Abstract:
An electric motor with a permanent magnet excitation comprises permanent magnets retained in a pole housing and associated with the flux-conducting element, wherein at least one permanent magnet and a flux-conducting element are arranged so that they are in positively interlocking engagement.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an electric motor with permanent magnet excitation. 
     DE 35 39 851 A1 has disclosed an electric starter motor for an internal combustion engine, said starter motor having permanent magnets on the pole housing which surround an armature shaft with an armature arranged thereon. The permanent magnets are fixed in the housing with the aid of holding springs, which are arranged between adjacent magnets and have spring arms bearing against one end side of the magnets in a resilient manner. In order to achieve a series characteristic, in which high torques are emitted even at low rotation speeds, flux guidance pieces are associated with the permanent magnets, said flux guidance pieces consisting of a material with good magnetic conductivity and serving to guide the magnetic flux. In accordance with DE 35 39 851 A1, the holding springs are additionally fastened on the housing with a rivet in order to be able to absorb the radial forces acting on the flux guidance pieces and to hold the magnets including the flux guidance pieces in the housing in the desired position. 
     In accordance with a further embodiment described in DE 35 39 851 A1, the permanent magnets including the flux guidance pieces are positioned on the pole housing with the aid of holding rings, which secure the permanent magnets and the flux guidance pieces radially with respect to the surrounding pole housing. Spring tongues which protrude out of the holding rings through 90° place the flux guidance pieces against the magnets in the circumferential direction. Both the holding via riveted-in holding springs and via the holding rings or in the form of welded-in or riveted flux guidance pieces with simple holding springs represent comparatively complex embodiments. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of forming an electric motor with permanent magnet excitation using simple design measures in such a way that the magnets, without any complex subsequent processing, for example, by grinding, and the flux guidance elements are held securely on the pole housing. 
     The invention is based on an electric motor with permanent magnet excitation, said electric motor being used for example as a starter motor for an internal combustion engine. The electric motor is in particular in the form of an internal-rotor motor, whose stator surrounds an armature shaft with an armature. The stator comprises a pole housing, with the permanent magnets and the flux guidance elements being fixed to the inner side of said pole housing with the aid of fastening means. 
     The invention provides that at least one permanent magnet and at least one associated flux guidance element are arranged so as to engage in one another in a form-fitting manner, the form-fitting connection being provided in the radial direction and preferably also in the circumferential direction. This is realized, for example, by means of a projection, which extends in the circumferential direction, which is arranged on one of the component parts and which protrudes into a complementarily shaped cutout in the other component part. In this case, both the embodiments in which the projection is arranged on a side face of the permanent magnet and the cutout is introduced into the facing side face of the flux guidance piece and reverse embodiments with a projection on the flux guidance element and a cutout in the permanent magnet are possible. In both cases, which may also be combined with one another, if appropriate, the projection and the cutout extend in the circumferential direction, with the result that a recess between the mutually engaging sections and therefore a form-fitting connection is provided in the radial direction. The side faces on which the projection or in which the cutout is formed delimit the respective component part in the circumferential direction. 
     Owing to the form-fitting connection, it is in principle sufficient for only one of the component parts, i.e. either the permanent magnet or the flux guidance element, to be fixed on the pole housing with the aid of the fastening means. The holding force is also transmitted, via the form-fitting connection, to the component part, on which the fastening means does not directly act, and secures said component part in the position on the inner side of the pole housing. It is therefore in principle not necessary for the component part which is not directly in contact with the fastening means to be secured via a further fastening means or for both component parts to be held directly in position by the same fastening means; although these variants are also possible. 
     A further advantage of the embodiment according to the invention can be considered to be one in which the side faces of the permanent magnets do not need to be subjected to any processing or only need to be subjected to less processing than is required in the prior art. In principle, it is sufficient to incorporate the magnets directly into the electric motor after production of said magnets without any processing of the lateral limiting faces, in particular without any grinding operations, since side faces on the magnets which are parallel by virtue of the production can also be used owing to the form-fitting engagement of the flux guidance element behind said magnet. The magnets provided after pressing with parallel side faces on the magnets can be inserted directly into the pole housing in this form. 
     If appropriate, the side faces are subjected to processing, but this may be less involved in comparison with embodiments from the prior art. For example, it may be sufficient to subject the side faces to partial grinding only over part of their radial extent, with the result that an angled lateral limiting face on the permanent magnet is produced. Owing to the angular alignment of the partially ground partial face, it is possible for the flux guidance element to engage in a form-fitting manner behind this, without it requiring any additional installation space when the permanent magnet and the flux guidance element engage in one another. 
     In addition to the form-fitted securing arrangement, the arched effect for holding the permanent magnets on the inner side of the pole housing is used by virtue of the permanent magnets being supported on the two opposite side faces, with a flux guidance element being arranged on one side. Since the flux guidance elements are subjected to increased forces acting radially inwards when the starter motor is switched on, improved mutual support to counteract radial shifting in the direction of the armature shaft is achieved via the form-fitting connection between the flux guidance element and the adjoining permanent magnet. 
     In accordance with an expedient embodiment, a gap is provided in the region of the contact surface between the flux guidance element and the permanent magnet, said gap extending in the radial direction at least over a subregion of the mutually facing side faces. The gap serves to compensate for manufacturing tolerances of the magnet and/or the flux guidance element, with the result that it is possible to dispense with grinding processing, in particular in the production of the magnet. In addition, the gap reduces the magnetic short circuit through the flux guidance element. 
     In accordance with a further expedient embodiment, the mutually facing side faces of the permanent magnet and the associated flux guidance element are provided with a geometry in which a radially outer edge of the permanent magnet, on the one hand, and an edge which is positioned further radially inwards of the flux guidance element, on the other hand, lie on different sides with respect to a radial which is passed through the contact region. This means that, given a planar embodiment, the contact face between the permanent magnet and the flux guidance element is at an angle with respect to the radial, which ensures a sufficient form-fitting connection. In this case, both embodiments in which the radially outer edge of the permanent magnet extends further in the circumferential direction than the radially inner edge of the permanent magnet and embodiments in which the radially outer edge extends less far in the circumferential direction than the radially inner edge on the same side face, wherein the facing side face of the flux guidance element has in each case a complementary cross-sectional geometry, are possible. Furthermore, angular side face geometries with two partial faces arranged at an angle with respect to one another both on the part of the permanent magnet and on the part of the flux guidance element are also possible. 
     In accordance with yet a further embodiment, in principle there is also the possibility of a contact face which extends in the radial direction, in which case the form-fitting connection is ensured via at least one projection extending in the circumferential direction on one of the component parts and via a correspondingly shaped cutout in the other component part. 
     In accordance with yet a further embodiment, the fastening means for holding the permanent magnet and the associated flux guidance element on the inner side of the pole housing is in the form of a holding spring, which, by virtue of the arched effect, bears against the pole housing and is held in position in the pole housing via tabs, for example. The holding spring preferably has a U shape, with a limb pressing against the flux guidance element or against the permanent magnet in the circumferential direction. The holding spring is associated with precisely one magnet/flux guidance element combination, wherein an embodiment of the holding spring in which two adjacent permanent magnets including flux guidance elements are secured via a common spring is also optionally possible. A limb of the holding spring presses against the flux guidance element or the permanent magnet either in punctiform fashion, in linear fashion or areally. 
     In principle, an embodiment of the fastening means in the form of end-side holding rings with an integrally formed collar which hold the flux guidance element and/or the permanent magnet in position radially with respect to the pole housing is also possible. 
     In accordance with yet a further expedient embodiment, provision is made for the flux guidance element to have a constant cross section, when viewed in the radial direction, apart from its base geometry, which may have a cross-sectional area which is in the form of a rectangle, in the form of a partial circle or with an annular form, for example. However, it is also possible to use flux guidance elements which have a varying cross section in the radial direction, in particular a cross section which increases in size from the inside outwards radially. 
     The flux guidance elements are preferably in the form of a sheet-metal part, a bent part, a stamped part or an extruded part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and expedient embodiments can be gleaned from the remaining claims, the description relating to the figures and drawings, in which: 
         FIG. 1  shows a starting apparatus for an internal combustion engine in a longitudinal section, 
         FIG. 2  shows the electric motor of the starting apparatus in section transversely to the longitudinal axis, 
         FIG. 3  shows an enlarged illustration of a view of a permanent magnet with a laterally adjoining flux guidance element, which has an angled form, 
         FIG. 4  shows a further exemplary embodiment, in which the two limbs of the angular flux guidance element are asymmetrical, 
         FIG. 5  shows a further exemplary embodiment in which the flux guidance element has a varying cross section in the radial direction, 
         FIG. 6  shows a further exemplary embodiment, in which a projection which extends in the circumferential direction on the flux guidance element protrudes into an associated cutout in the side face of the permanent magnet. 
     
    
    
     DETAILED DESCRIPTION 
     In the figures, identical component parts have been provided with the same reference symbols. 
       FIG. 1  shows a starting apparatus  10  in a longitudinal section. This starting apparatus  10  has, for example, a starter motor  13  and a starter relay  16 . The starter motor  13  and the starter relay  16  are fastened on a common input drive end frame  19 . The starter motor  13  has the functional purpose of driving a starter pinion  22  when it is engaged in the ring gear  25  of the internal combustion engine (not illustrated here). 
     The starter motor  13  is in the form of an electric motor with permanent excitation and has, as housing, a pole tube  28 , which bears permanent magnets  31  on its inner circumference, said permanent magnets each having associated flux guidance elements. The permanent magnets  31  in turn surround an armature  37 , which has an armature stack  43  comprising laminations  40  and an armature winding  49  arranged in slots  46 . The armature stack  43  is pressed onto an armature or input drive shaft  44 . Furthermore, a commutator  52 , which comprises, inter alia, individual commutator segments  55 , is fitted to that end of the drive shaft  44  which is remote from the starter pinion  22 . The commutator segments  55  are electrically connected to the armature winding  49  in a known manner in such a way that, when the commutator segments  55  are energized by carbon brushes  58 , a rotary movement of the armature  37  in the pole tube  28  results. A power supply line  61  arranged between the starter relay  16  and the starter motor  13  supplies current to the carbon brushes  58  in the switched-on state. The input drive shaft  44  is supported on the commutator side by a shaft journal  64  in a sliding bearing  67 , said sliding bearing in turn being held fixed in position in a commutator bearing cap  70 . The commutator cap  70  is in turn fastened in the input drive end frame  19  by means of tension rods  73 , which are arranged distributed over the circumference of the pole tube  28  (screws, for example 2, 3 or 4 pieces). In the process, the pole tube  28  is supported on the input drive end frame  19 , and the commutator bearing cap  70  is supported on the pole tube  28 . 
     In the output drive direction, the armature  37  is adjoined by a so-called sun gear  80  which is part of a planetary gear mechanism  83 . The sun gear  80  is surrounded by a plurality of planet gears  86 , usually three planet gears  86 , which are supported by means of a roller bearings  89  on axial journals  92 . The planet gears  86  roll in a hollow wheel  95 , which is mounted externally in the pole tube  28 . In the direction towards the output drive side, the planet gears  86  are adjoined by a planet carrier  98 , in which the axial journals  92  are accommodated. The planet carrier  98  is in turn mounted in an intermediate bearing  101  and a sliding bearing  104  arranged therein. The intermediate bearing  101  is configured in the form of a pot in such a way that both the planet carrier  98  and the planet gears  86  are accommodated in said intermediate bearing. Furthermore, the hollow wheel  95  is arranged in the pot-shaped intermediate bearing  101  and is ultimately closed by a cover  107  with respect to the armature  37 . The intermediate bearing  101  is also supported with its outer circumference on the inner side of the pole tube  28 . The armature  37  has a further shaft journal  110  on that end of the input drive shaft  44  which is remote from the commutator  52 , said shaft journal likewise being accommodated in a sliding bearing  113 . The sliding bearing  113  is in turn accommodated in a central bore in the planet carrier  98 . The planet carrier  98  is integrally connected to the output drive shaft  116 . This output drive shaft  116  is supported with its end  119  remote from the intermediate bearing  101  in a further bearing  122 , which is fastened in the input drive end frame  19 . The output drive shaft  116  is divided into various sections: a section with a so-called straight gearing  125  (inner gearing) which is part of a so-called shaft-hub connection thus follows the section arranged in the sliding bearing  104  of the intermediate bearing  101 . This shaft-hub connection  128  makes it possible in this case for a driver  131  to perform an axially linear sliding movement. This driver  131  is a sleeve-like protrusion, which is integral with a pot-shaped outer ring  132  of the freewheel  137 . This freewheel  137  (ratchet) furthermore comprises the inner ring  140 , which is arranged radially within the outer ring  132 . Clamping bodies  138  are arranged between the inner ring  140  and the outer ring  132 . These clamping bodies  138 , in interaction with the inner ring and the keyways of the outer ring, prevent a relative rotation between the outer ring and the inner ring in a second direction. In other words: the freewheel  137  provides the possibility of a relative movement between the inner ring  140  and the outer ring  132  only in one direction. In this exemplary embodiment, the inner ring  140  is integral with the starter pinion  22  and the helical gearing  143  (outer helical gearing) thereof. 
     For reasons of completeness, details are also given here on the meshing mechanism. The starter relay  16  has a pin  150 , which is an electrical contact and which is connected to the positive terminal of an electrical starter battery (not illustrated here). This pin  150  is passed through a relay cover  153 . This relay cover  153  seals off a relay housing  156 , which is fastened to the input drive end frame  19  by means of a plurality of fastening elements  159  (screws). Furthermore, a pull-in winding  162  and a so-called hold-in widening  165  are arranged in the starter relay  16 . The pull-in winding  162  and the hold-in winding  165  both each induce an electromagnetic field in the switched-on state, said electromagnetic field flowing both through the relay housing  156  (consisting of an electromagnetically conductive material), a linearly movable armature  168  and an armature magnetic return path  171 . The armature  168  bears a push rod  174 , which is moved in the direction toward a switching pin  177  during linear pull-in of the armature  168 . With this movement of the push rod  174  toward the switching pin  177 , said switching pin is moved out of its rest position in the direction toward two contacts  180  and  181 , with the result that a contact link  184 , which is fitted at that end of the switching pin  177  which faces the contacts  180  and  181 , electrically connects the two contacts  180  and  181  to one another. As a result, electric power is passed from the pin  150  beyond the contact link  184 , to the power supply line  61  and therefore to the carbon brushes  58 . The starter motor  13  is energized in the process. However, furthermore, the starter relay  16  and the armature  168  also have the object of moving, with a pull element  187 , a lever which is arranged in rotationally movable fashion on the input drive end frame  19 . This lever  190 , usually in the form of a forked lever, engages with two “prongs” (not shown here) on its outer circumference around two disks  193  and  194  in order to move a driver ring  197  which is trapped between said disks towards the freewheel  137  counter to the resistance of the spring  200  and thereby to mesh the starter pinion  22  in the ring gear  25 . 
     The stator  13  of the starter motor in the form of an electric motor is illustrated in section in  FIG. 2 . A plurality of permanent magnets  31  are arranged distributed over the circumference on the inner side of the cylindrical pole housing  28 , with in each case one flux guidance element  300 , directly adjoining in a circumferential direction, being associated with each permanent magnet  31 , said flux guidance element consisting of a material with good magnetic conductivity. The flux guidance element  300  is in contact with the associated permanent magnet  31 ; a small air gap produced during manufacture may be located between the facing side faces of the permanent magnet  31  and the flux guidance element  300 . 
       FIG. 3  shows the form-fitting connection between a permanent magnet  31  on the inner side of the pole housing  28  and the associated flux guidance element  300 . The permanent magnet  31  has a mirror-symmetrical configuration with respect to a magnet central plane and has cross-sectional configuration in the form of a partial circle, the radial outer side  308  bearing directly against the inner wall of the pole housing  28  and the radial inner side  307  having a small radial distance from the armature. The side faces  303  limiting the magnets in the circumferential direction are each angular and have a radially outer section  304  and a bevel  305  which extends at an angle thereto, the sections  304  and  305  having an angle of less than 60° with respect to one another, said angle being produced by virtue of pressing. A magnet foot  306  is provided in the transition between the bevel  305  and the radial inner side  307 . The radially outer sections  304  on the two mutually opposite side faces of the permanent magnet  31  are parallel to one another. 
     The flux guidance element  300 , whose cross-sectional geometry is matched to the side face geometry of the permanent magnet  31 , adjoins one of the lateral limit faces  303 . The flux guidance element  300  has a complementary geometry with respect to the side face  303  of the permanent magnet on the side facing the permanent magnet  31 . Accordingly, the flux guidance element  300  is also angled, with the two angle sections being of approximately the same length. A gap  309 , which acts as tolerance gap for compensating for manufacturing tolerances, in particular of the permanent magnet  31 , is located in the contact region between the associated side faces of the permanent magnet  31  and the flux guidance element  300 . The gap  309  does not extend over the entire axial length of the contact region; direct contact between the side face  303  of the permanent magnet and the flux guidance element  300  is provided adjacent to the radial inner side  307  and to the radial outer side  308  of the permanent magnet  31 . 
     In order to hold the permanent magnet  31  and the associated flux guidance element  300  on the inner side of the pole housing  28 , a fastening means in the form of a holding spring  301  is provided, said fastening means applying a holding force to the flux guidance element  300  in the circumferential direction. One limb of the holding spring  301  is supported on the inner side of the pole housing  28 , and the other limb of the holding spring is in linear contact with an edge of the flux guidance element  300  and exerts a force on the flux guidance element  300  in a circumferential direction and with an additional component in the radial direction onto the pole housing. The holding spring  301  is in the form of a U, with only one limb  302  being illustrated for reasons of simplicity. The holding spring  301  is symmetrical to the axis  315 , which extends in the radial direction. The holding spring is positioned in the pole housing via tabs (not illustrated). 
     The flux guidance element  300  and the permanent magnet  31  are coupled to one another in a form-fitting manner, when viewed in the radial direction, with the result that the holding of the flux guidance element  300  via the holding spring  301  is in principle sufficient for securing the permanent magnet  31  in its radial position on the inner side of the pole housing  28  as well. Owing to the angular embodiment of the side faces of the permanent magnet  31  and the flux guidance element  300 , a form-fitting connection between these component parts is provided in the radial direction. In this case, the radially outer edge  310  of the permanent magnet  31  in relation to a radial  312  through the contact region between the permanent magnet and the flux guidance element is on the opposite side with respect to a central edge  313 , which is formed at an angle between the two angular sections of the flux guidance element  300 . In the same way, the radial inner edge  311  of the flux guidance element  300  is on the opposite side of the radial  312  with respect to the central edge  314  of the permanent magnet  31  on the side face  303 . This ensures that the permanent magnet  31  and the flux guidance element  300  engage in one another in the circumferential direction and therefore a form-fitting connection is provided in the radial direction. 
     In the exemplary embodiment shown in  FIG. 3 , the two lateral sections  304  and  305  on the side face  303  of the permanent magnet  31  each have at least approximately the same length. In the exemplary embodiment shown in  FIG. 4 , on the other hand, the section  304  reaching as far as the radial outer side  308  is longer than the section  305  facing the radial inner side. The section  304  is at least twice as long as the section  305 . As in the preceding exemplary embodiment, the sections  304  pointing radially outwards on the opposing side faces are parallel to one another. 
     In the exemplary embodiment shown in  FIG. 5 , the permanent magnet  31  is provided with a geometry which corresponds to that in  FIG. 4 . The flux guidance element  301 , in contrast to the preceding exemplary embodiment, has a cross-sectional configuration which varies in the radial direction. On the side adjacent to the radial inner side  307 , the flux guidance element  300  has a smaller extent in the circumferential direction than on the side facing the radial outer side  308 . 
     A further difference with respect to the preceding exemplary embodiments consists in that a flat contact between the permanent magnets  31  and the flux guidance element  300  is provided on the section  304  which extends as far as the radial outer side  308 . On the other hand, an air gap  309  is provided between the radially inner section  305  on the side face  303  and the facing side face on the flux guidance element  300 . 
     In the exemplary embodiment shown in  FIG. 6 , the partially ground side faces  303  of the permanent magnet  31  are aligned radially and are provided with a cutout  316 , into which a complementarily shaped projection  317  on the flux guidance element  300  protrudes. In the region of the projection  317 , an air gap  309  can be formed toward the wall of the cutout  316 . 
     The flux guidance element  300  has, on its wall side opposite the projection  317 , a complementary cutout, which is produced by virtue of a metal sheet with a constant thickness, from which the flux guidance element is manufactured being provided with an embossed position in order to mold the projection  317 . As illustrated by the dashed line  318 , the flux guidance element  300  can also have a greater thickness in the circumferential direction, if appropriate. For large thicknesses, a cross section which increases as the diameter of the partial circle increases is advantageous. 
     As fastening means, a holding ring  319  is provided in  FIG. 6  which is arranged on the radial inner side  307  and supports the permanent magnet  31  and the flux guidance element  300  radially. The flux guidance element  300  is placed against the magnet  31  by spring tongues (not illustrated) in the circumferential direction.