Improvements in or relating to Electrical Transformers.
Year: 1897

page_text: Date of Application, 14th Dec., 1897-Accepted, 5th Feb., 1898
COMPLETE SPECIFICATION.
Improvements in or relating to Electrical Transformers.
We, THE BRITISH THOMSON-HOUSTON COMPANY, LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of this
invention (which has been communicated to us from abroad by Walter S. Moody,
of Lynn, in the County of Essex, State of Massachusetts, United States of
America), and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement:-
The present invention relates to electrical transformers, especially adapted for
three-wire circuits. It will be found useful with transformers which have their
primary winding connected to a two-wire circuit and their secondary windings
to a three-wire circuit as is now common practice, or conversely with transformers
whose primaries are connected to a three-wire circuit and secondaries to a two-
wire circuit, or with transformers having both primaries and secondaries con-
nected to three-wire circuits.
Three-wire transformers adapted for such service have heretofore been made
with a rectangular core having only a single magnetic circuit, with two primary
coils connected in series, and arranged one on each leg of the core, and two
secondary coils also arranged one on each leg of the core; ordinarily the primary
and secondary coils are superimposed. Such a transformer gives excellent regula-
tion for varying loads so long as the load is maintained balanced on the different
branches of the system, and is for its regulating qualities and other reasons a
very desirable form of construction. Our correspondent has found however that
when the secondary coils are arranged as described, that is, with the secondaries
for the different sides of the system arranged each on its own leg of the core and
adjacent to one of the primaries, the regulation is unsatisfactory when the load
becomes unequal to the different branches of the three-wire circuit. While the
load is equally divided the counter electromotive force of each secondary maintains
the proper relation of the electromotive force of the adjacent primary and the
current in each secondary is practically due to the induction of the adjacent primary,
the leakage of all the primaries and secondaries being a minimum. If now one
side is heavily loaded and the other lightly loaded, the heavily loaded secondary
will take current not only from the adjacent primary, but from both primaries,
and at the same time the normal relation between the counter electromotive force
of the lightly loaded secondary and the primary adjacent to it is distubed so
that the leakage increases due to the abnormal relation between the lightly loaded
secondary and the adjacent primary, and the greater distance of the heavily
loaded secondary from said primary. This increased leakage is found to shunt
the heavily loaded secondary and decreases its voltage while the leakage through
the lightly loaded secondary increases and its voltage rises. These, of course,
are exactly the opposite of the conditions requisite for good regulation for
unbalanced load in a three wire circuit.
To avoid this difficulty our correspondent has sub-divided the secondary coils

page_text: heretofore employed, and has distributed the coils feeding each branch of th
three-wire circuit on the two legs of the transformer core so that the secondar
winding connected to each side of the circuit receives induction equally from bot
of the primary coils. The total flux, therefore, will act equally on all th
secondary windings. He finds by test that in this way. the difficulties in regulation
heretofore existing on unbalanced load are overcome, and that a transformer s
constructed has excellent regulating properties, not only for varying total loads
but also for unequally distributed loads. It is convenient to thus describe the
transformer. as having its secondaries connected to a three-wire circuit, but a
will be hereinafter pointed out, the invention may equally well be embodied by a
like arrangement of the windings in cases where the primary coils are con
nected to a three-wire circuit and the secondaries to a two-wire circuit, or where
both primaries and secondaries are connected to three-wire circuits.
In the accompanying drawing, Fig. represents a two-wire system supplying
the primary of a transformer, the secondary of which is connected to a three-wire
system; Fig. 2. represents' a three-wire system supplying the primary of a trans-
former, with the secondary, connected to 'a-two-wire system; and Fig. 3 is a detail
of the mechanical construction."- • 1
In Fig. 1 the alternating current generator A supplies current to a two-wire
system by means of wires B and C. A single-magnetic circuit transformer Dis
provided, consisting of a laminated core E upon which are wound the primary
and secondary coils. The primary coils F and F are mounted upon separate
legs of the transformer core and connected in series with each other and in shunt
across the mains B and C. The secondary is divided into a number of
coils G G G2 G; these coils are connected in pairs, and the coils of each pair
are mounted upon separate legs of the core E; coils G, G³, forming one pair, are
coupled in series and connected between, the main H and neutral wire I;
coils G G2 forming the second pair, are coupled in series and connected between
the main J and neutral wire I. Coils G G³ being mounted on separate legs of the
transformer core and at a substantially equal inductive distance from both the
primary coils, will receive induction equally from both the primary coils; this
is also true of coils G1 G2 which are connected to the other side of the system. By
this arrangement each side of the three-wire system receives a substantially equal
amount of current from both of the primary coils, and when the lights K which
form the load on one side of the secondary system are cut out, this influences
practically alike the relation between the impressed and counter electromotive
forces in each of the primary coils and the relation between the impressed and
counter magneto-motive forces in each leg of the transformers, so that an unbalanced
load does not give rise to an unbalanced inductive action between the primaries
and secondaries, and good regulation is secured.
In Fig. 2 is shown a three-wire system supplying current to a two-wire system
by means of a transformer D¹. The alternating current generator A supplies
current to the three-wire system by means of a transformer T to which are con-
nected the mains a b and neutral wire c. The primary coils of the transformer.D'
are connected to the three-wire system by wires a b c¹. The transformer D¹ is
similar to the transformer D shown in Fig. 1,, except that the primary coils are
now arranged and connected in a manner similar to the secondary coils in Fig. 2,
and so also the secondary coils in Fig. 2 correspond in arrangement to the primary.
coils of Fig. 1.
While the primary of transformer D₁ is shown.subdivided and connected to a
three-wire system, and the secondary connected to a two-wire system, it is within
the scope of the present invention to. connect both primary and secondary to a
three wire system and to subdivide them in the manner above described.
Fig. 3 shows the relation which the coils bear to each other in practice. (For
the purpose of illustration only one leg of the transformer is shown). The core E 5
is made of laminated iron and surrounding it is the primary coil F' which is
quitably insulated therefrom. Superimposed on the primary coil F are the coils G

page_text: and G2; these are suitably insulated from each other and the primary; if desired,
the secondaries may be first applied to the core and then the primary.
It is preferable if coil G is next to the primary F on one side, to place its
companion coil G³ outside of the secondary coil G₁; by this means the coils of
both sides will subject to equal inductive effects.
Having now particularly described and ascertained the nature of the said
invention (as communicated to us from abroad) and in what manner the same is
to be performed, we declare that what we claim is :-
1. In a transformer, the combination of a core, a winding mounted thereon,
and a winding in inductive relation to the first named winding and connected
to a three-wire circuit comprising a plurality of coils connected to each side of
the three-wire circuit, said coils being so arranged on the core that they receive
substantially equal induction from the first mentioned winding under balanced
and unbalanced loads, as herein described.
2. The combination in a transformer of a core having a single magnetic circuit,
a primary comprising a plurality of coils, each mounted upon a separate part of
the core, a secondary subdivided into a number of coils and connected to a three-
wire circuit, the coils which are connected to each side of the three-wire circuit
being mounted on separate parts of the core, and receiving induction equally from
all of the primary coils, as herein described.
3. The combination in a transformer of a laminated core, primary coils mounted
on separate parts of the core, a subdivided secondary set of coils connected to a
three-wire circuit and superimposed upon the primary coils, the arrangement
being such that the secondary coil which lies next the primary on one part is
farther removed on the other, as described.
4. The combination in a transformer of a laminated core having a single
magnetic circuit, primary coils mounted on separate parts of the core and
connected in series, a three wire secondary circuit, and sets of secondary coils for
each branch of the three-wire circuit which are connected in series and are
arranged on the core so that the coils connected across each of the different
branches of the circuit are in substantially similar inductive relation to each of
the primaries, as set forth.
5. The combination in a transformer for three-wire circuits, of a core having
a single closed magnetic circuit, primary coils arranged at different points on
the core, secondary coils located on the core adjacent to the primary coils, and
connections between the secondary coils and the three-wire circuit, such that a
part of the secondary winding for each branch of the circuit lies adjacent to each
of the primary coils, and the total secondary winding for each of the different
branches of the circuit is in substantially similar inductive relation to each of
said primary coils, as set forth.
6. The combination in a transformer, of a core having a single closed magnetic
circuit, primary coils located at different points on the core and adapted to be
connected in series in the primary circuit, secondary coils connected in series in
pairs or sets, and arranged on the core with the different coils of each set located
respectively adjacent to the different primary coils, in such a manner that said
pairs or sets of coils collectively considered are in substantially the same inductive
relation to the different primary coils, as herein set forth.
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