Patent Publication Number: US-2012043846-A1

Title: Motor with impedance balanced winding

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a three-phase, electric motor assembly. More specifically, aspects of the present invention concern a three-phase, electric motor assembly that includes phase winding portions that are disposed within a stator core assembly to balance impedance, increase motor efficiency, and provide for mechanized insertion of the phase winding portions. 
     2. Discussion of the Prior Art 
     Those of ordinary skill in the art will appreciate that three-phase electric motors are known to be generally effective and are commonly used in a variety of industrial applications. For example, three-phase electric motors may be used to power industrial machinery, such as a drive system in a track-type tractor, among other things. Three-phase electric motors include at least three distinct phase windings, commonly referred to, and readily understood by one of ordinary skill in the art, as A-phase, B-phase, and C-phase windings. 
     Conventionally, the A-phase, B-phase, and C-phase windings are inserted within axial slots of a stator core assembly. One known technique for inserting the phase windings into the stator core assembly is lap winding, wherein an individual coil of wire comprising a number of turns is inserted into a selected pair of slots within the stator core assembly. Subsequent individual coils of wire are then inserted into adjacent pairs of slots within the stator core assembly, such that the coils overlap in a “shingled” arrangement. Lap winding is known to be generally effective in some ways, but the coils must be manually inserted and manipulated by hand This is a time-consuming and labor-intensive process that can be detrimental to large-scale production. 
     In order to facilitate large-scale production, phase windings have been inserted into the stator core assembly by machine, in an attempt to approximate the structure provided by manual lap winding. In a conventional machine-insertion technique, the coils of wire comprising each phase winding have been inserted in sequential order. In this way, the entire A-phase winding is inserted into selected ones of the slots within the stator core assembly first. Next, the entire B-phase winding is inserted into selected ones of the slots within the stator core assembly second. Finally, the entire C-phase winding is inserted into selected ones of the slots within the stator core assembly last. 
     While known machine-insertion techniques have been satisfactory in some respects, notably in that increased production can be realized, such machine-insertion techniques have also presented drawbacks in the structure of the total winding. For example, the variance in axial disposition between the phase windings created by sequential order insertion results in an impedance imbalance between the phases across an air gap between the rotor and the stator. Such an impedance imbalance detrimentally impacts overall efficiency of the motor and can create an inability to meet high efficiency demands. 
     In addition, the relative radially outer disposition of the first-inserted A-phase compared to the relative radially inner disposition of the last-inserted C-phase presents drawbacks in cooling performance. Since cooling systems for the phase windings, such as an oil spray, are frequently disposed along a radially outer margin of the total winding, the radially inner portion of the total winding tends to get hotter during operation than the radially outer portion of the total winding. Because all of the last-inserted C-phase is disposed along the radially inner portion of the total winding, this phase winding often runs hotter than the other phase windings. When the radially inner C-phase runs hotter, this phase ages faster and may create hot spots that can lead to premature failure of the motor, it may operate at a temperature over the induction class limit, and it may exhibit increased resistance within the phase and lead to further impedance imbalance. 
     Those of ordinary skill in the art will appreciate that known winding insertion techniques have essentially required a choice to be made between the increased performance but higher cost and low production capabilities of lap winding, and the impedance imbalance but higher production capabilities of known machine-insertion methods. Even with high volume production requirements, it remains undesirable to suffer an efficiency loss and a thermal performance detriment of known machine-insertion methods, especially when trying to meet high efficiency demands. 
     SUMMARY 
     According to an aspect of the present invention, an inventive machine-insertion technique has been developed that more closely resembles the structure provided by lap winding in a three-phase electric motor. The new insertion technique yields a motor with more balanced impedance, increased efficiency, and provides for mechanized insertion of phase winding portions for high production capabilities. The structure from the inventive insertion technique also results in better thermal performance of the motor, since at least a portion of each phase winding is disposed along both radially outer and radially inner portions of the total winding. Such an arrangement of the phase windings more directly exposes at least a portion of each phase winding to a cooling system for the phase windings, such as a cooling oil spray. 
     According to one aspect of the present invention, a three-phase electric motor assembly is provided. The motor assembly includes a stator core that presents circumferentially spaced axial slots and that defines a central bore for receiving a rotor configured to rotate about an axis. The motor assembly also includes a first phase winding that is received within and distributed generally across multiple ones of the axial slots of the stator core, a second phase winding that is received within and distributed generally across multiple ones of the axial slots of the stator core, and a third phase winding that is received within and distributed generally across multiple ones of the axial slots of the stator core. At least two of the phase windings each include radial inner portions within selected ones of the axial slots and radial outer portions within selected others of the axial slots. Each of the radial inner portions of the phase windings is positioned within the corresponding axial slot radially inward from the radial outer portion of another one of the phase windings. Each of the radial outer portions of the phase windings is positioned within the corresponding axial slot radially outward from the radial inner portion of another one of the phase windings. 
     According to another aspect of the present invention, a method of assembling components for a three-phase electric motor is provided, wherein the motor includes a stator core that presents circumferentially spaced axial slots and that defines a central axial bore for receiving a rotor configured to rotate about an axis. The method includes the steps of inserting initial portions of a first phase winding and a second phase winding into selected ones of the axial slots, such that the initial portions cooperatively define a part of a radially outermost margin of a generally axially concentric winding, and inserting a third phase winding into selected ones of the axial slots. At least some of the slots into which the third phase winding is inserted include the initial portions of the first and second phase windings, such that the portions of the third phase winding disposed in those slots are disposed radially inwardly from the initial portions of the first and second phase windings. The method also includes the step of inserting remaining portions of the first phase winding and the second phase winding into selected others of the axial slots, such that the remaining portions cooperatively define a part of a radially innermost margin of the generally axially concentric winding. 
     Another aspect of the present invention concerns a method of placing phase windings into a stator core for a three-phase electric motor to optimize impedance balancing, wherein the stator core presents circumferentially spaced axial slots and defines a central axial bore for receiving a rotor configured to rotate about an axis. The method includes the steps of inserting an initial portion of a first phase winding into selected ones of the axial slots, such that the initial portion of the first phase winding defines a part of a radially outermost margin of a generally axially concentric winding, and inserting an initial portion of a second phase winding into selected others of the axial slots, such that the initial portion of the second phase winding defines another part of the radially outermost margin of the generally axially concentric winding. The method also includes the steps of inserting an initial portion of a third phase winding into selected ones of the axial slots, such that at least some of the slots into which the initial portion of the third phase is inserted include one of the initial portion of the first phase winding and the initial portion of the second phase winding, to thereby define a part of a radially innermost margin of the generally axially concentric winding, and inserting a remaining portion of the third phase winding into selected others of the axial slots, such that at least some of the slots into which the remaining portion of the third phase is inserted include the other of the initial portion of the first phase winding and the initial portion of the second phase winding, to thereby define another part of the radially innermost margin of the generally axially concentric winding. The method also includes the steps of inserting a remaining portion of the first phase winding into selected ones of the axial slots, such that at least some of the slots into which the remaining portion of the first phase is inserted include the initial portion of the second phase winding, to thereby define another part of the radially innermost margin of the generally axially concentric winding, and inserting a remaining portion of the second phase winding into selected others of the axial slots, such that at least some of the slots into which the remaining portion of the second phase is inserted include the initial portion of the first phase winding, to thereby define another part of the radially innermost margin of the generally axially concentric winding. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description of the preferred embodiments. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     Various other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       A preferred embodiment of the present invention is described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  is a partial cutaway isometric view of a three-phase, electric induction motor assembly constructed in accordance with the principles of an embodiment of the present invention, illustrating a rotor assembly and a stator core assembly disposed within a motor case that includes opposite endshields, and a shaft partially extending through one of the endshields, depicting in detail internal components of the motor assembly including the stator core assembly comprising a plurality of axially stacked stator laminations presenting axial slots; 
         FIG. 2  is a generally schematic winding distribution diagram of a portion of a prior art three-phase motor, illustrating a stator core assembly with phase windings received within selected ones of the axial slots, and depicting relative dispositions of each of the phase windings with respect to one another; 
         FIG. 3   a  is a schematic phase winding insertion diagram, illustrating a winding insertion step for the prior art motor of  FIG. 2 , depicting winding coils to be inserted during a first insertion step wherein all of the A-phase winding is inserted into the selected axial slots; 
         FIG. 3   b  is a schematic phase winding insertion diagram, similar in many respects to  FIG. 3   a , illustrating a subsequent winding insertion step for the prior art motor of  FIG. 2 , depicting winding coils to be inserted during a second insertion step wherein all of the B-phase winding is inserted into the selected axial slots after placement of all of the A-phase winding; 
         FIG. 3   c  is a schematic phase winding insertion diagram, similar in many respects to  FIGS. 3   a  and  3   b , illustrating a subsequent winding insertion step for the prior art motor of  FIG. 2 , depicting winding coils to be inserted during a third insertion step wherein all of the C-phase winding is inserted into the selected axial slots after placement of all of the A-phase and B-phase winding; 
         FIG. 4  is a generally schematic winding distribution diagram of a portion of a three-phase electric induction motor constructed in accordance with the principles of a preferred embodiment of the present invention, such as the motor assembly of  FIG. 1 , illustrating a stator core assembly with initial portions of the A-phase and C-phase windings received within selected ones of the axial slots, and depicting relative dispositions of each of the portions of the phase windings with respect to one another; 
         FIG. 5  is a generally schematic winding distribution diagram of the portion of the three-phase electric induction motor shown in  FIG. 4 , illustrating the stator core assembly with the B-phase winding received within selected ones of the axial slots such that some portions of the B-phase winding are disposed radially inwardly from some of the initial portions of the A-phase winding and other portions of the B-phase winding are disposed radially inwardly from some of the initial portions of the C-phase winding, and depicting relative dispositions of each of the portions of the phase windings with respect to one another; 
         FIG. 6  is a generally schematic winding distribution diagram of the portion of the three-phase electric induction motor shown in  FIGS. 4 and 5 , illustrating the stator core assembly with remaining portions of the A-phase and C-phase windings received within selected others of the axial slots such that some portions of the remaining A-phase and C-phase windings are disposed radially inwardly from some of the portions of the B-phase winding, and depicting relative dispositions of each of the portions of the phase windings with respect to one another; 
         FIG. 7   a  is a schematic phase winding insertion diagram, illustrating a winding insertion step for a three-phase electric induction motor constructed in accordance with the principles of a preferred embodiment of the present invention, such as the motor assembly of  FIG. 1 , depicting winding coils to be inserted during first insertion steps wherein initial portions of the A-phase and C-phase windings are inserted into the selected axial slots such that the stator core assembly following these insertion steps corresponds in many ways with the diagram shown in  FIG. 4 ; 
         FIG. 7   b  is a schematic phase winding insertion diagram, similar in many respects to  FIG. 7   a , illustrating a subsequent winding insertion step for the three-phase electric induction motor, depicting winding coils to be inserted during second insertion steps wherein the B-phase winding is inserted into the selected axial slots such that the stator core assembly following these insertion steps corresponds in many ways with the diagram shown in  FIG. 5 ; and 
         FIG. 7   c  is a schematic phase winding insertion diagram, similar in many respects to  FIGS. 7   a  and  7   b , illustrating a subsequent winding insertion step for the three-phase electric induction motor, depicting winding coils to be inserted during third insertion steps wherein remaining portions of the A-phase and C-phase windings are inserted into the selected axial slots such that the stator core assembly following these insertion steps corresponds in many ways with the diagram shown in  FIG. 6 . 
     
    
    
     The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the preferred embodiments. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is susceptible of embodiment in many different forms. While the drawings illustrate, and the specification describes, certain preferred embodiments of the invention, it is to be understood that such disclosure is by way of example only. There is no intent to limit the principles of the present invention to the particular disclosed embodiments. 
     With initial reference to  FIG. 1 , a three-phase, electric induction motor assembly  20  constructed in accordance with the principles of an embodiment of the present invention is depicted for use in various applications. While the motor assembly  20  is useful in various applications, the illustrated embodiment has particular utility when the motor assembly  20  is configured to power industrial machinery. More specifically, the motor assembly  20  may include a digital controller  22 , and is notably advantageous when the motor assembly  20  is configured to power a drive system in a track-type tractor (not shown). 
     It is noted that in an industrial application, the digital controller  22  may be a separable component of the motor assembly  20  (as depicted), or may be integrated into either the motor assembly  20  or the device to be driven thereby without departing from the teachings of the present invention. Moreover, it is specifically noted that the motor assembly  20  need not take the form of an induction motor assembly (as shown in  FIG. 1 ), as various aspects of the present invention may also apply to switch reluctance and/or permanent magnet motor assemblies, as will be readily appreciated by one of ordinary skill in the art upon review of this disclosure. 
     As is generally customary, the motor assembly  20  broadly includes a rotor assembly  24 , which is rotatable about an axis  25 , and a stator assembly  26 . The rotor assembly  24  and the stator assembly  26  are both generally contained within an internal motor chamber  28  that is broadly defined by a motor case  30 . The rotor assembly  24  includes an axially disposed shaft  32  that projects outwardly from one end of the motor case  30 . 
     The illustrated motor case  30  is generally cylindrical and presents opposite axial margins  34 ,  36 . The motor case  30  comprises a shell element  38  that includes a plurality of vent openings  40  disposed around a radially outer margin of the shell  38  to present a vented shell  38 . It will be readily appreciated by one of ordinary skill in the art, however, that the alternative use of a non-vented shell (not shown) is clearly within the ambit of the present invention. The motor case  30  further comprises endshields  42 ,  44  disposed adjacent the axial margins  34 ,  36 , respectively, and secured to the shell  38 . In the illustrated embodiment, each endshield  42 ,  44  is secured to the shell  38  with a plurality of fasteners comprising bolt-and-nut assemblies  46 . It will be readily appreciated by one of ordinary skill in the art, however, that either or both of the endshields  42 ,  44  could be alternatively secured to the shell  38 , such as by welding or being integrally formed therewith, without departing from the teachings of the present invention. 
     With continued reference to  FIG. 1 , it is noted that the endshields  42 ,  44  are substantially similar in many respects, with the notable exception that the endshield  42  is predominantly solid, while the endshield  44  (not depicted in detail) includes a plurality of vent openings (not shown) defined therethrough. Such vent openings may permit vent air to flow in a generally axial direction from outside to inside the motor chamber  28  to cool the motor assembly  20  from heat generated during operation. As will be readily appreciated by one of ordinary skill in the art upon review of this disclosure, a fan (not shown) configured for rotation with the rotor assembly  24  may be used to pull cooling vent air through the vent openings, into the motor chamber  28 , and push the air out of the vent openings  40  in the shell  38  in order to provide a cooling effect to the motor assembly  20 . 
     While only one exemplary embodiment is depicted here, of course alternative cooling and/or venting arrangements, including a totally enclosed motor having a non-vented shell (not shown) and endshields without vent openings (such as the solid endshield  42 ), are contemplated and are clearly within the ambit of the present invention. Such alternative embodiments may include generally conventional cooling systems without departing from the teachings of the present invention. In one specific example, a totally enclosed motor having a non-vented shell and endshields without vent openings may include a cooling system comprising an oil spray, as will be readily appreciated by one of ordinary skill in the art. 
     As will be readily understood by one of ordinary skill in the art, a bearing assembly (not shown) is operably associated with a portion of each endshield  42 ,  44  for rotatably supporting the shaft  32 . Additionally, a cover  48  is operably secured to a portion of the endshield  42  to substantially separate the internal motor chamber  28  from outside elements. The cover  48  includes a hole extending therethrough to surround and facilitate passthrough of the shaft  32 . It is noted that a predominantly solid cover (not shown) is similarly operably secured to a portion of the endshield  44 , but without facilitating passthrough of the shaft  32 . 
     As will be readily appreciated by one of ordinary skill in the art upon review of this disclosure, many of the above-described general components of the motor assembly  20  are substantially conventional in nature, and various aspects of such components may take alternative forms and/or otherwise vary significantly from the illustrated embodiment without departing from the teachings of the present invention. Furthermore, it will be understood by one of ordinary skill in the art that several of the above-described general components (e.g., a shell, endshields, and/or covers) may not be included in some applications of the motor assembly  20 , such as where the motor assembly  20  is a totally enclosed motor having a non-vented shell and endshields without vent openings. Any such modifications to generally conventional components of the motor assembly  20  are not intended to impact the scope of the present invention, which is defined exclusively by the claims. 
     Turning briefly now to construction details of the stator assembly  26 , one of ordinary skill in the art will readily understand that the stator assembly  26  depicted in  FIG. 1  broadly includes a stator core  50  and a generally axially concentric winding  52 . The illustrated stator core  50  is comprised of a plurality of axially stacked stator laminations  54 , as is generally known in the art. It is noted that the winding  52  depicted in  FIG. 1  is shown in a conventional schematic form, but that additional details regarding the winding  52  are described below. As will be readily appreciated by one of ordinary skill in the art, the particular configuration of the winding  52  may directly impact the power, torque, voltage, operational speed, number of polls, etc. of the induction motor assembly  20 . 
     As is somewhat conventional in the art, each individual stator lamination  54  includes a substantially annular steel body, such that the plurality of axially stacked stator laminations  54  forming the stator core  50  cooperatively presents a generally central axial bore  56  for receiving the rotor assembly  24 . As will be readily understood by one of ordinary skill in the art, an air gap  58  extends radially between the stator core  50  of the stator assembly  26  and the rotor assembly  24 , such that the rotor assembly  24  is able to rotate freely within the stator assembly  26 . The plurality of axially stacked stator laminations  54  forming the stator core  50  also cooperatively presents a plurality of holes  60  extending axially therethrough, such that the bolt-and-nut assemblies  46  are passed through the holes  60  upon construction of the motor assembly  20 . 
     Additionally, the plurality of axially stacked stator laminations  54  forming the stator core  50  further cooperatively presents a plurality of generally arcuate slots  62  extending axially therethrough, with each depicted slot  62  being in communication with the air gap  58 . As will be readily understood by one of ordinary skill in the art, wires comprising the winding  52  pass through the slots  62  for receipt therein. It is noted that in the illustrated embodiment, the stator core  50  of the stator assembly  26  includes forty-eight slots  62 , although various numbers of slots may be alternatively provided without departing from the teachings of the present invention. 
     The rotor assembly  24  need not be described in detail herein, with it being sufficient for the understanding of one of ordinary skill in the art to note that the rotor assembly  24  may be of conventional construction as is generally known in the art. For example, the rotor assembly  24  may comprise an exposed bar, squirrel cage rotor, although one of ordinary skill in the art will readily appreciate that various configurations of rotor assemblies may be provided while remaining within the ambit of the present invention. 
     Shifting now to operation considerations of three-phase motors, and to details of the winding used therein, one of ordinary skill in the art will readily appreciate that three-phase electric induction motors are commonly used in a variety of industrial applications (such as to power a drive system in a track-type tractor, among other things). As is generally known, a three-phase motor is often more compact and can be less costly than a single-phase motor of the same voltage class and duty rating. In addition, many three-phase motors often exhibit less vibration and may therefore last longer than corresponding single-phase motors of the same power used under the same conditions. 
     Three-phase electric induction motors can be configured to operate multiple speeds, which may be desirable in certain applications where the load is to be driven at different speeds based upon operational requirements. In the exemplary embodiment described herein, the discussion will focus on a single-speed motor. The principles of the present invention, however, are not limited to a single-speed motor, but may alternatively be applied to a two-speed motor or a motor that includes additional operating speed modes. 
     If desired, a common way to change operating speed modes within a three-phase motor involves changing the number of effective poles that are generated for each operating speed mode. For the exemplary embodiment described herein, the three-phase, single-speed electric induction motor assembly  20  is configured as a 4n-pole motor, where n is an integer greater than or equal to one. More specifically, the detailed discussion herein focuses on an embodiment where n is equal to one, such that the motor assembly  20  is configured as a 4-pole motor. Such an exemplary embodiment, however, is not limiting on the principles of the present invention, as other integer values of n, corresponding to higher or lower multiples of effective poles generated, remain within the ambit of the present invention. 
     The three-phase electric induction motor assembly  20  is driven by energizing the winding  52  with three phases of alternating current from a power source (not shown), with the phases being commonly designated as A, B, and C phases (it is noted that such conventional phase notation is used consistently herein). Each of the A, B, and C phases are essentially equal in magnitude, but are offset from one another by 120° (2π/3 radians), as will be readily understood by one of ordinary skill in the art. 
     By dividing the winding  52  into at least one winding coil group for each phase—here, into four (4) groups of winding coil groups corresponding with each phase in the depicted 4-pole motor—the three offset A, B, and C phases cooperatively create a rotating magnetic field within the stator assembly  26 . In the three-phase electric induction motor assembly  20  depicted herein, the rotating magnetic field within the stator assembly  26  induces a corresponding rotating magnetic field within the rotor assembly  24 , thereby causing rotation of the rotor assembly  24 , as will be readily understood by one of ordinary skill in the art. 
     In order to facilitate operation of the three-phase motor assembly  20 , power leads of the coil groups comprising the winding  52  are connected to an appropriate controller that can connect the leads to the power source (not shown) to thereby drive the motor assembly  20 . With reference to  FIG. 1 , in the depicted embodiment of the present invention, the winding coil groups cooperatively present six leads  64 ,  66 ,  68 ,  70 ,  72 ,  74  that are connected to the controller  22  to be selectively connected to the power source (not shown), as will be readily appreciated by one of ordinary skill in the art upon review of this disclosure. 
     With attention first to the prior art winding distribution within a three-phase motor, schematically depicted in  FIGS. 2 ,  3   a ,  3   b , and  3   c , a known machine-insertion technique and the resultant motor structure will be described. It is initially noted that, for the sake of brevity and convenience, certain common motor elements depicted in  FIGS. 2 ,  3   a ,  3   b , and  3   c  that may be structurally similar to those of the motor assembly  20  depicted in  FIG. 1  and described above are identified by the same reference numbers as above, but include a prime to distinguish those elements shown in the figures depicting the prior art. 
     As can be seen in  FIG. 2 , a section of motor assembly  20 ′ depicts stator assembly  26 ′ including stator core  50 ′ presenting axial slots  62 ′ about central bore  56 ′, which contains axis  25 ′. Winding  52 ′ comprises phase windings described in detail below disposed within the slots  62 ′ to surround the central bore  56 ′, which is configured to receive a rotor assembly (not shown), as will be readily appreciated by one of ordinary skill in the art. 
     As is generally traditional in the art, the winding  52 ′ comprises A-phase winding  76 , B-phase winding  78 , and C-phase winding  80 . The axial slots  62 ′ present radially outermost back portions  82 , and the A-phase winding  76 , B-phase winding  78 , and C-phase winding  80  are all received within selected ones of the axial slots  62 ′. The winding  52 ′ presents a radially outer margin  84  and a radially inner margin  86 . 
     From a detailed review of  FIG. 2 , it will be readily appreciated that some of the axial slots  62 ′ include parts of multiple phase windings  76 ,  78 ,  80 . More specifically, some axial slots  62 ′ include part of an A-phase winding  76  and part of a B-phase winding  78 , with the part of the A-phase winding  76  being disposed radially outwardly from the part of the B-phase winding  78 . Some other axial slots  62 ′ include part of an A-phase winding  76  and part of a C-phase winding  80 , with the part of the A-phase winding  76  being disposed radially outwardly from the part of the C-phase winding  80 . Some other axial slots  62 ′ include part of a B-phase winding  78  and part of a C-phase winding  80 , with the part of the B-phase winding  78  being disposed radially outwardly from the part of the C-phase winding  80 . 
     It is specifically noted, however, that none of the slots  62 ′ that include parts of multiple phase windings  76 ,  78 ,  80  include any part of the C-phase winding  80  disposed radially outwardly from any part of either the A-phase winding  76  or the B-phase winding  78 . Moreover, none of the slots  62 ′ that include parts of multiple phase windings  76 ,  78 ,  80  include any part of the A-phase winding  76  disposed radially inwardly from any part of either the B-phase winding  78  or the C-phase winding  80 . Thus, only selected parts of the B-phase winding  78  are disposed one of radially inwardly and radially outwardly from parts of the A-phase winding  76  and parts of the C-phase winding  80 . 
     Given the relative dispositions of the multiple phase windings  76 ,  78 ,  80  within the axial slots  62 ′, it is generally referenced in the art that the A-Phase winding  76  are disposed predominantly in the backs  82  of the slots  62 ′, that the B-Phase winding  78  are disposed in a predominantly alternating arrangement between mid-slot and the back  82  of the slots  62 ′, and that the C-Phase winding  80  are disposed predominantly toward the central bore  56 ′. Thus, the radially outer margin  84  includes mostly A-phase winding  76 , while the radially inner margin  86  includes mostly C-phase winding  80 . 
     The generally alternating positions of the multiple phase windings  76 ,  78 ,  80  from the backs  82  of the slots  62 ′ to toward the central bore  56 ′ creates imbalanced reactance within the prior art motor assembly  20 ′. Moreover, the coils of the multiple phase windings  76 ,  78 ,  80  are typically graded from the A-phase winding  76  to the C-phase winding  80  in order to achieve a minimum end-turn package, as will be readily appreciated by one of ordinary skill in the art upon review of this disclosure. The grading of the coils of the multiple phase windings  76 ,  78 ,  80  from the A-phase winding  76  to the C-phase winding  80  means that the coils of the A-phase winding  76  are physically longer than the coils of the C-phase winding  80 . This coil grading creates imbalanced resistance within the prior art motor assembly  20 ′. 
     Turning briefly to  FIGS. 3   a ,  3   b , and  3   c , sequential schematic phase winding diagrams illustrating conventional machine-insertion steps for inserting the multiple phase windings  76 ,  78 ,  80  to arrive at the winding  52 ′ depicted in  FIG. 2  and described in detail above. As will be readily appreciated by one of ordinary skill in the art,  FIG. 3   a  illustrates winding coils to be inserted during a first insertion step wherein all of the A-phase winding  76  is inserted into the selected axial slots  62 ′. Next,  FIG. 3   b  illustrates the A-phase winding  76  disposed within the selected axial slots  62 ′ as described above, and further illustrates winding coils to be inserted during a second insertion step wherein all of the B-phase winding  78  is inserted into other selected axial slots  62 ′. Finally,  FIG. 3   c  illustrates the A-phase winding  76  and the B-phase winding  78  both disposed within the selected axial slots  62 ′ as described above, and further illustrates winding coils to be inserted during a third insertion step wherein all of the C-phase winding  80  is inserted into the selected axial slots  62 ′. 
     Those of ordinary skill in the art will appreciate that the generally alternating positions of the multiple phase windings  76 ,  78 ,  80  within the slots  62 ′, based upon conventional machine-insertion steps as shown in  FIGS. 3   a ,  3   b , and  3   c , and described in detail above, creates imbalanced reactance within the prior art motor assembly  20 ′. Additionally, the grading of the coils of the multiple phase windings  76 ,  78 ,  80  from the A-phase winding  76  to the C-phase winding  80  creates imbalanced resistance within the prior art motor assembly  20 ′. The imbalances within the prior art motor assembly  20 ′ detrimentally contribute to efficiency losses that can make it difficult for a manufacturer to met high customer efficiency demands while employing mechanized insertion steps to facilitate higher-volume production. 
     Turning briefly now to electric motor efficiency, it may be readily appreciated by one of ordinary skill in the art that an energy cost associated with the operation of an electric motor over the lifetime of the motor can amount to a significant financial burden for an end user. Thus, an improvement in overall motor efficiency, even if such an improvement is only a relatively small percentage, can result in significant savings in energy costs over the lifetime of the motor. An inventive improvement to motor design or construction resulting in an efficiency gain, therefore, may provide significant competitive advantage. 
     With attention specifically now to FIGS.  1  and  4 - 7 , the inventive three-phase induction motor assembly  20  and methods of inserting phase windings to produce such a motor assembly will be described in detail. Turning first to  FIGS. 4-6 , a section of the motor assembly  20  depicts the stator assembly  26  including the stator core  50  presenting axial slots  62  about the central bore  56 , which contains the axis  25 . The winding  52  comprises phase windings described in detail below disposed within the slots  62  to surround the central bore  56 , which is configured to receive a rotor assembly (not shown), as will be readily appreciated by one of ordinary skill in the art. 
     As is somewhat conventional in the art, the winding  52  comprises a phase winding for each of the A, B, and C phases. Unconventionally, as described in detail below, each of the phase windings for the A, B, and C phases comprises initial and remaining portions that are configured to be inserted into selected ones of the axial slots  62  separately, as will be readily appreciated by one of ordinary skill in the art upon review of this disclosure. 
     More specifically, the phase winding for the A phase comprises an initial portion  88  of A-phase winding and a remaining portion  90  of A-phase winding, with the respective portions  88 ,  90  cooperatively defining the whole A-phase winding. Similarly, the phase winding for the B phase comprises an initial portion  92  of B-phase winding and a remaining portion  94  of B-phase winding, with the respective portions  92 ,  94  cooperatively defining the whole B-phase winding. Finally, the phase winding for the C phase comprises an initial portion  96  of the C-phase winding and a remaining portion  98  of the C-phase winding, with the respective portions  96 ,  98  cooperatively defined the whole C-phase winding. 
     Additionally, the axial slots  62  present radially outermost back portions  100 , and the A-phase winding  88 ,  90 , the B-phase winding  92 ,  94 , and the C-phase winding  96 ,  98  are all received within selected ones of the axial slots  62 . The winding  52  presents a radially outer margin  102  and a radially inner margin  104 . 
     With reference first to  FIGS. 4 and 7   a , an initial portion  88  of the A-phase winding and an initial portion  96  of the C-phase winding are inserted during two passes of a first insertion step wherein the initial portion  88  of the A-phase winding and the initial portion  96  of the C-phase winding are inserted into the selected axial slots  62 . These initial portions  88 ,  96  of the A-phase and C-phase windings, respectively, are disposed predominantly in the backs  100  of the slots  62 . The initial portions  88 ,  96  of the respective A-phase and C-phase windings are received in different ones of the slots  62  from one another and cooperatively form a part of the radially outer margin  102 . It is noted that in the depicted embodiment, each of the initial portions  88 ,  96  of the respective A-phase and C-phase windings define approximately one-half of the total of each of the respective A-phase and C-phase windings, although alternative division between the portions of the phase windings is possible without departing from the teachings of the present invention. 
     With reference next to  FIGS. 5 and 7   b , an initial portion  92  of the B-phase winding and a remaining portion  94  of the B-phase winding are inserted during two passes of a second insertion step wherein the initial portion  92  and the remaining portion  94  of the B-phase winding are inserted into the selected axial slots  62 . Some of one of the initial and remaining portions  92 ,  94  of the B-phase winding is disposed within slots  62  that contain some of the initial portion  88  of the A-phase winding, and some of the other of the initial and remaining portions  92 ,  94  of the B-phase winding is disposed within slots  62  that contain some of the initial portion  96  of the C-phase winding. Some other of the initial and remaining portions  92 ,  94  of the B-phase winding is disposed within slots  62  that do not contain either of the initial portion  88  of the A-phase winding or the initial portion  96  of the C-phase winding, such that the other of the initial and remaining portions  92 ,  94  of the B-phase winding is disposed predominantly in the backs  100  of the previously empty slots  62 . 
     These other of the initial and remaining portions  92 ,  94  of the B-phase winding disposed in the previously empty slots  62  cooperate with the initial portion  88  of the A-phase winding and the initial portion  96  of the C-phase winding to form the radially outer margin  102 . It is noted that in the depicted embodiment, each of the initial and remaining portions  92 ,  94  of the B-phase winding defines approximately one-half of the total of the B-phase winding, although alternative division between the portions of the phase winding is possible without departing from the teachings of the present invention. 
     With reference next to  FIGS. 6 and 7   c , a remaining portion  90  of the A-phase winding and a remaining portion  98  of the C-phase winding are inserted during two passes of a third insertion step wherein the remaining portion  90  of the A-phase winding and the remaining portion  98  of the C-phase winding are inserted into the selected axial slots  62 . Some of the remaining portion  90  of the A-phase winding is disposed within slots  62  that contain some of the initial portion  96  of the C-phase winding, and some of the other of the remaining portion  90  of the A-phase winding is disposed within slots  62  that contain some of one of the initial and remaining portions  92 ,  94  of the B-phase winding. Some of the remaining portion  98  of the C-phase winding is disposed within slots  62  that contain some of the initial portion  88  of the A-phase winding, and some of the other of the remaining portion  98  of the C-phase winding is disposed within slots  62  that contain some of one of the initial and remaining portions  92 ,  94  of the B-phase winding. Some other of the remaining portions  90 ,  98  of the respective A-phase and C-phase windings is disposed within slots  62  that do not contain any of the initial portion  88  of the A-phase winding, the initial portion  92  of the B-phase winding, the remaining portion  94  of the B-phase winding, or the initial portion  96  of the C-phase winding, such that the other of the remaining portions  90 ,  98  of the respective A-phase and C-phase windings is disposed predominantly in the backs  100  of the previously empty slots  62 . 
     These other of the remaining portions  90 ,  98  of the respective A-phase and C-phase windings disposed in the previously empty slots  62  cooperate with the initial portion  88  of the A-phase winding, the initial portion  96  of the C-phase winding, and the other of the initial and remaining portions  92 ,  94  of the B-phase winding to form the radially outer margin  102 . It is noted that in the depicted embodiment, each of the remaining portions  90 ,  98  of the respective A-phase and C-phase windings define approximately one-half of the total of each of the respective A-phase and C-phase windings, although alternative division between the portions of the phase windings is possible without departing from the teachings of the present invention. 
     From the sequential schematic phase winding diagrams illustrating machine-insertion steps for inserting the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  shown in  FIGS. 7   a ,  7   b , and  7   c , and the detailed description above, it will be appreciated that methods have been disclosed for arriving at the winding  52  depicted in  FIG. 6 . With continued reference to  FIG. 6 , it will be readily appreciated that some of the axial slots  62  include parts of multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98 . 
     More specifically, some axial slots  62  include part of an initial portion  88  of the A-phase winding and part of a remaining portion  94  of the B-phase winding, with the part of the initial portion  88  of the A-phase winding being disposed radially outwardly from the part of the remaining portion  94  of the B-phase winding. Some other axial slots  62  include part of an initial portion  88  of the A-phase winding and part of a remaining portion  98  of the C-phase winding, with the part of the initial portion  88  of the A-phase winding being disposed radially outwardly from the part of the remaining portion  98  of the C-phase winding. 
     Also, some axial slots  62  include part of an initial portion  92  of the B-phase winding and part of a remaining portion  90  of the A-phase winding, with the part of the initial portion  92  of the B-phase winding being disposed radially outwardly from the part of the remaining portion  90  of the A-phase winding. Some other axial slots  62  include part of an initial portion  92  of the B-phase winding and part of a remaining portion  98  of the C-phase winding, with the part of the initial portion  92  of the B-phase winding being disposed radially outwardly from the part of the remaining portion  98  of the C-phase winding. 
     Additionally, some axial slots  62  include part of an initial portion  96  of the C-phase winding and part of a remaining portion  90  of the A-phase winding, with the part of the initial portion  96  of the C-phase winding being disposed radially outwardly from the part of the remaining portion  90  of the A-phase winding. Some other axial slots  62  include part of an initial portion  96  of the C-phase winding and part of a remaining portion  94  of the B-phase winding, with the part of the initial portion  96  of the C-phase winding being disposed radially outwardly from the part of the remaining portion  94  of the B-phase winding. 
     It is specifically noted, therefore, that within the slots  62  that contain part of the A-phase winding  88 ,  90  and part of the B-phase winding  92 ,  94 , for each slot  62  that contains the part of the initial portion  88  of the A-phase winding being disposed radially outwardly from the part of the remaining portion  94  of the B-phase winding, there is a corresponding slot  62  that contains the part of the initial portion  92  of the B-phase winding being disposed radially outwardly from the part of the remaining portion  90  of the A-phase winding. 
     Moreover, within the slots  62  that contain part of the A-phase winding  88 ,  90  and part of the C-phase winding  96 ,  98 , for each slot  62  that contains the part of the initial portion  88  of the A-phase winding being disposed radially outwardly from the part of the remaining portion  98  of the C-phase winding, there is a corresponding slot  62  that contains the part of the initial portion  96  of the C-phase winding being disposed radially outwardly from the part of the remaining portion  90  of the A-phase winding. 
     Furthermore, within the slots  62  that contain part of the B-phase winding  92 ,  94  and part of the C-phase winding  96 ,  98 , for each slot  62  that contains the part of the initial portion  92  of the B-phase winding being disposed radially outwardly from the part of the remaining portion  98  of the C-phase winding, there is a corresponding slot  62  that contains the part of the initial portion  96  of the C-phase winding being disposed radially outwardly from the part of the remaining portion  94  of the B-phase winding. 
     Given the relative dispositions of the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  within the axial slots  62  described above and shown particularly in  FIG. 6 , it will be readily appreciated that the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  are substantially balanced between the backs  100  of the slots  62  and toward the central bore  56 . In more detail, each of the A-phase winding  88 ,  90 , the B-phase winding  92 ,  94 , and the C-phase winding  96 ,  98  is substantially evenly distributed in disposition between the backs  100  of the slots  62  and toward the central bore  56 . More specifically, in the depicted embodiment, each of the initial portions  88 ,  92 ,  96  of the respective A-phase, B-phase, and C-phase windings defines approximately one-half of the total of each of the respective A-phase, B-phase, and C-phase windings, and each of the remaining portions  90 ,  94 ,  98  of the respective A-phase, B-phase, and C-phase windings defines approximately one-half of the total of each of the respective A-phase, B-phase, and C-phase windings, as described in detail above. 
     The balance within each of the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  between the backs  100  of the slots  62  and toward the central bore  56 , wherein each of the A-phase winding  88 ,  90 , the B-phase winding  92 ,  94 , and the C-phase winding  96 ,  98  is substantially evenly distributed in disposition between the backs  100  of the slots  62  and toward the central bore  56  balances impedance (both resistive and reactive components thereof) between the A-phase winding  88 ,  90 , the B-phase winding  92 ,  94 , and the C-phase winding  96 ,  98 . Balanced impedance between the A-phase winding  88 ,  90 , the B-phase winding  92 ,  94 , and the C-phase winding  96 ,  98  minimizes losses attributed to inter-phase circulating currents, as will be readily understood by one of ordinary skill in the art. Minimizing losses attributed to inter-phase circulating currents may increase the overall efficiency of the motor assembly  20 . 
     As noted above, the efficiency of an electric motor plays a large role in the energy cost associated with operation of the electric motor. Therefore, any improvement in overall motor efficiency, even if such improvement is only a relatively small percentage, can result in significant savings in energy costs over the lifetime of the motor, which can advantageously lower the financial burden on an end user. It is believed that the electric induction motor assembly  20  constructed in accordance with a preferred embodiment of the present invention, as described in detail above, including balance within each of the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  between the backs  100  of the slots  62  and toward the central bore  56  to optimize impedance balancing, provides a notable overall gain in efficiency within the range of approximately one-half to one percent (0.5-1%) compared with prior art electric induction motor assemblies constructed by previously-known insertion techniques, such as motor assembly  20 ′ described above. 
     Moreover, the balanced multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98 , as described above, may be mechanically inserted into the slots  62 , as described in detail above, with a mechanized process rather than insertion by hand The ability to insert the balanced multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  mechanically may lead to higher volume production and reduced labor cost compared with some conventional hand-insertion processes. 
     Furthermore, the coils of the balanced multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  are graded from a pass of the first insertion step of inserting the initial portion  88  of the A-phase winding to a pass of the third insertion step of inserting the remaining portion  98  of the C-phase winding in order to achieve a minimum end-turn package, as will be readily appreciated by one of ordinary skill in the art upon review of this disclosure. The balance within each of the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  between the backs  100  of the slots  62  and toward the central bore  56 , coupled with the coil grading yields balanced resistance at terminals of the motor assembly  20 . 
     Additionally, the balance within each of the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  between the backs  100  of the slots  62  and toward the central bore  56 , wherein each of the A-phase winding  88 ,  90 , the B-phase winding  92 ,  94 , and the C-phase winding  96 ,  98  is substantially evenly distributed in disposition between the backs  100  of the slots  62  and toward the central bore  56  provides increased thermal performance over prior art motor assemblies. In particular, and with continued reference to  FIG. 6 , it will be readily appreciated that the radially outer margin  102  of the winding  52  is cooperatively formed from substantially equal parts of each of the A-phase winding  88 ,  90 , the B-phase winding  92 ,  94 , and the C-phase winding  96 ,  98 . Furthermore, the radially inner margin  104  of the winding  52  is similarly cooperatively formed from substantially equal parts of each of the A-phase winding  88 ,  90 , the B-phase winding  92 ,  94 , and the C-phase winding  96 ,  98 . 
     From the above inventive construction, substantially equal parts of all of the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  are configured for exposure to a cooling system (not shown), such as fan air blown along the radially inner margin  104 , cooling oil spray applied along the radially outer margin  102 , or the like. By more effectively cooling substantially equal parts of all of the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98 , the inventive construction of the motor assembly  20  described herein enhances heat rejection from end coils, which may increase reliability and service life of the motor assembly  20 , as more uniform cooling among the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  may reduce any premature wear on any one phase winding. 
     It is noted that, as is somewhat conventional in the art, phase paper (not shown) or the like may be disposed between each of the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98 , as will be readily appreciated by one of ordinary skill in the art upon review of this disclosure. Phase paper may be particularly effective in isolating the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  from one another in high-voltage motor assemblies, such as motor assemblies rated at four-hundred sixty volts ( 460 V) and above. It is specifically noted that the inclusion or omission of phase paper within the motor assembly  20  is not intended to impact the scope of the present invention. 
     Finally, it is further noted that each of the A-phase winding  88 ,  90 , the B-phase winding  92 ,  94 , and the C-phase winding  96 ,  98  is described herein with particularity to correspond with the embodiment depicted in the drawings. However, as will be readily appreciated by one of ordinary skill in the art upon review of this disclosure, it may be possible to insert the multiple phase windings  88 ,  90 ,  92 ,  94 ,  96 ,  98  in a different order without departing from the teachings of the present invention. Therefore, the following claims recite the multiple phase windings more generally with reference to first, second, and third phase windings in the most broad recitations thereof, as will be readily understood by one of ordinary skill in the art. 
     The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and access the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention set forth in the following claims.