Abstract:
The invention relates to a rotor for an electric motor comprising an essentially cylindrical rotor core having a central aperture and comprising permanent magnets which are embedded in the rotor core and extend essentially like spokes through the rotor core, the rotor core being formed as an integral body and the selected permanent magnets being bridged at their radially inner or outer ends by recesses in the rotor core.

Description:
This application claims priority to the filing date of German Patent Application No. 103 18 624.7 filed Apr. 24, 2003, the specification of which is incorporated herein in its entirety. 
   FIELD OF THE INVENTION 
   The invention relates to a rotor for an electric motor comprising an essentially cylindrical rotor core having a central aperture, and comprising permanent magnets which extend essentially like spokes through the rotor core, the permanent magnets being embedded in the rotor core. 
   BACKGROUND OF THE INVENTION 
   More generally, the invention relates to the field of electric motors having permanent magnets such as brushless, electronically commutated DC motors and other permanent magnet motors, and in particular those configured as inner rotor motors. In general, inner rotor motors consist of a rotor arrangement which is mounted onto the motor shaft and includes one or more permanent magnets, as well as a stator arrangement, such as a stator core, which is built up of metal laminations that carry windings. The rotor arrangement is coaxially inserted into the stator arrangement. For outer rotor motors, the rotor arrangement encloses the stator. 
     FIG. 8  shows the basic construction of an electric motor having a housing  114  in which a stator arrangement  118 , a rotor arrangement  116  and bearings  126 ,  128  are accommodated to rotatably support the rotor arrangement. The stator arrangement  118  includes stacked metal laminations  155  and windings  160  and encloses an inner space into which the rotor arrangement  116  can be inserted. The rotor arrangement  116  includes the shaft  110 , a back iron yoke  112  and permanent magnets  122 . The bearings  126 ,  128  supporting the rotor arrangement can be integrated into a flange  124  in the motor housing  114 . 
     FIG. 8  serves to explain the basic construction of an electric motor. As explained in the opening paragraph, the invention relates to a rotor for such an electric motor, the rotor having an essentially cylindrical rotor core with a central aperture and the permanent magnets being embedded in the rotor core. 
   According to the prior art, rotors with embedded magnets are generally known. A rotor configuration having a multi-polar design resembling a spoked wheel with radially extending embedded magnets is revealed, for example, in “Design of Brushless Permanent-Magnet Motors”, J. R. Hendershot Jr. and T J E Miller, Magna Physics Publishing and Clarendon Press, Oxford, 1994. As shown in this publication, it is known to manufacture a rotor with embedded radially extending magnets that are protected by means of a ring or a tube surrounding the rotor. The rotor in which the magnets are embedded is used as a back yoke. 
   A conventional form of rotors with embedded magnets is also revealed in EP 0 691 727 A1. This publication shows a number of permanent magnets which are inserted into slots formed in the rotor allowing the permanent magnets to be inserted into the rotor from the outside. At their radially inner ends, the permanent magnets are enclosed by the material of the rotor core. 
   Rotors with embedded permanent magnets have the basic advantage that the magnets can be fully encapsulated so that the rotor can also come into contact with aggressive media without the magnet material needing special surface protection to prevent corrosion etc. However, the described rotor design has the disadvantage that stray flux is generated by the rotor core in the vicinity of the shaft. 
   To prevent such stray flux from arising, it has been suggested in the prior art to place a sleeve made of magnetically non-conductive or low-conductive material onto the shaft onto which the flux guide elements of the rotor core are then fixed, between which the permanent magnets in turn are embedded. Such a design is revealed, for example, in EP 0 641 059 A1; EP 0 803 962 A1; and DE 101 00 718 A1. Although this construction represents a great improvement on the prior art as described above in terms of the magnetic circuit and the distribution of magnetic flux density in the rotor, it is costly to manufacture and, due to the many individual parts, problems in the mechanical construction, such as an addition of tolerances, could arise. 
   EP 0803 962 A1 additionally shows that the slots to accommodate the permanent magnets have a bridge on their outer periphery to fully protect the permanent magnets from the outside. 
   WO 00/57537 describes a multi-polar permanent magnet rotor for an electric motor having embedded magnets which are disposed in such a way that a concentration of flux is produced. The permanent magnets are formed as flat cubes which are disposed like spokes radially to the rotor axis in recesses that are arranged between the flux guide elements which are fixed to the rotor. In assembling the magnets and the flux guide elements, the permanent magnets are formed as adjacent half-elements representing one pole respectively, and both the permanent magnets and the flux guide elements are attached to the shaft via a sleeve. 
   Another method of constructing a rotor having embedded magnets is shown in EP 0 872 944 A1. The magnets are arranged in a radial direction, or parallel to a radial direction, to the rotor. In EP 0 872 944, the permanent magnets are disposed in a so-called double-spoke configuration. Each of these “double magnets” consists of a pair of permanent magnets whose direction of magnetization is substantially the same. They can be arranged parallel to each other as in the cited publication or inclined at an angle to each other. This arrangement goes to improve the running performance of the electric motor and, in particular, to reduce cogging torque and torque ripple. 
   Other published patents in respect of rotors with embedded magnets include GB 1,177,247; EP 0 955 714 A2; and U.S. 2002/0067096 A1. 
   The rotor presented in the invention preferably finds application in a brushless DC motor or another permanent magnet synchronous motor. Such motors can be used in a great variety of applications, including spindle motors for disc drives, motor-assisted systems in motor vehicles such as steering and braking systems, electric tools and many other applications. 
   The radial arrangement of the permanent magnets embedded in the rotor core gives rise to the problem of stray flux in the region of the shaft onto which the rotor is mounted. The shaft is usually made from steel and acts as an extra back yoke for the magnetic flux through the rotor core. This gives rise to considerable magnetic stray. This problem can be countered by fitting a sleeve made from a magnetically non-conductive or low-conductive material to the shaft to which the flux guide elements of the rotor core are fixed, between which in turn the permanent magnets are embedded. This construction method is relatively costly and requires extra individual parts. 
   The object of the present invention is to submit a rotor for an electric motor which has embedded magnets and is simple to manufacture but nonetheless prevents the above problem of stray flux being generated in the region of the shaft. 
   SUMMARY OF THE INVENTION 
   This object has been achieved through a rotor having the characteristics described in claim  1 . The rotor presented in the invention has an essentially cylindrical rotor core with a central aperture. Permanent magnets are embedded in the rotor core and extend essentially like spokes through the rotor core. According to the invention, the rotor core is formed from a basically integral body, with selected permanent magnets being bridged at their radially inner ends by recesses in the rotor core. As described above, for rotor cores of the prior art the problem arises that considerable stray flux is generated in the region of the shaft at the radially inner ends of the permanent magnets. The basic idea behind the invention is to interrupt the rotor core there where the risk of forming undesirable stray fields is the greatest. The recesses which bridge the radially inner ends of the permanent magnets prevent fields from propagating at will in this area. By these means, stray can be reduced considerably. Moreover, the magnetic field lines are guided more intensively to the outer region of the rotor which increases the effectiveness of the electric motor. 
   In a preferred embodiment of the invention, the radially inner ends of two adjacent permanent magnets are bridged by a recess. Two like poles of the adjacent permanent magnets are preferably bridged in this way. 
   In another beneficial embodiment of the invention, the permanent magnets are arranged in the rotor core in pairs like double spokes, the permanent magnets of each pair being magnetized in the same direction. According to the invention, it is the directly adjacent permanent magnets of two adjacent permanent magnet pairs that are bridged at their radially inner ends. 
   It generally holds true that, depending on the special configuration of the permanent magnets embedded in the rotor core, the recesses are suitably selected and arranged so that the magnetic flux at the outer periphery of the rotor is intensified and undesirable stray flux in the region of the central aperture of the rotor core is prevented. 
   For the efficient functioning of the rotor presented in the invention, it is important that the rotor core is formed from an integral body so that the permanent magnets are fully enclosed by the rotor core at least at their radially inner ends or radially outer ends. 
   In a suitable and preferred embodiment of the invention, the recesses are bounded by bridges at the central aperture of the rotor core and also by the inner ends of two adjacent magnets. These bridges form a closed ring around the central aperture and bridge the radially inner ends of two adjacent permanent magnets. 
   By these means, the rotor core forms a closed surface at its central aperture which can be fitted onto the shaft. This means that the rotor presented in the invention can be directly fitted onto the steel shaft without the need to interpose a sleeve and without the risk of the shaft generating a magnetic reflux and significant stray flux being incurred. These are prevented by the described recesses which are enclosed by the bridges. 
   The invention thus reveals a rotor for an electric motor which is easily constructed and preferably has a closed surface at it central aperture so that it can be directly mounted onto a shaft. The construction presented in the invention minimizes the stray flux between the rotor and the shaft without needing to provide an extra sleeve made of a non-magnetic or low-magnetic material. 
   According to the invention, the bridges are preferably formed in such a way that they bridge the radially inner ends of two adjacent permanent magnets. By these means, two adjacent, like poles can be connected by a bridge. The technician will be aware that these could be either the N or the S poles. 
   The recess can be filled with air or any other gaseous medium. Alternatively, the recess can also be filled with another non-magnetic or low-magnetic material. 
   In a preferred embodiment of the invention, the permanent magnets are also fully enclosed by the rotor core at their radially outer ends. This produces a rotor with fully embedded rotor magnets allowing the rotor to come into contact with aggressive media as well without causing problems. A large variety of magnetic materials can be used and, in particular, those materials that would require extra surface protection if the magnets were exposed. 
   The rotor core consists of a ferromagnetic material, preferably of sheet metal laminations which are stacked to prevent eddy currents. As an alternative, ferrite materials can be used. The rotor core can be constructed in such a way that it has slots into which the permanent magnets can be inserted from either side. The rotor core is then sealed from both sides so that the magnets are hermetically sealed and do not require a surface coating. As magnetic materials, neodymium-iron-boron (NbFeB) or samarium-cobalt (SmCo) magnets can be used, for example. To prevent corrosion of these materials they would normally have to be coated. By embedding them fully into the rotor core, however, this is no longer necessary. Moreover, fully embedding the permanent magnets into the rotor core provides the permanent magnets with extra mechanical protection. 
   The invention can also be applied to an outer rotor motor. In this configuration, the recesses which bridge two adjacent permanent magnets are provided in the vicinity of the outer periphery of the rotor core. 

   
     SHORT DESCRIPTION OF THE DRAWINGS 
     The invention is described in more detail below on the basis of preferred embodiments with reference to the drawings. The figures show: 
       FIG. 1  a schematic sectional view through a rotor which is ideal in respect of the magnetic circuit; 
       FIG. 2  a schematic sectional view through a rotor in accordance with the invention with the stator enclosing the rotor also being shown; 
       FIG. 3  a similar view as in  FIG. 2  with flux lines being marked in; 
       FIG. 4  a schematic sectional view through a rotor in accordance with another embodiment of the invention which is fitted into a stator; 
       FIG. 5  a similar view as in  FIG. 4  with flux lines being marked in; 
       FIG. 6  a schematic sectional view through a rotor in accordance with another embodiment of the invention; 
       FIG. 7  a schematic sectional view through a rotor in accordance with another embodiment of the invention which is fitted into a stator; 
       FIG. 8  a sectional view through an electric motor in accordance with the prior art. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a schematic sectional view through a rotor which is ideally constructed in respect of the magnetic circuit. The rotor  10  is fitted onto a shaft  12  which is usually made of steel. For this purpose, a sleeve  14  is pressed or bonded onto the shaft  12  and flux guide elements  16  are fixed to the sleeve  14 , between which permanent magnets  18  are embedded. The sleeve  14  has the function of preventing magnetic stray flux between the flux guide elements  16  and the shaft  12 . For this purpose, it is made of a magnetically non-conductive or low-conductive material. With the aid of the sleeve  14 , it is possible to ensure that practically no magnetic losses are incurred in the region of the shaft  12 . This construction of the rotor  10  as illustrated in  FIG. 1  is thus ideal in terms of the magnetic circuit. It is disadvantageous, however, in that it requires many individual parts, making the mechanical construction both complicated and costly. 
     FIG. 2  shows a schematic sectional view through a rotor in accordance with the invention. The rotor  20  of the invention includes flux guide elements  22  which are joined together via outer and inner bridges  24 ,  26  at the outer periphery or at a central aperture  28  of the rotor  20  respectively. In the illustrated embodiment, the inner bridges  26  form a closed ring and thus enclose the central aperture  28 . Permanent magnets  30  are embedded between the flux guide elements  22  and extend like spokes in a radial direction through the rotor  20 . 
   The outer bridges  24  have the function of fully embedding and protecting the permanent magnets  30  in the rotor  20  from the outside so that the permanent magnets  30  cannot come into contact with the medium surrounding the rotor  20 . The inner bridges  26  have a similar function. The inner bridges  26  ensure that the rotor  20  is fixedly connected to the shaft  12 . The bridges  24 ,  26  connect the flux guide elements  22  so that the rotor  20  forms a single integral body. For the optimal functioning of the rotor  20  of the invention, at least the outer bridges  24  or the inner bridges  26  should connect each of the flux guide elements  22  to each other to form a magnetic connection between the like poles of the rotor  20 . 
   In the preferred embodiment of the invention illustrated in  FIG. 2 , the inner bridges  26  form a closed ring enabling the rotor  20  to be directly fitted, e.g. pressed or bonded, onto the shaft (not illustrated) without the need to interpose a sleeve. 
   The inner bridges  26  are connected to the flux guide elements  22  via short radial bridges  42  and each enclose a recess  32 . In the embodiment illustrated in  FIG. 2 , each recess  32  bridges two adjacent permanent magnets  30 , with the inner bridges  26  in this embodiment connecting like poles of the permanent magnets  30 . In the embodiment illustrated in  FIG. 2 , the bridges  26  connect the S poles. It is clear that the inner bridges could also connect only the N poles. This produces a rotor  20  design in which the bridges  26  bridge the radially inner ends of two permanent magnets  30  and thus enclose the recess  32  which can be filled with air or another magnetically non-conductive or low-conductive medium. In principle, bridges can connect all like poles, although an integral tube is formed when only two like poles are joined via the ring  26 . 
   Due to the design of the rotor  20  presented in the invention and in particular due to the specific design and arrangement of the recesses  32 , stray flux in the interior of the rotor  20 , that is to say near the inner bridges  26  and the central aperture  28 , can be largely prevented. This results in a considerably lower magnetic loss than in the case of conventional rotors with embedded magnets which are constructed without the sleeve  14  shown in  FIG. 1 . 
   The flux guide elements  22  are made of ferromagnetic material and are preferably formed from sheet metal laminations which are stacked to prevent eddy currents. As an alternative, they can be made from ferrite material. The flux guide elements  22  of the rotor  20  can be built as an integral component into which the magnets  30  are inserted from either side. The rotor  20  is then sealed so that the magnets are hermetically sealed and do not require a surface coating. 
   The rotor  20  is enclosed by a stator  34  which includes a stator core  36  and stator windings  38 . The stator core  36  can again consist of sheet metal laminations which are stacked as generally known in the prior art. 
   All permanently magnetic materials can be used as magnet materials. Examples include neodymium-iron-boron (NbFeB) and samarium-cobalt (SmCo). 
     FIG. 3  shows a schematic sectional view through the rotor according to the invention which is set into a stator. In  FIG. 3 , flux lines  40  have been marked in to explain the invention. Like components appearing in  FIG. 2  are indicated by the same reference numbers and are not described in detail again. 
   In  FIG. 3 , magnetic flux lines  40  are marked in, with the strength of the magnetic field being greater where the flux lines are more densely spaced so that it can be seen from the figure that the magnetic flux is very low in the region of the recesses  32 . This means that no stray flux between the rotor  20  and the shaft, on which it is placed, is incurred during operation. 
     FIG. 3  makes it clear that in the design of the rotor  20  presented in the invention in which the recesses  32  bridge like poles of adjacent permanent magnets  30 , practically no stray flux towards the shaft, in the region of the central aperture  28 , is incurred without the need to provide a special sleeve between the rotor and the shaft. In this way, losses can be kept low. 
     FIG. 4  shows a schematic sectional view through another embodiment of the rotor according to the invention, with the rotor being set into a stator. The rotor  44  includes flux guide elements  46  which are connected via outer and inner bridges  48  or  50  at the outer periphery or at a central aperture  52  in the rotor  44  respectively in order to form an integral body. In the illustrated embodiment, the inner bridges  50  form a closed ring and enclose the central aperture  52 . Permanent magnets  54 ,  54 ′ are embedded between the flux guide elements  46  and extend through the rotor  44  in an essentially radial direction like double spokes. Two directly adjacent permanent magnets  54 ,  54 ′ form a permanent magnet pair, the permanent magnets  54 ,  54 ′ of a pair being inclined at an angle to each other relative to the radius of the rotor  44 . In another embodiment which is not illustrated, the permanent magnets of a pair can also be arranged parallel to each other. The permanent magnets  54 ,  54 ′ of a pair have essentially the same direction of magnetization, i.e. the arrangement of the north and south poles as indicated by the arrows in  FIG. 4 . This means that between the permanent magnets  54 ,  54 ′ of a pair, no poles are formed but rather the magnetic field lines connect the inner sides of the permanent magnets of a pair at the shortest distance, as can be seen from  FIG. 5 . The permanent magnets  54 ,  54 ′ of a pair essentially act as a double magnet enabling the field generated by the magnets to be intensified compared to the embodiment shown in  FIG. 2 . The arrangement of the permanent magnets at an angle goes to improve torque and particularly to suppress cogging torque. 
   The function of the bridges  48 ,  50  in protecting the embedded permanent magnets  54 ,  54 ′ and in enabling the rotor  44  to be directly mounted onto a shaft is essentially the same as described in relation to  FIG. 2 . Moreover, the bridges  50  in combination with shorter radial bridges  56  enclose recesses  58  which bridge adjacent permanent magnets  54 ,  54 ′ of adjacent permanent magnet pairs. In the illustrated embodiment, the inner bridges  50  connect the spaces between the permanent magnets  54 ,  54 ′ of a permanent magnet pair. The effect of the recesses  58  thus formed is the same as described in reference to  FIGS. 2 and 3  and as explained below in reference to  FIG. 5 . 
   In the embodiment illustrated in  FIG. 4 , the flux guide elements  46  are also made of ferromagnetic material and are preferably formed from sheet metal laminations which are stacked to prevent eddy currents. The flux guide elements  46  of the rotor  44  are preferably made as an integral component. 
   The rotor  44  shown in  FIG. 4  is enclosed by a stator  60  with an air gap  62  being formed between the stator  60  and the rotor  44 . The stator  60  includes a stator core  64  with associated stator poles onto which phase windings  66 ,  66 ′ are wound. For its part, the stator core  64  can be made of sheet metal laminations which are stacked as is basically known in the prior art. 
     FIG. 5  shows a similar schematic sectional view through the rotor  44  of the invention which is placed into a stator  60 , with like components being identified by the same reference numbers as in  FIG. 4 . In  FIG. 5 , flux lines have been marked in to explain the invention. 
   In modification of the embodiment shown in  FIG. 4 , recesses  68  are provided in  FIG. 5  at the outer periphery of the rotor  44  in the region of the outer bridges  48 , which are evenly or unevenly distributed over the periphery of the rotor  44 . These recesses  68  improve the torque of the electric motor in operation and, in particular, reduce cogging torque even more than in the embodiments described above. 
   In  FIG. 5 , magnetic flux lines  40  are marked in and it can be seen from the figure that the magnetic flux is practically non-existent in the region of the recesses  58  so that no magnetic stray flux between the rotor  44  and the shaft, onto which it is mounted, is incurred during operation. 
     FIG. 6  shows a schematic sectional view through the embodiment of the rotor  44  illustrated in  FIG. 5 . Like components appearing in  FIG. 5  are identified by the same reference numbers and are not described again. It can be seen from  FIG. 6  that the rotor of the invention can be so constructed that slots  70  to accommodate the permanent magnets  54 ,  54 ′ can be formed in the rotor  44 , the permanent magnets being inserted into these slots  70  and the rotor  44  being then sealed. 
   Another embodiment of the rotor presented in the invention is shown schematically in  FIG. 7 . This embodiment substantially corresponds to the embodiment described in reference to  FIG. 2 , with the radially inner bridges, however, not forming a closed ring. Like components appearing in  FIG. 2  are identified by the same reference numbers as in  FIG. 2  or  FIG. 3 . 
   In the embodiment illustrated in  FIG. 7 , the recesses  32  are enclosed by the short radial bridges  42  as well as by bridge butts  72 , which adjoin the central inner aperture  28  of the rotor  20 . The outer bridges  24  ensure an integral rotor body  20  with all the flux guide elements  22  being connected. Although the radially inner bridges or bridge butts  72  are not connected to each other, the recesses  32  provide the same suppression of stray flux between rotor  20  and shaft as described above in reference to the previous embodiments. Only the mechanical strength of the rotor  20  at the central inner aperture  28  is somewhat less than in the embodiments described above. 
   The characteristics revealed in the above description, the claims and the figures can be important for the realization of the invention in its various embodiments both individually and in any combination whatsoever. 
   IDENTIFICATION REFERENCE LIST 
   
       
         10  Rotor 
         12  Shaft 
         14  Sleeve 
         16  Flux guide elements 
         18  Permanent magnets 
         20  Rotor 
         22  Flux guide elements, rotor core 
         24 , 26  Bridges 
         28  Central aperture 
         30  Permanent magnets 
         32  Recess 
         34  Stator 
         36  Stator core 
         38  Stator windings 
         40  Flux lines 
         42  Radial bridges 
         44  Rotor 
         46  Flux guide elements, rotor core 
         48 ,  50  Bridges 
         52  Central aperture 
         54 ,  54 ′ Permanent magnets 
         56  Radial bridges 
         58  Recesses 
         60  Stator 
         62  Air gap 
         64  Stator core 
         66 ,  66 ′ Phase windings 
         68  Recesses 
         70  Slots 
         72  Bridge butts 
         110  Shaft 
         112  Back iron yoke 
         114  Housing 
         116  Rotor arrangement 
         118  Stator arrangement 
         122  Permanent magnets 
         124  Flange 
         126 ,  128  Bearings 
         155  Metal laminations 
         160  Windings