Abstract:
A motor housing and cooling fin assembly for an electric motor comprises a stator support having a longitudinal base and a series of fin plates protruding radially from the base, the fin plates for directing fluid coolant through the motor, a cylindrical support sleeve comprising a thin wall, the thin wall containing a series of slots for receiving distal ends of the fin plates; and a motor housing having a cylindrical inner wall, the support sleeve positioned between the stator support and the motor housing in an interference fit. By reducing the thickness of the motor housing by more than the thickness of the added cylindrical support sleeve, the motor housing is lighter. Instead of machining slots directly into the motor housing, they are machined the cylindrical support sleeve, which is of simple geometry, thereby reducing the difficulty and cost of manufacturing.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to apparatus and methods for housing electric motors and, more particularly, to apparatus and methods for using low cost light weight motor housing for housing high power density electric motors. 
         [0002]    Electric motors with high power density require effective cooling to remove a large amount of heat away from the stator or stator assembly during operation. This requirement is in order to prevent the motor from overheating, which could burn out the motor. Cooling the motor can be accomplished by circulating liquid coolant or cold air through a number of radially constructed fin plates surrounding a stator. While this is necessary, introducing such a design feature significantly increases the cost and weight of a motor. 
         [0003]    In a conventional design of a motor housing having a cooling mechanism, the stator support assembly has fin plates which slide into longitudinal slots in the motor housing. In particular, the longitudinal slots are machined into the inner diameter of the motor housing. This machining process is costly because of the complex geometry of the motor housing. Because the slots that need to be machined into the motor housing have sharp corners, great care has to be taken while machining the slots to ensure that mistakes are not made that can predispose the motor housing to cracking at a later time during use. The fact that the exterior of the motor housing is irregular and is not a simple cylinder further increases the difficulty of machining the slots into the motor housing because the machining process typically requires the irregularly shaped motor housing to be positioned perfectly on the machines for milling. The increased difficulty adds to the time and cost of making the motor. 
         [0004]      FIG. 1A  and  FIG. 1B  show a high power density electric motor having a conventional motor housing  5 , a stator support  7 , a rotor assembly  8  and a stator assembly  9 . Typically, motor housing for a high power density electric motor has an outer diameter OD of approximately 5 to 9 inches. A “conventional” motor housing is the motor housing that would be utilized without considering the teachings of the present invention. In a conventional motor, for example, although the thickness of the motor housing is typically minimized due to space limitations, for example in an aircraft or other application, the motor housing must be thick enough to receive and securely maintain, without undue stress or cracking, the sharp cornered slots, which are typically square or rectangular, for insertion of the fin plates of the cooling mechanism. As seen from  FIG. 1A  and  FIG. 1B , the motor housing  5  also must have a thick wall in order to accommodate both the depth of the slots and the stress concentration effects at the inner corner of the slots. Since the longitudinal slots  6  in the motor housing have sharp corners, i.e. are rectangular or square, the mechanical stress associated with them is amplified by approximately two to three times at those local areas. 
         [0005]    It is therefore useful to have a lower cost design for a high power density electric motor while still allowing for a suitable cooling mechanism to keep such an electric motor from burning out due to its generation of heat. Furthermore, there is a need to reduce the weight of such a motor, especially in the aerospace industry. In addition, there is a need to have a way to reduce the maximum stress in the outer housing derived from the presence of the adjacent cooling mechanism. 
       SUMMARY OF THE PRESENT INVENTION 
       [0006]    In one aspect of the present invention, there is presented a motor housing and cooling fin assembly for an electric motor comprising a stator support having a longitudinal base and a series of fin plates protruding radially from the base; a cylindrical support sleeve having a wall, the wall containing a series of slots for receiving distal ends of the fin plates; and a motor housing having a cylindrical inner wall, the support sleeve positioned between the stator support and the motor housing in an interference fit. 
         [0007]    In another aspect of the invention, there is presented a method of cooling a high power density electric motor during its operation, the method comprising calculating a thickness of a motor housing so as to accommodate placement of a cylindrical support sleeve adjacent the motor housing; forming a motor housing whose inner wall is cylindrical; heating and enlarging the motor housing so that the inner diameter of the motor housing increases from being smaller than an outer diameter of the cylindrical support sleeve to being larger than the outer diameter of the cylindrical support sleeve; inserting the cylindrical support sleeve inside the motor housing; allowing the motor housing to cool so as to create an interference fit between the motor housing and the cylindrical support sleeve; and circulating a coolant through the motor during operation of the motor. 
         [0008]    In a further aspect of the invention, there is presented an assembly comprising a longitudinal outer housing having an irregularly shaped exterior and a cylindrical inner wall, a longitudinal internal support assembly inside the outer housing including a plurality of radially protruding elements; a cylindrical support sleeve containing a plurality of cornered longitudinal slots for receiving distal ends of the radially protruding elements; the cylindrical support sleeve positioned between the internal support assembly and the outer housing in an interference fit, and a combined thickness of the cylindrical support sleeve and outer housing being approximately 0.2 inches or less. 
         [0009]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, descriptions and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1A  is a horizontal sectional view of a high power density electric motor having a conventional motor housing of the prior art; 
           [0011]      FIG. 1B  is a longitudinal sectional view of the motor of  FIG. 1A ; 
           [0012]      FIG. 2  is a perspective view of a high power density electric motor in accordance with the present invention; 
           [0013]      FIG. 3  is a sectional view of the assembly of the present invention; 
           [0014]      FIG. 4  is a vertical sectional view of the motor of  FIG. 2  taken along line  4 - 4  of  FIG. 5 , including the assembly of  FIG. 3 ; 
           [0015]      FIG. 5  is a horizontal sectional view of the motor of  FIG. 2  taken along line  5 - 5  of  FIG. 4 ; 
           [0016]      FIG. 6  is an exploded view of the motor of  FIG. 2 ; and 
           [0017]      FIG. 7  is a flow chart showing a method of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0019]    The present invention generally provides a motor housing and cooling fin assembly for a high power density electric motor, such as those found in the aerospace industry and those used in industrial control machinery. Generally, in terms of electric motors, a power density exceeding six horsepower per pound of weight (hp/lb) may be considered a “high power density”. 
         [0020]    In both conventional motors and in the motor of the present invention, the high power density electric motor may generate a substantial amount of heat and may be cooled by circulating liquid coolant or cold air through a number of radially projecting fin plates of the stator support. Using the present invention, this cooling mechanism can be accomplished without significantly increasing the cost and weight of the electric motor. This is because the motor housing is reduced in thickness (as compared to the thickness of the motor housing of conventional high power density electric motors) and the fin plates fit directly into a series of slots that are more easily machined into a thin cylindrical support sleeve of simple geometry, which support sleeve is fitted between the thin motor housing and the stator support with an interference fit. Accordingly, the electric motor does not unnecessarily add to the weight of the aircraft. 
         [0021]    In contrast to the prior art, which utilizes a motor housing, if made of aluminum, of approximately 0.4 inches wall thickness for a high power density electric motor, the motor housing and cooling fin assembly of the present invention utilizes an motor housing combined with a cylindrical support sleeve which together have a thickness of less than 0.2 inches, less than half the conventional motor housing. As a result, in contrast to the prior art, the electric motor of the present invention may be light weight for a high power density electric motor. In further contrast to the prior art, which does not make use of a cylindrical support sleeve between the motor housing and the stator support, and requires the expensive and difficult process of machining slots for the fin plates directly into a thick motor housing of complex geometry, the motor housing and cooling fin assembly of the present invention may use a thin cylindrical support sleeve of simple geometry, i.e. a cylinder, into which the series of slots for the fin plates are machined. This design may reduce the overall cost of the electric motor and may make it more affordable though it contains a special cooling mechanism. 
         [0022]    As seen from  FIG. 2 , the high power density electric motor  200  (showing end cap  199 ) that uses the assembly  10  of the present invention may have a motor housing  40  whose exterior  47  may be irregular, or of complex geometry. As seen from  FIG. 3 , the motor housing and cooling fin assembly  10  for electric motor  200  may include a stator support  20 , a cylindrical support sleeve  30  and a motor housing  40 .  FIG. 5  also shows other elements of high power density electric motor  200 , including its stator/stator assembly  29  and its rotor assembly  28 . 
         [0023]    Stator support  20 , which may be located inside cylindrical support sleeve  30  and by definition also inside housing  40 , may comprise a longitudinal base  22  and a series of fin plates  24  protruding radially from base  22 . Base  22  may be generally cylindrical. Fin plates  24  may direct fluid coolant, i.e. air or a liquid, through the motor through a length of the stator support  20 . Fin plates  24  may direct the fluid coolant from one end of motor  200  to the other end. 
         [0024]    As seen from  FIG. 4 , cylindrical support sleeve  30  may include a thin wall that contains a series of slots  33  for receiving distal ends  24   a  of fin plates  24 . The exact thickness of cylindrical support sleeve  30  may vary with the application. In a high power density electric motor made of aluminum, for example, cylindrical support sleeve  30  may have a thickness of less than 0.1 inches since its thickness combined with the thickness of motor housing  40  may be less than approximately 0.2 inches. Cylindrical support sleeve  30  may be of uniform thickness, although in other cases cylindrical support sleeve  30  may be of varied thickness. 
         [0025]    Slots  33  may be longitudinal in that they may extend for all or most of the length of the motor  200 . Slots  33  may also be cornered and in particular may have sharp corners. Specifically, slots  33  may be rectangular or square, or may have a curved shape. Since cylindrical support sleeve  30  may be of simple geometry, machining slots  33  into cylindrical support sleeve  30  may be easier and less costly. 
         [0026]    Motor housing  40  may be generally longitudinal and may have an inner wall  41  that is cylindrical and may have an outer diameter OD that varies due to the irregularity or complex geometry of the exterior of motor housing  40  (see  FIG. 2 ). The exterior of motor housing  40  may be of any shape motor housing suitable for that particular high power density electric motor or other electric motor that it is serving to house. The thickness of motor housing  40  may have been reduced to accommodate at least the thickness of a cylindrical support sleeve  30 . In fact, motor housing  40  may be reduced more than that and may have a thickness such that its own thickness combined with the thickness of cylindrical support sleeve  30  is significantly less than the thickness of a conventional motor housing. 
         [0027]    As can be seen from  FIGS. 1A and 1B , the conventional motor housing is comparatively thick, and if one compares this thickness to the thickness of the motor housing  40  shown in  FIG. 4  and  FIG. 5 , one can see that the motor housing  40  of the present invention may be considerably thinner. 
         [0028]    In addition, one can see from  FIG. 4  and  FIG. 5  that the conventional motor housing of  FIG. 1A  and  FIG. 1B  may even be significantly thicker than the combined thicknesses, in the motor of the present invention, of cylindrical support sleeve  30  and motor housing  40 . For example, a combined thickness of cylindrical support sleeve  30  and outer housing  40  may be less than approximately 0.2 inches if housing  40  and ring  30  are made of aluminum, whereas the motor housing of a conventional high power density electric motor may be approximately 0.4 inches if made of aluminum. As a result, the overall outer diameter OD of motor housing  40  and the overall outer diameter OD of motor  200  may be reduced. In the process, moreover, motor housing  40  may be of lighter weight. 
         [0029]    Motor housing  40 , cylindrical support sleeve  30  and stator support  20  may be of approximately equal length since they may run the length of motor  200 . 
         [0030]    The motor housing and cooling fin assembly  10  may further comprise cylindrical support sleeve  30  positioned between stator support  20  and motor housing  40  in an interference fit. The interference fit may be created by using a motor housing that is thinner than it otherwise be, to the point of being able to accommodate insertion of cylindrical support sleeve  30  and still be less thick than a motor housing without cylindrical support sleeve  30 , and then inserting cylindrical support sleeve  30  into motor housing while motor housing is hot and enlarged. When motor housing  40  cools down, an interference fit may be created. 
         [0031]    As a result of the interference fit, motor housing  40  is at all times compressing support sleeve  30 . This constant tension from the constant hoop compression force that support sleeve  30  is under may mean that support sleeve  30  may not prone to cracking and may not crack. 
         [0032]    As seen from  FIG. 6 , motor  200  may comprise motor housing  40 , cylindrical support sleeve  30  and stator support  20  of assembly  10  and may also comprise stator  29  and rotor assembly  28 , which are all located between end caps  199 . Furthermore, the irregular exterior  47  of motor housing  40  is visible from  FIG. 6 . 
         [0033]    As seen from  FIG. 7 , the present invention may also be expressed as a method  100  of making a light weight low cost motor housing and cooling fin assembly for a high power density electric motor. In accordance with such a method  100 , when motor housing  40  is formed, in step  110 , its thickness may be calculated so as to accommodate placement of a cylindrical support sleeve  30  adjacent to the motor housing  40 . In effect, motor housing  40  may be reduced in the area of its inner wall  41  which means that the inner diameter ID of motor housing  40  may be increased without a corresponding increase in the outer diameter OD of motor housing  40 . 
         [0034]    The thickness of motor housing  40  may be calculated to accommodate placement of cylindrical support sleeve  30  while achieving a reduced overall size of the electric motor as measured by the outer diameter of the electric motor. This is because the thickness of motor housing  40  may be reduced by an amount that exceeds the thickness of cylindrical support sleeve  30 . 
         [0035]    In step  120 , motor housing  40  may be formed having an inner wall that may be cylindrical. Although the exterior of motor housing  40  may be of complex geometry and may be generally irregularly shaped, as seen in  FIG. 2 , inner wall  41  of motor housing  40  remains cylindrical. Thus, the inner diameter ID of motor housing  40  may be increased to accommodate more than the thickness of a cylindrical support sleeve. 
         [0036]    In a further step  130  of method  100 , motor housing  40  may be heated to an elevated temperature. In doing so, motor housing  40  may be enlarged. This step  120  of heating can be accomplished in any number of ways. Those skilled in the art, for example, may suggest accomplishing this by heating motor housing  40  in an oven. This heating of motor housing  40  to an elevated temperature may cause motor housing  40  to expand so that the inner diameter ID of motor housing  40  increases from being smaller than the outer diameter OD of cylindrical support sleeve  30  to being slightly larger than the outer diameter OD of cylindrical support sleeve  30 . 
         [0037]    Method  100  may further include a step  140  wherein while motor housing  40  is still heated and in an enlarged state, cylindrical support sleeve  30  may be inserted into motor housing  40  so that support sleeve  30  may be inside motor housing  40 . Stator support  20  may have already been placed inside cylindrical support sleeve  30  when inserting cylindrical support sleeve  30  inside motor housing  40 . Furthermore, stator  29  may have already been placed inside stator support  20  when stator support  20  is placed inside cylindrical support sleeve  30 . In step  150 , motor housing  40  may then be allowed to cool, for example to room temperature, so as to create an interference fit between motor housing  40  and cylindrical support sleeve  30 . Finally, in step  160 , coolant may be circulated through the motor during its operation. The coolant may be guided by fin plates  24  of stator support  20 . 
         [0038]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.