Core lamination for shell-type cores, preferably for transformers

The invention concerns a transformer lamination having a center leg, two outer legs and two yokes connecting these legs and having at least one joint between one side of the center leg and the adjacent yoke. Characteristic features are that the length of the joint at each side of the center axis is at least 0.75 times the center leg width and that the joint has at least one concave part and at least one convex part on at least one side of the center axis. A preferred embodiment contains two symmetrical joints, each of them comprising two sections which are parallel to the center axis and one section which is perpendicular to the center axis.

The invention relates to a core lamination for shell-type cores, preferably 
for transformers, repeaters, chokes and other similar devices, consisting 
of a plurality of alternately interleaved core laminations, which core 
lamination has a center leg, two outer legs parallel thereto at a certain 
distance, and two yokes connecting the ends of said legs, at least one 
joint being provided between one side of the center leg and the adjacent 
yoke for interleaving in the winding. 
In standard core laminations of this kind the width c.sub.1 of the 
jointlessly connecting yoke is equal to the width c.sub.2 of the parted 
yoke, and the sum of said two widths c.sub.1 and c.sub.2 is equal to the 
width f of the center leg. Transformers consisting of such core 
laminations do not make optimum use of the material, and their joints 
exhibit undesirably high reluctance. 
A catalog which was published by Kienle & Spiess GmbH, Grossachsenheim in 
1952, shows a core lamination section in which the joint extends within 
the yoke such that the center leg continues into the yoke by four-sixths 
of the yoke width. This does in fact reduce the reluctance of the joints 
as compared with standard cores, but it is still too high for exacting 
demands. German document laid open for inspection (DT-OS) No. 2,454,419 
refers to similar core lamination sections in which the joints extend 
within the yoke in such a way that the center leg continues into the yoke 
by three-sixths to five-sixths of the yoke width. 
To suppress the reluctance of the joints, it has been proposed, e.g. in 
German application print (DT-AS) No. 1,053,096, to use core laminations in 
which the yoke width amounts to 1.5 times or even 2 times half the width 
of the center leg and the joints represent a linear continuation of the 
center leg edges or extend diagonally from the inner window corners to the 
center of the outer yoke edge. However, yokes of such great width imply 
higher-than-normal material amounts and weights. Said application print 
also features a joint which extends within the yoke in such a way that the 
center leg continues well into the yoke. 
Core laminations have also been proposed, which have yokes approximately 
1.35 times wider than half the width of the center leg, with joints 
extending asymmetrically to the center axis. Yoke widths of this nature 
give excellent utilization of both grain-oriented and isotropic material. 
Besides, the reluctance of the joints in transformers consisting of such 
core laminations is extremely low when the core laminations in the stack 
comprise successive groups of four alternately interleaved layers. 
However, this interleaving process necessitates more expensive 
interleaving machinery than for core laminations interleaved 
conventionally in pairs of alternate layers. 
In contrast, the object of the invention is to improve the core lamination 
of the type named at the beginning in such a way that transformers or 
other kinds of induction devices that contain said core lamination unite 
the advantages of a small amount of material, high quality and ease of 
manufacture. 
According to the invention this object is achieved in that the sum of the 
width of the jointlessly connecting yoke and the width of the parted yoke 
is at least 1.25 times and max. 1.45 times the center leg width, in that 
the length of the joint at each side of the center axis is at least 0.75 
times the center leg width and in that the joint has at least one concave 
part and at least one convex part on at least one side of the center axis, 
relative to said center axis. 
At one time it was incorrectly thought that a jointless magnetic circuit of 
isotropic material was at its optimum when its cross-section was of equal 
size throughout. However, this opinion has been refuted by the subject of 
German patent No. 1,223,473 and by special publications made in connection 
therewith. In fact, a partly wound magnetic circuit reaches its highest 
performance per unit of material and also very low magnetic leakage and 
pick-up when its bare limbs are of greater width, i.e. when the yokes and 
outer legs of a shell-type core are wider than the covered center leg. 
Theory and practice demonstrate that when the joints are neglected, it is 
most beneficial -- with regard to both the reactive and active losses -- 
to increase the doubled yoke cross-section by approximately the factor 
1.25 to 1.45 as compared with the center leg cross-section. The optimum 
value for isotropic material is closer to the bottom limit whereas the 
best value for Goss grain-oriented material, which has its preferred 
direction of orientation parallel to the center leg, is closer to the top 
limit. With due allowance for the fastening holes, excellent values are 
roughly the factor 1.3 for isotropic material, and 1.4 for grain-oriented 
material. The range between these two values and particularly the value of 
approximately 1.35 are eminently suitable for universal cores of most 
standard materials. Another advantageous feature in this connection is 
also obtained when the width of the two outer legs together is roughly 1.2 
times to 1.3 times the width of the center leg. 
The shell-type core has to be capable of insertion in a finished winding 
for practical purposes. To this end, the core laminations forming the 
shell-type core need joints. It was commonly felt in the past that 
shell-type cores having said optimum yoke size increasing factor of 1.25 
to 1.45 have to put up with at least one of two disadvantages, either 
considerable reluctance due to the joints or increased expenditure on 
making the core from laminations arranged in groups of four alternately 
interleaved layers. The present invention achieves the surprising effect 
that it avoids both these disadvantages. 
Each joint in a shell-type core consisting of laminations arranged in pairs 
of alternately interleaved layers, overlaps once only. It is common 
knowledge that the ideal situation for singly overlapping joints is 
achieved when the entire joint is twice as long as the center leg is wide. 
However, more detailed investigations have demonstrated that the joint 
need not be so long in, for instance, mains transformers made of isotropic 
material. The saturation induction of currently common isotropic 
transformer laminations is roughly the factor .sqroot.2 above the 
induction at which the magnetization characteristic breaks down by a more 
or less severe degree. This is to be explained by the fact that the 
material consists of magnetic elementary regions which have to be 
magnetized chiefly in the (1,0,0) direction below the breakdown induction, 
yet also in the (1,1,0) direction above the breakdown induction and 
finally even in the (1,1,1) direction. In a shell-type core whose center 
leg is operated on the basis of said breakdown induction, the singly 
overlapping places thus reach the point of saturation precisely when the 
joint length is 2/ .sqroot.2 = .sqroot.2 times, and not 2 times, the 
center leg width. However, the operating induction of the center leg may 
be made moderately higher than the breakdown induction in shell-type cores 
which have yokes and/or outer legs wider than the wound center leg. This 
leads to the teaching disclosed by the invention, namely that in yokes 
with optimum increases in yoke size the joint length is to be moderately 
more than .sqroot.2 = 1.41 times the center leg width, namely 1.5 times at 
least. At a joint length of only 1.5 times the center leg width in 
conjunction with greater center leg induction, a small proportion of the 
magnetic flux can actually be passed through the joint itself. This can be 
accepted for not too exacting demands. 
The core laminations of the invention are designed primarily yet not 
exclusively for shell-type cores in which said core laminations are only 
ever arranged in pairs of two alternating layers. To keep the reluctance 
of the joints small on both sides, the invention teaches us not only that 
the entire joint length is to be at least 1.5 times the center leg width 
but that the joint length is also to be at least 1.5/2 = 0.75 times the 
center leg width on each side of the center axis in particular. As regards 
shell-type core laminations which have so far been made known and have 
yokes 1.25 to 1.45 times the width of the center leg width, the joint 
length is smaller than 0.75 times the center leg width on at least one 
side of the center axis. 
With regard to shell-type core laminations which have yokes equal in width 
to half the center leg width on both sides, a magnetic advantage is 
obtained when the path of the joint does not depart too much from the 
diagonal connecting the accompanying inner window corner to the center of 
the outer yoke edge. For if the joint of a core of such dimensions extends 
from the inner window corner by, for instance, a large amount as a linear 
continuation of the longitudinal center leg edge or perpendicular thereto, 
the magnetic flux -- in the yoke in the first case and center leg in the 
second -- will be forced partly away from the window corner, its density 
thus being increased locally. This is disadvantageous for grain-oriented 
material in particular. The invention shows that the situation changes 
when the yokes on both sides together are of substantially greater width 
than the center leg. 
Joints running along the diagonals connecting the inner window corners to 
the center of the outer yoke edge have a variety of disadvantages. For 
example, their length in a yoke that is not excessively wide is inadequate 
for very exacting demands. Besides, the flux in grain-oriented material 
must pass through the overlapping zone in the worst direction, namely at 
50.degree. to 55.degree. relative to the preferred direction of 
orientation. In addition, the sharp points at the yoke ends and center leg 
end are undesirable from the tooling point of view. Moreover, there are no 
semi- or fully-mechanical devices available for interleaving such 
laminations in windings. 
According to the conception underlying the invention it is important not 
only that the joints are of adequate length but also that the magnetic 
flux can reach and leave with little reluctance the overlapped joint that 
is to be crossed. This is not true of joints curved in one direction only 
-- in the event that such curvature is so great that the joint length is 
made substantially larger than that of straight joints. The example 
denoted by the short broken lines in FIG. 1 (these do not represent part 
of the invention) demonstrates that increases in flux density occur 
locally in the region of the yokes or center leg, so giving rise to 
greater reluctance. 
In contrast, the joints of the invention serve to minimize the reluctance 
both of the joints themselves and of the region upstream and downstream 
thereof in the direction of the flux; this also holds good for singly 
overlapped joints, that is when the core laminations in the shell-type 
core are interleaved in just two different layers. Thus, the joints of the 
invention afford a much greater advantage than might be expected from the 
resulting elongation of the joint. The joints of the invention would not 
give a substantial advantage or might not even give any advantage at all 
if the yoke widths were not made larger, as defined by the invention, than 
the center leg width. The unification of these elements thus gives an 
additional and surprising combination effect. 
An advantageous configuration of the invention is obtained when the width 
c.sub.1 of the jointlessly connecting yoke is greater than the width 
c.sub.2 of the parted yoke. Hence, less than 50% of the yoke material 
cross-section consists of parted core lamination yokes in the alternately 
interleaved core. Some of the magnetic flux emerging from the center leg 
can swing sideways along a shorter magnetic path into the jointlessly 
connecting core lamination yokes without travelling past a joint. It may 
be beneficial under these circumstances when the joint first follows a 
convex course -- relative to the center axis -- and then a concave course, 
starting from the inner corner of the accompanying core lamination window. 
It goes without saying that in the core lamination of the invention the 
width c.sub.1 of the jointlessly connecting yoke may also be the same size 
as the width c.sub.2 of the parted yoke. In this case it is very 
advantageous when a joint part which is concave relative to the center 
axis is followed by a convex part at a larger distance along the joint 
from the accompanying core lamination window corner. 
It is common practice nowadays, and also expedient in connection with core 
laminations according to the invention, to make the joint not as a plain 
parting but in the form of an air gap, for example as a hairline gap 0.05 
mm to 0.3 mm wide. This stops the end of the center leg from getting 
caught by the yoke, for this would be undesirable from the point of view 
of manufacture. When the joints are designed as air gaps, their minimum 
length and the suitable course thereof must meet very high requirements 
because the reluctance of the gap is proportional to its width. 
The joints according to the invention may be symmetrical or asymmetrical to 
the center axis. Asymmetrical joints in conjunction with groups of four 
alternately interleaved core laminations give three-fold overlapping of 
the joints in parts at least, which may be advantageous for grain-oriented 
material and very high demands. 
Joints of the invention provide good magnetic characteristics, for 
grain-oriented material too, when their overall length is at least equal 
to the length of the two diagonals running from the inner core lamination 
window corner to the center of the outer yoke edge. Said length is equal 
to .sqroot.4 c.sub.2.sup.2 + f.sup.2 when c.sub.2 denotes the width of the 
parted yoke and f the center leg width. Outstanding magnetic 
characteristics are obtained from joints according to the invention when 
their overall length is approximately equal to or even greater than 
2f-2(c.sub.1 -c.sub.2) where c.sub.1 is the width of the undivided yoke. 
In this case the flux density at the points of overlapping is not higher 
or not much higher than in the center leg, even when the joints overlap 
once only. Joints which are much longer, for instance larger than 1.1 
times or even 1.2 times 2f-2(c.sub.1 -c.sub.2), normally imply wastage and 
may even be disadvantageous. However, joints of such great length may be 
advantageous in cases where core laminations of different thickness and or 
different quality are used in the various layers of the shell-type core; 
for example, two oppositely directed core laminations of 0.35 mm thick, 
grain-oriented material for every core lamination of 0.75 mm thick, 
cold-rolled material. 
The core laminations of the invention may have two joints, each of which 
reaches the outer yoke edge, or a single joint within the yoke. In the 
first case, it is advantageous when each joint ends at a point on the 
outer yoke edge which is further away from the imaginary linear 
continuation of the longitudinal center leg edge on the same side than 
from the center axis, yet not reaching as far as the center axis itself. 
In the second case it is advantageous when the joint is straight in the 
region of intersection with the center axis, and when it is perpendicular 
to the center axis. 
The invention teaches that the joints are not to coincide along their 
entire length with the diagonals connecting the accompanying inner core 
lamination window corner to the center of the outer yoke edge. However, 
they may coincide partly with said diagonals. It is advantageous when the 
joints do not move too far away from the accompanying diagonals. For 
example, a beneficial configuration is obtained when, on at least one side 
of the center axis, no part of the joint is more than a quarter or much 
more than a fifth of the center leg width away from the accompanying 
diagonal line. As regards joints which are situated entirely within the 
yoke, it may be very advantageous when said distance is no more than 
one-sixth of the center leg width. It is advantageous to make the joints 
intersect the diagonal at least twice. 
The joints of the invention assume very favourable proportions when they 
comprise at least one section which is approximately parallel or 
approximately perpendicular to the center axis. Such sections help greatly 
to extend the joint and serve to promote magnetic flux in grain-oriented 
material. Sections of this kind may be joined, for instance, by sharp 
corners, by curves or short slanting portions. The more such successive 
sections there are, the closer the joint can get to said diagonal, e.g. in 
steps or meandering lines. However, too many sections of this kind do not 
afford any more advantages and may even give magnetic disadvantages in 
addition to manufacturing difficulties. For at the points where the joints 
overlap the magnetic flux has to diverge on both sides of the joint to the 
undivided lamination layers in a length of the order of 1 mm. This leads 
to local flux distrubances at each right-angled corner or each sharp bend 
of the joint, so adding up to a considerable overall effect. 
Very beneficial joints are those which contain, on at least one side of the 
center axis, one or two sections approximately parallel to the center axis 
and/or one or two sections approximately perpendicular to the center axis. 
A magnetic advantage is obtained, especially in core laminations with 
yokes of equal width on both sides (c.sub.1 = c.sub.2), when on at least 
one side of the center axis the joint forms, in the first fifth of its 
path from the accompanying window corner, an average angle of not much 
more than 45.degree. with the outer center leg edge; said angle is 
preferably between 0.degree. and 30.degree..

Further examples of core laminations according to the invention are 
obtained from FIG. 1 to 5, any side shown to the right of the center axis 
9 being complemented by the side shown to the left of the center axis 9 in 
any other of these five diagrams. 
The core lamination according to FIG. 5 is a preferred embodiment of the 
invention. It is of square design and consists of a center leg 1, two 
outer legs 2 and 3 parallel thereto at a certain distance, and two yokes 4 
and 5 connecting the ends of said legs, two joints 8a and 8b being 
provided between one end 7 of the center leg 1 and the adjacent yoke 4 to 
permit insertion in the winding not shown in FIG. 5. The sum of the width 
c.sub.2 of the parted yoke 4 and of the width c.sub.1 of the jointlessly 
connecting yoke 4 is larger than the width f of the center leg 1. 
Similarly, the sum of the widths b of the two outer legs 2 and 3 is 
greater than the width f of the center leg. The center leg 1, the outer 
legs 2 and 3 and the yokes 4 and 5 enclose the windows 10 and 11, being of 
length e calculated in the direction of the center axis 9. Since the width 
c.sub.1 of the jointlessly connecting yoke 5 is larger than the width 
c.sub.2 of the parted yoke 4 in this embodiment of the invention, the 
windows 10 and 11 are asymmetrical to the transverse axis 6. The outer 
edge parallel to the center leg 1 is of length a. 
The shell-type core according to FIG. 6 contains core laminations according 
to FIG. 5, which are interleaved alternately in two different layers. When 
both these opposing layers of equal numbers of core laminations are 
assumed to be of identical thickness, both yokes of the shell-type core 
have the same cross-section and this cross-section bears the relationship 
1/2 (c.sub.1 + c.sub.2)/f to the cross-section of the center leg of the 
shell-type core. The iner edges 12 of the jointlessly connecting yokes 5 
of the core laminations are practically touching the flanges 13 of the 
coil form 14 bearing the winding not shown here. In contrast, the inner 
edges 15 of the parted yokes 4 of the core laminations are spaced away 
from the flanges 13 of the coil form 14 by an amount larger by the 
difference between the core lamination yoke widths (c.sub.1 -c.sub.2). The 
inside length e.sub.K of the shell-type core window is shorter by the 
difference c.sub.1 -c.sub.2 between the core lamination yoke widths than 
the length e of the windows 10 and 11 of each individual core lamination. 
FIGS. 1 to 4 only show the parted yoke 4 of the width c.sub.2 of core 
laminations according to the invention. Said width c.sub.2 is greater than 
half the center leg width f. The width c.sub.1 of the yoke not shown here 
may be equal to, or different from, c.sub.2 ; it is advantageous to be 
greater than c.sub.2. Very beneficial and realistic relations are 
obtained, for instance, when c.sub.1 /f = 0.72 and c.sub.2 /f = 0.62, 
hence (c.sub.1 +c.sub.2)/f = 1.34, as is approximately true of the core 
lamination according to FIG. 5. 
Neither of the two joints denoted by short broken lines in FIG. 1 has both 
concave and convex portions, relative to the center axis 9. Hence, such 
joints do not form part of the invention. Although both these joints are 
longer than the diagonals denoted by long broken lines, they still do not 
represent a good solution. The left-hand one forms a very acute angle with 
the outer yoke edge 16, so being poor both magnetically and for the 
purposes of manufacture. The right-hand one is especially poor when the 
two yoke widths c.sub.1 and c.sub.2 are equal to one another, because some 
of the magnetic flux travelling close to the longitudinal center leg edge 
17 is forced away by the joint toward the center axis at a slanting angle. 
These disadvantages are avoided by the joints 8a and 8b according to the 
invention; these joints are denoted by solid lines in FIG. 1. Always 
viewing relative to the center axis 9, both joints 8a and 8b proceed from 
the accompanying window center 18, first following a concave and then a 
convex course. In the core lamination shown in FIG. 2 the right-hand joint 
8a first has a convex portion, then a concave section and a convex portion 
again. In FIG. 3, the convex portion of the joint 8 precedes the concave 
part on both sides of the center axis 9. In FIG. 4, the joint 8 has a 
convex portion between two concave parts on both sides of the center axis 
9. In FIG. 5 the convex portion of each joint 8a and 8b follows a two-part 
concave portion. The concave and convex portions of the joints 8, 8a and 
8b may be designed as corners, arcs or other kinds of curves. 
In the core laminations according to FIGS. 1, 4 and 5, the joints 8, 8a and 
8b according to the invention start at the accompanying window corner 18, 
representing a linear continuation of the longitudinal center leg edge 17 
and maintaining this direction exactly or roughly for a certain distance. 
this is very beneficial, especially for grain-oriented material, because 
the magnetic flux coming from the coil wound part of the center leg 1 can 
thus continue in the region of each inner window corner 18 without being 
disturbed much, partly retaining its direction and partly crossing at 
right-angles the overlapped joints 8, 8a and 8b. 
In the joints 8, 8a and 8b according to FIG. 4 and FIG. 5 the concave part 
is followed by a portion which is perpendicular to the center axis 9 and 
is roughly one quarter as long as the center leg width f. This affords, 
among other things, the advantages that the joints 8, 8a and 8b are long, 
that they intersect the imaginary diagonal (FIG. 1) and that they provide 
in the overlapping lamination layers the ideal crossing direction for 
grain-oriented material. 
In the joints 8, 8a and 8b according to FIG. 4 and 5, said portion is 
followed firstly by a convex corner and secondly by a section which is 
parallel to the center axis 9 and continues in FIG. 5 to the outer yoke 
edge 16 by a length amounting to roughly two-fifths of the center leg 
width f. The joints 8, 8a and 8b are thus made long and again intersect 
the imaginary diagonal. In the joint according to FIG. 4 there is one more 
portion consisting of a very shallow arc. This helps hold the yoke 4 
together mechanically. 
The right-hand joint 8a according to FIG. 2 and the joint 8 according to 
FIG. 3, proceeding again from the accompanying window corner 18, have a 
convex portion before the concave section. The convex portion of the joint 
8a in FIG. 2 is so shallow that it constitutes but little or no magnetic 
disadvantage, yet facilitates the operation of interleaving the core 
laminations in the winding. The joint in FIG. 3, that forms at the 
accompanying window corner 18 an angle of about 45.degree. relative to the 
center axis 9, is of a more serious nature. However, this slanting angle 
which is beneficial for interleaving the core laminations is of no 
disadvantage magnetically when the yoke width c.sub.1 is greater than the 
yoke width c.sub.2. A slight slant to facilitate the interleaving of the 
laminations is also provided in the joints 8a and 8b according to FIG. 5. 
In the core lamination according to FIG. 5 the joint corner 20, which is 
convex relative to the center axis 9, is designed in such a way that the 
distance at which it is set from the prolongation of the accompanying 
longitudinal center leg edge 17 is approximately half as large as the sum 
formed by the joint lengths between said corner 20 and accompanying inner 
core lamination window corner 18, and by the yoke width difference c.sub.1 
-c.sub.2. This sum is equal to the distance between the corner 20 of each 
joint 8a and 8b and the accompanying clear inner window corner 19 of the 
shell-type core, which distance is reckoned along the joint 8a and 8b and 
the subsequent portion of the longitudinal center leg edge 17. This means 
that in a shell-type core consisting of laminations interleaved in two 
different, opposing layers, the center leg 9 will not show any local flux 
density rises even when no flux at all crosses the actual gap. 
In the joints 8, 8a and 8b according to FIGS. 3, 4 and 5 the distances of 
the concave corners 21 of the joints 8, 8a and 8b from the center axis 9 
and outer yoke edge 16 are designed in such a way that between said 
corners 21 and the outer yoke edge 16 the flux density is reduced 
appropriately relative to the center leg 9 as a result of the joint 
lengths left between said corners 21 and the outer yoke edge 16 and center 
axis 9. The ideal flux density reduction factor f/(c.sub.1 +c.sub.2) is 
not quite reached here. However, the actual reduction factor of about 0.75 
to 0.8 is even adequate for very exacting demands. This is achieved in 
each of the two joints 8a and 8b according to FIG. 5 in that the distance 
between the outer yoke edge 16 and the concave corner 21 situated further 
away from the window corner 18 amounts to approximately 0.6 to 0.65 times 
the lengths of both joint portions left between said concave corner 21 and 
outer yoke edge 16. This relation applies analogously to that portion of 
the joint 8 in FIG. 3 and 4 that is to the right or left of the center 
axis 9. However, the joint lengths are then to be calculated to the center 
axis 9 and not to the outer yoke edge 16.