Abstract:
Rectangular conductor wires ( 38 ) are often used in alternator applications requiring a high slot fill to maximize output and efficiency. However for lower output and efficiency applications, round conductor ( 40 ) wire may increase cost competiveness in these alternators. A common lamination for a core ( 110 ) alternatively accommodates both rectangular conductor wires ( 38 ) and round conductor wires ( 40 ) for different applications without any other component changes. The lamina has a slot ( 112 ) that aligns round wire ( 40 ) in a single row within the slot and provides a predetermined clearance from the slot opening ( 126 ). A stator core ( 110 ) formed from these laminae has a relatively high slot fill factor when wound with the round wire ( 40 ). The same stator core ( 110 ) can be alternatively wound with square wire ( 38 ) to increase the slot fill factor even higher. The common lamination results in two stator configurations: a high slot fill version (round wire) and a very high slot fill version (square wire).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 62/323,313, filed Apr. 15, 2016, the contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    This application relates to the field of electric machines, and particularly to the accommodation of multiple conductor geometries in rotary electric machines. 
       BACKGROUND 
       [0003]    Rotary electric machines operate by exploiting the interaction of the magnetic fields of a rotor and a stator rotating relative to one another. In a common application, the rotor is disposed within and rotatable relative to the stator. The rotor is typically fixed to a shaft mounted for rotation centrally by bearings disposed in a casing that surrounds the stator. These machines include a configuration of insulated wire coils or windings in the stator, which are distributed about the stator central axis. The windings are typically arranged in a progressive sequence to define different electrical phases. The stator windings are typically wound around ferromagnetic poles of the stator core to enhance the strength of the stator&#39;s magnetic field. The stator poles generally are tooth-like cross sections that are usually rectangular or trapezoidal, and typically defined by slots in the stator core. 
         [0004]    In a polyphase electric motor, flowing current of different phases through a progressive sequence of wire windings in the stator generates rotating magnetic fields in the stator, which impart electromechanical torque to the rotor and its shaft. Conversely, in a polyphase electric generator or alternator, externally forced rotation of the shaft and rotor imparts rotation to magnetic fields that induce current flows in the stator windings. 
         [0005]    The stator core may be formed by a stack of interlocked, ferrous laminae, which are typically formed from electrical sheet steel. Each lamina has a central hole with the holes of all the laminae being aligned in the lamina stack to form a stator core central bore having a central axis. Thus, the stator core may be a unitary annular member with its central bore defining a radial internal bore face that is generally cylindrical and centered about the central axis. The radial internal bore face is provided with the generally axially extending, elongate slots formed by aligned, notched portions of the lamina holes that define the stator poles. The stator slots pass axially through the lamina stack adjacent the central bore since they extend over the entire axial length of the lamina stack and are open radially on an internal side and the two opposite axial ends. 
         [0006]    The slots formed by the lamina stack typically lie in planes that intersect along and contain the stator central axis, but the slots can also be inclined with respect to central axis. The stator slots are typically distributed at an even pitch about the stator central axis. Relative to the stator, radial and axial directions mentioned herein are respective to the stator central axis, and the stator slots generally extend radially from the central bore face into the stator core and axially along the bore length. Thus, each stator core slot has a generally axial length dimension extending along the length of the stator core bore, a width dimension extending circumferentially about the central axis between a pair of adjacent stator core teeth, and a radial depth dimension extending between the slot opening proximate the stator core central bore and the slot bottom. 
         [0007]    Elongate electrical conductors that define the stator coil windings are disposed in and extend along the stator slots. By virtue of the conductors being routed through the stator slots, they are wrapped about the stator poles. Typically, a stator slot insulator insert is located between the conductors and the walls of the stator slots to ensure electrical isolation of the stator windings from the stator core. Typically, the insulator insert is formed of a flexible, electrically insulative sheet material such as a paper or plastic that is inserted into the slot before a conductor is installed therein. The sheet material forms an electrically insulative layer between the conductors and their respective stator slot. 
         [0008]    In a polyphase rotary electric machine, the stator coil windings include a plurality (typically three, five, six, or seven) of different phase windings each formed of elongate electrical conductor material such as a copper magnet wire or bar. The conductor cross-section is typically circular or rectangular (including square), or oval. Round wire of conventional sizes may be used for the conductors. Optionally, thick bar conductors can be used for making a wire coil with a designed current-carrying capacity requiring fewer turns than is possible with smaller sized round wire. 
         [0009]    Each stator slot may accommodate multiple, small diameter wire segments that are wound in bulk and rather randomly oriented and located, and typically cross over each other, within the slot. Examples of such windings are well-known to those having ordinary skill in the relevant art. Alternatively, the stator slots may have a depth and/or width that is a multiple of the cross-sectional dimension of the conductor, in the slot&#39;s radial and/or circumferential direction. In the example of a three-phase stator, multiple electrical conductor segments may be housed within each of the stator slots with the electrical conductors arranged in a predetermined winding pattern to form the stator winding. 
         [0010]    The particular winding patterns of stator windings can vary considerably between different machine designs and include, for example, standard-wind configurations, S-wind configurations, or segmented conductor configurations. S-wind configurations typically include a continuous length of wire that is wound in an out of the various slots of the stator, where end loops connect a in-slot portions in one layer to an in-slot portion in the same layer, to form a complete winding. The wire includes relatively straight lengths that are positioned within the slots of the core portion and curved lengths that extend between in-slot portions at the ends of the core portion. Similarly, in a segmented winding configuration, the windings typically comprise a plurality of segmented conductors which include in-slot portions and ends that are connected together. The in-slot portions of the conductors are positioned in the stator slots, and the ends of the conductors are connected to form windings for the electric machine. 
         [0011]    It is known that increasing the fill of a conductor material in a stator slot improves both the performance and efficiency of an electrical machine. Such high slot fill stators often include rectangular shaped conductors that are aligned single file in one radial row in each slot and that fit closely to the width of the insulated, rectangular shaped core slots. The use of rectangular wires in high slot fill stator applications can, however, increase the complexity of placing the winding in the stator. In addition, the cost per kilogram of rectangular wire is significantly more than the cost per kilogram of round wire. Thus, for applications requiring less output and efficiency, the use of round wire instead of rectangular or square wire in the same electrical machine could provide significant cost savings by taking advantage of existing technologies, such as current S-wind technology. However, existing high slot fill applications incorporate stator slot geometries that often only accept rectangular wire. 
         [0012]    Accordingly, it would be advantageous to provide a common lamination component for electric machines which alternatively accommodates both rectangular or square wire and round wire for different applications without the need for any other component changes in the electrical machine. 
       SUMMARY 
       [0013]    A stator core for an electric machine in one embodiment includes a core body having a plurality of teeth, adjacent teeth of the plurality of teeth defining respective slots in the core body, each slot having a slot depth in a respective direction along which the teeth extend from the core body and a slot width in a respective direction along which the teeth are spaced circumferentially from each other, the core body has a configuration in which a plurality of elongate wire segments having a round cross section are arranged in single file within each slot, the round cross section having a diameter that approximates the slot width, the slot depth has a range defined by the equation (N*Ø)+0.2≦D C ′≦(N+1)*Ø where N equals the number of the wire segments in the slot and θ equals the diameter of the second wire segments. 
         [0014]    Two stator core assemblies for respective electric machines in one embodiment includes a first stator core and a second stator core that is identical to the first stator core, each stator core having a plurality of teeth with adjacent teeth of the plurality of teeth defining respective slots, the first stator core has a first plurality of elongate wire segments arranged in single file within in each slot, the first wire segments having a rectangular cross section, and the second stator core has a second plurality of elongate wire segments arranged in single file within each slot, the second wires segments having a round cross section. 
         [0015]    A method of producing a plurality of stator assemblies in one embodiment includes forming a plurality of identical cores, each core having a plurality of teeth, adjacent teeth of the plurality of teeth defining respective slots in the core body, winding a first core of the plurality of cores with a first plurality of elongate wire segments, the first wire segments having a rectangular cross section and are arranged in single file within in each slot, the rectangular cross section having a cross-sectional dimension that approximates a slot width of the slot, winding a second core of the plurality of cores with a second plurality of elongate wire segments, the second wires segments having a round cross section and are arranged in single file within each slot, the round cross section having a diameter that approximates the slot width, the wound first core has a first slot fill factor and the wound second core has a second slot fill factor, the second slot fill factor within 10 percent of the first slot fill factor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a perspective view of a high slot fill core of a prior art electric machine; 
           [0017]      FIG. 2  shows a cross-sectional view of a portion of the core of  FIG. 1  with at least one slot of the core including a plurality of rectangular conductors; 
           [0018]      FIG. 3  shows an enlarged view of the slot of  FIG. 2  illustrating the features of the slot and the rectangular conductors in more detail; 
           [0019]      FIG. 4  shows the core of  FIG. 2  overlaid with round conductors to illustrate the incompatibility of the core to alternatively accommodate both rectangular and round conductors; 
           [0020]      FIG. 5  shows a cross-sectional portion of a core in accordance with the present invention with at least one slot of the core including a plurality of round conductors; 
           [0021]      FIG. 6  shows enlarged view of the slot of  FIG. 5  illustrating the features of the slot and the round conductors in more detail; 
           [0022]      FIG. 7  shows an overlay of the core of  FIG. 2  and the core of  FIG. 5  with a bottom of the slots of the core of  FIG. 2  shown in phantom lines; and 
           [0023]      FIG. 8  shows a flow diagram of a method for producing a first core assembly with rectangular conductors and second core assembly with round conductors using the core of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIGS. 1 and 2  depict a prior art stator core  10  for use in a three-phase rotary electric machine. The core  10  has a core body  11  that includes a number of core slots  12  arranged about a central axis  14  with each of the core slots  12  associated with one of the three current phases. This association progressively repeats itself in sequence around a circumferential inner surface  16  of the core  10 , which defines a substantially cylindrical bore  18  through the core  10 . The core slots  12  extend in a direction, indicated by an arrow  13 , parallel to the central axis  14  of the core  10  between a first end  15  and a second end  17  thereof. As used herein, an “axially upward direction” is defined as moving toward the first end  15  of the core  10  and an “axially downward direction” is defined as moving toward the second end  17  of the core  10 . 
         [0025]    The core slots  12  are equally spaced around the circumferential inner surface  16  of the stator core  10  and respective inner surfaces  19  of the core slots  12  are substantially parallel to the central axis  14 . The core slots  12  have a depth D C  along a radial axis, indicated by an arrow  23 , and are configured to receive a stator winding, discussed in more detail below. As used herein, a “radial inward direction” is defined as moving towards the central axis  14  of the core  10  and a “radial outward direction” is defined as moving away from the central axis  14 . 
         [0026]    The core  10  is formed of a stack of aligned, interconnected electrical steel laminae, which define the circumferential inner surface  16  and the core slots  12 . The following features described with reference to the “core” or “core body” also describe features of individual lamina since the stack of laminae forms the core. Similarly, figures of the present application that depict cross-sections of the “core” or “core body” can be interpreted as depicting cross-sections of individual lamina. The core slots  12  are separated from one another by stator poles or teeth  20  formed by the lamina stack. As viewed axially along arrow  13 , the longitudinal inner surfaces  19  of the core slots  12  are generally U-shaped with approximately parallel sides  22 ,  24 . The core slot sides  22 ,  24  extend in the radial outward direction from a slot opening  26  in the circumferential inner surface  16 . As best shown in  FIG. 3 , the depth D C  of each core slot  12  extends from the slot opening  26  at the circumferential inner surface  16  to a core slot bottom  28  that is spaced in the radial outward direction from the slot opening  26 . 
         [0027]    With reference to  FIGS. 2 and 3 , the core slots  12  are each fitted with respective insulation sleeves  21  that electrically insulate one or more elongate segments of copper magnet wire conductors  38   a - h  positioned in the core slots  12  from the core  10 . The wire conductors  38   a - h  shown in  FIGS. 2 and 3  each have a rectangular cross-sectional shape with a length L RCS  extending substantially parallel to the radial axis  23  and a width W RCS  extending substantially perpendicular to the radial axis  23  between the parallel sides  22 ,  24 . The cross-sectional area of each of the rectangular conductors  38   a - h  is substantially equal. The rectangular conductors  38   a - h  are aligned in a single row by the respective parallel sides  22 ,  24  of the core slots  12 . As shown, it is common that the rectangular shaped conductors may include radii on the corners intermediate two adjacent edges. 
         [0028]      FIG. 3  shows an enlarged cross-sectional view of one of the core slots  12  of the core  10  with eight rectangular conductors  38   a - h  positioned therein. The rectangular conductors  38   a - h  may be positioned in any configuration, including S-wind or segmented conductor configurations. In the configuration shown, each conductor  38   a - h  is separated from neighboring conductors in the core slot  12  by at least one insulation layer  30  and from the core  10  by the insulation sleeve  21 . The insulation layer  30  and the insulation sleeve  21  each have a substantially uniform thickness. As used herein, “substantially uniform thickness” means a thickness in which deviations across integral surfaces of an element from which the thickness is measured are minimized by known manufacturing methods. The length L RCS  and the width W RCS  of each the rectangular conductors  38   a - h  referred to herein includes the thickness of the insulation layer  30 . As shown in  FIG. 3 , the insulation sleeve  21  is positioned along the parallel sides  22 ,  24  and the core slot bottom  28  so as to substantially surround the conductors  38   a - h  in each of the slots  12  and thus defines a sleeve slot  32  with a sleeve slot width W S  and a sleeve slot depth D S . 
         [0029]    The sleeve slot width W S  at the slot openings  26  is slightly larger than the width W RCS  of the rectangular conductors  38   a - h  so as to permit the conductors  38   a - h  to be inserted radially into the core slots  12 . The circumferential spacing between the adjacent teeth  20  may be consistent along the depth D C  or the circumferential spacing may widen slightly in the radial outward direction from the opening  26  to a width W C  of the core slot  12  defined between the interfacing parallel sides  22 ,  24  of the circumferentially adjacent teeth  20 . The stator winding may be prepared using any variation of a conventional technique suitable for rectangular wire, and the rectangular conductors  38   a - h  are inserted either individually or as a group into their respective core slot  12  through its opening  26 . 
         [0030]    When viewed along a cross-sectional plane situated perpendicular to the central axis  14 , each core slot  12  and sleeve slot  32 , and the common opening  26  thereto are centrally positioned about a slot radial centerline  34  ( FIG. 2 ). The difference in the core slot width W C  and the sleeve slot width W S  is substantially equivalent to twice the thickness t (i.e., 2t) of the insulation sleeve  21  that lines the core slot  12  and defines the interior of the sleeve slot  32 . The insulation sleeve  21  is a known, flexible, dielectric material layer having thermal properties suitable for conductively transferring heat between the rectangular conductors  38   a - h  and the core  10 . As mentioned above, the sleeve  21  may be made of plastic or paper sheeting, for example. As shown, each sleeve  21  extends continually along the perimeter of its respective core slot  12  and terminates at the circumferential inner surface  16 . 
         [0031]    The core slot width W C  and the insulation sleeve thickness t are such as to allow unrestricted radial insertion of the rectangular conductors  38   a - h  into each core slot  12 , between the slot walls defined by the interfacing, parallel surface portions of its respective insulation sleeve  21 . Thus, W S =Wc−2t, and approximates the width W RCS  of the rectangular conductors  38   a - h . There is typically a clearance of, for example, from about 0.1 to 0.8 mm between the sleeve slot width Ws and the width W RCS  of the rectangular conductors  38   a - h , the clearance being comparatively much smaller than the width W RCS  of the rectangular conductors  38   a - h . In the embodiment depicted in  FIGS. 2 and 3 , the insulation sleeve thickness t is about 0.125 mm and the core slot width W C  is about 2 mm such that sleeve slot width is 1.75 mm (2 mm−(2*0.125 mm)). A rectangular conductor with a width W RCS  of about 1.6 mm will have approximately 0.15 mm of clearance (1.75 mm-1.6 mm) between the parallel walls of the insulation sleeve  21 . Thus, a single file arrangement of the rectangular conductors  38   a - h  is maintained along the depth D S  of the sleeve slot  32  with the surfaces of the arranged rectangular conductors  38   a - h  extending parallel with the width W RCS  of the conductors in abutment with one another. 
         [0032]    One issue with the core  10  depicted in  FIGS. 1-3  is that the core slots  12  are specifically configured to accept a specific number of rectangular wire conductors to achieve a desired performance characteristic. This limitation is acceptable for some applications of S-wind electrical machines since rectangular wire is typically used for high slot fill applications in order to achieve maximized output and efficiency from the machine. However, there are many applications requiring lower output and efficiency in which round wire could be used instead of rectangular or square wire in order to take advantage of cost savings associated with use of common S-wind technology and lamination design. 
         [0033]    There are numerous design considerations in standardizing a lamination slot design that alternatively accommodates both rectangular wires and round wires. For instance, round wire can be desirable over square wires as it is much easier to insulate and therefore significantly less expensive to manufacture. As is known, square wire can be desirable over round wire in some applications because the cross-sectional area is higher and, therefore, the slot fill is higher, which improves performance and efficiency while lowering stator temperature. A lamination with a slot width that is too wide is not desirable because the teeth will be thin and become easily saturated with flux. A lamination with a slot with that is too narrow is not desirable because the wire will become too thin and the current density of the wire will be too high. 
         [0034]    It has been determined that a desirable number of wires per slot is five to seven. However, windings with odd numbers of wires can be difficult to manufacture, so a particularly desirable number of wires per slot is six. For a 12V system, the equation V=N*d(phi)/d(t), where V=induced voltage, N=number of electrical turns, phi=magnetic flux, and t=time, suggests that six electrical turns may be an excessive number of turns for an electrical machine. Moreover, the rotor poles are typically twelve to sixteen poles due to manufacturing limitations. As is known, the number of poles and the surface linear speed of the rotor determine d(phi)/d(t). Thus, to achieve the proper V for a 12V system, the number of poles times the number of turns for a high slot fill wye-wound electrical machine is typically around forty-eight, and the number of electrical turns is typically three or four. To achieve three or four electrical turns with a six wire-in-a-slot stator, the winding could be bifilar resulting in three turns, or the winding could be delta-connected, resulting in three and one-half effective-wye turns since delta effective wye turns equals turns/1.734. 
         [0035]    It has additionally been determined that for a six wire-in-a-slot stator with round wires, it is desirable to have about a 0.5 mm clearance from the circumferential inner surface  16  to the innermost conductor in the radial outward direction (i.e., rectangular conductor  38   a  in  FIGS. 2 and 3 ).  FIG. 4  depicts the core  10  of  FIG. 2  overlaid with six round conductors  40   a - f  in the sleeve slot  32 . For clarity, the rectangular conductors  38   a - h  are illustrated using solid lines while the round conductors are illustrated using dashed lines. The round conductors  40   a - f  each have a diameter Ø that is approximately equal to the width W RCS  of the rectangular conductors  38   a - h . As used herein, a first dimension that is “approximately equal to” or that “approximates” a second dimension means the first dimension is within a narrow dimensional range measured from the second dimension. For example, a round conductor  40   a - f  with a diameter Ø of 2.0 mm is not approximately equal to a rectangular conductor  38   a - h  with a width W RCS  of 1.6 mm, whereas a round conductor  40   a - f  with a diameter Ø of 1.575 mm is approximately equal to a rectangular conductor  38   a - h  with a width W RCS  of 1.6 mm. As is shown in  FIG. 4 , the innermost round conductor  40   a  has no clearance from circumferential inner face  16  in the radial outward direction. Instead, the innermost round conductor  40   a  extends in the radial inward direction from the circumferential inner face  16 . Thus, the slot design of the prior art core  10  does not sufficiently accommodate both rectangular wires and round wires in alternative applications under the aforementioned preferred conditions. 
         [0036]      FIGS. 5 and 6  show a core  110  for use in a three-phase electrical machine and configured to accept both rectangular conductors (i.e.,  38   a - h  shown in  FIGS. 2 and 3 ) and round conductors  40   a - f  in alternative applications. The core wound with rectangular conductors may sometimes be referred to herein as a “first configuration”, while the core wound with round conductors may sometimes be referred to herein as a “second configuration” although the structure of the core is identical in both the first and second configurations. The core  110  also has the advantage that a desired clearance between the circumferential inner surface  16  and the innermost conductor (i.e., rectangular conductor  38   a  in  FIGS. 2 and 3  and round conductor  40   a  in  FIGS. 5 and 6 ) results with either conductor geometry. In  FIGS. 5 and 6 , elements of the core  110  that are similar to those of the core  10  of  FIGS. 1-4  are identified with like numerals whereas new or changed elements are identified with a single prime symbol or by incrementing the prior reference number by 100. As used hereafter, the terms “rectangular conductor”, “rectangular wire”, or the like refer to a conductor with a rectangular, non-square cross-sectional geometry when viewed along a cross-sectional plane situated perpendicular to the central axis  14  of the core  110 . 
         [0037]    The core  110  has a core body  111  that includes a number of core slots  112  arranged about the central axis  14  with each of the core slots  112  associated with one of the three current phases. This association progressively repeats itself in sequence around a circumferential inner surface  16  of the core  110 , which defines a substantially cylindrical bore  18  through the core  10 . The core slots  112  extend parallel to the central axis  14  of the core  110  between the first end  15  and the second end  17  thereof. The core slots  112  are equally spaced around the circumferential inner surface  16  of the stator core  110  and are substantially parallel to the central axis  14 . The core slots  112  have a depth D C ′ ( FIG. 6 ) along the radial axis  23  ( FIG. 1 ). 
         [0038]    The core  110  in the illustrated embodiment is formed of a stack of aligned, interconnected electrical steel laminae, which define the circumferential inner surface  16  and the core slots  112 . The core in other embodiments can be formed in any other known manner. The following features described with reference to the “core” or “core body” also describe features of individual lamina since the stack of laminae forms the core  110 . The core slots  112  are separated from one another by stator poles or teeth  120  formed by the lamina stack. As viewed axially along the arrow  13  ( FIG. 1 ), the longitudinal inner surfaces  119  of the core slots  112  are generally U-shaped with approximately parallel sides  122 ,  124 . The core slot sides  122 ,  124  extend in the radial outward direction from the slot opening  126  in the circumferential inner surface  16 . As best shown in  FIG. 6 , the depth D C ′ of each core slot  112  extends from the slot opening  126  at the circumferential inner surface  16  to a core slot bottom  128  that is spaced in the radial outward direction from the slot opening  126 . 
         [0039]    The core slots  112  are each fitted with respective insulation sleeves  121  that electrically insulate the round conductors  40   a - f  positioned in the core slots  112  from the core  110 . As discussed with reference to  FIG. 4 , the diameter Ø of the round conductors  40   a - f  is approximately equal to the width W RCS  of the rectangular conductors  38   a - h  depicted in  FIGS. 1-3 . Similarly, the cross-sectional area of each of the round conductors  40   a - f  is substantially equal. The round conductors  40   a - f  are aligned in a single row by the respective parallel sides  122 ,  124  of the core slots  112 . 
         [0040]      FIG. 6  shows an enlarged cross-sectional view of one of the core slots  112  of the core  110  with the six round conductors  40   a - f  positioned therein. In the configuration shown, each conductor  40   a - f  is separated from neighboring conductors in the core slot  112  by at least one insulation layer  130  and from the core  110  by the insulation sleeve  121 . The insulation layer  130  and the insulation sleeve  121  each have a substantially uniform thickness. The diameter Ø of each of the round conductors  40   a - f  referred to herein includes the thickness of the insulation layer  130 . In the embodiment shown, the diameter Ø is approximately 1.6 mm. As shown in  FIG. 6 , the insulation sleeve  121  is positioned along the parallel sides  122 ,  124  and the core slot bottom  128  so as to substantially surround the conductors  40   a - f  in each of the slots  112  and thus defines a sleeve slot  132  with a sleeve slot width W S  and a sleeve slot depth D S ′. 
         [0041]    Since the diameter Ø of the round conductors  40   a - f  is approximately equal to the width W RCS  of the rectangular conductors  38   a - h , the sleeve slot width W S  at the slot openings  26  can be the same for both the cores  10  and  110 . Similar to the core  10 , the circumferential spacing between the adjacent teeth  120  of core  110  may be consistent along the depth D C  or the circumferential spacing may widen slightly in the radial outward direction from the opening  26  to a width Wc of the core slot  112  defined between the interfacing parallel sides  122 ,  124  of the circumferentially adjacent teeth  120 . The stator winding may be prepared using any variation of a conventional technique suitable for round wire, and the round conductors  40   a - fh  are inserted either individually or as a group into their respective core slot  112  through its opening  26 . 
         [0042]    When viewed along a cross-sectional plane situated perpendicular to the central axis  14 , each core slot  112  and sleeve slot  132 , and the common opening  26  thereto are centrally positioned about the slot radial centerline  34  ( FIG. 5 ). The difference in the core slot width W C  and the sleeve slot width W S  is substantially equivalent to twice the thickness t (i.e., 2t) of the insulation sleeve  121  that lines the core slot  112  and defines the interior of the sleeve slot  132 . The insulation sleeve  121  of  FIGS. 5 and 6  is formed from the same material as the insulation sleeve  21  of  FIGS. 2 and 3 . As such, the sleeve slot width W S  is approximately equal to the slot width W C  minus two times the thickness t of the insulation sleeve  121  (i.e., W S =Wc−2t), and approximates the diameter Ø of the round conductors  40   a - f  with similar clearances as were noted with the rectangular conductors  38   a - h . Thus, a single file arrangement of the round conductors  40   a - f  is maintained along the depth D S ′ of the sleeve slot  132  with circumferential surfaces of the arranged round conductors  40   a - f  aligned along the slot radial centerline  34  and in abutment with one another. 
         [0043]    As noted above, it is desirable to have about a 0.5 mm clearance from the circumferential inner surface  16  of core  110  to the innermost conductor in the radial outward direction (i.e., round conductor  40   a  in  FIGS. 5 and 6 ). To approximate this clearance in the core  110 , the sleeve slot depth D S ′ ( FIG. 6 ) is between N times the round wire diameter plus 0.2 ((N*wire diameter)+0.2) and N plus 1 times the round wire diameter ((N+1)*wire diameter) where N=the number of wires in the core slot  112 . Thus, the relationship between the diameter Ø of the round conductors  40   a - h  and the sleeve slot depth D S ′ can be stated as: 
         [0000]      ( N *wire diameter)+0.2≦ D   S ′≦( N+ 1)*wire diameter.
 
         [0000]    Thus, for the core  110  to be wound with a round conductor having a diameter of 1.6 mm with six conductors per slot, the sleeve slot depth D S ′ is between 9.8 mm ((6*1.6)+0.2) and 11.2 mm ((6+1)*1.6). Similarly, for the core  110  to wound with a round conductor having a smaller diameter, for example 1.3 mm, with fewer conductors per slot, for example 5 conductors per slot, the sleeve slot depth D S ′ is between 6.7 mm ((5*1.3)+0.2) and 7.8 mm ((5+1)*1.3). Based on this relationship, the core  110  including a round conductor with six conductors per slot and a diameter that is approximately equal to the width of a rectangular conductor that can also be accommodated has a sleeve slot depth D S ′ which is approximately 10% longer than the sleeve slot depth D S  in the core  10 . 
         [0044]    With this slot design, round wire can be inserted in a single row in the core  110  and the slot fill factor will remain rather high at approximately 0.56 (or 56% slot fill) ( FIG. 6 ) as compared to the core  10  with rectangular wire at approximately 0.62 (or 62% slot fill) ( FIG. 3 ). The slot fill factor of the core in the second configuration is 9.7% ((0.62−0.56)/0.62)—or within 10%—of the slot fill factor of the core in the first configuration. The slot fill factor in the embodiment described is determined without deformation of the wires in the core by external force. Thus, the slot design of the core  110  provides a single lamination design for two, separate stator designs: a high slot fill version using round wire and a very high slot fill version using square wire. As is known in the art, slot fill factor is equal to the ratio of the conductor area (or volume) over the total slot area (or volume). For example, a slot fill factor of 0.5 would signify that half (50%) of the slot area (or volume) is occupied by the conductors. The other half of the slot area (or volume) is occupied by conductor insulation, slot insulation, and gaps in between the conductors and between the conductors and the slots sides. 
         [0045]      FIG. 7  shows an overlay of the core  10  of  FIG. 2  and the core  110  of  FIG. 5  with the core slot bottom  28  of the core  10  shown in phantom lines. As shown in  FIG. 7 , the relationship between the sleeve slot depth D S  and the diameter of the round conductor established above results in an elongation of the core slot  112  in the outer radial direction. Specifically, the core slot bottom  28  of the core  10  is adjusted from the positioned depicted by the phantom lines to the position of the core slot bottom  128  of the core  110 . As discussed above, the elongated core slots  112  enable the core  110  to accept both rectangular conductors (i.e.,  38   a - h  shown in  FIGS. 2 and 3 ) and round conductors  40   a - f  (i.e.,  40   a - f  shown in  FIGS. 5 and 6 ) in alternative applications. 
         [0046]    A flow diagram of a method  200  for forming stator assemblies for electrical machines is shown in  FIG. 8  and described with reference to  FIGS. 2-6 . The method begins by forming a plurality of identical stator cores including a first stator core  110   1  and a second stator core  110   2 , each stator core  110   1 ,  110   2  defining a plurality of stator slots  112   1 ,  112   2  spaced circumferentially about a central axis  14  of the core and extending in a radial outward direction for a depth D S ′ from an inner circumferential surface  16  of the core to a core slot bottom  128   1 ,  128   2  in accordance with the equation (N*wire diameter)+0.2≦D S ′≦(N+1)*wire diameter (block  202 ). In this description of the method  200 , subscripts are used after the reference numbers to distinguish identical features of the first and second stator cores. 
         [0047]    First windings  38   a - h  are assembled on the first stator core  110   1  with the first windings having a rectangular cross-section when viewed along a cross-sectional plane situated perpendicular to the central axis  14  of the first core  110   1  (block  204 ). Second windings  40   a - f  are assembled on the second stator core  110   2  with the second windings having a round cross-section when viewed along a cross-sectional plane situated perpendicular to the central axis  14  of the second core  110   2  (block  206 ). The first windings  38   a - h  are disposed in a single row within the first slots  112   1  of the first core  110   1 , while the second windings  40   a - f  are disposed in a single row within the second slots  112   2  of the second core  110   2 . An innermost conductor  38   a ,  40   a  of the respective first and second windings  38   a - h ,  40   a - f  is spaced from the inner circumferential face  16  of the first and second core  110   1 ,  110   2  by a predetermined distance. The first stator core assembled with the first windings has a first slot fill factor and the second stator core assembled with the second windings has a second slot fill factor. The second slot fill factor is within 10% of the first slot fill factor. 
         [0048]    The foregoing detailed description of one or more embodiments of the stator core has been presented herein by way of example only and not limitation. It will be recognized that there are advantages to certain individual features and functions described herein that may be obtained without incorporating other features and functions described herein. Moreover, it will be recognized that various alternatives, modifications, variations, or improvements of the above-disclosed embodiments and other features and functions, or alternatives thereof, may be desirably combined into many other different embodiments, systems or applications. Presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the appended claims. Therefore, the spirit and scope of any appended claims should not be limited to the description of the embodiments contained herein.