Abstract:
A method of assembling a stator includes forming a plurality of cleats into a brazing tape, forming a U-shaped clip using the stamped brazing tape, and attaching the U-shaped clip to a stator conductor, where the attached U-shaped clip is self-secured to the conductor by the cleats. Apparatus for connecting a stator conductor pair includes a brazing clip shaped to conform to and fit over one of the conductors and having generally a U-shape with two sides each including at least one cleat configured to engage the one conductor and secure the respective side thereto. A system for brazing together adjacent pairs of conductors includes a self-securing, three-sided brazing clip having a plurality of cleats formed therein, a comb having a plurality of receptacles structured for retaining respective stator conductors, and electrodes radially aligned with one adjacent pair, where compression of the electrodes sandwiches the brazing clip therebetween.

Description:
BACKGROUND 
       [0001]    The present invention is directed to improving manufacture of an electric machine and, more particularly, to a brazing system and clip for reliably positioning objects during a resistance brazing operation. 
         [0002]    Various electric machines are assembled by fixing a copper conductor to one or more other conductor(s). For example, in a known “hot staking” process, a current is applied by a pair of welding electrodes, where at least one conductor is sandwiched between and engaged by the electrodes. The combination of heat and compressive force softens the copper conductor(s) and causes conductor deformation. After a period of time, current to the electrodes is terminated and the electrodes are removed. The copper conductor(s) re-harden and form a bond with the other conductor(s). Processing continues until all such weld connections are completed. 
         [0003]    Unfortunately manufacturing problems may occur in a hot staking operation. For example, it may be difficult to maintain a constant temperature in a tungsten electrode because the electrode typically becomes hotter with each successive weld when a same current is provided for each weld and insufficient time is provided for electrode cooling between the individual welds. When such electrode becomes sufficiently hot, it may cause damage by penetrating too far when it forcibly contacts the conductor and causes the conductor to completely deform and melt into a U-shape around the electrode. As a result, the weld is faulty and incapable of conducting current in the designed manner. 
         [0004]    Brazing is another technique for fixing and thereby electrically connecting a copper conductor to one or more other conductor(s). In a typical brazing operation, a filler material is positioned in the location where the conductors are to be joined, and heat is generated at a connection resistance to a current provided by electrodes, as in the hot staking process. As the temperature increases, the filler material begins to melt, for example at a temperature at least 500° F. lower than the temperature at which the copper conductor(s) begin to deform. As the filler material melts, it flows between the conductors by capillary action. 
         [0005]    In a typical brazing operation, a thin, flat brazing ribbon is placed between the faying surfaces of the conductors, the electrodes compress against the conductors, and current is applied through the electrodes and the work piece portions being brazed. After the brazing material melts, the remaining ribbon is withdrawn by hand. The brazing process may be applied as a series of successive events for effecting multiple connections. 
         [0006]    In comparison to a hot staking process, brazing advantageously avoids the problem of excessive heat causing conductor deformation. The brazed conductors are not thereby melted so they retain their original shapes, and the respective conductor edges and contours are also not changed by the formation of a weld fillet. Since less heat is required to heat the brazing material to its melting temperature, the brazing process is generally more efficient than a hot staking type welding process. 
         [0007]    In a brazing operation, the brazing material must be carefully positioned and held in place at the conductors until the process is completed. Conventionally, the brazing ribbon is inserted manually, which is inefficient and unsafe because it requires that the fingers of the operator be placed undesirably close to the electrodes, which may be pressed onto the conductors with a large force. Brazing clips may be fitted onto conductors to be brazed. However, conventional brazing clips can move from their proper position. In addition, such brazing clips may block or otherwise interfere with an electrode. The brazing ribbon cannot be allowed to contact the electrodes because the electrodes would become permanently welded to the copper conductor. 
       SUMMARY 
       [0008]    It is therefore desirable to obviate the above-mentioned disadvantages by providing apparatus, system, and method for brazing copper conductor(s). 
         [0009]    According to an exemplary embodiment, a method of assembling a stator includes forming a plurality of cleats into a brazing tape, forming a U-shaped clip using the stamped brazing tape, and attaching the U-shaped clip to a stator conductor, where the attached U-shaped clip is self-secured to the conductor by the cleats. 
         [0010]    According to another exemplary embodiment, apparatus for connecting a stator conductor pair includes a brazing clip shaped to conform to and fit over one of the conductors, the brazing clip having generally a U-shape with a bottom and two sides, the sides each including at least one cleat configured to engage the one conductor and secure the respective side thereto. 
         [0011]    According to a further exemplary embodiment, a system for brazing together adjacent pairs of stator conductor ends includes a self-securing, three-sided brazing clip having a plurality of cleats formed therein, the cleats each structured to engage one of the conductor ends and secure the clip thereto, a comb having a plurality of receptacles structured for retaining respective stator conductors, and first and second electrodes aligned respectively with a radially-inward and a radially-outward side of one of the adjacent pairs of stator conductor ends. Radial movement of one of the electrodes toward the other electrode compresses the one adjacent pair so that the brazing clip is sandwiched therebetween. 
         [0012]    The foregoing summary does not limit the invention, which is defined by the attached claims. Similarly, neither the Title nor the Abstract is to be taken as limiting in any way the scope of the claimed invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0013]    The above-mentioned aspects of exemplary embodiments will become more apparent and will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein: 
           [0014]      FIG. 1  is a schematic view of an exemplary electric machine having a stator core that includes stator windings; 
           [0015]      FIG. 2  is a perspective view of a stator core; 
           [0016]      FIG. 3A  is a cross-sectional view of an exemplary conductor bar used for forming stator windings such as those used in a motor/generator of an electric vehicle;  FIG. 3B  is a perspective view of an exemplary conductor bar segment having a bent “hairpin” shape for insertion into two slots of a stator body; 
           [0017]      FIG. 4  is a partial perspective view of the connection end of an exemplary stator assembly; 
           [0018]      FIG. 5  is a schematic top view of apparatus for a brazing operation, according to an exemplary embodiment; 
           [0019]      FIG. 6  is a perspective view of a cleated brazing clip, according to an exemplary embodiment; 
           [0020]      FIG. 7  is a perspective view of a cleated brazing clip, according to an exemplary embodiment; 
           [0021]      FIGS. 8A-8C  show three embodiments of brazing clips before they are shaped to fit over conductors; 
           [0022]      FIG. 9  is a schematic top view of apparatus for a brazing operation, according to an exemplary embodiment; and 
           [0023]      FIG. 10  is a schematic top view of a multiple position comb electrode, according to an exemplary embodiment. 
       
    
    
       [0024]    Corresponding reference characters indicate corresponding or similar parts throughout the several views. 
       DETAILED DESCRIPTION 
       [0025]    The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of these teachings. 
         [0026]      FIG. 1  is a schematic view of an exemplary electric machine  1  having a stator core  2  that includes stator windings  3  such as one or more coils. An annular rotor body  4  may also contain windings and/or permanent magnets and/or conductor bars such as those formed by a die-casting process. Rotor body  4  is part of a rotor that includes an output shaft  5  supported by a front bearing assembly  6  and a rear bearing assembly  7 . Bearing assemblies  6 ,  7  are secured to a housing  8 . Typically, stator core  2  and rotor body  4  are substantially cylindrical in shape and are concentric with a central longitudinal axis  9 . Electric machine  1  may be a motor/generator such as an automotive alternator/starter. Housing  8  may have a plurality of fins (not shown) spaced apart from one another on a housing external surface for dissipating heat produced in stator windings  3 . 
         [0027]      FIG. 2  is a perspective view of a substantially columnar stator core  2  having a center axis  9 . In an exemplary embodiment, stator core  2  has  108  radially and longitudinally extending slots  12  each extending radially outward of a circumferential inner surface  14  and each having a nominal width  13 . In various embodiments, the radially-inner openings of slots  12  may be narrowed to a width less than nominal width  13 . Slots  12  are shown by example extending axially between axial ends of stator core  2 . Stator core  2  may be formed as a stack of individual steel laminations that have been coated with a thin layer of insulation material. 
         [0028]      FIG. 3A  is a cross-sectional view of an exemplary conductor bar  15  used for forming stator windings such as those used in a motor/generator of an electric vehicle. Conductor bar  15  may be formed of copper, aluminum, or other conductive material. For example, in order to provide a higher output for a given motor size, solid copper wire may be selected because of its excellent conductivity and may have a substantially rectangular cross section, thereby maximizing the amount of copper per unit volume in a stator winding  3 . Conductor bar  15  has an approximately rectangular profile with two long surfaces  18 ,  19 , for example 0.120 inches, and two short surfaces  16 ,  17 , for example 0.060 inches. 
         [0029]      FIG. 3B  is a perspective view of an exemplary conductor bar segment  25  having a bent “hairpin” shape for insertion into two slots  12  of stator body  2 . A first insertion portion  30  and a second insertion portion  31  extend essentially axially outward from respective distal ends  32 ,  33  of conductor bar segment  25 . Bends  10 ,  11  are respectively formed to define obtuse angles at axially outward ends of insertion portions  30 ,  31 , so that the hairpin legs meet to form an obtuse angle defining a center apex portion  23  of conductor segment  25 . After the end turn portion has been formed into its desired shape, conductor bar segment  25  may be coated with an electrically insulating and/or other protective coating. An individual conductor bar segment  25  may for example be inserted into stator  2  so that end  32  is placed into a slot  12  at one radial slot position and end  33  is placed into a slot  12  at another radial position. Although shown with tapered portions, conductor ends  32 ,  33  may alternatively be formed without features. 
         [0030]    Typically, a number of hairpin conductor segments are inserted into slots of a stator core so that the apexes of all hairpins are on one axial end of the stator and the conductor ends are all on the other axial end.  FIG. 4  is a partial perspective view of the connection end of an exemplary stator assembly. Each stator slot may include one or more slot liners  24  that prevent respective hairpin segments  25  from contacting stator core  2  and/or other hairpin segments  25 . When a group of hairpin segments  25  have been inserted and seated into respective pairs of slots, the protruding portions of legs  30 ,  31  (e.g.,  FIG. 3B ) are grasped and bent according to a chosen wiring diagram. As a result, pairs of conductor ends are placed into a connection position. In particular, a short side  16  of a leg of one hairpin  25  is placed adjacent to a short side  17  of a leg of a second hairpin  25 , thereby forming an adjacent pair to be connected by brazing or welding. For example, a conductor end  61  placed in proximity to a conductor end  62  forms an adjacent pair, and a conductor end  63  placed in proximity to a conductor end  64  forms another adjacent pair. 
         [0031]      FIG. 5  is a schematic top view of apparatus for a brazing operation, according to an exemplary embodiment. When hairpin conductors  25  have been installed into stator core  2 , a plurality of adjacent pairs are aligned so that a short cross-sectional side  16  or  17  of one end portion  32  or  33  is in close proximity to a short cross-sectional side  16  or  17  of another end portion  32  or  33 . The two hairpins  25  of each hairpin pair are aligned substantially radially. A three-sided brazing clip  20  is attached to one hairpin  25  of each adjacent pair. Although clips  20  are shown as being alternately placed on the radially-inner hairpin  25  and radially-outer hairpin  25 , either hairpin  25  of an adjacent pair may have an attached clip  20 . When all adjacent pairs have an attached clip  20 , the corresponding stator core is secured for brazing. A radially inner brazing electrode  21  and a radially outer brazing electrode  22  are pressed against radially distal short sides  16  of an adjacent pair, so that brazing clip  20  is sandwiched between radially proximate short sides  17 . As force is applied to the conductors from electrodes  21 ,  22 , a voltage is applied across electrodes  21 ,  22 , causing a current to flow through the conductor ends  32 ,  33 . Electrical current is applied by a brazing power supply such as a mid-frequency inverter type machine. For example, a current of about seven thousand amperes may be applied for one-quarter second. Generally, the brazing current may be around 10,000 amps, and may be applied for about 1 second or less, thereby producing sufficient heat to melt the brazing material. The electric current causes the copper conductors and clip to heat up. The clip  20  has a melting temperature of about 1420° F., whereas copper hairpin conductors  25  have a higher melting temperature of about 2000° F., such that only the brazing material liquefies during the brazing operation. As the brazing material liquefies, the phosphorous component cleans the copper and the brazing alloy flows by capillary action into the spaces between the conductor ends being joined. 
         [0032]    The electrical current creates heat at the conductor interface that includes clip  20 , whereby brazing clip  20  melts and ends  32 ,  33  of the adjacent pair are resistance brazed to one another. The amount of radially-directed force pressing adjacent ends  32 ,  33  together, and/or the current level(s) and duration may be adjusted to optimize the quality of the brazed joint. The force being applied by electrodes  21 ,  22  is typically maintained for a period of time after the current is terminated. Thereafter, electrodes  21 ,  22  are removed and the brazing material hardens and forms a bond with the conductor ends being joined. For example, the brazing material may require 0.25 to 0.5 seconds to harden and electrodes  21 ,  22  may remain in biasing force position for one second. 
         [0033]    The alignment of short sides  16 ,  17  of adjacent hairpin ends  32 ,  33 , and the retention of brazing clip  20  on one hairpin  25  of each adjacent pair are each subject to problems. For example, when individual brazing clips  20  are placed onto a hairpin  25  for each of 108 total adjacent pairs to be joined, and when such subassembly is handled and moved within a manufacturing location, some of the  108  brazing clips  20  may be dislodged. In addition, the two adjacent hairpin ends  32 ,  33  of an adjacent pair may become misaligned before or during brazing. As a result, unless care is taken during manufacturing, one or more brazed joints may be defective. In such a case, repair is difficult, and defective connection joints reduce performance and decrease machine efficiency. 
         [0034]      FIG. 6  is a perspective view of a cleated brazing clip  26 , according to an exemplary embodiment. A brazing tape is punched to form cleats  27 , and is then formed into a three-sided shape. For example, the brazing tape may be about 0.004 inch thick and have a width of about 0.25 inch. A bottom portion  28  has a width dimension slightly greater than the width of a short side  16 ,  17  of conductor  15  ( FIG. 3A ). Inner side walls  34 ,  35  each contain an array of cleats  27 . Side walls  34 ,  35  are folded back on themselves at respective bends  36 ,  37 , thereby creating respective pull tabs  38 ,  39 . Bends  36 ,  37  may each be perforated. In an exemplary embodiment, brazing clip  26  may be formed of a thin brazing material, for example an alloy of copper, phosphorus and silver. There are a large number of commercially available brazing alloys that may be suitable for use with the present invention, and one suitable alloy is known by those skilled in the art as BCuP5, which has a composition of about 15% silver, 5% phosphorous, 80% copper and trace amounts of other materials. Alternatively, the brazing tape may be AgCuP or other suitable material. In use, cleats  27  keep brazing clip  26  secured in proper position on conductor end  32  or  33  prior to the brazing operation. Pull tabs  38 ,  39  may be held while electrodes  21 ,  22  are moved toward one another into the brazing position, and may then be pulled slightly to be broken away at folds  36 ,  37  during brazing. In this manner, a consistent brazed joint is obtained. 
         [0035]      FIG. 7  is a perspective view of a cleated brazing clip  40 , according to an exemplary embodiment. A brazing tape is punched to form a first series of cleats  41  and a second set of cleats  42 . Cleats  41 ,  42  may have a serrated profile to provide a number of very sharp edges, and may be formed in rows. Corner spaces  43  are provided without cleats to assure structural integrity when the brazing tape is formed into a clip. A bottom portion  44  has a length dimension slightly greater than the width of a short side  16 ,  17  of conductor  15  ( FIG. 3A ). Inner side walls  45 ,  46  each contain cleat series  41 ,  42 , and also contain an end cleat or inwardly bent tab that engages the conductor of an armature to hold it in place. Side wall  45  has an inwardly bent end corner  47  formed to be a pointed and sharp cleat. This allows corner cleat  47  to pierce the surface of conductor  15  and thereby “dig in” to the copper conductor and prevent movement. Similarly, cleat or inwardly bent tab  47  may also be positioned in an indentation (not shown) previously formed in conductor  15  to thereby resist movement of brazing clip  40  by engagement of cleat  47  with such indentation. In like manner, side wall  46  may have an inwardly bent corner  48  formed to be symmetrical respecting cleat  47 . Cleat or inwardly bent tab(s)  47 ,  48  may also biasingly engage conductor  15  to thereby increase the bearing pressure exerted between brazing clip  40  and conductor  15 . For example, when clip  40  is formed with only one bent corner cleat  47 , engagement of cleat  47  with conductor  15  forms a spring that exerts pressure on the side of conductor  15  opposite cleat  47  to thereby resist movement of brazing clip  40  on conductor  15  by frictional forces. In such a case, edge cleats  49  may be formed to be skewed respecting cleat series  41 ,  42  in order to create resistance to twisting torque and lateral movement. In a given embodiment, the number and shapes of cleats is a design variable. Since there are typically four corners on the material that is used to form the clip, there can be four cleats formed using bent corners. 
         [0036]    Additional features may be formed in a three-sided brazing clip. For example, in certain embodiments it may be desirable to form a depression on the conductors  15 , to assist the clip in remaining in its installation position. In other embodiments, a clip having a single cleat  47  in the form of a bent corner may be sufficient.  FIGS. 8A-8C  show three embodiments of clips  40  before they are formed into the shape to fit over the conductors. In  FIG. 8A , a clip  40  is shown having a single cleat  47 .  FIG. 8B  illustrates an option in which two cleats  47 ,  48  are provided.  FIG. 8C  shows a clip  40  having two cleats  47 ,  51 . One of skill in the art would recognize other configurations within the scope of these teachings. Advantageously, the inventive clips with cleats provided by these teachings are not limited to use with any specifically shaped brazing clip. Rather, the clips can be formed in any of a wide variety of shapes and still be formed with the inventive cleats. 
         [0037]      FIG. 9  is a schematic top view of apparatus for a brazing operation, according to an exemplary embodiment. As described above regarding  FIG. 5 , when hairpin conductors  25  have been installed into stator core  2 , a plurality of adjacent pairs are aligned so that a short cross-sectional side  16  or  17  of one end portion  32  or  33  is in close proximity to a short cross-sectional side  16  or  17  of another end portion  32  or  33 , and is aligned substantially radially. A three-sided brazing clip  20  is attached to one hairpin  25  of each adjacent pair being brazed together. A comb  50  is used to temporarily retain the radial outer edge  17  of the radially outer hairpin  25 , for each adjacent pair. For example, comb  50  may contain five slots  52 , or any appropriate number of slots  52 , each sized to have a circumferential width slightly greater than the width of a short cross-sectional side  16  or  17 . Comb  50  is curved in correspondence with the curvature of the array of adjacent hairpin pairs. Comb  50  may be structured as an electrode, or a separate radially-outer electrode  22  may be used. For example, radially-outer electrode  22  may be axially outward of comb  50  so that when comb  50  secures a number of radially-outer hairpins  25 , electrode  22  may be moved circumferentially to be aligned with and then contact an individual hairpin side  16  or  17 . 
         [0038]    In an exemplary embodiment, comb  50  receives and secures radially outer sides of hairpin conductors  25 , and inner electrode  21  moves circumferentially along the radially-inner portion of the adjacent pairs to sequentially resistance braze one pair at a time. A rotary indexer having a servomotor, such as an x-y-z table, may be used to position a moving electrode  21 . Typically electrodes  21 ,  22  contain about 25% tungsten, which acts to partially block heat transfer to water cooling channels, but such electrodes may be safely operated to remain at a temperature that does not damage contacting surfaces  16 ,  17 . For example, a mid-frequency inverter type of brazing machine (not shown) may supply current in bursts of about 0.25 second, but the total machine time between successive brazes may be about ten to twenty seconds when a stabilizing period, an unclamping time, an electrode rotation time, an indexing time, and a reclamping time are accounted for. In an alternative embodiment, radially inner electrode  21  may have a wide coverage so that it contacts more than one hairpin  25 , and radially outer electrode  22  may be moved circumferentially for sequentially clamping, brazing, and unclamping an outer hairpin surface  16  or  17 . The inner edges  53  of comb slots  52  may be curved or beveled to avoid unnecessarily torqueing or damaging hairpins  25 . Secure engagement of comb  50  with hairpins  25  allows radially outer electrode  22  to more quickly disengage from a radially outer surface  16  or  17  while still allowing the freshly brazed joint to stabilize, whereby sequential electrode positioning time is decreased. Comb portions  54  may incorporate individual electrode elements, as discussed further below. Comb teeth  55  may incorporate cooling channels, insulating portions, and/or alignment pins, where appropriate. 
         [0039]      FIG. 10  is a schematic top view of a multiple position comb electrode  60 , according to an exemplary embodiment. Comb receptacle portions  54  are separated from one another by teeth  55 , and may each contain a brazing electrode  56 . Electrodes  56  may be flush with a receptacle back surface  57  or they may be recessed slightly. When electrodes  56  are recessed, the intermediate material between surfaces  57  and electrodes  56  may be formed of an electrically conductive metal such as copper. When electrodes  56  are flush with respective surfaces  57 , the material of comb  60 , in whole or in part, may have properties of electrical insulation, heat conductance, rigidity, flexibility, and others. Comb  60  may alternatively be formed with a concave curvature for securing and providing electrodes to radially-inward hairpin surfaces  16  or  17 . For example, a brazing operation may utilize one or more inner and outer combs  60  for sequentially brazing individual adjacent hairpin pairs without moving electrodes, by sequentially applying the brazing electrical current to individual ones of electrodes  56 . In example, more than one adjacent pair may be brazed at the same time when both inner and outer comb electrodes  60  are used. Additionally, brazing of individual adjacent pairs may be performed in a skipping order, whereby freshly brazed joints are allowed to stabilize and cool while brazing continues for an adjacent pair located several slots away from the fresh braze. In such a case, the compression of electrodes being pushed toward one another may be maintained after the brazing electrical current has been turned off, allowing the freshly brazed joint to stabilize and harden rather than becoming separated. The melted brazing clip  20 ,  26 ,  40  may thereby be more accurately formed into a desired fillet around the joint. In a further example, a comb electrode  60  may secure the radially-outer hairpin surfaces  16  or  17  while radially-inner electrode  21  (e.g.,  FIG. 5 ) is sequentially stepped from one radially-inner hairpin surface  16  or  17  to the next. The amount of radially-directed force pressing adjacent ends  32 ,  33  (e.g.,  FIG. 5 ) together, and/or the electrical current level(s) and duration may be adjusted to optimize the quality of the brazed joint. 
         [0040]    While various embodiments incorporating the present invention have been described in detail, further modifications and adaptations of the invention may occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.