Abstract:
An electric motor for use in an electric vehicle, the electric motor includes: an inner housing which holds a stator; and an outer housing which defines a space through which cooling water is caused to flow between the inner housing and itself. The inner housing and the outer housing are connected by a plurality of columns disposed in the space.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electric motor for use in an electric vehicle. 
     2. Description of the Related Art 
     As the global warming issue is becoming more serious, more attention is paid to electric vehicles which provide less environmental load, and various technologies have been proposed as elementary technologies of electric vehicles. For example, in electric motors (hereinafter, referred to as a motor) for use in electric vehicles, a water-cooled motor has been proposed in which a cooling water jacket is provided on an outer circumference of a stator for circulation of cooling water for cooling the motor in order to maintain the temperature in the interior of a motor in operation to a permissible value. 
     As a construction of a cooling water jacket in a water-cooled motor, there has been proposed a construction in which a space between an inner housing into which a stator is inserted and an outer housing which configures a cooling water jacket is completely hollowed in order to ensure a required cooling capability. In addition, there has also been proposed a cooling water jacket construction for increasing further the cooling effect in which radiation fins are provided in a hollow portion in a cooling jacket while considering a flow of cooling water (refer to JP-A-8-19218). 
     The operation speed of motors for electric vehicles reaches about 10000 rpm in an attempt to obtain a highly efficient area. As this occurs, a high-frequency vibratory force by an electromagnetic force is applied to a stator, whereby an inner housing portion which holds the stator is caused to vibrate, and the whole of a motor case is vibrated by micro-amplitude high frequency waves, high-frequency vibration noise of several kHz being thereby generated. However, the motor in which the cooling water jacket is completely hollowed originally has difficulty in increasing the rigidity thereof, and in the event that a high-frequency vibratory force is applied to stator, the structural rigidity of the motor case is insufficient for the rigidity required for motor cases. In addition, in the case of the motor described in JP-A-8-19218 in which the radiation fins are provided in the hollow portion provided as the cooling water jacket and the inner housing portion and the outer housing portion are connected together by part of the radiation fins, since the radiating effect cannot be obtained when the radiation fins are made thick, the inner housing and the outer housing are connected together by thin plate-like members. Consequently, in the event that a high-frequency vibratory force is applied to the stator, the structural rigidity of the motor case is still insufficient for the structural rigidity required for motor cases, and the reduction in vibration noise level has still been difficult. Further, when an inner housing and an outer housing are connected together by radiation fins, in the event that a hollow portion residing between the inner housing and the outer housing is not increased, it becomes difficult to discharge core sand which is used as a core to form the hollow portion. Consequently, when attempting to connect the inner housing and the outer housing together by the radiation fins, there has been caused a problem that the external shape of the motor itself has to be enlarged. In this way, in the field of motors for electric vehicles, with a view to suppressing vibrations without enlarging the external shape of a motor, an increase in structural rigidity, in particular, an increase in rigidity of an inner housing which holds a stator has been desired. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an electric motor which can increase rigidity thereof and cooling efficiency. 
     In order to achieve the object, according to the invention, there is provided an electric motor for use in an electric vehicle, the electric motor comprising: 
     an inner housing which holds a stator; and 
     an outer housing which defines a space through which cooling water is caused to flow between the inner housing and itself, wherein 
     the inner housing and the outer housing are connected by a plurality of columns disposed in the space. 
     A plurality of first ribs which extend in an axial direction and a plurality of second ribs which extend in a circumferential direction may be provided on an outer circumferential surface of the outer housing. At least first one of the plurality of columns may be disposed in a position which corresponds to one of the plurality of first ribs and the plurality of second ribs. 
     At least second one of the plurality of columns may be disposed in a position which corresponds to a center of a portion surrounded by the plurality of first ribs and the plurality of second ribs. 
     The portion surrounded by the plurality of first ribs and the plurality of second ribs may have an area which can generate membrane vibration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are diagrams showing an embodiment of an electric motor according to the invention,  FIG. 1A  is a radial sectional view of the electric motor and  FIG. 1B  is an axial sectional view of the same motor. 
         FIGS. 2A and 2B  are diagrams showing a relationship between an external appearance and an interior flow path of the electric motor shown in  FIGS. 1A and 1B ,  FIG. 2A  is an external view of the electric motor shown in  FIG. 1A  as viewed from thereabove and  FIG. 2B  is an external view of the same motor as viewed from therebelow. 
         FIGS. 3A and 3B  are diagrams showing a relationship between the external appearance and the interior flow path of the electric motor shown in  FIGS. 1A and 1B ,  FIG. 3A  is an external view of the electric motor as viewed from the left and  FIG. 3B  is an external view of the same motor as viewed from the right. 
         FIG. 4  is a perspective view showing a configuration of the flow path in an interior of the electric motor shown in  FIGS. 1A to 3B . 
         FIG. 5  is a development of the flow path in the interior of the electric motor shown in  FIG. 4 . 
         FIG. 6  is a graph showing a relationship of magnitude of transmission of vibration relative to vibration frequency between a related-art example (without columns) and the invention (with columns). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, using  FIGS. 1A to 5 , an embodiment of an electric motor according to the invention will be described in detail. Note that the electric motor according to the invention is suitable for use in electric vehicles. 
       FIGS. 1A and 1B  are diagrams showing an embodiment of an electric motor according to the invention,  FIG. 1A  is a radial sectional view of the electric motor and  FIG. 1B  is an axial sectional view of the same motor. Firstly, the configuration of the electric motor according to the invention will schematically be described by the use of  FIGS. 1A and 1B . 
     As is shown in  FIGS. 1A and 1B , an electric motor (motor)  1  of this embodiment has a shaft  2  which is a rotational shaft, a rotor  4  which is supported concentrically on the shaft  2  and which has a plurality of permanent magnets  3  and a cylindrical stator  6  which is disposed on a circumference of the rotor  4  with an appropriate space held between the rotor  4  and itself and which has a plurality of coils  5 . The electric motor  1  is constructed in such a manner that the stator  6  is held inside an inner housing  10  and the shaft  2  and the rotor  4  are supported together with bearings by brackets  7 ,  8  which are disposed at both end portions of the inner housing  10 . 
     In addition, an outer housing  11  is provided on an outer circumferential side of the inner housing in such a manner as to define the inner housing  10  and itself a space through which cooling water is caused to flow. This is a construction which is a so-called cooling water jacket. In the motor  1  of the embodiment, a space between the inner housing  10  and the outer housing  11  is not hollowed completely, but, as is shown in  FIGS. 1A and 1B , the inner housing  10  and the outer housing  11  are connected together by a large number of columns  12   a ,  12   b ,  12   c  which are disposed in this apace. A flow path  20  (refer to portions shaded with dots) through which cooling water flows is formed by the inner housing  10 , the outer housing  11  and columns  12   a ,  12   b ,  12   c.  In addition, the columns  12   a ,  12   b ,  12   c  are not thin elongated columns like radiation fins but are each formed into a circular cylindrical shape. Note that while the circular cylindrical columns are depicted as an example in this embodiment, for example, an oval cylinder, a triangular prism, a quadrangular prism, a polygonal prism, a prism having a star-shape cross section and the like may be used as the columns. 
     In addition, in order to increase rigidity and prevent the occurrence of membrane vibrations, a plurality of axial ribs  13   a  and a plurality of circumferential ribs  13   b  are provide in axial direction and a circumferential direction, respectively, on an outer circumferential surface of the outer housing  11 . 
     Additionally, an inlet portion  14  and an outlet portion  15  for cooling water are provided in such a manner as to project in a tent-like fashion from the outer circumferential surface of the outer housing  11 , and an inlet port  14   a  and an outlet port  15   a  are provided in the inlet portion  14  and the outlet portion  15 , respectively, so as to supply and discharge cooling water into and from the flow path  20 . 
     In addition, a plurality of sand removal holes  16  are provided. These sand removal holes  16  are for removing sand which is used as a core after casting and are thereafter sealed with lids. 
     Here, referring also to  FIGS. 2A to 5 , the flow path  20  will be described. Note that for easy comparison of the drawings including  FIGS. 1A and 1B , angles are also shown on the drawings which are measured from a bottom portion of the motor as 0°. 
     In addition,  FIGS. 2A to 3B  are diagrams showing a relationship between an external appearance and an interior flow path of the motor  1  shown in  FIGS. 1A and 1B ,  FIG. 2A  is an external view of the motor shown in  FIG. 1A  as viewed from thereabove,  FIG. 2B  is an external view of the same motor in  FIG. 1A  as viewed from therebelow,  FIG. 3A  is an external view of the same motor in  FIG. 1A  as viewed from the left and  FIG. 3B  is an external view of the same motor in  FIG. 1A  as viewed from the right.  FIG. 4  is a perspective view showing a configuration of the flow path  20  in an interior of the motor shown in  FIGS. 1A to 3B , and  FIG. 5  is a development of the flow path  20  shown in  FIG. 4  in which the flow path is deployed from 0° in a direction indicated by an arrow. 
     In the motor  1  of the embodiment, the large number of columns  12   a ,  12   b ,  12   c  are disposed in the flow path  20  which is defined between the inner housing  10  and the outer housing  11 , and the inner housing  10  and the outer housing  11  are connected together by these columns  12   a ,  12   b ,  12   c.  As a result, the rigidity of the inner housing  10  is increased by the rigidity of the outer housing  11  which is connected thereto by the columns  12   a ,  12   b ,  12   c , whereby the high-frequency vibration of the stator  6  is suppressed by both the inner housing  10  and the outer housing  11 . Further, the plurality of axial ribs  13   a  and the plurality of circumferential ribs  13   b  are provided on the outer circumference of the outer housing  11 , so as to increase further the rigidity of the outer housing  11  itself, so that the rigidity of the inner housing  10  is also increased further, whereby the high-frequency vibration of the stator  6  is suppressed further. Note that increasing the rigidities of the inner housing  10  and the outer housing  11  means increasing the natural frequencies thereof, in the event that the natural frequencies of the inner housing  10  and the outer housing  11  are increased to a region where they do not resonate with the high-frequency vibrations of the stator  6 , a reduction in vibration noise can be realized. 
     In this way, by the inner housing  10  and the outer housing  11  being connected together by the large number of columns  12   a,    12   b ,  12   c , the rigidity of the inner housing  10  can be increased. In the motor  1  of the embodiment, however, a further increase in rigidity of the inner housing  10  is realized by devising disposing positions of the columns  12   a ,  12   b ,  12   c.  Referring to  FIG. 5 , although the columns  12   a ,  12   b ,  12   c  look as if they were disposed at random, basically, the columns are disposed based on the following rule. 
     Specifically, the columns  12   b  are basically disposed in positions which lie directly below at least one of the axial ribs  13   a  and the circumferential ribs  13   b , and when circumstances require, the columns  12   b  are disposed in positions which lie in the vicinity of at least one of the axial ribs  13   a  and the circumferential ribs  13   b  or in positions which lie directly below intersecting portions between the axial ribs  13   a  and the circumferential ribs  13   b . The ribs  12   b  are made to have an outside diameter of the order of twice the widths of the axial ribs  13   a  and the circumferential ribs  13   b, and based on the outside diameter of the columns  12   b , the outside diameters of the columns  12   a  and the columns  12   c  are set larger and smaller than the outside diameter of the columns  12   b , respectively. 
     In addition, although the columns  12   c  are also disposed in positions which lie directly below at least one of the axial ribs  13   a  and the circumferential ribs  13   b , the outside diameter of the columns  12   c  is smaller than that of the columns  12   b.  This is because the inlet portion  14  resides in the vicinity thereof and the columns  12   c  do not constitute resistance to a flow of cooling water supplied from the inlet port  14   a.  However, the outside diameter of the columns  12   c  is larger than the widths of the axial ribs  13   a  and the circumferential ribs  13   b.  Note that since in the event that there are provided any columns directly below the inlet port  14   a  and the outlet port  15   a , a pressure loss is increased, no columns are provided in those portions (refer to  FIG. 5 ). 
     On the other hand, the columns  12   a  are disposed in positions which lie directly below centers of portions which are surrounded by the axial ribs  13   a  and the circumferential ribs  13   b.  This is because in a case where the areas of the portions which are surrounded by the axial ribs  13   a  and the circumferential ribs  13   b  are wide, there is a fear that membrane vibration is generated. Consequently, in the event that the portions which are surrounded by the axial ribs  13   a  and the circumferential ribs  13   b  have such an area that causes membrane vibration, the columns  12   a , whose outside diameter is larger than that of the columns  12   b , are disposed in positions which lie directly below the centers of the portions surrounded by the axial ribs  13   a  and the circumferential ribs  13   b , that is, in positions where the rigidity is predicted to be reduced. By this configuration, the rigidity of the portions which are surrounded by the axial ribs  13   a  and the circumferential ribs  13   b  can be increased, so as to suppress the membrane vibration. Note that in this embodiment, in many cases, the columns  12   a  redisposed on a center line C of the development shown in  FIG. 5 . 
     In this way, by the columns  12   b ,  12   c  being disposed directly below or in the vicinity of the axial ribs  13   a  and the circumferential ribs  13   b , the columns  12   b ,  12   c  are made to be joined to the axial ribs  13   a  and the circumferential ribs  13   b,  and the columns  12   a  are disposed at the portions where the rigidity is predicted to be reduced. By adopting this configuration, the rigidity of an overall motor case including the inner housing  10  and the outer housing  11  can be increased further. Consequently, although also in the motor  1  of the embodiment, high-frequency vibrations are radially transmitted to the inner housing  10  and the outer housing  11  from the stator  6  which functions as the source of vibratory force, since the rigidity of the overall motor case is high, as is shown in  FIG. 6 , it could be confirmed that high-frequency vibrations could be dampened remarkably in vibration frequencies exceeding 2 kHz. In addition,  FIG. 6  shows in a graph the magnitude of transmission of vibration relative to vibration frequency between a related-art example (without columns) and the invention (with columns). 
     In addition, in the motor  1  of the embodiment, in addition to the increase in rigidity, a device is also made to realize an increase in cooling efficiency. 
     Specifically, the aforesaid columns  12   a ,  12   b ,  12   c  function not only to increase the rigidity of the motor case but also generate turbulent flows so as to increase heat transfer coefficient. This is because although in the event that there is stagnation in a flow of cooling water, heat stays or is confined in the motor case, whereby the heat transfer coefficient to cooling water is reduced, since turbulent flows (Karman vortexes) are produced on the peripheries of the columns  12   a ,  12   b ,  12   c , the stirring action is promoted by these turbulent flows so produced, so as to reduce the stagnation of cooling water, whereby the heat transfer coefficient from the stator  6  to the cooling water can be increased. In the motor  1  of the embodiment, since the large number of columns  12   a ,  12   b,    12   c  are disposed, many turbulent flows are produced, the stirring action is promoted largely, so as to increase the heat transfer coefficient, thereby making it possible to increase the cooling performance of the motor  1  itself. 
     In addition, orifice portions  21  which narrow the width of the flow path  20  are formed between the inlet port  14   a  (the inlet portion  14 ) and the outlet port  15   a  (the outlet portion  15 ), whereby the short-circuit of cooling water supplied from the inlet port  14   a  is suppressed so that the cooling water so supplied is not discharged directly from the outlet port  15   a.  The orifice portions  21  are formed in such a manner that a rate of direct flow to back flow (flow on a short-circuit side) becomes 7 to 3. 
     In addition, although recessed portions  22  are portions formed for fixing members and wiring members of the motor  1  to be disposed, in addition to functioning so, the recessed portions  22  also function to generate turbulent flows in the flow of cooling water which flows on both sides of the flow path  20  so as to reduce the stagnation of cooling water to thereby increase the heat transfer coefficient. 
     In addition,  FIG. 4  is a perspective view showing the configuration of the flow path  20  in the interior of the motor  1  as it is removed therefrom, and the configuration of the flow path  20  corresponds to a core configuration in casting a motor case part of the motor  1 . For example, in the electric motor described in JP-A-8-19218, unless the thickness of the core configuration is made thick, it becomes difficult to cast the thin elongated cooling fins, and this inevitably increases the outside diameter of the motor case. In contrast to this, in the motor  1  of the embodiment, since the circular cylindrical columns only have to be cast, the thickness of the core configuration does not have to be increased, and hence, there is no such situation that the outside diameter of the motor case is increased, and there is also no such situation that an increase in weight is called for. In this way, in the motor  1  of the embodiment, the core configuration in casting is taken into consideration, whereby it becomes easy to fabricate the motor case of the motor  1 , which contributes largely to mass production of motor cases. 
     According to an aspect of the invention, since the plurality of columns are disposed in the space defined between the inner housing and the outer housing, so that the inner housing and the outer housing are connected together by these columns, the rigidities of the inner housing and the outer housing can be increased without increasing the overall dimensions of the motor case and calling for almost any increase in weight. As a result, a remarkable damping effect can be exhibited in vibration frequency of several kHz, and the increase in cooling efficiency can be realized by turbulent flows produced by the large number of columns. 
     The electric motor according to the invention is suitable for use in electric vehicles.