Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to transformer cores
and more particularly to three phase transformers of the wound
core type.
There are a multitude of existing designs for trans~
former coreS which can be broadly categorized as rectangular or
cruciform. After examination of these existing types, it was
considered tha-t the cruciform -types were philosophically supe-
rior, but the rectangular types embodied production advantages,
-including simplicity.
10The object of the present invention is to improve
upon these existing cores.
A preferred object of the invention is to provide
a cruciform-like core with legs having a circular cross-section.
Another preferred object of the invention is to
provide a core that can be easily manufactured from an electrical
; steel strip without a large number of different widths of
electrical steel strip being required.
A further preferred object of the invention is to
provide a core of near optimum geometry according to a straight
~ forward procedure, by using a tapered electrical steel strip
or produclng a~hexagonal or better approximation to circular
cross-section for those portions of the core under the
windings.
In its broad aspect, the present invention proposes
a method, of manufacturing a transformer core of star or Y
configuration from an electrical steel strip, which core
:comprises three ~rames each o~ substan-tlally C-shape in side
~view, the f~rame~being arranged at substantially 120 apart.
This method comprises the steps of:
~; cutting a length of strip into a plurality of
laminatlon lengths, each group of three lamination lengths
being separated from -the adjacent group at each end by a cut
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at substantially 90 to its longitudinal axis, each said group
forming a layer of the core, with the three lamination lengths
in the group being separated from each other by cuts at
substantially 60 to the longi-tudinal axis;
butt joining the ends of two of the lamination
lengths cut at substantially 60 to the sides of a third
lamination length adjacent an end of the third lamination
length cut substantially 90 to form a lamination layer; and
assembling the lamination .layers to form the assem-
bled core.
In accordance with a preferred embodiment of the in-
ventlon the portion of the strip can be cut with an approxi-
mately linear taper to obtain C-shaped frames hexagonal or
substantially circular in cross-section.
In accordance with another preferred embodiment of
the invention, each layer is rotated one third of a turn from the
previous layer prior to assembling it.
III another aspect, the present invention proposes
a transformer core constructed according to the abo~e method.
The invention, in its preferred simplest form, namely
~the hexagonal form approximation, can be achieved in either
of~two~ways, both using a single wldth size of conventionally
slit steel strip~, which is then specially slit. For scrapless
production of corès, two identioal cores can be slit from the
normal parallel sided strip in such a manner that the tapered
pieces are complementary to each other. Two identical tapered
strips can readlly be cut from a suitable rectangular piece
by cutting it at an appropriate angle.
.
~ The invention will be better understood with reference
.
to the following general description of a method of manufacture
of a number of different transformer cores, and of a preferred
30 ~ embodiment of the invention applied to a three-phase, Y-shaped
transfGrmer core.
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In the accompanying drawings:
Fig. 1 is afront view oE a single phase core of
substantially hexagonal cross~section
Fig. lA is a cross--sectional view taken on line lA-lA
of Fig. 1,
Fig. lB shows the cutting plan of the strip for the
core of Fig. 1 L
Fig. ~ is a cross-sectional view of a modified form
of the core of Fig. l;
10- Fig. 2A shows the cutting plan of the strip for the
core of Fig. 2 L
Fig. 3 is a frontview of a single phase shell type
corei
Fig. 3A is a cross-sectional view taken on line 3A-3A
of Fig. 3;
Fig. 4 is a front view of a three pahse delta~> core
Fig. 4A is a cross- sectional view taken on line 4A-4A
of Fiy. 4;
;~ ~ Fig.~ 5A is a view of an assembled layer of a three-
phase star core according to the invention;
Fig. 5B is a plan view of the strips for the assembled
layer of Fig. SA; and
Fig. SC is a perspective ~view of the ~oint of the three-
phase~ star core~made from assembled layers as shown in Fig. 5A.
The production of the tapered electrical steel strip
used for manufacturing transformer cores of the wound core type
ncluding those according to the lnvention is implemented by
a suitable slitting machine. Because
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-the angle Or tclper is so smcll:L e.g. less tl-an 1 , the
ax,is of the sli-tting roller~s is set perperlcliclllar to the
5 trip of e.l,ectrical s-teel, and -the nece~sl:ry -tal)er is achieved
by f`orcing the ro:Llers ac.ross the shee-tO The need fo:r
precise con-t:rol of` the l~osi-tionir~ of` the slittin.g rollers
means thnt the slit-ti.n~.r mach:irle is best built with a single
pair of rollers :for thc slitti,ng operation. This means tllat
it only has I;o accomodate the wiclth of steel needed for the
largest core to be cut by the method~ The details of~ the
mettlod are mos-t e~sily illus-trated by a description of the
relevant parts Or the machine. After dereeling, the strip
is passed through a pair of plain rollers, comprising a clriven
roller to control the speed of the strip~ and its idler. The
strip then passes through two guides with tungsten carbide
wear parts which control its latercal position, and then -through
a second pair of rollers similar to the firstO The second
idler is identical to the first, but the other roller is
mQchined to have a circumference which matches the nurnber of
pulses ~r revolution of -the pulse generator (sha~t encode:r)
that it driYesO This unit thus measures the length along the
strip as it lS fed through, ancl the slitting roller assembly
: : ~ is immediately adjacent to it. The s,l,itting rollers are mo~mted
on a very rigid frame, a~d can be set so that they are pre~
Loaded to minimize deflection:and with the desired a~lount of over-
lap. fhe roller :frame is wlder than the strip~ since it mt~ t
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be able to move back and forth across the strip. The frame
is mounted on a machine bed and driven by a worm drive from
a direct current motor geared down by a large amount because
of the slow travel required. Included in this assembly is a
second shaft encoder with its own small roller which enables
the position of -the slitting rollers to be known and controlled.
The control system for the slitting roller assembly is straight-
forward in that the motion of the slitting rollers sideways
across the strip is directly proportional to the length of strip
passing through. This can be implemented by ordinary logic and
servosystem components, but is better and simpler done by a
microprocessor based computer which may control the rest of the
machine. Thus the tapered strip required for the core designs
can readily be produced.
Figs. 1, lA and lB show a core 10 which can be
continuously wound from strips produced as disclosed hereina-
boce. This core 10 has a substantially hexagonal cross-section
as shown in Fig. lA, comprising a section 10A of increasing
width, a central section lOB of maximum width, and a section
lac of decreasing width.
Referring to Fig. lB, section lOA is wound from strip
llA which is cut from a rectangular stock strip 12, the
; strip;llA having a linear taper from a su~stantially zero width
upto the~width of the stock-strip 12.
Section 10B is wound from a strip llB cut from a
length of the stock strip 12 and has parallel sides.
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Section lOC is wound from a strip llC cut from
stock strip 12 and has a linear taper from the width of the
stock strip 12 down to substantially zero width. As the
strip llA, llC are of the same width, they can be cut from
lengths of stock strip 12.
A core 20 having the modified form illustrated in
Figs. 2 and 2A can be obtained by using substantially the
same method of manufacture. The core 20 has a section 20A
of increasing width and a section 20B of decreasing width.
The sections both have a maximum width equal to the maximum
width of the core and a minimum width equal to one-half of the
core, the sections 20A, 20B being wound from strips 21A, 21B
respectively from rectangular stock strip 22 which has a width
equal to 1.5 times the maximum width of the core 20. As the
strips 21A, 21B are complementary, two transformer cores 20 can
be cut from a single length of stock strip 22 without scrap.
The single phase shell~type core 30 of figs. 3 and
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~ 3A has a pair of core frames 3IA each with a cross-section
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which is substantially identical with an isosceles trapezium.
When the two rames are placed back-to-back, the central leg
is substantially hexagonal in cross-section ~as shown in
Flg. 3A). ;Each~frame can be wound from a single strlp of
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increasing~width, such as strips 21A, 21B shown in fig. 2A.
As these strips are complementary, the core 30 can be produced
from a single piece oE rectangular stock strip i.e. strip 22.
; The three-phase delta core 40 shown in figs. 4 and
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4~ comprises three frames 41 each conjoined at their sidesto the other two frames, the legs of the core having a
substantially hexagonal cxoss-section. Each frame 41 is
continuously wound from a section 42 of fixed width (equal
to 0.5 times the diameter of the leg) cut from a length of
rectangular stock strip of that width, and then a section 43
of decreasing width. As the sections 43 of two of the frames
can be complementary, they can be cut from rectan~ular stock
strip having a width equal to 0.5 times the diameter of the
core legs. The core 40 is assembled by placing the frames ~1
in the configuration shown in figs. 4 and 4A and securing
the frames together.
All the cores shown in figs. 1 to 4A have been
disclosed by way of interest only, in order to better explain
how core legs having a substantially hexagonal or circular
cross-section can readily be obt~ained from steel strips of
increasing and/or decreasing taper and/or constant width.
The invention however is restricted to the manufacture
of transformer core of star or Y configuration.
~ Figs.5~, 5B an~-5C illustrate the manufacture of such a
three-phase astar or aY core 50 with legs of substantially
hexagonal cross-section. The core 50 comprises three frames
51A, 51B, 51C each of substantially C-shape in side view.
Each frame 51A, 51B, 51C is formed from a series of
lamination lengths A, B, C respectively cut from atapered strip
53.
The strip 53 is of increasing taper to form section
52A of each frame,and of decreasing taper to Eorm section 52B,
and may be cut as shown in Fig. 2A.
Lamination lengths A.B.C are cut to selected length
of square-ended strip 53 with two cuts at 60 to the longitu-
dinal axis of the strip 53. This cutting step for most size
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cores can i~nore the taper on the layers which come from the
tapered strip because of the small angle of taper.
Each layer is formed as shown in FIG. 5A where the
angled cut ends of each lamination length are butted to the
side of adjacent strip adjacent its free ends.
The method of assembling such a star core is to lay
together all the laminations so that one joint (e.g. the
bottom joint) is assembled and the core has the appearance Or
three radial arms. The joint is clamped and the lamination
arms A, s, C are bent upward until they are perpendicular to
the plane of the joint. The arms are secured hy claimping and
then the joint is unclamped enabling the clamped arms to be
separated from the joint. The top portion of each clamped
laminated arm is then folded to form a substantially C-shaped
laminated assembly of lamination lengths. The electromagnetic
properties of the joint are best when each layer is rotated one
third of a turn from the previous layer, that is when piece A
of FIG. 5A is placed one each core leg in turn. This core
joint~is the only straight cut, scrapless butt inter-leaved
~star or Y core joint, and applicable not only to cores with
tapered strip, but to any core of this type, also known as Y
~(wye) cores.~ The section labelled E can be seen to protrude
from the joint, but there is no advantage to be gained by
removing it.~
All of the new desings, being of the wound core type,
require annèaling after -they have been cut and formed to shape.
By using the tapered strip to form the cores, which can
be continuously wound ~except for the star core 50 of
~FIGS. 5A and 5B~, the transformer designer can achieve great
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flexibility in design~ By careflll selection of the taper
of the electrical strip, he cc~ achieve almost any core
cross-section W~iC~I he may recluire and can almost achieve the
theoretica.lly optirnum circular cross-sectiorlO ~or simplicity,
the hexagonal cross-sectiorl as an approximation of the circular
cross-section is readily achicvable~
The invention~ in addition to its application as
a means of producing the hexagorlal foIm approximation, can in
an analogous .iashion to the cruciform case, be used to produce
octagonal or hig~er order even regular sided approximations to
a circular cross-section. However, for each pair of sides in
excess of six an additional size of parallel strip is required
to allow scrapless production of the core.
Various changes and modifications may be made to
the methods described without departing from the scope of the
present invcntion~
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SUPPLEMENTARY DISCLOSURE
The present supplementary disclosure is intended
to better emphasize some advantageous and unique features
of the core according to the invention and its method of
manufac-ture~.
As indicated on page 8, last paragraph of the original
disclosure; the laminations lengths A, B and C used for the ~ -
manufacture o a transformer core according to the invention,
are to be cut to selected lengt~ of square-ended s~rip 53.
The so-selected lengt~sare, in practice, easily
determinable by one skilled in the art, simply by approximation.
However, a computerizable method for calculating
the non-lineax increase in length of the lamination lengths,
can be used if desired. Such a method is based on making an
initial estimate of the required total length and then using
the~appropriate taper for the estimated length to calculate
the dimensions of each piece. The error in the initial estima-
te~will then provide a correcting factor and the process can
, ~
; ; be repeated until the exact degree of accuracy required is
obtained. As aforesaid, this method can of course be
~ computerized.
For deslgn purposes;, a~method based on such an
nitial estimate can be used~with~an accuracy of about 1%. ~ ;
The ~following equations usable for determining the length,
volume~and ma~ss of a Star-shaped, tranaformer core with legs
of~hexagonal form, are illustrative of such a method.
1) n =~ ~INTEGE~ ~
5 / : -- ( 1 )
2 ~ * T~K
2~) ~ STR~ HT + 2 WID - ~ CLD ~ - (4 - ~f ) MBR
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;~ 3~) Length =~ 3m STR + 3 ~ n2THX - 0.491 n CLD
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4) Volume = 2 CLD * THK * Length
51 Mass = 0.97 * 7.65 x lO 6 * Volume
= 7.42 * lO 6 * Volume
wherein:
THK is the thickness of the electrical steel;
CLD is the core leg diameter (i.e.the maximum width of the core);
HT is the height of the winding window;
WID is leg centres (i.e. winding diameter);
STR is the centre line string length equivalent to first
laminations;
MBR is the minimum bend radlus of core steel; and
n 1s the number of layers of 1amlnations in the core.
As also indicated on page 9 at the end of the second
Full paragraph o~ the origlnal disolosure, the portlorsE of
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the lamination layers which are cut at 90 protude;from the
olnt~through the cores assembly.~ Thus, those portions form
20 ~ a wedge-llke~end portion on~each end of each core frame
assembly. ~This~wedge-llke and portlon greatly assists in ;
reassembling~t~he core;after the frames have been annealed
and/or wound~. Indeed, this~wedge joint helps in meshing
es~s~entially one~layer at a time.~ ;
This~sdditional feature will become more apparent
with reference to the accompan~ing, supplementary drawings
whereiD; ~
Fig. 6;is a perspective view of the joint of a
three-phase~star core; and~ ~ ~
3~0 ~ Flg. 6a is a partial view of one leg of the star
core shown in Fig. 6, when separated from the other.
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As c~ be seen, the profile of the wedge-shaped end portion
E of the laminations of the core according to the inven-tion,
which profile results from the shape of the lamination lengths
and the way they are assembled, advantageously permits to
achieve butt joining at either the top or bottom of the core
structure. If the three C-shaped frame are separated, then
the exposed end of each frame at the joint will have a wedge
shaped profile as shown in fig. 6A. This particular feature
makes it possible to separate the three legs of the core
in order to fit the windings, and then to reassemble the core
as a simple plug fitting assembly operation, because the
profile at the ends of each frame allows progressive engagement.
EXAMPLE:
A model transformer core was manufactured in
accordance with the present invention from 0.28 mm M4 grain
oriented electrical steel.
This model core had the following details.
Widest lamination: 60 mm (corresponding to core leg diameter)
Winding diameter: 130 mm (corresponding to the linear distance
between a pair of leg centres)
Winding height: 130 mm
Lamination thickness: 0.28 mm
Minimum bend radius: 3.0 mm
This model core comprised 187 layers and 561 pieces for a total
length of material of approximately 166 metres.
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