Abstract:
A method of forming a stator winding includes providing a stator core having a plurality of longitudinally extending slots formed about a circumference thereof, providing a plurality of hairpin conductors each having a substantially rectangular cross-section and each having an apex portion and a pair of legs that terminate at respective ends, cutting a bevel at each leg end, inserting the hairpin legs into respective ones of the slots so that the leg ends extend from an axial end of the stator core, bending the hairpin legs to form a plurality of adjacent pairs of leg ends with beveled cuts facing one another, compressing the beveled cuts of each pair together, and resistance welding the pairs to form a plurality of welded joints.

Description:
BACKGROUND 
       [0001]    The present invention is directed to improved reliability and manufacturability of an electric machine and, more particularly, to welded interconnections of a stator winding. 
         [0002]    Dynamoelectric machines in automotive applications include alternators, alternator-starters, traction motors, and others. The stator of an electric machine typically includes a cylindrical core formed as a stack of individual laminations and having a number of circumferentially spaced slots that extend longitudinally through the stator core. A rotor assembly includes a center shaft and is coaxial with the stator core. The stator core has wires wound thereon in the form of windings that extend axially through ones of the core slots. End turns are formed in the windings at the two axial ends of the stator core, whereby a given winding forms an end loop as it extends circumferentially to a different slot. 
         [0003]    Stator windings may be formed by inserting and then connecting together individual “hairpin” conductors each having a crown or apex portion and having two legs that extend in a same general direction. For example, hairpins may be formed from a heavy gauge copper wire with a rectangular cross section, into a predetermined shape for insertion into specific rectangular slots in the stator core. Hairpin conductors are typically coated with an insulating material prior to insertion, so that adjacent hairpin surfaces within a slot are electrically insulated from one another. 
         [0004]    Typically, the apex portions of the hairpins protrude from one axial end of the stator core and the leg ends of the hairpins protrude from the opposite axial end. After insertion, the portions of the legs protruding from the stator core are bent to form a complex weave from wire to wire, creating a plurality of adjacent wire end pairs. Adjacent paired wire ends are typically joined to form individual electrical connections, such as by a welding operation. In a given electric machine, it may be desirable to join together the cross-sectionally short sides of rectangular hairpins. Such short hairpin sides may also include rounded corners, so that the engagement surfaces of the adjacent pair that form the faying surfaces of a weld are difficult to align. As a result, the joinder of adjacent pairs of hairpins may result in a number of connections having increased resistance and/or defective joints. For example, the faying surfaces may slide laterally and become misaligned, and/or the contact surface area at a welded joint may be insufficient for reducing electrical resistance and improving electrical performance. 
       SUMMARY 
       [0005]    It is therefore desirable to obviate the above-mentioned disadvantages by providing a method and structure for joining hairpin type conductors. 
         [0006]    According to an exemplary embodiment, a method of forming a stator winding includes providing a stator core having a plurality of longitudinally extending slots formed about a circumference thereof, providing a plurality of hairpin conductors each having a substantially rectangular cross-section and each having an apex portion and a pair of legs that terminate at respective ends, cutting a bevel at each leg end, inserting the hairpin legs into respective ones of the slots so that the leg ends extend from an axial end of the stator core, bending the hairpin legs to form a plurality of adjacent pairs of leg ends with beveled cuts facing one another, compressing the beveled cuts of each pair together, and resistance welding the pairs to form a plurality of welded joints. 
         [0007]    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 
         [0008]    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: 
           [0009]      FIG. 1  is a partial perspective view of a connection end of an exemplary stator core populated with hairpin type conductors; 
           [0010]      FIG. 2  is a perspective view of a hairpin conductor, according to an exemplary embodiment; 
           [0011]      FIG. 3  is a partial perspective view of a section of conductor wire being formed in an exemplary hairpin manufacturing operation; 
           [0012]      FIG. 4  is a top plan view and  FIG. 5  is a partial perspective view of two exemplary hairpin conductor ends in position for being connected to one another; 
           [0013]      FIG. 6  is a partial perspective view of hairpin conductor ends joined together by an exemplary process described herein; 
           [0014]      FIG. 7  is a partial perspective view of the connection end of a fully populated stator having welded pairs of conductor ends, according to an exemplary embodiment; 
           [0015]      FIG. 8  is a schematic view of conductor ends during a process of being joined together, according to an exemplary embodiment; 
           [0016]      FIG. 9  is a schematic view of conductor ends during a process of being joined together, according to an exemplary embodiment; and 
           [0017]      FIG. 10  is a top plan view of two exemplary hairpin conductor ends in position for being connected to one another, according to an exemplary embodiment. 
       
    
    
       [0018]    Corresponding reference characters indicate corresponding or similar parts throughout the several views. 
       DETAILED DESCRIPTION 
       [0019]    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. 
         [0020]      FIG. 1  is a partial perspective view of an axial end  3  (connection end) of an exemplary stator core  1  populated with hairpin type conductors  2 . Core  1  has a plurality of longitudinally extending slots  4  spaced around a circumference thereof. The radially inner ends of slots  4  are partially closed so that only a narrow longitudinally extending slit  5  separates adjacent slots  4 . This partial closure of the radially inner portion of each slot  4  is defined by opposed shoulder portions  6 ,  7  of adjacent teeth  8 . Each leg portion of a respective hairpin conductor  2  contained within a slot  4  is surrounded by an electrically insulating slot liner  9  or similar sleeve that keeps the conductor portion from electrically connecting or shorting to other conductor portions or to stator core  1 . The conductor ends  10  of a radially outer layer may be connected to conductor ends  11  of an inwardly adjacent radial layer of conductors extending circumferentially around stator core  1 . Such connections may be implemented by welding, brazing, jumpers, and/or by other structure. 
         [0021]      FIG. 2  is a perspective view of a hairpin conductor  2 , according to an exemplary embodiment. Hairpin conductor  2  may be formed of substantially rectangular copper wire, such as wire having rounded corners and nominal dimensions of about one sixteenth inch by one eighth inch. Typically, the end turn portions of all hairpins are disposed at one axial end of stator core  1  and the connection ends of all hairpins are disposed at the opposite axial end. Prior to insertion into slots  4  of a stator core  1 , the leg portions  12 ,  13  are unbent and extend away from the end turn portions, each defined with an apex  14  and turning portions  15 ,  16 . After being inserted into respective slots  4 , legs  12 ,  13  are first bent at respective bends  17 ,  18  and then at respective second bends  19 ,  20 . The two distal ends  21 ,  22  each have respective tapered surfaces  23 ,  24  that are cut along one side thereof. 
         [0022]      FIG. 3  is a partial perspective view of a section of conductor wire  25  being formed in an exemplary hairpin manufacturing operation. A notch is stamped to provide a first taper face  26  and a second taper face  27  that meet at an intersection  28 . After the notch is stamped, the wire section is cut along intersection  28  to thereby provide taper faces  26 ,  27  for respective subsequent use as tapered surface  23  or  24  of hairpin  2 . 
         [0023]      FIG. 4  is a top plan view and  FIG. 5  is a partial perspective view of two exemplary hairpin conductor ends in position for being connected to one another. Conductor ends  21 ,  22  are placed into abutment or into close proximity at a location  29 . 
         [0024]    Two resistance welding electrodes, discussed further below, respectively contact conductor ends  21 ,  22  at engagement locations  30 ,  31 . The electrodes may be angled so that when they are moved toward one another during a welding operation, they push tapered surfaces  23 ,  24  toward one another. For example, when the gap between tapered surfaces  23 ,  24  is approximately one-quarter inch to one-half inch, and when the electrodes press toward one another with a force of approximately fifty to six hundred pounds of force, conductor ends  21 ,  22  are bent so that upper edges  32 ,  33  are made contiguous. When edges  32 ,  33  touch one another, welding current flows through such contiguous portions. As the welding current is gradually increased, the copper of conductor ends  21 ,  22  is annealed and softens. In order to prevent lateral movement of conductor ends  21 ,  22  during the welding, the electrodes may be provided with forked or notched surfaces that keep conductor ends  21 ,  22  in alignment with one another. Peak welding current may be 5,000 to 10,000 amperes, or any other suitable current. Typically, engagement locations  30 ,  31  are as close as practical to respective conductor ends  21 ,  22 . As the welding current and compressive force of the electrodes are maintained, the remaining portions of tapered surfaces  23 ,  24  are made flush and mated. The gradual increase in welding current allows the copper conductor material to soften and be easily compressed. A brazing alloy may be provided. For example, a brazing alloy may be a SIL-FOS composition primarily containing copper, silver, and phosphorous (SIL-FOS is a registered trademark of Handy &amp; Harmon Corp., White Plains, N.Y.). The brazing alloy may be placed to direct welding current there-through. In particular, the electrical resistance of the brazing alloy may be about five or ten times that of the hairpin conductor, whereby the alloy becomes much hotter than the hairpin during the initial welding period and creates a brazed joint, and where continued welding current directs the joinder of hairpin legs along the intersection of tapered surfaces  23 ,  24 . For example, brazing alloy may be formed or placed along outer edge surfaces  32 ,  33  so that the joinder of surfaces  23 ,  24  begins at an intersection of edges  32 ,  33  and proceeds along such joined surfaces toward location  29 . When surfaces  23 ,  24  are properly joined to be flush, the welding current is removed while the electrodes remain in place until the weld cools and is mechanically stable. It may be necessary to pull the brazing alloy tape to disengage it from the weld after a portion of the brazing tape has melted. The brazing alloy may be provided as a tape having lateral perforations that allow the tape to be easily broken away from the portion of the brazing tape being applied. In such a case, a measured, consistent amount of brazing alloy may be applied. 
         [0025]    Welding parameters such as time, incremental (e.g., 0.5 milliseconds) current levels, rise and decay times, pulse width, duty cycle, cooling time, and others may be accurately controlled with a mid-frequency resistance welding machine. For example, the various welding parameters may be controlled according to profiles based on any number of criteria. The non-destructive softening effected by controlled application of welding current allows the copper of conductor ends  21 ,  22  to soften and be more easily bent and compressed. Generally, longer weld periods having a more gradual rise in current and associated heat may allow use of a lesser compressive force. For example, when welding current rise time is increased, a small compressive force of approximately thirty to eighty pounds may be sufficient and this lower compressive force may reduce the possibility of misalignment respecting conductor ends  21 ,  22 . Similarly, the compressive force may be modulated to optimize the level of applied heat. For example, the electrical resistance at a welding target location may be increased by using a relatively smaller amount of compressive force. This increased electrical resistance provides increased localized heat during the application of welding current. The brazing alloy also facilitates rapid heat transfer, so that the softening of copper and the compression of softened copper into a joint may be performed without excessive melting and without effecting a destructive welding process. In particular, the conductivity of brazing alloy is low, for example eighteen percent, and such low conductivity acts as an electrical resistance that generates heat during the resistance brazing/welding. 
         [0026]    In an exemplary embodiment, a large initial compressive force acts to bend the copper so that edges  32 ,  33  come into contact; compressive force may then be reduced to assure that sufficient heat is then maintained for melting the brazing alloy and softening the copper, and where the compressive force may again be modified to assure that the weld is accurately formed. Typically, a fillet (not shown) is formed in a weld region having the highest peak temperature. In another example, a small compressive force can quickly provide sufficient heat to begin softening the copper in joint portions that are already in contact. A relatively large initial compression may bend conductor ends  21 ,  22  until the distal ends of edges  32 ,  33  come into contact; after coming into contact, a greatly reduced compressive force allows the electrical resistance between faying surfaces to remain high, whereby welding heat is maintained. By comparison, if the compressive force exerted by the electrodes is too great, the electrical resistance at the joint becomes too small for maintaining proper welding heat. In another example, a medium-sized initial compressive force, such as about two hundred pounds, and a small welding current, such as about 1,000 amperes for thirty to ninety milliseconds, may be utilized for engaging edges  32 ,  33  and then softening the copper conductor ends  21 ,  22 ; the compressive force may then be increased to about 300 pounds and the welding current may be increased to about 5,000 amperes until faying surfaces  23 ,  24  are fully flush and mated. The brazing alloy melts during this 5,000 ampere period. The associated temperature rise is nonlinear, as the increased heat causes the electrical resistance of the copper to increase. The welding current is turned off while the electrodes remain in their final position until the weld cools. Typically, the welding electrodes have a fluid cooling system in close proximity to the work surface, so that the cooling of the electrodes and weld requires only a small amount of time, such as one-quarter second to one second or more. 
         [0027]      FIG. 6  is a partial perspective view of hairpin conductor ends  21 ,  22  joined together by the exemplary process described above. The radially outer side of conductor end  21  and the radially outer side  35  of conductor end  22  are angled toward each other by the compressing and welding. The corresponding radially inner edges of conductor ends  21 ,  22  are joined along a seam/joint  36  that extends axially outward from axially inner location  29 . An axially inner space  37  may be formed between conductors. 
         [0028]      FIG. 7  is a partial perspective view of the connection end of a fully populated stator having welded pairs of conductor ends  10 ,  11 , according to an exemplary embodiment. Radially outer conductor ends  10  are each joined to a respective one of radially inner conductor ends  11 . The joinder may include brazing and/or welding. 
         [0029]      FIG. 8  is a schematic view of conductor ends  10 ,  11  during a process of being joined together, according to an exemplary embodiment. A radially outer electrode head  38  and a radially inner electrode head  39  are shown at a distance away from respective conductor contact areas  40 ,  41  for clarity of description. The illustrated view shows edges  32 ,  33  already bent, to a position where they are in close proximity, by the compressive force of electrode heads  38 ,  39 . A gap  42  is thereby created between faying surfaces  23 ,  24 , where gap  42  extends axially from location  29  to the distal end at the intersection of edges  32 ,  33 . Electrode head  38  is formed with a “V” shape having contact surfaces  43 ,  44 . Electrode head  39  is formed with a “V” shape having contact surfaces  45 ,  46 . A compressive force  47  urges surfaces  43 ,  44  of electrode head  38  against conductor contact area  40 . A compressive force  48  urges surfaces  45 ,  46  of electrode head  39  against conductor contact area  41 . 
         [0030]    Compressive forces  47 ,  48  may each be applied at an angle in order to optimize and direct the compression of surfaces  23 ,  24  toward one another. Such angle may be changed during a welding operation. For example, the angle may initially be chosen to press edges  32 ,  33  into contact in order to direct the welding current therethrough. Once edges  32 ,  33  are in contact and have an electrical resistance between them, localized heating begins and the angle(s) may be changed for more efficiently and precisely pressing surfaces  23 ,  24  together. In another exemplary embodiment, electrode heads  38 ,  39  may be stepped into position and then used to apply respective compressive forces  47 ,  48  in a first compression, and may then be stepped into another position for applying compressive forces  47 ,  48  in a second compression, etc. Various shapes may alternatively be implemented in forming electrode heads  38 ,  39 , although the respective sizes of electrode heads  38 ,  39  may not exceed available circumferential working space for each conductor pair being joined. For example, respective contacting surfaces of electrode heads  38 ,  39  may each be formed as single plane surfaces each having a groove formed therein for capturing the associated conductor wire targets  40 ,  41 . The shapes of electrode heads  38 ,  39  may be chosen for holding/retaining and/or aligning conductor ends  10 ,  11 . 
         [0031]    Even when the planes defined as tapered surfaces  23 ,  24  are properly aligned for being joined together, lateral movement may be possible, and such lateral movement may be reduced or prevented by forming mating feature(s) in conductor ends  21 ,  22 , discussed further below, and/or by utilizing electrodes having shapes that retain engagement locations  30 ,  31  and prevent lateral misalignment. In an exemplary embodiment, the welding electrodes may have a contact area of approximately one-quarter inch by one-quarter inch, for welding together each of  108  adjacent pairs of conductor ends  21 ,  22  in a fully populated six inch diameter stator where adjacent welds are approximately one-eighth inch apart. 
         [0032]      FIG. 9  is a schematic view of conductor ends  10 ,  11  during a process of being joined together, according to an exemplary embodiment. A radially outer electrode head  49  and a radially inner electrode head  50  are shown at a distance away from respective conductor contact areas  40 ,  41  for clarity of description. The illustrated view shows edges  32 ,  33  already bent, to a position where they are in close proximity, by the compressive force of electrode heads  49 ,  50 . 
         [0033]    Compressive forces  47 ,  48  may each be applied at an angle and/or may be moved axially in order to optimize and direct the compression of surfaces  23 ,  24  toward one another. Such angle may be changed during a welding operation. Electrode heads  49 ,  50  are moved axially into position and are then used to apply compressive forces  47 ,  48  in opposed radial directions. A brazing alloy tape  51  may be applied to the intersection of edges  32 ,  33  after they are bent to be contiguous. As shown by placement direction arrow  52 , brazing tape  51  is moved axially into the brazing location and is pulled away from the brazing location in order to break brazing tape  51  away from the target surface when sufficient brazing alloy has been applied. Brazing tape  51  may alternatively be applied at location  29 , and the brazing alloy may alternatively be utilized in the form of preformed brazing clips rather than as a tape. 
         [0034]      FIG. 10  is a top plan view of two exemplary hairpin conductor ends in position for being connected to one another, according to an exemplary embodiment. Conductor ends  21 ,  22  are placed into abutment or into close proximity at a location  29  (e.g.,  FIG. 4 ) axially inward of the outer axial extremity of conductor ends  21 ,  22 . Tapered surfaces  23 ,  24  are formed with features for maintaining alignment of conductor ends  21 ,  22  during the joinder operation. In the illustrated example, a tongue  53  is formed along tapered surface  23  and a groove  54  is formed along tapered surface  24 . As conductor ends  21 ,  22  are compressed together (e.g.,  FIG. 8  or  FIG. 9 ), tongue  53  mates with groove  54  and thereby aligns conductor ends  21 ,  22 . Features  53 ,  54  may be formed to be self-aligning. For example, tongue  53  and groove  54  may be formed so that as tongue  53  is inserted into groove  54 , the mating is made more precise. Tongue  53  and groove  54  may be formed at any corresponding portions along an axis between location  29  and the axial outer portions of conductor ends  21 ,  22 . Tongue  53  and groove  54  may be formed as projections that extend radially away from the otherwise planar faces of tapered surfaces  23 ,  24 . Tongue  53  and groove  54  may be formed as axial extensions of location  29 . Many other alternative mating/alignment features, such as interlocking, grooved, or barrier forms, may be formed along tapered surfaces  23 ,  24 . Similarly, conductor contact surfaces  40 ,  41  (e.g.,  FIG. 8 ) may be formed with features that engage or otherwise cooperate with respective electrode contacting surfaces  43 ,  45  so that conductor ends  21 ,  22  do not slip during the joinder operation. Such engagement aligns conductor ends  21 ,  22  and prevents lateral movement thereof until the weld has cooled in place. 
         [0035]    By the disclosed embodiments, it can be seen that the surface area of a welded joint is substantially increased by creating tapered faying surfaces  23 ,  24 . This increased joint surface area provides improved electrical performance of an electric machine, including a larger current path, improved machine efficiency, reduced operational temperatures, and reduced power losses. For example, typical electrical currents through a given conductor end joint of a hairpin conductor may be more than 300 amperes, and any improvement in current capability at hairpin joints results in substantial overall machine performance. In a worst case, a thin hairpin connection may act as a fuse and cause an electric machine to stop operating. 
         [0036]    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.