Abstract:
Disclosed is a liquid cooling system for an electric machine including a frame heat conductively attachable to a stator of an electric machine. The liquid cooling system further includes a cover mechanically attached to the frame and fluidly sealed to the frame, the cover and frame defining a cavity therebetween. The cover includes at least one protrusion extending substantially a distance between the cover and the frame. A method for constructing a liquid is also provided. The method includes forming at least one protrusion in the cover and structurally affixing the cover to the frame. The cover is fluidly sealed to the frame.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a non-provisional application of U.S. Ser. No. 60/895,241, filed Mar. 16, 2007, the contents of which are incorporated by reference herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to electric machines. More specifically, this invention relates to an improved liquid cooling system for an electric machine. 
         [0003]    As higher voltage and higher power electric machines are utilized in vehicles and the like, a problem regarding the fact that such electric machines produce an increasing amount of heat is realized. Excess heat must be dissipated to preserve the reliability and efficiency of the electric machine. In many applications, the amount of heat is great enough that a liquid cooling system is used to dissipate heat from the electric machine. 
         [0004]    Prior liquid cooling systems have utilized a cooling jacket in thermal contact with the machine, and a fluid is circulated through the cooling jacket to transfer heat from the jacket into the fluid, which then is carried from the cooling jacket to a heat loss device. One type of cooling jacket is a double-walled cast aluminum cooling jacket. The constraints of casting design and fabrication result in a cooling jacket of substantial thickness. Since the overall package size of the electric machine is usually restricted by available space in, for example, a vehicle, the cast cooling jacket thickness is disadvantageous because it limits a space available for the electric machine stator and thereby limits the performance of the electric machine. 
         [0005]    A second type of cooling jacket, a brazed steel assembly, has been used in an effort to reduce the cooling jacket thickness. The brazed joints, however, have low mechanical strength and are vulnerable to cracking under vibration, which will result in a fluid leak and potential failure of the electric machine. The brazed cooling jackets are less efficient at heat transfer because the interior of the jackets have a decreased surface area simply due to a smaller diametrical dimension of the outer surface of the cooling jacket as compared to that dimension of the cast jacket, which as noted must be thicker. Additionally, because the interior walls of the brazed cooling jackets are smooth compared to the cast cooling jacket, the result is a less turbulent flow of the cooling fluid through the jacket, and consequently less effective cooling. 
         [0006]    Although the normal systems do indeed reduce operating temperatures of electric machines, the art will nevertheless well receive alternative configurations and methods that improve cooling ability, reduce required footprint, reduce cost, or improve longevity. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention solves the aforementioned problems by providing a liquid cooling system for an electric machine including a frame heat conductively attachable to a stator of an electric machine. The liquid cooling system further includes a cover mechanically attached to the frame and fluidly sealed to the frame, the cover and frame defining a cavity therebetween. The cover includes at least one protrusion extending substantially a distance between the cover and the frame. 
         [0008]    A method for constructing a liquid is also provided. The method includes forming at least one protrusion in the cover and structurally affixing the cover to the frame. The cover is fluidly sealed to the frame. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of preferred embodiments when considered in the light of the accompanying drawings in which: 
           [0010]      FIG. 1  partial cross-sectional view of an electric machine with an improved liquid cooling system; 
           [0011]      FIG. 2  is an enlarged view of the liquid cooling system of the electric machine of  FIG. 1 ; 
           [0012]      FIG. 3  is a schematic axial view of a cavity of the liquid cooling system of  FIG. 2 ; 
           [0013]      FIG. 4  is a plan view of a cavity illustrating a first example of a protrusion configuration; 
           [0014]      FIG. 5  is a plan view of a cavity illustrating a second example of a protrusion configuration; 
           [0015]      FIG. 6  is a plan view of a cavity illustrating a third example of a protrusion configuration; 
           [0016]      FIG. 7  is a plan view of a cavity illustrating a fourth example of a protrusion configuration; 
           [0017]      FIG. 8  is a view of a first example of an end turn heat transfer enhancement; 
           [0018]      FIG. 9  is a view of a second example of an end turn heat transfer enhancement; and 
           [0019]      FIG. 10  is a view of a third example of a heat transfer enhancement. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Shown in  FIG. 1  is a fluid-cooled electric machine  10 . The type of electric machine shown in  FIG. 1  is a belt-driven alternator starter (BAS), but applications of this invention to other electric machines such as generators and/or alternators is contemplated. Electric machine  10  includes a rotor  12  disposed circumferentially about a shaft  14 , and rotable with the shaft  14  about a rotor axis  16 . Extending axially along the rotor axis  16  and surrounding the rotor  12  is a stator  18 . The stator  18  comprises a plurality of conductive windings (not shown) disposed on a stator core (not shown). Enclosing the electric machine  10  at each axial end is an end bell  20 , the shaft  14  extending through a bell hole  22  therein. 
         [0021]    A frame  24  is placed entirely around an outer surface of the stator  18  to maximize thermal contact between the frame  24  and the stator  18 . In one embodiment, this is accomplished by shrink-fitting the frame  24  to the stator  18 , but other means are contemplated within the scope of this invention. A cover  26  is then affixed to the frame  24  defining a cooling jacket  28 . In one embodiment, the cover  26  includes a forward cover flange  30  and an aft cover flange  32  that extend inwardly toward the frame  24  such that when the cover  26  is affixed to the frame  24 , a cavity  34  is defined between the frame  24  and the cover  26 . In the embodiment shown in  FIG. 2 , a mechanical or thermoset structural attachment means as well as a fluid sealing means are used to affix the cover  26  to the frame  24 . In one embodiment, welding, rolling, and brazing are used to affix the cover  26  to the frame  24  to ensure a leak-free cavity  34 . It is to be understood that while welding, rolling, and brazing are specifically identified, it is also possible to omit the welding step. Further, welding may be substituted for by other affixation means that effectively remove mechanical stress from the seal means. Too, the seal means may be other than by brazing while remaining within the scope of the invention. The frame  24  includes a forward frame flange  36  and an aft frame flange  38  that extend circumferentially around the frame  24 . Referring now to  FIG. 2 , the forward cover flange  30  is affixed to the frame  24 , such as by welding, for example, at a forward interface  40 , and similarly, the aft cover flange  32  is affixed to the frame  24 , such as by welding, for example, at an aft interface  42 . Such welding is to remove structural strain the components for sealing purposes and may be fully welded or spot welded. It is noted that since a rolling or folding process is also employed, it is expected that economy is served by merely spot welding. Indeed in some embodiments, welding is dispensed with altogether. The rolling or folding operation is similar to that employed commercially for cans. More specifically, the forward frame flange  36  is rolled or folded over the forward cover flange  30 , and the aft frame flange  38  is rolled or folded over the aft cover flange  32 . 
         [0022]    Additionally, a fluid seal is formed between the cover  26  and the frame  24  at a forward edge  44  and at an aft edge  46 . To accomplish formation of the fluid seal, a brazing material or other sealing material, heat activated or not, is put in place during the rolling or folding operation. If the material is heat activated, then heat is applied. In one embodiment, heat is supplied via an induction heating arrangement to reduce peripheral heating of the cooling jacket  28 . It is to be appreciated that one or the other of the edges  44 ,  46  may be brazed or both may be brazed. This joint configuration helps ensure the existence of a leak free cavity  34  between the frame  24  and the cover  26 . 
         [0023]    The cover  26  and the frame  24  in an exemplary embodiment are made from steel although it is to be appreciated that other materials may be substituted. When steel is employed, properties such as strength and stiffness are enhanced while maintaining a low cost. In some embodiments the steel may be coated with a corrosion inhibiting substance such as, for example, an aluminum coating to enhance corrosion resistance. Utilizing steel allows a thickness  48  of the cooling jacket  28  to be approximately 50% less than a similar cooling jacket formed from cast aluminum. Because a maximum diameter  50  of the machine  10  may be restricted by application, minimizing the thickness  48  allows for maximization of a stator diameter  52  of the stator  18  which subsequently can be translated into an increase in torque that can be produced by the electric machine  10 . For example, if the maximum diameter  50  is 172 mm, the thickness  48  of a steel cooling jacket  28  would be approximately 6 mm which accommodate a stator  18  with a stator diameter  52  of 160 mm. A cast aluminum jacket, however, would be approximately 14 mm thick which will accommodate a stator  18  with a stator diameter of 144 mm. This difference in stator diameters  52 , results in a torque advantage of 20-25% for the electric machine  10  produced with the steel cooling jacket  28 . 
         [0024]    Providing for the introduction of cooling fluid into the cavity  34 , an inlet connection  54  is disposed at an inlet hole  56  in the cover  26 , and is affixed to the cover  26  in a way similar to that described above. A connection flange  58  of the inlet connection  54  is affixed to an inlet hole boss  60  such as by welding, for example. Additionally, an inlet connection neck  62  is sealed to an inlet hole flange  64 . In one embodiment, inlet connection neck  62  is brazed to the inlet hole flange  64 . It is to be appreciated that one or the other of the inlet connection neck  62  and inlet hole flange  64  may be brazed or both may be brazed. As above, this configuration helps ensure that the joint between the inlet connection  54  and the cover  26  is leak free. 
         [0025]    Cooling fluid is urged through the inlet connection  54  and into the cavity  34 , circulating through the cavity  34 . As the fluid circulates, it conducts heat from the cooling jacket  28  which had conducted the heat from the stator  18  due to the frame  24  being in thermal contact with the stator  18 . The warmed cooling fluid exits the cavity  34  through an outlet connection  66 . As shown in  FIG. 3 , the outlet connection  66  is similarly disposed and sealed in an outlet hole  68  in the cover  26 , and in one embodiment is circumferentially adjacent to the inlet connection  54 . A connection flange  70  of the outlet connection  66  is affixed to an outlet hole boss  72  such as by welding, for example. Additionally, an outlet connection neck  74  is, in one embodiment, brazed to an outlet hole flange  76 . It is to be appreciated that one or the other of the outlet connection neck  74  and outlet hole flange  76  may be, for example, brazed or both may be, for example, brazed. In one embodiment, a barrier plate  78  is disposed in the cavity  34  blocking the cavity  34  between the inlet hole  56  and the outlet hole  68  to encourage 360 degree flow of cooling fluid around a circumference of the cavity  34 . 
         [0026]    In some embodiments, at least one protrusion  80  disposed in the cover  26  extends substantially a distance between the cover  26  and the frame  24 . In some embodiments, the protrusions  80  are drawn structures, meaning that while a protrusion is formed on one of an inner surface or an outer surface of the frame or the cover, a depression is formed on the other of the inner surface or the outer surface of the frame or the cover. For simplicity in the explanation of the invention in this application, the structures will be referred to as protrusions. It is to be understood that the type of “protrusion” can be any of the foregoing or equivalents thereof. The protrusions  80 , examples of which are shown in  FIGS. 4-7 , define a tortuous path for flow of cooling fluid through the cavity  34 . The protrusions  80  increase a surface area for dissipating heat from the stator  18  into the fluid and increase turbulence in the cavity  34  thereby decreasing convection resistance. The protrusions  80  additionally provide structural support for cover  26 , and increase stiffness of the cover  26  to protect it from potential handling damage. The protrusions  80  may extend from one or the other of the cover  26  and the frame  24 , or the protrusions  80  may extend from both the cover  26  and the frame  24  (although it is noted that the shrink fit used to secure the frame  24  to the stator  18  may be enhanced if protrusions are not formed in the frame  24  so that the frame  24  retains the greatest tensile integrity that the material from which the frame  24  is made affords), and extend at least partially across the cavity  34 .The protrusions  80  may be formed in the cover  26  and/or the frame  24  by for example, stamping, or alternatively by affixing the protrusions  80  to the cover  26  and/or the frame  24  by, for example, welding. When the cover  26  is then affixed to the frame  24  as described above, a labyrinthian flow path is defined in the cavity  34  as shown in  FIGS. 4-7 . 
         [0027]    An example of a protrusion  80  configuration is shown in  FIG. 4 . The protrusions  80  in this configuration are elongated structures disposed such that the elongation is in a substantially circumferential direction between the inlet hole  56  and the outlet hole  68  and extend into the cavity  34 . The protrusions  80  may be arranged in rows in an axial disposed between the forward cover flange  30  and the aft cover flange  32 . 
         [0028]    A second example of a protrusion  80  configuration is shown in  FIG. 5 . In the second example, the protrusions  80  are elongated structures disposed such that the elongation is in a substantially axial direction between the forward cover flange  30  and the aft cover flange  32  and extend into the cavity  34 . The protrusions  80  alternately extend from the forward cover flange  30  and the aft cover flange  32 . The result is a labyrinthian flow path for the cooling fluid. 
         [0029]      FIG. 6  illustrates a third example of a protrusion  80  configuration. In  FIG. 6 , the protrusions  80  comprise an array of structures with substantially circular cross sections arranged in the cavity  34 .  FIG. 7  illustrate yet another example of an arrangement of protrusions  80 . In  FIG. 7 , the protrusions  80  are elongated structures disposed such that the elongation is in a substantially circumferential direction between the inlet hole  56  and the outlet hole  68  and extend into the cavity  34 . The protrusions  80  alternately extend from either side of the barrier plate  78 . This creates a labyrinthian flow path for the cooling fluid through the circumferentially arranged protrusions  80 . It will be appreciated that the example protrusion  80  configurations other than those described above may be employed in the cavity  34 . 
         [0030]    Referring now to  FIGS. 8-10 , some embodiments include features to enhance heat transfer of stator end turns  82  to the cooling jacket  28 . In  FIG. 8 , the end turns  82  are encapsulated in, for example, a potting compound or plastic or other material which forms a potting layer  96 . A ring  84  made from a heat conductive material, for example, steel or aluminum, is fit onto the potting layer  96 . It will be appreciated that other materials may be employed for the ring  84 . The ring  84  may be contoured to better fit a shape of the potting layer  96  thus displacing substantially all of the air between the potting layer  96  and the ring  84  thus increasing thermal contact between the ring  84  and the potting layer  96 . Further the ring  84  may include slots or holes  86  to allow egress of lead wires  88  or the like. Utilization of the rings  84  reduces the amount of potting compound used, and increases the effectiveness of heat conduction from the end turns  82  into the cooling jacket  28 . 
         [0031]      FIG. 9  illustrates a second example. In  FIG. 9 , the end turns  82  are encapsulated, resulting in a potting layer  96 , and end bells  20  are fitted to the potting layer  96 . The end bells  20  may be contoured to better fit a shape of the potting layer  96  displacing substantially all of the air between the end bells  20  and the potting layer  96  thus increasing the thermal contact between end bells  20  and potting layer  96 . Additionally, a layer of thermal grease, sealant, or other thermally conductive compound may be interspersed between the potting layer  96  and the end bells  20 . 
         [0032]    A third example is illustrated in  FIG. 10 . In this example, the stator end turns  90  and  92  are encapsulated as described above. A first end bell  94  is fitted to first potting layer  96 , and a ring  84  is fitted to second potting layer  96 . The first end bell  94  and/or the ring  84  may be contoured to provide increased thermal contact with the potting layers  96 . 
         [0033]    While embodiments of the invention have been described above, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.