Abstract:
An electrical device operating on the principle of induction, such as a transformer. The device employs High Temperature Superconductors to build pancake coils having a very low height to diameter ratio. These pancake coils are placed around ferromagnetic core legs as in a conventional transformer. In multiphase applications, the low height to diameter ratio of the pancake coils causes the transformer to become quite wide. The present invention proposes overlapping the adjacent pancake coils in a multiphase induction device to reduce the width. A specific example of a 3-phase power transformer is presented.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Some of the research leading to the development of the proposed invention was sponsored by the Office of Naval Research, under CAPS Program N 00014-00-1-0778. 

   CROSS-REFERENCES TO RELATED APPLICATIONS 
   Not Applicable. 
   MICROFICHE APPENDIX 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the field of electrical devices. More specifically, the invention comprises a novel construction for an inductive device such as a transformer. 
   2. Description of the Related Art 
     FIG. 1  illustrates a prior art transformer  10 . The components are housed within tank  12 , the front half of which is cutaway in the view. This particular transformer is of the 3-phase variety, as is typically used in power transmission equipment. It has three vertical assemblies, one for each phase. The first phase is assembled upon phase  1  core leg  18 , the second upon phase  2  core leg  20 , and the third upon phase  3  core leg  22 . Upper yoke  14  and lower yoke  16  structurally support the three core legs. 
   The windings for all three phases are identical (phase  1  winding  28 , phase  2  winding  30 , and phase  3  winding  32 ). Those skilled in the art will known that many different types of transformer windings are employed for power transmission devices. While the particular type of winding is unimportant to the present invention, a concentric-type winding is illustrated. With reference to phase  1  winding  28 , the reader will observe that low voltage winding  26  is placed immediately around phase  1  core leg  18 . High voltage winding  24  is then placed around low voltage winding  26 . 
   Of course, a complete description of a transformer would include dielectric insulators, cooling fluids or gases, and various coil taps used to deliver the primary voltage and extract the secondary voltage. As these devices are well understood by those skilled in the art, they will not be illustrated nor described. However, in reviewing  FIG. 1 , the reader will be generally familiar with a typical construction for prior art 3-phase transformers. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is an electrical device operating on the principle of induction, such as a transformer. The device employs High Temperature Superconductors to build pancake coils having a very low height to diameter ratio. These pancake coils are placed around ferromagnetic core legs as in a conventional transformer. In multiphase applications, the low height to diameter ratio of the pancake coils causes the transformer to become quite wide. The present invention proposes overlapping the adjacent pancake coils in a multiphase induction device to reduce the width. A specific example of a 3-phase power transformer is presented. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a perspective view, showing a prior art transformer. 
       FIG. 2  is an isometric view, showing a pancake coil. 
       FIG. 3  is a perspective view, showing a transformer using pancake coils. 
       FIG. 4  is a perspective view, showing a transformer using overlapping pancake coils. 
   

   REFERENCE NUMERALS IN THE DRAWINGS 
   
     
       
             
             
             
             
           
         
             
                 
             
           
           
             
               10 
               transformer 
               12 
               tank 
             
             
               14 
               upper yoke 
               16 
               lower yoke 
             
             
               18 
               phase 1 core leg 
               20 
               phase 2 core leg 
             
             
               22 
               phase 3 core leg 
               24 
               high voltage winding 
             
             
               26 
               low voltage winding 
               28 
               phase 1 winding 
             
             
               30 
               phase 2 winding 
               32 
               phase 3 winding 
             
             
               34 
               low aspect pancake coil 
               36 
               low aspect transformer 
             
             
               38 
               high voltage pancake coil 
               40 
               low voltage pancake coil 
             
             
               42 
               turn 
               44 
               overlapped transformer 
             
             
                 
             
           
        
       
     
   
   DETAILED DESCRIPTION OF THE INVENTION 
   Those skilled in the art will know that “pancake” type coils have been in common use for several decades. These coils are formed in many different fashions. They all have the common trait of a low height to diameter ratio.  FIG. 2  shows low aspect pancake coil  34 . It is made of a plurality of turns  42 , which are electrically connected in series. While the method of connection is again unimportant to the present invention, the particular version shown in  FIG. 2  employs a spiral path. Such pancake coils may be stacked and connected in series in order to create additional turns. 
   Low aspect pancake coil  34  has an inner circumference, an outer circumference, and a height (The thickness of the “pancake”). It is said to be a “low aspect” pancake because the difference between the diameter at the outer circumference and the diameter at the inner circumference is significantly greater than the height of the coil. Thus, it has a thin “pancake” shape. 
   Transformers have traditionally been constructed with copper or aluminum conductors. However, recent advances in the field of superconductivity have created the opportunity to construct transformers using superconducting material. High Temperature Superconductors (“HTS”) are particularly advantageous, since they do not require the extreme cold traditionally needed in order for conventional materials to exhibit superconductivity (ordinarily around 4 degrees Kelvin, using coolants such as liquid Helium). Of course, the phrase “high temperature” is a subjective one. Cryogenic cooling—the use of liquefied gases—is still needed for HTS materials. However, this can be relatively “mild” cryogenic cooling using liquid Nitrogen (around 77 degrees Kelvin). 
   Although it has long been known to create pancake type coils, the efficiency of such devices was traditionally limited by the relatively small amount of Ampere-Turns they could create. It is now possible to construct a pancake coil using superconducting tape conductors (made of HTS materials). These are wound into a shape generally similar to the configuration shown in  FIG. 2 , although they would often employ many more turns. 
   The ability for a pancake coil such as low aspect pancake coil  34  to carry enormous current—via the use of HTS materials—means that inductive windings no longer have to have a significant axial height (like the prior art device shown in  FIG. 1 ). Instead, efficient inductive devices can be made using one or two pancake coils for each core leg. 
     FIG. 3  shows one such design. Low aspect transformer  36  contains the same major elements as the typical prior art design, also enclosed by a tank  12 . The first phase is assembled upon phase  1  core leg  18 , the second upon phase  2  core leg  20 , and the third upon phase  3  core leg  22 . Also like the prior art design, upper yoke  14  and lower yoke  16  structurally support the three core legs. The core legs are electrically insulated from each other, as well as the two yokes. 
   However, each phase uses only two sets of two pancake coils. The first phase, as an example, has two pancake coils connected in series to form high voltage pancake coil  38 . Immediately below this set lies low voltage pancake coil  40 —also comprised of two pancake coils connected in series. Those skilled in the art will know that the high voltage and low voltage coils include a different number of total turns in series (in order to use the induction phenomenon to step up or down the voltage). Nevertheless, dimensions of the low voltage and high voltage windings are nearly the same due to the fact that some turns are connected in parallel for the low voltage winding. The second and third phase assemblies are constructed in the same fashion as the first phase assembly. 
   The embodiment illustrated shows the high voltage and low voltage pancake coils being stacked one on top of the other. Using different winding techniques, it is also possible to create different mechanical constructions. However, as the stacked construction is easy to visualize, it has been employed here. 
   In order to maintain the pancake coils at the temperature needed for superconductivity, they are encased within cryostat  42  (shown cut away in the view for visualization of the internal components). Cryostat  44  is filled with a cryogenic fluid which is maintained at the appropriate temperature by external systems (not shown). It completely surrounds the sets of pancake coils. It also has three vertical openings, allowing the three core legs to pass through. Obviously, cryostat  42  must be insulated from the various conductors, and must also prevent leakage of the contained fluid. 
   In studying  FIG. 3 , the reader will observe that the use of the pancake coil construction has allowed a drastic reduction in the height of each of the three core legs, compared to a prior art transformer. The nature of the pancake coils has, however, also created a substantially greater overall width. This occurs for obvious reasons. Returning to  FIG. 2 , the reader will note that low aspect pancake coil  34  has an inner circumference and an outer circumference. The outer circumference is substantially displaced from the inner one. Thus, since the inner circumference must be placed next to the core leg, the overall diameter of the winding substantially increases over the prior art device shown in  FIG. 1 . 
   Returning to  FIG. 3 , it is obvious that this increase in diameter is tripled by the presence of the three phases.  FIG. 4  shows one embodiment of the present invention, denoted as overlapped transformer  44 . The elements contained within the device are identical to low aspect transformer  36 , including the use of cryostat  42 . However, in overlapped transformer  44 , adjacent sets of pancake coils have been overlapped to conserve width. The reader will observe that a portion of the pancake coil assembly centered on phase  2  core leg  20  lies over portions of the pancake coil assemblies centered on phase  1  core leg  18  and phase  3  core leg  22 . 
   The shape of cryostat  42  and the height of the core legs must be altered to accommodate this overlap. In comparison to low aspect transformer  36 , overlapped transformer  44  is somewhat taller, but also significantly less wide. 
   The type of overlapping shown is by no means the only possibility. The three assemblies could be overlapped in a staircase fashion just as easily. Likewise, although it is customary to arrange the three core legs in a line, this need not be the case. They could just as easily be placed at the corners of an isosceles triangle. The overlapping principle disclosed would be equally applicable to the triangular arrangement. Many other geometries are possible. 
   Although a three phase transformer has been shown, the principle also applies equally to two phase devices (where one coil assembly would overlap the other). Likewise, although transformers have been used to illustrate the invention, it would apply to virtually any type of device operating on the principle of electrical induction. 
   Those skilled in the art will know that many more internal elements are needed for the construction of a practical transformer than are illustrated in  FIGS. 3 and 4 . These elements, such as insulators and coil taps, are well known to those skilled in the art. As they are not relevant to the novel aspects of the invention, they have not been illustrated nor described in detail. 
   The preceding descriptions do contain significant detail regarding the novel aspects. They should not be construed, however, as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. Thus, the scope of the invention should be fixed by the following claims, rather than by the examples given.