Improvements in Electrical Transformers.
Year: 1897

page_text: Date of Application, 12th Nov., 1897-Accepted, 26th Feb.,
1898
COMPLETE SPECIFICATION.
Improvements in Electrical Transformers.
We, THE BRITISH THOMSON-HOUSTON COMPANY, LIMITED, and HENRY METCALF
HOBART, all of 83, Cannon Street, in the City of London, Electrical Engineers, 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:-
This invention relates to transformers for alternating electrical currents and has for
its objects the securing of a more perfect apparatus for the purposes for which
transformers are employed. The invention comprises novel and special means
whereby the heat generated during service may be readily emitted from the interior
of the coils and core to the surrounding air oil or other fluid or liquid insulating
and ventilating medium.
According to our method of construction, a core may conveniently be built up
to the desired depth by laying upon each other a suitable number of thin rectangular
sheet iron plates. These are at first held between thick wrought iron slabs slightly
narrower and longer than the rectangular sheets, and are drawn solidly together
under pressure and secured by bolts temporarily inserted in holes suitably located
in the rectangular sheets and thick wrought iron slabs. The projecting ends of
the thick wrought iron slabs which, as above stated, are somewhat longer than the
rectangular sheets, serve to secure the core in the winding machine. We employ
corner strips which are temporarily secured by twine or tape, and then the winding
is begun.
As the winding of the first layer progresses from one end to the other, the
temporary iron bolts are removed one by one as reached and replaced by wooden
pins, as the pressure of the copper conductor is sufficient to maintain the core at
its original dimensions, and to secure the said corner strips rigidly in place.
Successive layers serve to bind core, corner strips, conductor and insulation, into
one solid structure, and when the winding is completed, and there is no longer need
for holding the core in the winding machine, the thick wrought iron supporting
slabs are withdrawn lengthwise, leaving the two ducts which they have temporarily
occupied, free, like the other two ducts at the edges of the plates. This construction
leaves the iron bare, and the copper adjacent to the duct is also bare except for
whatever thin insulation is necessary for insulating adjacent turns from each other.
This is an important feature, and also other advantages hereinafter more particu-
larly ascertained accrue. Other methods of temporarily securing and of manipu-
lating the core during the process of construction, will be evident to those skilled
in the art, and we do not limit ourselves to the precise process hereinabove set
forth.
Our novel construction is in contradistinction from constructions (of which we
are aware) wherein the coils have been wound on formers and subsequently
assembled about the iron core and wherein also corner distance-pieces have been
employed, and our method secures advantages which are not obtainable with such
existing arrangements.

page_text: A further object of our invention is to allow of more heavily insulating the end
turn or turns of the windings without producing an unsightly and mechanically
objectionable bunching such as would result if coils wound in the ordinary way
were so insulated. For this purpose we wind each layer with one or more less
turns at the ends than there are at the ends of the adjacent layer below it and
nearer to the core, and we extend the interposed layers of insulating material so
as to project slightly and serve to ensure that a long leakage path is formed
between adjacent layers at the ends of the coil, and other advantages also accrue.
In order that our improvements may be fully understood we will now describe
by way of example a transformer of the single magnetic-circuit type, constructed
in accordance therewith. This is illustrated in the accompanying drawings in
which Figs. 1 and 2 show in sectional plan and side elevation respectively so much
of the said transformer as is necessary to properly explain its construction. LL are
the plates which constitute the core completing their magnetic circuit through the
end pieces L' (Fig. 2). Upon each leg of the core L is wound first a low potential
secondary S made up of a few turns of bare or but lightly insulated copper strips,
next a primary P for high potential made of many turns of small wire and upon it
a secondary S similar to the inner secondary. Between the first secondary S and
the core L an open duct or space I for air, oil or other insulating and ventilating
medium is provided by the use of the insulating corner strips C. These strips are
placed over the corners of the cores L before the winding commences and together
with a narrow insulating strip W (if used) as hereinafter referred to, make up such
a thickness that the said permanently open duct or space I is ensured between the
first secondary S and the core L. In manufacture these corner strips Chaving been
placed upon the core L the pressure of the coils as they are wound on in the manner
above described is sufficient to bind the coils, the corner strips, and the core all
solidly together. An important advantage of this improved construction and
method of building up is that it permits of leaving the iron of the core L and the
copper of the coil S entirely or nearly bare except at the points covered by the
corner strips, and consequently the heat generated during service is better emitted.
This emission of heat is further assisted by the natural current of air or oil circu-
lating through these open ducts II which are generally vertical. The corner
strips C are made of some insulating material and hence there is little or no need
of any insulating wrapping between the copper winding and the core. Further,
by this arrangement the conductor Slies rigidly and rectilinearly from corner
strip to corner strip, leaving the full predetermined width of the duct I for the
circulating medium. This is in marked contrast to constructions wherein
additional insulation is required under the conductors, as not only does this bag
inwardly and bulge, thus obstructing the ducts, but it also retains the heat of the
conductors, whereas when these are nearly or practically bare as above described
their emissivity is greater. In some cases it is or may be desirable to have
additional insulation between corner strip and conductor as shown in the figures
at W but this should not project beyond the edges of the corner strip more than
enough to give a reasonably long leakage path from the conductor over the insulat
ing strip around to the corner strip. The liability to leakage is reduced to a
minimum when the low voltage element is, as shown, wound immediately over the
corner strips. We are aware that natural-draft transformers have been constructed
before and therefore we do not broadly claim a transformer made on this principle,
except in combination with the special arrangements herein described."
.
Further, according to our present improvements in order to allow of more heavily
insulating the end turn or turns ??? of each layer of the coils, they are wound
so that each succeeding one or two layers has one, two or more turns less at each
end than the layer immediately preceding it, and this gives a tapered form to the
windings as plainly shown in the sectional part of Fig. 2, and also avoids the
unsightly bunching above referred to. The specific construction is shown in
Fig. 2. The inner secondary S is shown as consisting of one layer, next outside this
is a thick insulating wrapping D extending for slightly more than the width of

page_text: This
the conductor beyond the primary in the known manner, then upon this is wound
the primary P, its ends being stepped back as illustrated in Fig. 2 and the insula-
tion interposed between the layers projecting slightly beyond them.
insulation between adjacent layers of the primary coil is, as will be seen, gradually,
thickened in the known manner from end to end as the winding advances, to
meet the increased voltage as the turns proceed. Over the primary P is laid a
second thick layer of insulation D¹ extending well outward, and next follows the
outer secondary S¹ made up of the required number of turns, but stepped back
from the last turn of the primary. The end turns ss of both secondaries may be
insulated similarly to the primary. By these improved arrangements and by the
method of winding and insulating, long leakage paths in addition to ample thickness
of insulation are assured between the end turns of different layers (between which
the difference of potential is great) and also bulging at the ends of the coil is
effectually avoided.
'
Having now particularly described and ascertained the nature of our said
invention, and in what manner the same is to be performed, we declare that what
we claim is;
-
1. In constructing an electrical transformer, the use of corner strips on the core
for maintaining the coils at a proper distance therefrom and for providing permanent
open ventilating and insulating ducts between core and coils and the method of
building up which consists in placing such strips in position at the time of winding,
in order that the pressure of the wire as it is wound over same will serve to bind
the whole solidly together, by which construction the iron core and the coil wound
on it are left entirely bare except at the parts covered by the corner strips, and
the emission of heat during service is facilitated and aided by the free passage of
the ventilating and insulating medium through the said open duets, substantially
as described and illustrated.
2. In the construction of a transformer, winding each layer of wire with one or
a few turns less than the layer below it for the purpose of rendering it possible
to more heavily insulate the end one or more turns of each layer without causing
bunching at the ends, and for ensuring a minimum of leakage between the end
turns of adjacent layers of wire and also between primary and secondary sub-
stantially as herein described and best shown in Fig. 2 of the drawings.
3. In the construction of a transformer in combination with the arrangement of
winding referred to in Claim 2 extending the insulating material laid or wrapped
between the layers of the winding so as to project beyond the end turns thereof
whereby a long leakage path in addition to ample thickness of insulation is assured
between successive layers without bulging the ends of the coil.
4. In a transformer the use in combination of the special improvements herein.
set forth and illustrated in the drawings, and severally claimed above.
Dated the 12th day of November 1897.
For the Applicants,
J. C. CHAPMAN,
Chartered Patent Agent, 70, Chancery Lane, London, W.C..