Patent Publication Number: US-11381126-B1

Title: Electric motor with bar wound stator and end turn cooling

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/862,829, filed on Jan. 5, 2018, entitled “Electric Motor with Bar Wound Stator and End Turn Cooling,” which claims the benefit of U.S. Provisional Application No. 62/443,219, filed on Jan. 6, 2017, entitled “Electric Motor with Bar Wound Stator and End Turn Cooling.” The contents of the foregoing applications are incorporated herein by reference in their entireties for all purposes. 
    
    
     TECHNICAL FIELD 
     The application relates generally to electric motors. 
     BACKGROUND 
     Electric motor designs typically include a stator and a rotor. The stator is an annular structure that is fixed in a housing. The rotor is positioned within the stator along an axis and is supported with respect to the housing such that it is able to rotate relative to the stator. In some electric motor designs, permanent magnets are connected to the rotor, and windings are connected to the stator. The windings are energized to induce rotation of the rotor with respect to the stator in response to interaction of the magnetic fields created by energization of the phase windings with the permanent magnets. 
     The stator windings may be of the wire-wound type or of the bar-wound type. Wire-wound windings include bundles of conductors that are wrapped around stator teeth of the stator and disposed within the slots of the stator. Bar-wound windings include rigid bars, typically formed from copper, that are disposed within the slots of the stator. Both types of windings include end turns, which are portions of the windings located at the ends of the stator to interconnect portions of the windings that are located in different slots. 
     SUMMARY 
     One aspect of the disclosed embodiments is an electric motor that includes a stator body that defines slots. Winding bars are each disposed in one of the slots defined by the stator body. An end turn ring has an upper ring surface and a lower ring surface, and includes bus bars that are arranged in a circular array. Each bus bar of the bus bars has a first end portion that is connected to one of the winding bars and a second end portion that is connected to one of the winding bars. The first end portion has a lower bar surface that defines part of the lower ring surface. The second end portion has an upper bar surface that defines part of the upper ring surface. A cooling structure is disposed in a thermally conductive relationship with at least one of the upper ring surface or the lower ring surface for receiving heat from the end turn ring. 
     Another aspect of the disclosed embodiments is an electric motor that includes a stator body that defines slots. The electric motor also includes winding bars that are each disposed in one of the slots defined by the stator body. An end turn assembly has an upper surface and a lower surface, and is connected to the winding bars. A cooling structure has a first portion that is in a thermally conductive relationship with the lower surface of the end turn assembly and a second portion that is in a thermally conductive relationship with the upper surface of the end turn assembly. 
     Another aspect of the disclosed embodiments is an electric motor that includes a stator body that defines slots. The electric motor also includes winding bars that are each disposed in one of the slots defined by the stator body. Each of the winding bars defines an internal passageway. An end turn ring has an upper ring surface and a lower ring surface, and includes bus bars that are arranged in a circular array. Each of the bus bars has a first end portion that is connected to one of the winding bars, and a second end portion that is connected to one of the winding bars. The first end portion has a first upward-facing surface. The second end portion has a second upward-facing surface. An annular cooling manifold supplies a liquid to the internal passageway of each of the winding bars through liquid ports. The liquid ports are each sealed to one of the first upward facing surfaces of the bus bars or one of the second upward facing surfaces of the bus bars. 
     Another aspect of the disclosed embodiments is a three-phase electric motor that includes a stator body that defines slots, an upper end turn assembly and a lower end turn assembly each having bus bars that are arranged in a circular array, and winding bars. The winding bars are each disposed in one of the slots defined by the stator body. The winding bars are straight and each extend from one of the bus bars of the upper end turn assembly to one of the bus bars of the lower end turn assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side cross-section view of an electric motor. 
         FIG. 2  is an exploded view showing an upper end turn assembly, phase windings, and a stator of the electric motor of  FIG. 1 . 
         FIG. 3  is a perspective view showing a bus bar. 
         FIG. 4  is a perspective view showing a bus bar according to an alternative implementation. 
         FIG. 5  is a detail view showing the upper end turn assembly connected to the phase windings of the electric motor of  FIG. 1 . 
         FIG. 6  is a side cross-section view showing the upper end turn assembly and a cooling structure of the electric motor of  FIG. 1 . 
         FIG. 7  is a side cross-section view showing the upper end turn assembly and a cooling structure according to an alternative implementation. 
         FIG. 8  is a perspective view showing an annular cooling manifold. 
         FIG. 9  is a cross-section view showing the annular cooling manifold of  FIG. 8 . 
         FIG. 10  is an illustration showing assembly of an upper end turn assembly to winding bars, with the upper end turn assembly and the winding bars in a disconnected position. 
         FIG. 11  is an illustration showing assembly of the upper end turn assembly to the winding bars of  FIG. 10 , with the upper end turn assembly and the winding bars in a connected position. 
         FIG. 12  is an illustration showing winding bars located in a slot of a stator according to an example. 
         FIG. 13  is an illustration showing winding bars located in a slot of a stator according to an example. 
         FIG. 14  is an illustration that shows an end turn assembly and winding bars that are connected to the end turn assembly according to an example. 
         FIG. 15  is an illustration that shows an outer bus bar of the end turn assembly of  FIG. 14 . 
         FIG. 16  is a cross-section view of the end turn assembly of  FIG. 14  taken along line  16 - 16  of  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure herein is directed to bar wound electric motors that include end turn assemblies that allow for cooling to reduce the temperature of the end turns and phase windings of the electric motors. 
       FIG. 1  is a side cross-section view of an electric motor  100 . The electric motor  100  extends along a central axis  101  and has a rotor  102 , winding bars  105 , a motor housing  108 , a frame  110 , bearings  112 , a shaft  114 , an upper end turn ring  116 , and a lower end turn ring  118 . 
     The rotor  102  is connected to the shaft  114 . The shaft  114  is in turn supported with respect to the frame  110  by the bearings  112  to allow free rotation of the rotor  102  with respect to the frame  110 . In the example shown in  FIG. 1 , the rotor  102  is an interior permanent magnet type rotor that includes permanent magnets  120  that are disposed in internal slots  122  that are formed in the rotor  102 , radially inward from an outer periphery  124  of the rotor  102 . 
     The electric motor  100  is a bar-wound type motor, and the winding bars  105  serve as the phase windings of the electric motor  100 . The winding bars  105  are energized and de-energized to induce torque on the rotor  102  in a conventional manner by interaction of magnetic fields generated by the winding bars  105  with the permanent magnets  120  of the rotor  102 . The winding bars  105  are connected to the upper end turn ring  116  and the lower end turn ring  118 , which interconnect the winding bars  105  and supply electrical power to the winding bars  105 . 
     The stator  106  is a generally cylindrical structure that is centered on the central axis  101  such that the rotor  102  and the stator  106  are coaxial. The stator  106  may be a laminated structure that is formed from plates that are stacked axially and joined together. The stator  106  includes an outer periphery  126 , an inner periphery  128 , and slots  130 . In the illustrated example, the slots  130  are open-ended and extend radially outward from the inner periphery  128  of the stator  106 . 
       FIG. 2  is an exploded view showing the upper end turn ring  116 , the winding bars  105 , and the stator  106 . The upper end turn ring  116  and the lower end turn ring  118  are formed in the same manner, and the description of the upper end turn ring  116  is equally applicable to the lower end turn ring  118 . In some implementations, however, the electric motor  100  may include only the upper end turn ring  116 , with end turns at the opposite end of the electric motor  100  being formed by bending the winding bars  105 . 
     The upper end turn ring  116 , which may also be referred to as an end turn assembly, is an annular structure. The upper end turn ring  116  extends around the central axis  101  of the electric motor  100  ( FIG. 1 ) such that the upper end turn ring  116  is centered on the central axis  101  and is concentric with respect to the stator  106 , the rotor  102 , and the shaft  114  of the electric motor  100 . The upper end turn ring  116 , in cooperation with the lower end turn ring  118 , functions to receive electrical power from an external power source (not shown) and distribute the received electrical power to the winding bars  105  during energization and de-energization of the winding bars  105  during operation of the electric motor  100 . 
     The upper end turn ring  116  includes bus bars  232  that are arranged in a circular array. The bus bars  232  function to conduct electrical power such that they interconnect pairs of the winding bars  105 . Accordingly, the bus bars  232  are formed from an electrically conductive material, such as copper or another electrically conductive metal. At least a portion of each of the bus bars  232  is exposed at the exterior of the upper end turn ring  116 . 
     To connect the bus bars  232  with respect to one another and thereby define the upper end turn ring  116 , the upper end turn ring  116  may include a non-conductive body  234  that connects the bus bars  232  to each other. As an example, the bus bars  232  may be stacked in a circular array and embedded in the non-conductive body  234 . The non-conductive body  234  may be a solid body formed from a non-conductive material. As an example, the non-conductive body  234  may be molded onto the bus bars  232 . Non-conductive plastic materials may be utilized to form the non-conductive body  234  by molding. Other materials that are able to electrically insulate the bus bars  232  from one another and/or physically connect the bus bars  232  to one another may be utilized for the non-conductive body  234 . 
       FIG. 3  is a perspective view showing one of the bus bars  232 . The bus bar  232  includes a first end portion  336  and a second end portion  338 . The first end portion  336  and the second end portion  338  of the bus bar  232  are located at opposite longitudinal ends of the bus bar  232 , with the first end portion  336  being positioned at a first longitudinal end of the bus bar  232  and the second end portion  338  being positioned a second longitudinal end of the bus bar  232 . 
     The bus bar  232  may be a thin structure as measured in a top-to-bottom direction that corresponds to the direction of the central axis  101  of the electric motor  100 . For example, the bus bar  232  may be formed from metal that is bent, cast, stamped, or otherwise formed in a desired geometrical configuration for the bus bar  232 . As a result of the thin structure of the bus bar  232 , the bus bar  232  includes a first surface  340  and a second surface  342  that are opposed to one another in the top-to-bottom direction of the bus bar  232 . Each of the first surface  340  and the second surface  342  extend from the first end portion  336  to the second end portion  338  of the bus bar  232 . The first surface  340  and the second surface  342  may each include multiple portions separated by geometric features of the bus bar  232 , such as steps defined by the bus bar  232  in the example illustrated in  FIG. 3 . 
     The bus bar  232  includes an intermediate portion  344 . The intermediate portion  344  extends from the first end portion  336  to the second end portion  338 . The intermediate portion  344  functions to electrically connect the first end portion  336  and the second end portion  338 . The intermediate portion  344  also serves to allow an elevation difference to be defined between the first end portion  336  and the second end portion  338  when the bus bar  232  is incorporated in one of the upper end turn ring  116  or the lower end turn ring  118 . As will be described further herein, this allows the first end portion  336  and the second end portion  338  to each be disposed at upper or lower surfaces of the upper end turn ring  116  or the lower end turn ring  118 . In the illustrated example of  FIG. 3 , the intermediate portion  344  provides an elevation difference between the first end portion  336  and the second end portion  338  by incorporating the previously described stepped configuration between the first end portion  336  and the second end portion  338 . 
       FIG. 4  is a perspective view showing a bus bar  432  according to an alternative implementation. The bus bar  432  may be incorporated in the upper end turn ring  116  or the lower end turn ring  118  as previously described. As an example, the bus bars  432  may be disposed in a circular array and joined with the non-conductive body  234 , such as by molding, to define the upper end turn ring  116  or the lower end turn ring  118 . 
     The bus bar  432  is similar to the bus bar  232  except as otherwise described herein. The bus bar  432  includes a first end portion  436 , a second end portion  438 , a first surface  440 , a second surface  442 , and an intermediate portion  444 . The bus bar  432  differs from the bus bar  432  in that the intermediate portion  444  lacks the stepped configuration described with respect to the intermediate portion  344  of the bus bar  232 . In order to provide an elevation difference between the first end portion  436  and the second end portion  438  when the bus bar  432  is incorporated in one of the upper end turn ring  116  or the lower end turn ring  118 , the intermediate portion  444  is angled with respect to the first end portion  436  and the second end portion  438  in order to define a ramp configuration. Thus, the intermediate portion  444  of the bus bar  432  extends upward relative to a plane defined by the first surface  440  in the area of the first end portion  436  of the bus bar  432 , until reaching a plane defined by the second surface  442  in the area of the second end portion  438 , at which point the intermediate portion  444  terminates and no longer extends upwardly. 
       FIG. 5  is a detail view showing the upper end turn ring  116  connected to the winding bars  105  of the electric motor  100 . The bus bars  232  are positioned with respect to the non-conductive body  234  of the upper end turn ring  116  such that the first end portion  336  and the second end portion  338  of each of the bus bars  232  extends radially inward from the non-conductive body  234 . This configuration positions the first end portion  336  and the second end portion  338  of each of the bus bars  232  directly above one of the slots  130  of the stator  106 . In the illustrated example of  FIG. 5 , two of the winding bars  105  are disposed within each of the slots  130  of the stator  106 . For each of the slots  130  of the stator  106 , one of the winding bars  105  is connected to the first end portion  336  of a respective one of the bus bars  232  and the other one of the winding bars  105  is connected to the second end portion  338  of a different one of the bus bars  232 . Accordingly, each of the bus bars  232  is connected to a pair of the winding bars  105 , with the paired winding bars  105  being disposed within different ones of the slots  130  of the stator  106 . 
     The bus bars  232  may be disposed in the non-conductive body  234  such that the first surface  340  and the second surface  342  of the bus bars  232  form portions of the exterior of the upper end turn ring  116  or the lower end turn ring  118 . This allows heat to be dissipated from the upper end turn ring  116  and the lower end turn ring  118 , since both of the first surface  340  and the second surface  342  of each bus bar  232  is exposed to the exterior of the upper end turn ring  116  or the lower end turn ring  118 . Thus, the upper end turn ring  116  and the lower end turn ring  118  may be formed such that none of the bus bars  232  lacks exposure on the external surfaces of the non-conductive body  234 . In addition, the first surface  340  of each of the bus bars  232  may form a portion of an upper ring surface  546  of the upper end turn ring  116  or the lower end turn ring  118 , and the portion of the second surface  342  of each of the bus bars  232  may form part of a lower ring surface  547  (shown in  FIG. 6 ) of the upper end turn ring  116  or the lower end turn ring  118 . As will be explained further herein, forming the upper end turn ring  116  and the lower end turn ring  118  such that the first surface  340  and the second surface  342  of each of the bus bars  232  is exposed at the exterior of the non-conductive body  234 , heat may be efficiently absorbed from the bus bars  232  of the upper end turn ring  116  and the lower end turn ring  118 . 
       FIG. 6  is a side cross-section view showing the upper end turn ring  116  and a cooling structure of the electric motor  100 . The cooling structure is defined by the motor housing  108  and the frame  110  of the electric motor  100 . In particular, the relative geometric configurations of the motor housing  108  and the frame  110  define channels that extend annularly around the exterior of the motor housing  108 , between the motor housing  108 , and the frame  110 . A liquid  648  is introduced in the space between the motor housing  108  and the frame  110  to absorb heat from the motor housing  108 , which, in turn, absorbs heat from other portions of the electric motor  100 . The liquid  648  that is circulated around the motor housing  108  may be chilled before being introduced into the electric motor  100  and may then be removed from the electric motor  100  subsequent to absorbing heat from it in order to be chilled again. 
     The cooling structure defined in part by the motor housing  108  includes a liquid channel  660  that has a portion of the liquid  648  disposed within it. The liquid channel  660  may be circular and may be centered radially on the central axis  101  of the electric motor  100 . The liquid channel  660  is positioned adjacent to a wall  662  that is part of the motor housing  108  and is in contact with the liquid  648  that is disposed within the liquid channel  660 . This allows heat from the wall  662  to be transferred into the liquid  648 . The wall  662  is positioned between the liquid channel  660  and the upper end turn ring  116 . Since the upper end turn ring  116  is also centered radially on the central axis  101  of the electric motor  100 , the wall  662  and the liquid channel  660  are positioned adjacent to the upper end turn ring  116  continually around the periphery of the electric motor  100 . This allows thermal conduction from the lower ring surface  547  of the upper end turn ring  116  to the cooling structure, including the wall  662  and the liquid channel  660 . 
     The thermally conductive relationship between the lower ring surface  547  and the cooling structure may be accomplished through direct contact of the lower ring surface  547  with the wall  662  or by indirect contact in the form of thermal conduction through an intervening material. In some implementations, the wall  662  incorporates an insulating material  664  on its exterior surface. The insulating material  664  defines an electrical insulation layer that functions to electrically insulate the upper end turn ring  116  from the motor housing  108 . In some implementations, the insulating material  664  is formed separately from the wall  662  of the motor housing  108 . The insulating material  664  is a material selected so that it can electrically insulate the motor housing  108  from the upper end turn ring  116 , while allowing thermal conduction from the upper end turn ring  116  to the wall  662  of the motor housing  108 . Thus, the material selected for the insulating material  664  may be a good thermal conductor and is not intended to have thermal insulating properties. 
       FIG. 7  is a side cross-section view showing the upper end turn ring  116  and a cooling structure according to an alternative implementation. The cooling structure is defined by the motor housing  108 , an upper housing part  766 , and the frame  110 . As described with respect to the implementation shown in  FIG. 6 , the liquid  648  is disposed within the area between the motor housing  108  and the frame  110 . In addition, the liquid  648  is disposed in the area between the upper housing part  766  and the frame  110 . 
     The cooling structure of  FIG. 7  includes a first liquid channel  760  and a second liquid channel  761 . The first liquid channel  760  has a portion of the liquid  768  disposed in it and is defined by the motor housing  108  including a first wall  762  that is disposed between the first liquid channel  760  and the lower ring surface  547  of the upper end turn ring  116 . The second liquid channel  761  has part of the liquid  648  disposed in it and is defined by the upper housing part  766 , including a second wall  763 . The second wall  763  is in contact with the liquid  648  and is in a thermally conductive relationship with the upper ring surface  546  of the upper end turn ring  116 . The first wall  762  is located adjacent to the lower ring surface  547  of the upper end turn ring  116  and is in a thermally conductive relationship with the lower ring surface  547  of the upper end turn ring  116 . The first wall  762  may be in direct contact with the lower ring surface  547  or may contact the lower ring surface  547  in a thermally conductive manner through a first insulating material  764  that electrically insulates the upper end turn ring  116  from the motor housing  108 . The second wall  763  is located adjacent to the upper ring surface  546  of the upper end turn ring  116 , such that it is in a thermally-conductive relationship with the upper end turn ring  116 . The second wall  763  may be in direct contact with the upper ring surface  546  of the upper end turn ring  116  or may be in a thermally-conductive relationship through an intermediate structure such as a second insulating material  765  that is positioned between the upper housing part  766  and the upper end turn ring  116  in order to electrically insulate the upper housing part  766  from the upper end turn ring  116 . 
       FIG. 8  is a perspective view that shows an annular cooling manifold  870 . The annular cooling manifold  870  is operable to cool an end turn ring  816  and winding bars  805  that are disposed within slots  830  of a stator  806 . The annular cooling manifold  870  includes one or more ports  872  that function as inlets and/or outlets for supplying liquid to or removing liquid from a hollow interior of the annular cooling manifold  870 . By circulating liquid through the annular cooling manifold  870 , heat may be removed from the end turn ring  816 . 
       FIG. 9  is a cross-section view that shows the annular cooling manifold  870  disposed in a portion of an electric motor  800  that includes a motor housing  808  and a frame  810  that together define a cooling structure for circulating a liquid  848  within a space between the motor housing  808  and the frame  810  including a channel  860 . The liquid  848  is also introduced into the ports  872  of the annular cooling manifold  870 . The end turn ring  816  is in a thermally conductive relationship with the annular cooling manifold  870  and the motor housing  808 . The motor housing  808  may contact the end turn ring  816  directly or through an insulating material  864  that electrically insulates the motor housing  808  from the end turn ring  816 . 
     The end turn ring  816  includes bus bars  832  that are similar to the bus bars  232  as previously described. Each of the bus bars  832  is connected to a pair of the winding bars  805 . The winding bars  805  are hollow structures that each include an internal passageway  874 . In order to cool the winding bars, the annular cooling manifold  870  supplies the liquid  848  to the internal passageway  874  of each of the winding bars  805  through liquid supply ports  876  that are formed on the annular cooling manifold  870 . The arrangement and lengths of the liquid supply ports  876  are configured such that the liquid supply ports  876  engage the bus bars  832 . In particular, the winding bars  805  may be connected to the bus bars  832  such that the internal passageways  874  of the winding bars  805  are exposed to an upward-facing surface  878  of each of the bus bars  832 . This configuration allows the liquid supply ports  876  to be each sealed to one of the upward-facing surfaces  878  of the bus bars  832 , such that liquid may be transferred from the interior of the annular cooling manifold  870  into the internal passageways  874  of the winding bars  805 . 
     To further absorb heat from the upper end turn ring  116 , the annular cooling manifold  870  may be in contact with an annular upper surface  846  of the end turn ring  816 . 
     One implementation is an electric motor that includes a stator, winding bars, an end turn assembly, and a cooling structure. The stator body defines slots, and the winding bars are each disposed in one of the slots defined by the stator body. An end turn assembly has an upper surface and a lower surface and is connected to the winding bars. A cooling structure has a first portion that is in a thermally conductive relationship with the lower surface of the end turn assembly and a second portion that is in a thermally conductive relationship with the upper surface of the end turn assembly. 
     In this implementation, the electric motor may be configured such that the first portion of the cooling structure defines a first liquid channel having a liquid therein and the second portion of the cooling structure defines a second liquid channel having the liquid therein. In this implementation, the electric motor may be configured such that the first portion of the cooling structure includes a first wall and the second portion of the cooling structure includes a second wall, an exterior surface of the first wall is configured to absorb heat from the lower surface of the end turn assembly, an interior surface of the first wall is in contact with the liquid, an exterior surface of the second wall is configured to absorb heat from the upper surface of the end turn assembly, and an interior surface of the second wall is in contact with the liquid. 
     In this implementation, the end turn assembly of the electric motor may include bus bars that are arranged in a circular array. In addition, each of the bus bars may include a first end portion that is connected to one of the winding bars and has a lower bar surface that defines part of the lower surface of the end turn assembly, and a second end portion that is connected to one of the winding bars and has an upper bar surface that defines part of the lower surface of the end turn assembly. 
     Another implementation is an electric motor that includes a stator body, winding bars, an end turn ring, and an annular cooling manifold. The stator body that defines slots, the winding bars that are each disposed in one of the slots defined by the stator body, and each winding bar of the winding bars defines an internal passageway. The end turn ring has an upper ring surface and a lower ring surface, and includes bus bars that are arranged in a circular array. Each of the bus bars has a first end portion that is connected to one of the winding bars, and the first end portion having a first upward-facing surface. Each of the bus bars also has a second end portion that is connected to one of the winding bars, and the second end portion has a second upward-facing surface. The annular cooling manifold that supplies a liquid to the internal passageway of each of the winding bars through liquid ports, wherein the liquid ports are each sealed to one of the first upward facing surfaces of the bus bars or one of the second upward facing surfaces of the bus bars. In this implementation, the electric motor may be configured such that the annular cooling manifold is in contact with the upper ring surface of the end turn ring. 
       FIG. 10  is an illustration showing assembly of an upper end turn assembly, inclusive of an upper end turn ring  1016  and bus bars  1032 , to winding bars  1005 , with the upper end turn assembly and the winding bars  1005  in a disconnected position.  FIG. 11  is an illustration showing assembly of the upper end turn assembly to the winding bars  1005  of  FIG. 10 , with the upper end turn assembly and the winding bars  1005  in a connected position. The bus bars  1032  have end portions  1038  that extend radially inward from the upper end turn ring  1016 . Apertures  1039  are formed through the bus bars  1032 , are positioned on the end portions  1038 , and extend between the top and bottom surfaces of the bus bars  1032  in a generally axial direction relative to the end turn assembly. The apertures  1039  are configured such that they are complementary to end portions of the winding bars  1005  to allow connection of the winding bars  1005  to the bus bars  1032  by insertion. 
     Initially, as seen in  FIG. 10 , the winding bars  1005  are spaced from the bus bars  1032  to define the disconnected position, with the end portions of the winding bars  1005  aligned with the apertures  1039 . In one implementation, the winding bars  1005  are inserted into the apertures  1039  and held by a friction fit. In another implementation, the winding bars  1005  are moved to the connected position by a press fit, through application of force to the upper end turn ring  1016  while the winding bars  1005  are held stationary (or vice versa), with the amount of force applied causing localized deformation of the winding bars  1005  and/or the bus bars  1032  sufficient to rigidly connect the winding bars  1005  to the bus bars  1032 . For example, the winding bars  1005  may be held stationary while positioned in the slots of the stator of an electric motor (not shown in  FIGS. 10-11 ), such as the slots  130  of the stator  106  of the electric motor  100 . In another implementation, the winding bars  1005  are moved to the connected position by heat shrink fitting, by first heating the end portions  1038  of the bus bars  1032  to enlarge the apertures  1039 . The end portions of the winding bars  1005  are then inserted into the apertures  1039 . Cooling contracts the apertures  1039  to rigidly connect the bus bars  1032  to the winding bars  1005 . 
       FIG. 12  is an illustration showing windings bars  1205  located in a slot  1230  of a stator  1206  according to an example. Two or more of the winding bars  1205  are located in the slot  1230 , and other slots of the stator  1206  may also include two or more winding bars. In the illustrated example, four of the winding bars  1205  are located in the slot  1230 . Each of the winding bars  1205  is shown spaced from the internal walls of the slot  1230 , but, in some implementations, some or all of the winding bars  1205  may contact the internal walls of the slot  1230 . 
     Each of the winding bars  1205  has an insulation material  1280  that extends around it, and along its axial length. The insulation material  1280  is any suitable electrical insulator, such as fiberglass insulation or polyamide (e.g., nylon) insulation. The insulation material  1280  may be formed as one or more layers of a sheet material that are wrapped around or otherwise placed on each of the winding bars prior to insertion of the winding bars  1205  into the slot  1230 . The insulation material  1280  is applied to the winding bars  1205  individually, such as by wrapping the insulation material  1280  to the winding bars  1205 . 
     As one example, the insulation material  1280  may be applied to the winding bars  1205  first, then the winding bars  1205  are placed in the slot  1230 , then the upper and lower end turn assemblies (not shown in  FIG. 12 ) are connected to the winding bars  1205 . As another example, the insulation material  1280  may be applied to the winding bars  1205  first, then the winding bars  1205  are connected to either of the upper end turn assembly or the lower end turn assembly, then placed in the slot  1230 , then connected to the other of the upper end turn assembly or the lower end turn assembly. 
     In conventional electric motor designs, slot paper is stuffed into stator slots to create an interference fit that holds winding bars in place relative to a stator, and the slot paper is impregnated with an adhesive, such as epoxy resin, in order to fix the stator bars in place. In contrast, the insulation material  1280  may be wrapped or otherwise applied in layers, without creating an interference fit or fixing the winding bars  1205  with adhesives, because the end turn assemblies (e.g., as explained with reference to the electric motor  100 ) maintain the winding bars  1205  in position. Thus, the winding bars  1205  may be fixed against motion with respect to the stator  1206  by the end turn assemblies, and the winding bars  1205  may therefore be free from adhesive bonding to the stator  1206 . 
       FIG. 13  is an illustration showing windings bars  1305  located in a slot  1330  of a stator  1306  according to an example. The example shown in  FIG. 13  is similar to the example shown in  FIG. 12 , except that an insulation material  1380  is applied around all of the winding bars  1305  together, as opposed to being wrapped or otherwise applied to the winding bars  1305  individually. As an example, the insulation material  1380  may be applied to a group of two or more of the winding bars  1305  that are disposed in the slot  1330  (i.e., in a common slot) of the stator  1306  such that one or more layers of the insulation material extend around all of the winding bars  1305  in the group. Assembly and use of the winding bars  1305  is otherwise as described with respect to the winding bars  1205 . 
       FIG. 14  is an illustration that shows an end turn assembly  1482  and winding bars  1405  that are connected to the end turn assembly  1482 . The end turn assembly  1482  and winding bars  1405  may be incorporated in, for example, an alternating current electric motor that energizes the winding bars  1405  using three-phase electrical power. The winding bars  1405  may be straight bars that are connected to the end turn assembly  1482  at their upper ends and to an analogous end turn assembly at their lower ends. Features and details from electric motors and related structures from previously-described examples may be utilized with the end turn assembly  1482  and the winding bars  1405 , such as integration of these components into an electric motor, generally, and additional features such as cooling, insulation, and assembly features. 
     The end turn assembly  1482  includes an end turn ring  1416  (which may be an upper or lower end turn ring), inner bus bars  1432   a  that are embedded in the end turn ring  1416 , and outer bus bars  1432   b  that are embedded in the end turn ring  1416 . 
     The end turn ring  1416  is formed from a non-electrically conductive material, such as plastic, that is formed over the inner bus bars  1432   a  and the outer bus bars  1432   b , such as by molding. Surfaces of some of the inner bus bars  1432   a  and the outer bus bars  1432   b  may be exposed to the exterior of the end turn assembly  1482  to aid in heat dissipation, such as at upper and/or lower surfaces of the end turn ring  1416 . For example, the end turn ring  1416  may be molded onto the inner bus bars  1432   a  and the outer bus bars  1432   b , and excess material may be removed from the end turn ring  1416 , such as by machining, to expose portions of the inner bus bars  1432   a  and the outer bus bars  1432   b.    
     The inner bus bars  1432   a  and the outer bus bars  1432   b  are electrically conductive structures that electrically connect pairs of the winding bars  1405 . Thus, a first end and a second end of each of the inner bus bars  1432   a  and the outer bus bars  1432   b  are connected to respective ones of the winding bars  1405 . In the illustrated example, each of the inner bus bars  1432   a  is paired with a respective one of the outer bus bars  1432   b . Wither respect to each pair, the inner bus bars  1432   a  are shorter than the outer bus bars  1432   b , and the outer bus bars have a U-shaped or C-shaped configuration, such that the outer bus bars  1432   b  are positioned adjacent to and outward from three sides of the inner bus bars  1432   a , including first and second circumferential sides, and an outer radial side of the inner bus bars  1432   a    
     The inner bus bars  1432   a  and the outer bus bars  1432   b  are substantially planar structures that are arranged in layers. As will be explained further herein, all bus bars in a particular layer may be at substantially the same elevation relative to a fixed reference point, such as a top or bottom surface of the end turn ring  1416 . As best seen in  FIG. 15 , which is an illustration that shows one of the outer bus bars  1432   b , the planar structure of the outer bus bars is defined by a sheet of material having a substantially consistent thickness between top and bottom surfaces. For example, the outer bus bars  1432   b  may be formed by stamping. The outer bus bars  1432   b  each have a first end portion  1536 , a second end portion  1538 , and an intermediate portion  1544 . The first end portion  1536  and the second end portion  1538  each extend radially inward from the concave shape of the intermediate portion  1544 . The intermediate portion  1544  is arc-shaped and configured to extend circumferentially around the end turn ring  1416 . The first end portion  1536  and the second end portion  1538  may incorporate notches or apertures to allow connection to the winding bars  1405 , such as by welding, by press fit, by heat shrink fit, or by other suitable methods. 
     To simplify assembly, the inner bus bars  1432   a  and the outer bus bars  1432   b  may be generally flat. Thus, each of the inner bus bars  1432   a  and the outer bus bars  1432   b  could be disposed at a generally consistent elevation. As one example, the top and bottom surface elevations at the first end portion  1536  and the second end portion  1538  may be substantially the same. As another example, the top and bottom surface elevations of the inner bus bars  1432   a  and the outer bus bars  1432   b  may be substantially the same throughout. As another example, the inner bus bars  1432   a  and the outer bus bars  1432   b  may be substantially planar and free from bends in the axial direction of the end turn assembly  1482 . 
     The inner bus bars  1432   a  are configured similarly to the outer bus bars  1432   b , differing in geometry due to their position radially inward from and circumferentially within the extents of the outer bus bars  1432   b.    
     In  FIG. 14 , the inner bus bars  1432   a  and the outer bus bars  1432   b  that are depicted are from a top most layer of multiple layers of bus bars that are incorporated in the end turn assembly  1482 .  FIG. 16  is a cross-section view of the end turn assembly of  FIG. 14  taken along line  16 - 16  of  FIG. 14 . As shown in  FIG. 16 , there are four of the winding bars  1405  arranged radially, for disposition in a single slot of a stator (not shown in  FIG. 16 ). To provide electrical power to the winding bars  1405  that are depicted in  FIG. 16 , as well as the winding bars  1405  that are disposed at other locations around the end turn assembly  1482 , the inner bus bars  1432   a  and the outer bus bars  1432   b  are provided in multiple layers. 
     In the illustrated example, the layers of the inner bus bars  1432   a  and the outer bus bars  1432   b  include, in top-to-bottom order, a first layer  1684   a , a second layer  1684   b , a third layer  1684   c , a fourth layer  1684   d , a fifth layer  1684   e , a sixth layer  1684   f , a seventh layer  1684   g , and an eighth layer  1684   h . Thus, given the substantially flat configuration for the inner bus bars  1432   a  and the outer bus bars  1432   b , the end turn assembly  1482  includes multiple stacked layers of bus bars embedded in the non-electrically conductive material of the end turn ring  1416 , wherein each layer is substantially flat per the geometry of the bus bars included in the layer. The bus bars from the first layer  1684   a  and the second layer  1684   b  supply electrical power to radially innermost ones of the winding bars  1405  and successive pairs of layers of the bus bars supply electrical power to the successively radially outward ones of the winding bars  1405 .