US3551863A

US3551863A - Transformer with heat dissipator

Info

Publication number: US3551863A
Application number: US718972A
Authority: US
Inventors: Louis L Marton
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-03-18
Filing date: 1968-03-18
Publication date: 1970-12-29
Anticipated expiration: 1987-12-29
Also published as: FR2004169A1; FR2004169B1; DE1913163A1; GB1239342A; SE361972B; CH512135A

Description

Da. 29, 191e L.. L. MARON 355mm TRANSFORMER WITH HEAT DISSIPATOR Filed March 18, 1968 5 Sheets-Sheet l Bec. 29, 1970 L. a.. MAMON @,SSLM

TRANSFORMER WITH HEAT DISSIPATOR Filed March 18, 1968 5 Sheets-Sheet FIG.5

L. L Mmmm ipf@ TRANSFORIMR WITH HEAT DISSIPATOR.

med umn 1e, 196e s sheets-sheet 4 N E A M am LM TRANSFORMER WITH H EAT DISSIPATOH Filed Malcch 18. 1968 United States Patent O 3,551,863 TRANSFGRMER WITH HlEA'l` DISSIPATOR Louis L. Marton, 7424-1/ 4 Arizona Ave., Los Angeles, Calif. 90045 Filed Mar. 18, 1968, Ser. No. 718,972 Int. Cl. H01f 27/10 U.S. Cl. 336-58 9 Claims ABSTRACT F THE DISCLOSURE This invention relates to transformers equipped with more effective new heat dissipators for the purpose of reducing the external and/or internal resistances in the path of the heat iloW. This reduction is achieved by applying dissipators equipped with a multiplicity of fins having narrow surfaces exposed to the stream of the cooling medium. On surfaces having small dimensions in the direction of the flow, a negligible boundary-layer develops; thus the heat transfer can be substantially improved.

This invention relates generally to transformers equipped with heat dissipators and more particularly to improved and more efficient cooling arrangements for dissipating heat generated in the active parts of any stationary magnetic apparatus having at least one active part and being exposed to a stream of cooling medium.

A common characteristic of electric transformers and other magnetic apparatus is that a certain loss of energy occurs in its active parts (windin-gs, core, etc.) during operation causing a temperature rise, which is the main and often the only limiting factor as to the power handling capacity of the device. The temperature gradient in turn maintains a heat flow from the thermal center of the heat source toward the surfaces of the active part across its internal structure which has a degree of heat conductivity. The heat gets dissipated from the surface partly by the surrounding cooling medium, such as air, oil, etc., by natural convection or forced flow, and partly by radiation. In most cases the greatest resistance to the flow of the heat appears at the surfaces between the solid material and the gaseous or liquid cooling medium; consequently, this is the area where the largest part of the temperature gradient develops. The next largest gradient appears internally, in multilayer windings, where the internal structure of the winding contains a multiplicity of insulating layers with poor heat conductivity.

An object of the invention is to reduce the portion of the resistance to the flow of the heat which appears between the surface of the active parts of the transformer and cooling medium externally, and thereby to reduce the temperature gradient of these active parts which appears above the temperature of the cooling medium. This reduction can be achieved by applying surface heat dissipators.

An additional object of the invention, besides reducing the external resistance to the flow of the heat on the surface, is also to reduce the portion of the resistance which appears internally in the structure of the active parts by opening up internal transfer surfaces in them, bypassing most of the internal resistance in the way of the heat ow. This reduction can be achieved by applying extended internal heat dissipators.

Another object of the invention is to present different practical ways for utilizing the new heat dissipators in transformers of different designs, kva. ratings, and methods of cooling.

Yet another object of the invention is the extension of the economical application of simpler cooling methods toward higher kva. ratings; e.g., natural convection cooled 3,551,863 Patented Dec. 29, 1970 ICC dry type units can be built for ratings where presently forced air cooling is needed; forced air cooled units can be built for all ratings where presently only liquid cooled units can be used; etc.

An essential feature of the invention is its employment of heat dissipators in a transformer, said dissipators comprising at least one sheet of highly heat conductive material, connected mechanically with heat conductive means to at least one transfer surface of the transformer; transfer surface being defined as an external or internal surface of a heat generating active part, through which the heat leaves the active part and enters into the heat dissipator or directly into the stream of the cooling medium. The dissipator occupies a region which contains it, and comprises at least one sheet of highly heat conductive material, provided with a multiplicity of cooling fins by a prefabrication process.

According to a further feature of the invention, the dimension of the fins in an axis of orientation (in most cases in the direction of the stream) is less than one fourth the dimension of the region of the dissipator, measured in the same direction. According to a further feature of the invention, the multiplicity of fins are arranged in at least one of the zones defined by a subdividing of the region of occupancy along said axis into at least two zones with border lines substantially perpendicular to said axis. Since the dimensions of the ns are relatively small in the direction of the axis of orientation, the cooling medium has short contacts with the fins; therefore, a thick boundary-lever cannot build up on the fin surfaces. If the multiplicity of fins are arranged in several zones along the ow so that the necessary cooling surface area may be obtained, repeated contacts will occur. This arrangement results in a slightly increased resistance to the stream of the cooling medium, thus reducing the intensity of the stream; nevertheless, this disadvantage is compensated for in properly designed arrangements by the increased heat transfer on narrower iin surfaces due to the decreased boundarylayer thickness.

According to a further feature of the invention, a surface heat dissipator is connected to an exposed surface of the winding, core, enclosure, and/ or radiator of the transformer. According to another feature, an extended internal heat dissipator is adapted to be connected to an internal surface of a heat generating active part (winding or core), converting the contacted internal surfaces into transfer surfaces, and dissipating the heat via extended areas exposed to the stream of the cooling medium.

According to further features, at least two dissipator sheets can be employed over the same surface of an active part; two sheets may be employed in a cooling duct with fins oriented toward one another. All these sheets can be extended beyond at least one dimension of the active part.

The improved heat transfer that can be achieved by the application of the new heat dissipators is the result of the combined improvements in two factors of the heat transfer:

(l) The increase of the heat dissipating surfaces, due

(1.1) Extending existing transfer surfaces beyond their origonal dimensions by connecting dissipator sheets with large dimensions;

(1.2) Converting internal surfaces into transfer surfaces by introducing smooth portions of extended internal dissipator sheets into active parts, e.g., between winding layers, core laminations.

(2) The introduction of very effective, practically boundary-layer free surfaces as additional heat dissipating surfaces by use of additional surfaces such as fins, strips, or narrow sheet surface areas between punched holes, etc. with small dimensions in the direction of the liow. In such arrangements, the boundary-layer develops negligible thicknesses only.

It is a well known fact that the cooling medium tends to form a boundary-layer increasing in thickness with the ow along the surface, clinging to it, and preventing good heat transfer between the surface and the cooling medium. The boundary-layer begins to form at the leading edge of the surface, where, however, it is of negligible thickness. If the surface is subdivided into a multiplicity of leading ledges in accordance with the invention, the heat transfer will be substantially increased.

The increased heat transfer can be utilized in many different ways by the designer. One way leads to reduced sizes and weights of the units, through use of higher specific load figures, such as current density and llux density. This reduction can be achieved up to a certain kva. rating without increase of losses by eliminating the presently used internal coling ducts and using extended internal heat dissipators instead. This substitution leads to a reduction in the copper weight because of a decrease in mean turn length. As a consequence of a reduction in the copper weight, the core weight also can be reduced, to maintain an economical proportion. In cases where a slight increase in copper loss is permissible, further drastic weight reduction can be achieved; eg., if, in a 750 kva. dry type transformer, built with 6 kw. copper loss and 0.8 kw. core loss, resutling in 99.1 percent efficiency (because there was no economical way to dissipate more than 6.8 kw. total losses), the copper loss was increasedto 12 kw. by use of a new design with heat dissipator, the efficiency offers only a slight decline to 98.3 percent, but the total weight of the unit decreases by 50 percent.

Another way of utilizing the increased heat transfer is to lower the operating temperature for the same rating. Due to the lower operating temperature, the temperature dependent copper loss will be reduced and efficiency improved; furthermore, the costs may be reduced by using insulating material of lower thermal class. In some cases, a combination of the two approaches, i.e., a moderate reduction of both the weight and temperature along with the copper loss and the substitution of inexpensive lower thermal class insulation, offers the best solution.

In liquid cooled transformers, the heat dissipator can be utilized for increasing up to four times the convective heat transfer on the radiators and enclosure walls, thereby reducing the number of necessary radiators down to a fourth.

Further objects, features, and advantages of the present invention become apparent on the basis of the following description. Various exemplary embodiments are illustrated in the accompanying drawings.

FIG. 1 is a plan view of a preferred embodiment of a surface heat dissipator sheet;

FIG. 2 is a side elevation of the heat dissipator sheet in FIG. 1;

FIG. 3 is a longitudinal section of an illustrative embodiment of a transformer equipped with heat dissipators in several different arrangements;

FIG. 4 is a partial longitudinal section of a further illustrative embodiment of a transformer equipped with various extended heat dissipators;

FIG. 5 is a partial plan view of the transformer shown in FIG. 4;

FIG. 6 is a partial sectional view of a disc type transformer winding equipped with extended internal heat dissipators;

FIG. 7 is a partial sectional view taken along sectional plane AA in FIG. 6 showing a dissipator sheet in plan view;

FIG. 8 is a partial sectional perspective View taken along section plane BB in FIG. 6;

FIG. 9 is a partial sectional View of the cutting tool for the production of heat dissipators according to FIG. 8, shown in three consecutive phases of operation A, B, C;

FIG. 10 is a vertical longitudinal section Of a transformer equipped with pancake type coils and extended surface heat dissipators;

FIG. 11 is a horizontal cross section of the transformer taken along section plane CC in FIG. 10;

FIG. 12 is an enlargement of the upper end of the transformer winding shown in FIG. 11',

FIG. 13 is a variation of FIG. 12;

FIG. 14 is a longitudinal section of an encased air cooled transformer, with surface heat dissipators, illustrating arrangements for closed and open natural convection and forced air cooling;

FIG. 15 is a partial side elevation of a liquid cooled transformer with its enclosure and radiators equipped with surface heat dissipators;

FIG. 16 is a cross section of a single radiator element equipped with surface heat dissipators on both sides.

Referring in detail to the drawings, FIGS. 1 and 2 illustrate a preferred embodiment of a prefabricated surface heat dissipator sheet for improving the heat transfer on any surface exposed to a stream of cooling medium. Said sheet is made by taking a continuous sheet of metal; and using a suitable tool, a three edged, substantially rectangular fin is punched out of the metal and bent up substantially at right angle along the intact fourth edge. These fins are all bent so that they protrude on the same side of the sheet; their major surfaces become substantially parallel to an axis of orientation (generally representing the direction of the stream of cooling medium, with one exception detailed at the end of this paragraph) and substantially perpendicular to the closest area of said sheet. Although other forms of the dissipator sheets can be used (e.g., continuous sheets with welded on fins, or punched sheets with different patterns, etc.) according to the invention, the preferred arrangment shown has definite advantages, such as low production costs and in some cases additional heat transfer capability, as will be pointed out later in connection with FIGS. 3 and 16. It will be seen from FIGS. 1 and 2 that the metal sheet 1 has substantially rectangular fins 2 in pairs of rows according to a regular pattern, each pair contained by a zone. In the first row, representing the upper part of the rst zone, each lin has its holes (punched in the prefabrication process and referred to, hereafter, as punched holes) to the left; in second row, representing the lower part of the first zone, each iin has its holes to the right; in the third row, representing the upper part of the second zone, to the left; and so on, alternately. On sheets with a finless zone, the latter may have punched peep slots perpendicular to its edges to facilitate the alignment of the sheet in the winding operation. The holes and the lins can be tapered, symmetrically (A1 and A2) or unsymmetrically (B1, B2 and C1). In extreme cases, the holes can be tapered with a right angle at the top edge (B1) or at the lower edge (C1), the top edges for both remaining on zone border line 3. The distance dA between

border lines

4 and 5 is slightly smaller then the distance dB between zone border lines 3' and 6, while the surface areas of the fins remain the same. An increased distance dB is more advantageous in practical application, offering more space for bandaging and for cutting the sheet while retaining a smooth bordering edge. A slightly tilted arrangement, as shown by hole C1, introduces a small angle a between the iin surfacey 7 and the direction of the ow 8 of the cooling medium. lIf a larger angle is preferred in order to improve the heat transfer in some cases (see details later in connection with FIG. 15), the top edges of hole C1 become slanted and penetrate above the zone border line 3.

According to the invention, the cooling `tins on the heat dissipator are narrower than one fourth of the dimension of the region occupied by the dissipator, both measured in the direction of the stream of the cooling medium. The ns are arranged in at least one of the zones defined by a subdividing of the region occupied by the heat dissipator into at least two zones along the axis of orientation. Larger tin dimensions in the direction of the stream allow an increasingly heavy boundary-layer to develop along the iin surface, resulting in a diminishing heat transfer toward the rear edge of the iin and an increasing resistance to the iiow of the cooling medium (due to the longer friction area along the larger dimensions). On the other hand, if the stream passes along a number of narrow fins while crossing the subsequent zones, the resistance to the stream increases (especially in ducts); nevertheless, the heat transfer may still be improved due to smaller boundary-layer thickness on the narrower tins. The best compromise can be determined experimentally for each given condition; several results will be given shortly.

Further advantages of the pattern shown in FIG. l appear when the pattern is used in duct arrangements with two dissipator sheets facing each other `with tins being oriented to be exposed to the stream in the duct. It is easy to design patterns on the basis of FIG. l, where at least half of the iins find a solid metal surface in any relative position, preventing them from penetrating through the holes, thus avoiding possible injury to the insulating material below the metal sheet. Another advantage is a more uniform bending strength, the punched sheet having no tendency to become polygonal when bent, as in the case of sheets punched with holes all in the same direction.

Surface heat dissipators, e.g. as shown in FIGS. l and 2, are utilized on transfer surfaces of active parts normally exposed to a stream of cooling medium. The dissipator sheets are connected mechanically to the transfer surfaces with heat conductive means, and they can be extended beyond the dimensions of their transfer surface. In the following, their heat transfer rate in W/in.2 will be referred to the transfer surface area of the active part, which is in actual contact with the dissipator sheet, with an exception related t FIG. 16.

The optimum dimensions of the iin arrangement vary in accordance with the different heat dissipator applications. For natural convection and radiation, on outside surfaces in air, a heat transfer close to ZW/in.2 can be achieved with one of the best exemplary embodiments with no extensions when a 100 C. temperature gradient above 25 C. ambient temperature exists. This value represents an improvement close to double, compared to a iinless surface. The Idimensions of the embodiment are the following: Total vertical height (region of occupancy) HT="; other dimensions are as shown in FIG. l: X=3.375, Y=1.250", L=0.500, H=0.500, W= 0.032. On a surface with larger height, e.g. HT--l0, a decline of about 5 percent can be expected in the heat transfer, when the access of fresh air is unlimited.

If the same fin dimensions are used in combining two dissipator sheets which face each other in a duct, and if the same iin density is kept (by punching the iins in the first zone on the sheet placed on one side of the duct, in the second zone on the opposite side of the duct, and so on), the heat transfer will be about 1 W/in.2 under the i same conditions; this represents a 50 percent improvement over ducts of the same size with nless surfaces. The heat transfer can be improved by approximately percent to 1.15 W/in.2 through the use of 0.750 long fins in a wide duct; but by doubling the overall height of the duct to HT=10, the value of the heat transfer declines close to 0.95 W/in.2

If forced air cooling is applied, a value of 8 to l2 W/in.2 is feasible with the same arrangement even in long ducts, without exceeding the reasonable limits for pressure drop; e.g., in a 0.5 wide and 10" long duct, a pressure drop of 0.2 to 0.3 water column is sufficient. Consequently, simple propeller fans with venturi casing connected to an envelope to prevent the air from bypassing the heat dissipators offer quite satisfactory solutions, even in cases where alternative natural convection cooling is alsoy desired.

For natural convection cooled dry type units in air, up to approximately 100 kva. rating in single phase, the best arrangement is to build relatively short coils with no ducts,

having therefore a reduced mean turn and copper Weight. This arrangement also leads to short legged compact cores; further, to wound-in internal and/or wraparound external heat dissipators made of heavier, up to 0.063 thick or double aluminum sheets and extending considerably beyond the axial dimensions of the winding (as shown in FIG. 3).

Referring to FIG. 3, a transformer is shown comprising a core `9 and a winding 10 having extended heat dissipators, in several different versions. On the left side, two dissipator sheets are arranged on the outside surface over the top insulating layer 11 of the winding 10. Both of the sheets occupy a region longer than the axial length of the winding 10; the region of occupancy is subdivided into 9 zones, each containing a multiplicity of ns on the external sheet; but only the first two and the last two zones are containing fins on the internal sheet. The internal sheet 12.1 is wrapped around the insulating layer 11 with its central area without tins. It has fins on its extensions only, oriented toward the center of the winding. Directly on top of the internal sheet 12.1 the external sheet 12.2, with fins all over its surface, is wrapped and placed with its punched holes in alignment with the punched holes on the sheet 12.1, so that air can pass through the holes of both top and bottom extensions along the path of arrows 13.1 and 13.2, respectively. The fins of the second sheet 12.12 are oriented away from the fins of the first sheet 12.1.

The heat transfer on the extended areas of the dissipator is higher per surface unit, since all the extended areas consist of narrow strips with practically boundary-layer free surfaces all around, including the sheet itself between holes.

It is possible to improve the heat transfer even further, in the same arrangement, with no change in the surface areas, simply by cutting up the extended portions of the heat dissipators along the paths of the heat flow into strips and bending them apart;

The right hand side of FIG. 3 illustrates several versions of the heat dissipator created by this process. In order to maintain the clarity of the illustration, strips in the background are not shown.- Over the upper area of insulating layer 11, dissipator sheet `12.11 is placed, having prefabricated extensions. Its top extension 12.11A has punched out fins 12.11B and is cut up into strips containing two or four vertical rows of fins. After the sheet is in place, the strips are bent outward to facilitate the movement of air between them involving more air in the convection process. The bottom extensions of sheet 12.11 are cut up into simple narrow parallel strips 12.11C, and twisted so that the planes of the strips become oriented substantially perpendicular to the original plane of sheet 12.11; after the sheet is wrapped around the winding, the twisted strips 112.11 can be bent up into a position, close to horizontal, as shown, with their helical transient area close to the main body of the sheet wrapped around the winding.

This arrangement can be used on external or internal sheets at both ends. As the arrangement has no constrictions in the strips to restrict the heat flow, and as it makes available all the surfaces on both sides as a practically boundary-layer free dissipative surface, oriented substantially in the direction of the air ow, it achieves the best heat transfer among the various dissipator arrangements for short (below 12) windings; consequently, this method is preferred whenever the dimensions of the coil make its application economically sound.

A second sheet 12.12 covers the bottom area of the same layer 11. The extension strips 12.12A of sheet 12.12 are bent upward in this illustrative embodiment for the purpose of being accommodated in the window of core 9. Sheet 12.21 is wrapped directly over sheet 12.11, and sheet 12.22 over sheet 12.12. Both have punched out fins covering their entire surface area; sheet 12.21 has a top extension 12.21A, which is cut up and bent as shown, to allow the air to flow along both sides of the strips.

Since the application of strips as extensions provides an increased mobility for the cooling air, additional extended dissipator sheets, such as sheet 14, can be wrapped around one or more internal layers of the winding. The extensions 14B, 14C of sheet 14 in this example are similar to those of sheet 12.11, respectively. The internal sheet 14 creates internal transfer surfaces for the winding on both of its sides without the usual increase in the mean turn (such as in the case of conventional cooling ducts), decreasing the internal gradient and the winding hot spot temperatures effectively. To facilitate the winding operation and handling, two thinner sheets can be used in close contact with each other, having a prefabricated extension area (one oriented to the top, the other to the bottom), and a wider nless area, which can be sheared according to the axial dimension of each winding. The two sheets may be provided with an electrical insulating coating to prevent the flow of eddy currents between them, and with peep slots for alignment.

To allow the application of internal extended heat dissipators in larger transformers, the winding can be built up from sections, leaving enough space between them to accommodate bent out extensions such as 14C. There is a possibility of using extensions of this type also in long layer wound coils, by providing gaps periodically between turns to accommodate the bent up extensions.

An illustrative embodiment of a production tool for the prefabrication of sheets equipped with twisted strip extensions, such as 12.11C and 14C, will be discussed later in connection `with FIG. 9.

If heat dissipators are used on the winding surfaces, the heat transfer through the layers of the winding is also multiplied. To avoid the development of large internal temperature gradients, care should be exercised to keep the thermal resistance in the coil as low as possible. To decrease the part of the resistance due to the layerinsulation, special insulating sheets can be used with improved heat conductivity. This improvement can be achieved by embedding fine grains of solid, highly heat conductive insulating material (eg. fine silica sand) into the base material of the sheet. Another way to improve the degree of heat conductivity in the internal heat transfer is by winding tightly, combined with void free vacuum impregnation. Heat dissipators should have means for good heat conduction with the surfaces where they receive heat; they should, therefore, be bent carefully to follow the curvature of the surface and tightened. The tightness can be improved in rectangular windings by using a mandrel with a slight outward arch on all sides and a corner radius of not less than twice the wire dimension, as shown in FIG. 5. Any tendency of the first insulating layer to buckle (during impregnation) can be eliminated by winding a temporary reinforcing metal sheet (eg. cut out of core steel) under this layer and peeling it off after impregnation and oven treatment.

In coils, where pockets of impregnating material or air can be expected, the heat conductivity of these pockets can be drastically improved by filling up the pockets with silica sand composed of different sized very small grains. The silica filler, embedded in the impregnating material, is capable of effectively bridging any internal gaps. The best procedure for getting the silica into the gap is to mix the dry silica with a temporary liquid binder, obtaining a paste-like mixture which is then applied over each layer, as necessary, to fill up lower areas, and achieving even layer surfaces during the winding operation. After vacuum drying and vacuum impregnation, the liquid binder can be readily exchanged with the final liquid impregnating material. Where no vacuum impregnating equipment is available, fairly good results may be obtained by mixing the silica with the nal impregnating material and applying this mixture to the layers as needed, and curing it in an oven.

For larger dry type transformers, over 100 kva. ap proximately in single phase, dissipator arrangements in accordance with FIGS. 4 and 5 offer some advantages, especially when an alternative possibility for operation with forced air cooling with increased kva. rating is also desired. On leg 15 of a magnetic core, multilayer winding 16 is accommodated. Six heat dissipator sheets extending beyond the length of the winding are attached to the winding on each of its end portions protruding horizontally beyond the longer contour lines of the core. Sheet 17 is placed on the internal end surfaces of the winding, with ns penetrating into a duct between the leg 15 and the winding 16. To dissipate the core losses, the heat dissipators 18 are accommodated in the same duct and joined to the core 15 at its laminated surfaces. Underneath the last layer of the first winding (eg. primary), another duct 19 is formed on the end portions, by placing two

dissipator sheets

20, 21 along the outer surface of the second last layer of the winding, and sheet 22 along the inner surface of the top layer of the first (eg. primary) winding. Sheet 20, being closer to the winding surface, has fins in zones on its extended area only, which are oriented toward the core;

sheets

21 and 22 have fins all over their respective surfaces, the fins being oriented away from the core in the case of the former and toward the core in the case of the latter, and thus into the air stream owing in duct 19. The punched holes of

sheets

20 and 21 are inalignment to facilitate the movement of the cooling medium.

Where natural convection is the process for the heat transfer, the density of the fins in the ducts should be substantially reduced as compared to that on outside surfaces, in order to facilitate the movement of the cooling medium; e.g., with 1A ducts in air, when the duct is 6 long, 1/2 ywide fins Should not be closer vertically than 1.250, while the vertical rows should be about 1/2 apart horizontally. The main insulation 23 is placed over the last layer of the first 'winding and has to withstand the full test voltage between primary and secondary windings, On the top of the secondary winding,

dissipator sheets

24 and 25 are accommodated. All sheets are extended beyond the length of the winding. For

sheets

24 and 25, as in the case of

sheets

20 and 21, the ns are oriented away from one another and the punched holes are brought in alignment to facilitate the movement of the air.

The dissipator sheets are insulated from the next layer of the winding by the normal layer insulation, and can be connected to a point in the next layer where the voltage, due to the inductive voltage distribution, is closest to the voltage level of the dissipator sheet, due to capacitive voltage distribution. This arrangement allows the sheets to play a secondary role as surge shields, thus preventing high frequency oscillations on the winding if a sudden voltage change is generated by switching, 1ightning stroke, or any other surge source.

The best arrangement for internal heat dissipators is one where the heat does not have to flow through heavy electrical insulating layers. This flow pattern can be achieved in most cases by selecting for the internal heat dissipators layers that allow the peak internal temperature between two dissipators such as 22 and 24 to develop in the line 0f the heavy insulating layer (e.g. 23). In three phase core type units, the center leg is always 1n an inferior position regarding heat transfer. This situation can be readily corrected by using additional heat dissipators, most advantageously on the surface of the winding, e.g. applying outside dissipator sheet 26 (in FIG. 5 all around the coil, while omitting the same in the window area of coils on the outer legs.

The extended portions of the dissipator sheets can be cut and bent apart with the same advantages as described in connection with FIG. 3.

In arrangements where two dissipator sheets are used in a winding in close contact with each other, suflcient electrical insulation should be applied, most conveniently in the form of film coating, to reduce the ilow of eddy currents which may be generated by stray iiux between the sheets.

For windings of larger transformers, made in the form of plane horizontal discs stacked up along the legs, plane disc-like extended heat dissipators with a louVer-like iin structure can be used. In FIGS. 6 to 8, an illustrative embodiment of the disc-like heat dissipator is shown with a louver-like structure extending along the outside contour of the winding. On an insulating tube 27, between insulating rings 28 and discs 29, heat dissipators in the form of prefabricated segments of metal discs 30 are placed, stacked up between disc windings 31. On metal segments 30, a multiplicity of substantially rectangular iins 32 (arranged in a row along the contour line of the winding) are punched out along their two radial sides by a tool and twisted out of their plane close to a direction perpendicular to the plane of the dissipator sheet. This louver-like arrangement lets the cooling medium flow through, e.g. as shown along arrow 33, in FIG. 8, establishing very effective heat transfer on its relatively narrow, practically boundary-layer free fin surfaces.

In order to avoid excessive additional losses in the dissipator sheets due to eddy currents being generated by strong stray flux penetrating the sheets, the segments can be cut up, or narrow separating channels 34 can be punched out along the paths of the heat flow as close to one another as needed. If stray tiux is expected to occur between iins 32, channels 34 should be extended out to the edge, as shown by channel 34A in FIG. 7.

FIG. 9 shows an illustrative embodiment of a cutting tool to prefabricate the louver-like iin structures 32, 12.11C, and 14C (the latter two shown in FIG. 3). A multiplicity of cutting blades are arranged in alignment to form multiple shears. The blades 32.1 of the lower, stationary part of the shears are oriented with cutting edges upward; the vertically movable upper blades 32.2 are oriented with cutting edges downward. The edges of both groups of blades are in horizontal alignment; on FIG. 9, however, to demonstrate the cutting and twisting operation, the three upper blades are shown in three consecutive phases of their downward movement, A, B, and C. The metal sheet is introduced in position 32A between the cutting edges; the blades start cutting (phase A); then by further movement, the strips are twisted by the rounded rear edges of the blades (phase B), while the center line 32C of the strips moves down also; the jig supporting the sheet should follow this movement to produce a twisted tin with no bending at the base. At the lowest position of the upper blades (phase C), each lin is pressed between the vertical rear surfaces of the blades, acquiring its iinal shape, while the sheet itself moves to position 32B.

Two examples of the application of heat dissipators in plane form are presented in FIGS. 10 to 13. Referring to FIGS. 10 to 12, which show a shell type transformer having a horizontal leg 35 and pancake type rectangular windings, primary coils 36 and secondary coils 37 are arranged in pairs with the solid main insulation 38 between them; heat dissipators 39 are placed between two primary windings 36 and between two secondary windings 37, respectively, and at the end of the window, between the last winding and the core (39A in FIGS. 11 and 12), separated from the winding by layer insulation 38A only. In this arrangement, the ducts built up by the dissipators are placed between balanced leakage groups where no leakage flux develops. In an alternative arrangement, shown in FIG. 13, to achieve high internal impedance, greater distance is used between primary coils 40 and secondary coils 41. In the leakage duct, two heat dissipators 42 are therefore accommodated with the layer insulation 43 adjacent to the windings, separated by a stack, consisting of the main insulation 43A sandwiched between two metal sheets 44. Since the full leakage iiux appears between the primary and secondary coils, tending to generate eddy currents in loops composed of lins short-circuited by sheets 44, to provide the necessary elecl0 trical insulation needed for the prevention of eddy currents between

sheets

42 and 44, sheets 44 being coated with an insulating lm.

The unit shown in FIG. 10 is equipped with a group of propeller fans 45, accommodated in circular Venturitubes 46 which are connected to a casing 47, enveloping the lower part of the unit vertically. When the fans are not in operation, the unit can get cooling air through natural convection of the fan openings. When the fans are operating, at least four times as much heat can be removed and the kva. load increased to at least its double value.

Another arrangement is illustrated in FIG. 14 for air cooled transformers designed for operation with natural convection and alternative forced air cooling. A dry type transformer 45, with heat dissipators 46 on its windings, is equipped with fan 47 housed in a bypass-free envelope 48. Even when fan 47 is not in operation, the surface heat dissipators in the duct get cooling air through the fan opening, along arrows 49, while the external surface dissipators get their cooling air mostly along arrows 50, bypassing the envelope 48. The transformer-fan assembly is surrounded by enclosure 51, shown in two versions. On the right side of FIG. 14, the side wall 52 of the enclosure has louver-like openings 52.1 both on the top and the bottom, serving as entrance and exit for the fresh cooling air. On the left side of FIG. 14, a version is shown with a completely sealed enclosure. The sealed inner wall 53 is equipped with heat dissipators 54A on their internal surface and dissipators 54B on their external surface. The sealed wall 53 is surrounded by an outer wall 55 having built-in fans 56 on the bottom and louver-like exit openings 57 on top for the air drawn in by fan 56. Baffle 58, with its extension 58.1, coniines the air ow along dissipator 54A. The sealed part of the enclosure may be filled with dry air or a special insulating gas ('Freon etc.).

The cooling process of the sealed version consists of two cycles: in the internal cycle, the cold internal cooling medium moves upward along heat dissipators 46, picking up the heat, then returns to its starting level. While passing along internal heat dissipator 54A, the cooling medium, guided by baille 58, transfers its heat content to wall 53 through fins of dissipator 54A. In the external cycle, fresh air is blown by the fan 56 through the channel formed by the external wall and wall 53 from which it receives the heat via external heat dissipators 54B and leaves the unit through louvers 57.

When the fan 47 is in operation, all the dissipators 46 get accelerated cooling air from fan 47. In the narrow passage, however, created by baffle extensions 58.1, the injector effect of the high speed air stream 49.1 along the external dissipators creates a pressure .drop above the passage which accelerates the outside stream of air along arrow 50, preventing a downward flow along envelope 48.

Heat dissipators can be used also on sealed transformer enclosures, or any extensions thereof such as cooling tubes, radiators, etc., which contain insulating liquid. Since the largest gradient develops between the enclosure and the cooling air on the outside surfaces when liquid to air cooling is used, the application of heat dissipators can be limited to the outside surfaces in most cases.

An illustrative embodiment is shown in FIGS. 15 and 16. To the outside surfaces of the sealed enclosure 59 of a liquid cooled transformer, heat `dissipators 60 are connected mechanically. To obtain additional cooling surfaces, radiators 61 are connected to the enclosure by tubes 62 allowing the internal cooling liquid to circulate through the radiators. Heat dissipators 63 are connected to the outside surfaces of the radiators with heat conductive means.

The cooling air moves vertically up along the surfaces by natural convection, or is driven by fans 64. To allow more air to get involved in the cooling process, to shorten the average path length of the air along the radiator surface, and to provide fresh air also for the upper part of the radiators, the radiators can be built in pairs slightly tilted closer together at the top as shown by radiators 61.1 and 61.2 in FIG. 15. If the plane of the fins are tilted away from the closely arranged tops of the pair, as shown, the normally vertical path 65 of the cooling air can be deflected farther away from the center line along path 65.1. This results in a'substantial shortening of the original path length of the air from P1-P2 to P1P3. Tilted arrangements of radiators 61.1 and 61.2 are especially advantageous when they are used with alternative forced air operation and a common fan 64 as shown.

Since the temperature gradient in liquid cooled transformers is limited to a substantially lower value than with dry type units in accordance with transformer standards, the rate of heat transfer is fairly low as a result. The low heat transfer rate allows the use of dissipators prefabricated from steel sheets. These can be welded to the vertical ridges of the radiators, eg. by electric resistance welding. Because the sheet is not continuous, having a hole pattern similar to that in FIG. l, and as the original radiator surface is ridged, most of the dissipator sheet surface being exposed to the air flow on both sides (e.g. 63.1 and 63.2 in FIG. 16), practically without boundary-layer formation, the eiiiciency of the heat transfer is higher than in the previous cases.

Since the outside surfaces of the radiators are facing one another, only a very small part of the heat is dissipated by radiation. The heat is removed therefore mostly by convection. The average temperature gradient between radiator surfaces and cooling air can be estimated to -be 50 C, at the most, to remain within the limits of the present standards. The rate of convective heat dissipation on smooth, long surfaces for the above gradient is around 0.15 W/in.2. Application of heat dissipators according to the invention makes it possible to raise the rate of convective dissipation by about three to four times, up to values of OAS-0.6() W/in.2 (referring to the base area defined by the outline of the radiator), due to the high efficiency of the dissipator in this arrangement. Consequently, if heat dissipators are used, only one-third to onefourth of the presently used radiators are required. Additionally, because the ns and plane surfaces of the dissipator offer a practically boundary-layer free surface also for forced air cooling, the newly reduced radiator bank can remove about four times as much heat with forced air cooling as with convection cooling. With presently used large radiator banks, forced air cooling cannot achieve more than a 2 to 2.3 times increase; therefore, forced air kva. ratings of liquid cooled transformers can also be substantially increased.

Various additional modifications of the above described embodiments of the invention will occur to those skilled in the art, and therefore the invention should be broadly construed in accordance with its full spirit and scope.

I claim: 1. A transformer comprising: at least one active part in which heat is generated by electric and magnetic energy losses, said active part having an internal structure with a degree of heat conductivity for transferring said heat from the place of its generation to at least one transfer surface;

heat dissipator means defining a region of occupancy and including within said region at least one portion having an axis of orientation, said portion being subdivided along said axis into a plurality of zones, said heat dissipator means comprising:

at least one layer of highly heat conductive material;

means mechanically connecting said at least one layer to said transfer surface; and

a multiplicity of cooling fins each extending out of said at least one layer and connected in good heat conductive relationship therewith 'within at least one of said zones of said region portion, said fins each detining major surfaces bordered by free leading and trailing edges and a side edge substantially coincident Lll) with said layer, said major surfaces being spaced from said transfer surface and being oriented substantially parallel to the axis of orientation and having a dimension between the leading and trailing edges thereof in the direction of said axis less Vthan one-fourth of the dimension of said region of occupancy measured in the same direction;

said heat dissipator means comprising at least two layers, each being connected mechanically to one of the two opposing surfaces of a cooling duct penetrating at least one of the active parts, said iins protruding from both said layers into said duct, said fins being exposed to a stream of cooling medium flowing through said duct.

2. A transformer comprising:

at least one active part in which heat is generated by electric and magnetic energy losses, said active part 'having an internal structure with a degree of heat conductivity for transferring said heat from the place of its generation to at least one transfer surface;

at least one layer of highly heat conductive material;

a multiplicity of cooling fins each extending out of said at least one layer and connected in good heat conductive relationship therewith within at least one of said zones of said region potrion, said ns each definng maor surfaces bordered by free leading and trailing edges and a side edge substantially coincident with said layer, said major surfaces being spaced from said transfer surface and being oriented substantially parallel to the axis of orientation and having a dimension between the leading and trailing edges thereof in the direction of said axis less than one-fourth of the dimension of said region of occupancy measured inthe same direction;

at least one layer of said heat dissipator means being connected electrically to a point of a winding of said transformer where the voltage created by inductive voltage distribution is close to the voltage level of said at least one layer, created by capacitive voltage distribution, whereby the heat dissipator means play a secondary role as surge shield, t'hus preventing high frequency oscillations on the winding at a sudden voltage change.

3. A transformer comprising:

at least one active part in which heat is generated by electric and magnetic energy losses, said active part having an internal structure with a degree of heat conductivity for transferring said heat from the place of its generation to at least one transfer surface;

heat dissipator means defining a region of occupancy and including within said region at least one portion having an axis of orientation, said portion beng subdivided along said axis into a plurality of zones, said heat dissipator means comprising:

at least one layer of highly lheat conductive material;

a multiplicity of cooling fins each extending out of said at least one layer and connected in good heat conductive relationship therewith within at least one of said zones of said region portion, said fins each defining major surfaces bordered by free leading and trailing edges and a side edge substantially coincident with said layer, said major surfaces being spaced from said transfer surface and being oriented substantially parallel to the axis of orientation and having a dimension between the leading and trailing edges thereof in the direction of said axis less than one- Y fourth of the dimension of said region of occupancy measured in the same direction;

at least two layers of said heat dissipator means being arranged between two neighboring windings of said transformer, each heat conductive layer being insulated electrically with the normal layer insulation from the closer Winding and having insulation between said two layers to withstand the full test voltage required between said two windings.

4. A transformer comprising:

at least one layer of highly heat conductive material;

a multiplicity of cooling fins each extending out of said at least one layer and connected in good heat conductive relationship therewith within at least one of said zones of said region portion, said fins each defining major surfaces bordered by free leading and trailing edges and a side edge substantially coincident with said layer, said major surfaces being spaced from said transfer surface and being oriented substantially parallel to the axis of orientation and having a dimension between the leading and trailing edges therof in the direction of said axis less than one-fourth of the dimension of said region of occupancy measured in the same direction;

said heat dissipator means comprising at least two layers in close mechanical contact with each other, said layers having electrical insulation applied between them to prevent the iiow of eddy currents therebetween.

5. A transformer comprising:

at least one active part in which heat is generated by electric and magnetic energy losses, said active part having an internal structure with a degree of heat conductivity for transferring said heat from the place of its generation lto at least one transfer surface;

at least one layer of highly heat conductive material;

a multiplicity of cooling fins each extending out of said at least one layer and connected in good heat conductive relationship therewith Within at least one of said zones of said region portion, said iins each dening major surfaces bordered by free leading and `trailing edges and a side edge substantially coincident with said layer, said major surfaces being spaced from said transfer surface and being oriented substantially parallel to the axis of orientation and having a dimension between the leading and trailing edges thereof in the direction of said axis less than onefourth of the dimension of said region of occupancy measured in the same direction;

a sealing enclosure containing an internal cooling and insulating medium being circulated along the active sealed enclosure is extended by radiators characterized by heat dissipator means connected mechanically with heat conductive means to the outside surface of said radiators.

7. A transformer according to claim 6 wherein the planes of the iins of said heat dissipators are tilted away from the center lines of said radiators.

8. A transformer according to claim 7 wherein pairs of said radiators are tilted closer together at the top and the planes of said fins are tilted in a way opposite to the tilt of the radiators.

9. A transformer comprising:

at least one active part in which heat is generated by electric and magnetic energy losses, said active part having an internal structure with a degree of heat `conductivity for transferring said heat from the place of its generation to at least one transfer surface;

at least one layer of highly heat conductive material;

a multiplicity of cooling fins each extending out of said at least one layer and yconnected in good heat conductive relationship therewith within at least one of said zones of said region portion, said fins each defining major surfaces bordered by free leading and trailing edges and a side edge substantially coincident with said layer, said major surfaces being spaced from said transfer surface and being oriented substantially parallel to the axis of orientation and having a dimension between the leading and trailing edges thereof in the direction of said axis less than one-fourth of the dimension of said region of occupancy measured in the same direction;

said ns being substantially rectangular and extending substantially perpendicular to said layer said iins being arranged in at least one zone containing a pair of rows oriented in opposite directions, the tins in one row having punched holes one side thereof and the fins in the other row having punched holes to the other side thereof.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 2/1966 Belgium 336-136 THOMAS I. KOZMA, Primary Examiner U.S. Cl. X.R.