Abstract:
A synchronous machine ( 10 ), in particular, for a starter-generator of an internal combustion engine, includes a stator, a rotor ( 22 ) having a rotor shaft ( 24 ), a stack of sheets ( 28 ), and squirrel cage ( 30 ) non-rotatably connected with the rotor shaft ( 24 ) and the stack of sheets ( 28 ). The squirrel cage ( 30 ) has a short circuit ring ( 36 ) on its opposite front ends, which is secured to an annular reinforcement element ( 40, 42 ). The reinforcement elements ( 40, 42 ) do not overlap adjacent, outer circumferential surfaces of the squirrel cage ( 32 ) or the short circuit rings ( 36 ).

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an asynchronous machine for a starter-generator of an internal combustion engine. 
   Asynchronous machines of this type are of interest for many uses, such as, for example, machine tools, electric tools, or in the motor vehicle sector for hybrid drives, electrically-driven turbochargers, or starter-generators for internal combustion engines, which must be designed partly for very high maximum engine speeds. 
   If an asynchronous machine with a short circuit rotor is used as a starter-generator, in which the short circuit cage, or “squirrel cage” of the rotor is made as one-piece from aluminum die casting, the standards based on performance and efficiency frequently are not fulfilled, on account of which pure copper is used not infrequently as the cage material. This squirrel cage made of pure copper comprises generally cold-formed copper bars, which are inserted in receiving grooves of the stack of sheets of the rotor and are hard-soldered on their front ends, respectively, with one of two short circuit rings, which generally are either cast from copper or are stacked from multiple stamped copper sheets. As a result of the soldering of the short circuit bars with the short circuit rings, however, the physical properties of the copper material that is utilized are drastically decreased in the area of the solder points and around these points. As a result, with high speeds, the elastic limit of the material is exceeded, which leads to plastic deformation. This deformation takes the form of bowing out or warping, which begins in the inner diameter of the short circuit rings, and in extreme cases, can lead to tearing of the short circuit rings and to destruction of the asynchronous machine. 
   In order to make the rotors sufficiently speed-fixed, additional features are utilized, such as, for example, the mounting of so-called reinforcement rings made from a metal with high resistance, which encompasses the adjacent short circuit ring on its front side facing away from the stack of sheets, so that it outwardly supports the short circuit ring and absorbs a portion of the centrifugal forces acting on the short circuit ring with high speeds. 
   The Applicants&#39; DE 199 55 050 A1 disdose such a reinforcement ring, which is attached with screws on the stack of sheets and has a circumferential groove open on one side in the axial direction, which accommodates the front end of the squirrel cage overlying the adjacent face surface of the stack of sheets with the short circuit ring. If this type of reinforcement ring is made from sheet metal, which has magnetic properties, this leads to a good magnetic inference for the stray field on the front end of the rotor, which results in an improper enlargement of the rotor dispersion and therewith, associated power losses of the asynchronous machine. Basically, these disadvantages could be avoided if a non-magnetic type of steel is used when manufacturing this type of reinforcement ring. However, this leads to significantly higher material costs and makes more difficult the mechanical machining of the reinforcement ring. 
   SUMMARY OF THE INVENTION 
   In contrast, the asynchronous machine of the present invention offers the advantage that with the both reinforcement elements opening to an air gap, a magnetic inference for the stray field on the front ends of the rotor can be avoided, so that the reinforcement elements can be manufactured from cost-effective, magnetic steel that is easier to machine, but which nevertheless enables sufficient protection against centrifugal force-related deformation of the short circuit rings. Stability analyses surprisingly have provided that the highest material demands occur on the inner diameter of the short circuit rings, which concurs with the observed forms of damage with warping or bending out of the short circuit rings beginning at the inner diameter. In order to prevent plastic deformation of the short circuit rings on these points of the highest material demand, according to the analyses of the present invention, it suffices that the reinforcement elements are mounted only in the area of the face surfaces of the short circuit rings and serve purely as axially supports of the short circuit rings. 
   In a preferred form of the present invention, it is provided that the reinforcement elements are flush on their outer circumference with adjacent, outer circumferential surfaces of the short circuit rings, while they can overlie on their inner circumference adjacent, inner circumferential surfaces of the short circuit rings, so that the entire face surface of the short circuit rings and in particular, the area of the highest material demand, is supported by the respective reinforcement element, without interfering with insertion of the rotor in the housing through the outer circumference of the front reinforcement ring. 
   According to a further preferred embodiment of the invention, the short circuit rings or squirrel cage are braced between the reinforcement elements, in which at least one of the reinforcement elements is pressed in the axial direction against the adjacent short circuit ring, so that also with high speeds, it cannot be pressed away in the event the yielding point of the copper in the short circuit ring should be exceeded in the area of the solder points. 
   This axial pressing of the reinforcement element can be ensured, for example, in that the reinforcement element between the short circuit bars of the squirrel cage is screwed, riveted, or welded with the stack of sheets of the rotor, or is shrunken on or pressed on or otherwise fixedly connected with the rotor stack of sheets. This also makes possible a balancing of the rotor by selective material removal of the reinforcement elements, since these are coupled non-rotatably with the stack of sheets by the fixed connection. 
   Advantageously, the two reinforcement elements additionally serve for axially fixing the stack of sheets and the squirrel cage on the rotor shaft. The reinforcement elements contacting the front sides of the short circuit ring, after sliding on the stack of sheets and the squirrel case onto the rotor shaft, perferably support the front element against an annular shoulder of the rotor shaft, while the rear element is fitted by shrinking or pressing on the rotor shaft. 
   In order to facilitate the manufacture of the reinforcement elements and to minimize the material costs as much as possible, the reinforcement elements perferably are made from commercial magnetic steel, whose tensile strength and bending strength exceed that of the copper to a sufficient extent. 
   BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1  shows a longitudinal sectional view of an asynchronous machine. 

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The asynchronous machine  10  shown in FIG.  1  and embodied as a short circuit motor or squirrel cage motor comprises essentially a stationary housing  12 , a support or stator  14  with a stator or support stack of sheets  16  and a stator or support coil  18 , which supplies a three-phase rotating current via a connection cable  20 . In the housing  12 , an armature of the rotor  22  is rotatably supported, which essentially comprises a rotor shaft  24  overlying a front housing end, a stack of sheets  28  non-rotatably connected with the rotor shaft  24  by means of a fit-in key  26 , as well as a squirrel cage  30  forming the armature coil. The stator  14  and the rotor  22  are separated by an air gap L. 
   The squirrel cage  30  of the rotor  22  comprises a plurality of short circuit bars  32 , which penetrate the axially oriented grooves (not visible) of the armature stack of sheets  28  and which are fixedly connected on their opposite front ends overlying the front faces  34  of the stack of sheets  28  to a short circuit ring  36 . The short circuit bars  32  comprise cold-formed copper and are inserted with their front ends in axial recesses of copper rings  36  cast from copper or formed in layers from stamped copper sheets, before they are connected with them by hard-soldering. Both short circuit rings  35  rest against the adjacent front surfaces  34  of the stack of sheets  28  and are arranged with their inner circumferential surfaces  38  at a radial distance from the adjacent circumferential surface of the rotor shaft  24 . Since the physical properties of the copper material of the short circuit bars  32  and the short circuit rings  36  are drastically impaired as a result of the solders in the area of the solder points  39 , the rotor  22  is provided with two reinforcement elements in the form of contact or pressure discs  40 ,  42 , which are pressed from opposite sides against the flat front surfaces, which face away from one another, of the two short circuit rings  36 . Both contact discs  40 ,  42  are limited in the axial direction by flat front surfaces, while they are limited in the radial direction by inner, cylindrical circumferential surfaces resting against the rotor shaft  24  or an outer, cylindrical circumferential surface that is flush with the outer circumferential surfaces of the short circuit rings  36  and the stack of sheets  28 . 
   Both contact disks  40 ,  42  overlie somewhat radially inward the inner circumferential surface of the adjacent short circuit ring  36  and are made from common, non-alloyed, magnetic steel. Since they do not overlap the two short circuit rings  36  outwardly in the radial direction, a magnetic inference from the stray field on the front end of the rotor  22  can be avoided, in spite of the use of this type of material, and therewith, an enlargement of the rotor dispersion and the power loss of the asynchronous machine  10  connected therewith can be prevented. 
   While the front contact disk  40  adjacent to a front housing end is placed with play on the rotor shaft  24  until it braces against an annular shoulder  46  of the rotor shaft  35  behind the sheet stack  28  in the area of a front rotational bearing  44  of the rotor shaft  24 , formed as a fixed bearing, the rear contact disk  42  is mounted non-rotatably and axially non-displaceable on the rotor shaft  24 , whereby it presses the stack of sheets  28  with a prescribed pressing force in the direction of the front contact disk  40 , so that it is braced between the two contact disks  40 ,  42 . The mounting of the rear contact disk  42  can take place by means of pressing on or shrinking on onto the rotor shaft, whereby in the latter case, the contact disk  42  is re-pressed after cooling off with a prescribed pressing force axially in the direction of the contact disk  40 , in order to provide sufficient bracing of the stack of sheets  28 . By the play-free arrangement of the two contact disks  40 ,  42  against the respective, adjacent front surface of the squirrel cage  30 , the stack of sheets  28  is fixed with the squirrel cage  30  non-displaceably in the axial direction with reference to the fit-in key  26 . 
   In order to permit a secure seating of the rear contact disk  42 , this disk  42  has an axially overlying ring-shaped projection  50  with a smaller outer diameter for enlargement of the contact surface between its inner circumferential surface and the circumferential surface of the rotor shaft  24  on its rear front side. 
   The front and rear contact disks  40 ,  42  can be screwed, riveted, or welded to the stack of sheets  28 , so that they can be mounted together with the stack of sheets  28  and the squirrel cage  30  onto the rotor shaft  24 . In this case, the front contact disk  40  is provided on its inner circumference at one point with a receptacle  52 , so that it can be shifted over the fit-in key  26 , which before the mounting of the stack of sheets  28  and the squirrel cage  30 , is inserted in an axial groove  54  of the rotor shaft  24  and is attached with a screw  56  in the front and back. 
   In addition, a fixed, non-rotatable connection between the contact disks  40 ,  42  on the one hand, and the stack of sheets  28  with the squirrel cage  30  on the other hand, has the advantage that the rotor  22  can be balanced before insertion into the housing  12  by means of selective material reduction MA on the front sides of the contact disks  40  facing away from one another, which is shown in dashed lines. With the rear contact disk  42 , the material reduction takes place on a part  58  of the contact disk  42  overlying the ring-shaped projection  50  outwardly in the radial direction. 
   The front rotational bearing  44  for the rotor  22  is formed from a bearing assembly comprising two roller bearings  60 ,  62  in the area of the front end of the rotor shaft  24 , while a rear rotational bearing  64 , formed as a floating bearing, is made up of a single roller bearing  66  on the back end of the rotor shaft  24 . The two roller bearings  60 ,  62 , placed from the forward direction onto the rotor shaft  24  are secured by a safety ring  68  against an annular shoulder  70  of the rotor shaft  24  resting on the rotor shaft  24 . A retaining ring  71  screwed with attachment screws  69  from the inside to the housing  12  serves as a reverse fixing of the fixed bearing  60 ,  62 . The roller bearing  66  is pressed from behind until contacting an annular shoulder  72  of the rotor shaft  24  on its back end after the mounting of the stack of sheets  28  with the squirrel cage  30 . 
   After the balancing of the rotor  22 , the rotor is inserted with the front roller bearings  60 ,  62  and the rear roller bearing  66  into the housing  12 , until the outer ring of the front-most roller bearing rests again an annular shoulder  74  of the housing  12 . Subsequently, the open back end of the housing  12  is closed by a bearing cover  76 , which is provided on its inner side with a cylindrical seat  78  for the roller bearing  66 . To balance a different heat strain of the housing  12  and the rotor shaft  24 , a corrugated spring washer (not shown) is provided between the outer ring of the back roller bearing  66  and the bearing cover  76  within the seat  78 . After feeding through the connection cable  20  through a side opening  80  between the housing  12  and the bearing cover  76 , the latter is screwed tightly with multiple attachment screws  82  on an adjacent, overlying annular flange  84  of the housing  12 . 
   It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above. 
   While the invention has been illustrated and described herein as a lubricating device with pressure equalization, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
   Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention. 
   What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.