Abstract:
An electric motor for high speed operation use and a rotor which enables use of common parts with electric motors for low speed operation use and which thereby enables reduction of the manufacturing costs. The rotor is provided with a shaft, a rotor core which is fastened to the shaft at the outside in the radial direction and has a first end face at one end in the axial direction and a second end face at the other end in the axial direction, a plurality of conductors which are arranged at the rotor core, and a pair of end rings which are respectively arranged adjoining the first end face and the second end face and which short-circuit the plurality of conductors with each other. The shaft has an outer circumference, while the end rings have outer circumferences which are arranged concentrically with respect to the outer circumference of the shaft.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotor which is provided with end rings and to an electric motor. 
     2. Description of the Related Art 
     Known in the art is a rotor which is provided with end rings which short-circuit a plurality of conductors which are arranged at a rotor core, wherein reinforcing members for preventing deformation of the end rings when driving rotation of the rotor are attached to the outer circumferences of the end rings (for example, Japanese Patent Publication No. 2013-090447A). 
     In the above-mentioned rotor, a shaft, the end rings, and the reinforcing members are generally fabricated from different materials, so due to the difference in coefficients of thermal expansion among these members, sometimes the shaft ends up deforming at the time of operation of the rotor. According to the above patent publication, the reinforcing members are divided into a plurality of members to prevent such deformation of the shaft. However, according to this configuration, there is the problem that the number of parts which form the rotor increases and the work steps at the time of production increase, so the manufacturing cost rises. 
     Further, end rings easily deform when operating an electric motor at a high speed, so in the past, reinforcing members were assembled together with the end rings only in an electric motor for high speed operation use. Therefore, it had been difficult to make use of common parts for an electric motor for high speed operation use and an electric motor for low speed operation use. 
     SUMMARY OF INVENTION 
     In one aspect of the present invention, a rotor of an electric motor is provided with a shaft; a rotor core which is fixed to radially outside of the shaft and which includes a first end face at one end in the axial direction and a second end face at the other end in the axial direction; a plurality of conductors which are arranged at the rotor core, and each of which extends from the first end face to the second end face; and a pair of end rings, one of which is arranged adjacent to the first end face, and the other of which is arranged adjacent to the second end face, the end rings short-circuiting the plurality of conductors with each other. The shaft has an outer circumference which is centered about an axis of rotation, while the end ring has an outer circumference which is arranged concentrically with respect to the outer circumference of the shaft. 
     The rotor may be further provided with a reinforcing member which includes an inner ring part fitted over the outer circumference of the shaft; and an outer ring part connected to the inner ring part and fitted over the outer circumference of the end ring. The inner ring part may be housed in a space defined between the outer circumference of the shaft and the inner circumference of the end ring opposite to the outer circumference of the end ring. 
     The inner ring part may have a first inner circumference which contacts the outer circumference of the shaft. The outer ring part may have a second inner circumference which contacts the outer circumference of the end ring. The first inner circumference and the second inner circumference may be concentric with each other. The reinforcing member may further include a ring-shaped connecting part which extends from the inner ring parts to the outer ring parts in the radial direction. The end ring may be housed in a space defined by the inner ring part, the outer ring part, and the connecting part. 
     The inner ring part may include a plurality of weights arranged in the circumferential direction. Each of the plurality of weights may be inserted into a tap hole formed in the inner ring part. The reinforcing member may be made from a nonmagnetic metal which has a higher rigidity than aluminum. The reinforcing member may be fastened to the shaft by shrink fitting or screwing. In another aspect of the present invention, an electric motor is provided with the above-mentioned rotor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, and advantages of the present invention will become further clearer by the following description of the preferred embodiments given while referring to the attached drawings, in which: 
         FIG. 1  is a cross-sectional view of an electric motor according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the rotor shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of a rotor according to another embodiment of the present invention; 
         FIG. 4  is a cross-sectional view of the reinforcing member shown in  FIG. 3 ; 
         FIG. 5  is a view of the reinforcing member shown in  FIG. 4  as seen from an arrow V in  FIG. 4 ; and 
         FIG. 6  is an enlarged view of the region VI in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Below, embodiments of the present invention will be explained in detail based on the figures. First, referring to  FIG. 1 , an electric motor  10  according to an embodiment of the present invention will be explained. Note that, in the following explanation, the “axial direction” is the direction along an axis O of rotation of the shaft  32  of the electric motor  10  shown in  FIG. 1 , the “radial direction” indicates the radial direction of a circle centered about the axis O, and the “circumferential direction” is the circumferential direction of the circle centered about the axis O. Further, “front in the axial direction (axially front)” shows the leftward in  FIG. 1 . 
     The electric motor  10  includes a housing  14  which defines an inside space  12 ; a stator  16  fixed in the inside space  12  of the housing  14 ; and a rotor  30  rotatably arranged radially inside (inside in the radial direction) of the stator  16 . The stator  16  includes a stator core  18  and a coil  20  wound around the stator core  18 . The rotor  30  includes a shaft  32  extending in the inside space  12  in the axial direction; a rotor core  34  fixed radially outside of the shaft  32 ; a plurality of conductors  36  arranged at the rotor core  34 ; and a pair of a first end ring  38  and a second end ring  39  short-circuiting the plurality of conductors  36 . 
     Next, referring to  FIG. 2 , the configuration of the rotor  30  according to the present embodiment will be explained in more detail. The shaft  32  has a first outer circumference  40 ; a second outer circumference  42  adjoining the first outer circumference  40  at the front in the axial direction; and a third outer circumference  44  adjoining the first outer circumference  40  at the rear in the axial direction. 
     The first outer circumference  40  is configured from a cylindrical surface extending in the axial direction. The rotor core  34  is fixed radially outside of the first outer circumference  40 . The second outer circumference  42  is constituted by a cylindrical surface having a diameter D 1  and extending in the axial direction. Similarly, the third outer circumference  44  also is constituted by a cylindrical surface having the same diameter D 1  as the second outer circumference  42  and extending in the axial direction. 
     The rotor core  34  is fabricated from a plurality of magnetic steel sheets stacked in the axial direction, and has an end face  50  at the front in the axial direction and an end face  52  at the rear in the axial direction. The rotor core  34  includes a center bore  46  and a plurality of through holes  48  formed radially outside of the center bore  46 . These through holes  48  are arranged to be aligned in the circumferential direction at substantially equal intervals. The conductors  36  arranged at the through holes  48 , the first end ring  38 , and the second end ring  39  are integrally cast by aluminum die casting. Each of the conductors  36  extends from the axially front end face  50  of the rotor core  34  to the axially rear end face  52 . 
     The first end ring  38  is a ring-shaped member arranged on the end face  50  of the rotor core  34  so as to extend in the circumferential direction. For example, the first end ring  38  is fabricated from a conductive material such as aluminum. The first end ring  38  short-circuits the axially front end of the plurality of conductors  36 . The first end ring  38  has an outer circumference  54  and an inner circumference  56  opposite to the outer circumference  54 . The outer circumference  54  is constituted by a cylindrical surface having a diameter D 2 , while the inner circumference  56  is constituted by a conical surface. 
     The second outer circumference  42  of the shaft  32  and the inner circumference  56  of the first end ring  38  define a space  58  therebetween. More specifically, the space  58  is a ring-shaped space defined by the second outer circumference  42  of the shaft  32 , the inner circumference  56  of the first end ring  38 , and the end face  50  of the rotor core  34 . 
     In the present embodiment, the second outer circumference  42  of the shaft  32  and the outer circumference  54  of the first end ring  38  are concentric with each other about the axis O. The second outer circumference  42  of the shaft  32  and the outer circumference  54  of the first end ring  38  can be made concentric, e.g. by cutting the outer circumference  54  of the first end ring  38  after assembling the rotor  30  as shown in  FIG. 2 . 
     The second end ring  39  has a configuration the same as the first end ring  38 . Specifically, the second end ring  39  is arranged on the end face  52  of the rotor core  34 , and short-circuits the axially rear end of the plurality of conductors  36 . The second end ring  39  has an outer circumference  60  and an inner circumference  62  opposite to the outer circumference  60 . The outer circumference  60  is constituted by a cylindrical surface having a diameter D 2  the same as the outer circumference  54  of the first end ring  38 , while the inner circumference  56  is constituted by a conical surface. 
     A space  64  is defined between the third outer circumference  44  of the shaft  32  and the inner circumference  62  of the second end ring  39 . Specifically, the space  64  is a ring-shaped space defined by the third outer circumference  44  of the shaft  32 , the inner circumference  62  of the second end ring  39 , and the end face  52  of the rotor core  34 . Here, the third outer circumference  44  of the shaft  32  and the outer circumference  60  of the second end ring  39  are concentric with each other about the axis O. 
     Next, referring to  FIG. 3 , a rotor  70  according to another embodiment of the present invention will be explained. Note that, the same elements as the above-mentioned rotor  30  will be assigned the same reference numerals and detailed explanations thereof will be omitted. The rotor  70 , in the same way as the above-mentioned rotor  30 , is rotatably arranged radially inside of the stator  16  shown in  FIG. 1 , whereby an electric motor is constituted. 
     The rotor  70  includes a shaft  32 , a rotor core  34 , a plurality of conductors  36 , a first end ring  38  and a second end ring  39 , and a pair of a first reinforcing member  72  and second reinforcing member  74 . The first reinforcing member  72  is fitted over the shaft  32  so as to be arranged at the radially front end face  50  of the rotor core  34 . On the other hand, the second reinforcing member  74  is fitted over the shaft  32  so as to be arranged at the radially rear end face  52  of the rotor core  34 . 
     Next, referring to  FIG. 4  and  FIG. 5 , the configuration of the first reinforcing member  72  will be explained. The first reinforcing member  72  is a monolithic ring member having an end face  88  at the front in the axial direction and an end face  90  at the rear in the axial direction. The first reinforcing member  72  is made from nonmagnetic metal, such as nickel or titanium, which has higher rigidity than aluminum or copper. The first reinforcing member  72  includes an inner ring part  76 ; an outer ring part  78 ; and a ring-shaped connecting part  80  extending between the inner ring part  76  and the outer ring part  78 . 
     The inner ring part  76  has an inner circumference  82 ; an outer circumference  84  opposite to the inner circumference  82 ; and an end face  92  at the rear in the axial direction. The axially rear end face  92  of the inner ring part  76  is positioned slightly frontward in the axial direction relative to the above end face  90 . A plurality of tap holes  86  are formed at the inner ring part  76  so as to extend from the end face  88  to the end face  92 . 
     These tap holes  86  are arranged in the circumferential direction at substantially equal intervals. A weight  94  is inserted into each of the tap holes  86 . In the present embodiment, the inner ring part  76  is formed with a total of 16 tap holes  86  into which weights  94  are inserted. These weights  94  function to adjust the rotational balance of the rotor  70  when driving the rotor  70  to rotate. 
     The outer ring part  78  is a cylindrical member arranged so as to be separated from the inner ring part  76  at radially outside of the inner ring part  76 . Specifically, the outer ring part  78  has an inner circumference  96  and an outer circumference  98  opposite to the inner circumference  96 . The connecting part  80  extends from the inner ring part  76  to the outer ring part  78  in the radial direction. More specifically, the connecting part  80  has the end face  100  at the rear side in the axial direction, and connects the axially front end of the inner ring part  76  and the axially front end of the outer ring part  78  each other. 
     A ring-shaped space  102  is defined by the outer circumference  84  of the inner ring part  76 , the inner circumference  96  of the outer ring part  78 , and the end face  100  of the connecting part  80 . In the present embodiment, the inner circumference  82  of the inner ring part  76  and the inner circumference  96  of the outer ring part  78  are concentric with each other about the axis O. 
     The second reinforcing member  74  has the same configuration as the above first reinforcing member  72 . That is, the second reinforcing member  74  is a monolithic ring member including an inner ring part  76 , outer ring part  78 , and connecting part  80 . Further, the inner ring part  76  is formed with tap holes  86 , into each of which a weight  94  is inserted. 
     The second reinforcing member  74  includes a ring shaped space  102  defined by the outer circumference  84  of the inner ring part  76 , the inner circumference  96  of the outer ring part  78 , and the end face  100  of the connecting part  80 . Further, in the second reinforcing member  74 , the inner circumference  82  of the inner ring part  76  and the inner circumference  96  of the outer ring part  78  are concentric with each other. 
     Next, referring to  FIG. 3  to  FIG. 6 , the mounting structures of the first reinforcing member  72  and second reinforcing member  74  will be explained in detail. As shown in  FIG. 6 , the first reinforcing member  72  is fitted over the shaft  32  from the axially front side, and fastened on the axially front end face  50  of the rotor core  34  by e.g. screwing or shrink fitting. In this state, the inner ring part  76  of the first reinforcing member  72  is fitted over the radially outside of the second outer circumference  42  of the shaft  32 , thereby the inner circumference  82  of the inner ring part  76  contacts the second outer circumference  42 . 
     Further, the outer ring part  78  of the first reinforcing member  72  is fitted over the radially outside of the outer circumference  54  of the first end ring  38 . In this state, the inner circumference  96  of the outer ring part  78  contacts the outer circumference  54  of the first end ring  38 . Further, the inner ring part  76  of the first reinforcing member  72  is housed in the space  58 , and the first end ring  38  is housed in the space  102 . 
     Similar as the first reinforcing member  72 , the second reinforcing member  74  is also fastened to the rotor core  34  at the rear in the axial direction by e.g. screwing or shrink fitting. Specifically, as shown in  FIG. 3 , the inner ring part  76  of the second reinforcing member  74  is fitted over the radially outside of the third outer circumference  44  of the shaft  32 , thereby the inner circumference  82  of the inner ring part  76  contacts the third outer circumference  44 . 
     Further, the outer ring part  78  of the second reinforcing member  74  is fitted over the radially outside of the outer circumference  60  of the second end ring  39 . In this state, the inner circumference  96  of the outer ring part  78  contacts the outer circumference  60  of the second end ring  39 . Further, the inner ring part  76  of the second reinforcing member  74  is housed in the space  58 , while the second end ring  39  is housed in the space  102 . 
     The rotor  30  shown in  FIG. 2  can be used as an electric motor for low speed rotation. The reason for it is that, it is not necessary to attach reinforcing members  72 ,  74  as in the rotor  70  shown in  FIG. 3 , since the end rings  38 ,  39  hardly deform when the rotor  30  rotates at a low speed. 
     On the other hand, the rotor  70  shown in  FIG. 3  can be used for an electric motor for high speed rotation. It is necessary to attach reinforcing members  72 ,  74 , since the end rings  38 ,  39  easily deform due to the centrifugal force when the rotor  70  rotates at a high speed, as explained above. 
     According to the present embodiment, it is possible to constitute a rotor  70  suitable for high speed rotation by just attaching reinforcing members  72 ,  74  by screwing etc. to a rotor  30  suitable for low speed rotation. Therefore, it is possible to make use of common parts for almost all of the parts of the rotor  30  for low speed rotation and rotor  70  for high speed rotation. Due to this, it is possible to efficiently produce a rotor in response to the required application (rotation speed) while decreasing manufacturing costs. 
     Further, according to the rotor  70  shown in  FIG. 3 , the inner ring part  76  of the reinforcing members  72 ,  74  is fastened to the shaft  32 , while the outer ring part  78 , which is integrally connected to the inner ring parts  76  via the connecting part  80 , can hold the end ring  38 ,  39  from the radially outside. Due to this, it is possible to effectively prevent the end rings  38 ,  39  from deforming toward radially outside when the rotor  70  rotates at high speed. 
     Further, according to the rotor  70 , the second outer circumference  42  of the shaft  32  and the outer circumference  54  of the first end ring  38  become concentric, and also the inner circumference  82  of the inner ring part  76  of the first reinforcing member  72  and the inner circumference  96  of the outer ring part  78  of the first reinforcing member  72  become concentric. Similarly, the third outer circumference  44  of the shaft  32  and the outer circumference  60  of the second end ring  39  become concentric, and also the inner circumference  82  of the inner ring part  76  of the second reinforcing member  74  and the inner circumference  96  of the outer ring part  78  of the second reinforcing member  74  become concentric. 
     According to this configuration, when assembling the rotor  70  in the electric motor shown in  FIG. 1  and operating it and the rotor  70  becomes high in temperature, the degrees of heat expansion of the shaft  32 , reinforcing members  72 ,  74 , and end rings  38 ,  39  can be made uniform, whereby it is possible to prevent the shaft  32  from deforming. 
     Further, in the rotor  70 , the inner ring parts  76  of the reinforcing members  72 ,  74  are housed in the spaces  58 , and the end rings  38 ,  39  are housed in the spaces  102 . According to this configuration, it is possible to reduce the dead space formed between the reinforcing member  72 ,  74  and the rotor core  34  when attaching the reinforcing member  72 ,  74  to the rotor core  34 , whereby it is possible to make the rotor  70  more compact. 
     Note that, in the above-mentioned embodiment, the case was explained where the second outer circumference  42  and the third outer circumference  44  of the shaft  32  and the outer circumferences  54 ,  60  of the end rings  38 ,  39  were configured from cylindrical surfaces. However, the invention is not limited to this. These outer circumferences may also, for example, be defined by tips of pluralities of projecting parts which are arranged in the circumferential directions. Similarly, the inner circumference  82  of the inner ring part  76  and the inner circumference  96  of the outer ring part  78  are not limited to cylindrical surfaces. For example, they may also be defined by tips of pluralities of projecting parts which are arranged in the circumferential directions. 
     Above, the present invention was explained through embodiments of the present invention, but the above embodiments do not limit the invention relating to the claims. Further, all combinations of features which were explained in the embodiment are not necessarily essential for the invention. Further, the above embodiments can be changed or improved in various ways as clear to a person skilled in the art. Such changed or improved embodiments are also included in the technical scope of the present invention as clear from the claim language.