Abstract:
A stator includes a cylindrical core with a plurality of longitudinally extending slots, a first winding set formed as a first cascaded wire in two radial layers of the slots, a second winding set formed as a second cascaded wire in two other radial layers of the slots, and a jumper connecting the first and second wires. A first winding set may have three cascaded phase wires in first and second layers of the slots, and a second winding set may have three cascaded phase wires in third and fourth layers of the slots. A winding set of a first phase may have a first cascaded wire in two radial layers of a first one of the slots, and a winding set of a second phase may have a second cascaded wire in two other radial layers of the first one of the slots.

Description:
BACKGROUND 
       [0001]    Exemplary embodiments pertain to reducing costs and simplifying manufacturing of stator windings of electric machines and, more particularly, to achieving high machine efficiency and high manufacturing efficiency with cascaded wiring 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 axially 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, a given winding having an end loop as it extends circumferentially to a different slot. In this general manner, a stator winding extends axially from end to end in selected ones of the plurality of stator core slots and extends circumferentially between slots, according to a chosen wiring pattern. 
         [0003]    The stator may be formed with any number of separate phase windings, such as three-phase, five-phase, six-phase, etc., and such determines the general wiring pattern to be implemented when winding the stator core. Since most applications emphasize reducing the size of the electric machine while improving efficiency, it is desirable to utilize the available slots in a manner that maximizes the filling of the stator core slots. High slot fill stators generally produce more electrical power with increased machine efficiency. Use of rectangular conductor wire may achieve a slot fill ratio of 75% or greater. Hairpin conductors are U-shaped solid wires having a substantially rectangular cross-sectional profile that are inserted into two slots at one axial end of the stator core and that are twisted and then welded to other hairpins at the other axial end of the stator core, as part of a phase winding. However, use of hairpin conductors may require a tradeoff between achieving a high slot fill ratio and reducing undesirable AC performance characteristics such as skin effect and others. Skin effect reduces the effective cross-sectional area of a conductor in a slot as the thickness of the conductor increases. Therefore, generally, the thickness of rectangular wires in a slot should be made as small as possible. Alternatively, a given wiring configuration may be designed to greatly reduce undesirable performance, for example by placing more than one phase in a slot. 
         [0004]    Manufacturing problems and associated increased costs may also be encountered when forming and welding hairpin conductors. For example, connecting the ends of hairpins at one axial end of a stator typically requires a large number of welds. In addition, each hairpin may be required to be staggered or interleaved with respect to adjacent hairpin end portions, and the insertions, bending, and routing of individual hairpins necessitate a large number of manufacturing steps. 
       SUMMARY 
       [0005]    It is therefore desirable to obviate the above-mentioned disadvantages by providing a stator winding configuration that may be employed in a simple, distributed manner. The disclosed exemplary embodiments utilize cascaded continuous wire in forming two or more distributed sections, whereby the sections may be joined together with a minimum of interconnections such as welds. 
         [0006]    According to an exemplary embodiment, a stator includes a substantially cylindrical core having two axial ends and a plurality of longitudinally extending slots formed therebetween, a first winding set formed as a first cascaded wire in two radial layers of the slots, a second winding set formed as a second cascaded wire in two other radial layers of the slots, and a jumper connecting the first and second wires. 
         [0007]    According to another exemplary embodiment, a stator includes a substantially cylindrical core having two axial ends and a plurality of longitudinally extending slots formed therebetween, a first winding set formed as three cascaded phase wires in first and second layers of the slots, and a second winding set formed as three cascaded phase wires in third and fourth layers of the slots. 
         [0008]    According to a further exemplary embodiment, a stator includes a substantially cylindrical core having two axial ends and a plurality of longitudinally extending slots formed therebetween, a first winding set of a first phase formed as a first cascaded wire in two radial layers of a first one of the slots, and a second winding set of a second phase formed as a second cascaded wire in two other radial layers of the first one of the slots. 
         [0009]    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 
         [0010]    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: 
           [0011]      FIG. 1  is a perspective view of an exemplary stator core; 
           [0012]      FIG. 2  illustrates an exemplary distributed winding pattern for a stator core having four radial slot layers; 
           [0013]      FIG. 3  illustrates another exemplary distributed winding pattern for a stator core having four radial slot layers; 
           [0014]      FIG. 4  shows a wiring schematic for a distributed cascaded stator winding prior to insertion, and a slot fill pattern for one phase, according to an exemplary embodiment; 
           [0015]      FIG. 5  shows slot locations for one phase of an exemplary stator winding; 
           [0016]      FIG. 6  is a top plan view of a fully populated stator core, according to an exemplary embodiment; 
           [0017]      FIG. 7  is a top plan view showing a cascaded winding for a single phase, according to an exemplary embodiment; and 
           [0018]      FIG. 8  illustrates exemplary flux paths in a partial top plan view of the stator winding of  FIG. 2 . 
       
    
    
       [0019]    Corresponding reference characters indicate corresponding or similar parts throughout the several views. 
       DETAILED DESCRIPTION 
       [0020]    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. 
         [0021]      FIG. 1  is a perspective view of a generally cylindrically-shaped stator core  10 . Stator core  10  includes a plurality of core slots  12  extending radially outward of a circumferential interior surface  14  thereof. Core slots  12  extend in a direction, indicated by an arrow  16 , relative to the central axis  17  of stator core  10  between a first axial end  18  and a second axial end  20  thereof. An axially upward direction is defined as moving toward first axial end  18  and an axially downward direction is defined as moving toward second axial end  20 . Core slots  12  are typically equally spaced around circumferential inner surface  14 , and inner surfaces  14  between core slots  12  are typically substantially parallel to the central axis  17 . A circumferential clockwise direction is indicated by an arrow  21  and a circumferential counterclockwise direction is indicated by an arrow  23 . Core slots  12  define a depth  25  along a radial axis, indicated by an arrow  24 , and are adapted to receive a stator winding, discussed in more detail below. A radial inward direction is defined as moving towards central axis  17  of stator core  10  and a radial outward direction is defined as moving away from central axis  17 . 
         [0022]      FIG. 2  illustrates an exemplary distributed winding pattern for a stator core having four radial slot layers. A phase “A” wire pair  1 ,  2  are disposed in layer one at slots six and seven and in layer two at slots thirteen and fourteen, thereby spanning a pitch of seven slots. A phase “B” wire pair  3 ,  4  are disposed in layer two at slots seven and eight and in layer one at slots twelve and thirteen, thereby spanning a pitch of five slots. This pattern for phases A and B is duplicated for layers three and four. A phase A wire pair  5 ,  6  are disposed in layer three at slots six and seven and in layer four at slots thirteen and fourteen, thereby spanning a pitch of seven slots. A phase B wire pair  7 ,  8  are disposed in layer four at slots seven and eight and in layer three at slots twelve and thirteen, thereby spanning a pitch of five slots. Since this first exemplary distributed wiring pattern may have different pitches for separate phase windings, it is herein referred to as being non-symmetric or unequal. 
         [0023]      FIG. 3  illustrates another exemplary distributed winding pattern for a stator core having four radial slot layers. A phase A wire pair  26 ,  27  are disposed in layer one at slots six and seven and in layer two at slots twelve and thirteen, thereby spanning a pitch of six slots. A phase B wire pair  28 ,  29  are disposed in layer two at slots six and seven and in layer one at slots twelve and thirteen, thereby spanning a pitch of six slots. This pattern for phases A and B is duplicated for layers three and four. A phase A wire pair  30 ,  31  are disposed in layer three at slots seven and eight and in layer four at slots thirteen and fourteen, thereby spanning a pitch of six slots. A phase B wire pair  32 ,  33  are disposed in layer four at slots seven and eight and in layer three at slots thirteen and fourteen, thereby spanning a pitch of six slots. Since this second exemplary distributed wiring pattern may have substantially the same pitches for separate phase windings, it is herein referred to as being symmetric or equalized. 
         [0024]    The exemplary stator winding patterns illustrated in  FIGS. 2 and 3  are each amenable to being placed by use of separate wiring magazines, thereby effecting a distributed winding. For example, the windings of layers one and two may be placed onto a stator core and then the windings of layers three and four may be placed in a separate winding operation. However, these patterns each require interlacing of conductors as the conductor pairs are sequentially placed during assembly. Such interlaced windings are typically implemented using hairpin type conductor segments that are connected at one axial end of stator core  10 . The hairpin manufacturing process includes welding each conductor leg to an adjacent conductor leg, and the large number of individual welds increases the probability of defects and reduces reliability and consistency of the winding. The use of hairpin conductor segments also typically requires bending and routing of hairpin legs in a staggered or interleaved pattern that may add axial length to a stator assembly and that may cause additional likelihood of shorts or other product defects. 
         [0025]      FIG. 4  shows a wiring schematic for a distributed cascaded stator winding  34  laid out in a linear manner for ease of visualization, and a slot fill pattern for phase A that references slots numbered one, two, and three, according to an exemplary embodiment.  FIG. 5  is a wiring schematic for the exemplary distributed cascaded stator winding pattern of  FIG. 4 , except that only one phase (phase A) is shown with slot numbers for each layer, for phase A wires passing through the core. The illustrated pattern is designed for a stator core having 48 circumferential slots, but any other multiple of six slots may alternatively be utilized. A phase A wire pair  35 ,  36  are respectively disposed in layer one at slots one and two, and the wires respectively extend to layer one at slots seven and eight, each wire thereby spanning a pitch of six slots. A subsequent circumferential portion of the cascaded wiring of phase A pair  35 ,  36  extends at a pitch of six slots, except where noted. Specifically, a 5-7 pitch wiring section  37  is implemented two times as phase A wires  35 ,  36  complete two revolutions around stator core  10 . For example, wire  35  has a pitch of seven slots in layer one by being sequentially placed in slot thirty-seven and slot forty-four, and wire  36  has a pitch of five slots in layer one by being sequentially placed in slot thirty-eight and slot forty-three. Wires  35 ,  36  pass through 5-7 pitch wiring section  37  in layer one and are then configured as a seven pitch wiring section  38  at a transition between layer one and layer three. Similarly, wires  35 ,  36  pass through 5-7 pitch wiring section  37  in layer four and are then configured as a seven pitch wiring section  38  at a transition between layers four and two. The combination of 5-7 pitch wiring section  37  and seven pitch wiring section  38  serves to transition between a layer having conductor  35  as an outer conductor, for a group of six wires, and a next layer having conductor  36  as the outer conductor. A reversing section  45  is provided for changing the circumferential direction of the cascaded winding between layer three and layer four. Reversing section  45  may be formed as integral continuous wires, or it may be a section of pairs of wires joined together, such as by welding or any appropriate joining procedure. 
         [0026]    The reference numbers  35 ,  36 ,  39 - 44  refer to conductor portions in a section view; for example, conductor portions  35  and  43  are portions of the same wire. Conductor portion  39 , at slot one in layer two, may be an extension of conductor  36 . Conductor portion  40 , at slot two in layer two, may be an extension of conductor  35 . Conductor portion  41 , at slot two in layer three, may be an extension of conductor  36 . Conductor portion  42 , at slot two in layer four, may be an extension of conductor  35 . Conductor portion  43 , at slot three in layer three, may be an extension of conductor  35 . Conductor portion  44 , at slot three in layer four, may be an extension of conductor  36 . This pattern repeats around slots  12  of stator core  10 , whereby phase A fills all layers in slot two, fills the radial inner half of a left adjacent slot (i.e., slot one), and fills the radially outer half of a right adjacent slot (i.e., slot three). In general, the distributed winding is characterized in that each phase has “X” number of radial slot positions/segments in a “full” slot (i.e., a slot filled with conductors of one phase), X/2 slot positions/segments in the radially outward portion of the left adjacent slot, and X/2 slot positions/segments in the radially inward portion of the right adjacent slot. 
         [0027]    Each of six phase wires  35 ,  36 ,  46 - 49  of stator winding  34  is cascaded, meaning that at least three consecutive conductor portions of one conductor are housed in a same layer of the slots. This allows, for at least a portion of the windings, each of the conductors to be placed into slots of stator core  10  in a sequential order. The conductors are, therefore, not interleaved. The conductors may also be formed of a continuous wire. For example, conductors  35 ,  36 ,  46 - 49  may be formed into a zig-zag shape while being positioned as shown for layer one and layer three as a first distributed winding  50 , and then conductors  35 ,  36 ,  46 - 49  may be positioned on top of winding  50  (note: for clarity, winding  51  is shown below winding  50 ) for layers two and four as a second distributed winding  51 . Individual slot sections of the winding may be placed into the appropriate slots of stator core  10 . 
         [0028]    Reversing section  45  may be formed for interconnecting distributed windings  50 ,  51 , or an interconnection may be formed at one or more different location(s). The circumferential direction of winding installation is reversed, such as by changing from a counter-clockwise circumferential direction to a clockwise circumferential direction. Reversing section  45  may change the relative orientation of phase wires  35 ,  36 ,  46 - 49  by forming one or more loops whereby, for example, a cascade wiring order defined by the aforementioned zig-zag shaping may be changed between layers three and four. Any radial adjustments for conductor portions may be readily implemented for a chosen wiring pattern. 
         [0029]      FIG. 6  is a top plan view of a fully populated stator core  10 , according to an exemplary embodiment. Conductor wire(s) for phase “A” are illustrated using light hashed symbols  52 , conductor wire(s) for phase “B” are illustrated using medium density hashed symbols  53 , and conductor wire(s) for phase “C” are illustrated using dark hashed symbols  54 . Phase A conductor segments  40 - 42 ,  56  fill the four layers of a slot  57 . Phase A conductor segments  39 ,  55  fill the radially outer two layers of left-wise adjacent slot  58 , and phase A conductor segments  43 ,  44  fill the radially outer two layers of right-wise adjacent slot  59 . Phase B conductor segments  60 - 63  fill the four layers of a slot  64 . Phase B conductor segments  65 ,  66  fill the radially outer two layers of left-wise adjacent slot  67 , and phase B conductor segments  68 ,  69  fill the radially outer two layers of right-wise adjacent slot  70 . Phase C conductor segments  71 - 74  fill the four layers of a slot  75 . Phase C conductor segments  76 ,  77  fill the radially outer two layers of left-wise adjacent slot  78 , and phase C conductor segments  79 ,  80  fill the radially outer two layers of right-wise adjacent slot  81 . The distributed winding for a three phase stator as shown in  FIG. 6  thereby implements the general slot fill pattern of  FIG. 4 . Specifically, each full slot  57 ,  64 ,  75  has “X” radial slot positions each filled with conductors of one phase; each respective radially outer two layers of a corresponding left-wise adjacent slot  58 ,  67 ,  78  have “X/2” conductor segments of the same one phase; and, each respective radially inner two layers of a corresponding right-wise adjacent slot  59 ,  70 ,  81  have “X/2” conductor segments of the same one phase. 
         [0030]      FIG. 7  is a top plan view showing a cascaded winding for a single phase (e.g., phase A), according to an exemplary embodiment. The first wire of the single phase is shown as a round conductor and has extending lead wires  87 ,  83 . The second wire of the same single phase is shown as a rectangular conductor having extending lead wires  86 ,  82 . Although shown for illustration purposes in  FIG. 7  as having different cross-sectional shapes, the actual wires in most applications have the same shape. For purposes of increasing slot fill ratio, the shape of the wire is typically rectangular, but may alternatively be square, round, or any other shape. Once the winding is completed, these two wires are connected in parallel. For example, jumpers or the like may be used to connect extending leads  86 ,  87  together and to connect extending leads  82 ,  83  together, in a parallel configuration. In an alternative embodiment, the two wires may be connected in series by connecting together extending leads  83 ,  87  and by connecting together extending leads  82 ,  86 . For ease of description, each conductor segment shown in  FIG. 7  is not uniquely identified, but the associated groups of conductor segments are identified, and each such group represents a slot fill pattern as described above in the immediately preceding paragraph and in  FIGS. 4 and 6 . Three-slot groups  90 - 97  are located about the circumference of stator core  10 , and represent portions of the phase A winding that pass through slots in stator core  10 . 
         [0031]    The circumferential span between groups  91  and  97  may be formed without connections, and the wire routing between sequential groups alternates between stator core axial end  18  and stator core axial end  20  ( FIG. 1 ). Specifically, wiring section  98  is routed along stator core bottom  20  as a pair of wire portions respectively formed between layer one slot segments  88 ,  89  and layer one slot segments  99 ,  100 . The next sequential wiring section  101  is routed along stator core top  18  as a pair of wire portions respectively formed between layer one slot segments  99 ,  100  and layer one slot segments  102 ,  103 . This same wire routing pattern continues in a clockwise direction, so that wiring section  104  passes along stator core bottom  20  as a pair of wire portions respectively formed between layer one slot segments  102 ,  103  and the pair of layer one slot segments in group  94 . Wiring section  105  passes along stator core top  18  as a pair of wire portions respectively formed between layer one slot segments of group  94  and the pair of layer one slot segments in group  95 . Wiring section  106  passes along stator core bottom  20  as a pair of wire portions respectively formed between layer one slot segments of group  95  and the pair of layer one slot segments in group  96 . Wiring section  107  passes along stator core top  18  as a pair of wire portions respectively formed between layer one slot segments of group  96  and the pair of layer one slot segments in group  97 . 
         [0032]    The top/bottom alternating pattern described in the preceding paragraph is inverted for layer two sections, is repeated for layer three sections, and is inverted for layer four sections. Specifically, wiring section  108  is routed along stator core top  18  as a pair of wire portions respectively formed between layer two slot segments  109 ,  110  and layer two slot segments  111 ,  112 . The next sequential wiring section  113  is routed along stator core bottom  20  as a pair of wire portions respectively formed between layer two slot segments  111 ,  112  and the two respective layer two slot segments of group  93 . This same wire routing pattern continues in a clockwise direction, so that wiring section  114  passes along stator core top  18  as a pair of wire portions respectively formed between layer two slot segments of group  93  and the pair of layer two slot segments in group  94 . Wiring section  115  passes along stator core bottom  20  as a pair of wire portions respectively formed between layer two slot segments of group  94  and the pair of layer two slot segments in group  95 . Wiring section  116  passes along stator core top  18  as a pair of wire portions respectively formed between layer two slot segments of group  95  and the pair of layer two slot segments in group  96 . Wiring section  117  passes along stator core bottom  20  as a pair of wire portions respectively formed between layer two slot segments of group  96  and the pair of layer two slot segments in group  97 . 
         [0033]    Regarding layer three wire routing, wiring section  118  is routed along stator core bottom  20  as a pair of wire portions respectively formed between the two layer three slot segments of group  91  and the two layer three slot segments of group  92 . The next sequential wiring section  119  is routed along stator core top  18  as a pair of wire portions respectively formed between the two layer three slot segments of group  92  and the two layer three slot segments of group  93 . This same wire routing pattern continues in a clockwise direction, so that wiring section  120  passes along stator core bottom  20  as a pair of wire portions respectively formed between layer three slot segments of group  93  and the pair of layer three slot segments in group  94 . Wiring section  121  passes along stator core top  18  as a pair of wire portions respectively formed between layer three slot segments of group  94  and the pair of layer three slot segments in group  95 . Wiring section  122  passes along stator core bottom  20  as a pair of wire portions respectively formed between layer three slot segments of group  95  and the pair of layer three slot segments in group  96 . Wiring section  123  passes along stator core top  18  as a pair of wire portions respectively formed between layer three slot segments of group  96  and the pair of layer three slot segments in group  97 . 
         [0034]    Regarding layer four wire routing, wiring section  124  is routed along stator core top  18  as a pair of wire portions respectively formed between the two layer four slot segments of group  91  and the two layer four slot segments of group  92 . The next sequential wiring section  125  is routed along stator core bottom  20  as a pair of wire portions respectively formed between the two layer four slot segments of group  92  and the two layer four slot segments of group  93 . This same wire routing pattern continues in a clockwise direction, so that wiring section  126  passes along stator core top  18  as a pair of wire portions respectively formed between the two layer four slot segments of group  93  and the pair of layer four slot segments in group  94 . Wiring section  127  passes along stator core bottom  20  as a pair of wire portions respectively formed between layer four slot segments of group  94  and the pair of layer three slot segments in group  95 . Wiring section  128  passes along stator core top  18  as a pair of wire portions respectively formed between layer four slot segments of group  95  and the pair of layer four slot segments in group  96 . Wiring section  129  passes along stator core bottom  20  as a pair of wire portions respectively formed between layer four slot segments of group  96  and the pair of layer four slot segments in group  97 . 
         [0035]    A 5-7 pitch wiring section  130  is formed between the two layer one slot segments of group  97  and the two level two slot segments of group  90 . As a result, a seven-slot pitch exists between group  97  slot segment  131  and group  90  slot segment  134 , and a five-slot pitch exists between group  97  slot segment  132  and group  90  slot segment  133 . A 5-7 pitch wiring section  135  is formed between the two layer three slot segments of group  97  and the two level four slot segments of group  90 . As a result, a seven-slot pitch exists between group  97  slot segment  136  and group  90  slot segment  137 , and a five-slot pitch exists between group  97  slot segment  138  and group  90  slot segment  139 . A 5-7 pitch wiring section  140  is formed between the two layer one slot segments of group  90  and the two level two slot segments of group  91 . As a result, a seven-slot pitch exists between group  90  slot segment  84  and group  91  slot segment  110 , and a five-slot pitch exists between group  90  slot segment  85  and group  91  slot segment  109 . A 5-7 pitch wiring section  141  is formed between the two layer three slot segments of group  90  and the two level four slot segments of group  91 . As a result, a seven-slot pitch exists between group  90  slot segment  142  and group  91  slot segment  143 , and a five-slot pitch exists between group  90  slot segment  144  and group  91  slot segment  145 . 
         [0036]    A reversing loop  146  is formed between the two layer four slot segments  147 ,  148  of group  97  and two corresponding level four slot segments  139 ,  137  of group  90 . As a result, the clockwise wiring direction of the wire pair at wiring section  129  changes, at slot segments  139 ,  137 , to a counter-clockwise wiring direction at 5-7 pitch wiring section  135 . Such also effects a wiring transition between layers three and four. 
         [0037]    A seven pitch wiring section  149  is formed between the two layer two slot segments  150 ,  151  of group  97  and the two level three slot segments  142 ,  144  of group  90 , thereby effecting a wiring transition between layers two and three. A seven pitch wiring section  152  is formed between the two layer two slot segments  133 ,  134  of group  90  and the two level three slot segments  153 ,  154  of group  91 , thereby also effecting a wiring transition between layers two and three. 
         [0038]      FIG. 8  illustrates exemplary flux paths in a partial top plan view of the stator winding of  FIG. 2 . In a typical bifilar machine (i.e., having two wires in parallel), it is desirable for each filar to have substantially the same average radial position within the slot segments as the other filar of the bifilar winding pair. This is so because as flux travels down a stator tooth toward the back iron portion of a stator core. For example, an aggregate  155  of flux lines at a given slot of stator core  10  includes flux  156  that travels down a stator tooth  157  towards the back iron  158 , and also includes flux  159 ,  160  that jumps across the slot to an adjacent stator tooth  161 . As a result, the wires closer to the stator core interior surface  162  are linked by more flux than the wires located closer to the back (i.e., radially outer portion) of a slot. In such a case, there is a different voltage generation in each wire, and this variation causes circulating currents to travel around a given wire instead of being directed through the wires for providing useable electric power. The flux leakage thereby reduces the efficiency of an electrical machine. 
         [0039]    Conventionally, the stator windings may be formed in an interlaced pattern that routes conductors between layers so that the average radial distances for the respective conductors is approximately the same. Another conventional stator winding may route conductors so that each slot contains currents of different phases and skin effects are thereby reduced. By comparison, the cascaded wires of the presently disclosed embodiments are not interlaced and therefore are not oriented with different radial offsets. For example, a bifilar winding in an exemplary embodiment may have a first filar circumferentially routed along layer one and a second filar circumferentially routed along layer two. By extending the cascaded layer one windings sequentially to layers three, four, and two (e.g.,  FIG. 4 ), and by selective placement of 5-7 pitch portions, seven pitch portions, and a reversing loop, a slot segment pattern is provided where the average radial distance of the slot segments of filar one is roughly the same as the average radial distance of the slot segments of filar two. As a result of providing the same average radial distance for each filar of a winding, the associated voltage generation becomes equalized and machine efficiency is improved. For example, pulsations in stator current and deviations in voltage across a winding are reduced because any differences in flux leakage between filars produce associated voltage fluctuations that become normalized as they are distributed around the circumference of the stator as a bifilar winding. 
         [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.