Three-phase and three-leg core of core-type transformer

A three-phase and three-leg core of a core-type transformer comprising three main legs formed of a plurality of steel sheets stacked in the form similar to a circle in cross section and spaced each other, and upper and lower yokes formed of a plurality of steel sheets stacked in the form similar to a circle in cross section for magnetically connecting the main legs. The steel sheets for forming each main leg are cut diagonally at opposite longitudinal ends thereof, and each yoke is formed of two types of steel sheets, one type being of diagonal cuts disposed at opposite longitudinal ends thereof to provide steel sheets of the trapezoidal shape and the other type being of a diagonal cut disposed at one of opposite longitudinal ends thereof and a right angle cut disposed at the other longitudinal end thereof to provide steel sheets of the trapezoidal shape. The steel sheets for forming the upper and lower yokes have a width greater than the width of the steel sheets for forming the main legs. The opposite longitudinal ends of the steel sheets for forming the center main leg are cut diagonally at an angle less than 45 degrees and joined diagonally and at a right angle to the steel sheets for forming the upper and lower yokes through the entire surfaces. The steel sheets for forming the two outer main legs are cut diagonally at opposite longitudinal ends thereof at 45 degrees and joined diagonally to the steel sheets for forming the upper and lower yokes in an area in which the yoke steel sheets are cut diagonally. This construction is conducive to reduced iron loss and to form the main legs of a small diameter.

BACKGROUND OF THE INVENTION 
This invention relates to three-phase and three-leg cores of core-type 
transformers, and more particularly, to a three-phase and three-leg core 
wherein steel sheets or laminations having longitudinal opposite ends 
thereof cut diagonally are used for forming each main leg and steel sheets 
having longitudinal opposite ends thereof cut diagonally and steel sheets 
having one longitudinal end thereof cut diagonally while having the other 
longitudinal end thereof cut at a right angle are used for forming upper 
and lower yokes. 
Generally, a joint system shown in FIG. 1 or 3 has been in use for a 
three-phase and three-leg core of a core-type transformer of the prior 
art. By using these joint systems, a plurality of steel sheet groups 
differing from one another in width are stacked in laminations, and each 
main leg is arranged in a core circle contacting the outer ends of the 
core legs, to provide a three-phase and three-leg core. 
Losses suffered by the three-phase and three-leg core of a core-type 
transformer include an eddy current loss caused by a magnetic flux flowing 
through the steel sheets, a hysteresis loss, a loss caused by a 
disturbance of the flow of the magnetic flux through the gaps of the steel 
sheets, and a loss which is increased by the three-phase magnetic flux in 
the revolving magnetic field and caused to occur in the joints between the 
main leg located in the center and the upper and lower yokes. The loss in 
the joints disposed between the main leg located in the center of the 
three-phase and three-leg core and the upper and lower yokes is maximized 
because the conditions referred to hereinabove all occur in these joints 
and the temperature therein rises to a high level. Thus, limitations are 
imposed by these conditions on the number and size of cooling oil ducts in 
the three-phase and three-leg core and the mean magnetic flux density in 
each main leg. Therefore, in the three-phase and three-leg core, the space 
factor of the steel sheets with respect to the core circle gets worse than 
is necessary. 
FIG. 1 shows one example of the three-phase and three-leg core of the prior 
art in which main legs 3, 4 and 5 parallelly spaced and upper and lower 
yokes 1 and 2 magneticatly connecting the main legs 3, 4 and 5 to one 
another are formed by stacking a plurality of steel sheet groups. The main 
legs 3, 4 and 5 and the yokes 1 and 2 are connected together by diagonal 
joints, and the steel sheets are stacked in such a manner that the joints 
between the main leg and the yoke alternately move in parallel 
relationship as indicatd by solid and broken lines. 
In this construction, steel sheets 1A and 2A shown in FIG. 2(a) which are 
cut diagonally at opposite longitudinal ends thereof as shown and in which 
a portion D disposed in the center and forming the joint with the main leg 
5 is formed by cutting the material in triangular form are used for 
forming the upper and lower yokes 1 and 2. Steel sheets 3A and 4A of the 
trapezoidal shape having longitudinal opposite ends thereof cut diagonally 
as shown in FIG. 2(b) are used for forming the main legs 3 and 4 located 
on the left side and the right side respectively, and steel sheets 5A 
having opposite longitudinal ends thereof cut diagonally in triangular 
form as shown in FIG. 2(c) are used for forming the main leg 5 located in 
the center. 
In this joint system, there are the disadvantages that the upper and lower 
yokes 1 and 2 and the main legs 3, 4 and 5 located at opposite ends and in 
the center are formed of steel sheets of the same width, so that scrap 
part is caused at a portion (D) regarding the steel sheets 1A and 2A for 
forming the upper and lower yokes 1 and 2 and other scrap parts are caused 
at portions (B and C) regarding the steel sheets 5A for the main leg 5. 
On the other hand, a joint system disclosed in U.S. Pat. No. 3,283,281 
which is generally referred to as a scrapless system for a three-phase and 
three-leg core is also used. FIG. 3 shows one example of this system in 
which steel sheets used for forming the upper and lower yokes 1 and 2 and 
the main legs 3, 4 and 5 are of the same width, and steel sheet 
laminations are provided by stacking the steel sheets in such a manner 
that the diagonal joints are alternately moved as indicated by solid and 
broken lines. In this core, the upper and lower yokes 1 and 2 are formed 
of steel sheets 7A and 7A' of the trapezoidal shape having opposite 
longitudinal ends thereof cut diagonally at 45 degrees and steel sheets 8A 
and 8A' of the trapezoidal shape having one longitudinal end thereof cut 
diagonally at 45 degrees and the other longitudinal end thereof cut at a 
right angle as shown in FIGS. 4(a) and 4(b). The steel sheets 7A and 8A 
have a greater length than the steel sheets 7A' and 8A' by an amount 
corresponding to each of the overlapping diagonal joints of the yokes 1 
and 2 with the main legs 4 and 3 located outwardly of the center main leg 
5. 
The main legs 3, 4 and 5 of the core are formed, as shown in FIG. 4(c), of 
steel sheets 3A, 4A and 5A of the trapiezoidal shape having longitudinal 
opposite ends thereof cut diagonally at 45 degrees. As a result, the 
diagonal joints between the upper and lower yokes 1 and 2 and the outer 
main legs 3 and 4 move in parallel relationship in an overlapping face 11 
of an area defined by solid and broken lines, and the diagonal joints 
between the upper and lower yokes 1 and 2 and the center main leg 5 have 
inner and outer end edges 9 and 10 reversed as indicated by solid and 
broken lines in FIG. 3. 
The joint system for the three-phase and three-leg core shown in FIG. 3 can 
obviate the disadvantage of causing scrap end portions in the steel sheets 
that must be cut off and wasted, as shown in FIG. 1. However, the system 
shown in FIG. 3 suffers the disadvantage that losses increase at the 
joints between the upper and lower yokes and each of the main legs. 
The joint systems shown in FIGS. 1 and 3 share a defect in that the 45 
degree joint faces of the upper and lower yokes 1 and 2 and the outer main 
legs 3 and 4 are displaced from each other at the overlapping faces 11 due 
to the fact that the steel sheets forming the upper and lower yokes 1 and 
2 and the outer main legs 3 and 4 are of the same width. The result of 
this is that, in section A in FIGS. 1 and 3, a cutout is formed which 
causes disturbance of a flow of magnetic flux from the outer main legs 3 
and 4 to the upper and lower yokes 1 and 2 to increase. The overlapping 
face 11 at each joint cannot have its area reduced more than is necessary 
for maintaining the enough strength of the core, and this tendency exerts 
greater influences on steel sheets of smaller width. To minimize the 
disturbance of the magnetic flux, the overlapping face 11 is proportioned 
such that one-half thereof is disposed on the main leg side and one-half 
thereof is disposed on the yoke side. This results in projections 12 and 
13 extending outwardly of the three-phase and three-leg core as shown in 
FIG. 3. Of these projections, the projection 13 does not become an 
obstacle because it extends into a space located inwardly of the diameter 
of winding not shown. However, the projection 12 extends above and below 
the three-phase and three-leg core, so that it has to be severed when 
conditions for transportation of the transformer are severe. 
Further, as disclosed in above-noted U.S. Pat. No. 3,283,281, it has been 
proposed to reduce the width of the steel sheets for forming the main legs 
of a core as compared with the width of the steels sheets for forming the 
upper and lower yokes. In such three-phase and three-leg core, the faces 
cut by 45 degrees causes distinctions in accordance with the width of the 
steel sheets used, with the result that recesses or cutouts are formed in 
a section corresponding to section A of FIGS. 1 and 3 and in the joints 
between the center main leg and the upper and lower yokes. Thus, the 
three-phase and three-leg core would suffer the disadvantage that the 
upper and lower yokes are not satisfactorily utilized as magnetic flux 
passages and that iron loss is high at the joints between the center main 
leg and the upper and lower yokes. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a three-phase and three-leg core 
of a core-type transformer capable of reducing iron loss at the joints 
between the main legs parallelly spaced and the upper and lower yokes and 
at the same time capable of avoiding a rise in temperature at the joints. 
Another object is to provide a three-phase and three-leg core of a 
core-type transformer capable of increasing magnetic flux density in each 
main leg and improving the space factor of the iron core, whereby an 
overall compact size can be obtained in a core-type transformer and iron 
loss can be reduced. 
Additional and other objects of the invention will become apparent from the 
description set forth hereinafter when considered in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings wherein like reference numerals are used 
throughout the various views to designate like parts and, more 
particularly, to FIG. 5, according to this figure, a three-phase and 
three-leg core of a core-type transformer includes upper and lower yokes 1 
and 2, and three spaced main legs 3, 4 and 5 disposed in parallel and 
magnetically connected to one another by the yokes 1 and 2. Each of the 
yokes 1 and 2 and the main legs 3, 4 and 5 is formed of a plurality of 
layered steel sheets stacked into a shape similar to a circle in cross 
section as subsequently to be described, and they are magnetically 
connected together by using known diagonal miter joints. In the 
three-phase and three-leg core according to the invention, the steel 
sheets for forming the main legs 3, 4 and 5 each have a width Wc which is 
smaller than the width Wy of the steel sheets for forming the upper and 
lower yokes 1 and 2, and the upper and lower yokes 1 and 2 are connected 
to the outer main legs 3 and 4 by diagonal miter joints of 45 degrees 
while the upper and lower yokes 1 and 2 are connected to the center main 
leg 5 by a diagonal miter joint of less than 45 degrees and a right angle 
miter joint. 
The upper and lower yokes 1 and 2 and the main legs 3, 4 and 5 of the 
three-phase and three-leg core shown in FIG. 5 are formed of steel sheets 
shown in FIGS. 6(a), 6(b), 6(c) and 6(d). More specifically, steel sheets 
3A, 4A and 5A for forming the outer main legs 3 and 4 and the center main 
leg 5 each have a width which is smaller than the width of each of steel 
sheets 7A, 7A', 8A and 8A' for forming the upper and lower yokes 1 and 2, 
and the steel sheets for forming the outer main legs 3 and 4 and the 
center main leg 5 are of the same width. 
For forming the upper and lower yokes 1 and 2, steel sheets 7A and 7A' are 
used having a trapezoidal shape with one longitudinal end thereof cut 
diagonally at 45 degrees and the other longitudinal end thereof cut 
diagonally at an angle 90-.alpha. degrees and the steel sheets 8A and 8A' 
of the trapezoidal shape having one longitudinal and thereof cut 
diagonally at 45 degrees and the other longitudinal end thereof cut at a 
right angle as shown in FIGS. 6(a) and 6(b). For forming the outer main 
legs 3 and 4, there are used steel sheets 3A and 4A of the trapezoidal 
shape each having longitudinal opposite ends thereof cut diagonally at 45 
degrees as shown in FIG. 6(c). For forming the center main leg 5, steel 
sheets 5A of the trapezoidal shape having longitudinal opposite ends 
thereof cut diagonally at an angle .alpha. which is less than 45 degrees 
as shown in FIG. 6(d) are used. 
Steel sheet laminations are stacked to constitute the outer main legs 3, 4 
and the upper and lower yokes 1, 2, alternately in overlapping 
relationship as indicated by solid and broken lines in FIG. 5. The outer 
main legs 3 and 4 are connected diagonally to the upper and lower yokes 1 
and 2 at an angle of 45 degrees. In this construction, overlapping faces 
14 at the diagonal joints are formed such that upper and lower ends of the 
steel sheets constituting the outer main legs 3 and 4 do not extend above 
and below the upper and lower yokes 1 and 2. The diagonally cut portions 
of the outer main legs 3 and 4 are formed such that they are miter-jointed 
to the upper and lower yokes 1 and 2 within the range of the diagonally 
cut portions of the yokes 1 and 2 each of which portions is cut diagonally 
at 45 degrees. Thus, the three-phase and three-leg core according to the 
invention is free from the defect that the disturbance of the magnetic 
flux is caused by the cutouts as is the case with a three-phase and 
three-leg core of the prior art. 
The steel sheets used for forming the center main leg 5 have their 
longitudinal opposite ends cut diagonally at an angle .alpha. which is 
less than 45 degrees, and the steel sheets for forming the upper and lower 
yokes 1 and 2 to be connected to the aforesaid steel sheets have their 
longitudinal opposite ends cut at 90-.alpha. degrees. Thus, by connecting 
these steel sheets by diagonal miter joints, the joints become longer in 
length than the joints obtained by connecting the steel sheets cut 
diagonally at 45 degrees. 
Assume that the steel sheets for forming the center main leg 5 are cut 
diagonally at 42.5 degrees, for example. Then the ratio of the width Wy of 
the steel sheets for forming the yokes 1 and 2 to the width Wc of those 
for forming the main legs 3, 4 and 5 is Wy/Wc=1.09. When the ratio becomes 
smaller or 0.06 in value, for example, the forward end of the steel sheets 
for forming the center main leg 5 extends beyond the steel sheets for 
forming the yokes 1 and 2, provided that the diagonal cut angle .alpha. 
remains unchanged. When the projections have electrically adverse effects, 
portions thereof that extend beyond the yokes may be cut off without any 
trouble. 
In the three-phase and three-leg core of the first embodiment of the 
invention, the steel sheets are prepared for each component part of the 
core on a mass production basis and they are free from scrap parts that 
should be cut off as waste material. Since the steel sheets for forming 
the upper and lower yokes 1, 2 each have a width greater than that of the 
steel sheets for forming the main legs 3, 4, 5, there increases the length 
of the diagonal joints between the upper and lower yokes and the main legs 
3, 4, 5, thereby reducing magnetic flux density. Since the reduction rate 
of iron loss is proportional to several powers of the magnetic flux 
density, losses and a rise in temperature in the joints are reduced. Thus, 
the three-phase and three-leg core according to the invention is more 
advantangeous than that of the prior art wherein the density of the 
magnetic flux must be reduced due to increased losses and a rise in 
temperature in the joints. Thus, in the present invention, it is possible 
to keep the magnetic flux density high in the main legs 3, 4, 5 and to 
reduce the number and size of the cooling ducts, in comparison with the 
conventional core. Thus, the space factor of the iron core of the present 
invention can be improved. This means that if the amount of magnetic flux 
remains equal to that of the conventional core, the main legs can have 
their diameters reduced, thereby making it possible to obtain an overall 
compact size in a three-phase and three-leg core and to reduce iron loss. 
Meanwhile, the steel sheets for forming the outer main legs 3, 4 are 
connected to those for forming the yokes 1, 2 at their joints in an area 
in which they are cut diagonally at 45 degrees, and no projections extend 
from the diagonal joints above and below the yokes 1, 2. So long as no 
special problems arise, the need to cut off the end portions of the steel 
sheets is eliminated. Moreover, since the width of the main legs 3, 4, 5 
is smaller than that of the upper and lower yokes 1, 2, the main legs 3, 
4, 5 are in contact with the yokes 1, 2 through the entire length of the 
cut of 45 degrees or less, so that losses in the joints can be minimized. 
In the embodiment shown and described hereinabove, the center main leg 5 is 
formed of steel sheets of the trapezoidal shape having longitudinal 
opposite ends thereof cut diagonally at an angle .alpha., as shown in FIG. 
6(d). It is to be understood that the same effects as achieved by the 
embodiment shown in FIGS. 5 and 6 can be achieved by the embodiment shown 
in FIG. 7 in which steel sheets, in the form of a paralleloogram angled at 
opposite ends thereof at .alpha. degrees, are used for forming the center 
main leg 5. In this construction, no trouble occurs in the production 
process because the difference between the embodiment shown in FIG. 5 and 
the embodiment shown in FIG. 7 is only in the point that the arrangement 
of the steel sheets for forming the lower yoke 2 in FIG. 7 is reversed at 
right and left from that in FIG. 5. 
FIG. 8 shows a still another embodiment of the three-phase and three-leg 
core in conformity with the invention in which steel sheets of a larger 
width and steel sheets of a smaller width are used in combination for 
forming the upper and lower yokes 1 and 2 and the main legs 3, 4 and 5, to 
obtain predetermined dimensions. This construction enables a core-type 
transformer of a large capacity with yokes 1, 2 and main legs 3, 4, 5 of 
large sizes to be readily produced without any trouble. 
FIG. 9 shows a further embodiment of the three-phase and three-leg core in 
conformity with the invention in which a gap G is formed in each of the 
upper and lower yokes 1 and 2 to allow insulating oil to flow therethrough 
for cooling the iron core. The gap G is formed by forming each of the 
yokes 1 and 2 with two steel sheets spaced with a suitable spacing 
therebetween. The provision of the gap G in each of the yokes 1 and 2 
enables a rise in temperature to be effectively suppressed in the core. 
From the foregoing description, it will be appreciated that according to 
the invention a three-phase and three-leg core of a core-type transformer 
is produced by using steel sheets of larger width for forming the upper 
and lower yokes 1, 2 and steel sheets of smaller width for forming the 
main legs 3, 4, 5, the steel sheets for forming the center main leg 5 
disposed between the outer main legs 3, 4 have their longitudinal ends cut 
at an angle less than 45 degrees and joined to the steel sheets forming 
the upper and lower yokes 1, 2 diagonally and at a right angle 
substantially through the entire surfaces, and the steel sheets for 
forming the outer main legs 3, 4, or the main legs disposed on the right 
and left of the center main leg 5 have their longitudinal opposite ends 
cut diagonally at 45 degrees and joined diagonally to the steel sheets for 
forming the yokes 1, 2 in an area in which the yokes 1, 2 are cut 
diagonally. By virtue of this construction, it is possible to reduce the 
magnetic flux density in the diagonal joints formed in the core, thereby 
reducing iron loss and avoiding a rise in temperature. Thus, the 
three-phase and three-leg core according to the invention can increase the 
magnetic flux density in the main legs as compared with the same type of 
core of the prior art. This is conductive to reduced dimensions of the 
main legs 3, 4, 5 and the yokes 1, 2 and reduced coils, thereby enabling a 
compact overall size to be obtained in a core-type transformer.