type of boiler in ordinary use. Boilers may
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826
ELECTRIC TRANSMISSION OF POWER.
be generally divided into three classes: Shell boilers, in which
the water is contained in a plain cylindrical tank heated
on the outside; tubular boilers, in which there are one or
many tubes running lengthwise of the boiler shell, and serving
as channels for the heated gases from the fire; and water-tube
boilers, in which the water is contained in a group of metallic
tubes, around which the heat of the fire freely plays. Fig. 188
shows a cross section through the furnace of a bank of boilers
of the first class. In this case, three shells were placed over
each furnace, commimicating with a common steam drum.
Each shell was 30^ in diameter and 30' long. Fig. 189 rep-
resents one of the many forms of tubular boiler. In this the
structure is vertical, with a furnace at the bottom, and the
Fia. 188.
tubes are numerous and rather small, giving a large heating
surface. Tubular boilers are very often arranged horizontally,
and in one very excellent and common type (return tubular),
the flame and heated gases pass horizontally under the boiler
shell and then back through the tubes to the furnace end
and thence upward into the stack. A typical water-tube
boiler is shown in Fig. 190. Here the furnace is at the left
of the cut and the stack at the right. The tubes are inclined as
is usual in water-tube boilers, and steam space is secured by
the drum above. Each class of boiler has nearly as many
modifications as there are makers, most of them being with
relation to the arrangement of the fire with respect to the
boiler proper.
As to the merits of the different classes, opinions differ very
widely. It is clear from experience that the simple shell
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ENGINES AND BOILERS.
827
boiler is decidedly inferior to either of the others in econ-
omy, in spite of its simplicity and cheapness. Of late years it
has been the fashion to employ water-tube boilers under all
Fio. 180.
sorts of conditions, on account of their supposed great effi-
ciency as steam producers, safety, and compactness. Purely
experimental runs with such boilers often show phenomenal
efficiency, but teste under working conditions sometimes r^*
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828
ELECTRIC TRANSMISSION OF POWER,
sidt otherwise. It is important to note that not only does
skill in firing produce a great improvement in boiler economy,
but that by influencing the firing different kinds of coal give
very different results quite independent of their theoretical
value as fuel. The thermal value of coal, or other solid fuel,
is almost directly as the proportion of carbon contained in it,
and for comparative purposes boiler tests are generally re-
duced to evaporation of water from and at 212° F. per pound
of combustible used, i.e., per pound of carbon. However, the
firing in different furnaces is differently affected by changes in
mmmm?///i/////m//mm
FlO. 190.
fuel, so that it is impossible to predict by tests on one boiler
what a similar one will do imder other conditions.
Altogether, the subject of boiler efficiency is a difficult and
tangled one, since the conditions are constantly changing,
and the best guide is fomid in the general result of a long series
of tests rather than in theories of combustion. Forcing the
output of a boiler usually injures its efficiency by compelling
the combustion of an abnormal amount of coal for the grate
surface of the furnace. It follows that a boiler is apt to be
more efficient at moderate loads than at very high ones. In
marine practice, boilers may sometimes have to be forced to
a high output to save weight and space. In electric stations
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ENGINES AND BOILERS.
829
it is sometimes better to force the boilers at the hours of heavy
load, than to keep a relay of boilers banked in readiness for
use, but except for this, the boilers, like the rest of the plant,
should be worked as near their maximum efficiency as possible.
The best fuel to use is not at all invariably that of the
highest thermal value, in fact with the proper furnace a grade
of coal only moderately good is very often the most economic
cal. In starting a steam plant of any kind comparative tests
of various coals should generally be made, and are more than
likely to pay for themselves many times over. In absolute
heating value various kinds of fuel compare about as follows:
Kind of Fuel.
Heat of Combiution.
Evaporatien.
15,260
15.8
14,600
16.0
14,875
14.9
18,750
14.2
12,760
18.2
12,500
18.0
11,750
12.2
9,650
10.0
7,250
7.6
Best anthracite . .
Welsh steam cosd
Pocahontas
Cumberland
Coke, ordinary . .
Cape Breton
Lignite
Peat, dry
Wood, dry
The heat of combustion is per pound of fuel, and is given in
thermal imits, this imit being the heat required to raise 1 lb.
of water 1^ F.
The evaporation gives the poimds of water which can be
evaporated from and at 212° F. by the complete utilization
of the annexed heats of combustion. In other words, no more
than 15 lbs. of water can possibly be evaporated by 1 lb.
of coal of the thermal value of 14,500. Extravagant claims
are sometimes made for patented boilers of strange and
imusual kinds, so it is well to bear these figures in mind and
to remember that you cannot evaporate more water than
the figures indicate, any more than you can draw a gallon
out of a quart bottle. In practice coal is likely to fall perhaps
10 per cent below the thermal values given above. Good
boilers with careful firing will utilize from 70 to 75 per cent
of the thermal value of the coal. Occasional experimental
runs may give slightly higher figures, but only under very
exceptional circumstances.
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ELECTRIC TRANSMISSION OF POWER.
Now as to actual tests under boilers. Examinations of
more than a hundred carefully conducted tests by various
authorities show from 8 to 13 lbs. of water evaporated
from and at 212° per poimd of combustible. As average good
steam coal contains from 8 to 15 per cent of ash and mois-
ture, these results correspond to from 7 to 11 J lbs. of water
per pound of coal. Now and then a single test gives a
result a few hundredths of a pound better than 13 lbs. per
•pound of combustible, and an occasional poor boiler shows less
than 8 lbs. Generally from 10 to 16 lbs. of coal are consumed
per hour per square foot of grate surface. The following
table gives a general idea of the results of boiler tests, good,
bad, and indifferent.
Kind of Boiler.
Kind of Coal.
Evaporation.
Return tubular
Welsh steam
18.12
Water-tube
( Bituminous, 8 parts 1
pea and dust, 1 part j
Cumberland
18.01
Return tubular
12.47
Vertical tubular
Cumberland
12.29
Return tubular
Cnmh^rlftTld
12.07
Return tubular
Cumberland
12.03
Return tubular
Anthracite
11.68
Marine
Newcastle
11.44
Water-tube
Anthracite
11.81
Water-tube
Cumberland
10.98
Plain tubular
Anthracite
10.88
Water-tube
CnvnY>p,rland . . .
10.79
Marine. ,
10.44
Return tubular
Anthracite
10.48
Liooomotive
Coke
10.39
Water-tube
Anthracite
10.00
Return tubular
Anthracite
9.55
Cylinder
Anthracite
9.22
Cylinder
Cumberland
8.74
Cylinder
Anthracite
8.44
The evaporation is per pound of combustible. The most
striking feature of this table is the small difference in efficiency
between the various kinds of boiler. Putting aside the cylin-
drical shell boilers, which are distinctly inferior to the others,
it appears that in other types of boiler there is little to choose
on the score of economy alone. The difference between the
better and worse boilers of each class, due to difference of
design, condition, and firing, is much greater tlwi th^ differ-
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ENGINES AND BOILERS, 881
ence between any two classes. Even the same boUer with
different fuels, firing, or when in different condition, may give
evaporative results var3dng by 30 per cent. Economy de-
pends vastly more on careful firing and proper proportion-
ing of the grate and heating surfaces to the fuel used, than
upon the kind of boiler. In fact, judging from all the available
tests, the differences between various types of boiler when
properly proportioned are quite small.
The most that can be said is that plain shell boilers are de-
cidedly inferior to the other forms, of which the horizontal return
tubular and the water-tube have ^ven slightly higher results
than the others. Water-tube boilers are generally rather com-
pacter and stand forcing better than ordinary tubular boilers.
They also are less likely to produce disastrous results if they
explode. On the other hand, they are more expensive, and are
as a class hard to keep in good condition, particularly if the
water supply is not of good quality.
Probably under average conditions a well-designed horizon-
tal return tubular boiler will give as great evaporative effi-
ciency as can regularly be attained in service, and the choice
between it and a water-tube boiler is chiefly in economy of
space and capacity for forcing. There is no excuse for the
explosion of any properly cared for boiler.
The actual evaporation secured per poimd of total fuel is
something quite different from the figures in the table just
given. In the first place, allowance must be made for ash and
fuel used for banking the fires. In the second place, in regular
running the firing is seldom as careful as in tests.
On account of these the evaporations per poimd of com-
bustible given in the table must be reduced from 15 to 20 per
cent to correct the result to pounds of coal used in actual
service.
Ten poimds of water or over, evaporated from and at 212® per
pound of total fuel may be regarded as exceptionally good prac-
tice in every-day work. Nine to 10 lbs. under the same con-
ditions represents fine average results, and 8 to 9 lbs. is much
more common. In fact, 8 lbs. is an unpleasantly frequent figure,
particularly in boilers operated under variable load, such as is
generally found in electric plants of moderate size. All these
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ELECTRIC TRANSMISSION OF POWER.
facts point out the necessity of thorough and careful work in
every part of a power plant. Bad design or careless opera-
tion anywhere plays havoc with economy. In most instances
far too little attention is paid to the adaptation of the furnace to
the particular fuel used. In case of attempting power trans-
mission from cheap coal at or near the mines, the furnace and
firing problem is of fimdamental importance. Most furnaces
are constructed to meet the requirements of high grade fuel
and are quite likely to work badly with anything else. In
transmitting power from cheap coal the grate surface, draft,
and so forth must be carefully arranged with reference to the
grade of fuel to be used and not with reference to standard
coals used elsewhere. The methods of firing, too, require
careful attention.
At the present time mechanical stokers are in very extensive
use in some parts of the country. The reports from them are
of varying nature, but the consensus of opinion seems to be
that they are very advantageous in working medium and low
grade coals, but of less utility in the case of high grade
coals. They are somewhat expensive and require intelligent
care now and then like all other machiner}'^, but when it comes
to firing large amounts of cheap fuel at a fairly regular rate
they do most excellent work. When coal is dear, careful hand *
firing is probably more economical than any mechanical
method. With first-class coal and boilers one good fireman
and a coal-passer can take care of 2,000 KW in modem appara-
tus, so that the total cost of firing is not a very serious matter.
A poor fireman is dear at any price, and quite as disadvantage-
ous to the station as a poor engineer.
Kind of Engine.
Coal per IHP Hour.
GondenBing.
Non-Condensing.
Simple, dependent valve,
2.77
2.33
2.22
2.00
1.88
1.66
1.44
3.66
Simple, independent valve
3.11
Compound, dependent valve
2.66
Compound, independent valve
2.44
Ti'inle. denendent valve
Triple, independent valve
Triple, independent valve large
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ENGINES AND BOILERS. 833
Reverting now to engine performances, wo may form a fairly
definite idea of what may ordinarily be expected in the way of
coal consumption per indicated horse-power hour.
The foregoing table shows the coal consumption of the
various kinds of engines, based on the burning of 1 lb. of
coal for each 9 lbs. of feed water used. Although greater
evaporation can often be obtained, 9 lbs. of water per pound
of coal is a very good performance indeed, decidedly better
than is found in general experience. It presupposes good
boilers, good coal, and skilful firing, such as one has a right
to expect in a large power plant.
The figures apply only to engines of several hundred HP,
at or near their points of maximum economy, and operated
from a first-class boiler plant.
They can be and are reached in regular working, and are
sometimes exceeded. A combination of great efficiency at
the boilers and small steam consumption in the engine some-
times gives remarkable results. The best triple expansion
condensing engines worked under favorable conditions can be
coimted on to do a little better than 1.5 lbs. of coal per IHP
hour, occasionally even in the neighborhood of 1.25 lbs. Even
with compound condensing engines, tests are now and then
recorded, showing below 1.5 lbs. of coal per IHP hour. But
these very low figures are the result of the concurrence of
divers very favorable conditions, and those just tabulated
are as good as one should ordinarily expect. It must not be
supposed that the weight of coal used per HP hour necessarily
determines the economy of the plant. The cost of fuel of
course varies greatly, and its price in the market is by no
means proportional to its thermal value. As a rule, the coals
which give the best economic results are not those of the
greatest intrinsic heating power. On the contrary, dollar for
dollar, the best results are veyy frequently obtained from cheap
coal, or mixtures of inferior coal with a portion of a better
grade. Hence, the boilers of a plant which is a model of econ-
omy may show an evaporation of only 7 or 8 lbs. of water
per pound of coal. Boiler tests with the conditions of economy
in view are of great importance, and are likely to pay for them-
selves tenfold in even a few months of operation.
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334 ELECTRIC TRANSMISSION OP POWER.
A word here about fuel oil. Petroleum has, weight for
weight, much greater heating power than coal. Its heat of
combustion is about 20,000 to 21,000 thermal units, it costs
little to handle and fire, leaves no ash and refuse to be taken
care of, produces little smoke, and is generally cleanly and
convenient.
It has been thoroughly tried by some of the largest elec-
trical companies in this coimtry, and at moderate prices, a
dollar a barrel or less, is capable of competing on fairly even
terms with coal. But experience has shown some curious
facts about its performance. The amount of steam or equiva-
lent power required to inject and vaporize the oil in one of
the most skilfully handled plants in existence amoimts to no
less than 7i per cent of the total steam produced. And
curiously enough, the cost of oil for firing up a fresh boiler,
and the time consumed, compare unfavorably T^ith the results
obtained from coal. In spite of the great amount of heat
evolved from fuel oil, it appears to be less effective than coal
in giving up this heat to the boiler by radiation and convection.
There is good reason to believe that more than half the total
heat of combustion of incandescent fuel is given off as radiant
heat, and most of the remainder is of course transferred by
convection both of heated particles of carbon and of molecules
of gas.
It is not imlikely, therefore, that a petroleum fire with its
small radiating power and comparative absence of incan-
descent particles, fails in economy through inabiUty to give
up its heat readily. This view of the case is bonie out by the
facts above cited and by the abnormally high temperature of
the escaping gases often foimd in boiler tests with petroleum
fuel. At all events it is clear from such tests that the evapo-
ration obtained from fuel oil is not so great as would be ex-
pected from its immense heat of combustion, and unless at
an exceptionally low price, its use is less economical than that
of coal.
The most striking innovation of recent years in the genera-
tion of mechanical power by steam is the development of the
steam turbine. Year by year during the past decade it has
slowly grown from experiment to realization, until at the pres-
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ENGINES AND BOILERS. 335
ent time it has reached a position that demands for it most
serious consideration. It looks very much as if, for many pur-
poses, the reciprocating steam engine might be hard pushed.
Strangely enough the steam turbine or impulse wheel is the
earliest recorded form of steam engine, dating clear back to
Hero of Alexandria, who flourished about 130 B.C. The engine
which Hero suggested was merely a philosophical toy, and it
took nineteen centuries beyond his day to produce any engine
that was not a toy, but now after two thousand years Hero's
idea has borne fruit.
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Fio. 191.
The fundamental principle of the steam turbine is just that
of the water turbine — directing fluid pressure against a series
of rotating buckets. The first practical steam turbine, devised
by De Laval, is very closely akin to the Pelton water-wheel
and to the little water-motors sometimes attached to faucets
for furnishing a small amount of power. The essential fea-
tures of his apparatus are well shown in Fig. 191. It consists
of a narrow wheel A with buckets around its periphery, re-
volving within a housing B and supported by a rather long and
slender shaft. Bearing upon the buckets at an acute lateral
angle is the steam jet Ey in this case one of three equidistant
jets playing on the same wheel. To obtain the most efficient
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836 ELECTRIC TRANSMISSION OF POWER,
working of the jet the steam nozzle is somewhat contracted at
D, a little way back from the buckets. The steam is discharged
on the other side of the wheel as shown. It strikes the buckets
as a jet at great velocity, and should, if the conditions were
just right, expend nearly all its energy in driving the wheel and
should itself leave it at or near zero velocity. Of course thio
condition does not hold in practice, but still a steam turbine of
this De Laval construction is capable of doing marvellously well.
The main objection to this form is the enormously high rotative
speeds necessary for efficient running. Here, as in hydraulic
impulse wheels, the peripheral velocity should be about one-half
the spouting velocity of the fluid. With high pressure steam
this is, when one works the turbine condensing, 3,000 to 5,000
feet per second. In practice these De Laval wheels have usu-
ally been geared to a driving shaft, but the wheel itself has run
at 10,000 to 30,000 r.p.m., seldom below the former figure even
in large sizes. But the economy reached has sometimes been
very high, as in some tests a few years ago in France, when,
with an initial pressure of 192 lbs., a 300 HP turbine showed
a steam consumption of only 13.92 lbs. per effective HPH —
a figure seldom reached with engines. The governing is by
throttling the steam supply in response to the movement of
a fly-ball governor of the kind generally familiar in steam
engineering.
The inconvenience of the very high rotative speed of such
turbines has led to the development of forms working more
along the lines of hydraulic turbines, of which by far the best
known is the Parsons turbine, which has recently made so
striking a record in marine work, having been applied to sev-
eral British torpedo-boats and even to larger vessels. In this
remarkable machine the passage of the steam is parallel to
the axis of rotation instead of tangential, and its hydraulic
prototype is the parallel-flow pressure turbine, shown in dia-
gram in Fig. 200. In fact the steam is passed successively
through a large number of such parallel-flow turbines, gradually
expanding and giving up its energy to the successive runners
located, of course, on the same shaft. The course of the ex-
panding steam is well shown in Fig. 192, which gives in diagram
its progression through four sets of vanes, two, 1 and 3, being
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ENGINES AND BOILERS.
837
rings of guide blades, and the others, 2 and 4, rings of ninner
blades. The steam, starting at pressure P, expands succes-
sively to Pi, Pii, Piii, Piv, expanding sharply against the runner
blades and giving them a reactive kick as it leaves. In this
case the steam velocity against, say the runner blades 2, is
not, as in the De Laval form, the full spouting velocity due
to the initial head of steam, but merely that corresponding to
the differential pressure P-Pi. This enables the peripheral
speed of the rimner to be kept within reasonable limits without
violating the conditions of economy, but the turbine at best
is not a slow-speed machine.
Fig. 193 is a longitudinal section through the Parsons
type of steam turbine as developed in this country by the
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Fig. 192.
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pipe controlled by the governor and comes first into the annu-
lar chamber A at the extreme left-hand end of the runner.
The runner blades are graduated in size so that the expanding
steam may give nearly a uniform useful pressure per blade,
and to this end the diameter of the runner hub is twice in-
creased as the steam expands towards the exhaust chamber B.
The endwise thrust of the steam entering the turbine from A
is balanced by its equal pressure on the balancing piston C,
which revolves with the runner. To the left of this is an
annular steam space and a second balance piston C. Now,
when the expanding steam has passed the first section of the
runner into the steam space E, it can flow back through the
channel F and the post D against this second balance piston.
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ELECTRIC TRANSMISSION OP POWER,
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Still further to the left is a second steam space and a third
piston C, which is similarly exposed to the pressure from G,
where the third stage of expansion begins. The effect of this
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ENGINES AND BOILERS, 889
balancing system is to render the end thrust negligible what-
ever may be the ratio of expansion in the turbine. A thrust
bearing at H keeps the working parts positioned and takes up
the trivial thrusts which may incidentally be present. J, J, J
are the bearings, which are out of the ordinary in that within
the gun-metal sleeve that forms the bearing proper are three
concentric sleeves fitting loosely. The clearance between
them fills with oil, cushioning the bearings. Now if the run-
ner is not absolutely in balance there is a certain flexibility in
the bearings so that the runner can rotate about its centre
of gravity instead of its geometrical centre, thus stopping
vibration. An equivalent expedient is found in the De Laval
turbine, the shaft of which is deliberately made slightly flexible
so that it may take up rotation about its centre of gravity.
If is a pipe which again takes up the work of keeping the
pressure balanced by connecting the exhaust chamber B \nth
the steam space behind the last balance piston. At M is a
simple oil pump taking oil from the drip tank N and lifting it
to the tank 0, whence it is distributed to the bearings. A by
pass valve P turns high pressure steam directly into the steam
space E, in case a very heavy load must be carried, or a con-
densing turbine temporarily rim non-condensing, i? is a
flexible coupling for the driving shaft, and at that point too is
the worm gear that drives the governor. The governor in
its operation is somewhat peculiar. Instead of throttling the
steam supply so as to reduce the effective pressure, the steam
is always sent to the turbine at full boiler pressure, but discon-
tinuously. The main steam valve is controlled by a little
steam relay valve which is given a regular oscillatory motion
by a lever driven from an eccentric. The steam is thus ad-
mitted to the turbine in a series of periodic puffs. Now the
fulcrum of this valve lever is movable and is positioned by
the fly-ball governor, so that the position of the valve with rela-
tion to the port is varied without changing the rate or ampli-
tude of the valve motion. Hence the length of the puffs is
changed so that while at full load steam is on during most of
the period, at light load it is on for only a small part of each
period. This is well shown graphically by Fig. 194, which is
self-explanatory.
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ELECTRIC TRANSMISSION OF POWER.
The governor balls are so arrajiged that they work both
ways, their mid-position corresponding to full admission of
steam, so that a violent overload can be made to shut off steam,
and a break in the governor driving gear will do the same
instead of letting the turbine run away.
These turbines are capable of operating with really remark-
able efficiency. Fig. 195, from the makers' tests, gives the
performance, both condensing and non-condensing, of a West-
inghouse-Parsons turbine directly coupled to a 300 KW
quarter-phase alternator giving 440 volts at 60*^, the speed be-
ing 3,600 r.p.m. Operated condensing, the steam consumption
WHEN RUNNING LIGHT LOAD
Fio. 194.
at full load falls to about 16.4 lbs. per electrical HPH, and is
below 20 lbs. from 125 HP up. This extraordinary uniformity
of performance at large and small loads is mainly due to the
very small frictional losses in the turbine, although it is helped,
perhaps, by the load curve of the generator. The results when
working non-condensing are very much inferior to these, rela-
tively worse than in an ordinary steam engine.
Altogether it is an admirable showing for the steam turbine.
The writer believes Fig. 195 to be entirely trustworthy, as it
corresponds very closely ^vith certain independent tests now
in his possession, from another turbine of the same capacity
and speed, in which tests the makers of the turbine had no
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PLATE XI.
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ENGINES AND BOILERS,
841
part. The substance of the matter is that the steam turbine
will work just about as efficiently as a first-class compound
condensing engine, and can not only be more cheaply made,
but takes up much less room. In the same way, for electri-
cal, purposes a directly connected generating set with steam
turbine is, or ought to be, much cheaper and smaller than
those now in use, while retaining equally high efficiency.
High rotative speed is by far the cheapest way of getting out-
as so
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ELECTRJOAL HOME POWER
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put, and when, as in this case, no heavy reciprocating parts
are involved, there is no good reason for objecting to high
speed. The present fashion for low speed dynamos is largely
a fad, having its origin in direct coupling to Corliss engines,
and with the modem stationary armature construction there is
no reason why high rotative speed should not be used, at least
in alternators.
In Plat« XI is shown the first large turbine-driven generator
installed for regular commercial service in this country. Smaller
ones had been in use in isolated plants for some time, but
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342 ELECTRIC TRANSMISSION OF POWER,
this 1,500 KW set. installed for the Hartford Electric Light
Co., was the first important installation of this kind. The
turbine is designe<l for a maximum output of 3,000 HP at a
speed of 1,200 r.p.m., and the complete set, weighing only
175,000 lbs., takes a floor space of but 33' 3^^ X 8' 9^. The
generator is a quarter-phase machine at 60^^ frequency. This
outfit should be capable of giving an efficiency rather better
than that shown in Fig. 195 — probably less than 15 lbs. of
steam per electrical HPH at steady full load; in other words, it
should do nearly as well as a triple expansion engine. This
machine has now been in successful operation for some four
years.
Alternators may be conveniently and cheaply built for the
speed implied in steam turbine practice, but continuous cur-
rent generators involve some difficulties. For power trans-
mission work turbo-generators have much to recommend them
as auxiliaries, and there is a strong probability of their taking an
important place in the development of the art. There has not
yet been accumulated enough experience with them to enable
a final judgment of their practical properties to be formed,
or to justify an unqualified indorsement of their economy.
Another successful form of steam turbine, now consider-
ably used in units of output as great as several thousand KW,
is the Curtis, which differs radically in several respects from
that just described. In the first place it is not a pressure
turbine but an impulse turbine, in which the steam is expanded
in the admission nozzles to a high jet velocity, the kinetic
energy of which is then utilized in the runner buckets. It
is thus dynamically more akin to the De Laval than to the
Parsons turbine, but differs from it much as a true impulse
turbine differs from a Pel ton wheel.
In the second place it is regularly built in all the larger
sizes as a vertical shaft machine carrying the generator arma-
ture on its upper end and being supported below by an inge-
niously contrived water step kept afloat by a pressure pump.
In the Curtis turbine, however, the expansion takes place
not in a single nozzle as in the De Laval type but in several
success' ve stages so that the jet velocity and with it the neces-
sary peripheral speed is very considerably reduced. Fig. 196
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ENGINES AND BOILERS,
843
shows the course of the steam and is almost self-explanatory.
The upper section is the first stage, the lower the second stage,
and in some of the larger units three or four stages are used.
Their effect is akin to that of compounding an engine in the
better temperature distribution and proportioning of parts.
The governing in this turbine is exactly in line with the prin-
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ciples of hydraulic impulse turbines; the numerous admission
nozzles being fitted with independent valves as sho\vn in the
cut. These are in succession opened or closed in accordance
with the requirements of the load, by the action of a sensitive
fly-ball governor which controls a series of relay valves, in turn
working the admission valves. In the earlier and some of
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344 ELECTRIC TRANSMISSION OF POWER.
the later turbines these relay valves have been electrically
actuated, but at present both this and purely mechanical con-
trol are used. As each admission valve is either fully open or
closed there is no throttling of the steam, which gives a material
gain in efficiency.
Plate XII shows a 500 KW Curtis turbo-generator which
is peculiarly interesting as being a direct current machine
designed for railway purposes. The rotative speed is 1,800
r.p.m., but in spite of this, the problem of commutation has
been successfully met. In this, as in all the large Curtis turbines,
there is but one supporting bearing on which the moving parts
spin top fashion kept in line by a pair of small guide bearings.
The structure is thus wonderfully compact, but it is still an
open question among engineers as to the advisability of a
vertical shaft. It gains a little in friction and a certain amount
in space together with immunity from flexure of shaft, while
on the other hand it loses greatly in accessibility, and exposes
the generator portion to certain risks from heat, steam, and oil
that are not altogether negligible. From a practical stand-
point the Curtis turbine has made a good record, and many
large units, even up to 5,000 KW, are in successful use, to no
small extent in large stations designed for railway service,
polyphase current being generated and transmitted to con-
verter stations. The large polyphase turbo-generators are all
of the vertical type closely resembling Plate XII.
The strong points of steam turbines are cheapness for a
given output, economy of floor space, freedom from vibration,
uniform efficiency at various loads, and light friction. Of
these the first is the direct result of high speed both in tiu"bine
and generator. For some years there was a strong tendency
toward very low engine speeds, which produced generating
units of needlessly great weight and cost. The turbine goes
to the other extreme of speed, and while it runs at speed too
high for the most economical construction, can be built in-
cluding the generator at a cost probably below that of any
other direct connected unit. The current price is relatively
high, being adjusted to that determined by engines, but this
condition is of course temporary. Economy of floor space is
very marked, especially in the vertical shaft type, but it
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PLATE XII.
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ENGINES AND BOILERS, 845
actually is much less than at first appears, since the location
of the boilers generally determines the area of the plant, and
turbines cannot conveniently be huddled into the space which
their dimensions would suggest. They are remarkably free
from vibration, due to the necessity of avoiding centrifugal
strains by extremely careful balancing, and their friction is
very light indeed. The uniform economy at various loads is
partly due to small friction and partly to the fact that the
expansion of the steam is substantially fixed by the construc-
tion and does not vary materially with the load. Nevertheless,
as has already been shown, an engine properly designed for
varying load will show at least equally good practical results.
Compare in this the lowest curve of Fig. 186, and Fig. 195, to
say nothing of Fig. 187.
The actual efficiency of the steam turbine is good without
being in any way phenomenal. At equal steam pressure, super-
heat, and vacuum, the steam turbine, in its present stage of
development, is as a rule slightly less efficient per brake horse-
power than a first-class compound condensing engine. Tur-
bines, however, suffer extremely, like other engines in which
there is very great expansion, from diminished vacumn and
only by using a vacuum of 28^^ and 100° to 150° F. superheat-
ing, can they be brought up to a performance just about equiva-
lent to a compound condensing engine. No tests of turbines
made under any conditions have yet been able to equal or very
closely approach the best results from compound or triple ex-
pansion reciprocating engine in steam per brake horse-power.
Whether they will do so in the future remains to be seen, but
at present there is no reason to predict it.
As a practical matter, however, one can very comfortably
stand some loss in efficiency if thereby the fixed charges and
maintenance can be kept down. The greater the necessary
proportion of such items, the more complacently can one
stand a slight loss in steam economy. In the case of engines
which from the conditions of their use give a rather small
annual output for their capacity, reduction of fixed charges
and maintenance is of great importance. Hence for auxiliary
plants in power transmission which are idle a large part of the
time, there is a great deal to be said for the turbo-generator if
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ELECTRIC TRANSMISSION OF POWER.
it can be had at a reasonable figure. It can also be put into
action more quickly than ordinary engines, and handles heavy
overloads well beside running easily in parallel by reason of
the uniform rotative effort.
Considerable space has here been given to describing some
of the details of steam turbines, in the belief that they are of
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sufficient importance to warrant it even in a chapter not
intended to be in the least a compendium of steam practice,
but a mere outline of the essential facts. They certainly have
already proved their right to a place, and the question is now
merely that of the probable limitations of their usefulness,
which only protracted experience can disclose.
For an electrical power station operated by steam power the
vital economical question is the cost of fuel per kilowatt hour.
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ENGINES AND BOILERS.
847
rather than the performance of engines and boilers alone.
This final result involves the performance of the station appa-
ratus imder varying loads, too frequently rather light, and,
implicitly, the skill of the operator in keeping his apparatus
actually running as near its point of maximum economy as
possible, in spite of changes in the electrical output. This
personal element forbids a reduction of the facts to general
laws, but a concrete example will be of service in showing what
may be expected in a well-designed and well-operated power
plant. Fig. 197 shows a pair of *' load lines," from a large and
particularly well-operated power plant. The solid line shows
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Fxo. 198.
the variations of load throughout a day in the latter end of
January, and the broken line the variations of load during a
day early in April.
The early darkness of a winter's day is very obvious in the
former line. The station carries in addition to lights a heavy
motor service that keeps up a fairly miiform load through-
out the day, until the sudden call for lights in the early
evening. The load factor shown by the solid line is .35 (i.e.,
this is the ratio between maximum and average load). The
second load line gives a much better relation between these
quantities, the load factor being .64, which is quite usual in
this station during the spring and summer. Of course every
effort is exerted to keep the machines which are in use as fully
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848 ELECTRIC TRANSMISSION OF POWER.
loaded as possible. In spite of this the small output during
the early morning hours, coupled with the losses due to circu-
lating pumps and other minor machinery, and the fuel used for
banking and starting fires, brings the cost of fuel during this
period far above the average for the day. The curve in Fig.
198 shows roughly the variation in the cost of fuel per KW-
hour throughout the day, taken from the average of a nimi-
ber of tests. As the fuel cost in a large central station is a
considerable portion of the total expense, it is evident that the
result is an excellent one. During all the hours of heavy load
the cost of fuel is less than six-tenths of a cent per KW-hour,
and the total cost of production but little more. This result
will give an excellent idea of the cost of generating power on
a large scale with cheap coal. It is, however, exceptionally
good, and can only be equalled by a very well managed plant
with the best modem equipment both electrical and mechanical.
Of course the expenses of distribution, administration, and
the like must be taken into account in considering the cost
per KW hour delivered. The general question of station ex-
penses cannot be here investigated, but this brief digression
gives some idea of the necessary relation between the character
of the work and the commercial results in generating electric
power on a large scale, so far as the use of steam engines as
prime movers is concerned.
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CHAPTER IX.
WATER-WHEELS.
The importance of the development of water-powers for
electrical purposes we have already come fully to realize. The
lessons of the last few years have been exceedingly valuable
ones, and it is safe to say that the utilization of water-powers
for electrical transmission will be kept up until every one
which is capable of commercially successful development is
worked to its utmost capacity. In spite of the length of time
that water-wheels of various sorts have been used, it is only
very recently that these prime movers have been brought to a
stage of development that renders them satisfactory for elec-
trical purposes. The old water-wheel was even more trouble-
some as a source of electrical power than the old slide valve
steam engine.
The customary classification of water-wheels for many years
has been into overshot, undershot, and breast-wheels, and
finally tiu-bines. Various modifications of all these have, of
course, been proposed and used. Of these classes, the first
three may be passed over completely as having no importance
whatever in electrical matters, save in certain modifications so
different from the original wheel as to be scarcely recognizable.
To all intents and purposes they are never used for the pur-
pose of driving dynamos, although occasionally an isolated
instance appears on a very small scale.
It is the turbine water-wheel which has made modem
hydraulic developments possible, and more particularly elec-
trical developments. The turbine practically dates from 1827,
when Foumeyron installed the first examples in France,
although it is interesting to know that a United States patent
of 1804 shows a wheel of somewhat similar description, never
so far as is known used. The modem turbine consists of two
distinct parts, the system of guide blades and the runner. The
runner is the working part of the wheel, and consists of a
S49
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ELECTRIC TRANSMISSION OF POWER.
series of curved buckets so shaped as to receive the water with
as little shock as practicable, and to reject it only after having
utilized substantially all of its energy. These buckets are
arranged in almost every imaginable way around the axis of
the runner, but always symmetrically.
Sometimes the curvature of the buckets is such that the
water after having passed through them leaves the wheel
parallel to its axis; sometimes so that the water flows inward
and is discharged at the centre of the nmner; sometimes so
that it passes outward and is discharged at the periphery.
FlO. 199.
Fio. 200.
More often the buckets have a double curvature so that the
water flows along the axis and at the same time either inwardly
or outwardly. It is not unusual, moreover, to have two sets of
buckets on the same shaft for various purposes. The growth
of the art of turbine building has made any classification of
turbines depending on the direction of the flow of the water
very uncertain, as in nearly every American turbine this flow
takes place in more than one general direction, usually inward
and downward. Aside from the runner the essential feature
of the modem turbine is the set of guide blades which sur-
round the runner, and which are so curved as to deliver the
water fairly to the buckets in such direction as will enable it
to do the most good. Accordingly these blades are ciu-ved in
all sorts of ways, according to the way in which the water is
intended to be utilized.
Fig. 199, taken from Rankine, shows a species of idealized
turbine which discloses the principles very clearly. In this fig-
ure A is the guide blade system and B the runner.- The flow
is entirely along the axis, forming the so-called parallel flow
turbine, a form not in general use in America.
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WATER-WHEELS. 861
Fig. 200 shows the sort of curvature which is given to the
guide blades and to the buckets of the runner. The axis of
this or any other kind of turbine may be horizontal or vertical,
as convenience dictates. As may be judged from the illustra-
tion, the water acts on the runner with a steady pressure, and
the buckets of the runner are always filled with the water
which drives them forward. Working in this way by water
pressure due to the weight of the water column, it is not
necessary that the turbine should be placed at the extreme
bottom of the fall, provided an air-tight casing is continued
below the runner so as to take advantage of the solid water
column below the turbine. Such an arrangement is called a
draft tube, and may be of any length up to the full column
which may be supported by atmospheric pressure, provided
the body of water shall be continuous so that there shall be no
loss of head due to the drop of the water from the wheel to the
level of the water in the tail-race. It is as if the column below
were pulling and the column above pushing, the ruimer being
in a solid stream extending from the highest to the lowest
level of water used. As a matter of practice the draft tube is
generaUy made considerably shorter than the column of water
which might be supported by atmospheric pressure, generally
less than 20 feet, depending somewhat on the size of the wheel.
With longer tubes it is difficult to preserve a continuous column,
which is necessary in order to utilize the full power of the
water.
Nearly all American turbines are of this so-called "pressure"
type. There is, however, another type of turbine wheel used
somewhat extensively abroad, and occasionally manufactured
in this country, which without any very great change in
character of the structure operates on an entirely different
principle. There are present, as before, guide blades deliver-
ing the water to the buckets of the ruimer, but the spaces be-
tween these blades are so shaped and contracted as to deliver
the water to the runner as a powerful jet. The energy of
water pressure is converted into the kinetic energy of the
spouting jet, and the buckets of the nmner are not filled solidly
and smoothly with the water, but serve to absorb the kinetic
energy of the jets, and discharge the water below at a very
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ELECTRIC TRANSMISSION OF POWER.
low velocity. Such turbines are known as impulse turbines,
from the character of their action. In the pressure turbines the
full water pressure acts in the runner and in the space between
the guides and the runner. In the pressure turbine each space
between the guide blades acts so as to form a water jet, which
fmpinges fairly on the bucket of the runner without causing a
uniform pressure either throughout the bucket spaces or in the
space between runner and guides. It is not intended that the
passages of the wheel should be, as in the pressure turbine,
entirely filled with the water, nor is it best that they should
be. Fig. 201 gives a sectional view from Unwin showing the
arrangement of the guide blades and buckets of an impulse
Fio. 201.
turbine, in which the flow is, as in the pressure turbine previ-
ously shown, in general along the axis of the wheel. An
impulse turbine necessarily loses all the head below the wheel
and cannot be used with a draft tube.
Occasionally an attempt is made, in the so-called limit tur-
bines, so to design the guides and buckets that the jets may
completely fill the buckets, which are adapted exactly to the
shape of the issuing stream. In such case the turbine works
as an impulse wheel or as a pressure wheel, according as the
draft tube is or is not used.
A modified impulse turbine, largely used for very high heads
of water, is found in the Pel ton and similar wheels, in which
the impulse principle is used through a single nozzle acting in
succession on the buckets of a wheel which revolves in the
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WATER-WHEELS.
353
same plane with the issuing jet. Such a Pelton wheel is shown
in Fig. 202. Occasionally two or more nozzles are used, de-
livering water to the same wheel. Impulse wheels of this class
are exceedingly simple and efficient, and work admirably on
high heads of water. They are, moreover, very flexible in the
matter of obtaining efficiently various speeds of rotation from
the same head of water, as the whole structure is so simple
and cheap that it can be modified easily to suit varying condi-
tions.
It is obvious that the operation of such an impulse wheel is
similar to that of a true impulse turbine, in which only one,
Fig. 202.
or at the most three or four jets from the gmde blades are util-
ized. Most of the hydraulic work done in this country is ac-
complished with pressure turbines, which are worthy, therefore,
of some further description. A small but important por-
tion is accomplished by Pelton and other impulse wheels, and
in a very few instances the impulse turbine proper has been
used.
There are manufactured in this country more than a score
of varieties of pressure turbines. They differ widely in de-
sign and general arrangement, but speaking broadly it is safe
to say that most of them are of the mixed discharge type, in
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ELECTRIC TRANSMISSION OF POWER.
which the water passes away from the buckets of the runner
inward and downward with reference to the axis of the
wheel. It would be impossible to describe even a considerable
part of them without making a long and useless catalogue.
The essential points of difference are generally in the con-
struction of the runner and in the mechanism of the guide
blades. In a good many turbines regulation is accomplished
by shifting the guide blades so as to deliver more or less water
to the runner. A few types will serve to illustrate the general
character of some of the best-known American wheels. Fig.
FlOS. 203 AND 201.
203 shows the so-called Samson turbine of James Leffel &
Co., and Fig. 204, the rimner belonging to it. This wheel is of
the class which regulates by shifting the guide blades, which
are balanced and connected to the governor by the rods at the
top of the casing shown. The water enters the guide blades in-
wardly, and the runner is provided as sho\Mi with two sets of
buckets; the upper set discharging inwardly, the lower and
larger set downwardly. The action of the wheel is almost
equivalent to two wheels on the same shaft, the intention being
to secure an unusually large power and speed from a given
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WA TER-W HEELS.
355
head of water on a single wheel structure. This result is,
as might be anticipated, accomplished, and for a given diam-
eter the Samson turbine has a speed and power consider-
ably greater for a given head than found in the usual standard
single wheels. As before remarked, however, it is almost,
mechanically speaking, equivalent to two wheels through its
peculiar feature of double discharge through independent
buckets.
Another very excellent and well-known wheel is the Victor
turbine, shown in Fig. 205. In this wheel the gate is of the so-
Fia. 205.
called cylinder type, which lengthens or shortens the apertures
admitting water to the guide blades. The runner of this
wheel is so shaped that the water is discharged inwardly
and downwardly. The area of the rimner blades exposed
to the full water pressure is notably great. The cylinder
form of gate is rather a favorite with American wheel man-
ufacturers, and is intended to secure a somewhat uniform
efficiency of the wheel, both at full and part load, although
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ELECTRIC TttANSMlSStON OF POWER.
how completely it does this is a matter which, of course, is still
in dispute. The wheel shown, however, is an exceptionally
good and efficient one, so far as can be judged from general
practice. The same makers also manufacture a wheel with
shifting guide blades.
Another excellent wheel of the cylinder gate type, the
McCormick, is showTi in Fig. 206. The runner of this wheel has
its main discharge downward. It has a rather large power for
its diameter, owing to the proportion of the runners, and is
Fig. 206.
well known as a successful wheel considerably used in driving
electrical machinery.
These turbines are typical of the construction and arrange-
ment used by first-class American manufacturers. They are
all arranged for either horizontal or vertical axes, and for
purposes of driving electrical machinery are whenever possible
used in the horizontal form. All of them, particularly the two
first mentioned, have been widely used for electrical piu'poses.
They are all practically pure pressure turbines and are installed
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857
usually with draft tubes of appropriate length. They are
often, too, installed in pairs, two wheels being placed on the
same shaft, fed from a common pipe but discharging through
separate draft tubes. The arrangement of these draft tubes
is very various, as they can be placed in any position convenient
for the particular work in hand. Fig. 207 shows a common
arrangement where a single wheel is to be driven. The water
Fig. 207.
enters through the penstock, passing into the wheel case,
through the wheel, which has, as is generally the case except
with very low heads, a horizontal axis, and thence passes into
the tail-race through the draft tube, shown in the lower part of
the cut. The full head in the particular case shown is 43 feet,
so that the draft tube is fairly long. Where double wheels are
employerl, there is no longer any necessity of taking up the
longitudinal thrust of the wheel shaft, and an arrangement
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358 ELECTRIC TRANSMISSION OF POWER,
frequently followed is shown in Fig. 208, which gives a very
good idea of the general arrangement of the pair of horizontal
turbines, which may be directly coupled to the load or, as in
the case just mentioned, drive it through the medium of belts.
In many instances it is found cheaper and simpler to moimt
the two wheels together in a single flume or wheel-case, so as
to discharge into the same draft tube. Fig. 209 shows an
arrangement which is thoroughly typical of this practice,
applied in this case to a low head. The pair of wheels are
here arranged so as to discharge into a common draft tube
Fio. 208
between them, while they receive their water from the timber
penstock in which they are inclosed. Such wooden penstocks
are generally very much cheaper than iron ones and for low
heads have been extensively used.
The central draft tube here shown need not go vertically
downwards, but may take any direction that the arrangement
of the tail-race requires. Whether the draft tube is single or
double is determined mainly by convenience in arranging the
wheel and its foundations, and the tail-race. The use of a pair
of turbines coupled together is not only important in avoiding
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WATER-WHEELS,
359
end thrust, but it also enables a fair rotative speed to be
obtained from moderate heads, which is sometimes very im-
portant in driving electrical machinery.
For example, suppose one desired to drive a 500 KW gener-
ator by turbines from a 25 foot head. Allowing a little margin
for overload the turbine capacity should be in the neighbor-
hood of 750 HP. Now turning to a wheel table applying, for
instance, to the "Victor" wheel, one finds that a single 54"
wheel would do the work, but at the inconveniently low speed
of 128 r.p.m. But under the same head a pair of 39^ wheels
Fig. 209.
would give a little larger margin of power at 180 r.p.m., and
hence would probably enable one to get his dynamo at lower
cost, as well as to avoid a thrust bearing. Often such a change
of plan will allow the use of a standard generator where a
special one would otherwise be necessary.
Wherever possible it is highly desirable to employ these hori-
zontal wheels for electrical purposes, inasmuch as power has, in
most cases, to be transferred to a horizontal axis, and the use
of a vertical shaft wheel necessitates some complication and
loss of power in changing the direction of the motion. Occa-
sionally a vertical shaft wheel is used for electrical purposes,
driving a dynamo having a vertical armature shaft. This prac-
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360 ELECTRIC TRANSMISSION OF POWER,
tice is not generally to be recommended, as it involves special
dynamos, and a somewhat troublesome mechanical problem
in supporting the weight of the armature, which is generally
carried by hydraulic pressure. A fine example of this arrange-
ment is to be found in the great Niagara Falls plant.
The use of pressure turbines for driving electrical machinery
is exceedingly convenient on low or moderate heads, say up to
50 or 100 feet. With higher heads frequently the rotative speed
becomes inconveniently great; for example, under 100 feet
head, 150 HP can be obtained from a wheel a little more than
15 inches in diameter, at a speed of more than 1,000 revolu-
tions per minute. At 200 feet head, the power for the same
wheel will have risen to about 400 HP and the speed to nearly
1,300. This is a rather inconvenient speed for so large a power,
and it is necessitated by the fact that a pressure turbine to
work under its best conditions as to efficiency, must rim at
a peripheral speed of very nearly three-quarters the full
velocity of water due to the head in question. If, therefore,
turbines are used for high heads, either the dynamos to
which they are coupled must be of decidedly abnormal design,
or the dynamo must be run at less speed than the wheels.
The former horn of the dilemma was taken in the Niagara
plant, and involved some very embarrassing mechanical ques-
tions in the construction of the djmamos. Where belts are
permissible the other practice is the more usual, of which a
good example is found in the large lighting plant at Spokane
Falls, Wash., where the wheels were belted to the dynamos
for a reduction in speed instead of an increase, as is usually
the case.
Impulse turbines are little used, although manufactured
to some extent by the Girard Water Wheel Co., of San Fran-
cisco, Cal. The wheel manufactured by them is one with a well-
known foreign reputation. Its general arrangement is well
shown by the diagram, Fig. 210. The Girard impulse turbine is
of the outward flow type, a form rather rare in pressure tur-
bines. The water enters the wheel centrally through a set of
guide blades, which form a series of nozzles from which the
water issues with its full spouting velocity and impinges on the
buckets of the nuiner, which siu'romids the guide blades.
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WATEEr-WHEELS. 861
The discharge is virtually radially outward. Regulation is
secured by a governor which either cuts ofif one or more of the
nozzles or may be arranged by swinging guide blades to con-
tract all or a part of the nozzles. In either case, there is no
water wasted, and the wheel works efficiently at practically all
loads.
Like others of the impulse type, the peripheral speed of the
wheel when worked \mder its best conditions for efficiency, is
very nearly one-half the spouting velocity of the water as it
Fio. 210.
issues from the nozzle. This produces for a wheel of given
diameter a lower speed for the same head than in the case of
pressure turbines, while the use of a larger number of nozzles
working simultaneously on the runner gives a higher power for
the same diameter than in the case of the Pelton or similar
wheels, which use only a few nozzles with jets applied tan-
gentially; hence, such impulse turbines occupy a useful place
in the matter of speed, aside from all questions of efficiency.
Under moderately high heads, from 100 up to 300 or 400
feet, they give a much greater power for a given rotative
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speed than impulse wheels emplo3dng only two or three
nozzles. On the other hand they do not run inconveniently
fast, as is the case with pressure turbines under such heads.
At extremely high heads they give, unless operated with
only one or a few nozzles, so great power as to be inconve-
nient for the high speed attained, so far at least as the oper-
ation of dynamos is concerned. At very low heads there
is material loss from the fact that the wheel cannot be used
with the draft tube, and consequently a certain amount of the
head must be sacrificed to secure free space from the wheel to
the tail-water. These Girard turbines are made with both
vertical and horizontal axes, and are applicable to electrical
work with the same general facility which applies to other
types of wheel. Their strong point is economical and effi-
cient regulation of the water supply, together with high effi-
ciency at moderate loads.
The Pelton wheel, already shown in Fig. 202, may be regarded
as an impulse turbine having a single nozzle, and that applied
tangentially. These wheels have proved immensely effective
for heads from several hundred up to a couple of thousand feet.
Like the true impulse turbines, the peripheral speed should be
half the spouting velocity of the water, hence, by varying the
dimensions of the wheel a wide range of speed can be obtained,
which is exceedingly convenient in power transmission work,
permitting direct coupling of the dynamos under all sorts of
conditions. They are not infrequently made with two or three
nozzles, which give, of course, correspondingly greater power
for the same speed. At heads of only 100 or 200 feet these
wheels with their few nozzles give "an inconveniently low rota-
tive speed for the power developed, and are at their best in
this respect between 300 and 1,000 feet. The Pelton wheel
is usually regulated by deflecting the nozzles away from the
buckets of the wheel, a very effective but most inefficient
method, so far as economy of water is concerned. The wheel
has, however, under favorable conditions, a very high efficiency,
certainly as high as can be reached with any other form of
hydraulic prime mover. The practical results given by this
class of wheel are admirable under circumstances favorable to
their use, and the Pelton and Doble wheels have played a very
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WATER-WHEELS. 868
important part in the great power transmission works which
have placed the Pacific coast in the van of modem engineering.
A recent improvement in impulse wheel practice is the
development of a successful "needle valve" for the nozzles^
which obviates the waste of water due to the use of deflecting
nozzles. The needle valve is simply a nozzle which can be
closed at will by a central plunger, moving axially in the
stream just behind the nozzle. The plunger and its seat are
given surfaces curved in such wise that in all positions of the
plunger a smooth emergent stream is produced, and the effi-
ciency of the wheel is very Uttle changed.
This is upon the whole, a better method of regulation than
the deflecting nozzle in that it is economical of water, but
shutting off the stream quickly produces very severe strains
in the pipe line and in most instances some form of relief valve
is desirable to reduce the pressure. To use any form of nozzle
valve, too, the water must be thoroughly freed from sand,
which at the stream velocities often used, cuts even the toughest
metal with great rapidity.
Another wheel of this class is the Leffel "Cascade" water-
wheel. Two complete rings of buckets are employed for this
wheel, and the wheels are arranged to be supplied from several
nozzles, of which one or more are put into use according to
the necessities of regulation. The cascade wheel therefore
occupies a place, as it were, between the ordinary impulse
wheel and the impulse turbine, resembling the former in the
arrangement of its multiple jets, and the latter in the method
of regulation by cutting off completely some of the nozzles.
From the foregoing it will be appreciated that each of the
three general classes of wheels described, pressure turbines,
impulse turbines, and tangential impulse wheels, has a sphere
of usefulness in which it can hardly be approached by either
of the others. It is worth while, therefore, to examine some-
what in detail the conditions of economy under various cir-
cumstances.
The pressure turbine has its best field under relatively low
and imiform heads. By means of the draft tube no head is
lost, as is the case with that portion of the head which lies
between the turbine and the tail-water in the use of impulse
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ELECTRIC TRANSMISSION OF POWER.
wheels of any description. Further, the pressure turbine under
all heads gives a higher relative speed than the impulse wheels,
whether of the tangential or turbine variety, and under low
heads is apt to be of less bulk and cost and to give a more con-
venient speed for electric work; hence, these pressure turbines
have been more extensively used than any other variety of
water-wheels in the enormous hydraulic developments of the
last quarter of a century. Furthermore, the pressiu^ turbine
has, under favorable conditions, as high efficiency as any known
.7 .8
PROPORTIONAL DI8CHARQE
Fio. 211.
JT
variety of water-wheel. The losses of energy are mainly of
four kinds.
1. Friction of bearings, usually small.
2. Friction and eddying in the wheel and guide passages.
3. Leakage, and
4. Unutilized energy of other kinds, largely owing to imper-
fect shaping of the working parts, or loss of head.
With the best construction these losses aggregate 15 to
20 per cent. Of them the shaft friction is the smallest and
the loss from friction and eddies in the wheel the largest,
probably fully half of the total loss, particularly under high
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WATER-WHEELS. 865
heads. This efficiency is approximately true of the better
class of turbines, whether of the pressure or impulse variety.
Under low and uniform heads the pressure turbine probably is
capable of a little better work than the impulse variety, but both
suffer if the head varies. The curves, Fig. 211, show efficiency
tests made with the greatest care on four first-class pressure tur-
bines at the Holyoke testing flume, probably the best equipped
place in the world for making such tests. It should be noted
that the efficiency of all the wheels shown is good; over 80
per cent at full admission of water; at partial admission the
efficiencies vary more between the individual wheels. This
variation is largely due to the methods of regulating the flow
employed. These are in general three:
1. Varying the number of guide passages in use.
2. Varying the area of these guide passages by moving the
guide blades.
3. Var3ang the admission to the guide passages by a gate
covering the entrance to all of them.
The first method is particularly bad, as the buckets are at one
moment exposed to full water pressure and then come opposite
a closed passage, setting up a good deal of unnecessary shock
and eddying. It is a method that is scarcely ever used in this
country. Between the other two it is not so easy to choose.
Both have strong advocates among wheel makers; some com-
panies building both types, and the others only one of them.
The curves shown represent both these methods of regulation.
The truth probably is that the relative efficiency of the two
depends more on the design of the wheel with reference to its
particular form of regulation, than on the intrinsic advantages
of either form. Turbines are generally constructed so that
the point of maximum efficiency is rather below the maxi-
mum output, as a little leeway is desired for purposes of regu-
lation under varying heads, so that the design is arranged to
give the best efficiency of which the wheel is capable at a point
a little below full admission. These efficiency curves were
taken at heads of from 15 to 18 feet and show what can be regu-
larly accompUshed by good wheel design. They are neither
phenomenally high nor unusually low. Occasionally efficiencies
are recorded slightly better than those shown. In this conneo-
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ELECTRIC TRANSMISSION OF POWER.
tion it is desirable to state in the way of warning, that there
was obtained at the Holyoke flume some years ago a series
of tests of turbines of more than one make, which showed
enormously high efficiency, afterward traced to a constant
error in the experiments. As the fact of the error was not
so generally known as the result of the tests, occasionally
reports are heard of phenomenal turbine efficiencies which
are given in entire good faith, but based on errors of experi-
ment. It is only fair to say that the tests now made at the
Holyoke flume are worthy of entire confidence.
As regards impulse turbines, data are hard to obtain.
Those which are available indicate, however, that with an effi-
ciency probably a little less at full load than that of pressure
turbines under moderate heads, the half-load efficiency is
generally considerably higher. This is owing to the fact that
the buckets of the rimner work entirely independently of each
other, and the water acts in precisely the same way on each
bucket whether it is received from all the nozzles formed by the
guide blades, or from a part of them. The impulse turbuies are
generally regulated by cutting off more or less of the nozzles.
The shaping of the surfaces in the runner and guide blades,
and the smoothness of the finish, are of more importance
in these wheels than in the ordinary pressure turbines. The
impulse turbines are, as has already been stated, peculiarly
adapted in point of speed and general characteristics for use
on moderately high heads, and in this work they give a better
average efficiency and more economical use of water than any
of the pressure turbines. For low heads their advantages are
far less marked, and the pressure turbines are generally
preferred.
The tangential impulse wheels are, at full admission of water,
of an efficiency quite equal to that of the best turbines. At
partial admission they cannot be expected to give the same
results as do the best impulse turbines, inasmuch as they regu-
late generally by deflecting the nozzle away from the buckets,
and hence wasting water. The variation of the stream by a
needle valve considerably relieves this difficulty in cases where
it can be successfully applied. For very high heads, however,
the tangential wheels are preferable to any turbines, as they
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WATER-WHEELS, 867
give a better relation between power and speed, so far as driv-
ing electrical machinery is concerned, and their extreme
simplicity is favorable to good continuous working under the
enormous strains produced by the impact of water at great
spouting velocities.
To summarize, pressure turbines are admirable for low and
uniform heads, particularly where the load is steady. The
impulse turbines give more efficient use of water at part load,
and a more convenient speed on moderately high heads. The
tangential impulse wheels do relatively the best work under
very high heads, and where water does not have to be rigidly
economized. Each of the three classes has decided advantages
over the others in particular situations, and the full load effi-
ciency of all three is approximately equal. The choice of
either one of these types should be made in each individual
case in accordance with the hydraulic conditions which are to
be met. The choice between particular forms of each type is
largely a commercial matter, in which price, guarantees,
facility of getting at the makers in case of repairs, standard
sizes fitting the particular case in hand, and similar considera-
tions are likely to determine the particular make employed,
rather than any broad difference in construction or operation.
The success of a power transmission plant depends quite as
much on careful hydraulic work as on proper electrical instal-
lation. The two should go hand in hand, and any attempt,
such as is often made, to contract for the two parts of the
plant independently of each other, or to engineer them inde-
pendently, generally results in a combination of electrical and
hydraulic machinery that is far from being the best possible
under the conditions, and is quite likely to be anything but
satisfactory.
The hydraulic and electrical engineers should go over
the arrangement of the plant together with a view to adapting
each class of machinery to the other as perfectly as possi-
ble, in order to get a symmetrical whole. Many troublesome
questions have to be encountered, and only the closest study
will lead to perfectly successful results.
One of the commonest and most serious difficulties met with
in la3dng out an electrical and hydraulic plant for transmission
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368 ELECTRIC TRANSMISSION OF POWER,
work, lies in the variability of the head of water. There are
comparatively few streams from which can be obtained an in-
variable head practically independent of low water or freshets.
The usual condition of things is to find a fairiy uniform head
for nine or ten months in the year, and rather wide variations
during the remainder of the time. It is not at all uncommon to
meet a water-power which, even when very skilfully developed,
will stiU entail upon the user a variation of 25 or 30 per cent
in the available head.
At the time of high water this appears as a rise of water
level in the tail-race, so as to diminish the head available for the
wheels. In times of low water, the head might be normal, but
the quantity of water altogether insufficient. Any variations
of this kind are of a very serious character, because they not
only vary the amount of power which is available, but they
change the speed of the wheels so that the dynamos no longer
wiU operate at their proper speed and hence will change in
voltage, and if alternating apparatus is used, in frequency also,
which is even more serious. For example: Under 24 feet
head one of the well-known standard wheels gives nearly
650 HP at 100 revolutions per minute. Under 16 feet head
the same wheel would give only 352 HP at 82 revolutions.
The lack of power occurring at the time of high water is
serious. The change of speed, although not great, is very
annoying, and should be avoided if possible. Changes much
greater than this are common enough. The season of reduced
head is generally short, not over a couple of months, often
only a week or two, and this renders the situation doubly
embarrassing, because during a large part of the year the same
wheel must be able to operate economically. The methods
taken to get out of this difficulty of varying head are various;
most of them bad. One of the commonest is to arrange the
wheels to operate normally at partial gate, then on the low
heads to throw the gate wide open and obtain increased power.
On the high heads the wheel is throttled still more. Such an
arrangement works the dynamo in a fairly efficient fashion,
but the wheel, as a rule, quite inefficiently a large portion of
the time, as may be seen by reference to the efficiency curves
of the wheels just given. It is a practice similar to that which
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WATER-WHEELS. 869
one would find in working an engine at part load. For moder-
ate variations of head, not exceeding 10 per cent, the loss of
efficiency is not so serious as to bar this very simple plan, but
under conditions too frequently encountered, these variations
of efficiency would be so great as to make the method exceed-
ingly undesirable.
Hydraulic plants are occasionally operated without any
reference to economy of water, and in such cases the practice
of operating normally at part gate is . frequently followed,
but it must be remembered that as water powers are more
and more developed, economy of water becomes more and
more necessary, and in every case should be borne in mind
even if it is not rendered necessary by conditions actually
existing. In thoroughly developed streams it is generally
important to waste no water.
Another method of overcoming the difficulties due to
variations of head, is the installation of two wheels on the
same shaft, one intended to give normal power and speed at
the ordinary head, the other at the emergency head. This
practice is carried out in variou? forms. Sometimes two
wheels may be moimted on the same horizontal or vertical
axis, and one of them is disconnected or permitted to run
idle except when actually needed. Another modification of
the same general idea is the use of a duplex wheel with the
runner and guides arranged in two or three concentric sets of
buckets, which can be used singly or together according to
the head which is available.
A fine example of this practice is found in the great power
plant at Geneva, to which reference has already been made,
where the head varies from 5^ to 12 feet. Here the turbines
have buckets arranged in three concentric rings, the outermost
being used at the highest head and all three at the lowest head.
Under the latter condition, the average radius at which the
water acts upon the wheel is diminished and the speed is
therefore increased, while the greater volume of water keeps
up the power. The various combinations possible with the
rings of buckets are so effective in keeping the speed imiform
that the extreme variation of speed under the maximum varia-
tion of head is only about 10 per cent. Such a triplex turbine
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is of high first cost, but is decidedly economical of water at
normal load. Still another variation of the double turbine
idea consists of installing two turbines for each unit of power,
one acting directly, the second through the medium of belts.
The direct-acting turbine is intended for normal load, the
belted turbine of larger dimensions for use during the periods
of low head.
This arrangement is used in the large power transmission
plant at Oregon City, Ore. It is economical of water, but is
mechanically somewhat complicated. It is probably on the
whole less desirable than the installation of two turbines on
the same shaft, and much less desirable than the duplex or
triplex arrangement just referred to. Where two turbines are
operated on the same shaft, it is generally possible to arrange
the turbine designed to operate on the lower head so as to run
at a disproportionately high velocity with some loss of effi-
ciency, and so to hold the speed fairly uniform.
Still another method of counteracting the variation of
head is applicable only where the power is transmitted from
the turbine by gears or belts. In this case it is always possi-
ble to operate the machinery under the reduced head with
some loss of output, but still at or near the proper speed.
Whatever way out of the difficulty is chosen, it should be
borne in mind that the most desirable, on the whole, is the
one which will work the wheels during the generally long
period of fairly steady head at their best efficiency. If there
is to be any sacrifice of efficiency, it should by all means be
for as short a time as possible, and, therefore, should be at
the periods of extreme low head. At such times water is
generally plenty, while at the higher heads economy in its
use is more necessary.
REGULATION OF WATER- WHEELS.
For many years there have been bad water-wheel governors
and worse water-wheel governors, but only recently have there
appeared governors which may be classified as good from the
standpoint of the electrical engineer. It has been necessary
to go through the same tedious period of waiting and experi-
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WATER-WHEELS. S71
mentation that was encountered before dynamo builders could
find engines which would hold their speed at var3dng loads.
Until the advent of electrical transmission work, water-wheels
were most generally employed for certain classes of manufac-
turing, such as textile mills, where the speed must be quite
uniform, but where at the same time the load is almost imiform;
or, on the other hand, for saw mills and the like, where constant
speed is of no particular importance.
The action of water-wheel governors as regards the way in
which they vary the supply of water is very different; some
merely act to open or close the head gates; others to work a
cylinder gate immediately around the wheel, and still others
to vary the area of the guide passages, as in the so-called
register gate turbines.
In whatever way the governing action takes place, its
result is too often unsatisfactory, due to the great difficulty
that has to be encountered in the great inertia of the water
and of the moving parts of the wheel. Both water and wheel
are sluggish in their action, and as a result some time elapses
after the governor has produced a change of gate, before
that change becomes effective. Meanwhile, the speed has
fallen or risen to a very considerable extent, and perhaps in
addition the load has again changed so that by the time the
speed of the wheel has been sensibly affected by the governor,
the direction of the governing action may be exactly opposite
to that which at the moment is desirable. Even if this is not
the case, the governing is usually carried too far, being con-
tinued up to the time at which the wheel is affected and reacts
on the governing apparatus, hence another motion of the
governor becomes necessary to counteract the excess of dili-
gence on the part of the first action. In other words, the
governor "hunts," causing a slow oscillation of the speed
about the desired point, an oscillation of decreasing amplitude
only if the new load on the wheel be steady.
This sluggishness of reaction to changes indicated by the
governor is the most formidable obstacle to the proper control
of the water-wheels. To overcome it, even in part, it is
necessary that the movement of the gates be comparatively
active, if the changes of load are frequent, and this entails still
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872 ELECTRIC TRANSMISSION OF POWER.
further difficulty by caiLsing severe strains on the mechanism
and the gates, particularly if the water is led to the turbine
through a long penstock. In the latter case, the variations in
pressure produced by rapid governing are often dangerous,
and have to be counteracted by air chambers, stand pipes, or
the like, and aside from all this there is a still further difficulty
in the considerable weight of the gate and the pressure against
which they have to be operated, so that the amount of mechani-
cal power controlled by the governor must be very consider-
able.
A very large variety of governors have been designed to
meet the serious difficulties just set forth. Most of them
have been abject failures, aud those that may be really reck-
oned of some considerable value for electrical work may be
counted on the fingers of one hand.
Water-wheel governors may be roughly divided into two
classes. First, come those regulators in which the wheel itself
supplies power to the gate-shifting mechanism, which is con-
trolled by a fly ball governor through more or less direct
mechanical means. Second, comes the relay class of governors,
wherein all the work possible is taken off the centrifugal
governor, and its function is reduced to throwing into action
a mechanism for moving the gates which may be quite inde-
pendent of any power transmitted from the wheel to the gov-
erning mechanism. The various classes of hydraulic, pneu-
matic, and electric governors are worked in this way. Their
general characteristic is that their sole function in governing
is to work the devices which control the secondary mechanism,
which consists, in various cases, of hydraulic cylinders oper-
ating the gates, pneumatic cylinders serving the same pur-
pose, or electric motors which open or close the gates by
power derived from the machines operated by the turbines.
A vast amoimt of ingenuity has been spent in trying to
work out regulators of the first mentioned class. Almost
every possible variety of mechanism has been employed
to enable the governor to apply the necessary power to the
mechanism operating the gates. The general form of most of
these governors is as follows: Power is taken from the wheel
shaft by a belt to the governor mechanism, where it serves at
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WATER-WHEELS. 873
once to drive the governor balls, and to work the gates when
the governor connects the gate-controlling gears to the pulley
whicji supplies the power. This is generally done by friction
cones or their equivalent, thrown into action in one direction
when the governor balls rise, and in the other direction when
they fall.
Sometimes this mechanism is varied by employing a pair
of oscillating dogs, one or the other of which is thrown
into appropriate gearing by the governors. There are many
governors of this kind on the market, and where the load is
fairly steady and no particular accuracy of regulation is neces-
sary, they have given good satisfaction. The fault with all
governors of this sort is that the centrifugal balls either lack
sensitiveness or lack power. If the governor works at all
rapidly in moving the gates, too heavy a load is tlu*owii on
the governor for any but a massive mechanism, and the cen-
trifugal device becomes insensitive; or, on the other hand, if
the gates are worked slowly, the governor in itself is sluggish
and ineffectual.
In most cases the gates are made to move quite slowly.
In the attempt to get sensitiveness, the friction wheels or
dogs are often adjusted so closely that the governor is in a
constant slight oscillatory motion, but when its action is
really needed, as in the case of a sudden change of load,
response generally does not come quickly enough. It is of
course possible to construct a mechanical relay which would
possess both power and sensitiveness, but nearly all the
governors made on this principle lack one or the other, and
sometimes both.
The second type of governor, as mentioned, is not open to the
objections noted, if properly designed, inasmuch as it is a
comparatively easy matter to make a balanced hydraulic or
pneumatic valve which can be worked even by the most sensi-
tive of governors, and yet can apply power enough to move
heavy gates as rapidly as is consistent with safety. In ad-
dition, such governors can be made to work with a rapidity
depending on the amount of change in speed, so that if a
heavy load is thrown on the wheel, the relay valve would
be thrown wide open, and consequently bring a great and
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374 ELECTRIC TRANSMISSION OF POWER.
immediate pressure to bear upon the gates. In the so-called
electric governors, the function of the governor balls is merely
to make in one direction or the other the electrical connec-
tions to a reversible motor which handles the gates. This
relay class of governors has been recently worked out with
considerable care, and is capable of giving surprisingly close
regulation even under widely varying loads, results compar-
able even with those obtained from a steam engine governor.
A third type of water-wheel governor is independent of
any centrifugal device and operates by a differential speed
mechanism, so that wherever the speed of the wheel varies from
a certain fixed speed maintained by an independent motor,
the gates are opened or closed as occasion demands. The
difficulty here is to get a constant speed which will not be
sensibly altered when the load of working the gates is thrown
on the governor mechanism. Some species of relay device
is almost necessary to the successful operation of a differential
governor, but with such an adjunct very close regulation
can be and is obtained.
Up to the past few years almost all hydraulic governing has
been by mechanisms of the first class, and it is only recently
that the relay idea has been worked out carefully, both for
centrifugal and differential mechanisms, so as to obtain any-
thing like satisfactory results for electrical work where close
regulation of speed over a wide variation of load is very
necessary.
For electrical purposes, several rather interesting governing
mechanisms have been tried, which do not fall into any of these
classes, inasmuch as their function is to keep the load con-
stant and prevent variations of speed instead of checking these
variations after they have been set up. Such governors (load
governors they may properly be called) operate by electric
means, throwing into circuit a heavy rheostat or a storage
battery when the electrical load falls off, and cutting these
devices out again when the load in the main circuit increases.
These governors have in several instances been applied
with success to controlling the variable loads found in electric
railway stations operated by water-wheels. But they waste
energy in a very objectionable manner, and at best can only
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WATER-WHEELS. 375
be regarded as bad makeshifts, out of the question when there
must be any regard for economy of water, and only to be
tolerated in the lack of an efficient speed regulator.
Occasionally electric governors operated by the variations
in the voltage of the circuit supplied have been tried, but
these are open to two serious objections: In the first place,
they do not hold the voltage steady for the same reasons that
most speed regulators do not hold the speed steady. Secondly,
they regulate the wrong thing. In transmission plants, most
of which are and will be operated by alternating currents, it
Pig. 212.
is important that the frequency be kept uniform. If the vol-
tage is kept constant by varying the speed, the frequency is
subject to enough variation to be very annoying in the opera-
tion of motors. Automatic voltage regulators, working through
variation of the field excitation of the generator, belong in a
different category and have come into considerable and suc-
cessful use.
To pass from the general to the special, Fig. 212 shows a
typical water-wheel governor of the first class, that is, of the
kind operated directly by the wheel through a system of dogs
worked by a fly ball governor. There is here no attempt at
delicate relay work, and the resulting mechanism, while quite
good enough for rough-and-ready work, is of little use for
any case where a variable load must be held to its speed with
even a fair degree of accuracy. The cut shows the construction
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376 ELECTRIC TRANSMISSION OF POWER,
well enough to render further description superfluous. Gov-
ernors like these were practically the best available for many
years, and proved to be cheap and durable, but they seldom
governed much more than to keep the wheels from racing
dangerously when the load was thrown off, or from slowing
down permanently when it came on. It is not too much to
say that they never should be used in connection with an
electrical station, unless combined with intelligent hand regu-
lation — which at a pinch is not to be despised.
Of the indirect acting and relay governors there are many
species, most of which had better be consigned to the oblivion
of the scrap heap. But out of the manifold inventions and
experiments good has come, so that at the present time there
are a few delicate relay governors capable of holding the wheel
speed constant within a very narrow margin indeed. Others
of similar excellence will probably be evolved, but just now
three, the Lombard, Replogle, and the Faesch-Piccard, together
with one or two electrical governors, are decidedly the best
known. The first named has given very remarkable results
in many transmission plants in which it has been employed —
results quite comparable with those obtained from a well-gov-
erned steam engine. The second has given excellent results
in the Oregon City transmission and elsewhere, while the last
was adopted for the original transmission at Niagara Falls
and has done its work well, although in the extension of the
plant an hydraulic relay governor designed by Escher, Wyss
& Co., was installed. They are suitable types of the hydraulic,
electric, and mechanical relay governors.
The Lombard governor, Plate XIII, is an hydraulic relay in
principle. The gate-actuating mechanism is a rack gearing
into a pinion, and driven to and fro by the piston of a pressure
cylinder. The working fluid is thin oil, kept under a pressure
of about 200 lbs. per square inch. This pressure is supplied
by a pump driven by the pulley shown in the figure and
operating to keep up a 200 lb. air pressure in the pressure
chamber at the base of the governor, above the oil that par-
tially fills it. This chamber is divided into two sections, the
one holding the oil under pressure, the other being a vacuum
space kept at reduced pressure by the pump system.
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878 ELECTRIC TRANSMISSION OF POWER.
The circulation of oil is from the pressure chamber through
the piping system and valves to the working cylinder, and
thence into the vacuum chamber, whence it is pumped back
into the pressure chamber again. The governor proper con-
sists of a sensitive pair of fly balls operating a balanced piston
valve in the path of the pressure oil. A motion of ^^ of an
inch at the valve is sufficient to put the piston into full action
and open or close the gates. Sensitive as this mechanism is, it
would not govern properly without the addition of an ingenious
device, peculiar to this governor, to take account of the inertia
of the system. The weakest point of all such governing
mechanisms has been their helplessness in the matter of inertia.
If a governor even of the sensitive relay class be set to regu-
late a wheel, we encounter the following unpleasant dilemma:
If the mechanism moves the gates quite slowly, it will not be
able to follow the changes of load. If it moves them rapidly
the governing overruns on account of the inertia of the whole
wheel system, so that the apparatus "hunts," perhaps the
worst vice a governor can have when dynamos are to be gov-
erned. Hence most governors have either been unable to
follow a quickly varying load at all, or they have made matters
worse by hunting.
In the liombard governor, special means are provided to
obviate hunting. The bell-crank lever seen in the background
of Plate XIII is actuated by the same movement that works the
wheel gates, and moves the governor valve independently of
the fly balls. Its office is promptly to close the valve far enough
ahead of the termination of the regular gate movement to
compensate for inertia. For example, if the speed falls and
the fly balls operate to open the gate Avider, the lever in ques-
tion closes the governor valve before the fly balls are quite
back to speed, so that instead of overrunning and hunting, the
governing is practically dead beat.
The result obtained with this governor is well seen in Fig. 213.
This diagram is taken from a plant operating an electric street
railway — perhaps the worst possible load in point of irregu-
larity. The diagram shows a maximum variation of 2.1 per
cent from normal speed, lasting less than one minute, imder
extreme variation of load. These results are entirely authen-
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PLATE Xin.
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WATER-WHEELS.
379
tic, the readings having been taken jointly by the represen-
tatives of the governor company and the local company. Speed
was taken by direct reading tachometer and load from the
station instruments.
Fig. 214 shows a small governor of the same make intended
for use with impulse wheels, and for similar light work under high
Fiu. 214.
heads. It works on precisely the same principle as the larger
governor, save that the power is derived from a water cylinder
taking water from the full head of the plant. The work of
this little governor is 5.4 foot-pounds for each foot of working
head, quite enough to handle the deflecting nozzles or needle
valves used for regulating impulse wheels. The larger governor
of Plate XIII has a very different task in moving the heavy
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880 ELECTRIC TRANSMISSION OF POWER,
gates of turbine wheels and is designed to develop more than
10,000 foot-pounds. For still heavier duty a vertical cylinder
type of Lombard governor, somewhat simplified from the
forms here shown, has recently been introduced.
A gate gauge is generally attached to the bed plate of the
Lombard governor, so that the excursions of the piston plainly
show the exact extent of gate opening. The mechanism of
the governor is decidedly complicated, but it is extremely
well made and fitted, so that it seldom gets out of order. It
permits readily of all sorts of adjustment with respect to the
speed, but for power transmission work one needs constant
speed only, except when vaiying speed temporarily in syn-
chronizing a generator. The invariable rule, therefore, should
be to adjust the governor carefully for the exact speed required,
and thereafter to let its adjustments alone as long as it
continues to hold that speed. In power transmission work and
in railway plants, this governor is at present used probably
more than all others combined.
The Faesch & Piccard governor has taken several forms,
the idea of a sensitive relay mechanism being carried through
all of them. An hydraulic relay has been successfully em-
ployed abroad. In this the function of the fly balls is reduced
to moving a balanced valve controlling hydraulic power
derived from the natural head, or from a pressure cylinder.
There is no mechanical provision against hunting, but the
speed of governing is adjusted as nearly as possible to the re-
quirements of the load, and the results are generally good. In
the great Niagara plant the governor is situated on the floor
of the power house, nearly 140 feet above the wheel. It is a
very sensitive mechanical relay, in which the motion of a pair
of fast running fly balls puts into operation through a system of
oscillating dogs a brake-tightening mechanism, which in its turn
permits power to be transferred from pulleys driven from the
turbine shaft through a pair of dynamometer gears, to the sjrstem
of gearing that works the balanced gate at the end of the lever
system 140 feet below the governor. This governor was guar-
anteed to hold the speed constant within 2 per cent under ordi-
nary changes of load, and to limit the speed variation to 4 per
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WATBB-WHBELS.
881
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S82
ELECTRIC TRANSMISSION OF POWER.
cent for a sudden change of 25 per cent in the load. Fig. 215
gives a good notion of the principles of this apparatus, which
is fairly satisfactory. The Replogle governor is an electro-
mechanical relay shown in Fig. 216, whch exhibits its general
arrangement very well. The work done by the fly balls is
very trifling and the mechanism is both sensitive and powerful.
Fio. 216.
Fig. 217 shows its performance in governing a railway load
under conditions of unusual severity. As in Fig. 213, 20 min-
utes of operation are plotted and the maximum variation from
105 revolutions per minute, the normal speed, is less than 10
revolutions, and that variation lasted less than 20 seconds and
was due to the opening of the circuit-breaker. Such work is
quite good enough to meet all ordinary conditions.
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884
ELECTRIC TRANSMISSION OF POWER.
To a very different type of mechanism belongs the diflferen-
tial governor shown in Fig. 218. It has been applied widely to
the governing of Pelton impulse wheels, with very excellent
results. The principle involved is very simple. Two bevel
gears, each carrying on its shaft a pulley, are connected by a
pair of bevel gears on a crosswise shaft, forming a species of
dynamometer gearing. Normally the main gears are driven
in opposite directions, the one at a constant speed by a special
FlO. 218.
source of power, the other from the shaft to be governed.
So long as the speeds of these wheels are exactly equal and
opposite, the transverse shaft remains stationary in space and
the gate moving mechanism attached to it is at rest. When,
however, the working shaft changes speed under the influence
of a change in load, the transverse shaft necessarily moves in
one direction or the other and keeps on moving until the
working shaft gets back to speed.
In practice the main difficulty is to hold the constant speed
necessary for one of the bevel gears, and the governor works
admirably or badly as this constancy is or is not maintained.
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WATER-WHEELS. 385
A heavy fly wheel on the constant speed side is desirable, and
its motive power should be quite independent of the main
drive. Perhaps the best result is obtained by using a second,
small, differential governor to hold the speed uniform at the
main governor. With the high heads and balanced deflecting
nozzles usual in Pelton wheel practice, this form of governor
is very sensitive and does not hunt noticeably, owing to the
small inertia of the moving parts. It gives good regulation
under all conditions except extreme variations of load where
the wheel is loaded beyond the power of the jet to enforce
prompt recovery of speed, and is well suited to the conditions
under which Pelton wheels are generally used.
The greatest difficulty in hydraulic governing is that of
hydraulic inertia. Water moves sluggishly through long and
level pipes, and its velocity does not change promptly enough
for good governing, unless the waterways are planned with
that in view. If a wheel is at the end of a long and gently
sloping penstock it takes a certain definite amomit of time to
get that water column under way or checked in response to
the movement of the gates. And the longer this time con-
stant of the water colunm the more difficult it is to get accu-
rate governing, however good the governhig mechanism may
be. For by the time the water gets fairly into action the load
conditions may have changed, and the governor may be again
actively at work trying to readjust the speed.
In order to get accurate governing it is absolutely necessary
to keep the time constant of the waterways as small as pos-
sible. To accomplish this the regulating gate should obviously
be right at the wheel and the penstock should be as short and
as nearly vertical as possible. The most favorable condition
for governing is when the wheel is practically in an open
flume. If steel penstocks are used they should pitch as
sharply down upon the wheels as conditions permit, some-
thing after the manner of Fig. 208. If long head pipes must
be used governing will become difficult, although much help can
be obtained from an open vertical standpipe connected with
the penstock close to the wheel. The contents of this pipe
serve as a pressure colunm if the gate is suddenly opened and
as a relief valve if the gate suddenly closes, averting the some-
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S86 ELECTRIC TRANSMISSION OF POWER.
times serious pressure due to the violently checked stream.
Plate XIV shows such a standpipe in action just after a heavy-
electric load had been thrown off. The water normally stands
very near the top pf the pipe, which begins to overflow with a
slight increase of the hydraulic pressure.
Under high heads such a standpipe is of course impracti-
cable, and although some forms of relief valve are of use, the
conditions of governing are not easy until one comes to the
impulse wheel with a deflecting nozzle.
Not all water-wheels are governed with equal ease. If the
gates are properly balanced a comparatively small amoimt of
power will manage them promptly, and the wheel is governed
without trouble. But there are some wheels on the market
with gates under so much unbalanced pressure that proper
governing is difficult or impossible. There is no excuse for
the existence of such wheels, for they do not have compensat-
ing advantages, and they should be shunned. All the typical
wheels which have been described in this chapter govern
easily, however, as do many others. It is worth while to re-
member that good governing is absolutely indispensable for
good service, and although one finds cases in which the load is
so steady that the wheels can almost go without governing,
such are rare exceptions to the general rule.
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PLATE XIV.
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CHAPTER X.
HYDRAULIC DP^VELOPMENT.
So much electrical transmission work depends on the utiliza-
tion of water-powers that it is worth while briefly to consider
the subject of developing natural falls for such use. The
subject is a large one, quite enough to fill a volume by itself,
and the most that can be done here is to point out the salient
facts and put the reader in possession of such information as
will enable him to avoid serious blimders and to take up the
subject intelligently.
Natural water-powers of course vary enormously in their
characteristics. In our own country, where water-power is
very widely distributed, we find three general classes of powers,
often running into each other but still sufficiently distinct to
cause the methods of developing them to be quite well defined.
By far the best known class of powers are those derived
from the swift rivers that are found in New England and
other regions in which the general level of the country changes
rather rapidly. They flow through a country of rocky and
hilly character, and large or small, are still swift, powerful
streams, with frequent rapids and now and then a cascade.
Such rivers are generally fed to no small extent by springs
and lakes far up toward the mountains, and catch in addition
the aggregated drainage of the irregular hill country through
which they flow. Types of this class are the Merrimac and
the Androscoggin among the New England rivers, the upper
Hudson, and many others. Another and quite different class
of powers are those derived from the slow streams that flow
through a flat or rolling alluvial country — the Mississippi
valley and the lowlands of the Southern Atlantic States. Al-
though possessed of many tributaries that spring from among
the mountains, the great basins which they drain form the
main reliance of rivers of this kind — immense areas of fertile
country the aggregated rainfall of which supports the streams.
387
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/ 388 ELECTRIC TRANSMISSION OF POWER.
Finally, there are many fine water-powers that come from
mountain streams, fed from little springs among the rocks,
from the melting of the winter snows and the drainage of
heights which the snow never deserts, and from the rain
gathered by desolate gorges.
These moimtain rivers often furnish magnificent powers,
easy and cheap to develop, but very variable. In summer the
stream may dwindle to a mere brook, while in spring, from
the combined effect of rains and melting snow, it may suddenly
increase even many thousand fold, becoming a tremendous
torrent that no works built by man can withstand. The
available heads are often prodigious, from a few hundred to
more than a thousand feet, and the volume of water may seem
at first sight absurdly small, but when, as in the Fresno (Cal.)
plant to be described later, each cubic foot flowing per second
means 140 mechanical HP delivered by the wheels, large
volume is needless.
Upland rivers like those common in New England, seldom
give opportunity for securing high heads. Most of the powers
developed show available falls ranging from 20 to 40 feet.
Unless the stream has considerable volume, such low heads do
not yield power enough to serve anything but trivial purposes —
only two or three HP per cubic foot per second. Upland rivers,
however, furnish the great bulk of the water-power now utilized,
for they furnish fairly steady and cheap power under favorable
conditions. Although subject to considerable, sometimes formi-
dable, freshets, when the snow is melting or during heavy rains,
they are generally controllable without serious difficulty.
Lowland streams seldom offer anything better than very
low heads, rarely more than 10 to 15 feet, and consequently
demand an immense flow to produce any considerable power.
They are, however, as a class rather reliable. The size and
character of the drainage basin makes extremely low or ex-
tremely high water rare, and only to be caused by very great
extremes in the rainfall. Such streams furnish a vast number
of very useful powers of moderate size, forming a large aggre-
gate but seldom giving opportunity for any striking feats of
hydraulic engineering, at least in our own coimtry, where
fuel is generally cheap.
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HYDRAULIC DEVELOPMENT. 889
In taking up any hydraulic work with reference to electrical
power transmission, or any other purpose in fact, the first
necessary step is to make a sort of reconnoissancey to ascertain
the general topography of the region, the available head, and
the probable flow. The first two points are generally easy to
determine from existing surveys or by a brief series of levels,
the last named requires a combination of educated judgment
and careful engineering. The U. S. Geological Survey maps
are invaluable, when available, for getting a preliminary idea
of the topography and the probable drainage basin. The facts
are not really very difficult to get at, but guesswork is emphati-
cally out of order and heresay evidence even more worthless
than usual. The author has seen more than one mighty tor-
rent dwindle into a trout brook when looked at through
untinted spectacles.
The only way to find out how much flow is available is to
measure it carefully, if it has not already been measiu^d in a
thorough and trustworthy manner — not once or twice or a
dozen times, but weekly or, better, daily, for an entire season at
least; the more thoroughly the better. A knowledge of the
absolute flow at one particular time is interesting, but of little
value compared with a knowledge of the variations of flow
from month to month, or from year to year.
Such a series of measurements tells two very important
things — first, the minimum flow, which represents the max-
imum power available continuously without artificial storage
of water; and second, the aggregate flow dining any specified
period, which shows the possibilities of eking out the water
supply by storage.
The methods of measurement are comparatively simple.
For small streams the easiest way is to construct a weir across
the stream and measure the flow over a notch of known dimen-
sions in this weir. Such a temporary dam should be tight and
firmly set, and high enough to back up .the water into a quiet
pool free from noticeable flow except close to the edge of the
weir. There should be sufficient fall below the bottom of the
notch in the weir to give a clear and free fall for the issuing
water — say two or three times the depth of the flow over the
weir itself.
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ELECTRIC TRANSMISSION OF POWER.
Fig. 219 shows clearly the general arrangement of a measur-
ing weir. Here A shows the end supports of the weir, here
composed of a single plank, while B is the lower edge of the
notch through which the water flows. This edge 5, as well as
the sides of the notch, should be chamfered away to a rather
sharp edge on the upstream side, which must be vertical.
Back some feet from the weir so as to be in still water, should
be set firmly a post E, the top of which is on exactly the same
level as the bottom of the weir notch B. D shows this level,
while the line C shows the level of the still water. The quan-
tities to be exactly measured are the length of the notch B
mi^m
¥lQ, 219.
and the height from the level of the edge of 5 to the normal
level surface of the water in the pool. This can be done
generally with sufficient accuracy by holding or fixing a scale
on the top of the post E. If we call the breadth of the notch
by and this height A, both measured in feet, the flow in cubic
feet per minute is
Q^ 40 cbh yJ2gh.
Here g is 32.2 and c is the "coefficient of contraction," which
defines the ratio of the actual minimum area of the flowing
jet to the nominal area 6 A.
This coefficient varies slightly with the width of the notch
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HYDRAULIC DEVELOPMENT.
391
as compared with the whole width of the weir dam. Calling
this w, the value of c is approximately
b
c = 0.57 + 10 - •
w
This gives c = .62 for a notch half the width of the weir and
c = .67 for the full width of the weir. For notches below one-
quarter the width of the weir the values of c become somewhat
uncertain, and as a rule 6 should be over half of w. Further,
the notch should not be so wide as to reduce the water flowing
over it to a very thin sheet. It is best to arrange the notch so
that the depth of water h may be anywhere a tenth to a half of
b. For purposes of approximation weir tables are sometimes
convenient. These give usually the flow in cubic feet per
minute corresponding to each inch in width b, for various
values of h. Such a table, condensed from one used by one of
the prominent turbine makers, is given below. Where quite
exact measurement is required the constant c should be deter-
mined from the actual dimensions and a working table de-
duced from it.
Table of Wbirb.
Incbee and Fractions Depth on Weir.
0
4
h
1
1
0.40
1.14
2.09
3.22
4.61
6.92
7.46
9.12
10.88
12.76
14,71
16.76
18.89
21.12
23.42
26.80
28.26
30.78
0.66
1.36
2.36
3.63
4.86
6.30
7.87
9.55
11.34
18.23
16.21
17.28
19.44
21.68
24.01
26.41
28.88
31.43
0.74
1.59
2.64
3.86
5.25
6.68
8.28
9.99
11.80
13.72
16.72
17.82
20.00
22.26
24.60
27.02
29.51
82.07
0 97
2
1 84
3
2.93
4
4 17
6
5 56
6
7 07
7
8 70
8
10 43
9
12 27
10
14 21
11
16 24
12
18 35
13
20 56
14
22.83
15
25 19
16
27 63
17
30 14
18
82 73
Cubic feet per miDute per iucli of width.
West of the Rocky Mountains a special system of measuring
water by ''miner's inches" has come into very extensive use.
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ELECTRIC TRANSMISSION OF POWER.
It originated in the artij&cial distribution of water for mining
and irrigating purposes, and has since extended to a conven-
tional measurement for streams. The miner's inch is a unit
of constant flow, and varies somewhat from State to State, its
amount being regiJated by statut-e in various States. It is
the flow through an aperture 1 inch square under a specified
head, frequently 6 inches. The method of measurement is
shown in Fig. 220. The water is led into a measuring box
closed at the end except for an aperture controlled by a slide.
The end board is IJ inch thick, and the aperture is 2 inches
Fio. 220.
wide, its bottom is 2 inches above the bottom of the box, and its
centre 6 inches below the level of the water. Each inch of
length of the aperture then represents 2 miner's inches.
Under these conditions the flow is 1.55 cubic feet per minute for
each miner's inch. Under a 4J inch effective head, which is
extensively used in southern California and the adjacent
regions, the miner's inch is about 1.2 cubic feet (9 gallons)
per minute.
For streams too large to be readily measured by the means
already described, a method of approximation is applied as
follows:
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HYDRAULIC DEVELOPMENT,
398
Select a place where the bed of the stream is fairly regular
and take a set of soundings at equal intervals, a, h, c, d, Fig.
221, perpendicular to the direction of flow, using a staff rather
than a sounding line, as it can more easily be kept perpendicu-
lar. Ascertain thus the area of flow. Then establish two lines
across the stream, say 100 feet apart and nearly equidistant
from the line of soundings. Then throw floats into the
stream near the centre and time their passage across the two
reference lines. This establishes the velocity of the flow
across the measured cross section. As the water at the bottom
and sides of the channel is somewhat retarded, the average
velocity is generally assumed to be 80 per cent of that mea-
sured as above in the middle of the stream.
The more complete the data on variations of flow, the
Fia. 221.
better. The most important point to be fixed is the flow at
extreme low water, both in ordinary seasons and seasons of
unusual drought. Except on very well-known streams pre-
vious data on this point are generally not available. The
flow should therefore be measured carefiJly through the usual
period of low water during at least one season. From the
minimum flow thus obtained there are various ways of judging
the miniminn flow in a very dry year. Sometimes certain
riparian marks are known to have been uncovered in some
particiJar year, and the relative flow can be computed from the
difference thus established. Again, the records of a series of
years may be obtained from a neighboring stream of similar
character, and the ratio between ordinary and extraordinary
minima assumed to be the same for both. This assumption
must be made cautiously, for neighboring streams often are
fed from sources of very different stability.
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394 ELECTRIC TRANSMISSION OF POWER.
Failing in these more direct methods, recourse may be
taken to rainfall observations. For this purpose the rainfall
in the basin of the stream shoiJd be measured during the con-
tinuance of the observations on flow. By noting the effect of
known rainfall on the flow of the stream, one can make a
fairly close estimate of the flow in a very dry year in which
the rainfall is known by months, or for an assumed minimum
rainfall. In a similar way can be ascertained the probable
high water mark, record of which is often left by debris on
the banks.
In a fairly well-known country the conditions of flow can be
approximated by reference to rainfall alone. The area drained
by the stream down to the point of utilization can be closely
estimated. If rainfall observations in this district are avail-
able, or can be closely estimated from the results at neighbor-
ing stations, one may proceed as follows: The total water
falling into the basin is 2,323,200 cubic feet per square mile
for each inch of rainfall. Only a portion of this finds its way
into the streams, most of it being taken up by seepage, evapo-
ration, and so forth. The proportion reaching the streams
varies greatly, but is usually from .3 to .6 of the whole. If this
proportion is known from observations on closely similar
basins and streams the total yearly flow can be approximated,
and if the distribution of flow on a similar stream is known,
one can make a tolerable estimate of the amount and condi-
tions of flow in the stream under investigation.
This process is far from exact, since the proportion of the
total water which is found in the streams varies greatly from
place to place, and with the total rainfall in any given week or
day. The sources of loss do not increase with the total pre-
cipitation, and the only really safe guide is regular observa-
tion of the rainfall and the flow ddring the same period. At
times, however, rainfall estimates are about the only source
of information available and when made with judgment are
decidedly valuable. In a well investigated country they are
sometimes surprisingly accurate.
A good idea of the uncertainties of hydraulic power can be
gathered from the recorded facts as regards the Merrimac,
one of the most completely and carefully utilized American
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HYDRAULIC DEVELOPMENT, 896
streams, which has been under close observation for half a
century. The area of its watershed above Lowell, Mass., is
4,093 square miles and the mean annual rainfall of the region
is about 42 inches. The observations of many years indicate
that the maximum, minimum, and mean flows are on approxi-
mately the following basis:
(Spring) Maximum, 90 cubic feet per minut/e per square mile
(June) Mean, 65 cubic feet per minute per square mile
(August, September) . . Minimum, 30 cubic feet per minute per square mile
The annual rainfall, if it all coiJd be reckoned as in the
stream and uniformly distributed, would amount to very
nearly 180 cubic feet per minute per square mile of watershed.
In fact, this flow is reached or passed only on occasional
days of heavy freshets during the spring rains, when the snow
is melting rapidly. The normal maximum flow is just 50 per
cent of the conventional average, while the real average falls
to about 30 per cent and the minimum to less than 17 per
cent. Of late years this minimum has sometimes been still
smaller, little over 10 per cent instead of 17, a state of things
due to the destruction of the forests on the upper watershed.
In a heavily wooded country the rainfall is long retained and
finds its way to the streams slowly and gradually. When
the forests are cut off the water runs quickly to the streams,
and the result is heavy seasons of freshets when the snow is
melting — all the more rapidly because of lack of forest shade
— and extreme low water dming the dry months. In a bare
country the variations of flow are often prodigious, and with-
out storage one can safely reckon only upon the minimum
flow of the dryest year. As the denudation of the uplands goes
on hydraulic development will steadily grow more expensive.
One cubic foot per second per square mile of drainage
area is a figure often used to determine the average flow for
which development should be planned and in streams like
those of New England this estimate is not far from the truth.
In some streams, generally in hot climates, no small part
of the flow is during the dry season in the strata underlying
the apparent bed of the stream, and can be in part, at least,
captured by carrying down the foundations of the permanent
works.
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896 ELECTRIC TRANSMISSION OF POWER.
When the flow has been ascertained the available HP is
easily computed. The practicable head can be easily deter-
mined by a little leveling. If H is this head in feet and Q
the flow in cubic feet per minute, then the theoretical HP of
the stream is
62.4 H Q
HP =
33,000
The mechanical HP obtained by utilizing the stream in water-
wheels is this total amount multiplied by the efficiency of
the wheels, usually between .75 and .85. At 80 per cent
efficiency the proceeding formula reduces to
^^-"650-'
which gives the available mechanical horse-power directly. In
many streams the available head is limited by the permissible
overflow of the banks as determined by the rights of other
o\vners, or by danger of backing up the stream to the detri-
ment of powers higher up. These conditions must be deter-
mined by a carefiJ survey.
Before taking up seriously the development of a water-power
it is advisable to enter into an examination of the legal status
of the matter, which is sometimes very involved. The gen-
eral principle of property in streams is that the water belongs
in common to the riparian owners, and cannot be employed by
one to the detriment of another. But each State has a set of
statutes of its own governing the use of water for power and
other purposes, often of a very complicated character, involved
with special charters to storage and irrigation companies and
other ancient rights, so that the real rights of the purchaser of
a water privilege are often limited in curious and troublesome
ways, especially when the stream has been long utilized else-
where.
Generally the riparian owners have fiJl rights to the nat-
ural flow of the stream, which is often by no means easy to
determine. The laws of various States regarding the matter
of flowage vary widely, and altogether the intending purchaser
will find it desirable to investigate carefully not only the title
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HYDRAVLtC D^VtlLOPMENT, 397
to the property, but the limitations of the rights which he
would acquire.
In streams of small volume carried through pipe lines the
effective head is diminished by friction in the pipes. This
loss has already been discussed in Chapter II. In any case
where water is carried in canals or open flumes there will be,
too, a slight loss of head, generally trivial.
It often happens that there is so great a difference between
the normal flow of the stream during most of the year and its
minimum flow during a few weeks as to make it highly desira-
ble to store water by impounding it, so as to help out the
sometimes scanty natural supply. With mountain streams
under high head this is frequently quite easy. Even when it
is impracticable to impound enough to help out during the
whole low water period, it is sometimes very useful to impound
enough to last for a day or two in case of necessary repairs.
A certain reservoir capacity is quite necessary, so as to per-
mit the storage of water at times of light load for utilization
at times of heavy load. This process is carried out on a vast
scale on the New England rivers, where the water, used during
the day in textile manufacturing, is stored in the ponds at
night as far as possible. While electric transmission plants do
not offer the same facilities for storage, since they generally
run day and night, the application of the same process would
often greatly increase their working capacity and greatly lower
the fixed charges per hydraulic HP. Such storage is espe-
cially valuable in cases where the water supply is limited, as it
often is in plants working under high heads. Every cubic foot
of water is then valuable and should be saved whenever pos-
sible. Regulation by deflecting nozzles, which is very generally
employed in this class of plants, is particularly objection-
able on the score of economy, and ought to be replaced by
some more efficient method.
As an example of what can be done with storage under high
heads, it happens that at 650 feet effective head oae mechani-
cal HP requires almost exactly one cubic foot of water per
minute at 80 per cent wheel efficiency. For a 500 HP plant,
then, the water required is 30,000 cubic feet per hour.
One can store 43,560 cubic feet per acre per foot of depth,
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398 ELECTRIC TRANSMISSION OF POWER.
so that a single acre 10 feet deep would store water enough to
operate the plant at full load for 14^ hours, or under ordinary
conditions of load for a full day. If the flow in the stream
were only 15,000 cubic feet per hour in time of drought, the
acre would yield two days supply and 15 acres would carry the
plant for a month. Such storage is common enough in irriga-
tion work, and is capable of enormously increasing the work-
ing capacity of a transmission plant, even at a head much less
than that mentioned.
With even 100 feet available head, it is comparatively easy to
impound water enough to assist very materially in tiding over
times of heavy load and in increasing the available capacity.
Fig. 22,2.
A survey with storage capacity in view should be made when-
ever storage is possible, and the approximate cost of storage
determined. A little calculation will show in how far it can
be made to pay.
In general the utilization of a water-power consists in lead-
ing the whole or a part of a stream into an artificial channel,
conducting it in this channel to a convenient point of utiliza-
tion, and then dropping it back through the water-wheels
into the channel again, usually via a tail-race of greater or less
length.
Except where there is a very rapid natural fall a sub-
stantial dam is necessary, which backs up the water into a
pond, usually gaining thus a certain amount of head, whence
the water is led in an open canal to some favorable spot from
which it can be dropped back into the channel at a lower level.
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HYDRAULIC DEVELOPMENT,
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The canal may vary in length from a few rods to several miles,
according to the topography of the coimtry. The tail-race lead-
ing the water from the wheels back to the stream is short, except
in rare instances like the great Niagara plant. In this case,
shown somewhat roughly in Fig. 222, the usual construction
was reversed. To obtain ample clear space for manufacturing
sites and the like, the water was utilized by constructing above
the cataract an artificial fall at the bottom of which the wheels
were placed. From the bottom of this huge shaft, cut 178 feet
deep into the solid rock, the water is taken back into the
Fig. 223.
river through a tunnel 7,000 feet long, which constitutes the
tail-race.
In the case of mountain streams having a very rapid fall,
the dam is often quite insignificant, serving merely to back up
the water into a pool from which it may be conveniently drawn,
and in which the water may be freed of any sand that it carries,
or even to deflect a portion of the water for the same purpose.
In such cases the water is usually carried in an iron or steel
pipe, following any convenient grade to the bottom of the fall
chosen, at which point its full pressure becomes available.
In ordinary practice at moderate heads the volume of water
has to be so considerable for any large power as to make a
long canal very expensive. Further, it usually happens that
the topography of the coimtry is such as to make it very diffi-
cult to gain much head by extending the canal. Thus the
points chosen for power development must be those where
there is a rather rapid descent for a short distance — falls or
considerable rapids. Then a dam of moderate height gives a
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400 ELECTRIC TRANSMISSION OF POWER.
fair head by simply carrying the canal to a point where the
water can be readily returned to the stream below the natural
fall. The more considerable this fall the less need for an
elaborate dam, which may become simply a means of regulat-
ing the flow of water without noticeably raising the head.
A fine example of this sort of practice is shown in Fig. 223,
which shows a plan of the hydraulic development of the falls
of the Willamette River at Oregon City, Ore. The river at
this point gives an estimated available HP of 50,000 under 40
feet head. The stream plimges downward over a precipitous
slope of rough basalt, and the low dam which follows the some-
what irregular shape of the natural fall, is hardly more than an
artificial crest to guide the water toward the canal on the west
Fia. 224.
bank of the stream. This canal has recently been widened,
and both constructions are shown in the figure. The fine three-
phase transmission plant of the Portland General Electric Com-
pany now faces on the new canal wall near the section G. At
the end of the canal downstream a series of locks lead down to
the lower river, making the falls passable for river craft. Only
a small part of the available power is as yet used.
Almost ever}'^ river presents peculiarities of its own to the
hydraulic engineer. Generally the dam is a far more promi-
nent part of the work than at Oregon City, and adds very
materially to the head. Choosing a proper site for the dam,
and erecting a suitable structure, requires the best skill of the
hydraulic engineer. Bearing in mind that the function of a
dam is to merely retain and back up the flowing water, it is
evident that it may be composed of a vast variety of materials
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HYDRAULIC DEVELOPMENT.
401
put together in all sorts of ways. Stone, logs, steel, all come
into play combined with each other and with earth.
The character of the river bed which furnishes the founda-
tion is a very important factor in determining the material and
shape of the dam used. When the bed is of rock or of that hard
packed rubble which is nearly as solid, a well-built stone dam
is the best, as it is also the costliast, construction. For such
work the way is cleared by a coffer dam and the masonry is
TOP OF EHUTTES AND
^TOP OF OAy IK IHUTTER
OPEhthi::,&MuTTEFi r
j BY Q HVDRAUUC HAMS
SKCTION OP DAM
(thrust — IQTI TOKS. \
1 STABILITY— 7070 TONS, f
Containing 37,000 cu. yaras of masonry.
Fig. 226.
laid, if possible, directly upon the bedrock. When the bottom
is hard pan a deep foundation for the masonry is almost as
good as the ledge itself, while on a gravel bottom sheet piling
is sometimes driven and the stone work built aroimd it. The
ground plan is very frequently convex upstream, giving the
effect of an arch in resisting the pressure of the water. Fig.
224 shows a section of a typical masonry dam, built over sheet
piling in heavy gravel. This particular dam is 22 feet 6 inches
high and nearly 300 yards long. The coping is of solid granite
slabs a foot thick. Below the dam lies the usual apron of
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402 ELECTRIC TRANSMISSION OF POWER,
timber and concrete, with timber sills anchored into the dam
itself. The flooring of the apron, of 12 x 12 inch timbers laid
side by side, is bolted to the fomidation timbers laid in the
concrete. The purpose of this apron, as of such structures in
general, is to prevent imdermining of the dam by the eddies
below the fall.
A still finer example of the masonry dam is shown in Fig.
225 — the great dam of the Folsom Water Power Company
across the American River at Folsom, Cal. It is built of
hewn granite quarried on the spot, and is founded on the same
ledge from which the material was taken. The abutments
likewise are built into the same ledge. On the crest of the
dam proper is a huge shutter or flash board, 185 feet long,
capable of being swiuig upward into place by hydraulic power.
When thus raised it gives an added storage capacity of over
13,000,000 cubic yards of water in the basin above. This
dam furnishes power for the Folsom-Sacramento transmission,
now part of the immense network of the California Gas and
Electric Co., and it ranks as one of the finest examples of
hydraulic engineering in existence. Including the abutments
it is 470 feet long, and the crest of the abutments towers
nearly 100 feet above the foundation stones. Its magnificent
solidity is not extravagance, for the American River carries
during the rainy season an enormous volume of water, filling
the channel far over the crest of the dam when at its maximum
flow. There are few streams where greater strains would be
met.
While these masonry dams are splendidly strong and endur-
ing, they are also very expensive, and hence unless actually
demanded for some great permanent work are less used than
cheaper forms of construction. In many situations these are
not only cheaper in first cost, but even including deprecia-
tion. There are divers forms of timber dam which have given
good service for many years at comparatively small expense.
Of such dams, timber cribs ballasted with stone are probably
under average conditions the best substitute for solid masonry.
These crib dams when well built of good materials, are very
durable and need few and infrequent repairs. Some such
dams, replaced after twenty-five or thirty years in the course of
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HYDRAULIC DEVELOPMENT.
408
tKry LimAjt I
changing the general hydraulic conditions, have shown timbers
as solid as the day they were put down, and capable of many
years' further service.
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404 ELECTRIC TRANSMISSION OF POWER.
A fine example of such construction is the dam of the Con-
cord (N. H.) Land & Water Power Company, at Sewall's
Falls on the Merrimac. A section of this structure is shown
in Fig. 226. The foundation is in the main gravel, in which the
dam is made secure by sheet piling and stone ballast. The
structure is essentially a very solid timber crib with a very long
apron. The total head is 23 feet, of which more than half is
due to the dam, as shown in the levels. The apron is armored
with five-sixteenths inch steel plate, the better to withstand the
bombardment of stray logs to which it is sometimes subjected.
The abutments are of granite. It has proved very serviceable,
having successfully withstood several tremendous freshets with
no damage save some undermining of one of the abutments,
- •- }i Bods
I Hkh W>wr I^TCl
CnMoTfiKm
^
fe^*
IX lUck a ft OmttM
iiMMfntkDiiii
toterSafe
rmg;^f!sm0^'^^
^iilSK^Xf4eikii^^
1* ^
1
Fia. 237.
which has been repaired with crib work. Considering the
character of the river bed, this dam is probably as reliable as
one of masonry, and its cost was little over half that of a
masonry dam.
For small streams these ballasted timber dams are admir-
able, and Uttle more is needed in most cases.
Another very convenient and useful form of dam, of which
many examples have of late been, erected, is shown in Fig. 227.
It is a concrete and steel dam of the gravity type in which the
dam is given stability far above that due to its structural
weight, by the weight of the superincumbent water. It is
unusual in that it really follows modem architectural lines
instead of conventional hydraulic construction, and the form
here shown, devised by Mr. Ambursen, of Watertown, N. Y.,
involves a good many novel features. Fig. 228 gives a section
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HYDRAULIC DEVELOPMENT. 405
of the dam which is essentially an inclined concrete and steel
floor supported by concrete buttresses. The dam here shown
is about 12 feet high and the supporting buttresses are of the
section indicated in the figure, 12 inches thick and spaced 6
feet between centres. Along the upstream slope of these
buttresses there are set in the concrete, connecting the buttresses,
a series of f inch twisted steel rods about 8 inches between
centres, and just below them covering the whole slope are
sheets of heavy "expanded metal." Over and about this steel
substructure is laid a tight concrete floor about 6 inches thick,
merging at the toe of the dam into a massive concrete shoe
filling the space between the buttresses and built upon the
foundation ledge. Near the top of the dam the slope is made
with a hard finish of rich concrete and the top itself is made
extra heavy to resist the rush of the water.
These dams are sometimes built with concrete downstream
faces and aprons, and in fact may take any form that occasion
requires. They are tight and strong, and ought to prove
durable, while the cost is usually little more than that of a
timber crib. They are, like concrete work generally, quickly
erected, and seem specially adapted to long runs of moderate
height, although they are being used for heights of 30 feet
and more, and when properly designed would appear to be as
generally applicable as any other construction. A similar
construction can sometimes be advantageously used for flumes
and canal walls, since concrete work can be done with material
easily available, and with a very small proportion of skilled
labor, and when well done is both strong and durable.
The type of dam selected for any particular case is governed
by the hydraulic requirements and the conditions at the
proposed site, and the relative costs can only be settled by
close estimates. Sometimes massive rubble masonry is about
as cheap as anything else, while in other circumstances con-
crete or timber would show the minimum cost.
The canals leading the wat^r to the wheels are of construc-
tion as varied as the dams, depending largely on the nature of
the ground. Sometimes they are merel}'' earthwork, oftener
they are lined with timber, concrete, or masonry. Canal con-
struction is a matter to be decided on its merits by the
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ELECTRIC TRANSMISSION OF POWER,
hydraiJic engineer, and very little general advice can be
given. For low heads wooden pipes made of staves like a bar-
rel and hooped with iron every 3 or 4 feet are sometimes used.
In many situations this construction is cheaper than steel pipe
and answers admirably. Such wooden pipes are considerably
employed in the West, the material being generally redwood,
and have proved remarkably durable, some having been in use
Fio. 228.
for more than twenty years. Open timber flumes are also
widely used.
For very high heads, canals and flumes are almost univer-
sally replaced by iron or steel riveted pipe taken by the nearest
rout<j to the wheels below. This practice has been general
on the Pacific coast and has given admirable resiJts. The
pipes are asphalted inside and out to prevent corrosion, and
some pipe lines have been in service for a quarter of a cen-
tury without marked deterioration. Large pipes and those for
very heavy pressures are usually made of mild steel. The
pipes are castomarily made in sections for shipment, from
20 to 30 feet long, and the slip joints are riveted or packed on
the ground. For transportation over very rough country and
for very large pipes, the sections may be no more than 2 or
3 feet long. The joints are then asphalted on the ground.
Fig. 228 shows several of these short sections joined together,
exhibiting the nature of the riveting and the terminal slio
joint.
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HYDRAULIC DEVELOPMENT,
407
In running such a pipe line it is usually taken in as straight
a course as possible, and is laid over, on, or under the ground as
occasion requires, usually on the surface, conforming to its gen-
eral contour. In long lines the upper end is somewhat larger
and thinner than the lower, which has to withstand the heavy
pressure. Fig. 229, which is a profile to scale of the pipe line
of the noted San Antonio Canon plant in southern California,
gives an excellent idea of good modem practice in this sort of
work. There is here a total fall of about 400 feet in a distance
of 2,000 feet. The main pipe is 30 inches in diameter, and the
steel is of the gauges indicated on the various sections. At
p»Jt*itii«uiD^ Of IM4 ua illp julfiti ,
FlO. 230.
the crests of two undulations, air valves are placed to ensure
a solid and continuous column of water in the pipe. The last
540 feet of pipe is reduced to 24 inches and the gauge of steel
is somewhat heavier. The total length of the pipe line is 2,370
feet. To protect the pipe against great changes of tempera-
ture it was loosely covered with earth, rock, and brush when-
ever possible. At two sharp declivities the pipe was anchored
to the rock.
The general method of anchoring on a steep incline is shown
in Fig. 230. In this case the slip joint is simply calked, and
where consecutive sections are at an angle, a short sleeve is
fitted over the joint and lead is run in as shown in the cut.
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408
ELECTRIC TRANSMISSION OF POWER.
Often a packed slip joint is used very freely, thereby gaining
in flexibility, and riveted joints may be only used occasionally.
The line is generally started from the lower end and the joints
or the whole interiors of the sections asphalted as they are laid.
The following table gives the properties of steel hydraulic
pipe of the sizes in common use, and double riveted:
Diameter in inches
10
12
14
16
18
20
84
30
36
42
Area in square inches.
78
113
153
201
254
314
452
706
1,017
1,385
Cabic feet per minute at
three foot per second
100
142
200
255
320
400
570
890
1,300
1,760
Weight in pounds per
foot
19.25
22.76
26
29.6
34
36.5
43.6
54
67
74.6
Safe head in feet
900
750
650
560
600
460
375
300
150
135
Change in safe head for
each gauge number. .
100
90
80
70
00
55
45
35
20
20
The pipe is assumed to be of No. 10 gauge steel, and the
changes in safe head are of course approximate only, but hold
with sufficient exactness for a variation of four or five gauge
numbers. It is better to use a pipe too thick than one too
thin, and to use extra heavy pipe at bends. Where the ground
permits, the water can often be carried to advantage in a flume
or ditch, and then dropped through a comparatively short pipe
line. For heads approaching or surpassing 1,000 feet it is prob-
ably safer to use lap-welded tube for the lower portion of the
run. In every case suspended sand must be kept out of the
water, else it will cut the wheels and nozzles Ijke a sand blast.
When one remembers that under 400 feet head the spouting
velocity of the water is about 160 feet per second, the need of
this precaution is evident. A large settling tank is usually
provided at the head works, spacious and deep enough to let
the pipe draw from the clear surface water. At its lower end
the pipe line terminates in a receiver — a heavy cylindrical
steel tank of considerably larger diameter than the pipe prop)er,
from which water is distributed to the wheels.
On very high heads a relief valve is attached at or near the
receiver to avert danger from a sudden increase in pressure in
the pipe, such as might be caused by some sudden obstruction
at the gate.
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HYDRAULIC DEVELOPMENT, 409
This pipe line method of supply is considerably used for
turbines of moderate size on heads as low as 75 to 100 feet, in
cases where the natural fall of the stream is rather sudden. It
really amounts to a considerable elongation of the iron pen-
stock which is in common use. Whenever there is a sharp
declivity in difficult country, piping is often easier and cheaper
than constructing a sinuous flume or canal. In such situa-
tions the pipes may be 5 or 6 feet in diameter or even more,
and being under very moderate pressure, may be comparatively
light and cheap.
In cold climates ice is one of the difficulties most to be
dreaded in hydraulic work. In high-pressure pipe lines there
is little to fear, for fast-running water does not freeze easily
and the pipes can generally be readily covered, as in the San
Antonio Canon plant, enough to prevent freezing. Large
canals simply freeze over and the interior water is thus pro-
tected. But in cold climates there is considerable danger of
the so-called anchor-ice. This is, in extremely cold weather,
formed on the bed and banks of rapid and shallow streams.
The surface does not freeze, but the water is continually on
the point of freezing and flows surcharged with fine fragments
of ice that pack and freeze into a solid mass with the freezing
water rapidly solidifying about it. When in this condition it
rapidly clogs the racks that protect the penstocks, and even
the wheel passages themselves. In extremely cold climates
xmder similar circumstances the water becomes charged with
spicular ice crystals known as frazil in Canada, far worse to
contend with than ordinary anchor-ice.
The best protection against ice is a deep, quiet pond
above the dam, in which no anchor-ice can form, and which
will attach to its own icy covering any fragments that drift
down from above. In case of trouble from anchor-ice, about
the only thing to do is to keep men working at the racks with
long rakes, preserving a clear passage for the water. If the
wheel passages begin to clog there is no effective remedy.
The most dangerous foe of hydraulic work is flood. The
precautions that can be taken are, first, to have the dam and
head-works very soKd, and second, so to locate them if possible
as to have an adequate spillway over which even a very large
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410 ELECTRIC TRANSMISSION OF POWER.
amount of surplus water can flow without endangering the
main works. If a pipe line is used it must be laid above high
water mark, else the first freshet will probably carry it away.
The power station must likewise be out of reach even of the
highest water.
Closely connected with the subject of floods is that of varia-
ble head, which in many streams is a constant source of diffi-
culty. In times of flood the extra height of the water above
the dam is generally useless, while the tail-water rises and
backs up into the wheels, cutting down their power and speed,
often very seriously. This matter has already been discussed
in Chapter IX, in so far as it is connected wdth the arrange-
ment of the turbines. At very high heads this trouble van-
ishes, as no possible variation of the water level can be a
considerable fraction of the total head.
The most delicate questions involved in hydraulic develop-
ment are those connected with variable water supply. Having
ascertained as nearly as possible the minimum flow, the mini-
mum natural continuous supply of power is fixed, but it remains
to be determined how the water in excess of this shall be
utilized, if at all.
Three courses are open for increasing the available mer-
chantable power. First, water can be stored to tide over the
times of small natural supply. Second, a plant can be installed
to utilize what water is available for most of the year and can
be curtailed in its operation during the season of low water.
Third, the service can be made continuous by an auxiliary
steam plant in the power station. Storage of water can
obviously be used in connection with either of the other
methods.
Under very high heads storage is always worth undertaking
if the lay of the land is favorable. This of course means a
dam, but not necessarily a very high or costly (me. If possible
the storage reservoir should be a little off the main flow of the
stream so as to escape damage from freshets. Reverting to
our previous example of storage, suppose we have 500 HP
available easily for nine months of the year, but a strong
probability of not over 250 HP for the remaining three months.
We have already seen that under these circumstances 15
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HYDRAULIC DEVELOPMENT. 411
acres flooded 10 feet deep wUl keep up the full supply for a
month. If say 50 acres can be thus flooded, the all-the-year-
round capacity of the plant will be doubled. In the moun-
tainous localities where such heads are to be found, land has
usually only a nominal value, and impounding the equivalent
of this amoimt of water is frequently practicable. If it can be
done at say a cost of $75,000, the annual charge per HP stored,
coimting interest and sinking fund at 8 per cent, will be $24,
and the investment would generally be a profitable one. If
the storage cost $100,000, the annual charge would be $32,
and this would not infrequently be well worth the while, when
power could be sold for a good price.
At lower heads the annual charge per HP stored would be
considerably greater for the same total expenditure. Some-
times, however, storage capacity can be much more cheaply
gained for both high and low heads, at for instance not more
than half the charge just mentioned. The matter is always
worth investigating thoroughly when there is doubt about
supplying the power market with the natural flow. The points
to be looked into are the nature and extent of the low water
period, and the cost of developing various amounts of storage
capacity. Sometimes the period of extreme low water is
much shorter than that assumed, and storage is correspond-
ingly cheaper.
There are some cases in which it is possible to supply cus-
tomers with power for nine or ten months in the year, falling
back on the individual steam plants in the interim. When
transmitted power can be cheaply had, it is worth while for
the power user who is paying say $100 per HP per year for
steam power, to take electric power at $50 per HP per year for
nine months, and to use steam the other three months. Certain
industries, too, are likely to be comparatively inactive in mid-
summer, or may find it worth while to force their output
during the months when cheap power is obtainable, and shut
down or run at reduced capacity when the power is unavail-
able. This is a matter very dependent on local conditions,
and while the demand for such partial power supply is gener-
ally limited, there are many cases in which it would be advan-
tageous for all parties concerned. In some mountainous
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412 ELECTRIC TRANSMISSION OF POWER.
regions, winter is the season of low water owing to freezing,
and vanous industries are suspended which may be profitably
supplied with power when the winter unlocks its gates.
Eking out the water supply by an auxiliary steam power
station is likewise not of general appUcabiUty, but sometimes
may prove advantageous. It is most likely to prove useful in
locaUties where a steam power plant would pay by virtue of
the economy due to production on a large scale and distribu-
tion to small users. Cheap water-power a large part of the
year then abundantly justifies adjunct steam-power when
necessary. The moral effect of continuous power supply
is valuable in securing a market. Whether such a supply is
profitable depends on the ratio between the cost of water-
power and the cost of steam-power. And it must not be for-
gotten that steam-power for two or three months in the year is
relatively much more costly than continuous power.
The general charges are the same, although labor, coal, and
miscellaneous supplies decrease nearly as the period of opera-
tion. Consequently, since there is this large fixed item,
amounting to from 20 to 40 per cent of the total annual cost,
the cost of power in a plant operated only three months will be
relatively at least 50 per cent greater than if it were in con-
stant operation. There must be a large margin in favor of
water-power to justify this auxiliary use of steam, imless the
latter would pay on its own account, as for instance in a plant
used largely for lighting, which would be the most profitable
kind of electric service were there a sufficiently large market.
A large lighting load increases the peak considerably, but, as
compensation, drops the peak notably during summer when
low water generally comes. Particularly is this the case with
pubUc lighting which in summer does not overlap the motor
load.
The fundamental questions to be asked in taking up the
supplementary steam plant are first, how large a plant is it
advisable to install, and second, how much energy will it have
to contribute to the common stock. To determine the answers,
the distribution of flow must be pretty closely kno\vn. The
hydraulic power may fail either by drought or by flood. If
the former, there is likely to be a period of a couple of months
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HYDRAULIC DEVELOPMENT.
413
in which the power will be subnormal, perhaps half to two-
thirds of the average supply. To carry a full load over this
period, implies a steam plant of say half the full capacity of the
hydraulic plant, in operation during a portion of the time for
two months. To put things on a concrete basis, suppose a
2,000 HP hydraulic plant, with a 1,000 HP supplementary
steam plant. As stations ordinarily run, the load for a con-
siderable part of the day is much less than the maximum load.
Fig. 231 shows the actual load curve for three successive days
on a high voltage transmission plant doing a mixed power and
8S
I"
_L
t^%^
-t ^xt
it \
^ -^ ^
% - - t ' ^h 4%
\ - - 5 - - ^= ..=^/j
>^x -A
S^ 4t
^S ">=. ^t'
^"--^^3?
10
apjiL
6P.M.
Fig. 231.
8 A.M.
6A.M.
lighting business. It shows a load factor of very nearly 50 per
cent, and if the peak of the load corresponded to the full capacity
of the hydraulic plant, during the period of low water under
oiu* assumed conditions, the steam plant would have to fur-
nish, not one-half the full capacity of the plant, but merely
the energy above the half load ordinate of the load diagram.
Considering this portion of Fig. 231, it will be seen that while
at the peak of the load the steam plant would be working at
full capacity, it would have to be in use only about half the
time at a load factor again of about 50 per cent. So it appears
that for about two months the 1,000 HP steam plant will be
called upon for only about one-fourth of the actual energy
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414 ELECTRIC TRANSMISSION OP POWER.
delivered by the plant, say between four and five per cent of
the year's output.
Therefore, the economic question is the cost of furnishing
5 per cent of the yearly output of a steam plant of half the
total hydraulic capacity worked at about half load. This is
a very different case from any ordinary estimate on the cost
of steam-power.
Taking the cost of the supplementary steam and electric
plant in this case at $60,000 and counting interest, depreciation,
and other fixed charges at 10 per cent, there is an annual
charge of $6,000 against the plant even if it be not nm at all.
At the assumed load conditions it will cost at least $25 per day
for fuel, supplies, and extra labor during the two months of
low water so that the upshot of the matter is, that it will cost
not less than $7,500 yearly to raise the limit of capacity of
the plant from 1,000 HP to 2,000 HP.
But since the plant load factor is 50 per cent, the average
output is raised only from 500 to 1,000 HP. Even on this
basis the supplementary plant evidently will pay at the ordi-
nary prices of fuel and of electric energy, but it is equally
clear that as the required supplementary plant grows rela-
tively larger, and the proportion of steam generated output
increases, a point will soon be reached at which the added
annual cost cannot be compensated by increased possible
sales of power; provided of course, that the price of power
sold is below that at which it could profitably be generated
by steam alone on a similar scale.
To look at the matter from another side, in the case we
have been considering, the total effect of supplying part of
the output by steam would probably be to increase the cost of
the year's output by less than 15 per cent, and the supplemen-
tary plant would pay handsomely. Probably in most cases
it would still be profitable even if 10 per cent of the total out-
put were due to steam. As this proportion increases, the
advantage diminishes, and finally a point is reached at which
any further use of steam cuts down profits rapidly. Each
case must be worked out by itself, as the result depends upon
local conditions, and generalizations are therefore unsafe.
It is usually the fact however that a stream can be developed
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HYDRAULIC DEVELOPMENT. 415
for its flow in early sunfimer with entire safety, leaving the
minimum flow to be cared for by a supplementary plant.
In a few instances it is practicable to connect the power
station generators so as to be operated either by steam or by
water-power, thus saving the cost of extra generators. But
it is generally better to place the supplementary plant in a
substation at the receiving end of the line, thus allowing it
to serve as an auxiliary plant in case of accident to the line or
needed repairs to the hydraulic works. Turbo generators from
their economy of space and their readiness for operation are
well adapted for use in supplementary plants, and for such
use it does not pay to install a costly boiler plant.
Steam-power on a 12 hour basis at steady full load varies
according to the size and kuid of plant, cost of fuel, and so
forth, from a little under $20 per HP year to $125 or more,
with an increase of one-third to one-half in case of variable
loads. Water-power fully developed rents for from $5 to $50
or more per HP year, and may cost to develop anywhere from
$20 to $150 per HP. At the former price it is cheaper than
steam under any circumstances ; at the latter it is dearer than
steam unless the fuel cost is abnormally high.
If the cost of hydraulic development can be kept below
$100 per HP, water-pc^wer can nearly always drive steam-power
out of business.
With respect to the prime movers to be employed in a
hydraulic development, one must be governed largely by cir-
cumstances. The choice in general lies between turbines and
impulse wheels, the properties of which have been fully dis-
cussed in Chapter IX. Without attempting to draw any hard
and fast lines, turbines are preferable up to about 100 feet head,
unless very low rotative speed is desirable, or very little power
is to be develop)ed. Above that, the impulse wheels grow
more and more desirable, and above 200 feet head the field is
practically their o^n. It is generally practicable and desirable
to use wheels with a horizontal axis. Only in a few instances is
it necessary to resort to a vertical axis, as when there is consid-
erable danger of the tail-water rising clear up to the wheels, or
when, as at Niagara, a very deep wheel pit is employed.
The line of operations in developing a water-power subse-
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416 ELECTRIC TRANSMISSION OF POWER.
quent to the reconnoissance has already been indicated. After
the more general considerations have been determined, comes
the question of utilization.
It may seem needless to suggest that the first thing neces-
sary is an actually available market, but the author has more
than once had imparted to him, under solenm pledge of secrecy,
the location of "magnificent" water-powers which could be
developed for a mere song, located a hundred miles from
nowhere — out of effective range even of electrical transmission.
Having a possible market, the next thing is to investigate
it thoroughly. The actual amount of st«am-power must be
found, together with its approximate cost in large and in
small imits. This information ought to be extended to at
least an approximate list of every engine used and the nature
of its use, whether for constant or variable load, whether in
use throughout the year or only at certain seasons. These
more mmute data are not immediately necessary, but are
inmiensely useful later. If it is proposed to include electric
lighting in the scheme, an estimate of the probable demand
for lights should be carefully made. A fair guess at this can
be made from the number of inhabitants in the city or town
supplied. Where there is comp)etition only with gas, experi-
ence shows that the total number of incandescent lights installed
is likely to be, roughly, from one-fourth to one-sixth of the
population, occasionally as many as one-third, or as few as
one-eighth. In cities of moderate size it is usually foimd that
even with competition fn)m gas, the annual sales of electricity
for all purposes can with proper exploitation be brought up to
from $1.50 to $2.00 per capita. This amoimt may be increased
by 50 per cent under favorable conditions.
From the data thus obtained one can estimate the general
size of the market, and hence the approximate possible demand
for electrical energy. With this in mind, further plans for the
hydraulic development can be made. It may be that the
water-power is obviously too small to fill the market, if so, it
shoulil be developed completely. If not, much judgment is
necessary in determining the desirable extent of the develop-
ment. Probable growth must be taken into account, but it
cannot safely be counted upon. If steam-power is very
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HYDRA ULIC DEVELOPMENT. 417
expensive most of the engines can probably be replaced by
motors. The replacement of one-half of them is, under
average circumstances, a sufficiently good tentative estimate.
With this as a basis, approximate estimates of the hydraulic
development can be made. This should be done by a compe-
tent hydraulic engineer. If the developemnt is easy it is well
to make estimates for a liberal surplus power also. At this
stage it is best to have the hydraulic and the electrical engi-
neer work hand in hand to estimate on the delivery of the
assumed amount of power. From these estimates the general
outlook for returns can be reckoned.
Before actually beginning work it is advisable to make a
pretty thorough preliminary canvass of the market, to see
what can be done immediately in the sale of power and light.
With the certain and the probable consumption ascertained,
the hydraulic and electrical engineers can work their plans
into final shape and prepare final estimates.
All this preliminary work may at first sight seem rather
unnecessarily exhaustive, but mistakes on paper are corrected
more easily than any others, and the investigation is likely to
save many times its cost in the final result.
Whatever is done should be done thoroughly. Poor work
seldom pays anywhere, least of all in a permanent installation,
and it should be conscientiously avoided.
Above all, continuity of service has a commercial value that
cannot be estimated from price lists. If it anywhere pays to
be extravagant, it is in taking extreme precautions against
breakdowns and in facilities for quick and easy repairs in case
of unavoidable accident. This applies alike to the hydraulic
and the electrical work. If the first severe freshet demoral-
izes the hydraulic arrangements, or the plant runs short of
water at the first severe drought, a damage is done that it
takes long to repair in the public mind. On the other hand,
careful, thorough work, coupled with intelligent foresight,
insures that complete reliability that is the mint mark of
honest and substantial enterprises.
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CHAPTER XI.
THE ORGANIZATION OF A POWER STATION.
The first thing to be determined in planning a power station
is the prop)er site, which should, if steam be the motive power,
be settled by convenience with respect to the supply of coal
and water. In using water-power the position of the station
should be determined in connection with the hydraulic devel-
opment. Near the foot of the working fall is the natural site,
but, particularly in mountainous regions, it may be quite im-
practicable on account of lack of available space, unsuitable
ground for foundations, inaccessibility, or more often danger
of flood. Under high heads where a pipe line is used, one
has a considerable amount of freedom in determining the site,
since the pipe can be extended and led around to convenient
locations at moderate expense, say not more than $3 or $4 per
foot. A relatively small sacrifice of head, too, may enable one
to secure an admirable location.
On low heads there is far less latitude permissible, since the
canal and tail-race are relatively costly, and a change of level
is a serious matter.
The proper location and design of a power house calls for
great tact and judgment. Often hampered by the topographi-
cal conditions, the site selected must be such as to secure good
operative conditions at minimum cost. It is well in approach-
ing the subject to put aside all preconceived notions as to
how a plant should look, and to remember that it is a strictly
utiUtarian structure. On the hydraulic side it should have
easy access and exit of the water with the minimum loss of
head, the shortest feasible penstocks, and the greatest security
from variations of head. On the electrical side it must be
dry and clear of floods, conveniently arranged for all the appa-
ratus, and with an easy entrance for the transmission lines.
Withal it must have solid foimdations, must often be capable
of easy future extensions and must meet all these require-
ments at the minimum expense.
418
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THE ORGANIZATION OP A POWER STATION. 419
Some of these conditions tend to be mutually exclusive.
When a plant is built at once to the full capacity of the hydrau-
lic privilege, the conditions are considerably simplified, but
this is not the usual case. The plants incidentally described
in this chapter have been chosen as illustrative of some of the
problems of power-house organization rather than as models
of any recognized canons of design. There are no such, save
in the very general way already indicated, and few of the
plants erected are not in some particular open to severe criti-
cism.
But their design has been necessarily a compromise, and
more often than not, the objectionable features have resulted
from following some fashion set by some conspicuous plant
working imder different conditions. The best watchwords in
power-house design are, safety, operative simplicity, and
accessibiUty. Heeding these, with a keen eye to local peculi-
arities one is not likely to go far astray.
If possible, the power station should be placed well off the
main line of flow, or with the main floor well above high water
mark. The foimdations must be of the best to secure safety
from floods and a proper support for the moving machinery.
To meet these conditions is not always easy, particularly
when the available head is low, and sometimes extreme artificial
precautions have to be taken against flood. Such a case is
found in the Oregon City plant already mentioned, of which a
sectional view is given in Fig. 232, showing the foundations, a
single generator, its wheels, and their appurtenances. The
inner wall of the station is here the outer wall of the canal,
and both walls and foundations are built very solidly of
masonry and concrete. In the cut A and B are the draft
tubes belonging respectively to the wheel cases D and F, which
are supplied by the penstocks C and E. F contains the regu-
lar service turbine, a 42 inches Victor wheel coupled direct to
the generator at P. On the pedestals G above this wheel is a
ring thrust bearing at / and an hydraulic thrust bearing K.
Above this is a pulley y, 6 feet in diameter, and still above
this the upper bearing support, the bearings N and 0, the
coupling M, and pedestals Q.
The wheel case D contains a 60 inch wheel with bearings, pul-*
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420
ELECTRIC TRANSMISSION OF POWER.
iki. 'JS^I,
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THE ORGANIZATION OF A POWER STATION. 421
ley, and so forth, R, S, W, T, U, The function of this wheel and
its attachments is to supply power at the seasons of very high
water, sometimes several years apart. When the tail-water
backs up so far that the smaller wheel is no longer equal to the
work, the generator shaft is arranged to be uncoupled just
above the wheel. Then the belt tightener X can be brought
into use, the large wheel started, and the generator driven by
the horizontal belt. The belt tightener is operated by hand
wheels at E2 and Dj, while similar hand wheels at C^ and B^
enable the wheels to be regulated by hand when desirable.
The governing is normally accomplished by the automatic
regulator A2. F^ is one of the main race gates, lifted by the
mechanism at Gj. The wheel room is Hghted by water-tight
heavy glass bulls eyes at Z, each three feet in diameter. The
d^Tiamo room is lighted by side windows and monitor roof, and
is fitted with a twelve ton travelling crane K^j carried on the
supporting column M^ and N^. The penstocks pass through
the hea\y cement floor of the wheel room, J^j with water-tight
joints. The main point of interest in this station for our pres-
ent purpose is not the very complicated and cumbersome
hydraulic plant but the structure of the wheel room, which
forms a massive permanent coffer dam securing the motive
power against all direct interference by even the fiercest
floods. Such a construction is somewhat inconvenient, but in
some instances is almost absolutely necessary. The design of
this plant is imique, in some respects uniquely bad from the
standpoint of general practice, but many of its peculiarities are
the result of its situation and of imusual conditions of water
supply which forced the use of uncommon remedies. Generally
such extreme measures need not be taken, although since it is
usually desirable to have the dynamos on a level with the
wheels, and coupled to them, a water-tight wall between the
d3aiamo room and the wheel room is rather common. Quite
as often, however, full reliance is placed on the strength and
tightness of the penstocks and wheel cases, and wheels and
djoiamos are placed in the same room. A plant so arranged
is cheap and simple, and where there is no unusual danger
of flood is sufficiently secure. Fig. 233 shows a good typical
plant of this sort, consisting of three double horizontal tur-
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ELECTRIC TRANSMISSION OF POWER.
bines under 50 feet head, each directly coupled to its generator.
Each pair of wheels gives 560 HP at about 430 revolutions per
minute. This represents construction as straightforward and
simple as that of Fig. 232 was difficult and intricate. It is
specially interesting in the arrangement of several wheels to
discharge into a common tail-race, instead of into several
Fig. 233.
costly arched tail-races extending imder the dynamo room, a
construction sometimes quit€ unnecessarily employed.
The hydraulic conditions may drive the engineer to all sorts
of expedients, but the main points are security against being
drowned out, and good foundations. If the dynamos and
wheels can be given direct foundations of masonry and con-
crete, such as the former have in Fig. 233 and the latter in Fig.
232, so much the better. If moving machinery must be carried
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THE ORGANIZATION OF A POWER STATION.
423
on beams, support these beams as in Fig. 233, directly mider the
load, J)y iron pillars or masonry piers. For direct coupling it
is preferable to have foundations entirely secure from vibration.
If such cannot be had one may resort successfully to a flexible
coupling, very often desirable in driving from water-wheels,
and sometimes rope or belt driving is advisable.
The proper site having been selected, the next consideration
is the form of the structure itself. As a rule, whatever the
O A N A L
IIUT
»«l .* ■'■»>" ■'■■ ^Jg^^
Fio. 234.
nature of the power units, they are most conveniently put, in
a water-power plant, side by side in a single row with their
shafts parallel. This placing enables the hydraulic plant to be
simply and conveniently arranged, and enables the operator to
take in the whole plant at a glance and watch all the apparatus
simultaneously. Fig. 234 shows the original ground plan of
the great Niagara station, well exemplifyiag this arrangement.
In stations employing horizontal turbines such a distribu-
tion of units has even greater advantage in avoiding long and
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ELECTRIC TRANSMISSION OF POWER.
crooked penstocks. Fig. 233 forcibly suggests the difficulty of
setting the generators otherwise than in a single row.
There are, however, not infrequent cases in which the gen-
erators can be more conveniently placed otherwise. Some-
times the site desirable for hydraulic reasons is cramped so
that a power house cannot readily be lengthened enough to
place the machines in a single row, and even when there is
space enough considerations of speed may compel a greater
number of units than can readily thus be accommodated.
Fig. 235 which is a floor plan of the great Canadian plant at
"Ar^MUra
9S00 to M.OOO VoIU MOO to 60,000 Volti
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ighUnTDi
FlO. 235.
Shawinigan Falls is a case in point. Here the units are dis-
posed in Echelon, which gives a shorter and wider power house
than usual, and room for extension lengthwise. It should be
noted here that the switchboard is in a raised gallery over-
looking the station as in the Niagara plant, and that the
raising transformers are in a separate adjacent building, to-
gether with the lightning arresters.
In some cases the generators may well be put in two short
rows facing each other, an arrangement sometimes giving a
far more compact power house than the single line, which is
inconveniently long when many machines are installed.
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THE ORGANIZATION OF A POWER STATION. 425
The main thing is to get the generators so placed as to be
easily watched when in operation and extremely accessible in
case of accident or of necessary repairs, while the hydraulic
arrangements are still as simple as possible. Sometimes the
power house can be greatly cheapened by avoiding the common
arrangement which calls for tail-races, usually of arched mas-
onry extending clear imder the building. It is not uncom-
mon to find the foundation masonry in such cases costing very
much more than the superstructure, and involving much
expense really needless.
In general the building erected for a power station should
be light, dry, fireproof, and well ventilated. Dynamos usually
run hot enough, without boxing them up in a close room.
There should be plenty of space back of the row of dynamos, so
that if machinery has to be moved there will be ample room.
On the other hand the row of dynamos should be fairly compact,
as a needless amount of scattering of the machines makes them
hard to look after. In very many cases one story in height is
quite sufficient, and in all cases it is preferable to more, so far
as working apparatus is concerned. Sometimes a second story
can be well utilized for store rooms, transformer room, and
quarters for the operating force, but as a rule a single story
allows more complete accessibility — one of the most important
features in station design. As land is seldom dear around a
station for power transmission ample floor space is easily
obtained, except in occasional cramped localities. A brick
structure with iron roof is perhaps the most satisfactory kind
of station. In some situations rubble masonry or concrete
and steel constructions are convenient. Avoid wood as far
as is practicable, at least in every place near dynamos, or
wiring of any kind. A second story, if used at all, should
have a fireproof floor. Sometimes from temporary necessity
a frame building is used, but even this can be made fairly safe
by keeping the machines and wiring clear of wood. In any
case the floor is the most troublesome part of a station to fire-
proof. Probably the best material is hard finished concrete,
or artificial stone with only so much wood covered space as is
needed to keep it from being too cold, or slipper}', or to pro-
tect it temporarily in moving about machines. Window space
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426 ELECTRIC TRANSMISSION OF POWER.
should be large and arranged so as to avoid leaving dark
comers around the apparatus. There should be, too, ample
door space to facilitate replacing apparatus — nothing is
more aimoying than to be short of elbow room when moving
heavy machinery.
For the same reason a good permanent road should be built
to the power station if one is not already in existence. In
moimtainous regions this is sometimes impracticable, but
money spent in improving the road is better invested than
when put into special sectionalized apparatus. It is quite
possible so to sectionalize a generator of several himdred
KW that the parts can all be carried on mule back, but the
expense is considerably increased, and the great advantage of
having a standard type of apparatus has to be abandoned.
Hence, imless the cost of improving the road to admit of trans-
porting ordinary apparatus is decidedly greater than the differ-
ence in cost between regular and sectionalized machinery, the
former procedure is advisable. Of course when it comes to a
question of long mountain trails, sectionalized machinery some-
times has to be employed. The armature of a polyphase
machine for use with transformers can very easily be section-
alized, but if for high voltage or of very large size it is better
to send in core plates and other material in bundles and wind
the armature on the spot.
Having determined the general location and nature of the
power station, one may take up further arrangements as
follows :
I. Motive Power.
II. Dynamos.
III. Transformers.
IV. Accessories.
The fundamental question is the proper size and character
of power units. In direct coupled work, prime mover and
generator must be considered together. In steamdriven
stations for power transmission the boiler plant may be
determined by itself, but dynamos and engines should be taken
up conjointly.
There is at present rather too strong a general inclination
to use direct coupled units at any cost. Direct driving i^
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THE ORGANIZATION OF A POWER STATION. 427
beautifully simple and efficient when conditions are favorable,
and for large units is necessary, but belt and rope driving gives
singularly little trouble, and when well engineered wastes very
little energy — not over 3 to 5 per cent for a single direct
drive, which can almost invariably be used. It is very easy
to lose far more than this in using a d3mamo designed for a
speed imsuited for its output, or wheels working under dis-
advantageous conditions. Cases of such misfit combinations
are not imcommon, and while the workmanship and results
are often good the engineering is faulty. A very character-
istic example is shown in Fig. 236, from the power plant of
an early single phase transmission for mining purposes. The
generator selected was a 120 KW Westinghouse machine of
standard form and excellently adapted for its purpose. Its
speed was 860 revolutions per minute, and to obtain this from
a working head of 340 feet a battery of four 21 inch Pelton
wheels was required. Now the Pelton wheel under favorable
conditions is imexcelled as a prime mover in convenience and
efficiency, but these conditions were distinctly unfavorable.
The same work could have been done by a single wheel four or
five feet in diameter at not over one-third the initial expense
for wheels and fittings, and at enough higher efficiency to
more than compensate for the slight loss of energy in a
simple belt drive. In this case wheel efficiency was sacrificed
to the speed of the generator. An error quite as common
is to sacrifice generator efficiency to the speed of the prime
mover.
The most flagrant case of this kind that has come to the
author's notice, was a polyphase machine of less than a hundred
KW output direct coupled to a vertical shaft turbine at 20
revolutions per minute. This was of course a low frequency
machine, but an instance nearly as bad may be foimd in the
case of a 75 KW alternator for 15,000 alternations per minute
direct coupled to an engine at a little less than 100 revolutions
per minute. These are extreme examples, of course, such
machines costing several times more than normal generators
of the same capacity, and having probably fully 10 per cent
less efficiency. It is, however, not rare to find costly direct
coupled units which gain no efficiency over belted combinar-
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ELECTRIC TRANSMISSION OF POWER.
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THE ORGANIZATION OF A POWER STATION, 429
tions, have little to recommend them save appearance, and pay
dearly for that.
The best way to avoid such mistakes is to put aside preju-
dice and let the makers of generators and prime movers put
their heads together in consultation and work out the problem
together. Both are usually anxious to do good work, and will
arrive at a judicious conclusion.
Alternating work is sometimes difficult in this respect on
account of the requirements of frequency, but at the present
time all the large makers of hydraulic and electrical machinery
have a sufficient line of patterns to meet most cases easily
without involving special work to any considerable extent.
In deciding on the number of units to be employed several
things must be taken into account. The number should not
be so small that the temporary crippling of a single imit will
interfere seriously with the work of the plant. This deter-
mines the maximum permissible size of each unit. The
nearer one can copie to this without involving difficulties in
the way of proper speed or serious specialization, the better.
It is seldom advisable to install less than three units, while in
some cases a considerably larger number must be used to suit
the hydraulic conditions.
To illustrate this point, suppose we are considering a trans-
mission of 3,000 KW from a water-power with 16 feet available
head. One would naturally like to install three 1,000 KW
generators or four of 750 KW. But trouble is encoimtered at
once in the wheels. The 1,000 KW machine should have, say
1,500 HP available at the wheel, and the 750 KW about 1,100.
Even assuming at once the use of double turbines the highest
available speed for an output of 1,500 HP would be about 75 to
80 revolutions per minute, too low for advantageous direct
coupling at any ordinary frequency; 1,100 HP can be obtained
at a speed perhaps 10 revolutions per mmute higher — not
enough to be of much service. It is a choice between evils at
best, either generators of speed so low as to be both expensive
and difficult to get up to normal efficiency, or belting, when one
would much prefer to couple direct. At lower heads, say 12
feet, one would be driven from direct connection; at 30 to 40
feet head it would be comparatively easy. In the case in hand
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430 ELECTRIC TRANSMISSION OF POWER,-
we are near the dividing line, and it would require very close
figuring to get at the real facts, figuring which would have to
be guided by local conditions. The chances are that six 500
KW generators at about 125 r. p. m., would give a good
combination of efficiency and cost. As an alternative, one
might use 750 KW generators either coupled to 3 water
wheels or rope driven. Each case of this kind has to be
worked out on its merits. Since the dynamos cost far more
than water-wheels for the same capacity, if there is any special-
izing to be done it is cheaper to do it at the wheels. If, how-
ever, it proves convenient to change the dynamo speed a trifle,
most generators can be varied 5 per cent either way without
encountering any difficulties.
Now and then it becomes necessary to plan for vertical
wheel shafts. This, unhappily, Ls apt to confront one at very
low heads, and leads to immediate difficulty. Direct coupling
is usually impracticable since the speed is very low, double
wheels being out of the question, and even if the dyiiamo could
be economically built the support of the revolving element
would be very troublesome. The usual arrangement is to use
bevel gears, and this is generally the only practicable course.
It is desirable in any case to operate each dynamo by its
own special wheels, to avoid complication. Hence the con-
siderations which determine the number of dynamos also de-
fine the number of wheels. It is very seldom expedient to
use more than a single pair of wheels for driving a single
generator, on account of difficulties in alignment and regula-
tion and consequent tendency to work inharmoniously. This
tendency is stronger in impulse wheels than in turbines, on
account of the very small volume of water generally employed,
and consequent hypersensitiveness to small changes in the
amount, pressure, and direction of the stream. So, usually, a
single wheel or pair of wheels, equal to the task of handling a
single generator, may be taken as the hydraulic unit.
For simplicity and economy one should keep down the
number of generators to the limit already imposed, except as
special cases may call for an increase. If the plant is to feed
several transmission lines it is sometimes best to assign separ-
ate dynamos to each line for one purpose or another, and this
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THE ORGANIZATION OF A POWER STATION. 431
may make it necessary to increase the total number. The
requisite security from accident can be in such cases ob-
tained by one or two spare units, or by shifting a generator
from a lightly loaded line to a heavily loaded one. In point
of fact the modem generator is a wonderfully reliable machine,
and it is not imusual to find a machine that has nm day and
night, save for a few hours in the week, for many months with-
out any reserve behind it. The author saw recently a small
incandescent machine which had run some hours per day in an
isolated plant, for fourteen consecutive years without a failure
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432 ELECTRIC TRANSMISSION OF POWER.
of any kind. During that time the armature had been out of
its bearings but once, to have the commutator turned down.
In steam driven plants, as in water-power works, the most
convenient arrangement of generators is generally side by
side in a single line. So placed they are easy to take care of,
and the spare room is more available than when it is irregularly
disposed. In case water-wheels are the prime movers a water-
tight bulkhead is generally placed between them and the d3nia-
mos, so that leaks or overflows will be confined to the wheel
pit, where they can do no harm. Through this bulkhead the
shafts should pass if the units are directly coupled. In case
of a belt or rope drive it is frequently convenient to place
wheels and dynamos on different levels, thus obtaining similar
security. Fig. 237 shows a well-arranged small plant of this
sort, driven by a pair of Pelton wheels. The plant is so small
that both dynamos can be conveniently driven by pulleys on a
very short extension of the wheel shaft.
In a larger plant each wheel imit would drive a single dyna-
mo, and the receiver and wheels with their fittings would
occupy one-half of the station, while the dynamos would be
placed in the other half, following the same general plan shown
in Fig. 237. The main point is to get good foimdations for
the dynamos while keeping them out of reach of stray water.
In an alternating current station it is advisable to drive the
exciters from special prime movers, so that a change of speed,
even momentary, in the main machine may not change the
exciter voltage and thus make a bad matter worse. This is par-
ticularly necessary in water-power plants, where the governing
is apt to be none too close or prompt. It is a good thing also
to have plenty of reserve capacity in the exciters, so as never
to be caught with insufficient exciting power, even in case of
accident to one exciter.
Both wheels and dynamos should be thoroughly accessible,
and wheel and dynamo rooms must be well lighted, naturally
and artificially. A dark and slippery wheel pit, without suffi-
cient space around the wheels, is sure to prove a source of
annoyance and sometimes of serious delays. It should be
possible to get at every wheel and its fittings and to work
around them freely when all the other wheels are in full use.
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THE ORGANIZATION OF A POWER STATION. 438
Sometimes it is useful to separate wheels by bulkheads, pref-
erably movable, and there should always be floor space
enough to stand and work on without putting up temporary
stagings and loose boards. There should always be electric
lights ready for use aroimd all the working machinery, arc
lamps or incandescents as may be most convenient, but plenty
of them. Around the wheels it may sometimes be necessary
to use incandescents in marine globes to protect them from
the water, and to install waterproof flexible cable for the mov-
able lights.
As an example of good practice in a plant for heavy power
transmission, operated by turbines imder a moderate head,
the Folsom, Cal., installation shown in Plate XV is worth
studying. Fig. 1 shows the general character of the power
house and its relation to the forebay, penstocks, and tail-race.
The forebay itself is double, being divided lengthwise by a
wall, on each side of which are the gates and penstocks for
two double turbines. The tail-races are foiur masonry arches
under the power house, uniting then into a single channel.
The tubular steel penstocks are 8 feet in diameter, and
the relief pipes above them, 4 feet in diameter. The gates
are handled by hydraulic cylinders, like the head gates at the
dam. It will be observed that the wheel pit is not in the
power house, but in the clear space between the rear wall of
the power house and the end wall of the forebay, which like
the other masonry work in this plant, is of granite blocks.
The power hoiuse itself is a spacious two-story brick structure
on granite foimdations. The lower floor is the dynamo room
while the upper floor contains the transformer room, storage
space, and so forth, together with the high tension switch-
board, the lines from which are shown running out from the
end of the building. The wheels are 30 inch double horizontal
turbines of the McCormick type, giving about 1,250 HP per
pair at 300 revolutions per minute under the available normal
head of 55 feet. There are, besides, two small single horizon-
tal wheels for driving the exciters. Each of the main wheel
units carries on its shaft a 15,000 lb. fly-wheel to steady its
operation under varying loads.
The arrangement of the wheels and generators is admirably
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434 ELECTRIC TRANSMISSION OF POWER.
shown in Fig. 2, Plate XV, from a photograph taken during
the process of construction.
This gives a view of one complete unit: generator, coupling,
governor, turbines, and fly-wheel, and includes also an exciter
and its wheel, not yet aligned and coupled. Four such main
units and the two exciters, all placed side by side in a single
row, make up the plant.
The generators are three-phase machines, of 750 KW
capacity, at 60*^. Each has 24 poles, runs at 300 revolutions
per minute, and weighs about 30 tons. They are of very low
inductance, with polyodontal bar-wound armatures designed to
give normally 800 volts between lines, and to produce a very
close approximation to a true sinusoidal wave form. They
are normally intended to run in parallel, although there is
actually a complete circuit per machine available when wanted.
The wheels were originally installed with Faesch-Piccard
governors, which functioned fairly well but were not strong
enough for the heavy service, and have now been replaced.
When the heavy apparatus was all in place and connected,
the arched spaces shown in Fig. 2 were walled up except for
shaft holes, and the wheel pit permanently separated from
the dynamo room. From the dynamos the current is taken
to the low tension switchboard facing the row of generators.
Thence it passes to the transformer room on the second floor
of the station. Here is a bank of twelve raising transformers,
of the air blast substation type largely used in the practice of
the General Electric Company. These raise the working pres-
sure to 11,000 volts. At this potential the current passes to the
high tension switchboard and thence to the line. A second
switchboard in the transformer room serves to distribute the
low tension current received from the dynamos.
The general arrangement of this station is excellent, for the
installation as made. Were the plant to be worked at higher
voltage for which the air blast transformers would be inad-
visable, it would be wise not to put the transformer room in
a second story, but to locate the oil transformers in a fireproof
space by themselves.
The line consists of four complete three-phase circuits each
of No. 0 B. & S. wire. There are two independent pole lines
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Fig. 1.
Pio. 2,
PLATE XV.
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THE ORGANIZATION OF A POWER STATION. 435
running side by side a few rods apart, constructed of red-
Wood poles 40 feet long. Each pole line carries two circuits
symmetrically arranged on two cross arms, one circuit being
on each side of the pole, the wires arranged so as to form an
equilateral triangle, with an angle downward. One of the
pole lines carries an extra cross arm a few feet below the
main circuits, to accommodate the telephone circuit. All wires
are transposed at fre(][uent intervals to lessen induction. The
pole line is on the southern side of the American River and
follows in the main the coimtry roads clear into Sacramento,
the two lines being on opposite sides of the road. The route
thus followed is a trifle longer than the actual linear distance,
but the gain in accessibility more than counterbalances the
extra mile or so of line. The high tension line is carried
along the river through the northern edge of the city fairly
into the district of load, and is then terminated in a handsome
brick substation containing the transformer and dynamo
rooms and the offices of the company. The distribution system
is mixed in character owing to the operation of the existing
railway and lighting loads.
The main distribution circuit is a three-phase four-wire
circuit worked at 125 volts between the active wires and the
neutral. This gives an admirable network for lighting and
motor work, very economical of copper, easy to wire and to
operate. All the transformers in the substation are arranged
for a secondary voltage of 125, 250, or 500 as may be desired,
so as to be ready for any kind of service.
This plant first went into operation in July, 1895, and has
since then been in continuous service day and night. No seri-
ous trouble has been encountered, the high voltage line has
performed admirably, and there has been no difficulty due to
inductance, lack of balance, resonance, or any of the other
things that used to be feared in connection with long distance
polyphase work. Furthermore the plant is a success financially
as well as electrically. Apart from Niagara, which even now
is only beginning long distance work, it has been one of the
most valuable of the pioneer plants in establishing confidence
in power transmission, and in putting the art upon a sub-
stantial basis.
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436 ELECTRIC TRANSMISSION OF POWER.
Another fine example of three-phase work, of especial interest
as being operated under a very exceptionally high head,
is the plant utilized at Fresno, Cal. Fresno is a flourishing
city of 15,000 inhabitants at the head of the magnificent
San Joaquin valley in central California. Like other Cali-
fomian cities, it has been hampered in its development by
the very high cost of coal — $8 to $10 per ton in carload lots,
and some of its active citizens cast about for an available
water-power to develop electrically. Such a one was found on
the north fork of the San Joaquin River very nearly 35 miles
from the city. At a point where this stream flows through a
narrow canon it was diverted, and the stream was carried in a
series of flumes and canals winding along the hillsides for
seven miles to a point where it could be dropped back into the
river bed, 1,600 feet below.
At this point an emergency reservoir was formed in a natural
basin, which by an expenditure of less than $3,000 was devel-
oped into a pond capable of holding enough reserve water for
several days' rim at full load.
The minimum flow of the stream is 3,000 cubic feet per
minute, capable of giving between 6,000 and 7,000 HP off the
shafts of the wat^r-wheels when fully utilized. In the initial
plant only a small portion of this power is employed. From
the head works at the reservoir a pipe line is taken down the
hillside to the power house. The pipe is 4,100 feet long. At
the upper end for 400 feet a 24 inch riveted steel pipe is used,
then lap-welded steel pipe is employed diminishing in diameter
and increasing in thickness toward the lower end, where it
is 18 inches in diameter, of five-eighths inch mild steel, and
terminating in a tubular receiver 30 inches in diameter, of
three-fourths inch steel. The vertical head is 1,410 feet. This
corresponds to a pressure of 613 lbs. per square inch, while the
emergent jet has a spouting velocity of 300 feet per second.
To withstand and utilize this tremendous velocity unusual
precautions were necessary. The main Pel ton wheels, designed
for 500 HP at 600 revolutions per minute, have solid steel plate
centres with hard bronze buckets. Each carries on its shaft a
steel fly-wheel weighing 3 tons, and 5 feet in diameter. With
their enormous peripheral speed of over 9,000 feet per minute,
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Fio. 1.
Fio. 2.
FlQ. 3.
PLATE X¥L T
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THE ORGANIZATION OF A POWER STATION. 437
these have a powerful steadying effect on the speed of the
generators. There are four of these wheels, each directly
coupled to a 360 KW General Electric three-phase generator,
giving 700 volts at 60^-'. There are also two 20 HP Pelton
wheels, each 20 inches in diameter, and each direct coupled to
a multipolar exciter. All the wheels are controlled independ-
ently by Pelton differential governors.
On the main floor of the power house opposite the generators,
is the bank of raising transformers. These are of 125 KW
capacity each, of the ordinary air blast type. Space is
provided for additional transformers more when the load
demands them.
These transformers raise the pressure to 19,000 volts
between lines and from the high tension section of the switch-
board the current passes to the transmission line. This con-
sists of two complete three-phase circuits which can be worked
together or independently. They are of No. 00 bare copper
wire carried on special double petticoat porcelain insulators,
all tested at 27,000 volts alternating pressure.
The pole line is of 35 foot squared redwood poles set 6 feet
deep. Each pole carries four cross arms. Three of these at
the top of the pole are for the transmission circuits. These
are at present confined to the two upper cross arms, leaving
space for additional circuits below. A fourth short cross arm
about 4 feet below the others carries the telephone wires.
Plate XVI gives a good idea of the general arrangement of
the Fresno plant. Fig. 1 gives a glimpse of the storage
reservoir at the upper end of the pipe line. Fig. 2 shows the
situation of the power house below, which is built of native
granite on a solid rock foundation, with a wooden roof. It is
75 X 30 feet in size. The wheel pit is seen nmning along one
side of the station just outside the wall, through which pass the
wheel shafts driving the dynamos inside. In the foreground
appears the beginning of the transmission line.
Fig. 3 shows the interior of the power house with the d3ma-
mos and transformers in place and the switchboard at the
further end of the room.
In the city of Fresno the transmission lines are taken to a
substantial brick substation in the centre of the city. Here
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438 ELECTRIC TRANSMISSION OF POWER.
are situated the reducing transformers and accessory apparatus,
including two 80-light arc dynamos direct coupled to 60 HP
induction motors.
The distribution system is threefold. In the central dis-
trict of the city a three-phase four- wire network is employed,
supplied from three 125 KW reducing transformers, and
worked at 115 volts between active wires and neutral. For
the outlying residence region three 75 KW transformers supply
current at 1,000 volts for use with secondary transformers.
Finally, for reaching neighboring towns, three 40 KW trans-
formers feed a 3,000 volt subtransmission system. The oper-
ation of this plant, like that of the Folsom plant, has been
highly successful from the start, and the electrical troubles
that have often been feared on long lines at high voltage have
been conspicuous by their absence.
Both these plants represent even to-day, first-class practice
in general equipment and arrangement, save that the voltages
of transmission have now become ultra conservative, and
while differing conditions bring their own necessary modi-
fications, these examples may be regarded as thoroughly typi-
cal. They have incidentally demonstrated the thorough prac-
ticability of general distribution of energy for lighting and
power by polyphase currents under large commercial condi-
tions, and at distances great enough to involve all the elec-
trical difficulties likely to be met at the voltage employed. A
more recent plant of peculiar interest in some of its engineer-
ing features is that of the Truckee River General Electric
Company near Floriston, Cal., shown in Plates XVII and
XVIII. This plant was erected to supply power to the mines
of the famous Comstock Lode, where it is used for mining
hoists, milling and pumping, which had formerly been done
almost entirely by steam provided by burning pine at $8.50
to $15 per cord.
The source of the water-power is the Truckee River, an
unusually steady stream rising among the snows of the Sierra
Nevada. At the head works is a timber crib dam about 50
yards long and only 7 feet in height, serving mainly to back
the water into a wide, slow running canal a couple of hundred
yards long, which serves also as a settling pond. Thence the
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FlO, 1.
Fio. 2.
PLATE XVIL
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PLATE XVIII.
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THE ORGANIZATION OF A POWER STATION, 439
water passes through the racks into a timber flume, 10 feet deep
and 6 feet 8 inches wide inside, the entrance being widened to
28 feet at the racks and tapered to the normal width in a rim
of 40 feet.
This timber flume, a portion of which is well shown in Plate
XVII, Fig. 1, winds along the hillsides for a distance of a little
more than a mile and a half. It carries 300 cubic feet of water
per second at a depth of 6 feet in the flume, the corresponding
velocity being 7.5 feet per second. This flume is carried on
heavy timber frames 16 feet between centres, with two inter-
mediate sets of four posts each. Along the line of the flume
are two spiU gates each in the side of a sand box dropped
below the bottom of the flume.
This flume terminates in a timber penstock 36 feet long and 21
feet wide, f mnished with a central bulkhead and strongly stayed
with iron rods. Back of the penstock a spill flume is carried
for 200 feet alongside the main flume. From the penstock two
pipes, taking their water through head gates, run to the wheels.
These pipes are of redwood staves, hooped with |-inch roimd
steel, 6 feet in diameter inside, and 160 feet long. The working
head is 84.5 feet, and a few feet from the power house the
wooden pipes are wedged into the steel pipes that lead to the
wheel-cases. Plate XVII, Fig. 2, shows the power house, pen-
stocks, pipes, and tail-races. The power house itself is 88 X 31
feet, of brick, with roof of corrugated galvanized iron, and has
concrete foundations.
Plate XVIII shows the arrangement of the wheels and gener-
ators. The wheel plant consists of two pairs of 27-inch McCor-
mick horizontal turbines, each pair giving 1,400 HP at 400
r. p. m. Each pair discharges into a central cast-iron draught
box continued by a 20-foot draught tube. Each pair of wheels
is directly coupled to a 750 KW, 600 volt three-phase Westing-
house generator. But instead of the arrangement shown in
Plates XV and XVI the shafts of both sets of wheels and of
the generators are in one straight line, with the wheels at its
extremities. This gives space for a very solid foimdation for
the generators between the arched tail-races, and if need be the
generators can be directly coupled together so as to run both
from a single wheel or as a single unit. Arrangements of this
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440 ELECTRIC TRANSMISSION OF POWER.
kind may be very freely adopted where the units are few, and
the plant is not built with probable extensions in mind. Each
generator has a separate multipolar exciter driven by a small
separate turbine, and each of these receives the water from the
case of its main wheel and discharges into the corresponding tail-
race. Each exciter is of sufficient capacity for both generators.
Each main pair of wheels is regulated by a Lombard gov-
ernor, one of which appears in the foregroimd. But to in-
sure close regulation an unusual device is installed in connec-
tion with the governors. The supply pipes are too long and
the head too high to permit the installation of efficient relief
pipes, as in the Folsom plant, and the enormous inertia of the
water in the supply pipes was consequently both an incon-
venience and a menace. Hence relief was provided by a
huge balanced Ludlow valve connected with the wheel-case and
the tail-race. This valve is operated by wire ropes and sheaves,
so connected with the gate shaft of the wheel that when the
governor closes the wheel gate it opens the relief valve and
vice veraa, thus keeping the velocity of the water nearly con-
stant. The effect is closely similar to that obtained with the
deflecting nozzle used with Pelton wheels, and while it wastes
water, that is of small moment compared with the necessity
for regulation.
The 500-volt current from the generators is raised by oil
insulated transformers to 22,000 volts for the 33-mile trans-
mission to Virginia City, Nev., which is the centre of utili-
zation. The pole line is of square sawed redwood poles 30 feet
long, 11 inches square at the butt and 7 inches square at the top.
These poles carry two cross arms on which the two three-phase
circuits of bare No. 4 B. & S. wire are arranged as usual, forming
an equilateral triangle on each side of the poles. The insula-
tors are porcelain on oil-treated eucalyptus pins. The poles
are spaced about 40 to the mile, and carry a couple of brackets
for the telephone line below the cross arms. The three-phase
lines are transposed every 144 poles.
The distribution in Virginia City is at 2,250 volts three-
phase over a maximum radius of about 2 miles. This plant is
a good example of recent practice in dealing with moderately
high heads. The timber flume in particular strikes Eastern
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THE ORGANIZATION OF A POWER STATION. 441
engineers unfavorably at first, but the irrigation companies of
the Pacific slope have had many years of experience in that
sort of construction, and have learned that it is easy, cheap,
and durable when properly cared for. There are hundreds of
miles of it used for various purposes in California, and in
many instances it is the only practicable means of water
delivery. Altogether this particular plant teaches a useful
lesson in hydraulic construction, and like those just described
is a very good example of modem engineering.
At present long-distance plants are rather the exception, and
in the natural course of events there must be developed a great
number of power transmissions at quite moderate distances,
under ten miles or so. Such plants as regards general organi-
zation do not possess any special peculiarities. The dynamos,
however, may often be wound for exceptionally high voltage.
Dynamos for use with raising transformers should be of
moderate voltage, not much over 2,000 volts unless the units are
of immense size, or must furnish local power in addition to
their regular function.
At moderate voltage the generators gain in cost per unit of
output, in simplicity, and in comparative immunity from acci-
dents. They are also likely to be designed for lower arma-
ture reaction. Nevertheless, there are many cases in which
it is advisable to install generators for 5,000 to 12,000 volts
for the sake of economy and simplicity of plant. In fact, it
is questionable whether it is ever worth while to use raising
transformers in work at these very moderate transmission
voltages. As already indicated, such generators should always
have stationary armatures, and should, and Ho, have extraor-
dinarily good insulation. When installed they are sometimes
insulated from the foimdations with scrupulous care, and
if direct coupled they may be provided with insulating coup-
lings. Small high-voltage machines have been supported
on porcelain insulators. Large generators may be carried on
hardwood timbers thoroughly treated with insulating material,
and bolted to the foundation cap stone. As the art of insula-
tion has progressed, such precautions have become less and
less necessary, and at the present time generators for 10,000
volts and more are often installed and successfuUv used
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442 ELECTRIC TRANSMISSION OF POWER.
without any such general insulation at all. It is desirable to
surround such machines with an insulated platform a few inches
above the floor, and to protect the leads with vulcanite tubes.
It is well also to shield 'the terminals so that only one can be
manipulated at a time when the machine is in action. These
high-voltage generators have proved to be entirely reliable, do
not seem to be more subject to accident than other generators,
and if injured are rather more easily repaired than transformers.
In all plants employing more than a single generator, — ^and
this means nearly all power transmission plants of every kind,
— the generators should be arranged to run in parallel, and in
most instances should be so operated regularly. Now and
then generators may advantageously be operated on separate
lines, as when these lines must be run under diflFerent condi-
tions of regulation, or when a line must be isolated for the
purpose of carrying a very severe fluctuating load, but for the
vast majority of plants these expedients are totally unneces-
sary, and only complicate the operation of the system without
any material compensating advantage.
Plants operated for lighting alone can get along after a
fashion by shifting load quickly from one machine to another,
an operation quite familiar to most people who have been cus-
tomers of such a system; but for the general distribution of
lights and power this procedure is inadmissible, for it usually
means stopping some or all of the motors. Moreover, it is a
clumsy method at best, abandoned long ago by continuous
current stations, and without any excuse for existence save
villainously bad generator equipment or incompetence in the
operation of the station.
All modem generators of good design are capable of running
in parallel without the slightest difficulty, provided they have
somewhere nearly similar magnetic characteristics and are
intelligently operated.
It is inadvisable to attempt running a smooth-core and an
iron-clad armature in parallel, or two machines which are very
different in regulation or which give very different wave shape,
but on the other hand such machines ought not to be installed
together on general principles. The nearer alike the machines,
the better they will run in parallel.
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THE ORGANIZATION OF A POWER STATION, 443
No subject has been oftener a topic of fruitless discussion
than the paralleling of alternators. As a matter of fact, any
two similar alternators will go into parallel and stay there with
very little difficulty, at least if driven from water-wheels, as is
nearly always the case in transmission plants.
High-inductance machines have been supposed to be some-
what easier to put and work in parallel than those of low
inductance. They certainly can be thrown together carelessly
with less likeUhood of a large synchronizing current flowing
between them, but with low-inductance machines a little more
care, or an inductance temporaril)'- inserted between the
machines, leads to the same end.
In throwing two alternators of any kind in parallel, they
should be in the same phase, rimning at the same speed and at
approximately the same voltage. The more nearly these con-
ditions are fulfilled, the less synchronizing current will flow
between the machines, and hence the more smoothly will they
drop together.
The ordinary arrangement of phase lamps shows the relation
of both speed and phase with ample exactness. When the
indicator lamp is pulsating at the rate of one period in four or
five seconds, it is evident that the relative speeds of the
machines are very nearly right, and it is quite easy to cut in
the new machine when its phase is very nearly right. One
soon gets the swing of the slow pulsations, and can catch the
middle point of the interval of darkness with great accuracy.
The pulsations can in fact be easily reduced to a ten-second
period or even longer. It is, on the whole, best to reverse the
phase lamp connections so that concordance of phase will be
marked by the lighting up of the phase lamps. The lamps
should be of such voltage that they will come merely to a
bright reil when the machines are in phase. This arrange-
ment averts the possibility of a lamp burning out during
phasing and giving apparent concordance of phase. This
accident has actually happened — with spectacular results. In
large stations special synchronism indicators, of which more
in the next chapter, frequently replace phase lamps to good
purpose. It is not a bad idea to provide both as a safety
precaution.
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444 ELECTRIC TRANSMISSION OF POWER,
It is obviously necessary that the speeds of the two machines
should be normally alike, and that the speeds should have a
certain slight flexibility. When belt-driven from the same
shaft, the various generators to be put in parallel must be run
very accurately at the same speed, else one of the belts will
constantly slip and there will be considerable synchronizing
current. When properly adjusted, the machines should be so
closely at speed that the phase lamps will have a period of
from 20 to 30 seconds. This is not a difficult matter when
driving from the same shaft. In direct-coupled units, or in
general those driven from independent prime movers, it is best
to let one governor do the fine adjustment of speed, the others
being a little more insensitive. Otherwise the governors are
likely to fight among themselves and be perpetually see-sawing.
With respect to equality of voltage, the better the regula-
tion of the generators in themselves, the more necessary it is to
have them closely at the same voltage when put into, or when
running in, parallel. Two generators with bad inherent regu-
lation will divide the load with approximate equality, even if
put in parallel with a noticeable difference in voltage, since the
machine that tends to take the heavier current will promptly
have its voltage battered down and the tendency corrected —
at the expense, however, of accurate regulation in the plant.
With machines of low inductance and good regulation, the
voltages should be very closely the same before putting into
parallel, to avoid heavy synchronizing current, and they will
then divide the load correctly with a very slight adjustment
of the voltage. If the characteristics of the machines are
known, as they should be, the voltages can be arranged so
that they will fall together as accurately as if the added
machine had been put on an artificial load before parallelizing.
If these precautions are observed, no difficulty will be experi-
enced in parallel nmiiing, and machines in stations many
miles apart will work together in perfect harmony. This is
sometimes necessary in large central station work, when a
portion of the power is transmitted from a distance and a
portion generated on the spot. It sometimes happens, too,
that to obtain the amount of water-power that is desired, it
must be taken from a group of falls. In point of fact, it is a
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THE ORGANIZATION OF A POWER STATION, 446
perfectly simple matter to operate a number of transmission
plants in parallel, rather easier than so to operate the machines
in a single station. The inductance in a long line acts as an
electro dynamic buffer.
The magnitude of the transformer units, when transformers
are used, should be determined by the same considerations that
apply to generators, except that questions of speed do not
have to be considered. The smallest number of transformers
that it is desirable to use is that number which will permit
the disuse of a single unit without inconvenience. Above this
number one must be guided by convenience, but in general
the fewer units the better, since transformers such as are
used in large transmission work vary very little in efficiency
under varying load, and hence there is no considerable gain
in usmg small units so as to keep them fully loaded. When
using large transformers the difference in efficiency between
full load and half load should be no more than two or three-
tenths of a per cent, and as a rule the general efficiency can-
not be sensibly improved by using smaller units.
In polyphase transmission the transformer imit must be
taken to include all the phases, so that this unit will usually con-
sist of two or three allied transformers. In three-phase work
the circuit can be operated either with two or three trans-
formers, so that in a measure each transfonner group contains
a reserve of capacity, since, if a transformer fails, the remain-
ing pair can be connected to do nearly two-thirds of the work.
It is inadvisable, however, to try the resultant mesh on a large
scale save as an emergency expedient, and the raising and
reducing transformers should regularly be in groups of three for
three-phase work, connected star or mesh as occasion requires.
Very recently combined three-phase transformers have begun
to come into use, but there has not yet been experience enough
with them to give them a definite place in the art. For very
high voltage each phase may have several transformers in
series, although, since single transformers are now made for
60,000 volts, such a step is needless unless as an emergency
measure.
It is advisable in arranging the transformer plant, to bear
contingencies in mind. Spare transformers are a good form
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446 ELECTRIC TRANSMISSION OP POWER.
of insurance. In the station raising transformers alone are
concerned. These are likely to be of large capacity and high
voltage. The individual transformers will very seldom be as
small as 50 KW, and the voltage is sure to be from 5,000 volts
upward to 10,000, 20,000, or 30,000 volts, and sometimes even
more up to 60,000.
For large transformers, both the air-cooled and the oil-
insulated types are in common use. The former depend
wholly on solid insulating material and are cooled by a forced
blast through the ventilating spaces. The heat is so effectively
carried off by this means that the output can be readily forced
without any material loss of efficiency, and these transformers
are therefore somewhat less expensive than others. For
work up to 10,000 volts or so they are much used and prove
very satisfactory. For the higher transmission voltages, the
oil insulation gives a much larger factor of safety, and is there-
fore usually preferred. The smaller sizes are often merely
enclosed in a sheet-iron or cast-iron tank with very deep corru-
gations to gain radiating surface, and which is filled with pet-
roleum oil carefully freed from the least residual traces of acid
and water.
It is a curious fact that such oil seems sometimes to absorb
moisture from the air, and may even have to be given a sup-
plemental drying by blowing air through it. Generally, how-
ever, the oil furnished for this purpose by the manufacturing
companies is in good condition if kept in closed tanks or
barrels, but it is advisable to test for moisture in undertaking
any large use of it.
In the "self-cooled" transformers just referred to, the radia-
tion from the case is enough to keep the oil from getting too
hot, and for sizes up to two or three hundred kilowatts this
form of cooling is very generally used. For larger units the
natural circulation of the oil as it is warmed by the coils and
core is hardly sufficient, and forced cooling has to be employed.
This is generally accomplished by putting just inside the
boiler iron case of the transformer a coil of brass pipe through
which cool water is pumped or allowed to flow. This furnishes
excellent facilities for cooling, and is the plan very generally
followed in all the larger station transformers.
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THE ORGANIZATION OF A POWER STATION 447
Plate VI shows the general appearance of these artificially
cooled transformers, while Plate XVI, Fig. 3, shows a bank of
the air-cooled type, installed above a common ventilating flue
which receives air from a motor blower. Fig. 238 shows the
appearance of a self-cooled oil transformer for three combined
phases.
Although transformer oil has so high a flashing point as to
be practically non-inflammable under any ordinary provo-
cation, it may still be a source of danger when in considerable
quantity, and exposed to great and continued heat. It is
FlO. 238.
therefore wise to install oil transformers in such wise as to
prevent the spread of burning oil in case of serious fire. They
should, therefore, be isolated from inflammable material, and
provision should be made for draining off the oil in case of
necessity. The cases are usually provided with heavy cast-
iron covers through which the terminals come and which
protect the oil from access of flames or of air in case of short
circuits, which, by the way, very rarely ignite the oil.
It is a good plan to locate the transformers with drainage
spaces around them and exits through which the oil can harm-
lessly flow if, from a combination of accidents, it escapes from
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448 ELECTRIC TRANSMISSION OF POWER.
•
the transformer case. A sloping concrete floor recessed or
with low barriers to prevent spreading of oil, and a drainage
flue opening outside the building, is effective, as is also a large
drainage flue from the bottom of the case, capable of being
opened without going too near the transformer, which, of course,
should be cut out of circuit before attempting to drain it.
Another plan carried out in the Shawinigan Falls plant is
to make the transformer cover oil-tight, and to provide it with
a large pipe extending to a sewer. At the bottom of the case
is another large pipe connected with the water supply, so that,
in case of combustion inside, the water may be turned on and
force out the oil through the top. This avoids access of air,
and the temporary presence of water is not likely to do much
additional damage.
Air blast transformers also involve some risk, as the blast
faus any burning insulation, and once started combustion may
go slowly on long after the blast is shut off. They should
therefore always be installed on a non-combustible floor.
It is not a bad plan to insulate air blast transformers from
the earth somewhat carefully as an additional precaution, but
one should not place too much reliance upon this, since the
vital thing is the insulation of the evils themselves. In case
of oil-insulated transformers this precaution is rarely taken,
and when the water-cooled form is used the transformer
case is generally effectively grounded by the cooling
pipes. It is well to surround high-voltage transformers,
unless the outer cases are grounded, as in the water-
cooled type, with an insulated platform, and in general
high-voltage transformers should be treated with extreme
respect. The high-voltage leads in particular are likely to
require pretty close watching where they emerge from the
case, and should be taken out of the transformer house by a
simple and direct route. High-voltage transformers are gen-
erally given a room by themselves, sometimes a separate
building as in Figs. 234 and 235, but even if, as in many small
stations, they are located in the general apparatus room, they
should be scrupulously railed off and given the place where
they will do the least mischief in case of accident. As regards
the connections employed for the transformers, most American
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THE ORGANIZATION OF A POWER STATION, 449
plants employ three-phase transmission with a separate trans-
former for each phase. Whether these or combined three-
phase transformers are used, there are obviously many possible
methods of connection, since the primaries and secondaries of
each group may be either in star or in mesh, and the generator
may also be in star or in mesh.
In the star connection the voltage between either wire and
1
the neutral point is —i= or about 58 per cent of the working
voltage between wires, and if the neutral point be grounded the
maximum voltage between a wire and the ground is limited to
this amount. The voltage demanded of each transformer is
therefore reduced, and the strain upon the insulators likewise.
Operating with a grounded neutral, however, implies a more
or less serious short circuit of one phase in case of a ground
elsewhere upon the line, together with a flow of current
through the earth which may and sometimes does cause seri-
P
FlO. 239.
ous trouble to any grounded telegraph or telephone wires in
the vicinity.
With the mesh connection the strains upon the insula-
tion of transformers and line are higher in the proportion of
1
—7= : 1, but no single ground causes a short circuit or heavy
earth currents, and if one transformer of the trio is crippled the
other two can be connected in resultant mesh so as to deliver
somewhat more than half the original capacity of the bank.
An ungroimded star connection is very sensitive to grounds
and other faults, and the neutral point easily drifts so as to
greatly disturb the phase relations and voltages. This dis-
turbance affects the whole system, and may cause dangerous
rise of potential if the conditions are favorable.
The sort of thing which may happen is well exemplified in
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450
^LtlCTRtC TRANSMlSSlO]^ OF POWER.
Fig. 239. Here a generator voltage nominally 1,000 is raised
to 10,000 volts by a star-mesh combination, and lowered by
a mesh-star to a nominal 1,000 volts for distribution. The
raising and lowering star neutral points are grounded, but
the generator neutral is not. The diagram gives the distri-
bution of voltages when there is a ground on the low-tension
side of one raising transformer of which the high-tension coil
is open. The result evidently might be disastrous, even as
it is, and such an electro dynamic wrench would very possibly
provoke resonance or start formidable surging. On the face
of things such an accident would seem to be very improbable,
but it might easily happen if one undertook to cut out the
high-tension side of a damaged transformer during the prog-
ress of a bum-out m the coils.*
On account of such possibilities all three phases should be
opened and closed simultaneously and never by single switches,
unless in changing connections when all the lines are dead.
As regards the possible abnormalities of voltage due to accident,
the whole matter may be summed up by saying that the
regular mesh sjrstem is safe from them, and star connections
are also safe when grounded at the neutrals throughout the
system indnding the generator. Mixed connections of star and
mesh are likewise safe when grounded at the neutrals of every
star. The following combinations are commonly found in
practice.
Connections Thbouoh System.
Generator.
Raising
Transmission.
(Low tension.)
Raising
Transmission.
(High tension.;
Reducing
Transmission.
(High tension.)
Reducing
Transmission.
(Low tension.)
Mesh
Mesh
Star
Mesh
Mesh
Mesh
Mesh
Star
Mesh
Mesh
Mesh
Mesh
Star
Star
Star
Mesh
Mesh
Star
Star
Star
Mesh
star
Star
Mesh
Star
All these can be made thoroughly operative if all the neutrals
indicated are grounded. The first and fourth on the list are
perhaps rather more used than the others. The last three
• For valuable iuformation aloDg this line, see Peck. Trans. A. I. E. E.,
Vol. XX, p. 1248.
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THE ORGANIZATION OF A POWER STATION. 451
have a star connection on the main transmission circuit,
which considerably lessens the strain on the insulation, and
somewhat simplifies protection against lightning and static
disturbances, but at the cost of heavy short-circuiting in case
of a ground. Choice of a system depends very much on the
particular kind of risks likely to be locally met, and in many
plants the various connections are used just as occasion dictates.
The real question involved is the desirability of working with
a grounded neutral, which varies very much according to cir-
cumstances. In many cases it is advantageously used on a
large scale, while now and then the conditions seem decidedly
adverse.
It should of course be understood that transformers in power
transmission work can be, and very often are, worked in par-
allel with the greatest facility. Transformers to be so used
must have closely similar magnetic characteristics, and par-
ticularly must regulate alike under varying loads. They must
also have independent fuses or other safety devices, so that
each can take care of itself. In all cases it is highly desir-
able to have one or more spare transformers, ready to be cut
in at a moment's notice anywhere that may be necessary.
Where transportation is difficult, the installation of trans-
formers is rather a serious problem. Generally speaking, it is
best to sectionalize the coils, each section being independent
and fully insulated. The core plates caii then be taken in in
bundles and the transformers built up on the spot, with what-
ever additional insulation may be necessary. Of course
means must be at hand for the final testing, including a small
testing transformer to obtain the necessary voltage.
The most important accessories of a plant pertain to the
switchboard, which in high- voltage transmission work has to
be planned and constructed with extraordinary care. The
component apparatus will be considered in the next chapter.
The location of the board involves some troublesome con-
siderations. In small plants by far the best place for it is at
the general level of the generators and midway the power-
house wall opposite them. In plants with only three or four
generators it can sometimes well be placed at one end of the
building, as in Plate XVI, Fig. 3. Generally speaking, the best
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462 ELECTRIC TRANSMISSION OF POWER.
location is the most accessible, for in case of trouble it is usually
necessary to reach the switchboard on the instant. Hence
it should be close to and easily visible from the line of generators.
It should also be set so as to have good light both before and
behind, with plenty of room in the rear. All combustible
material should be eliminated from its vicinity.
The modem board is generally built up of marble panels, each
containing the apparatus for a single generator, with supple-
mentary panels for the apparatus pertaining to feeders and to
the plant as a whole. Behind the board are the necessary trans-
formers for the instruments, all the wiring, and all the high-
voltage connections. This arrangement implies ample room,
which is not always allowed for in designing the power
house.
In large plants it is now common to install the switchboard
in an elevated gallery overlooking the generator room, as in
Fig. 235. This, of course, necessitates a special attendant
constantly on the watch and alert. It is a very pretty arrange-
ment when everything is going well, but in case of extremity
the switchman cannot either see or hear as well as if he were
nearer the seat of trouble, and it necessitates a great deal of
heavy wiring, and high-voltage concealed wiring at that, which
is a source of some danger. On the whole, it seems inadvis-
able to use the gallery switchboard unless one is prepared at
the same time to use a complete system of remote control
switches, reducing the elevated board to a mere control desk
overlooking and facing the generators. If the switches are
also arranged for manual control in emergencies, such an
arrangement has much to commend it, but as a rule a board of
the moderate degree of complexity usual to power transmission
plants is none the better for being in a relatively inaccessible
gallery.
Whether the board is an elevated control board or a manual
board on the floor, there are certain often neglected precautions
which should be insisted upon. Whatever other switches are
installed, each generator should be equipped with a switch
betw^een it and the general connections of the board, as near
the generator as possible in fact, and able to break the cir-
cuit under the severest conditions. This should have both
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THE ORGANIZATION OF A POWER STATION. 463
manual and remote control, if for large output. There have
been a great many costly accidents in power houses because
somebody wanted a compact and handsome board, which
eventually short-circuited inside the switches.
The present tendency is to do as much of the switching
work as possible on the low-tension wiring, and to leave the
high-tension side of the transformers pretty much to itself.
The tendency is a healthy one, but in many cases there must
be provision for opening the high-tension circuits imder load.
Switches for such work are readily available at least up to 50,000
or 60,000 volts. The wiring of a transmission plant should be
kept as simple as feasible. In so far as is possible, aU the main
leads should be kept in full view, and when they must pass
out of view they should be insulated with extreme care, and
preferably carried in separate ducts which can be kept dry and
clean. One of the very common sources of trouble is found
in the cables, which some one with an obsession of neatness
hfts stored away too compactly. The worst shut-down which
has occurred in the great Niagara plant since it went into
operation was due to this cause. Cables running under the
floor to the switchboard are fertile soiu'ces of trouble and
should be avoided when possible.
In any event, the high-voltage wires should be in plain sight
all the way from the transformers to the exit from the build-
ing. It is far better to do without a permanent travelling
crane in handling the transformers, than to take the chances
that come with concealed high-voltage wires. In the Shaw-
inigan Falls plant already referred to, the transformers are
arranged so that they can be slid upon rails under a fixed
tackle, and a little ingenuity will usually make it possible to
locate the high-voltage wires and preferably the generator
leads also where they shall be in full sight.
All these things must be taken into account in building a
power station, since afterthoughts are apt to be costly and
ineffective. In designing a power transmission plant, every-
thing about the station should give way to utility, and the
aim of the designer should be to produce a building that shall
be convenient, accessible in every part, well lighted, and fire-
proof. If at the same time it is cheap to construct and of
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454 ELECTRIC TRANSMISSION OF POWER.
pleasing exterior, so much the better, but stations are not
intended for decorative purposes.
In the way of mechanical fittings, the first place is generally
given to a travelling crane, capacious enough to move every-
thing which is likely to need moving about the plant. Not
only is it exceedingly useful in installation, but it may be
needed for repairs, and in such case may save much valuable
time. It need not be of the most elaborate construction,
being only intended for occasional use, and, in view of possible
interference with the wiring, may sometimes well be reduced
to the simplest possible terms, merely a bridge to which
tabkle can be affixed when needful.
It is very important to have at least one man about the
plant who is a good practical mechanic, and to provide a work-
room and tool equipment enough to enable small repairs to
be made on the spot. In most cases material and tools for
minor electrical repairs are necessary, and they are always
desirable, for they make it possible to forestall further repai^p,
and often will tide over an emergency, even if outside help
has finally to be called in. The more isolated the station, the
more necessary it is to make such provisions, and the more
spare parts must be at hand. Of line material there should
always be plenty in stock to repair breaks, and this stock
should never be allowed to get low.
Finally, as regards attendance, incompetent men are dear at
any price. It pays to employ skilled men and to make it
worth their while to settle down to permanent work. They
are valuable all the time, and can be depended upon in an
emergency when less competent ones would fail. In this as
in other things, avoid the fault stigmatized in the vernacular
as "saving at the tap and spilling at the bimg-hole."
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CHAPTER XIL
AUXIUARY AND SWITCHBOARD APPARATUS.
In this category one may properly place a wide variety of
apparatus employed in generating and substations for all
sorts of purposes. A station implies far more than generators
and prime movers, although the choice and placing of these
with relation to the work to be done is the chief consideration
in station design.
After the generators the most important items in station
design are the exciter equipment and switchboard, subjects
merely outlined in the previous chapter. As regards the first,
certainty in operation is the main requisite. In some of the
earlier plants it was the custom to provide each generator
with an individual exciter generally belted to a pulley on the
generator shaft. This plan is objectionable in that any trivial
failure in the exciter may put the generator out of service,
unless an additional source of excitation is provided. Grant-
ing the necessity of such other source, one naturally falls into
the judicious present practice of providing two or more exciters
driven from independent prime movers, and each large enough
to supply, if occasion requires, exciting current for all the
generators or for a considerable group of them.
Speaking in general terms, the exciting energy required is
from 1 to 3 per cent of the full generator output, and as it is
good policy never to work an exciter very hard, a con-
siderable margin of capacity is desirable.
As a i-ule, it is well to install exciters of moderate speed,
directly coupled to independent water-wheels in case of hydrau-
lic stations. The wheels should be provided with first-class
regulators and installed in such wise that they ^vill not be
interfered with by any ordinary hydraulic difficulties. Of
late it is not unusual to find one or more motor-driven exciters,
a motor generator set or sets being installed in addition to
the wheel-driven equipment. The motors are induction mo-
465
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456 ELECTRIC TRANSMISSION OP POWER,
tors designed for very small variation of speed with load, and
supplied with current from the general bus bars of the system.
This practice has both good and bad features. Its strong point
is that in case of hydraulic troubles, anchor ice for instance,
the small wheels may be considerably affected, making it very
difficult to hold up the voltage. On the other, hand, in case
of trouble on the lines, one is better off with an independent
drive for the exciter. In any case, the exciter fields should
be given a liberal margin of capacity, so that in case of reduced
speed the voltage can be easily kept up.
In steam-driven stations the motor-driven exciter is a source
of some economy, since small steam engines are generally
uneconomical, while the losses in the motor are comparatively
small. However driven, the exciters should be so connected
as to allow them to be interchanged at a moment's notice.
To facilitate this, one should never rely on a single exciter in
operation, but should keep a spare exciter ready for action,
even if it is not actually at speed and running in parallel
with the one in use.
The exciter panels on'the switchboard should occupy a prom-
inent and accessible position, since, if an3rthing goes wrong
with the exciting circuits, it must be remedied at once. The
location of the exciters themselves is immaterial, save as
they should be placed where they can be easily inspected
and cared for.
The subject of exciters naturally leads to that of voltage
regulation. The division of the total regulation between the
generating plant and the substations is always a somewhat
dubious matter. On long lines in which the total loss is con-
siderable, the work is generally divided, the coarse regulation
of the plant as a whole being done at the power plant, and the
feeder regulation at the substation. If the power station can
hold constant voltage at the secondary bus bars of the reduc-
ing transformers, a long step toward good regulation will have
been taken. This can be done by hand regulation, but at the
present time there are several automatic regulators quite
capable of doing the work with sufficient precision. Several
forms of automatic compounding have already been considered,
but the regulators proper are instruments responsive to varia-
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AUXILIARY AND SWITCHBOARD APPARATUS. 457
tions of voltage from whatever cause, and operating to compen-
sate for variations of speed as well as of load and power factor.
They may be worked by pressure wires coming back from the
load, or by the station secondary voltage compensated for
variations in load.
They are essentially voltmeter relays acting on the excita-
tion of the generators or exciters. One of the best known
forms is the Chapman regulator, of which the typical connec-
tions are shown in Fig. 240. The relay, it will be noted, is
FlO. 240.
compensated by a variable winding carrying current from a
series transformer, which enables the voltage to be held con-
stant, through to the end of the line, and to, or even beyond,
the reducing secondaries if necessary. The main automatic
rheostat is controlled by this relay, and is placed ordinarily
in the field circuit of the generator, or in the field of the exciter
if the load variations are not likely to be extreme. It is very
prompt in action, and nearly dead beat.
The relay is ordinarily adjusted to hold the equivalent
secondary voltage constant to about one-half of one per cent,
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458
ELECTRIC TRANSMISSION OF POWER,
or doser if necessary, and the whole arrangement is simple and
effective.
Another excellent and very ingenious voltage regulator is
made by the General Electric Co. Its connections are shown
in diagram in Fig. 241 as applied to a single generator and its
exciter. The fundamental method employed is the opening
FiO. 241.
and closing of a fixed shimt around the field of the exciter.
At first thought, this would seem to vary the excitation by
leaps, but the play of the relays keeps the magnetization steady
by catching it before it has gone further than needful, the
natural inertia of the magnetic changes giving the requisite
amoimt of stability to the process. The d.c. control magnet
checks too extensive changes without throwing the work on
the voltage relay proper, which is provided with an adjust-
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AUXILIARY AND SWITCHBOARD APPARATUS. 459
able compensating winding derived from a series transformer.
The office of the condenser is to relieve sparking at the main
contacts.
Both these regulators give excellent results in practice and
have proved thoroughly reliable. They take care of both
variations in speed and in load, but under extreme variations
of power factor the series windings on the relays may require
some readjustment. They can be applied to many problems of
automatic regulation with admirable results. In somewhat
simpler forms they have been considerably used in regulating
direct current generators, particularly when driven from water-
wheels, in which case the effect of small changes of speed may
have to be guarded against. Their use is extending, although
many large plants depend entirely upon hand regulation,
which, if carefully carried out, gives first-claas results when the
load does not vary too erratically.
Some general suggestions on switchboards have already
been given, in addition to which it is worth while to examine
the principles which underlie switchboard design.
The whole purpose of a switchboard is to make easy the
changes in connections necessary in the practical operation
of a station. It is not to supply architectural effects in polished
brass and marble, or to furnish employment for extra attend-
ants. The fundamental switching operations are compara-
tively simple, and beyond these there are some which are
desirable and others which are of more or less fanciful value.
Likewise, in the matter of switchboard instruments, some are
necessary, others desirable, and still others mere casual conven-
iences. In the interest of economy and easy operation, it is
well to keep the design simple, for it is a perfectly easy matter
to double the necessary cost of a board without making any
compensating gains.
The first thing for which provision must be made is the
connection of the several generators to the bus bars. The
next is for the connection of these to the lines or to the low-
tension sides of the several transformers. If the latter, the
third requisite is the connection of the high-tension sides of the
transformers to the several lines. As a general rule, the less
switching done at high tension the better, but it is at times
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ELECTRIC TRANSMISSION OF POWER,
necessary. Beyond this, provision must be made for the
excitation of the generators, their operation in parallel, and
the measurement of their output to guard against overloads.
Then, in addition, there may be an almost endless array of
accessories and provisions against more or less remote con-
tingencies. Fig. 242 gives diagrammatically the elementary
switching connections for a transmission power station feeding
Li
!i
X.
— j_j III
i/
r/jf xM )(j(p
Fig. 242.
duplicate lines. Here E^ E^ are exciters, G^ Gj ^z *^® genera-
tors, T^ r, Tj banks of transformers, and Lj Lj the lines.
A is the row of excitation switches, B the generator switches,
C the low-tension transformer switches, D the high-tension
transformer switches, and F the high-tension line switches.
H, /, J, are the generator, transformer, and exciter bus bars
respectively.
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AUXILIARY AND SWITCHBOARD APPARATUS. 461
The plant is operated in parallel throughout, and the switch-
ing requirements are of the simplest kind. The moment
parallel operation is abandoned and separate generators are
assigned to separate duties, switchboard complication begins.
Thus, if one introduces the requirement that the lines Lj Lj
shall be operated entirely independently of each other, in order
that the service shall be interchangeable in the station the
bus bars H, /, must be in duplicate, the switches B, C, Z>,
must be double-throw or in duplicate, and the switches F must
be in duplicate. If still more lines are to be entirely indepen-
dent, the complication increases at a frightful rate, and can
only be avoided at the cost of lessened interchangeability.
As high-tension switching devices are expensive, the expense
incurred for complete interchangeability may nm up to a
good many thousand dollars, and the case is generally com-
promised.
When there are many transformers located at some distance
from the generators, the connections are often made by dupli-
cate cables from H to a low-tension distributing bus bar at
the transformer board. The main thing is to make the switch-
ing connections as simple as is consistent with security of
operation under the required conditions.
Even in Fig. 242 there are required 11 heavy switches, 5
of them being for the full line voltage, and in theory any one
of the 11 may have to open its circuit on an overload.
In most transmission plants, switching at the high voltage
under load is avoided as far as possible, and the switches at D,
and also sometimes at F, are merely disconnecting switches
often with fuses as a protection against overloads. Such dis-
connecting switches should always for polyphase circuits
make a complete break of all phases to lessen danger from
surging, single-pole switches being reserved for cases safe from
the necessity of opening under load.
Fuses for transmission circuits are somewhat troublesome
at the higher voltages, but serve a useful purpose in an emer-
gency, by cutting off severe overloads. They are disadvan-
tageous in that they may open but a single leg of the circuit,
and that in a way to provoke surging, but they only come into
play in extreme cases when there is trouble ahead, anyhow.
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462
ELECTRIC TRANSMISSION OF POWER.
The types used most are expulsion fuses, enclosed powder
fuses, and very long wire fuses often enclosed in glass tubes.
Moderate voltage plants, say up to 20,000 volts, are readily
safe-guarded by fuses, but at higher pressures more caution
is necessary. But fuses are so much cheaper than any form
of high- voltage overload switch yet devised — a few dollars
Fig. 243.
as against a few hundred — that they cannot be lightly put
aside.
As to switches, the oil break type is by far the most reliable
for breaking considerable currents at high voltage. The
arcing power of a high-pressure alternating circuit is so tremen-
dous that in air switches the gaps must be very long, even to
a good many feet, to entirely prevent any chance of the arc
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AUXILIARY AND SWITCHBOARD APPARATUS. 463
holding. Now and then the opening switch may catch the
circuit nearly at zero current and open with very little disturb-
ance, although as a rule the effects are somewhat pyrotechnic
in appearance. The switch breaking under oil on the contrary
very rarely makes any noticeable disturbance and seldom
starts severe surging. It is the best means of opening a high-
voltage circuit, and is almost imiversally used for the principal
work of the power house.
Fig. 243 is a typical oil switch for voltages of 13,000 and
below. It is a three-pole double break switch of the plunger
form, shown in the cut with the oil tank in which the contacts
Sonrce
OverloAd CqH
FlO. 2M.
are submerged, removed for inspection. The plunger is operated
by means of a toggle-joint from a hand lever on the front of
the board, and the example here shown is also fitted with an
electro-magnetic release which can be operated from a remote
point, or which, if energized by series transformers in the main
circuit controlled, can convert the switch into an automatic
circuit breaker for overloads. Such switches are very prompt
and certain in their action, and serve admirably for the main
generator switches, or for line switches at moderate voltage.
As generator switches the automatic overload device is not
needed in most instances, as station operators ordinarily pre-
fer to keep the generators in action through any but very
severe and prolonged overloads.
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ELECTRIC TRANSMISSION OF POWER.
Not infrequently, time limit relays are applied to such
automatic switches, arranged with an adjustable dash pot so
that an overload lasting less than a predetermined number of
seconds will not cause the opening of the switch. In still
another modification, this limit is automatically shortened in
case of very severe overloads. The ordinary connections of
Fig. 243 as an automatic switch are given by Fig. 244.
At the present time there is a rather strong tendency, often
carried to an extreme, to arrange these important switches for
remote control, the switches being located back of or otherwise
away from the board and operated by a small actuating switch
FlO. 245.
on the board. This has been a natural outcome of placing
large and complicated boards in a gallery with limited room
at the immediate back of the board. It is often inconvenient
to place large oil switches on the board itself, and it is not a
bad plan to connect them to the board by operating levers
which will allow the switches to be placed where they have
ample room, as in Fig. 245.
Less commonly, the conditions call for a switch entirely
operated by electric power. Such switches are generally those
for very high voltage or very large output, which are bulky
and require elaborate insulation. They are intended to be
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AUXILIARY AND SWITCHBOARD APPARATUS, 466
Fig. 246.
operated from the board or by hand if necessary, and are
placed in a safe and convenient position quite irrespective of
the board.
Such a remotely controlled switch as made by the General
Electric Company, for voltages even up to 60,000 and at
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ELECTRIC TRANSMISSION OP POWEH,
lower voltage for very large outputs, is shown in Fig. 246. It
is a three-pole double break affair, with each break in a sepa-
rate oil tank, and each phase in a separate brick compart-
ment. It is operated by a d.c. series motor generally worked
from the exciting circuit and controlled from the switchboard
to which its operation is signalled back. Fig. 247 shows the
connections of this form of apparatus, which is used chiefly
rCuc.
5v
■«i
Red ladlcailBK Lump
.y^ (Oil Switch CIOMd)
CIoalDg CoBUct
Op«BlnK ConUet '
>Gre«B ladicatiBK Lamp
(Oil Switch OpcD)
lis Volt BtiBM
Sarlti Motor
'^OponllBic Of 1 Switch
g Clutch UacBct Coll
AatoniKlic Coatacl Flagtra
Caaa Actuated
on Switch IB CIoBod Pmttion
Fig. 247.
for very heavy work. One great advantage of remote con-
trolled switches, seldom, however, realized, is for control of
the individual generators as at 5, Fig. 242. By going to
remote control it is easy to locate the switches right at the
generators, so that no trouble at the board can short-circuit'
the generator inside the switches. This accident has happened
a good many times with disastrous results, for the switch-
board and its connecting cables are by no means an insignifi-
cant source of danger.
In Plate XIX is shown a Westinghouse remote control switch
for large work at 60,000 volts. It is operated not by a motor
as in Fig. 246, but by powerful solenoids actuating the switch
mechanism directly. Switches for such high voltage should
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PLATE XIX.
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AUXILIARY AND SWITCHBOARD APPARATUS. 467
be operated very carefully, and the leads to and from them
must on account of the voltage be elaborately insulated and
located with extreme caution. By the use of electrically
operated switches, it is sometimes possible to simplify the
high-tension wiring very considerably. They can obviously be
made automatic if necessary or desirable. They are, however,
very expensive, and on this accoimt should be used only where
there is very good cause for choosing them as against hand-
operated switches. Electric actuation in itself is an additional
complication, rendering switching easier but somewhat less
direct and certain, and should be resorted to only when the
rqpz^^rz^^pirj
FlO. 248.
total complication is thereby materially diminished. In
installing high-voltage oil switches, they should be provided
at some point with disconnecting switches to facilitate inspec-
tion and repairs by cutting them clear from the circuits.
A capital. air switch for disconnecting duty and for opening
lines under moderate load was described recently by Professor
Baum, as in successful use up to 60,000 volts by the California
Gas and Electric Co. It is a three-blade switch mounted on
high-tension insulators and arranged for operation by a lever
and long connecting rods. Fig. 248 shows the detail of a
single blade. An outdoor switch of similar construction,
made double break, is shown in Fig. 249. This is used for dis-
connecting load up to say 1,000 K.W, while the former pattern
is used for connecting the high-tension side of transformers to
the bus bars. The main thing in designing such switches is^^
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ELECTRIC TRANSMISSION OF POWER,
to give them ample insulation and a long, quick break. In
the practice of The California Gas and Electric Co., no fuses
or circuit-breakers are used at the power plant, but the trans-
formers are fused at the substations.
In the operation of important stations the tendency is to
hold on as long as practicable in case of a short circuit on the
lines, on the chance of the line clearing itself without com-
pelling a shut-down. Hence automatic safety devices are
rather sparingly employed. In many respects the working of
long-distance power transmission plants is peculiar. With
many miles of lines in circuit, a fault, even if it could be quickly
located^ cannot often be quickly reached. The lines are few in
^m
g ^^ "5
c
Fig. 249.
number and heavily loaded, each supplying energy over a great
area. From the consumer's standpoint it is quite as bad to
have a line switch opened as to have a line burned off by
keeping the switch shut. There is a chance of burning off a
short-circuiting twig, for instance, before the line itself fails,
and that chance is usually taken — and wisely.
The instruments used in power transmission stations are
kept almost entirely on the low- voltage side of the equipment.
Those needed for each generator are customarily located on
a standard panel, and these panels are imited into a complete
section of the switchboard. As the generators are in plants
using raising transformers seldom for higher voltage than 2,000'
to 2,500, the provision of instruments involves no difficulties.
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AUXILIARY AND SWITCHBOARD APPARATUS, 469
and the equipment of each panel is about as follows in case of a
three-phase machine:
3 Ammeters.
1 Voltmeter.
1 Plug for connecting voltmeter to any phase.
1 Main Switch.
1 Plug for connecting synchronizing device.
1 Disconnecting Switch for isolating main switch.
1 Field Rheostat.
1 Field Switch.
1 Field Ammeter.
To this list is sometimes added a wattmeter, either indicat-
ing or integrating, as the case may be. In assembling these
into a generator board, there are added the exciter panels,
each carrying a voltmeter, ammeter, field rheostat, and main
switch, and in case of motor-driven exciters extra motor panels
with the appropriate instruments. The necessary potential
transformers, current transformers, oil switches, and heavy
accessories are located behind the board, no high-voltage
parts being allowed upon the front of it.
For the whole plant there is provided a suitable sjmchroniz-
ing device, and to this may be added a frequency indicator
and a power factor indicator, both of which are convenient,
although neither is necessary. Besides the panel carrying
these, there may be others in large systems providing for
switchmg groups of generators upon a general main set of bus
bars, perhaps itself sectionalized. These latter complications,
however, are seldom found in power transmission plants
engaged in ordinary general service.
At present, the once universal phase lamps are commonly
used only as auxiliaries, the real work falling upon the "sjm-
chroscope" or "synchronism indicator" which is far more con-
venient. Such a device is manufactured both by the Westing-
house and General Electric companies. The latter form is
shown in Fig. 250. Externally it consists of a case with a dial
and pointer. When connected to the bus bars and to the in-
coming machine, the direction of rotation of the pointer shows
whether the incoming machine is rimning too fast or too slow,
a complete revolution meaning the gain or loss of one cycle.
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ELECTRIC TRANSMISSION OF POWER,
The amount of displacement therefore indicates the phase angle
of the incoming machine, and when the pointer is steady at
zero the machines may be thrown together. Internally the de-
vice is essentially a pair of rudimentary induction motor fields,
each energized from one of the machines to be parallelized, and
acting in opposite directions upon a common armature attached
to the pointer. In polyphase circuits the arrangement is very
simple. For application to single phases the fields are con-
FlG. 2B0.
nected as split-phase motors by means of a combination of
resistance and reactance. Obviously the device can then be
used with either single-phase or polyphase circuits, and such
is the usual form of the instnunent, which is often mounted
adjacent to a pair of voltmeters, one on the main circuit and
the other capable of connection to any machine by means of
a system of potential plugs.
In the practice of the Westinghouse Co., a very ingenious
scheme of automatic synchronization has been worked out,
whereby at the proper moment a relay closes the actuating
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AUXILIARY AND SWITCHBOARD APPARATUS, 471
circuit of an electrically operated main switch. The arrange-
ment is shown diagrammatically in Fig. 251 as applied to syn-
chronizing a rotary converter. The relay scheme is very in-
genious, being a balanced and slightly damped lever actuated
by opposed coils from the synchronizing circuits. As syn-
chronism is approached, the resulting pulls alternate at lower
and lower frequency and are more effective against the damp-
ing, until finally, when synchronism is reached, the relay cir-
cuit closes and the machines are thrown together with the
utmost precision. In many cases a refinement of this sort is
Thr*t-PbaM Bai Ban
CBB^rou.Tawucii^ljr
1W7a1i*jl»r«rt Curmrt
folrtitttil TraDtfoTiEwn
Pig. 251.
quite unnecessary, in others it is likely to prove exceedingly
convenient.
The power factor indicator is a comparatively recent addition
to station equipment, but one that is most serviceable in station
operation. In particular it gives a very valuable check on the
regulation which we have seen varies greatly with the power
factor. It also enables one very readily to adjust rotary con-
verters and synchronous motors for minimum input. It is
essentially an instrument for balanced circuits, and is graduated
to read power factors directly on such circuits. It is essentially
a differential combination of wattmeter and volt-ampere meter.
The frequency indicator is very convenient in detecting any
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ELECTRIC TRANSMISSION OF POWER.
tendency to vary from the normal periodicity. In principle it
is a voltmeter specialized so as to be hypersensitive to varia-
tions of frequency, and with a scale graduated to these varia-
tions while being relatively insensitive as a voltmeter when
near its rated normal voltage by reason of relatively great
reactance. It might well be given an extra scale, in stations
having imiform generators, fitted to read generator speed
directly.
Ground detectors used to be a regular part of station equip-
ment, but as transmission voltages have risen these instruments
FlO. 252.
have become more and more difficult safely to apply, so that
as regards high-voltage circuits they are little used, grounds
making themselves all too obvious without special instruments.
Up to 10,000 volts or so, and especially on cable circuits, they
may be of considerable service. Fig. 252 is one of the common
forms for a three-phase circuit. It, like most of its class, is an
electrostatic instrument with an electrometer leaf and pomter
for each phase.
Most station instruments are now made with illuminated
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AUXILIARY AND SWITCHBOARD APPARATUS.. 473
dials, and are very frequently put up in edgewise form so as
to economize space on the switchboards. They should be
checked by standard instruments at frequent intervals, as
even the best of them are liable to get out of order occasionally.
The lighting of a station is a simple enough matter, the
main consideration being to leave no dark comers. It is good
policy, whatever the ordinary source of current, to have inde-
pendent means of throwing part at least of the lights upon an
exciter circuit so that an accident will not leave the station in
darkness.
As already intimated, there is a wide range of possibility in
supplying stations with instruments, switches, and auxiliary
equipment generally. With respect to automatic devices of
various kinds especially there is much room for difference of
opinion. If too liberally supplied, there may be reached a
point where the care of them is more onerous than the func-
tions which they assume. There is also some danger that
the station staff in depending upon them will lose something
of alertness. And in any case, it must be remembered that
even the best automatic devices may go wrong, and that
manual means of control should always be ready for use if
necessary, and that without delay.
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CHAPTER XIII.
THE LINE.
The line is a very important part of a power transmission
system, for on its integrity depends the continuity of service
without which even the most perfect apparatus is commer-
cially useless. In most cases the customer who uses electrical
power neither knows the efficiency of his motor nor cares
much about it, so long as the machine goes steadily along
without the annoyance and expense of frequent repairs. But
if the service frequently fails, suspending the operation of all
his machinery while repairs are being executed, the electric
motor, so far as he is concerned, is a commercial failure, and
a auisance to boot, and no representations of cheap power
can be of much avail when a single stoppage may cause more
loss than could be recompensed by free power for a month.
Modern dynamos and motors of almost every class are
reasonably efficient and reliable, so that as a rule the line is
the weakest portion of the system. More particularly is this
the case when the distance of transmission is great and many
miles of line must be guarded, inspected, and kept in perfect
working order. In such long lines, not only is the actual labor
of maintenance great, but the principal engineering difficulties
will there be encountered. With apparatus of the character
even now available, the future of electrical power transmission
depends in very large measure on the development that takes
place in the construction, insulation, and maintenance of the
line, together with the solution of certain electrical problems
that arise as the line grows longer. It is therefore important
to go into the matter very carefully, as regards not only the
general arrangements and the electrical details of the work,
but with respect to methods of construction.
We may then with advantage divide our consideration of the
line into three heads. First, the line in its general relations
to the plant, considering it merely as a conductor. Second,
the line as a special problem in engineering. Third, the line
474
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THE LINE. 476
as a mechanical structure. Of these heads the first has to
do with such questions as the proper proportioning of the line
as a part of the system, its function as a distributing con-
ductor, and its bearing on the general efficiency of the plant
of which it is a part. Next come up for examination the
electrical difficulties that appear m the line, and finally the
materials of construction and the methods of applying them.
One of the first questions that arises in designing a plant
for the transmission of power, is the character and dimensions
of the conducting system in their relation to the rest of the
plant. Efficiency is generally the first thing considered — cost
comes as a gloomy afterthought; and between these two, good
service is only too frequently neglected. In taking up a trans-
mission problem, the layman's first quer}'^ generally is, "How
much power will be lost in the line?" and when the engineer
answers, "As much or as little as you please," the subject of
line design is opened up in its broadest aspect.
Whenever an electrical current traverses a conductor, there
is a necessary loss of energy due to the fact that all substances
have an electrical resistance which has to be overcome. The
energy so lost is substantially all transformed into heat, which
goes to raising the temperature of the conductor, and indirectly
that of surrounding bodies. The facts in the case are put
E
in their clearest and most compact form by Ohm's law, C = — .
R
This states that the current is numerically equal to the elec-
tromotive force between the points where the current flows,
divided by the resistance. Hence, this E. M. F. equals the
current multiplied by the resistance between these points.
Thia tells us at once the loss in E. M. F. between the ends
of any line, provided we know the current flowing and the
resistance of the line. And inasmuch as the energy trans-
mitted by the same current varies directly with the working
E. M. F., a comparison of the loss in volts determined as above,
with the initial E. M. F. applied to the circuit, shows the per-
centage of energy lost in the line. Obviously its absolute
amount in watts is equal to the volts lost, multiplied by the
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476 ELECTRIC TRANSMISSION OF POWER.
current; i.e., CEj or from the last equation (PR if we prefer to
reckon in terms of resistance. As the loss of energy varies
with the square of the current, halving the current would
divide the absolute loss by four, and the percentage loss by
two, since the total energy is proportional to the current, the
E. M. F. being fixed.
A glance shows that the voltage employed is the determin-
ing factor in the cost of the lines. For a fixed percentage of
voltage loss, doubling the working voltage will evidently divide
the amount of copper required by four, since the current for
a given amount of energy will be reduced by one-half, while
the actual volts lost will be doubled in maintaining the fixed
percentage.
So in general the amount of copper required for transmit-
ting a given amount of energy a given distance at a fixed effi-
ciency, will vary inversely as the square of the voltage.
If the distance of transmission is doubled, the area of the
conductor will evidently have to be doubled also; conse-
quently, since the length is doubled, the weight of copper will
be increased four times. That is, for the same energy trans-
mitted at the same per cent efficiency and the same voltage,
the weight of copper will increase directly as the square of the
distance. The advantage and, indeed, necessity of employing
high voltages for transmissions over any considerable distance
is obvious. In fact, it will be seen that by increasing the
voltage in direct proportion to the distance, the weight of
copper required for a given percentage of loss will be made a
constant quantity independent of the distance.
If one were free to go on increasing the voltage indefinitely
without enormously enhancing the electrical difficulties, power
transmission would be a simple task, but unfortunately such
is not the case. With very high voltages we meet difficulties
both in establishing and maintaining the insulation of the line,
and in utilizing the power after it is successfully transmitted.
The specific character of these limitations will be discussed
later, but enough has been said to render it evident that in
establishing a power transmission system, both the working
voltage and the volts lost in the line must be determined with
great judgment.
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THE LINE. 477
In the matter of economy in the line, high voltage is desir-
able — first, last, and always. In systems where the voltage
undergoes no transformation, its magnitude is somewhat arbi-
trarily fixed by the practicable voltage which can be employed
in the various translating devices, motors, lamps, and the like.
For example, in a system at constant potential wherein incan-
descent lamps are an important item, 125 volts, or 250 volts
as an extreme figure, would be the highest pressure advisable
for the receiving system in the present state of the art; or in
certain cases where cheap power can be had, these voltages
might be doubled, and 220 to 250 volt lamps used on ^ three-
wire system. For a direct-current-motor system the corre-
spondmg figure would be 500 to 600 volts or 1,000 to 1,200
worked three-wire. Similar limitations indicated elsewhere
will hold for other classes of apparatus.
When there is a transformation of voltage m the system,
whether direct or alternating current, so that the line voltage
is not fixed by that of the translating devices, it is advisable to
raise the voltage of transmission as high as the existing state
of the art permits. It must be borne in mind, however, that
this general rule is subject to modification by circumstances.
It would be bad economy, for instance, to use very high pres-
sures and costly insulation for a transmission of moderate
length and trifling magnitude. Such practice would result in
sending perhaps 100 KW over a line or through a conduit
which could as easily serve for ten times the power without
great additional cost for copper. It is well, however, not to
stop at half-way measures, but, if transforming devices are to
be used at all, to go boldly to the highest voltage which experi-
ence has shown to be safe on the line, or in the generators, if
only reducing transformers are used.
For example, in most cases . of alternating current work,
1,000 volts is entirely obsolete; if the line voltage has to be
reduced at all, it is better to get the advantage of 2,000 to
12,000 volts on the line; if raising and reducing transformers
are employed, the latter figure might as well be increased to
20,000 or 40,000, unless climatic or other special conditions are
unfavorable.
It will be seen that, quite aside from engineering details,
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478 ELECTRIC TRANSMISSION OF POWER.
divers really commercial factors must enter into any final
decision regarding the voltage to be used. And these com-
mercial factors are the final arbiters as to the working voltage,
and even more completely as to the proportion of energy
which it is desirable to lose in the line. Power transmission
systems are installed to earn money, not to establish engineer-
ing theses.
It is evident, to start with, that whatever the voltage, high
efficiency of the line and low first cost are in a measure mutu-
ally exclusive. The former means large conductors, the
latter small ones; the former delivers a large percentage of
salable energy, with a high charge for interest on line invest-
ment; the latter a smaller amount of energy, with a lessened
interest accoimt against it. At first sight it would seem easy
to establish a relation between the cost of energy lost on the
line and the investment in copper which would be required to
save it, so that one could comfortably figure out the conditions
of maximum economy.
In 1881, Lord Kelvin, then Sir William Thomson, attacked
the problem and propoimded a law, known often by his name,
which put the general principles of the matter in a very clear
light, but which indirectly has been responsible for not a little
downright bad engineering.
He stated, in eifecrt, that the most economical area of con-
ductor will be that for which the annual interest charge equals
the annual cost of energy lost in it.
While it is true that for a given current and line, Kelvin's
law correctly indicates the condition of minimum cost in trans-
mitting said current, this law has often caused trouble when
misapplied to concrete cases of power transmission, in that
it omits many of the practical considerations. It involves
neither the absolute value of the working voltage nor the dis-
tance of transmission, and for long transmissions at moderate
voltage often gives absurd values for the energy lost. Indeed,
as it deals directly only with the most economical condition
for transmitting energy, it quite neglects the amount of energy
delivered. In fact, one may apply Kelvin's law rigidly to a
concrete and not impossible case, and find that no energy to
speak of will be obtained at the end of J;he line.
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THE LINE, 479
In other words, Kelvin's law, while a beautifully correct
solution of a particular problem, is in its original form totally
inapplicable to most power transmission work.
Various investigators, notably Forbes, Kapp, and Perrine,
have made careful and praiseworthy attempts so to modify Kel-
vin's law as to take account of all the facts; indeed, nearly every
writer on power transmission has had a shy at the problem.
Perhaps the commonest attempt at improvement is to follow
the general line of the original law, but to equate the interest
charge on copper to the annual vcdv^ of the power lost; in
other words, to proportion the line by increasing the copper
imtil the annual net value of a horse-power saved in the line
would be balanced by the interest charge on the copper re-
quired to save it. This proposition Soimds specious enough
at first hearing. Practically, it produces a line of greater
first cost than is usually justified. It is evident that the pos-
session of a little extra power thus saved brings no profit
unless it can be sold, and in few cases is a plant worked close
enough to its maximum capacity during the earlier years of
its existence to render a trifling increase in output of any
commercial value, especially in the case of transmission from
water-power. When the plant is worked at a very high cost
for power, or soon reaches its full capacity, a few horse-power
saved in the line will be valuable; but far oftener, particularly
in water-power plants, it would be cheapcyr to let the addi-
tional copper wait until the necessity for it actually arises.
Furthermore, it evidently does not pay to so increase the line
investment that the last increment of efficiency will bring no
profit.
As an example, let us suppose the case of a 1,000 HP trans-
mission so constituted that the line copper costs $10,000 with
10 per cent loss of energy in the line, and suppose in addition
that the net value of 1 HP at the receiving end is $50 per
annum. It is evident that by decreasing the loss in the line
to 2i^ per cent there would be available 75 additional HP
worth $3,750 per annum. The cost of this addition to the
line would be $30,000, on which interest at 6 per cent would
be $1,800. So long as the plant is not worked up to 90 per
cent of its maximum capacity of 1,000 HP, there will be a
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480 ELECTRIC TRANSMISSION OF POWER.
steady charge of $1,800 plus depreciation, if the additional
copper be installed at the start. A few months' loss at this
rate would more than cover the labor of reinforcing the line
when needed, even supposing that installing the additional
copper at the start would not have involved extra labor in
construction.
Various formulae for designing the line so as to secure the
minimum cost of transmission have been published, derived
more or less directly from Kelvin's law, and attempting to
take into account all the various factors involved in line effi-
ciency. They all contain quantities of very uncertain value,
and hence are hkely to give correspondingly inexact results.
More than this, they are founded on two serious misconceptions.
First, they generally give the minimum cost of transmission,
which is not at all the same thing as the maximum earning
power on the total investment. Second, however fully they
take account of existing conditions, the data on which they
are founded refer to a particular epoch, and are very unre-
liable guides in designing a permanent plant.
A few years or even months, may and often do so change the
conditions as to lead to a totally dififerent result. In the vast
majority of cases it is impossible to predict with any accuracy
the average load on a proposed plant, the average price to be
obtained for power, or the average efficiency of the translating
devices which will be used. So probable and natural a thing
as competition from any cause, or adverse legislation, will
totally change the conditions of economy.
For these reasons neither Kelvin's law nor any modification
thereof, is a safe general guide in determining the proper
allowance for loss of energy in the line. Only m some specific
cases is such a law conveniently applicable. Each plant has
to be considered on its merits, and very various conditions are
likely to determine the line loss in different cases. The com-
monest cases which arise are as follows, arranged in order of
their frequency as occurring in American practice. Each case
requires a somewhat different treatment in the matter of line
loss, and the whole classification is the result not of a priori
reasoning, but of the study of a very large number of concrete
cases, embracing a wide range of circumstances and covering
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THE LINE. 481
a large proportion of all the power transmission work that has
been accomplished or proposed in this country.
Case I. General distribution of power and light from water-
power. This includes something like two-thirds of all the power
transmission enterprises. The cases which have been investi-
gated by the author have ranged from 100 to 20,000 HP, to be
transmitted all the way from one to one hundred and fifty miles.
The market for power and light is usually uncertain, the propor-
tion of power to light unknown within wide limits, and the total
amount required only to be determined by future conditions.
The average load defies even approximate estimation, and as
a rule even when the general character of the market is most
carefully investigated little certainty is gained.
For one without the gift of prophecy the attempt to figure
the line for such a transmission by following any canonical
rules for maximum economy is merely the wildest sort of
guesswork. The safest process is as follows: Assume an
amount of power to be transmitted which can certainly be
disposed of. Figure the line for an assumed loss of energy at
full load small enough to insure good and easy regulation,
which determines the quality of the service, and hence, in
large measure, its growth. Arrange both power station and
line with reference to subsequent increase if needed. The
exact line loss assumed is more a result of trained judgment
than of formal calculation. It will be in general between
5 and 15 per cent, for which losses generators can be con-
veniently regulated. If raising and reducing transformers are
used the losses of energy in them should be included in the
estimate for total loss in the line. In this case the loss in the
line proper should seldom exceed 10 per cent. A loss of less
than 5 per cent is seldom advisable.
It should not be forgotten that in an alternating circuit two
small conductors are generally better than one large one, so
that the labor of installation often will not be increased by
waiting for developments before adding to the line. It fre-
quently happens, too, that it is very necessary to keep down
the first cost of installation, to lessen the financial burden
during the early stages of a plant's development.
Case II. Delivery of a known amount of power from ample
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482 ELECTRIC TRANSMISSION OF POWER.
water-power. This condition frequently arises in connection
with manufacturing establishments. A water-power is bought
or leased in toto, and the problem consists of transmitting
sufficient power for the comparatively fixed needs of the works.
The total amount is generally not large, seldom more than
a few hundred horse-power. Under these circumstances the
plant should be designed for minimum first cost, and any loss
in the line is permissible that does not lower the efficiency
enough to force the use of larger sizes of dynamos and water-
wheels. These sizes almost invariably are near enough to-
gether to involve no trouble in regulation if the line be thus
designed. The operating expense becomes practically a fixed
charge, so that the first cost only need be considered.
Such plants are increasingly common. A brief trial calcula-
tion will show at once the conditions of economy and the way
to meet them.
Case III. Delivery of a known power from a closely limited
source. This case resembles the last, except that there is a
definite limit set for the losses in the system. Instead, then,
of fixing a loss in the line based on regulation and first cost
alone, the first necessity is to deliver the required power.
This may call for a line more expensive than would be indicated
by any of the formulae for maximum economy, since it is far
more important to avoid a supplementary steam plant entirely
than to escape a considerable increase in cost of line. The data
to be seriously considered are the cost of maintaining such a
supplementary plant properly capitalized, and the price of the
additional copper that render it unnecessary. Maximum
efficiency is here the governing factor. In cases where the
motive power is rented or derived from steam, formulae like
Kelvin's may sometimes be convenient. Ix)sses in the line
will often be as low as 5 per cent, sometimes only 2 or 3.
Case IV. Distribution of power in known amoimt and units,
with or without long-distance transmission, with motive-power
which, like steam or rented water-power, costs a certain
amount per horse-power. Here the desideratum is minimum
cost per HP, and design for this purpose may be carried out
with fair accuracy. Small Hue loss is generally desirable
imless the system is complicated by a long transmission.
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THE LINE. 488
Such problems usually or often appear as distributions only.
Where electric motors are in competition with distribution by
shafting, rope transmission, and the like, 2 to 5 per cent line
loss may advantageously be used in a trial computation.
The problem of power transmission may arise in still other
forms than those just mentioned. Those are, however, the
commonest types, and are instanced to show how completely the
point of view has to change when designing plants imder vari-
ous circumstances. The controlling element may be minimum
first cost, maximum efficiency, minimum cost of transmission,
or combinations of any one of these, with locally fixed require-
ments as to one or more of the others, or as to special con-
ditions quite apart from any of them.
In very many cases it is absolutely necessary to keep down
the initial cost, even at a considerable sacrifice in other respects.
Or economy in a certain direction must be sought, even at a
considerable expense in some other direction. For these
reasons no rigid system can be followed, and there is constant
necessity for individual skill and judgment. It is no uncom-
mon thing to find two plants for transmitting equal powers
over the same distance imder very similar conditions, which
must, however, be installed on totally different plans in order
to best meet the requirements.
As regards the general character of transmission lines the
most usual arrangement is to employ bare copper wire sup-
ported on wooden or iron poles by suitable insulators. Now
and then underground construction becomes necessary owing
to special conditions. Not infrequently an aerial transmission
line must be coupled with underground distribution, owing
to municipal regulations. Occasionally insulated line wire is
used. It is frequently employed in cases where the transmis-
sion lines are continued for purposes of distribution through
the streets of a town, in fact, is usually required. As such
lines are generally of moderate voltage, very seldom exceed-
ing 3,000 volts, good standard insulation may often be effec-
tive in lessening the danger to life in case of accidental contacts,
and in reducing the trouble from crossing of the lines with
other lines, branches of trees, and the like. In case of really
high voltages, 10,000 and upward, no practicable insulation
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484 ELECTRIC TRANSMISSION OF POWER.
can be trusted for the former purpose, and may in fact create a
false sense of security, while it is far better practice to endeavor
to avert the danger of short circuits than to take extraordi-
nary precautions to mitigate their momentary severity. Hence
bare copper is to be preferred both on the score of safety and
of economy. Now and then at some particular point a high
grade of insulation may minimize local difficulties.
Much can be said in favor of placing a transmission line
underground, but there are also very strong reasons against it.
Such a li»e is eminently safe, and free from danger of acci-
dental injury. At the same time it is very difficult to insulate
properly, and if trouble does arise it is exceedingly hard to
locate and difficult to remedy. In addition, there are serious
electrical difficulties to be encountered, which often can be
reduced only by very costly construction. The chief objec-
tion aside from these is the expense, which in very many cases
would be simply prohibitive.
In cities there is an increasing tendency on the part of the
authorities to demand underground construction. Overhead
wires are objectionable on account of their appearance, danger
to persons and property, and their great inconvenience in
cases of fire, and these objections apply with almost equal
total force to all such wires, whether used for electric light or
power, or for telegraphic and telephonic purposes, the latter
more than making up by their number for any intrinsic advan-
tage in the matter of safety. The future city will have its
electric service completely imderground, at least in the more
densely inhabited portions. It must be said, however, that it
is far more important for a city to have electric light and.
power than to insist on having it in a particular way, and
unless the service is very dense, so as to abundantly justify
the very great added cost of underground work, private
capital will hesitate to embark in an enterprise so financially
overloaded.
Fortunately, for city distribution moderate voltages must be
employed on account of the intrinsic limits of direct current
circuits employed for general distribution, and the undesira-
bility of distributing transformers of moderate size on very
high pressure alternating circuits. More than 2,000 to 2,500
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THE LINE. 485
volts, save on arc circuits, can seldom be used advantageously
in general distribution, and such voltages can be and are suc-
cessfully insulated without prohibitive expense. They work
well in practice, and have stood the test of considerable experi-
ence. Moreover, with proper care the cables employed as
conductors, when thoroughly protected and inspected, probably
have a slightly less rate of depreciation than overhead insulated
lines, and are much less liable to interruption. As the district
within which undergroimd service is necessary is usually of no
great extent, the electrical difficulties that are to be dreaded
in attempting long underground transmissions are not here of
so serious magnitude.
For this limited service, then, in districts where both popula-
tion and service are dense, there is no serious objection to
underground lines, and many who have used them are decided
in commending them as on the whole more convenient and
reliable than aerial lines. Besides, a large proportion of under-
ground work is done at low voltages, less than 250 volts, with
which the difficulties of insulation except at joints are really
trivial. Such work does not belong so much to power trans-
mission proper as to distribution from centres after the trans-
mission is accomplished.
• With high voltages and long distances the case is very dif-
ferent. Not only are the difficulties of insulation great, but
electrical troubles are introduced of so severe a character as to
make success very problematical, even in cases where the cost
alone is not prohibitive. The feat of cable insulation for pres-
sures as great as 25,000 volts has been accomplished, and this
limit could probably be exceeded, but the cost of such work is
necessarily extremely high, and the location and repair of
faults is troublesome. An overhead line is so much easier to
insulate and to maintain that nearly all power transmission
will probably continue to be carried on by this method for some
time to come, until, indeed, there are revolutionary changes
in underground work of which we now have no suggestion.
The possibility of a long interruption of service while a fault
is found and repaired is too unpleasant a contingency to be
incurred. Duplicate lines are a natural recourse in such case,
effective, but very costly. Aerial lines are much cheaper to
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486
ELECTRIC TRANSMISSION OF POWER,
duplicate, and the labor of finding and repairing faults is com-
paratively light. Finally, when it comes to the question of
really high voltages like those now coming into frequent use,
say 40,000 volts to 60,000 volts, it must be admitted that in the
present state of the art of insulation underground cables, if
possible at all, are absolutely prohibitive in cost.
For these reasons underground transmission lines should be
avoided, certainly until we have had a long experience with
high voltages overhead.
Throughout the foregoing it has been assumed that the con-
ducting line is composed of the best quality of commercial
copper wire. Inasmuch as other materials are occasionally
proposed, it it worth while saying something about the relative
properties of certain metals and alloys as conductors. Aside
from silver, pure copper is intrinsically the best conductor
among the metals. In fact, it is hard to say that it is not the
equal of silver. Commercial copper wire is of somewhat vari-
able conductivity, since this property is profoundly affected by
very small proportions of certain other substances. An ad-
mixture, for instance of one-tenth of one per cent of iron
reduces the conductivity by about 17 per cent. It used to
be a most difficult matter to procure commercial wire of good
quality, and in the early days of telegraphy much annoyance
was experienced on this score. At present the best grades of
standard copper wire have a conductivity of fully 98 per cent
that of chemically pure metal, and even this figure is not
Material.
Ck)iiductivity.
Strength, Lbs.
Commercial copper wire
Good hard-di-awn copper
(1) Silicon broDze
98-99
97-98
97
05
80
69-60
46
26
14
10-12
86,000
60,000-66,000
63,200
Magnesium bronze
73,000
(2) Silicon bronze
76,000
Aluminiam
32,900
(3) Silicon bronze
110,000
Phosphor bronze
101,000
Iron annealed wire
65,000
High carbon steel wire
120,000-130,000
infrequently exceeded. On account of the comparatively low
tensile strength of copper, ordinarily about 35,000 lbs. per
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THE LINE. 487
square inch, very vigorous efforts have been made to exploit
various alloys of copper on the theory that their greater
strength would more than overbalance the lessened conduc-
tivity and increased cost, by enabling less frequent supports to
be employed. Aluminium bronze, silicon bronze, and phos-
phor bronze have been tried, together with some other alloys
of a similar character exploited imder various trade names.
The whole matter of high conductivity bronzes has been so
saturated with humbug that it is very hard indeed to get at
the facts in the case. Most of them are tin bronzes carrying
less than 1 per cent of tin, of which even one-tenth per cent
will raise the tensile strength by more than 40 per cent, lower-
ing the conductivity, however, more than hard drawing to the
same tensile strength. Copper which is hard -drawn probably
has greater tensile strength than any so-called bronze of sim-
ilar conductivity, from 60,000 to 65,000 lbs. per square inch,
with an elastic limit of about 40,000 lbs. per square inch and
a resistance less than 3 per cent in excess of that of ordi-
nary copper. The foregoing table gives the conductivities and
tensile strengths of some of the various materials used or pro-
posed for line wire, pure copper being taken as the standard
at 100 per cent conductivity.
It is sufficiently evident from this table that where the best
combination of strength and conductivity is wanted, hard-
drawn copper is unexcelled. For all oi'dinary line work good
annealed copper wire is amply strong, and is, besides, easier to
manipulate than wire of greater hardness. Occasionally,
where it is desirable to use extra long spans, or excessive
wind pressure is to be encountered, the hard-drawn wire is
preferable. Not uncommonly a medium hard-drawn copper
is used having a tensile strength of about 50,000 lbs. per
square inch and a conductivity of about 98 per cent. Now
and then, in crossing rivers or ravines, spans of great length
are desirable — several hundred yards — and in these cases
one may advantageously employ silicon or other bronze of
great tensile strength, or as an alternative, a bearer wire,
preferably a steel wire cable, carrying the copper conducting
wire or itself serving as the conductor. Where mechanical
strains are frequent and severe, bronzes are somewhat more
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488 ELECTRIC TRANSMISSION OF POWER,
reliable than hard-drawn copper of equal tensile strength,
since *they are homogeneous, while the hard-drawn copper
owes its increase in tenacity to a hard exterior shell, the core
of it being substantially like ordinary copper. If the prop-
erties of this skin ma}'^ be judged by its proportion of the
total area of the wire, the tensile strength must rise to nearly
150,000 lbs. per square inch, with a conductivity lowered 10
to 15 per cent.
Compoimd wires have now and then been used, consisting of
a steel core with a copper covering, but these are costly and
no better than hard-drawn copper for line use. Iron alone
replaces copper to any extent. It is cheaper for equal conduc-
tivity, but in wire is far less durable, and in rods cannot be
strung overhead conveniently, while, even were this possible,
the difficulty of making and maintaining joints is most serious.
Very recently aluminium has been successfully used as a line
conductor. At present prices (1905) it is materially cheaper
than copper for equal conductivity, but its bulk and the diffi-
culty of making joints are sometimes objectionable. Alu-
minium has about six-tenths the conductivity of copper, the
resistance of one mil-foot of pure aluminium wire being 17.6
ohms at 25° C. Owing to its very low specific gravity its
conductivity is very high when compared on the basis of
weight. It has very nearly one-half the weight of copper for
the same conductivity, to be exact 47 per cent, so that as a
conductor aluminium wire at 30 cents per poimd is a little
cheaper than copper wire at 15 cents per pound. The tensile
strength of the aluminium is slightly less than that of copper,
being a Uttle less than 33,000 lbs. per square inch as a max-
imum, and ui commercial wire usually between 25,000 and
30,000, while soft-drawn copper is about 34,000 lbs. Like
soft copper, the aluminium wire takes permanent set very
easily, having a very low elastic limit, about 14,000 lbs. per
square inch, so that at about half its ultimate strength it is
apt to stretch seriously. Comparing wires of equal conduc-
tivity the aluminium has absolutely greater strength, since
its cross section is about 1.64 times that of the corresponding
copper wire. If, however, the copper be hard drawn, the
aluminium wire of the same conductivity has only about 60
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THE LINE. 489
per cent of the strength, but having only half the weight of
the copper, still retains a slight advantage in relation of weight
to strength.
Being somewhat larger, the aluminium wire has a trifle
greater inductance and capacity than the copper and is more
exposed to the effect of storms. It has about 1.4 times the
linear coefficient of expansion of copper, so that there is more
tendency to sag in hot weather and to draw dangerously taut
in cold weather. This property has caused some practical
trouble in aluminium lines, and has to be met by great atten-
tion to temperature and uniform tension in stringing the wire.
In practical line construction, aluminium is always now used
in the form of cables laid up of wire, generally No. 8 to No. 12.
Such cables show somewhat more tensile strength than solid
wires of similar area and are very much more reliable. They
have come to be rather widely used and have given excellent
results.
Joints in aluminium wire are, as already indicated, a very
serious problem. In contact with other metals aluminium is
attacked electrolytically by almost everything, even zinc. A
successful soldered joint for aluminium has not yet been pro-
duced, and in line construction recourse has to be taken to
mechanical joints. One of the most successful of these is that
used in several California lines. It consists of an oval alumin-
ium sleeve, large enough to slip iji the two wire ends side by
side, and for No. 1 wires about 9 inches long. In making the
joint the ends of the wires were filed rough, the wires were
slipped side by side through the sleeve, and then by a special
tool, sleeve and wires were twisted through two or three com-
plete turns. The result was a johit practically as strong as
the original wire, and electrically good. There is considerable
danger of electrolytic corrosion in any such mechanical joint,
and lines exposed to salt fogs would probably suffer rather
severely in this way, but with care in making, and regular
inspection, these joints serve the purpose well. Very re-
cently a process of cold welding a sleeve joint under great
pressure has given excellent results.
Altogether it seems clear that aluminium is a most useful
substitute for copper for transmission lines, and it will cer-
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490 ELECTRIC TRANSMISSION OF POWER.
tainly be used extensively whenever copper rises to a price
above 15 to 16 cents per pound for bare wire. Not only is
the aluminiun cheaper in first cost, but its lesser weight
means a great decrease in cost of freights as well. It cer-
tainly makes an excellent line when carefully put up, and there
is no good reason why it should not be freely used whenever
the price of copper throws the balance of economy in favor of
aluminium. There have been attempts to improve the strength
of aluminium wire by alloying it, but as in the case of bronzes
the gain in strength is at the expense of conductivity. Such
alloy wire should be very cautiously investigated before use.
Before taking up the practical task of line calculation it is
necessary to consider somewhat at length the electrical diffi-
culties that must be encountered, and which impose limitations
on our practically achieving .many things that in themselves
are desirable and useful. We have seen already that the secret
of long distance transmission lies in the successful employ-
ment of very high voltages, and whatever the character of the
current employed, the difficulties of insulation constantly con-
front us. These are of various sorts, for the most part, how-
ever, those that have to do with supporting the conducting
line so that there may not be a serious loss of current via the
earth. Next in practical importance come those involved in
insulating the conductor as a whole against, first, direct earth
connections or short circuits in underground service, and
second, grounds or short circuits, if the line is an aerial one.
In a very large number of cases no attempt is made to
insulate the wire itself by a continuous covering, and reliance
is placed entirely on well-insulated supports. In most high
voltage lines this is the method employed, partly for economy
but chiefly because there is well-grounded distrust in the du-
rability of any practicable continuous covering under varying
climatic conditions and the constant strain imposed by high
voltage currents.
So far as supports go, it is evident that while the individual
resistance of any particular one may be very great, the total
resistance of all those throughout the extent of a long line to
which they are connected in parallel to the earth, may be low
enough to entail a very considerable total loss of energy. The
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THE LINE. 491
possibility of such loss increases directly with the number of
supports throughout the line. The most obvious way of
reducing such losses would be to considerably increase the
distance between supports as in some recent constructions.
This process evidently cannot go on indefinitely, from mechan-
ical considerations, and hence the greatest advance can be
made in reducing the chance of loss in individual supports.
Most of the present practice consists merely of an exten-
sion of the methods that were devised for telegraphic work.
These were quite sufficieut for the purpose intended, but are
inadequate when applied to modem high voltage work.
The ordinary line consists, then, of poles, bearing on pins of
wood or metal secured to cross arms, bell-shaped glass or por-
celain insulators. To grooves on or near the top of these the
line wire is secured by binding wire. Loss of current to earth
in a line so constituted takes place in two ways. First, the
current may pass over the outer surface of the insulator, up
over the interior surface, thence to the supporting pin and so
to earth. Second, it may actually puncture the substance of
the insulator and pass directly to the supporting structure.
The first source of trouble is the commoner, and depends on
the nature and extent of the insulating surface, and even more
on climatic conditions. The second depends on the thickness
and quality of the insulating wall which separates the wire
from the pin. To avoid leakage an insulator should be so
designed that the extent of surface shall be as long and narrow
as practicable; also, this surface must be both initially
and continuously highly insulating. The first condition is
met by making an insulator of comparatively small diameter,
and adding to the length of the path over which leakage must
take place by placing" within the outer bell of the insulator
one or more similar bells (usually called petticoats). These
not only help in the way mentioned, but they are likely to
stay tolerably dry even when the exterior surface is wet, and
thus help to maintain the insulation.
A good glass or porcelain insulator made on these general
lines gives excellent results with ordinarily moderate voltages,
say up to 5,000 volts. When the insulators are new and clean
they will quite prevent perceptible leakage, and for the vol-
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492 ELECTRIC TRANSMISSION OF POWER.
tages mentioned are satisfactory under all ordinary conditions.
When higher voltages are employed the results may be at first
good, but they are unUkely to stay so unless the climatic con-
ditions are exceptionally favorable. Most glass permits a
certain amount of surface leakage, even when new, although
generally not enough to be of practical importance, but even
the best commercial glass weathers when exposed to the
elements, so that in time the surface becomes slightly rough-
ened and retains a film of dirt and moisture that is a very
tolerable conductor. Even while perfectly free from this
deterioration at first, it is generally hygroscopic, because it is
in a trifling degree soluble even in rain water, and tends to
retain a slight amount of moisture. Thus in damp climates
glass is likely to give trouble when used on a high voltage
line. As regards temporary fall in insulating properties, a
searching fog or drizzling rain is much worse in its effects on
insulators than a sharp shower or even a heavy rain, which
tends^ to wash the outer surface free of dirt, and affects the
comparatively clean interior but little.
Much cheap porcelain is also hygroscopic owing to the poor
quality of the glaze, and it has the considerable added dis-
advantage of depending on this glaze for much of its insulat-
ing value.* Glass is homogeneous throughout its thickness,
while porcelain inside the glaze is often porous and practically
without insulating value. Nevertheless, porcelain which is
thoroughly vitrified, the ordinary glaze being replaced by an
actual fusing of the surface of the material itself, is decidedly
preferable to ordinary glass, being tough and strong, quite
non-hygroscopic, and of very high insulating properties.
The surface does not weather, and the insulation is well kept
up under all sorts of conditions. If the vitrification extends,
as it should, considerably below the surface, the insulator will
resist not only leakage, but punctiu-e, better than any glass.
The process of making this quality of porcelain is somewhat
costly, since the baking has to be at an enonnous temperature
* Much American porcelain will absorb 1 to 2 per cent of its weight of
water, a sign of poor insulating properties. The best porcelain should
absorb no water and should show a brilliant vitreous fracture which will
take no flowing stain from ink.
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PLATK XX
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THE LINE. 493
and long continued, but the result is the most efficient insulat-
uig substance in use. Glass, however, is better than ordinary
grades of porcelain.
Surface discharge is more to be feared than puncture at all
voltages, since the absolute insulation strength of the material
can be made high enough, by careful attention to quality and
sufficient thickness, to withstand any practical voltage contin-
uously, barring mechanical injury. But leakage is a function
of moisture, drifting dust, and things meteorological generally,
besides which, it may take place in serious amount at voltages
which otherwise would be very easy to work with.
Up to about 20,000 volts the familiar types of insulator
of good material and size prove adequate. At higher pressures,
however, a different state of affairs is encountered, since the
pressures become sufficient to break down the air as a dielec-
tric over distances great enough to be inconvenient.
At about 20,000 volts the lines begin to show a quite per-
ceptible luminous coating of faint blue at night, little brushes
spring from the tie wires and sometimes stream from the
insulators, and as the pressure rises still further these pheno-
mena become more and more marked. The appearance is
quite similar to that presented by the high tension leads from
a large induction coil in a darkened room.
At 50,000 volts or so the effect is somewhat menacing, and
unless the lines are well separated there may be considerable
loss of energy, and it is possible for arcs to strike from wire
to wire, producing temporary short circuits of most formid-
able appearance. Plate XX is from a photograph of this
phenomenon, taken on the lines of the Provo transmission
where they run through the old basin of the Great Salt Lake.
A heavy wind will raise clouds of saline dust which is very
trying to the insulation of the pole tops, and it was during
such a "salt-storm" that the picture was taken. Since the
wires were some six feet between centres, the arc must have
flamed ten feet high, having been coaxed into action by brush
discharges over the saline coating of the cross arms. The
conditions were of course unusual, but at voltages exceeding
20,000 or 25,000 volts the failure of the air as a dielectric
introduces an element of difficulty which must be reckoned
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ELECTRIC TRANSMISSION OP POWER.
with in trying to maintain the insulation of the lines. As
a preliminary to the design of high voltage lines, therefore,
it is necessary to know approximately the dielectric strength
of air under practical conditions. This is practically measured
by the striking distance over which various voltages will
leap in ordinarily dry air, between sharp points. The strik-
0 lu au 8u 40 oo eu 7o so 9o luo no lao iso i40 iM
EFFECTIVE AINUSOIDAL VOLTAGES IN KILOVOLT8
Fig. 2S3.
ing distances thus taken are greater than between rounded
surfaces but since the presence of sharp edges and burrs upon
the line wires or tie wires must be taken into account the
point distances form a safer guide.
Fig. 253 shows graphically the relation between effective
voltage and striking distance, the points used being sharp
sewing needles. Curve A is from the recent experiments of
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THE LINE, 496
Fisher*, which were particularly directed to the measurement
of high voltages by their striking distance, while curve B is
from the researches of Steinmetz. Below 30,000 volts the
two are in sufficiently close agreement, but above this point
large divergences appear in these, and in fact all other experi-
ments, not too large, however, to make these curves a valu-
able guide for general purposes.
In wet air, or at high elevations, and at high temperatures
to a lesser degree, the striking distances are increased con-
siderably. Experiments on insulators tested wet by a spray
and also dry, show that under practical conditions the increase
due to moisture may be twenty-five or thirty per cent.
There is, however, suflficient loss of energy and liability to
trouble on high tension lines to make necessary a consider-
able factor of safety in the aerial insulation strength. The
brushes and the hissing sound at the insulators at very high
pressures speak heavy static discharges and impending trouble,
even when the air insulation is very thick. In a closed space
these discharges would quickly so ionize the air as to cause
discharges, but in the open there is much less danger of this
occurrence.
In ordinary practice the diameter of the line wire p^'oduces
very little effect upon the matter of a break down of the air
although under test conditions in a confined space the size is
a very important factor. The reason for this discrepancy
is very simple — in actual lines the weakest point as regards
breaking down is at the insulators, and the transmission wires
on lines long enough to require very high voltage are usually
for commercial reasons \ inch or more ui diameter, and never
over i inch except in the case of cables built up of smaller
wires.
In other words the practical variation hi the radius of the
wires used is not great, and if they can be made safe at the
poles there will be little chance of trouble elsewhere.
The general leakage of a line is the summation of the brush
leakages at every point. So far as the line wires are concerned
they are customarily kept far enough apart to avoid direct
leakage in any amount. The effect of increasing the distance
* Trans. Int. Elec. Cong. St. Louis, II, 2M.
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496
ELECTRIC TRANSMISSION OF POWER.
and also the magnitude of the practical energy losses is well
shown in Fig. 254, which gives the result of tests by Mr.
Mershon on one of the early lines at Telluride, Colo., 2.25
s
1
4B0O
4000
8600
a"
1
8000
I K
Loss on Circuit with Wires at
Different Distances.
Frequency 60; Slotted Armature
Wires 15, 28, 35 and 52 inches
apart.
15
1
8900
1
MOO
1
1
lam
/
fiMA
/
/i
f
f
MM
/
//
/
y
y
r /
/
_
,__
=
^
^
r-
18 aO 34 28 88 96 40
Thousands of Volts
Pio. 254.
miles long. The conspicuous thing is, that after the energy
loss exceeds say 100 watts per mile, the breaking down of
the insulation resistance is very rapid indeed. The breaking
down point is determined by the height of the peak of the
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THE LINE, 497
voltage wave, so that in further experiments at Telluride it
was found that sinusoidal voltages showed less tendency to
break down the line than indicated in these curves.
If the pole tops are kept safe from flashing across, the free
wires will take care of themselves and the weakest point in
the line insulation is at the insulators themselves, granting
as we may, that one can get glass or porcelain to resist punc-
ture at pressures far above the highest now practically used.
It should be added that Mershon's tests were on No. 6 wire
and with air somewhat rarified by the elevation, the barom-
eter reading in the neighborhood of 20 inches. It has been
shown by Ryan* that the barometric height and the tempera-
ture greatly influence the point at which the air gives way and
a coronal discharge sets in. From a considerable series of
tests Ryan has deduced the following formula for the voltage
E required to start a coronal discharge between two wires of
radius r in inches, spaced s inches, when the barometer reads
h iiiches and the temperature is <° F. •
„ 17.94 h
459 + t
X 350,000 log,o(-)(r + .07).
This agrees fairly with experimental results on lines and
applies to wires from No. 4 B & S up. For smaller wires
Ryan foimd values of E much lower than the formula indicates,
possibly for reasons connected with his method of experiment-
ation, but the cause of the aberrancies is of small practical im-
portance since wires smaller than No. 4 are very rarely used
in transmission work. E it must be remembered is not the
rated voltage but the peak of the voltage wave, and for sinu-
soidal waves must be divided by v2 to reduce it to rated
voltage.
Moisture seems to produce small effect on the critical vol-
tage which corresponds with the sharp upward turn in the
curves of Fig. 254, and the main thing practically is to space
the lines sufficiently to give a liberal factor of safety between
E and the working voltage. It would hardly be wise to
allow a value of E less than double the working pressure,
•Trana. A. I. E. E., Feb. 26, 1904.
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498
ELECTRIC TRANSMISSION OF POWER,
but even so it would certainly be safe so far as coronal dis-
charge is concerned to work wires of ordinary size up to
100,000 volts when spaced six feet or so. Stranded cable
should give slightly lower values for E than solid wire of
similar size, but whether materially lower is dubious, and so
far as practical values of s are concerned, the main problem
is to resist flashing over the insulator surface in one way
or another to the cross arm. .
As a matter of fact it is found that when such discharges
take place they do not follow the insulator surfaces, but jump
Fio. 255.
the spaces from petticoat to petticoat. For instance in Fig.
255 which shows in section a glass insulator designed for use
at 40,000 volts, the air space which serves as a defence against
break down is the distance A from upper to lower petticoat,
plus a small distance B to the pin. Insulators faU by this
direct discharge and not by a creeping discharge along the sur-
face. High voltage insulators do not much tend to accumu-
late moisture which is either repelled or dried off pretty effec-
tively, and in a rather open construction which favors keeping
the surfaces free from dirt and moisture, the upper surfaces of
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THE LINE, 499
petticoats must be regarded in damp weather at least as
fairly conducting, leaving the sparking distances as shown.
In Fig. 255 the sparking distance neglecting B is about 6.5
inches, so that turning to Fig. 253 it appears that if there
is a difference of 40,000 volts from wire to ground, the given
insulator has a factor of safety of about 2.5.
Now considering the fact that in wet weather all pole tops
may be regarded as giving fair surface conduction, it is clear
that the working air insulation between two wires of a cir-
cuit carried on insulators like Fig. 255, has an aggregate thick-
ness of only 13 inches or so, and that this and not the spacing
of the wires is the real limitation upon the voltage. The
insulators are always the weakest points of the line both as
regards general insulation, and danger of arcing. Without
going into details of insulator construction which will be
taken up in the next chapter, it may be said that insulators
of first-class material and of dimensions that should give a
factor of safety of not less than 2.5 on a working voltage of
60,000 are now commercially obtainable.
This factor of safety is none too large, and when one
considers that very high voltage renders a line particularly
liable to interruption from accidents which at moderate vol-
tages would be trivial, it is a wonder that transmission lines
perform as well as they do.
As to voltages for such lines great progress has been made.
10,000 to 15,000 volts is a conservative pressure now used
only for short distances. A common rough and ready rule
for voltage is a thousand volts per mile or as near it as you
dare on the longer distances. The present tendency is to use
not less than 20,000 to 30,000 volts for all serious transmission
projects. Such pressures have now been in regular service
with excellent results for half a dozen years past, and that
in many cases. It may be fairly said that they may be regarded
as not only completely reliable, but rather conservative.
They are in operation in all parts of the country, under all
sorts of climatic conditions, without experiencing any diffi-
culties which would not be equally in evidence at half the
voltage.
In other words, a line can be built and operated at 20,000
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600 ELECTRIC TRANSMISSION OP POWER.
to 30,000 volts, without trespassing on entirely safe values of
the factors of safety in the various parts.
From 30,000 to 45,000 volts there are now in operation
more than a score of plants and the reports from them are
uniformly rather favorable. There is no doubt that certain
classes of line troubles become more prominent, especially in
reaching the neighborhood of 40,000 volts and above. The
root of these troubles is the relatively low factor of safety at
the insulating supports. All may go well under normal con-
ditions but there is always danger that deterioration or abnor-
mal pressures arising from one cause or another may break
down the air gap. If insulators are worked at a voltage which
is near the sparking distance voltage, dirt, and moisture par-
ticularly from sea fogs, may so much reduce the surface resis-
tance as to lead a discharge over and start an arc. It sometimes
happens that insulators individually tested with a good factor
of safety will later break down without any adequate electrical
cause, probably from the starting of cracks from mechanical
strain. Pins may break or bend thus letting the wire down
upon or near the cross arm, and many minor faults not con-
spicuous at 25,000 volts may become serious as the voltage
nears that at which current will jump the insulators.
Nevertheless, a good many plants have been working at
these high pressures with relatively small trouble. Now and
then temporary shut downs occur, as upon plants at lower
voltage, but on the whole accidents are few and even these
are seldom fairly chargeable to the unusual voltage.
Transmission plants working at 45,000 to 60,000 volts are
few in number, but are generally of considerable magnitude,
and have probably been as reliable as those in the class just
considered. They have had at times trouble with insulators,
but as they have not temporized with the problem, and have
used the very best insulators obtainable, they are working
upon a factor of safety quite as large as that found in many
plants of much lower voltage, and consequently have not
experienced unusual difficulties. Certainly several plants arc
doing good commercial work at voltages falling little short of
60,000.
Let us now sum up our present knowledge of the transmis-
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THE LINE. 601
sion of electrical energy over high voltage lines. From a con-
siderable amount of experience, we are sure that there is no
real difficulty whatever in establishing and maintaining ade-
quate insulation up to an effective pressing of 25,000 volts.
Above this the plants are less numerous, but it is quite
certain that satisfactory results can regularly be reached up
to 30,000 without very extraordinary precautions. With
good cHmatic conditions 40,000 or 50,000 may be considered
entirely practicable, with reasonable precautions, and 60,000
has now been reached without any signs of impending failure.
At still higher voltages the difficulties ^re likely to multiply
more rapidly, and a point will ultimately be reached at which
the cost of insulating devices wiU overbalance the saving of
copper due to increased voltage. This point is at present inde-
terminate, and will always depend on the amount of power
to be transmitted, the permissible loss in the line, and un-
known variables involving repairs and depreciation, cost and
depreciation of transformers and so on. It is quite impossible
from present data to set such a limit even approximately, for
we know as yet nothing of the relative difficulty of insulat-
ing voltages considerably above the range of our experience.
In cases where continuous insulation is employed, it is
for one of two purposes, chiefly to prevent interference with
the circuit by such accidents as twigs or wires falling across
the Une, and either short circuiting the lines or grounding
them. Aside from this, the only other object in insulation
is to lessen the danger to persons accidentally touching the
wires and to prevent the current stra3ring to other circuits.
With moderate voltages both these ends can be reached with
a fair degree of success. With high voltages it is very diffi-
cult, and in many cases well-nigh impossible.
Nearly all materials which are available for insulation
deteriorate to a very marked extent when exposed to the
weather. Those substances which are the best insulators,
such as porcelain, glass, mica, and the like, cannot be used
for continuous insulation, and, in fact, our best insulators
are mechanically so bad as to be impracticable. There
is a large class of insulators complicated in chemical consti-
tution, but mechanically excellent; these are the plastic or
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502 ELECTRIC TRANSMISSION OF POWER.
semi-plastic substances like gutta-percha, India rubber,
bitumen, paraflBn, and the like. All of these are subject to
more or less decomposition, more particularly those which
are, through good mechanical quaUties, desirable for insula-
tion. All which have been mentioned are sufficiently good
insulators to answer every practical requirement, if they do
not deteriorate.
Gutta-percha and India rubber are decidedly the best of
these; but gutta-percha is too plastic at anything excepting
low temperatures to be mechanically good. Gutta-percha fills,
however, an unique place on account of its remarkable ability
to withstand the action of salt water, and it is the most reliable
insulator for submarine work. For overhead work it is nearly
useless, as the heat of the sun softens it so as to endanger its
continuity, and even a moderate increase in temperature may
decrease its specific resistance to a tenth of its ordinary value.
India rubber is, by all odds, the best all around insulator
for overhead lines. In its pure state it deteriorates with very
great rapidity; but when vulcanized by the addition of a small
amount of sulphur, its chemical character is so changed as to
resist both spontaneous changes and those due to the atmos-
phere to a very considerable extent, without injury to its
insulating properties. It is, however, costly, and is eventually
affected by the weather. To cheapen the manufacture of
insulated wire a large variety of rubber compounds are em-
ployed, consisting of mixtures of rubber with various other
substances intended to give the material good mechanical and
insulating qualities at less expense. These rubber compounds
are much inferior to pure vulcanized rubber in point of specific
resistance, but make a good and substantial covering for
ordinary purposes, sometimes more durable than the purer
material. They are very generally employed for commercial
work.
Insulated wires for overhead work may be divided into two
classes. First, those which are so prepared as to withstand
the weather to a considerable extent and to retain high insu-
lating properties even in bad weather. Such wires are usually
covered with compoimd fairly rich in vulcanized rubber, com-
monly protected outside with a braiding of cotton saturated
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THE LINE. 608
with some insulating compound, and serving to protect the
main insulation from mechanical injury.
The second class of wires includes those in which no solid
insulating material is used, but which are thoroughly protected
by a covering of fibrous material saturated with compounds of
rubber, bitumen, or the like. These wires are most exten-
sively used; th^ insulation is good in dry weather, and fair
under most ordinary circumstances, but generally greatly in-
ferior to those wires which are given a coating of rubber.
So far as protection of the wire from accidental contacts is
concerned, either class of insulation is tolerably effective at
moderate voltages until the covering becomes worn or
weathered by long or hard usage.
As regards danger in touching such wires, at moderate
voltages both khids of insulation afford a fair degree of pro-
tection. At high voltages neither can be trusted, in spite of
the apparently high insulation resistance. There is good
reason to believe that any insulation employed on wires is
greatly affected by the strain of high voltage. Tests made
with the ordinary Wheatstone bridge give us no useful inform-
ation as to the action of the same insulation imder continued
stresses of 5,000 or 10,000 volts. Tests made with pressures
ranging up to even 500 volts show generally a noticeable,
although very irregular, falling off in resistance, and the
higher the voltage is carried the more likelihood of complete
breaking down of the insulation and the more irregular the
results.
It is improbable that even the most careful insulation with
vulcanized rubber of any reasonable thickness would give a
wire which, under a pressure of 10,000 volts, could be long
depended on to remove all danger to persons from accidental
contact. Even if entirely safe at first, it would be unlikely
to remain so for any great length of time. A rubber covered
lead sheathed cable with the sheath thoroughly grounded is
probably the nearest approximation to safety. So serious is
the difficulty of continuous insulation of high pressures, that
it is best not seriously to attempt it; but either to fall back
upon bare wire with very complete insulation at the supports,
or, if insulated wire be employed at all, to use an insulation
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504 ELECTRIC TRANSMISSION OF POWER.
intended only to lessen the danger of short circuits from
falling objects, and always to treat the line, so far as personal
contact goes, precisely as though it were bare wire.
Information regarding the insulation of lines, whether of
bare or insulated wire, under high voltage, is very scarce; but
all such lines should be treated at all times as if they were
grounded, in spite of any teste of the insulation that may have
been made. Theoretically, one should be able to touch a
completely insulated circuit without danger save from static
charge; but, practically, it is suicidal so to treat any high
voltage circuit.
The writer calls to mind one case in which a man was
instantly killed, while standing on a dry concrete floor, by
contact with a 10,000 volt circuit. He probably touched a
bare portion of the wire, but so far from the general insulation
of the circuit saving him, the current which he received was
sufficient to bum into the concrete floor the print of the nails in
one of his shoes. The ordinary tests on the line made shortly
afterward showed no particular groimd, nor was there any
reason to believe that one existed at the time of the accident.
Other accidents, under similar conditions, have occurred with
arc light circuits of lesser voltage, on which there was a similar
absence of perceptible ground. It is advisable, therefore, that
all high voltage circuits should be treated as uninsulated, so far
as contact is concerned, at all times, and if insulation tests are
to be made upon them to determine the resistance to ground,
these tests should be made with, at least, the full voltage of the
circuit. It is quite as well not to place too much reliance on
insulation of any kind; but to regard a high voltage electrical
circuit as dangerous, and to be treated with the same respect as
is due to other useful, but dangerous, agents, like high pressure
steam and dynamite, neither of which is likely to be abandoned
on account of the danger that comes from careless use. The
precautions taken, either with these or with high voltage cur-
rents, should be in the direction of preventing such careless-
ness as might result disastrously.
An electrical circuit should be so installed that no material
risk can be run by any person who is not indulging in wilful
interference with the line, and in such case, if an accident
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THE LINE. 505
occurs, the victim is deserving of no more sympathy than one
who deliberately stands in front of an express train.
If the circuit is of bare wire, there can be no doubt in the
mind of any one as to its dangeroiLS character, whereas, if
insulated wire is employed, there is likely to be established a
certain false sense of security. There is no good reason,
therefore, for advising the extensive use of insulated wire for
high voltage lines.
The ideal overhead circuit is one in which the conductor is
thoroughly insulated as regards leakage, carefully protected
from danger of wires or branches falling acn)ss it, and placed
out of the reach of anything except deliberate interference of
human beings. There may be places at various points along
the line where insulation would be desirable, in order to avoid
extensive cutting away of trees, branches of which might fall
upon the line, or where local regulations require the use of
insulated wire. Except under these circumstances continuous
insulation increases the cost and maintenance of the line
without giving any adequate returns in security. On rare
occasions, portions of the high voltage circuit may have to be
placed underground. Here only the very best quality of
insulation should be employed, thoroughly protected by an
outside sheathing of lead against the effects of moisture, and
installed in smooth, clean, dry, and accessible conduits with
especial attention to insulation at the joints. Of this, more
in Chapter XIV.
From what has been said, it should be understood that while
the problem of installing high voltage lines is unquestionably
a difficult one, we have not yet had sufficient experience to be
able to say definitely how difficult it may be. It is very cer-
tain that much more can be done than has been accomplished.
It seems probable that so far as overhead work is concerned,
it will before lorig be practical to employ voltages considerably
greater than those now in ase. Before any limit can be set
to the progress in this direction, we need ample experimental
data, not only on the behavior of insulation at a very high
pressure, but on the maximum voltage which is likely to be
encountered when a certaui effective voltage is to be employed.
This opens up a wide field for investigation, involving con-
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606 ELECTRIC TRANSMISSION OF POWER.
ditions of unknown seriousness, connected especially with the
electrical peculiarities of alternating currents, which there is
every reason to believe will be employed almost exclusively on
high voltage work.
The special difficulties to be met in working with alternating
currents are two — inductance in the line and apparatus, and
electrostatic capacity, accompanied by the very serious phe-
nomena of electrical resonance. In addition to these, what-
ever the character of the current used for transmission purposes,
there is danger of getting accidentally upon the line a voltage
much higher than the normal. Inductance is met with to a
very considerable extent in all alternating circuits; resonance
in a small degree is probably much commoner than is generally
supposed, and abnormal voltage, due to the generators them-
selves, must always be guarded against.
Passing at once to the practical side of the question, we
find that when an alternating current is sent through any
Figs. 266 and 267.
conductor, it has to deal not only with the electrical resis-
tance of that wire, but with a virtual resistance due to the
fact that the electro-magnetic stresses set up at any point
of the conductor set up electromotive forces at other points
in the same conductor, which oppose and retard the passage
of the current.
These matters have been fully discussed theoretically in
Chapter IV, and hence will be here but briefly mentioned.
For example, if a wire be bent into a coupFe of spiral coils
like Fig. 256, the electro-magnetic field of one coil will affect the
other, just as we have induction from one separate ring to
another in Fig. 4, page 13. If such a spiral has an iron core,
this self-inductance will be much increased. Even if only a
straight wire be concerned in the carrying of current, there
will be a similar inductive relation between the inner and outer
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THE LINE. 607
portions of the wire at any point, since the electro-magnetic
stresses exist inside the wire as well as outside.
Let Fig. 257 represent a circuit carrying an alternating cur-
rent, which at a given moment is flowing as shoT^n by the
arrows. The electro-magnetic field set up by this current in
the loop has a direction perpendicular to the plane of the
paper, and sets up an E. M. F. opposing that of the wire. The
greater the area of the loop, i.e., the farther apart the two
wires, the greater proportion of the electro-magnetic field will
pass within the loop and produce self-induction.
Similarly, the larger the wires for a given distance between
them, the less effective field within the loop to set up induc-
tance. In fact, the amoimt of inductance in the circuit
depends directly on the ratio between the radii of the wires
and the distance between them. So if the diameter of the
wire is decreased to one-half the original amount, the wires
must be strung only half as far apart in order to retain the
same inductance.
The practical effect of inductance in the line is to neces-
sitate the use of an initial E. M. F. large enough to overcome
the inductive loss of voltage, as well as that due to resistance,
and so keep the E. M. F. at the receiving end of the line up
to its proper value. To undertake in an orderly way the
design of a ix)wer transmission line we may consider seriatim
the effects of resistance, inductance, and capacity as determin-
ing the losses and the precision of regulation and as related
to the abnormal values of the voltage which determine the
real factor of safety in the insulation.
To begin with, Ohm^s law is the basis of all computations
regarding the line, and lies behind all the formulae Uvsed for
this piu-pose. The most obvious way of applying it would
be to find the resistance of the whole line corresponding to
the required current and loss in voltage, and then to look
up in a wire table the wire which taken of the recjuired length
would give this resistance.
As a matter of convenience in computation, various formulae
have been devised to include in simple form the factors of
distance, voltage, power transmitted and loss in the line, and
giving the area weight or cost of the conductors.
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608 ELECTRIC TRANSMISSION OF POWER.
The area of wires is in English speaking countries expressed
in terms of the circular mil (cm.), which is the area of a circle
0.001 inch in diameter, a barbarous imit, which, however,
by merest chance leads to formulae nimierically simple.
The following formulae are perhaps the most convenient
of those in use. They are derived as follows. Starting with
E
Ohm's law -B ==- — and remembering that for any wire
C/
Total length in ft. X resistance of 1 ft . of wire 1 mil in diameter
Area in circular mils
we obtain since the resistance of 1 mil-foot of copper wire is
very nearly 11 ohms,
11 L
R =
cm.
or taking the total length of wire as twice the distance of
transmission in feet, since this distance is the thing immedi-
ately concerned we have
^ 2D X 11
R = ,
cm.
Now substituting this value of ft in the expression for Ohm's
law we have
2DxllxC
cm. = (1)
This gives the area of the wire for delivering any current
over any distance with any loss, E in volts. The correspond-
ing sizes and weights of wire can be looked up in any wire
table.
As a matter of convenience the following table gives for
the sizes of wire likely to be used in power transmission the
area in circular mils, the diameter, resistance per thousand
feet, weight per thousand feet bare, and weight also with
insulation of the so-called weather-proof grade, commonly
used on distributing circuits. The diameters are given to
the nearest mil, the areas to the nearest 10 cm. and weights
to the nearest pound. No wires larger than 0000 are here
considered, since even this size of copper is seldom used, and
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THE LINE.
509
Circular
Gauge No.
Diameter
OhmB per M
ft. at 70° P.
Ohma
Wt. per M
feet Bare.
Wt. per M
ft. We*th»r
proof.
Mils.
B.*&8.
in Mils.
and 98% Con-
ductivity.
per mil.
211,600
0000
460
.06026
.26637
640
725
167,800
000
410
.06337
.33459
608
580
183,100
00
365
.07991
.42182
403
480
105,600
0
326
.10077
.53196
320
376
83,690
1
289
.12707
.67093
263
307
66,370
2
258
.16024
.84606
201
246
62,630
3
229
.20206
1.06687
159
196
41,740
4
204
.25479
1.34629
126
147
33,100
5
182
.32129
1.69651
100
121
26,250
6
162
.40616
2.13924
79
99
on the other hand wires smaller than No. 6 are mechanically
weak and rarely would be advantageous. In fact the sizes
No. 00 to No. 2 inclusive include the wires commonly used.
The actual value of the mil-foot constant at ordinary tem-
peratures is approximately 10.8, but is here taken as 11 ohms
to take account of the ordinary contingencies of irregular
diameter, slight variation in conductivity, and the effect of
hard drawing.
The next step in simplifying the computations, is to find a
simple expression for the weight of the wire required. Now
it chances that a copper wire 1,000 cm. in area, weighs very
nearly 3 lbs. per thousand feet, and hence we can get a very
simple formula giving directly the weight in pounds per thou-
sand feet. Taking D in thousands of feet and expressing this
fact by writing it D^ we have
^ 2i)^x33 XC
^"*"^ E '
or for the total weight of the wire
4 Z)«» X 33 X C
W ^
E
(2)
(3)
This applies to ordinary direct current or single-phase circuits.
Now we have already seen that each conductor of a three-
phase line has one-half the area of one wire of the equivalent
single-phase line, so that by dropping the factor 2 in (1) we
have for the required area of one wire
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610 ELECTRIC TRANSMISSION OF POWER.
-. w
Dx llx-^
cm. = (4)
Herein -— is taken to avoid any possible confusion as to the
value of C, w being the watts at either end of the line and
V the working voltage at the same end, while E as before
is the loss in volts. Then proceeding as before we get for
the total weight of the three-phase lines,
'^'^4 (5)
W =^
E
the constant being taken as 100, instead of 99, to compensate
for a minute deficit, in the assumption of 3 lbs. per thousand
feet for a wire of 1,000 cm.
This particular simplification lends itself very readily to
a cost formula in which P is the total price in dollars when
p is the price of copper wire taken in cents per pound; as
follows:
P -_= —L (6)
E
Finally, since a power factor less than unity implies the deliv-
ery of increased current for the same energy and voltage, we
can take accoimt of this factor of increase by writing
w
P^-7 (7)
£cos ^
with analogous expressions in the case of the previous formula.
For aluminium wire, insert the factor 2 in the denominators
of the weight formulae.
Another convenient empirical formula for the total weight
of copper in a three-phase circuit is the following
M^Kw
TF= 300,000,000 — -— •
a y
In which M is the distance of transmission in miles, K w the
kilowatts of energy transmitted at voltage V, and a is the
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THE LINE. 611
percentage of loss expressed as a whole number. This for-
mula gives results a few per cent larger than (5) and like it,
can be made to take account of lagging current so far as ohmic
drop is concerned by putting cos ^ in the denominator. For
aluminium wire, put 2 in the denominator as in the other
formulfiB.
It frequently happens in using these formulse that the
wire indicated by the assumed data, falls between two of the
ordinary sizes. In large work one can have wire or cable
made very nearly to the desired size without increased expense,
but ordinarily one chooses the nearest standard size, prefer-
ably the next larger.
So far then as computing the copper required for trans-
mitting the energy goes, it is a very simple matter to figure
a transmission line. But inductance is another matter.
The simplest way of treating it is to deal with it as an addi-
tional resistance, causing no increased loss of energy for the
same current, but demanding increased E. M. F. at the gener-
ator, and affecting consequently the regulation. The resis-
tance of the line determines the energy loss, the impedance
the limits within which the impressed E. M. F. must be regu-
lable.
For any system of given size and distance of wires worked
at a given frequency, the inductance like the resistance in-
creases directly with the length of circuit so that the ratio
between them is constant, and one can express the impedance
in terms of resistance by multiplying the resistance by the
proper impedance factor, when once this ratio is ascertained.
The same factor converts the ohmic drop into the impedance
drop which is the quantity here sought.
In a circuit with wires spaced 8 inches between centres,
and each r inches in radius, the self-induction in henrys per
mile is
L =- 0.000322 [2.303 log- + .25]
which results from translation of the C. G. S. formula
-Kva
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4t^'
612
ELECTRIC TRANSMISSION OF POWER.
into English measure and common logarithms. From this
as a basis and from the known resistances, is constructed the
curves of Fig. 258. They give graphically the impedance
factors at 60-*- for wires from No. 0000 to No. 4 string .24, 48,
and 72 inches between centres.
The impedance factor increases with the size of wire at any
given spacing because the resistance decreases in proportion
to the area, and the length of the circuit is not concerned
since both resistance and inductance mcrease directly with
the length so that they remain proportional. For the most
part the value of the factor ranges from 1.5 to 2.5 so that
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THE LINE. 613
considering only this matter, the total drop is seldom much
above twice the ohmic drop. To compensate for the inductive
drop, then, the generator must have a margin of voltage
correspondingly greater than that required by the ohmic loss
alone.
It cannot be too strongly impressed upon the reader, how-
ever, that in actual practice these line constants are greatly
modified by the character and amount of the translating
devices at the receiving end of the line. To determine the
actual drop in the line, and the regulation required, one must
take into account both the line and the load. As a rule the
inductances and capacities of high voltage apparatus are
rather large compared with those of Imes of moderate length,
but large or small they modify the regulation, for the final
impedance of the system is the geometrical sum of its com-
ponents.
As a practical matter it is the constant effort of the engi-
neer to keep the power factor of the transmission circuit high,
so as to avoid the loss due to generating and transmitting a
large useless component of the current chargeable to lag.
In working at a bad power factor, not only does the impedance
ratio rise, but the resistance drop increases for the same energy,
so that the regulation quickly goes from bad to worse.
As a general rule the impedance due to the line and load is
likely to introduce a total line drop two to three times the ohmic
drop for the same line current, imless helped out by capacity.
If then the full load drop due to resistance be 10 per cent,
one must be prepared at the station to furnish 10 to 20 per
cent extra voltage to compensate for inductive drop. It is
therefore especially desirable to obtain a high power factor
at and near full load, to avoid using generators of abnormal
capacity. The light load power factors cause little trouble.
The fimdamental requirement is that the station should be
able to hold uniform voltage at the receiving end of the line
under all circumstances of load. To give good commercial
results the service voltage should be kept within 2 per cent
of normal if lighting by incandescents is important, and within
4 or 5 per cent for satisfactory motor service.
This means that the conditions of regulation must be thor-
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614
ELECTRIC TRANSMISSION OP POWER,
oughly investigated. At any state of load the regulation is
determined by the vector sum of the impedances in circuit,
which for regulation at the receiving substation means sum-
iWSOO
0 1
QAuoe (a A 8)
Fio. 269.
ming the impedances of the line and of the receiving circuit
under various conditions of load.
First in order comes the actual inductance of the line wires.
Here as elsewhere in this discussion, the line is assumed to be
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THE LINE!, 6l6
three-phase with the wires symmetrically arranged at the
comers of an equilateral triangle. The formula for the co-
efficient of self-induction, L, which depends entirely on the
dimensions of the system, has been given, but for convenience
the values per mile of complete circuit for wires from No. 0000
to No. 4, strung 24, 48, and 72 inches apart are shown graphi-
cally in Fig. 259. To reduce to self-induction per wire divide
by V3. Multiply by 2 irn to obtam the inductance in ohms
from the values of L given by the curves. For 60-^, 2 ^ n ==
377.
The curves show that for the wires in common use in trans-
mission work, L does not vary over a wide range, being com-
monly 3 to 3.5 milli-henrys per mile of circuit.
There is another cause of increased drop of voltage m alter-
nating current circuits quite apart from ordinary inductance.
Some years ago Lord Kelvin pointed out that in the case of
alternating and other impulsive currents the ohmic resistance
of conductors is slightly increased. This is for the reason
that in such cases the current density ceases to be uniform
throughout the cross section of the conductor. The instan-
taneous propagation of any current is primarily along the sur-
face of the conductor, and only after a measurable, though
short, time is the condition of steady flow reached.
When the current rapidly alternates in direction the interior
of the conductor is thus comparatively unutilized, for before
the flow has settled into uniformity its direction is changed,
and the original surface flow is resumed. The larger the wire
and the greater the frequency the more marked this effect.
Fortunately, with the common sizes of wire and the frequencies
ordinarily employed for power transmission work, it is quite
negligible. At 60 periods the increase of resistance due to
this cause, in a conductor even half an inch in diameter, is less
than one-half of 1 per cent. Any line wire that is allowable
on the score of its impedance factor will be unobjectionable
on this account as well. Only occasionally, as in bus bars for
low voltage switchboards, is it worth considering, and in such
cases the use of flat bars, half an inch or less thick, or tubular
conductors, will obviate the difficulty.
In computing the sum of the impedances, it is sometimes
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616 ELECTRIC TRANSMISSION OF POWER,
convenient to include with the line the impedances of raising
and reducing transformers reduced to terms of the full line
pressure, the primary resistance being increased by the secon-
dary resistance multiplied by the square of the transformation
ratio to form the equivalent total resistance, visually not far
from twice the primary resistance; and the inductance being
determined from the inductive drop when loaded.
At the receiving secondary terminals, the measured angle
of lag due to the load at once tells the story of the relation
between the power and the idle component of the current.
For the purpose of determining regulation the items of the
load need not be considered, if we know the lag angle which
determines the current components which haye been fur-
nished over the line. For short lines overhead, the line impe-
dance and the lag angle determine the regulation, but on very
long overhead lines, and in underground cables, capacity
plays an important part.
The capacity of overhead circuits like the self-induction is
determined by the dimensions of the system, except as there
may be localized capacity. For the customary three-phase
overhead circuits the situation has been simplified by Perrine
and Baum,* who showed that for such circuits the capacity
acted as if concentrated in three condensers at the middle of
the line, and star connected to a common neutral point. Upon
this hypothesis, which leads to sufficiently precise results for
all cases now practical, the capacity C in microfarads reckoned
between one wire and netural point for wires r inches in radius
and spaced d inches apart becomes, per mile,
.0776
2log.o(^
Fig. 260 shows graphically the values of C for wires of the usual
sizes spaced respectively 24, 48, and 72 inches between centres.
Here again there is a considerable degree of imiformity, C
ranging ordinarily between .014 and .018.
The corresponding current equivalent, or charging current,
* Trans. A. I. E. E. May, 1000.
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THE LINE.
617
if depends like the inductance on the frequency, but also,
unlike the inductance, upon the voltage; and in general
i = pCnV.
Here, for one wire of a three-phase line, p has the value
p = .000,003,627.
For a 60 cycle line, multiply the values from the curve by
I 0 00
WIRE OAUOE (B A 8)
FlO. 260.
2.18 for 10,000 volts line pressure and proportionately more
for higher pressures.
As for our purpose the capacity is taken as if localized at
the centre of the line, i must be regarded as flownig through
one-half the line impedance. If there is localized capacity
elsewhere, as in case of cables, its charging current, determined
from the capacity of the cable as above, must be taken as flow-
ing over the actual length up to the capacity and forms a
geometrical addition to the capacity just considered.
We now have in hand the data for figuring the terminal
voltage of a transmission line from the impressed voltage by
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518 ELECTRIC TRANSMISSION OF POWER,
summing the several impedances, and computing their resultant
in view of the current and energy values disclosed by the
lag (or lead) angle 4> at the terminus.
For the present purpose the most simple and elegant method
of making this summation is that of Perrine and Baum (loc.
cU) * which takes as a starting point the receiver voltage
which is to be held steady, and treats the power component
and the idle component of the receiver current as if they
flowed independently through all the impedances up to the
receiver as, in effect, they do.
Let us start then with the receiver voltage and lay out
Oa, Fig. 261, equal to this voltage on any suitable scale. Here
the receiver voltage is taken at 10,000. The ohmic drop at
full non-inductive load we will take as 2,000 volts which lay
off as an extension of Oa to h. The total current in the system
is composed of the true energy current, the idle current, and
the charging current, if any, each of which consumes voltage
in being forced through the line impedance. Taking them
up successively, the energy current is / cos ^, ^ being the
angle of lag or lead at the receiver, and since we are here
considering full load energy
a 6 = / iJ cos ^
i.e.y the ohmic drop of the energy current. Now proceed to
form the ordinary impedance triangle a & c as follows. From
h erect a perpendicular such that
6 c = / (L «) cos ^
on the same scale as a 6, L„ being the inductance in ohms.
This can be done by computing the actual inductance from
the data assumed. Then a c, on the working scale, gives in
magnitude and direction the total volts consumed over the
line by the energy-current. If transformers or other apparatus
are included in this estimate for the line, this fundamental
triangle a h c must be built of its components geometrically
as shown in Fig. 56. If the line only is concerned, the point
♦ See also Baum, Elec. World & Eng., May 18, 1901, and Trans. Int.
Elec. Cong., 1904, Vol. H, p. 243.
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THE LINE.
519
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Fig. 961,
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520 ELECTRIC TRANSMISSION OF POWER,
c is at once located by striking from a as a centre an arc with
a radius equal to the impedance factor on the scale of a b, and
erecting a perpendicular from b to meet it.
The next step is to determine the magnitude and direction
of the pressure consumed over the line by the idle component
of the current. This is at right angles to a c, hence draw a
line perpendicular to a c, as g c d. Then lay off the angle <^
from a as a centre to the right if lagging, to the left if leading.
The intercept c d is the pressure required; for dropping the
perpendicular d e upon c b, the triangles a b c and c d e are
similar, with their corresponding sides by construction respec-
tively proportional to cos ^ and sin <t>. Thus
c 6 == / iJ sin ^
ed =^ I (L„) sin <^
c d = / sin <^ V/22 H- (L„)»
Now draw Od which is the geometrical sum of Oa, ac, cd and
we have E, the impressed E. M. F. necessary to give 10,000
volts at the receiver under the assumed conditions. With E
as radius, draw the arc d f and J? is at once seen to be 14,200
volts. The point d corresponds to cos ^ = .90. For other
values lay off the appropriate angles and treat as before.
For angles of lead lay off the angles on the other side of a c,
as a g for cos ^ = .90. This gives B, which thrown down
upon the voltage axis gives 11,200 volts at the point b. This
shows less than the normal drop, since a leading current at
the load can only exist concurrently with condenser effects.
And capacity in the line remains to be considered. From
d lay off 0 fc = —and fc; = — -^, when / d becomes the
z ^
impedance for the charging current and 0 j the new impressed
E. M. F.
If capacity is an important item, it is easier, since it is
constant for all values of the load, to lay out dk and A;; at
the start, making d coincide with a and then starting the
fundamental power triangle from the new position of j as in
Fig. 262.
As a matter of fact, line capacity is not an important factor
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THE LINE.
621
in transmission, save in rather long lines at high voltage.
For example, in a 20-mile line at 20,000 volts, of No. 00 wire
spaced 24 inches, the charging current is about 1.6 amperes
per wire, the impedance factor of the line nearly 1.7, the
resistance of half a wire a little over 4 ohms, and the resulting
E. M. F. for capacity impedance only some ten or a dozen
volts.
For partial load regulation note that ah c, Fig. 261, holds
its shape for all loads and merely changes in magnitude. For
half load therefore, go half-way up along a c to the point Z,
which corresponds to c of the full-load diagram. Draw the
perpendicular corresponding to cd through I. Then for any
power factor, as .8, the intersection m gives the end of the
corresponding impressed E. M. F. as before. A system of
Fio. 262.
lines parallel with I m and for every tenth of a c, intersecting
all the power factor lines, makes it easy to determine the
regulation for almost any sort of load.
The rise of E. M. F. at the end of a line containing capacity
is one of the most striking features of alternating current
working, and while the constructions just given show its
amoimt, they do not at first sight disclose its physical signifi-
cance. The fact is, however, that a condenser is a device for
storing electrical energy, which is returned to the line in such
wise that its voltage is added (geometrically of course) to the
line voltage. It simply amounts to an electrostatic booster
of enormous efficiency, close upon 100 per cent, taking energy
from the line and utilizing it in raising the voltage. If the
capacity is distributed along the line, it takes a very long
line to do much boosting. If it is concentrated and consider-
able, as in a cable, the effect may be very striking.
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622
ELECTRIC TRANSMISSION OF POWER,
An over-excited synchronoiis motor, as we have already
seen, can be made to act like a condenser in the system, although
Table oi
f Natural Tangents, Sinks and Cosines.
o
Diff.
9
Diff.
!
Tan.
Sin.
Cofl.
Sin.
W
1
Tan.
Sin.
Cob.
Sin.
o«
.0000
.0000
1.0000
1
.0175
.0175
.9998
29
46°
11.0355
1.7193
.6947
20
2
.0349
.0349
.9994
29
47
1.0724
.7314
.6820
20
8
.0524
.0523
.9986
29
48
1.1106
.7431
.6691
20
4
.0699
.0698
.9976
29
46
1.1504
.7547
.6561
19
6
.0875
.0872
.9962
29
60
1.1918
.7660
.6428
19
6
.1051
.1045
.9945
29
61
1.2349
.7771
.6293
18
7
.1228
.1219
.9925
29
62
1.2799
.7880
.6157
18
8
.1405
.1392
.9903
29
63
1.3270
.7986
.6018
18
9
.1584
.1564
.9877
29
64
1.3764
.8090
.5878
17
10
.1763
.1736
.9848
29
66
1.4281
.8192
.5736
17
11
.1944
.1908
.9816
29
66
1.4826
.8290
.5592
17
12
.2126
.2079
.9781
28
67
1.5399
.8387
.5446
16
13
.2309
.2250
.9744
28
68
1.6003
.8480
.5299
16
14
.2493
.2419
.9703
28
69
1.6643
.8572
.5150
15
16
.2679
.2588
.9659
28
60
1.7321
.8660
.5000
15
16
.2867
.2756
.9613
28
61
1.8040
.8746
.4848
14
17
.3057
.2924
.9563
28
62
1.8807
.8829
.4695
14
18
.3249
.3090
.9511
28
63
1.9626
.8910
.4540
13
19
.3443
.3256
.9455
27
64
2.0503
.8988
.4384
13
20
.3640
.3420
.9397
27
66
2.1445
.9063
.4226
12
21
.3839
.3584
.9336
27
66
2.2460
.9135
.4067
12
22
.4040
.3746
.9272
27
67
2.3559
.9205
.3907
12
23
.4245
.3907
.9205
27
68
2.4751
.9272
.3746
11
24
.4452
.4067
.9135
27
69
2.6051
.9336
.3584
11
26
.4663
.4226
.9063
26
70
2.7475
.9397
.3420
10
26
.4877
.4384
.8988
26
71
2.9042
.9455
.3256
10
27
.5095
.4540
.8910
26
72
3.0777
.9511
.3090
9
28
.5317
.4695
.8829
26
73
3.2709
.9563
.2924
9
29
.5543
.4848
.8746
25
74
3.4874
.9613
.2756
8
30
.5774
.5000
.8660
25
76
3.7321
.9659
.2588
8
31
.6000
.5150
.8572
25
76
4.0108
.9703
.2419
7
32
.6249
.5299
.8480
25
77
4.3315
.9744
.2250
7
33
.6494
.5446
.8387
24
78
4.7046
.9781
.2079
6
34
.6745
.5592
.8290
24
79
5.1446
.9816
.1908
6
36
.7002
.5736
.8192
24
80
5.6713
.9848
.1736
5
36
.7265
.5878
.8090
24
81
6.3138
.9877
.1564
5
37
.7536
.6018
.7986
23
82
7.1154
.9903
.1392
4
38
.7813
.6157
.7880
23
83
8.1443
.9925
.1219
4
39
.8098
.6293
.7771
22
84
9.5144
.9945
.1045
3
40
.8391
.6428
.7660
22
86
11.430
.9962
.0872
2
41
.8693
.6561
.7547
22
86
14.300
.9976
.0698
2
42
.9004
.6691
.7431
21
87
19.081
.9986
.0523
1
43
.9325
.6820
.7314
21
88
28.636
.9994
.0349
1
44
.9657
.6947
.7193
21
89
57.290
.9998
.0175
0.5
46
1.0000
.7071
.7071
20
90
00
1.0000
'.0000
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THE LINE. 628
at less efficiency, and the angle of lead which it must give to
the receiver current in order to produce any desired effect
on the voltage can be deduced from the construction of Fig.
261. Practically, therefore, synchronous machines are very
valuable adjuncts in regulation. If rotary converters are
used, as in handling a railway load, they can be so compound
wound from the direct current side as to compensate for the
effect of their o^vn changing load upon the receiver voltage.
The same 'thing can be even more easily done with a motor
generator. Now and then on very long high voltage lines
it is desirable to add inductance at light loads to preserve the
regulation. For if one lays out the light load conditions in
Fig. 261 the capacity triangle, dkj becomes relatively* im-
portant. For convenience in computations a table of natural
sines, cosines, and tangents is annexed. The colunrn of dif-
ferences for the sines holds good for cosines of the same numer-
ical values.
Really the most serious practical difficulties in an ordinary
alternating plant are those in which the generator is involved
by inductances in the system. These are often of far greater
moment than the impedance factor of the line. An inductance
in the system produces two effects on the generator — first,
as just noted, it demands a larger current to deliver the same
energy; second, it tends to beat down the E. M. F. of the
machine. This effect is analogous to that produced by shift-
ing the brushes of a continuous current generator away from
the position of maximum E. M. F. (See Chapter V.)
This reaction of the armature is serious in that it not only
demands a considerable increase in the exciting current, but
causes a severe stram on the insulation when it suddenly
ceases. It is not uncommon to find an alternator that requires
on a heavy inductive load double the light-load excitation of
the field. For instance, if the voltage be 2,000 on open cir-
cuit, the excitation may have to be increased on inductive
load to a point that on open circuit would give 4,000 volts.
If, now, this load is cut off, or the line is broken, the insula-
tion will be exposed, momentarily, at least, to double the
normal voltage.
Such generators should not be i^sed oo inductive loads or
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624 ELECTRIC TRANSMISSION OF POWER.
in any case where the extra strain on the insulation is impor-
tant. It is perfectly easy to build a generator which requires
only 10 to 15 per cent more excitation at fuU and inductive
load than at no load, and such machines should be used in all
cases where a steady voltage under all working conditions is
needed. The other type has its uses, but the general work of
power transmission is not one of them. With a properly
designed machine, inductive load is little to be feared.
Another possible source of danger is that under certain
conditions of inductive load, the reaction of the load on the
generator, without materially lowering its effective voltage,
may so change the shape of the E. M. F. wave as to give to it
an abnormally high maximum, and thereby greatly to increase
the strain on the insulation. This effect may readily occur,
but usually u^ so small a degree as to be of little moment.
Occasionally, owing to a combination of severe inductive load
and badly designed generator, the results may be somewhat
formidable, the more so as the change takes place under
heavy load and not, as in the case just treated, only on open
circuit or a sudden light load. The rise in pressure thus pro-
duced may amount to several times the nominal voltage. The
same sound principles of design that insure good regulation
under changes of load will obviate any danger of this kind.
In fact, most of the possible disturbing factors in alternating
current work become negligible in an installation carried out
with regard for the general principles of good engineering.
These abnormalities of voltage lead natiu-ally to the con-
sideration of another far more serious, due to the static capac-
ity of the system. Of course, the fact that imder certain
circumstances capacity in the system will cause a lessening
of the apparent **drop" on the line, or even overcome it alto-
gether and show a higher voltage at the receiving end than
at the generator, is already well known to the reader. Under
certain conditions, however, this rise may become cumula-
tive, producing electrical resonance, the fundamental prin-
ciples of which have already been described.
Every electrical system has as we have already seen, a
definite period of oscillation determined by its particular prop-
erties. If we could apply an instantaneous electromotive
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THE LINE. 526
stress to any point of it, the effect would be that the result-
ing strain would travel back and forth with a definite fre-
quency until its energy would be completely exhausted by
doing work on various parts of the system. The action
resembles that which takes place when we strike the end of
a long rod with a hammer. An impulse is sent out at a rate
depending on elasticity, density, and so forth, travels to the
end of the rod, is reflected, and so goes on swinging back and
forth until the energy is frittered away. This corresponds
to electric oscillations on open circuit.
The two properties of an electrical system which determine
its vibration period are its self-induction, which is analogous
to inertia, and its capacity, which resembles elasticity in the
dielectric, capable of taking up and returning energy. Resis-
tance, like intermolecular friction in the rod just referred to,
determines the rate at which the vibrations will die out by
yielding up their energy to the system, but has ordinarily
a negligible effect on the vibration period.
This period in an electric circuit is given by the formula:
r = . 00629 Vi;c = ^Vlc.
In this T is the natural time period of the circuit expressed
in seconds, L is the coefficient of self-induction in henrys,
and C the capacity in microfarads. For example, suppose we
are dealing with a circuit of which the capacity is two micro-
^
mpsraimnr
Fig. 263.
farads and the self-induction one henry. Let it be arranged
as in Fig. 263. For simplicity the inductance and capacity are
shown localized and in series as would happen if a line ran
through a group of series transformers and thence into a cable.
If the line were open-circuited beyond the cable, we might find
a very severe strain on the cable insulation. The period of
this line would be .00887 second — about 113 cycles per second.
If this should chance to be the frequency of the generator it
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626 ELECTRIC TRANSMISSION OF POWER.
would be in resonance with the line, and each wave of E. M. F.
sent out by the generator would add itself to another wave just
starting out in the same direction. A period later these two
added E. M. F.'s would be reinforced by the next generator
wave, and so on indefinitely.
The only thing which prevents the resultant voltage from
rising mdefinitely is the effect of energy losses in causing each
wave to die out gradually as it contmues its oscillations, so that
only a limited number of waves can add materially to the
resultant E. M. F. across the terminals of the capacity.
In a given circuit the relation between the initial voltage
and the voltage of resonance can be easily determined to
a fair degree of approximation. It is, neglecting minor reac-
tions, as we have already seen,
^. ^-^ ^
R
In this equation E' is the rise of E. M. F. due to resonance,
n the frequency, L the self-induction in henrys, R the ohmic
resistance, and E the initial voltage. Applying this formula
to the case just discussed, and assuming the resistance of the
line to be 15 ohms and the initial voltage to be 2,000, we find
„, 113 X 1 X 2,000 .^_^ , ,
E^ = = 15,066 volts. A very moderate Ime
15
voltage might thus, in a resonant line, give rise to a pressure
quite capable of rupturing any ordinary cable, or causing serious
trouble (m an overhead line, to say nothing of greatly increas-
ing the danger to persons and property. If the working
pressure were 10,000 or 15,000 volts, the E. M. F. of resonance
might theoretically rise to an appalling amount.
Fortunately the theoretical value is in practice much reduced
by hysteretic losses and Foucault currents in any iron-cored
coils in circuit, waste of energy in the dielectric, and other
minor causes of damping the electrical oscillations, even when
resonance is complete. Still, dangerous rises in voltage are
very possible. When the frequency of the applied E. M. F.
differs somewhat from the natural period of the line, resonant
effects can evidently still take place, but in a rapidly lessen-
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THE LINE. 627
ing degree; when the oscillations are strongly damped by the
presence of iron, the total resonant rise is considerably dimin-
ished, but it varies less rapidly as the resonant frequency is
departed from.
A resonance curve for various capacities shows that the
rise of voltage extends over quite a wide range of variation
of capacity, but is large over but a small range. The shape
of such a curve necessarily varies widely, as the resonance is
more or less damped by resistance, iron-cored coils and so
forth; but we may be quite sure that the maximum resonance
will occur at not far from the point indicated by our equa-
tion for the vibration period of the circuit, and that the maxi-
mum E. M. F. of resonance will usually be considerably less
than that given by the theoretical equation.
In practical alternating circuits the current wave is never
truly sinusoidal, but consists of a main or fundamental wave
with the odd (i.e., 3d, 5th, 7th, etc.) harmonics of various
amplitudes superimposed upon it. In nearly every case the
third harmonic is the most prominent and is quite capable of
causing resonance, even to a dangerous degree, if it happens to
fall in with the frequency of the system. The point at which
resonance occurs and the rise of E. M. F. are found for the
harmonics by the formulae already given.
So far as the line is concerned, the facts regarding resonance
can be easily computed with tolerable accuracy. From well-
established data it is evident that the line capacities and
inductances are generally so small as to make the oscillation
period so short as not to correspond with the frequencies in
ordinary use except in the upper harmonics, which are generally
of small moment, although one case of severe resonance from
a higher harmonic (probably the 7th) has come to the author's
notice. For example, with a 7th harmonic of 1,000 volts
amplitude on a 10,000 volt line at 60-w, having an inductance
of .2 henry and a resistance of 20 w, the rise due to resonance
might be some 40 per cent of the line voltage.
It must be remembered that not only the line capacity, but
the capacity of the sending and receiving apparatus, must be
considered. The former is but small, except in the case of
underground or submarine cables, for which the capacities are
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528 ELECTRIC TRANSMISSION OF POWER,
likely to be from 4 to ^ microfarad per mile, as ordinarily
manufactured. High-voltage devices, like synchronous motors,
generators, and transformers, often may have static capacities
of several tenths of a microfarad, and inductances of several
hundredths of a henry. Resonance may involve the whole
system, or may at times be started in a minor degree in some
branch in which the natural oscillation period happens to be
just right.
As a matter of fact, experience seems to show that one is
not likely to stumble upon very serious resonance in overhead
lines, although in cables it is easily possible. On the other
hand, it is more than likely that resonance of a minor kind,
mostly from harmonics, is far commoner than is generally sup-
posed. It will be noted from the data given that L and C
on simple overhead lines do not vary over a wide range in dif-
ferent sizes of wire at the ordinary spacings. Both increase
directly with the length of the line, and so of course does y/LC
on which the natural frequency of the line depends. Bearing
this in mind, one can get a roughly approximate idea of the
natural frequency on which resonance depends. For fairly
long lines, say between 50 and 100 miles, N, the frequency in
question is likely to fall between 300^ and 500^, being
proportionately less for longer lines and greater for shorter
lines.
Obviously this value makes resonance with the funda-
mental generally out of the question, but gives a good chance
for the 5th and 7th harmonics.
Pure resonance with a periodic E. M. F. due to the generator
is therefore practically confined to harmonics, but there are
other sources of abnormal pressure on a transmission line.
Chief among these is surging^ which is due to the oscillations
of energy when a circuit which contains inductance and capa-
city is broken. It is a resonant phenomenon, depending as
it does on the line period, but ordinarily it falls in with no
source of cumulative impulses, which separates it from reso-
nance, ordinarily so called.
The theory of surging is comparatively simple. When a
circuit containing inductance and capacity is broken when
carrying a current /, a certain amount of energy is left momen-
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THE LINE. 529
tarily stored in the form of the electro-magnetic stresses in the
system. The energy thus cut off in transit, as it were, is
LP
2 '
This, for lack of other outlet, is thrown into the capacity, and
then, thrown back by it spring-wise, goes on thus oscillating
with decreasing amplitude until it is frittered away by ohmic
and other sources of loss.
But the energy stored in a condenser is
2
where E^ is the voltage across its terminals. And since in
surging, the energy in the condenser is that received from the
electro-magnetic storage in the line,
LP E^C
2 ~ 2 "
C is here taken in farads. The frequency of the oscillation is
evidently that naturally belonging to the system. Now this
frequency involves a relation between L and C, being 2 «• iV =
— = ; and now solving the energy equation just given for E^,
VLC
the E. M. F. of the surge, one obtains two correlated expres-
sions for E, one involving L and the other C, and both in terms
of the frequency and current, as follows:
E, = 2^NLI, (1)
Knowing / the current broken, L and C, the value of E is
obtained at once by substitution in either above equation (1)
or (2).
The Ej thus obtained is the alternating voltage as ordinarily
reckoned. Its crest is approximately E^ V2 volts, more if
the wave be peaked, and the maximum strain tending to
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680 ELECTRIC TRANSMISSION OF POWER.
break down insulation is this phis the crest of the impressed
E. M. F. It is not uncommon to find waves sufficiently peaked
to give E^^ = 1.6 E,
Based on somewhat rough approximations to the average line
constants, Baum has given an approximate equation E^ = 200 /
which is sufficient to give a working relation between the
current in amperes broken and the voltage rise of the surge.
This of course does not hold with cables in circuit or when the
inductance and capacity of apparatus are taken into account.
In any case there is a good chance of opening the circuit
at some other instant than that of maximum current. When
ordinary switching is going on, especially with oil switches,
there is rarely much surging, but a short circuit, particularly
in a line containing cables, is likely to make mischief.
Still apart from surging, is the group of impulsive disturbances
loosely classified as "static." They are exceedingly common,
since they result from all sorts of sudden changes of load,
switching on feeders, cutting in transformers, and so forth.
Suppose, for example, a long line is thrown on. There is
a sudden rush of current sending an impulse along the line.
This wave may be very abrupt, and, at the end of an open
line or at any electrical obstacle like inductance or a sudden
reduction in capacity, is wholly (for open circuit) or partially
reflected, and as the phase changes suddenly during reflec-
tion there is an impulsive rise in pressure, up to double the
wave voltage for total reflection with its phase change of a
quarter cycle.
The reflected wave in running back may coincide with the
crest of a secondary disturbance, or in very extreme cases
may fall into resonance; but as a general rule, the effect is
merely a sharp rise of pressure at the reflecting point, amount-
ing to an increase of perhaps 50 to 100 per cent in the nominal
pressure. In one particular the results may be serious, for
the wave front in thus charging a Ime may be so abrupt as
to be equivalent with respect to self-induction to a current
of enormous frequency. Reaching an obstacle like the pri-
mary of a high-tension transformer, the full crest of the wave is
upon it before the front has had time to penetrate far into
the coil, and there may thus result a dangerous concentration
Digitized by VjOOQIC
THE LINE. 531
of potential in the outer layers of the coils, sufficient to cause
punctures of the insulation. Grounds, short circuits, induced
or direct lightning discharges, or any sudden and violent
change of potential from any cause, may start a potential
wave abrupt enough to produce breaking down of insulation.
In practice ** static" comes thus from a wide variety of causes,
and, being impulsive, seldom is so much a source of danger
as a heavy surge or true resonance. Yet it sometimes pro-
duces punctures that are followed by the line current with
serious results. A very good account of " static " may be
found in two papers by Thomas.* In point of fact, resonance,
surging, and static may cooperate in the same phenomenon,
and it is generally difficult to analyze the result on the avail-
able evidence. The moral of all this is that, in the insulation
of high-voltage apparatus and lines, a considerable factor of
safety must be allowed, since the insulation may be subjected
to strains considerably greater than those due to the rated
voltage. Probably the most dangerous condition is a surge
following the breaking of a short circuit. With the relations
existing on overhead lines, between L C and R one is not
likely to get more than 3 to 4 times normal voltage. It is
well to estimate the surge for a short circuit midway the
line, and use the factor of safety thus indicated, bearing in
mind, as a favoring factor, the fact that the arc from a short
circuit softens the suddenness of the break, and lets down the
current. The worst cases will be met on underground systems,
and it is worth noticing that for a given amount of energy
transmitted the higher the voltage the less the current, and
the less the voltage rise due to interrupting that current.
On the other hand, near the highest voltages now in use there
is a tendency to trench on the factors of safety in insulation.
We have now investigated all the important factors that
enter into the design of a transmission line, whether for direct
or alternating currents. Let us review them with the idea of
seeing how they enter into practical cases. First comes the
all-important question of initial voltage, involving the choice
between the direct generation of the working pressure or its
derivation from transformers, if alternating currents are used.
* Trans. A. I. E. E., March, 1902, and June, 1905.
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532 ELECTRIC TRANSMISSION OF POWER.
We have already seen the practical limitations of voltage for
direct currents. With alternators the commutator troubles
are absent, and the limitations are those imposed by generator
design. The higher the voltage of a dynamo, the more space
on the armature must be allowed for insulation, thereby cut-
ting down the output of the machine. Hence the practicable
voltage depends on the size of the generator.
In a general discussion it is difficult to make exact state-
ments as to what can or cannot be done, but experience seems
to show that at present 10,000 to 13,000 volts are the
greatest pressures that can economically be derived from the
generator, even in very large units, while in units of 100 or
200 KW it is seldom advisable to go above 3,000 to 5,000.
Higher voltage than this has been attempted, but there is good
reason to believe that, except in very large machines, the loss
due to increased space required for insulation outweighs the
possible gains.
As to loss in the line, much has been said already, and the .
best advice that can be given is to make a few trial computa-
tions along the general lines indicated. Almost every case
will require special treatment in certain particulars, depending
on the conditions of service. For example, a common com-
plication is the supply of power or light, or both, at a point
perhaps half-way along the line. Then, accordiug to the
amount and kind of service, it may be desirable simply to
tap the line for power and use a motor generator for
lights, to establish a substation with regulating apparatus, to
compound the generator for the point in question and use
either of the above methods at the end of the line, to install
rotary transformers, or to run a separate line with regulators
at the generating station. Such details will be treated at
length later.
The line structure is generally of bare copper wire carried
on strong wooden poles. Do not put it underground unless
you have to do so for reasons now obvious. It may be
necessary to insulate portions of the wire, but it is best not
to put much faith in an insulating covering. Instead, it is
desirable to make a very thorough job of insulation at the
supports, and provide for the easy inspection of the line.
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THE LINE. 533
In using alternating currents, inductance in the line must
always be considered. Practically it means raising the voltage
of the generator or raising transformers, unless a fair part of
the load is in synchronous motors which can be employed
to counteract the inductive drop. In nearly every case its
real importance is small, in spite of its scaring the uninitiated
now and then.
So, too, with the inductive load. Its real effect is merely
to increase the current in the line by a small amount, usually
less than 20 per cent, and to demand increase of excitation
at the dynamo. If this is so designed as to regulate badly,
an inductive load will render it difficult or impossible to keep
a uniform voltage. On the other hand, a generator capable
of holding its voltage from no load to a full and inductive
load with an increase of only 10 or 15 per cent in the exciting
current, will usually give no trouble whatever with reasonable
attention to the regulators.
The total net result of inductance in line and load is to
call for a well-designed generator with good inherent regula-
tion and a reasonable margin of capacity. One who know-
ingly installs anything else deserves all the troubles that
inductance can produce.
Rise in voltage, on throwing off the load or through distor-
tion of the current wave by an inductive load, can be reduced
to insignificance by employing a proper generator, as just
noted. Aside from this, a mixed load, particularly if it con-
sists in part of synchronous motors, seldom has a bad power
factor or great and sudden changes in its amount. Exception
must here be made with respect to the constant current trans-
former systems exploited of late in connection with series
alternating arc lamps. These, unless fully loaded, give a
severely inductive load, and must be thrown upon the circuit
very carefully to avoid serious fluctuations of voltage.
As regards static disturbances, few overhead systems have
capacity enough to give cause for alarm. Difficulties are to
be looked for chiefly on very long lines, and those composed
in part of undergroimd or submarine cables. In these cases
one may sometimes knov»r the conditions well enough to cal-
culate the actual result in rise of voltage. More often the
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5S4 ELECTRIC TRANSMISSION OF POWER.
data are incomplete, and the simplest way out of the diffi-
culty is to try the effect of varying the capacity of the system
before it goes into regular operation. If the addition of a
condenser, say of one-third microfarad, makes a sharp varia-
tion in the voltage, look out for resonance and investigate the
capacity of the system, step by step. A change of capacity
or inductance can be made sufficient to avert any serious
danger of resonance under ordinary conditions. Resonance
chargeable to the variation of harmonics under changes of
load and to changes in inductance and capacity due to appara-
tus used on the system, is hard to foresee, and must be treated
symptomatically when it chances to appear.
In the practical computation of a line, the question of
allowable drop is generally settled by the regulation desired.
Too much loss makes it impossible to give good ser\'ice, and
a loss at full load of 10 to 15 per cent in the line and trans-
formers is about as much as can be endured, save on very long
lines where one has to make a \drtue of necessity. Eight or
ten per cent loss in the line proper is a common figure unless
power commands a very high price, or a limited source must
be fully utilized. As to voltage, 2,000 to 3,000 is the max-
imum which can conveniently be used in a general distribu-
tion without step-doAvn transformers. Hence many little
plants senduig power only two or three miles use such
voltage.
For serious transmission work, nothing less than 10,000
volts is worth considering. For 10,000 to 14,000 volts excel-
lent high-voltage generators are available, and save some-
thing in cost and efficiency. Roughly one can say that the
use of the high-voltage generator saves about $6 per kilowatt
transmitted. On going to a higher voltage with transformers
then one must be able to save $6 per KW in the line out of the
cost for the line at 10,000 to 12,000 volts, in order to make
the change worth the while. For any proposed voltage and
distance one can readily settle the economics of the question.
The nominal saving of $6 should, however, be verified for the
machines and equipment considered, since prices of generators
sometimes vary very irregularly. I^t us suppose, for example,
that we are investigating the advisability of using 12,500 volts
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THE LINE, 636
from the generator or 25,000 from raising transformers. The
latter will save 75 per cent of the copper required by the
former, which for equality should cost $6. The two schemes
will then be equal in cost at a distance for which the lower
voltage demands $8 per KW for copper, the percentage losses
being taken as the same. Now reduce the total copper cost
to pounds, insert in equation (5), page 510, and solvefor D^, or
put the total cost in (6) and solve for Dm-
With 15 cent copper, 1,000 KW, and 10 per cent loss, the
critical distance is just over 5 miles. In general, there will
be few transmissions over half a dozen miles in which it will
not pay to raise the voltage and install transformers. But
in any case where one for any reason does not wish to go to
the neighborhood of 20,000 volts on the line, the high-voltage
generator is preferable.
In leaving generator voltage, therefore, go to at least 20,000
volts and preferably to 30,000. Above that there should be
more caution in examining adverse conditions; but with a
reasonably good climate and topography, 40,000 to 60,000
volts are entirely practicable pressures, and in a few years
we shall probably be working at 80,000 to 100,000.
These extreme pressures will, however, seldom be needed
for ordinary transmission work.
Voltage and loss being settled, the next thing is to lay out
the line conductors, following the copper formulae already
given. Then with the approximate dimensions found, con-
stnict the regulation diagram, and plan in so far as may be
the load to aid the regulation. It is seldom that you cannot
find at least one big synchronous machine, the excitation of
which can be controlled. On very long lines look out especially
for the effects of capacity at light loads. Sometimes a few
large induction motors steadily loaded prove good counter
irritants. With the load roughly blocked out, look into the
conditions of resonance, surging, and so forth, and plan the
insulation precautions, keeping a special eye on cables and
their junctions to aerial lines. Line material, the mechanical
design of the line, and its construction, will next occupy our
attention.
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CHAPTER XIV.
LINE CONSTRUCTION.
The first consideration is the general question of location.
Other things being equal, it is obvious that a direct line is the
best, but as a matter of fact it is seldom altogether practicable.
A line must above all things be secure against interruptions,
and with this in view, both the location and the constructional
f eatings should be determined.
In smooth and easy country, a nearly straight line can usu-
ally be laid out. For large plants carrying large amounts of
power at high voltages, it is often desirable to buy the right of
way outright. Such has mainly been the policy pursued in the
transmission from Niagara to Buffalo, and, while expensive, it
gives an absolute command of the situation. In some States
electric light and power companies are given the right of
eminent domain to make such ownership possible.
In cases wherein the purchase of such a location is imprac-
ticable or would, as often happens, involve very serious expense,
the best thing is to secure right of way along the pubUc roads,
so far as they can be conveniently utilized, and right of way
for the pole line through such private property as may be in
the contemplated route. Rights along the public roads are
very desirable, as giving capital facilities for line inspection
and repair without added expense. It is well, in addition to
securing rights from the local governing body, to establish
friendly relations with the abutters and to secure a definite
understanding as to interference with trees, proximity to build-
ings, and the like. Right of way merely for the line across
private lands, with proper facilities for access, can generally
be cheaply secured. Many owners are public-spirited enough
to give it for the asking, or for very reasonable compensation,
when a strip of land has to be taken for a roadway.
In small transmissions the public roads are most desirable
as a route, using private lands only for occasional shorts cuts.
58C
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LINE CONSTRUCTION. 637
Since a good road along the pole line is highly desirable,
the route should be taken through clear and accessible country,
so far as is possible.
Places to be avoided when possible, even by a detour, are
marshes, where poles are always hard to set and maintain, and
roads are difficult to construct; heavily wooded country,
where there is constant danger to the line from falling
branches and the like; and rough rocky slopes, where construc-
tion is difficult, and the line, when constructed, is highly inac-
cessible. Sometimes the topographical conditions are such
that these difficulties have to be met, but they are always
serious.
In a wooded region the only proper plan is to seciu-e right
of way broad enough to permit clearing away the trees so
that they cannot interfere with the line wires, even were
branches to be blown off in a storm. Nothing short of a hurri-
cane sufficient to blow down large trees should possibly be able
to cause trouble; and when the neighboring trees are danger-
ously high, careful watch should be kept, and any weak or de-
caying tree at once cut down. The right of way may be some-
what expensive, but the service must not be liable to inter-
ruption by so probable a thing as the breaking of a branch.
It must be remembered that in high-voltage transmissions a
twig as big as a lead-pencil may, by falling across the line, start
an arc that will shut down the plant. Sometimes the use of
extra long poles may enable one to carry the wires clear of pos-
sible obstructions of this sort.
In mountainous regions poles may have to be set in very
bad locations, and sometimes for long stretches every hole
may have to be blasted at a cost of $5 to $10 per hole, but
such contingencies are not very common, and may often be
avoided by a moderate detour. It is better to go around a
mountain than over it, unless the distance is considerably
greater. When these questions arise they should be answered
by preliminary estimates. The country should be carefully
inspected and the relative costs of various routes looked into.
For a uniform coimtry the cost of poles and construction is
directly as the distance, and the cost of copper directly as the
square of the distance.
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638 ELECTRIC TRANSMISSION OF POWER.
In case the direct line leads into difficult country — over, for
example, a rocky hill where the poles would be hard to place
and much blasting would have to be done — a detour often may
cheapen construction. A brief computation will give the
facts. Suppose a 10-mile transmission of about 500 KW at
10,000 volts, for simplicity assumed to be on the monophase
system. The line would have to be about No. 0 wire for, say, 6
per cent loss, and the total weight of copper would be about
33,000 lbs. Suppose the average cost of poles and insulators
in position to be $5 in the open country, but that the direct
route lies for a mile over a rough hill, where holes would have
to be blasted and poles would be difficult to place. The extra
cost of this mile might readily be $500 to $600. Now if a devia-
tion of a mile would clear this hill, it would probably pay to
abandon the direct route. By taking the shortest available
course, the actual increase in the length of the route would
probably not exceed half a mile. This would increase the
weight of copper for the same loss by about 10 per cent, $495
at 15c. per lb., and would increase the cost of the pole line by
about $250 more. In such a case the increased accessibility
of the line, and the lessened cost of providing a road for inspec-
tion and repairs, would more than compensate for the small
difference in expense.
The same reasoning holds with respect to avoiding other
obstacles by making detours. It often pays to go somewhat
out of the way to utilize the public roads, to cross rivers on
existing bridges, and so forth. A few experiments on the route
constructed on paper, after careful inspection of the country,
will usually show the most advantageous line to follow. The
old and simple process of sticking pins in the map and follow-
ing up the line with thread is generally the easiest way of getting
the approximate distances.
In mountainous country a direct line is often out of the
question, and the line has to conform to existing trails with
such shorts cuts as may be possible. An occasional long span
will sometimes lessen the cost of the line materially. Rivers
and lakes often form very serious obstacles to line construc-
tion and call for much skilful engineering. The former can
often be crossed on existing bridges or by long ppans, which
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LINE CONSTRUCTION.
689
will be discussed later, but the latter usually have to be gone
around, although sometimes cables may have to be carried
under water, or long suspension spans erected carrying the
conductors clear across. The latter plan is preferable in most
cases, and a cable should be taken only as a last resort, unless
in the rare case of the obstacle being near one end of the line,
so that the cable may be for the originating or the receiving
voltage.
Nearly all long lines have to encounter more or less serious
obstacles of the sorts mentioned, and as a rule they cause con-
siderable deflections from a straight course. Sometimes devia-
tions are desirable merely as the cheapest way of reaching en
route localities where power is to be distributed, a matter
which a few trial computations will settle.
LINE WIRE.
As already mentioned, copper is the best and most usual
material for conductors; soft-drawn copper under ordinary
circumstances, hard-drawn when extra strength is desirable.
No other material gives so advantageous a combination of con-
ductivity and tensile strength for nearly all purposes. The
tensile strength of the copper is raised by hard drawing from
about 34,000 to 35,000 lbs. per square inch to 60,000 or even
70,000, and the resistance is only raised 2 to 4 per cent, the
latter amount only in small sizes. Often a medium hard-drawn
Tensile Strength
Permissible
Gauge
B.&B.
Diameter
Aree Circu-
Wt., Lbs., per
(Ultimate)
Tension
Mils.
lar Mils.
1,000 Feet.
Based on ai,000
with Factor of
Lbs. per Sq. In.
Safety 6.
0000
460,000
211,600
640.73
5,640
1,128
000
409,640
167,806
508.12
4.480
896
00
304,800
183,079
402,97
8,553
711
0
824,960
105,592
319.74
2,819
564
1
289,300
83.684
253.43
2,236
447
2
257,630
66,373
200.88
1,772
344
8
229,420
62,633
150.38
1,405
281
4
204,310
41,742
126.40
1,114
223
6
181,040
33,102
100.23
884
177
6
162.020
26,250
79.49
700
140
7
144,280
20,816
63.03
556
111
8
128,490
16,509
49.99
440
88
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540 ELECTRIC TRANSMISSION OF POWER.
wire is used having a tensile strength of, say, 45,000 to 50,000
lbs. per square inch. Such wire is materially stronger than
the annealed wire, and yet is much easier to handle than such
hard-drawfi wire as is used for trolley wires.
For line copper the wire should be free from scale, flaws,
seams, and other mechanical imperfections. It should be very
close to its nominal gauge, variations of 1 to 2 mils being the
largest which should be tolerated, and should be within 2 per
cent or less of standard conductivity, as given for pure copper
in tables of wire.
The foregoing table gives the standard mechanical constants
of the sizes of wire commonly used in power transmission work.
The various constants should none of them fall short of these
tabulated values by more than 2 per cent.
For hard-drawn copper wire the tensile strength should not
fall short of 1.75 times the values given for annealed wire, save
in case of wires intentionally drawn only to medium hardness,
in which case the factor is generally about 1.5. Medium hard-
drawn copper is strongly to be recommended for transmission
work, and has to a great extent replaced ordinary soft-drawn
copper. The elastic limit of hard-drawn copper wire ranges
from 30,000 to 40,000 lbs. per square inch according to the
nature of the drawing.
When in use, wire is subject to serious mechanical strains,
due in the first place to its weight and normal tension, second
to variations in tension by change of temperature, and third
to extraneous loads like ice and wind pressure, separately or
combined. These last-mentioned strains are sometimes for-
midable and must be carefully taken into account, particularly
in cold climates.
When a wire is suspended freely between supports, it takes
a curve known technically as the catenary. The exact solution
1
D
Fio. 264.
of its properties is very difficult, but for the case in hand the
catenary comes very close to the parabola, a much simpler
curve to compute ; and based on this approximation the follow-
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LINE CONSTRUCTION. 641
ing simple deductions can be made: If a wire be stretched
between points A and B, Fig. 264, it assumes the curve A D B.
The thing to be determined is the relation between the length
A B (which we may call L, the length of span), the vertical
deflection d, at the middle point of the span, and the tension
on the wire at il or B as a function of its weight. This relation
is as follows:
or transposing,
Here L is the length of the span in feet, d the central deflec-
tion in feet, w the weight of the wire in pounds per foot, and
T the maximum tension on the wire in pounds.
These equations show that with a given wire the tension
varies inversely as the deflection for a given span, and that for
a given tension and wire, the deflection must increase with the
square of the span. Obviously, shortening the span and in-
creasing the deflection eases the strain on wire and renders
the construction more secure, but shortening the span adds
considerably to the cost, and increasing the deflection increases
the danger of the wires swinging in the wind and touching each
other. To prevent this, the deflection should not much
exceed twice the horizontal distance between wires.
The application of the formulae can be shown by an ex-
ample. Suppose we are stringing No. 00 wire on poles 100 ft.
apart. What is the least deflection allowable with a factor of
safety of 4? This means that T must not exceed one-fourth
the breaking strain of the wire, which fraction from the table
is 888 lbs. The weight per foot from the table is .4 lb. Sub-
stituting in equation (2) we have:
d = ^^r^^'o^J^ = .57 foot = 6.8 inches.
This minimum deflection should not be exceeded in this case,
and hence must be applicable to the lowest temperature to
which the line is to be exposed. At whatever temperature
the wire is strung, enough deflection should be allowed so that,
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642 ELECTRIC TRANSMISSION OP' POWER.
as the wire contracts in cold weather, the above minimum
should not be passed.
The total length of wire in the catenary is approximately
£' = i + 3^; (3)
or transposing for the value of d,
■ ,.^!l^E5. (4)
wherein U is the actual length of wire, and L the span.
From these formulae we can figure d for any temperature.
The coefficient of expansion of copper is .0000095 of its
length per degree Fahrenheit, so that we can get at once the
length for any temperature.
If the wire we are considering is stnmg at 75° F. and is to
encounter a minimum temperature of — 5° F., enough deflec-
tion must be allowed at the former temperature to bring the
deflection at —5° F. to the value just obtained. The length
of wire at the lower temperature is from (3),
Z* = 100 + ^4S^ = 100.0096.
At 75° F. this length would be increased by 100.0086 X
.0000095 X 80 ft., and hence the new value of L^ would be
100.076 ft. The deflection corresponding to this is found from
(4) as follows:
:=V/
•^^ _ 1.69 ,t, . 20.28 i„„k„.
A large allowance in deflection must, therefore, be made for
such variations in temperature as are likely to be encountered
in northern climates.
The changes in deflection due to changes of temperature are
found in practice to be somewhat lessened by the fact that the
wire as strung is under tension due to its weight, which modi-
fies its expansion and contraction. The actual coefficient for
copper wire under various tensions has never been properly
investigated. It undoubtedly is subject to considerable vari-
ations, and .000005 is perhaps a fair approximation.
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LINE CONSTRUCTION. 64S
This matter of temperature is imfortunately not all that
must be looked out for. We have fully taken care of the
weight of the wire itself, but it is exposed to other and some-
times dangerous forces in the weight of the ice coating that
is to be feared in winter, and the strain of wind pressure on
the wire either bare or ice-coated.
Taking up these in order, let us suppose the wire to become
coated with ice to the thickness of half an inch, quite a pos-
sible contingency in severe winter storms. A layer of ice of
this thickness would weigh 0.54 lb. per linear foot, thus loading
the wire with more than its own weight. Assuming this load
at the minimum temperature of —5° for which the assumed
deflection was 0.57 ft., the tension of the ice-loaded wire be-
comes from (1),
^^gooyxf, 2,051 lbs.
8 X .57 '
This is dangerously large, far beyond the elastic limit of the
wire, and more than likely to bring down weak joints.
And beyond this the wind pressing must be considered.
This may be taken as acting at right angles to the weight of
the wire and adding materially to the resulting total stress.
The total pressure P on a wire is, per foot, approximately
P ■■= .05 p D, where p is the normal pressure of the wind per
square foot, and D is the diameter of the wire in uiches. p
varies from a few ounces per square foot in light breezes to 40
or 50 lbs. in a hurricane.
Assuming 40 lbs. as the greatest pressure likely to be en-
countered, we can at once find its effect on the line under con-
sideration. For our No. 00 wire,
P = .05 X 40 X .364 = .728 lbs.
This pressure is combined with the weight of the wire as a
force acting at right angles; hence the resultant stress, which
we may call W, is
W = \ly^ + P^ = V(.4)2 + (.728)' = .83.
This, from the example given, is obviously a dangerous strain
on the wire. But the combination of even half the normal
wind pressure just assumed with an ice-coated wire would be
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644 ELECTRIC TRANSMISSION OF POWER.
disastrous. Taking the ice as half an inch thick as before,
D = 1.36 P = .05 X 20 X 1.36 = 1.36,
and
W = V(.94)2 _,. (1.36)3 ^ 1 65.
Substituting in (1),
This is over the breaking weight of the wire, which must con-
sequently give way, and would almost infallibly wreck the line
in so doing. This means that the factor of safety of 4, assumed
at the start, is too small for due security. It is sufficient for a
moderate climate, where high wmds are rare, but 5 is generally
preferable, while 7 or 8 should be used in cold and exposed
regions. It must be remembered that joints are weak points
in the wire; a carefully soldered Western Union joint has only
about 85 per cent the strength of the wire. Fort-imately, trans-
mission lines seldom accumulate half an inch of ice. One-
quarter of an inch is an unusually thick coating, and with very
high-tension lines there seems to be a tendency to check the
formation of a sleety covering. For extreme tensions a
stranded conductor of hard-drawn copper is advisable as being
more reliable than a single wire, and possessing a much higher
available elastic limit.
The same process that served to take account of an ice coat-
ing, 2.^., adding the distributed load to the weight of the wire,
can be readily applied to finding conditions of safety in the use
of bearer wires carrying the conductor suspended from them.
An interesting corollary to these computations is finding the
maxinmm length of span which can safely be used in an emer-
gency such as crossing a river or canon. Suppose we use
simply hard-drawn copper wire of the same size as before. Its
ultimate tenacity is about 6,270 lbs. Using it with a factor
of safety of 6, the permissible value of T becomes 1,045 lbs.
TT is as before 0.4 lb., and we will assume that for the purpose
in hand the wires are spread and the deflection is permitted to
be 10 ft. From (1) we have for the permissible length of span
V w
(6)
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LINE CONSTRUCTION. 545
Substituting the above values of the known quantities, we have
/8 X 104i
1046x10 ,K^ . « ^
— = 467 + feet.
Ten, however, is a preferable factor of safety, which corresponds
to a length of span of 354 ft. In extreme cases a bearer of
steel cable may be used, of the highest available tenacity, and
carrying the copper line wire to secure the requisite conduc-
tivity, or a steel or silicon bronze wire may be used alone; the
conductivity being made up elsewhere in the line to the de-
sired general average. The steel is rather the more reliable of
the two, but is more likely to deteriorate through rusting. An
ultimate tenacity of 150,000 lbs. per square inch is the limit for
either material, with factor of safety of 10 for practical working.
Now assuming No. 00 silicon bronze or its equivalent in steel
cable and the same factor of safety as before, the working ten-
sion rises to 2,612 lbs., and allowing 20 ft. deflection, the pos-
sible length of span is
v^
8 X 2612 X 20 . ^oo . * 4.
— -: = 1,022 + feet
Spans of even this length can be managed without any very
elaborate terminal supports. When the line wires are heavy
and numerous, or longer spans must be used, it may be neces-
sary to use stout bearer cables, arranged like a rudimentary
suspension bridge with a footpath, to facilitate inspection and
care of the conductors. The expense of such a structure is
sometimes justified by enabling one to avoid long and expen-
sive detours. When a simple long span of conductors is used,
the support of the ends and the proper insulation of the tense
wires require care. A timber truss well guyed will answer
in most cases, and the strain may be distributed among several
stout insulators. The conductors should always be in duplicate
across such a span. Increasing the deflection is the simplest
and most effective way of securing a proper factor of safety
in the conductors. Line construction for power transmission
was originally patterned after the construction usual with
telephone and telegraph wires, and followed with little modi-
fication hi early electric light plants. The spacing of the poles
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646
ELECTRIC TRANSMISSION OF POWER.
follows the precedent established for poles loaded with cn)ss
arms and wires strung so close that it was necessary to pull
the wires taut to avoid fouling them. There is no reason for
using anything but very moderate tensions on lines with the
wires spaced 3 to 6 ft. apart.
Of course, to the eye of the old telephone constructor, a
line with large deflections looks very slip-shod, but actually
it is far safer and more desirable than a taut line. From
Al
T^fw
8 9 lU 11 U 18 14 16 l(i 17
TJTTrTTTTTTTTTTTTTTTTrTTTTTTTq
TTTTTTT
0.3 0.4 0.& 0.« 0.7
percent increase in length of conductor
Fig. 286.
T1m-^Tt;^t-1
the properties of the catenary, it follows that tension in-
creases very rapidly as the deflection decreases, while the
length of the conductor between supports changes very little.
Fig. 265 shows graphically the relations between the deflec-
tion as a fraction of the span, tension in terms of the weight
of conductor in the span, and variation in the length of the
catenary. It will at once be seen that in reducing the de-
flection below about 2 per cent of the span length, the ten-
sions increase very rapidly, while the change in the length of
the conductor is very trifling, hardly more than a few tenths
of a per cent.
It pays, therefore, to use fairly large deflections in all cases
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PLATE XXI.
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LINE CONSTRUCTION, 647
where the line is exposed to severe strains. The curves of
Fig. 265 give sufficient data for the solution of most line prob-
lems, save in the case of very long spans in which generally
copper would not be used, and which demand precise calcu-
lation.
One need merely make the span tight enough to avert risk
of swinging together, and to keep the wires from being too
near the ground at the centre of the catenary. Any more
than this is a concession to appearances, harmless enough if
not carried too far. Deflections of 2 to 3 per cent of the span
at normal temperatures are none too great for most situations.
Bearing in mind that the variation in the length of con-
ductor between supports changes with the temperature to the
extent of probably about one one-hundredth per cent for each
20° F. when under strain, one can quickly approximate the
variations in the factor of safety from Fig. 265. Of course
there are small variations in the strength and elasticity of the
wire which might be taken into consideration, but there is so
much uncertainty about the actual coefficient of copper wire
when changing temperature under strain, that the most one can
do is to keep on the safe side, perhaps even to the extent of
using the coefficient imreduced for strain, which then amounts
to about one-hundredth per cent elongation for 11° F. In the
140-mile transmission of the Bay Counties Power Co. to Oak-
land, Cal., an extremely long span became necessary in cross-
ing the Straits of Carquinez. The problem was to cross a
deep, swift, navigable waterway, 3,200 ft. wide at the narrow-
est point. Submarine cables were out of the question, and
the United States Government required 200 ft. above high-water
mark for the lowest point of any suspended structure.
On the north shore, on a point 160 ft. above high water, was
erected the skeleton steel tower shown in Plate XXI. On
the south shore there was higher land, and a similar tower
65 ft. high sufficed. The construction adopted was that gen-
erally used for steel tower work; and each tower bore near its
top, four massive wooden out-riggers surmoimted by the insu-
lated saddles that carried the weight of the cable spans.
As in suspension bridge work, the cables rest upon rollers
upon the saddles, and then extend far shoreward to the anchor-\
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ELECTRIC TRANSMISSION OP POWER.
ages, where the strain is taken. From anchorage to anchor-
age the span is 6,200 ft. Each cable consists of nineteen
strands of steel, galvanized, is seven-eighths of an inch in dia-
meter over all, and has the electrical conductivity of No. 2
copper. The breaking strain of the cable is 98,000 lbs., each
span weighs 7,080 lbs., and, as suspended with 100 ft. dip, has
a factor of safety of 4.
Two difficult problems of insulation were presented. First,
the great weight of the cable must be supported at the saddle
Fio. 2e6.
with insulation adequate for 60,000 volts. Second, the pull
must be taken at the anchorage with equally high insulation.
The pull of the cable being 12 tons, the task at the anchorage
was by far the more difficult of the two.
At the saddle the weight is taken upon huge triple petticoat
porcelain insulators, each built up of four great nested porce-
lain cups, the inner being filled with sulphur, securing a large
steel pm. Six such insulators, each 17 in. in diameter over the
outer petticoat, cooperate to sustain the pressure at each
saddle. Fig. 266 shows a cross section through insulators,
supports, frame, and saddle. The heads of the insulators are
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LINE CONSTRUCTION.
649
built into a timber platform which serves at once as a rain
shed and a base for the cast-iron saddle proper. This carries
in line five steel grooved sheaves over which the cable passes.
Fig. 267 shows the structure in longitudinal section, together
with the suspended platform beneath it, for ease of access.
The strain insulators for the anchorage are of highly ingen-
ious construction. Micanite seemed to be the only insulating
substance possessing the necessary mechanical strength, and
to prevent surface leakage across it the surface exposed to
BadikyvfuUMdwlA
Hlo If
Fig. 2C7.
leakage was enclosed in an oil tank. Fig. 268 shows the struc-
ture of the completed insulator more plainly than description.
Two of these insulators are put in series and enclosed m a
shelter shed to keep off water, for each cable, the pair being
secured to a long tie rod anchored in a massive bed of concrete.
Great care was taken in all the details of the structiu-e to
secure all the insulation practicable, even the timber out-
riggers carrying the saddle insulators being filled and varnished,
and the foundation timbers proper being boiled in paraffin.
The use of four cables gives one reserve conductor in case of
accident. The total length of the span from tower to tower
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ELECTRIC TRANSMISSION OF POWER.
is 4,427 ft., 2i times the span of the Brooklyn Bridge. It is
one of the most striking enghieering feats in the records of
electrical power transmission, and has withstood successfully
very severe tests.
Such structures are necessarily costly, but they are more
reliable than submarine cables and cheaper than long detours.
It should be noted that the deflection of the cables is over 2
per cent of the span length, and that even so the factor of
FlQ. 268.
safety is not so great as is generally advisable. But this
merely means that the span is nearing the maximum lengtji
advisable without a greater deflection, which could well have
been given had it been necessary, by making the towers some-
what higher. It represents a rather extreme but necessary
construction, and has done its work admirably.
When bodies of water too wide for a suspended structure
must be crossed, there is trouble ahead. In marshy shallows
a timber trestle is perhaps the best way out of the difficulty,
but in deeper water cables may occasionally have to be used,
although rarely in view of the possibilities of very long spans
like the one just mentioned.
Cables can be obtained that will stand 5,000 to 10,000 volts
alternating current under water with a fair factor of safety.
Above this pressure success is problematical. Near the ends
of the line before the raising or after the reducing transformers,
cables may be successfully used ; but when the obstacle is in the
middle of a long line, the choice is between evils, reducing the
pressure locally by an extra transformation, or going the long
way around. Either expedient is costly and to be avoided if
possible. It is almost needless to say that when cables are
used they should be in duplicate.
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LINE CONSTRUCTION.
561
POLES.
As a rule, all aerial lines in this country are carried on wooden
poles. Iron poles are used much for railroad work, and abroad
considerably for miscellaneous work, including power trans-
mission.
GENERAL DIMENSIONS OF POLES.
Total
Length in
Feet.
Diameter
at Top-
Inches.
Diameter 6*
from Butt,
in Inches.
Depth of
Setting
Approximate
Weight,
in Pounds.
Number that Can
Be Loaded on
a Pair of Cars.
35
40
46
60
n
n
8
8
12i
13
14
16
5' 6''
6'
6'6"
r
650
900
1,000
1,300
90
76
66
50
In the eastern and central parts of the United States, white
Northern cedar, chestnut, and Northern pine are the most
desirable woods for poles, in the order named. West of the
Rocky Mountains, redwood is a favorite, and stands even ahead
of cedar in estimation. Abroad, Norway fir is highly valued.
For power transmission work the poles should be both long
and strong — long to carry the wires well out of reach and
often above other circuits; strong to stand the pressure of the
often heavy wires and the wind. In open coimtry the length
is less important, and it is sometimes well to use rather stubby
poles, say not over 35 ft., but extra stout. The poles should
be straight and free from knots, of soimd, live wood, and the
bark should be peeled and the poles trimmed and shaved.
The foregoing table gives the size and other characteristics
of the poles most likely to be used on power transmission work.
This is based on cedar poles, and the dimensions given are the
minimum to be p^mitted in first-class line construction.
Pine and redwood and chestnut are somewhat lighter than
poles of the weight given. For the best utilization of the
lumber, the top diameter of the pole should be about J of the
diameter at the ground. Natural cedar poles commonly show
rjither more taper than this, natural chestnut poles rather less,
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552 ELECTRIC TRANSMISSION OF POWER.
It will be noted that poles of these lengths have generally to
be carried on two cars, one being too short. Various preserva-
tive processes are used to increase the life of wooden poles.
Of these, "creosoting" is generally preferred. The process
consists of stowing the poles in an air-tight iron retort, treat-
ing with dry steam for several hours, and then forcing in the
preservative fluid, preferably tar oil from coke ovens, under
heavy hydraulic pressure. Creosoting is more effective on
open-grained timber than on harder woods, and when properly
performed will give a pole life three or four times longer than
if untreated. The process does not weaken the wood imless
the preliminary steaming is at too high temperature or too long
continued. Cross arms, pins, and the like are best treated by
the vacuum process at a moderate temperature.
When not specially treated, the poles should be coated
heavily with pitch, tar, or asphalt on the portion to be buried
up to and fairly above the ground level.
The pole top is usually wedge-shaped or pyramidal, and this
roof should be painted or tarred. Before the pole is erected,
the gains for the cross arms are cut, and the cross arms them-
selves should be bolted in place and the pins set for the insu-
lators. The upper cross arm centre should be 10 to 18 inches
below the extreme apex of the pole, and the lower cross arms 18
to 36 inches further down. In power transmission work em-
ploying heavy wires, the spacing of the cross arms should be
guided by the arrangement of the circuits, there being no
standard practice.
The cross arms themselves are of wood, having the same
characteristics of strength and durability as the poles; hard
yellow pine being rather a favorite. They are, of course, of
such length as the work demands; m power work, generally
from 4 to 8 ft. There are two sectional dimensions in common
use, 4} X 3t in., and 4J x 3 J in., also a 4 X 5 in. section for heavy
work. The latter should be used for the longer cross arms and
those carrying heavy cables or the like, while the former serve
for 4 or 5 ft. arms not heavily loaded. The cross arms are best
secured in their gains by a strong iron bolt passing through both
the pole and the cross arms in a hole bored to fit, and set up
hard with wide washers under head and nut. This construc-
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LINE CONSTRUCTION. 653
tion makes a cleaner job than the practice of fastening the
cross arm with two lag screws, and permits of easier changes
and repairs. The bolt should be about three-quarters of an
inch in diameter, and the gain is from 1 to 2 in. deep, according
to the size of the pole. Lag screws are cheaper, however, and
are, as a rule, employed in ordinary work. Cross arms 6 ft. long
or more should be braced.
In ordinary transmission circuits about 50 poles per mile
are used, 110 ft. apart, or 48 per mile, being a common spacing.
The setting should be carefully done. The earth should not be
disturbed more than enough to make easy room for the pole,
and the earth and gravel filled in aroimd the pole should be
heavily tamped. When setting poles in soft groimd, it is some-
times impossible to give them stability enough merely by tamp-
ing, and the best procedure is to fill in concrete about the pole,
using one part of Portland cement to three or four parts of sand
and heavy gravel or broken stone.
The stresses to which a pole line is exposed may be classi-
fied as follows: 1. The direct weight of the wire and the down-
ward component of the wire tension. 2. Bending moment due
to the pull of the wires at turns in the line. 3. Wind pressure
on poles and wires. 4. Wind pressure plus ice.
1. In power transmission lines built as has been indicated,
the crushing stress is completely negligible. The ultimate
resistance against crushing amounts in the woods used for poles
to at least 5,000 lbs. per square inch. The ordinary pole, there-
fore, has a factor of safety of several hundred, and the danger
of crushing, even from tense and ice-laden wires, has no real
existence.
2. Bending moment is more serious, since the forces acting
have a long lever arm. The ultimate eifect of this stress is to
break the pole, generally near to the surface of the ground, by
crushing the fibres on the side next the stress and pulling apart
those on the other side. The pull or push necessary to break
a round pole by bending is approximately
where A is the area of the pole section at the ground, S the
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554 ELECTRIC TRANSMISSION OF POWER.
strength per unit area, R the radius at the ground, and D the
distance between the ground and the centre of pressure.
For example, take a 40 ft. pole, 13 in. in diameter at the
ground. Taking S = 7,500 lbs. per square inch and the centre
of pressure as 32 ft. above the ground, (6) becomes
The factor of safety allowed should be never less than 5, and up
to 8 or 10 in cases where high winds are to be expected. Square
sawed poles are relatively weaker than natural poles, and may
be approximately figured by the same formula, taking R as half
the side at the ground. The values of S are rather uncertain,
but the figure given is about right for the woods customarily
used in large sticks. Small samples run relatively higher
from bemg of selected material.
The following table gives the commonly received tensile
strengths for the American woods generally used hi electric
construction, the figures being derived from small samples, and
hence to be taken with reservations in the case of poles, while
fairly applicable to cross arms and pins.
^T _ 1 Value of S per
W^- square incfiT
Cedar 11,000
Chestnut 10,000
Yellow Pine 12,000
Hickory 14,000
Redwood 11,000
White Oak 14,000
Locust 20,000
Practically, poles at angles should always be guyed, like ter-
minal poles. This is best done with a steel rope one-quarter
to one-half an inch in diameter, taken from as near the centre of
the stress on the pole top as the position of the circuits permits.
The guy rope should extend downward at an angle of from 45**
to 60® with the pole, directly back from the direction of the
pull on the pole, and should be drawn taut and securely fas-
tened to a tree or a firmly set post. Where there are three or
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LINE CONSTRUCTION, 556
four cross arms, what is known as a y guy is often used, con-
sisting of a guy roj)e attached near the pole top and another
just below the cross arms. These divide the tension and are
moored by a single guy rope in the ordinary manner. This
arrangement is not commonly needed in transmission work
save when the circuits are numerous or the strain exception-
ally severe, and in any case great care should be taken to keep
the guy wires well clear of the high-voltage lines. Sometimes
two or more light guys in different directions are valuable in
securing a pole, when proper setting is very difficult, and may
save expensive blasting.
The bendmg moment due to an angle is normally 2 T cos
^ where T is the tension as already determined and o is the
angle made between the wires at the turn. For the simple
circuit of No. 00 wire already discussed and a turn with 120°
between the wires, taking a factor of safety of 7 on the wire, the
tension per wire is 507 lbs. The total pull for the two wires
forming the circuit is then 2,028 lbs. x cos 60° = 1,014 lbs.,
a pressure rather greater than would be permissible without
guying.
3. The wind pressure on the wires has already been com-
puted, and the same formula serves for figuring the pressure
on the poles, using the mean diameter in inches, and for the
total pressure, multiplying by the feet of pole exposed. For
example, assuming a pole of 34 ft. out of ground, 7 in. diameter
at the top and 13 in. at the ground, the average diameter is
10 in , and for a storm giving a normal wind pressure of 40 lbs.
per scjuare foot,
P = .05 X 40 X 10 X 34 = 6801b8.
This acts virtually at the middle point of the pole, hence it is
equivalent to 340 lbs. at the pole top, to which must be added
the pressure on the wire itself, which for the circuit in ques-
tion amounts to about 145 lbs. more, making a total of 485 lbs.
This is well within the safety limit, and would remam so even
if there were half a dozen wires instead of two. As 40 lbs. per
square foot is an extreme wind pressure, never met in most
localities at all, it is safe to say that a well-set line of the poles
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556 ELECTRIC TRANSMISSION OF POWER.
assumed, loaded with any power transmission circuit likely to
be met in practice, is perfectly secure so far as wind pressure
alone is concerned, unless the line is literally struck by-
a cyclone.
4. The most dangerous stresses on an aerial line come from
sleet storms that load the wires with ice, increasing the weight
and the lateral thrust due to wind pressure. On rare occasions
ice may be formed on wires to the depth of a couple of inches.
Such a coating on a No. 00 wire would weigh about 5.9 lbs. per
lineal foot. The mere weight of this would produce a ten-
sion, assuming d = 2 ft., and No. 00 wire as before,
which is well above the tensile strength of the wire if soft-
drawn. Allowing a wind pressure of 20 lbs. per square foot,
the pressure on a single span of 100 ft. would be
P = .05 X 20 X 4 X 100 = 400 lbs.
Adding to this 170 lbs. pressure on the pole itself, the total for
a single circuit of 2 wires would be 970 lbs. total thrust, which,
while high, is not likely to carry down the pole. Even 6 No. 00
wires would give a total thrust of only 2,570 lbs., which is still
below the ultimate strength of the pole. The pole line is there-
fore stronger than the wires. If a line is to stand such extreme
stresses, which are far beyond really practical requirements,
the only safe plan would be to string hard-drawn wire, shorten
the poles and increase the diameter, and guy frequently. As a
matter of fact, the insulators and their pins are quite sure to
give way before the wires or poles imder these extreme stresses,
and in most transmission lines are the greatest source of
anxiety.
The insulators themselves can be made strong enough to
stand the greatest stresses to which they will be subjected, but
it is not easy to so support them as to give ample strength
without endangering the insulation. The ordinary wooden
pin answers well if the circuits are not very heavy or likely to
be weighted with ice.
By common consent, locust is the wood best suited for pins,
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LINE CONSTRUCTION.
567
which for general line work are about 12 ft. long and 2 in. in
extreme diameter at the shoulder, below which the pin is
cylindrical and 1^ in. in diameter. This fits a hole bored in
the cross arm and is secured by a nail driven through arm and
pin. The top of the pin is threaded for the insulator to be
used. Under extreme forces these pins are liable to break at
the shoulder; and for transmission circuits carrying very
heavy wire, for long spans, and for cases where special insula-
tors demand extra long pins, a variation of this construction
A I
1— 1— r-'-r
ir •■^~*
a
-ii Ll a.
In II: II
U. ii
B
-Li A
*
^IK-
^m
III ^
?~in
Fio. 269.
is desirable. On the Pacific coast excellent results have been
obtained from eucalyptus pins, which are even tougher and
stronger than locust, but unfortunately not readily obtainable
in the East. Lacking both locust and eucalyptus, a fair pin
may be made from seasoned oak. Pins for heavy transmis-
sion work may with advantage be made much heavier than
ordhiary up to 2^ in. at the shoulder and up to 2 in. in the
cylindrical base, the standard pin-hole in the corresponding
insulators being If in.
In ordinary line work, the pins are set 12 to 14 in. between
centres. With heavy wires this distance may advantageously
be increased to 18 to 24 in. At very high voltage these dis-
tances must be increased farther, perhaps up to 48, 60, or
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ELECTRIC TRANSMISSION OF POWER.
sometimes to 72 inches and more in dealing with voltages in
the uncertain region beyond 50,000 volts.
When the lines have to be traiisi^)sed, as in long parallel
alternating power circuits, this transix)sition involves some
careful work, for the wires must be kept well clear of each
other. Heavy strain pins will generally answer the purpose
and allow the transp<^sition to be safely made. Such trans-
position should not be made at an angle or elsewhere where
the tension on the insulators is imusually great.
FlO. 270.
A good example of line construction for hea\'y transmission
work is found in the Hne constructed a few years ago for the
Niagara-Buffalo power circuit. Fig. 269 shows the pole head.
The cedar poles, intended ultimately to carry 12 cables each
of 350,000 cm., are extra heavy, varying from 35 to 50 feet
in length with tops 9 and 10 in. in diameter. The two mahi
cross arms are of yellow pine, 12 ft. long and 4 X 6 in. in sec-
tion, fastened to the ix)le with long lag screws, and braced by
an angle iron diagonal } X 2i in., bolted to the pole and to
the bottom of the cross arm at each side. Each side of each
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LINE CONSTRUCTION, 659
arm is bored for three pins spaced 18 in. apart. The trans-
mission is three-phase, and one complete circuit is on each side
of each cross arm. The cross arms themselves are 2 ft. apart.
The pins and insulators, Fig. 270, are special, the pins being
much heavier than usual, and the insulators of dense porcelain
formed in the usual double petticoat design. They have one
peculiar feature: a gutter is formed on the external surface,
leading to diametrically opposite lips so placed as to shed
dripping water clear of the cross arm, thus lessening the dan-
ger of ice formations. Each of the main circuits is designed to
transmit 5,000 HP. A short cross arm below the others carries
a private telephone line. The right of way is in part owned by
the operating company and fenced in, and in part along the
Erie Canal. The line is elaborately transposed every five
poles to annul induction. So frequent transposition is un-
usual and generally needless. Transposition every 20 to 40
poles is ample for ordinary cases, and on long lines in open
country it is enough to transpose once in a couple of miles.
This line is admirably constructed, but it is a grave question
whether all the circuits should be carried on a single pole line
on account of the difficulty of executing repairs, and the insu-
lators are rather closer to the cross arms than seems safe in
view of the climate and the high voltage to be employed. Cer-
tainly at voltages above 10,000 a duplicate pole line is prefer-
able to running two circuits on one pole line. It is, however,
entirely feasible to execute repairs on one side of a pole like
Fig. 269 while the circuit on the other side is in use, although
it is a careful job, and should not be attempted unless, as in
this case, the cross arms are unusually long.
Another admirable type of high-tension line construction
is found in the lines of the Missouri River Power Co., of which
Fig. 271 shows the pole head and detail of pin and msulator
construction. This line, it should be said, is 65 miles long,
and has been in regular service for four years at 57,000 volts
with unusual immunity from interruptions of service.
The poles are of cedar, varying from 35 to 75 ft. according to
the necessities of the case, with tops from 9 to 12 in. Poles are
normally spaced 110 ft. The cross arms are of Oregon fir, and
the pins of oak boiled in paraffin. The insulators are glass,
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ELECTRIC TRANSMISSION OF POWER.
with an additional glass sleeve surrounding the pin almost
down to the cross arm. An especial feature of this system is
the use of white oak braces for the cross arms instead of the
usual metal, the change being in the interest of insulation be-
tween wire and wire.
This principle has been carried still further in the long trans-
mission line from T^gan to Ogden and Salt liake City, Utah,
in which case the entire pole construction is wood, the cross
arms being mortised through the pole. Locust pins, paraffin-
treated, are used of extra length so as to carry the insulators
Fio. 271.
well above the cross arm. The change from metal braces and
cross arms was made as the result of bitter experience, it hav-
ing been found that in wet weather these metal parts became
the seat of trouble by burning the adjacent wood, especially
in case of a broken insulator producing considerable leakage to
the cross arm. On the other hand, iron braces and iron or
steel pins are in common use on some very large high-voltage
systems with apparently excellent results.
At times wooden pins have given much trouble from burn-
ing, owing to leaky insulators, and show a strong tendency to
"mould" and soften in the thread and at the cross arm. This
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LINE CONSTRUCTION. 661
has been traced to the action of the bnish discharge at high
tension, probably setting free nitric acid from the moisture
present. In certain locaUties a combination of moisture and
dirty insulators has been very destructive of wooden pins, in
one plant causing 26 shut-downs from burning, in a single
month. The truth seems to be that untreated or imperfectly
treated wood may be rapidly attacked in a moist atmosphere
by the discharge at high voltage over dirty insulators. With
thoroughly treated wood this difficulty disappears. On the
Logan line referred to, the pins are treated as follows. The
locust pins are heated with stirring in vats of hot paraffin at
150° C. for 6 to 12 hours, and then are kept submerged during
gradual cooling. Thus treated, the paraffin saturates the pins
Pig. 272.
clear to the core, and they give practically no trouble from
burning or "moulding." There seems to be no good reason
why a pin thus treated should not stand up well in almost any
climatfe.
Steel or iron pins, however, are very advantageous in the
matter of strength, and give admirable service. They are sub-
ject to the difficulty of putting severe strain on the insulator
thread if used alone, so that it is desirable to use a lead bushing
around the steel pin to furnish the thread, or otherwise to
interpose soft material. Steel pins are now made with sleeves
of treated wood for the thread portion, and with porcelain
sleeves covering the shaft of the pin clear down to the cross
arm after the idea of Fig. 271. Such a composite pin is shown
in Fig. 272. The wood, of course, may be replaced by lead if
anybody objects to wood, and the' porcelain sleeve retained.
As between wooden and steel pins, the mechanical advantage
when the strains are severe is with the latter, both on accoimt
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562 ELECTRIC TRANSMISSION OF POWER.
of intrinsic strength and less wea .ening of the cross arm <5n
account of smaller diameter. Electrically, the advantage lies
on the side of well-treated wood. Either can and does give
good service at the highest voltages yet employed.
Much the same sort of question arises as between wooden
and iron poles and cross arms. Iron poles are considerably
used abroad, although not on such high voltage as is used in
the large American systems. They are excellent mechanically,
and have a very long life. On the other hand, they cost sev-
eral times as much as wooden poles and when used with iron
cross arms as usual, carry the earth potential squarely up into
the interior of the insulator itself. Any failure of the latter
means an instantaneous and complete shut-down of the line,
which is a very serious contingency.
As a rule, failure of an insulator on a wooden pole line does
not do this. It may cause progressive burning and leakage,
which gives warning of trouble and eventually may become
serious, but often gives ample opportunity for temporary re-
pairs before it puts the line out of service. With these condi-
tions it seems like taking unwarrantable chances in the present
state of insulator construction, to replace wooden poles by
iron in the ordinary form of line construction.
An altogether different question is raised by the introduction
of the tower construction which has been in successful use for
a year or so in the Guanajuato transmission plant in Mexico.
The plan here followed was to employ steel towers of sufficient
height and stability, not only to replace wooden poles but to
admit the use of very long spans, thus greatly reducing the
number of insulating supports which are by common experi-
ence, the weakest points in the line. The tower and tower head
is shown in Fig. 273. The structure is a standard 45 ft. gal-
vanized steel windmill tower anchored at each comer to a
concrete foundation. The span employed is 500 to 600 ft., the
conductors being of hard-drawn copper cable equivalent to No. 1
B & S. The deflection in the centre of the span is variously
stated at from 7^ to 18 ft. At the former figure, the factor of
safety would be less than 3 as regards the ultimate strength of
the cable, and less than 2 on the elastic limit. At the latter
figure the conductors would be less than 20 ft. above the ground
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LINE CONSTWCTION.
563
at the centre of the span. The actual deflection is probably
intermediate between the reputed figures.
These towers cost laid down from Chicago between $60 and
$70 each, and from 9 to 12 would be required per mile. Assem-
bling and erecting amounted to about $7 each, bringing a con-
1 \4'ft.ft^Ch«aBtf
».«.J^
»«»»3it.
««««^L
/i
/ ■
JU M
4 6.ft^ChaaMl
^
e*ttlii( at top of Ifti
.8«XL
m
PiO. 273.
servative estimate of the line structure to about $750 per mile,
including insulators.
The construction is a very ingenious one, and possesses great
convenience for regions where poles are scarce or where the
ravages of insects are to be feared. If constructed, however,
with the factors of safety generally to be recommended in over-
head lines, the cost would run materially higher than that just
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664
ELECTRIC TRANSMISSION OF POWER.
stated, by reason of the use of more or higher towers, to allow
shorter spans or more deflection. Employed with caution the
plan is mechanically good. Electrically, it is open to the ob-
jection to all metal structures of complete dependence on the
insulators, in this case subject to more than usual strain.
The reduction of the number of insulators is a material gain
over ordinary practice. It should, however, be pointed out
that the long-span principle in a less extreme form can readily
be carried out with wooden poles, employing conservative
FlO. 274.
factors of safety and still giving a material gain in cost and in
the number of insulating supports.
A stout 40 or 45 ft. pole will carry 3 cables, such as are used at
Guanajuato, on a spacing of, say, 20 poles to the mile with a
considerable improvement in the factor of safety and at about
half the cost of construction under ordinary conditions. The
difference in annual charges on the cost is great enough to pro-
vide for the replacement of wooden construction every dozen
years, even assuming eternal life for the steel.
Barring local conditions, modifying considerably the rela-
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LINE CONSTRUCTION. 605
tive costs, or affording special reasons for discrimination, ex-
pense is about a stand-oflF between the two, save as a low first
cost is sometimes important. Either can be made mechani-
cally secure, and the real question between them is whether,
in the present state of the art in insulators, one is justified in
constructing a line in such wise that failure of an insulator
implies a complete and instantaneous shut-down of the line.
If one is so justified, then the steel structure has much in favor
FlO. 276.
of it, especially on large systems — if not, then the use of steel
involves taking long chances for dubious benefits.
Insulators for high-tension work are now generally of j)orce-
lain, although glass is being successfully used, as in the Missouri
River Power Co. plant just referred to, and in the great Utah
system. The form of insulator used in the latter case is shown
in Fig. 274. It is only 7 in. in diameter, but with the long
paraffined pins and wooden construction there used, it has
given good service for half a dozen years past at 40,0(K)
volts.
Porcelain insulators, although more costly than gla.sH and
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ELECTRIC TRANSMISSION OF POWER.
requiring individual testing, are mechanically stionger, and
probably stand weathering more successfully. As has already
been indicated, the path of the discharge when an insulator
"spills over" is from edge to edge of petticoats and thence to
pin or cross arm by the nearest course, so that, irrespective of
other things, the insulator must have a long sparking distance
if it is to be successful at high voltage. With either glass or
first-class well- vitrified porcelain the insulation strength of the
material is ample, and insulators rarely fail by puncture unless
Pig. 278.
mechanically defective.
Fig. 275 shows a typical insulator of the kind used for very
high-voltage. It is made in three pieces to insure proper
baking of the porcelain, which is difficult in large masses. The
diameter of the upper petticoat is 14 in., and the sparking dis-
tance is 8i in. The test voltage is about 150,000 and the
line voltage 60,000.
Fig. 276 shows a somewhat different design of about the same
dimensions, but with a sparking distance increased to 9J in.
These big insulators are shipped in pieces and cemented when
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LINE CONSTRUCTION. 667
one is ready to test them prior to installation. Fig. 277 is a
somewhat smaller design, intended for 50,000 volt work, with
an upper petticoat llj in. in diameter and a sparking distance
of 6 J in. All these have pin holes If in. in diameter to take an
extra heavy pin, either steel with bushing, or wooden. An ex-
cellent example of glass insulator for medium voltages, say up
to 20,000, is shown in Fig. 278. This is 7 in. in extreme dia-
meter, with a sparking distance of 3 in., and a If in. pin hole
like the others. All have top grooves, which are preferable
for high voltage.
Now, all these insulators are well made and pretty well de-
FiG. 277.
signed, and have been used with success, but none of them has
a high factor of safety. If one glances at the curve of striking
distances already given, it is apparent that the sparking dis-
tances for the insulators are for the higher voltage barely twice
the possible striking distance of the normal voltage. This is
not enough, considering the possible rises of potential due to
surging, resonance, and static effects. To increase the size of
the insulators means increased difficulty of supporting them,
increased cost, and greatly increased difficulty in getting first-
class porcelain. A porcelain or glass sleeve over the pin seema
a very desirable safety precaution.
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668 ELECTRIC TRANSMISSION OF POWER.
The factor of safety, as regards both dielectric strength and
sparking distance, should be raised to not less than 3 and pre-
ferably higher.
One of the undetermined points in power transmission is the
degree of immimity from line troubles which may fairly be
called successful operation. All systems suffer more or less,
but it seems to be impossible to give actually continuous ser-
vice at the higher voltages wdthout duplicate lines. One
experienced engineer regards the conditions as very good if
one per cent of the insulators do not have to be replaced yearly.
If the line were of steel poles, cross arms, and pins, this would
insure an abundant supply of shut-downs not to be averted
unless by a complete duplicate pole line, and not certainly
then. In case of a wooden construction, complete failure
from broken insulators can nearly always be avoided if a
spare line is available.
Fig. 278.
All lines alike are liable to be the victims of casual accidents,
like branches falling or being blown across the circuit, large
birds flying into the wires, lightning, and all sorts of curious
and apparently trivial causes.
The chief trouble, however, is in the failure of insulators
from one cause or another, and next to that stands lightning.
Lightning, although fortunately not a continuous risk like
insulators, is a very dangerous contingency, the more so since
no system of defence has proved entirely effective. Lightning,
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LINE CONSTRUCTION, 569
SO far as transmission systems are concerned, is of two sorts.
First in danger comes the lightning discharge, which actually
strikes, fully or by a branch discharge, the lijie structure itself.
This is, luckily, an imusual happening, but a very serious one.
Second comes the induced discharge along the circuit, due to the
tremendous effect of a lightning discharge at a greater or less
distance. These induced discharges constitute the vast ma-
jority of all so-called cases of lightning upon the circuits. They
are in effect surges of potential of sometimes very great vio-
lence, but never comparable with even a minor direct stroke.
These surges seem to be enormously abrupt, and when
checked as by an inductance, their sparking power is somewhat
formidable. It is very difficult to form any proper idea of the
potentials concerned in an actual lightning stroke, but they
run to many millions of volts, probably several hundred mil-
lions at times. The induced discharges which make up prob-
ably 99 per cent of so-called lightning, are of a very different
order of magnitude, but they certainly give rise to sparks hav-
ing striking distances corresponding to voltages up to consider-
ably above 100,000 volts. The majority of such discharges,
however, are of minor violence, but quite sufficient to pimcture
insulation and cause serious damage to apparatus.
An actual stroke of lightning upon a line is to be dreaded. It
frequently shatters insulators and poles, and may break down
apparatus as well, especially if near the station. It will some-
times distribute its effect for a quarter mile or so from the
striking point, doing more or less damage at every pole. It does
not, upon a wooden pole line, necessarily shut down the line,
although of course it may do so. A duplicate pole line is the
best safeguard against lightning, since it is highly improbable
that in a single storm both lines will be hit hard enough to put
them out of action.
That component of a direct stroke which follows the lines
to the station is like an unusually severe induced surge, and
must be dealt with as best one can.
Lightning arresters, so called, are merely devices for giving
induced or other discharges an easy path to groimd, while
checking the tendency of the line current to follow them. They
consist essentially of spark gaps connecting the line with a
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ELECTRIC TRANSMISSION OF POWER.
ground wire and means for checking the following rush of cur-
rent from the line. In addition, reactive coils are usually
placed between the machines and the arresters to check the
surge, and throw it over the gaps to earth.
As now generally arranged, lightning arresters consist of a
series of short spark gaps bctw^een metal cylinders, m series
FlO. 279.
with resistance enough to attenuate the folio whig line current
sufficiently to allow the arcs across the spark gaps to go out.
As the line voltage increases, more and more gaps and more and
''Xiiiilniiiir
PlO. 280.
more resistances are put in series. For convenience, the gaps
are arranged in groups and assembled as required. Fig. 279
shows a Westinghouse unit, and a General Electric unit respect-
ively. The former has six gaps, the latter four and two high-
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PLATE XXIL
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LINE CONSTRUCTION.
671
resistance carbon sticks. The Westinghouse Company as-
sembles its resistances separately.
Either can be arranged to work double pole for low voltage,
and the General Electric unit is shown so arranged. Both
companies use reactance coils next to the apparatus. Fig. 280
shows the General Electric form of coil. The individual gaps are
about -g^ in., and for high voltage enough units are assembled
to aggregate the required striking distance. The Westinghouse
Pig. 281.
cylinders forming the gaps are made of an alloy which pro-
duces a non-conductive oxide when vaporized by the passage
of an arc, while the General Electric cylinders are more massive
and of somewhat different alloy, believed to act mainly by
chilling the feeble arc permitted by the series resistances.
Either is effective under not too severe conditions.
These components are assembled as convenient, generally
in regular panels. Plate XXII shows a recent type of West-
inghouse arrester for moderately high voltage including re-
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ELECTRIC TRANSMISSION OF POWER.
actance coil arrester cylinders and resistances. Fig. 281 shows
a General Electric arrester set for a three-phase line, equipped
with cut-off switches and static protectors in the form of the
intermediate high resistance carbons, shown as uniting the
phases midway the groups of arrester units. The office of
these is to allow minor rises of potential to be equalized through
the auxiliary resistances, without having to leap the whole
series of spark gaps, while a heavy lightning surge will go to
LINE
Fig. 282.
ground in the ordinary way. The arrester of Plate XXII has a
similar function, the connection being diagrammatically as
shown in Fig. 282. Minor disturbances are eliminated via the
high-shunt resistance, the series gaps being proportioned to
spill over on comparatively small rises above the line voltage.
Heavy discharges are sent to earth over the full series of
gaps and the relatively low series resistance.
The Westinghouse Company also employs, especially for the
protection of very high-voltage apparatus, a device shown in
Fig. 283, known as a "static interrupter.'' It consists of a
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USE CONSTRUCTION,
673
reactance coil in the line, and a condenser connected between
the coil and the apparatus. One pole of the condenser is
grounded, and the other is brought to the coil, both being en-
closed in an oil-filled case. This device very considerably
attenuates static waves from any source, the wave being, as it
were, checked by the coil and soaked up by the condenser,
to be frittered harmlessly away in minor oscillations.
The devices here described are the best yet devised for deal-
ing with lightning. They are imdoubtedly effective against a
PlO. 283.
very large proportion of the induced class of discharges, but
any and all protection is liable to failure in case of a heavy
direct stroke.
As the line voltage is raised, it becomes increasingly difficult
to deal with the tendency to ^hort-circuit after a heavy dis-
charge over the arresters. On the other hand, a system that
is insulated for 60,000 volts with a factor of safety of, say, 2^
has a margin of some 90,000 volts insulation to protect
it against the effect of static surges. Hence, such a system is
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674 ELECTRIC TRANSMISSION OF POWEIt.
fairly immune as regards most induced lightning discharges,
although perhaps in increased danger from direct strokes.
At the power station and at sub-stations it is wise to install
as complete lightning arrester systems as the state of the art
permits. If, as is desirable on long lines, section houses are pro-
vided with means for cutting out and switching the lines, and
to serve as headquarters for line inspection, arresters may be
provided there also. But the custom of installing arresters
on the line at frequent intervals has been abandoned on account
of the elaborate nature of the protection required for high vol-
tage and the likelihood of trouble after a severe discharge.
At one time grounded conductors stretched along the pole
line were considerably used, but in high-tension work they are
generally considered as of dubious utility, ineffective against
the class of lightning strokes most to be feared. If used at all,
they should be of strong stranded steel cable such as is used for
guy wires, the barbed wire sometimes used being too weak for
safety. The most that can be said for the groimded wire is
that it may sometimes be of use locally as auxiliary to other
lightning protection.
Experience indicates that the best way of stringing a three-
phase transmission line is in the usual form of an equilateral
triangle with the apex uppermost. It is undesirable to run
more than two circuits per pole line at high voltage; and for
security, at pressures of 50,000 volts and upwards, it is highly
desirable to run but a single circuit per pole line unless in very
large plants.
Circuits should be transposed at convenient intervals to keep
down mutual indirection^ especially at the higher voltages.
Practice varies in this respect, from transposing every mile or
less, to making only a few transpositions in the entire length of
the line. If section houses are used, these afford convenient
points for spiralling the lines, which at high voltage is some-
what troublesome.
It pays to make a very careful and thorough job of the line
and to use only the best material.
The life of a pole line is a varying quantity, according to the
character of the material and the soil in which it is fixed. With
wooden poles the chief danger is of course rotting at and below
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LINE CONSTRUCTION.
676
the surface of the ground. If one starts with a dry pole,
thorough and repeated painting with tar-oil, asphalt, or the
like, from the butt to a little above the ground line, will greatly
increase the life of the pole. A well set and treated line should
last at least 15 years, and if the poles were actually "creo-
soted," as railway ties are, this life should be extended for
another decade. But lines set with green poles in damp earth
are likely to require heavy repairs within 6 or 8 years, if not
sooner. Thorough treatment at the start and judicious inspec-
tion are necessary to keep a line in proper shape. The me-
chanical danger points in a line are changes in direction,
whether horizontal or vertical, and changes in lengths of span.
Double arms and proper guying will make these points safe.
One of the minor difficulties in line work is making a safe
entrance into the power station and other buildings which the
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ELECTRIC TRANSMISSION OF POWER.
high-tension lines have to enter. The chief requirement is
ample space around the conductors. Up to 10,000 volts or so,
long porcelain wall tubes set through the walls with a slight
downward slope toward the outside do very well, the wires
being supported on each side of the wall by line insulators.
At really high voltages the striking distance is so consider-
able as to call for better insulation around the wires, and many
plans have been tried, mainly based on a wide hole with the
wire held centrally in it by suitable insulators. Two of the
best schemes for entrance hitherto tried are those shown in
Figs. 284 and 285.
These are nearly self-descriptive. In Fig. 284 the purpose
2 D X S {^"k s']|Kri>lll
t"x u'aiaat.Tabe
FlO. 285.
of the perforated plate glass cover at the inner end of the
tile is to keep out the cold. Otherwise, the tile might as well
be open, and open tiles are frequently used. The little rain
fehed and the downward slope of the wire to keep away
dripping water are of obvious use.
Fig. 285 is an excellent construction for cold climates. A
long tube of high-grade porcelain may well replace the glass,
and the tube should be given a slight slope as in the previous
case.
High- voltage wires should never be brought through a roof,
or into any contracted place. Allow plenty of space about
them. Inside the building they are sometimes insulated, but
should be treated with the same respect as if they were bare.
In certain cases it becomes necessary to carry high-tension
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LINE CONSTRUCTION.
677
conductors underground. This is always to be avoided if
possible, but if necessary it can be done at present up to pres-
sures of about 25,000 volts. The best plan is to use a three-
conductor lead-covered cable run in vitrified clay ducts, well
drained, and with frequent manholes of ample size so that the
cable joints can be carefully made and supported.
Fig. 286 gives a full size section of the class of cable most
commonly used for high-tension undergroimd work. The
insulation is of paper, well impregnated with insulating com-
pound. Each conductor is served with a heavy coating of this,
the interstices are packed with jute and insulating compound.
BUmndO
MOOOVoiti
Fia. 286.
and the whole is given an external wrapping and then leaded.
This insulation is wholly dependent on the integrity of the lead
covering, and hence the joints must be made and protected
most carefully, but it has proved very reliable. Cables are
used mostly below 12,000 volts, for which the insulation need
not be as thick as that shown. To a certain extent leaded
cables insulated with rubber and with varnished cambric are
also used.
The jimctions between cables and overhead lines are danger
spots with reference to lightning and static surges generally,
and should be protected by static dischargers and lightning
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578 ELECTRIC TRANSMISSION OF POWER.
arresters on just the same plan as high- voltage apparatus in the
stations. Underground cables should be laid in duplicate,
since a fault cannot generally be quickly found and repaired.*
A well-nigh indispensable accessory of every, power trans-
mission line is a private telephone line connecting the power
house with the sub-stations and with intermediate points.
Such a line is usually carried on side brackets attached to the
poles six or eight feet below the power wires. This line is most
often a metallic circuit of galvanized iron wire, about No. 12
in size or larger on long lines, carried on ordinary glass insu-
lators and transposed every twenty poles or so. Such lines
can be made to give fair service, but the transposition of the
wires has to be very carefully adjusted to suppress induction.
The lengths of wire under induction must agree, not within a
few poles merely, but within a few feet, to avoid annoying sing-
ing. The two sets of insulators should be kept at a uniform
distance from the main line, and the wires should be drawn
imiformly tight and so transposed that taking the line from end
to end, each wire shall have just half its length on the upper
and half on the lower bracket, or on the right and left insu-
lators if a short cross arm is used.
The wires, too, must be kept clear of grounds from foliage
and other interference, in order to keep the inductive balance
perfect. With care in stringing, the line can easily be kept in
good operative condition, but is seldom free from some residual
induction. Such lines should be fused, protected with light-
ning arresters, and provided with insulated platforms for those
using the instruments.
A far better although considerably more expensive line is
obtained by using the twin-wire insulated cable made for tele-
phonic purposes.
On long lines it is good policy to make provision, say every
10 miles or so, for getting at the high- voltage line for repairs.
» For much valuable though sometimes discordant information on
modern line construction, see the Trans. Int. Elec. Congress, St. Louis, 1904,
Section D, Vol. II, especially the papers of Baum, Gerry, Converse, Buck,
Blackwell, and Nunn, to whom the author stands indebted. It will be
sufficiently evident that the problems encountered are complicated and
difficult.
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LINE CONSTRUCTION.
679
If the line is in duplicate, it should be so arranged that at these
junctions jumpers can easily be put on or switches closed
between wires in the same phase, and a section of one of the lines
cut loose so that it can be readily handled. At such points
there should be opportunity for cutting in a portable instru-
ment on the telephone line. Telephone boxes, Fig. 287, much
like the ordinary police signal box, can be obtained, and may
advantageously be permanently installed at the ends of these
FlO. 287.
line sections. These are good points, too, for installing line
lightning arresters and making provisions for testing.
The commonest accidents on high- voltage lines are short
circuits from branches of trees and broken insulators. The
effect of the first is to start an arc that is likely to bum down
the line, if the branch is more than a mere twig. There are
great fluctuations of current and voltage, and the character of
the accident is generally evident. Broken insulators may in
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580 ELECTRIC TRANSMISSION OF POWER.
dry weather produce no sensible effect at all, but if the cross
arms are damp there may be serious leakage between line and
line that sometimes ends by burning up the cross arm or even
the pole top. Broken insulators can be replaced if necessary,
while the line is "alive," even when carrying pressures as high
as 15,000 or even 20,000 volts. The line affected can be pulled
or pushed clear of the cross arm and held clear while the line-
man puts on a new insulator, preferably one with a top groove
for ease of manipulation. Then the line can be pulled back
into position and an insulated tie wire put in place, if needful,
with long rubber-handled pliers. It takes a skilful and cau-
tious lineman to do the job, but it can be done if necessary. It
is best not to trust to rubber gloves, as they are seldom in good
condition, and there is nearly always enough leakage around
the pole top to give a powerful shock. Sometimes, when work-
ing at such a job, a nail is driven into the pole well below the
workman, and a temporary jumper thrown from it over the wire
under repair so that the lineman will be less likely to get leak-
age shocks, or the cross arm is temporarily grounded by a wire
for the same purpose.
Duplicate lines are much easier to repair, since one can then
work on dead wires, and for very high voltages duplicate pole
lines are better still; but, with care, it is far safer and easier
to work on high-voltage lines than is generally supposed.
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CHAPTER XV.
METHODS OF DISTRIBUTION.
In most cases of power transmission, the primary object is
the supply of power and light in various proportions through-
out a more or less extended region. Therefore, the question
of methods of distributing electric energy, after it has been
received from the transmission line, must often be carefully
considered. The subject may conveniently be treated in three
divisions: First, distribution direct from the transmission cir-
cuit without the use of special reducing transformers or sub-
stations. Second, distribution from scattered sub-stations.
Third, distribution from a main reducing station. These divi-
sions do not have rigid boundaries and often overlap, but they
involve three quite diverse sets of conditions.
Into the class first mentioned fall all the ordinary electrical
installations wherein the power station is separated from its
load by a transmission line. This line is usually of moderate
length, for otherwise the voltage used would need to be reduced
for the workmg circuit, and the region supplied is generally a
town or city of moderate size. Such cases are common enough,
and generally arise from the existence of a convenient water-
power half a dozen miles, more or less, from a town that needs
light and power, or that has already a central station which
from motives of economy it is desirable to operate by water-
power. The power is therefore developed and new distribution
lines are erected, or the old ones reorganized. The whole con-
dition of things is closely similar to ordinary central station
practice, save that the load is all at a considerable distance
from the station. Only the use of alternating current need be
considered, since this current alone is practically employed for
general purposes at distances above a mile or two.
The rudimentary map, Fig. 288, gives a case typical of many.
The power station is at 4, with a line across country to the
town which is to be supplied with light and power. The dis-
681
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ELECTRIC TRANSMISSION OF POWER.
tance to the touTi, A B,\& perhaps four miles. Now the prob-
lem is to distribute the energy derived from A over the town
in the best and most economical way. Since much lighting as
well as motor service is to be done, good regulation is essential,
while abimdance of water makes small v&riations in efficiency
of little moment. The town is scattered, with a main business
street C, running lengthwise through it.
BUKKVJLLE
FlO. 288.
There is here little object in a sub-station, for the distances
are too great for convenient distribution at low voltage, and
the short transmission makes it desirable to avoid raising and
reducing transformers. The choice of a system is the first con-
sideration. This is not a question of such vital importance as
the average salesman hastens to proclaim. The skilful organi-
zation of the installation will make much more difference in
the general success of the plant, than the particular species of
apparatus used. This should, however, be determined with
due regard to the local conditions.
Any altemattrig system except plain monophase can be con-
veniently used, and monophase is inapplicable only in default
of suitable motors, which are not at the present time available
in this country, at least in any form which warrants their use
in cases where motors are to form any considerable portion of
the total load. With a moderate amount of motor service in
small imits, the monophase system answers the purpose excel-
lently. Something depends on the character and amoimt of
the motor service. If it be very considerable and in all sorts
of service, general experience both in this country and abroad
indicates some advantages in triphase apparatus. This advan-
tage, however, depends more on the ease and economy with
which a triphase distribution can be carried out, when motors
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METHODS OF DISTRIBUTION. 583
and lights are to be served in the same territory, than on any
intrinsic advantages in the motors. When made with equal
care and skill, all polyphase motors are substantially alike in
their properties. Details of the various systems of distribu-
tion will be given in treating sub-station work. Where the
motor service consists of a few large units, even the monophase
system with synchronous motors is entirely practicable,
although seldom advisable. Diphase and triphase systems can
be advantageously applied to any case that is likely to arise,
and which one will best fit it is a matter that only a trained
engineer with full knowledge of the local conditions can prop-
erly decide.
Of far more importance are the general methods employed in
carrying out the electrical distribution, and these are applicable
with almost equal force to any sort of altematmg system.
First in importance is the maintenance of a uniform voltage
on the primary service lines. This voltage should, as far as
possible, be the same at every transformer and should be con-
stant, save as it may be raised to compensate for the loss in the
secondaries.
The first step toward obtaining this imiformity is to assume
a fictitious centre of distribution as at Z), Fig. 288. This should
be chosen at or near the centre of load, generally in the business
centre of the city. If the office of the operating company is
conveniently situated, it should be used as a habitation for the
centre of distribution, at which supplies can be kept and meas-
urements made. jD is taken as the termination of the trans-
mission line proper, and acts in the capacity of a central station
toward the primary service wires. As a preliminary toward a
more exact regulation, there must be means for keeping the
voltage at this point D up to the normal under all conditions of
load. The most obvious suggestion is overcompounding the
generators for constant voltage at D, and this is often advisable,
though it must be remembered that compound winding is by
no means the only and not always the best means of securing
constant voltage at a point distant from the generator.
When the circuit is nearly non-inductive, and the current
therefore very nearly in phase with the E. M. F., or when the
power factor can be kept very nearly constant, compounding
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684 ELECTRIC TRANSMISSION OF POWER.
works admirably, and so is readily applicable to cases where
lighting is the main work to be done, or where s3^chronous
motors keep up the power factor of the system.
If, however, the load is largely of induction motors, running
at all sorts of loads, or is otherwise of strongly inductive char-
acter, compound winding alone wDl not suffice to keep constant
voltage at the point D, It will fail in proportion to the amount
of compoimding necessary to be employed, and for two reasons:
first, because of the direct effect of the laggmg current on the
excitation necessary; and second, because, as has already been
pointed out in Chapter VI, the lagging in phase of the current
disturbs the functions of the commutator. It is, therefore,
desirable to bring "pressure wires" back from D to show at the
station exactly the condition of things at the load, so that the
voltage may be maintamed by hand regulation, if necessary.
This is, of course, a temporary expedient with a compound-
wound machine, but it may avert frequent bad service. The
pressure wires may come either from the primary circuit at the
centre of distribution, or from some point of the secondary
system which is chosen to represent average conditions of load.
The latter is the preferable method, if there is a fairly complete
system of secondary mains. The pressure wires may be taken
as a guide for close hand regulation, or may operate some form
of automatic control of the field rheostat. Neither hand nor
automatic control is very satisfactory, if the generator requires
great change of excitation mider change of load. For the class
of power transmission under consideration, it is therefore better
to use a generator of moderate inductance and armature re-
action, whether it be compounded or otherwise regulated.
For the pressure wires may be substituted a compensated
voltmeter, arranged to take accoimt of the drop in the line
and show at the station the real voltage at the centre of dis-
tribution, provided the power factor does not change so errati-
cally as to vitiate the compensation.
Granted now that means are taken to regulate the voltage
at D as it would be regulated if the generator were at that
point, the distribution problem is the same as that in an ordi-
nary central station. Most alternating stations, however, are
far from well organized in this respect. Nothing is at present
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METHODS OF DISTRIBUTION.
586
commoner than to find an alternating station which receives
pay for not more than one-half of the energy delivered to the
lines, and sometimes this low figure falls to one-third or even
a quarter. This imhappy state of things is due mainly to
badly planned secondary circuits and to the indiscriminate
use and abuse of small transformers. The alternating current
transformer is a marvellously efficient and trustworthy piece
of apparatus, and, perhaps in part for this very reason, it has
been often the victim of wholesale misuse. Without going in
detail into the case of sub-station vs, house-to-house distribu-
tion, it is sufficient to say that the essential thing for efficiency
is to keep the transformers in use well loaded and hence at
J
1
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r
Fio. 289.
their best efficiency, and that for this purpose a few large trans-
formers are, on the whole, much better than many small ones.
The reason for this may be best shown by taking the following
practical example:
A given region requires, let us say, 250 incandescent lamps
or thereabouts, together with fan motors and perhaps an occa-
sional large motor. These are distributed among a score of
customers scattered over a couple of blocks. Fig. 289. The
blocks are, say, 200 ft. long, with alleys cutting them in two.
Now these customers may be supplied from individual trans-
formers, or all may be supplied from one transformer. In
either case the lines should be carried in the alley. In the
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586
ELECTRIC TRANSMISSION OF POWER.
former case 20 transformers would be connected to service
wires attached to the primary service main a b. These trans-
formers would average, say, 12 lights capacity each (600 watts).
In the latter case, a b would be a secondary main supplied from
a single transformer of 12,000 watts capacity. Now, assuming
a load such as would be met in ordinary practice, let us examine
the transfor merlosses in each case. The day may conven-
iently be divided into three periods in considering load: 7 a.m.
to 5 P.M. forms the day load of motors and a few lights; 5 p.m.
to 12 night, the evening load; and 12 to 7 a.m., the morning
load. During the first period we may assume 15 transformers
to be quite unloaded, 2 to be three-quarters loaded on motor
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FlO. 290.
400
500
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work except during the noon hour, and 3 transformers to be
one-quarter loaded on day lights.
During the second period we will assume the motors to be
off, 8 transformers to be three-quarters loaded on the average
from 5 until 7 p.m., and the rest one-quarter loaded from 5 until
midnight.
For the third period, it is safe to assume 15 transformers to
be unloaded and the other 5 one-sixth loaded from midnight
imtil 7.
Now the efficiency curve of a 500 or 600 watt transformer at
various loads is appnjximately as shown in Fig. 290, derived
from a consideration of several transformers of diflf^r^nt make^.
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METHODS OF DISTRIBUTION. 687
The constant loss, when the transformer is rim imloaded, is
about 30 watts.
On the above assumptions, and knowing the efficiency of the
transformer at various loads, it is easy to calculate for each
period the total energy supplied and the transformer output
which is delivered and paid for. The result of this calculation
is as follows:
l8t Period. 2d Period. 3d Period. Total.
Energy Supplied, Watt Hours, 18,480 20,420 10,060 48,060
Energy Delivered, " ** 10,460 17,700 8,600 81,660
Therefore, barely six-tenths of the energy supplied to the
transformers is delivered by them to the consumers. And this
is a condition of things more favorable than is usually found in
stations of moderate size, using, as many of them do, small
transformers.
The other method of distribution is to use a single large trans-
former m place of the small ones, and distribute to all the dis-
trict by secondary mains.
Now the efficiency of a 10-12 KW transformer is very closely
that shown in Fig. 291. Moreover, the energy consumed when
running without load is hardly more than 150 watts, so that
the transformer, when absolutely unloaded, wastes only one-
fifth of the energy wasted by the small transformers of the same
total capacity. Taking the output for the same periods as
before, a much better result is reached, as follows:
l8t Period.
2d Period.
3d Period.
Total.
Energy Supplied, Watt Hours,
11,800
19,660
6,830
37,290
Energy Delivered, »* **
10,460
77,700
3,500
31,660
With the single large transformer, more than 80 per cent of
the energy supplied to it is delivered on the customers' circuits.
This means that for a given amount of energy supplied from
the station, one-third more revenue will be obtained if the dis-
tribution be accomplished by a large transformer as against
quite small ones. Such a difference is important, even in a
plant driven by cheap water-power. Besides, for a given
amount of energy delivered to the customers, high-plant effi-
ciency means, smaller first cost of plant. With distribution
by secondary mams, not only will smaller djmamos at the
power station suffice for the work, but the cost of the trans-
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688
ELECTRIC TRANSMISSION OF POWER.
former capacity necessary is enormously reduced. In the
house-to-house distribution it is quite possible for any trans-
former to be loaded with all the lights connected to it. When
twenty customers are supplied from a single transformer, the
chance of such an occurrence is almost nil. In the hypotheti-
cal case just discussed, certain of the transformers would be
called on for full output almost daily, while all of them would
be subject to such a demand. The largest total regular out-
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Fig. 291.
8
10 II
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put, however, would be not much over one-half the aggregate
transformer capacity. So, instead of using a 12 KW trans-
former to replace 20 small ones, in reality a smaller one, say
one of 10 KW, would be ample.
In point of cost, the single transformer would have the ad-
vantage by not less than $250, enough in most cases to pay for
the difference in secondary wiring. In regulation, too, the
single transformer has the advantage, for the load is less liable
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METHODS OF DISTRIBUTION. 589
to sudden fluctuations, and the transformer itself regulates
more closely.
Ill practice it is best to go a step further than shown in
Fig. 289, and connect the secondary mains at a and b to the next
section of secondary just across the street, and also c with the
main in the next alley, so as to form, at least in the region of
dense load, a complete secondary network. Thus, each trans-
former can help out its neighbor, in case of need. The second-
ary mains should, in so far as is practicable, be designed for
the same loss of voltage, and the compounding and other regu-
lation applied to the generator should be arranged to compen-
sate for the loss of voltage in the transformers, and to hold the
voltage as steady as possible in their secondary mains. The
perfection of such regulating arrangements depends, of course,
on the uniformity of the distribution of load; but with a little
tact in arranging the circuits, variations in voltage at the lamps
can often be kept within 2 per cent of the normal pressure.
In large systems, as will be presently shown, even better work
can be done.
An essential point in the use of secondary mains is the em-
ployment of fairly high voltage. The general law, that the
amount of copper necessary in a given distribution varies
inversely as the square of the voltage, applies here with great
force.
In the early stages of alteniating work, when small trans-
formers were nearly always used and regulation was generally
bad, the favorite voltage for incandescent lamps was about
50 volts. The main reason for continuing this practice was
the fact that it is not difficult to make a 50- volt lamp that will
stand much abuse in the way of varying voltage. With good
regulation, this pressure can now be more than doubled with
equal security from breakage and great advantage to the dis-
tributing system. Not less than 110 volts should be used, and
a pressure of 115 to 120 volts is better, as it gives equally good
service with a quarter less weight of copper. From the present
outlook, even higher voltage is becoming practicable.
It is not always advisable to do all the work of distribution
by secondary mains. In districts where the service is scat-
tered, a few small transformers of various sizes can be very
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690 ELECTRIC TRANSMISSION OF POWER,
advantageously used, but should be generally employed as
a temporary expedient only, and shifted to another field of
usefulness when the service grows heavy enough or stable
enough to justify installing secondary mains.
Recurring now to Fig. 288, we have found that the best pro-
cedure is to use an alternating system, compoimded or other-
wise regulated so as to hold the voltage as nearly as possible
constant at the secondary terminals of the transformers.
These should be large enough to do all the work within a dis-
tance of 200 ft., more or less, and should feed secondary mains
at a pressure of, say, 115 volts. When these mains are more
than usually long, it is best not to feed current directly into
them, but to employ feeders connectirife, for instance, c. Fig. 289,
with points midway between c and a, and c and 6, respectively.
Neighboring secondaries may often be interconnected with
great advantage.
As to the primary distribution, we have assumed a centre
at D, Fig. 288. From this point feeders should extend to
primary mains connecting the transformers more or less com-
pletely, preserving nearly equal drop in voltage from D to
each transformer. The degree of elaboration in this primary
network is a matter to be determined by local conditions. If,
for example, the plant is of rather small size and the drop from
B to C, Fig. 288, is not above 1 or 2 per cent, the transformers
may be connected to short branch lines crossing B D C at vari-
ous points, without any further complications, or the main
line may be branched at B, each branch having short cross
feeders, while with other distributions of load the primary
lines may be quite completely netted, with regular feeders
at D.
The motor service may often require special treatment. It
often happens that it is best to feed large single motors or
groups of motors from special transformers, which will gener-
ally be large enough to avoid the objections adduced against
a general house-to-house transformer system. Such special
transformers avoid throwing a large and varying load on
the secondary lighting mains during the hours of "lap-load"
when it might be objectionable, and thereby avoid needlessly
heavy mains and annoying variations of voltage.
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METHODS OF DISTRIBUTION. 591
It must be remembered that Ohm's law is a very stubborn
fact. Any apparatus that takes a large and variable current is
liable to interfere with regulation. There is no such thing as
a motor, either for continuous or alternating currents, which
will not affect the lighting service. The nearest approach to
such a motor is obtained by arranging the distributing system
so that the largest current taken by the motor will be insuffi-
cient noticeably to disturb the regulation of the lamps.
This means that care should be taken, in arranging the dis-
tribution, to avoid overloading the lighting mains with motors.
It is an easy mattei* to determine the effect of the motor cur-
rent by calculation if the current is continuous, and by experi-
ment or calculation for alternating current. In the latter case
the easiest way is to connect the motor with any convenient
main and put on load with a brake — even a plank held agamst
the pulley will do. Put an ammeter in circuit, and if at the
rated amperage of the motor the fall in volts at the transformer
is enough to endanger regulation, the motor should be put on
transformers of its own. Generally the likelihood of trouble
can be judged from the size of the motor and the load on the
mains, without experiment. One of the advantages of regula-
tion by secondary pressure wires is the easier handling of an
mductive load of which compounding alone generally takes in-
sufficient account.
One of the nice questions to be decided, in such a plant as is
under discussion, is arc lightlag. The most obvious method
of arc lighting from a transmission plant is to use alternating
motors to drive arc dynamos, either belted or directly coupled.
This method is in use in a good many plants, and works ad-
mirably, although the efficiency is not all that could be desired,
being probably about 70 per cent at full load, reckoning from
the energy received by the motor to that delivered at the lamps
under the most favorable commercial conditions. That is,
for the operation of each 450 watt (nominal 2,000 c. p.) con-
tinuous current arc, at least 650 watts would have to be deliv-
ered to the motor. In working commercial circuits, on which
the number of lights varies greatly, the efficiency at light loads
would be greatly reduced, and might easily fall to between 50
and 60 per cent.
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592
ELECTRIC TRANSMISSION OF POWER.
This is not a cheerful showing, and much ingenuity has been
spent in attempting to remedy such a state of things. For
street lighting, the scheme is reasonably good, but it breaks
down in commercial lighting.
In cases where plants operate low-tension direct current
systems via motor-generators or rotary converters, the solu-
tion of the difficulty is simple, since all the commercial arcs
can readily be worked at constant potential, using preferably
enclosed arc lamps taking from 5 to 7 amperes. The effi-
ciency of the rotaries is high and the loss in the mains is not
great, so that by this means the only circuits that need be
worked on the series system are the street lights, which form
a nearly constant full load. When the distributing system is
alternating, one can still use constant potential lamps for the
commercial circuits with fairly good results.
Alternating constant potential enclosed arc lamps have at
the present time been brought to a state that justifies their
extensive use, and yet it must be admitted that they are some-
what less satisfactory than the direct current arcs. Taking
lamps as they are found commercially, and comparing direct
current with alternating current enclosed constant potential
arc lamps, the following results were obtained by a committee
of the National Electric Light Association appointed to deal
with arc photometry:
^
Mean
Spherical C. P.
Watta
Per M. 8. C. P.
Opal
Globe.
Clear
Globe.
Opal
Globe.
Clear
Globe.
Direct
4.90
629
155
182
3.41
2.90
Average of
8 lamps.
Alternating . .
6.29
417
114
140
8.66
2.98
Average of
7 lamps.
These figures show that even considering the energy abso-
lutely wasted in dead resistance in the direct current constant
potential lamp, it still has a slight advantage in efficiency over
the alternating lamp, not enough, however, to compensate in
addition for the loss of energy incurred in changing from
alternating to direct current.
The alternating lamp is at a slight further disadvantage in
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METHODS OF DISTRIBUTION. 698
that it requires rather the more careful handling, and a rather
better grade of carbons. It also is liable to give trouble from
noise, although the best recent lamps are comparatively free
from this defect, which has in the past been a serious objec-
tion to this type of lamp. Some noisy lamps are stUl to be
found on the market, and a hard carbon will make almost any
lamp sing. Most of the noise originates in the arc itself, and
it is considerably reduced by enclosing the arc and using a
non-resonant gasket on the outer globe. Good lamps care-
fully operated are capable of giving very excellent service,
and are entirely adequate for commercial circuits under ordi-
nary circumstances.
It would appear at the first glance at the table just given, that
enclosed arcs of either type are little, if at all, more efficient
than good incandescent lamps.
The difference between them is in fact not very great, but
incandescent lamps are generally rated on horizontal candle
power and on their initial, not their average, efficiency. With
due allowance for this, the efficiency of the best commercial
incandescent lamps per mean spherical candle power ranges
from 4 to 4.5 watts per candle power, so that the arcs have
still a fair margin of advantage, increased by the better color
of their light. In using alternating arcs for commercial work,
their performance is much improved by pushing the current up
to about 7.5 amperes, at which point the lamp is more efficient,
more powerful, and, if properly adjusted, steadier.
In street lighting, the best results have so far been obtained
by operating alternating arcs in series on the constant current
system. This requires special lamp mechanism and special
devices for changing the constant potential alternating current
of the transmission line to a constant alternating current of
voltage automatically varying with the requirements of the
circuit.
Such constant current regulators vary considerably in detail,
but the underlying principle of all of them is as follows: A
heavy laminated iron core is surrounded by a movable, counter-
balanced coil, through which the cmrent to be regulated flows.
Any change in the position of this coil changes the reactance
of the combination, and hence varies the current. The coil
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694 ELECTRIC TRANSMISSION OF POWEtt,
is counterbalanced, until, when the normal current is flowing,
the coil floats in equilibrium, the opposing forces being gravity
and the attraction or repulsion between the coil and the mag-
netized core. Then any change of current due to varying con-
ditions in the external circuit establishes a new position of
equilibrium for the coil in which the changed reactance brings
the current back to its normal amount. Sometimes the regu-
lator is combined with a transformer which receives current
from the transmission system at any convenient voltage, while
the floating coil acts as a secondary and delivers constant
current to the lighting circuits.
This is the arrangement of the constant current transformer
system as operated by the General Electric Company. Fig. 1,
Plate XXIII, shows the internal arrangements of the trans-
former. It has two fixed primary coils at the top and bottom
of the structure, receiving current from the main line, and two
floating secondary coils counterbalanced against the repulsion
of the primaries, and balanced against each other by the double
system of rocker arms visible at the top of the cut, which are
supported on knife edges. The short balance lever in the fore-
ground is attached to the rocker arms by a chain, and is like-
wise pivoted on knife edges, while it carries on its longer sector,
suspended by a chain, the adjustable counterbalance weights
The current can be adjusted at will within reasonable limits
merely by adding or taking off counterbalance weights. The
whole apparatus is enclosed in a deeply fluted cylindrical cast-
iron case filled with paraffin oil, which serves the double pur-
pose of giving high insulation and also damping the oscillations
of the floating coils. Hence, the system has been jocularly
dubbed the "tub" system, and the name has every appearance
of sticking. Transformers of this type are built for as large a
load as 100 series arc lights, in which instance they are usually
arranged on the multi-circuit plan, with two 50-light circuits.
Some of these big tubs have four primaries and four second-
aries, while the smallest sizes have but one of each.
The diagram, Fig. 292, gives a very clear idea of the circuits
of this simpler form of the apparatus, and of the shifting of the
secondary coil under changing load. The secondary is shifted
by hand into the short-circuit position and a plug short-cir-
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Fio. 1.
A,
\
Fig. 2.
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FLATE XXIII. O
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METHODS OP DISTRIBUTION.
695
cuiting switch inserted just prior to starting up, and when the
primary current is on, the short-circuiting switch is withdrawn,
throwing the current upon the lamps.
These transformers regulate promptly and steadily, and hold
the current quite accurately constant from full load down to
as low as one-quarter load. Their efficiency as transformers
of energy is high in spite of a rather unfavorable form of the
magnetic circuit, being 95-96 per cent at full load in the aver-
age sizes. From the nature of the case, however, the power
factor of such apparatus is not as high as would be desirable,
being about 80 per cent at full load, and falling off in practically
Fio. 292.
linear proportion as th load decreases. For this reason the
apparatus, when put into action, throws a nasty inductive load
upon the system, and it is good policy to cut it in with a water
rheostat in the primary circuit so that the load may go on
gradually. An ordinary barrel nearly filled with pure spring
water, with a fixed electrode at the bottom and a movable one
at the top, each a little smaller than the barrel head, makes
a very efficient rheostat for an ordinary circuit of 2,000 volts
or so.
Owing to the low-power factor, such apparatus should not
be used on circuits likely to be worked much at partial load.
It is very well suited, however, to street lighting, and has come
into very extensive use.
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696 ELECTRIC TRANSMISSION OF POWER.
A similar device has been considerably used in the practice
of the Westinghouse Company, differing, however, hi that the
transformer and regulator functions are not combined. The
regulator is shown in Fig. 2, Plate XXIII, and in virtue of what
has already been said the cut is self-explanatory. The balance
floating coil inserts automatically the reactance necessary to
hold the current constant. Regulators are made for a range
of action varying from 25 per cent to 100 per cent of the whole
load, according to the requirements of the case, and, of course,
are of greater size and cost according to the range required.
They hold the current closely at a uniform value, and have,
probably, at full load a somewhat better power factor than the
combined apparatus just described. They have the same
rapid falling off of power factor at low loads, however, and
must be used in connection with a separate static transformer
to give the required voltage on the arc circuit. They could, of
course, be installed directly on the distribution circuit, but at
the risk of a ground on the arc circuit involving the whole sys-
tem in trouble, so that practically they are regularly used with
transformers. The efficiency of this system does not differ
materially from that of the tub system already discussed, and
the operative qualities of both are much the same.
The series alternating arc lights thus operated have come
into large use, and are rapidly driving out the open continu-
ous current arcs for street lighting. They are, of course, always
enclosed, and give a very steady and evenly distributed light,
free from shadows and bright zones, which has proved highly
satisfactory for street lighting.
The ordinary series alternating arc takes about 6.5 amperes
and 425 watts, and at this input is materially better as an
illuminant, light for light, than the so-called 1,200 c.p. open arc
or the enclosed continuous current arc taking 5 amperes or
thereabouts. To compete on favorable terms with the 9.5
ampere open arcs, or the 6.5 ampere enclosed continuous cur-
rent arcs, the current in the alternating system should be
carried up to about 7.5 amperes, at which point it is fully
the equal of the others in practical street lighting.
Used thus on the street, the slight noise of the alternating
arc is not noticeable, and experience has shown the operation
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Fia. 2.
PLATE XXIV.
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METHODS OF DISTRIBUTION. 597
of the system to be eminently satisfactory. For commercial
lights the series alternating arcs are not to be commended,
since the power factor of the regulating devices is objection-
ably low at partial loads. The choice for such work lies be-
tween conversion to continuous current and the use of con-
stant potential alternating arcs, with the advantage, at the
present time, rather in favor of the former expedient. Re-
cently some very good results have been obtained in arc light-
ing by means of the mercury rectifier already described. It is
rather early to judge of the results to be obtained on a large
scale, but the method is decidedly promising.
For much commercial work there is no need of using arcs at
all, and often incandescent lamps may replace arcs to advan-
tage. In cases where fairly powerful radiants are necessary,
and particularly where the color of the light is important, very
good results can be obtained by the use of Nernst lamps.
The Nernst lamp is a modified incandescent in which the
light-giving body is not a filament in vacuo, but a stick of re-
fractory material driven to high incandescence in air. The
material used is akin to that used in Welsbach gas mantles,
mainly thorium oxide. A non-conductor when cold, it must
be artificially heated to start the current, when it becomes a
tolerable conductor and can be successfully worked at a higher
temperature, and hence a higher efficiency, than an ordinary
incandescent lamp. The principle involved is simple, but the
accessory parts needed produce a lamp which, while less com-
plicated than an arc lamp, requires more attention than an
ordinary incandescent.
The Nernst lamp as used in practice is shown in Plate XXIV,
of which the upper figure shows the connections of the 3-glower
lamp which is the ordmary form, and the lower, the 3-glower
lamp complete. The essential parts are the glower, the heater,
the heater cut out, and the ballast.
The first named is a stick of oxides, in a 220-volt lamp about
an inch long and ^V inch in diameter, into the ends of which
are fused tiny platinum balls connected to the lead wires.
The heater is a spiral of fine platinum wire embedde<i in a tube
of refractory enamel, and the magnetic cut-out merely serves
to prevent this from remaining in circuit after the current is
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598 ELECTRIC TRANSMISSION OF POWER.
fairly established through the glower. The ballast is a resis-
tance of fine iron wire in series with the glower, and enclosed
in an oxygen free tube to prevent oxidation. As iron rapidly
increases in resistance when heated it prevents too large a cur-
rent through the glower, which decreases its resistance when
hot, in case of a rise in voltage, and thus tends to steady the
action of the glower. The cuts speak for themselves.
The lamps suffer seriously from a species of electrolytic
action on the glower when used on direct current, and thus
are essentially alternately cun*ent lamps doing well at periodi-
cities from 25^^ up, with rather better life at the higher fre-
quencies. The life of the glowers is 600 to 800 hours, some-
times more, and the mean spherical efficiency when used with
a light-diffusing globe, as is usually necessary on account of
the high intrinsic brilliancy, is slightly better than that of an
a.c. arc lamp. The Nernst lamp, however, is arranged inten-
tionally for a strong downward distribution of light, and hence
in many situations does considerably better, watt for watt,
than the enclosed arcs. Figures on the maintenance of these
Nernst lamps vary widely, but the best information at hand
indicates that it is relatively rather less than for arcs. The
color of the light is almost pure white, very conspicuously
better than that of any form of enclosed arc, and the illumina-
tion is beautifuHy steady. The lamps start rather slowly,
rising to full brilliancy in forty seconds or so. They have
come into considerable use already, and for commercial work
on alternating current transmission systems have much to
recommend them.
The general principles of distribution laid down hold what-
ever alternating system is used. Polyphase and other modified
alternating systems require special treatment in the details of
distribution, but not in the broad methods employed.
Motor service should generally be cultivated as a desirable
source of profit and an excellent way of raising the plant effi-
ciency. A motor load, if of numerous units or a few steadily
loaded ones, is remarkably uniform. Fig. 293 shows the load
line of a three-phase power transmission plant. The motor
load consisted of about fifty induction motors of various makes,
aggregating nearly 360 rated HP. The curve shows the prim-
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METHODS OF DISTRIBUTION,
699
ary amperes in one leg of the circuit throughout the twenty-
four hours. It was taken on a day in early August when the
lamp load was very light and reached its maximum as late as
8 P.M. The motor load, save for the sharp decline during the
noon hours, was very steady, although there were frequent
variations through a range of a few amperes, too brief to appear
on the diagram. In this case and at this season of the year
there is no "lap load." The distribution is, as far as possible,
from secondary mains, and even in winter, when the lap load
1 — 1
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o
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6AM
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FlQ. 293.
6 PM
12
is prominent, although the motors still require the major part
of the output, the regulation of the system is admirable.
Thus, even a heavy motor load gives very little trouble with
a properly designed system of distribution and judicious hand-
ling. The things to be feared are large motors running on very
variable load, motors with bad power factors carried by over-
loaded transformers, and overloaded conductors during the
period of lap load. Now and then a system is installed for
motor service only or with special motor circuits. In this casQ
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Fia. 294.
in different directions, and generally at diflFerent distances.
Fig. 294 shows the character of the conditions thus met. A is
a generating station, the position of which may be determined
by various reasons — the existence of valuable water-power
being the commonest; S, C, Z), E, F, are the various points to
be supplied with power. They may be at any distances, and
of any sizes or natures. Usually the greatest distance involved
will not be coupled with the greatest load, and the situation
is otherwise inconvenient. If all the loads were large, the
I
600 ELECTRIC TRANSMISSION OF POWER.
it should be remembered that there is no need for any very
close uniformity of voltage throughout the S3^tem, and that to
attempt it means waste of time and money. The circuits can
be laid out with reference to the desired efficiency alone, for j
in most cases even 10 per cent variation in voltage between one |
motor and another is of little consequence.
The distribution of power from scattered sub-stations fed
by a common line, involves some of the most intricate and
puzzling problems to be found in power transmission. Such
distributions generally arise from an attempt to supply from a
common power plant, energy for divers purposes to several
separate towns or regions, having different requirements. In
the main such plants require special treatment in order to
secure decent service. A great variety of cases may arise,
almost every plant having peculiarities of its own, but in
general they will fall into one of the three following categories:
1. Radial distribution from a centrally located station.
2. Radiating distribution from an eccentric station.
3. Linear distribution.
1. The first-mentioned class consists of those plants which
supply from a single station power to different localities lying
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METHODS OF DISTRIBUTION. 601
simplest procedure would be to install one or more generators
for each circuit and operate them independently. Or if by
good luck two or more load points were' of similar size, distance,
and character, they would naturally be operated as if they
were one.
To consider methods of operation more in detail, imagine a
system consisting of the station Aj a load at B consisting of
150 KW in lights and motors, largely the latter, distant 3
miles; and a load at E, 6 miles away, of 250 KW, mostly incan-
descent lamps. At both B and Ej it would be desirable to
distribute at the voltage of transmission without a general
reducing station. In such a plant it might be possible to
operate B and E from separate generators, compounding
them or using the regulating methods already described. But,
if day lighting at E is to be attempted, it would be necessary
either to run one dynamo all day at a trivial load, or to throw
this day work in on the other circuit and take the voltage
as it chanced to come.
With the ordinary amount of loss in the line A By the re-
sult would be decidedly bad regulation at i7, with only motors
at B or ^ the case would be very simple; the station would be
regulated with reference to the lighting load alone, but with
lights at both places there must be good regulation at both.
During the day at least it would be desirable to work both lines
from the same generator. The first step in this direction
would be to install at il a hand regulator to control the line
A E. As already pointed out, a motor load is often fairly
steady except at certain times, so that the regulator would
require little attention save in the early morning and at noon.
Before the motor load fell off in the afternoon, it would prob-
ably be desirable to start a separate d3mamo for E,
In operating both lines regularly from the same generators,
hand regulation on at least one of them would become nece?-
sary; on which one is a matter of relative convenience. If
the distances A By AE were much smaller, not more than two
or three miles, it might be feasible to install both lines for
small and equal drop — not over 2 per cent — so that, if the
dynamo were compounded for an equal amount, the possible
variations of voltage would be trifling. Such a plan cannot,
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602 ELECTRIC TRANSMISSION OF POWER.
however, give really good regulation over any save very short
distances without inordinate expense for copper. This sort of
regulation by general average has been tried too often already,
with disastrous results, and is quite out of place in serious
power transmission work.
li A E were ten or fifteen miles long the manner of opera-
tion would become a still more troublesome question. Raising
and reducing transformers would generally be used, and the
best plan would probably be to install a pressure regulator in
connection with the raising bank of transformers, and let the
shorter line be taken care of by the compounding of the gen-
erator or by a pressure regulator of its own. The latter pro-
cedure is somewhat preferable. For if the drop in the lines be
5 or 10 per cent and the loads variable, the work of regulation
will be lessened by compounding the generator, if at all, for
constant potential at its own terminals. The range of the
hand regulation is thus lessened, since there is no attempt at
over compounding; and two regulators requiring occasional
adjustment are easier to handle than one which requires con-
tinual juggling to produce indiflferent results.
In certain cases of heavy load there may be a regular sub-
station at B or at E, the distribution at the other point being
direct. Then the regulation question is better transferred to
the sub-station, the generator being regulated for the loss in the
other line, which as its load will usually be relatively small,
should have a comparatively small drop.
The most troublesome case that can arise is when power is
to be furnished to a street railw^ay at B or E, in addition to a
general lighting and motor service. A railway load is so vio-
lently variable that it cannot be operated in direct connection
with an incandescent service unless this latter with the general
motor load is so great as to quite dwarf the variations of rail-
way load. Frequently, therefore, a separate generator should
be devoted to the railway work. In case this cannot be done
without great inconvenience, it may become necessary to
install a sub-station at which the lighting circuits can be regu-
lated either by hand or automatically.
Suppose now that the problem is complicated by the addi-
tion of loads at C, D, and F. These lines will be treated on
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METHODS OF DISTRIBUTION. 603
the same general principles as the first two. To begin with,
any line operating motors alone can be worked direct from
the generator. Even if all the loads be mixed in character,
two or more can often be found which through similarity of
conditions can be worked together in parallel, either by a
common regulator or by compounding the generator. The
other? should be treated as already indicated. At the worst,
it might be necessary to install a regulator for each line. This
is not really so burdensome as might be supposed, since several
of the regulators will usually require infrequent attention, so
that one man can manipulate the whole set. This line of action
is similar to that followed in most large central stations, where
feeder regulation, although rather a nuisance, is successfully
accomplished without any particular difficulty. Feeder regu-
lators for alternating circuits have, however, by no means
received the attention that is their due.
Pressure wires from each load point are desirable, though,
if the load is such that the inductive drop is small or quite
steady, the regulator can be as easily adjusted in accordance
with the current on the line, or in accordance with the indica-
tions of a compensating voltmeter.
In the transmission and distribution of power from an eccen-
tric station, the difficulties are many unless recourse be taken
to a regulator sub-station. Fig. 295 shows a typical situation.
Here A is the generating station and B, Cj Z), Ej F, are the
load points. If the distance from A to the nearest load is
A*--
Fia. 295.
great enough to require raising and reducing transformers, it
is generally best to install a reducing sub-station worked like
the central station A^ Fig. 294. Sometimes, however, it is only
half a dozen miles or so from A to the group of load points.
The case is similar to that discussed in the first part of this
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604 ELECTRIC TRANSMISSION OF POWER,
chapter, save that the load is in several distinct localities in-
stead of being generally distributed. From this difference
the complication arises. A certain proportion of cases can be
treated readily, however, by choosing a point G near the centre
of load and then running the lines G B,G C,GD,G E,G F with
1 or 2 per cent loss wherever lights are to be furnished. Then
by holding the voltage constant at G or slightly over compoimd-
ing at that point, sufficiently good service can often be given.
If the loads are very imequal in size, G may be chosen at or
near the most important point and lines run to the others as
before, with the regulation question confined practically to the
first. K the load points are quite numerous and scattered,
FlO. 296.
Fig. 296 may be a preferable plan. Here two lines A B and
A C are run and a group of load points is served from the ter-
minal of each line. The groups shown are about equal, but
sometimes it would be desirable to run a separate line for a
single point where the load was peculiarly heavy or trouble-
some.
These scattered distributions are fortunately mostly for
motor service, so that regulation, in practice, is often easier
than the situation indicates. They sometimes run naturally
into the linear distribution, which, imless of trivial size, is a
thorn in the flesh of the engineer.
Fig. 297 is a type of this linear distribution, which is often
met with in large transmission work and especially in long
distance cases.
The power station A is mainly intended to supply lights and
power at B, which may generally be supposed to be the largest
town in the immediate region. Incidentally it is highly desir-
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METHODS OF DISTRIBUTION, 605
able to supply lights and power to C, D, B, F, G, towns or
manufacturing points at which electric power is needed. The
main line A B may be taken as 20 miles, which is enough to
disclose most of the difficulties.
Of course, the line must be operated at high voltage with
raising and reducing transformers. In neariy every case the
latter would be placed in a regular sub-station, with appro-
priate regulating apparatus for keeping uniform voltage
throughout the primary and secondary networks in B, The
loss of voltage in the line above may be assumed at 10 per
cent, and the primary pressure at B as 20,000 volts. As B
PlO. 297.
comprises by far the largest and most important part of the
load, attention should be first directed to complete regulation
at that point.
This can be best attained by first holding the primary pres-
sure at B constant by compounding or other regulation at A,
and second, by careful regulation of the primary and secondary
feeders in the sub-station. In fact the whole transmission
must first be treated with respect to results at B, while never-
theless it is necessary to scatter power along the line at the
points indicated. There may be present all sorts of require-
ments. For example, at C there may be required 1,000 in-
candescent lamps and a few motors; at Z), 500 incandescents;
at Ej a 50 HP motor and 300 incandescents ; at F, 300 incan-
descents; and at G, 200 HP in motors and 200 lamps.
Frequently the load at one or more points may consist of
motors only. This case is not included above, since no special
regulation is needed; the power has only to be transformed
from the line voltage to that of the motors, neglecting the
effect of varying loss in the line.
Each of the cases noted involves the question of regulation
in a somewhat troublesome form; at Z), for example, the con-
ditions xmder which incandescent lamps must be supplied are
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606 ELECTRIC TRANSMISSION OF POWER.
most severe. To begin with, at the nearest point of the main
line, A B, the voltage may change by about 6 per cent, owing
to varying loss in the line; the branch to D causes a trifle more
variation, the drop in the transformers still more, and finally
there must be added the loss in the secondaries up to the lamps.
In all, these cumulative variations in voltage may be 10 per
cent or more. At best, this means 5 per cent change of vol-
tage above and below the normal. This is too great to allow
what can be called good service, although worse is sometimes
given. In fact such variation ought to be classified as out-
rageously bad. To better matters, two methods are available.
First, one may use a hand regulator in connection with the
reducing transformers; for, in so lai^e a system as that in-
volved, the changes in voltage are relatively slow, and the con-
ditions of load may be such that over compounding on the
main line may partially compensate for the losses elsewhere.
Or second, the lights may be operated by a dynamo driven by
a s3mchronous motor. This procedure adds somewhat to the
expense and trouble, but completely eliminates the loss in the
line, since the speed of the motor is independent of the applied
voltage, and incidentally, of the load.
For small outputs a good induction motor serves the purpose
well, for it is simpler to operate than the synchronous variety
and can be made remarkably insensitive to changes of load
and voltage. This motor generator device is an admirable
resource when a very variable line voltage must be dealt with.
In making the installation for a point like D, the actual varia-
tion of the pressure at the point of tapping the main line should
be ascertained, and the effect of the subsequent losses up to
the lamps should be computed. If the resultant changes are
frequent and considerable, a motor generator gives the best
result. For gradual and moderate changes, an occasional
touch at a regulator may be all that is needed, and now and
then the resultant variation will prove to be not more than 2
per cent above or below an assumed normal for the lamps, in
which case the regulation often may take care of itself.
At C there is a distribution equivalent to that from a small
central station. The line pressure will generally have to be
twice reduced before feeding the lamps. The choice of
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MtJTHODs Of distribution. 607
methods is the same as in the case just discussed; except that,
with the losses of a double transformation and rather scattered
service, regulation caimot be left to chance. Generally in a
station of this size some regulation due to the distribution itself
will have to be provided for, and the simplest course is to es-
tablish a sub-station with one or more pressure regulators.
This is operated just like the sub-station at S, being merely on
a much smaller scale. A careful study of local conditions,
however, is needful to enable one to discriminate between the
two methods mentioned.
At the station E the motor wUl take care of itself, but the
lamps might give trouble owing to variations in motor* load.
If these are great and sudden, nothing save rimning from the
motor a generator for the lights will answer, and even that will
not be entirely satisfactory. If the load is steady and the
lights regularly in use, as would be common in factory service,
the loss in the branch line to E and the secondaries can be
adjusted so that if the voltage at B is kept constant by regula-
tion at A, that at E will be nearly so. This device is probably
the one best suited to give good service at F. For G the same
method holds, but with so large a proportion of motor load,
separate transformers for the lights are almost necessary. In
cases where there is no regulation at A for the loss in the line,
pressure regulators or sometimes motor generators will have
to be used at E, F, G,
The various cases of linear distribution just considered are
of necessity treated little in detail, since they are so much
modified in practice by special circumstances. Enough has
been said, however, to indicate the methods to be followed and
to show how tactfully this class of problems must be treated.
Finally comes that very important class of cases which
involves the distribution of transmitted energy from large
reducing stations. Such is the normal condition of affairs
whenever power is transmitted to a city in large amounts for
lighting and motor service. Passing over a few instances in
which this power may be mainly utilized for driving by motors,
or replacing by rotary transformers, existing central stations,
orie is confronted by the problem of constituting a great dis-
tributing system for alternating currents; a system general
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608 ELECTRIC TRANSMISSION OF POWER.
enough to be available for every service, and perfect enough
to compare favorably with the great networks now worked by
continuous currents. Until very recently this problem would
have been insoluble in any practicable way, but to-day, thanlcs
to the modem alternating systems and to the intelligent use
and arrangement of large transformer imits, it is possible sub-
stantially to duplicate in convenience and efficiency the best
direct current systems, while retaining the enormously valuable
advantage of using high tension feeders. It must not be sup-
posed, however, that the same procedure must suit both cases —
the results but not necessarily the methods, must be in full
accord.
The basis of each system must be a carefully laid out network
of working conductors, giving throughout the area of service
a substantially uniform voltage as high as can conveniently be
employed in the various receiving apparatus — lights, motors,
and so forth. This voltage is practically determined by that
of the incandescent lamps which are available. A few years
ago 100 to 110 volts was the working limit of effective voltage
between incandescent service wires (not of course the extreme
voltage to be found between any two wires of the system). Of
late the majority of important stations employ lamps of 115-
120 volts. Now and then 120-130 volts is reached, and very
recently there has been a strong movement toward boldly
doubling the usual voltages and employing lamps made for
200-250 volts.
A considerable number of scattered small plants use such
lamps, and in a few cases central stations have adopted them
in connection with three-wire systems, usmg thus about 440
to 500 volts between the outside wires. There is a decided
tendency in this direction, and occasional stations have imder-
taken to change to this double voltage, at least to the extent of
trying 220 volt lamps extensively. At present these lamps are
of somewhat imcertain quality and rather high price, but they
have been rapidly improved, both here and abroad.
It is undoubtedly much harder to get an efficient and durable
filament for 220 than for 110 volts at a given candle-power.
Such a filament is necessarily very slender and correspond-
ingly fragile. If two 110 volt filaments mounted in series
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METHODS OF DISTRIBUTION. 609
would answer, the task would be simple, but such a combina-
tion gives double the required candle-power, which is gener-
ally undesirable. The net result of present experience is that
while 220 volt lamps can be made to give excellent results in
efficiency and life they are, as a rule, both poorer and costlier
than the corresponding lamps of half the voltage. From the
nature of lamp manufacture this condition is likely to remain,
in perhaps lessened degree, even when the production of these
high voltage lamps is extensive. The question between the
two from a commercial standpoint will ultimately be a close
one, although at present the advantage is altogether on the
side of the lower voltage in most instances. The high voltage
lamps are most satisfactory when of 20 to 32 candle-power and
forked at 3.5 to 4 watts per candle. Under such conditions
the filaments, being somewhat thicker than in a 16. c. p. lamp
of similar voltage, and being worked at a lower temperature
than the high efficiency lamps, give a reasonably good life.
Until much experience has been accumulated with reference
to the high voltage lamps, their use in any considerable under-
taking cannot safely be recommended. It would be particu-
larly imwise to attempt it in a large transmission plant, where
any trouble with the lamps would inevitably be charged against
the general system. It is better, then, to select for incandes-
cent lighting a voltage only so high as has been thoroughly
tried — say 115 to 125.
The resulting service voltage on the secondary network
depends on the system of distribution employed. There are
actually employed for primary or secondary distribution with
alternating currents about a round dozen of distinct methods,
more or less convenient and inconvenient, and requiring very
various amoimts of copper for distributing the same amoxmt
of energy at the same loss and distance. Several of them are
very convenient and valuable, others have as their only excuse
for existence the desire to exploit a novelty or to evade some-
body's patent.
The simplest of them all is the ordinary two-wire system
worked with alternating currents. In this the maximum vol-
tage of the lamps is the maximum voltage of the secondary
system. To avoid this limitation and to secure the ability to
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610
ELECTRIC TRANSMISSION OF POWER.
run motors is the principal function .of the various modifica-
tions, polyphase and other, which make up the remainder. As
these various systems are often exploited, it is worth the while
to review them briefly, with special reference to economy of
copper and convenience of installation on a large scale for the
purpose we are considering. The two-wire system is shown
diagrammatically in Fig. 298. Its main advantage is extreme
simplicity. It requires the same amount of copper as a two-
wire direct current system at the same effective voltage, and
Fig. 298.
is installed in the same general way, except that, owing to the
peculiarities of alternating currents already explained, very
large single wires are imdesirable and armored conduits must
be used with great caution, if at all.
As to motors for such a system, the case is not altogether
what one would desire. Alternating monophase motors are
not yet so satisfactory for general service as those of some
other types, more particularly as regards starting and severe
service, and, mitil considerable improvement is made in them,
the pure monophase system is severely handicapped. The two-
wire arrangement is always at rather a disadvantage in the
amoimt of copper required both for feeders. and service mains.
The most obvious modification of this distribution is its evo-
lution into a three-wire system such as is familiar in Edison
stations. The extreme working voltage is at once doubled^
; 1
Fia. 299.
and thus with the same voltage at the lamps, the cost of copper
is greatly reduced. If the copper for a given two-wire system
be taken as 100, that for the corresponding three-wire system is
31.25, assuming that the so-called neutral wire is of one-half
the cross section of either of the others. Fig. 299 shows this
familiar arrangement in diagram. Like every other system
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METHODS OF DISTRIBUTION. 611
which saves copper, a three-wire distribution is subject to cer-
tain inconveniences. In the first place, it is necessary to carry
three wires instead of two over substantially the whole working
area. Secondly, the lamps must be nearly equally divided
between the two sides of the system. This balancing of the
load is not particularly troublesome in a well-managed plant,
and general experience has sho>vn that the gain in copper far
outweighs this disadvantage.
This three-wire distribution has been largely used for alter-
nating current work. It is sometimes very convenient when
applied to single or grouped transformers for the lighting of
large buildings and regions in which balance of load is easily
preserved. In such case the transformers are supplied from
high voltage feeders, generally arranged on the two-wire sys-
tem. As a rule, however, proper balancing is not easy in iso-
lated districts, and the best use of the three-wire system is for
I
Fig. 300.
a general network of secondary mains, the voltage upon which
can be controlled from a central station. In an ordinary
direct current plant, the feeders are of course at low voltage,
and a great advantage is gained for the alternating arrange-
ment by feeders at two or more thousand volts supplying the
mains through transformers. As regards motors, the alter-
nating current three-wire system is on substantially the same
basis as the alternating two-wire system.
More complicated pure monophase systems are seldom used,
although there is an instance at Portland, Ore., of a four-wire
feeder system; derived, however, from polyphase generators.
Fig. 300 shows the arrangement of the lines, which are operated
in general like a three-wire plant and require similar care in
balancing, with the additional complication of nmning four
wires and balancing three branches. The saving in copper is
of course very great, the amount needed, allowing half the area
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612 ELECTRIC TRANSMISSION OF POWER,
of the outside wires for each neutral, being about 16.6 against
100 for the two-wire plant. The corresponding five-wire S3n3-
tem may be passed over, as it is not used at all for alternating
currents, nor extensively in any way.
Next in proper order comes the so-called monocyclic system,
which is essentially a monophase system, but heterophase with
reference to the operation of motors. Its principal features
have already been explained. So far as lights are concerned,
it is simply the monophase system already described in both
the two-wire and three-wire forms. The "power wire," which
supplies magnetizing current for the fields of the motors, is
only used itt so far as is necessary for its special purpose, and
Cj
u
Fig. 801.
may or may not form part of the regular network. The two-
wire monocyclic system shown in Fig. 301 describes itself.
The expense and trouble of installing the "power wire" is the
price paid for the ability to run motors. The total amount of
copper is, of course, governed by the size and extent of the
power wire. The main wires must accommodate the full
current of the generator, for motors and lights must often be
operated together, and at all events the machine must be fully
utilized. The power wire, on the other hand, has to carry only
a part of the current used in the motors. In a system heavily
loaded with motors, the power wire might be one-half the cross
section of each of the main wires. If then it extended over
the entire system, it would add 25 per cent to the copper re-
quired for the main circuit. Generally its size or extent would
be less than that just noted. The total copper required for a
monocyclic system is then variable. Its relative amount may
vary from 100, when the system is operating lights alone, to 125
for rather extreme cases of motor load.
The same general properties hold good for the three-wire
monocyclic system shown in Fig. 302. It is treated like any
other three-wire system, except for the addition of the power
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METHODS OF DISTRIBUTION. 613
wire wherever required. There is evidently a great saving of
copper over the two-wire monocyclic, secured at the cost of
running an extra wire as a neutral and balancing the load on
the two branches. The relative weight of copper varies from
31.25 for lights only to say 40 when the motor system is exten-
sive. Either form of the system is singularly easy to install
and operate in plants already having a considerable network
of lines, since there need be no rearrangement or balancing of
circuits, but only an additional line wire running to the motors
installed and extended hand in hand with the motor service.
The monocyclic system is now very little used in practice,
however, since it possesses no important advantages over
fiN^
FlO. 302,
ordinary polyphase systems and is decidedly less satisfactory
for motor service.
Passing now to the polyphase systems, it is well to reiterate
what has already been stated in explainhig them, viz., that
they all involve about the same principles and lead djniamically
to about the same results. They do, however, differ consider-
ably in their characteristics as applied to a general system of
distribution, and in rather interesting ways*
The diphase system can be worked either with four wires,
i.e., a complete and independent circuit for each phase, or with
three wires. The former arrangement is the one almost invari-
ably used. The two circuits can be worked independently for
lights, but must be united to allow the operation of diphase
motors. For the former purpose the two windings of the gen-
erator may be treated, save in one important respect, like sepa-
rate monophase alternators. For the latter purpose they must
work conjointly. Fig. 303 shows the relations of the two cir-
cuits. In a general system it is the best plan to carry the two
circuits throughout the territory to be covered. In this way
motors can be run anywhere. Otherwise, if the main circuits
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ELECTRIC TRANSMISSION OF POWER.
covered different districts, connecting lines might have to be
run at considerable expense for copper and labor, uniting the
two systems. Further, when the two circuits are together, it
is easier to divide the load evenly between them; which is desir-
able to prevent one circuit of the generator being overloaded
before the other is fully used. Incidentally, hand regulation
must sometimes be used for one or both circuits, unless the
loads are equal as regards drop in the lines. If the generator
is to be compound wound, the two phases must be equally
loaded in order that the compounding may be able to hold the
voltage on both phases alike. It must not be understood that
unequal loads affect the voltage as in a three-wire S3rstem —
1
X
Fio. ao3.
they merely produce different "drops" in the two S3rstemB,
which cannot be equalized by the generator.
As to the relative amount of copper required, it is, when
both phases are run together, 100. If separated, this may be
slightly increased by cross connections for motors.
A diphase system can be organized with each phase form-
ing a three-wire system like Fig. 299. This doubles the work-
ing voltage and so saves copper, but at the cost of very serious
complication. The full distribution requires six wires, three
per phase, and these must be carried together or cross-con-
nected for motors, if sej)arated. The first procedure — nm-
ning two three-wire systems side by side over the same dis-
trict — involves frightfully complicated wiring; and the second
if the motors are at all numerous, requires a troublesome
system of subsidiary lines. In either case, not only would
each three-wire system have to be balanced in itself, but the
two must be mutually balanced unless hand regulation is
resorted to for one or both.
Altogether the diphase system with separated phases,
does not lend itself readily to distribution for lights and motors
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METHODS OF DISTRIBUTION. 615
on a large scale, save in changing over existing monophase
or other two-wire systems, for which it happens to be exceed-
ingly well suited. Its worst features are the large amount of
copper required for secondary mains, and the forbidding com-
plication of any attempt to secure economy by using the three-
wire distribution. Like the diphase interconnected system
about to be described, and certain forms of the three-phase
system, it is most practicable in plants of moderate size not
requiring a complete sub-station with a full system of secon-
dary mains.
The interconnected diphase system, Fig. 304, employs a
common return for the two phases. It has been often proposed
but seldom used, for a good practical reason. The com-
bined phases are unsymmetrical with respect to the inductance
g
Fio. 304.
of the system, so that, even when the two sides of the system
are equally loaded, the voltages between the common wire and
the mains are unequal by an amount proportional to the induc-
tive loss in the lines. Hence, it is unsuited for long lines
either primary or secondary, overhead or underground. The
lamps on the two sides of the circuit are at nearly the same
voltage, but the voltage between the mains is so compounded
of the two phases as to give increased working pressure enough
to reduce the relative amount of copper to 72.8 under the most
favorable circumstances. The system need scarcely be con-
sidered further, since it is more curious than valuable, and
is imlikely to be employed in large sub-station work.
Three-phase circuits are variously arranged, as has been
already indicated. The phases are very seldom separated, for
a six- wire circuit is too complicated for general use, but are
usually interconnected. The commonest and simplest con-
nection is shown in Fig. 305. This consists of only three wires
each rimning from the termhial of a phase winding on the
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616 ELECTRIC TRANSMISSION OF POWER.
armature. Motors are connected to all three wires, and
lamps between any two wires. The voltage is the same be-
tween each pair of wires^ provided each pair be equally loaded.
The relative amount of copper required is 75, as explained
elsewhere. Here, as always, the uniting of circuits to save
copper is accompanied by the need for balancing the loads.
Not only does change of load on one branch change the drop
in the other two, but interacts with them in the transformers
and generators. The disturbance, however, is fortunately trivial
in amount, except for very great inequalities of load or for
abnormally large line loss. With ordinary losses in the line
it is absolutely negligible when the circuits at full load are
balanced within 10 or 15 per cent, and at light loads far greater
inequality will have no perceptible effect. With ordinary care
FlO. 306.
in arranging the installation the question of balance never as-
sumes any considerable importance, and need not do so even
when very close regulation is desired, although extra care is
necessary in reaching first-class results. The main objection
to the system of Fig. 305 is the considerable amoimt of copper
required for a distribution by secondary mains as compared
with the ordinary three-wire systems. Its salient advantage
is its ability to handle motors and lights with equal facility on
a system composed of only three wires, and with some saving
of copper. The trouble of approximately balancing the three
branches is regarded as insignificant by those who are operat-
ing such s)^tems. This three-phase distribution is often taken
from the three common jimctions of a mesh connection, while
for motors the connection is a matter of indifference.
A far better system for sub-station distribution is that
shown in Fig. 306. It is a three-phase system with a neutral
wire connected to the neutral point of the three-phase wind-
ings. The lamps are connected between this neutral wire and
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METHODS OF DISTRIBUTION. 617
the several main lines. The result is that the working voltage
of the lamps is the voltage from either line to the neutral point,
while the working voltage of the system is 1.73 times greater,
being the voltage between line and line. Hence, there is a great
reduction in the amount of copper required, the relative weight,
as compared with the two-wire monophase system, being only
29.2 if the neutral wire is taken of cross section equal to one-
half that of either of the other wires. This system must be
balanced approximately, but requires less care in this respect
than the ordinary three-phase connection just described. It is
on the whole, better adapted for large distributions of mixed
lighting and power than any other of the modem alternating
systems, since it combines a fairly simple arrangement of wiring
with very great economy of copper. It lends itself readily even
to underground service, giving a rather simple cable construe-
"p
Fia. 906.
tion and facilitating testing. It is used with excellent results
in the Folsom-Sacramento, the Fresno, and other important
transmission plants, for the main work of distribution.
An interesting modification of the three-phase system is that
used in the city of Dresden and shown in Fig. 307. Here the
system is constituted in the ordinary way, but two of the leads,
a and 6, are arranged to carry all the lighting, while the third
wire c, which may be of much less area, is used only in connec-
tion with the motors. It may even sometimes be advantage-
ous to increase the cross section of two of the armature wind-
ings at the expense of the third. A machine so constituted
would have fully as great capacity as a monophase machine
of the same dimensioiis, and still would be amply able to carry
any ordinary motor loads. Even with the ordinary three-
phase winding this connection may be used without serious
reduction of output as compared with a monophase generator
of the same cost. Obviously the relative copper required may
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618
ELECTRIC TRANSMISSION OF POWER.
vary from 100, when the load is of lights only, to 75 for the
other extreme case. With half lights and half motors it would
require 80-90 relative copper, according to the allowances
made for drop, inductance, etc. In point of convenience it is
»€t^
FlQ. 307.
very similar to the "monocyclic" system, and like the latter
may be used with great ease in remodeling monophase S3rstems
for motor work, without requiring special generators of a type
which is tending to obsolescence.
A natural derivative of this mixed system is shown in Fig.
308. It is a combination of Figs. 306 and 307; a and b being
the mains, c the motor wire and d the neutral wire. The rela-
tive copper required naturally varies with the proportion of
motors and lights; 36 representing that necessary for an ap-
-^^
4^
.5
FlO. 806.
proximately equal division under ordinary conditions. Fig.
308 may be compared with Fig. 302, the monocyclic three-wire
sjrstem. It is about the same in effect as the three-phase
system with neutral, having but two branches instead of three
to balance, and paying for this privilege with about 20 per cent
more copper.
There is thus a liberal choice of methods more or less avail-
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METHODS OF DISTRIBUTION. 619
able for the general distribution of power and light. Any one
of them may prove to be the most useful in particular situa-
tions. Now and then it may be worth while to use more than
one of them in the same plant, as, for example, monophase two-
wire and monophase three-wire or three-phase and three-phase
with neutral.
It must be borne distinctly in mind that one cannot organize
a large sub-station distribution successfully on any substan-
tially two-wire system — the cost of copper is too great. If
work akin to that of a large central station is to be done,
methods must be used akin to those which have proved suc-
cessful in such work. The methods of distribution must be
those which are capable of giving a secondary network of mod-
erate cost, easy to install and maintain. The use of alternating
current gives a great advantage in the use of high tension
feeders and in efficient methods of regulation, and there is at
present no difficulty in furnishing a reliable and efficient motor
service; but to secure the full advantage of all this, one must
cut loose from the traditions of alternating current service.
A transformer must be looked upon not merely as a device for
lowering the voltage to a point available for direct consump-
tion, but as a generator of extreme simplicity and enormous
efficiency that operates without attention, can be started and
stopped from any convenient point, and may be regulated
without material loss of energy. That it receives current from
a transmission line instead of energy of rotation from a steam
engine is clear gain in simplicity, not a marvel to be looked at
askance. On the contrary, the transmission plant is usually
quite as manageable and trustworthy as a steam plant.
Approaching the sub-station from this standpoint, the prob-
lem of effective distribution becomes tolerably straightforward.
Given the transmitted energy, it must be distributed over a
known area cheaply and efficiently, with the smallest feasible
loss of energy at all loads, and the best regulation attainable.
It will not do to plead transformer losses when the lights bum
dim, or the depravity of alternating motors when they flicker.
First, as to locating a sub-station. On general principles
any station should be placed as nearly as may be at the centre
of its load, and inasmuch as a transformer station requires
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620
ELECTRIC TRANSMISSION OF POWER,
little space and makes no noise, there are few limitations to its
position save the ability to bring to it the transmission lines,
which, being generally at very high voltage, will be eyed cau-
tiously by the municipal authorities. The main district to be
covered is generally quite definite, and the next thing to be
done is to reach every part of it with a network of working
conductors proportioned to the service. The nature of the
8\
4
f
€t^
FlO. 310.
wiring will vary, according to the system employed; but the
generally accepted principles are, save for inductance, the in-
fluence of which has already been considered, the same that
are familiar in contmuous current work.
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METHODS OF DISTRIBUTION. 621
The problem is to supply a certain amoiint of energy at a
given loss over a known area, and the formnli© already stated
give the key to the solution. Working out the details, how-
ever, is a somewhat complicated matter, requiring great judg-
ment and finesse, and to be accomplished properly only by an
experienced engineer, working on the spot. The intricacies
of the problem are too great to be treated in an elementary
treatise like the present. The general situation, however, is
something as follows: A city. Fig. 309, is to be supplied with
light and power from a transmission plant. Let A be the
centre of load at which the transmission lines terminate. At
this point can most advantageously be located the reducing
sub-station, lowering the voltage of transmission to perhaps
2,200 volts for feeders, or to a tenth of this for direct supply.
The centre of load considered is not the geographical centre
of the district to be supplied, but the centre of gravity of the
load. This is determined just as if the electrical loads at
various points were weights fastened on a rigid weightless
framework. For example, suppose there are given the loads
of Fig. 310, five in number and in relative magnitude as shown
by the figures. Connect any two of them, as 1 and 2. These
would balance as weights at the point a, which acts with re-
spect to other points as if 1 and 2 were concentrated at it.
Now connect a and 3. These weights are equal, hence the
point of balance is the middle point of a 3, b, at which the
weight is evidently 6; 6 4 balances at c, where the weight is 10,
and finally the whole system balances at d, which is the centre
of gravity. The points may be taken in any order, but each
line must be divided so that, for instance, the length a 1, mul-
tiplied by weight 1, shall equal the length a 2 multiplied by
weight 2.
The centre of load thus found should be the centre of dis-
tribution to secure maximum economy in copper. The fact
that distribution lines usually run in a rectangular street sys-
tem renders the solution thus obtained merely approximate,
but it is nevertheless close enough for purposes of station loca-
tion.
Recurring to Fig. 309, several methods of arranging the
service are available. The simplest is, if the load is tolerably
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622 ELECTRIC TRANSMISSION OP POWER.
concentrated, to institute a secondary network about il so as
to include a good part of the load and then pick up the out-
lying load by transformers, placed where they can do the most
good, fed from high tension feeders. Sometimes, however,
there will be no heavy service near the centre of load, so that
the whole work of the station will be done through high ten-
sion feeders, each supplying through its transformers a more
or less extensive system of secondaries. Standard trans-
formers are conmionly wound for about 2,200 volts primary,
but 2,400 volts and 3,100 volts are also regular primary pres-
sures and the former is in considerable use. Above these
figures small transformers are rather expensive, but if necessary
the standard transformers can be used in star connection.
As has already been pointed out, there is every reason for
using a secondary network, connected directly to the reducing
transformers, at the sub-station if possible, thereby avoiding
the expense of transformers for a second reduction in voltage
and the loss of efficiency involved in such a reduction. The
house-to-house transformer distribution should be shujmed
as one would shun the plague, if there is any expectation of
securing an efficient station, capable of giving first-class
service.
It must be remembered that to be successful, a modem plant
for distributing power and light throughout a city must be
able to compete with the best that can be done, not with the
precarious and shiftless service of a dozen years ago.
It is possible with a modem alternating plant, to equal the
best service given by a continuous current central station, but
the feat can be accomplished only by the study of central
station practice..
The sub-station at A, Fig. 309, should be treated, so far as
distribution is concerned, as if the reducing transformers were
ordinary generators. The transformer units should be of the
size that would be convenient if they were generators, and the
bank should be so managed as to keep the transformers in use
as thoroughly loaded as possible. From the transformer bank
should run feeders to the principal sub-centres of distribution
in the network, with pressure regulators in such of the feeders
dis require them. From these sub-centres, pressure wires should
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METHODS OF DISTRIBUTION. 628
run back to the station whenever needed for the guidance of
the operator in charge of the regulators.
Outside the effective radius of distribution of the principal
secondary network will come the independent sub-centres
referred to, with their high tension feeders and subsidiary net-
works. These latter should be, so far as possible, interlinked
so that, at times of light load, only the transformers actually
needed shall be in service. If secondary pressure wires are
brought home from the subsidiary networks, all the regulation
can be done on the high tension feeders, thereby giving equally
good service all over the plant. Most continuous current sta-
tions extend their lines far beyond the radius that is economi-
cal for low tension currents, and often have to depend on
boosters with feeders worked at a heavy loss for service in
the outlying districts. With an alternating system this diffi-
culty is avoided, and the loss in transformers and regulators
is far less than that incurred with boosters and long low ten-
sion feeders.
As for the motor service in such a system, it should be treated
by common sense, as it would be in a central station distribut-
ing continuous current.
Alternating motors, polyphase or other, can be connected
to the secondary mains up to the point at which their demands
for current become burdensome. At that point the mains
must be reinforced or special feeders run, just as would be the
case with continuous current motors. The only difference is
that produced by the so-called idle current in the alternating
motors, which simply means that the point in question is
reached a little sooner than with continuous current motors.
In practice this difference need not be enough to be of serious
moment in plants having the ordinary proportions of lights
and motors. In case of large motor plants in which the ser-
vice is severe, the use of special high tension feeders will relieve
the trouble that might be experienced with the lights, but this
expedient is one to which recourse would seldom have to be
taken on a large scale.
The greatest difficulty in such sub-station distribution is, as
has been already indicated, the arc lighting. At present the
alternating arc lamp is hardly adequate to meet all conditions,
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624 ELECTRIC TRANSMISSION OF POWER.
although it is coming gradually into more and more extended
and successful use.
In cases where power is to be supplied for railway purposes,
there are few difficulties in the way. Existing railway gen-
erators can readily be utilized by driving them from syn-
chronous motors. This is the method employed in the old
transmission to Sacramento, Cal., and elsewhere not infre-
quently. Where the utilization of the old machine is not
important, or in new plants, the tendency is to use the rotary
converter, which has been already fully discussed. Such
apparatus was first put into extensive use in the Portland
(Ore.) transmission plant, and is now largely and very suc-
cessfully employed everywhere. Continuous current for other
purposes may be obtained with ease by the various methods
described in Chapter VII. A very instructive example of
recent practice in sub-station distribution may be found in
Salt Lake City, Utah. This city is supplied with electric
power from a group of transmission plants, the general loca-
tion of which is shown in Fig. 311. These plants were started
independently, but later were consolidated with the local light-
ing interests and are operated together. The Big Cottonwood
plant, started in 1896, contains four 450 KW, three-phase
generators, and has a double 10,000 volt circuit 14 miles
long into Salt Lake City. The Ogden plant, started the suc-
ceeding year, has five 750 KW three-phase generators, at
2,300 volts, at which pressure energy is supplied in the city
of Ogden. The rest of the output is raised to 16^000 volts
and sent into Salt Lake City over a pair of circuits 36 i miles
long.
The third plant, that of the Utah Power Co., is like the
first, in the Big Cottonwood Canon, but is two miles nearer
the city, and contains two 750 KW two-phase generators, with
a two-phase-three-phase raising bank of transformers to 16,000
volts, feeding duplicate three-phase circuits.
These are now (1905) also interlinked with the Provo system
with its plants on the Provo River and with a 2,000 KW plant
at Logan, some 40 miles to the north of Ogden. The longer
lines are worked at 40,000 volts. The whole system comprises
six hydraulic plants, two auxiliary steam plants, and 420 miles
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METHODS OF DISTRIBUTION.
625
Fig. 811.
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626
ELECTRIC TRANSMISSION OP POWER.
of high voltage lines covering a territory 160 miles long. The
entire network is successfully operated in parallel.
The Utah plant, with one generator and line of the Big
Cottonwood plant are put in parallel on the high tension side,
and nm two-phase rotaries in a sub-station near the centre of
the city. This sub-station supplies power to the electric rail-
Fig. 312.
way system, and is entirely separate from the lighting distri-
bution.
The Ogden lines and the remaining Big Cottonwood line are
put in parallel on the low tension 2,300- volt side, at a centrally
located sub-station devoted to lighting and power. From this
sub-station is carried out a system of three-phase primary
feeders and mains serving the entire city. This network is
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Methods of distribution.
627
well shown in Fig. 312. The primaries are connected in mesh,
but the secondaries have the star connection with neutral,
forming a regular three-phase four-wire distribution, with 115
volts betw^een the neutral and either phase-wire. Motors are
connected to the three-phase wires, giving about 200 volts, and
all motors over 10 HP are put on transformers of their own.
II_ll_J 1 — rl I, r- I r.-_^
JDnnnannn
Secondary M«fn8 -
Secondary Mains •
4 wire •
2wtre
Tranaformers repmenied by dotti*
nnnQDDorjTL
3ng
ULfUUUi-IUUUiUU
□CGGDODnDDD
lacaQuDnLraDD
Fig. 313.
As appears from the cut, Fig. 312, the primary network is
quite symmetrically arranged with reference to the extension
of service.
The secondary service is developed into a systematic net-
work of mains, well shown in Fig. 313. Where the service is
dense there is a regular four-wire network. Each block is served
by two groups of three transformers at the opposite comers.
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628 BLECTRtC TRANSMISSION OP POWER.
from which secondary mains are carried around the block and
tied by fuse boxes to the secondary mains of adjacent blocks.
When a distribution of this sort is used, fuse boxes and cut-outs
should be judiciously employed on both primary and secondary
networks, so that in case of severe short circuits or of fire the
district affected can be promptly cut clear of the rest of the
system.
Where service is less dense, as in residence districts, the first
step is to put in a two-wire secondary main from the trans-
formers, consisting of a neutral and one phase- wire, street being
balanced against street in this light service. Then when busi-
ness demands, the other phase-wires are carried into the street,
lights balanced upon them, and the completed four-wire system
is then tied to the network already completed.
Commercial arc lighting is by constant potential alternating
arcs, of which some 500 are in use in Ogden and Salt Lake
City. Street lighting is at present supplied from continuous
current series arc machines driven by S3mchronous motors.
These motors are located in an old electric light station near
the sub-station, and can be driven as generators in case of
need, while the sub-station itself has a small reserve steam
plant and generator equipment. The s3mchronous motors are
useful in regulating the voltage at Salt Lake City, being capable
of accomplishing a variation of 10 per cent when the lines are
heavily loaded.
This scheme of sub-station distribution is admirably con-
ceived, and works out very simply and neatly. The trans-
mission system itself is decidedly complex, owing to the vari-
ous and diverse power houses, but it works well and has done
excellent service. It is interesthig to note that no trouble
is experienced in running these distant and diverse plants in
parallel. At light load there is some interchange of current,
but at heavy loads everything settles down to business.
All the stations are connected by telephone, and by a little
intercommunication the generators can be put in parallel in
the ordinary manner either at a station or at the sub-station in
Salt Lake City. The record of the system for continuity of ser-
vice has been good, and it is worth noting that mo-?t instances
of trouble on the lines have been due to malicious interference,
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METHODS OF DISTRIBUTION. 629
such as shooting ofF insulators and throwing things across the
lines. Altogether the system is a notable instance of the flexi-
bility and convenience of modem power transmission methods,
as well as a good example of a systematic and logical develop-
ment of the distribution system. As the service grows various
refinements will doubtless suggest themselves, but the system
is correctly started and there will be little work to undo. It is
in striking contrast with some transmission systems which
could be named, in which the operators, less skilled in dealing
with modem methods, have blundered around trying to give
good service in an unsystematic and helter-skelter fashion,
getting deeper into trouble at every jump, and then blaming
the state of the art for the results of their own lack of discretion.
The most delicate and important work in connection with
heavy sub-station service is that involved in the proper regu-
lation of the voltage. The sub-station receives its supply of
energy often from a long transmission line in which there is
considerable drop, to say nothmg of that encountered in the
generators and two banks of transformers.
It must distribute this energy throughout a complicated
network, so that the variations in pressure at the lamps shall
not exceed two or three volts at the outside. This is never an
easy task — it tries the ingenuity even of the best central
station engineers.
In connection with a transmission plant, probably the best
plan is to divide the regulation into two stages: first, that con-
cemed with the transmission proper, and second, that concemed
with the distribution. By compounding the generators, or by
hand or automatic regulation of generators having good inher-
ent regulation, it is certainly possible to hold the voltage closely
constant up to the primary terminals of the reducing trans-
formers. In large altemating generators ordinary compound-
ing is seldom or never attempted, and in many cases the sole
reliance is hand regulation, which is by no means to be despised
in the absence of other means.
Within the last few years several automatic regulators
capable of giving excellent service have been brought out, and
they are coming into somewhat extensive use. The two prin-
cipal forms have already been described.
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630
ELECTRIC TRANSMISSION OF POWER,
None of these regulators are arranged automatically to take
care of the line drop when the power factor varies considerably,
but they are amply sufficient to provide for the general regula-
tion up to the sub-station, at which point it may be taken up
as a separate problem. This residual regulation ordinarily
consists of the drop in the reducing transformers, which should
be not over 2 per cent; the drop in the feeders and secondary
mains; in high tension feeders and transformers when em-
ployed ; and finally in the house wiring. These losses will aggre-
FlQ. 314.
gate generally less than 10 per cent, and are best cared
for in the sub-station. As the variations in load, and hence
in loss, are generally rather slow, this regulation should be
accomplished without difficulty. In some cases it may be
advantageously reduced in amount by carrying the primary
regulation through to the secondary terminals of the reducing
transformers.
However this may be, the regulation of the voltage on the
secondary lines must be carried out with the utmost care-
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METHODS OF DISTRIBUTION. 681
The apparatus employed for this purpose is both very simple
and exceedingly efficient. It is in every case a transformer
arranged to give a variable ratio of transformation and adding
its E. M. F. to that of the working circuit.
The best known form of this device is probably the Stillwell
regulator, which has for some years past been very successfully
used by the Westinghouse Company. It is, in effect, a trans-
former, from the secondary coil of which leads are brought out
to terminals so arranged as to enable one to vary the number
of secondary turns, and so to vary the E. M. F. added to the
working circuit. Fig. 314 shows a diagram of the connections
by which this result is effected. The diagram is self-explana-
tory, except that it should be noted that the "preventive coil"
is intended to avert the necessity of breaking circuit or short
circuiting a secondary coil in passing from one contact to the
CifeNERATORTv^ D I Ib Load
\ ^ —
FlO. 315.
next, and that the reversing switch enables the regulator to
diminish the voltage on the working circuit, which may now
and then be convenient. In the ordinary practice of the West-
inghouse Company, this regulator is installed in the generating
station and used to vary the voltage on the primary line. In
sub-station work it can be applied either to the primary or sec-
ondary side of the reducing transformers; practically the latter
is the working connection. These regulators are made to have
a range of action of 10, 15, and 20 per cent of the working vol-
tage. They are generally employed with a very ingenious
device known as the "comp)ensator," the fimction of which is
to indicate the pressure at the end of the line or feeder without
the use of pressure wires. The principle of this is shown in Fig.
315. The voltmeter V is in circuit with the opposed E. M. F.'s
of two secondaries C and D, of which the primaries A and D
are respectively in series and in shunt with the load. The
voltage of D is proportional to the main primary E. M. F., that
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632 ELECTRIC TRANSMISSION OF POWER.
of C to the primary current strength, so that the difference
between C and D, which shows on the voltmeter, can be made
proportional to the voltage as reduced by the drop due to the
current in the line. The compensator is in addition provided
with a series of contacts by which the E. M. F. of C is adjustable
for any given percentage of loss in the line.
The practice of the General Electric Company is somewhat
different. The generator is generally over-compounded for a
fixed loss in the line at full load, or hand regulation is effected
by the field rheostat. For sub-station purposes a variable
transformer is employed to vary the working voltage. The
principle of this voltage regulator is the variation of the induc-
tive relation of primary and secondary instead of varying the
number of secondary turns. The apparatus itself is made in
several forms, one of which, used in a number of three-
phase plants, is shown in Fig. 262. It is essentially a trans-
former with a movable secondary, and serves either to raise or
lower the working voltage, as occasion requires. The grada-
tion of voltage is not by definite steps, but by continuous varia-
tion. The apparatus is made for substantially the same range
of action as the Stillwell regulator just described, and accom-
plishes the same result. The General Electric Company also
makes a voltage regulator with a variable number of secondary
windings.
It should be stated that neither over-compounding nor any
similar devices can deal successfully with a load of very variable
power factor such as is often found in motor service. They
can be made to work well on either non-inductive or inductive
load, but are not well adapted for a load of which the power
factor varies much. For this condition nothing has yet been
devised so good as pressure wires combined with intelligent
hand regulation.
Various attempts have been made to employ pressure wires
in conjunction with automatic regulators, but none have yet
met with very encouraging success. Automatic control of
alternating current sub-station regulators is by no means so
simple a matter as pressure regulation applied to the generator.
Apparatus of the type of the Stillwell regulator has to deal
with fairly large currents, and the contact arm^ to prevent undue
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METHODS OF DISTRIBUTION. 633
load on the "preventive coil/' should be quickly moved from
segment to segment. This involves various mechanical diffi-
culties and the expenditure of some power. Apparatus like
Fig. 316 works none too easily, on account of the magnetic
forces involved. Such regulators are sometimes motor-driven
and thus readily controllable from the switchboard. In fact,
it is safe to say that the problem of working sub-station regu-
lators automatically involves the use of powerful relay mechan-
FlG. 316.
ism akin to that used for water-wheel governors, although on
a very much smaller scale.
No such apparatus is just at present in practical use,
although if succcvssful it would be in considerable demand. Still
less progress has been made toward the development of an
automatic balancing device for polyphase circuits. Given a
good automatic sub-station regulator, and its application to
preserving accurate balance in a two-phase or three-phase
distributing system is an obvious extension of its general use.
Balance is not difficult to secure with a little tact in arranging
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634 ELECTRIC TRANSMISSION OF POWER.
the load, but sometimes when there is a particularly heavy
lap load, there will be sensible unbalancing while this load is
coming on and going off. This is taken care of sometimes by
having certain loads that can be switched at will upon any leg
of the circuit, and sometimes pressure regulators are installed
for manual operation. A good automatic balancer and pres-
sure regulator would often be of very considerable service, but
it is not yet forthcoming. It must not be supposed that the
lack of it is a very grave deficiency, since practically all ordi-
nary central station regulation is manual save in so far as it
can be accomplished by over-compounding the generators.
The devices just described are amply competent to furnish
very exact regulation for sub-station purposes. Its complete-
ness depends in the last resort on the skill with which the dis-
tributing system is designed. If this is carefully done, the
sub-station regulation should hold the voltage within very
narrow limits clear up to the lamps.
As regards the best system of transmission to employ in con-
nection with heavy sub-station work, there is naturally a wide
diversity of opinion. In the author's judgment, there is at
present no distributing system for large sub-station work in
connection with long-distance transmission so generally advan-
tageous as the three-phase distribution with neutral wire shown
in Fig. 306. It is remarkably free from trouble as regards
balancing, and extraordinarily economical of copper. With
further advance in the development of single-phase alternat-
ing motors, the single-phase three-wire system shown in Fig.
299 will do admirable work when the motor service is rather
light. The di phase system has been installed in some central
stations and the "monocyclic" in others, so data will eventu-
ally be available regarding each of these systems, but there is
little reason to expect as good general results as could be ob-
tained by the systems mentioned above. Diphase, mono-
cyclic, and the Dresden three-phase systems are, however, very
much easier to adapt to the circuits of present stations than is
the three-phase system with neutral wire.
When a large part of the output of a transmission plant is
required for railway w^ork and other motor service of extreme
severity, and a lighting system is also to be operated, it is ft
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METHODS OF DISTRIBUTION. 635
wise precaution to work the two services normally over sepa-
rate lines and from separate generators as is done in the Salt
Lake City system just described. Otherwise the variations
of load may be so great and so rapid that no care in regulation
could prevent serious fluctuations in voltage. A small rail-
way load and all ordinary motor service can be worked from
the same circuits as lamps without much difficulty. These
limitations are not peculiar to transmission plants — no Edison
station, for instance, would dare to attempt working a low
voltage conduit railway from its lighting mains. In these, as
in many similar matters, a little common sense will prevent
serious mistakes and show the necessity of working every sys-
tem so as to obtain the best possible results, and not to discover
what it will endure without giving intolerably bad service. Of
late storage batter)'' auxiliaries have often been suggested, and
sometimes have been employed, in connection with power
transmission plants. Some reference has already been made
to storage in Chapter II, but the matters here to be consid-
ered are of a different character. In transmission work a
battery may be used for two entirely distinct purposes. In the
first place it may be used, as it sometimes is in steam-driven
stations, for the purpose of storing energy at times of light load
to be used in making up deficiency of power at times of heavy
load.
In steam-driven stations the installation of a batter}^ effects
a considerable economy by enabling the engines to be run at
all times at the points of maximum economy, and an additional
saving, in first cost, by reducing the capacity of the steam plant
and generators required. The conditions of economy depend
mainly upon local circumstances, but a material saving can
be made in many instances by using the battery.
In hydraulic practice the case is different. In the average
water power plant the main hydraulic works should generally
be installed for the full available capacity, save in the few in-
stances when a partial fall can be economically utilized. As
a rule the dam will be substantially the same for a partial de-
velopment as for a complete one, and the latter can be carried
out more cheaply at the start than when added as patchwork
later. Consequently there is seldom or never any saving in
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636 ELECTRIC TRANSMISSION OF POWER.
installing a costly battery subject to heavy depreciation in
order to avert the first cost of a larger plant. Further, the loss
of energy in the battery is much greater than the loss ordi-
narily incurred hi the line at full load, so that the total saleable
power for a given first cost would in nearly every case be re-
duced \Sy installing a battery. The one case in which a battery
can advantageously be used in connection with power trans-
mission for the purpose indicated, is that in which the total
hydraulic power available is actually insufficient to carry the
required maximum load. Storage may then be very advan-
tageous, since is enables the imutilized power at light load to
be applied to the peak. Especially will it be advisable when
the peak is high and the load factor rather poor, under
which conditions a battery may raise the possible max-
imum output by 30 to 50 per cent, sometimes even a little
more.
The second use of a battery is as a reserve to tide over a brief
break down. The question of reserve against accident in trans-
mission work is always a troublesome one. In the author's
opinion the need of a complete reserve located in the sub-
station is overestimated. Experience clearly indicates that of
the interruptions of service occurring on the system of a trans-
mission plant with sub-station distribution, only a very small
minority occur on the transmission line proper. The distribu-
tion lines throughout an average city are peculiarly exposed
to interruption from limbs of trees, which in residence streets
can never be adequately trimmed; from the fall of foreign
wires; from necessary cutting off in case of fire, and from other
causes. A high voltage transmission is neither more nor less
likely to encounter trouble on its distributing system than an
ordinary central station. So far as these causes of trouble go,
the transmission plant's sub-station is exactly on a par with any
other central station in requiring special precautions. Now
while central stations always should have more or less reserve
apparatus to use in case of break down, it is not required on
account of possible trouble on the line except as such trouble
may injure apparatus. A short circuit on the feeding S3^tem
will not be removed by the presence of a spare engine and
dynamo in the station. Hence, the need of reserve in the sub-
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METHODS OF DISTRIBUTION, 637
station of a power transmission system bears relation simply to
the accidents which may affect continuity of service as regards
the main transmission line, and particularly accidents producing
more than momentary interruptions. Such accidents are rare
on properly designed and erected lines, and save on extremely
long lines of which the cost is a considerable part of the total
cost of the system, it is generally true that a fraction of the
cost of a complete reserve plant at the sub-station would pro-
vide a duplicate line so guarded that reserve apparatus would
be practically needless. With well-built duplicate pole lines
and proper switching arrangements, serious trouble on the lines,
save under conditions which would also paralyze the service
on the distributing system, and thus cripple the plant in any
event, becomes almost impossible.
Sometimes, however, a partial auxiliary plant is extremely
useful, but it is rather for its convenience in case of repairs
to apparatus at the generating station or sub-station than as a
safeguard to the main line. In working a large sub-station,
a storage battery may be of considerable use in this way, par-
ticularly if the system is being pushed near to its capacity. It
is decidedly not good policy, however, to use a battery unless
the station is upon a scale large enough to warrant the employ-
ment of an especial man skilled in handling batteries and im-
burdened with other duties. Charged and discharged through
motor generators or rotaries, a storage battery can be put into
service on a moment's notice, and is far less troublesome to
keep up than any other auxiliary for temporary use.
In some localities a generator coupled to a gas or oil engine
makes an admirable auxiliary. Such engines can now be ob-
tained of large output and very high economy, and form a re-
serve almost as convenient as a battery. Steam reserves are
not large in first cost, unless high economy in operation is
attempted, but cannot be put quickly into action unless the
fires are kept banked, which is a very considerable expense.
However, by keeping a banked fire under threatening climatic
conditions the reserve can be ready when it is likely to be
needed, and if apparatus needs repair there is generally notice
enough given to get steam up. Power of quick firing is of
great importance in boilers for an auxiliary plant, and with
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636 MLECTRIC TltAmMISSlON OP POWER.
tactful treatment a steam reserve is probably the most satis-
factory for plants of moderate size.
In an increasing number of cases a steam auxiliary plant is
used to supply a deficit of power at times of low water. The
more use required of such a plant the more regard must be
had for high economy, in which respect it must be sharply
distinguished from an auxiliary used merely to tide over emer-
gencies and accidents.
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CHAPTER XVI.
THE COMMERCIAL PROBLEM.
Power transmission is of little avail if it does nob pay, and
the chances of commercial success form the first subject of
investigation in the development of any power transmission
enterprise. Reduced to its lowest terms, the question presents
itself thus: Can I profitably furnish power at a price which will
enable me to undersell the current cost of power production?
Evidently this question cannot be answered a prioriy but must
be thoroughly investigated in each particular case.
The first thing to be determined is the existence of a suffi-
cient market, the second thing is the price current in this
market. It is not difficult to find out the gross amount of
power used in a given region, but it is exceedingly hard to dis-
cover the real cost of production. Even if all men were strictly
veracious it is a fact that very few users of power have any
clear idea of what they pay for it. Coal bills and wages are
tangible and men realize them, but interest, depreciation,
repairs, miscellaneous supplies, water, taxes, insurance, and
incidentals, are seldom rigorously charged up to the power
account, and these are large items when power is used irregu-
larly.
Further, the cost per HP is often computed from the nominal
HP of the engine, without exact knowledge of the real average
yearly load. Hence, people often think that they are produc-
ing power at $15 or $20 per HP per year when the real cost is
$30 to $50.
The most exhaustive researches as yet made on this subject
are those of Dr. C. E. Emery. The accompanying table gives
a summary of his results, based on 500 net HP delivered for
ten hours per day, 308 days in the year. The power is sup-
posed to be derived from a single engine worked continuously
at its normal capacity. These figures represent results much
better than are generally reached in practice, since most en-
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640
ELECTRIC TRANSMISSION k^F POWER.
gines are not worked continuously at full load. In a large
majority of cases the real cost exceeds that given in the table,
even for engines of similar size. For the rank and file of small
engines used for miscellaneous manufacturing purposes, cheaply
built and generally underloaded, the tabular figures should
Kind of Engine.
Goal
$2 per T.
Coal
$3 per T.
Coal
$4 per T.
Coal
$5perT.
Simple high speed ....
Simple low speed ....
Simple low speed, condensing
Compound condensing, low
speed
Triple expansion condensing,
low speed
$29.81
28.46
22.82
21.97
22.36
$;^.17
84.20
26.77
26.63
25.32
$42. 6t
89.94
30.73
29.09
28.28
$48.90
46.67
34.69
32.66
31.26
be nearly doubled. In regions where coal is unusually dear
the cost in units of 50 HP and upward may range from $100
to $150 per HP year for a ten-hour day. Costs considerably
below those in the table are now and then reported, particu-
larly from engines in textile mills where the load is especially
favorable. Some of the reduction is undoubtedly due merely
to bookkeeping, a portion of the expense properly chargeable
to power being taken care of elsewhere, but some very low
genuine costs have certainly been secured. Dr. Emery's
tables are based on costs which can be materially lowered at
present prices as regards certain items, and they include some
items of expense which in favorable cases can be reduced.
For example, in the case of large engines the labor cost is
materially less than with the 500 HP assumed, and the inter-
est charge for an engine considered as part of a manufactur-
ing plant might properly be reduced to 5 per cent.
Then the table is based on average steam consumption,
while in recent mill engines a better figure is justified.
Assuming a power of 1,000 BHP and coal at $2.00 per long
ton, and making the necessary modifications in the data as
just indicated, the cost of the HP year on the basis of 308 days
of 10 hours each per year, with first-class compound condens-
ing engines, falls to about $17 to $18. These figures have un-
questionably been reached in actual practice, although rather
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THE COMMERCIAL PROBLEM. 641
seldom. They must, however, now and then be reckoned
with, and can be met only by very carefully planned trans-
mission from an imusually cheap water-power. As a rule, even
in large engine plants, the cost per HP year of 3,080 hours runs
above rather than below $20. On variable load the costs are
likely to run 20 or 25 per cent higher. There are few cases in
which transmission from cheap water-power on a large scale
cannot beat out steam power even in large units.
In units under 50 HP one is very unlikely to find the HP
year, reckoned on the above basis of 10 hours per day, costing
less than $50, even with coal as low as $2 per long ton. These
are the facts in the case; the fancies will be duly appreciated
if one canvasses for electric power. Not more than one man
in six knows and will admit that his power is costnig him as
much as the table would indicate. The process of reasoning
(so called) is often about as follows: "I paid for my engine and
boiler house when I built the factory, and I do not propose to
charge my engine rent. It has been running ten years and is
just as good now as it ever was; has not depreciated for my
purpose a cent. If any repairs were needed, the engineer and
one of my men have made them and they haven't cost me any-
thing but my material. My fireman I have to have anyhow,
for I heat by steam, and my taxes and insurance I have to pay
anyhow: that is a 200 HP engine; my coal cost me $2,450 last
year, and oil and stuff $70. I pay my engineer $60 a month;
that's $16.20 per horse-power per year; if you can furnish
electric power for $15 per year perhaps we can trade." This
theme, with variations, is famiUar to anyone who has had
practical experience in power transmission work, and although
the more intelligent and able class of manufacturers are quite
too keen not to see the facts when properly presented, a cer-
tain amoimt of this ignorant short-sightedness is always met
in investigating the power market.
With a working year as above of 3,080 hours, the cost of
steam power is actually very seldom as low as 1 cent per HP
hour, and in units below 100 HP is not very often below 2
cents. In imits of less than 20 HP it is quite certain to be 5
cents or more. These figures are based on continuous work-
ing. If the use of power is intermittent, the cost per HP hour
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642 ELECTRIC TRANSMISSION OF POWER.
is increased, by an uncertain but always large amount, depend-
ing on the nature of the service. For highly intermittent
service, gas engines are undoubtedly cheaper than steam, and
in ordinary units the cost of operating these is seldom less than
10 cents per HP hour of use. Used continuously at full load
or thereabouts, the gas or petroleum engine is the most formid-
able competitor of electric motors, since the actual cost of fuel
is low — from 2 to 5 cents per HP hour — and the atten-
dance required is trifling. Such engines, however, are high
in cost and are inefficient at low loads, besides being subject
to relatively large depreciation.
These peculiarities are well shown in a recent test of a 6 HP
gas engine in which the following facts appeared: The cost of
operation, including maintenance, was at full load 41 cents per
hour, and at no load 20 cents per hour; the cost of gas being
$1.70 per M feet.
We may easily find from this the cost of power under given
circumstances of use; $10 per HP per year may fairly be
charged up to interest and depreciation. Suppose, now, power
is used for 10 hours per day 308 days in the year, the engine
being fully loaded all the time. The cost can be made up as
follows for 6 HP:
3,080 hours ^ 41 cents » 11,262.80
Interest and depreciation «= 60.00
Total cost « $1,322.80
Cost per HP hour =7.15 cents, of which the interest and
depreciation amounts to but 0.31 cents per HP hour.
Second, suppose the engine is in full use 3 hours per day, and
running idle the rest of the time, or is in equivalent partial use
for 10 hours. We then have
924 hours to) 41 cents = $378.81
2,166 " »» 20 »» - 431.20
Interest and depreciation ^ 60.00
- fSTOToi
This is 12.08 cents per HP hour actually used, and is a fair
type of present practice as gas engines are generally used. It
will hold for the average engine used for small power purposes.
In regular running such engines consume from 25 to 35 cubic
feet of average illuminating gas per brake HP and, when run-
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THE COMMERCIAL PROBLEM.
643
ning light, take nearly half as much gas as at full load. In
careful experimental running these results can be bettered 10
to 20 per cent, but in regular work and with only ordinary care,
the gas consumption given is correct.
Petroleum engines give rather less fuel expense, but lose in
extra care and repairs nearly or quite all the gain in fuel.
These figures must not be understood as applying to large gas
engines of 100 HP and upward, worked on cheap "producer"
or fuel gas. It is reasonably certain that such engines give
results better than any save the most economical steam en-
gines, if worked at or near full load. The dubious point about
such large gas engine plants is the maintenance, particularly
in case a producer is installed. In the small sizes above con-
sidered the gas engine is a considerably cheaper source of power
than steam engines, probably by not less than 30 per cent. It
must not be forgotten also that the cost of power from small
gas engines is steadily being reduced owing to the great
stimulus given to engine design and operation by the develop-
ment of the automobile industry.
In a general way we may summarize these facts regarding
cost of power as follows, coal being taken at $3 per ton:
Kind of Engino.
Go8t per HPH, 10-
Hour Day,
Oo«t per HPH, Inter-
mittent Use, Partial
Large compouod cond. . . .
Simple, 100 HP and less . . .
Gas, 20 60 HP
Gas, small
Steam, small
0 8c. to Ic.
1.6 " 2.6
2.0 •• 4.0
6. ** 8.0
7. ** 12.
Ic. to 1.6c.
3. ** 6.
8. *' 7.
10. '^le.
12. "20.
By small engines are meant those not over 15 to 20 HP, such
as are used in large numbers for light manufacturing work.
These figures are of course only approximate, and must be
modified by the cost of fuel and labor in any particular locality.
They take no account of the efiiciency lost between the en-
gine and its work, which has been already discussed in Chapter
II, and which gives motor service some of its greatest commer-
cial advantages.
They show plainly, however, that electrical energy delivered
to the consumer at 4 to 5 cents per kilowatt hour has the com-
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644
ELECTRIC TRANSMISSION OF POWER,
mercial advantage in small work of all kinds, and in competi-
tion even with fairly large engines used at light load or inter-
mittently. In addition there is, in favor of electricity, the
generally considerable saving in waste power, and the greater
cleanliness and convenience of the motor. At equal prices
electric power will pretty effectively keep steam out of all new
work, but the cost of changing from one motive power to the
other demands some concessions on the part of electricity.
This cost of change is rather uncertain, for not only do elec-
tric motors vary very widely in price, owing to differences in
size, speed, and construction, but the net value of engines and
boilers replaced may vary from two-thirds to three-quarters
of their cost down to little more than scrap.
In both engines and motors the cost of the smaller sizes is
disproportionately large, owing to the relatively large percen-
tage of labor in their construction. Gas engines are even more
expensive than a steam boiler and engine in ordinary sizes.
In replacing engines by motors, the selling value of the former,
including boilers, if steam is used, may be anything, say from
$10 to $25 per HP, and the market is rather uncertain at best.
A little time will generally effect a sale on tolerable terms.
The following table gives the approximate cost of electric
motors installed and ready to run, based on motors of ordinary
speeds and voltages, with the usual accessories and with a
moderate amount of wiring. No useful iSgures can be given
on the cost of special installations with complex wiring.
HP.
Cost.
1
9 76 to 9 125
8
150 '' 260
6
200 '* 276
10
300 '« 460
16
350 '' 450
20
400 <' 600
25
500 '^ 700
30
600 '' 800
40
700 «' 900
60
800 " 1,100
76
1,200 '» 1,600
100
1,500 »' 2,000
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THE COMMERCIAL PROBLEM, 646
From this it appears that while large motors, 50 HP and
upward, can generally be counted on at not over $20 per HP,
the smaller sizes are much more costly. Below 20 HP the
net cost of changing from steam or gas engines to motors is
pretty certain to be $20 to $30 per HP. Taking interest and
depreciation at 10 per cent, the annual charge amounts to $2
or $3 per HP, which must be increased to $5 or $6 to cover
maintenance and miscellaneous expenses. Hence, for steady
use 10 hours per day, there should be charged to general cost
about 0.2 cent per HP hour, which is equivalent to perhaps
0.5 cent for intermittent use.
In changing motive power, then, electric service must gen-
erally be cheaper than what it replaces by about the amounts
mentioned.
As to the cost of furnishing electric power figures are a little
deceptive, since from place to place the conditions vary. It is
safe to allow about one KW at the station for one HP actually
delivered and paid for.
Now with steam for a motive power, the data already given
for mechanical power can readily be reduced to kilowatt hours,
assuming the dynamos to have as usual 92 to 95 per cent effi-
ciency at full load. But a steam station for power transmis-
sion has the advantage of nearly or quite continuous running,
thereby reducing general expenses, and besides, on a large scale,
the load can be kept at an efficient point most of the time.
In fact, in large railway power stations — the only steam-driven
stations for power transmission on a large scale — the machines
can be worked very efficiently most of the time, and power can
be, and is, very cheaply produced.
Fig. 317 shows graphically the approximate variation of
total cost with output in well-designed power stations, the
figures given being based on $3 per ton for coal and power
delivered at the station bus bars. Anything imder one cent
per KW hour including interest, depreciation, superintendence,
and general expense is good practice, even for a very large
station. Steam is not likely to be often used as a motive power
for power transmission work, except in working a very cheap
coal supply.
Dr. Emery has worked out at considerable length, the prob-
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646
ELECTRIC TRANSMISSION OF POWER.
lem of the cost of steam power on a very large scale and with
the most economical modem machinery. He assumed a
20,000 HP plant, worked 24 hours per day, on a variable load
averaging 12,760 HP, 63.8 per cent of the maximum. This
load factor is judiciously estimated and could certainly be
realized in a plant of such size, employed in the general dis-
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Fig. 317.
tribution of power. Taking coal at one mill per pound, $2.24
per long ton, and entering every item of expense, he found the
total cost per HP per year to be $33.14. If the plant were
established at the mouth of the coal mine, fuel should be ob-
tained at not over one-third the above cost. This advantage
would bring the cost per HP per year down to $24.89. Taking
now 15,000 KW in dynamo capacity in large direct coupled
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THE COMMERCIAL PROBLEM. 647
units, say five in number, the electrical plant would cost,
installed with all needful accessories and ready to run,
$200,000. Taking interest, taxes, and depreciation together
at 10 per cent, which is enough, since a 3 per cent sinking
fund would amply allow for depreciation; allowing $15,000 per
year for additional labor and superintendence and $10,000 more
for maintenance and miscellaneous expenses, brings the total
annual charge for the electrical machinery to $45,000. Add-
ing this to the steam power item and reducing the whole to
cost per KW hour, assuming 94 per cent average dyiiamo effi-
ciency, the total cost per KW hour delivered at the station
switchboard becomes 0.436 cent. Working, then, on an im-
mense scale from cheap coal, it is safe to say that less than half
a cent per KW hour will deliver the energy to the bus bars.
The next step is the cost of delivering it to the customer.
This varies so greatly, according to circumstances, that an
average is very hard to strike. A plant such as we are con-
sidering will usually be installed only when the radius of dis-
tribution is fairly long. Taking the transmission proper as
50 miles, the line and right of way, using 30,000 volts, may be
taken as about $25 per KW; the raising and reducing trans-
formers with sub-station and equipment would cost perhaps
$15 per KW, and the distributing circuits, with a fair propor-
tion of large motors, about $10 per KW additional. The com-
plete distributing system for 15,000 KW would then cost about
$750,000. Figuring interest and depreciation roundly at 10
per cent, the annual charge is $75,000. Add now $15,000 for
labor in sub-station and distributing system, $10,000 for gen-
eral administrative expense, and 5 per cent on the cost for
maintenance and miscellaneous expenses, and we reach a total
annual charge for distribution of $137,500. The average out-
put being almost exactly 9,000 KW, the cost of distribution per
KW hour is 0.174 cent. The actual cost of generating and dis-
tributing the power then becomes 0.610 cent per KW hour.
This is probably pretty nearly a minimum for distribution of
power from coal mines. It supposes a very large plant in-
stalled for cash and operated for profit. It makes no allow-
ance for the floating of bonds at 60 to 80 cents on the dollar,
the operations of a construction company, the piu'chase of
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648 ELECTRIC TRANSMISSION OF POWER.
coal from the directors, the payment of big salaries to the pro-
moters, or any of the allied devices well-known in financial
circles.
Under favorable circumstances a materially better result
can be reached with hydraulic power.
These figures mean that power could be sold at an average
of 1 cent per KW hour at a good profit, aggregating for the
plant in question more than a quarter of a million dollars per
year.
Only the largest plants, skilfully handled, can approach
such figures for cost of power as have just been given.
It should be possible, however, to bring the cost of distribu-
tion per KW hour in a well-designed transmission plant of
1,000 HP or more down to less than 0.5 cent per KW hour.
Less than this may indeed be found in practice, while figures
approaching 0.25 cent may be found in good central station
working.
The cost of producing power in steam-driven plants of vari-
ous sizes has already been given ; that in water-power plants is
far less definite, but on the whole lower. In some hydraulic
plants where development has been costly, the cost of water-
power rises to $20 or $25 per net HP year, while on the
other hand water-power has been leased at the canal for as
little as $5 per year per hydraulic HP in the canal, equivalent
to about $6.50 per available HP at the wheel shaft. The
investment per effective HP at the wheel ranges from nearly
$150 to as low as $30 or $40. This includes both the hydraulic
rights and work and the wheels themselves.
A typical estimate for a water-power plant under fairly fav-
orable conditions, derived from actual practice, runs about as
follows, for a 1,000 HP plant working at, say, 3,000 volts, so
Hydraulic works ^0,000
Wheels and fittings 12,600
Power station 2,500
Pole line, 8 miles 4,000
Transmission circuit 16,000
Dynamos and equipment, 750 K W 15,000
Transformers, 750 KW 7,600
Distributing lines 16,000
Miscellaneous 5,000
Total 9116,600
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THE COMMERCIAL PROBLEM. 649
Operating expense:
Interest and depreciation, 10 per cent Ill, 650
Attendance at plant 4,000
Linemen and team 2,000
Office expense 8,600
Rent, taxes, and incidentals 1,000
Maintenance and supplies 4,000
Total i26;i60
that there is no reducing sub-station, but only an ordinary
distribution.
The full capacity of the plant is about 750 KW. Supposing
the plant to be worked somewhere near its capacity at maxi-
mum load, and to be in operation on a mixed load 24 hours
per day, we may estimate the daily output about as follows:
KW KWH
9 hours (a) 600 4,600
6 *» *» 260 1,250
3 '* " 100 300
6 *» " 60 300
Total (5,360
This should be taken for 300 days in the year. The other
65 days, Sundays, holidays, and occasional periods of unusually
small motor loads, it is not safe to count on more than 1,000
KW hours per day. Taking account of stock, we have for the
year,
1,970,000 KWH,
and the net cost per kilowatt hour becomes 1.33 cents. It is
worth noting that the distribution of power for the day is taken
from a transmission plant in actual operation.
Of the above total cost, 0.47 cent is chargeable to distribution
expenses and 0.86 to power production. Doubling the cost of
the hydraulic works would raise the generating cost to 1.07 cents
and the total cost to 1.54.
It is evident in this case that power could be sold at 2 cents
net per HPH with a good profit, assuming the smaller total
cost, and at 2.5 cents, even with the greater hydraulic cost.
Even if the total investment were as great as $250,000, the
plant would pay fairly well at 3 cents per HPH.
The fact is, hydraulic transmission plants generally will pay
well if a good load can be obtained. The above example does
not show a cheap plant nor a remarkable load factor. In
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650 ELECTRIC TRANSMISSION OF POWER.
fact, the cost per KW in this case runs to about $155, while at
present prices of material, many plants are installed at a
considerably less figure even when as here the cost of the dis-
tributing system is large. In really favorable cases the cost of
power distributed will not exceed 1 cent per HP hour, and
in comparatively few plants will it rise to 2 cents, unless the
market for power is grossly overestimated.
This is one of the commonest troubles with plants that do
not pay well. A costly hydraulic development is undertaken,
resulting in rendering available several times as much power
as can be utilized; a portion of this is then transmitted and
sold, but the plant is burdened with heavy initial expense, and
struggles along as best it can. It is not safe to count on the
stimulation of industrial growth by cheap power unless the
situation is exceptionally fortunate, or cost of producing power
is so small that the plant will pay tolerably well on the ex-
isting market.
A careful canvass for power is a necessary part of the pre-
liminary work for a power transmission, and the more com-
plete it can be made the better. Reference to the table of
p. 643 shows that, at a selling rate of 2 to 4 cents per HP
hour, the cost of power can be reduced for all small consumers
and a good many rather large ones. If the cost of coal is
high, $5 per ton or more, nearly all consumers will save by
using electric power, while with favorable hydraulic conditions
money can be saved by transmission even when replacing very
cheap steam power.
Take, for example, a large manufacturing plant requiring
1,000 HP steadily, 12 hours a day. At a distance of, say, 8
miles, is a hydraulic power that can give, say, 1,200 HP, and can
be purchased and developed for $100,000. The cost of gener-
ating and transmitting power will be about as follows:
Hydraulic work 8*100,000
Wheels and fittings 15,000
Power house 8,000
Pole line 4,000
Dynamos and equipment 20,000
Transmission circuit 15,000
Motors and equipment 16,000
Miscellaneous 10,000
Total 1182,000
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THE COMMERCIAL PROBLEM. 651
and the operating expenses would be about as follows:
Interest and depreciation 818,200
Attendance at plant 2,600
** " motors 1,800
Other labor '. 1,000
Maintenance, supplies, etc 5,000
Total §28,500
This would furnish, taking the working year as 308 days,
3,696,000 HP hours at a cost of 0.77 cent per HP hour. With
a low cost of hydraulic development and a short line, say not
over three miles, the above figures for cost could be brought
down to about $130,000. Now, allowing 5 per cent for inter-
est, and setting aside 3 per cent for sinking fund, which allows
for complete replacement in less than 20 years, we may figure
the annual cost of power again thus:
Interest and sinking fund $10,400
Attendance at plant 2,600
*» ** motors 1,800
Maintenance and incidentals 6,000
Total §19,700
This is $19.70 per HP year, or 0.53 cent per HP hour, or
$15.80 per HP year omitting the sinking fund, which very sel-
dom is allowed to creep into estimates on the cost of steam
power. This is certainly cheaper than power can be generated
by steam, save in very exceptional instances, provided proper
account be taken of interest, depreciation, and repairs. As a
matter of fact, the cost just given has been reached, in practice,
in transmission work at moderate distances. On a larger scale,
slightly better results can be attained. These figures take no
account of the saving in actual power obtained by distributed
motors, always an important matter in organizing a transmis-
sion for manufacturing purposes. This can generally be
counted on to make it possible to replace 1,000 HP in a steam
engine by not over 750 HP in electric motors, with a corre-
sponding reduction in the aggregate yearly cost of power.
Speaking in a general way of costs at the present time (1906),
dynamos and their equipment may safely be taken at $10 to
$20 per kilowatt, raising and reducing transformers at from
$4 to $8 per KW, line erected at from $10 to $30 per KW,
water-wheels and governors at $10 to $20 per HP, and steam
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652 ELECTRIC TRANSMISSION OF POWER.
plant, when used, at from $40 to $60 per net HP, Under fav-
orable conditions the total cost per KW of capacity can be
brought to $50 or $60 excluding all questions of steam plant
and of hydraulic development.
The line is always a rather uncertain item, on account of its
variations in cost at different distances, and in meeting local
conditions of distribution. The pole line itself will cost from
$250 to $500 per mile, according to circumstances, but the
copper must be figured separately, as already explained.
No account is here taken of freaks in design — d3mamos of
special design for peculiar speeds or voltages, extraordinary
line voltages, unusual frequencies, or eccentric methods of dis-
tribution like the wholesome use of rotary converters and stor-
age batteries. The figures are intended to represent ordinary
good practice as it exists to-day.
One of the nicest points in operating a transmission plant is
the proper adjustment of the price of power to the existing
market. It is no easy matter to strike the point between the
cost of other power and the cost of generating and distributing
electric power, which will give the maximum net profit. In
general it is best to work entirely on a meter basis, for the
customer then pays simply for what he uses, and the station
manager knows the exact distribution of his output.
The generating station or the sub-station should be equipped
with a recording wattmeter that will show the actual output,
and from this measurement much valuable information can be
obtained.
Knowing the investment and the approximate operating
exj)ense, it is easy to figure, as we have just done,the total cost
of delivering energy per KW jK)wer at various outputs. This
is the basis of operations. The next thing is to estimate as
closely as possible the average local cost of power in units of
various sizes. These two quantities form the possible limits
of selling price. One must keep far enough above the first
to insure a good profit, and enough below the second to cap-
ture the business. It is convenient to plot these data as in
Fig. 318, which is based on the table of p. 643, and the plant
discussed on p. 649. Curve 1 shows the effect of change in the
aiuiual output on the net cost per KWH. Curve 2 shows the
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THE COMMERCIAL PROBLEM,
653
approximate existing cost of steam or other power, the points
from which the curve was drawn being shown by crosses.
Curve 3 shows the same for intermittent loads, the points being
indicated by circles. It is evident that for yearly outputs less
than 1,000,000 KWH, the plant would be in bad shape to
get busmess. At 2,000,000 KWH good profits are m sight,
ar
ANNUAL, OUTPUT THOUSANDS OF K.W. HOURS
Fig. 318.
while at 3,000,000, the electric plant can meet all cases at a
profit.
At the given output of 1,970,000 KWH, it would be possible
to charge 2 cents per KWH as a minimum without losing busi-
ness, while all the smaller customers could gain by changing
to electric power at 4, 5, or 6 cents per KWH.
When a few consumers are generating power at an unusually
low figure, there is always the temptation to obtain them at a
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664 ELECTRIC TRANSMISSION OF POWER,
special out rate. As a rule, this is bad policy unless they are
desirable for some particular reason aside from increase of out-
put, for the moral effect of special low price contracts is always
bad, and in the long run it is best to make standard rates and
to adhere to them.
The best prices can always of course be obtained from small
consumers, and these are also specially desirable in that they
tend to keep a uniform load on the system. Not only do 50
10 HP motors yield ordinarily several times as much revenue
as one 500 HP motor, but they will call for power very steadily
all day long and keep the regulation excellent, while the large
motor may be off and on in the most exasperating way and
cause great annoyance at the time of the "lap load," when
lights and motors are all in use. Large motors running inter-
mittently are especially disadvantageous, for they do not
greatly increase the aggregate station output and pay relatively
little.
In general, the best schedule of prices can be made up by
starting with a rate arranged to get all the powers below, say,
4 or 5 HP, and then for larger powers arranging a set of dis-
counts from this initial rate. These discoimts, however,
should be based, not exclusively upon the size of the motors,
but on the monthly KW hours recorded against them. In one
respect, charging by wattmeter alone is at rather a disadvan-
tage. A large motor running at variable load, and much of
the time at light load, is far less desirable as a station load than
a small and steadily running motor using the same number of
KW hours monthly. The former demands far greater station
capacity for the same earning power, and also inflicts a bad
power factor upon the system at times of light load if the dis-
tribution is by alternating current. It is not easy to avoid this
difficulty, although various devices to that end have been intro-
duced. In one large plant, recording ammeters are installed
for each motor, and the largest demand for current lasting two
minutes or more during a given month is made a factor in de-
termining the price paid for that month's supply of power, so
that large demands for station capacity must in part be paid
for by the consumer.
Another de\dce for the same purpose is a combination of the
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THE COMMERCIAL PROBLEM, 665
flat-rate and meter methods of charging. A fixed monthly-
charge per horse-power of the motor connected is made, and in
addition the consumer pays for his energy by wattmeter, of
course at a somewhat lower rate than in using the meter alone.
A rough illustration of the effect is as follows. Suppose a flat
charge of $1 per month per HP of the motor installed and a
meter rate of 3 cents per KWH. One customer has a 10 HP
motor worked steadily at full load 10 hours per day for 30 days.
Another has a 50 HP motor which runs at full load for 2 hoiu^
Ijer day. Each may, for example, use 3,000 KWH per month,
and pay by meter $90 therefor; but the former pays a flat
charge of $10, the latter one of $50, so that the monthly bill is
in the former case $100, in the latter $140. The extra $40 may
be regarded as the payment of rent for station capacity, and
capacity of lines and transformers, to be held at the cus-
tomer's call at all times. It is, in fact, a very genuine expense
to the station. The whole question of equitable charging for
current used for light and power is a very puzzling one. Tak-
ing the coimtry through, there has been a tendency for basic
rates to cluster about 20 cents per KWH for lighting and 10
cents per KWH for power. This difference has no logical
reason for existence, and merely represents the natural ten-
dency to get business by trying to keep below each consumer's
supposed cost of production. The present tendency is to put
current for lighting and power upon nearly the same basis,
letting a sliding scale of discounts take care of the generally
smaller output purchased bj'' the lighting customer. These
discounts vary greatly from place to place, but they generally
run up to 50 to 70 per cent for large consumers, and are com-
monly less for lighting than for power. On the whole, the
simpler the system of rates and discounts, the better.
It must not be forgotten that an electric supply company
is a public service corporation doing business in virtue of fran-
chise rights, and consequently it must tread softly and circum-
spectly in its dealings with the public. Special contracts, save
for an open and general reason, like the use of power during
restricted hours, are from this point of view particularly to be
avoided, and the whole rate system ought to be as open and
above board as possible.
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656
ELECTRIC TRANSMISSION OF POWER.
A complicated system of discounts is not at all necessary to
financial success, as witness the results obtained by the gas
companies from a nearly fiat rate. A simple and obvious dis-
count scale generally is acceptable to the public, and the trouble
really begins when one attempts to take account of differences
in demand of the sort already mentioned. A minimum bill
per lamp or HP installed plus a simple meter schedule can prob-
KW. HR. KR MONTH
19000 ia»0 9MI0C
^000 ' gpow
FlO. 319.
ably take account of varying conditions as satisfactorily as any
system yet devised.
The exact form of the rate schedule can best be determined
after looking over the local conditions. As an example of
how the thing can be done, let us start with the data of Fig. 318
on local costs of power. On the basis of curves 2 and 3, lay
down a tentative curve of a sort fitted to get the business.
Let this be, for example, curve 1 of Fig. 319. This naturally
falls pretty near curve 2 of Fig. 318. Now at near full load a
10 HP motor should take not far from 2,000 KWH per month.
Hence, one can set down a rough scale of outputs correspond-
ing to the horse-power of the several motor sizes. Of course
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THE COMMERCIAL PROBLEM. 667
very few motors run fully loaded, and as they fall below full load
their condition of economy approaches curve 3, Fig. 318, and
their bills slip up along our present curve. By the curve a man
with a 20 HP motor fully loaded will pay for 4,000 KWH a
month at 6 cents, while if he consumes but 2,000 KWH he will
pay for it at the rate of 6.7 cents per KWH. The man with a 100
HP motor will get his power at 2.4 cents per KWH at full
load, or at half load for 3.8 cents. These figures, although
based on reasonable data, are, as compared with actual motor
rates, rather high for the small motors and low for the
large ones. The difference perhaps indicates that in general
it has been found wise to encourage small motor business, as
has already been indicated.
Now to apply the combination of flat rate and meter to en,
courage steady loads. A regular charge of $1 per HP of
motor per month will mean, on a basis of 200 KWH monthb
per HP, a fixed charge of 0.5 cent per KWH on the fully loaded
motor. Therefore curve 1, Fig. 319, should be dropped down
0.5 cent to get the new meter rates. Here is the chance for
equalizing a bit, by dropping the upper end of the curve and
letting its lower end alone. Curve 2 shows a curve thus low-
ered. At about 50 HP the drop compensates for the fixed
charge, and the total rates rise above that point, and below it,
fall. Now for the 20 HP motor the monthly bill is $20 +
4,000 KWH @ 5 cents = $220, and for the 100 HP motor
$100 + 20,000 @ 2.3 cents = $560. The former pays $11
per horse-power per month, the latter $5.60. At half input
these figures rise to about $14 and $9.40 respectively.
To simplify the discounts, curve 2 is commonly made up of
a series of arbitrary steps such as are shown. They can be
arranged to suit any case, the one shown being merely a simple
example. Based upon it the discounts from the basic price of
10 cents per KWH are:
KWH Monthly Consumption Srcent'
Below 400 0
400- 1,000 20
1,000- 8,000 40
8,000- 5,000 60
6,000- 9,000 60
9,000-16,000 70
16,000 and over 80
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658 ELECTRIC TRANSMISSION OF POWER,
If there is an unusually good market for small motors, the steps
can be arranged to favor them a bit, as, for instance, by giv-
ing a 10 per cent discount from 200 to 400 KWH. It will
be seen that the whole scheme is frankly empirical, although
based on premises which are not without reason. Some dis-
count schedules are far more complex than that shown, while
others are rather simpler. The prices given here are fairly
high save on the large motors. Many plants give an extra
discount of 5 or 10 per cent for prompt settlement. In selling
current for lighting, the discounts are generally less variable
with the consumption than here shown, and a flat service rate
in addition is of rather dubious expediency considering the
policy of the gas companies. The discount schedule here
given would do very well for the lighting output as well as
for the motor load.
Charging by a recording ammeter instead of a wattmeter
will reach the users of motors that injure the power factor of
the system, and, combined with the flat rate just mentioned,
would probably give a really fairer system of payment for the
customer's demand upon the station than either of the schemes
just described, but the wattmeter is so generally used and
undei-stood that it can hardly be escaped.
Methods of selling and charging, however, must be modified
to suit local conditions and customs. Each community has
peculiarities of its own that must be studied and reached.
Sometimes a flat rate, objectionable as it often is, will secure
a more remunerative business than any system of metering,
whDe elsewhere a meter system, however intricate, may work
better than a flat rate. As a rule, however, metering is the
best method of charging for all parties.
A water-power transmission plant has the peculiarity when,
as usual, the water is owned outright, of showing a nearly
constant operating expense, irrespective of output. Hence,
after the receipts exceed this expense, all additional load, at
any price, means profit. But it means profit precisely in pro-
portion to its price, so that taking on large consumers at a
very low price is usually bad policy, it being better to encour-
age small consumers by giving what is to them a very reason-
able figure.
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THE COMMERCIAL PROBLEM, 659
After the maximum output comes near to the capacity of
the plant, the total yearly output for the given plant is diffi-
cult to increase. Hence, it is desirable persistently to culti-
vate the use of power at such times as will not increase the
maximum load. This can best be done by offering liberal dis-
coimts for power used only between, say, 8 p.m. and 8 a.m.
There is at best rather a small amount of this, and it is all
worth getting even at a.low rate. After getting all the avail-
able night power, the next step should be to get whatever day
business is possible for hours restricted to the period prior to
the beginning of the peak, say at 4 p.m., again at special dis-
counts. Now and then a customer can be picked up on this
basis to the great advantage of the station.
In stations using rented water-power at a fixed price per
HP, or employing steam, the operating expense is of course
variable, and this variation will influence greatly the adjust-
ment of prices, although the general principles are unchanged.
Experience has now shown that electric power transmission
may generally be made a profitable enterprise.
If a transmission is planned and executed on sound business
l)rinciples and with ordinary forethought, it is well-nigh cer-
tain to be a permanent and profitable investment.
Failure is generally chargeable to attempts to work with
altogether insufficient capital, leading to ruinous actual rates
of interest; the purchase of material at extortionate prices
due to various forms of credit; and huge commissions to
promoters.
Organized in such wise, almost any enterprise becomes
merely speculative, and its failure should produce neither sur-
prise nor sympathy, for such a course is the broad highway that
leads straight into the ever ready clutches of a receiver. Hon-
esty is the best policy in power transmission, as elsewhere.
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CHAPTER XVII.
THE MEASUREMENT OF ELECTRICAL ENERGY.
The basic fact regarding the measurement of electrical
power is the stress between a magnetic field and a coil carry-
ing a current. Obviously such a coil produces of itself a mag-
netic field, but it is the proportionality of this field to the cur-
rent rather than its mere existence that gives it importance in
measuring instruments.
The fimdamental measurements which have to be made in
ordinary practical engineering are three — current, electro-
motive force, and electrical energy, which is their co-directed
product. In continuous current work, while mere readings
of the first two give the energy as their numerical product, it is
generally desirable to have instruments which measure energy
directly and which integrate a varying output continuously,
so that one may at all times keep track of the output of the
station, a single circuit, or the energy supplied to a single cus-
tomer. In alternating current work a wattmeter is doubly
necessary, first because the product of volts and amperes does
not give the real energy, but the apparent energy, as has
already been explained; and, second, because the true energy
divided by the apparent energy equals the power factor, which
should be looked after very carefully in an alternating station.
Any effect of electric current which is proportional to or
simply related to that current may obviously be used for its
measurement, and in laboratory measurements instruments
based on almost every imaginable property of electric current
have been used with more or less success. But for every-day,
practical purposes instruments must possess qualities not so
important in the laboratory, so that the possible types of meas-
uring instrument have simmered down to a very few, with
respect to the principles concerned.
So far as continuous currents are involved, nearly all prac-
tical instruments are electro-magnetic, as has already been
660
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THE MEASUREMENT OF ELECTRICAL ENERGY. 661
indicated — almost the sole exception being the Edison chemi-
cal meter, which need not here be described, since it is passing
rapidly out of use.
The simplest electrical measuring instrument is the ammeter,
designed for the practical measurement of current strength.
In its commonest forms, as used for continuous current, it
consists of a fixed coil of wire carrying the current to be meas-
ured and a pivoted magnetic core, to which is attached a pointer
sweeping over a fixed scale. The force on this core varies with
the current, and is resisted by some opposing force that brings
the pointer into a new point of equilibrium for each value of
the current. Sometimes this opposing force is the magnetic
field of the earth, as in the ordinary laboratory galvanometer,
%^ 300 mo
FlO. S20.
but in practical instruments it is generally gravity, a spring,
or a relatively powerful permanent magnet.
Most of the numerous varieties of ammeter have been pro-
duced in the effort to secure a permanent and constant control-
ling force, and uniformity of scale; that is, such an arrangement
of parts as will make the angular deflection of the pointer
directly proportional to the amperes flowing through the coil.
The result has been all sorts of curious arrangements of the
coils and the moving armature with respect to each other, and
the upshot of the matter generally is that the scale has to be
hand- calibrated for each instrument, the divisions of the scale
being fairly uniform through the parts of the scale most often
used, but varying somewhat near its ends. In first-class mod-
em instruments, a remarkably even scale is attained. Fig. 320
is a good example taken from the scale of a regular station
ammeter. Gravity is far and away the most reliable control-
ling force, but it is also highly inconvenient in instruments
intended for portable use or for a wide range of action while
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662
ELECTRIC TRANSMISSION OF POWER,
still preserving small inertia in the moving parts, so that springs
or permanent magnetic fields form the main reliance in practice.
In some admirable instruments the well-known principle of the
D'Arsonval galvanometer is employed. In this instrument, of
which a famiUar laboratory type is shown in Fig. 321, a light
Fig. 321.
movable coil is suspended between the poles of a very powerful
permanent magnet, shown in the cut as built up in circular
form. Current traversing the coil through the suspension
wires sets up a field, which, reacting with the magnet, pro-
duces a powerful deflecting force on the coil, controlled by the
torsion suspension. In commercial instruments the suspen-
sion is replaced by jeweled bearings, and the current is led in
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THE MEASUREMENT OF ELECTRICAL ENERGY, 663
through the controlling hair springs or by very flexible leads.
The resulting instrument is very sensitive and accurate for the
measurement of small currents or known fractions shmited
from larger ones. The famous Weston direct current instru-
ments, together with others less well known, are constructed
along this general line.
The sources of error even in the best commercial ammeters
are many. Permanent magnets and springs do not always
hold their strength precisely, jeweled bearings wear, and break
if the instruments are roughly handled, pointers get bent,
dust sometimes gets in, and on these accidental errors are
superposed those due to errors in scale and calibration.
Nevertheless, the best station and portable ammeters possess
and maintain a very commendable degree of accuracy. When
carefully handled and used well within their working range
they can be trusted to within about one or two per cent. If
of the highest grade and frequently verified, they can be relied
on in the best part of the scale down to, say, half the above
amount, and under circumstances exceptionally favorable will
do even a little better. In laboratory work, where they are
merely used as working instruments and 'often checked, it is
possible to nurse them into still higher accuracy, but one cannot
depend upon it for long at a time under commercial condi-
tions. For relative measurements only, made within a short
time, high-grade ammeters are very accurate, but the hints
already given should make it clear that when in regular use one
must not expect to use them for absolute measurements with
a great degree of precision. The cheaper class of instruments
is likely to show double the errors just noted.
For the measurement of alternating currents only a few of
the types of ammeter used for continuous current are appli-
cable. Hysteresis in the iron parts and reactance in the coils
are likely to incapacitate them, but some of the forms can
readily be modified to give good results, and certain others are
specially suited to alternating currents. In this work a new
class, having a fixed field coil reacting on an armature
coil capable of rotation, and spring controlled, has been
made generally useful. These instruments are derived
from the laboratory electro-dynamometer much as those pre-
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664 ELECTRIC TRANSMISSION OF POWER.
viously mentioned are derived from the D'Arsonval galvano-
meter, and are capable of similar precision in practice. On
accoxmt of their extremely small reactance, hot wire instru-
ments have retained still some slight measure of their one-time
popularity. In this class of instruments the current is passed
through a fine suspended wire of rather large resistance, which
is thereby heated and expands, carrying with it the pointer to
which it is attached, usually by means of multiplying gear.
Such instruments require correction for the temperature of the
air, but are capable of very good accuracy if carefully handled.
They are ''dead beat," i.e., the pointer comes to rest without
oscillation, a very useful property, which is secured to a cer-
tain extent in most instruments by various damping devices.
Instruments having a powerful permanent magnet often are
supplied with a copper damping vane, which checks oscillations
by virtue of the eddy currents stirred up in it by the magnet;
and sometimes air vanes in a close-fitting recess or light me-
chanical stops, which can be brought up against the moving
parts, are used for this purpose. For high-voltage generators
the current for the instruments is derived from a current trans-
former, as the instfuments themselves are difficult properly
to insulate for more than 2,000 to 2,500 volts. They are used
with instruments graduated to show the primary current, a
known fraction of which is actually derived from the secondary.
Such a current transformer for moderate currents is shown in
Fig. 322. It is designed merely to furnish current for the
ammeter and wattmeters.
Voltmeters for measuring the electromotive force ate in all
general points constructed precisely like ammeters, save that
the working coil, whether fixed or movable, is wound with very
fine wire in many turns, so as to be adapted to work with very
small currents, and usually has in series with it a resistance of
several thousand ohms. Voltmeters are in fact ammeters
having so much resistance permanently in circuit that the
current which flows through them is substantially proportional
to the voltage across the points to which the instrument is con-
nected, irrespective of other resistances which may casually be
in circuit. Only in rare instances, as sometimes in incandes-
cent lamp testing, is the current taken by the voltmeter a
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THE MEASUREMENT OF ELECTRICAL ENERGY. 666
source of perceptible error, and in such cases it is readily al-
lowed for. Voltmeters are more difficult to construct than
ordinary ammeters, owing to the fine wire windings and the
high resistance, and are generally rather more expensive.
They are capable of just about the same degree of precision
as ammeters, being subject to about the same sources of error.
When used for alternating current, the large auxiliary resist-
ance is wound non-inductively, and the working coil is propor-
tioned for as low reactance as may be possible with the required
sensitiveness. For measuring very high alternating voltages,
a "potential transformer," shown in Fig. 323, as adapted for
high-voltage transmission systems, is used. These transformers
Fio. 322.
have usually a capacity of from 50 to 250 watts, and are used
for the instruments only. They are wound with an accurately
known ratio of transformation, receive the high-pressure current,
and deliver it to the voltmeter at a more reasonable voltage. In
dealing with continuous currents the problem is more difficult.
Sometimes a very sensitive voltmeter is provided with a sep-
arate high-resistance box, reducing the scale readings to some
convenient fraction of their real value, so that the instrument is
used with a constant multiplier to transform its readings to
the corresponding voltage. This is a useful device for obtain-
hig the voltage of arc circuits and the like.
In default of high- voltage instruments, a rack of incandescent
lamps may be wired in series and voltmeter readings taken
across a known fraction of the total resistance thus inserted;
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666
ELECTRIC TRANSMISSION OF POWER
250- volt lamps in sufficient number not to be brought up to full
candle-power are convenient for this purpose, and the volt-
meter should be of so high resistance that its presence as a shunt
around part of the lamps \vill not introduce material error.
A generating station should be liberally equipped with am-
meters and voltmeters. Besides the ordinary switchboard
instruments, usually an ammeter for each machine and each
FlO. 32S.
feeder, it is desirable to have several spare instruments which
can be temporarily put in for testing purposes. Station in-
struments should have large, clearly divided scales and con-
spicuous jjointer, so that the readings can be seen at a distance
from the switchboard. The large illuminated dial instruments
are excellent for the principal circuits, and the main station
voltmeters may well be of similar type. To save space such
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THE MEASUREMENT OF ELECTRICAL ENERGY. 667
instruments are very commonly made with scales arranged
edgewise as in the station voltmeter shown in Fig. 324.
Voltmeters are ordinarily not numerous in a station, and
are usually arranged with changeable connections, so that
they may be plugged in on any circuit and mounted on
swinging brackets so as to be readily visible from various
directions. There should, however, always be at least one
conspicuous voltmeter permanently connected to show the
working pressure on the main circuits. In polyphase work,
this should be capable of being plugged in on each phase,
although it is preferable to have a voltmeter permanently
on each phase in large transmission work. At least two other
Fio. 324.
voltmeters should be available for connection to such circuits
as may be desirable, in testing circuits, parallelizing machines,
and the like. These ought to be small switchboard instru-
ments of the highest grade, mounted side by side to enable
comparative readings to be readily made. As potential
transformers for high voltage are decidedly costly, a simple
and safe arrangement for plugging in the primary side of such
a transformer on any high voltage connection is much to be
dcvsired. A duplicate or spare potential transformer should
always be kept in stock, since it is most inconvenient to have
a voltmeter thrown out of action. In stations having high
voltage generators it is sometimes practicable to connect for
the voltmeters around a single fixed armature coil in each
generator, which much simplifies the transforming arrange-
ments.
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668 ELECTRIC TRANSMISSION OF POWER.
Indicating wattmeters reading the output directly are not in
by any means as general use as ammeters and voltmeters,
but are highly desirable in portable form for motor and lamp
testing, and should be seen upon the switchboard far oftener
than they are. These instruments follow the same general
line of design as ammeters and voltmeters, but are provided
with two working coils or sets of coils. One takes the current
of the line on which the output is to be measured either directly
or through a current transformer, and the other is a voltmeter
coil suspended so as to turn in the field due to the current coil.
The torque produced obviously dejxjnds on the product of
the two fields due to the coils respectively, which is propor-
tional to the energy delivered. If the two fields are in the
same phase, as in continuous current practice, or at times
of unity power factor in alternating circuits, the numerical
product of the two field strengths is proportional to the total
energy; but if there is difference of phase, then the co-directed
components of the two fields are proportional to the energy.
The controlling and damping forces are like those in ammeters
and voltmeters, and the wattmeters differ little from them
in general arrangement save for having two sets of terminals,
one for current and the other for potential, and in the gradua-
ation of the scale. An indicating wattmeter is at times
a valuable addition to a generator or feeder panel, but it is
not necessary in the same sense as ammeter or wattmeter.
A well-equipped station should also have two or three such
instruments in portable form, one for the testing of incandes-
cent lamps and such small outputs, and others capable of
taking the output delivered to the ordinary sizes of motors
and recording wattmeters. It should also have a set of por-
table ammeters capable of reading the ordinary range of cus-
tomer's currents without getting off the good working portions
of their respective scales. For instance, if one ammeter will
read with good accuracy from 1 to 10 amperes, the next might
go effectively from 5 to 25 amperes, and the next from 20
to 60.
Of portable voltmeters there should be enough to measure
accurately the voltages used for the distribution, and a por-
table potential transformer to enable primary voltages to be
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ths measurement of electrical energy. 669
dealt with in an alternating system. It is desirable to have
a pair of exactly similar voltmeters to use in simultaneous
readings for drop, and to check each other and the station
instruments.
Another form of voltmeter regarded by the author as a
necessity in every power transmission plant is a recording
instrument keeping a continuous permanent record of the
voltage and its variations. Such a record is shown reduced
Fig. 325.
in Fig. 130, page 234. The Bristol voltmeter is the form of
instrument most commonly seen, and is shown in Fig. 325.
It is merely a strongly made voltmeter with a long pointer
carrying a pen, and swinging from centre to circumference of
a paper disk driven by a clock and ruled in circles for the volts
and radially for time. A variable resistance permits it to be
accurately adjusted to agree with a standard voltmeter, and
when carefully managed it is quite reliable. As a check on
the operation of the station and for reference in case of dispute
it is invaluable, since it shows every variation of voltage,
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670
ELECTRIC TRANSMISSION OF POWER.
and the time at which it occurred. In using it the pen should
be kept clean and smooth running, bearing just heavily enough
to leave a sharp, thin line, and the clock should be very care-
fully adjusted to keep correct time. The chart should be
changed at the same time each day and put on so as to record
the correct time.
Recording ammeters and steam gauges are made upon a
similar principle, but for power transmission plants the volt-
meter is the most important instrument. Installed in the
Fig. 3-26.
generating station it keeps accurate record of the regulation,
and in the sub-station it serves a similar purpose.
An instrument sometimes used of late is a frequency meter,
showing on its dial the periodicity at any time just as an
ammeter shows the current. Its principle is very simple.
Any voltmeter having some considerable reactance will change
its reading with change of frequency. If furnished with a
scale empirically graduated for different frequencies, it be-
comes a frequency meter, and if installed where the voltage
is fairly constant and designed so as to be hypersensitive to
changes of frequency, it serves a useful purpose in telling
whether the machines are at the exact speed intended. In
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THE MEASVREMEST OF ELECTRICAL ESEROW 671
fact it could in any gi\'en situation bo graduattnl for s|mhh1 as
well as for frequency.
Occasionally recording ^Yattmote^s, similar to tl\o recHmling
voltmeters already doscril^ed. are ustnt; but it is dillicult to gt^t
accurate readings over a wide enough rang<^ to Im> of much
use, and the more usual instrument is the integrating watt-
meter, sometimes referred to as recording, which registers the
output in watt hours continuously. Instruments of this class
are used both to register the energy supplied to customers and
to take account of the energy generated. Daily readings of
the switchboard instruments give by difference the daily
output in KW hours, and in steam driven stations are most
important in keeping record of the station efliciency and its
variations. It is needless to say that instruments used for
this purpose should be kept in especially careful calibration
since errors in the whole output are dealt with. lOven in
hydraulic stations they give a useful chock on station ojK^ra-
tion and cm the energy sold.
Integrating wattmeters are essentially motors whose s|hmh1
is proportional to the output. Like indicating wattmeters
they produce a torque due to the co-action of current and
potential coils, and the armatures revolving under this stresH
are furnished with an automatic drag due to a disk revolving
between magnet poles or to air vanes, so that the hjxhhI shall
be proportional to the output on the circuit in watts. Prob-
ably in principle the simplest of these instruments is the widely
known Thomson recording wattmeter. Fig. 326 shows the
general appearance of this meter with the cover removed, and
Fig. 327 gives its connections in the ordinary two-wire form.
Essentially it consists of the following parts: a pair of fi(»ld
coils of thick wire, in series with the load; an armature, drum
wound, of very fine wire, in series with a large resistance and
placed across the mains; and a copjKir disk on the armature
shaft revolving l:)etween the poles of three drag magnets.
The fields and armature are entirely without iron, the arma-
ture shaft rests on a sapphire or diamond jewel lx*aring, and
its upper end carries a worm to drive the recording gear.
The commutator is of silver of which the oxide i» a fair
conductor so that the commutator does not easily get out of
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672
ELECTRIC TRANSMISSION OF POWER,
condition, and current is taken to it by slender copper brushes
restingtangentiallyupon it. The drag magnets are artificially
aged, so that they remain very permanent and are adjustable
to regulate the meter, if necessary. The resistance of the
potential circuit is several thoiLsand ohms, and the loss of
energy in the meter at full load does not often exceed 5 to 10
watts. As the static friction of the armature is considerably
greater than the running friction, the "shunt" in the poten-
tial circuit is made part of the field, so as to help the meter
in starting. Doubling the current evidently doubles the
torque in such a motor meter, but since the work done in eddy
FlO. 327.
currents in the drag increases as the square of the speed, the
armature will run at a speed directly proportional to the
energy, which is the speed desired.
In point of fact, such meters are capable of giving very great
accuracy — within two per cent under ordinary good commer-
cial conditions, and very uniform results under different con-
ditions of load.
Such meters are suited for use on both continuous and alter-
nating circuits, and are remarkably reliable in their indications
under all sorts of conditions, save with very low power factors.
Another and very beautiful group of meters is designed espe-
cially for use on alternating circuits only, and follows the prin-
ciple of the induction motor, just as the Thomson meter is a
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THE MEASUREMENT OF ELECTRICAL ENERGY. 678
commutating motor. The pioneer of this class was the famous
Shallenberger meter, an ampere-hour meter, which has been
very widely used and would be extremely useful where am-
peres rather than watts- are to be measiu-ed, although now prac-
tically abandoned.
A fair type of the induction wattmeter is shown in Fig. 328,
the Scheefer meter, one of the earliest of the class, although
here shown in a recent form. It consists of a finely laminated
FlO. 328.
field magnet energized by a current coil and a potential coil,
an aluminium disk armature, and the magnetic drag which has
come to be generally used in meters. A priori one would sup-
pose that so simple a structure could hardly be made to give
an armature speed proportional to the energy in the circuit;
and in fact it takes great finesse to design it so as to accomplish
this result, but it can be successfully done, and meters of this
class turned out by various manufacturers are capable of doing
very accurate work.
As a class they develop very small torque, but in part make
up for this failing by the very small weight of armature and
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674 ELECTRIC TRANSMISSION OF POWER.
shaft. The speed is seldom accurately proportional to the
energy over a very wide range of load, but day in and day out
the small errors generally tend to balance each other, so that
the total reading at the end of a month varies but little from
the facts. The induction meters are liable to material errors
in case of large change of voltage, power factor, or frequency,
but within the range of these factors in ordinary service they
do sufficiently accurate work for all commercial purposes,
and the best of them are substantially as accurate as the com*
mutating meters.
All types of meters are made suitable for switchboard work
in measiuing large outputs, and in alternating stations can be
fitted for use on primary circuits, although this is seldom nec-
essary, and should not be attempted at any but moderate vol-
tages without the use of transforming apparatus for the meter.
Most switchboard meters for such work as power transmission
are of special designs, modified for the particular work in hand.
Monophase alternating circuits and continuous current cir-
cuits are measured in the most direct way possible, the am-
meters being put in the mains, and the voltmeters across them,
through a potential transformer if need be, as it is somewhat
troublesome to wind voltmeters for use directly upon circuits
above 2,000 to 2,500 volts.
Wattmeters are connected to such circuits in a similar
straightforward way, shown for continuous or secondary alter-
nating current in Fig. 327 and for primary alternating circuits
in Fig. 329. In these and other cuts of wattmeter connections
the circuits of the Thomson meter are shown, but they must
be regarded as merely typical, since in using other meters the
arrangement of circuits follows the same principle, the field or
current coil being put in the mains and the armature or poten-
tial circuit across them. The former is wound with coarse wire
or copper strips, the latter with very fine wire, so that they can
very easily be told apart even at a casual inspection.
Ordinary two-phase circuits are measured in a precisely simi-
lar fashion, each pair of phase-wires being treated as a separate
circuit and supplied with its own instruments. The metering
likewise, whether of primary or secondary circuits, is gen-
erally accomplished by the use of two wattmeters, each con-
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THE MEASUREMENT OF ELECTRICAL ENERGY. 676
nected to its own pair of mains. In case of motors in which
the two phases may be regarded as substantially balanced and
equal, it is only necessary to put the instruments in one of the
phases and to multiply the readings of energy by 2 to get the
total input. If the two-phase circuits are imbalanced, two sets
of instruments are absolutely necessary for a simultaneous read-
ing on both phases, unless some form of combined instrument
takes their place.
With three-phase circuits the case is rather more compli-
cated. The simplest to manage is a star-connected three-
phase balanced ci]:cuit, as found iii some motors. Here the
GEff£RhTDft
FlO. 829.
ammeter or current coil of the wattmeter goes directly into
one lead, and the voltmeter or potential coil of the wattmeter
is connected between thai lead and the neviral point of the star.
The instrument then gives correctly one-third of the energy.
Therefore, the wattmeter reading multiplied by 3 gives the
energy on the circuit. On some of the early three-phase motors
of which the primaries were star-woimd, an extra lead was
brought from the neutral point to the connection board to
facilitate measurements. On a circuit mainly of motors fair
balance usually exists.
If the circuit is balanced it is not necessary that a star con-
nection at the generator or transformers should either be easily
accessible or exist in order to use the method of measure-
ment just described. For if the circuit is balanced the am-
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676
ELECTRIC TRANSMISSION OF POWER.
meter or current coil of the wattmeter may be put in a lead,
and the voltmeter or potential coil of the wattmeter be con-
nected between the same lead and the neutral point formed
by three equal high resistances connected to the three leads
respectively, and with their three free ends brought to a com-
mon juijction. Such an artificial neutral is very commonly
used in connecting wattmeters on the secondary circuits for
motors, and may be applied to primary circuits as well. The
writer has sometimes constructed such a neutral t)y connect-
ing three strings of incandescent lamps to the three leads and
FlO. 330.
to a conmion junction. Then connecting the potential coil of
a wattmeter around one lamp and its current coil in the lead to
which the string containing this lamp ran, it became possible
to make a closely approximate measurement of the primary
energy with only an ordinary 110 voltmeter and such appli-
ances as can be picked up around any station. This device
Of an artificial neutral as Applied to secondary circuits is well
shown in Fig. 330.
The measurement of energy on an unbalanced three-phase
circuit is a very different proposition. Of course three watt-
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THE MEASUREMENT OF ELECTRICAL ENERGY. 677
meters with their three potential coils respectively in the three
branches of a star-connected resistance, such as has just been
shown, would do the work, but at a very undesirable cost and
complication.
If, however, two wattmeters are used with their current coils
in two phase-wires respectively, and their potential coils re-
spectively between their own phase-wites and the remaining
wire of the three, the sum of the readings of these two meters
records correctly the total energy of the circuit. Such an
arrangement of meters is shown in Fig. 331, as commonly
Fl(4. 331.
applied to three-phase secondary circuits. A precisely simi-
lar arrangement with the addition of potential and current
transformers is used for primary circuits.
In a similar connection two indicating wattmeters will give
the energy of the circuit at any moment. An indicating
wattmeter with its current coil in one phase-wire of a three-
phase system, will give three diverse readings according as its
potential coil is connected between its own wire and each
of the other phase-wires, or finally across the two other phase-
wires. The latter reading is dependent on the angle of lag,
being zero for unity power factor, and a wattmeter so con-
nected can be used as a phase meter, while the other read-
ings will be respectively increased and diminished to an amount
dependent on the lag.
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678 ELECTRIC TRANSMISSION OF POWER.
Many attempts have been made to combine the two watt-
meters necessary to measure correctly the energy on an im-
balanced three-phase circuit into a single instrument, and
recently with considerable success. Fig. 332 shows a com-
bined induction wattmeter, for two- or three-phase circuits,
balacend or unbalanced, and Fig. 333 its connections with
current and potential transformers as viewed from the front
when used on a three-phase, or three-wire two-phase circuit.
If the three-phase circuit to be measured be a balanced one,
such a composite wattmeter need merely have a current coil
connected in either lead and a pair of potential coils connected
from this to the adjacent leads respectively. In testing motors
FlO. 332.
one can readily get the same result if the load be uniform,
by using an indicating wattmeter with one lead connected
through its current coil and then switching the potential con-
nection successively to the adjacent leads, and adding the two
readings. Instruments for unbalanced circuits should have
two currents and two potential coils, as already indicated.
It should be noted that in balanced mesh-connected circuits
one can measiu^ the energy correctly by putting the current
coil of the wattmeter into one side of the mesh inside the joint
connection to the lead, and the potential coil across the same
side of the mesh. This gives a reading of one-third the total
energy. Ammeter and voltmeter similarly connected give
readings showing one-third the apparent watts.
In ordinary three-phase distributing systems the actual
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THB MEASUREMENT OF ELECTRICAL BNEROW 679
metering is much simpler than would appear at first sight.
Motors are provided with a single meter, usually connected as
shown in Fig. 330. Much of the lighting is from a pair of
phase-wires or from one phase-wire and the neutral, in which
case the secondary service is a simple two-wire distribution
measured like any other monophase system. In cases where
all three wires are taken into the same service, the energy
can be measured by two meters, as shown in Fig. 331, or by
a meter like Fig. 332,
The induction type of meter is sometimes liable to consider-
cu
Line
e
LoAd
"Bta, 888.
able errors on motor circuits where the power factor is subject
to large variations, and should therefore be used with caution.
Before purchasing meters it is advisable to ascertain by actual
tests how they will perform on circuits of varying power factor.
In the case of large station meters especially in polyphase
stations, it is necessary as already indicated to take especial
precautions. In the first place, the meter in such ca«» han
a large constant, since it is operated from current and poten-
tial transformers, each of which transforms down to the meter.
Assuming that the transformation ratios of these transformers
are correct, there are still some residual errors that must be
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660 ELECTRIC TRANSMISSION OF POWER,
looked out for. Unless the potential transformer has a negli-
gible drop of potential under load, which is never really the ease,
the voltage supplied by it to the wattmeter will vary slightly
with the load upon the transformer. Hence, for station watt-
meters, separate transformers should be used if high precision
is expected.
Second, there will be some variation of the phase displace-
ment between primary and secondary E. M. F. due to the trans-
former reactance, which in a wattmeter is combined with
another slight phase shift between primary and secondary
current in the current transformers due to the magnetizing
current required for the transformer core, and varying with
the power factor. There may also be slight error from the
wmng loss between the transformer and meter. None of these
items would be of practical accoimt in oixiinary meter work,
but in a station or other large meter they may be significant.
They are of a combined magnitude which may be two or three
per cent, in other words larger than any ordinary errors in
the meter and may at times be additive with respect to these,
so that they must be taken account of. Special transformers
for the main wattmeter, and careful meter calibration for an
average value of the power factor, will go far toward reducing
such errors to a negligible amount.
Stray magnetic fields about the switchboard may also cause
very appreciable instrument errors, and should be looked out
for assiduously.
Meters should be installed where they will be free from vibra-
tion, extreme heat, and dampness, chemical fumes, and dust.
To a less extent the same rules apply to other instruments, but
'meters with their constantly moving parts and very light
torque should be looked after with particular care. They
should be thoroughly inspected every few months, and at less
frequent intervals should be carefully tested in situ, which can
very readily be done by the aid of an indicating wattmeter
connected to the same load. The following formula serves for
this test:
3600 X Constant of meter (if any)
Watts in use "*
seconds per revolution of armature. ,
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THE MEASUREMENT OF ELECTRICAL ENERGY. 681
Nearly all meters use the magnetic drag, and a light mark
near the periphery of the meter disk timed for a few revolu-
tions with a stop watch, gives the right hand side of the equa-
tion, while the watts input is checked by the indicating watt-
meter. The constant of the meter by which its reading must
be multiplied to give the true energy recorded is nearly always
plainly marked as an integral number upon the meter. If a
meter shows material error it can be brought to the correct
rate by slightly shifting the position of a drag magnet or adjust-
ing the dragging device, whatever it is. This adjustment up
to a reasonable amount is provided for in meters of all types,
and if the error is more than can be thus comi)ensated the
meter should be thoroughly overhauled, particularly as to the
armature bearings. For the details of meter inspection and
adjustments reference should be had to the instruction books
issued by the manufacturers, as many types and forms of meters
are in use, and no generalized directions can fit them all.
With proper care, meters in commercial service can be kept
correct within two or three per cent year in and year out.
They are more apt to run slow than fast, so that the consumer
seldom has just ground for complaint. For the best work
meters should be installed with the idea of keeping them gen-
erally working near their rated loads. The greatest inaccura-
cies are at light loads, and part of the inspector's duty should
be to make certain that the consumer's meter will start promptly
on, say, a single 8 c.p. incandescent lamp. Otherwise the
consumer can, and usually finds out that he can, get a certain
amount of light without paying for it. Electric meters nearly
always are read on their dials in exactly the manner that gas
meters are read. With unskilled or careless men reading the
meters there is some chance for mistake. To avert this some
companies furnish their meter readers with record books hav-
ing facsimiles of the meter dials plainly printed on the pages.
The reader then merely marks on these with a sharp-pointed
pencil the position of the hand on each dial of the consumer's
meter, and the record thus made is translated deliberately at
the office. Part of a page from such a record book is shown in
Fig. 334.
A direct-reading meter, arranged somewhat after the manner
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682 ELECTRIC TRANSMISSION OF POWER.
of a cyclometer, showing the total reading in plain figures, is
A highly desirable instrument, but although several such meters
have been brought out they have not as yet come into a secure
place in the art. The difficulty is mainly a mechanical one.
The meter can easily move one number disk, but, as it runs on,
an evil time comes when it has simultaneously to move two,
three, four, or five disks, and at one of these points it is likely
to balk. Such a meter would be particularly hard to adapt to
the induction type now widely used, and, desirable as it would
he, the time of its coming is not yet.
Customer Meter No.
Meter Capacity Rate Constant-
Mar.
Jan. f* o jnc o fir* o 3C © ZXZ ®
Feb.
Fig. 33i.
For special purposes a considerable variety of meters are
used, all, however, being made and applied on substantially
the lines already described. In some cities prepayment meters
with an attachment for switching on the current worked like
A slot machine, are finding a foothold, particularly in the
poorer quarters. Elsewhere two-rate meters with a clock-
work attachment to cut down the rate of running between
<«rtain hours of relatively light station load, and some other
Automatic discount meters, have been employed. But all
these are peculiar in their special attachments rather than in
Any fundamentals.
The chemical meters of which the early Edison meter was a
type, have passed quite out of use in this country. In spite
of certain advantages, the demand for a meter which can be
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THE MEASUREMENT OF ELECTRICAL ENERGY. 68S
Tead by the consumer and the use of alternating current grew so
overpowering that the chemical meter had to go. It survives
in various forms abroad, some of them rather successful, and
even arranged for direct reading upon an easily observed scale.
Whatever meters and instruments are used, it is of primary
importance that they be kept alwa3rs in the best working order.
Most of the measurements w^ith which the supply station
has to do are those connected with metering, but at times
more difficult problems arise. Most of these are due to the use
of unusually high voltage. The exact determination of high
primary voltages is rather troublesome when one gets lx»yond
the potential transformer and desires to obtain independent
voltage measurements. The most available method of work
is to use a high-grade voltmeter, very carefully insulated, in
connection with very high and nearly non-inductive resis-
tances, of which the impedance has been carefully determined
before hand. It must be remembered that such impedances
must be added to the voltmeter impedance geometrically, as
is generally the case in alternating current measurements.
For a check upon such devices electrostatic instruments
may sometimes be used to advantage. The best known of
these is the Kelvin electrostatic balance shown in Fig. 31^5,
and in its simpler forms well known in laboratories. It is
merely a quadrant electrometer reduced to a practical form,
and is obtainable for voltages of even 50,000 and more. It
is not a very convenient instrument to use, but at times serves
a useful purpose in keeping track of errors, being free from all
those associated with the amount and phase of the current
necessary in working electro-dynamic instruments.
Very earnest efforts have been made to obtain a close mea-
surement of voltage by its sparking distance between points.
As appeared from the previous discussion of this matter, the
measurement is a somewhat troublesome one, but it has a value
in that it measures the very effect that is sometimes most
important in keeping track of abnormalities of line pressure.
From the work thus far done it appears that by careful atten-
tion to detail, fair precision may be reached, but that it is
unsafe to rely upon tabular values unless for the apparatus
and conditions of use these values are checked at a few points.
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ELECTRIC TRANSMISSION OF POWER,
Within limits the method is useful, and anyone interested
in trying it will find a good account of the details in a paper
by Fisher.*
The measurement of line insulation on high tension systems
is another troublesome matter. In fact, very little has beea
Fig. 335.
done on this problem beyond the ordinary resistance mea-
urements that may be made with the "bridge-box/' which
should form a part of every station equipment. The capa-
city of a long line is so considerable as to introduce great diffi^
culties in testing with high alternating voltages, and direct
* Trans. Int. Elec. Cong., 1904, Vol. II., p. 294.
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THE MEASUREMENT OF ELECTRICAL ENERGY. 685
current voltages high enough to be of much use in testing are
difficult to attain. For such measurements, for capacity
measurements, and the like, one has to revert to strictly labo-
ratory processes, since no commercial apparatus is up to the
present available.
No useful general method of locating faults on high tension
overhead lines has yet been devised. They occur under so
various conditions and for so different causes that they cannot
be treated in any systematic way. The telephone and friends
along the line is the winning combination in case of trouble.
As a rule line troubles are not instantaneous in their occurence,
and serious results can often be averted by starting a prompt
inquiry over the wires. On high voltage systems any abnor-
mal loss of energy means mischief in the very near future, so
that there is the constant necessity of keeping the insulation
at the highest attainable figure. If it is low enough to measure
readily, it is too low for safety. On underground circuits
which are usually of rather moderate voltage and length,
troubles assume a more definite character and can generally
be located when the cable can be put out of service and tested.
Insulation tests are here of more service and should be periodi-
cally made. In case of grounds the following adaptation of
the loop test, described by Ferguson in a valuable paper on
xmderground work,* may be found useful. Fig. 336 shows
the arrangement of the apparatus. A £ is a slide wire bridge
or its equivalent, carefully calibrated. C is the moving con-
tact connected to a grounded battery. Let a conductor be
grounded at 0. Join the remote end of it to the end of a
• TranB. Int. Elec. Cong., 1004, Vol. II., p. 683.
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686 ELECTRIC TRANSMISSION OF POWER.
sound conductor and the near ends respectively to the ends
of the bridge. Then balance in the ordinary way. Let L
be the total length of the joined conductors. Then
"^^ CB •
L may be taken directly as linear distance if the loop is of .
the same cross section throughout, but if the sizes dififer L
should be taken as resistance and A 0 should be reduced to
distance from the known size of the conductors. The result
does not involve the resistance of the fault, but this should
be low enough, or the testing voltage high enough, to get proper
deflection of the galvanometer. This test is reported to give
location within one or two hundred feet, in lines from 1 to
5 miles in length. If the conductor is burned off, the fault
can sometimes be located by capacity tests from the two
ends, if there is not too much leakage. Similar tests can in a
certain number of cases be used for overhead lines, for which
reference should be had to the general testing methods used
on telegraph systems, but as before noted a fault on a high
tension line is generally of so pyrotechnic a character that it
can be located closely enough for the repair gang long before
the hne can be cleared and the necessary measurements made.
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CHAPTER XVIII.
PBESBNT TENDENCIES IN HIGH VOLTAGE TRANSinSSION.
It is now nearly five years since the third edition of this work
appeared, and during that time there has been a great advance
in the freedom with which high voltage is employed, although
there have been no sensational changes. Improvement has.
come through gradual progress along lines which had already
been pretty well mapped out.
In fact, the list of high voltages in existing plants runs no
higher to-day than it did five years ago, albeit the average
working voltage, if one may be permitted to speak of so vague
a thing, has been nearly doubled within the same period. The
list of high voltage transmissions which appears at the end of
this chapter, tells the story clearly enough. It has proved so-
hopeless a task even to catalogue the 10,000 volt plants that
it has been necessary to confine the list to those plants oper-
ated at 20,000' volts or more. There are about 95 such plants-
in the United States, Canada, and Mexico, as against 70 plants
working at or above 10,000 volts five years ago. And of the
95, 20 are working at or above 50,000 volts, in contrast with
the single plant of the earlier date.
The longest distance of transmission in the earlier list is
145 miles, on the same great system which has now carried
commercial transmission up to 232 miles. The region between
40,000 and 60,000 volts has now been pretty thoroughly ex-
plored, and may be entered without fear. The difficulties-
encountered there are, as was to be expected, connected with
the line insulation. So far as transformers are concerned,
higher voltages than 60,000, perhaps up to 80,000 or even
100,000, might be commercially employed, but the insulator
has not kept pace with the transformer, and while excellent
insulators have been made for use at 60,000 volts and even a
little higher, the factors of safety are not yet as great as con-
servative engineering should demand. That this condition
687
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688 ELECTRIC TRANSMISSION OF POWER.
will be improved there is little reason to doubt, but for the
present great caution is desirable in going to or above 60,000
volts.
The most radical innovation in high voltage construction
is the introduction of the tower construction with spans of
500 ft. or more. At the time of writing, this is on trial with no
definite verdict yet in sight. If it fulfils the hopes of its advo-
cates, the great reduction in the number of insulation points
will prove to be highly advantageous. Meanwhile, high vol-
tage transmission is going steadily along, and will not be
checked by the failure of this or any other experiment.
As to distance, the question is now as it always has been,
a commercial one. The higher the available voltage, at least
within wide limits, the greater distance can be covered with
a given capital and maintenance charge per kilowatt trans-
mitted. Certain elements of cost like right of way. poles,
insulators, and line construction depend mainly upon the dis-
tance alone and not upon the output, so that in a general way
the larger the amount of power to be transmitted the farther
it will pay to transmit it irrespective of voltage, which in every
case of long transmission is likely to be pushed up as far as the
state of the art permits. At the present time power is regu-
larly transmitted 100 miles or more from some eight or ten
plants, but the ordinary requirements are, and are likely to
remain, very much below this figure.
As a matter of fact, there are comparatively few sources of
power which are compelled to find a ftiarket at a great distance
or are large enough to warrant a very long transmission. In
most cases the power can be sold within a radius of much less
than 100 miles. Still, there are instances in which conditions
demand a far greater distance of distribution. At the present
time enough experience has actually accumulated to justify
transmissions of several hundred miles, so far as the engineer-
ing side of the matter is concerned.
Few data on the economic performance and cost of mainte-
nance of very long lines are available. The latter item un-
doubtedly increases considerably faster than the length of
the line since the actual number of troubles increases, other
things being equal, about as the number of insulators, while
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HIGH VOLTAGE TRANSMISSION. 689
they are scattered over a large territory that must be watched
This fact has a bearing on the advantage gained where very
large amounts of energy are transmitted.
In the matter of commercial frequency there is small ten-
dency toward change. A large majority of all the high tension
plants, including six of the eight operating over 100 miles or
more, are worked at 60^. One of the remaining two is worked
at 50-w, the other at 25^. In the case of a transmission of sev-
eral hundred miles involving say 50,000 or 100,000 KW, a
lower frequency than OO-^would certainly be advisable, but
for the rank and file of plants there is a tendency to standard-
ize at eO-^unless there is some very good reason to the con-
trary.
As to generator voltage, practice has not been much changed
recently. With the increasing use of 20,000 volts and upwards
there is perhaps somewhat less incentive to use high voltage
generators, which now show an ecocLomy only on lines of a few
miles in length. Nevertheless, many generators of 10,000,
12,000, and 13,500 volts are in use, the first mentioned having
been superseded in new plants by the others. For use with
raising transformers of course any voltage will serve, but prac-
tice is now gravitating toward about 2,200 to 2,400 volts which
is standard for local lighting and power distribution.
Occasionally a somewhat higher voltage is chosen on account
of a more extensive local load than can be conveniently man-
aged at 2,200 volts, but such instances are exceptional.
The most striking and important feature in recent power
transmission work is the growing tendency to unite the power
generating plants of a single district into a coherent system.
This means far more than the fusion of the business of several
stations into a single administration — it implies as well the
physical organization of a group of plants into a single dynami-
cal unit. It must not be confused with the tendency to replace
a group of stations by a common central plant, a practice often
carried to unwise extremes.
The development of the transmission network is carrying
out upon a gigantic scale the same organization that has proved
so valuable in low tension distribution networks. It consists
in linking together into a network the transmission lines of all
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690 ELECTRIC TRANSMISSION OF POWER.
the power plants of a large region, so that each may reinforce
the others in capacity and in the market for output. The
region covered may amount to thousands of square miles, and
the stations linked may be half a dozen or more, scores of miles
apart and located on different streams, and even upon differ-
ent watersheds. It has proved feasible to operate many plants
in parallel on such a network whether large or small, driven by
water- or by steam-power.
The essential feature is that the network voltage shall be high
enough to enable the plants to work together without a loss
in the lines sufficient to imperil regulation.
If the network be wisely laid out it will Uke low tension net-
works, enable the territory to be covered at a lower cost for
lines than if independent feeding systems were employed, or
for equal costs it will give a lower average loss of energy.
There is also a considerable gain in the matter of establish-
ing reserve capacity, since in case of accident the several plants
can help each other out. In the same way, with a properly
arranged network one line so serves as a relief for another as
to obviate the necessity for duplicating lines.
The details of networks are very various and there are no pre-
cise rules to be laid down, but the general principles are shown
in Figs. 337 and 338. The former shows the ordinary arrange-
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HIQH VOLTAGE TRANSMISSION. 691
ment of independent stations, the latter the effects of intelli-
gent Unkage. In Fig. 337 there are four generating stations
1, 2, 3, 4, and three load points A, B, C. We will assume 1
and 4 to be the most important stations, and B the largest load
point. Now as power plants are conmionly installed by di-
verse interests and somewhat at haphazard, one would gen-
erally find say three companies working, one supplying A
from station 4, another suppl3ang B and C from station 1, and
a third supplying B from stations 2 and 3. The lines cer-
tainly, and the pole lines generally, would be in duplicate, and
the voltages would differ according to the period of the re-
spective installations. In point of fact the largest network
in existence is the result of a consolidation which left the oper-
ating company in the proud possession of lines at 50,000,
40,000, 23,000, 16,000, 10,000, and 5,000 volts, and it is small
wonder that the work of standardization has been long de-
layed.
Now, were the situation of Fig. 337 developed in accordance
with the methods now becoming current, the result would be
something like Fig. 338 modified more or less by topographical
and commercial requirements. Here there are no duplicate
lines as such, but each load point is supplied by two or more
lines through each of which all the generating stations can
deliver current.
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692 ELECTRIC TRANSMISSION OF POWER.
The security against interruption is further increased by
the fact that the several supply lines to each load point follow
different routes. As regards auxiliary plants and spare capa-
city the network of Fig. 338 is highly advantageous, since a
single auxiliary plant, say at B, will serve for the whole system,
and the reserve generator capacity can be located wherever
it seems best.
The price paid for these advantages is some additional loss
in the lines at times when the longer routes are in action, and
some additional care in operation.
The latter requirement comes not so much from any one
cause as from a variety of causes. As a general proposition,
a group of stations can be operated in parallel without much
difficulty provided the stations individually are well designed.
The first requirement is stability of voltage at the several
stations, which implies in turn generators giving close regula-
tion, especially under changes of lag, and operated at constant
speed. Second, there should not be excessive drop in the
lines, for changes of terminal voltage due to this cause make
it difficult to equalize the loads between the stations. Third,
there should be means for governing the power factors so as
to steady the inductive drop and to keep down cross currents
between the stations.
The best modern practice tends toward throwing the bur-
den of regulation upon the sub-station, the endeavor of the
power houses being to preserve uniform voltage at the ends
of the respective lines. The details of regulation are then
looked out for by the sub-Soation regulating apparatus, voltage
regulators, or synchronous motors at adjustable excitation^ In
this operation the power factor meter plays an important
part.
Obviously a scheme of regulation such as this cannot be
carried out effectively unless it is accomplished at a single
point and under systematic direction. The regulating point
is naturally the main substation as at B, Fig. 338. For such
a single point the regulation can be reduced to a rather regular
programme, but the wandering of the load which takes place
on every large system complicates the situation. For ex-
ample, at certain times of the day. A, Fig. 338, may require
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HIGH VOLTAGE TRANSMISSION. 693
an abnormal proportion of the total load, and the voltage
regulators must be adjusted accordingly; the exact programme
being determined by experience. The problem is akin to
voltage regulation upon a large distributing system, and must
be solved by the same general process.
As in distributing networks, too, means must be provided
for promptly isolating lines on which there is trouble, and to
this end it is often necessary to do a certain amount of switching
at high tension. If, for example, the line B 3, Fig. 338, begins
to show signs of trouble, quick work in cutting it clear may
often save a short circuit that would seriously disturb the
voltage of the whole system.
When a group of stations of very diverse character is to be
operated in parallel great care must be taken with the gov-
erning. If a sudden variation of load occurs, the natural
tendency is for the shock to be taken up by the plant equipped
with the most sensitive governor. This would practically
mean that if a steam plant were one of the group it would
have to stand the worst of the blow, which would then fall in
succession upon the hydraulic plants in order of the rapidity
of their governing. As it is undesfrable generally to make
the steam plant take up such variations, the governers in the
several plants should be adjusted for rapidity of action in
such a manner if possible, as to throw the shock on the plant
best able to stand it.
The large networks of the country have grown up rather
gradually so that they have not been arranged as yet to oper-
ate in the fullest harmony, but they are being steadily im-
proved.
The most striking single example of a great and far-reaching
system is that of the California Gas and Electric Co., of which
much has been heard. It is shown roughly in Fig. 339, which
gives not only the system but the parts from which it has
been aggregated. It operates about 700 miles of line at
S0,000 volts, besides several himdred miles at lower voltages,
and has an aggregate capacity of about 50,000 KW. There
are all told 14 power houses, the most recent being a huge
gas-engine auxiliary plant in San Francisco, with 4,000 KW
units, the first of which has just been installed. As will be
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ELECTRIC TRANSMISSION OF POWER.
seen from the map, the fusion of the whole into a network is
not yet complete, but it is being done as occasion offers. The '
transmission from the De Sabla power house to Sausalito,
OH
& Jl S S 811
FlO. 339.
232 miles, is the longest yet attempted in the world although,
on occasion, power has been commercially transmitted be-
tween points on the system nearly 350 miles apart. The
main transmission may be reckoned at about 150 miles from
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HIGH VOLTAGE TRANSMISSION. 696
the chief power houses at Colgate and Electra. On this sys-
tem have been worked out some very important problems
in power transmission. The long cable span over Carquinez
Straits has already been described, and it need only be added
that during more than three years of operation it has given
no trouble. Long spans are freely used on the more recent
parts of the system with a strong wooden pole construction.
Another interesting feature of the system is the considerable
use of long break open-air disconnecting switches on lines
up to more than 60,000 volts, and also the practical abandon-
ment of ordinary lightning arresters in favor of open-air horn-
gap arresters of the. simplest possible description. The whole
system spans a space of about 240 miles in length by about
half that breadth, and constitutes altogether the most exten-
sive power-transmission yet undertaken, supplying light and
power at nearly a hundred distribution points. The uniform
frequency is 60-w and the voltage is tending toward 60,000 as
the general limit for the present.
Second only in magnitude to this S3rstem is that of the
Los Angeles Edison Co., in southern California. This was
earlier than the northern system in its inception, containing
among its constitutents not only the first polyphase trans- *
mission plant operated in America, but the first long distance
plant operated at an3rthing like the voltages now common.
It is less characteristically a network than the system just
described, being essentially a long trimk line through the
splendid valley that lies south of the Sierra Madre, beginning
in the mountains just east of Redlands and running clear
through to the sea, with numerous branches, the main point
of supply being Los Angeles itself. The system operates 8
plants, five hydraulic and three steam, with several more hy-
draulic plants under construction. Fig. 340 shows in outline the
group of plants and transmission lines at present constituting
the system. The beginning of the network was the Redlands
plant, known on the map as Mill Co. Hyd. P.H. 1, the first
polyphase transmission plant, started as a 2,500 volt trans-
mission into Redlands in 1893. Three years later the Edison
Company started with a steam station in Los Angeles, and
in 1898 it acquired the Southern California Power Corn-
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ELECTRIC TRANSMISSION OF POWER.
§
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HIGH VOLTAGE TRANSMISSION, 69T
pany, which then owned the original Redlands plant and the
Santa Ana canon plant with its 80 mile transmission at 33^000
volts into Los Angeles, the first of the very long high voltage
Unes. Since then its growth has been rapid. In 1896 the
Mill Creek plant was changed by extending the pipe line, from
377 to 530 ft. head, and raising transformers to 10,000 volts
were installed, although it is interesting to note that the origi-
nal generators are still in use after more than 12 years of ser-
vice. Later, Mill Creek power houses No. 2 and No. 3 were
added higher up the canon. Also in Santa Ana canon a
plant, No. 2, has been added, and No. 3 and No. 4 are
projected. In the same region the Lytle Creek Power House
has been added on a branch of the Santa Ana River. A
point worth noting in several of these later plants is that
the receiver, a usual feature of the earlier hydraulic plants,
and already mentioned, has been abandoned in favor of
branches spreading finger-like from the end of the main
pressure pipe, cast-steel Y's being used for the division.
This arrangement averts some loss of pressure otherwise
incurred.
Another feature lately introduced in the hydraulic .construc-
tion, is the use of concrete pipe on the slight grades leading to
the steel pressure pipes. This pipe is moulded on the ground
of heavy gravel 2 parts, and Portland cement 1 part, made up
in very short sections and united by concrete collars. It is
laid in trenches and back filled. Depressions across which this
pipe cannot be conveniently laid are spanned by steel pipe in
inverted siphons. Aside from the hydraulic plants here noted,
the system is to be supplied with a very large additional amount
of power from points on the Kern River far to the northward,
over a transmission system of nearly 150 miles in length. The
number of power sites available is 6, aggregating some 87,000
HP at the minimmn, but of these only plant No. 1 is needed for
immediate use, and that with some 24,000 HP capacity is near-
ing completion. A notable fact is that the whole Kern River
district is across the Sierra Madre Mountains, on a watershed
of its own covering some 2,000 square miles, and has a higher
and more wooded region upon which to draw than is possessed
by the streams earher developed.
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ELECTRIC TRANSMISSION OF POWER.
The whole system is operated at 50'*', which was the fre-
quency adopted for the original Redlands plant.
As in the work of the California Gas and Electric Company,
there is here also a tendency to use much longer spans than are
common in regions where transmission lines are less familiar.
Fig. 341 shows the pole head used for some miles of recently
constructed line. Of course, anyone who takes the trouble to
design a pole line instead of guessing at it knows that a 225 ft.
Fig. 341.
span such as is here used is entirely feasible with light high
voltage wires, but it is a rather striking exhibit to see the plan
carried out with a 12 ft. upper cross arm. The line thus con-
structed stood up against wind storms of exceptional violence
without the slightest damage. It is generally preferable to set
the wire triangles with the points up instead of down as here
shown, and in most cases so long a cross arm as here shown
would scarcely be needed.
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HIGH VOLTAGE TRANSMISSION, 699
The important group of plants near Salt Lake City, Utah,
has already been noticed. The system just over the moun-
tains near Tclluride, Colorado, although much less extensive, is
memorable as the scene of the first serious work on transmission
at high voltage from which was derived much of the data and
experience which has made modern transmission practicable.
The transmission network fed from many stations must
certainly be ranked as the most considerable advance in power
transmission made within recent years. As at present carried
out it is mostly concerned with large powers, but the same
principle applies whatever the scale of the operations.
There are many small powers which can well be utilized in
a similar manner. The tendency is to work up the large
powers and to neglegt the lesser ones.
Another line of operations which is beginning to be pressed
is the creation of artificial powers. In regions of fairly con-
siderable rainfall the aggregate amount of water received by
a given watershed may be large, while the distribution of run-
off is very unfavorable. If the situation is such that even a
few square miles of watershed can be made to contribute to
a storage system at high head, a very considerable permanent
power can be developed.
The problem is akin to the ordinary one of providing water
supply for city use or for irrigation, with the exception that
for a power plant the available head should be as great as
possible.
Oddly enough the most t3rpical case of the kind here con-
sidered is to be found in the State of Vermont. In the south-
em reaches of the Green Mountains there is much high country
over which the rainfall is heavy, averaging nearly 45 inches per
annum, and at a point in this region about eleven miles from
Rutland on East Creek, a water storage for power purposes has
been created. The area of the pond is about 800 acres, secured
by a dam 750 ft. long and 54 ft. in maximum height. The
storage here secured amounts to 435,000,000 cubic feet and
by a pipe line 35,000 ft. long the water can be delivered under
a head of 697 ft. At the present time the water from the large
reservoir is used to reinforce the supply in a second reservoir
5 miles down stream having a capacity of about 63,000,000
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700 ELECTRIC TRANSMISSION OF POWER.
cubic feet, and this lower reservoir supplies the power station
under a head of 222 ft., through a pipe line 8,000 ft. long. Thus
at present only about the lower third of the fall is utilized,
though the rest can readily be made available.
The drainage area of the upper reservoir is only 15 square
miles, but of a total rainfall of 45 inches only 11.5 inches need
reach the pond to fill it, while the actual run-oflf from the water-
shed has been found by measurement to be nearly 70 per cent,
so that there is a large margin of safety in reckoning the stor-
age given.
At the full head given the 435,000,000 cubic feet of storage
would give more than 6,000,000 KWH per year, available as
desired, and equivalent under ordinary conditions of use to
an installation of more than 2,000 KW. Even now 1,200 KW
of generator capacity is in place, and the maximum output
provided for is merely a question of profitable use.
The cost of purely artificial storage is generally high and
only in case of very great heads is it likely to pay at present.
If topographical conditions are favorable, however, there is
no reason why impounding the rainfall cannot be made profit-
able. The actual amount of land diverted from its ordinary
uses is in such a case only that used for the reservoir, in which
this power storage has the advantage of storage for water
supply, and stands in exactly the position of storage for irriga-
tion. In the Chittenden reservoir only some 800 acres of
upland was removed from employment as farm, forest, or pas-
ture. As regards the rest of the watershed, it is rather improved
than injured by the pond.
Looking at the proposition broadly, one can under a head
of 650 to 700 ft. store power on the basis of 1 mechanical horse-
power hour from the wheels for each 60 cubic feet of water, or
about 4,800 KWH for each acre per foot of depth. And for
each acre of watershed one should be able at ordinary values
of the run-off, to store this foot of depth. Mountain land,
therefore, may easily be worth more for storage than for any-
thing else. At present, water-powers can generally be de-
veloped from natural falls more easily than they can be thus
created, but as the natural powers are taken up and fuel rises
in price storage will become more and more profitable.
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HIGH VOLTAGE TRANSMISSION. 701
In the inevitable struggle of industry against increasing
scarcity, of fuel, every source of hydraulic power must be de-
veloped to the fullest extent. It is idle to speculate on the
date of probable exhaustion of the coal supply since we know
not what stores are hoarded unknown in the great unused con-
tinents of Africa and South America, but the fact remains
that to-day the energies of the human race are being expended
in regions in which fuel is necessary not only for industry but
for artificial heat, and that the supply readily obtainable is
being rapidly depleted.
At no distant day increasing cost of fuel will compel either
A drift of civilization southward or the utilization of every
continuous source of energy available. The key to the situa-
tion lies in the transmission of power at high voltage and in the
union of all the available powers of a large district into a coher-
ent network. The steady rise in working voltage during recent
years makes possible an ever increasing area over which net-
works can be made effective.
The following list of plants now operating at 20,000 volts
and more, tells more plainly than any general statement the
tendencies now prominent. At the head of the list comes a
^roup of plants rated at 60,000 volts, led by the great Cali-
fornia network already described. It is doubtful whether at
the present moment any single plant is regularly worked at
•60,000 volts, but the plants so rated are rapidly moving to-
ward that limit which will be reached as the load conditions
force upward the initial voltage. Probably the 60,000 volt
plants now ordinarily work at 55,000 to 58,000, awaiting the
need of more. But steps are now going forward toward still
higher voltages, 70,000 to 80,000 being already in view, and
for some of the transmissions now being seriously considered,
' like that from Victoria Falls on the Zambesi to the Rand, some-
thing like double even these figures is imperative for commer-
cially profitable work.
The present list shows how fearlessly voltages considered
extreme a few years since, are now employed under all sorts of
cUmatic conditions, and this fact is the best possible augury for
greater achievements in the future.
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702
ELECTRIC TRANSMISSION OF POWER.
American PlanU Worked at £0,000 VoUs or
More.
Name.
Location.
Capa-
Vol-
tage.
»l
k\
California Gas & Electric Go
American Biver Electric Co
Ontario Power Go
Colgate, Eleetra, De
Sabla, and else-
where. Gal.
Plaoerrille, GaL
Niagara Falls, Can.
LacDu Bonnet, Man-
itoba
Niagara Falls, Ont...
Niagara Falls, Ont...
Spokane, Wash
Guanajuato, Mejdco.
Salisbury, N. C
Wisconsin
Kern Biver, Cal.....
Duluth, Minn
Helena, Mont
Canyon Ferry, Mont.
Seattle, Wasn
Taylors Falls, Minn..
Montreal, P. Q
rShawinicnin Falls).
Anderson, Ind
Durango, Colo
.Hamilton, Ont
(Decew Falls)
Lewiston, Id
Clarkston. Wash...
(Asotin, Wash
Provo, Utah
Norris, Montana
Kalamazoo, Mich. . .
Kalamazoo, Mich...
Plainwell, Mich
Logan, Utah
Near New Milford,
Conn.
American FaUsJdaho
Auburn, N.Y
Cedar Bapids, la.. . .
Sacramento, Cal
Ft. Wayne, Ind
Honolulu, T.H
Indianapolis, Ind...
Lebanon, Ind
Los Anffeles, Cal....
Snoqualmie, Wash..
I^ma, Ohio
Joplin, Mo
Bedlands, Cal
Bedlands, Cal
Grafton, Cal
Lockport, 111
Visalia, Cal
60,000
3/)00
80,000
6,000
88,000
9,000
2,400
25,000
2,000
20,000
22,600
4,600
7,600
23,000
10,000
16,000
3,000
4,600
{2 900
900
1,600
2,000
1,600
600
2,600
2,000
600
1,300
1,160
690
6,900
1,360
8,000
2,000
760
11,000
1,900
3,000
3,000
4,600
3,000
1,876
1,360
14,000
2,625
4,600
1,600
1,000
3,000
3,760
1,360
15,000
60,000
60,000
60,000
60,000
60.000
22,000
60I0OO
60,000
60,000
60,000
60,000
60,000
60,000
60,000
67,000
166,000
1 40,000
60,000
60,000
60,000
60,000
{45,000
46,000
40,000
40,000
40,000
40^000
40.000
40,000
33,600
38,600
33,000
33,000
33,000
33,000
33,000
33,000
33/N)0
33,000
33,000
33,000
83/)00
33,000
33,000
38.000
32,000
30,000
80,000
30,000
26,400
26,400
25,000
26.000
25,000
25,000
IS
(232
90
"67
90
98
110
104
76
60
139
'60
66
Is
80
"66
66
67
46
32
75
160
80
26
■46
"28
80
80
80
87
36
36
80
28
60
18
21
40
40
60
OO'
26
Winnipeg Oeneral Power Go
Electrical Development Co. of
Ontario.
Washington Water Power Co.. . .
Ouanajuato Electric & Power Go.
Whitney Beductlon Co
60
25-
25.
60>
OO*
6a
Madison Blver Power Co
60*
TjOh Angeles Rdfson Go
60
Great Northern Power Co
Missouri Biver Power Co
7S
60>
MiMOuri Biver Power Co
60
Columbia Improvement Co
Columbia Improvement Co
Shawinigan Water ft Power Co. .
Union Traction Go. of Indiana. .
Animas Canal Co
Hamilton Cataract Power Light
ft Traction Go.
60-
60
8»
27
60
{os
60
Telluride Power Transfer Go
Power Go. of Montana, The
Detroit and Chicago By Go
Plainwell Construction Go
Hercules Power Go ,.
60
60
60
60
60
60
New Milford Power Go
00
American Falte Power Light ft
Water Go.
Auburn and Syracuse El.B. B. Co.
Cedar Bapids Electric Light ft
Power Co.
Central California Electric Co.. .
Fort Wayne ft Wabash Valley
Tr.Co.
Hawaiian Electric Co
00
25>
60
60
26
00
Indianapolis ft Cincinnati Tr. Co.
Indianapolis ft Northern Tr. Co..
San Gabriel Electric Co
25
25
60
Seattle-Tacoma Power Co
Western Ohio By. Go
Spring Bi ver Power Co
Southern Cal. Power Go
Bedlands Elec.Llght ft Power Go.
Edison Electric Co
00
26
60
60
Economy Light ft Power Co.,
Joliet, 111.
Mt. Whitney Power Go
60
60
Hudson Biver Electric Go
Montgomery Water Power Co...
Aurora, Elgin & Chicago By
Columbus, London ft Springfield
Columbia Mills Co
Spiers Falls, N.Y...
Tallassee, Ala
Wheaton, 111
Medway, O
Columbia, S.C
Butte, Mont
Ogden, Utah
Big Bar Bridge, Cal.
40
60
26
26
40-
Montana Power Trans. Co
Pioneer Electric Power Co
Blue Lakes Water Co
60-
60*
eo»
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HIGH VOLTAGE TRANSMISSION.
708
AmerUMn Plants Worked at ZO.OOO VoUa or Mtn-e —
Continaed.
Name.
Location.
Gapar
Vol
tage.
^1
4
Montreal Cotton Co
Montreal Light, Heat St Power Go.
Snoqualmie Falls Power Co
St. Crolz Power Go
Valleyfleld, Que
Chambly Falls, St.
Therese
Snoqualmie Falls,
Wash.
St. Paul, Minn
Salt Lake City, (Bear
River) U.
Belton, 8. C
2,800
11,000
6,000
3,000
24»0
8,100
1,100
2,000
16,000
10,600
2,660
4,000
4,500
600
78,750
1,600
900
1,000
1,200
2,260
87,600
2,260
6,370
760
2,000
600
1,760
760
1,600
4,000
2,600
600
1,600
6,250
1200
1,000
15,000
1,125
1,600
4,600
6,000
600
1,410
600
" 1,666
1,500
250
1,500
1,500
26,000
25,000
26,000
26,000
23,000
22,600
22,600
22,600
22,000
22,000
20,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22,000
22.000
20,000
20,000
20.000
20,000
17
19
46
31
28
90
"24
33
*'i2
30
12
■*32
22
83
10
27
**i6
26
28
35
100
22
12
40
60
"22
38
17
"is
60
60
60
60
Utah Sugar Co. . ^ . . x . ^
60
Belton Power Co
60
Phcaniz Lighting ft Fuel Co
Cataract Power Go
Allegheny County Light Co
Atlanta water & Elec. Power Co.
West Kootnay Electric Power Co.
Utica Electric Light & Power Co.
G. AO.Braniff &Co
Siskiyou Electric Power Co
Niagara Falls Power Co ^ . . .
Truckee River Power Go
Phceniz, Ariz
Hamilton, Ont
Pittsburg, Pa
Morgan Falls, Ga....
West Kootnay, B. G.
Utica, N.Y....
Tlalnepantla, Mez..
Yreka,Gal
Niagara Falls, N.Y.
Truckee River, Gal..
Boise, Id
Boise. Id
60
66
60
26
60
60
'60*
26
60
Barber Lumber Co
60
Boise-Payette Electric Power Co.
Big Creek Power Co
Cascade Water Power ft Lighting
Co.
Cataract Power ft Conduit Co.,
(Niagara Falls Power Ck>.)
da. Electrical Irragadora
Gia. Explotadora de las Fuerzas
Hidro-Electricas de San Ilde-
fonso.
Condor Water ft Power Co
Cleveland ft Southwestern Trac-
tion Go.
Corporation of Orilla
.I>etroit, Ypsilantl ft Ann Arbor
R. R. Co.
Grande Consolidated N.S. ft P.Ck>.
Highland Park Mfg. Co
60
San Jose, Gal
Cascade, B. G. (Co-
lumbia River)
Niagara Falls, N. Y.
Pachuca, Mez
Mezloo, Mez
Tolo, Ore
Elyria,Ohio
Orilla, Can
Ypsilantl, Mich
Grand Forks, B.C...
Charlotte, N. G
S. Chicago, 111
Scranton, Pa.
St. Catherine, Can.. .
Myrtle Falls, Wash.
Redding, Gal
Portsmouth, Ohio...
Alliance, Ohio
Massena SprIngs,N. Y.
Silver City, Id
Floriston, Gal
Vancouver, B. G.
(Lake Beautiful).
Connellsville, Pa
Youngstown, Ohio .
Colo. Springs, Colo..
Mercur, Utah
Deronica, Mez
Jenison, Mich
Anderson, Cal
Lowville,N.Y
Quebec, P. Q
Canada
60
60
26
60
60
25
60
26
60
60
Illinois Steel Co..:.
26
Lackawanna ft Wyoming Valley
Rapid Trans. Go.
Niagara, St. Catherine ft Toronto
T^.Co.
Nooksack Falls Power Go
Northern California Power Co.. .
Portsmouth St. Ry. Go
25
26
60
60
60
Stark Electric Ry. Co
25
St. Lawrence River Power Co.. . .
Trade Dollar Consolidated Min-
ing Ck).
Truckee River Wr. Power Co. . . .
Vancouver Power Go
25
60
60
60
W. Penn. Ry. ft Ltg. Syndicate. .
Youngstown ft Sharon Railroad
ft Lighting Co.
(Colorado Electric Power Co
Consolidated Mercur Gold Mines
Co.
G. ft 0. Braniir ft Co
Grand Rapids, Holland ft Lake
Mich. Railway Co.
Keswick Electric Power Co
Wetmore Electric Co
60ft2&
60
80
60
50
26
60
60
Jacques Cartier Power Co
International Hydraulic Co
60
60
This list cannot claim to be complete, but it is approzimately so at the date of writ-
ing. So many transmissions at 20,000 volts and thereabouts are now being Installed that
it 18 almost impossible to keep track of them even by the help of the Targe manufac-
turers, through whose courteous assistance this list has been made up.
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INDEX,
Air:
oompreanon of, efficiency, 51.
oompresaor, 48, 50.
gap in induction motors, 251.
reheater, 53.
Alloys, relative properties of, 486.
Alternating currents:
characteristics of, 125.
circuits, properties of, 125.
compared with d. c, 125.
Alternating va. d. c. machinery,
120.
Alternators. (See Generators.)
Aluminum:
conductor joints, 489.
electrol3rtic corrosion of, 489.
vs. copper, 489.
Anmietero, 661.
a. c, 663.
recording, 670.
sources of error in, 663.
Ampere:
definition of, 21.
hour meter, 673.
Analysis of wave form, 169.
Anchor ice, 409.
Angle of lag, 131, 133.
method of measuring, 136.
Arc motor, 89.
lamps, power, current, and candle
power of, 592.
lighting, commutating apparatus
for, 285.
Armature:
(a. c.) iron dad, 162.
(a. c.) loss of e.m.f. in, 164.
four coil drum, 78.
inductance, ways of reducing,
165.
Armature, continued.
of 5,000 p. p. Niagara generator,
180.
reaction, 96, 167.
effect of, 168, 170.
slots (a. c), arrangement and in-
sulation of, 163.
winding, bar type, 81.
cdmparison of Gramme and
drum types, 81.
Gramme type, 80.
iron-dad drum type, 82.
modem ring type, 82.
polyodontal, 179.
prindple of, 78.
turns per coil, 79.
Arresters, 569.
Auto-converter, 249.
Auto-starter, 249.
Baism's method of alternator regu-
lation, 197.
Barlow's wheel, 237.
Barometeric height effect on strik-
ing distance, 497.
Battery, installation of, in water-
power plant, 635.
Belting, loss in, 64, 427.
Biberest Paper Mills plant, 117.
Boiler capacity for engines, 322.
Boilers, 309.
classification of, 325.
effidency of, 328.
evaporating power of various
types, 330.
firing of, 331.
fire-tube vs, water-tube t3rpe, 331.
forcing of, 329.
fueb for, 329.
705
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706
INDEX,
BoQers, eorUinvj&d.
furnaces for, 332.
mechanical stokers, 332.
merits of different daases, 326.
results of tests of, 330.
Booster transformer, 213.
electrostatic, 521.
Bradley split phase connection, 215.
Bridges for damping fluctuations,
236.
Bristol voltmeter, 669.
CaUe:
capacity of, 150.
for long spans, 548.
high tension underground, < 577.
insulation of, 485.
methods of locating faults in, 685.
submarine, 550.
California Gas and Electric Co. sys-
tem, 693.
Canals, construction of, 405.
Capacity:
for splitting phases, 215.
in actual circuits, 150.
in circuits, 143.
of armored cables, 150.
of overhead circuits (formulas
and curves), 516, 517.
imit of, 144.
Catenary curve, formulas for, 540.
C.Q.S. system, 2.
Chapman regulator, 457.
Charge, electric, definition of, 9.
Charging current for line (formula),
517.
Circuit breakers, 463.
with time limit relay, 464.
Circuits:
a. c. inductance, 130.
a. c. phase displacement, 131.
a. c. properties of, 125.
angle of lag, in 133.
method cl measuring, 136.
capacity and inductance in actual
circuits, 150.
capacity in, 143.
Circuits, continued.
carrying leading current, 146.
coefficient of self-induction of, 137.
condensanoe in, 145.
effect of energy losses on phase
position, 153.
energy losses in, 153.
impedance, 134.
diagram with oonden8anoe,149.
impedances in parallel, 142.
in series, 141.
increase of e.m.f. by conden-
anoe, 154.
inductive e.m.f.'s in, 133.
power factor in, 139, 149.
resonance, 155.
t*me constant of, 155.
Circular coil, definition of, 508.
Clearance in induction motors, 251.
Coal, as fuel, 24.
fields, extent and capadty of, 24.
per i. h. p. with various types of
engines, 332.
utilization of, 32.
Coefficient of self-induction, defi-
nition of, 137.
Combe-Garot constant current
transmission S3rstem, 109.
Conmiercial problem, 639.
Commutation, process of, 78.
Commutator, multi-segment, 78.
PoUock, 284.
piinciple of, 18.
sparking at, 79.
synchronizing, 281.
two-part, 18.
volts per segment, 79.
Conunutators, rectifying, maximum
output of, 287.
Compound alternators, 174.
wires, 488.
Compoimding arrangement for al-
ternators, diagram of, 196.
for inductive loads, 175.
for various power factors, 176.
of alternators on inductive load,
196.
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INDEX.
707
Compressed air transmission, 48.
Compressor, fur, 48, 50.
hydraulic efficiency of, 57.
Taylor hydraulic, 55.
Compressoro, air, efficiency of, 51.
Condensance, definition of, 145.
Condenser, effect of frequency upon
current received and deliv-
ered by, 144.
nature of, 143.
used to increase power factor, 152.
used to increase e.m.f., 154.
Conductivity of various metals and
alloys, 486.
Conductors, 539.
compound, 488.
high tension undergroimd, 576.
tons of energy in, 475.
Connections commonly found in
practice (table), 450.
Constant current plants in Genoa,
104.
Continuous current, 17.
production of, 77.
vs, a. c. machinery, 120.
Converter, mercury vapor, 304.
efficiency of, 307.
for constant current, 307.
Copper, hard drawn, tensile
strength of, 540.
losses, 200.
required by various transmis-
sion systems, 186.
V8. aluminium, 488.
wire, mechanical constants of,
539.
wire, properties of (table), 509.
Corliss valve gear, 313.
Cosines, sines, and tangent (table
of), 522.
Cost formula, 510.
Cotton mill drive, 67.
Counter e.m. f ., 87.
Cross-arms, 552.
steel and iron iw. wooden, 562.
Culm, utilization of, 33.
Current:
continuous, 17.
electric, 10.
generation of polyphase, 177.
leading, 146.
monophase, 158.
polyphase, 158.
reorganizers, definition of, 280.
three-phase, 182.
transformers, 664.
unit of, 21.
value of polyphase for motor pur-
poses, 179.
Currents (a. c), characteristics of,
125.
Cycle, definition, 136.
Dampers:
on synchronous machines, 236.
Damping, 235.
Dams, 399.
concrete steel, 404.
construction of, 399.
masonry, 401.
materials for, 400.
timber, 402.
D' Arson val galvanometer, 662.
Delta connections, 185, 209.
Depreciation charges, 647.
Dielectric constant, definition of,
143.
Discounts, 656.
Distribution:
arc lighting, 591.
centre of load, 621.
constant potential, 119.
efficiency of, 120.
current and power taken by
lamps, 592.
desirability of motor service, 598.
diphase system, 613.
direct from transmission circuit,
581.
efficiency of, 63.
example of, 440.
substation system, 624.
few V8. many transformers, 587._
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708
INDEX.
Distribution, continued,
from eccentrically located station,
603.
large reducing stations, 607.
scattered substations, 600.
substation, 621.
heavy substation, 629.
interconnected diphase system,
615.
interdependent d3mamos and
motors, 115.
maintenance of uniform voltage,
583.
methods of, 581.
monocyclic system, 612.
monophase system, 611.
motor generator device to com-
pensate for losses, 606.
motor power, 62.
. motor service, 690.
of power by shafting, belts, etc., 65.
polyphase system, 613.
primary, 590.
problem, 482, 621.
radial from centrally located
station, 600.
radius of operation of trans-
formers, 590.
' railway load in addition to motor
and lighting service, 602.
regulation of voltage on secon-
dary lines, 630.
relative importance of polyphase,
heterophase and single-phase
systems, 277.
secondary mains, 589.
substation vs, house-to-house, 585.
three-phase system, 615.
three-wire system, 114, 610.
two-wire system, 610.
voltage, 608.
Doble water-wheel, 362.
Draft tube, 351.
Drive:
choiee of, 427.
from vertical shafts, 430.
Dynamos. (See Generators.)
Dynamotor, 103.
Dyne, definition of, 20.
Eddy current loss, 201.
Edison three-wire system, 113.
Electric charge, definition of, 9.
current, definition of, 10.
current, propagation of,
transmission. CSee Transmission.)
Electricity:
flow of, 7.
nature of, 1.
principles of, 1*
static, 8.
Electro-magnetic induction, 13.
strains, 11.
Electrolytic strain on insulation,
123.
Electrostatic booster, 521.
instruments, 683.
E.M.F. automatic regulation of
polyphase generators, 194.
effective, 188.
generation of, 127.
impressed, 131.
increase of, by use of condenser,
154.
induced, direction of, 14.
inductive, 131.
in resonant circuit, 156.
loss in a. c. generator, 164.
teaser, 189.
unit of, 20.
waves, 128.
Energy:
apparent, 136.
classification of, 4.
conservation of, 3.
definition of, 2.
electrical, 7.
electrical measurement of, 660.
internal heat of earth, 31.
losses, efifect on phase pomtioQ
of current, 153.
luminous, 5.
measurement of, on three-phase
circuit, 676.
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INDEX,
709
Energy, ccmtinued.
potential and kinetic, 2.
sources of, 24.
transfonnation of, 3.
transformation, efficiency of, 4.
wave, 5.
Eng;ine:
and dynamo, combined efficiency
of, 64.
choice of, for given service, 324.
steam, boiler capacity necessary,
322.
boiler pressure, 318.
choice of, for power service, 320.
choice of, for railway service,
320.
classification, 312.
coal per i.h.p. for various types
of, 332.
compound vs» simple, 315.
condensing V8. non-condensing,
316.
effect of varying load on econ-
omy, 319.
performance at different loads
of various types, 321.
performance of, 333.
piston speed, 316.
principle of, 309.
speed of, 325.
steam consumption, of different
types, 318.
thermal efficiency, expression
for, 310.
use of, superheated steam in,
322.
valves, 312.
Engines, 309.
gas, 323.
cost of fuel for, 642.
cost of operation, 642.
economy of, 25.
thermal efficiency of, 323.
solar, 27.
Ether, 5:
Evaporation, definition of, 329.
in various t3rpes of boilers, 330.
Exciter equipment, 455.
Exciters, choice of, drive for, 432.
connection of, 456.
f'aesch & Piccard governor, 380.
Farad, definition of, 144.
Faults, method of, locating, 685.
Field:
about current carrying coi^
ductor, 11.
distortion of, 167.
windings, 83.
compound type, 85.
series tjrpe, 84.
shunt type, 84.
Fire risk of transformers, 447.
Fire-tube boilers, 326.
Flat rate, 655.
Flume, timber, 439.
Flumes, 357.
loss of head in, 397.
Frazil, 409.
Frequency:
choice of, 278.
formula for, 160.
indicator, 471.
meter, 670.
used in rotaries, 300.
Fuel:
coal, 24.
gas, 24.
oil, 334.
. variation of cost of throughout
day per k.w., 348.
Fuels, heat of combustion and
evaporative power of va-
rious, 329.
Furnaces for boilers, 332.
Fuse», 461.
Galvani constant current plant,
104.
Galvanometer, 662.
Gas as fuel, 24.
Gas engine. {See Engine.)
economy of, 25.
Gearing, bevel, loss in, 41.
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710
INDEX,
G. E. voltage regulator, 458.
Generator and engine, combined
efficiency of, 64.
Generators :
a. c, armature reaction, 167.
as effected by inductance, 523.
Baum's method of regulation,
197.
compound wound, 85, 174.
compounding for inductance
loads, 175.
connections commonly found
in practice, 450.
constitutional features of, 159.
device for over-compounding
on inductive load, 195.
direct connection of, 193.
efficiency of, 59.
field. {See Field.)
formula for frequency, 160.
G. E. compensated field alter-
nator, 196.
general construction of, 190.
heterophase, 189.
inductor type, 192.
methods of reducing induct-
ance in, 165, 166.
monocyclic system, 188.
polyphase, regtilation of, 194.
practical limits of voltage,
532.
pxinciple of, 126.
regulation of, 173.
relation between poles, speed,
and frequency, 160.
revolving field, 191.
revolving field, advantages of,
194.
series wound, 84.
shunt wound, 85.
star and delt connections, 184.
theoretical e.m.f . generated by,
164.
three-phase, 181.
three-phase, efficiency of, 194.
wave forms, 128.
windings, 161.
Generators, continued.
advantages of moderate voltage,
441.
arrangement of, in power station,
432.
choice of drive, 427.
comparison of a. c. and d. c, 18.
commutators. (jScc Commutator.)
cost of, 651.
design, principles of, 18, 19.
energy required for excitation,
455.
high voltage, 689.
high voltage, d. c, 109.
inductance of, 150.
insulation of, from floor, 441.
location of, 424.
operated in parallel, 442.
principle of, 14.
regulation of compounding, 116.
turbo, 342.
two-phase, 178.
Glass vs. porcelain, 492.
Governors:
action of, 371.
classification of, 372.
F»sch and Piccard type, 380.
hydraulic, disadvantages of, 385.
load type, 374.
Lombard type, 376.
on Pelton wheel, 384.
water-wheel, 370.
Replogle t>T)e, 382.
Gramme ring, 80.
Ground detector, 472.
Grounded conductors for light-
ning protection, 574.
neutrals, 450.
Gutta-percha, 502.
Heat:
radiant, 5.
of combustion, definition of, 329.
fuel oil, 334.
Heating value of various fuels, 320.
Henry, definition of, 138.
Heterophase systems, 189.
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INDEX.
711
High Voltage:
measurement of, 665.
measurements, 683.
Hoist motor, 94.
Huntng, 233, 235.
Hydraulic :
development, 387.
maximum allowable, cost of, 415.
plants, description of various, 41,9.
power, price of, 43, 46.
Hydro-electric plant, efficiency of,
67.
Hysteresis losses, 200.
Ice:
on wires, 543.
Idlers for rope drive, 38.
Impedance:
definition of, 134.
diagram, 135.
factor, 511.
Impedances:
addition of, 15.
in parallel, 142.
in series, 141.
Incandescent lamps:
220 volt, 608.
watts per candle-power, 593.
India rubber, 502.
Individual drive, 62.
Inductance:
armature, ways of reducing, 165.
effects on generator, 523.
for splitting phases, 216.
in actual circuits, 150.
line, 506.
nature of, 130.
of generators and transformers,
150.
of generators, method of redu-
cing, 166.
of line (cur\'e8), 514.
on line, 121.
troubles caused by inductive
drop, 141.
unit of, 138.
used to preserve regulation, 523.
Induction:
electromagnetic, 13.
motor, 237.
advantages of, 266«
arrangement of windings, 253.
auto starter, 249.
choice of, 276.
comparative qualities of differ-
ent types, 270.
construction of, 239.
depth of air gap, 251.
form of slots, 253.
maximum torque, 273.
performance curves, 266, 268.
primary winding, 247.
principle of, 239, 244.
relation between static and
running torque, 274.
relation between resistance and
reactance in, 273.
secondary winding, 239.
single-phase, 258.
single-phase, characterist ic
curves, 260, 262.
single-phase, principle of, ^55,
slip as affected by resistance,
273.
slip in, 241.
slow speed, 268.
speed regulation, 275.
starting current, 271.
starting torque, 271.
use of resistance, in secondary,
272.
wattmeter, 673.
Inductive drop, 141.
Inductor type alternator, 192.
Instrument equipment of generat-
ing station, 666.
Instruments:
continuous current, 660.
edgewise type, 473.
electrostatic, 683.
used in power transmission,
468.
Insulated wires, classification of,
502.
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712
INDEX,
Insulation:
continuous, 501.
materials available for, 501.
of a. c. and d. c. lines, 121, 123.
of a. c. armature slots, 163.
of bar-woimd armatures, 81.
of constant current line, 109.
of lines, 122.
of machines operated on cor.8tant
current line, 101.
tests, 685.
Insulator pins. (See Pins.)
Insulators:
factor of safety, 499.
for high tension work, 565.
line, 491.
number replaced yearly, 568.
porcelain, 565.
sparking, distance for, 567.
support of, 556.
strain (novel type), 549.
Interrupter static, 572.
Joule:
definition of, 21.
Kelvin:
balance, 683.
Kelvin's law, 478.
modifications of, 479.
Kinetic energy, 2.
Lahmeyer rotary, 294.
Lamps, 220 volts, 608.
Leakage, 491.
Lentz's law, 16.
Light, electromagnetic theory of, 6,
energy, 5.
Lighting, lamps in series, 102.
Lightning, 568.
arresters, 569.
danger from — with a. c. and
d. c. apparatus, 123.
protection, grounded wire, 574.
Line, 474.
amount of copper required, 476.
Line, continued.
calculation of terminal voltage,
517.
calculations of losses, etc., 508.
^ capacity of (formula and curves),
516, 517.
charging current (formula), 517.
choice of initial voltage, 531.
.conductors, 539.
conductors, loss in, 475.
construction used on Missouri
River Power Co., 559.
continuous insulation of, 501.
cost formula, 510.
energy losses in (curves), 496.
entrance into buildings, 575.
(erected), cost of, 651.
fonnula for self-induction in, 511.
formula for weight of wire re-
quired, 509.
grounded wires for lightning pro-
tection, 574.
impedance factor, 511.
inductance in, 506.
insulation, 490.
insulators. {See Insulators.)
its general relation to the plant,
474.
junctions between cables and
overhead lines, 577.
lightning arresters on, 574.
lightning stroke, 569.
long, cost of maintenance, 688.
long spans, 698.
loss of current to earth, 491.
maximum loss in, 534.
mil-foot constant, 509.
overhead, 505.
pms. {See Pins.)
poles. {See Poles.)
provision for repairs, 578.
river-crossings, 550.
skin effect, 515.
static disturbances, 530.
steel towers, 547.
surging, 528.
telephone, 578.
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INDEX.
718
Line, continued.
three-phase, fonnula for weight
of wire, 610.
tower construction, 562.
tower, total cost of, 563.
towers, cost of, 563.
voltages, 499.
wave form, 127.
way of treating inductance, 511.
wire, 639.
choice of deflections, 546.
copper, mechanical constants
of, 639.
deflection due to temperature,
542.
factor of safety, 544.
ice loaded, 543.
maximum deflection of, 541.
maTJmiim length of span, 544.
relation between deflection,
tension, and length of span,
546.
wind-pressure on, 543.
wires, transposition of, 558.
Lines, duplicate, 485.
general character of, 483.
sines. {See Sines.)
Load governors, 374.
lines (curves), 347.
synchronous motor, disturbing
effect of, 171.
Lombard governor, 376.
Loop test, 685.
Los Angeles Co., system, 695.
Magneti solenoid, 12.
Magnetic field, 11.
Market, estimate of, 639.
McCormick turbine, 356.
Measurements, electrical, 660.
Mechanical drive, efficiency of, 65.
Mercury rectifier in arclighting, 597.
vapor converter, 304.
Mershon's tests, 497.
Mesh connections, 185, 209.
Metals, relative properties of, 486.
Meters, chemical, 682.
reading of, 681.
testing of, 680.
Microfarad, definition of, 144.
Miner's inch, definition of, 391,
Monocyclic system, 188.
Motor:
generator, 103, 288.
advantage of, 289.
d. c. loss in, 117.
disadvantages of, 290.
efficiency of, 290, 302.
efficiency of large sizes, 292.
synclironous, operation of, 221.
in transmisdon, 221.
maximum power factor, 225.
output, input, etc., 6f, 223.
principles of, 217.
vector diagram of, 224.
water, 44.
impulse, 45.
oscillating, 44.
Motors:
electric,
a. c, 217.
arranged for wide speed range,
100.
classification of operating con-
ditions, 89.
oommutating a. c. (See Series
a. c.)
commutator. {See Commutator.)
compared with mechanical
drive, 66.
constant speed series, 95.
cost of — installed ready to
run, 644.
current taken by, 87.
differential shimt motor, 99.
drive, choice of, 62.
effect of S3mchronous — on
wave form, 171.
efficiency of, 59.
efficiency of system, 63.
efficiency, for different
(table), 62.
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714
INDEX.
Motors, continued,
efficiency, variation of, with
load, 63.
field. (See Field.)
fundamental principle, 237.
high voltage, d. c, 109.
induction. (See Induction
Motor.)
installation of — for constant
current systems, 107.
performance of, 87.
principle of, 14, 17.
puU on armature conductors,
86.
self-regulating series, 94.
series a. c. (See series, Motor.)
series driven by series dynamo,
95.
series for constant potential,
91.
series-woimd constant current,
89.
shunt-wound constant poten-
tial, 96.
single-phase (See Single-phase
Motors.)
synchronous. (See Synchro-
nous Motor.)
torque at armature surface, 86.
voltage of, 113.
with one meter, 679.
working of, 86.
hydraulic efficiency of, 44.
wave, 31.
Heedle-valye for water-wheels, 363.
Nemst lamps, 697.
Ohm, definition of, 21.
Ohm's law, 475.
Oil fuel, 334.
Overload circuit-breaker, 463.
Padnotti constant current {dant,
106.
Parallel, operation, switching re-
quirements for, 461.
Paralleling of alternators, 443.
Pelton wheel, 44, 352.
governing of, 384.
Pendulum, oscillation of, 155.
Periodicity. (See Frequency.)
Petroleum as fuel, 24.
Phase displacement, 131.
lamps, 443.
Pilot wires, 457.
Pins, 556.
burning of, 560.
composite, 561.
metal, 561.
treated, 561.
Pipe line:
concrete, 697.
cost of, 418.
lines, construction of, 406.
loss of head in, 397.
steel hydraulic, properties of
(table), 408.
Plant, location of, 23.
Plants, in parallel, 461.
Pneumatic transmission. (See
Transmission.)
Pole-head, used on Niagara-Buffalo
line, 558.
line, life of, 574.
cost of, 652.
Poles, 551.
at angles, 554.
bending moment of, 553.
classification of, stresses on^
553.
creosoting of, 552.
cross-arms, 552.
crushing resistance of, 553.
general dimensions of, 551.
guying of, 554.
number per mile, 553.
stresses from sleet-storms, 55G»
wind-pressure on, 555.
wood for, 551.
Pollak commutator, 284.
Porcelain vs. glass, 492.
Potential energy, 2.
transformer, 665.
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INDEX.
715
Power:
eentralization of, 33.
cost at customer's meter, 647.
cost at switchboard, 647.
cost of, when developed by va^
nous t3rpes of steam-engines,
640.
cost of, when developed by divers
engines, 643.
cost of per k.w. for different
capacities, 646.
definition of, 35.
determination of price of, 654.
estimate of cost of, 639.
estimate of market for, 416.
factor, 149.
definition of, 139.
increase of, with condenser, 152.
indicator, 471.
plant, choice of power units,
426.
load curves, 347.
organization of, 418.
station. {See Power-station.)
transportation of materials for,
426.
variation of cost of fuel per
k. w. throughout day, 348.
plants, description of various,
419.
list of — operating at more
than 20,000 volts, 702.
station, at Folsom, Cal.
at Fresno, Cal., 436.
building, 425.
design of, 418.
foundations for, 422.
general arrangement of typi-
cal station, 434.
lighting-arrester system for,
574.
location of, 418.
location of high voltage wires,
453.
* location of generators, 424.
number of units, choice of,
429.
Power, carUinued,
of Truckee River, G. E. Co.,
438.
operated in parallel, 693.
reserve apparatus, 636.
structure, 423.
switchboard. (See Switch-
board.)
traveling crane in, 454.
steam, cost of, 415.
steam electric, cost of, 69.
Prime movers, classification of,
309.
gas-engines. (See Engines.)
steam-engine. (See Engine.)
steam-turbines. (See Turbines.)
water-wheels. (iSee Water-wheels.)
Pumpmg, 233, 235.
Railway:
a. c. transmisnon d. c. distri-
bution efficiency of system,
111.
motor, 91.
Railways with three-wire system,
114.
Rainfall observations, 394.
Rates, determination of, 654.
Ratio of transformation, definition
of, 200.
Reactance, negative, 145.
Rectifiers, 280.
commutating, 280.
electrolytic, 303.
Rectifying commutator, advantages
of, 287.
commutators, maximum output
of, 287-
Regulation:
Bamn's method, 197.
best modem practice, 692.
close, 302.
diagram, 519.
of alternators, 173.
of alternators with inductive
load, 174, 177.
. of polyphase generators, 194.
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716
INDEX.
Regulation, continued,
of three-phasere, 184.
of voltage, 456.
of voltage on secondary lines, 630.
of water-wheels, 370,
preserved by use of inductance,
523.
speed. (See Speed.)
Regulator:
Chapman type, 457.
G. E. type, 458, 632.
Stillwell type, 631.
Regulators for constant current,
principle of, 593.
Relay, time limit, 464.
Reoiganizers, definition of, 280.
Replogle governor, 382.
Resistance :
apparent, 134.
increase of — by alternating cur-
rent, 515.
of copper wire, 500.
unit of, 21.
Resonance, 170.
as afifected by armature reaction,
168.
dynamics of, 155.
testing for, 168.
River crossings, 550.
Rivers:
low land, 388.
measurement of flow, 392,
mountain, 388.
slow, 387.
swift, 387.
upland, 388.
Rope drive, cost of, 41.
cost of plant, 70. •
cost of plant operation, 70.
efficiency of, 38.
idlers, use of, 38.
losses in, 427.
multiple sheaves, 38.
multiple sheaves, efficiency of,
39.
power, size of rope, diameter of
pulley and speed (table), 42.
Rope drive, continued,
straightaway, 37.
wire, 35.
construction of rope, 36.
efficiency of, 42.
span, 36.
speed of, 36.
Rotary converter. (See Synchro-
nous Converter.)
Rotating field, 240, 244.
Samson turbine, 354.
Scott system of connections, 210.
Self-induction in a circuit, 511.
Series motor:
a. c, 262.
commutation sparking, 265.
compensating winding, 264.
efficiency and power factor of,
265.
Westinghouie, 264.
Shafting, losses in, 65.
Shaflenbeiger meter, 673.
Sheaves, rope, construction of, 37.
Sheefer meter, 673.
Shell-boilers, 326.
Shields, for damping fluctuations,
236.
Sines, tangents, and cosines, table
of, 522.
Single-phase motors, characteristic
curves, 260, 262.
efficiency and power factor, 250.
induction. (Seelnductionmotors.)
power factor of, 277.
uses of, 258.
Wagner, 259.
Skin effect, 515.
Slip in induction motor, 241.
Slots in induction motors, 253.
Solar-engines, 27.
cost of, 28.
Speed:
constant — motor, 96, 97.
of engines and dynamos, choice
of, 325.
regulation for series motors, 93.
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INDEX,
717
Speed, continued.
regulation of constant-current
series motors, 90.
of induction motors, 275.
of shunt-motor, 98.
variable — motor, 92, 93, 98.
wide-range, motor, 100.
iSplit-phase connections, 215.
Squirrel cage, secondary, 239.
Stanley motor, 255.
Star connection, voltage to neutral,
449.
connections, 185, 209.
Static, 530.
electricity, 8.
interrupter, 572.
Steam and water-power, relative
cost of, 33.
auxiliary, 412.
consumption of, dififerent tjrpes
of engines, 318.
electric plant, cost of, 69.
efficiency of, 66.
engine. (/See Ekigine.)
gauges, recording, 670.
plant, cost of, 651.
power of, cost of, 415.
superheated, use of, 322.
turbine. (See Turbine.)
Staiwell regulator, 631.
Stokers, mechanical, 332.
Strain insulators, novel, 549.
Street lighting, 593.
Strength of various metals and
alloys, 486.
Striking distances, 494.
Substation:
reserve apparatus in, 636.
Surging caused by lightning, 569.
definition and theory of, 328.
e.m.f. of, 529.
relation of voltage rise to cur-
rent broken, 530.
Switchboard apparatus, 455.
equipment of panels, 469.
location of, 451.
purpose of, 459.
Switchboards, 455.
Switches:
air-break for high voltage, 467.
electrically operated, 464.
for remote control, 464.
oil-break, 462.
Switching connections, elementary,
460.
Synchronous converter, 203.
action in, 297.
and transformers, combined
efficiency of, 302.
connection of, 188.
effects of line loss, inductance
and resonance upon d. c.
e.m.f., 301.
efficiency of, 300.
frequencies used, 300.
in railway operations, 299.
ratio between a. c. and d. c.
e.m.f., 301.
winding of, 297.
motor, advantages of, 229.
disadvantages of, 230.
hunting or pumping of, 233.
load, disturbing, effect of, 171.
minimum, practical size of,
233.
power factor of, 228.
polyphase power factor of, 232.
polyphase, 232.
regulation of line by, 227.
* self -starting, 231.
starting of, 230.
uses of, 277.
with solid poles, 235.
Synchronism, definition of, 218.
indicators, 443.
Synchronization, automatic, 470.
Synchronizing commutator, 281.
Synchronscope, 409.
Tangents, sines and cosines, table
of, 522.
Taylor hydraulic air-compressor, 55.
Teaser, 189.
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718
INDEX,
Three-wire system, 113.
for railways, 114.
Thury system, 109.
Tidal energy, 28.
cost of utilizing, 30.
Time, constant of, electric circuit,
155.
Torque:
constant, 89.
maximums in induction motor,
273.
relation between static and run-
ning in induction motor,
274.
Towers, 562.
Transmission:
a. c. and d. c. compared, 121.
analysis of, 68.
comparison of commercial pos-
sibilities of different systems,
68, 76.
comparison of rope and electric,
39.
electric, a. c. 158.
a. c, classification, 158.
a. c, material of, 159.
at high voltage, present ten-
dencies, 687.
best system for heavy sub-
station work, 634.
constant current, 101.
constant potential. 111.
voltage control, 112.
continuous current voltage
control, 103.
copper required by various
systems, 186.
cost of operation, 71.
cost of plant, 71.
d. c. system, 77.
development of network, 689.
delivery known power from
limited water-power, 482.
delivery of known power from
ample water-power, 481.
effect of distance and voltage
on copper required, 476.
Transmission, continued.
efficiency of, 110.
efficiency of, at full and half-
load of different S3rstems, 74.
efficiency of system, 61, 66.
general distribution from water-
power, 481.
heterophase systems, 189.
installations, 102, 103.
line efficiency, 61.
line insulation. (jSee Insulation.)
lightning protection. (See
Lightning.)
longest distance, 687.
monocyclic system, 188.
poljrphase, efficiency of, 111.
polyphase, with rotary con*
verter, efficiency of. 111.
problems, 480.
study of various cases, 481.
synchronous motor for regu-
lation, 228.
the line. (See line.)
voltage to be used, 477.
V8. all other systems, 58.
underground, 484.
gas, 58.
gearing bevel, efficiency, 41.
general conditions of, 23.
hydraulic, 42.
allowable velocity in pipes, 46*
efficiency of, 44, 47.
high artificial pressure, 45.
loss of head in pipe (table), 47.
medium pressure, 43.
methods, classification of, 35.
of coal energy, efficiency of, 32.
Pneumatic, 48.
allowable velocity, in pipes, 52.
cost of plant, 70.
cost of operation, 70.
efficiency of, 54.
loss of head in pipes (table),
52.
Paris system, 54.
price of power, 54.
process of, 50.
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Google
INDEX.
719
Transmission, continued,
reheater, use of, 53.
rope. {See Rope Drive.)
shafting, belting, etc., losses in,
65.
sphere of application of different
systems, 75.
straightaway, 37.
cost of plant, 70.
cost of operation, 70.
wire-rope, 35.
efficiency of, 38.
Transformers, 198.
a. c. to d. c, choice of apparatus,
303.
air-blast, 448.
artificial cooling of, 204.
choice of, 446.
connection of, 208.
connected for split single-phase
into three-phase, 215.
connections commonly used in
practice, 450.
constant current, 504.
constant loss in, 587.
construction of, 190.
core type, 203.
cost of, 651.
current, 664.
data, 201.
determination of magnitude of
units, 445.
duplex machine. {See Syn-
chronous Converter.)
efficiency of, 59, 201, 205, 206,
588.
fire protection, 447.
fire risk in, 447.
high voltage, location of, 448.
inductance of, 150.
installation of, 451.
losses in, 200.
maximum, practicable voltage of,
687.
maximum, size of, self-cooled, 446.
motor-generator. {See Motor
Generator.)
Transformers, continiLed.
polyphase, 206.
principle of, 130.
ractius of operation of, 500.
ratio of transformation of, 200.
rectifiers. {See Rectifiers.)
rotary converter. {See Syn-
chronous Converter.)
shell type, 202.
star, mesh and resultant mesh
connections, 209.
static converter. {See Converter.)
two to three-phase and vice versa,
210.
two to three-phase and vice versa,
without special transform-
ers, 212.
used as boosters, 213.
working of, 199.
Turbines:
steam, 334.
actual efficiency of, 345.
advantages of, 344.
Curtis, principles of, 342.
De Laval, principles of, 335.
De Laval, steam consumption
of, 336.
for high heads, 360.
for low heads, 360.
governors for. {See Gov-
ernors.)
impulse type, 351.
impulse type, efficiency of, 366.
impulse type, maximum effi-
ciency of, 361.
installation of, 357.
McCormick type, 356.
methods of regulating, 365.
multiplex types, 369.
Parsons, principles of, 336.
Parsons, efficiency of, 340.
Parsons, steam consumption
of, 340.
Parsons' performance curves
6f, 341.
Pelton and Doble, efficiency
of, 362.
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INDEX,
Turbines, amtinued.
pressure t)rpe, 351.
piessure type, losses in, 364.
pressure type, efficiency of, 364.
Samson t3rpe, 354.
Victor type, 355.
water, choice of, 367.
Turbo-generator, 342.
Umformer, 2d4.
Units, 20.
Valve:
needle for water-wheels, 363.
Valves, engine, dependent tjrpe,
313.
independent type, 312.
Victor turbine, 355.
Volt, definition of, 20.
Volta, constant current plant, 106,
108.
Voltage:
choice of, initial, 531.
diagram, 519.
regulator, of, G'. E. Co., 632.
regulation for lighting and motor
service, 513.
rise at end of line containing
capacity, 521.
Voltages, striking distance at vari-
oui^, 494.
Voltmeter relays, 457.
Voltmeters, 664.
a. c, 665.
classification of, 387.
connections for, 667.
.cost of, 648.
recording, 669.
Water-power:
and steam, relative cost of, 33.
canals, 405.
creation of artificial, 699.
dams. {See Dams.)
development of, 387.
development, maximum allowable
cost of, 415.
difficulties from ice, 409.
Water-power, amtinued,
distribution of, 25.
estimate of market for, 416.
formula for available h. p., 396.
formula for mechanical h. p., 396.
measurement of flow, 389.
. mountain streams, development
of, 399.
pipe line, cost of, 418.
plant, itemized cost of, 648.
cost of generating and trans-
mitting power, 650.
operating expenses of, 651.
protection against ice, 409.
questions involved in develop-
ment of, 410.
rainfall observations, 394.
reconnoissance of, 389.
settling tanks for sand, 408.
steam auxiliary, when to install,
414.
steel and iron pipe lines, 406.
storage, when to provide, 410.
storage reservoir, 397.
utilization of, 416.
varying head, how to deal with,
368.
with steam auxiliary, 412.
wooden pipe lines, 406.
Water-tube boilers, 326.
Water velocity, allowable, 46.
Water-wheels, 349.
classification of, 349.
cost of, 651.
drive from vertical, 430.
governors. {See Governors.)
installation of, 357.
Leffel cascade type, 363.
Pelton, 352.
principles of, 350.
regulation of, 370.
timber flumes, 439.
turbine. {See Turbines.)
Watt, definition of, 21.
Wattmeter:
connection to three-phase drcuit,
676.
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INDEX.
721
Wattmeter, continued.
for two or three-phase circuits,
678.
induction type, 673.
integrating, 671.
recording, 671.
Wave energy, 5.
form analysis of, 169.
as afiFected by inductive load,
524.
as afiFected by synchronous
motor, 171.
device to obtain sine, 164.
in practical circuits, 527.
of three-phasers, 184.
Wave forms, 128.
Wave motors, 31.
Waves, irregular forms of, 278.
Weston, d. c. instruments, 663.
Wiers:
coefficient formula for, 391.
formula, 390.
table, 391.
Wind-power, windmills as prime
movers, 26.
pressure on line wire, 543.
Winding armature. (See Armature
Winding.)
Wire-rope, 36.
Woods, tensile strength of various,
554.
WoA, unit of, 21.
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