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THIS invention is covered by United States
Patent No. 274,290, issued to Edison
on March 20, 1883. The object of the
invention was to provide for increased economy in
the quantity of copper employed for the main
conductors in electric light and power
installations of considerable extent at the same
time preserving separate and independent control
of each lamp, motor, or other translating
device, upon any one of the various distribution
circuits.
Immediately prior to this invention the highest
state of the art of electrical distribution was
represented by Edison's feeder system, which
has already been described as a straight parallel
or multiple-arc system wherein economy of copper
was obtained by using separate sets of
conductors--minus load--feeding current at
standard potential or electrical pressure into
the mains at centres of distribution.
It should be borne in mind that the incandescent
lamp which was accepted at the time as a standard
(and has so remained to the present day) was a
lamp of 110 volts or thereabouts. In using
the word "standard," therefore, it is
intended that the same shall apply to lamps of
about that voltage, as well as to electrical
circuits of the approximate potential to operate
them.
Briefly stated, the principle involved in the
three-wire system is to provide main circuits of
double the standard potential, so as to operate
standard lamps, or other translating devices,
in multiple series of two to each series; and
for the purpose of securing independent,
individual control of each unit, to divide each
main circuit into any desired number of derived
circuits of standard potential (properly
balanced) by means of a central compensating
conductor which would be normally neutral, but
designed to carry any minor excess of current
that might flow by reason of any temporary
unbalancing of either side of the main circuit.
Reference to the following diagrams will
elucidate this principle more clearly than words
alone can do. For the purpose of increased
lucidity we will first show a plain
multiple-series system.
In this diagram G and G represent two
generators, each producing current at a
potential of 110 volts. By connect- ing
them in series this potential is doubled, thus
providing a main circuit (P and N) of 220
volts. The figures marked L represent eight
lamps of 110 volts each, in multiple series
of two, in four derived circuits. The arrows
indicate the flow of current. By this method
each pair of lamps takes, together, only the
same quantity or volume of current required by a
single lamp in a simple multiple-arc system;
and, as the cross-section of a conductor
depends upon the quantity of current carried,
such an arrangement as the above would allow the
use of conductors of only one-fourth the
cross-section that would be otherwise required.
From the standpoint of economy of investment
such an arrangement would be highly desirable,
but considered commercially it is impracticable
because the principle of independent control of
each unit would be lost, as the turning out of a
lamp in any series would mean the extinguishment
of its companion also. By referring to the
diagram it will be seen that each series of two
forms one continuous path between the main
conductors, and if this path be broken at any
one point current will immediately cease to flow
in that particular series.
Edison, by his invention of the three-wire
system, over- came this difficulty entirely,
and at the same time conserved approximately,
the saving of copper, as will be apparent from
the following illustration of that system, in
its simplest form.
The reference figures are similar to those in
the preceding diagram, and all conditions are
also alike except that a central compensating,
or balancing, conductor, PN, is here
introduced. This is technically termed the
"neutral" wire, and in the discharge of its
functions lies the solution of the problem of
economical distribution. Theoretically, a
three- wire installation is evenly balanced by
wiring for an equal number of lamps on both
sides. If all these lamps were always lighted,
burned, and extinguished simultaneously the
central conductor would, in fact, remain
neutral, as there would be no current passing
through it, except from lamp to lamp. In
practice, however, no such perfect conditions
can obtain, hence the necessity of the provision
for balancing in order to maintain the principle
of independent control of each unit.
It will be apparent that the arrangement shown
in Fig. 2 comprises practically two circuits
combined in one system, in which the central
conductor, PN, in case of emergency, serves
in two capacities--namely, as negative to
generator G or as positive to generator G,
although normally neutral. There are two sides
to the system, the positive side being
represented by the conductors P and PN, and
the negative side by the conductors PN and N.
Each side, if considered separately, has a
potential of about 110 volts, yet the
potential of the two outside conductors, P and
N, is 220 volts. The lamps are 110
volts.
In practical use the operation of the system is
as follows: If all the lamps were lighted the
current would flow along P and through each pair
of lamps to N, and so back to the source of
energy. In this case the balance is preserved
and the central wire remains neutral, as no
return current flows through it to the source of
energy. But let us suppose that one lamp on the
positive side is extinguished. None of the
other lamps is affected thereby, but the system
is immediately thrown out of balance, and on the
positive side there is an excess of current to
this extent which flows along or through the
central conductor and returns to the generator,
the central conductor thus becoming the negative
of that side of the system for the time being.
If the lamp extinguished had been one of those
on the negative side of the system results of a
similar nature would obtain, except that the
central conductor would for the time being become
the positive of that side, and the excess of
current would flow through the negative, N,
back to the source of energy. Thus it will be
seen that a three-wire system, considered as a
whole, is elastic in that it may operate as one
when in balance and as two when unbalanced, but
in either event giving independent control of
each unit.
For simplicity of illustration a limited number
of circuits, shown in Fig. 2, has been
employed. In practice, however, where great
numbers of lamps are in use (as, for instance,
in New York City, where about
7,000,000 lamps are operated from
various central stations), there is constantly
occurring more or less change in the balance of
many circuits extending over considerable
distances, but of course there is a net result
which is always on one side of the system or the
other for the time being, and this is met by
proper adjustment at the appropriate generator in
the station.
In order to make the explanation complete,
there is presented another diagram showing a
three-wire system unbalanced:
The reference figures are used as before, but
in this case the vertical lines represent
branches taken from the main conductors into
buildings or other spaces to be lighted, and the
loops between these branch wires represent lamps
in operation. It will be seen from this sketch
that there are ten lamps on the positive side and
twelve on the negative side. Hence, the net
result is an excess of current equal to that
required by two lamps flowing through the central
or compensating conductor, which is now acting
as positive to generator G The arrows show the
assumed direction of flow of current throughout
the system, and the small figures at the
arrow-heads the volume of that current expressed
in the number of lamps which it supplies.
The commercial value of this invention may be
appreciated from the fact that by the application
of its principles there is effected a saving of
62 1/2 per cent. of the amount of copper
over that which would be required for conductors
in any previously devised two-wire system
carrying the same load. This arises from the
fact that by the doubling of potential the two
outside mains are reduced to one-quarter the
cross-section otherwise necessary. A saving of
75 per cent. would thus be assured, but the
addition of a third, or compensating, conductor
of the same cross-section as one of the outside
mains reduces the total saving to 62 1/2 per
cent.
The three-wire system is in universal use
throughout the world at the present day.
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