Note: Descriptions are shown in the official language in which they were submitted.
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TWO PART TRANSFORMER CORE, TRANSFORMER AND
METHOD OF MANUFACTURE
TECHNICAL FIELD
The present invention relates in general to power
transformation, and in particular to a two part transformer
core and transformers made with such a core.
BACKGROUND OF THE INVENTION
Transformers for distribution and power have improved
greatly in the last decade due to improved materials, and
sophisticated design tools for optimizing performance, cost
and size. Recent energy saving legislation in North
America commonly known as "Energy Star" in the USA and "C
802" in Canada drive the issues of cost and energy savings,
which has spawned significant developments in the art of
transformer design. Manufactures are faced with an ever-
increasing competitive market and stringent power
efficiency requirements for their products.
A large part of transformer costs is based on
material, such as the copper/aluminum (for windings) and
steel (for magnetic cores). Magnetic materials available
to the transformer industry have been designed for known
transformer topologies. The producers of 'soft magnetic
materials' for the transformer industry, have consequently,
made it difficult to realize new transformer topologies.
Transformers can take many forms. Some are applied to
single phase or three phase applications and others provide
a multitude of voltages and phases depending on the need
and application. Known transformer topologies can take
various forms, for example the most common single or 3-
phase transformers are classified as 'core type' or 'shell
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type' transformers. The core type transformer is
recognizable by external windings surrounding a magnetic
core, whereas a shell type transformer is recognized by a
core extending around a part of the windings.
Transformer size dictates the power handling capacity
of the transformer and its ability to dissipate transformer
generated heat produced as a result of transformer energy
or power losses. Usually, the two greatest loss components
are contributed by the resistive losses in the transformer,
hysteresis and eddy current loss in the core. A cooling
mechanism is needed to dissipate the heat maintaining a
thermal equilibrium of the transformer, as otherwise
'thermal runaway" occurs and the transformer fails.
Thermal runaway occurs when the energy or power losses
of the transformer produces more heat than can be
dissipated by the transformer. The ability to dissipate
heat of a transformer is a function of many things,
including: thermal resistance of the windings/core to a
cooling medium (e.g. oil or air), a dissipation constant, a
thermal coefficient of resistance of windings, core
properties, a thermal resistance of an electrical
insulation system used to electrically insulate the
windings, a physical geometry, and enclosure type, if used.
Transformers most commonly used in the power and
distribution industry are of 'dry type', i.e. where air is
used as the cooling medium. As such, cooling of these
transformers is predominantly performed by air passing
around the windings.
For this reason, prior art transformer designs include
portions of the windings and/or parts of the magnetic core
that protrude or are exposed to the surrounding air (or
other medium). This exposure to the medium permits the
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required cooling to prevent thermal runaway, and also
compensates for an imperfect optimization between steel and
copper content within available magnetic laminations or
strip steel assembly configurations. In dry type
transformers, the windings are normally configured to allow
air to flow between the winding layers thus effectively
increasing the cooling surface area. This is very wasteful
in terms of winding wire material content since winding
wire is expensive and can contribute to over half the total
material content. Also the exposure of the windings and
core brings about external leakage of flux. Furthermore,
the thermal transfer between the copper winding and air is
best when the winding is directly exposed to the air, but
cannot exceed a certain thermal transfer rate. Typically
20uW per mm2 per degree Centigrade rise.
In reality the minimum material content of
transformers are not materialized because of the thermal
dissipation requirements, and because the costs of
materials, practical constraints on construction methods,
etc. The toroidal transformer, which has the
characteristics of minimizing materials and magnetic
leakage losses, is generally the most optimum core type
transformer design currently available. However, toroidal
transformers cannot be easily configured into 3-phase
transformers where portions of the core can share and
partially cancel magnetic flux vectors.
The technical challenge in designing transformers is
only exacerbated with increase in power losses due to the
winding current. Larger power transformers produce more
heat. The relationship between dissipation and temperature
rise as a function of transformer dissipating surface area
is not a linear function, and below a certain critical
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surface area, losses and temperature rise vs. winding
current increase exponentially. This critical surface area
is a constraint on the size of the transformer.
Furthermore, as cores get larger the ratio of surface area
to volume of material decreases, thus the capacity to
dissipate heat becomes more of a problem for a certain
dissipation per cubic meter. In high power transformers
cores can be large enough to cause very high temperature
rises inside the core causing dimensional distortion and
mechanical stresses that affect magnetic properties of the
core. Also, for very large transformers, the core heat
affects the winding adjacent to the core requiring extra
spacing to cool the core and winding. This further
decreases efficiency of the transformer, and increases
material costs, noise and vibration of the transformer.
Accordingly, a topology for a transformer is required
that can reduce material costs, improve efficiency, or
provide a compact arrangement with acceptable thermal
dissipation for a given power requirement.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide
an improved transformer topology for providing step-up or
step-down voltage transformation for the electrical
distribution and power industries.
It is a further object of the invention to provide a
transformer with improved efficiency, and a more compact
arrangement using less and/or lower cost materials, in
comparison with standard known transformers.
In accordance with the invention there is provided, in
accordance with an aspect of the invention, a transformer
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comprising: a two-part core composed of a magnetic
material, including a toroidal piece having an inner wall,
and an outer wall, and a shell piece having an inner wall
and an outer wall, the toroidal piece being concentrically
disposed within the shell piece; and at least two windings
disposed in a space formed between the outer wall of the
toroidal piece, and the inner wall of the shell piece.
The invention further provides a method for designing
a transformer of a given power for a predetermined
application, the method comprising: selecting dimensions of
a toroidal piece and a shell piece of a two-part core to
provide balanced magnetic flux paths on either radial side
of a space between the toroidal piece inserted within the
shell piece, which space houses at least two windings;
solving equations 1 and 2 to compute a surface area of the
core required to ensure thermal equilibrium of the
transformer under specified operating conditions:
equation l:
~, - (RT,, . A . 25.4z . f3 + 1) {[(20 - tomb )Q.' -1]IPZ . Ro - PcoreLoss } -
Iamb ' A ~ 25.4z . ,f3)
(Ro .cz.RT~, .IPZ -1)A.25.4z . j3+Ro .a,IPz
equation 2:
~Rth ~(A 25.4z~(i~ + 1'~~(20 - t_amb )~a - I~~IZ~Ro - Core loss'
[+t amb~~A-25.4z~(3)
Pd = IZ~Ro~ 1 + a~ + t_amb - 20 + Core_loss
~Ro a~Rth~Iz - 1~~A~25.42~(3 + IZ~Ro~a
wherein A represents an area of vertical heat dissipative
surface of the two-piece core (square inches), a
represents the temperature coefficient of a resistance of a
particular winding, j3 represents a dissipation constant of
the two-part core (uWImm2/°C), IP represents a total
current referred to the windings, P~o,ecoss represents total
losses contributed by the core, Pp represents a power
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dissipation of the transformer, Ro represents a total
resistance of the windings referred to a particular
winding, RTh represents a thermal resistance in (°C/W)
between the windings and an external cooling medium, t
represents temperature, and tamb represents the ambient
temperature of the cooling medium; and providing cooling
fins in thermal contact with the two-part core for
providing the required effective surface area of the
transformer.
The invention likewise provides a method of
manufacturing of toroidal transformer, comprising: winding
a strip of magnetic material around a spindle to form a
toroidal piece having an inner wall and an outer wall; heat
annealing the toroidal piece and removing a sector there
from; applying a layer of insulation to the outer wall of
the toroidal piece; winding a primary winding over the
insulation on the toroidal piece; applying a layer of
insulation over the primary winding; winding a secondary
winding over the insulation applied to the primary winding;
applying a layer of insulation of the secondary winding;
winding a strip of magnetic material over the insulation
applied to the secondary winding to form a shell piece
having an inner wall contacting the insulation applied to
the secondary winding and an outer wall; heat annealing the
shell piece, and removing a sector there from.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the present
invention will become apparent from the following detailed
description, taken in combination with the appended
drawings, in which:
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FIG. 1 is a schematic drawing of a core topology in
accordance with an embodiment of the invention;
FIGs. la and lb are cross-sections of FIG. 1 taken
along lines AA', and BB', respectively;
FIGS. 2a-f is a schematic illustration of a method of
manufacturing a two-part core for a transformer in
accordance with an embodiment of the invention;
FIG. 3 is a more detailed, partially exploded
illustration of a transformer in accordance with an
embodiment of the invention;
FIGS. 4a and 4b are exploded and assembled views of a
multi-phase transformer consisting of three axially aligned
transformers in accordance with the invention;
FIGS. 5a, 5b and 5c are three embodiments of
transformers provided with cooling members; and
FIGs. 6a is an exploded view of a sealed transformer
in accordance with the invention; and
FIG. 6b is an exploded view of the transformer shown
in FIG. 6a in an assembled condition.
It should be noted that throughout the appended
drawings, like features are identified by like reference
numerals.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides a topology for a
transformer, transformers and methods of designing and
constructing a transformer to produce efficient
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transformers with significantly lower material costs,
improved efficiency and improved thermal dissipation.
FIGS. 1, la and lb schematically illustrate a
transformer topology in accordance with an embodiment of
the invention. The transformer topology shown includes a
two-part core 10, consisting of a shell piece l0a and a
toroidal piece lOb made of a magnetic material, such as
magnetic steel. Both the shell piece l0a and the toroidal
piece 10b are hollow cylindrical pieces, with an inner
radius of the shell piece l0a being greater than the outer
radius of the toroidal piece 10b, although it will be
appreciated that other shapes of the shell piece 10a and
toroidal piece 10b are equally possible if they permit the
toroidal piece to be disposed concentrically within the
shell piece lOb providing a space between the two. Because
the toroidal piece lOb is concentrically disposed within
the shell piece 10a, the space 12 is formed between an
outer wall 14 of the toroidal piece lOb, and an inner
wall 16 of the shell piece 10a.
The toroidal piece lOb has an inner wall 18 that
defines an inner cooling duct 20 for the transformer. As
shown, the inner cooling duct 20 may be a cylindrical
opening; however it will be appreciated by those skilled in
the art that other shapes for this opening are possible.
The primary function of the cooling duct 20 is to permit
the cooling of the toroidal piece 10b, by increasing a
surface area of the two-part core 10.
Dimensions of the shell piece l0a and toroidal
piece lOb are preferably chosen to compensate for the fact
that while the flux passing through the toroidal piece 10b
equals the flux passing through the shell piece 10a, the
flux density of the toroidal piece lOb is equal to that of
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the shell piece 10a because of the cross-sectional area of
the shell piece l0a with respect to the toroidal piece lOb.
The compensation is effected by providing a radial
thickness of the toroidal piece lOb that is greater than
that of the shell piece 10a. In this manner an area of the
flux path through the shell piece 10a is equal to that of
the flux path through the toroidal piece lOb.
Mathematical optimization techniques can be used to
derive the optimum dimensions of the two-piece core for a
given power rating of the transformer, and the associated
temperature limit. In this optimization an assumption is
made that all heat produced by the windings is passed
through the core structure to the surrounding air. Results
obtained by this optimization clearly demonstrate that the
quantity of material employed by this transformer topology
is substantially less than a core-type or shell-type
transformer for the same losses and temperature rise.
The space 12 is of a dimension to receive at least two
windings. The windings are disposed between the shell
piece l0a and the toroidal piece lOb separated only by any
required insulation. This is shown in FIG. 1a. The outer
wall 14 of the toroidal piece lOb is covered with a layer
of suitable electrical insulator 22a, over which a primary
winding 24 is wound. The primary winding 24 is insulated
with a layer of the electrical insulator 22b, over which a
secondary winding 26 is wound. A third layer of the
electrical insulator 22c provides a dielectric barrier
between the shell piece l0a and the secondary winding 26.
It will be evident to persons skilled in the art that
at least one aperture is required either through the shell
piece 10a, through the toroidal piece lOb, or elsewhere for
permitting terminals of the windings to pass from the
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space 12 to an exterior of the transformer. This aperture
may be provided in any suitable manner, and may be provided
by a yoke used to cap opposite ends of the two-part core,
or may be provided by both the yokels) and caps in the
shell piece l0a or toroidal piece lOb.
As a thermal model, the total heat flow capacity from
the windings 24, 26 to the outside cooling medium is far
greater than if the windings are exposed to air alone.
This is for two reasons:
~ the surface area of contact between the windings 24,26
and the two-part core 10 (along the outer wall 14 and
inner wall 16) is sufficiently large to permit heat
conduction to be superior to the air convection
processes of regular transformers; and
~ the thermal flow is radial so the effective cooling
surface area exposed to the ambient cooling medium is
larger than if the windings were directly exposed to
air.
As steel is a much better conductor of heat than air,
the transfer of heat from the winding to the core is more
effective, and as the radiative outer surface of the two-
part core is of a much greater surface area than the
windings, there is also improved heat dissipation from the
core with respect to the ambient medium.
Another thermal advantage of this transformer topology
is that inexpensive cooling fins can be added to an outer
wall 32 of the shell piece 10a, and/or to the inner
wall 18. Such cooling fins are in thermal equilibrium with
the two-part core 10, and can dramatically increase a
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surface area of the core for cooling purposes, to further
augment the heat dissipation capability of the transformer.
The electrical insulation layers 22 present an
impediment to the radial thermal conductivity of the two-
part core that induces a corresponding temperature rise
within the transformer. In general, the temperature rise
within the transformer follows formula 1.
Formula 1:
~,' (RTE ~A~25.42 ~/3+1){[(20-tp",6)a-1]IPZ ~Ro-PCoreGoss]-la~n'A'25.4z ~/3)
(Ro ~cz-RTh ~Ip2 -1)A~25.42 ~~3+Ro ~a~lpZ
where: A represents the vertical dissipating surface
(square inches); a represents the temperature coefficient
of the resistance of the windings; ~i represents the
dissipation constant of the core (uW/mm2/°C); IP represents
the total current referred to the primary winding; I'~oreLoss
represents the total power losses contributed by the core;
Ro represents the total resistance referred to the primary
winding; R~, represents the thermal resistance in (°C/W)
between the windings and the cooling medium; and, tai,
represents the ambient temperature of the cooling medium.
The dissipation of the two-part core 10 follows
formula 2.
Formula 2:
r[Rth ~~A~25.4z~p~ + 1]~~(20 - t amb )~a - l~~Iz~Ro - Core loss , ...~
ILL+t amb ~~A~25.4z~p~
Pd = Iz Ro ~ 1 + a ~ + t_amb - 20 + Core_loss
~Ro~n Rth~lz - I)~A~25.4z~j3 + IZ Ro~a
It will be appreciated by those skilled in the art
that the dissipation and temperature rise functions
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demonstrate that below a certain critical dissipative
surface area, losses in transformers increase
exponentially. In accordance with the invention,
additional winding material is not required and no changes
in the configuration of the core are required, and all core
material is used to create the magnetic flux path for the
transformer.
If additional cooling is required, cooling fins, such
as an aluminum sheet may be placed in thermal contact with
the two-part core to provide cooling similar in principle
to that of baseboard heaters. This minimal use of core
material is a very important feature for designs that
comply with recent legislation governing transformers.
Canadian bill C802.2 dictates that transformer efficiency
of 30KVA sized units must be 97.58 at 0.35 p.u., whilst the
U.S Department of Energy is pursuing efficiency figures at
0.5 p.u.
It is a well known rule of thumb that transformer
efficiency peaks when the core losses are substantially
equal to the winding losses. When designing transformers
to comply with these energy efficiency standards, the
development engineer is faced with the dilemma of basing
his design on core loss at 0.35 p.u. - 0.5 p.u. while
maintaining reasonable copper losses at full load. This
results in a design that is more expensive to construct due
to increase in material costs. Using the transformer
topology shown in FIG. 1, the enhanced conduction cooling
of the windings through the reduced volume core eliminates
this problem because cooling fins can be added to dissipate
losses for the full power load.
An example illustrates the thermal dissipation
properties of the current transformer topology. A
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transformer with a two-part core was loaded with 20A input
current (20% above the calculated rating). At 20A input
current and a surface area of 500 sq. in. (for the same
insulation thermal resistance), the power dissipation
was 774W, maintaining an efficiency of almost 94% at the
20% overload. At full load, efficiency is preserved at
over 95%. The transformer therefore surpasses government
legislated energy efficiency requirements in North America
and Europe, which is typically 95% efficiency at 0.35 p.u.
for transformers of 30KVA.
Once a transformer has been designed, the manufacture.
of a transformer may be effected according to the method
schematically illustrated in FIGS. 2a-f. The method begins
with the fabrication of the toroidal piece lOb. In the
illustrated embodiment, the toroidal piece lObl is formed
by rolling a strip or laminations of a steel supply 40 to a
predetermined thickness. The steel 40 is preferably
laminated to reduce eddy currents within the resulting
toroidal piece lOb, when in use. FIG. 2a shows the initial
forming of the toroidal piece lObl. It will be appreciated
by those skilled in the art that the application of the
steel supply 40 may be performed by winding the steel strip
or laminations about a spindle that defines the inner
wall 18 of the toroidal piece lOb, while supplying
sufficient tension to provide the desired density of the
core.
Once the desired thickness of the toroidal piece lOb2
is achieved (FIG. 2b) , the steel supply 40 is cut and the
now cylindrical toroidal piece lOb2 is subjected to a heat
annealing treatment, schematically illustrated in FIG. 2b,
in a manner well known in the art.
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In FIG. 2c a sector 42 has been removed from the
treated piece. The sector or gap 42 is cut through the
toroidal piece lOb3 to prevent magnetic flux (which travels
in the axial direction) from inducing an electric field in
the direction of the strip steel forming the core (an
azimuthal direction). The induced electric field would
otherwise cause a thin insulating coating of the steel to
break down to connect an adjacent strip, in which case
effectively each turn of steel would then act as a poorly
coupled turn of a winding. It will be appreciated by those
skilled in the art that if the toroidal piece 10b2 is
created from a powdered steel with resistive properties,
for example, the tendency to induce current in an azimuthal
direction is significantly reduced, and accordingly a
sector need not be removed from the toroidal piece 10b.
However, the apertures) for the winding terminations have
to be provided in the transformer and the aperture (s) may
be in part or in whole supplied by a channel through the
toroidal piece lOb.
In FIG. 2d the electrical insulator 22a is applied to
the toroidal piece lOb, and on of a secondary or a primary
winding 24 is wound about the toroidal piece 10b. In
general, secondary windings of a transformer are wound
first, however the transformers in accordance with the
invention permit the primary or the secondary to be would
in either order. In this example, however, the primary
winding 24 is applied to the toroidal piece lOb.
Terminations 44a, 44b of the primary winding 24 in
accordance with the illustrated embodiment are drawn away
from the primary windings 24 on the outer wall 14 of the
toroidal piece lOb adjacent the sector 42 (or other passage
for the terminations).
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Subsequently, the exposed surface of the primary
winding 24 is covered with the electrical insulator 22b, in
preparation for applying the secondary winding 26. As is
shown in FIG. 2e, the secondary winding 26 is applied about
the toroidal piece lOb.
Steps shown in FIGs. 2a, 2b, and 2c are repeated to
produce the shell piece l0a in a like manner. The outer
surface defined by the secondary winding 26 is covered with
the insulating material 22c, and the shell piece 10a is
wound concentrically over the secondary winding 26, as
shown in FIG. 2f and then annealed before the sector is
removed. The terminations 44a, 44b, 46a, and 46b of the
primary and secondary windings 24, 26 are insulated and
passed through the sector 42 removed from the shell
piece 10a, completing the manufacture of a transformer
module 48.
FIG. 3 is a partially exploded view of a
transformer 50 manufactured using the transformer module 48
manufactured according to the method shown in FIGs. 2a-f.
The transformer 50 consists of the module described above
and top and bottom yokes 52. The yokes 52 are laminated
annular pieces having an exposed surface 54, a core-
contracting surface 56, and a passageway 58 between the
exposed and core-contracting surfaces 54, 56 that extend
the cooling duct 20 of the transformer 50.
The yoke 52 is made of magnetic material and is
designed so that the yokes 52 and the two-part core 10
provide a closed magnetic flux path that is minimally
separated from the windings 24, 26. Accordingly the
yokes 52 are of a dimension to cover the top 34a of the
shell piece 10a, and the top 34b of the toroidal piece IOb,
and the core-contracting surface 54 is designed to
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electromagnetically couple the yoke 52 with the toroidal
piece lOb and the shell piece 10a. A thickness of the
yokes 52 separating the meeting and exposed surfaces 54, 56
is preferably chosen to be approximately equal to the
radial thickness of the toroidal piece lOb.
The yokes 52 may be.constructed from strip steel and
are preferably configured to minimize eddy currents. As
illustrated a yoke designed to minimize eddy currents may
be constructed from strip steel by securing equal length
pieces 60 of the strip steel in a jig having a core
defining the passageway 58. With the pieces 60 secured in
the jig, an azimuthal force is applied to the free ends of
the strips, in order to rotate the free ends. Such
rotation radially compacts and densifies the yoke 52.
After the yoke 52 is compacted, it is annealed.
The yokes 52 serve to sealably enclose the
transformer 50. Certified sealing materials are known in
the art for sealably enclosing transformers. Accordingly
the transformer 50 designed in accordance with the present
invention is suitable for use in damp, wet or hazardous
environments. For example, construction method can be used
for transformers of 1000VA to over 20MVA and when sealed
using proper compounds do not require enclosures, as will
be described in detail below with reference to FIGS. 6a and
6b. The transformer 50 can operate in damp or wet
conditions when sealed, without expensive NEMA 3 and
higher-rated enclosures.
The shell piece l0a also serves to reduce noise and
vibration. Vibration is further reduced by the fact that
the windings are tightly restrained between the toroidal
piece lOb and shell piece l0a without spacers etc.
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FIGS. 4a and 4b are exploded and assembled views of a
multi-phase transformer 65 consisting of three axially
aligned transformers modules 48 with respective top and
bottom yokes 52, and yokes 52 between each transformer
module 48. The transformer modules 48 may be stacked to
provide a magnetic flux conserving arrangement for multi-
phase applications. The yokes 52 separating transformers
modules 48 may be thinner than the top and bottom yokes 52
to obtain more material savings, because the flux density
in the three-phase transformer 65 is lower.
FIGs. 5a, 5b and 5c are three embodiments of
transformers equipped with cooling fins of different types.
Because of the closure of the transformers 50, 65 the core
can be cooled with the addition of one or more cooling
fins 66 which can be of various shapes, including
longitudinal fins 66a affixed to an outer surface of the
transformer 50; a sheet 66b folded to form fins 66 wrapped
around an outer surface of the transformer 50; or disc-
shaped fins 66c affixed to the outer surface of the
transformer 50. Other shapes that effectively increase the
surface area of the two-part core 10 to increase the
efficiency of the heat dissipation may also be used. The
discs 66c shown in FIG. 5c are particularly useful for
horizontally oriented transformers 50, 65.
FIG. 6a is an exploded schematic view of a sealed
transformer 50 in accordance with one embodiment of the
invention. The transformer 50 is economically sealed
without an expensive NEMA 3, or higher, enclosure. In this
embodiment, sealing of the transformer is accomplished by
sealing the sector 42 removed from the toroidal piece lOb,
and the sector 48 removed from the shell piece 10a. Sealing
the sector 42 may be accomplished by, for example, placing
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insulation 70 in the sector 42 and applying a bead of
sealant 72 where the sector 42 intersects the inner wall 18
of the cooling duct through the toroidal piece lOb. The
sealant 72 can be the same sealant used to seal the yokes
52 to the two-piece core, as described above. Sealing the
sector 48 may be accomplished by sealingly securing a
connector box 80 to the outer sidewall of the shell piece
l0a so that it covers the sector 48, after the yokes 52 are
sealed to the top and bottom ends of the shell piece l0a
and the toroidal piece lOb, as also described above. The
connector box can be of any size to permit easy bending and
termination of the wires. The connector box 80 supports two
or more power source feed-throughs or connectors 82, which
are commercially available and well known in the art. The
assembled transformer is shown in FIG. 6b.
As will be understood by those skilled in the art, the
transformer shown in FIG. 6b is economically constructed
and can be used in exposed weather conditions or damp
environments.
It should be noted that less expensive magnetic
materials can be used to create the two-part core to
achieve performance comparable to prior art transformers,
at a lower cost. The magnetic material grading system well
known in the art (the 'M' grading system) characterizes
materials according to maximum magnetic material losses per
pound weight at 50Hz or 60Hz, usually for flux densities of
15,0.00 Gauss or 1.5 Tesla(T). For example, M6 grade
specifies that losses shall be below 0.6W per pound at 1.5T
(60Hz), and M19 grade gives a maximum loss of 1.9W per
pound under the same conditions. The better grades M6, M4
and so on, are usually grain orientated, so that the losses
are guaranteed only in one particular flux direction,
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OR File No.11171-18CA
defined with respect to the rolling direction of the steel.
M19, M22 and lower grades are usually not grain orientated
and give substantially equal losses in either direction of
flux flow.
To account for imperfect orientation of the grain with
respect to the flux, loss figures are also commonly given
for 75~ flux in grain and 25% cross grain conduction and
typically, effective losses for M6 are approximately 1W per
pound. The cost of these materials varies with the grade.
M19 grade, for example, is 150 ~ 25% less expensive than M6
grade, and certain grades of M4 gauges can be almost twice
as expensive as M6. Manufacturing cares with grain-
orientation constraints increases the complexity and cost
of the designs.
The transformer topology shown also minimizes joints
in the transformer core and accordingly losses associated
with the core joints are reduced.
This invention is not restricted to transformers and
transformer manufacture processes but can also be applied
to ballasts and inductive devices which also use windings
and magnetic cores. For example, chokes are commonly used
for arc discharge lamp lighting or for application to motor
start in large industrial machines.
The invention may advantageously be applied to air-
cooled transformers but is not restricted to "dry-type"
transformers, as the same principles of the topology apply
to oil-cooled transformers, Sulphur Hexafluoride (SF6)
cooled transformers, etc. Dry-type transformers can be
used for applications with extremely small power e.g.
fractions of a watt or for very large power applications
exceeding 20 MW.
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The transformer topology can apply to the most common
power frequencies (from 30Hz to 400Hz), however the theory
and practice of the transformer 50 can be applied at any
frequency deemed appropriate for the materials chosen to
form the transformer in accordance with the invention.
Transformer 50 provides a transformer topology where
the theoretical minimum material content can be very nearly
be realized. The transformer 50, by its topology, has a
high surface area to volume ratio, and in addition, the
effective cooling surface area for the windings and the
core is easily increased. The windings and core of the
transformer 50 are concentric so that heat from the
windings is conducted radially away from the windings and
radiated by exposed surface of the core l0a,b.
The design for the transformer 50 permits the use of
steel as a primary thermal transfer medium to a larger
surface area. Since steel is a much better conductor of
heat than air, this improves the heat dissipation of the
transformer.
The transformer 50 has windings that are substantially
radially outwardly enclosed by the shell piece l0a of a
core 10, substantially enclosed on a top and bottom by
respective yokes 52, and substantially enclosed radially
inwardly by the toroidal piece lOb of the core 10. The
core 10 and yokes 52 provide a shortened magnetic flux path
and eliminates material waste by maximizing the utilization
of materials such as winding wire and the magnetic core
material. The enclosure of the windings also effectively
eliminates external flux leakage.
The transformer 50 operates more quietly at elevated
flux levels. Transformers in general have noise problems
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OR File No.11171-18CA
associated with their operation due to magnetostriction and
coil vibration. Magnetostriction is the elongation and
contraction of the magnetic core due to the magnetic flux
flowing through it, the problem is worse in transformers
having long core structures as vibration increases with
length and flux density.
As the windings 24, 26 are enclosed in the shell piece
10a; the leakage of flux is limited to within the
transformer structure. Consequently vibration by magnetic
coupling to an enclosure is eliminated. The windings 24,
26 may be better constrained in accordance with the
invention as they are in contact with the core via a
compliant insulator and therefore vibrate Less than
comparable transformers when the transformer is on load.
The invention also provides heat dissipation,
minimization and loss prediction algorithms for designing
transformers having the two-part core.
The transformer 50 exhibits improved heat dissipation
efficiency, requires substantially less core and winding
material, and/or may be constructed of material of a lower
cost, while enabling similar or improved performance in
comparison with prior art transformers.
The embodiments of the invention described above are
intended to be exemplary only. The scope of the invention
is therefore intended to be limited solely by the scope of
the appended claims.
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