Method of making a ducted dry type transformer

The method making a dry type, air cooled transformer having a mitered magnetic core and a compression bonded coil with air ducts formed in the end portions of the coil. The coil on each of the core legs is generally rectangular in shape and comprises a plurality of layers of wound conductor with the conductor layers in the end portions of the coil being spaced apart to form a plurality of air ducts for the passage of cooling air therethrough. The conductor layers in each of the side portions of the coil are compressed and then bonded together in their compressed state by means of a heat cured adhesive coated on opposite sides of the sheets of insulation between adjacent layers. This compression bonding of the coil sides squares up the inner and outer surfaces of the coil so as to improve the coil and core space factors thereby allowing a smaller core. Conversely, compression bonding allows higher output power ratings to be achieved by packing added conductor material through the same size core window. The air ducts in the coil are formed by inserting temporary and permanent duct spacers in the coil during winding thereof, wherein the temporary duct spacers are located at the corners of the conductor layers. Following winding, the temporary duct spacers are loosened and removed from the coil thereby leaving only the permanent duct spacers in place.

CROSS REFERENCE TO RELATED APPLICATIONS 
The subject matter of this application is related to the following commonly 
assigned applications which were all filed on the same day with the 
respective disclosures being incorporated herein by reference: 
Ser. No. 512,735, filed July 11, 1983, Dry Type Transformer and Method of 
Making Same, Leo C. Rademaker, Philip J. Hopkinson, Noah D. Hay, Gordon M. 
Bell. 
Ser. No. 512,737, filed July 11, 1983, Ducted and Compression Bonded 
Transformer and Method of Making Same, Gordon M. Bell, Philip J. 
Hopkinson, Noah D. Hay. 
Ser. No. 512,736, filed July 11, 1983, Dry Type Transformer Having Improved 
Ducting, Noah D. Hay. 
Ser. No. 512,886, filed July 11, 1983, Transformer Having Improved Space 
Factor and Method of Making Same, Philip J. Hopkinson, Gordon M. Bell. 
FIELD OF THE INVENTION 
The present invention relates to a method for making a single phase or 
multiple phase electrical transformer of the dry type, that is, the 
transformer is not immersed in oil or another cooling medium, but is 
exposed to ambient air in use. More particularly, the invention relates to 
a method of forming the air ducts in the ends of the coil resulting in a 
transformer having improved cooling characteristics. 
BACKGROUND OF THE INVENTION 
In general, a transformer of the type disclosed in the present application 
is concerned comprises a magnetic core having a plurality of leg pieces 
and yoke pieces connecting the leg pieces to form a generally rectangular 
flux path surrounding a window. In the case of a three phase transformer, 
the magnetic core will comprise three leg pieces and four yoke pieces and 
will have two core windows. Supported on each of the leg pieces will be a 
coil having a high voltage winding and a low voltage winding each 
comprising one or more layers of aluminum or copper conductor wound around 
a coil window that is dimensioned to be mounted on the respective core 
leg. Electrical connections are made to the high voltage and low voltage 
windings to accomplish the desired step up or step down in voltage between 
the input and output. 
From the standpoint of cost, it is highly desirable to achieve an output of 
the transformer, which is typically expressed in kilovolt-amperes (KVA), 
with a minimum of material. The output in terms of kilovolt-amps from a 
transformer is defined by the following formula: 
##EQU1## 
Where f=frequency, hz 
B=flux density in the core, kl/in.sup.2 
J=current density in the conductor, amp/in.sup.2 
A.sub.1 =core window cross section, in.sup.2 
S.sub.1 =coil space factor within core window 
A.sub.2 =coil window cross section, in.sup.2 
S.sub.2 =core space factor within coil window 
Basically, the flux density is the amount of flux per cross sectional area 
flowing through the core, the current density is the amount of amperage 
per cross-sectional area flowing in the wound conductor in the coil, and 
the space factors are measures of the utilization of the space within the 
core and coil windows. More specifically, the coil space factor is a 
measure of the utilization of the space within the core window by the 
coil, and this factor is maximized when all of the available space within 
the core window is either conductor or layer insulation. The core space 
factor is the measure of the utilization of space within the coil window 
and would be maximized if all of the space in the window is occupied by 
the core leg and the core insulation. 
Since the frequency is established at 60 hertz, the KVA output per parts 
size of the transformer is maximized when the flux density, current 
density and space factors are maximized. Conversely, improvement in these 
factors will enable the physical size of the transformer to be reduced for 
a given KVA output rating because of better utilization of the magnetic 
core and coil material. 
A significant factor which limits the output of a transformer is the 
current density within the coil. Heat buildup inside the copper or 
aluminum conductor of a transformer dictates that a short circuit or 
severe overload such as fifty times normal current for two seconds and/or 
two times normal current for thirty minutes will cause the conductor to 
melt. In order to drive the current density as high as possible, it is 
necessary to conduct heat away from the conductor to the ambient so that 
the temperature of the conductor will stay within acceptable limits. As 
the cooling of the conductor within the coil is increased, the current 
density can be concomitantly increased thereby resulting in an increase in 
the KVA output of the transformer. 
Typical prior art dry type transformers are rectangular in shape with the 
conductor layers in the side portions in close overlapping relationship 
and most or all of the conductor layers in the end portions being spaced 
apart so as to form air ducts therebetween to permit air to flow through 
the conductor layers thereby conducting heat away from the coil. Although 
the temperature of the conductor within the coil end portions can be 
maintained at an acceptably low level quite easily due to the presence of 
the air ducts, there has been a problem in conducting heat away from the 
tightly wound layers in the sides of the coil. A portion of the heat is 
conducted inwardly to the core, which functions as a large heat sink, but 
the majority of the heat must be transmitted down to the air ducts in the 
ends of the coil for dissipation into the ambient air surrounding the 
coil. 
In order to space apart the conductor layers in the ends of the coil, duct 
spacers of various types have been used in the past. Basically, duct 
spacers are elongate elements made of a material which is not electrically 
conductive, such as a glass filled high temperature polyester. In oil 
filled transformers, there are a series of closely spaced duct spacers 
within each duct, and because the oil surrounding the coil is such an 
effective conductor of heat, the problem of providing sufficient breathing 
space within the ducts is not nearly the problem that it is in air cooled 
dry type transformers wherein maximum exposure of the conductor layers to 
air is such a high priority. In prior art dry type transformers, the air 
ducts in the ends of the coil are formed by inserting elongate duct 
spacers between adjacent conductor layers during winding of the layers, 
and by locating the duct spacers at the corners of the conductor layers so 
that as the next layer is wound thereon, it will be bent along the duct 
spacers to form corners and will be spaced from the preceeding layer by 
the duct spacers. Although locating the duct spacers at the corners of the 
coil is useful to space the end conductor layers the entire width of the 
coil, and to maintain the structural integrity of the coil after winding 
to prevent collapsing of the coil during further assembly of the 
transformer and during use, particularly under short circuit conditions, 
the corner duct spacers act as thermal barriers inhibiting the flow of 
heat from the sides of the coil to the air ducts in the ends. The heat 
generated within the tightly wound sides of the coil tends to flow along 
the conductor layers toward the cooler end portions of the coil and the 
corner duct spacers act to insulate the corner portions of the conductor 
layers from the ambient thereby maintaining the corners at relatively high 
operating temperatures, which impedes the flow of heat from the coil sides 
past the conductor layer corners. The inability to more efficiently 
conduct heat away from the transformer coil imposes a constraint on the 
maximum current density for the coil, thereby necessitating more conductor 
to achieve the same power rating. 
In order to realize better heat conduction away from the sides of the coil 
into the ducted end portions, there exists a need for an efficient method 
of forming cooling ducts in the ends of the coil. 
SUMMARY OF THE INVENTION 
The method according to one form of the present invention results in a 
ducting arrangement wherein there is more surface area conductor exposed 
to ambient air flowing through the air ducts, and better conduction of 
heat from the conductor and coil sides to the ducted end portions. As 
discussed earlier, prior art transformers of this general type typically 
provided a series of elongate duct spacers at the corners of the coil 
around which the conductor is wound. Although locating the duct spacers at 
the corners enables the transformer to be wound in a rectangular shape and 
enables the conductor layers in the end portions to be spaced along the 
entire width of the coil, confinement of the conductor in the corners by 
the duct spacers forms a thermal block which impedes the flow of heat from 
the tightly wound sides to the air ducts in the end portions of the coil. 
This impaired cooling of the transformer necessitates a lower current 
density limit thereby requiring more conductor for a given KVA rating 
which increases the cost of the coil. The larger coil also necessitates a 
larger core window and a larger core so that there is an increase in coil 
material as well. 
By eliminating the corner duct spacers and locating only one duct per coil 
in the center portion of the ducts and away from the corners, the corners 
of the conductors can be maintained at a lower temperature because they 
can immediately transmit their heat to the ambient air within the ducts. 
It has been found that locating a single duct spacer in the center 
portions of the ducts results in very minimal decrease in breathing of the 
ducts, yet is sufficient to maintain the structural integrity of the coil, 
even during short circuit conditions. It is the compression bonding which 
makes this possible by bonding the conductor layers together so that they 
cannot shift relative to each other either during subsequent assembly of 
the transformer or in use. Thus, the location of the duct spacers away 
from the corners of the coil made possible by the compression bonding 
permits heat generated within the coil sides to be conducted much more 
readily to the conductor in the ends of the coil and from there to the 
convection ambient air flowing through the ducts. 
In order to maintain the structural integrity of the coil, it is preferable 
that the duct spacers be aligned along respective lines intersecting the 
coil window, and preferably along a line intersecting the axis of the 
coil. 
In accordance with one form of the present invention, the ducts having 
improved air contact with the conductor are formed by placing temporary 
duct spacers at the corners of the conductor layers during winding and a 
permanent duct spacer between the temporary duct spacers, and then winding 
another layer of conductor on top of the duct spacers so placed. Following 
winding, the corner, temporary duct spacers are removed by loosening them 
so as to leave the permanent duct spacers in place. 
Compression bonding of the coil sides bonds together the conductor layers 
thereby maintaining the structural integrity of the coil even though the 
corner duct spacers have been removed. 
An object of the present invention is to provide a method for making a dry 
type air cooled transformer and a method for making same wherein the 
corner duct spacers which are emplaced during winding can be removed after 
winding of the coils so as to expose the corners of the conductors 
directly to the ambient air in the cooling ducts. 
A further object of the present invention is to provide a method for making 
a dry type air cooled transformer wherein temporary duct spacers emplaced 
in the coil during winding at the corners of the conductor layers can be 
easily removed after winding, thereby leaving only permanent duct spacers 
in place. 
A still further object of the present invention is to provide a method of 
making a dry type air cooled transformer wherein effective ducting of the 
coil can be achieved through an efficient, reproducible manufacturing 
process. 
In one form of the invention, there is provided a method for making a dry 
type air cooled transformer comprising forming a coil by winding a 
plurality of turns of conductor about a coil axis to form a first coil 
layer, placing first and second duct spacers over the first coil layer on 
one side of the coil, the duct spacers being spaced apart in the 
circumferential direction relative to the coil axis, and tightly winding a 
plurality of turns of conductor on the first coil layer over the duct 
spacers to form a second coil layer. The duct spacers space the second 
layer from the first layer to form an air duct on the coil side. 
Subsequently, third and fourth duct spacers are placed over the second 
coil layer direction over the first and second spacers, respectively, and 
a plurality of turns of conductor are wound on the second coil layer over 
the third and fourth duct spacers to form a third coil layer wherein the 
third and fourth duct spacers space the third coil layer from the second 
coil layer to form a second air duct. Subsequently, the first and third 
duct spacers are loosened and slid axially out of the coil thereby leaving 
the second and fourth duct spacers in place. The coil is subsequently 
placed on a magnetic core. 
In another form of the invention, there is provided a method of making a 
dry type air cooled transformer comprising forming a coil including a 
plurality of superimposed layers of wound conductor by winding a conductor 
around a form and placing duct spacers between at least certain of the 
layers during winding to place the certain layers apart thereby forming a 
plurality of air ducts extending through the coil, there being at least 
two duct spacers in each air duct. Then at least one duct spacer in each 
duct is loosened and removed therefrom while leaving a duct spacer in each 
of the plurality of ducts. The coil is subsequently mounted on a magnetic 
core. 
The invention also provides, in yet another form thereof, a method of 
making a coil for a dry type air cooled transformer having a plurality of 
superimposed layers of wound conductor surrounding a coil window and a 
pair of opposite end portions with a plurality of air ducts extending 
through the coil between certain ones of adjacent layers in the end 
portion. The method comprises winding the conductor about an axis around a 
form and placing a pair of elongate temporary duct spacers and a permanent 
duct spacer between each of the adjacent layers where an air duct is to 
occur, the duct spacers for a duct being placed on a layer already wound, 
and the adjacent layer is then wound on top of the duct spacers for the 
pertaining duct. After forming the coil, the temporary duct spacers are 
loosened and removed from the coil thereby leaving only the permenent duct 
spacers in place. 
In still another form of the invention, there is provided a method of 
making a coil for a dry type air cooled transformer wherein the coil 
comprises a plurality of superimposed layers of wound conductor 
surrounding the coil window. The method comprises the steps of winding a 
conductor around a form and placing duct spacers between at least certain 
of the layers during winding to space the certain layers apart thereby 
forming a plurality of air ducts extending through the coil in a direction 
generally parallel to the coil window, there being at least two duct 
spacers in each duct. Then, at least one of the duct spacers is loosened 
and removed from each of the plurality of ducts while leaving the other 
duct spacer in the pertaining duct. 
Still further, the invention, in another form thereof, provides a method of 
making a coil for a dry type transformer having a plurality of layers of 
superimposed wound conductor wherein adjacent layers on an end of the coil 
are separated from each other to form an air duct extending through the 
coil. The method comprises winding a first layer of conductor along a form 
about a winding axis, the first layer having a peripheral outer surface 
facing radially outward from the winding axis, placing an elongate 
temporary duct spacer on the outer surface of the first layer, the duct 
spacer extending generally parallel to the winding axis and having a 
fulcrum supported on the outer surface of the first layer, the spacer 
being rotatable about its fulcrum and placing an elongate permanent duct 
spacer on the outer surface of the first layer circumferentially spaced 
from the temporary spacer. Then, a second layer of conductor is wound 
around a first layer and duct spacers about the winding axis, the second 
layer tightly engaging the duct spacers and being spaced from the first 
layer by the spacers to form the air duct, the second conductor layer 
exerting inward force on the temporary spacer urging the temporary spacer 
to rotate about its fulcrum. During winding, the temporary spacer is 
restrained against rotating about its fulcrum, and then subsequently to 
winding the second layer, the temporary spacer is permitted to rotate 
about its fulcrum thereby becoming loosly received in the duct. The 
temporary duct spacer is then slid out of the coil, but the permanent duct 
spacer is left in place. 
In yet another form of the invention, there is provided a method for making 
a coil for a dry type air cooled transformer having an air duct therein. 
The method comprises winding a first conductor layer having a plurality of 
turns, placing elongate an permanent duct spacer and an elongate temporary 
duct spacer on the first winding, the duct spacers being spaced apart from 
each other, and then winding a second conductor layer having a plurality 
of turns over the duct spacers in the first layer, the duct spacers being 
tightly clamped by the first and second layers and spacing the first and 
second layers apart to form an air duct. Then the temporary duct spacer is 
loosened within the coil whereby it is no longer tightly clamped by the 
first and second layers, and the loosened temporary duct spacer is slid 
out of the coil with the first and second layers continuing to clamp the 
permanent duct spacer to retain it in the coil. 
The invention is also applicable, in one form thereof, to transformers 
employing annular ducting wherein duct spacers may also be provided 
between certain of the layers in the coil sides.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to the drawings, and in particular to FIGS. 1-4, a preferred 
embodiment of the transformer made by a method according to one form of 
the present invention is illustrated. The transformer 34, which is a three 
phase transformer, comprises a stacked lamination magnetic core 36 having 
three legs 38, 40, and 42 on which are placed three wound coils 44. 
Transformer 34 is housed within a cabinet having side panels 46 and 48, a 
back panel 50 and a base 52. Base 52 comprises side flanges 54 to which 
the sides 46 and 48 and back 50 are bolted or otherwise secured. A pair of 
support rails 56 are connected to base 52 by bolts 58 (FIGS. 3 and 4), and 
serve to raise base 52 off of the surface on which it is supported so that 
cooling air can flow beneath base 52 and upwardly through openings 60 
therein so as to cool transformer 34. 
Magnetic core 36 is mounted to base 52 by a pair of L-shaped core clamps 
62, that are connected to base 52 and rails 56 by bolts 64, nuts 66, 
washers 68 and resilient isolation pads 70. Bolts 72 extend through 
openings in core clamps 62 and clamp core 36 in place. It will be noted 
that core 36 and coils 44 are mounted in the lower portion of cabinet 51 
and in relatively close proximity to base 52 so that transformer 34 is 
exposed to cooler ambient air than would be the case if it were mounted in 
the upper portion of the cabinet, as in some prior art transformers. Leads 
108 and 110 from coils 44 are connected to bus bar 76, and bus bar 76 is 
grounded to base 52 by grounding strap 78 and to core clamp 62 by 
grounding strap 80. 
As is customary in prior art dry type transformers, coils 44 are provided 
with a plurality of taps to the outermost winding so that the transformer 
34 can be connected in a number of different configurations in use. Rather 
than welding terminals directly to the conductor, as is done in many prior 
art transformers, termination loops 82 may be formed in the conductor for 
coils 44 as the coils are wound. The termination loops 82 are twisted, the 
insulation removed and then dipped in solder so that connections to leads 
84 can be made directly by a simple nut and bolt assembly 86. A more 
detailed description of the formation of termination loops 82 will be 
provided at a later point. Termination loops 82 are located at selected 
positions on the outermost conductor layer in coils 44 so as to change the 
ratio of the input and output voltages. Although twisted termination loops 
82 have been used in the past on dry type transformers, they have not been 
used on compression bonded transformers as described in the present 
application. 
Leads 88 are the end portions of the actual conductors of the high voltage 
windings. In order to permit the user to make connections to transformer 
34 in a convenient manner, three bus bars 90 for the low voltage winding, 
and three bus bars 92 for the high voltage winding are provided. Bus bars 
90 and 92 are mounted to bus bar board 94, which is made of an 
electrically insulating material, and board 94 is connected to one of the 
upper core clamps 96 by support bars 98 and bolts 100. Upper core clamp 96 
and rear upper core clamp 102 are connected to core 36 by bolts 104 and 
nuts 106. Core clamps 96 and 102, like lower core clamps 62, compress the 
laminations in magnetic core 36 as bolts 72 and 104 and their respective 
nuts are tightened. Conductor ends 88 are connected to bus bars 90, leads 
84 from selected termination loops 82 are connected to bus bars 92, the 
ends 108 of the high voltage winding of coils 44 and conductor ends 110 of 
the low voltage winding of coils 44 are connected to ground bus bar 76. 
The user makes connections to bus bars 76, 90 and 92 by means of 
conventional terminals (not shown). Support bars 112 pass through slots in 
core clamps 62 and support the lower surface 114 of core 36. 
Referring now to FIGS. 6 and 7, magnetic core 36 will be described. FIG. 6 
illustrates a flat stacked laminated magnetic core 36 having a thickness 
which is determined by the number of lamination layers therein. The 
lamination layers are divided into a set 116 of odd numbered layers of 
which the single exposed layer 118 at the front of FIG. 6 is 
representative, and a set 120 of different even numbered layers of which 
layer 122 shown in FIG. 7 is representative. All of the layers 116 and 120 
are preferrably edgewise coincident in the sense that neither set has 
projections which protrude beyond the other set, or voids or recesses 
which do not extend out to the edge of the other set. Each layer comprises 
a plurality of separate sheets of laminations, and the presently preferred 
arrangement is four sheets per layer, although FIG. 6 has not been drawn 
with sufficient lines to show each individual sheet, for purposes of 
clarity of illustration. 
Referring to the front sheet 124 of the odd numbered layers 116, it 
comprises a center leg piece 126, and two outer leg pieces 128 and 130, 
which are identical to each other. The ends of leg pieces 126, 128 and 130 
are beveled or mitre cut at an angle of 45.degree. to their lengthwise 
dimension with the end tips or points cut off to produce square corners, 
such as corner 132 on leg piece 130, for example. Each of leg pieces 126, 
128 and 130 are made of conventional grain oriented magnetic steel wherein 
the grain orientation, which is also known as the favored magnetic 
direction, is along the longitudinal direction of each lamination. As is 
well known, magnetic steel of this type presents less reluctance to the 
magnetic flux in directions parallel to the favored magnetic direction 
than in directions transverse thereto. Such types of steel are well known 
so that further discussion of them is not necessary. 
Each sheet 124 in the odd numbered layers 116 also comprises two yoke 
pieces 134 and 136, which are identical and have their ends mitered or 
bevel cut at an angle of 45.degree. to their lengthwise dimension except 
that those bevel cuts are square notched on the same side of the piece or 
part as indicated at corners 140 so that the cuts do not extend in a 
straight line entirely across the ends of the pieces. Sheet 124 also 
comprises two yoke pieces 142 and 144, which are identical to each other 
and have one end 146 which is cut square and the other end 148 which is 
mitered or bevel cut at 45.degree. to the longitudinal direction of the 
lamination 142 or 144, which is a direction perpendicular to the 
longitudinal dimensions of leg pieces 126 and 128. Thus, the major portion 
of ends 148 is cut at a bevel relative to the favored magnetic direction, 
and ends 146 are cut perpendicular to this favored magnetic direction. In 
the case of yoke pieces 134 and 136, the major portions of both ends are 
beveled at 45.degree. to the favored magnetic direction. As indicated 
earlier, each of the odd numbered layers 116 comprises four such sheets 
124 comprising leg pieces 126, 128 and 130 and yoke pieces 134, 136, 142, 
and 144. 
FIG. 7 illustrates one of the sheets 122 of an odd numbered layer 120 and 
will be seen to comprise a center leg 150 and a pair of outer legs 152 and 
154, all of which are identical to each other. Each of leg pieces 150, 152 
and 154 have the major portions of their ends mitered or bevel cut at an 
angle of 45.degree. to the longitudinal dimension of the lamination, which 
is also the direction of the grain orientation. Yoke pieces 156 and 158 
are identical to each other and have ends 160 and 162, respectively, which 
are cut perpendicular to the direction of grain orientation, and comprise 
ends 164 and 166, the major portions of which are cut at an angle of 
45.degree. to the direction of grain orientation. Yoke pieces 168 and 170 
have their ends cut at beveled angles of 45.degree. to the longitudinal 
dimension of the laminations 168 and 170, which coincides with the 
direction of grain orientation. All of the laminations 150, 152, 154, 156, 
158, 168 and 170 of each of sheets 122 of the even numbered lamination 
layers 120 is made of a grain oriented magnetic steel commonly used in 
transformer manufacture. 
In stacking the laminations to form magnetic core 36, the leg and yoke 
pieces are arranged end to end so that they circumscribe two vacant core 
windows 172 and 174 which receive the sides of coil 44. As will be 
described at a later point, upper yoke pieces 142, 168 and 134, 160 can be 
assembled to legs 128-152, 126-150 and 130-154 after placement of the 
coils on core 36. 
Although a particular pattern of lamination arrangement has been 
illustrated, the invention is not limited to a magnetic core having this 
particular structure. In designing core 36, priority is given to obtaining 
the maximum area of mitered core joints between abutting laminations yet 
producing as little scrap as possible in stamping out the laminations. 
Although all of the core laminations are shown to have equal thickness and 
width, which results from their being cut from the same strip of magnetic 
material, it would also be possible to have a core whose legs are not 
equal in width in some applications. 
In operation, it will be seen that the core joints across which the 
magnetic flux flows are, for the most part, beveled so that the flux is 
not required to travel cross-grain in moving from one lamination to an 
abutting lamination. Although there are two butt joints in each sheet 124 
and 122, and portions of the other joints are along directions 
perpendicular to one of the laminations, the existence of butt joints has 
been minimized in a scrapless process for producing the laminations of the 
core 36. Core 36 is of such a design that the flux density can be driven 
to approximately 129 kilolines per square inch within acceptable noise 
levels. By increasing the flux density, the size of core 36 can be reduced 
yet achieve the same total flux which is necessary for the particular 
output of the transformer. 
Referring now to FIG. 5, the structure of coils 44 in the disclosed example 
will be described. Each of coils 44 comprises a low voltage winding 176 
comprising two layers 178 of either aluminum or copper conductor wound in 
a rectangular shape, and a high voltage winding 180 comprising four 
conductor layers 182 also wound in rectangular shapes and being made of 
either copper or aluminum. The conductors forming low voltage and high 
voltage windings 176 and 180 have ends 88, 110, and 108 which connect to 
bus bars 90 and 76 as illustrated in FIG. 1. Low voltage winding 176 is 
specifically made of a conductor having a larger cross sectional area 
because of its higher current carrying requirements, and the cross 
sectional shape of the conductor is often rectangular. The invention is 
not limited to transformers having rectangular conductors, however, but 
also covers smaller size transformers that utilize round cross section 
conductors. 
Conductor layers 178 and 182 are superimposed on one another about a coil 
window 184 within which the respective magnetic core leg 128, 152 or 126, 
150 is received. The geometrical center of coil window 184 is at the 
geometrical centers of the respective conductor layers 178 and 182, and 
this geometrical center is referred to in the present application as the 
coil axis 186. 
Postioned around the core legs 128-152, 126-150 and 130-154 are layers of 
electrically insulating sheets 188, which have single thicknesses on two 
sides and double thicknesses on the other two sides. The purpose of 
insulation 188 is to prevent a short circuit from developing between the 
innermost conductor layer 178 and core 36. Each coil 44 comprises a pair 
of opposite side portions 190 and a pair of opposite end portions 192. In 
side portions 190, conductor layers 178 and 182 are tightly packed 
together, whereas in end portions 192, the conductor layers 178 and 182 
are spaced apart, with the exception of the two outermost layers, which, 
like the layers in side portions 190, are wound very close together. 
Inbetween adjacent conductor layers 178, 182 in side portions 190 are 
positioned one or more sheets of electrically insulating material 194, and 
a sheet of this material 194 is positioned between the two outermost 
conductor layers 182 in both the side portions 190 and the end portions 
192. 
Core insulation 188 and conductor layer insulation 194 in the disclosed 
embodiment are preferably sheets of aromatic polyamide insulation 
material, which is available from the E.I. Dupont DeNemours Company under 
the trademark NOMEX 410. Both sides of the NOMEX paper insulation are 
coated with an adhesive, such as epoxy, that is B-staged thereon at a 
thickness of approximately 0.2 to 0.3 mil on each side. B-staged epoxy is 
epoxy which has been deposited on the NOMEX in a liquid form and the 
solvents driven off by heat so that the epoxy is left on the NOMEX sheets 
in a solid form but not completely cured. NOMEX sheets 194 between 
adjacent conductor layers 178 and 182 are coated on both sides with the 
epoxy material, but only the sides of the layers of the core insulation 
188 which face radially outward are so coated with the epoxy so that the 
inner sides do not adhere to the clamping fixture during the bonding 
operation, as will be described below. Insulation sheets 194 extend the 
full width of the conductor layers on the side portions 190 where there 
would be any possibility of conductor to conductor contact. 
As will be described in greater detail hereinafter, the B-staged adhesive 
on the NOMEX sheets 194 and 188 is utilized to bond together the conductor 
layers 178, 182 in the coil side portions 190. The conductor layers 178 
and 182 are tightly compressed together so that they and the insulation 
layers 194 are in a tightly packed condition. When the adhesive is cured 
by heating, it exerts retentive forces on the conductor layers to maintain 
them in their compressed state after the clamping forces are removed. The 
compression and subsequent bonding squares up the outer surfaces 196 of 
the coil side portions 190 so that the thickness of the coil sides 190 is 
smaller and the coils 44 occupy less space. This permits smaller core 
windows 172 and 174 so that core 36 may be made smaller thereby enabling 
realization of the benefit of the increased flux density benefits obtained 
by the mitered core design and reducing losses in core 36 so that the 
amount of magnetic steel and conductor for the same size output 
transformer can be reduced. Because of the much flatter outer coil 
surfaces 196, the coil 44 can be moved closer together in core 36 so that 
more of the space within core windows 172 and 174 is occupied by coil 
conductor thereby improving the space factor of the coil within the core 
window. This improvement in space factor produces an increase in output 
or, alternatively, enables a smaller transformer to be utilized for the 
same output, in accordance with the formula for transformer output 
discussed earlier. 
Compression bonding of the coil sides 190 also results in an improvement in 
the core space factor, that is, the utilization of the space within coil 
window 184 by core 36. Due to springback following winding, the inner 
surface 200 defined by the innermost conductor layer 178 tends to bow 
outwardly in the side portions 190 of coil 44, thereby producing a slight 
air gap between it and the leg of core 36. By squaring up this inner 
surface 200, the core space factor can be improved, thereby also resulting 
in an improvement in output characteristics. Compression bonding also 
assists in the transfer of heat from coil sides 190 to the ambient end to 
magnetic core 36. In uncompressed coils, there are slight air spaces 
between adjacent layers and the side portions of the coil, and these air 
spaces act as thermal barriers to the conduction of heat through the coil 
sides. By compressing and then bonding the coil sides 190, however, the 
sides 190 are compressed into a nearly solid block of conductor and 
insulation, which permits the more efficient conduction of heat both 
inwardly into core 36, which acts as a heat sink, and directly outwardly 
through the outermost conductor layer 182 to the ambient. As discussed 
earlier, an improvement in the ability to cool coils 44 results in an 
increase in the available current density which can be tolerated, thereby 
increasing output of the transformer. 
The layers of conductors 178 and 182 are spaced apart in end portions 192 
of each coil 44 to form a plurality of air ducts 202 therein. Air ducts 
202 extend completely through coils 44 in a direction parallel to coil 
axis 186. As will be noted, the two outermost conductor layers 182 are not 
spaced apart because adequate cooling can be achieved by virture of the 
outermost layer being in direct contact with the ambient completely around 
its periphery. Conductor layers 178 and 182 are spaced apart to form duct 
202 by a plurality of duct spacers 204, which are elongate stick-like 
members extending completely through coils 44 in directions parallel to 
coil axis 186. Each duct spacer 204, which is permanently retained within 
coil 44, is made of a high temperature polyester and glass fiber 
combination, and are generally H-shaped in cross section having a pair of 
spaced apart legs 206 joined by a connecting segment 208. The ends 210 of 
each of legs 206, which form elongate ridges, are the only points in 
contact with adjacent conductor layers 178 or 182 so that maximum exposure 
of conductor layers 178 and 182 to the ambient air can be achieved. 
Duct spacers 204 are preferably located at the centers of ducts 202 and are 
aligned along respective lines intersecting coil window 184. The alignment 
of duct spacers 204 is preferred because each spacer 204 supports the next 
outward spacer 204 against compression forces acting on coil end portions 
192, as would be the case under short circuit conditions. Although it is 
preferred that duct spacers 204 be located at the centers of ducts 202, 
they could also be located anywhere within the generally central portions 
of ducts 202 away from the corners 212 of conductor layers 178 and 182. 
Also preferably, duct spacers 204 are aligned along a single line 
intersecting coil axis 186, but again, this is not critical to the 
invention, but only a preferred arrangement. 
As discussed earlier, prior art dry type transformers typically have duct 
spacers located in the corners of the ducts so that the conductor, as it 
is wound, will be bent around the corner duct spacers thereby forming 
corners such as corners 212 illustrated in FIG. 5. By permitting the duct 
spacers to remain at the corner portions, however, thermal barriers are 
produced at the corners, which maintains the temperature of the conductor 
at the corners at a much higher level due to the insulating effect of the 
corner duct spacers. This prevents the conduction of heat along coil sides 
190 into the end portions 192, where it can be removed by cooling ambient 
air flowing through air ducts 202. In accordance with the present 
invention, however, duct spacers 204 are located inwardly toward the 
center portions of ducts 202 away from corners 212 so that heat can much 
more easily flow from coil sides 190, where the temperature is higher due 
to the compression of conductor layer 178 and 182, to end portions 192 
having cooling ducts 202 therein. It has been found that the presence of a 
single duct spacer 204 in each duct, if located inwardly away from corners 
212 has very little effect on preventing heat dissipation. Although a 
single duct spacer 204 in each duct 202 is preferred, more than one duct 
spacer could be used, but it is important that the additional spacers also 
be located inwardly away from the corners 212 of ducts 202. 
Insulation layers 194 extend at least to the point where the adjacent 
conductor layer 178 or 182 nearest coil window 184 is bent so that, when 
the adhesive cures, there will be some bonding of the layers together, 
although the bonding will not be effective as in the area of coil window 
184 where the compression forces during clamping are the greatest. 
Insulation layers 194 can terminate directly at the point where the next 
inner conductor layer 178 or 182 is bent, but may also extend further 
along the adjacent outer conductor layer 178 or 182 without substantially 
affecting the cooling of the conductor layers 178 and 182 in ducts 202. 
Compression bonding of coils 190 permits the prior art corner duct spacers 
to be completely eliminated in the final coils 44 because the bonding 
holds conductor layers 178 and 182 together in their wound shape and 
prevents one conductor layer 178 or 182 shifting relative to the others in 
directions parallel and perpendicular to coil axis 186. The function of 
center duct spacers 204 is to provide stuctural rigidity in directions 
normal to conductor layers 178 and 182 in coil end portions 192 during 
subsequent assembly of the transformer 34 and in use, particularly under 
short circuit conditions. Duct spacers 204 also serve to maintain proper 
spacing of conductor layers 178 and 182 within the ducted end portions 
192. 
Of course, the number of conductor layers 178 and 182 and the number of 
ducts 202 may vary depending on the size and particular design of the 
transformer 34. Furthermore, while a three phase transformer has been 
illustrated, the invention is applicable to other than three phase 
transformers. 
With reference now to the remainder of the figures, a method for making 
transformer 34 in accordance with one form of the invention will be 
described. Copper or aluminum conductor 216, which may be either round or 
rectangular in cross section, is wound on a rectangular winding form or 
mandrel 218, which is rotated in the direction indicated in FIG. 8. Form 
218 comprises a pair of end blocks 220, which are also driven in unison 
with form 218. End blocks 220 have provided therein a pair of center 
grooves 222 extending from the outer edges 224 substantially inwardly to 
winding form 218, and also four corner grooves 226, which also extend 
inwardly from outer edges 224 to form 218, and terminate at form 218 near 
the respective corners 228 thereof. Center grooves 222 are oriented 
radially with respect to winding axis 230, and are narrower than grooves 
226 for reasons which will be described hereinafter. 
As illustrated in FIGS. 8 and 9, conductor 216 is started on form 218, 
which is then rotated in the direction shown under the control of the 
person operating the winding machine. Once the innermost layer 178 has 
been wound, a temporary, corner duct spacer 232 is slid inwardly along 
grooves 226 in each of end blocks 224 into contact with the previously 
wound conductor layer 178. FIGS. 16, 17 and 18 illustrate the structure of 
temporary corner spacers 232 and the manner which they are retained in 
place during winding. Each corner spacer 232 is generally elongate in 
shape having a shank portion 234 and a pair of notched end portions 236. 
End portions 236 have a width dimension 238 between parallel sides 240 and 
242 which is substantially equal to the distance 244 between sides 246 and 
248 of the respective groove 226 so that duct spacer 232 will be locked 
against rotation about its longitudinal axis when it is slid in place 
within groove 226. 
Referring now to FIGS. 10 and 11, form 218 together with its end blocks 224 
is rotated slightly further and a permanent, center duct spacer 204 is 
slid into place along grooves 222, and a further corner duct spacer 232 is 
slid into place along its respective groove 226. Each of the corner duct 
spacers 232 is substantially identical to that just described, and are 
locked against rotation about their longitudinal axis by the capturing of 
their end portions 236 within grooves 226. As illustrated in FIG. 11, an 
insulation sheet 194 is then placed against the conductor layer 178 just 
wound on the side portion 190 of coil 44, and form 18 is further rotated 
to wind the next conductor layer tightly on insulation layer 194. 
In the disclosed example, at some time prior to winding of the coil, the 
insulation sheets 194, which are made of Dupont NOMEX 410 aromatic 
polyamide paper, are coated with an epoxy that is B-staged on both sides. 
The epoxy is a bis-phenol-A epoxy commercially available from the Sterling 
Chemical Company under the designation Y-663M. The epoxy is coated to a 
thickness of approximately 0.2 to 0.3 mil, and the solvents are driven off 
by heat so that the epoxy is left on the NOMEX sheets in a solid form, but 
not completely cured. It has been found that this epoxy is very compatible 
with the insulation on the conductor, which may be GEMIDE insulation 
produced by the General Electric Company, or other insulation materials, 
such as NOMEX wrap. 
Alternative bonding material is a polyamide-imide coating which is also 
B-staged on the NOMEX insulation. With this material, however, bonding is 
preferably accomplished by resistance heating of the conductor, rather 
than oven heating, as in the case of the epoxy bonding material. 
The present invention is not limited to a particular type of bonding 
material, and other alternatives may exist. 
Returning now to FIGS. 11 and 12 of the drawings, once insulation layer 194 
has been laid in place, form 218 is further rotated to wind the second 
conductor layer 178 thereon, two more corner duct spacers 232 are slid 
into place together with a permanent center duct spacer 204 and the 
conductor 216 is wound thereon. This operation is repeated as illustrated 
in FIG. 12, until coil 44 has been nearly completely wound, as illustrated 
in FIG. 13. Between each of the conductor layers 178 and 182 in coil side 
portions 190 there is inserted a sheet or sheets of insulation 194, and 
between all or some of the conductor layers in end portions 192, there are 
inserted center duct spacers 204 and temporary corner duct spacers 232. As 
can be appreciated, as conductor 216 is wound over duct spacers 204 and 
232, it will be spaced apart in coil end portions 192 so as to form air 
ducts 202. 
FIGS. 13, 14, and 15 illustrate the manner of forming termination loops 82 
in coils 44. Since these termination loops are normally formed in the 
outermost conductor layer 182, the entire coil 144 is wound up to the 
point of winding the last conductor layer 182 in the forward facing end 
portion 192 of coil 44. At this point, the rotation of form 218 has 
stopped and a loop 250 is formed in conductor 216 by means of a suitable 
tool, such as a hydraulically operated loop former, or the hand operated 
former 252 shown in FIG. 13. Such tools are only exemplary, however, and 
loop 250 may be formed by any suitable tool. If using a hand tool such as 
tool 252, when hand grip portions 254 are squeezed together, peg 256 is 
pulled in one direction and a loop is pulled between pegs 258. Form 218 is 
then further rotated to position the loop at the appropriate place on coil 
44 as shown in FIG. 14. A plurality of such loops 250 are formed in coil 
44, and after the coil is wound, loops 252 are twisted as shown in FIG. 15 
to form termination loops 82. The twisted portion 260 of each loop 82 
serves to prevent the loop 82 from untwisting and to provide an opening 
262 into which can be inserted a bolt 86 or other fastener for the purpose 
of connecting loop 82 to a lead 84 (FIG. 1). 
Referring now to FIGS. 16 through 20, the method to one embodiment of the 
present invention will be described. Each of the corner duct spacers 232 
has a longitudinal fulcrum point 264 which runs along its entire length, 
at least in the shank portion 234 thereof, so that spacer 232 is capable 
of rotation about fulcrum 264 in a direction generally indicated by arrow 
266 (FIG. 17). Fulcrum point 264 is supported either directly on form 218, 
as in the case of winding the second innermost conductor layer 178, or on 
a previously wound layer. Although no insulation is provided between 
layers of conductor and coil end portions 192, there may be some 
application of the present invention where insulation layers would be 
provided, in which case duct spacers 232 would pivot on these insulation 
layers rather than directly on the conductors themselves. 
As discussed previously, notched end portions 236 of duct spacers 232 are 
locked against rotation by virtue of grooves 226 in end blocks 224 so that 
the tendency to rotate duct spacer 232 in the direction indicated by arrow 
266 as the next succeeding conductor layer 182 is wound thereon is 
resisted. The next succeeding conductor layer engages duct spacer 232 at 
corner 268 and exerts a generally inward force thereon, and the spacing 
between two adjacent conductor layers, such as layers 178 and 182, is 
determined by the distance between fulcrum point 264 and corner 268 
projected in a direction parallel to the previously wound conductor layer 
178. 
As succeeding conductor layers 178 and 182 are wound, the spacing between 
adjacent layers provided by duct spacers 232 is maintained because they 
are all locked against rotation by grooves 248. Subsequent to winding and 
the formation of termination loops 82, however, end blocks 224 are moved 
apart as illustrated in FIG. 19, or one end block 224 is moved relative to 
the other, so that the notched ends 236 of duct spacers 234 are no longer 
captured within their respective grooves 226. This permits duct spacers 
232 to rotate in the direction of arrow 266 generally to the position 
shown in FIG. 18 where duct spacers 232 are now loosely received within 
ducts 202. As coil 44 is slid off form 218, corner duct spacers 232 can 
easily be slid out of coil 44 as illustrated in FIG. 20, yet the permanent 
center duct spacers 204 will remain in place due to the tension of winding 
exerting compressive forces on duct spacers 204. 
Although a particular form of corner duct spacers 232 has been illustrated, 
other arrangements could also be used to enable the corner duct spacers 
232 to be removed following winding. For example, duct spacers 232 could 
be expandable slightly in the dimension of their thickness during winding, 
and then relaxed or retracted following winding to enable removal. 
Moreover, even when using the technique of locking duct spacers 232 
against rotation and then permitting rotation as described above, the 
particular diamond shape is not necessary, and other shapes could be used 
yet still accomplish the same result. To enable coil 44 to be slid off 
winding form 218, winding form 218 is contracted as in prior art winding 
machines used for winding the coils of transformers. 
FIG. 21 illustrates coil 44 subsequent to winding and removal of corner 
duct spacers 232. It will be noted that the conductor layers 178 and 182 
are rectangular in shape in planes perpendicular to the axis 186 of coil 
44, and that air ducts 202 extend completely through coil 44. Although 
coil 44 is sufficiently tensioned to maintain center duct spacers 204 in 
place and to retain the shape of the coil 44, springback following the 
release of the tension which was on conductor 216 during winding will 
cause side portions 190 of coils 44 to bow outward as illustrated in FIG. 
21, thereby increasing the thickness dimension of the side portions 190 in 
the area of coil window 184. 
Before or after termination loops 282 are twisted, they are dipped in a hot 
salt stripping bath 274 that is agitated by an ultrasonic generator 276. 
Receptacle 278 holds a hot salt bath 280 having a composition which is 20% 
sodium hydroxide (NaOH) and 80% potassium nitrate (KNO.sub.3), which is 
operating at a temperature of 400.degree. Celsius. The liquid 280 is 
agitated by an ultrasonic generator 276, which speeds the stripping action 
of the hot salt. The hot salt removes the wire insulation on the aluminum 
conductor, and has proven to be an effective wire insulation stripper on 
esterimide, amideimide and LO imide. The advantages of the salt stripping 
is that no additional mechanical stripping is needed, and there is no 
significant attack on the magnet wire substrate. Furthermore, the reaction 
gases formed are non-toxic and non-corrosive. The reaction takes place 
with only water vapor being given off as a byproduct, and the bath 
decomposes into non-degrading nitrates, nitrites, carbonates and 
bicarbonates. 
One major problem with the burning of insulation off wire is that of the 
time necessary to accomplish the stripping. A major advantage of using the 
ultrasonic agitation with a fused salt bath is the decrease in the 
stripping time due to the ultrasonic cavitation in the molten salt 
creating a scrubbing action. This mechanical motion helps to remove the 
magnet wire insulation residues, because instead of simply permitting the 
salt to float away, the residues are mechanically removed. The second 
beneficial affect is the reactivity of the salt itself. The byproducts 
form on the insulation surface and act as contaminants, but the formation 
of water vapor, potassium nitrite and sodium bicarbonate as byproducts 
change the reaction site composition and act to retard the removal rate. 
Rapid and continuous elimination allows the base material to be wetted 
with the fused salt. The expected benefits of this process is less damage 
to the coil because of long heat exposure, faster stripping on the large 
magnet wire giving better utilization of equipment and possible stripping 
on copper substrates because of the faster reaction times thereby making 
copper oxidation less of a problem. 
Following stripping of the wire insulation, termination loops 82 are dipped 
into a bath 282 of molten solder to prevent oxidation of the wire 
substrate and to provide a good electrical connection with the leads (FIG. 
23). 
With reference to FIG. 24, the next step in the manufacturing process is to 
insert core insulation 188. The core insulation is preferably two sheets 
of NOMEX insulation 286 one of which is coated on its outer surfaces with 
the B staged epoxy or other bonding material described above in connection 
with the conductor insulation layers 194. 
Core insulation sheets 286 are bent in U-shapes as shown in FIG. 24 and are 
inserted in cores 44 prior to compression bonding. A feasible alternative 
is to use two uncoated channels and insert them when the coils are placed 
on the core 36. NOMEX insulation may be wrapped around the end turns 192 
in order to prevent electrical breakdowns over the edges of channels 286, 
and duct spacers (not shown) may be inserted between core 36 and end 
portions 192 of coils 44, if necessary to obtain clearance between core 36 
and coil 44. 
FIGS. 25 and 26 illustrate the clamping fixture 292 for compressing coil 
sides 190 prior to the bonding step. Fixture 292 comprises a pair of 
tapered form elements 294 having end bars 296 connected thereto. Form 
elements 294 are substantially the same length as the height of coil 44, 
so that when they are placed in overlapping arrangement within coil window 
184, end bars 296 will abut against the top and bottom of coil 44. The 
thickness of the assembled form elements 294 is approximately equal to the 
width of coil window 184. 
Once form elements 294 have been inserted into coil window 194, plates 298 
are placed over the ends of end bars 296 so that bars 296 enter slots 300 
in plates 298. Then, tie rods 302 are inserted into notches 304 in plate 
298 and locked into place by tightening nuts 306 thereon to form the 
assembled clamping fixture 292 shown in FIG. 26. 
Coil 44 and fixture 292 are placed in a hydraulic press 308 having bolster 
310 and ram 312. Ram 312 engages top plate 292 at substantially the center 
of coil window 184, and a pad or block 314 on bolster 310 engages lower 
plate 298, again in the area of coil window 184. Preferably, plates 298 
are wider than coil window 184 so that there is some compression of 
conductor layers 178 and 182 in areas beyond coil window 184. Hydraulic 
press 308 is then activated and coil sides 190 are clamped and compressed 
between tapered form elements 294 within coil window 184 and end plate 298 
at a pressure of approximately 500 pounds per square inch. Because end 
bars 296 are slidable within slots 300, end plates 298 can move inwardly 
so as to compress the conductor layers and insulation layers in coil sides 
190. This reduces the thickness dimension of coil sides 190 and tightly 
packs and compresses the conductor layers 178, 182 and insulation layers 
194 together. When proper compression has been reached, nuts 306 are 
tightened down to take up the clearance between them and end plates 298, 
and fixture 292 and compressed coil 44 are removed from press 308. By 
compressing coil 44, its overall thickness can be reduced to approximately 
75% of what it was prior to compression. 
Then, fixture 298 and compressed coil 44 is placed in an oven 320 
illustrated diagrammatically in FIG. 27. Coil 44 is heated at a 
temperature of approximately 160.degree. C. for approximately thirty 
minutes to cure the epoxy bonding material thereby permanently bonding the 
compressed conductor and insulation layers together. During heating, the 
adhesive, such as epoxy, first goes through a liquid stage so that it can 
make intimate contact with the conductor layers, and during subsequent 
heating cures to a final, solid state. As it cures, it bonds the NOMEX 
insulation 194 and conductor layers 178, 182 together. Alternatively, if a 
polyamide-imide coating is utilized, resistance heating of the coils to 
obtain temperatures of 220.degree. Celsius to 240.degree. Celsius in 
approximately sixty seconds drives off the remaining solvents and bonds 
the material. Fixture 292 is then removed from coil 44. 
FIGS. 28 and 29 illustrate diagrammatically the changes that occur in each 
of the coils 44 by virtue of the compression bonding process. As is 
illustrated, there is a slight spacing between adjacent conductor layers 
178, 182 and the insulation layers 194 so that the side portions 190 of 
coil 44 are in a relatively loosely wound state, although sufficiently 
tight to enable coil 44 to hold its shape. This is caused by springback of 
coil 44 following winding, and causes side portions 190 to bow outwardly, 
and the innermost conductor layer 178 to be slightly concave in a 
direction facing coil window 184, also due to a bowing out effect. 
FIG. 29 illustrates coil 44 subsequent to compression bonding wherein it 
can be seen that all of the conductor layers 178, 182 and insulation 
layers 194 in side portions 190 are in a tightly packed, compressed state 
so that the outer surfaces 196 of coil side portions 190 are essentially 
flat and squared off, and inner surfaces 200 of the innermost conductor 
layer 178 are also essentially flat thereby taking up substantially all of 
the clearance between it and core 36. 
After the bonding step, coils 44 are assembled to partially completed core 
36 as illustrated in FIGS. 30-32. Core 36 at this stage of the assembly 
process comprises three legs 39, 40 and 42, and the two lower yoke pieces 
144, 170 and 136, 158. Compressed and bonded coils 144 having core 
insulation 188 therein are placed over the core legs such that the legs 
enter the respective coil windows 184, and the compressed and bonded coil 
side portions 190 are disposed within core windows 172 and 174 as shown in 
FIG. 31. Then, upper yoke pieces 168, 142 and 134, 156 are stacked in 
place, and upper core clamps 96 and 102 (FIG. 3) are mounted in place and 
tightened so as to clamp core 36. Assembled transformer 34 may then be 
mounted to base 52. 
In view of the foregoing, it is apparent that a novel transformer 34 and 
method of making the same have been described meeting at least some of the 
objects and advantages set out herein, as well as others. It is 
contemplated that changes as to the precise arrangements, shapes, details 
and connections of the component parts of such transformer, as well as the 
precise steps and order thereof of such methods, may be made by those 
having ordinary skill in the art without departing from the spirit of the 
invention or the scope thereof as set out by the claims which follow.