Abstract:
The apparatus for cooling a high power electrical transformer and electrical motors uses thermally conductive material interleaved between the turn layers of a high power transformer and iron core laminates to provide a low resistant thermal path to ambient. The strips direct excess heat from within the interior to protrusions outside of the windings (and core) where forced air or thermally conductive potting compound extracts the heat. This technique provides for a significant reduction of weight and volume along with a substantial increase in the power density while operating at a modest elevated temperature above ambient.

Description:
This application is a Divisional Application of the application having the Ser. No. 08/940,179 filed Sep. 30, 1997 now U.S. Pat. No. 6,259,347. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains generally to electrical power devices and more particularly to an apparatus for cooling electrical power devices. 
     2. Description of the Related Art 
     The power rating of present-day electrical devices, such as power transformers and motors, is limited by heat accumulation due to resistive losses in the copper windings and, in the case of power transformers, to losses from eddy currents and hysteresis within the iron or ferrite cores. It is not generally recognized that the magnetic flux within a transformer core remains approximately constant when the power output is increased. It is therefore unnecessary to increase the amount of iron or ferrite core material to increase the size of the transformer core in order to deliver more power. The trapped heat produced by the windings while operating at high power is the major limiting factor for high power transformers. 
     Different approaches have been attempted to try and remove heat from the core of power transformers. Some of these are the increasing of wire size to reduce resistive losses; immersion of the transformer in circulating coolant oil; air cooling of the transformer windings; increasing the operating frequency of the transformer to reduce windings; and increasing the thermal conductivity of the insulating potting compound around the transformer windings. All of these, however, impact on the mechanical size and weight of the transformer designs limiting the use of these applications. Without proper cooling the efficiency and reliability of these transformers and motors are considerably reduced. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide an apparatus for cooling high power electrical devices. 
     Another object of this invention is to provide a cooler operating high power electrical device that is of light weight, low cost, higher power density, and highly efficient design. 
     These and other objectives are obtained by placing thermal conductive strips between the turn layers along the axis and perpendicular to the turns of an high power electrical device, such as a transformer or motor, which extends outside of the windings or between the laminates of the core. The excess heat is conducted outward from the interior of the device along the strips to the outside of the device&#39;s windings where it is extracted from the protrusions by means of a highly thermal-conductive potting compound that has a short thermal path to a small heat sink. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cutaway view of a transformer with a thermal conductive strip between layers of wire turns around the transformer core. 
     FIG. 2 shows the temperature gradient for a transformer constructed utilizing current state-of-the-art techniques. 
     FIG. 3 shows the temperature gradient for a transformer constructed utilizing a thermal conductive strip technique. 
     FIG. 4 shows a cutaway view of a transformer with a thermal conductive strip between layers of wire turns around the transformer core and a thermocooler. 
     FIG. 5 a  shows an electric motor with a thermal conductive strip between windings of the motor. 
     FIG. 5 b  shows a cutaway of a motors laminations with thermal conductive strips interleaved between laminations. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The apparatus for cooling a high power electrical device, such as a transformer  10 , as shown in FIG. 1, comprised of various core materials such as laminated iron, ferrite, and other core materials known to those skilled in the art. The transformer core  12  is comprised of windings of electrical conducting material  14 ; preferably copper wire, preferably electrically insulated with a flexible, high dielectric material such as KAPTON® type 150FN019, manufactured by DuPont of Wilmington, Del., or similar material, wrapped around the transformer core  12 . KAPTON® FN film is a DuPont KAPTON® HN film coated on one or both sides with a TEFLON® FEP (fluorinated ethylene propylene copolymer) fluorocarbon resin to impart heat sealability, to provide a moisture barrier and to enhance chemical resistance. The KAPTON® prevents electrical shorts between conductors and adjacent layers. Heat is dissipated from the transformer core  12  to ambient through a heat sink  17  such as a base plate. 
     A thermally conductive material, or strip,  16  placed in preselected locations between the windings of electrically conductive material  14 , the ends of which protrude outside of the area covered by the conductive material  14 . In the example shown in FIG. 1 of a completed transformer  10 , the thermally conductive material  16  is inserted between every other layer of electrically conductive material  14 . The thermally conductive strip  16 , is preferably a high modulus carbon graphite laminate material, such as an Amoco type K1100X pitch fiber processed by Composite Optics of San Diego, Calif. The laminate of the conductive strip  16 , is an ansotripic material that is highly efficient in conducting heat along the fiber orientation which is unidirectional. An alternative material for the thermally conductive strip  16  is copper or a ceramic, however these have not been found to be as efficient in conducting heat away from the center of a device, such as the transformer  10 , as the high modulus carbon graphite laminate material. 
     The thermally conductive strip  16  normally has a smooth epoxy surface finish. To improve the thermal interface by as much as 10%, the strips  16  must be lightly scraped with a sharp instrument, such as a razor blade, to remove a small portion of the residual epoxy and fibers left over from the manufacturing process. After scraping, the strip  16  will appear dull with a graphite appearance. 
     Because the thermally conductive strip  16  normally will have sharp edges on the sides, a narrow glass tape (not shown), approximately 0.005 inches thick, 0.250 inches wide, and having a voltage breakdown of approximately 5 kV, such as 3M glass cloth tape No. 361, a pressure sensitive, 7.5 mil tape good to a temperature of 235° C., manufactured by 3M Electrical Products Division of Austin, Tex., is used to buffer the layers of the windings  14  from the thermally conductive strip  16  to prevent damage to the winding  14  coating thereby shorting out the transformer. 
     The glass tape (not shown) is placed on the edge of the thermally conductive strip  16  on both sides of the strip  16  and offset by one-half the tape width parallel to the strips  16 . In the art this technique is commonly referred to as “butterflying.” The application of the glass tape (not shown) forms a wedge adjacent to the edge of the strip  16 . 
     A thermally conductive grease, such as type 120-8, manufactured by Wakefield of Wakefield, Mass., is placed in the wedge formed by the tape (not shown) and the strip  16 ; a technique well known to those skilled in the art. The strip  16  is installed into the core  12  on top of the thermal grease and a second application of the thermal grease is used to cover the strip  16 . The thermal grease is placed between the two layers of glass tape (not shown) and a second piece of glass tape (not shown) is placed over the first by starting at one edge and lowering the tape (not shown) to the strip  16 . A light pressure is uses to encompass the two glass tapes (not shown) together and make contact with the strip  16  sealing the thermal grease inside of the structure. This is accomplished on both sides of the strip  16 , as previously stated. Heat generated within the transformer by resistive losses in the windings of electrically conductive material  14  and due to eddy currents within the core  12  is conducted to the portions of the thermally conductive strip  16  protruding outside of the electrical windings of conductive  14  and in contact with the ferrite core or iron laminates  12 . 
     Surrounding the transformer  10  is a high thermal-conductivity potting compound  22 , such as STYCAST® 2850, or similar material. STYCAST® 2850 is a highly filled, castable epoxy system manufactured by Emerson &amp; Cumming, Inc. of Lexington, Mass. Potting of the transformer core  12  is accomplished by placing the completed wound copper-core in a mold (not shown) in which potting compound  22  is molded around the transformer core  12  to provide a short thermal path to a base-plate main heat sink  17  where excess heat is dissipated to surround atmosphere. The mold (not shown) with the transformer  10  and potting compound  22  is placed into an evacuated chamber (not shown) until the potting compound  22  expands to the top of the mold (not shown) and cured for approximately two hours at approximately 100 degrees centigrade. The vacuum atmosphere within the chamber (not shown) further forces the thermally conductive epoxy (not shown) in and around the windings  14  of the completed copper core and the mold profile, thereby, further enhancing the heat dissipation of the strips  16 . The vacuum is applied and released a number of times until the potting compound  22  stops expanding to insure that very little air remains within the windings  14  or mold assembly (not shown). This will eliminate core failures due to corona. Additional potting compound  22  may have to be added to the mold (not shown) so as to cover completely the windings  14  when done. 
     The potting compound  22  on a transformer  10  is extended to the outer edge of the transformer core  12  on the base plate side only. On the other side the potting compound  22  need extend only past the outer edges of the thermally conductive strip  16 . 
     To prevent mechanical stresses on the transformer core  12  due to the expansion of the potting compound  22 , the mold assembly should be designed so as to provide a “head space” or gap  23  between the potting compound  22  and the transformer core  12 . In assembly this space is filled with a thermal heat sink strip , such as SIL-PAD® 2000, manufactured by Berquist of Minneapolis, Minn. 
     Alternatively, in place of the potting compound  22 , the heat may be conducted from the ends of the thermally conductive strips  16  by the use of a fan (not shown), a technique that is well known to those skilled in the art. 
     In a design of a test transformer, a 2 kva (2 kW) power transformer providing 1.2 lb/kW was constructed using modern state-of-the-art techniques well known to those skilled in the art. The design measures 3.02 inches by 3.17 inches by 2.22 inches, and weighed 2.4 pounds. In tests, the transformer constructed according to state-of-the-art techniques, after 40 minutes, showed a windings temperature of 200° C. at the center of the windings and suffered catastrophic failure due to excess heat (FIG.  2 ). 
     A duplicate transformer  10  weighing approximately 0.21 lb/kW was constructed utilizing the technology set forth in this invention with the K1100 conductive strips  16  placed within the windings  14  of the transformer. The design measured 3.02 inches by 3.17 inches by 2.22 inches and weighed 2.4 pounds. In tests, the transformer  10  with the thermally conductive strips  16  placed alternately between windings (FIG. 1) showed, after approximately 40 minutes, a windings  14  temperature of approximately 70° C. without failure (FIG.  3 ). 
     This invention allows for the reduction in size of a high power transformers by a factor of 4 to 8 and a reduction in weight by a factor of 4 to 6, and an increase in power density by 5 to 10 in power. The efficiency of the transformer is improved by maximizing the heat transfer from the transformers interior and minimizing voltage breakdown. The thermal properties of each core  12  will dictate the quantity of thermally conductive strip  16  material required to lower the transformer temperature to a predetermined level, some testing may be required to established the optimal amount needed to provide proper cooling. 
     When additional cooling is required or to raise the power of a transformer  20 , a thermocooler  18 , such as model CP2-127-06-7 made by Melcon of Trenton, N.J., may applied to the outside of the transformer  20 . The thermocooler  18 , with or without a cooling fan (not shown). Control of the thermocooler  18  may be such that it could be turned on and off as cooling demands raise and lower. The thermocooler  18  may be attached to the outer portions of the transformer  20  where it could be easily removed for replacement, if required. In some instances it may be desirable to selective control the operation of the thermocooler  18 , therefore a control device such as a timer (not shown) or thermal switch (not shown) may be integrated into the transformer  20  package to either increase the thermal conductivity or decrease it by switching the thermocooler on or off, as desired. 
     Although this embodiment has been described in relation to exemplary device such as a transformer, the claimed invention may equally well be utilized in other types of electrical devices where internal heat is a problem, such as motors, modulation transformers, etc. The size of the transformer is not of concern, it may vary from a small transformer used in switching power supplies to power transformers used in electrical distribution systems. Further, the frequency of the electrical current within the devices to be cooled is irrelevant, e.g., 60 cycles to 400 cycles operate the same thermally. High frequency transformers have higher copper losses due to skin effects. This additional heat may also be removed by the thermally conductive strip as set forth in this invention. 
     When applied to electrical motors  30 , as shown in FIG. 5 a , pieces of thermally conductive strip  16  are placed between windings of the motor  30  or interleaved into vertically stacked motor laminations  32 , as shown in FIG. 5 b . The internal heat from the motor laminations  32  and windings  36  is conducted from the interior of the motor  30  to the outer portions where the heat is then dissipated through the motor case  34  to ambient atmosphere. 
     Although the invention has been described in relation to the exemplary embodiment thereof, it will be understood by those skilled in the art that still other variations and modifications can be affected in the preferred embodiment without detracting from the scope and spirit of the invention as stated in the claims.