Patent Publication Number: US-2009224628-A1

Title: Twin rotor type motor

Description:
TECHNICAL FIELD 
     The present invention relates to a motor provided with a twin rotor. 
     BACKGROUND ART 
       FIG. 8  shows a cross-sectional view of a conventional twin rotor motor. A related art is disclosed for instance in Japanese Unexamined Patent Application No. 2001-37133. 
       FIG. 8  shows a conventional toroidal brushless motor including stator  60  and outer rotor  70  and inner rotor  75 .  FIG. 8  shows a 4-pole/12-slot motor with the combination ratio of pole-pair number P to slot number Ns of 1:6 that is used in normal distributed winding arrangement. 
     Stator  60  includes: annular yoke  61 ; a plurality of outer teeth  62  and outer slots  63  provided on an outer circumference of yoke  61 ; and a plurality of inner teeth  65  and inner slots  66  provided on an inner circumference of yoke  61 . Yoke  61  is provided with a plurality of coils  64  wound in toroidal shape across outer slots  63  and inner slots  66 . 
     Meanwhile, two rotors  70  and  75  are provided outside and inside of stator  60 . Outer rotor  70  includes outer permanent magnet  72  facing outer teeth  62  and outer yoke  71 , and is held outside of stator  60  rotatably. Inner rotor  75  includes inner permanent magnet  77  facing inner teeth  65  and inner yoke  76 , and is held inside of stator  60  rotatably. 
     Coils  64  are formed in three-phase star or delta connection. When coils  64  are energized, outer rotor  70  and inner rotor  75  start rotating all together to a predetermined direction by a torque generated by magnetic field by current-flow through coils  64 . 
     Stator  60  is provided with a plurality of mounting-holes  80  at intersections of outer teeth  62  and inner teeth  65  on annular yoke  61 . Mounting-holes  80  run through yoke  61 . 
     Since a twin rotor motor has outer rotor  70  and inner rotor  75  on outside and inside of stator  60  respectively, stator  60  cannot be fixed by usual fitting methods such as shrink-fitting or press-fitting. Therefore, mounting-holes  80  are provided so as to enable stator  60  to be fixed on a machine by inserting bolts or the like (not shown) through mounting-holes  80  as shown in  FIG. 8 . 
     The conventional fitting method is disclosed in a technical document: IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 40, NO 3, MAY/JUNE 2004. In “Section A. Flux Distribution” of the paper: “Design and Parameter Effect Analysis of Dual-Rotor, Radial-Flux, Toroidally-Wound, Permanent-Magnet Machines” on page 774 in the document, it is described that the bolt-holes will increase leakage flux, causing stator core to decrease in flux density by 15%. 
     The conventional twin rotor motor can be provided with mounting-holes  80  to fix stator  60  on the machine, which however requires areas for forming mounting-holes somewhere in yoke  61 . 
     To minimize influences on the motor performance, mounting-holes  80  should preferably be formed at intersections of outer teeth  62  and inner teeth  65  on annular yoke  61  as shown in  FIG. 8 . Especially, along with an increasing motor torque, bolts are required to have a higher fixing strength, which will need a larger diameter for mounting-hole  80 . 
     In cases like this, it sometimes occurs that the width required to keep mechanical strength for fixing is larger than widths of yoke  61 , outer teeth  62  or inner teeth  65  that are required for magnetic circuit designing. Therefore, because for instance the larger the width of outer teeth  62  or inner teeth  65  the smaller the area of slots become, the current density will increase to cause reliability problems, and then, enlarging the slot area to restrict the increase in current density will shorten the outer diameter of inner rotor  75 , causing the motor to decrease in torque force. Enlarged yoke width will also shorten the outer diameter of inner rotor  75 , causing the motor to decrease in torque force. As a result, forming mounting-holes  80  will cause difficulty in obtaining full performance of the motor. 
     SUMMARY OF THE INVENTION 
     A twin rotor motor of the present invention has the following configuration. 
     A stator comprising: an annular yoke; a plurality of outer teeth and outer slots provided on an outer circumference of the yoke; a plurality of inner teeth and inner slots provided on an inner circumference of the yoke; and a plurality of toroidally-wound coils on the yoke. 
     A rotor comprising: an outer rotor provided with an outer permanent magnet facing the outer teeth; and an inner rotor provided with an inner permanent magnet facing the inner teeth. 
     Moreover, the stator is provided with a resin mold that seals the coils and has a mounting-tab. The configuration can provide the twin rotor motor with high performance without causing poor properties such as decrease in motor efficiency or torque force. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  shows a cross-sectional view of a twin rotor motor used in preferred embodiment 1 of the present invention. 
         FIG. 2  shows a detailed cross-sectional view of a stator of the above. 
         FIG. 3  shows an axial cross-sectional view taken along a plane A-O in  FIG. 1 . 
         FIG. 4  shows an axial cross-sectional view of a twin rotor motor used in preferred embodiment 2 of the present invention. 
         FIG. 5  shows an axial cross-sectional view of a twin rotor motor used in preferred embodiment 3 of the present invention. 
         FIG. 6  shows a cross-sectional view of a twin rotor motor used in preferred embodiment 4 of the present invention. 
         FIG. 7  shows a detailed cross-sectional view of a stator of the above. 
         FIG. 8  shows a cross-sectional view of a conventional twin rotor motor. 
     
    
    
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           10 . stator 
           11 . yoke 
           12 . outer teeth 
           13 . outer slot 
           14 . coil 
           15 . inner teeth 
           16 . inner slot 
           20 . outer rotor 
           21 . outer yoke 
           22 ,  22 B outer permanent magnet 
           24 ,  29 ,  41  output shaft 
           25  inner rotor 
           26  inner yoke 
           27 ,  27 B inner permanent magnet 
           30 ,  30 A resin mold 
           31  mounting-tab 
       
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     A preferred embodiments of the present invention are described below with reference to the drawings. 
     Preferred Embodiment 1 
       FIG. 1  shows a cross-sectional view of a twin rotor motor used in preferred embodiment 1 of the present invention, illustrating a brushless motor provided with three phase toroidally-wound coils. The twin rotor motor of the present invention includes stator  10 , outer rotor  20  and inner rotor  25  as shown in  FIG. 1 .  FIG. 1  shows a 4-pole/12-slot motor with a combination ratio of pole-pair number P to slot number Ns of 1:6. 
     Stator  10  includes: annular yoke  11 ; a plurality of outer teeth  12  and outer slots  13  provided on an outer circumference of yoke  11 ; and a plurality of inner teeth  15  and inner slots  16  provided on an inner circumference of yoke  11 . Yoke  11  is provided with a plurality of coils  14  wound toroidally across outer slots  13  and inner slots  16 . Resin mold  30  is provided to seal coils  14  as indicated by hatching in  FIG. 1 . 
     Meanwhile, two rotors  20  and  25  are provided outside and inside of stator  10 . Outer rotor  20  includes outer permanent magnet  22  facing outer teeth  12  and outer yoke  21 . Inner rotor  25  includes inner permanent magnet  27  facing inner teeth  15  and inner yoke  26 . Both outer permanent magnet  22  and inner permanent magnet  27  are magnetized to form four poles (pole-pair number=2). 
       FIG. 2  shows an extracted cross-sectional view of an area sealed by resin mold  30  in stator  10  in  FIG. 1 . Coils  14  include individual coils of U 1 , V 1 , W 1 , U 2 , V 2 , W 2 , U 3 , V 3 , W 3 , U 4 , V 4  and W 4  toroidally-wound across respective outer slots  13  and inner slots  16  in yoke  11 , and they are connected in three-phase star or delta connection. When energized coils  14  generate torques, then outer rotor  20  and inner rotor  25  start rotating all together to a predetermined direction. 
       FIG. 3  shows a cross-sectional view taken along the plane A-o in  FIG. 1 . Outer yoke  21  of outer rotor  20  extends toward inner circumference and is attached to outer rotor output shaft  24 , which is supported rotatably. Inner rotor  25  is attached to inner rotor output shaft  29  via inner yoke  26 , and shaft  29  is supported rotatably. Outer teeth  12  face outer permanent magnet  22  via air-gap G 1  and inner teeth  15  face inner permanent magnet  27  via air-gap G 2 . Each of two output shafts  24  and  29  can be allowed to rotate in different directions or in different speeds respectively. 
     Stator  10  is provided with resin mold  30  so as to seal coils  14 . Resin mold  30  improves the heat dissipation property of coils  14  effectively. 
     Furthermore, resin mold  30  has mounting-tab  31 . Mounting-tab  31  should preferably be molded integrally with resin mold  30 . Mounting-tab  31  can be fixed on a machine for instance by using bolts or the like into through-holes drilled axially. The fixing method on the machine is not limited to bolting only but crimping or welding are available. Though pedestal-shaped mounting-tab  31  is shown in  FIG. 3 , the mounting-tab is not limited only to such a shape. Eliminating the pedestal-shaped portion, resin mold  30  for sealing the coils can be for instance bolted directly or be fixed on the machine by welding a plurality of projections formed beneath. 
     As described above, mounting-tab  31  molded integrally with resin mold  30  can eliminate through-holes provided on yoke  11  of the stator core and the shape of stator  10  can be determined by the design factors of the magnetic circuit only that have decisive influences on the motor properties. The configuration, therefore, can realize the twin rotor motor with a high performance without causing any decrease in motor properties due to taking the fixing method on the machine into account. 
     Preferred Embodiment 2 
       FIG. 4  shows an axial cross-sectional view of a twin rotor motor used in preferred embodiment 2 of the present invention. Elements similar to embodiment 1 have the same reference marks and detailed descriptions are omitted. In embodiment 2, the stator is the same as in embodiment 1, only the rotor is different. 
     In  FIG. 4 , outer yoke  21 A of outer rotor  20 A extends toward inner circumference and is attached to inner yoke  26 A forming inner rotor  25 A to form inner/outer rotor yoke  40 . Inner/outer rotor yoke  40  is attached to output shaft  41 . 
     In this way, being the rotor output shafts united into a single output shaft, the motor output shaft will have the sum of output torques of outer rotor  20 A and inner rotor  25 A consequently. The motor, therefore, will have a higher output torques. 
     As described above, inner/outer rotor yoke  40 , which is necessary for attaching outer rotor  20 A with inner rotor  25 A, is formed so as to cover one side in axial direction of stator  10 . Therefore, it is very difficult to form through-holes on yoke  11  of stator core for fixing rotors on the machine. As described in this embodiment, however, the motor can be fixed on the machine very easily by using mounting-tab  31  molded on resin mold  30  integrally. 
     Preferred Embodiment 3 
       FIG. 5  shows an axial cross-sectional view of a twin rotor motor used in preferred embodiment 3 of the present invention. Elements similar to embodiment 1 and 2 have the same reference marks and detailed descriptions are omitted. In this embodiment, the rotors are the same as in embodiment 2, only the stator is different. 
     In this embodiment, the shape of resin mold  30 A differs from embodiment 1 and 2. Namely, the structure is such that resin mold  30 A is shortened radially so as to expose the respective tops of outer teeth  12  and inner teeth  15 . The exposing structure can be applied to either or both of outer teeth  12  and inner teeth  15 . 
     The resin mold  30 A sealing coil  14  improves heat dissipation and acts to restrict temperature increase in coil  14 . In this embodiment, however, the effect can be obtained sufficiently by resin mold  30 A only that has the minimum necessary amount of resin, thus reducing material usages. 
     Moreover, the tops of outer teeth  12  and inner teeth  15  in the exposed structure can be used for positioning in motor assembling. Especially, the twin rotor motor such as described in the present invention needs a high accuracy in coaxiality or the like between stator and outer and inner rotors compared with normal motors. At this time, both radial surfaces of outer teeth  12  and inner teeth  15  and an axial end surface can be used for positioning in assembling, causing a great effect on assembling the motor highly accurately. 
     Preferred Embodiment 4 
       FIG. 6  shows a cross-sectional view of a twin rotor motor used in preferred embodiment 4 of the present invention.  FIG. 7  shows an extracted cross-sectional view of an area sealed by resin mold  30  in stator  10 B. Elements similar to embodiment 1 have the same reference marks and detailed descriptions are omitted. In this embodiment, the pole-pair number of permanent magnets and the coil winding way differ from embodiment 1. 
     The embodiment shows a 20-pole/12-slot twin rotor motor with the combination ratio of pole-pair number P to slot number Ns of P:Ns=5:6. 
     Two rotors  20 B and  25 B are provided outside and inside of stator  10 B respectively. Outer rotor  20 B includes outer permanent magnets  22 B facing outer teeth  12  and outer yoke  21 . Inner rotor  25 B includes inner permanent magnets  27 B facing inner teeth  15  and inner yoke  26 . Outer permanent magnets  22 B and inner permanent magnets  27 B are both magnetized to have 20 poles (pole-pair number P=10). 
       FIG. 7  shows a coil arrangement to realize this embodiment. Toroidally-wound individual coils: U 1 , V 1 , W 1 , U 2 , V 2 , W 2 , U 3 , V 3 , W 3 , U 4 , V 4  and W 4  are arranged in a reverse direction to the rotation compared with  FIG. 2 . 
     In case of the combination ratio of pole-pair number P to slot number Ns of P:Ns=5:6 as described in this embodiment, the torque constant of the motor is the same as that of P:Ns=1:6 in embodiment 1. Meanwhile, increase in pole-pair number causes stator core  12  to decrease in flux density. This means that iron loss generated in the stator core decreases, thus improving the motor efficiency consequently. 
     Meanwhile, coil  14 B provided on yoke  11  can be formed by changing the coil arrangement only with the toroidal shape kept unchanged as shown in  FIG. 7 , causing no increase in copper loss or the like which causes poor efficiency, or causing no torque fluctuation or irregular rotation which generates noises. 
     Furthermore, the fact that the flux density decreases in the stator core means that teeth  12  and  15  or yoke  11  can be designed narrower in width without any decrease in torque force. When for instance teeth  12 ,  15  and yoke  11  are designed narrower in width while keeping the same slot area, the outer diameter of inner rotor can be enlarged to improve torque property or motor efficiency. Narrowing the width of yoke  11  is able to have an effect of a shorter perimeter length of coil  14 B and accordingly a decrease in copper loss due to reduced coil resistance. 
     As described above, the configuration of this embodiment can realize the twin rotor motor with a high efficiency. 
     INDUSTRIAL APPLICABILITY 
     The twin rotor motor is provided with a resin mold to seal coils on a stator, and a mounting-tab formed integrally on the resin mold can eliminate through-holes in a yoke of stator core to fix the stator on a machine. The configuration can provide the twin rotor motor generating a high torque force efficiently without causing poor motor properties due to taking the fixing way on the machine into account.