Patent Publication Number: US-2021184551-A1

Title: Squirrel-cage rotor and associated asynchronous electrical machine

Description:
The present invention concerns squirrel-cage asynchronous rotating electrical machines and relates more particularly to a device for retaining the short-circuit rings incorporated in a rotor of the machine. 
     The present invention further relates to a rotating electrical machine comprising such a rotor. 
     In general, the rotor of an asynchronous rotating electrical machine operating at peripheral speeds of less than 200 m/s comprises short-circuit rings connected to conductive bars inserted into the magnetic mass of the rotor to form a squirrel cage, the short-circuit rings and the conductive bars generally being made of copper or copper alloy. 
     Reference is made to  FIG. 1 , which shows a partial view of a rotor  1  comprising a magnetic mass  2  clamping a shaft  3 . The magnetic mass  2  comprises magnetic sheets  4  clamped between clamping plates  5  and conductive bars  6 . 
     The rotor  1  further comprises a short-circuit ring  7  in contact or not in contact with the face of the clamping plate  5  opposite that in contact with the magnetic sheets  4 . 
     The conductive bar  6  is inserted into the short circuit ring  7  to hold the ring  7  and to form a squirrel cage. 
     A retaining ring  8  binds the short-circuit ring  7  to prevent the short-circuit ring from being projected outside of the rotor  1  under the effect of centrifugal force. 
     The retaining ring  8  is generally made of non-magnetic steel to prevent the retaining ring from heating up under the effect of the magnetic fields generated by stator coils. 
     The retaining ring  8  is generally made of stainless steel. 
     Reference can also be made to the European patent document EP2149970 which describes such a rotor. 
     However, stainless steel is an expensive material that is difficult to machine. 
     Stainless steel has very low non-magnetic properties and high mechanical properties, in particular a high yield strength, such that the stainless steel retaining ring  8  retains the short-circuit ring  7  which is far heavier under the effect of centrifugal force than the retaining ring  8 . 
     The radial thickness of the retaining ring  8  is thinner than the radial thickness of the short-circuit ring  7  to allow the short-circuit ring to have sufficient radial thickness for the passage of an electric current originating from the conductive bars  6 . 
     Reference can be made to the patent documents U.S. Pat. No. 7,919,895 and EP2866335, which describe a clamping plate comprising a recess for housing the short-circuit ring. 
     To improve the loopback of the flux paths, the conductive bars are disposed as close as possible to the peripheral surface of the magnetic mass. Since the conductive bars are in contact with the short-circuit discs, the radial outer thickness of material available to hold the short-circuit discs under the effect of centrifugal force in a radial direction is reduced. 
     As a result, the peripheral speed of the rotor is limited to avoid breakage of the material keeping only the short-circuit rings on the outer diameter thereof under the effect of centrifugal force. 
     Moreover, the short-circuit rings do not include a retaining shoulder under the conductive bars. 
     The patent documents EP2849320 and U.S. Pat. No. 9,935,533 disclose short-circuit rings held by pins engaged in a part acting as a mould when the short-circuit ring is cast by a casting process. 
     However, the holding surface of the rings is limited, reducing the peripheral speed of the rotor to prevent the pins from breaking by shearing. 
     The international patent document WO2016055199 discloses short-circuit rings held by metal rods passing through the magnetic mass. 
     However, the through-hole in the rod weakens the magnetic mass. 
     Moreover, the radial holding surface of the short-circuit ring is limited to prevent the rod from breaking by shearing and bending in the radial direction under the effect of centrifugal force. 
     Reference can be made to the international patent document WO2015188985 which discloses a rotor comprising a disc clamping a shaft. 
     The short-circuit ring is mounted on the disc. 
     However, under the effect of centrifugal force, the short-circuit ring is not held. 
     The patent documents WO2014124762, WO2016055186 and U.S. Pat. No. 9,130,434 disclose short-circuit rings held by tabs. 
     However, the peripheral speed of the rotor is limited so as not to damage the holding tabs by shearing and bending under the effect of centrifugal force. 
     The devices for holding the short-circuit rings known in the prior art are adapted for peripheral rotor speeds of about 110 m/s. 
     The invention thus proposes overcoming the drawbacks of the rotors for a squirrel-cage asynchronous rotating electrical machine according to the prior art, in particular by increasing the peripheral speed of the rotor without using a non-magnetic steel retaining ring. 
     In light of the above, the invention proposes a rotor for a squirrel-cage asynchronous rotating electrical machine comprising two compaction elements clamping a cylindrical magnetic mass, short-circuit rings facing the face of the compaction elements opposite that in contact with the magnetic mass, and conductive bars housed in recesses in the magnetic mass and distributed evenly over at least one diameter of the magnetic mass such that the short-circuit rings and the conductive bars form a squirrel cage. 
     Retaining means distributed over at least one diameter of each short-circuit ring and over at least one diameter of each compaction element interact so as to secure the short-circuit rings and the compaction elements together, the pitch circle diameters of the retaining means on the rings and the compaction elements being smaller than the pitch circle diameter of the conductive bars. 
     According to one feature, the retaining means comprise a groove in the at least one diameter of the compaction element, a lug on the at least one diameter of the short-circuit ring such that the lug fits into the groove to form a shoulder, and screws evenly distributed over at least one diameter of the short-circuit ring to secure the short-circuit ring and the compaction element together. 
     Preferably, the retaining means further comprise a second groove in a second diameter of the compaction element and a second lug on a second diameter of the short-circuit ring such that the second lug fits into the second groove to form a second shoulder, the second diameters being smaller than the first diameters. 
     Advantageously, the rotor further comprises screws distributed over a second diameter of the short-circuit ring, the screws on said second diameter passing through the lug. 
     According to another feature, the retaining means comprise a groove in the at least one diameter of the short-circuit ring, a lug on the at least one diameter of the compaction element such that the lug fits into the groove to form a shoulder, and screws evenly distributed over at least one diameter of the short-circuit ring to secure the short-circuit ring and the compaction element together. 
     Preferably, the retaining means further comprise a second groove in a second diameter of the compaction element such that one end of the short-circuit ring fits into the second groove to form a second shoulder, the second diameter being smaller than the first diameter. 
     Advantageously, holes are evenly distributed over a diameter of the short-circuit ring to house the conductive bars, the pitch circle diameter of the bars in the short-circuit ring being smaller than the pitch circle diameter of the conductive bars in the magnetic mass in order to produce a radial bending preload of the conductive bars. 
     Preferably, holes, advantageously circular holes, are evenly distributed over a diameter of the short-circuit ring to house the conductive bars in the magnetic mass, the holes being coaxial with the recesses of the conductive bars, the holes having a dimension that is smaller than a dimension of the conductive bars such that when the bars are inserted into the holes, a knurled end of each conductive bar deforms to create an electrical contact between said bar and the short-circuit ring. 
     Advantageously, the retaining means at each end of the rotor are of different types. 
     Preferably, the retaining means comprise a groove in the at least one diameter of the short-circuit ring, a retaining ring comprising, on at least one diameter, a lug such that the lug fits into the groove to form a shoulder and such that the face of the retaining ring opposite that facing the short-circuit ring is in contact with the compaction element, and screws distributed evenly over at least one diameter of the retaining ring to secure the retaining ring and the compaction element together. 
     According to another feature, the retaining means comprise a retaining ring secured to the compaction element by screws distributed evenly over at least one diameter of the retaining ring and comprising a groove in at least one diameter, the short-circuit ring comprising a lug on at least one diameter located on the face opposite that facing the compaction element such that the lug fits into the groove so as to hold the short-circuit ring. 
     Preferably, the short-circuit ring further comprises a second retaining lug opposite the retaining lug and the compaction element comprises a second groove interacting with the second retaining lug. 
     Advantageously, the retaining means further include a lug on a diameter of the retaining ring which fits into a groove in the compaction element so as to form a shoulder, the lug being located between the axis of rotation of the rotor and the pitch circle diameter of the fixing screws. Preferably, the retaining means further comprise a second groove in at least one diameter of the retaining ring and a second lug on a second diameter of the short-circuit ring located on the face opposite that facing the compaction element such that the second lug fits into the second groove so as to hold the short-circuit ring. 
     Advantageously, the retaining means comprise a retaining ring secured to the compaction element by screws distributed evenly over at least one diameter of the retaining ring and comprising a groove in at least one diameter, the short-circuit ring comprising a groove in at least one diameter located on the face facing that of the compaction element, and a holding ring comprising a face in contact with the compaction element and a groove in the face opposite that in contact with the compaction element such that the ends of the holding ring fit into the grooves of the short-circuit ring and of the retaining ring. 
     Preferably, the ends of the conductive bars are brazed on the short-circuit rings. 
     According to another feature, the rotor further comprises a binding band surrounding the short-circuit ring, the binding band preferably being non-magnetic, for example made of stainless steel. 
     Advantageously, electrical insulation means are disposed under the head of the screw and/or along the screw body and/or between the short-circuit ring and the compaction element. 
     Preferably, the compaction element comprises a clamping plate or a compaction flange of a non-through half-shaft. 
     According to yet another aspect, the invention proposes a squirrel-cage asynchronous rotating electrical machine comprising a rotor as described hereinabove. 
    
    
     
       Other features and advantages of the invention will be better understood upon reading the following description given of embodiments of the invention, provided as non-limiting examples and with reference to the drawings, in which: 
         FIG. 1 , which has already been mentioned, shows a rotor comprising a retaining ring according to the prior art; 
         FIG. 2  shows one embodiment of an asynchronous rotating electrical machine; 
         FIGS. 3 and 4  show a partial sectional view along an axial direction of a first embodiment of the rotor; 
         FIG. 5  shows a partial sectional view along an axial direction of a second embodiment of the rotor; 
         FIG. 6  shows a partial sectional view along an axial direction of a third embodiment of the rotor; 
         FIG. 7  shows a partial sectional view along an axial direction of a fourth embodiment of the retaining means; 
         FIG. 8  shows a partial sectional view along an axial direction of a fifth embodiment of the retaining means; 
         FIG. 9  shows a partial sectional view along an axial direction of a sixth embodiment of the retaining means; 
         FIG. 10  shows a partial sectional view along an axial direction of a seventh embodiment of the retaining means; 
         FIG. 11  shows a partial sectional view along an axial direction of an eighth embodiment of the retaining means; 
         FIG. 12  shows a partial sectional view along an axial direction of a ninth embodiment of the retaining means; 
         FIG. 13  shows a partial sectional view along an axial direction of a tenth embodiment of the retaining means; 
         FIG. 14  shows a partial sectional view along an axial direction of an eleventh embodiment of the retaining means; 
         FIG. 15  shows a partial sectional view along an axial direction of a twelfth embodiment of the retaining means; 
         FIG. 16  shows a partial sectional view along an axial direction of a thirteenth embodiment of the retaining means; 
         FIG. 17  shows a partial sectional view along an axial direction of a fourteenth embodiment of the retaining means; 
         FIG. 18  shows a partial sectional view along an axial direction of a fifteenth embodiment of the retaining means; 
         FIG. 19  shows a partial sectional view along an axial direction of a sixteenth embodiment of the retaining means; and 
         FIG. 20  shows a partial sectional view along an axial direction of a seventeenth embodiment of the retaining means. 
     
    
    
     Reference is made to  FIG. 2 , which shows one embodiment of an asynchronous rotating electrical machine  9  comprising a stator  10 , bearings  11  and a rotor  12  inserted into the stator  10  and the bearings  11 . 
     The rotor  12  comprises a rotor shaft  13  made, for example, of steel, having an axis (A) that is coincident with the rotational axis of the rotor  12 . 
     Reference is made to  FIGS. 3 and 4 , which show a partial view of a first embodiment of the rotor  12  and a partial sectional view along an axial direction of the rotor. 
     The rotor  12  comprises a cylindrical magnetic mass  14  clamped between two compaction elements comprising compaction plates  15 , and short-circuit rings  16  in contact with the face of the compaction plates  15  opposite that in contact with the magnetic mass  14  comprising laminated magnetic sheets  18 . 
     The thickness of the magnetic sheets  18  is preferably less than 2 mm, preferentially 0.65 mm or 0.5 mm. 
     Alternatively, the magnetic mass  14  can comprise metal plates, the thickness of the metal plates preferably being more than 5% of the outer diameter of the magnetic mass  14 . 
     According to yet another alternative embodiment, the magnetic mass  14  can comprise a one-piece steel body. 
     The shaft  13  passes through the magnetic mass  14 , the compaction plates  15  and the short-circuit rings  16 . 
     Conductive bars  17  are housed inside recesses in the magnetic mass and are evenly distributed over at least one diameter d 17  of the magnetic mass such that the short-circuit rings  16  and the conductive bars  17  form a squirrel cage. 
     The short-circuit rings  16  and the conductive bars  17  are made, for example, of copper or copper alloy. 
     The short-circuit ring  16  comprises a hole  16   a  coaxial with the recess for the conductive bar  17  such that the bar  17  is inserted into the ring  16 . 
     The rotor  12  further comprises retaining means comprising a groove  19  in a diameter d 19  of the compaction plate  15 , a lug  20  on a diameter d 20  of the short-circuit ring  16  such that the lug  20  fits into the groove  19  to form a shoulder, and screws  21  evenly distributed over a diameter d 21  of the short-circuit ring  16  to secure the short-circuit ring  16  and the compaction plate  15  together. 
     The pitch circle diameters d 20  of the lug  20  and d 19  of the groove  19  are smaller than the pitch circle diameter d 17  of the conductive bars  17 . 
     The screws  21  are located between the lug  20  and the pitch circle diameter of the conductive bars. 
     Reference is made to  FIG. 5 , which shows a partial sectional view along an axial direction of a second embodiment of the rotor  12 . 
     The rotor  12  with a non-through shaft comprises two half-shafts  22  comprising compaction elements comprising compaction flanges  23  clamping a cylindrical magnetic mass  24  and the short-circuit rings  16  in contact with the face of the compaction flanges  23  opposite that in contact with the magnetic mass  24  comprising laminated magnetic sheets  25 . 
     The thickness of the magnetic sheets  25  is preferably less than 2 mm, preferentially 0.65 mm or 0.5 mm. 
     Alternatively, the magnetic mass  24  can comprise metal plates, the thickness of the metal plates preferably being more than 5% of the outer diameter of the magnetic mass  24 . 
     According to yet another alternative embodiment, the magnetic mass  24  can comprise a one-piece steel body. 
     The conductive bars  17  are housed inside recesses of the magnetic mass  24  and are evenly distributed over a diameter d 17   a  of the magnetic mass  24  such that the short-circuit rings  16  and the conductive bars  17  form a squirrel cage. 
     The conductive bar  17  is inserted into the hole  16   a  in the short-circuit ring  16 . 
     The magnetic mass  24  further comprises tie rods  26  connecting the two half-shafts to keep the magnetic mass  24  compacted. 
     The tie rods  26  are evenly distributed over a diameter d 26  of the magnetic mass  24 . 
     The rotor  12  further comprises retaining means comprising a groove  27  in a diameter d 27  of the compaction flange  23 , a lug  28  on a diameter d 28  of the short-circuit ring  16  such that the lug  28  fits into the groove  27  to form a shoulder, and screws  29  evenly distributed over a diameter d 29  of the short-circuit ring  16  to secure the short-circuit ring  16  and the compaction flange  23  together. 
     The pitch circle diameters d 28  of the lug  28  and d 27  of the groove  27  are smaller than the pitch circle diameter d 17   a  of the conductive bars  17 . The pitch circle diameter d 26  of the tie rods  26  is smaller than the pitch circle diameter of the retaining means. 
     The screws  29  are located between the lug  28  and the pitch circle diameter of the conductive bars. 
     The conductive bars  17  are preferably inserted with radial clearance into the holes  16   a  so as to allow for the free axial thermal expansion of the conductive bars. 
       FIG. 6  shows a partial sectional view along an axial direction of a third embodiment of the rotor  12  comprising the short-circuit ring  16  and a compaction element  30 . 
     The compaction element  30  can comprise the compaction plate  15  if the rotor  12  has a through-shaft or the compaction flange  23  if the rotor  12  has a non-through shaft or one end of the metal body if the magnetic mass of the rotor  12  is made in one piece. 
     If the rotor  12  is made in one piece, i.e. the shaft and the magnetic mass are a single unit, the compaction element  30  comprises an end of the magnetic mass. 
     The third embodiment of the rotor  12  further comprises a second embodiment of the retaining means. 
     The retaining means comprise a groove  31  in a diameter of the compaction element  30 , a lug  32  on a diameter of the short-circuit ring  16  such that the lug  32  fits into the groove  31  to form a shoulder, and screws  33  evenly distributed over a diameter of the short-circuit ring  16  to secure the short-circuit ring  16  and the compaction element  30  together. 
     The screws  33  pass through the lug  32 . 
     The pitch circle diameters of the lug  32  and of the groove  31  are smaller than the pitch circle diameter of the conductive bars  17 . 
     In the embodiments of the retaining means described hereinabove, the lug and the groove have a rectangular cross-section. 
     The cross-section of the lug and of the groove can take various shapes, in particular a trapezoidal shape as shown in  FIG. 7  showing a partial sectional view along an axial direction of a fourth embodiment of the retaining means. 
     It shows the short-circuit ring  16  and the compaction element  30 . 
     The retaining means comprise a groove  34  having a trapezoidal cross-section in a diameter of the compaction element  30 , a lug  35  having a trapezoidal cross-section on a diameter of the short-circuit ring  16  such that the lug  35  fits into the groove  34  to form a shoulder, and screws  36  evenly distributed over a diameter of the short-circuit ring  16  to secure the short-circuit ring  16  and the compaction element  30  together. 
     The pitch circle diameters of the lug  35  and of the groove  34  are smaller than the pitch circle diameter of the conductive bars  17 . 
     The trapezoidal cross-section of the lugs  35  and groove  34  procure self-centring of the lug  35  in the groove  34  when assembling the short-circuit ring  16  in the compaction element  30 . 
     The screws  36  are located between the lug  35  and the pitch circle diameter of the conductive bars. 
     In an alternative embodiment not shown, the screws  36  pass through the lug  35 . 
       FIG. 8  shows a partial sectional view along an axial direction of a fifth embodiment of the retaining means. 
     It shows the short-circuit ring  16  and the compaction element  30 . 
     The retaining means comprise a groove  37  in a diameter of the compaction element  30 , a lug  38  on a diameter of the short-circuit ring  16  such that the lug  38  fits into the groove  37  to form a shoulder, and screws  39  evenly distributed over a diameter of the short-circuit ring  16  to secure the short-circuit ring  16  and the compaction element  30  together. 
     The screws  39  are located between the lug  38  and the pitch circle diameter of the conductive bars. 
     Each head of the screws  39  is housed in a counterbore  40  in the short-circuit ring  16  such that the head of the screw is held in a radial direction under the effect of centrifugal force. 
     The retaining means further include screws  41  distributed over a second diameter of the short-circuit ring  16 . The screws  41  distributed over the second diameter pass through the lug  38  and are housed in counterbores in the short-circuit ring in order to be radially held under the effect of centrifugal force. 
     The short-circuit ring  16  is held by two rows of screws  39  and  41  distributed over different diameters increasing the holding pressure of the ring  16  against the compaction element  30  compared to the embodiments of the retaining means described hereinabove. 
     The short-circuit ring  16  can comprise a hollowing  42  on the surface thereof in contact with the element  30  which is located on a larger diameter than the lug  38  so as to increase the contact pressure of the lug  38  in the bottom of the groove  37 . 
     It goes without saying that the embodiments described hereinabove can be combined, the embodiments described in  FIGS. 4, 5, 6 and 7  can further include screws distributed over a second diameter of the short-circuit ring and/or a hollowing  42  as shown in  FIG. 8 , and the embodiments shown in  FIGS. 4, 5, 6 and 8  can include a trapezoidal cross-section as shown in  FIG. 7 .  FIG. 9  shows a partial sectional view along an axial direction of a sixth embodiment of the retaining means. 
     It shows the short-circuit ring  16  comprising the lug  35 , the compaction element  30  comprising the groove  34  and the screws  36  according to the fourth embodiment of the retaining means described in  FIG. 7 . 
     The retaining means further comprise a second groove  42  in a second diameter of the compaction element  30  and a second lug  43  on a second diameter of the short-circuit ring  16  such that the second lug  43  fits into the second groove  42  to form a second shoulder, the second diameters being smaller than the first diameters. 
     The short-circuit ring  16  is held in the compaction element  30  by two shoulders increasing the radial holding of the short-circuit ring  16  under the effect of centrifugal force. 
     Moreover, since the ring  16  is held by two shoulders, the depth of the grooves  34  and  42  can be reduced compared to the embodiments describing a single groove in order to increase the stiffness of the compaction element  30 . 
     It goes without saying that the retaining means can comprise more than two shoulders. 
     The shoulders have, for example, a trapezoidal or rectangular cross-section. 
     In the embodiments described hereinabove, the compaction element  15 ,  23 ,  30  has the same outer diameter as the outer diameter of the short-circuit ring  16 . 
     According to other embodiments, the outer diameter of the compaction element  15 ,  23 ,  30  is smaller than the outer diameter of the short-circuit ring  16 . 
       FIG. 10  shows a partial sectional view along an axial direction of a seventh embodiment of the retaining means. 
     It shows the short-circuit ring  16  and a compaction element  44  which differs from the compaction element  30  in that the outer diameter d 44  thereof is smaller than the outer diameter of the short-circuit ring  16 . 
     The retaining means comprise a groove  45  in a diameter of the short-circuit ring  16 , a lug  46  on a diameter of the compaction element  44  such that the lug  46  fits into the groove  45  to form a shoulder, and screws  47  evenly distributed over a diameter of the short-circuit ring  16  to secure the short-circuit ring and the compaction element together, the screws being engaged inside the lug  46 . 
       FIG. 11  shows a partial sectional view along an axial direction of an eighth embodiment of the retaining means. 
     It shows the short-circuit ring  16  and the compaction element  44  comprising the groove  45 , the lug  46  and the screws  47 . 
     The retaining means further comprise a second groove  46   a  in a second diameter of the compaction element  44  such that one end  48  of the short-circuit ring  16  fits into the second groove to form a second shoulder, the second diameter being smaller than the first diameter. 
     In an alternative embodiment not shown, the retaining means further comprise screws distributed over a second diameter of the ring to secure the short-circuit ring and the compaction element together, the second pitch circle diameter of the screws being smaller than the first pitch circle diameter of the screws. 
     The screws on the second pitch circle diameter are engaged inside the groove  46   a.    
       FIG. 12  shows a partial sectional view along an axial direction of a ninth embodiment of the retaining means. 
     It shows the cylindrical magnetic mass  14 , the short-circuit ring  16  comprising the lug  20  and the screws  21 , and the conductive bar  17  described in  FIGS. 3 and 4 . 
     A compaction element  49  is inserted between the magnetic mass  14  and the short-circuit ring  16  differing from the compaction element  44  in that it comprises a groove  50  in a diameter of the compaction element  49  such that the lug  20  fits into the groove  50  to form a shoulder. 
     The pitch circle diameter d 16   a  of the holes  16   a  in the short-circuit ring is smaller than the pitch circle diameter d 17  of the conductive bars  17  in the magnetic mass  14 . 
     The holes  16   a , which are preferably circular in shape, are not coaxial with the pitch circle diameter of the conductive bars, which are preferably cylindrical in shape. 
     The conductive bar  17  is subjected to a bending preload in the hole  16   a  by the force of the screws  21  in order to establish an electrical contact with the short-circuit ring and prevent sparks when starting up the rotating electrical machine  9 , since the centrifugal force during start-up is not sufficient to establish a good electrical contact. 
     In an alternative embodiment not shown, if the holes  16   a , which are preferably circular in shape, are coaxial with the conductive bars  17 , which are preferably cylindrical in shape, the end of the conductive bars  17  inserted into the holes  16   a  includes a knurling such that the ends of the knurling have a larger diameter than the diameter of the holes  16   a.    
     When inserting the bars  17  into the holes  16   a , the ends of the knurling are upset by the force of the screws  21  and procure the electrical contact between the ring  16  and the bars  17 . 
     Alternatively, the holes  16   a  and the conductive bars  17  are rectangular or oblong in shape. The ends of the bars  17  can include a knurling with a cross-section that is larger than the cross-section of the holes  16   a.    
       FIG. 13  shows a partial sectional view along an axial direction of a tenth embodiment of the retaining means, which differs from the ninth embodiment of the retaining means shown in  FIG. 12  in that the short-circuit ring  16  comprises a counterbore  51  wherein the head of the screw  21  is housed to ensure that the head of the screw is held radially under the effect of centrifugal force, and the hole  16   a  and the conductive bar  17  comprise a sloping side  16   b  and  17   a  such that when inserting the bar  17  into the hole  16   a , the sloping sides come into contact to establish an electrical contact between the short-circuit ring  16  and the bar  17  and to prevent sparks when starting up the rotating electrical machine  9 . 
     The bar  17 , which preferably has a rectangular cross-section, is subjected to a gradual bending preload inside the hole  16   a  by the force of the screws  21  and the sloping sides  16   b  and  17   a.    
     In  FIGS. 12 and 13 , the bending preload of the bars  17  is produced by the means for retaining the lug  20  in the groove  50  preventing the short-circuit ring  16  from deforming radially outwards under the effect of the bending forces of the bars  17 . 
     In the embodiments described hereinabove, the conductive bars  17  are inserted into the short-circuit rings  16 , thus allowing for the free axial thermal expansion of the bars  17 . 
     The conductive bars  17  can be brazed on the short-circuit rings  16 . 
     However, under the effect of the temperature generated by the brazing operation, of up to 700° C., the copper lug of the short-circuit ring  16 , which expands, breaks by radial shearing inside the groove of the steel compaction element under the effect of the different coefficients of thermal expansion of copper and steel, and of the ring which is at a higher temperature than the compaction element during the brazing operation. 
     The embodiments described hereinabove are not suitable for conductive bars brazed on the short-circuit rings  16 . 
     The retaining means described hereinbelow are adapted to the brazing of the conductive bars  17  on the short-circuit rings or to the insertion of the conductive bars  17  into the holes  16   a.    
       FIG. 14  shows a partial sectional view along an axial direction of an eleventh embodiment of the retaining means wherein the conductive bars  17  are brazed on a short-circuit ring  51 . 
     The short-circuit ring  51  is no longer in direct contact with a face of a compaction element  52 . 
     The outer diameter d 52  of the compaction element  52  is determined such that the outer periphery of the compaction element is not in contact with the conductive bars  17 . 
     The retaining means comprise a groove  53  in a first diameter of the short-circuit ring, a retaining ring  54  comprising, on at least one diameter, a lug  55  and screws  56  evenly distributed over at least one diameter of the retaining ring to secure the retained ring and the compaction element together, the screws  56  being placed under the inner diameter of the short-circuit ring. 
     The retaining ring  54  is inserted between the short-circuit ring  51  and the compaction element  52 . 
     The lug  55  fits into the groove  53  to form a shoulder and such that the face of the retaining ring  54  opposite that in contact with the short circuit ring  51  is in contact with the compaction element  52  and secured by the screws  56 . 
     During the operation for brazing the bars  17  on the short-circuit ring  51 , the screws  56  are not present, which allows for the free radial thermal expansion of the short-circuit ring during the brazing operation. 
     When the assembly obtained is at ambient temperature, the screws  56  are assembled in order to secure the retaining ring  54  and the compaction element  52  together, and retaining the ring  51  by the lug  55  fitted into the groove  53 . 
       FIG. 15  shows a partial sectional view along an axial direction of a twelfth embodiment of the retaining means wherein the conductive bars  17  are brazed on a short-circuit ring  51 . 
     This embodiment differs from the preceding embodiment in that the heads of the screws  56  are each disposed in a counterbore  57  in the retaining ring  54  for radially holding the heads of the screws  56  under the effect of centrifugal force, the retaining ring includes a groove  58  in one diameter, and the short-circuit ring  51  includes a lug  59  on a diameter located on the face opposite that in contact with the compaction element  52  such that the lug  59  fits into the groove  58  to form a shoulder so as to hold the short-circuit ring  51  under the effect of centrifugal force. 
     The retaining ring  54  is retained under the effect of centrifugal force by the end thereof in contact with the compaction element  52 , the retaining ring  54  being fitted into a counterbore  52   a  in the compaction element  52 . 
       FIG. 16  shows a partial sectional view along an axial direction of a thirteenth embodiment of the retaining means wherein the conductive bars  17  are brazed on a short-circuit ring  51 , or alternatively, the conductive bars  17  are inserted into the holes  16   a  in the short-circuit ring  16 . 
     This embodiment differs from the preceding embodiments in that the short-circuit ring  16 ,  51  comprises a second retaining lug  59   a  opposite the retaining lug  59  and the compaction element  52  comprises a second groove  58   a  interacting with the second retaining lug  59   a.    
     The second groove  58   a  further interacts with the end of the retaining ring  54  which is housed in the groove  58   a.    
     The retaining ring  54  is fixed by the screws  56  to the compaction element  52 , which ring radially bears against the lug  59   a.    
     In an alternative embodiment not shown, the short-circuit ring  16 ,  51  comprises two opposite retaining lugs  59  and  59   a  which fit into the respective grooves  58  and  58   a  in the retaining ring  54  and the compaction element  52 . Contrary to  FIG. 16 , the end of the retaining ring  54  comprises a retaining lug  62  which fits into a second groove  63  in the compaction element  52 . The fixing screws  56  are located on a larger diameter than the diameter of the second groove  63 . 
     The advantage of a short-circuit ring comprising two opposite retaining lugs  59  and  59   a , as shown in  FIG. 16 , is that the short-circuit ring is better retained by the two lugs thereof under the effect of centrifugal force, allowing the rotor  12  to rotate at higher rotational speeds. 
       FIG. 17  shows a partial sectional view along an axial direction of a fourteenth embodiment of the retaining means wherein the conductive bars  17  are brazed on a short-circuit ring  51 . 
     This embodiment differs from the preceding embodiment in that the retaining means include a groove  60  on a diameter of the face of the short-circuit ring  51  opposite the compaction element  52 , and the retaining ring  54  includes a lug  61  which fits into the groove  60  to form a retaining shoulder, the head of the screws  56  not being housed in a counterbore in the retaining ring  54 . 
     The retaining means further include a lug  62  on a diameter of the retaining ring  54  which fits into a groove  63  in the compaction element  52  so as to form a shoulder, the lug  62  being located between the axis of rotation of the rotor and the pitch circle diameter of the fixing screws  56 . 
     Moreover, the inner diameter  51   a  of the short-circuit ring  51  is tapered and interacts with the tapered outer diameter  54   a  of the retaining ring  54 , procuring a self-centring of the lug  61  and  62  in the groove  60  and  63  when assembling the retaining ring  54  in the compaction element  52 . 
       FIG. 18  shows a partial sectional view along an axial direction of a fifteenth embodiment of the retaining means wherein the conductive bars  17  are brazed on a short-circuit ring  51 . 
     This embodiment differs from the embodiment shown in  FIG. 15  in that the retaining means further comprise a second groove  64  in a second diameter of the retaining ring  54  and a second lug  65  on a second diameter of the short-circuit ring  51  located on the face opposite that in contact with the compaction element  52  such that the second lug  65  fits into the second groove  64  so as to form a second shoulder and to hold the short-circuit ring. 
     Alternatively, after the brazing operation, the retaining grooves can be machined to obtained an improved coaxiality with the two lugs of the retaining ring  54 . 
       FIG. 19  shows a partial sectional view along an axial direction of a sixteenth embodiment of the retaining means wherein the conductive bars  17  are brazed on a short-circuit ring  51 . 
     It shows the retaining ring  54  secured to the compaction element  52  by the screws  56  and the lug  62  fitting into the groove  63 , the groove  58 , the short circuit ring  51  comprising the groove  53 . 
     The retaining means further include a holding ring  66  comprising a face in contact with the compaction element  52  and a groove  67  on the face opposite that in contact with the compaction element such that the ends  68  and  69  of the holding ring fit into the grooves  53  and  58  of the short-circuit ring  51  and of the retaining ring  54 . 
     This embodiment, as with those described in  FIGS. 14 to 18 , allows the bars  17  to be brazed on the short-circuit ring  51  with free radial thermal expansion of the short-circuit ring during the brazing operation, the means for retaining under centrifugal force being produced by assembling the retaining ring  54  in the compaction element  52  after the brazing operation when the temperature of the short-circuit ring has returned to ambient temperature. 
     According to other embodiments, each end of the rotor can comprise retaining means of the same type or retaining means of different types. 
     The retaining means including two grooves or two fixing lugs or two rows of fixing screws on the short-circuit ring are particularly adapted to a peripheral speed of the rotor of less than 160 m/s. 
     When the peripheral speed of rotation exceeds 160 m/s and reaches up to 200 m/s, the short-circuit rings must be bound, which complements the retaining means described hereinabove. 
     According to other embodiments not shown, if the short-circuit ring is large in size, the ring is segmented, the segments of the ring being separated by a circumferential expansion clearance. The segments are electrically connected to one another for example by brazed, welded or preferably screwed connections on the ring. 
       FIG. 20  shows a partial sectional view along an axial direction of a seventeenth embodiment of the retaining means including a binding of the short-circuit ring  51 . 
     This embodiment differs from that shown in  FIG. 17  in that the short-circuit ring comprises a recess  70  opening out onto the outer periphery of the ring. 
     A binding band  71  is inserted into the recess  70  to hold the short-circuit ring  51  under the effect of centrifugal force. 
     The binding band  71  is made of a non-magnetic material to prevent it from heating up under the effect of the magnetic field induced by the stator coils. 
     The binding band  71  is, for example, made of stainless steel. 
     It goes without saying that the example retaining means described hereinabove can comprise a binding band  71 . 
     According to other embodiments, electrical insulation means are disposed beneath the head of the screw and/or along the screw body and/or between the short-circuit ring and the compaction element. 
     The insulation means prevent an interfering electric current from flowing through the fixing screws in the magnetic mass  14 . 
     The radial thickness of the retaining lug is preferably equal to a value in a range from 10% to 40% of the radial thickness of the short-circuit ring and the length in an axial direction of the retaining lug is preferably equal to a value in a range from 15% to 50% of the axial thickness of the short-circuit ring. 
     Preferably, the axial thickness of the compaction elements is greater than twenty times the axial thickness of the magnetic sheets  18  or  25 . 
     The retaining means described allow the peripheral speed of the rotor to be increased without the use of a non-magnetic steel band for binding the short-circuit ring up to a peripheral speed of 160 m/s, thus reducing expensive operations for machining, binding the binding band and supplying non-magnetic materials with high mechanical properties.