Note: Descriptions are shown in the official language in which they were submitted.
~2~3~
ENERGY SAVING
WOUND CORE TRANSFORMER
Field of Invention
.
T~is invention relates, in general, to electrical
transformers and, more particularly, to the magnetic core
structure of wound core transformers.
05
Background of the Invention
Present energy costs have forced utilities to take a
new look at the way in which they evaluate transformer losses.
Braunstein, "The Way You Buy Transformers Affects the Price",
Electrical Wvrld, July 1932, pg. 123. The evaluation of bids
for transformers is an involved study encompassing many facets
of engineering economics. When all factors are evaluated and
tabulated, it is common to find that the lowest-priced item
will cost more than the others in the long run. It is common,
for example, to find that transformer losses will cost more
over the life of the transformer than the original price of
the transformer. Chartier, "The Economics of Major Equipment
Evaluation", The Line 74-2 (1974) page 20. Thus, transformer
losses frequently be~ome the most significant factor in the
buying decision. A change in a few percent can spell the
.~
difference between a successful bid and a rejected bid.
The transformer in~ustry in the United States is
highly developed. There have been few major breakthroughs
over the last ten years or so~ This does not mean that there
is no need for improvement or that further improvement cannot
be made. However mature and sophisticated the design of
3~
transformers may be, the problem of finding an optimum design
is far from obvious. The engineer is often faced with con-
flicting alternatives and limited choices.
The design of successful commercial transformers
05 requires the selection of a simple structure or form, so that
the conductor coils and insulation are easy to wind and the
magnetic (iron) circuit is easy to build. At the same time,
the mean length of the coil windings and the magnetic circuit
must be short as possible for a given cross seGtional area so
that the amount of material required and the losses are kept
as low as possible. It is also desirable to operate at the
highest flux density consistant with low losses in order to
reduce the amount of iron and conductor. Two basic designs
have emerged from these considerations.
When the magnetic circuit takes the form of a single
ring encircled by two or more groups of primary and secondary
windings distributed around the periphery of the ring, the
transformer is termed a core-type transformer. When the
primary and secondary windings take the form of a common ring
which is encircled by two or more rings of magnetic material
distributed around its periphery, the transformer is termed a
shell-type transformer. One characteristic feature of the
shell-type transformer is the short mean length of the
magnetic circuit and the relatively long mean length of the
windings. Because of these differences in shape and form,
design ~lprovements to one are not necessarily adaptable
to the other.
As another example of the difficulty facing the
`transformer engineer, consider what might be done to reduce
the iron loss (no-load loss). ~he simplest solution is to
reduce the voltage and leave the physical design the same.
Since the iron loss will decrease approxima~ely as the square
of the voltage, one needs only to reduce the voltage by 5~ to
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~et a 10~ reduction in loss. However practical this solution
may be at first sight, one must realize that a 5~ reduction in
voltage is accompanied by a 5% loss in the effective trans-
former capacity. Furthermore, the load loss in percent of
05 this reduced rating has increased by 5~' Thus, one ends up
sacrificing transformer capacity and increasing the per unit
load loss by about half the percentage o~ reduction in the no-
load loss.
It is conventional wisdom that iron loss will vary
with the weight of the iron. ~husl another approach to lower
no-load loss is to change the physical design of the trans-
forme to reduce the cross-section of the iron core, while
increasing the number of conductor turns in the coil to keep the
flux density constant. If this is done, the core window
opening will have to be increased to accommodate the higher number
of turns, but there may still be a substantial decrease in
the weight of the core and the consequent no-load loss. The
conductor loss (or load loss), however, will increase with the
number of turns. For example, if one reduces the cross-
section of the core by 5~ and increases the turns in the coilby 5%, one can expect to obtain about a 5% reduction in core
loss, but at the cost of an increase of about 5% in the load
loss. Moreover, the reactance will increase nearly in pro-
portion to the square of the turns, or in this case by about
10%. Therefore, reducing the core section and increasing the
turns have the effect of: reducing the no-load loss by the per-
centage amount that the load loss increases; increasing the total
loss (if the load loss was originally greater than the no-load
loss); increasing the reactance; and decreasing the weight of
iron by approximately the same percentage amount that the weight
of conductor increases. Simply stated, low reactance does not
necessarily go with high load loss or low iron loss; similarly
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high reactance does not necessarily go with low load loss. In
fact, it is generally considered to be unreasonably expensive
to try to design for a low iron loss and low reactance in the
same transformer. Yet, low reactance is a real advantage in a
05 distribution transformer, because it is necessary to have the
lowest possible regulation in these transformers. One author
concludes that: If low iron loss is important, it will be
more economical to use as small a transformer as possible, and
to load it (high load loss) as heavily as possible using
forced cooling; and if load loss is important it may be more
economical to simply use a larger transformer (high no-load
loss). Bean, Trans~ormers for the Electric Power Industry,
McGraw-Hill Book Company, Inc. (1959)
However good this recommendation may be, the typical
]5 distribution transformer (particularly the pole-mounted
transformer in the 5-to-167-KVA range) is lightly loaded for
an appreciable portion of the 24-hour day. Because of this,
the loss in the core is a significant portion of the total
daily loss. Cores for these units are, therefore, designed
for low exciting current (low reluctance) and for reiatively
low core loss to minimize the operating cost. Fink, Standard
Handbook for Electrical Enqineers, Eleventh Edition, Section
10, Paragraph 158. Thus, for distribution transformers no-load
losses are important and a design that lowers core losses by
increasing the mass of iron in the core without increasing the
reactance and no-load losses is by no means obvious. It
should be equally clear that those prînciples which apply to
large power transformers do not necessarily apply to small
distribution transformers.
From the foregoing, it should be appreciated that
the design of transformers, and distribution transformers in
particular, still leaves room for improvement. An improved
distribution transformer which would allow one to reduce the
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P06-2639
core losses by an amount in excess of what the gain would be in
the size of the core, and ~ithout having a proportional effect
on the reactance of the transformer and without increasing the
load loss, would go far in achieving the ultima~e in a distri-
05 bution transformer. It would also ha~e the advantage ofsatisfying the continued long felt need, by utilities and
other purchasers of distribution transformers, in reducing
life-cycle costs. Finally, if this energy saving improvement
could be easily and readily adapted to existing transformer
designs, the improved transformer could be made available
quickly to customers, and without developing special e~uipment
or procedures, or extensive capital investment. Heretofore,
no one distribution transformer design, particularly one of
the wound core design, has been able to achieve these advan
tages and features in a simple construction.
Summ ry of the Inventio_
In accordance with the present invention a unique
laminated transformer core is described which features lower
core losses th~n that of a conventional design without a cor-
responding increase in load loss. In particular, a wound core
transformeL is described which includes a base-core formed
from two strips of magnetic material of generally uniform
thickness, and at least one filler strip. One of the strips
of the base-core is shaped in a substantially close~,
generally rectangular loop so as to define an inner surface
and an outer ~urface. The second strip of the base-core is
wrapped about the outer surface of the first strip in a sub-
stantially closed, generally rectangular loop with its ends off-
set aspaced distance from each other and from theendsofthefirst
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strip. The filler strip is also formed from magnetic
material. It is interleafed between the two strips forming
the base-core with the ends of the base-core strips disposed
along the base of the filler strip. An electrical conductor
is then wrapped about that side of the wound core opposite to
that of the base of the filler strip so as to form a coil.
Since that side of the wound ~ore containing the base of the
filler strip is thicker than that side of the wound core
surrounded by the coil, the magnetic induction formed in that
side of the wound core containing the base of the filler strip
is lower than that side of the wound core around which the
conductor is wrapped when the coil is energized. Preferably
the filler strip is of sufficiqnt length so that it occupies
the three sides of the wound core that are disposed outside of
the coil, whereby the three thickest sides of the GOmpOS ite
laminated wound core~which includes the base-core and the
filler strip)are disposed outside of the coil and the magnetic
induction formed in the three thickest sides is lower than
that of the remaining side when the coil is energized. In the
case of a wound core of generally rectangular cross section,
the filler strip, in its simplest embodiment, is U-shaped.
In another embodiment of the invention the filler
strip is formed from two strips of magnetic material of
generally uniform thickness which are wrapped about each okher
so as to form a filler strip of non-uniform thickness.
Specifically, the first strip is wrapped about itself so as to
form a substantially closed, generally rectan~ular loop having
its ends substantially overlapping eachother, with one of its
ends disposed opposite the center axis of the coil and with
its other end generally at right angles to the center axis of
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the coil. The second strip is wrapped about itself and the
first strip so as to form a substantially closed, generally
rectangular loop with its ends substantially overlapping each-
other, with one of its ends disposed opposite the center axis
05 of the coil and with its other end disposed generally opposite to
the other end o~ the first strip (i.e., generally at right
angles to the center axis of the coil on the other side of the
transformer core thereformed). In this embodiment the three
thickest sides of the transformer core formed by the base core
and the two filler strips are disposed outside of the coil and
the magnetic induction formed in the three thickest sides upon
the energization of the conductor or coil is lower than that
of the remaining side ti.e., the one around which the coil is
wrapped).
Significantly, it has been observed that despite the
insertion of extra magnetic material into the base core, the
percent reduction in core losses exceed the percent increase
in core material. Other advantages and features of the inven-
tion will become readily apparent from the following detailed
description of the invention, the embodiments thereof, from
the claims, and from the accompanying drawings.
Brief Description of the Drawinqs
Xn order that the invention maybe clearly understood
and readily carried into effect, devices in accordance there-
with will now be described by way oE example, with reference
to the accompanying drawings in which:
Fig. 1 is a perspective view of an electro-
magnetic induction apparatus, such as a wound-core trans-
former, having a winding and two wound iron cores which areformed according to my invention;
Fig. 2 is a cross sectional view of the trans-
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former illustrated in Fig. 1 as viewed along a reference plane
2-2;
Fig. 3 is a side elevational view of a repre-
sentative portion of one of the cores shown in Fig. l;
05 Fig. 3a is a diagramatic representation of the
filler strip shown in Fig. 3;
Fig. 3b illustrates still another embodiment;
Fig. 4 is a side elevational view of a portion
of the core shown in Fig~ 1, illustrating a second embodiment
of the invention;
Fig. 4a is a diagramatic representation of the
filler strips shown in Fig. 4;
Fig. 4b illustrates yet another embodiment; and
E~ig. 5 is a graph illustrating core losses vs.
voltage for an ordinary core and for one following the
principles of my invention.
Detail of the Preferred Embodiments
While ~his invention is susceptable of embodiment in
many different forms, there is shown in the drawings and will
herein be described in detail two preferred embodiments of the
invention. It should be understood, however, that the present
disclosure is to be considered an exemplification of the
principles of the invention and is not intended to limit the
invention to the specificate embodiments illustrated.
Turning to the drawings, Fig. 1, illustrates a
shell-type transformer 10 consisting of a coil winding 12
(shown in phantom to hetter illustra~e the unique concept of
the invention) and two laminated wound magnetic cores 14. It
should be understood that the cores 14 and coil 12 may be of
any cross section and shape. Here they are shown with a quad-
angular cross section - either square Gr rectangular havng
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rounded-o~f corners (See Fig. 2). The cores 14 form a closed
path around the coil winding 12 of the transformer 10. The
wound coil 12 as such forms a rectangular window 16 into which
two legs or sides 18 of the two rectangular cores 14 are
05 snugly fitted.
Turning now to Fig. 3, it should be understood that
each core 14 is formed from a plurality of individual strips
or laminations of suitable sheet material having high perme-
ability favoring flow of magnetic flux in a direction running
lengthwise of the strip. As many of these strips are pre-cut
to length as required for 'DU ilding the core of the desired
thickness and number of laminations. Each strip is of a
length sufficient for it to be wrapped about the coil 12
through one turn. In the embodiment illustrated in Fig. 3,
]5 the core 14 is formed from a base-core comprising five con-
centrically wound strips 20a, 20b, 20c, 20d, and 20e and from
an interleafed U-shaped filler strip 22. In Fig. 4, the base-
core 14' is formed from three concentrically wound strips
20a', 20b', and 20c' and from an interleafed pair of filler
2~ strips 22a', and 22b~o In Fig. 3, the ends of each strip form-
ing the base-core abutt one another with small gaps and with
ends of adjacent strips at a spaced distance apart from each
other. The abutting ends of each of the five strips forming
the base-core are disposed on one side (the right side accord-
ing to the orientation shown in Fig. 3) of the base-core and
staggered from each other so as to form an echelon. In Fig. 4,
the ends of each strip forming t~e base-core slightly overlap
eachother such that one of each strip abuts one end of the
adjacent strip (i.e., the outer end of strip 20a' abuts the
inner end of strip ~Ob'). J~st as in the embodiment
illustrated in Fig. 3, the ends of the strips forming the
base-core are on one side (the right-hand side according to
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the orientation illustrated in Fig. 4) of the base-core and
are staggered from each other so as to form an echelon.
In Fig. 3, the filler strip 22 is U-shaped so as to
define a base portion 24 and two oppositely disposed legs 26L
05 and 26U generally at right angles to the base portion 24. The
base portion of the filler strip 22 is on that side of the
base-core across which the ends of the indi~idual strips 20a,
20b, 20c, 20e, and 20d are staggered. Thus, a core 14 is
formed which has three sides which are thicker than the
remaining side. The thicker sides are disposed outside of the
coil window 16. Thus, when the coil 12 is energized, the
magnetic induction in the core 14 is lower in the three sides
disposed outside of the coil window. In Fig. 3b a sub-
stantially longer ~iller strip 22' is illustrated which
]5 achieves the same effect of a U-shaped filler strip 22. Thus,
the filler strip can be longer or shorter than the base-core
strips.
Turning to Fig. 4, each of the two filler strips
22a' and 22b' is formed from a strip of magnetic material
which is generally longer in length than the strips 22a', 22b'
and 22c' which form the base-core. Each filler strip 22a' or
22b' has its ends substantially overlapping each other with
one of its ends disposed along that side of the core to which
the ends of the base-core strips 20a', 20b'l and 20c' are dis-
posed (i.e., the right-hand side using the orientation of Fig.
4). The opposite end of each of the two filler strips 22a' and
22b' is disposed generally at right angles to that side of the
core 14' c~ntaining the overlapping ends of the base-core
strips (i.e., one end at the top and one at the bottom).
Fig. ~a is a simplified diagram of the manner in which
the two filler strips 22a' and 22b' are wrapped about each
otherO Since each filler strip has one end disposed along
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P06~2639
that side of the core (i.e., the right hand side) containing
the ends of the base-core strips, that side of the core 14'
thereformed and the two adjacent sides li.e., the upper side
and the lower side), are thicker than that side of the core
05 disposed within the coil window 18. Thus, when the coil 12 is
energized, the magnetic induction in those sides disposed out-
side of the coil window 16 is less than that of the side dis-
posed within the coil window. Fig. 4a diagrams the filler
strips 22a' and 22b' shown in Fig. 4. In Fig. 4b a one piece
filler strip 22' is illustrated having the same effect in
Yarying the thickness of the sides of the core thereformed.
Thus, one or two strips can be used for the filler strip.
In one specific embodLment, a wound-core was formed
from about 168 full str ~ teach 0.011 inches thick, grade M- p
]5 4) of oriented silicon~ and one filler strip of the same ~ /
material for ever~ 10 full strips much as that illustrated in
Fig. 3. The thickness of the leg without filler strips was
about 1.625 inches and the thickness of the leg with filler
strips was 1.78 inches. The core weight was 84 1/2 lbs. A
coil was wound around the leg without filler strips, the coil
was energized at various voltages, and the core loss was
measured with a precision wattmeter. The results are
illustrated in Fi~. 5 (i.e., the curve labeled "with extra
laminations'l). 100% rated voltage corresponded to an induc-
tion of 15.9 kilogauss in the side without filler stri~s.
Next, the filler strips were removed, reducing the core weight
to 80 lbs. Tests on the core without filler strips resulted in
the curve labe~ed "without extra laminations" in Fig. 5. It
can be seen that the use of filler strips ~ecreased the core loss
by three watts at 100% voltage, corresponding to 7.3~ reduction,
while the weight increased by only 536%. ThuS, a signifi-
cant reduction in core loss was achieved without increasing the
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P06-263~
size of the leg around which the coil is wound. Signifi-
- cantly, it was also discovered that the filler strips a~e
effective in reducing the core loss only if they are inserted
in the leg which contains the ends of the fu11 strips.
05 Apparently the extra reluctance of the gaps in the full strips
forces the magnetic induction into the filler strips.
From the foregoing, it will be observed that
numerous variations and modifications may be affected without
departing from the true spirit and scope of the novel concept
of the invention. For example, although two specific embodi-
ments of the filler strip have been illustrated and described
in detail, the filler strip may ass~me the form of a simple
flat lamination disposed on that side of the core opposite the
coil. It should be understood that no limitations with res-
pect to the specificate apparatus illustrated herein isintended or should be inferred. It is, of course, intended to
cover by the appended claims all such modifications as falls
within the scope of the claims.
