Patent Publication Number: US-2018041090-A1

Title: Water-Cooled Motor Structure and Water-Cooled Housing

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 14/432,904, filed on Apr. 1, 2015, which is a 371 of PCT/JP2013/076530, filed on Sep. 30, 2013, which claims priority to Japanese application 2012-221082, filed on Oct. 3, 2012, the entire contents of each of which are herein incorporated by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a water-cooled motor structure and a water-cooled housing that are readily produced and that have a high capability for cooling. 
     BACKGROUND 
     In a small and high-power motor (an electric motor), heat generation when it is in operation is disregarded, and sometimes a malfunction such as a burned-out winding of the motor occurs. 
     Conventionally, as a water-cooled structure for a motor, suggested is a cooling system for an electric motor in which a passage for cooling water is provided in a space between an inner cylinder and an outer cylinder of a cylindrical frame for which a stator and a rotator are fitted therein. This system includes a configuration in which a plurality of partition walls is provided on the passage for the cooling water along the axial direction of the cylindrical frame such that the cooling water supplied from an inlet conduit for the cooling water is guided to an outlet conduit for the cooling water, while the direction of the flow of the cooling water is alternately changed between the side of one end and the side of the other end of the cylindrical frame. 
     SUMMARY 
     In some examples, to achieve the configuration in which the cooling water supplied from an inlet conduit for the cooling water is guided to an outlet conduit for the cooling water, while the direction of the flow of the cooling water is alternately changed between the side of one end and the side of the other end of the cylindrical frame, the partition walls that extend along the axial direction to partition the space between the inner cylinder and the outer cylinder is configured such that they are alternately retracted between the side of one end and the side of the other end of the frame. With such a configuration, the retracted portions of the partition walls at the one end and the other end of the cylindrical frame act as return passages for returning the cooling water. 
     In addition, the cylindrical frame has a structure in which the inner cylinder, the outer cylinder, and the partition walls that partition the space therebetween are integrated. In such systems, such a frame structure is formed by a casting using a mold. 
     In such water-cooled motor structures, because the frame of the motor should be a specialized structure, there is an inconvenience in that it cannot be applied to a conventional motor. 
     Further, in the structure of the frame of the motor, the shape of the cross section of the passage for the cooling water is not uniform. So it is difficult to produce such a structure by a manufacturing process such as a cutting operation or extrusion. 
     Within examples herein, a water-cooled motor structure and a water-cooled housing is provided for which the components can be readily manufactured, and wherein the cooling performance is very efficient. 
     Examples of a water-cooled motor structure described herein comprises a water-cooled housing for containing a motor such that cooled water flows through the water-cooled housing to cool the motor. The water-cooled housing comprises a tubular passage section that has a plurality of partition walls erected in the direction of the central axis at prescribed angular intervals on a substantially cylindrical inner wall to form a plurality of outward passages and a plurality of inward passages extending parallel to each other along the central axis between the plurality of partition walls and the outer peripheral surface of the motor, a first cover section, which closes a first opening of the tubular passage section while having a plurality of return passages that connects inward passages to the following outward passages, a second cover section, which closes a second opening of the tubular passage section and has a plurality of return passages that connects outward passages to the following inward passages, a water supply section that is provided on the first cover section and connected to the entrance of a first outward passage, and a water drain section that is connected to the outlet of the last inward passage of the first cover section or is connected to the outlet of the last outward passage of the second cover section. 
     Each passage of the plurality of the outward passages and the plurality of the inward passages includes an inner wall of the tubular passage section, the partition wall, and a peripheral surface (an outer wall) of the motor. 
     The shape of the cross section of the inner wall of the tubular passage section is typically uniform at any point between the one end and the other end in the axial direction. 
     Each of the return passages that are provided on the first cover section and the second cover section consists of a cavity. It connects two adjacent passages of the tubular passage section that has the interior of the corresponding cover section. Namely, a connection between the adjacent outward passage and the inward passage is achieved by the return passages, which are provided with the first cover section and the second cover section. 
     The first cover section can be provided with the water supply section configured for receiving supplies and cooling-water from the exterior. The supplied cooling-water flows into the water-cooled housing. The water drain section is configured for externally draining the cooling water through the water-cooled housing. Alternatively, while the first cover section can be provided with the water supply section configured for receiving supplies and cooling water from the exterior, the supplied cooling-water flows into the water-cooled housing. And the second cover section can be provided by the water drain section configured for externally draining the cooling water through the water-cooled housing. 
     The water-cooled housing may include a first space corresponding to a first motor and a second space corresponding to a second motor such that the first space and the second space are connected in parallel to each other. The first space and the second space share a pair of the water supply section and the water drain section. The cooling-water supplied to the water supply section flows into a plurality of passages of the first space and then flows into a plurality of passages of the second space, and is drained from the water drain section. 
     Other examples of a water-cooled housing described herein contain a motor such that cooling water cools the motor. The water-cooled housing comprises a tubular passage section, which has a plurality of partition walls erected in the direction of the central axis at prescribed angular intervals on a substantially cylindrical inner wall to form a plurality of outward passages and a plurality of inward passages extending parallel to each other along the central axis between the plurality of partition walls and the outer peripheral surface of the motor, a first cover section, which closes a first opening of the tubular passage section while having a plurality of return passages that connects inward passages to the following outward passages, a second cover section, which closes a second opening of the tubular passage section while having a plurality of return passages that connects outward passages to the following inward passages, a water supply section that is provided on the first cover section and connected to the entrance of the first outward passage, and a water drain section that is connected to the outlet of the last inward passage of the first cover section or is connected to the outlet of the last outward passage of the second cover section. 
     Thus, a water-cooled motor structure is described whose components can be readily manufactured. And the water-cooled motor structure and a water-cooled housing have a high efficiency of cooling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the motor water-cooled structure, according to one example embodiment. 
         FIG. 2  is another perspective view of the motor water-cooled structure in a view from an angle different from that of  FIG. 1 . 
         FIGS. 3A and 3B  illustrate examples of the appearance of the motor, according to one example embodiment. 
         FIGS. 4A and 4B  show a front view of the tubular passage section in a view from the axial direction and a perspective view of it in a view slanted from the outside. 
         FIGS. 5A-5C  shows a front view of the first cover section according to an example embodiment in a view from the axial and inner direction, a cross-sectional view along arrows A-A therein, and a perspective view of it in a view from an inward angle. 
         FIGS. 6A-6C  shows a front view of the second cover section of an example embodiment in a view from the axial and inner direction, a cross-sectional view along arrows A-A therein, and a perspective view of it in a view from an inward angle. 
         FIGS. 7A and 7B  shows a front view of the motor contained in the housing of an example embodiment and a cross-sectional view along arrows B-B therein. 
         FIG. 8  is a perspective view of the first cover section of an example embodiment, wherein the first cover section is cut away by a plane orthogonal to the shaft of the motor. 
         FIG. 9  is a perspective view of the water-cooled motor structure as shown in  FIG. 2 , in the side of the first cover section, wherein the tubular passage section is cut away by a plane orthogonal to the shaft of the motor, 
         FIG. 10  is a perspective view of the water-cooled motor structure as shown in  FIG. 2 , in the side of the tubular passage section, wherein the second cover section is cut away by a plane orthogonal to the shaft of the motor. 
         FIG. 11  is a perspective view of the second cover section that was cut away in  FIG. 9 , in a view from the inner side. 
         FIGS. 12A-12C  illustrates the flow of the cooling water within the water-cooled motor structure corresponding to  FIGS. 8, 9, and 10 . 
         FIG. 13  shows an example of an application of example embodiments. 
         FIGS. 14A-14C  illustrates perspective views, which are cut out in three portions, of the application as shown in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Below the example embodiments will be described in detail in reference to the drawings. 
       FIGS. 1 and 2  are perspective views of the water-cooled motor structure of example embodiments in a view from different angles. 
     The water-cooled motor structure contains a motor  40  in a water-cooled housing  100 . In  FIGS. 1 and 2 , the rotary shaft  43  of the motor  40  and an end cover  42  are visibly exposed from. the housing  100 . The housing  100 , broadly described, comprises a centered tubular passage section  20 , and a first cover section  10 , and a second cover section  30 , that are coupled to both ends of the tubular passage section. The tubular passage section  20 , the first cover section  10 , and the second cover section  30 , can all be produced from any materials having rigidity, such as a metal, a polymer, or ceramic. 
     The tubular passage section  20  has a shape substantially like a cylinder. The inner wall of the substantially cylindrical body is provided with a plurality of partition walls erected in the direction of the central axial at prescribed angular intervals to form a plurality of outward passages and a plurality of inward passages extending parallel to each other along the central axis of the cylindrical body between the plurality of partition walls and the outer peripheral surface of the motor. As used herein, the term “outward passage” refers to a passage wherein cooling water flows from the side of the cover section that has a water-supply conduit as described below, in the direction toward the other cover section. A passage wherein the cooling water flows in the opposite direction refers to the “inward passage.” These passages can also be interpreted as being a plurality of concave slots that are formed in the inner wall of the cylindrical body that extend parallel to each other along the central axis of the cylindrical body. 
     The first cover section  10  blocks a first opening of the tubular passage section  20  and has a plurality of return passages that connects the inward passages to the following outward passages. In this embodiment, the first cover section  10  is provided with the water-supply conduit  16   a  (the water supply section) to receive the cooling water (liquid) from the exterior and to supply it to the tubular passage section  20 , and a drainage conduit  17   a  (a water drain section) to externally drain the cooling water that flows through the tubular passage section  20 . 
     The water-supply conduit  16   a  is provided with the first cover section  10  and is connected to the entrance of the first outward passage of the plurality of passages of the tubular passage section  20 . 
     The drainage conduit  17   a  is connected to an outlet of the last inward passage of the first cover section  10 . Alternatively, the drainage conduit  17   a  may be provided with the second cover section  30  such that the drainage conduit  17   a  is connected to an outlet of the last outward passage of the second cover section  30  (such an arrangement is not shown). 
     The second cover section  30  has a shape that is substantially like a doughnut. In the second cover section, the shaft  43  and the cover  42  of the motor  40  are projected from an opening  35  (see  FIG. 2 ) to block a second opening of the tubular passage section  20 , and a plurality of return passages are provided for connecting the outward passage of the tubular passage section to the following inward passage over the entire inside peripheral edge of the second opening. 
     In addition, a wire (a cable) to supply power to the motor  40  is, but is not so limited, to one externally derived from the end cover  42 . 
       FIG. 3  shows an example of the appearance of the motor  40 . The housing  100  is configured to adapt the external profile of the motor  40 . The rotary shaft  43  projects from one end of the substantially cylindrical motor  40 . The plane in which the shaft  43  is projected is provided with a plurality (in this example, four) of threaded holes  44 . As shown in  FIG. 1 , the first cover section  10  is fixed to the motor  40  by threading threads (not shown) into the threaded holes  44  of the motor  40  through threaded holes  18  that are provided on an outer plane of the first cover section  10 . Also, the curved side surface of the cover section  10  is provided with a threaded hole  19 . Through the threaded hole  19 , a thread is threaded into a threaded hole  46  that is provided on the outer periphery of the motor  40 . The fixing of the cover section  10  to the motor  40  with the threads described herein is intended to illustrate just one example of a fixing means of the present invention, and is not intended to be a limitation. The end cover  42  is fixed such that one end thereof abuts an end  47  of the motor  40 . 
       FIGS. 4A and 4B  show a front view of the tubular passage section  20  in a view from the axial direction and a perspective view of it in a view from an outward angle. 
     As will be appreciated from these figures, the tubular passage section  20  is provided with the plurality of partition walls  21 - 28  that are elongated along the axial direction and erected in the direction of the central axis of the cylindrical body at the prescribed angular interval on the inner wall of the cylindrical body. As shown in  FIGS. 4A and 4B , a dashed-line circle  29  illustrates a cylindrical space corresponding to the outer peripheral profile of the motor  40 . When the motor  40  is contained in the tubular passage section, free ends of the partition walls  40  are configured such that they abut the outer peripheral surface of the motor  40 . With this configuration, the substantially cylindrical inner wall of the tubular passage section  20 , the partition walls  21 - 28 , and the outer wall (the outer peripheral surface) of the motor  40  form a plurality (in this example, eight) of passages  21   a - 28   a.  The respective partition walls  21 - 28  have the same sizes and the same plate-like shapes. To evenly cool the outer peripheral surface of the motor  40 , preferably the thickness of each of the partition walls that separate the adjacent passages is as thin as possible. Basically, the cooling water can evenly and stably flow in the respective passages by having the sizes of the passages  21 - 28 , i.e., cross sections or angles at the circumference, be uniform. 
     In this embodiment, however, temperature sensors not shown) are positioned within the passage  21  that communicates with the water-supply conduit  16   a  of the housing  100  and the passage  28  that communicates with the drainage conduit  17   a  such that the cross sections of both passages are greater than those of the remaining passages in consideration of the respective flow resistances of these sensors. One of the two temperature sensors is a backup sensor. 
       FIGS. 5A-5C  show a front view of the first cover section  10  in a view from the axial and inner direction, a cross-sectional view along arrows A-A therein, and a perspective view of it in a view from an inward angle. It should be noted that these figures are shown upside down in relation to  FIG. 1 . 
     The first cover section  10  is joined in one end of the tubular passage section  20  so as to cover the opening of that one end of the tubular passage section  20 . Namely, the cylindrical end of the tubular passage section  20  is fitted and joined to a shoulder  10   a , which is formed on the end of the cover section  10 , while the rotational angles of the cylindrical end and the shoulder  10   a  are aligned with each other. In this embodiment, such a joining is achieved using a water-resistant adhesive. But it is not limited to that adhesive. The method of joining can utilize any conventional means. Maintaining the water tightness may be done by using an elastic packing and the like. 
     The side of the cover section  10  is provided with a water-supply inlet port  16  that is to be coupled to the water-supply conduit  16   a  and a water-drain outlet port  17  that is being coupled to the drainage conduit  17   a.    
     In addition, the cover section  10  is provided with partition walls  11 ,  12 ,  13 ,  14 , and  15 , corresponding to the partition walls  24 ,  26 , and  28  of the tubular passage section  20 , respectively. These partition walls  11 - 15  are erected on the inner wall of the ring-like cavity of the cover section  10  along the central axis direction at prescribed angular intervals. When the motor is contained in the housing  100 , the free ends of the partition walls  11 - 15  are configured such that they abut the outer peripheral surface of the motor  40 . In the cover section  10 , a water-supply chamber  11   a  is formed between the partition wall  11  and the partition wall  12 . The water-supply inlet port  16  opens outwardly from the water-supply chamber  11   a . The water-supply chamber  11   a  corresponds to the first passage  21   a  of the tubular passage section  20 , to guide the cooling water supplied from the water-supply conduit  16   a  to the passage  21 . The water-supply chamber  11   a  also constitutes a part of all of the passages of the water-cooled housing  100 . 
     A return passage  12   a  is formed between the partition wall  12  and the partition wall  13 . The “return passage” consists of a cavity that connects the two adjacent passages to each other of the tubular passage section  20  that is provided inside the cover section  10 . The return passage  12   a  opposes the passages  22   a  and  23   a  of the tubular passage section  20  such that the return passage  12   a  returns and guides the cooling water flowing from the passage  22   a  to the passage  23   a.    
     A return passage  13   a  is formed between the partition wall  13  and the partition wall  14 . The return passage  13   a  opposes the passages  24   a  and  25   a  of the tubular passage section  20  such that the return passage  13   a  returns and guides the cooling water flowing from the passage  24   a  to the passage  25   a.    
     A drainage chamber  15   a  is formed between the partition wall  15  and the partition wall  11 . The water-drain outlet port  17  opens outwardly from the drainage chamber  15   a . The drainage chamber  15   a  also constitutes a part of all the passages of the water-cooled housing  100 . 
       FIGS. 6A-6C  show a front view of the second cover section  30  in a view from the axial and inner direction, a cross-sectional view along arrows A-A therein, and a perspective view of it in a view from an inward angle. 
     The second cover section  30  is joined with the tubular passage section  20  to cover the opening of the other end of the tubular passage section  20 . Namely, the cylindrical end of the tubular passage section  20  is fitted and joined to a shoulder  30   a,  which is formed on the end of the cover section  30 , while the rotational angles of the other cylindrical end and the shoulder  30   a  are aligned with each other. In this embodiment, such a joining is achieved using a water-resistant adhesive. The method of the joining can utilize any conventional means, and is not limited to the adhesive. Maintaining the water tightness may be carried out using an elastic packing and the like. 
     In addition, the cover section  30  is provided with partition walls  31 ,  32 ,  33 . and  34 , corresponding to the partition walls  21 ,  23 ,  25 , and  27  of the tubular passage section  20 , respectively. These partition walls  31 - 34  are erected on the inner wall of the ring-like cavity of the cover section  30  along the central axis direction at prescribed angular intervals. When the motor is contained in the housing  100 . the free ends of the partition walls  31 - 34  are configured so that they abut the outer peripheral surface of the motor  40 . 
     In the cover section  30 , a return passage  31   a  is formed between the partition wall  31  and the partition wall  32 . The return passage  31   a  opposes the passages  21   a  and  22   a  of the tubular passage section  20  such that the return passage  31   a  returns and guides the cooling water flowing from the passage  21   a  to the passage  22   a.    
     A return passage  32   a  is formed between the partition wall  32  and the partition wall  33 . The return passage  32   a  opposes the passages  23   a  and  24   a  of the tubular passage section  20  such that the return passage  32   a  returns and guides the cooling water flowing from the passage  23   a  to the passage  24   a.    
     A return passage  33   a  is formed between the partition wall  33  and the partition wall  34 . The return passage  33   a  opposes the passages  25   a  and  26   a  of the tubular passage section  20  such that the return passage  33   a  returns and guides the cooling water flowing from the passage  25   a  to the passage  26   a.    
     A return passage  34   a  is formed between the partition wall  34  and the partition wall  31 . The return passage  34   a  opposes the passages  27   a  and  28   a  of the tubular passage section  20  such that the return passage  34   a  returns and guides the cooling water flowing from the passage  27   a  to the passage  28   a.    
     The outer end of the cover section  30  is provided with an opening  35 . The end of the motor  40  is fitted to the opening  35 . In the opening  35 , the end of the motor  40  and the cover section  30  are coupled so that they are kept watertight by means of an adhesive or packing or the like. 
       FIGS. 7A and 7B  show a front view of the motor  40  contained in the housing  100  and a cross-sectional view along arrows B-B therein. 
     As is well shown in  FIG. 7B , the cover section  10  is coupled to the tubular passage section  20  by fitting the peripheral end of one end of the tubular passage section  20  to the shoulder  10   a  of the end of the cover section  10 . Similarly, the cover section  30  is coupled to the tubular passage section  20  by fitting the peripheral end of the one end of the tubular passage section  20  to the shoulder  30   a  of the end of the cover section  30 . The inner wall of the passages  13   a,    25   a,  and  33   a  consists of the outer peripheral surface of the motor. This arrangement is similar to the remaining passages, the water-supply chamber  11   a,  and the drainage chamber  15   a.    
       FIG. 8  is a perspective view of the first cover section  10 , wherein the first cover section  10  is cut away by a plane orthogonal to the shaft  43  of the motor. This cross section also illustrates cross sections of the water-supply conduit  16   a  and the drainage conduit  17   a.    
       FIG. 9  is a perspective view of the water-cooled motor structure as shown in  FIG. 2 , in the side of the cover section  10 , wherein the tubular passage section  20  is cut away by a plane orthogonal to the shaft  43  of the motor. In  FIG. 9  and the following figures, the motor  40 , for convenience, is shown as a dark member. 
       FIG. 10  is a perspective view of the water-cooled motor structure in  FIG. 2 , in the side of the tubular passage section  20 , wherein the second cover section  30  is cut away by a plane orthogonal to the shaft  43  of the motor.  FIG. 11  is a perspective view of the second cover section  30  that was cut away in  FIG. 9 , in a view from the inner side. 
       FIGS. 12A-12C , corresponding to  FIGS. 8, 9, and 10 , respectively, illustrate the flow of the cooling water within the water-cooled motor structure. 
     The cooling water, supplied from an external water-supplying tank (not shown) flows from the water-supplying conduit  16   a  through the water-supply chamber  11   a  to the passage  21   a  (the inward passage) of the tubular passage section  20 . The cooling water within the passage  21   a  arrives at the return passage  31   a  of the cover section  30  and is guided to the passage  22   a  (the outward passage) of the tubular passage section  20  therefrom. 
     The cooling water within the passage  22   a  arrives at the return passage  12   a  of the cover section  10  and is guided to the passage  23   a  (the outward passage) of the tubular passage section  20  therefrom. The cooling water within the passage  23   a  arrives at the return passage  32   a  of the cover section  30  and is guided to the passage  24   a  (the outward passage) of the tubular passage section  20  therefrom. 
     The cooling water within the passage  24   a  arrives at the return passage  13   a  of the cover section  10  and is guided to the passage  25   a  (the inward passage) of the tubular passage section  20  therefrom. The cooling water within the passage  25   a  arrives at the return passage  33   a  of the cover section  30  and is guided to the passage  26   a  (the outward passage) of the tubular passage section  20  therefrom. 
     The cooling water within the passage  26   a  arrives at the return passage  14   a  of the cover section  10  and is guided to the passage  27   a  (the inward passage) of the tubular passage section  20  therefrom. The cooling water within the passage  27   a  arrives at the return passage  34   a  of the cover section  30  and is guided to the passage  28   a  (the outward passage) of the tubular passage section  20  therefrom. 
     The cooling water within the passage  28   a  arrives at the drainage chamber  15   a  of the cover section  10  and is drained from the drainage conduit  17   a.  Because the drained cooling-water has absorbed heat generated from the motor and thus the temperature of it is increased, the drained cooling-water releases heat by means of a radiator (not shown) or the like, and is returned to the water-supplying tank. 
     The embodiment assumes an application to a motor used by a robot. For instance, in a robot to be moved, a robot wherein it is desired that it output a range of power while conserving space, a robot to work in cooperation with a human, with a rating of 80 W under the industrial guidelines, or another robot, because there is a requirement to perform a control process using a possibly compact body to achieve power as significant as possible, the present invention is useful. The present invention, however, is not limited to the application for a robot. 
     With the embodiment, the following example advantages can be obtained. 
     (1) Because water flowing through the tubular passage section evenly sweeps over the surface of the motor, the motor can be cooled. This can provide a more efficient cooling. In particular, because the outer wall (the outer peripheral surface) of the motor in itself constitutes the inner wall of the passages, the cooling water can directly contact the motor, and thus the efficiency of the water cooling can be enhanced.
 
(2) Because the cross-sectional area of the passage can be freely designed, the effective cross-sectional areas of the respective passages can be made uniform. The term “effective cross sectional area” refers to a cross-sectional area that considers the resistance in the event a blockage, such as by the temperature sensor in the above embodiment, occurs midway in the passage, rather than the actual cross-sectional area. However, a temperature sensor need not be used. If used, the number of temperature sensors is not limited to a specific number.
 
(3) Because the inner walls of the passages utilize the outer surface of the motor and the sides of the cover sections are provided with the return passages, water can simply flow straight through the passages of the tubular passage section from the respective one ends to the respective other ends. This results in that the cross-sectional shapes of the tubular passage section being capable of being uniform at any point between the one end and the other end in the axial direction. Therefore, the configuration of the tubular passage section can be significantly simplified, the tubular passage section can be readily manufactured, and the number of components to be required can be reduced.
 
     For instance, the tubular passage section can be manufactured with relative ease by cutting or extrusion, and so on. The tubular passage section can be manufactured by injection molding. The cover sections can also be readily manufactured by cutting or injection molding, and so on. Therefore, the manufacturing cost can be reduced. 
     Now an applied example of the embodiment will be described in reference to  FIGS. 13 and 14 .  FIG. 13  shows an example of the water-cooled motor structure of the applied example.  FIGS. 14A-14C  illustrate perspective views of the water-cooled motor structure, which are cut out in three portions. 
     In this applied example a plurality (in this example, two) of motors are contained in a single water-cooled housing. In the housing, a first cylindrical space for containing a first motor and a second cylindrical space for containing a second motor are provided in parallel. Like the above description, the housing  100   a,  broadly discussed, comprises a centered tubular passage section  20   b,  and a first cover section  10   b  and a second cover section  30   b  that are coupled to both ends of the tubular passage section. Each of the cross sections, corresponding to the respective first and second motors of these components, is substantially shaped as a pair of binoculars. However, a water-supplying conduit  16   a  (a water supply section) and a drainage conduit  17   a  (a water drain section) that are both provided on the first cover section  10   b,  are provided just one pair such that both motors are to be shared. 
     The tubular passage section  20   b  has a shape in which two of the substantially cylindrical tubular passage sections  20  in the above embodiment are connected in parallel. A plurality of passages a-n (outward passages and inward passages) extending parallel to each other along the central axis is provided between a plurality of partition walls and the peripheral surfaces of the respective motors. The passages a-f are for the first motor, while the passages h-n are for the second motor. The center passage g is for both the first and second motors. 
     The first cover section  10   b  is provided with return passages bc, de, fg, hi, jk, and lm. As denoted herein, each of the return passages as shown in  FIG. 14  is denoted by the two characters (letters). This denotation means that the cooling water is guided from the passage denoted by the first letter to the passage denoted by the second letter. For instance, the return passage bc guides the cooling water from the passage b to the passage c. Similarly, the second cover section  30   b  is provided with the return passages ab, cd, ef, gh, ij,kl, and mn. In the depicted example, the passage a communicates with the water-supply section  16   a,  while the passage n communicates with the drainage conduit  17   a.    
     To share the pair of the water-supply conduit  16   a  and the drainage conduit  17   a for the first and second motors, the cooling water introduced into the water-supply conduit first flows through all the passages corresponding to the first motor, then flows through all the passages corresponding to the second motor, and then arrives at the drainage conduit  17   a.  To achieve this, the passages for both motors are connected in the middle portion of the housing. This connecting method may include the two following methods. 
     (1) One linear passage of the tubular passage section  20   b  is shared by the first and second motors. In the example in  FIG. 14 , such a one linear passage matches the passage g. The cooling water flowing through that passage contacts the outer walls of both motors. The cooling water to the first motor of the tubular passage section  20   b  then flows into the passage g, and then is guided to the passage of the second motor by the first cover section  10   b  or the second cover section  30   b  (in the depicted example, the second cover section  30   b ). The example in  FIG. 14  corresponds to this configuration.
 
(2) The tubular passage section  20   b  has a passage independent from the first and second motors. The cooling water is guided to the passage of the second motor by the first cover section  10   b  or the second cover section  30   b.  
 
     Other configurations, the operations, and the functions of the applied example are described above. 
     While the example embodiments have been described, various modifications and permutations are possible other than the forgoing. For instance, the number of the outward passages and the inward passages are not limited to the depicted examples. While the example has both the water supply conduit and the drainage conduit provided on a single cover section, an arrangement may be possible in which one cover section is provided with the water supply conduit and another section is provided with the drainage conduit. In such an arrangement, the number of the outward passages may total one more than those of the inward passages.