Abstract:
An electrical rotating machine rotor, in particular for a motor vehicle, comprising a central shaft, an annular core coaxial with the shaft and two polar wheels which are axially arranged on either side of the core, of the type wherein the shaft includes at least one drive section which is axially force-fitted into a fixing bore of at least one component of the rotor so as to secure in rotation at least one of the two polar wheels of the rotor to the shaft, an intermediate sleeve being radially interposed between each polar wheel and the central shaft, and on which sleeve is mounted said polar wheel. The invention also concerns a method for making such a rotor.

Description:
FIELD OF THE INVENTION 
     The invention concerns a rotor for a rotary electrical machine, such as an alternator or an alternator-starter, in particular for a motor vehicle. 
     DESCRIPTION OF THE RELATED ART 
     The invention concerns more particularly a rotor for a rotary electrical machine, such as an alternator or an alternator-starter, in particular for a motor vehicle, which comprises: 
     a central shaft; 
     an annular core coaxial with the shaft; 
     a coil that extends radially around the core; 
     and two pole pieces that are arranged axially on each side of the core and coil; 
     of the type in which the shaft comprises at least one driving portion whose cross section, along a radial plane, is not smooth, and which is axially force-fitted in a bore for fixing at least one component of the rotor so as to rotationally fix at least the two pole pieces of the rotor to the shaft. 
     Rotors of this type are already known in which the core is divided axially into two distinct portions and each portion is produced in one piece with one of the pole pieces. 
     When the rotor is assembled, the two pieces are first of all pressed against each other on each side of the coil so that the facing internal radial faces of the two portions of the core are pressed against each other, in order to ensure a good abutment of the two faces so as to allow optimum passage of the magnetic flux through the core. 
     The angular positioning of one piece with respect to the other is achieved by means of locating fingers interposed between the two pole pieces in a temporary fashion during pressing. 
     In addition, the two core portions have a conical shape so that, under the pressing force, the coil is deformed radially and thus clamped around the core so as to be kept rotationally integral with the core. 
     Next, the shaft is force-fitted in the bores in the pole pieces. A driving portion of the shaft, which is received in the bores, comprises reliefs, for example knurling in the form of flutes or serrations, in order to rotationally connect the shaft and pole pieces. 
     According to another known embodiment of the rotor, the core is formed in a single piece that is distinct from the two pole pieces. The coil is wound around the core. The pieces are then positioned axially on each side of the core. Then an axial pressing force is applied to the pieces so that their internal radial faces are pressed against the core so as to ensure optimum passage of a magnetic flux between the core and each pole piece. 
     While the pole pieces are under stress, the driving portion of the shaft is force-fitted in the pieces and in the central tube. 
     However, in these two known embodiments, the driving portion of the shaft, and in particular the serrations, do not make it possible to obtain a sufficiently precise concentricity of the pole pieces and/or of the core with respect to the rotation axis of the shaft. The central bore of each pole piece is in fact deformed plastically in a radial direction in an uneven manner. This is because it is found that the serrations do not uniformly penetrate the central bores of the pole pieces and core. 
     To correct this lack of centering, it is necessary to perform an operation of machining the external peripheral face of the pole pieces after they are mounted in order to guarantee concentricity between the external periphery of the rotor and the rotation axis of the shaft. 
     During this machining operation, hot shavings are liable to be thrown onto the winding, which thus risks being damaged. 
     In addition, this machining operation cannot be carried out using a lubricant, which also risks damaging the coil. The machining operation is therefore made more lengthy and more expensive. 
     Moreover, the force necessary for axially pressing the pole pieces against each other does not make it possible to obtain a precise axial positioning of the pieces with respect to each other. 
     In addition, the serrations on the driving portion of the shaft are never oriented in a perfectly rectilinear fashion in an axial direction, but are generally slightly helical in shape around this shaft, which constitutes a defect. Thus, when the driving portion of the shaft is fitted in the pole pieces pressed against each other, the helical shape of the serrations causes the appearance of a torsion force between the bore of each pole piece and the shaft, which is liable to cause a relative rotation movement of the two pole pieces with respect to each other when the axial force is released. 
     This relative movement causes a circumferential misalignment between the pole pieces and in particular the teeth thereof. 
     Finally, pressing the pole pieces against each other prior to the fitting of the shaft has the effect of making their material flow towards the internal bore. This has the consequence of reducing the inside diameter of the bore of the pole pieces. The fitting force necessary for fitting the shaft in the pole pieces is therefore increased accordingly and the shaft is liable to buckle, since the rear end of the shaft has a smaller diameter in order in particular to mount collecting rings. 
     SUMMARY OF THE INVENTION 
     In order to resolve these problems, the invention proposes a rotor and further comprising at least one intermediate sleeve that is interposed radially between each pole piece and the central shaft, and on which the pole piece is mounted, and in that the driving portion of the shaft is fitted axially in a fixing bore produced in the intermediate sleeve or in the core. 
     By virtue of the invention the operation of machining the external peripheral face of the pole pieces, that is to say the operation of machining the external periphery of the claw rotor, is carried out in advance without the presence of the excitation coil of the rotor so that this excitation coil does not risk being damaged by the projection of shavings. This machining operation can be carried out with the help of a lubricant so that the service life of the machining tool is increased. 
     The air gap between the internal periphery of the stator body, which the rotary electrical machine has, and the external periphery of the rotor of this machine is therefore carried out without subsequent machining of the rotor. 
     In one embodiment it is possible to machine helical grooves in advance at the external periphery of the rotor in order to cut the eddy currents developed at the external periphery face of the pole pieces, as described in the document FR 2 774 524. 
     In addition it is possible easily to machine the external periphery of the intermediate sleeve or sleeves in order to have good concentricity of the sleeve or sleeves with respect to the axis of the shaft. The pole pieces are being machined before they are mounted on the sleeve or sleeves, which constitute centering sleeves. 
     In addition it is possible to standardize the outside diameter of the intermediate sleeve or sleeves in order to mount pole pieces of different sizes. 
     In one embodiment the intermediate centering sleeve is distinct from the central shaft. 
     Thus, by virtue of the invention, it is possible to use the same central shaft as in the prior art and the force for fitting the central shaft in the intermediate sleeve or sleeves is reduced, so that the rear end of the shaft is preserved. 
     In another embodiment each pole piece has a portion of core associated with the smooth centering sleeve issuing from the central shaft, each sleeve being extended by a non-smooth driving portion of the central shaft, the said driving portion being offset axially with respect to the sleeve. 
     In this case the outside diameter of a first one of the two sleeves of the central shaft is greater than the outside diameter of the second sleeve of the central shaft and the same applies to the complementary inside diameters of the first portion of the core and of the second portion of the core. 
     By virtue of these stagings in diameter it is possible to force-fit each non-smooth portion of the shaft in the portion of the core concerned, the sleeve with the smallest outside diameter and the associated non-smooth driving portion of the shaft passing through the core portion with the largest inside diameter. 
     The non-smooth portions of the shaft have a short axial length so that the outside diameters of the pole pieces can be machined in advance. 
     According to other characteristics of the rotor, taken in isolation or in combination:
         the intermediate sleeve comprises a cylindrical surface coaxial with the central shaft that is associated with each pole piece and that is received in a complementary central cylindrical bore in the associated pole piece, so as to position the associated pole piece coaxially with the shaft;       

     the intermediate sleeve is interposed between the core and the shaft; 
     the core is at least partly formed in one piece with one of the pole pieces; 
     the core is produced in at least two distinct portions axially and in that each portion of the core is formed in one piece with the adjacent pole piece; 
     the core and the pole pieces are distinct elements; 
     the rotor comprises a single sleeve; 
     the rotor comprises a plurality of sleeves; 
     the shaft is fitted directly in the core; 
     the rotor comprises at least two sleeves, each of which is associated with a pole piece and which are arranged axially on each side of the core; 
     the sleeves are produced in one piece with the core so as to form a single hub; 
     the rotor comprises means of axial positioning of the pole pieces with respect to each other along the shaft; 
     each pole piece comprises an internal radial face that is in abutment axially against a facing external radial face of the core; 
     each core portion comprises an internal radial face that are axially in abutment against each other; 
     the rotor comprises means for connecting the pole pieces with the intermediate sleeve in rotation about the axis of the central shaft; 
     the pole piece and the intermediate sleeve are fixed together by welding; 
     the pole piece and the sleeve are fixed together by crimping; 
     the pole piece or the sleeve comprises a bevel in an arc of a circle for crimping the other part; 
     the sleeve is produced from a ferromagnetic material. 
     The invention also proposes a method of producing such a rotor, of the type that comprises: 
     a step of mounting the core; 
     a step of mounting the coil; 
     a step of mounting each pole piece; 
     a step of adjusting the concentricity of each pole piece with respect to the shaft; 
     a step of adjusting the axial position of the pole pieces with respect to each other. 
     This method is characterized in that it comprises a step of mounting the intermediate sleeve that is prior to the step of mounting the pole piece or pieces associated with the intermediate sleeve. 
     In one embodiment the intermediate sleeve is distinct from the pole pieces and the mounting step consists of axially force-fitting the driving portion in the bore of the intermediate sleeve. 
     According to another characteristic of the method: 
     the step of adjusting the concentricity comprises a first operation of machining the sleeve, in order to form the cylindrical surface or surfaces able to receive each pole piece; 
     the operation of machining the sleeve is carried out subsequently to the step of mounting the sleeve; 
     the step of mounting each pole piece consists of fitting the associated cylindrical surface of the sleeve in the bore of each piece; 
     the step of adjusting the concentricity of each pole piece comprises a second operation of machining the bore of each pole piece; 
     the step of adjusting the concentricity comprises a third operation of machining the external peripheral cylindrical face of the pole piece; 
     the step of adjusting the axial positioning of each pole piece comprises a first operation of machining the internal radial face of the piece; 
     the step of adjusting the axial positioning comprises a second operation of machining each external radial face of the core prior to the step of mounting each pole piece; 
     a second operation of machining the external radial face of the core is subsequent to the step of mounting the core; 
     the step of mounting the core is carried out simultaneously with the step of mounting the pole pieces; 
     the step of mounting the intermediate sleeve is carried out simultaneously with the step of mounting the core and consists of axially fitting the driving portion of the shaft in the bore of the core; 
     the method comprises a final step of making each pole piece integral with the associated sleeve, with respect to rotation and translation with the shaft. 
     Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages will emerge during a reading of the following detailed description, for an understanding of which reference should be made to the accompanying drawings, among which: 
         FIG. 1  is a view in axial section that depicts an alternator provided with a rotor of the prior art; 
         FIG. 2  is a view in axial section that depicts a rotor produced according to the teachings of the invention; 
         FIG. 3  is the same view as that in  FIG. 2  in which the pole pieces of the rotor are fixed to the intermediate sleeve by crimping; 
         FIG. 4  is a side end view of the rotor of  FIG. 2 ; 
         FIG. 5  is a view in axial section that depicts a second embodiment of the invention; 
         FIG. 6  is a view in axial section for a third embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the remainder of the description, analogous, similar or identical elements will be designated by the same reference number. 
     In the remainder of the description an axial and radial orientation will be adopted, indicated by the arrows ‘A’ and ‘R’ in  FIG. 1 . 
     In addition, radial faces oriented towards the middle of the core will be termed internal faces while the faces oriented in an opposite direction will be termed external faces. 
     The internal and external radial faces are therefore axial end faces of the core. 
     Likewise, axial faces oriented towards the rotation axis of the shaft will be termed internal faces while axial faces oriented in an opposite direction will be termed external faces. 
     Referring to  FIG. 1 , this depicts a rotary electrical machine of the prior art, in the present case an alternator with internal ventilation of the polyphase type for a motor vehicle with a thermal engine functioning in alternator mode. Naturally the alternator can also be reversible and consist of an alternator-starter also functioning in electric motor mode in particular in order to start the thermal engine of the vehicle as described in the document FR A 2 745 445 (corresponding to U.S. Pat. No. 6,002,219). 
     When the machine is functioning in alternator mode it converts mechanical energy into electrical energy like any alternator. When the machine is functioning in electric motor mode, in particular in starter mode for starting the thermal engine of the vehicle, it converts electrical energy into mechanical energy. 
     This machine comprises essentially a casing  10  and, inside it, a rotor  12  rotationally integral with a shaft, rotor shaft, single-piece shaft or central shaft  14 , referred to as the rotor shaft, and a stator  16  that surrounds the rotor  12  and comprises a body in the form of a packet of metal sheets provided with recesses, for example of the semi-closed type, for mounting a stator coil  18  forming, on each side of the stator  16  at each axial end thereof, a coil end. 
     This stator coil  18  comprises for example a set of three-phase windings in a star or delta, the outputs of which are connected to a bridge rectifier (not shown) comprising rectifying elements such as diodes or transistors of the MOSFET type, in particular when the machine is of the reversible type, and consists of an alternator-starter as described for example in the document FR-A-2.745.445 (U.S. Pat. No. 6,002,219). 
     The windings are obtained by means of a continuous electrically conductive wire covered with an insulating layer and mounted in the relevant recesses in the body of the stator  16 . 
     In a variant that is not shown, for better filling of the recesses of the body of the stator  16 , the windings are produced by means of conductors in the form of bars, such as pins, connected together for example by welding. 
     According to another variant that is not shown, in order to reduce the degree of ripple and magnetic noise, the stator coil  18  comprises two sets of three-phase windings to form a composite stator winding device, the windings being offset by thirty degrees electrical as described for example in the documents US-A1-2002/0175589, EP-0.454.039 and FR-A-2.784.248. In this case two bridge rectifiers are provided and all combinations of three-phase windings in star and/or delta are possible. 
     In a variant the stator winding is of the pentaphase type. 
     In general terms the alternator is of the polyphase type and the bridge rectifier or rectifiers in particular rectify the alternating current produced in the windings of the stator  16  to a DC current in particular in order to charge the battery (not shown) of the motor vehicle and supply the loads and electrical consumers in the onboard system of the motor vehicle. 
     The rotor  12  is produced in the example shown in the form of a claw rotor, as described for example in the documents US-A1-2002/0175589 and EP-A1-0.454.039, comprising two pole pieces  20 ,  22 , here axially juxtaposed and each having an annular-shaped transverse flange  24  provided at its external periphery with claws  26 . 
     Each claw  26  comprises an implantation portion  28  transversely oriented in the plane of the flange  24  concerned. This implantation portion  28  is extended at its external periphery by a tooth  30  of axial orientation overall. 
     An annular air gap exists between the external peripheral face  32  of the teeth  30  and the internal periphery of the body of the stator  16 . 
     The teeth  30  are overall trapezoidal or triangular in shape and are directed axially towards the flange  24  of the other pole piece  20 ,  22 , the tooth  30  of one pole piece  20 ,  22  penetrating the space existing between two adjacent teeth  30  of the other pole piece  20 ,  22 , so that the teeth  30  of the pole pieces  20 ,  22  are interleaved. 
     An excitation coil  34  is located axially between the flanges  24  of the pole pieces  20 ,  22 . It is carried by a part of the rotor  12  in the form of a cylindrical annular core  36  coaxial with the shaft  14 , which comprises a central bore  37 . The core  36  here consists of two axially distinct portions, each of which is produced in one piece with associated pole pieces  20 ,  22  as shown in  FIG. 1 . 
     According to a variant that is not shown, the central core  36  consists of a single piece and is distinct from the pole pieces  20 ,  22 , which are arranged axially on each side of the core  36 . 
     In the remainder of the description, the term ‘coil’ without qualification will be understood to be the excitation coil  34  rather than the stator coil  18 . 
     The excitation coil  34  is therefore located in the space delimited radially by the claws  26  of the pole pieces  20 ,  22  and the central core  36 . 
     The pole pieces  20 ,  22  and the core  36  are preferably made from ferromagnetic material and have the rotor shaft  14 , also made from ferromagnetic material, passing through it coaxially. For this purpose, each pole piece  20 ,  22  comprises a central or fixing bore  38  that passes axially through the flange  24  and extends the central bore  37  of the part of the core  36  concerned. 
     The wire of the excitation coil  34  is in  FIG. 1  wound on a support made from electrically insulating material (not shown) mounted, preferably forcibly, on the external periphery of the core  36 . 
     This support here has a cross section roughly in the shape of a U in order to isolate the excitation coil  34  from the flanges  24  of the pole pieces  20 ,  22 . 
     According to a variant that is not shown, when the core  36  is in one part, the wire of the excitation coil  34  is wound on an insulator fixed to the core  36  and is conformed so as to prevent any contact with the flanges  24  and the teeth  30  of the pole pieces  20 ,  22 . 
     When the excitation coil  34  is activated, that is to say supplied electrically, the pole pieces  20 ,  22  and the core  36 , which are produced from ferromagnetic material, are magnetized and the rotor  12  becomes an inducing rotor with the formation of magnetic poles at the claws  26  with teeth  30  on the pole pieces  20 ,  22 . 
     This inducing rotor  12  creates an induced alternating current in the stator  16  induced when the shaft  14  turns. 
     The shaft  14  of the rotor  12  carries at its front end a pulley  40  belonging to a device for transmitting movements by means of at least one belt (not shown) between the alternator and the thermal engine of the motor vehicle, and carries at its rear end collecting rings  42  connected by cabled connections (not shown) to the ends of the excitation coil  34  of the rotor  12 . 
     Brushes belong to a brush holder shown in a general fashion at the reference  44  and are disposed so as to rub on the collecting rings  42  so as to supply the excitation coil  34  with electric current. The brush holder  44  is connected to a voltage regulator (not shown). 
     The casing  10  is here in two parts, namely a front bearing  46  adjacent to the pulley  40  and a rear bearing  48  carrying the brush holder  44  and usually the bridge rectifier or rectifiers and the voltage regulator. The bearings  46 ,  48  are hollow in shape and each carry centrally a ball bearing respectively  50  and  52  for the rotational mounting of the shaft  14  of the rotor  12 . 
     The alternator also comprises means for cooling it. 
     For example, as illustrated in  FIG. 1 , the bearings  46 ,  48  are perforated to allow the cooling of the alternator by the circulation of air. For this purpose the rotor  12  carries at least at one of its axial ends a fan intended to provide this circulation of air. In the example shown, a first fan  54  is provided on the forward frontal face of the rotor  12  and a second fan  56 , more powerful, at the rear face of the rotor  12 . Each fan  54 ,  56  is provided with a plurality of blades or sleeves  58 ,  60  that are fixed to the external radial faces of the flanges  24 . 
     According to a variant that is not shown, the alternator can also be cooled by a heat-transfer fluid, the casing  10  then being configured so as to comprise an appropriate circulation channel for the heat-transfer fluid. 
     It should be noted that, in the example embodiment described, the rotor  12  comprises eight teeth  30  per pole piece and therefore eight pairs of poles. Forty eight recesses are therefore provided in the body of the stator in the case in which one set of three-phase windings or two sets of three-phase windings are provided as described in the aforementioned document FR-A-2.737.063, or ninety six recesses in the solutions described in the aforementioned documents US-A1-2002/0175589 and EP-A1-0.454.039. Naturally the rotor  12  can, depending on the application, comprise a different number of pairs of poles. For example, each pole piece can comprise in a variant six teeth so that the rotor comprises six pairs of poles and the stator  16  or  72  recesses. 
     According to a variant that is not shown, the performance of the machine, namely its power and efficiency, can also be increased using a rotor  12  that comprises, in a known fashion and for example as described in the French patent FR-2.784.248, a certain number of permanent magnets interposed between two adjacent teeth  30  at the periphery of the stator  16 , choosing the number of these magnets so that it is equal to or less than the number of poles on the rotor and their arrangement is symmetrical with respect to the axis of the rotor. For example, four, six or eight pairs of magnets are provided for eight pairs of poles. 
     In a known fashion, the shaft  14  comprises portions with a non-smooth radial driving section or portion  57 , which are here knurled portions with axial serrations, as visible in  FIG. 1 , for fixing and driving the pole pieces  20 ,  22  and the core  36 . The pole pieces  20 ,  22  and the core  36  are thus mounted by force-fitting on the shaft  14 , so that the latter, by means of its serrations, cuts furrows in the central bore of the pole pieces  20 ,  22  and in the core  36  when they are force-fitted for rotational connection of the shaft with the core  36  and the pole pieces  20 ,  22 . 
     As described previously, such a design of the rotor  12  poses problems when the rotor  12  is produced. 
     The invention therefore proposes a rotor  12 , as shown in  FIGS. 2 to 6 , that comprises at least one intermediate sleeve  58  interposed radially between each pole piece  20 ,  22  and the central shaft  14 , and on which the said pole piece is mounted. 
     In  FIGS. 2 to 5  the driving portion  57  is force-fitted in a fixing bore  59  produced in the intermediate sleeve  58  distinct from the shaft  14 . 
     In the embodiment in  FIG. 6  the shaft is axially force-fitted in a fixing bore produced in the core, the intermediate sleeve issuing from the shaft and being offset axially with respect to the fixing bore. 
     In  FIGS. 2 to 5  the intermediate sleeve  58  is interposed radially between the non-smooth driving portion  57  of the central shaft  14  and the fixing bore  38  of at least one pole piece  20 ,  22 . This intermediate sleeve comprises, as described below, a cylindrical surface  60  coaxial with the shaft  14  and received in a complementary central cylindrical bore  38 ,  138  of the associated pole piece  20 ,  22  so as to position the pole piece coaxially with the shaft  14 . 
     The portion  57  is here provided with reliefs so that it is non-smooth as in  FIG. 1 . The reliefs consist here of a knurling with serrations as in  FIG. 1 . 
     According to a first embodiment of the invention that is depicted in  FIG. 2 , the rotor  12  is similar to the rotor  12  depicted in  FIG. 1 . The rotor  12  thus comprises a central shaft  14 , two pole pieces  20 ,  22  that are arranged axially on each side of a core  36 , and an excitation coil  34  that extends radially around the core  36 . 
     However, the rotor  12  comprises here an intermediate sleeve  58  that comprises a central fixing bore  59 . The driving portions  57  of the shaft  14  are force-fitted in the fixing bore  59  of the sleeve  58 . 
     These portions  57  comprise reliefs in the form of serrations belonging to a knurling. 
     The sleeve  58  comprises a cylindrical surface  60 , here axially oriented, that merges here with the external peripheral cylindrical surface of the sleeve  58 . The cylindrical surface  60 , which is by virtue of the invention coaxial as described below with the rotation axis of shaft  14 , is intended to receive the pole pieces  20 ,  22 . 
     According to this embodiment of the invention, the sleeve  58  is also fitted in the core  36 , so that the sleeve  58  is interposed radially between the shaft  14  and the core  36 , here in two portions, that is to say in two halves, each of which is made in one piece with its associated pole piece,  20 ,  22 . 
     For this purpose, the radial section of the cylindrical surface  60  has a shape complementary to the radial section of the central bore  38  of the pole pieces  20 ,  22 . More particularly, the surface  60  has in radial section a circular shape and is in close contact with the internal periphery of the pole pieces  20 ,  22  delimiting the axially oriented central bore  38 , which is therefore coaxial with the axis of the shaft  14 . 
     The cylindrical surface  60  ( FIG. 2 ) of the sleeve  58  is fitted in each central bore  38  of the pole pieces  20 ,  22 , and consequently in the central bore of the core  36 , here in two halves and which is therefore made in one piece with the pole pieces  20 ,  22 , so that the sleeve  58  is interposed radially between the driving portion  57  of the shaft  14  and the pole pieces  20 ,  22 . Here the sleeve  58  is also interposed radially between the driving portion  57  and the core  36 . 
     More precisely, in  FIGS. 2 and 3 , as in  FIG. 1 , two driving portions  57  are provided, with different axial lengths, namely a front portion longer than a rear portion. 
     The front axial end of the front portion  57  extends in axial projection with respect to the front axial end of the front pole piece  20  and the sleeve  58 . This front portion extends to the rear through its rear end roughly as far as the internal end face or the internal radial face  62  of the core half  36  issuing from the front pole piece  20  and constituting the rear axial end of this core half and of the front pole piece. 
     The rear end of this rear portion  57  extends roughly as far as a shoulder or collar  114  belonging, as in  FIG. 1 , to a collar of the shaft  14  adjacent to the rings  42  and therefore to the rear end of the shaft  14 . 
     Naturally in a variant the rear driving portion is omitted, all this depending on the torque to be transmitted. 
     The rear end of the sleeve  58  is in abutment on the shoulder  114 , or more precisely on the front face of the collar  114 . The front end of the sleeve  58  is intended to come into abutment on the annular strut  150  in  FIG. 1  interposed axially between the front end of the sleeve  58  and the bearing  50 . 
     The sleeve  58  is therefore intended to be mounted for axial clamping between the shoulder  114  and the strut  150  so that it makes it possible to reduce the stresses in the pole pieces  20 ,  22 . 
     In this example embodiment the front end of the shaft  14  is fitted in the sleeve  58  according to the method described below. The collar  114  limits the relative axial movement of the shaft with respect to the sleeve  58  and has here an outside diameter less than that of the tubular-shaped sleeve  58  in  FIGS. 2 and 3 . 
     The axial length of the sleeve  58  is at least equal to the sum of the axial lengths of the core  36  and the fixing bores  38  of each pole piece  20 ,  22 . Here each axial end of the sleeve  58  extends in the same radial plane as the external radial face of the flange  24  of the associated pole piece  20 ,  22 . 
     It should be noted that the strut  150 , in a variant, can also come into abutment on the internal periphery of the flange  24  of the front pole piece  20 . 
     The sleeve  58  here consists of a single piece. However, the rotor  12  can also comprise a plurality of sleeves  58  that are arranged end-to-end around the shaft  14 . 
     Advantageously, the sleeve  58  is produced from a ferromagnetic material and preferably from the same material as the pole pieces  20 ,  22  and the core  36 . 
     The rotor  12  also comprises means for the axial positioning of the pole pieces  20 ,  22  with respect to each other axially along the shaft  14 . 
     In the first embodiment, the axial positioning of the pole pieces  20 ,  22  is achieved by the facing internal radial faces  62 ,  64  each core half  36 . These faces  62 ,  64  respectively delimit the rear axial end of the pole piece  20  and the front axial end of the pole piece  22 . This is because, when the pole pieces  20 ,  22  are mounted on the shaft  14 , the internal radial faces  62 ,  64  of each core half  36  are in abutment against each other, thus allowing the axial positioning of the pole pieces  20 ,  22 . 
     It should be noted that each pole piece  20 ,  22  comprises respectively at its rear axial end and at its front axial end a mounting bevel to facilitate its mounting on the sleeve  58 . These bevels are advantageously continuous. 
     In a variant, the pole pieces have no mounting bevel. 
     In addition, the rotor  12  comprises means for rotationally connecting the pole pieces  20 ,  22  with the intermediate sleeve  58 , which is itself rotationally connected with here the driving portions  57  of the shaft  14 . 
     Here ( FIGS. 2 to 4 ) each pole piece  20 ,  22  also comprises respectively at its front axial end and at its rear axial end a connection bevel or annular bevel part  66 . 
     As shown in  FIGS. 2 and 3 , each pole piece  20 ,  22  therefore comprises two bevels, namely a connection bevel  66  and a continuous mounting bevel. 
     Thus at least one arc of the external circular rim of the central bore  38  of each pole piece  20 ,  22  comprises a connection bevel  66 . 
     As shown in  FIG. 4 , the connection bevel  66  is able to receive by crimping a deformed material part of the sleeve  58 . For this purpose, the sleeve  58  is advantageously produced from a ductile ferromagnetic material such as soft iron, which is particularly suited to crimping. The hardness of the sleeve is less than that of the central shaft  14  made from ferromagnetic material. 
     In addition, each bevel  66  is here in two annular parts, roughly semicircular, delimited angularly by two radial end faces  68  that make it possible to rotationally lock the pole pieces  20 ,  22  around the shaft  14  with respect to the sleeve  58 . It is therefore essential, in order to rotationally fix together each pole piece  20 ,  22  and the sleeve  58 , for each annular bevel part  66  to extend only over an arc of the circumference of the central bore  38  of each pole piece  20 ,  22  rather than over the entire circumference of the bore  38 . 
     Each annular bevel part  66  is filled with the material of the sleeve  58 , which flows, following the crimping operation, into the cavities formed by the bevel  66 . 
     In a variant the bevel  66  comprises a number of parts greater than two, for example 3 or 4 parts. 
     The crimping also makes it possible to lock axially, that is to say in translation, the pole pieces  20 ,  22  with respect to the sleeve  58 . 
     According to a variant of the invention that is not shown, the connection bevel is replaced or supplemented by recesses into which the material of the sleeve  58  flows. The circumferential length of the bevel  66  therefore depends on the application, in particular on the presence or not of recesses located between each bevel part. 
     According to a variant of the invention that is not shown, the structures are reversed so that the bevel  66  in at least two parts and/or the recesses are carried by an external ridge of each axial end of the surface  60  of the sleeve  58 , and each pole piece  20 ,  22  is crimped in the bevel  66  and/or the recesses. 
     In  FIGS. 2 to 4 , by virtue of the sleeve  58 , the thickness of the rotor, that is to say the distance between the front face of the front pole piece  20  and the rear face of the rear pole piece  22 , is controlled precisely. 
     According to yet another variant of the invention depicted in  FIG. 5 , each pole piece  20 ,  22  is fixed to its intermediate sleeve  58  by welding. Thus a weld  69 , preferably continuous, is produced between the periphery of the external ridge of the central bore  38  of the pole piece  20 ,  22  and the relevant sleeve  58 . It is possible for example to carry out welding of the TIG type or welding of the laser type. 
     It should be noted that, compared with the embodiments in  FIGS. 2 and 3 , the core  36  is in this case integrated in the sleeve and constitutes the central part thereof. 
     More precisely, in this embodiment a hub  158  is provided that replaces the sleeve  58  and the core  36  in  FIGS. 2 and 3 . This hub  158  comprises a central core  36  that therefore extends in radial projection with respect to a front intermediate sleeve  58  and a rear intermediate sleeve  58 . 
     The front sleeve  58  is dedicated to the mounting of the front pole piece  20  and the rear sleeve  58  to the mounting of the rear pole piece  22 . 
     These intermediate sleeves  58  are arranged axially on each side of the core  36 . 
     The pole pieces  20 ,  22  are thus simplified and the central bore, here referenced  138 , of each pole piece is shorter axially than the central bore  38  in  FIGS. 2 and 3  since it affects only the flange  24  of each pole piece. 
     In addition, for the same outside diameter of the rotor, the height of the flanges  24  of the pole pieces  20 ,  22  is reduced since in this embodiment the outside diameter of the surfaces  60  is greater than that in  FIGS. 2 and 3 . 
     It will be appreciated that, in this embodiment, the thickness of the rotor is controlled even more precisely. 
     It will also be appreciated that wear on the excitation coil  34  is prevented since the distance between the pole pieces is precise, the pole pieces being in abutment against the projecting radial end faces  70 ,  72  of the core  36 . 
     In addition the external periphery of the core  36  can be of any shape, for example cylindrical, rectangular or polygonal in shape. 
     Naturally, in  FIGS. 2 to 4 , the pole pieces can be fixed to the sleeve by welding. Likewise in  FIG. 5  it is possible to fix the pole pieces to the sleeve by crimping. 
     The invention also proposes a method of producing such a rotor  12 . 
     Hereinafter, the steps described and the operations that constitute them are numbered for reasons of clarity of the description; however, these steps and these operations can be performed in any order unless mentioned otherwise. 
     According to the teachings of the invention, the method of producing the rotor  12  of  FIGS. 2 and 3  comprises a first step E 1  of mounting the intermediate sleeve  58  that is prior to the mounting of each pole piece  20 ,  22 . 
     Step E 1  of mounting the sleeve  58  consists of axially force-fitting the driving portion or portions  57  of the shaft  14  in the fixing bore  59  of the sleeve  58  until the collar  114  comes into abutment on the sleeve  58 . 
     Here the shaft  14  is fitted from the front in the bore  59  in order to cut furrows in the latter by means of reliefs, here in the form of serrations, on the portion or portions  57 . 
     This fitting is achieved with reduced forces compared with those used in  FIG. 1  so that the rear end of the shaft  14  with a smaller cross section for mounting the rings  42  is preserved. These rings, in one example embodiment, belong to a collector fitted on the smaller-diameter rear end of the shaft  14 . Such a collector is described for example in the document FR A 2 710 200, to which reference can be made. 
     The production method also comprises a second step E 2  of adjusting the concentricity of each pole piece  20 ,  22  with respect to the rotation axis of the shaft  14 . 
     A first operation E 21  of step E 2  is implemented after the first step E 1  of mounting the sleeve  58 . 
     During this first operation E 21 , the surface  60  of the sleeve  58  is produced before the mounting of each pole piece  20 ,  22 . The surface  60  is machined in the external peripheral face of the sleeve  58  so that the cylindrical surface  60  is concentric with the rotation axis of the shaft  14 . 
     This operation E 21  makes it possible to correct the lack of concentricity of the surface  60  of the sleeve  58  with respect to the rotation axis of the shaft  14 , due to the uneven plastic deformation of the fixing bore  59  of the sleeve  58  around the driving portion or portions  57  with reliefs, here knurled, of the shaft  14 . 
     It will be appreciated that the sleeve  58  can be standardized in diameter and therefore that an assembly of shaft  14  and sleeve  58  can serve for mounting pole pieces  20 ,  22  of different sizes in terms of diameter. The length of the sleeve  58  can easily be adjusted according to the application, the sleeve being obtained in  FIGS. 2 and 3  from a tube. 
     The second step of adjusting the concentricity E 2  also comprises a second operation E 22  of the machining the central bore  38  each pole piece  20 ,  22 , and a third operation E 23  of machining the external peripheral cylindrical face of each pole piece  20 ,  22 , that is to say here the external surface of the teeth  30 , in order to define a precise air gap between the rotor and stator of the rotary electrical machine. 
     These last two operations E 22 , E 23  make it possible to adjust the concentricity of the external peripheral face  32  of each pole piece  20 ,  22  with respect to the central bore  38  before the mounting of the pole piece  20 ,  22  on the shaft  14 . These machining operations E 22 , E 23  can therefore be performed without any risk of damaging the excitation coil  34 . In addition, it is possible to use a lubricant during these machining operations, which makes the machining more rapid and less expensive. 
     Then a third step E 3  of adjusting the axial positioning of each pole piece  20 ,  22  one with respect to the other is performed. This third step E 3  comprises here only an operation E 31  of machining the internal radial face of the associated half core  36 . 
     This operation E 31  makes it possible to adjust the axial positioning of the pole pieces  20 ,  22  with respect to each other. 
     During a fourth step E 4 , the excitation coil  34  is positioned on the core  36  and then a fifth step E 5  of mounting the pole pieces  20 ,  22  is performed. 
     During this fifth step E 5 , the surface  60  of the sleeve  58  is fitted in the central bore  38  of the pole pieces  20 ,  22  so that the internal radial faces  62 ,  64  of each half core  36  are in abutment against each other. The pole pieces  20 ,  22  are thus pressed against each other in order to ensure the passage of a magnetic flux between them through their internal radial faces  62 ,  64 . 
     This fifth step E 5  is implemented after the first four steps E 1 , E 2 , E 3 , E 4 . 
     Finally, a final sixth step E 6  of connection is performed after the step E 5  of mounting each pole piece  20 ,  22 . During this step E 6 , the pole pieces and the sleeve  58  are connected together with respect to rotation about the axis of the shaft  14  and with respect to translation, for example by welding or crimping. 
     According to a first variant, depicted in dotted lines in  FIG. 3 , of the first embodiment of the rotor  12 , the core  36  forms a single piece which is distinct from the pole pieces  20 ,  22 . 
     The sleeve  58  is interposed radially between the shaft  14  and each pole piece  20 ,  22  on the one hand and between the shaft  14  and the core  36  on the other hand. 
     The core  36  comprises two external radial end faces, delimiting the axial ends thereof, against each of which the internal face of the flange  24  of each pole piece  20 ,  22  is pressed. 
     In this variant, the axial positioning of one pole piece  20 ,  22  with respect to the other is achieved by the internal radial face of each flange  24 , which is in abutment axially against a facing external radial face of the core  36 . 
     The production method is modified accordingly. 
     Thus, after easily fitting the excitation coil  34  on the core  36 , the fifth step E 5  is broken down into a step E 51  of mounting the core  36  on the shaft  14  during which the sleeve  58  is fitted in the core  36  and a step E 52  of mounting each pole piece  20 ,  22 . 
     Preferably, the step E 51  of mounting the core is performed after the first step E 1  of mounting the sleeve  58  and before the fifth step E 52  of mounting each pole piece  20 ,  22 . 
     Moreover, during the third step E 3  of adjusting the axial positioning, the operation E 31  of machining the internal faces  62 ,  64  of the half core  36  is here replaced by an operation of machining the internal face  62 ,  64  of each flange  24 . 
     In addition, to allow a precise axial positioning of one pole piece with respect to the other, the third step E 3  of adjusting the positioning comprises a second operation E 32  of machining each external end radial face of the core  36 . 
     Then the step E 52  of mounting each pole piece  20 ,  22  is implemented, after the implementation of the first five steps E 1 , E 51 , E 2 , E 3 , E 4 . The pole pieces  20 ,  22  are axially fitted on each side of the core  34  on end positions of the cylindrical surface  60  of the sleeve  58 , which thus form cylindrical surfaces able to receive the pole pieces  20 ,  22 . 
     According to a second embodiment of the invention depicted in  FIG. 5 , the shaft  14  is directly fitted in the hub  158  and therefore in the central part thereof formed by the core  36 . 
     The rotor  12  comprises here, in the aforementioned manner, two intermediate sleeves  58  that are arranged axially on each side of the core  36  and that are produced in one piece with the core  36 , and in which the driving portion or portions  57  of the shaft  14  is or are fitted. 
     The outside diameter of the core  36  is greater than the outside diameter of the surface  60  of each sleeve  58 . Thus the core  36  comprises two external end radial faces  70 ,  72  that project radially with respect to the sleeves  58  and delimit the axial ends of the core  36 . 
     The surface  60  of each sleeve  58  is fitted in the central bore  138  of the associated pole piece  20 ,  22  so that the internal radial face  74  of the flange  24  of the pole piece  20 ,  22 , delimiting respectively the rear axial end and the front axial end of the pole piece  20 ,  22 , is in abutment against the projecting external radial face  70 ,  72  facing the core  36 . The pole pieces  20 ,  22  are thus positioned axially with respect to each other. 
     In other words, the surfaces  60  are machined in a simple manner in each external cylindrical face of the axial end portions of the hub  158  comprising the core  36 . 
     In the embodiment depicted in  FIG. 5 , each pole piece  20 ,  22  is fixed to its associated intermediate sleeve  58  by welding  69 . For this purpose, two bevels  76 ,  78  are produced so as to coincide respectively in the external radial face of the flange  24  of the pole piece  20 ,  22  and in the external radial face of the associated sleeve  58 . 
     According a variant shown in  FIG. 4 , the connection between each pole piece  20 ,  22  and its sleeve  58  is effected by crimping. 
     The production method described above is modified in order to be adapted to this second embodiment of the invention. 
     Thus the fifth step E 5  is broken down into a step E 51  of mounting the core  36  on the shaft  14 , during which the sleeve  58  is fitted in the core  36 , and a step E 52  of mounting each pole piece  20 ,  22 . 
     However, the step E 51  of mounting the core is performed concomitantly with the first step E 1  of mounting the sleeve since the sleeves  58  and the core  36  form a single piece in the form of a hub  158 . 
     The step E 51  of mounting the core is therefore performed before the fifth step E 52  of mounting each pole piece  20 ,  22 . 
     In addition, during the third step E 3  of adjusting the axial positioning, the operation E 31  of machining the internal faces  62 ,  64  of the half core  36  is here replaced by an operation of machining the internal face of each flange  24 . 
     In addition, to permit a precise axial positioning of one pole piece with respect to the other, the third step of adjusting the positioning E 3  comprises a second operation E 32  of machining each external end radial face of the core  36 . 
     Advantageously the second operation E 32  of machining the core  36  is performed after the step E 51  of mounting the core  36 . This is because, when the sleeve  58  is force-fitted on the core  36 , each end face of the core  36  is liable to be deformed. Thus, in order to guarantee a precise axial positioning of the pole pieces  20 ,  22  and so as to permit optimum contact between each pole piece  20 ,  22  and the core  36 , it is preferable to carry out the machining of the core  36  after it is mounted on the sleeve  58 . 
     Then the step E 52  of mounting each pole piece  20 ,  22  is implemented, after the implementation of the first five steps E 1 , E 51 , E 2 , E 3 , E 4 . The pole pieces  20 ,  22  are fitted axially on each side of the core  34  on end portions of the cylindrical surface  60  of the sleeve  58 , which thus form cylindrical surfaces able to receive the pole pieces  20 ,  22 . 
     According to a variant, not shown, of the rotor  12 , the core  36  and the two sleeves  58  are distinct elements. The shaft  14  is therefore fitted directly in the core  36 , and the driving portions of the  57  of the shaft  14  are force-fitted in the sleeves  58 , which are distributed axially on each side of the core  36 . 
     The shaft  14  comprises in one embodiment a smooth portion between its two driving portions  57  associated with the sleeves  58 . This smooth portion serves as a centering device for the core  36 , the external periphery of the smooth portion of the shaft being in close contact with the internal periphery of the core  36  delimited by the internal bore of the core  36 . 
     It should be noted that the driving portions  57  are shorter axially than in the embodiments in  FIGS. 2 ,  3  and  5  so that the sleeves  58  are less deformed than in these figures. 
     The method of implementing this variant is similar to the method of implementing the second variant of the first embodiment. However, the step E 5  of mounting the core  36  precedes here the first step E 1  of mounting the sleeve  58  since the core  36  is mounted directly on the shaft  14  rather than on the sleeve  58  as is the case in the first embodiment. 
     In a third embodiment depicted in  FIG. 6 , the two driving portions  157 ,  257  of the central shaft  14  are shorter axially as in the aforementioned variant and, as in  FIGS. 2 and 3 , the core of the rotor  12  comprises two portions  36   a ,  36   b.    
     A sleeve respectively  258 ,  358  is associated with each portion of the core  36   a ,  36   b.    
     The sleeves  258 ,  358  are externally smooth and are in a single piece with the central shaft  14 . These sleeves  258 ,  358  therefore issue from the shaft  14  and have different outside diameters, just like the driving portions  157 ,  257 . 
     Each portion  157 ,  257  axially extends respectively the sleeve  258 ,  358 . 
     The portion  157  is adjacent to the collar  114 . 
     The outside diameter of the driving portion  257  is roughly equal to the outside diameter of the sleeve  258 . 
     The outside diameter of the sleeve  258  is less than the outside diameter of the driving portion  157 . 
     The outside diameter of the sleeve  358  is less than the outside diameter of the portion  257  and is therefore less than the outside diameters of the sleeve  258  and of the portion  157  with the largest outside diameter. 
     The central shaft  14  is therefore stepped in diameter. 
     The internal bores of the portions  36   a ,  36   b  are also stepped in diameter. 
     Each core portion has internally two portions with different diameters in order to cooperate respectively each with one of the portions  157 ,  257  and the associated sleeve  258 ,  358  of the shaft  14 . 
     As can be seen in this  FIG. 6  the portion  36   b  has a first part with an inside diameter less than that of its second part. This first part is delimited centrally by a cylindrical centering bore intended to come into close contact with the smooth external periphery of the sleeve  358  thus constituting a centering sleeve for the portion  36   b  and the pole piece  20 . The second part of this portion  36   b  is shorter axially than the first portion and is intended to come into engagement with the driving portion  257  that is harder than the core portion  36   b  so that, when the shaft  14  is force-fitted in the portion  36   b , the portion  257  cuts furrows in the second part for rotational connection of the shaft  14  with the pole piece  20 . The second part is therefore delimited centrally by a fixing bore. 
     Likewise the portion  36   a  has internally two parts, referred to as the third and fourth parts, with different diameters. The third part has an inside diameter less than that of the fourth part. This third part is delimited centrally by a cylindrical centering bore intended to come into close contact with the smooth external periphery of the sleeve  258 , thus constituting a centering sleeve for the portion  36   a  and the pole piece  22 . The fourth part of this portion  36   a  is shorter axially than the third part and is intended to come into engagement with the driving portion  357  that is harder than the core portion  36   a  so that, when the shaft  14  is force-fitted in the portion  36   a , the portion  357  cuts furrows in the fourth part for rotational connection of the shaft  14  with the pole piece  22 . The fourth part is therefore delimited internally by a fixing bore. 
     Thus the inside diameter of the first part is less than the inside diameter of the second part, roughly equal to the inside diameter of the third, which is less than the inside diameter of the fourth part. 
     The result of the above and  FIG. 6  is that each intermediate sleeve  258 ,  358  comprises at its external periphery a cylindrical surface coaxial with the central shaft  14 , which is associated with each pole piece  20 ,  22  and which is received in a complementary central cylindrical bore of the associated pole piece  20 ,  22 , so as to position the associated pole piece  20 ,  22  coaxially with the shaft  14 . 
     Each intermediate sleeve  258 ,  358  is interposed radially between the core portion  36   a ,  36   b  and the shaft  14 . 
     In this  FIG. 6  the second part emerges at the internal end face  62  of the portion  36   b  while the fourth part emerges at the external end face of the portion  36   a.    
     A groove  300  is provided at the front end of the shaft  14 . 
     This groove  300  is, in one embodiment, continuous. 
     During an operation of crimping the external face of the pole piece  20  the material of the pole piece  20  is made to flow into this groove  300  so that the pole pieces  20 ,  22  are finally locked axially between the collar  114  and the material  301  of the pole piece  20 . 
     Thus the sleeves  258  and  358  and the pole pieces  20 ,  22  are machined in advance. 
     The step E 1  of mounting the sleeves  258 ,  358  consists of producing a single-piece shaft with the sleeves  258 ,  358 . 
     The second step E 2  of adjusting the concentricity is easier since the first operation E 21  of step E 2  consists of machining the external periphery of the sleeves  258 ,  358  of the single-piece shaft  14 , steps E 22 , E 23 , E 3 , E 4  being performed in the aforementioned manner. 
     It should be noted that, during step E 23 , helical grooves  302  are produced at the external periphery of the rotor  12  in order to cut the eddy currents developed at the external peripheral face of the pole pieces as described in the document FR 2 774 524. Naturally this is applicable to the other embodiments. 
     Step E 5  consists of fitting the sleeves  258 ,  358  in the central bore of the pole pieces so that the internal faces of the portions  36   a ,  36   b  are in abutment against each other and pressed so as to ensure passage of the magnetic flux. 
     This step is performed easily since the sleeve  358  and the driving portion  257  pass through the cylindrical central bore of the pole piece  22  without any problem, the sleeve  258  coming into centering contact with the pole piece  22  before the driving portions are force-fitted in the second and fourth portions of the pole pieces  20 ,  22 . 
     Afterwards the crimping in the groove  300  is carried out, the collar  114  being in abutment on the pole piece  22  so that step E 6  is simplified. 
     For the requirements of the description, the rotor  12  has been described here arranged in an alternator. However, the rotor  12  is not limited to this application. 
     In a rotor produced according to the prior art, the force that is necessary for mounting the pole pieces directly on the driving portion of the shaft is very high. Consequently the axial distance between the two pole pieces is poorly controlled and it is necessary to provide a wide tolerance gap. 
     By virtue of the teachings of the invention, the force sufficient for mounting the pole pieces  20 ,  22  on their associated intermediate sleeve  58  is sufficiently reduced to substantially decrease this tolerance gap. 
     In a rotor according to the prior art, it is thus necessary to provide a large clearance between each axial end of the excitation coil and the flange of each pole piece. The rotor  12  produced according to the teachings of the invention makes it possible to obtain a more precise axial positioning of the pole pieces  20 ,  22  with respect to each other. It is therefore possible to install a longer excitation coil  34  between the two pole pieces  20 ,  22 , which makes it possible to increase the power of the alternator. 
     Advantageously, the precision of the axial distance between the two pole pieces  20 ,  22  of a rotor  12  according to the invention is improved compared with that of a rotor according to the prior art. It is thus possible to provide a excitation coil  34  that best occupies the space between the external periphery of the core  36  and the claws of the pole pieces, particularly in the context of the embodiment in  FIG. 5 . 
     It is also possible to better adjust the axial length of the stator body with respect to the axial length between the two pole pieces. 
     Likewise, the axial distance between the two pole pieces  20 ,  22  being better controlled, a rotor  12  according to the teachings of the invention makes it possible to arrange more powerful fans at the two ends of the rotor  12  without increasing the axial size of the alternator. Thus the rear fan  56  in  FIG. 1  is in a variant a ventilation device comprising two superimposed fans as described for example in the document WO 2004/106748 (corresponding to U.S. Patent Application 2007/041843), to which reference should be made. This ventilation device with two superimposed fans makes it possible to properly cool the excitation coil ends of the stator coil  18 , in particular when this stator coil  18  comprises, in the aforementioned manner, two three-phase windings in a delta offset by 30° and each connected to a bridge rectifier. This arrangement, associated with a mounting of permanent magnets between the teeth  30 , when the number of pairs can be less than or equal to the number of pairs of poles of the rotor, makes it possible to properly adjust the characteristic curve of the alternator (current intensity according to the number of revolutions per minute of the alternator) according to the application. 
     Another advantage is that it is also possible to reduce the axial size of the alternator. 
     The ratio between the outside diameter of the core and the outside diameter of the rotor can also be better controlled. 
     In general terms the power of the alternator is better controlled and the losses thereof are reduced. 
     By virtue of the invention it is possible not to modify the shaft  14  of the rotor  12  and therefore to use a shaft  14  of the standard type. 
     Likewise the core  36  is not modified profoundly. 
     Naturally all combinations are possible. Thus in  FIG. 5  the internal bore of the hub  158  can be configured like that in  FIG. 6  and therefore comprise four parts for receiving the shaft  14  in  FIG. 6 . 
     The groove  300  can comprise annular sectors separated by bands of material, each sector being filled by the material of the pole piece, which flows following the crimping operation so that a rotational locking is achieved as in  FIGS. 3 and 4 . 
     The presence of the collar  114  is not obligatory, the movement of the shaft being able to be programmed by means of a device. 
     The driving portions  57 ,  157 ,  257  can have another shape and comprise for example a plurality of projections. 
     While the form of apparatus herein described constitute a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.