Abstract:
There is provided an enameled flat wire, in which a flat wire conductor having a rectangular cross section composed of four flat surfaces and four rounded corners has an enamel coating with a predetermined thickness for electrical insulation. In the enameled flat wire, a difference in a thickness of the enamel coating on the flat surfaces between a maximum thickness and a minimum thickness is equal to or less than 25% of the predetermined thickness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. application Ser. No. 12/977,627 filed on Dec. 23, 2010, the contents of which are hereby incorporated by reference, which claims priority from Japanese patent application serial no. 2010-002605 filed on Jan. 8, 2010, the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to dies for applying insulation enamel coatings to wires (hereinafter referred to as “coating dies”), and particularly to dies for coating flat wires (which are advantageously used to increase the filing factor (space factor) of coils). Furthermore, the invention relates to enameled flat wires and methods for manufacturing enameled flat wires using the invented dies. 
         [0004]    2. Description of Related Art 
         [0005]    Enameled wires (enamel covered insulated wires) are widely used for coil wires in electrical equipment such as motors and transformers. Such enameled wires are formed by covering an insulation coating around a metal conductor having a desired cross section (such as circular or rectangular) depending on the shape and application of the coil. With the current trend toward small and high power vehicle motors (such as alternators), increasing demands exist for enameled flat wires (having a rectangular cross section) that can be wound into a coil more densely than enameled round wires (having a circular cross section). In order to achieve more accurate coil winding and a higher coil filling factor, there also exist demands for thinner and more uniform insulation enamel coatings. 
         [0006]    Known coating dies for applying an insulation enamel (varnish) around a conductor are classified into die assemblies and solid dies. Die assemblies will be explained first.  FIG. 1  is a schematic illustration showing, an example of a die part of a die assembly for flat wire coating, front, side and top views and an enlarged view of the principal portion.  FIG. 2  is a schematic illustration showing a top view of a die assembly assembled from four die parts shown in  FIG. 1 . 
         [0007]    As illustrated in  FIG. 1 , the die part  1  includes a cylindrical die body  2  and an approximately cubic die base  3 . The die base  3  has an insertion hole  4  for insertion of the die body  2  of another die part  1 . The cylindrical surface of the die body  2  has numerous annular coating grooves  5  for supplying an insulation varnish therethrough in order to apply the varnish around a flat wire conductor. As illustrated in  FIG. 2 , a die assembly  6  for flat wire coating is assembled from four  FIG. 1  die parts  1 . The space surrounded by the four die parts  1  forms a die hole  7  for insertion of a flat wire conductor in order to apply an insulation varnish around the conductor. 
         [0008]    Such die assemblies for enameling flat wires (also referred to as “flat wire coating die assemblies”) as shown in  FIGS. 1 and 2  have an advantage of being adaptable to different size flat wire conductors. That is, the die hole size of a die assembly can be changed by adjusting the insertion depth of each die body into the corresponding die base hole when assembling the die assembly from a set of die parts (four parts in  FIG. 2 ). Also, die assemblies have another advantage that a die assembly can be readily assembled and disassembled, and therefore it can be replaced with another die assembly without the need for cutting a flat wire conductor. 
         [0009]    Although die assemblies have the above advantage of being adaptable to different size flat wire conductors, they have the following disadvantage. It is difficult to accurately and controllably adjust the gap between the die hole of a die assembly and a flat wire conductor to be coated. Therefore, die assemblies cannot be used to form enameled wires requiring a very small allowable error (or tolerance) for the coating thickness or the finished dimensions. 
         [0010]    Next, solid dies will be explained. Solid dies include: A die body (typically, approximately cylindrical) and a die hole of a fixed shape and dimensions formed through the die body. As just noted, the die hole of solid dies has a fixed shape and dimensions. Therefore, the gap between a die hole and a conductor to be coated can be accurately adjusted, and as a result an insulation varnish can be uniformly applied around the conductor. Thus, solid dies are suitably used to manufacture enameled wires requiring a small tolerance for the coating thickness or the finished dimensions. 
         [0011]    Besides the above two types of coating dies, JP-A 2003-297164 discloses an assemblable/disassemblable coating die which combines the advantage of the accurate coating capability of solid dies with the advantage of the assemblability/disassemblability of die assemblies. Needless to say, the die hole of solid dies for enameled flat wires (also referred to as “flat wire coating solid dies”) has a rectangular cross section. 
         [0012]    There still remains a yet-unsolved problem shared by all of the above listed types of coating dies—circumferential nonuniformity in coating thickness. More specifically, a coating formed around a flat wire by a conventional coating die is prone to be selectively thinner on each rounded corner of the flat wire and thicker on the both sides of the corner. That is, an undesirable local thickening/thinning phenomenon (what is called a dog-bone phenomenon, see later-described  FIG. 7 ) is prone to occur on each rounded corner of the flat wire. Conventional thinking has been that this phenomenon is caused by the surface tension difference of an applied insulation varnish and is unavoidable in coatings formed by conventional coating dies. 
         [0013]    There has been a strong demand for solutions to this problem because thickness nonuniformities of an insulation coating (such as a dog-bone surface) may degrade the high voltage electrical insulation and also may make accurate coil winding difficult. In order to prevent such a undesirable local thickening/thinning phenomenon, JP-A 2004-134113 discloses an insulated flat wire in which each corner of a flat wire conductor to be coated is shaped in such a manner that the cross section has a polygonal shape (i.e., has two or more vertices). The total number of vertices per flat wire conductor is 8 or more, and each vertex angle is 120° or more. According to this JP-A 2004-134113, the corners of the flat wire conductor are not exposed to the coating surface, the coating thicknesses on the corners of the conductor are sufficiently thick, and the resulting insulated flat wire has excellent electrical insulation properties. 
         [0014]    As described above, there is an increasing demand for enameled flat wires in terms of the filling factor of coil windings as well as for uniform insulation enamel coatings in terms of breakdown voltage and coil winding accuracy. The technology disclosed in the above JP-A 2004-134113 has an advantage that an insulation coating can be uniformly formed. However, the cross section of the flat wire conductor used in this technology is close to a race-track shape (or an elongated circle) rather than a rectangle; therefore, coils wound from such a flat wire may not have a satisfactorily high filling factor. 
         [0015]    As for solid dies, they can be advantageously used to form uniform insulated round wires. However, even solid dies cannot be effectively used to form uniform enameled flat wires because the above-described undesirable local thickening/thinning (dog-bone) phenomenon inevitably tends to occur. And, there have not yet been proposed any practical solutions to this problem of local thickening/thinning (dog-bone) phenomenon accompanying the flat wire enameling processes. 
       SUMMARY OF THE INVENTION 
       [0016]    In view of the foregoing, it is an objective of the present invention to provide an enameled flat wire having a uniform thickness enamel coating formed on the entire surface of a long flat wire conductor without any undesirable significant local thickening/thinning. 
         [0017]    (I) According to one aspect of the present invention, there is provided an enameled flat wire, comprising: a flat wire conductor having a rectangular cross section composed of four flat surfaces and four rounded corners; and an enamel coating formed on an entire surface of the flat wire conductor with a predetermined thickness for electrical insulation, 
         [0018]    wherein a difference in a thickness of the enamel coating on the flat surfaces between a maximum thickness and a minimum thickness is equal to or less than 25% of the predetermined thickness. 
         [0019]    As used herein and the appended claims, the term “rectangle (rectangular)” refers to a rectangle (including a square) whose four corners are rounded. 
         [0020]    In the above aspect (I) of the present invention, the following modifications and changes can be made. 
         [0021]    (i) A difference in a thickness of the enamel coating between on the flat surfaces and on the rounded corners is equal to or less than 20% of the predetermined thickness. 
         [0022]    (ii) A difference in a thickness of the enamel coating between a maximum thickness and a minimum thickness is equal to or less than 25% of the predetermined thickness. 
         [0023]    (iii) The predetermined thickness of the enamel coating is 20 μm or more, and 100 μm or less. 
         [0024]    (iv) In the cross sectional view, a width and a length of the enameled flat wire are within a range from 0.5 to 17 mm. 
       ADVANTAGES OF THE INVENTION 
       [0025]    According to the present invention, it is possible to provide a flat wire coating die, a manufacturing method of enameled flat wires, and an enameled flat wire, by which the entire surface of the entire surface of a long lat wire conductor is stably coated with a uniform thickness insulating coating without any undesirable significant local thickening/thinning. Also, by using enameled flat wires in accordance with at least some embodiments, accurate coil windings having a high filling factor can be provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic illustration showing, an example of a die part of a die assembly for flat wire coating, front, side, and top views, and an enlarged view of the principal portion. 
           [0027]      FIG. 2  is a schematic illustration showing a top view of a die assembly assembled from four die parts shown in  FIG. 1 . 
           [0028]      FIG. 3  is a schematic illustration showing, an example of a conventional solid die for flat wire coating, a cross sectional view, and longitudinal sectional views along lines A and B. 
           [0029]      FIG. 4  is an enlarged schematic illustration showing a cross sectional view of a coating portion of a conventional solid die for flat wire coating, where a flat wire conductor is passing through the coating portion. 
           [0030]      FIG. 5  is a schematic illustration showing a cross sectional view of a case in which an insulation coating is formed around a flat wire conductor in a tilting relationship with each other when a conventional solid die is used for the flat wire coating. 
           [0031]      FIG. 6  is a schematic illustration showing a cross sectional view of an example of an enameled flat wire formed by using a conventional solid die that suffers from an undesirable local thickening/thinning phenomenon. 
           [0032]      FIG. 7  is a schematic illustration showing a cross sectional view of another example of an enameled flat wire formed by using a conventional solid die that suffers from an undesirable local thickening/thinning phenomenon. 
           [0033]      FIG. 8  is a schematic illustration showing, an example of a solid die for flat wire coating according to the present invention, a cross sectional view, and longitudinal sectional views along lines A and B. 
           [0034]      FIG. 9  is an enlarged schematic illustration showing a longitudinal sectional view of a die hole of an invented solid die for flat wire coating. 
           [0035]      FIG. 10  is an enlarged schematic illustration showing a cross sectional view of a bearing portion (having a constant cross section) of a coating portion of a die hole of an invented solid die for flat wire coating, in which a flat wire conductor is passing through the bearing portion. 
           [0036]      FIG. 11  is a schematic illustration showing a cross sectional view of an example of a flat wire covered with an insulation coating by using an invented solid die. 
           [0037]      FIG. 12  is a schematic illustration showing a cross sectional view of another example of a flat wire covered with an insulation coating by using an invented solid die. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    The present inventor has extensively investigated the above-described undesirable local thickening/thinning phenomenon which occurs in flat wire enameling processes. In particular, the inventor has intensively investigated, for various cases, the insulation varnish flow just after the varnish application until the completion of the baking. The present invention was developed based on these results. 
         [0039]    First, undesirable local thickening/thinning phenomena which occurs in insulation coatings formed by conventional solid dies for flat wire coating will be explained.  FIG. 3  is a schematic illustration showing, an example of a conventional solid die for flat wire coating, a cross sectional view, and longitudinal sectional views along lines A and B. As illustrated in  FIG. 3 , a conventional solid die  10  for flat wire coating includes a die body  8  and a die hole  9  for passing flat wire conductors therethrough. The die hole  9  includes: An entry portion having a cross section monotonically decreasing along the conductor insertion direction; a coating portion having a constant cross section; and an outlet portion having a monotonically increasing cross section. 
         [0040]      FIG. 4  is an enlarged schematic illustration showing a cross sectional view of a coating portion of a conventional solid die for flat wire coating, where a flat wire conductor  20  is passing through the coating portion. As illustrated in  FIG. 4 , the coating portion of the die hole  9  has a rectangular cross section having four straight sides  11  and four rounded corners  12 . And, the flat wire conductor  20  to be coated also has a rectangular cross section having four straight sides  21  and four rounded corners  22 . 
         [0041]    The die hole  9  is slightly larger than the flat wire conductor  20 . And, the flat wire conductor  20  is coated with an insulation varnish by supplying the varnish into a gap between the die hole  9  and the flat wire conductor  20 . Thus, accurate positioning (centering) of the conductor  20  in the hole  9  (i.e., accurate alignment between the hole  9  and the conductor  20 ) is necessary to evenly apply the varnish around the conductor  20 . 
         [0042]    In many cases, such an insulation varnish application and baking process as described above is repeated several times using different dies until a desired enamel coating thickness is obtained. Therefore, the above-described centering procedure also needs to be repeated for each varnish application process, thus incurring a problem of an increased overall centering procedure time (therefore, the problem of increased cost). Meanwhile, the degree of centering during each insulation varnish application and baking process is typically estimated by observation of a cross section of the resulting enameled wire. 
         [0043]      FIG. 5  is a schematic illustration showing a cross sectional view of a case in which an insulation coating is formed around a flat wire conductor in a tilting relationship with each other when a conventional solid die is used for the flat wire coating. As illustrated in  FIG. 5 , the flat wire conductor  20  is covered with the applied varnish  23  (the insulation coating  24 ) in a tilted and/or off-centered relationship both after the varnish application and after the baking. This phenomenon occurs relatively often in conventional processes of manufacturing flat wires. 
         [0044]    A probable cause of such significant tilting and/or off-centering as shown in  FIG. 5  is as follows: As already described, the cross section of the die hole  9  of the solid die  10  has the four straight sides  11  and the four rounded corners  12 . Therefore, an insulation varnish supplied does not evenly flow around the flat wire conductor  20  because the supplied varnish tends to flow into and accumulate on the corners  12 . As a result, once the conductor  20  is misalignedly inserted into the hole  9  or is caused to be misaligned by vibration or other factors after insertion, the conductor  20  cannot readily self-align relative to the hole  9 . 
         [0045]      FIG. 6  is a schematic illustration showing a cross sectional view of an example of an enameled flat wire formed by using a conventional solid die that suffers from an undesirable local thickening/thinning phenomenon. As illustrated in  FIG. 6 , the flat wire conductor  20  is evenly covered with the applied varnish  23  just after the varnish application. However, after the baking, the insulation coating  24  becomes thinner on the rounded corners  22  of the flat wire conductor  20  and becomes thicker than designed on the straight sides (flat surfaces)  21 . 
         [0046]    As previously described, one commonly employed solution to this problem is that an insulation varnish is applied thicker on the rounded corners  22  than on the flat surfaces  21 . The effectiveness of such solutions was evaluated.  FIG. 7  is a schematic illustration showing a cross sectional view of an example of an enameled flat wire formed by using a conventional solid die and the above-described non-uniform varnish application method which still suffers from an undesirable local thickening/thinning phenomenon. 
         [0047]    As illustrated in  FIG. 7 , the varnish applied on each rounded corner flows toward the neighboring flat surfaces as expected, but a thicker coating region is formed on both sides of the rounded corner, as a result exhibiting a more distinct dog-bone surface. Thus, the above-described non-uniform varnish application method still cannot offer a truly effective solution. Further, the dependency of the dog-bone surface formation on the thickness of the varnish  23  applied on the rounded corners  12  was examined. The results show that the smaller the minimum curvature radius of the rounded corners  12  of the die hole  9  is than the minimum curvature radius of the rounded corners  22  of a flat wire conductor  20 , the more pronounced the dog-bone phenomenon becomes. 
         [0048]    A probable cause of such an undesirable local thickening/thinning phenomena as shown in  FIGS. 6 and 7  is as follows: The surface curvatures (the inverses of the curvature radiuses) of an insulation varnish applied on each rounded corner of a flat wire conductor are different from (larger than) the surface curvatures on the adjacent flat surfaces. Thus, a surface curvature (surface tension) effect (a driving force of mass transfer caused by curvature difference) is exerted on the applied varnish during the coating process (just after the varnish application until the completion of the baking). And, such a surface curvature effect drives the varnish applied on each rounded corner of the conductor to flow toward the neighboring flat surfaces of the varnish. According to this assumption, it can be naturally inferred that the driving force of mass transfer in the  FIG. 7  case (in which the minimum curvature radius of the rounded corners  12  of the die hole  9  is smaller than that of the rounded corners  22  of the flat wire conductor  20 ) is stronger than that in the  FIG. 6  case (in which the minimum curvature radius of the corners  12  is approximately the same as that of the corners  22 ). 
         [0049]    Preferred embodiments of the present invention will be described below. However, the invention is not limited to the specific embodiments described below, but various combinations and modifications are possible without departing from the spirit and scope of the invention. 
         [0050]    (Outline Structure of Solid Die for Flat Wire Coating) 
         [0051]      FIG. 8  is a schematic illustration showing, an example of a solid die for flat wire coating according to the present invention, a cross sectional view, and longitudinal sectional views along lines A and B. As illustrated in  FIG. 8 , an invented solid die  30  for flat wire coating includes a die body  31  and a die hole  32  for passing flat wire conductors therethrough. The die hole  32  includes: An entry portion having a cross section monotonically decreasing along the conductor insertion direction; and a coating portion that at least includes a sub-portion having a constant cross section. The die hole  32  has a rectangular cross section, and has a plurality of inwardly projecting protrusions  33  on its inner surface. Meanwhile, as is often employed in wire drawing dies, the die body  31  may include, as a peripheral part of the die hole  32 , a nib and a nib holder for housing the nib. 
         [0052]    Next, each part of the invented solid die for flat wire coating will be explained in detail. 
         [0053]    (Die Hole) 
         [0054]      FIG. 9  is an enlarged schematic illustration showing a longitudinal sectional view of a die hole of an invented solid die for flat wire coating. As illustrated in  FIG. 9 , the entry portion has a monotonically decreasing cross section. The entry portion of  FIG. 9  has front and back entry portions each having a different average taper angle. However, the entry portion may be configured with only the back entry portion. The back entry portion preferably has an average taper angle θ for example of 10° to 16°. The coating portion has at least a bearing portion having a constant cross section. The coating portion may include, on the conductor inlet side, a front streamlining (laminarizing) portion having a monotonically decreasing cross section and/or, on the conductor outlet side, a back streamlining (laminarizing) portion having a monotonically increasing cross section. Or, the coating portion may be configured with only the bearing portion. 
         [0055]    Although, for simplicity of description, the taper angle of the inner surface of the  FIG. 9  die hole abruptly changes at each boundary between adjacent die hole portions, the die hole inner surface is preferably formed to have a taper angle that gradually changes at each boundary. The protrusions  33  will be detailed later. 
         [0056]      FIG. 10  is an enlarged schematic illustration showing a cross sectional view of a bearing portion (having a constant cross section) of a coating portion of a die hole of an invented solid die for flat wire coating, in which a flat wire conductor is passing through the bearing portion. As illustrated in  FIG. 10 , similarly to the die hole  9  of the solid die  10  in  FIG. 4 , a bearing portion of a die hole  32  of an invented solid die  30  for flat wire coating has a rectangular cross section having four straight sides (flat surfaces)  34  and four rounded corners  35 . However, the bearing portion of the solid die  30  has protrusions  33  on each flat surface  34 , unlike the solid die  10  in  FIG. 4 . 
         [0057]    The cross section of the bearing portion of the die hole  32  is not limited to any particular size, but is determined based upon the size of the flat wire conductor  20  to be coated and the desired coating thickness. Preferably, for example, the length W1 is from 1 to 17 mm, and the length W2 is from 0.5 to 4 mm. 
         [0058]    (Protrusion) 
         [0059]    As illustrated in  FIGS. 9 and 10 , each protrusion  33  on the inner surface of the coating portion of an invented solid die  30  is an elongated ridge which preferably runs parallel to the conductor insertion direction. Furthermore, each protrusion  33  preferably extends along the entire length of the coating portion. 
         [0060]    These protrusions  33  work as a streamlining plate for streamlining (laminarizing) the flow of an insulation varnish supplied, thus suppressing nonuniform (turbulent) varnish flow and as a result suppressing misalignment between the flat wire conductor  20  and the insulation coating around the conductor  20 . The protrusions  33  also mechanically suppress significant off-centering and/or tilting of the conductor  20 , thus reducing unevenness in coating thickness. 
         [0061]    The top contour (perpendicular to the conductor insertion direction) of each protrusion  33  is preferably a circular arc, an elongated circular arc or an elliptical arc. In addition, the maximum curvature of the top contour of the protrusions  33  is preferably larger than that of the rounded corners  35 . Thus, the surface of the top portion of each protrusion  33  is free from sharp edges. Therefore, even when the flat wire conductor  20  contacts a protrusion (protrusions)  33  during the coating process, it will not be damaged. As used herein, the term “top portion of a protrusion” refers to a portion of the protrusion from about half of the height to the peak height, and the term “top contour of a protrusion” refers to the contour (perpendicular to a conductor insertion direction) of the top portion of the protrusion. 
         [0062]    As just described, the maximum curvature of the top portion of the protrusions  33  is formed to be larger than that of the rounded corners  35 . This has the following effects and advantages: As described before, conventional solid dies for flat wire coating have a problem in which an insulation varnish applied on the rounded corners of a flat wire conductor is prone to flow into the flat surfaces thereof. And, this is probably caused by a difference between the surface curvature of the varnish applied on the rounded corners and that of the varnish applied on the flat surfaces. 
         [0063]    According to the present invention, in order to solve this problem, an insulation varnish is applied around a flat wire conductor in such a manner that regions having a maximum surface curvature larger than the maximum surface curvature of the varnish applied on the rounded corners of the conductor are intentionally and optimally formed in the applied varnish (these large curvature regions are actually depressions  36  created by the protrusions  33 , see later-described  FIG. 11 ). These depressions  36  provide a varnish surface curvature difference larger than the difference between the varnish surface curvature on the rounded corners and the varnish surface curvature on the flat surfaces. This feature of the invention probably produces the following effects: Mass transfer into the above-described large curvature depressions caused by a surface curvature difference predominantly occurs rather than mass transfer from the varnish surfaces on the rounded corners to the varnish surfaces on the flat surfaces. That is, mass transfer from the varnish surfaces on the rounded corners can be retarded to some extent. As a result, an insulation varnish applied on a flat wire conductor can be baked before the varnish applied on the rounded corners of the conductor starts to deform. 
         [0064]    Preferably, each protrusion  33  is positioned within a certain distance from a rounded corner  35  nearest to the protrusion  33 . More specifically, the distance L between each protrusion  33  and the nearest rounded corner  35  (see  FIG. 10  for the precise definition of the distance L) is preferably equal to or shorter than the minimum curvature radius r of the nearest rounded corner  35  (i.e., L≦r). The height h of the protrusions  33  is properly sized based on the desired coating thickness, the number of coatings, and other factors. For example, the height h is preferably from 50% to 90% of the gap t between the die hole  32  and the flat wire conductor  20 , and more preferably from 50% to 75%. The cross sectional area of the protrusions  33  is properly sized based on the desired total coating thickness and other factors. 
         [0065]      FIG. 11  is a schematic illustration showing a cross sectional view of an example of a flat wire covered with an insulation coating by using an invented solid die. As illustrated in  FIG. 11 , just after the varnish application, an insulation varnish  23  is evenly applied around a flat wire conductor  20  except for the depressions  36  caused by the protrusions  33 . On the other hand, after the baking, the depressions  36  disappear and an insulation coating  24  having a uniform thickness (e.g., 20 μm) is formed around the entire surface of the conductor  20 . 
         [0066]    As has been described, the coating portion of the invented solid die for flat wire coating has a plurality of inwardly projecting protrusions on its inner wall. This configuration enables an insulation coating having a uniform thickness to be controllably formed around a flat wire conductor. Also, according to the invention, the coating thickness on each flat surface of a flat wire conductor can be independently adjusted by changing the height of the protrusions formed on the corresponding die hole surface. In view of various specifications of enameled flat wires, it is preferable that the protrusions on at least two of the four flat inner surfaces of the coating portion of an invented solid die have the same height (or, the highest heights of the protrusions on at least two of the four flat inner surfaces are the same). 
         [0067]      FIG. 12  is a schematic illustration showing a cross sectional view of another example of a flat wire covered with an insulation coating by using an invented solid die. As illustrated in  FIG. 12 , just after the varnish application, the insulation varnish  23  applied on three flat surfaces of the flat wire conductor have the same thickness, but the varnish  23  applied on the other flat surface has a thicker thickness. Similarly to the  FIG. 11  varnish application, the varnish applied on each flat surface has the depressions  36 . Also, the flat wire conductor  20  did not suffer from any twisting. After the baking, the depressions  36  disappear similarly to the  FIG. 11  case. As a result, around the flat wire conductor  20  is formed an insulation coating  24  having the same desired thickness (e.g., 20 μm) on three of the flat surfaces and a desired thicker thickness (e.g., 100 μm) on the other surface. Such an enameled flat wire as illustrated in  FIG. 12  can be formed by using an invented solid die  30  in which the protrusions  33  on three flat inner surfaces  34  of the coating portion have the same height and the protrusions  33  on the other surface  34  has a higher height. 
       Examples 
       [0068]    The present invention will be more specifically described below by way of examples. However, the invention is not limited to the specific examples below. 
         [0069]    Two types of insulation coatings of a designed thickness of 20 μm were formed around a flat wire conductor having a cross section of 1.0 mm×5.0 mm. The insulated flat wire of Comparative Example 1 was formed by applying an insulation varnish around the flat wire conductor using a conventional solid die (see, e.g.,  FIG. 3 ) and by baking it. The insulated flat wire of Example 1 was formed by applying an insulation varnish around the flat wire conductor using an invented solid die (see, e.g.,  FIG. 8 ) and by baking it. 
         [0070]    The thickness distribution of each insulation coating after the baking was measured by optical microscopy. The breakdown voltage of each enameled flat wire was measured according to Method B of JIS C 3003: 1999 (Methods of test for enameled wires). These results are summarized in Table 1. The above thickness distribution measurement was conducted as follows: Each enameled flat wire was transversely cut at five positions. Then, for each cut surface, the coating thickness was measured on eight different points of the flat surfaces of the flat wire conductor and on four different points of the rounded corners. The breakdown voltage of each example given in Table 1 was determined by averaging the breakdown voltages measured on ten specimens. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Coating thickness distribution and measurement 
               
               
                 results of breakdown voltage of coating. 
               
             
          
           
               
                   
                 Coating Thickness 
                   
               
             
          
           
               
                   
                 On Flat 
                   
                 Difference between 
                   
               
               
                   
                 Surface 
                 On Rounded 
                 Max and Min 
                 Breakdown 
               
               
                   
                 (μm) 
                 Corner (μm) 
                 Thicknesses (μm) 
                 Voltage (kV) 
               
               
                   
                   
               
             
          
           
               
                 Comparative 
                 15-26 
                 12-13 
                 14 
                 3.26 
               
               
                 Example 1 
               
               
                 Example 1 
                 20-25 
                 23-24 
                 5 
                 4.71 
               
               
                   
               
             
          
         
       
     
         [0071]    As is apparent from Table 1, in the conventional coating of Comparative Example 1, the coating is thinner on the rounded corners of the conductor and is thicker on part of the flat surfaces (clearly indicating the formation of a dog-bone surface along and near the rounded corners). The thickness difference between the thickest and thinnest points was as large as 14 μm. By contrast, in the invented coating of Example 1, the difference between the resulting thickness and the designed thickness is smaller on both regions (on the rounded corners and on the flat surfaces). And, the thickness difference between the thickest and thinnest points is as small as about ⅓ of that of the conventional coating. Also, the breakdown voltage of the enameled wire of Example 1 is improved to about 1.4 times that of the enameled wire of Comparative Example 1. 
         [0072]    The results described above demonstrate that, by using the invented solid die for flat wire coating, the entire surface of a long flat wire conductor can be stably coated with an insulation coating having, on each straight side of the conductor, a uniform desired thickness without any undesirable significant local thickening/thinning. In addition, an insulation coating having the features just described above can be formed by both vertical and horizontal coating apparatuses employing the invented solid die. 
         [0073]    Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.