Patent Publication Number: US-2016241113-A1

Title: Rotor having flow path of cooling fluid and electric motor including the rotor

Description:
BACKGROUND ART 
     1. Technical Field 
     The present invention relates to a rotor and an electric motor including the rotor. 
     2. Description of the Related Art 
     A known rotor of an electric motor has a cooling structure for supplying a cooling fluid into the rotor. For example, a known spindle apparatus is configured to cool a rotor from the inside by circulating a cooling fluid through a flow path formed inside the spindle (see JP H01-092048 A and JP H04-164548 A). 
     In order to cool the entire rotor evenly, it may be preferable that a flow path has branch paths.  FIG. 3  is a longitudinal sectional view illustrating a rotor  100  of an electric motor according to related art. The rotor  100  includes a rotational axis  104  rotatable around a rotational axis line  102 , and a rotor body  106  for generating power to rotate the rotational axis  104 . A flow path  110  is formed in the rotational axis  104  so as to allow a cooling fluid to be circulated therethrough. The flow path  110  has a supply path  112  extending parallel to the rotational axis line  102 , branch paths  114  branching off from the supply path  112 , and return paths  116  extending from the branch paths  114 . 
       FIGS. 4A and 4B  are cross sectional views taken along lines  4 A- 4 A and  4 B- 4 B of  FIG. 3 , respectively. The branch paths  114  of the flow path  110  are formed radially from the supply path  112  and distant from each other by an angle of 90 degrees around the rotational axis line  102 . The return paths  116  extend from the respective branch paths  114 . According to the configuration in which the flow path  110  is divided into a plurality of branch paths distant from each other by an angle of a certain degree around the rotational axis line  102 , the inside of the rotational axis  104  can be cooled evenly. 
     However, the aforementioned configuration may result in a sharp drop in the pressure of the cooling fluid since the cross section area of the flow path  110  is sharply increased at the divergence points of the flow path  110  at which the branch paths  114  are provided. The sharp drop in the pressure may cause cavitation. The cavitation is the formation of small bubbles in a liquid and generates noise or vibration, or causes corrosion of parts. In particular, in the case where one supply path  112  is divided into a plurality of return paths  116 , a sharp drop in the pressure tends to occur. 
     Therefore, there is a need for a rotor which can provide a sufficient cooling effect and prevent cavitation from occurring. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a rotor formed with a flow path through which a cooling fluid is supplied, wherein the flow path includes a plurality of branch paths branching off from the flow path in the rotor, and the plurality of branch paths are provided distant from each other in a direction parallel to a rotational axis line of the rotor. 
     According to a second aspect of the present invention, in the rotor according to the first aspect, the plurality of branch paths extend from angular positions different from each other around the rotational axis line. 
     According to a third aspect of the present invention, the electric motor comprising the rotor according to the first or second aspect is provided. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the detailed description of exemplary embodiments thereof as illustrated in the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a longitudinal sectional view illustrating a rotor of an electric motor according to one embodiment. 
         FIG. 1B  shows the rotor viewed from an angle of 90 degrees relative to  FIG. 1A . 
         FIG. 2A  is a cross sectional view taken along a line  2 A- 2 A of  FIGS. 1A and 1B . 
         FIG. 2B  is a cross sectional view taken along a line  2 B- 2 B of  FIGS. 1A and 1B . 
         FIG. 2C  is a cross sectional view taken along a line  2 C- 2 C of  FIGS. 1A and 1B . 
         FIG. 2D  is a cross sectional view taken along a line  2 D- 2 D of  FIGS. 1A and 1B . 
         FIG. 2E  is a cross sectional view taken along a line  2 E- 2 E of  FIGS. 1A and 1B . 
         FIG. 3  is a longitudinal sectional view illustrating an electric motor according to the related art. 
         FIG. 4A  is a cross sectional view taken along a line  4 A- 4 A of  FIG. 3 . 
         FIG. 4B  is a cross sectional view taken along a line  4 B- 4 B of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described with reference to the accompanying drawings. Constituent elements of the illustrated embodiments may be modified in size in relation to one another for better understanding of the present invention. The same or corresponding constituent elements will be designated with the same referential numerals. 
       FIGS. 1A and 1B  are longitudinal sectional views illustrating a rotor  10  of an electric motor according to one embodiment.  FIG. 1B  shows the rotor  10  of  FIG. 1A  viewed from an angle of 90 degrees relative to  FIG. 1A  around a rotational axis line O. 
     The rotor  10  includes a rotational axis  12  rotatable around the rotational axis line O, and a rotor body  14  fitted onto an outer circumferential face of the rotational axis  12 . The electric motor further includes a stator, which is not shown, provided on an outer side of the rotor body  14 . The rotor body  14  is configured to cooperate with the stator so as to provide the rotational axis  12  with rotational power. Various types of electric motors are known in the art, any type of which may be used to implement the present invention. The electric motor may be a synchronous electric motor or an induced electric motor. 
     The rotor body  14  is formed from stacked electromagnetic steel plates, for example. The rotor body  14  is a substantially cylindrical hollow member formed with a shaft hole sized so as to be fitted onto the outer circumferential face of the rotational axis  12 . The rotor body  14  is fitted onto the outer circumferential face of the rotational axis  12 , for example, by interference fit, so that the rotational axis  12  and the rotor body  14  can rotate together when the electric motor is in operation. 
     The rotational axis  12  is a substantially cylindrical member made of metal. The rotational axis  12  is supported by a bearing, which is not shown, so as to be rotatable around the rotational axis line O. A flow path  30  for supplying a cooling fluid such as cooling oil is formed inside the rotational axis  12 . The cooling fluid is supplied to the flow path  30  with the aid of a pump or the like, which is not shown, and discharged to the outside of the rotor  10  through the inside of the rotational axis  12 . The cooling fluid discharged from the rotor  10  is supplied to the flow path  30  again through a circulation path, which is not shown. In this way, the cooling fluid is circulated and thus a stable cooling effect can be achieved. 
     The flow path  30  has a supply path  32  substantially extending in a direction parallel to the rotational axis line O of the rotor  10  (The direction may also be referred to as “the axial direction” hereinafter.), branch paths  36   a  to  36   d  branching off from the supply path  32  and radially outwardly, and return paths  34   a  to  34   d  extending from the branch paths  36   a  to  36   d  toward a base end side of the supply path  32  (upstream of the flow of the cooling fluid) in a direction substantially parallel to the supply path  32 . The flow path  30  may be formed by drilling, for example. 
     Also with reference to  FIGS. 2A to 2E , the detailed configuration of the flow path  30  according to the present embodiment will be described.  FIGS. 2A to 2E  are cross sectional views taken along lines  2 A- 2 A,  2 B- 2 B,  2 C- 2 C,  2 D- 2 D and  2 E- 2 E of  FIGS. 1A and 1B , respectively. 
     The supply path  32  extends toward a terminal end side opposite of the base end side (downstream of the flow of the cooling fluid) and is in communication with a first return path  34   a  through a first branch path  36   a . The first branch path  36   a  extends in a direction perpendicular to the supply path  32 , or radially outwardly. The first return path  34   a  extends from the first branch path  36   a  toward the base end side of the supply path  32  in a direction parallel to the supply path  32 . 
     Referring to  FIG. 1A , a second branch path  36   b  extends radially outwardly from a position distant from the first branch path  36   a  in the axial direction. A second return path  34   b  extends from the second branch path  36   b  toward the base end side of the supply path  32  in a direction substantially parallel to the supply path  32 . As shown in  FIGS. 2A and 2B , the first branch path  36   a  and the second branch path  36   b  are provided at angular positions distant from each other by 180 degrees around the rotational axis line O. 
     Referring to  FIG. 1B , a third branch path  36   c  extends radially outwardly from a position distant from the first branch path  36   a  and the second branch path  36   b  in the axial direction. A third return path  34   c  extends from the third branch path  36   c  toward the base end side of the supply path  32  in a direction substantially parallel to the supply path  32 . Also with reference to  FIG. 2C , the third branch path  36   c  is provided at an angular position distant from the first branch path  36   a  and the second branch path  36   b  by 90 degrees or −90 degrees around the rotational axis line O. 
     A fourth branch path  36   d  extends radially outwardly from a position distant from the first branch path  36   a , the second branch path  36   b  and the third branch path  36   c  in the axial direction. A fourth return path  34   d  extends from the fourth branch path  36   d  toward the base end side of the supply path  32  in a direction substantially parallel to the supply path  32 . Also with reference to  FIG. 2D , the third branch path  36   c  and the fourth branch path  36   d  are provided at angular positions distant from each other by 180 degrees around the rotational axis line O. 
     In the rotor  10  according to the present embodiment, the cooling fluid supplied into the rotational axis  12  through the supply path  32  flows through the branch paths  36   a  to  36   d  and into the return paths  34   a  to  34   d  from the different angular positions, thereby preventing the temperature of the rotational axis  12  from increasing due to the heat generated by the rotor body  14  or due to friction between the rotational axis  12  and the bearing. 
     In addition, according to the present embodiment, by virtue of the branch paths  36   a  to  36   d  provided at the different positions in the axial direction, the cross section area of the flow path  30  increases in a stepwise manner, thereby preventing a sharp drop in the pressure at one particular site. Therefore, the cavitation can be prevented from occurring in the flow path  30 . 
     In the illustrated embodiment, the four return paths  34   a  to  34   d  are provided at angular positions distant from each other by 90 degrees around the rotational axis line O. However, according to another embodiment, more return paths may be provided, or, for example, six return paths may be provided at angular positions distant from each other by 60 degrees. Alternatively, less return paths may be provided, or, for example, three return paths may be provided at angular positions distant from each other by 120 degrees. According to yet another embodiment, the branch paths  36   a  to  36   d  may be provided at angles relative to a direction perpendicular to the rotational axis line O. 
     The flow path of the cooling fluid for cooling the rotor  10  may be formed in the rotor body  14 , instead of or in addition to the flow path  30  which is formed in the rotational axis  12 . In this case, the alternative or additional flow path can be easily formed in the rotor body  14  by perforating the electromagnetic steel plates of the rotor body  14 . By virtue of the cooling fluid supplied through the inside of the rotor body  14 , heat generated by the rotor body  14  can be directly dissipated. 
     Effect of the Invention 
     According to the rotor and the electric motor of the present invention, the divergence points of the flow path of the cooling fluid are provided at positions distant from each other in a direction parallel to the rotational axis line of the rotor. This configuration allows the cross section area of the flow path to be increased in a stepwise manner, and reduces a pressure drop in the fluid resulting from the division of the flow path. 
     Although various embodiments and variants of the present invention have been described above, it is apparent to a person skilled in the art that the intended functions and effects can also be realized by other embodiments and variants. In particular, it is possible to omit or replace a constituent element of the embodiments and variants, or additionally provide a known means, without departing from the scope of the present invention. Further, it is apparent for a person skilled in the art that the present invention can be implemented by any combination of features of the embodiments either explicitly or implicitly disclosed herein.