Abstract:
A stator ( 100 ) of an electric motor includes a coil ( 12 ), a stator core ( 10 ) which supports the coil, and covers ( 22   a,    22   b ) which are attached to the stator core so as to surround coil ends ( 13   a,    13   b ) of the coil, wherein the covers have outside diameters which are at least partially smaller than the outside diameters of the stator cores, the covers have coefficients of linear expansion which are larger than the coefficient of linear expansion of the stator core, and the covers expand due to heat whereby the covers closely contact the housing which is arranged around the stator.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a stator of an electric motor. In particular, the present invention relates to a stator of an electric motor which has covers which cover coil ends which stick out from end faces of a stator core. 
     2. Description of the Related Art 
     In general, high output electric motors are being sought. However, to increase the output of electric motors, it is necessary to efficiently discharge the heat which is generated from the coil. For this reason, in the past, various electric motors have been proposed which efficiently discharge the heat which is generated from the coil to the outside. 
     Japanese Patent No. 3262716, Japanese Patent Publication No. 2008-178190, and Japanese Patent No. 3775348 disclose to insert a stator of an electric motor into a tube member and to fill an insulating resin with a high heat conductivity into the clearance between the stator and the tube member. In this case, however, the stator is increased in outside diameter by the amount of the tube member, so there is a possibility that the stator which is inserted into the tube member is not able to be placed in an existing housing. 
     Furthermore, Japanese Patent Publication No. 2010-68699 discloses a configuration where two covers surround the coil ends. In this case, the stator does not change in outside diameter, so the above-mentioned problem does not arise. Further, it is sufficient to prepare just covers, so the result is cheaper than with the case of using a tube member. 
     However, in Japanese Patent Publication No. 2010-68699, a material with a high heat conductivity is used to prepare the covers, so there is the problem that the covers more easily rise in temperature compared with the stator core. 
     Further, a stator is often fastened to a housing by shrink fitting. Therefore, if the stator is inserted into a heated housing etc, the covers are heated more rapidly than the stator core. As a result, sometimes the covers expand by heat and are tightly fit to the housing. 
     The present invention was made in consideration of such a situation and has as its object the provision of a stator of an electric motor which is provided with covers which are never tightly fit with the housing. 
     SUMMARY OF THE INVENTION 
     To achieve the above-mentioned object, according to a first aspect, there is provided a stator of an electric motor comprising a coil, a stator core which supports the coil, and covers which are attached to the stator core so as to surround coil ends of the coil which stick out from end faces of the stator core in the axial direction, wherein the covers have outside diameters which are at least partially smaller than an outside diameter of the stator core, the covers have coefficients of linear expansion which are larger than a coefficient of linear expansion of the stator core, and the covers expand by heat whereby the covers are made to closely contact a housing which is arranged around the stator. 
     According to a second aspect, there is provided the first aspect wherein cutaway parts are formed at outside edge parts of distal ends of the covers. 
     According to a third aspect, there is provided the first aspect wherein slanted parts which extend from proximal ends to distal ends of the covers are formed at outer surfaces of the covers. 
     According to a fourth aspect, there is provided the first aspect wherein curved parts are formed at outside edge parts of distal ends of the covers. 
     According to a fifth aspect, there is provided the first aspect wherein curved parts which extend from proximal ends to distal ends of the covers are formed at outer surfaces of the covers. 
     According to a sixth aspect, there is provided the first aspect wherein outer surfaces of the covers are formed with step parts and the step parts have outside diameters which are equal to the outside diameter of the stator core at proximal ends of the covers and which are smaller than the outside diameter of the stator core at distal ends of the covers. 
     According to a seventh aspect, there is provided the sixth aspect wherein outside edge parts of distal ends of the covers are formed with cutaway parts or curved parts. 
     These and other objects, features, and advantages of the present invention will become clearer from the detailed description of typical embodiments of the present invention which are shown in the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view which shows the configuration of a stator of an electric motor according to the present invention. 
         FIG. 2A  is a first enlarged view of a coil end part which is shown in  FIG. 1   
         FIG. 2B  is a second enlarged view of a coil end part. 
         FIG. 3A  is a third enlarged view of a coil end part. 
         FIG. 3B  is a fourth enlarged view of a coil end part. 
         FIG. 4A  is a fifth enlarged view of a coil end part. 
         FIG. 4B  is a sixth enlarged view of a coil end part. 
     
    
    
     DETAILED DESCRIPTION 
     Below, embodiments of the present invention will be explained with reference to the attached drawings. In the following figures, similar members are assigned similar reference notations. To facilitate understanding, these figures are suitably changed in scale. 
       FIG. 1  is a cross-sectional view which shows the configuration of a stator of an electric motor according to the present invention. As shown in  FIG. 1 , the stator  100  of the electric motor is provided with a stator core  10 , a coil  12 , mold resin  14 , and end covers  22   a ,  22   b . Further, the stator  100  is arranged inside of a housing  110  by shrink fitting etc. 
     The stator core  10  is a cylindrically shaped core and is produced by stacking ring-shaped magnetic steel sheets. At the inside of the stator core  10 , a hollow hole  11  is formed. In this hollow hole  11 , a fixture which is used when injecting the mold resin  14  (resin filler) or a rotor (not shown) of the electric motor is inserted. 
     The stator core  10  supports the coil  12 . The end parts of the coil  12 , constituted by the coil end parts  13   a ,  13   b , stick out from the end faces of the stator core  10  in the axial direction to the outside. Further, as shown in  FIG. 1 , from the coil end part  13   a , a lead wire  18  for supplying power to the coil  12  is led to the outside. 
     The end covers  22   a ,  22   b  in the present embodiment are substantially cylindrically shaped members and are formed from a high heat conductivity material, for example, a resin. As shown in  FIG. 1 , the end covers  22   a ,  22   b  are attached to the stator core  10  so as to cover the coil end parts  13   a ,  13   b  in the circumferential direction. Further, as can be seen from  FIG. 1 , the end covers  22   a ,  22   b  have maximum outside diameters H0 which are equal to the outside diameter W0 of the stator core  10 . 
     The mold resin  14  is injected and filled into the spaces between the coil end parts  13   a ,  13   b  and the end covers  22   a ,  22   b  where it is solidified. The mold resin  14  and the end covers  22   a ,  22   b  are in close contact with the outer circumferential surface of the mold resin  14 . For this reason, the heat which is generated from the heat coil end parts  13   a ,  13   b  is discharged through the mold resin  14  and the end covers  22   a ,  22   b  to the outside housing  110 . 
       FIG. 2A  is a first enlarged view of a coil end part. In  FIG. 2A , the one coil end part  13   a  is shown, but the same is true for the other coil end part  13   b  as well. Furthermore, for the purpose of simplification, in  FIG. 2A , illustration of the housing  110  is omitted. The same is true for the later explained  FIG. 2B  to  FIG. 4B . 
     As shown in  FIG. 2A , at the outside edge part of the end cover  22   a , a cutaway part  41  is formed. Therefore, at the distal ends of the end covers  22   a ,  22   b , the distal end outside diameters H1 of the end covers  22   a ,  22   b  are smaller than the outside diameter W0 of the stator core  10 . At the remaining parts of the end covers  22   a ,  22   b  where the cutaway parts  41  are not formed, the end covers  22   a ,  22   b  have maximum outside diameters H0 which are equal to the outside diameter W0 of the stator core  10 . 
     In such a configuration, when placing the stator  100  inside the housing  110 , slight clearances are formed between the distal ends of the end covers  22   a ,  22   b  and the housing. Further, the stator  100  is fastened to the housing  110  by shrink fitting. In the present invention, even if shrink fitting is used to fasten the stator  100  to the housing  110 , there is a slight clearance present, so the end covers  22   a ,  22   b  can be kept from rapidly rising in temperature. For this reason, it becomes possible to prevent the covers from ending up being tightly fit against the housing in the middle of shrink fitting. 
     Further, if the housing reaches a certain temperature at the time of operation of the electric motor, the end covers  22   a ,  22   b  will expand by the heat and the end covers  22   a ,  22   b  will have distal end outside diameters H1 at the distal ends which are equal to the outside diameter W0 of the stator core. For this reason, the outer circumferential surfaces near the distal ends of the end covers  22   a ,  22   b  also closely contact the inner surfaces of the housing. Therefore, at the time of operation of the electric motor, the heat generating members constituted by the end covers  22   a ,  22   b  closely contact the heat removing part constituted by the housing  110 . For this reason, at the time of operation of the electric motor, the heat from the end covers  22   a ,  22   b  can be efficiently conducted to the housing  110 . In other words, in the present invention, it is possible to obtain a heat removal effect similar to the stator  100  of the prior art where the outside diameters of the end covers  22   a ,  22   b  and the outside diameter of the stator core  10  are equal across the entire end covers  22   a ,  22   b.    
     In this regard, the end covers  22   a ,  22   b  have distal end outside diameters H1 which are determined by the following equation (1):
 
 H 1 =W 0×{1+σ0( T−Tr )}/{1+σ1( T−Tr )}  (1)
 
     In equation (1), σ0 is the coefficient of linear expansion (1/° C.) of the material of the stator core  10 , σ1 is the coefficient of linear expansion (1/° C.) of the material of the end covers  22   a ,  22   b , T is the temperature (° C.) of the housing  110  when the distal ends of the end covers  22   a ,  22   b  expand due to heat to be in close contact with the housing  110 , and Tr is the temperature (° C.). 
     The coefficients of linear expansion σ0, σ1 are determined in accordance with the materials which are used. Therefore, in the present invention, if the temperature at which the housing  110  to which the stator  100  is arranged and the end covers  22   a ,  22   b  come into close contact is determined, the distal end outside diameters H1 can be calculated. This temperature T is found in advance by experiments etc. considering the coefficients of linear expansion σ0, σ1. 
     In the embodiment which is shown in  FIG. 2A , the electrical steel sheets which form the stator core  10  have a coefficient of linear expansion σ0 of about 11.2×10 −6  [1/° C.] and have an outside diameter of W0=about φ180.0 [mm]. The end covers  22   a ,  22   b  are for example formed from aluminum alloy and have coefficients of linear expansion σ1 of 23×10 −6  [1/° C.]. Furthermore, the housing temperature Tr at the time of not operating is set to 25° C. while the temperature T at the time when the distal ends of the end covers  22   a ,  22   b  closely contact the housing  110  is set to T=50[° C.]. In such a case, equation (1) may be used to calculate that the end covers  22   a ,  22   b  have distal end outside diameters of H1=179.947 [mm]. By utilizing equation (1), the distal end outside diameters H1 of the end covers  22   a ,  22   b  are easily found. Therefore, it is possible to produce suitable shapes of end covers  22   a ,  22   b.    
     Further, to ease the stress which is applied to the coil  12  of the stator  100  when the end covers  22   a ,  22   b  and the mold resin  14  expand and contract, the end covers  22   a ,  22   b  and the mold resin  14  preferably have coefficients of linear expansion which are close to each other. Further, for similar reasons, the end covers  22   a ,  22   b  and the mold resin  14  are required to have coefficients of linear expansion which are larger than the coefficient of linear expansion of the material of the stator core  10  and coefficient of linear expansion of the housing. 
       FIG. 2B  is a second enlarged view of a coil end part. In  FIG. 2B , the end covers  22   a ,  22   b  have slanted parts  42  which are formed at the outer circumferential surfaces of the end covers  22   a ,  22   b  and which extend from the proximal ends to the distal ends. As can be seen from  FIG. 2B , the slanted parts  42  are formed so as to taper from the proximal ends toward the distal ends. Further, the slanted parts  42  are formed so that the distal ends of the end covers  22   a ,  22   b , which are also the distal ends of the slanted parts  42 , have outside diameters the same as the above-mentioned distal end outside diameters H1. In this case, compared with the cutaway parts  41  which are shown in  FIG. 2A , it is possible to more easily form the slanted parts  42 . 
       FIG. 3A  is a third enlarged view of a coil end part. In  FIG. 3A , the end covers  22   a ,  22   b  have distal ends at the outside edges of which the curved parts  43  are formed. As can be seen from  FIG. 3A , the curved parts  43  are formed so as to taper toward the distal ends of the end covers  22   a ,  22   b . Further, the curved parts  43  are formed so that the distal ends of the end covers  22   a ,  22   b , which are also the distal ends of the curved parts  43 , have outside diameters the same as the above-mentioned distal end outside diameters H1. Note that as shown in the fourth enlarged view of a coil end part of  FIG. 3B , the curved parts  44  may be formed so as to extend from the proximal ends to the distal ends of the end covers  22   a ,  22   b . It will be understood that the curved parts  44  can be easily formed compared with the curved parts  43  which are shown in  FIG. 3A . 
       FIG. 4A  is a fifth enlarged view of a coil end part. In  FIG. 4A , step parts  45  are formed at the outer circumference near the proximal ends of the end covers  22   a ,  22   b . As can be seen from  FIG. 4A , the step parts  45  at the proximal ends of the end covers  22   a ,  22   b  have outside diameters H0 which are equal to the diameter W0 of the stator core  10 . Further, the step parts  45  at the distal ends of the end covers  22   a ,  22   b  have outside diameters which are smaller than the outside diameter W0 of the stator core  10  and which are equal to the above-mentioned distal end outside diameters H1. 
     Furthermore,  FIG. 4B  is a sixth enlarged view of a coil end part. The end covers  22   a ,  22   b  which are shown in  FIG. 4B  are formed with step parts  45  in the same way as explained above. Further, the end covers  22   a ,  22   b  have distal ends with outside edges where curved parts  46  are formed. The curved parts  46  differ from the above-mentioned curved parts  43 . The curved parts  46  have outside diameters at the distal ends which are smaller than the above-mentioned distal end outside diameters H1. Note that instead of the curved parts  46 , it is also possible to form cutaway parts (not shown). Furthermore, while not shown in the drawings, in  FIG. 4A , the entire regions L of the end covers  22   a ,  22   b  with outside diameters which correspond to the distal end outside diameters H1 may be formed with slanted parts or curved parts. 
     In this way, in the embodiments which are shown in  FIG. 2B  to  FIG. 4B  as well, it will be understood that similar advantageous effects are obtained as explained with reference to  FIG. 2A . Note that the proximal ends have outside diameters which are equal to the outside diameter W0 of the stator core  10  and the distal ends have outside diameters which correspond to the distal end outside diameters H1. All shapes of end covers  22   a ,  22   b  are included in the scope of the present invention. 
     Advantageous Effects of Invention 
     In the present invention, when inserting the stator in the housing, fine clearances can be formed between the covers and the housing. Therefore, due to the presence of the clearances, the covers can be kept from rising in temperature. For this reason, it is possible to prevent the covers from being tightly fit against the housing in the middle of shrink fitting. 
     Furthermore, at the time of operation of the electric motor, if the housing reaches a certain temperature, the covers will expand due to the heat and the covers will become equal in outside diameters with the outside diameter of the stator core. Thus, the cover is in close contact with the inner surface of the housing and it is possible to improve a heat conduction efficiency. 
     Typical embodiments were used to explain the present invention, but a person skilled in the art would understand that the above-mentioned changes and various other changes, deletions, and additions may be made without departing from the scope of the present invention.