Abstract:
The invention relates to an electric machine comprising a stator ( 10, 40 ) provided with a plurality of teeth ( 11, 41 ), each tooth supporting at lest one individual coil ( 13, 46, 47 ), an external rotor ( 30 ) which is radially arranged outside the stator and provided with constant magnets, an internal rotor ( 20 ) radially arranged inside the stator, provided with constant magnets and connected to the external rotor ( 30 ). At least one of the external ( 30 ) and internal ( 20 ) rotors is disposed in a flux concentration.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to electrical machines and more particularly to those having a stator, an outer rotor and an inner rotor fastened to the outer rotor. 
   A machine with two concentric rotors is described in International Application WO 91/06147. The stator has teeth provided at their ends with pole shoes. The coils have heads onto which a heat-transfer fluid, such as oil, is sprayed so as to remove the heat from the windings. Such a machine is relatively complicated to manufacture. 
   The electric motor described in Application WO 91/06147 is also relatively long in the axial direction, especially because of the coil heads and the means of cooling them. 
   BRIEF SUMMARY OF THE INVENTION 
   There is a need to benefit from a machine of simplified construction and reliable operation without the necessity of cooling the coils by spraying a heat-transfer fluid onto them. 
   There is also a need to have a compact machine capable of operating with a high torque and/or at high speed. 
   The subject of the invention, according to one of its aspects, is an electrical machine comprising:
         a stator comprising a plurality of teeth, each supporting at least one individual coil;   an outer rotor placed radially on the outside of the stator and comprising permanent magnets; and   an inner rotor placed radially on the inside of the stator, comprising permanent magnets and being fastened to the outer rotor.       

   Thanks to the use of individual coils associated with the teeth of the stator rather than distributed windings, the construction of the machine is simplified and its reliability is thereby increased. 
   The presence of the outer and inner rotors makes it possible to virtually double the torque for the same volume, compared with a motor having a single rotor. 
   Likewise, the stator does not have a fixed yoke which serves electrically only to close the magnetic flux path and is a source of considerable iron losses. Its omission therefore results in the elimination of these losses and in considerable improvement in efficiency, particularly at high speed. 
   Preferably, at least one of the outer rotor and the inner rotor is a flux-concentrating rotor, and preferably both rotors are flux-concentrating rotors, that is to say two consecutive magnets of a rotor have faces of the same polarity that are placed so as to face a common adjacent pole piece placed between said magnets. This may allow the number of magnets used to be reduced without thereby degrading the performance of the machine. This may also allow the machine to operate at a high rotation speed, the magnets of the inner rotor being able to be retained by the pole pieces, if required. 
   The two rotors have the same number of poles and the two rotors may or may not be angularly offset. 
   In embodiments in which the rotors are angularly offset, when the number of phases m is even, the two rotors are advantageously offset by an angle of approximately π/S, for example to within 10%, where S=mp, S being the number of teeth of the stator and p being the number of pairs of poles of a rotor, and when m is odd, the two rotors are advantageously offset by an angle α of approximately π/2S, for example to within 10%. 
   The outer rotor may have pole pieces each having at least one recess on the radially outer side. This may allow the rotor to be lightened without thereby unduly reducing the efficiency of the machine, the magnetic flux lines being particularly concentrated on the radially internal side of the pole pieces of the outer rotor. The pole pieces of the outer rotor may for example pass through a minimum cross section at mid-length along the circumferential direction. 
   The magnets of at least one of the inner rotor and the outer rotor may have a wedge shape when observed along the axis of rotation of the machine, its width increasing upon moving away from the stator. Such a shape of the magnets allows retention by the pole pieces at a high rotation speed of the rotor, without it being necessary, for example, to cement the magnets. 
   In one particular embodiment, the inner rotor has pole pieces linked through shape complementarity to a shaft of the machine. For example, the pole pieces of the inner rotor may have slots and may be engaged via these slots on ribs of the shaft. Such an arrangement may make it easier to construct the rotor and especially to avoid having to produce pole pieces with apertures for engaging rods independent of the machine&#39;s shaft, for example, into them. 
   Again, according to one particular embodiment, at least one of the rotors may have pole pieces placed between the permanent magnets and each having, on their side turned toward the stator, a convex domed face turned toward the stator. Such a shape of the pole pieces makes it possible to minimize the difference L d −L q , and therefore avoids having to use the reluctance in order to generate the motive force, and makes it possible to reduce torque ripple. 
   In one particular embodiment, the stator having n teeth  teeth, each of the rotors having n pairs  pairs of poles and the current having n phases  phases, the number n teeth  of teeth of the stator may be chosen according to the relationship; n teeth =n pairs ×n phases . When this relationship is met, it is possible for the stator not to be subjected to stresses that tend to ovalize it. 
   In another particular embodiment, the stator may have 6n teeth and each of the rotors may have 6n±2 poles, n being greater than or equal to 2. This makes it possible to have a high winding factor, reflecting the efficiency of use of the windings, and thus the machine is more efficient and more compact. 
   In one particular embodiment, the teeth of the stator may each have a free first end located facing one of the rotors. The teeth may be fastened via a second end, opposite the first end, to to a nonmagnetic support. The support may for example be made of nonmagnetic steel or of aluminum or even an insulating material. 
   Such a configuration is most particularly suitable when each tooth of the stator serves as a core for a winding, by supporting a single individual coil. This is referred to as a “concentrated winding”. 
   As a variant, the teeth of the stator may each have two opposed free ends facing the inner and outer rotors, respectively. 
   The teeth may be held in place, for example at approximately mid-length by a nonmagnetic support. This support may be of tubular general shape. Such an arrangement is most particularly suitable when each of the teeth of the stator has two individual coils, which are not electrically together, so as to have a stator comprising two independent electrical circuits, where appropriate. 
   The stator may as a variant have a yoke made as a single part with the teeth. The yoke may be made of a magnetic material. 
   Preferably, the teeth of the stator are devoid of pole shoes, thereby allowing the individual coils to be fastened by engaging them on the teeth, the coils being produced separately. 
   As a variant, the teeth of the stator may have pole shoes. 
   The teeth of the stator may have notches near their free end facing one of the rotors. The coils may be held in place on the teeth by nonmagnetic blocks engaged in these notches. 
   The outer rotor may be surrounded by a casing, for example made of nonmagnetic steel or aluminum, which may be fastened onto the shaft of the machine, which is made for example of aluminum. 
   For machines having a relatively large axial dimension, it may prove desirable to duplicate the structure so as to reduce the cantilevered length of the stator and the rotors. 
   The stator may thus have a double structure, as may the inner rotor or the outer rotor. 
   The machine may constitute a synchronous motor or a generator, or even both in succession, and it may be used for example in an electric vehicle for driving the wheels and for recovering energy when braking. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood on reading the detailed description that follows of non limiting illustrative examples thereof and on examining the appended drawing in which:
           FIG. 1  is a schematic cross-sectional view of one example of a machine according to the invention;     FIG. 2  is a partial schematic view in axial section of the machine of  FIG. 1 ;     FIGS. 3 to 6  are partial views similar to  FIG. 1  of alternative embodiments of the invention;     FIG. 7  is a partial schematic view in cross section of one embodiment of the stator;     FIG. 8  shows, in isolation, a support piece for the teeth of the stator of  FIG. 7 ;     FIG. 9  is a view similar to  FIG. 7  of an alternative embodiment of the stator;     FIG. 10  is a circuit diagram illustrating the possibility of the stator having two independent electrical circuits so as to easily form a neutral point;     FIGS. 11 and 12  show two examples of alternative embodiments of the invention; and     FIGS. 13 and 14  are views similar to  FIG. 7  of alternative embodiments of the stator.       

   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   The electrical machine  1  shown in  FIGS. 1 and 2  comprises a stator  10 , an inner rotor  20  and an outer rotor  30  that are fastened together by a mechanical linkage  2  between them. 
   The stator  10  has a plurality of teeth  11  formed by a stack of magnetic laminations electrically isolated from one another, these teeth being fastened to a support piece  12  made of a nonmagnetic material, for example a nonmagnetic steel or aluminum, or made of an insulating material. 
   The support piece  12  is fastened, in the example described, to a frame  3  of the machine, as shown schematically in  FIG. 2 . 
   In the example in question, the teeth  11  are fastened via their radially innermost end to the support piece  12 , which has a generally tubular shape. The teeth  11  may be fastened by any means to the support piece  12 , for example they may be welded thereto. 
   Each tooth  11  carries an individual coil  13 , which comprises one or more electrical conductors wound around the axis of the corresponding tooth. 
   The coils  13  are connected to electrical conductors (not visible). A conventional device makes it possible, when the machine is used as a motor, to generate a rotating magnetic field and, when the machine is used as a generator, to collect the induced current. 
   The coils  13  may for example be connected to partially stripped cables, as described in Patent Application EP-A-1 251 623. 
   The teeth  11  are, as may be seen, devoid of pole shoes at their radially outer end, so as to allow the coils  13  manufactured beforehand to be put into place. 
   The teeth  11  may have parallel or nonparallel lateral faces, especially faces that diverge on moving away from the outer rotor  30 , so as for example to allow a certain amount of blocking of the coils  13  on the teeth, as described in the aforementioned Patent Application EP-A-1 251 623. 
   The teeth may each have, in the vicinity of their free end, two small notches  14  so as for example to allow the coil-retaining blocks  15  to be fastened onto the teeth. These blocks  15  may be made of an insulating material, for example a plastic. 
   The inner rotor  20  has a nonmagnetic shaft  21 , for example made of aluminum or an alloy of this metal, made of nonmagnetic steel or made of a composite. 
   The shaft  21  has ribs  22  serving for catching pole pieces  23 , each consisting of a packet of superposed identical magnetic laminations. The use of superposed magnetic laminations helps to reduce the induced current losses. Each rib  22  has a cross section in the general form of a T. The pole pieces  23  are not magnetically connected together, owing to the use of a nonmagnetic material to produce the shaft. 
   In the example in question, the pole pieces  23  have a convex domed face  24  turned toward the stator  10 . 
   Permanent magnets  25  are placed radially between the pole pieces  23 . Each magnet  25  has, when observed along the axis of rotation X of the machine, a slightly tapered shape, its width decreasing toward the stator  10 . 
   Each magnet  25  has a transverse magnetization and may be a one-piece magnet or may consist of several individual magnets place end to end. 
   The magnetic poles of the same polarity of two adjacent magnets  25  are directed toward the pole piece  23  lying between these two magnets, as illustrated in  FIG. 1 . In the example shown, the magnets  25  extend over practically the entire radial dimension of the sides of the pole pieces  23  and in contact with them. 
   The housings formed between the pole pieces  23 , in which the magnets  25  are placed, tend to widen under the effect of the centrifugal force when the inner rotor  20  is rotated at a speed greater than a predetermined speed, owing to the elasticity of the materials used. This widening tends to decrease when the rotation speed decreases. 
   In general, the inner rotor  20  may be similar to the rotor described in Patent Application EP-A-1 249 919. 
   The outer rotor  30  has permanent magnets  31  placed between pole pieces  32 , the rotor being surrounded by a nonmagnetic casing  33 , for example as described in Patent Application EP-A-1 251 023. 
   In the example described, the magnets  31  have a wedge shape when observed along the axis of rotation X of the machine, its width increasing upon moving away from the stator. 
   In the example in question, the poles of the two rotors are not angularly offset. Two consecutive magnets  25 ,  31  of the outer  30  and inner  20  rotors have faces of the same polarity that are placed so as to face a common adjacent pole piece  23 ,  32  placed between said magnets. Two pole pieces  23 ,  32  of each of the inner  20  and outer  30  rotors, located on any one radius, are of opposite N, S polarity. 
   In the example shown in  FIGS. 1 and 2 , the stator  10  has twelve teeth  11  and twelve coils  13 , while each of the rotors has eight poles, but the number of teeth or the number of poles may be different without thereby departing from the scope of the present invention. 
   Again in the example shown in these figures, the pole pieces  32  of the outer rotor  30  have radially internal faces  32   a  and radially external faces  32   b  that are cylindrical. 
   It would not be outside the scope of the present invention if the pole pieces were to have a different shape. 
   The pole pieces of the outer rotor may for example have a convex, domed, radially internal face turned toward the stator, as shown in  FIG. 3 . 
   The outer rotor  30  may have pole pieces each having at least one recess on their radially external side. 
   As an example,  FIG. 3  shows an outer rotor having recesses  34  between each of the pole pieces  32  and the nonmagnetic casing  33 . In this example, the pole pieces  32  of the outer rotor  30  each pass through a minimum cross section at mid-length along the circumferential direction. 
   Of course, it would also be possible for the outer rotor  30  not to have recesses  34  and for the pole pieces  32  not to have convex domed faces  32   a  turned toward the stator, without thereby departing from the scope of the present invention. 
   In the examples shown in the  FIGS. 1 to 3 , the permanent magnets  25  and  31  of the inner  20  and outer  30  rotors are of trapezoidal general shape. It would not be outside the scope of the present invention if the shape of the magnets were to be different. 
   By way of example,  FIG. 4  shows part of a machine having permanent magnets of parallelepipedal general shape. It would not be outside the scope of the present invention if the magnets of only one of the two rotors were to be trapezoidal and the magnets of the other rotor were to be parallelepipedal. The pole pieces  23  of the inner rotor  20  may then have shoulders  26  for the retention of the magnets between two successive pole pieces. 
   The inner rotor  20  that has just been described with reference to  FIGS. 1 to 3  has pole pieces  23  fastened by shape complementarity onto ribs  22  of the shaft  21 . It would not be outside the scope of the present invention if the pole pieces  23  were to be fastened in another manner to the shaft  21 . 
   The pole pieces  23  shown in  FIG. 4  are fastened by rods  27  that pass through the pole pieces and they are connected at each of their ends to retaining flanges (not shown). The shaft  21  is in this example of cylindrical general shape. 
   In the examples that have just been described, the poles of the inner and outer rotors are not angularly offset. 
   It would not be outside the scope of the resent invention if the poles of the two rotors were to be angularly offset, as shown by way of example in  FIG. 5 . In this figure, the poles are offset by an angle α, which has been intentionally exaggerated for the sake of clarity of the drawing. 
   When the number of phases m is even, the two rotors may be offset by an angle α of approximately π/S, where S=mp, S being the number of teeth of the stator, p being the number of pairs of poles of a rotor, and when m is odd, the two rotors may be offset by an angle α of approximately π/2S. Such an offset makes it possible to reduce, or even eliminate, torque ripple. 
   In the examples that have just been described, the number of teeth n teeth  of the stator is equal to 12, the number of poles of each of the rotors is equal to 8, with the number of pairs of poles n pairs  being equal to 4 and the number of phases being equal to 3, thus satisfying the relationship n teeth =n phases ×n pairs . 
   It would not be outside the scope of the present invention if the number of teeth of the stator or the number of poles of each of the rotors were to be different. 
   As an example,  FIG. 6  shows a machine having twelve teeth and ten poles at each of the rotors. The stator thus has 6n teeth and each of the rotors has 6n±2 poles, n being equal to 2 in this example, but it would not be outside the scope of the present invention if n were to be greater than 2. 
   In the examples that have just been described, each of the teeth of the stator carries a single individual coil, but it would not be outside the scope of the present invention if each of the teeth of the stator were to carry more than one, and especially two, individual coils. 
   By way of example,  FIG. 7  shows, schematically and partially, a stator  40  having teeth  41  that are held in place substantially at mid-length along their radial edges  42  by a nonmagnetic support  43  or a support made of insulating material, or else a combination of the two, said support having an annular shape closed on one or both sides. As a variant, the support  43  may be magnetic. 
   The teeth  41  have free ends  44  and  45  facing the inner rotor  20  and the outer rotor  30  respectively, each tooth being devoid of a pole shoe. 
   Coils  46  and  47  are placed respectively on either side of the support  43  on each tooth  41  in order to create a rotating magnetic field in the inner and outer rotors respectively, or to recover the current induced by the inner and outer rotors. 
   In the example shown in  FIG. 7 , the edges  42  of each tooth  41  are parallel, but the teeth  41  could, where appropriate, have nonparallel edges  42 , the teeth  41  widening for example toward the support  43  so that the coils  46  and  47  can be engaged on the teeth  41  with a slight clamping effect. 
   The teeth  41  could also have, at each of their free ends  44 ,  45 , two small notches intended for the fastening, onto the teeth, of coil-retaining blocks on the teeth, in a manner similar to that described with reference to  FIGS. 1 and 2 . 
   In what has just been described, the teeth are devoid of pole shoes, but it would not be outside the scope of the invention if the teeth were to have pole shoes  70 , making it possible for example to fasten coil-retaining blocks  71  onto the teeth, as illustrated schematically in  FIG. 13 . 
   The support  43  is shown schematically in  FIG. 8 . It has an annular general shape with openings  48  intended to house the teeth  41 . 
   The teeth  41  may be fastened by any means to the support  43 , for example by force-fitting, by welding or by bonding. 
   It would not be outside the scope of the present invention if the stator were to be produced differently. 
   For example, the stator may have coils  46 ,  47  placed on a yoke  73  made as a single part with the teeth  41 , for example made of a magnetic material, as illustrated schematically in  FIG. 14 . The stator shown in  FIG. 14  may for example be produced by molding, or else by the stacking of laminations. 
   The stator may also be produced differently. 
   As an example,  FIG. 9  shows a stator having teeth  41  that include, in the middle of their edges  42 , notches  50  capable of housing ends  52  of elements  53  that join the teeth together. 
   Each of the elements  53  has a curved general shape, being provided at its ends with raised features designed to cooperate with the notches  50  in order for two successive teeth  41  to be firmly held in place. 
   The elements  53  may be nonmagnetic or, as a variant, magnetic. 
   The coils carried by any one tooth may be electrically connected together, but it would not be outside the scope of the present invention if the two coils on any one tooth were not to be electrically connected together. 
   In this case, as shown in  FIG. 10 , the inner and outer coils of the stator may form two independent three-phase electrical circuits  61  and  62  connected, at the output of the machine  1 , to respective rectifier circuits  63 ,  64  and then to rising-edge or falling-edge voltage choppers  65 ,  66 , with the formation of a neutral point  67 . 
   The two electrical circuits  61  and  62  may or may not have a common neutral point  69 , shown by the dotted lines, which may or may not be connected to the neutral point  67  of the two rising-edge or falling-edge voltage choppers  65  and  66 . 
     FIGS. 11 and 12  show other possible configurations. 
   In particular, as illustrated in  FIG. 11 , the inner rotor  20  may be connected to the outer rotor  30  via a mechanical link  2 ′ that extends radially between two parts  10   a  and  10   b  of the stator  10 , the latter having a double structure. 
   Each part  10   a  or  10   b  has teeth, each tooth carrying one or two individual coils, in the manner described above, and the inner and outer rotors also each have a double structure, with respective first parts  20   a  and  30   a  designed to cooperate with the part  10   a  of the stator and respective second pats  20   b  and  30   b  designed to cooperate with the part  10   b  of the stator. 
   In the alterative embodiment shown in  FIG. 12 , the stator  10  again has a double structure with two parts  10   a  and  10   b , as do the inner and outer rotors. 
   The parts  10   a  and  10   b  of the stator are connected to the frame  3  via a mechanical linkage  70 , which is joined to the facing ends of the parts  10   a  and  10   b  of the stator. The part  20   a  of the inner rotor is joined, via a mechanical linkage  2 ″ similar to that shown in  FIG. 2 , to the part  30   a  of the outer rotor and the same applies to the other part  20   b  of the inner rotor, which is connected via a mechanical linkage  2 ″ to the other part  30   b  of the outer rotor  30 . 
   In the examples shown in  FIGS. 11 and 12 , the stator and the inner and outer rotors are generally symmetrical with respect to a mid-plane M perpendicular to the axis of rotation X, but this is not necessarily so and it would not be outside the scope of the present invention if double structures were to be produced with unsymmetrical parts. 
   Of course, the invention is not limited to the examples that have just been described. In particular, the features of the various embodiments described may be combined together. 
   Throughout the description, including the claims, the expression “having a” must be understood as being synonymous with “having at least one”, unless specified to the contrary.