Abstract:
In one exemplary embodiment, an electric machine includes a stator having a plurality of axial protrusions forming a plurality of stator cooling channels on a radially outer surface of the stator and a tapered portion located adjacent a distal end of at least one of the plurality of protrusions. Additionally, the disclosure includes a method for cooling the electric machine.

Description:
BACKGROUND 
     Electric machines typically comprise a stator element and a rotor element that interact electro-magnetically to convert electric power to mechanical power or to convert mechanical power to electrical power. For example, a conventional stator element comprises an annular housing having windings of copper coils circumferentially oriented. A conventional rotor element is mounted on a shaft for rotation. Electric current is passed through the stator windings to generate an electro-magnetic field that causes the rotor and shaft to rotate about an axis of rotation of the shaft. The electric current causes resistance heating of the coils, which heats the entire electric machine including the rotor. In particular, high power electric motors that operate at high speeds and are compact in size generate high heat densities. 
     Conventional schemes for cooling electric machines involve passing cooling fluid over the stator, which is typically easy to accomplish because of the stationary and exterior nature of the stator assembly. Rotors are typically cooled by passing cooling fluid between the stator and rotor. Such cooling schemes, however, often provide inadequate cooling for rotors used in high power motors due to high rotor heat density. In addition, as the size of high power motors decreases, the available area for passing cooing air between the stator and rotor also decreases. There is therefore a need to improve cooling efficiency in electric machines. 
     SUMMARY 
     In one exemplary embodiment, an electric machine includes a stator having a plurality of axial protrusions forming a plurality of stator cooling channels on a radially outer surface of the stator and a tapered portion located adjacent a distal end of at least one of the plurality of protrusions. Additionally, the disclosure includes a method for cooling the electric machine. 
     These and other features of the disclosed examples can be understood from the following description and the accompanying drawings, which can be briefly described as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a fan assembly having an electric machine. 
         FIG. 2  is an enlarge view of the electric machine of  FIG. 1 . 
         FIG. 3  is an example ring from the electric machine of  FIG. 1 . 
         FIG. 4  is a partial perspective view of the ring of  FIG. 3  on an example stator. 
         FIG. 5  is partial perspective view of another example ring on the example stator. 
         FIG. 6  is a partial perspective view of another example stator. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIG. 1 , a fan assembly  20  is driven by an electric machine  22 . The fan assembly  20  includes a fan impeller  24 , an outer housing  26 , an inner housing  28 , a cooling air tube  30 , and an electrical conduit  32  in electrical communication with the electric machine  22 . Although the electric machine  22  shown in the illustrated examples is an electric motor, this disclosure also applies to cooling an electric generator or another device that includes a stator. 
     The electric machine  22  includes a shaft  34  spaced radially inward from a rotor  36  and a stator  44  spaced radially outward from the rotor  36 . The shaft  34  is attached to the rotor  36  such that the shaft  34  and the rotor  36  rotate together. The shaft  34  includes a forward shaft portion  34 A located axially forward from an aft shaft portion  34 B relative to an axis of rotation A of the electric machine  22 . The forward shaft portion  34 A is mounted to a forward motor support  38 A located within the inner housing  28  by a forward bearing assembly  40 A and the aft shaft portion  34 B is mounted to an aft motor support  38 B located within the inner housing  28  by an aft bearing assembly  40 B. 
     The outer housing  26  forms an annular duct  42  with the inner housing  28  in which the fan impeller  24  is disposed to drive air through the fan assembly  20 . The electrical conduit  32  extends through the annular duct  42  and allows access to the stator  44  by passing through the inner housing  28  and outer housing  26  to provide an electrical connection with the stator  44  and a power source. The cooling air tube  30  also extends through the annular duct  42  and allows cooling air C from a cooling air source outside of the fan assembly  20  to reach the electric machine  22 , which is surrounded by the annular duct  42 . The cooling air source could include room air or another air source. 
     The inner housing  28  is concentrically mounted within the outer housing  26  by a support structure  46  which extends radially between a radially outer facing surface of the inner housing  28  and a radially inner facing surface of the outer housing  26 . The forward and aft motor supports  38 A and  38 B are mounted to a radially inner facing surface of the inner housing  28  and include portions for supporting the stator  44  and the forward and aft bearing assemblies  40 A and  40 B. The stator  44  and the forward and aft bearing assemblies  40 A and  40 B are mounted to radially inward facing surfaces of motor supports  38 A and  38 B. The forward and aft shaft portions  34 A and  34 B are positioned within the forward and aft bearing assemblies  40 A and  40 B and extend axially and concentrically with the axis of rotation A. 
     A tie rod  33  secures the forward shaft portion  34 A relative to the aft shaft portion  34 B through the application of a tensile force through the tie rod  33 . The tie rod  33  extends through a center of the forward shaft portion  34 A and a center of the aft shaft portion  34 B and engages a forward end cap  48 A adjacent the forward shaft portion  34 A and an aft end cap  48 B adjacent the aft shaft portion  34 B. The forward end cap  48 A connects the fan impeller  24  to the forward shaft portion  34 A so that the fan impeller  24  will rotate with the forward and aft shaft portions  34 A and  34 B and the rotor  36 . 
     The rotor  36  is mounted to a radially outward facing surface of the aft shaft portion  34 B and faces toward the stator  44 . A small gap between the rotor  36  and the stator  44  forms a rotor cooling passage  50  that permits the cooling air C from the cooling air tube  30  to flow around the rotor  36  and through the electric machine  22 . 
     Electrical wiring  32 A extends through the electrical conduit  32  and connects to the stator  44  to energize coil windings with electrical current. The energized coil windings exert an electro-magnetic flux field on the rotor  36 . The flux field causes the rotor  36  to rotate about the axis of rotation A on the shaft  34 . The tie rod  33  rotates with the forward and aft shafts portions  34 A and  34 B and the rotor  36 . The forward and aft shaft portions  34 A and  34 B and the rotor  36  rotate on the forward and aft bearing assemblies  40 A and  40 B and cause the fan impeller  24  to rotate in the annular duct  42  and push air between the inner and outer housings  28  and  26 . 
     The electric current provided to the stator  44  generates heat within the electric machine  22 . Stator cooling channels  56  allow the cooling air C to flow around a radially outer surface or back iron of the stator  44  between the stator  44  and the inner housing  28  to provide convective cooling to the stator  44 . The stator cooling channels  56  extend an entire length of the stator  44 . 
     A shaft cooling passage  54  also permits the cooling air C to enter the shaft  34  through a shaft inlet  52 B that extends through the aft end cap  48 B. The cooling air C exits at the interior cavity of the shaft  34  through a shaft outlet  52 A in the forward end cap  48 A. The shaft cooling passage  54  allows the cooling air C to flow along the interior of the forward and aft shaft portions  34 A and  34 B to provide convective cooling to the forward and aft shaft portions  34 A and  34 B as well as the rotor  36 . 
     The tie rod  33  extends through the shaft cooling passage  54  concentrically with the forward and aft shaft portions  34 A and  34 B along the axis of rotation A. The diameter of the tie rod  33  is smaller than that of shaft cooling passage  54  such that a spacing is present between the tie rod  33  and the forward and aft shaft portions  34 A and  34 B, which permits the cooling air C to pass through the shaft cooling passage  54 . 
     As shown in  FIG. 2 , the electric machine  22  is cooled by the cooling air C flowing through the shaft cooling passage  54 , the rotor cooling passage  50 , and the stator cooling channel  56 . Although the invention is described with respect to the use of cooling air C, other cooling fluids, such as liquid or gas, may be used in other embodiments. 
     As shown in  FIG. 2 , the electric machine  22  is cooled by the cooling air C flowing through the shaft cooling passage  54 , the rotor cooling passage  50 , and the stator cooling channel  56  as parallel flow circuits. The incoming cooling flow C is divided amongst the shaft cooling passage  54 , the rotor cooling passage  50 , and the stator cooling channel  56  based on their contribution to the total flow resistance. Therefore, in order to increase cooling of the stator  44  adjacent the stator cooling channel  56 , a flow rate of the cooling air C through the stator cooling channel  56  would need to increase. 
     The stator cooling channels  56  are formed by pairs of protrusions  60 , such as fins, that extend from a radially outer side of the stator  44  and form channels that defines each of the stator cooling channels  56 . In one example, a radially outer side of the stator cooling channel  56  is defined by the radially inward facing surface of the motor support  38 B and in another example, the radially outer side of the stator cooling channel  56  is defined by the inner housing  28 . The perimeter of the stator cooling channels  56  are a feature of the motor laminations (thin iron plates), and the channels achieve their length when numerous laminations are stacked together to form the motor core. Because the stator cooling channels  56  are formed by a series of stacked laminations that are usually die-punched from sheet stock raw material, the entrances and exits of each stator cooling channel are largely sharp-edged. The sharp-edge orifices at each cooling channel may lead to significant pressure-drop losses that reduce the amount of cooling flow that actually enter them. 
     In the illustrated example, the distal ends of the stator cooling channel  56  taper adjacent distal ends of the protrusions  60  to form an inlet  62  and an outlet  64 . The taper at the inlet  62  of the stator cooling channel  56  creates an area of decreasing cross-sectional area as the cooling air C enters the stator cooling channel  56 . The change in cross-sectional area in the inlet  62  provides a gradual transition for the cooling air C entering the stator cooling channel  56  which increases the flow rate of the cooling air C into the stator cooling channel  56  by reducing the pressure drop associated with the entrance losses of sharp-edge orifices 
     Conversely, the outlet  64  includes an increasing cross-sectional area as the cooling air C moves from the stator cooling channel  56  out of the stator  44 . The change in cross-sectional area in the outlet  64  provides a gradual transition for the cooling air exiting the stator cooling channel  56  which increases the flow of the cooling air C into the stator cooling channel  56  by reducing the pressure drop associated with the exit losses of sharp-edge orifices. 
       FIG. 3  illustrates a ring  66  formed by two separate ring halves and  68 B that can be located on opposing ends of the stator  44  to form the inlets  62  and the outlets  64  to the stator cooling channels  56 . Although the ring  66  is formed by the two ring halves  68 A and  68 B in the illustrated example, the ring  66  could be formed from a single piece of material or more than two pieces of material. 
     As shown in  FIGS. 3 and 4 , the ring  66  includes a ring portion  68  and a multitude ring protrusions  70  that extend radially outward from the ring portion  68  and are circumferentially spaced around an outer perimeter of the ring portion  68 . The ring  66  attaches to a distal end  44 A of the stator  44 . The ring protrusions  70  are circumferentially aligned with a corresponding one of the protrusions  60  on the stator  44 . 
     Each of the ring protrusions  70  include a first axial face  72  that engages one of the protrusions  60  on the stator  44  and tapers toward a second axial face  74  spaced from the first axial face  72 . The first axial face  72  has a corresponding cross-sectional area to the distal end of the protrusion  60 . The ring protrusions  70  taper at an angle α as shown in  FIG. 4 . In the illustrated example, the angle α is approximately 35 degrees. In another example, the angle α is between approximately 25 degrees and approximately 45 degrees. The angle α of the taper continues from the ring protrusion  70  onto the ring portion  68  such that the ring portion  68  also includes a taper adjacent the stator cooling channel  56  at the angle α. 
       FIG. 5  illustrates another example ring  66 ′. The ring  66 ′ is similar to the ring  66  except where shown in the Figures or described below. The ring  66 ′ includes a ring portion  68 ′ and a multitude of ring protrusions  70 ′ extending from the ring portion  68 ′. The ring  66 ′ attaches to the distal end  44 A of the stator  44  and includes the ring protrusions  70 ′ that extend radially outward from the ring portion  68 ′ that are circumferentially aligned with a corresponding one of the protrusions  60  on the stator  44 . 
     Each of the ring protrusions  70 ′ include a first axial face  72 ′ that engages one of the protrusions  60  on the stator  44  and tapers toward a second axial face  74 ′ spaced from the first axial face  72 ′. The first axial face  72 ′ has a corresponding cross-sectional area as the distal end of the protrusion  60 . The ring protrusions  70 ′ taper with a radius of curvature R 1 . The radius of curvature R 1  continues onto the ring portion  68 ′ such that the ring portion  68 ′ also includes a taper adjacent the stator cooling channel  56  with a radius of curvature of R 1 . 
       FIG. 6  illustrates another example stator  144  with an inlet  162  formed into a first distal end lamination  166  and an outlet  164  formed into a second distal end lamination  167 . Each of the first distal end lamination  166  and the second distal end lamination  167  includes a disk  168  and a multitude of protrusions  170  that extend radially outward from the disk  168  and are circumferentially spaced around an outer perimeter of the disk  168 . The disk  168  includes multitude of winding openings  169  for accepting stator windings. The protrusions  170  are circumferentially aligned with a corresponding one of a multitude of protrusions  160  on the stator  144 . 
     Each of the protrusions  170  include a first axial face  172  that engages one of the protrusions  160  on the stator  144  and tapers toward a second axial face  174  spaced from the first axial face  172 . The first axial face  172  has a corresponding cross-sectional area to the distal end of the protrusion  160 . The protrusions  170  taper at an angle α. In the illustrated example, the angle α is approximately 35 degrees. In another example, the angle α is between approximately 10 degrees and approximately 60 degrees. The angle α of the taper continues from the protrusion  170  onto the disk  168  such that the disk  168  also tapers toward stator cooling channels  156  at the angle α. Additionally, the multitude of protrusions  170  and the disk  168  could have a radius similar to the ring protrusions  70 ′ extending from the ring portion  68 ′ as shown in  FIG. 5 . 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.