Method of manufacturing wound transformer core

A transformer core is made by winding a strip of ferromagnetic material, such as amorphous metal or silicon iron, on a winding mandrel to form a first annulus and cutting once through this annulus to create a plurality of individual laminations which are then assembled in packets about a nesting mandrel of a smaller diameter than the winding mandrel to form a second annulus. Each packet consist of a predetermined number of groups of laminations, with the ends of each lamination group lapping each other to form a lap joint. The lap joints of each packet are arranged in staggered positions to create a repeating step-lap joint pattern confined within a predetermined joint region. By decreasing the lap joint dimension and increasing the number of groups in successively assembled packets, the increase in build of the joint region over that of the remainder of the second annulus is minimized. The completed transformer core uniquely characterized by its variable lap joint dimension and the absence of short sheets.

BACKGROUND OF THE INVENTION 
The invention herein disclosed is based upon work sponsored in part by the 
Electric Power Research Institute, Palo Alto, Calif. 
The present invention relates to transformer cores and particularly to 
transformer cores wound from a strip of ferromagnetic material. 
A wound core is the typical configuration utilized in high volume 
transformers, such as distribution transformer, as it is conducive to 
mechanized, mass production manufacturing techniques. Although equipment 
has been developed to wind a ferromagnetic core strip around and through 
the window of a preformed, multiturn coil to produce a core and coil 
assembly, the most common manufacturing procedure is to wind the core 
independently of the preformed coil or coils with which it will ultimately 
be linked. This means that the core must be formed with a joint at which 
the core laminations can be separated to open the core and thus 
accommodate insertion of the core into the coil window(s). The core is 
then closed to remake the joint. This procedure is commonly referred to as 
"lacing" the core with a coil. It is of course desirable from the 
standpoint of operating efficiency that the magnetic reluctance of this 
core joint be as low as possible. Moreover, the core joint should not 
unduly alter the distribution of the flux flowing through the joint 
region. 
One common type of wound core joint is the so-called step-butt joint 
wherein the ends of each individual lamination are butted together. Thus 
the plural laminations are all concentrically arranged. The positions of 
these individual butt joints are typically staggered throughout the core 
build, and thus the overall core joint has the appearance of flights of 
stairs, hence the term "step". While this type of core joint is convenient 
to produce, it results in relatively high core losses. Moreover, since the 
flux in each lamination, in completing its closed loop path, prefers to 
cross over into adjacent laminations rather than jump the high-reluctance 
air gap of its butt-jointed ends, the flux density in the joint region 
rises above the flux density existing elsewhere in the core. As a result, 
the core material in the joint region can become saturated since the most 
economical core design calls for the operating flux density to closely 
approach the saturation level of the core material in order to minimize 
the amount of core material required. In the case of amorphous metal 
cores, the joint configuration becomes a significant limiting factor, as 
the flux saturation level of amorphous metal is approximately 75% that of 
silicon iron. 
Another joint configuration commonly utilized in wound core constructions 
is a step-lap joint, wherein the ends of each lamination are lapped with 
each other. Again, the positions of these lap joints are typically offset 
or staggered repeating in stairstep fashion. This joint configuration 
produces an extra build-up in the cross sectional area of the core in the 
joint region, which appears as a bump. To avoid this bump, manufacturers 
have added a so-called "short sheet" to the core build each time the step 
pattern of lap joints is repeated. This short sheet is a partial length 
lamination having one of its ends butted with the overlapping end of the 
last lamination of one step pattern of lap joints and the other of its 
ends butted with the underlapping end of the first lamination of the next 
step lap joint pattern. The presence of these short sheets builds up the 
cross section of the rest of the wound core to equal the cross section of 
the joint region. With the presence of these short sheets, the plural 
laminations appear as a continuous spiral from the inside to the outside 
of the core. It is also characterized with lap joints of a constant lap 
dimension throughout the core. The step-lap core joint has a similar flux 
saturation limitation to that of the step-butt core joint in that the flux 
in the short sheets must cross over into adjacent, full length laminations 
in order to complete their closed loop paths. This crossover flux adds to 
the flux already flowing in these adjacent laminations and can drive the 
core material in the joint region into saturation. An additional drawback 
to this step-lap joint construction is the additional core material 
represented by the short sheets. In the case of amorphous metal cores, 
additional material is already required to compensate for its lower 
saturation level as compared with silicon iron, and thus a step-lap joint 
with short sheets implemented in amorphous metal represents a significant 
cost penalty for the sake of achieving the lower core loss characteristics 
afforded by this material. 
It is accordingly an object of the present invention to provide an improved 
wound transformer core. 
A further object is to provide a wound transformer core having a more 
efficient joint configuration. 
Another object is to provide a wound transformer core of the 
above-character having a step-lap joint wherein the extra build-up of the 
core cross section in the joint region is minimized. 
An additional object is to provide a transformer core of the 
above-character whose joint is configured such that the saturation level 
of the joint region is substantially equal to that of the remainder of the 
core. 
Yet another object is to provide a wound transformer core of the 
above-character which is constructed to make efficient use of core 
material. 
Another object of the present invention is to provide a method for 
manufacturing a wound transformer core of the above-noted character. 
Other objects of the invention will in part be obvious and in part appear 
hereinafter. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided an improved 
wound transformer core of a generally rectangular shape having four 
interconnected sides circumscribing a core window. The core sides comprise 
individually nested strips of a ferromagnetic material arranged in 
packets; each packet comprising a predetermined number of lamination 
groups, with each group consisting of at least one lamination strip. Each 
lamination group is arranged with its ends in lapped relation to form a 
lap joint. Within each packet, these lap joints are circumferentially 
offset by essentially butting together the ends of the immediately 
adjacent lamination groups to create a step-lap joint pattern which is 
repeated within each lamination packet. This repeating step-lap joint 
pattern is located in a joint region confined to one of the core sides. To 
economize on the amount of material in the core, partial length 
laminations or short sheets are dispensed with. The resulting additional 
build-up in the joint region is however minimized by decreasing the lap 
dimension of the lap joints and increasing the number of lamination groups 
in the lamination packets as the packet positions progress outwardly from 
the core window, thus reducing the number of lamination packets required 
to complete the core build. Thus, the core is characterized as having lap 
joints with varying lap dimensions from the inside to the outside of the 
core. The resulting wound core is less bulky and thus utilizes less core 
material, and the joint region thereof has a magnetic saturation level 
comparable to that of the other three core sides. 
To manufacture the wound core generally described above, a strip of 
ferromagnetic material, which can either be highly grain oriented silicon 
iron or amorphous metal, is tightly wound about a winding mandrel to form 
a first annulus. This first annulus is then cut through at one location 
along a single radial line to create a multiplicity of separate lamination 
strips which are then tightly formed about a nesting mandrel of a smaller 
diameter than the winding mandrel to produce a second annulus. In the 
process, these lamination strips are arranged in lamination groups and the 
lamination groups in lamination packets to create the above-described 
joint region consisting of repeating step-lap joint patterns. 
The second annulus is then formed into a rectangular shape and annealed to 
produce the four-sided wound core of the invention. 
The invention accordingly comprises the features of construction of an 
article of manufacture and the method step for manufacturing said article, 
all of which will be exemplified in the Detailed Description hereinafter 
set forth, and the scope of the invention will be indicated in the claims.

DETAILED DESCRIPTION 
Referring first to FIG. 1, the wound transformer core of the invention is 
produced by first tightly winding a strip 10 of ferromagnetic material, 
which may be highly grain oriented silicon iron but preferably is 
amorphous metal, on a winding mandrel 12 of a diameter 12a to create a 
first annulus 14. A suitable amorphous strip material is one marketed by 
Allied Corporation of Morristown, N.J. as its METGLAS Type 2605-S2 
material. Annulus 14 is then severed at one location along a single radial 
line 15 by a thin rotating cutting wheel 16 to produce a multiplicity of 
separate lamination strips 18 which fall into a stack, indicated at 19 in 
FIG. 1A. Preferable, annulus 14 is removed from mandrel 12 prior to its 
severance by cutting wheel 16. 
The cut laminations 18 are then tightly formed about a nesting mandrel 20, 
seen in FIG. 2, whose diameter 20a is smaller by a predetermined amount 
than the diameter 12a of mandrel 12, seen in FIG. 1, to create a second 
annulus 22. This nesting procedure may be performed manually or by 
suitable machinery, not shown. Consequently, the end portions of each 
lamination 18 are lapped with each other to create a lap joint, indicated 
at 24. In addition, the laminations are arranged into multiple packets, 
three of which are shown at 26 in FIG. 2. Each packet includes a 
predetermined number of laminations relatively positioned such that the 
overlapped end portion of one lamination is butted, as indicated at 25, 
with the underlapped end portion of the immediately adjacent, overlying 
lamination. Thus, the laminations within each packet are effectively 
arranged end-to-end in a coil or spiralled configuration about mandrel 20. 
The net result is that the lap joints 24 within each packet 26 are 
angularly offset to create a stairstep pattern, and thus the series of lap 
joints within a packet may be considered as constituting a step-lap joint. 
The laminations of the various packets 26 are arranged such that this 
step-lap joint pattern is repeated within each packet while being confined 
to a predetermined joint region 28 whose boundaries are essentially 
defined by lines 28a and 28b. 
Annulus 22 is then removed from mandrel 20 and formed into the generally 
rectangular shape of a typical wound transformer core, indicated at 30 in 
FIG. 3, by conventional means, not shown. Suitable annealing plates (not 
shown) are applied to the core, following which it is heated in a suitable 
oven at temperatures of about 360.degree. C. for approximately two hours 
while being subjected to a magnetic field in the presence of a nitrogen 
gas atmosphere. As is well understood, annealing acts to relieve stresses 
in the core material, including those imparted during the winding, 
cutting, lamination arranging and nesting, and core shaping steps. 
Following the annealing step, the step-lap joints in the joint region 28, 
which is seen in FIG. 3 to be confined to one of the four sides of core 
30, are separated to open the core and allow insertion of the core into 
the window of a preformed coil (not shown). The step-lap joints are then 
reclosed. The opening, inserting, and reclosing steps are often commonly 
referred to as "lacing" the core into the coil or coils. 
By referring to FIG. 3, wherein joint region 28 of annulus 22 of FIG. 2 is 
shown in enlargement, the arrangement of the laminations 18 into packets 
can be more clearly seen. While core 30 is depicted as including three 
lamination packets 26a, 26b and 26c, in practice the number of packets 
would be greater. Also more clearly seen in FIG. 3 is the lapping of the 
end portions of each lamination to create the individual lap joints 24 and 
the end-to-end butting relationship at 25 of the adjacent laminations 
within each packet. The extent of lamination end lapping is determined by 
the difference in the diameters of mandrels 12 (FIG. 1) and 20 (FIG. 2) 
and the relative space factors of the annuluses 14 and 22. As is well 
understood in the art, space factor is largely a function of the tightness 
at which strip 10 is wound to form annulus 14, the tightness at which the 
laminations 18 are formed about nesting mandrel 20 to create annulus 22, 
the surface smoothness of strip 10, and the uniformity of thickness of the 
strip from one lateral edge to the other. The transition from packet to 
packet is characterized by the presence of a pair of voids 32, one at the 
trailing end of the outermost lamination of one packet and the other at 
the leading end of the innermost lamination of the immediately adjacent, 
overlying packet. Normally, these voids are eliminated by the inclusion of 
a partial length lamination or "short sheet" in each packet-to-packet 
transition. As will be explained in conjunction with FIG. 4A, the presence 
of these short sheets causes an undesirable increase in the flux density 
within joint region 28, and thus short sheets are purposely avoided in 
core 30 of the present invention. 
Still referring to FIG. 3, it is seen that, due to the utilization of lap 
joints 24 at the ends of the laminations, there is an additional build-up 
of the core cross section in the side including joint region 28 as 
compared to the other three sides. This additional build-up increases the 
bulk of the core and represents additional core material and associated 
costs. While an increase in the core cross section in the joint region is 
unavoidable where lap joints without short sheets are involved, it is an 
important object of the present invention to minimize this increased cross 
section in the joint region relative to the other three core sides. It can 
be seen that each packet contributes to this additional build-up in the 
joint region by an amount equal to the thickness of one lamination 18. 
Thus, in the illustration of FIG. 3, the additional build-up of the joint 
region beyond that of the other three core sides is the thickness of three 
laminations 18. To minimize this additional build-up in accordance with 
the present invention, a fewer number of lamination packets are utilized 
in completing the core build. This is achieved by increasing the number of 
laminations 18 in the packets as their positions become more remote 
relative to core window 30a. Thus, as seen in FIG. 3, lamination packet 
26a includes five laminations, packet 26b includes six laminations, and 
packet 26c includes seven laminations. The inclusion of increased numbers 
of laminations in the outer packets is made possible because the joint 
region 28 can be of a keystone configuration, i.e., the length of the 
joint region can be expanded as it progresses outwardly from window 30a 
without conflicting with the corner regions. Also, by virtue of the 
additional build-up in the joint region, the extent of overlap of the end 
portions of the laminations, i.e., the lap dimension of the lap joints 24, 
progressively decreases from the innermost to the outermost packets, 
assuming the space factors of annuluses 14 and 22 to be substantially 
equal. In this connection, the diameter of the smaller nesting mandrel 20 
(FIG. 2) relative to the diameter of the larger winding mandrel 12 (FIG. 
1) is selected in order to achieve a minimum lap dimension of the lap 
joints in the outermost packet in the range of 0.3 to 0.5 inches. The 
number of packets utilized is selected in order to bring the maximum lap 
dimension of the lap joints in the innermost packet within the range of 
0.5 to 0.9 inches. 
It will be appreciated that in practice, the increase in laminations per 
packet may not be effected from packet to packet in uniform progression, 
as illustrated in FIG. 3. That is, the increase in the number of 
laminations per packet may be accomplished with every other packet or 
every third packet as the core build progresses outwardly from the core 
window. 
As indicated above, it is preferred that core 30 be formed of ferromagnetic 
amorphous metal. Amorphous metal in strip form, at present, is producible 
only in a very thin guage, nominally one mil thick. Silicon iron strips 
utilized in winding transformer cores are typically in the range of seven 
to twelve mils thick. Moreover, amorphous metal strip material is quite 
brittle and must be handled with extreme care to prevent chipping and 
fracturing during the core manufacturing process. As a consequence, 
amorphous metal strips are best handled in groups. Thus the laminations 18 
illustrated in FIGS. 2 and 3, are each comprised of a group of from five 
to thirty and preferable from ten to twenty amorphous metal strips or 
laminations, as indicated at 18a in FIG. 3. Reference may be had to the 
commonly assigned, copending Ballard et al. application entitled Amorphous 
Metal Transformer Core and Coil Assembly and Method of Manufacturing Same, 
Ser. No. 804,412, filed Dec. 4, 1985, which discloses a method for 
manufacturing a transformer core wound from an amorphous metal strip. It 
will be appreciated that if core 30 is wound with the thicker silicon iron 
strip, each lamination 18 illustrated in the drawings would typically 
consist of a single strip, although several such strips may be grouped 
together to form each illustrated lamination. 
To appreciate the benefits afforded by the present invention insofar as 
joint region flux density is concerned, reference is made to FIGS. 4A and 
4B. The former figure illustrates a core 40 constructed with a step-lap 
joint, generally indicated at 42, plus the inclusion of a partial length 
lamination or short sheet 44 in each packet-to-packet transition. It is 
seen that with the inclusion of these short sheets, the cross section or 
build of the core 40 is uniform throughout. The lamination 46 is a 
continuous spiral starting from the inside to the outside of core 40. 
Moreover, the individual full length laminations 46 together with the 
short sheets 44 are arranged in a continuous spiral throughout the core 
build. With regard to the flux flowing in these short sheets, it will be 
noted that this flux must cross over into the adjacent full length 
laminations 46 in order to complete its closed loop path between the 
widely separated ends of the short sheets. This short sheet flux thus adds 
to the normal flux flowing in these adjacent lamination, thus increasing 
the flux density in the portions of these laminations within the joint 
region. If the core 40 is operating close to the flux density saturation 
level of the core material, as is typically desired from a design economy 
standpoint, the addition of this crossover flux will cause the core 
material in the joint region to go into saturation. For example, in the 
case of a core 40 having seven lamination plus a short sheet in each 
packet 48, the flux density in the joint region is increased by the factor 
8/7 or 14%. It is seen that this presents a significant restraint on the 
allowable induction level of the core in order to avoid saturating the 
core material in the joint region. This situation is exacerbated where the 
core material is amorphous metal rather than silicon iron, since, as noted 
previously, the former has approximately a 25% lower flux saturation 
level. 
The same situation pertains in the core 50 of FIG. 4B which is illustrated 
as being constructed with a step-butt joint, generally indicated at 52. 
Thus, the laminations 54 are concentrically arranged with the two ends of 
each lamination in abutting relation. The flux flowing in each lamination 
crosses over into the adjacent laminations lapped therewith as this 
typically constitutes a lower reluctance path than the high reluctance of 
the inevitable air gap in the butt joint. This crossover flux increases 
the flux density in the joint region in the manner and substantially to 
the same degree as in the case of core 40 in FIG. 4A. 
As is readily seen in FIG. 3, there is no crossover flux in joint region 28 
of core 30 to increase the flux density therein. The flux flowing in each 
lamination 18 simply completes its loop path by flowing through the low 
reluctance lap joint 24 interconnecting its two ends, and thus has no 
tendency to cross over into adjacent laminations. Thus, core 30 of the 
present invention may be operated at flux density levels approaching the 
saturation level of the core material without fear of saturating the joint 
region. A more economical core construction is thus provided, since less 
core material is required to operate at optimum design levels of magnetic 
induction. 
The following table illustrates additional benefits (based on actual test 
results using model cores) of the present invention in terms of reductions 
in core loss (C/L) in watts/kilogram and exciting power (E/P) in volt 
amperes/kilogram at various levels of magnetic induction in teslas (T) for 
both silicon iron (SiFe) and amorphous metal (AM) cores. The various core 
loss and exciting power values for a core having a step-lap joint and 
short sheets, e.g., core 40 of FIG. 4A, and a core having a step-butt 
joint, e.g. core 50 of FIG. 4B, are expressed in per units of the 
corresponding values for core 30 (FIG. 3) of the present invention. 
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Mat'l Flux 
Core 30 Core 40 Core 50 
Density C/L E/P C/L E/P C/L E/P 
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SiFe 1.5T 1.0 1.0 1.05 1.11 1.09 1.18 
1.7T 1.0 1.0 1.07 1.49 1.10 1.67 
AM 1.3T 1.0 1.0 1.18 2.60 1.37 2.94 
1.4T 1.0 1.0 1.17 2.81 1.40 3.94 
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As can be readily seen from this table, the joint configurations of cores 
40 and 50 result in consistently higher levels of core loss and exciting 
power at the indicated induction levels, as compared to the joint 
configuration of core 30, and thus the latter offers rather dramatic 
improvements in these very important design parameters. 
It is thus seen that the objects of the present invention set forth above, 
including those made apparent from the preceding description, are 
efficiently attained and, since certain changes may be made in the above 
construction and method of achieving same without departing from the scope 
of the invention, it is intended that all matters contained in the above 
description and shown in the accompanying drawings shall be interpreted as 
illustrative and not in a limiting sense.