Abstract:
The invention relates to a transversal flux machine having a stator ( 13 ) and an external rotor ( 16 ) disposed about the stator ( 13 ), the stator ( 13 ) comprising two axial end faces ( 273, 276 ), having an inner core ( 107 ) of the stator ( 13 ), having a cooling path disposed radially within the inner core ( 107 ), characterized in that a cooling path either a) protrudes out of the transversal flux machine ( 10 )on the axial end face ( 273 ) of the stator ( 13 ), wherein said transversal flux machine faces away from an inlet side ( 303 ), or b) the cooling path protrudes out of the transversal flux machine ( 10 ) on the axial end face ( 273 ) of the stator ( 13 ), wherein said transversal flux machine faces away from the inlet side, and wherein the cooling path runs between the inlet and the outlet in an intermediate space ( 265 ) between the stator ( 13 ) and the external rotor ( 16 ). of the stator ( 13 ) facing the inlet side, wherein the cooling path between the inlet and the outlet runs in an intermediate space ( 265 ) between the stator ( 13 ) and the external rotor ( 16 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The design of a transverse flux machine is known from the dissertation “Entwicklung and Optimierung einer fertigungsgerechten Transversalflussmaschine” [Development and optimization of a transverse flux machine suitable for manufacture], author Mr. Michael Bork, Shaker-Verlag, publication year, in particular page 84 therein. 
         [0002]    The problem consists in improving the cooling of the machine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Exemplary embodiments of the method according to the invention as well as a control apparatus and a system with a control apparatus and a start apparatus are illustrated in the drawings, in which: 
           [0004]      FIG. 1  shows a longitudinal section through a first embodiment of an electric machine, 
           [0005]      FIG. 2  shows a front view of an electric machine on a flange without electronics, 
           [0006]      FIG. 3  shows a three-dimensional view of the machine shown in  FIG. 2 , 
           [0007]      FIG. 4  shows a three-dimensional view of the machine shown in  FIG. 1 , 
           [0008]      FIG. 5  shows a rear view of the electric machine on a flange with dismantled electronics, 
           [0009]      FIG. 6A  shows a longitudinal section through the stator, 
           [0010]      FIG. 6B  shows a detail of the stator shown in  FIG. 6A , 
           [0011]      FIG. 7  shows a three-dimensional view of the stator, 
           [0012]      FIG. 8  shows a further three-dimensional view of the stator shown in  FIG. 7 , 
           [0013]      FIG. 9  shows a fan, 
           [0014]      FIG. 10  shows a view into a cavity of an external rotor, 
           [0015]      FIG. 11  shows two views of the external rotor with an external view being illustrated in the upper half of the picture and a longitudinal section view being illustrated in the lower half of the picture, 
           [0016]      FIG. 12  shows a three-dimensional view of a stator winding, 
           [0017]      FIG. 13A  shows a cross section through the stator winding, 
           [0018]      FIGS. 13B and 13C  show further possible cross sections through the stator winding, 
           [0019]      FIG. 14  shows a cross section through a wire of a litz wire, 
           [0020]      FIG. 15  shows a further cross section through the stator winding, 
           [0021]      FIG. 16  shows a detail of the stator winding, 
           [0022]      FIGS. 17   a ) to  g ) show various method steps for producing a stator winding, 
           [0023]      FIGS. 18   a ) to  e ) show two different stator windings, different method steps for producing a stator winding and two different cross sections through the stator windings, 
           [0024]      FIG. 19  shows three connection parts of the stator windings which are star-connected, 
           [0025]      FIG. 20  shows a variant of a stator, 
           [0026]      FIG. 21  shows a second embodiment of an electric machine with the configuration of a transverse flux machine. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIG. 1  illustrates an electric machine with the configuration of a transverse flux machine  10 . As is the case for many electric machines, this electric machine also has a stator  13  and a rotor, in the form of a so-called external rotor  16 . Both parts are arranged in a housing  19 . The external rotor  16  is driven by means of a shaft  22 . In this case, the shaft  22  is driven by means of a pulley or a gearwheel or another torque transmission part. Electronics  28 , for example a passive rectifier or an active rectifier, are arranged beneath a cover  25  on the left-hand side in  FIG. 1 . 
         [0028]    An approximately pot-shaped housing shell  34 , which has a ring-shaped collar  37  with a central opening  40 , is supported on a flange  31 . The shaft  22  extends through the opening  40 . 
         [0029]    Furthermore, the stator  13  is also supported on the flange  31 . The stator  13  is fastened indirectly to the flange  31  by means of seven screws  43 . The screws  43  protrude through in each case one through-opening in the flange  31  and engage in in each case one threaded bore  46  of a further flange  49 . This further flange  49  is integrally connected to a central sleeve  52 , which likewise performs central tasks. 
         [0030]    The sleeve  52  bears, over its inner contour  54 , two rolling bearings  55  and  56 , which in this case are in the form of deep groove ball bearings. The inner contour  54  has two ring webs  59  and  60 . The ring web  60  serves as a stop for the rolling bearing  56 . Between the ring web  59  and the rolling bearing  55 , in order to produce an axial prestress on the two rolling bearings  55  and  56 , a disk spring  57  is clamped between the ring web  59  and the rolling bearing  55 . The shaft  22  is mounted fixed in position and rotatably via these rolling bearings  55  and  56 . The shaft  22  is placed between the bearing seats for the rolling bearings  55  and  56 . A spacer sleeve  63  is pushed onto the shaft  22  between the two rolling bearings  55  and  56 , in order that a defined distance is set between the rolling bearings  55  and  56 . The two rolling bearings  55  and  56 , or their inner rings (not illustrated here in any more detail), are braced with one another and against a shoulder  69  by means of the spacer sleeve  63 , a tensioning sleeve  66  and a tensioning screw  67 . An outer ring  70  of the rolling bearing  56  is secured in position by an inner securing ring  71 . 
         [0031]    The shoulder  69  also has the task of protecting the rolling bearings  55  and  56 , as well as the task of forming a stop for the external rotor  16 . The external rotor  16  has a pot-like configuration. A section  73  in the form of a cylinder lateral surface of the external rotor  16  bears permanent magnets  77  on its cylindrical inner side  75  in three rows arranged axially successively. A type of housing base  80 , which rests with a central bore  83  on the shaft  22 , adjoins the section  73  in the form of a cylinder lateral surface of the external rotor  16 , extending radially inwards at an axial end. A radially acting fan  86  is fastened on an inner side of the housing base  80 . A further fan  89 , which is in the form of a narrow ring in the radial direction, is fastened on the section  73  in the form of a cylinder lateral surface which is directed towards the electronics  28 . This fan  89  rotates in a groove which is incorporated in a front side of the flange  31  which is directed towards the fan  89 . Radially outside this fan  89 , a series of ventilation openings  90  is arranged all the way round in the housing shell  34 . 
         [0032]    The stator  13  is arranged radially within the section  73  in the form of a cylinder lateral surface. This stator  13  comprises three individual special ring systems  92 . Each ring system  92  has two half-rings  94  and  96 , which, between them, accommodate a ring coil as stator winding  98 . The stator winding  98  is surrounded or encompassed in each case by two half-yokes  100  and  101 , two ring walls  102  and  103  and claw poles  104  and  105 ; see also  FIG. 6A . The claw poles  104  and  105  in this case alternate with one another in the circumferential direction. In this case, an accommodating area  106  for the stator winding  98  is formed. The accommodating area  106  has a specific cross section, delimited by the half-yokes  100  and  101 , the two ring walls  102  and  103  and the claw poles  104  and  105 . The stator winding  98  with a preformed cross section rests in this accommodating area  106 , which is rectangular in this example. The cross section of the stator winding  98  is matched to the cross section of the accommodating area  106 . 
         [0033]    Arranged in concentrated fashion, three outputs  108 ,  109  and  110  of the in total three stator windings  98  are located radially inside the stator  13 , i.e. between the half-yokes  100  and  101 , which in total make up an inner yoke  107 , and the sleeve  52 . Each output  108 ,  109  and  110  is in this case associated with a stator winding  98 . The three stator windings  98  are star-connected to one another, which will be discussed in more detail further below. 
         [0034]    Ventilation channels  113 , which are part of a ventilation system which will likewise be described in more detail below, are located radially inside the stator  13 , i.e. likewise between the half-yokes  100  and  101  and the sleeve  52 . 
         [0035]      FIG. 2  shows a front view of the electric machine on the flange  31  with dismantled electronics. The flange  31  has a through-hole  115  for a respective fastening eyelet  114  at the clock positions “half one”, “six o&#39;clock” and “half ten”. These through-holes  115  are preferably equipped with internal threads  116 , as illustrated here, and are used for fastening the electric machine to its surroundings. Six further through-holes  118 , of which in each case two times two through-holes  118  are arranged in the fastening eyelets  114  and in each case one through-hole  118  is arranged in individual fastening eyelets  119 , are used for fastening the housing shell  34  to the flange  31 . For this purpose, correspondingly six tie rods  121 , in the form of long screws (see also  FIG. 3 ), are plugged through further through-holes  122 , which are incorporated in ring segments  123 . By virtue of applying a sufficient torque to the tie rods  121 , the housing shell  34  is held reliably in position on the flange  31 . 
         [0036]    Three threaded bores  125  located at the clock positions “two o&#39;clock”, “six o&#39;clock” and “ten o&#39;clock” are used for fastening a cooling plate  127  (illustrated by way of example in  FIG. 1 ) by means of screws  130 . The cooling plate  127  itself serves to cool the electronics  28 . 
         [0037]    Five of the seven screws  43  protrude into a groove  133  in the form of a ring segment, which extends approximately 270° about an axis of rotation  136 . Four slots  142  which are likewise in the form of ring segments protrude from a base  139  of the groove  133 , through which slots a view of the claw poles  104  and  105  and a casting compound  144  is free. The casting compound  144  covers the stator windings  98 . A ring-shaped web  147  delimits the flange  31  radially inwards and delimits a circular central through-opening  149  radially outwards. Inlet openings of the ventilation channels  113  are illustrated radially within the web  147 . In the background, fan blades  152  of the fan  86  can be seen through the ventilation channels  113 . 
         [0038]    In the foreground at the “twelve o&#39;clock” position, three connecting lugs  155  with cable sleeves  156  are illustrated. These connecting lugs  155  serve the purpose of making contact with the three stator windings  98  (see also  FIG. 3 ). In this example, three-phase current can be understood as a specific form of an alternating current. In contrast to the illustration in  FIG. 1 , the current of the three stator windings  98  can be guided by means of the connecting lugs  155  to so-called “path-building” electronics (for example a passive rectifier or an active rectifier), which are not arranged on the cooling plate  127 . 
         [0039]    The individual ventilation channels  113  are separated by radial webs  158 . These webs  158  protruding from the inner yoke  107  act as cooling ribs, cool the stator  13  and extend from the inner yoke  107  radially inwards. These webs  158  or cooling ribs are integrally formed on the inner yoke  107 . The webs  158  shown directly in  FIG. 2  are moreover webs  158  which are incorporated in the flange  49 . The same arrangement of webs  158  and ventilation channels  113  is also implemented in the half-rings  94  and  96 . While the flange  49  merges with a tubular section  160  of the sleeve  52  after the webs  158 , when viewed radially from the outside, the webs  158  of the half-rings  94  and  96  merge with a thin ring region  162 . The webs  158  or cooling ribs which are spaced apart in the circumferential direction are connected integrally to one another radially on the inside by the ring region  162 . In the axial direction, a plurality of ring regions  162  a plurality of half-rings  94 ,  96  are braced with one another. 
         [0040]      FIG. 3  shows the abovementioned ventilation openings  90  over the outer circumference of the housing shell  34 . Furthermore, a fan blade  164  of the fan  89  is shown within the ventilation openings  90 , in a manner representative of the entire fan  89 . 
         [0041]    Against the background of  FIG. 1 ,  FIG. 4  shows the technical solution with built-on electronics. Thus, the current produced by the outputs  108 ,  109 ,  110  is conducted via three conductor rails  166 ,  167  and  168  to the connections  169 ,  170 . A third connection is provided, but this is not illustrated in  FIG. 4  because it is hidden by the conductor rail  167 . Protruding from the cover  25 , a positive connection  173 , for example for supplying power to a power supply system of a motor vehicle (not illustrated) is shown. 
         [0042]    In addition, the connecting lugs  155  can also be fastened at the outputs  108 ,  109 ,  110 . 
         [0043]      FIG. 5  shows a rear view of the electric machine on the flange  31  with the electronics  28  dismantled or not fitted and also without the housing shell  34  and without the shaft  22  fitted. It can clearly be seen that the webs  158  are integrally formed on the half-ring  96 . The same also applies to the other half-ring  94 . This integral formation of this structure comprising the webs  158  and the ventilation channels  113  with the ring region  162  is technically less complex when the material from which the half-rings  94  and  96  are manufactured is a so-called ferromagnetic powder composite material (SMC, i.e. “soft magnetic composite”). In view of the fact that this material is at present very costly, the structures comprising the webs  158  and the ventilation channels  113  can be produced in a less complex manner, which will be discussed in more detail further below. 
         [0044]    As can already be seen from  FIG. 1 , the half-rings  94  and  96  are pushed onto the sleeve  52  with the stator windings  98  until they hit against the flange  49 . The half-rings  94  and  96  are in this case centered by a recess  176  ( FIG. 6A ). By virtue of two different form-fitting elements in the form of knobs  179  and corresponding depressions  180 , the half-rings  94  and  96  are centered with respect to one another. A pressure and centering ring  182  firstly results in an assembly comprising the half-rings  94  and  96  being centered around the sleeve  52  and a compressive force (generated by a tightened shaft nut  184 ) is applied to the SMC material without or virtually without a transverse force. A corresponding transverse force would be transmitted to the SMC material if the shaft nut  184  were to transmit the frictional force produced by it being tightened between itself and a body to be clamped directly to the SMC material. 
         [0045]    The rolling bearing  56  is inserted into the sleeve and secured by the inner securing ring  71 . The flange  31  has a recess  186  in the outer edge region on that side of said flange which is directed towards the stator  13 . This recess  186  serves the purpose of centering a housing shell  34  ( FIGS. 5 and 6A ). 
         [0046]    On its side directed towards the viewer, i.e. on the side pointing away from the flange  31 , the half-ring  96  has a groove  189 . This groove serves the purpose of being able to allow the casting compound  144  to flow between the two radial sides of the half-rings  94  and  96 .  FIG. 6B  shows a detail, in this case a section through two half-rings  94  and  96 . As can be seen in said figure, the two half-rings  94  and  96  have a notch  190  and  191 , respectively, whose profiles run radially inwards, are at right angles and supplement one another to form, overall, a rectangular overall profile. Special connecting parts of the stator winding  98  run in these notches  190  and  191 . Owing to the corresponding similarity, all of the half-rings  94  and  96  have a notch  190  and  191 , respectively. 
         [0047]      FIG. 7  shows a three-dimensional view of the stator  13 . As can already be seen from  FIG. 5 , the half-rings  94  and  96  have a further notch  194 , at which there are no webs  158 . The outputs  108 ,  109  and  110  are guided axially in the region of or in this notch  194  (see also  FIG. 1  and  FIG. 6A ). 
         [0048]    The claw poles  104  and  105  of each ring system  92  engage alternately in claw pole gaps  196  and claw pole gaps  198 , respectively, between the respective other claw poles. The claw pole gaps  196  are between claw poles  104 , and the claw pole gaps  198  are between the claw poles  105 . As can be seen from  FIG. 7 , a claw pole  104  of a ring system  92  bears against a claw pole  105  of another ring system  92 . The claw poles  104  and  105  of the three ring systems  92  are in this case arranged in such a way that undulating paths  200  and helical paths  201  are produced between the claw poles  104  and  105 . These paths  200  and  201  serve the purpose of allowing cooling air to pass through. 
         [0049]      FIG. 8  shows a further three-dimensional view of the stator  13 . The three connections  108 ,  109 ,  110  of the three stator windings  98  extend through an opening  203  in the flange  49  of the sleeve  52 . In each case one insulating layer  205  or  206 , which is produced from a polyamide film, for example, is located between the three connections  108 ,  109 ,  110 , i.e. between the connection  108  and  109  and between the connection  109  and  110 . As is also apparent in this regard from  FIG. 1 , functional sections of the connections  108 ,  109 ,  110  are of different lengths: measured from the first end face  209 , which is directed towards the flange  31 , those parts or sections of the connections  108 ,  109 ,  110  which are directed towards the stator windings  98  have approximately a ratio of 1:2:3 with respect to one another. That is to say that the corresponding section of the connection  108  is only approximately a third as long as the corresponding section of the connection  110 . On the other hand, those sections of the connections  108 ,  109 ,  110  which are directed towards the electronics  28  have a different ratio with respect to one another, for reasons of space. Thus, the lengths of the sections of the connections  108 ,  109 ,  110  from the end face  211  illustrated in  FIG. 6A  is approximately 3:5:3. That is to say that the section of the central connection  109  protrudes beyond the two other sections of the two other connections  108  and  110 . 
         [0050]    The insulating layer  205  extends at least from the outermost end face  213  of the connection  108 , said end face  213  pointing away from the stator  13 , as far as at least the outermost end face  215  of the connection  109 , said end face  215  pointing away from the end face  213  of the connection  108 . 
         [0051]    In more general terms, the insulating layer  205  extends between two directly adjacent connections  108  and  109  at least over the length which is between two end faces  213  and  215  pointing away from one another. 
         [0052]    The insulating layer  206  extends at least from the outermost end face  218  of the connection  110 , said end face  218  pointing away from the stator  13 , as far as at least the outermost end face  220  of the connection  110 , said end face  220  pointing away from the end face  218  of the connection  110 . 
         [0053]    In more general terms, the insulating layer  206  extends on or at the connection  110 , which makes contact with the stator winding  98 , which is furthest removed from the end face  209  of the ring system  92 , which is positioned closest to a connection side  217  of the stator  13 , over at least the entire axial length of the connection  110 . 
         [0054]    The three or the connections  108 ,  109 ,  110  are provided with a through-hole  224  on the side pointing away from the stator windings  98  at a point  222  which overall overlaps with respect to the connections  108 ,  109  and  110 . The insulating layers  205  and  206  are likewise perforated at this point  222 . A sleeve  225  consisting of an insulating material is plugged into the five holes. This sleeve  225  protrudes beyond the structure comprising the connections  108 ,  109 ,  110  and insulating layers  205  and  206  on both sides; see inter alia  FIG. 6A  and  FIG. 8 . The conductor rail  168  which is centered by the sleeve  225  rests on the upper side of this structure, i.e. on the connection  108 ; the conductor rail  166  centered by the sleeve  225  rests on the lower side of this structure, i.e. on the connection  110  ( FIG. 1  and  FIG. 4 ). Two insulating disks  227  resting thereon form bottom layers or top layers for a fastening means such as, for example, a screw and a nut, which are not illustrated here and press the structure with high contact stability. 
         [0055]      FIG. 9  illustrates the fan  86  illustrated in  FIG. 1  as an individual part in a three-dimensional view. This fan  86  is fastened on the inner side of the housing base  80  of the external rotor  16  by means of a few fastening elements. The fan  86  has a central opening  230 , whose diameter is greater than an outer diameter of the shaft nut  184  ( FIG. 1 ). 
         [0056]      FIG. 10  shows a view into the cavity of the external rotor  16 . The illustration clearly shows the fan blades  152  of the fan  86  and inner ends of the fan blades  164  of the fan  89 , which are prevented from bending radially outwards by a stabilizing ring  233 . The external rotor  16  is constructed from various component parts (see also  FIG. 11 ): the housing base  80  is pressed against the shoulder  69  of the shaft  22 . In the process, a screw  234  which is screwed into a shaft end presses a sleeve  235  against a stabilizing plate  237 , which in turn transmits the compressive force onto the housing base  80 . In order that the shaft  22 , the plate  237  and the housing base  80  can be fitted in the correct position with respect to one another, dowel pins  238  are inserted into bores in the shoulder  69 . The housing base  80 , the plate  237  and the sleeve  235  are positioned onto these dowel pins  238 . 
         [0057]    The permanent magnets  77  are fastened on the substantially cylindrical inner side. The fan  89 , which is produced from plastic, for example, is plugged on at that end  240  of the section  73  in the form of a cylinder lateral surface which is remote from the housing base  80 , by means of a snap-action connection. For this purpose, a ring section  243  which is integrally formed on the fan  89  engages around the cylindrical outer side of the section  73  in the form of a cylinder lateral surface. 
         [0058]      FIG. 12  shows a three-dimensional view of a stator winding  98 , in this case the stator winding  98  which is positioned closest to the connection side  217 . The stator winding  98  comprises precisely one turn  245 . However, the stator winding  98  is in this case in the form of a litz wire  244 , the litz wire  244  having a plurality of individual wires  245  ( FIG. 13A ). In accordance with a specific design, provision is made for the litz wire  244  to have 1000 wires with a diameter of in each case 0.2 mm. All of the individual wires  245  of the litz wire  244  are thus wound once (slightly less than 360°, i.e. not quite)360°. The individual wires  245 , i.e. each individual wire cross section consisting of copper, for example, of the litz wire  244  are also insulated with respect to one another by a layer of enamel as insulating layer  247 . Then, the litz wire  244  is additionally insulated at its outer circumference, with this being performed by banding  249 , for example. The litz wire  244  is first prepared in linear form. The individual wires  245  of the litz wire  244  are then all next to one another linearly. If it were then desired to wind such a linear banded litz wire  244 , this would result in considerable non-uniform changes in length and internal stresses (between the individual wires) over the cross section of the litz wire  244 . Alternatively, the individual wires  245  of the litz wire  244  can also be embodied without insulation  247 . Then, the possible disadvantage of relatively high current displacement stands against the possible advantage of relatively high copper cross section. In the exemplary embodiment described here, the litz wire  244  has a current displacement of 1.14 at 10 000 revolutions per minute. 
         [0059]    Following this step, the litz wire  244  is circumferentially compressed in the region of an intended abutment (the two ends of the litz wire  244  are opposite one another there) and the litz wire  244  is provided with two straight end faces. In the region of said end faces, the insulating layer  247  is removed and the wires are connected to one another by a solder. The stator winding  98  now has an open circular ring form of approximately 360°, with the stator winding  98  having two mutually opposite ends  250 ,  251 , the end  251  with a connection part  108  and the end  250  with a connection part  253  (eyelet connection) being cohesively connected to one another. In this case, an end  250 ,  251  is connected to a connection part  108 ,  253  in such a way that a rim  254  of a connection part  108 ,  253  surrounds the end ( FIG. 16 ). The connection part  108 ,  253  is in this case pressed against the end  250 ,  251  while a solder for the cohesive connection is still fluid. Furthermore, an insulating material  256 , for example an insulating plate, is introduced between the connection part  108  and the connection part  253  (eyelet connection) in order that no short circuit is produced between the connection part  108  and the eyelet connection  253 . Then, the stator winding  98  is banded. Preferably, in the process a neck section  258  is also banded. This neck section  258  comprises, both of the connection part  108  and the connection part  253  (eyelet connection), in each case one section which protrudes radially inwards. This neck section  258  protrudes into the notches  190  and  191  in the fitted machine (see also  FIG. 6B ). 
         [0060]    As part of the production method, a plurality of method steps are provided. First, a litz wire  244  is provided ( FIG. 17   a )). In a further step, the litz wire  244  is compacted, i.e. the litz wire  244  is provided approximately with a cross section which corresponds to the stator winding  98  ( FIG. 17   b )). By way of example, three different form cross sections after compacting or embossing of the litz wire  244  are illustrated in  FIG. 17   b ). During this method step, bundling of the litz wire  244  is expedient (possibly even by means of possibly only one banding) in order to avoid any movement of the wires, in particular in the following winding step. In  FIG. 17   c ) below, further embodiments of the litz wire  244  are illustrated, in which not only the actual turn section is embossed, but also both ends  246  which extend in addition in the axial direction. While  FIG. 17   c ) shows a litz wire  244  with a round cross section, the litz wire in  FIG. 17   d ) has a rectangular cross section.  FIG. 17   e ) illustrates a litz wire  244  or stator winding  98  which has been embossed with a rectangular form, with two laminations  248  as connections having been soldered or welded onto a radial outer side or the two ends  246  of the litz wire  244 .  FIG. 17   f ) illustrates a litz wire  244  or stator winding  98  which has been embossed with a rectangular form, with two laminations  248  as connections having been soldered or welded onto a radial inner side or the two ends  246  of the litz wire  244 .  FIG. 17   g ) illustrates a litz wire  244  or stator winding  98  which has been embossed with a rectangular form, with two laminations  248  as connections having been soldered or welded onto a radial outer side or the two ends  246  of the litz wire  244 . In this case, the litz wire  244  has been embossed in advance in such a way that a notch has been embossed into the annular cross section of the stator winding  98 , with the laminations  248  having been fitted into said notch. 
         [0061]    With reference to  FIG. 13A , in the exemplary embodiment a stator winding  98  has an unbanded cross section A 1  with a radial height H in the direction towards an axis of rotation of the external rotor  16  and an axial width B in the direction of the axis of rotation of the external rotor  16 . In the example, B is approximately 10 mm and H is approximately 7 mm. The unbanded cross section is therefore approximately 70 mm 2 . An individual wire of the litz wire  244  has a cross section A 2  of 0.1 2 *Π mm 2  and therefore approximately 0.0314 mm 2 . A ratio A 1 /A 2  is in this case approximately 2228. In the context of the design of the stator winding  98 , provision is made for the ratio in a first approximation to be less than 2500, and in a further approximation to be less than 2000 or less than 1500. 
         [0062]    As a further ratio, a quotient of the cross section A 1  and a circumference U of an individual wire of the litz wire  244  can be determined. A quotient A 1 /U of approximately  111  mm results from the example, where U is equal to the product of Π*0.2 mm. In a first approximation, it is desirable for the ratio A 1 /U to be greater than 40, and in a second approximation greater than 80, preferably greater than 120.  FIG. 13B  illustrates an alternative cross-sectional form for the stator winding  98 . This cross-sectional form is a total area (“house with pitched roof” form) comprising a rectangle as in  FIG. 13A  and a triangle on top. The triangle represents a gain with respect to the cross-sectional form in  FIG. 13A  resulting from optimized matching of the accommodating area radially beneath the claw poles  104  and  105 . In  FIG. 13C , a trapezoid is provided as further cross-sectional form of the stator winding  98  as a basic shape, with the sloping faces being oriented substantially in the axial direction. In addition, the trapezoidal form can in total be supplemented by a triangular cross-sectional area beneath the claw poles  104  and  105 . 
         [0063]    A further exemplary embodiment of a stator winding  98  is shown in  FIG. 18   a ). This stator winding  98 , in contrast to the previously described variant, is a stator winding  98  comprising a litz wire  244  with more than only one turn  245 . This has the advantage that the current displacement is further reduced. Furthermore, greater flexibility as regards the matching of the number of conductors in the stator winding  98  is provided. In addition, optimum use can be made of the so-called winding window. As already described in respect of  FIG. 17   a ), first a winding phase of litz wire  244  comprising a large number of insulated individual wires  245  ( FIG. 14 ) is provided. This winding phase is then insulated, for example by means of banding  249  ( FIG. 13A ). The winding phase is arranged in a plurality of turns  245  prior to or after the insulation is provided (see also  FIG. 18   a ) and  FIG. 18   b )) (forms in ring form, further exemplary embodiment). In  FIG. 18   a ), the turns  245  are wound or layered radially (axially in  FIG. 18   b )) one on top of the other.  FIG. 18   c ) is a schematic illustration showing individual steps. In step S 1 , the insulated litz wire  244  is first compressed in order to flatten the litz wire  244  (axial direction); preferably an inner diameter of the stator winding  98  or the litz wire  244  is already preset. Then, the flattened litz wire  244  is shaped (step S 2 ); possibly not only an outer diameter but also the inner diameter is adjusted or shaped. Then, the stator winding  98  or the litz wire  244  is embossed, with the result that the width B is also set (step S 3 ). Possibly, in a further step S 4 , a fixed structure is then produced, i.e. the stator winding  98  or the litz wire  244  is coated or impregnated with a preferably thermally curable resin (baked enamel), possibly heated in a form and thus a solid stator winding  98  is produced. 
         [0064]      FIG. 18   d ) shows a possible cross section through the stator winding  98  or the litz wire  244 , as is illustrated by the winding process shown in  FIG. 18   a ) and is produced after compacting or embossing. 
         [0065]      FIG. 18   e ) shows a possible cross section through the stator winding  98  or the litz wire  244 , as is illustrated by the winding process shown in  FIG. 18   b ) and is produced after compacting or embossing. 
         [0066]    A stator winding  98  for a transverse flux machine  10  is therefore disclosed, wherein the stator winding  98  is in the form of a litz wire  244  and the litz wire  244  has a plurality of individual wires  245 , the stator winding  98  being in the form of a coil with more than one turn  245 . Finally, prior to or following curing, connections are fitted to the stator winding  98  in one of the described ways. 
         [0067]    A method for producing a stator winding  98  comprising a litz wire  244  is thus disclosed, wherein first a litz wire strand is provided and, in later steps S 1 , S 2 , S 3 , the stator winding  98  is shaped into a ring form, insulated and a cross section of the stator winding is reshaped. Provision is made for more than only one turn to  245  to be wound in a circumferential direction. 
         [0068]    Provision is furthermore made for the stator winding  98  to be coated with a curable material, preferably resin or baked enamel, and later for this material to be cured. 
         [0069]      FIG. 19  illustrates three connection parts  253  of the three stator windings  98  arranged one after the other. In this case, these connection parts are the parts which act as eyelet connection. The three connection parts  253  are spaced apart from one another. In each case one metal bush  260  is located between two connection parts  253 . A screw bolt  262  of a screw  264  is plugged through the connection parts  253  and the bushes  260 . The connection parts  253  and the bushes  260  are braced with one another, with the result that an electrical connection is provided between connection parts  253  and bushes  260 . This arrangement is the neutral point of the three stator windings  98 . A transverse flux machine is therefore disclosed, wherein in each case one of the connections of the stator windings  98  has a hole  263  and these connections or one of the connection parts  253  of the stator windings  98  are arranged axially one behind the other in the direction of rotation of an external rotor  16 , these connection parts  253  being mechanically and electrically connected to one another by a bolt (screw bolt  262 ) positioned in the holes  263  and a neutral point thus being formed. Against the background of a generalization: this arrangement, either so as to form the neutral point or so as to pass out the connections  108 ,  109  and  110 , is independent of the selection of the embodiment of the stator winding  98 . It is merely important that, in order to form the neutral point, one end of a stator winding  98  is embodied with a connection part  253 , with preferably all of the stator windings  98  being embodied in such a way. In order to pass out the connections  108 ,  109  and  110  or to arrange said connections with respect to one another, provision is merely made for one end of the stator windings  98  to be connected to one of the connections  108 ,  109  and  110 . 
         [0070]    The text which follows will describe the cooling of the transverse flux machine  10 , which is described in  FIGS. 1 and 4  (with attached electronics). By virtue of a rotation of the external rotor  16  and therefore also of the fan  86 , a negative pressure is produced in the machine. This negative pressure results in air being transported radially outwards through the fan  86 , i.e. between the housing base  80  and the half-ring  96  of the ring system  92 , which is positioned closest to the housing base  80 . This cooling air is deflected by the external rotor  16  and, as shown in  FIG. 7 , is pressed between the claw poles  104  and  105  and therefore into an interspace  265  in the axial direction. The cooling air flows around all of the three ring systems  92  and is then pressed by the fan  89  radially outwards through the ventilation openings  90  into the surrounding environment. 
         [0071]    The negative pressure produced by the fan  86  means that a negative pressure is produced at that end of the ventilation channel  113  which is directly opposite the fan  86  and therefore cooling air then flows through the ventilation channel  113 . At that end of the ventilation channel(s)  113 , which is remote from the fan  86 , cooling air is sucked from the surrounding environment, for example in the region of the connections  108 ,  109  and  110 , through the flange  31  and therefore through the through-opening  149  ( FIG. 4 ). In addition, cooling air is sucked into the machine through openings  270  in the cover  25  in order first to cool the electronics  28  and then to flow through openings (not shown in  FIG. 1 ) in the cooling plate  127  to the through-opening  149  ( FIG. 2 ) and into the ventilation channels  113 . In addition, the fan  89  sucks additional cooling air for cooling the electronics  28  through the slots  142  and the groove  133  and openings (not shown) in the cooling plate  127 . 
         [0072]    A transverse flux machine  10  with a stator  13  and an external rotor  16 , which is arranged around the stator  13 , is thus disclosed, the stator  13  having two axial end sides  273 ,  276  remote from one another, with an inner yoke  107  of the stator  13 , with a cooling path which is arranged radially within the inner yoke  107 , the cooling path emerging from the transverse flux machine  10  on that axial end side  276  of the stator  13  which faces the inlet side, the cooling path running between the inlet and the outlet in an interspace between the stator  13  and the external rotor  16 . 
         [0073]      FIG. 20  shows a detail of a view of a variant of the stator  13 . In contrast to the previous variant, the half-rings  94 ,  96  are delimited radially inwards by the inner yoke  107 , i.e. the half-rings  94 ,  96  do not have any webs  158 . Instead, the half-rings  94 ,  96  have a central, preferably round opening  279 . A cooling rib element  280  consisting of a less expensive material such as an aluminum alloy, for example, is inserted into this opening  279 , i.e. adjacent to the inner yoke  107 , said cooling rib element  280  enabling heat emission from the half-rings  94 ,  96  by means of cooling ribs  283  and enabling centering of the half-rings  94 ,  96 , preferably by means of an inner ring  285 , on the sleeve  52 . The cooling rib element  280  can be an extruded profile, for example. 
         [0074]      FIG. 21  shows a sketch of a further exemplary embodiment of a transverse flux machine  10 . Identically functioning component parts are denoted by the same reference numerals. Thus, a three-phase stator  13  with three ring systems  92  is fastened on a housing inner wall  290  on a housing  19 . A shaft  22  is mounted both in the housing  19  and radially within the stator  13 , for which purpose the rolling bearings  55  and  56  are used. A supporting plate  293  is fastened with concomitant rotation on that end of the shaft  22  which is remote from the housing inner wall  290 . This supporting plate  293  bears fan blades  152  radially and axially on the outside. A section  73  in the form of a cylinder ring is mounted on that side of the supporting plate which is opposite the fan blades  152 . As was previously the case, permanent magnets  77  are likewise fastened in three rows on the cylindrical inner side of said section  73 , said permanent magnets magnetizing the ring systems  92  with their magnetic field. An end plate  296  between the shaft  22  and fan blades  152  serves to improve the fan efficiency. By virtue of rotation of the shaft, for example by means of a pulley (not illustrated) at the left-hand end of the shaft  22 , the fan  86  brings about a negative pressure at the outer edge of the fan  86 . A draught of air or cooling air is thus produced through the machine, said draught being described by the two long arrows, beginning at the cooling air inlet  300 . The cooling air therefore moves first from an inlet side  303  on one side of the stator  13  radially inwards in order to be deflected there in the axial direction (axis of rotation of the external rotor  16 ). Then, the cooling air flows past webs  158  in the axial direction in the interior of the stator  13 . Then, the cooling air emerges from that side of the stator  13  which is remote from the inlet side in order to be deflected radially outwards and to be passed out of the machine by the fan blades  152 . 
         [0075]    A transverse flux machine with a stator  13  and an external rotor  16  which is arranged around the stator  13  is thus disclosed, the stator  13  having two axial end sides  273 ,  276 , with an inner yoke  107  of the stator  13 , with a cooling path, which is arranged radially within the inner yoke  107 , the cooling path emerging from the transverse flux machine  10  on that axial end side  273  of the stator  13  which is remote from an inlet side  303 .