Improvements in Systems of Distribution of Alternating Electric Currents.
Year: 1896

page_text: Date of Application, 12th May, 1896-Accepted, 13th June, 1896
COMPLETE SPECIFICATION.
Improvements in Systems of Distribution of Alternating Electric
Currents.
I, JAMES DEVONSHIRE, Secretary of The British Thomson-Houston, Limited,
of 38, Parliament Street, Westminster, Lond in (a communication from abroad by
Charles P. Steinmetz, Electrical Engineer, of Schenectady, in the State of New
York, United States of America), do hereby declare the nature of this invention
and in what manner the same is to be performed to be particularly described and
ascertained in and by the following statement :-
My invention relates to the distribution of alternating electric currents,
particularly to polyphase distribution. It has its most important application to
three-phase systems, but others are not excluded. It has for its object to provide
a means of equalizing the voltages upon the several sides of the system, or of
"balancing the lines," as it is often called.
It has been proposed to connect transformers wound for three-phase work with
the Y system of connection, and to run a neutral wire from the common junction
of the coils back to the generator. Where the secondaries are also connected with
the Y system, this neutral wire on the primary side is a necessity, because, although
a neutral may also be run from the secondary side, this will not equalize the load,
but with an unequal distribution of load, the three secondary voltages will become
unbalanced and greatly unequal. Nothing holds them at an equality, but on the
contrary they change and adjust themselves so as to be proportioned to the three
secondary currents. The neutralizing wire on the primary side running from the
generator to the transformer must necessarily be of sufficient cross section to
equalize the load, and still it is normally an idle wire. This is of course
objectionable, and deprives the three-phase system when so installed, of one of its
characteristic advantages, to wit, economy in copper cost. The generator also
must be specially constructed where this is done, inasmuch as an additional sliding
contact must be provided for the neutral wire.
To obviate the difficulties thus pointed out I have devised my invention, which
consists in winding the primaries of the transformers with delta connection, and
the secondaries with Y connection, and using a neutral or equalizing wire on the
secondary side of the system only; and I have found that by this arrangement
so long as the primary voltages are constant, the secondary potentials also are
constant, irrespective of the balance of load upon the secondaries.
The accompanying drawing shows a diagrammatic embodiment of my invention,
A being a generator of three-phase current, a b and c being the mains therefrom;
D, E, F are the primary coils connected in delta, G, H, I, the secondary coils of
the transformer. G', H', I' are the inducing coils of the three phase motor M ;
g, h, i are the leads connecting the secondary coils and the motor coils together
and k is the neutral or equalizing wire.
I thus not only obtain the advantage in transmission of current at long distance
of confining the equalizing wire to the secondary distribution only, but inasmuch
as this may be connected to fixed terminals upon the motors, I avoid the additional
sliding contacts already referred to.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed as communicated to me
from abroad, I declare that what I claim is:
1. An alternating current system of distribution comprising transformers baving

page_text: their primaries connected in delta, and their secondaries in Y, and a neutral or
equalizing wire extending from the common junction of the secondaries only.
2. A generator of three-phase electric currents, lines leading therefrom, a
transformer having its primary connected in delta between the lines, a secondary
for the transformer having the coils connected in Y, and an equalizing wire
extending from the common junction of the secondary coils.
For the Applicant,
J. C. CHAPMAN,
70 Chancery Lane, London, W.C., Chartered Patent Agent.