Abstract:
A rotor ( 10 ) for a rotary electric machine ( 1 ) comprises a rotation shaft ( 11 ), and a plurality of rotor cores ( 120 - 129 ) fixed to the rotation shaft ( 11 ) and axially split. The rotor cores ( 120 - 129 ) have outer peripheral surfaces ( 120 A- 129 A) with a circular cross section. Permanent magnets ( 13 ) extending through the rotor cores ( 120 - 129 ) are arranged at equal circumferential intervals. Voids ( 120 B- 129 B) extending axially through the rotor cores ( 120 - 129 ) are formed between the outer peripheral surfaces ( 120 A- 129 A) and the permanent magnets ( 13 ). The voids ( 120 B- 129 B) of two adjacent rotor cores ( 120 - 129 ) are formed at circumferentially different positions, thereby being capable of suppressing the cogging torque without introducing a reduction in output torque.

Description:
FIELD OF THE INVENTION 
   This invention relates to a rotor for a rotary electric machine such as an electric motor, in particular, a rotor having permanent magnets near its outer periphery. 
   BACKGROUND OF THE INVENTION 
   A rotary electric machine using a rotor with permanent magnets embedded therein involves little energy loss and provides large output, so it is used in great numbers. However, a rotor with permanent magnets involves generation of a cogging torque. A cogging torque is a retention torque generated when a rotor is rotated slowly. In other words, it is a torque generated when an electric machine is rotated by an external force in a non-energized state. When the cogging torque is large, noise and vibration are generated under low load. Further, when the cogging torque is large, a large torque ripple is involved during normal operation. In particular, the torque ripple is large in the case of a concentrated winding. 
   In view of this, as shown in  FIG. 12 , according to JP 2003-32927-A issued by the Japan Patent Office in 2003, short permanent magnets are arranged on a rotor  100  so as to be circumferentially staggered, whereby torque concentration is prevented and the cogging torque is suppressed. 
   Further, as shown in  FIG. 13 , according to JP 2003-23740-A issued by the Japan Patent Office in 2003, the outer peripheral surface of a rotor  101  is formed in an approximately arcuate configuration providing an induction voltage having a substantially sinusoidal wave form, whereby the cogging torque is suppressed. 
   SUMMARY OF THE INVENTION 
   However, in the construction in which the permanent magnets are staggered, the stator and the rotor differ in polarity, with the result that the general magnetic flux amount decreases, and magnetic flux short-circuiting occurs between the staggered magnets, resulting in a reduction in output torque. Further, the manufacturing step for staggering the magnets is rather complicated. 
   In the construction in which the outer peripheral surface of the rotor is formed in an approximately arcuate configuration, the overall magnetic resistance increases, and the magnetic flux amount decreases, resulting in a reduction in output torque. Further, since the outer peripheral surface of the rotor is not circular, its production is rather difficult. 
   It is therefore an object of this invention to provide a rotor for an electric machine which suppresses the cogging torque without involving a reduction in output torque. 
   In order to achieve the above object, this invention provides a rotor ( 10 ) for a rotary electric machine ( 1 ) comprises a rotation shaft ( 11 ), a plurality of rotor cores ( 120 - 129 ) fixed to the rotation shaft ( 11 ) and axially split, each of the rotor cores ( 120 - 129 ) having outer peripheral surfaces ( 120 A- 129 A) with a circular cross section and permanent magnets ( 13 ) arranged at equal circumferential intervals and extending through the rotor cores ( 120 - 129 ), wherein voids ( 120 B- 129 B) which axially penetrate the rotor cores ( 120 - 129 ) are formed between the outer peripheral surfaces ( 120 A- 129 A) of the rotor cores ( 120 - 129 ) and the permanent magnets ( 13 ), and the voids ( 120 B- 129 B) of two adjacent rotor cores ( 120 - 129 ) are formed at circumferentially different positions. 
   The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a rotor for a rotary electric machine according to a first embodiment of this invention. 
       FIGS. 2A and 2B  are enlarged partial sectional views of the rotor respectively taken along the lines IIA-IIA and IIB-IIB of  FIG. 1 . 
       FIG. 3  is a cross-sectional view of the rotary electric machine with the rotor incorporated therein. 
       FIG. 4  is a longitudinal sectional view of the rotary electric machine taken along the line IV-IV of  FIG. 3 . 
       FIG. 5  is a diagram showing fluctuations in cogging torque in the first embodiment and a prior art. 
       FIG. 6  is a diagram showing fluctuations in output torque in the first embodiment and a prior art. 
       FIG. 7  is a perspective view of a rotor for a rotary electric machine according to a second embodiment of this invention. 
       FIGS. 8A and 8B  are enlarged partial sectional views of the rotor of  FIG. 7 , respectively taken along the lines VIIIA-VIIIA and VIIIB-VIIIB thereof. 
       FIG. 9  is a perspective view of a rotor for a rotary electric machine according to a third embodiment of this invention. 
       FIG. 10  is a perspective view of a rotor for a rotary electric machine according to a fourth embodiment of this invention. 
       FIGS. 11A and 11B  are enlarged partial sectional views, similar to  FIGS. 2A and 2B , of a rotor for a rotary electric machine according to a fifth embodiment of this invention. 
       FIG. 12  is a perspective view of a rotor according to a prior art. 
       FIG. 13  is a sectional view of a rotor according to another prior art. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a rotor  10  comprises a shaft  11 , a rotor core unit  12 , and permanent magnets  13 . 
   The rotor core unit  12  is provided on the shaft  11 . The rotor core unit  12  comprises a first rotor core  120  and a second rotor core  121 . The axial lengths of the first rotor core  120  and of the second rotor core  121  are the same or substantially the same. The first rotor core  120  has gaps  120 B formed as grooves in an outer peripheral surface  120 A thereof. The second rotor core  121  has similar gaps  121 B formed as grooves in an outer peripheral surface  121 A thereof. 
   The permanent magnets  13  are provided near the outer peripheral edge of the rotor core unit  12 .  FIG. 1  shows only one of the permanent magnets  13 , indicating it by the broken line. 
   Referring to  FIGS. 2A and 2B , the first rotor core  120  has holes  120 C, and the second rotor core  121  has holes  121 C, with the permanent magnets  13  being inserted into these holes  120 C and  121 C. 
   The gaps  120 B and the gaps  121 B are circumferentially staggered with respect to each other. In other words, when seen through from the axial direction, the gaps  120 B and  121 B are arranged alternately at equal intervals. Two gaps  120 B and two gaps  121 B are formed per permanent magnet. 
   While the first rotor core  120  and the second rotor core  121  may be constructed separately from one another, it is also possible to use rotor cores of the same construction and to arrange them oppositely in the axial direction, with their gaps being staggered circumferentially. This makes it possible to manufacture the rotor cores by a single mold, thereby being capable of decreasing the production cost for the rotor. 
   The rotor  10  is a component constituting a rotary electric machine  1 . Here, a three-phase 8-pole 12-slot concentrated-winding permanent-magnet-type rotary electric machine will be taken by way of example. 
   Referring to  FIG. 3  and  FIG. 4 , the rotary electric machine  1  comprises the rotor  10 , a stator  20 , and a case  30 . 
   The shaft  11  of the rotor  10  is supported by bearings  31  of the case  30  so as to be free to rotate. The rotor core unit  12  is formed by stacking together electromagnetic steel plates. Eight permanent magnets  13  are uniformly arranged near the outer peripheral edge of the rotor core unit  12 . The permanent magnets  13  extend substantially over the entire length of the rotor  10 , and, unlike those of the prior art as disclosed in JP-2003-32927-A, involve no reduction in output torque. The permanent magnets  13  are arranged such that the magnetic poles of the adjacent permanent magnets differ from each other. When an electric current flow through windings  23 , a magnetic flux is generated, and a reaction force is generated in the permanent magnets  13 . As a result, the rotor  10  rotates around the shaft  11 . Further, since the outer periphery of the rotor  10  is circular, there is no increase in the general magnetic resistance as in the case of the prior art JP 2003-23740-A. Further, due to its simple configuration, it is easy to manufacture. 
   The stator  20  is held by the inner wall of the case  30 , and is arranged on the outer side of the rotor  10 . The stator  20  has twelve teeth  21 . The windings  23  are wound around the teeth  21  with insulators  22  therebetween. 
   Next, referring to  FIG. 5  and  FIG. 6 , the effects of this embodiment will be described. In the drawings, the thick solid lines represent this embodiment, and thin solid lines represent the prior art as disclosed in JP 2003-32927-A, in which no gaps are formed in the outer peripheral surface. The alternate long and short dashed line in  FIG. 5  indicates the cogging torque generated by the first rotor core  120 , and the dashed line indicates the cogging torque generated by the second rotor core  121 . 
   Referring to  FIG. 5 , the cogging torques inherent in of the first rotor core  120  and the second rotor core  121  are both larger than those of the prior art as disclosed in JP 2003-32927-A. However, through a combination of both, the cogging torques are canceled out, so the cogging torque as a whole is smaller than that in the prior art. 
   Referring to  FIG. 6 , according to this rotor core unit  12 , the torque ripple is reduced as compared with the prior art as disclosed in JP 2003-32927-A although the average torque is the same as in the prior art. 
   Referring to  FIG. 7  and  FIGS. 8A and 8B , a second embodiment of this invention will be described. In this embodiment, no gaps are formed in the outer peripheral surface of the rotor  10 . Instead, as shown in  FIGS. 8A and 8B , through holes  122 B are formed between an outer peripheral surface  122 A of a first rotor core  122  and the permanent magnets  13 , and through holes  123 B are formed between an outer peripheral surface  123 A of a second rotor core  123  and the permanent magnets  13 . When seen through from the axial direction, the through holes  122 B and  123 B are arranged alternately at equal intervals. 
   In this embodiment also, it is possible to reduce cogging torque and torque ripple. Further, since no gaps are formed in the outer peripheral surface of the rotor  10 , the outer peripheral surface can maintain a circular section, making it possible to prevent generation of noise during rotation or occurrence of energy loss due to air resistance. 
   In this embodiment also, rotor cores of the same construction may be used as the cores  122  and  123 , and arranged axially in opposite directions, with the through holes  122 B and  123 B being circumferentially staggered with respect to each other, thereby being capable of reducing the production cost for the rotor. 
   Next, referring to  FIG. 9 , a third embodiment of this invention will be described. This embodiment employs two rotor cores  124  and two rotor cores  125 , arranged alternately along the direction of the shaft  11  of the rotor  10  such that circumferential staggering of gaps  124 B and  125 B with respect to each other occurs at three or more axial positions. In this embodiment also, it is possible to use rotor cores of the same kind as the rotor cores  124  and  125 , and arrange them axially in opposite directions, with the gap positions being circumferentially varied. 
   By thus arranging a number of small-sized rotor cores  124  and  125  along the shaft  11 , it is possible to reduce the unbalance in weight. 
   Next, referring to  FIG. 10 , a fourth embodiment of this invention will be described. In this embodiment, rotor cores  126 ,  127 ,  128 , and  129  are used and arranged along the direction of the shaft  11 , with their gaps  126 B,  127 B,  128 C, and  129 B being gradually staggered with respect to each other in their circumferential positions. 
   This makes it possible to adjust the cogging torque more accurately and to reduce in cogging torque and torque ripple. In this embodiment, it is possible for the rotor cores  126  through  129  can be of two kinds in view of the constructions thereof. In other words, two rotor cores are arranged oppositely in the axial direction to thereby form the rotor cores  126  and  129 . Other two rotor cores are arranged oppositely in the axial direction to thereby form the rotor cores  127  and  128 . Due to this construction, it is possible to keep the production cost for the rotor low. Further, while in this embodiment the gap positions are gradually staggered circumferentially, it is also possible to arrange the rotor cores  126 ,  128 ,  127 , and  129  in that order from the axial forward end. In other words, it is not always necessary to effect gradual staggering along the axial direction as long as the rotor cores are arranged such that their circumferential gap positions differ. 
   Further, while in this embodiment four kinds of rotor cores  126  through  129  are used, a similar arrangement is also possible with three or more kinds of rotor cores. 
   Next, referring to  FIG. 11 , a fifth embodiment of this invention will be described. In this embodiment, in a rotor core unit  12  similar to that of the first embodiment, the gaps formed in the outer peripheral surfaces of the first rotor core  120  and the second rotor core  121  are respectively filled with non-magnetic resins  120 D and  121 D. 
   In this embodiment, it is possible to enhance the strength of the rotor. Further, it is possible to prevent generation of noise during rotation and generation of energy loss due to air resistance. 
   It should be noted that such filling with resin is also possible in the rotor of the second embodiment, in which the through holes  122 B and  123 B are formed in the first rotor core  122  and the second rotor core  123 . By thus filling the through holes  122 B and  123 B with resin, it is possible to enhance the strength of the rotor  10 . 
   In the above-described embodiments, two gaps per permanent magnet are formed in the outer peripheral surface of the rotor. By increasing the number of gaps, it is possible to reduce the width of each gap. Further, by adjusting their number, it is possible to perform fine adjustment on the cogging torque. 
   Further, while in the above-described embodiments the number of poles of the rotor is eight, this invention is also applicable to cases in which the number of poles is other than eight. 
   Further, the rotor core is not restricted to one obtained by stacking together electromagnetic steel plates. It may also consist of a dust core. 
   The term “rotary electric machine” used in the above description generally refers to an electric motor and/or a power generator. 
   The gaps and through holes  120 B through  129 B in the above embodiments constitute the voids as referred to in the claims.