Abstract:
A synchronous rotating electrical machine is disclosed, of the type including a stator ( 10 ) and a rotor ( 11 ). The rotor is of the flux concentration type and includes a plurality of alternate North and South poles formed from permanent magnets (PM). The magnets are housed in slots (E 1 ) arranged in the magnetic body of the rotor. The rotor includes, for each permanent magnet, a magnetic circuit allowing the circulation of a defluxing magnetic flux. This magnetic circuit has a magnetic reluctance of the defluxing circuit (R f ) determined as a function of an internal magnetic reluctance of the magnet (R a ) such that the ratio (R f /R a ) of the magnetic reluctance of the defluxing circuit on the internal magnetic reluctance of the magnet (R a ) is within a range of predetermined values guaranteeing the magnet against a risk of demagnetization. This range of predetermined values is from approximately 0.3 to approximately 3 depending on the type of magnet.

Description:
This application is a US Utility Patent Application, and claims priority to French patent application Ser. No. 10/52448 filed Mar. 31, 2010. 
     FIELD OF THE INVENTION 
     The present invention relates to a synchronous rotating electrical machine with permanent magnets and flux concentration. More particularly, the invention relates to a rotating electrical machine of this type for applications such as generators or electric traction engines in electric and hybrid automotive vehicles. 
     BACKGROUND OF THE INVENTION 
     Thanks to their improved performance in terms of output and power-to-weight and power-to-volume ratios, synchronous engines with permanent magnets are widely used today in the field of traction in automotive vehicles. Moreover, the availability of rare-earth permanent magnets on a large scale and under acceptable economic conditions makes the choice of such electric engines viable for new generations of automotive vehicles. 
     Such electric engines can be produced in a wide range of powers and speeds and will find applications in all-electric vehicles and in low CO 2  vehicles of the types known as “mild-hybrid” and “full-hybrid”. 
     “Mild-hybrid” applications generally concern electric engines of the order of 8 to 10 KW, for example, an electric engine mounted at the front of a heat engine and coupled to it by a drive belt. It is possible with such an electric engine to reduce the thermal cubic capacity (engine downsizing) by providing torque electrical assistance which supplies auxiliary power especially when accelerating. Moreover, low-speed traction, for example in an urban environment, can also be provided by this same electric engine. Applications of the “full-hybrid” type generally concern engines of 30 to 50 KW for architectures of the series and/or parallel type with a more successful level of integration of the electric engine or engines in the vehicle&#39;s traction chain. 
     Among the known different synchronous engines with permanent magnets those of the flux concentration type are of particular interest owing to their excellent performance. In these flux concentration engines, the magnets are buried in the magnetic body of the rotor and arranged according to a roughly radial configuration. 
     In an automotive vehicle, an electric engine used in traction on all of the vehicle&#39;s circulation tasks is subject to variable conditions of speed and charge. A strategy of maximum torque control complemented by a defluxing strategy (also called “demagnetisation strategy”) to achieve the high speed zone seems to be a good solution for controlling the electric engine. However, defluxing in engines with permanent magnets presents a risk of demagnetisation of the magnets and even more so when the temperature of the magnets is high. Flux concentration engines, thanks to the arrangement of their magnets in the rotor, are particularly exposed to this risk of demagnetisation. 
     It is therefore desirable to propose synchronous rotating electrical machines with permanent magnets and flux concentration in which the risk of demagnetisation of the magnets is controlled or even totally eliminated. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention relates to a rotating electrical machine comprising a stator equipped with stator windings and a rotor, the rotor comprising a plurality of alternate North poles and South poles which are formed from permanent magnets arranged according to a roughly radial configuration known as with flux concentration, the permanent magnets being housed in first respective slots arranged in the magnetic body of the rotor. 
     In accordance with the invention, the rotor comprises at least, for each permanent magnet, a defluxing magnetic circuit allowing the circulation of a defluxing magnetic flux, the defluxing magnetic circuit having a magnetic reluctance of the defluxing circuit determined as a function of an internal magnetic reluctance of the permanent magnet such that the ratio of the magnetic reluctance of the defluxing circuit to the internal magnetic reluctance of the magnet is within a range of predetermined values, of between approximately 0.3 and approximately 3, guaranteeing the permanent magnet against any risk of demagnetisation when a current having an intensity equal to that of a short-circuit current of the machine is injected into the stator windings so as to oppose the magnetic flux generated by the permanent magnets. 
     Preferably, the permanent magnets are roughly rectangular in shape. 
     According to particular forms of embodiment of the invention, the permanent magnets are of the NdFeB type and the range of predetermined values is from approximately 0.7 to approximately 1.5. 
     According to a particular characteristic of the invention, the defluxing magnetic circuit comprises a second slot arranged in the magnetic body of the corresponding magnetic pole, this second slot being situated in a lower central part of the pole, this second slot being centred at roughly equal distances between lower extremities of the two consecutive radial permanent magnets delimiting the pole and having a first reluctance contributing to the magnetic reluctance of the defluxing circuit. 
     According to a further particular characteristic of the invention, the defluxing magnetic circuit comprises a constriction in the magnetic body of the corresponding magnetic pole, the constriction being situated in the lower central part of the pole, between a lower extremity of the corresponding permanent magnet and the second slot, and the constriction having a second reluctance contributing to the magnetic reluctance of the defluxing circuit. 
     According to a further particular characteristic of the invention, the defluxing magnetic circuit comprises a recess in the magnetic body of the rotor which emerges at the external circumference thereof and at the interior of the first slot forming a permanent magnet housing, the recess being roughly radial and parallel to the axis of the rotor, and the recess having a third reluctance contributing to the magnetic reluctance of the defluxing circuit. 
     According to a further particular characteristic of the invention, the first slot forming a permanent magnet housing comprises in its bottom part, at least one first space left vacant by the permanent magnet and forming a first reluctant space which opposes a local demagnetisation of the permanent magnet in its bottom part. 
     According to a further particular characteristic of the invention, the first slot forming a permanent magnet housing comprises in its upper part, at least one second space left vacant by the permanent magnet and forming a second reluctant space which opposes a local demagnetisation of the permanent magnet in its upper part. 
     According to a further particular characteristic of the invention, the rotor comprises, in each of its poles, a third slot arranged radially in the magnetic body of the pole concerned, between the two consecutive permanent magnets delimiting the pole, the third slot having a bottom part which participates in the control of the defluxing magnetic flux through the central part of the rotor and a top part which participates in a reduction in the mechanical inertia of the rotor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention will become apparent on reading the following description of a number of particular forms of embodiment, with reference to the figures below, in which: 
         FIG. 1  shows, in a simplified fashion, the structure of a particular form of embodiment of a defluxable rotating electrical machine with permanent magnets and flux concentration according to the invention; 
         FIG. 2  shows the different slots embodied in the magnetic body of the rotor of the machine of  FIG. 1 ; 
         FIG. 3  shows a configuration of mounting of the permanent magnets, incorporating a strip, at the top extremity of the magnets; 
         FIG. 4  shows a network of reluctances of a defluxing magnetic circuit for a permanent magnet of the rotor of the machine of  FIG. 1 ; 
         FIG. 5  is a magnetisation curve of the magnets showing the working point of these latter at the intersection of their straight line of charge; 
         FIGS. 6   a  and  6   b  show respectively first and second embodiments of a portion forming a bridge between two adjacent parts of a magnetic body of the rotor, on either side of a slot E 3 ; 
         FIGS. 7 and 8  are respectively torque and inertia curves as a function of a height H of the portion forming a bridge of  FIGS. 6   a  and  6   b ; and 
         FIGS. 9 and 10  are respectively torque curves of the machine and inertia curves of the rotor as a function of a length L of the portion forming a bridge of  FIGS. 6   a  and  6   b.    
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows the structure of a particular form of embodiment 1 of a defluxable rotating electrical machine according to the invention. The machine  1  is of the type with buried magnets and flux concentration and comprises a stator  10  and a rotor  11 . 
     A concrete embodiment of such a machine according to the invention is for example a traction engine of 8 to 10 KW for applications in automotive vehicles of the type known as “mild-hybrid”. A particular form of embodiment of this engine comprises a stator  10  having 60 recesses  101  and a rotor  11  comprising 10 alternate North poles (Np) and South poles (Sp) as illustrated in  FIGS. 1 and 2 . The rotor  11  has a diameter of the order of 100 mm and an axial length of the order of 50 mm. The rotor  11  comprises 10 permanent magnets PM having a roughly rectangular shape and with the dimensions: length (L zs )×height (h a )×width (l a )=50 mm×25 mm×5 mm. 
     The stator  10  and the rotor  11  are traditionally equipped with packets of metal sheets forming magnetic bodies. 
     The recesses  101  of the stator  10  are provided to receive stator windings (not shown) and form between them a plurality of stator teeth. Depending on the forms of embodiment, the recesses  101  will be provided to house concentrated windings, wound on large teeth, or distributed windings. 
     The rotor  11  has the general shape of a multi-lobed cylinder defining a magnetic body thereof, each of the lobes corresponding to a magnetic pole of the rotor. 
     The magnets PM are arranged radially so as to obtain a rotor structure of the flux concentration type. In certain forms of embodiment, the magnets PM may be slightly unbalanced in relation to the radius of the rotor  11 . The magnets PM are preferably rare-earth permanent magnets such as magnets of the Neodymium Iron Boron (NeFeB), Samarium-Iron (SmFe) or Samarium-Cobalt (SmCo) type or magnets obtained from sintered or bonded ferrites. 
     The rotor  10  comprises a central bore emerging at its two facial extremities and designed to receive its drive shaft A. It will be noted that in the present invention the shaft A can be made of a magnetic or non-magnetic material according to the application envisaged. 
     The rotor  10  also comprises slots E 1 , E 2  and E 3  which are repeated for each pole and extend axially over roughly the entire length of the rotor. 
     Closing metal sheets, without the slots E 1 , E 2  and E 3 , may be provided at the facial extremities of the rotor  11  in order to contribute to the assembly of the rotor  11 . Soldering points (not shown) at the edge of the packet of metal sheets and through tie-rods (not shown), parallel to the central axis, are also provided for the assembly of the rotor  11 . The through tie-rods are made of a magnetic or non-magnetic material, depending on the applications. Advantageously, the passage of the tie-rods through the packet of metal sheets of the rotor  11  can be made through the slots E 3 . 
     In this particular form of embodiment, the slots E 1 , E 2  and E 3  are each 10 in number, this number corresponding to the number N=10 of magnetic poles of the rotor  11 . 
     The slots E 1 , E 2  and E 3  are now described in detail with reference also to  FIGS. 2 and 3 . 
     The slots E 1  form quasi-rectangular housings for the permanent magnets PM. The slots E 1  are not occupied entirely by the magnets PM and comprise parts left vacant which fulfil functions of reluctance and magnetic barrier for controlling the passage of the magnetic flux in the magnetic body of the rotor  11  and the magnets PM. 
     The slot E 1  emerges on the external circumference of the rotor  11  by means of a recess E 10 . The recess E 10  is extended axially along the length of the rotor  11  and thus delimits the pole in relation to the neighbouring pole. First and second tips B 1 , B 2  are thus formed in adjacent poles, which are opposite each other and designed to retain the permanent magnet PM in its housing, by opposing the effect of the centrifugal force on the magnet PM. 
     A reluctant space E 11  is also provided between the top edge of the magnet PM and the lower face of the tips B 1 , B 2 . This reluctant space E 11  is a space of the slot E 1  left vacant by the magnet PM. 
       FIG. 3  shows details of a mounting of the permanent magnet PM in the tips B 1  and B 2 . This mounting in  FIG. 3  corresponds to a particular form of embodiment in which a strip LM is provided. 
     The strip LM is interposed between the upper face of the magnet PM and the lower faces of the tips B 1 , B 2 . The strip LM is for example made of charged plastic, in a resin of the glass fibre charged epoxy resin type, a composite material or a deformable non-magnetic metallic material. The strip has the function of spreading the mechanical efforts being exercised on the top of the magnet PM and the tips B 1 , B 2  and of absorbing by deformation any displacement of the magnet PM. The tests carried out by the inventive entity have shown that the centrifugal forces being applied to the magnet PM can be significant. If the machine is subjected to a very high speed of rotation, the magnet PM tends to move away from the axis of rotation of the rotor  11 , under the effect of the centrifugal force, and deformation of the tips B 1 , B 2  towards the exterior of the rotor  11  could occur. The strip LM contributes to a better spread of the mechanical stresses between the tips B 1 , B 2  and by absorbing by deformation any displacement of the magnet PM thus reduces the risk of a breakage of the magnet PM and/or of deformation of the tips B 1 , B 2 . The tests carried out have shown that a strip thickness of at least 0.1 mm is desirable for it to correctly fulfil its function. 
     In accordance with the invention, there is also provided an angle of inclination a of the lower face of the tip B 1 , B 2 , in relation to the upper face of the magnet PM. This inclination can for example be obtained by means of a chamfer which according to the application will have an angle of between α=0.1° and α=15°. 
     Preferably, as shown in  FIG. 3 , there is associated with this chamfer of angle α a mechanical reinforcement obtained by means of a rounding of radius R. This rounding is made between the lower chamfered face of the tip B 1 , B 2  and the related face, roughly radial, forming an interior wall of the magnet housing. The radius R will preferably have a value of between 0.2 and 0.9 times the depth L B  of the tip B 1 , B 2 . For example, the radius R will be between approximately R=0.5 mm and approximately R=1.5 mm. As can be seen in  FIG. 3 , chamfers or equivalents (roundings) CF are provided in this form of embodiment on the upper part of the magnet PM so as to avoid contact on the edge of the magnet PM with the rounding of radius R. Reference will be made solely to the chamfers CF in the rest of the description and in the attached claims, in the knowledge that the term “chamfer” also covers the equivalents such as roundings. 
     The tests carried out by the inventive entity have shown that for the class of machines to which the invention applies, an angle α and a radius R in the ranges indicated above make it possible to obtain satisfactory results in terms of optimum resistance to the centrifugal forces being exercised in the range of speeds from 0 to 20000 rpm. 
     The strip LM allows a distribution of the mechanical efforts on the two tips B 1  and B 2  and can in certain cases make it unnecessary to chamfer the edges of the magnets PM when its thickness is sufficient. 
     It will be noted that the magnetic reluctances introduced by the recess E 10  and the reluctant space E 11 , as well as participating in the general polarisation of the magnet PM, described later, oppose local demagnetisation of the magnet PM at the edges. 
     As can be seen more particularly in  FIG. 2 , the slot E 1  comprises in its bottom part, near to the shaft of the rotor  11 , reluctant spaces E 12  which are, in this form of embodiment, spaces filled with air, left vacant by the magnet PM and which on introducing a magnetic reluctance prevent local demagnetisations of the magnet PM. 
     The slots E 2  essentially have a function of reluctance and magnetic barrier for controlling a defluxing magnetic flux through the central part of the rotor, in other words, in the magnetic body between the bottom of the magnets PM and the shaft A of the rotor. It will be noted that these slots E 2  are filled with air in this particular form of embodiment. In certain applications, they can be filled with magnetic or non-magnetic materials with a low relative permeability. 
     The slots E 3  fulfil a number of functions. Generally speaking, their function is essentially to contribute, like the slots E 2 , to the control of the defluxing magnetic flux, through the central part of the rotor, and to reduce the inertia of the rotor  11 . In this form of embodiment, like the recesses E 10 , the reluctant spaces E 11 , E 12  and the slots E 2 , these slots E 3  are filled with air. In certain applications, they can also be filled with non-magnetic, or magnetic, materials but with low density. 
     Although the slots E 3  are represented here as being arranged radially in the rotor  11 , centred between the consecutive magnets PM and symmetrical in shape, it will be noted that in other forms of embodiment of the invention, the slots E 3  can, especially in their top part, be neither centred nor symmetrical in shape. 
     As shown especially in  FIG. 2 , other slots, labelled E 4 , are arranged in each pole, on either side of the slot E 3 . The slots E 4  contribute to reducing the inertia of the rotor  11  and are situated within the alignment of the field lines, between them, so as to oppose to the least extent possible the passage of the magnetic flux of the magnets PM. In this form of embodiment, the slots E 4  are two in number on either side of the slot E 3 . Generally speaking, the number of the slots E 4  can vary according to the application and the room available. For example, their number can vary from 1 to 8, although 2 to 3 for the present invention is a good compromise. 
     Furthermore, it will be noted that the practical rules for cutting metal sheets impose the use of a width of material depending on the case of between 1 and 2 times the thickness of the metal sheet. In other words, this means, for example, that between two neighbouring slots of the rotor or between a slot and an external circumference of the rotor, there must be at least a width of material of between 1 and 2 times the thickness of the metal sheet. Thus, for example, for a metal sheet 0.35 mm thick, the minimum width of material to be conserved will be between 0.35 mm and 0.7 mm. 
     In this form of embodiment, the slot E 3  comprises an upper trapezium E 30  and a bottom part E 31 . Generally speaking, the upper trapezium E 30  makes it possible to reduce the inertia of the rotor  11 . However it has an effect on the magnetic reaction of the armature and can also be dimensioned so as to participate in the control thereof. The bottom part E 31  is the part of the slot E 3  which participates in the control of the defluxing magnetic flux through the central part of the rotor  11 . The part E 31 , jointly with the reluctant spaces E 12  and the slot E 2 , makes it possible to control the passage of the magnetic field lines in the central part of the rotor  11 . 
     In accordance with the invention, the definition of the slots E 31  and E 2  from the study of the spread of the magnetic field lines in the pole makes it possible to obtain a defluxing magnetic circuit with optimum dimensions. This defluxing magnetic circuit must be dimensioned so as to permit a passage of the magnetic flux surrounding the magnet PM on the appearance of a situation of a short circuit in the stator windings, or when in high speed a current equal as a maximum to the short-circuit current is injected into the stator windings so as to oppose the magnetic flux generated by the permanent magnets. 
     Such a defluxing magnetic circuit prevents the appearance in the magnet PM of a demagnetising field with too high an amplitude, capable of producing irreversible demagnetisation of the magnet PM. The defluxing magnetic circuit must therefore be calculated so as to obtain correct polarisation of the magnet PM. 
     The equivalent magnetic circuit around a permanent magnet PM, in a situation of a short circuit in the stator windings, is represented in  FIG. 4 . 
     In a situation of a short circuit in the stator windings, the field lines LC (shown in  FIG. 2 ) of the magnet PM do not pass through the stator of the machine. These field lines LC are then enclosed by means of a high magnetic reluctance Rh and low magnetic reluctances Rb 1  and Rb 2  of the magnetic circuit formed around the magnet PM. 
     The high magnetic reluctance Rh is that of the passage of the field lines LC through the tips B 1 , B 2  and the recess E 10 . The low magnetic reluctance Rb 1  is that of the passage of the field lines LC through the slots E 2  on either side of the bottom of the magnet PM, in the two adjacent poles. The low magnetic reluctance Rb 2  is that of the passages of the field lines LC through the ferrous constrictions S of the two poles, between the slots E 2  and the opposing edges of the slots E 1 . 
     The constrictions formed by the portions B 1 , B 2  and S function overall in saturation mode. 
     As shown in  FIG. 4 , the magnetic circuit comprises the source FMM in series with the internal reluctance R a  and a magnetic reluctance of the defluxing circuit R f  roughly equivalent to the three reluctances Rh, Rb 1  and Rb 2  in parallel. 
     With reference also to  FIG. 5 , a description is now given of the functioning of the permanent magnet PM in a short circuit situation. 
     For modern permanent magnets, for example of the NdFeB type, the magnetisation curve is roughly the shape of a straight line as shown in  FIG. 5 . This straight line is represented by the equation:
 
 B   a =μ a   ·H   a   +B   r   (1)
 
     in which B a  is the magnetic induction of the magnet PM, H a  is the magnetic field applied to PM, B r  is the residual magnetic induction of PM and μ a  is the PM magnetic permeability. 
     The tests carried out by the inventive entity have shown that in order to avoid irreversible demagnetisation of the magnet PM, it is desirable to have a working point P satisfying the equation:
 
 B   a   =B   r /λ  (2)
 
     with λ having a minimum value between λ min  and a maximum value λ max , which, for magnets PM of the NdFeB type, have been determined to be equal to:
 
λ min =1.7 and
 
λ max =2.5
 
     More generally, depending on the type of magnet, λ can be between 1.3 and 4. 
     The close value of λ=2 seems to be a good compromise in a short-circuit situation for a magnet PM. 
     For simplicity, it is assumed here that the permeability μ a  of the magnet PM is roughly equal to the absolute permeability μ o =4·π·10 −7  H/m. 
     The reluctance of a rectangular magnet such as the magnet PM is given approximately by the formula:
 
 R   a   =l   a /(μ o   ·h   a   ·L   zs )  (3)
 
     in which, as shown in  FIG. 2 , l a  is the width of the magnet PM, h a  is the PM height and L zs  is the PM length. 
     The coercive field H e  producing final demagnetisation of the magnet PM is given in first approximation by:
 
 H   e   =B   r /μ a   (4)
 
     The magnetic flux   a  generated by the magnet PM is given by the product of the magnetic induction and the surface, namely:
 
φ a   =B   a   ·L   zs   ·h   a   (5)
 
     The magnetic flux φ a  is also given by the ratio:
 
φ a   =FMM /( R   a   +R   f )  (6)
 
     The magnetomotive force of the magnet PM is given in first approximation by:
 
 FMM=B   r   ·l   a   /μhd a   (7)
 
     From equations (5), (6) and (7), one obtains the equation:
 
 B   a   ·L   zs   ·h   a =( B   r   ·l   a /μ a )/( R   a   +R   f )  (8)
 
     It follows from equation (8):
 
 R   a   +R   f =( l   a /(μ a   ·L   zs   ·h   a ))·( B   r   /B   a )  (9).
 
     By introducing equations (2) and (3) into (9), one obtains:
 
 R   a   +R   f   =R   a λ  (10)
 
     This gives the ratio:
 
 R   f   /R   a (λ−1)  (11)
 
     In the knowledge that λ must be between λ min  and λ max , one obtains the in equation:
 
(λ min −1)≦ R   t   /R   a ≦(λ max −1)  (12)
 
     For magnets PM of the NdFeB type, with λ min =1.7 and λ max =2.5, one obtains the in equation:
 
0.7 ≦R   f   /R   a ≦1.5  (13)
 
     More generally, depending on the type of magnet, one obtains the in equation:
 
0.3 ≦R   f   /R   a ≦3  (14)
 
     Preferably, one will choose R f /R a =1 which corresponds to λ=2. 
     In accordance with the invention, it is possible to determine values λ min  and λ max  for each type of magnet. Then, knowing the reluctance R a  of the magnets PM chosen for the machine, an overall defluxing circuit reluctance R f  can be determined from the in equation (12). Once the value of the overall defluxing circuit reluctance R f  has been determined, it is possible to optimise the spread thereof between Rh, Rb 1  and Rb 2  so as to obtain the desired performance. 
     With reference now to  FIGS. 6 to 10 , a description is given below of the optimisation of a portion BR forming a bridge between two adjacent parts of the magnetic body of the rotor  11  on either side of the slot E 3 . 
     The tests carried out by the inventive entity have shown that the dimensioning of this bridge BR of the packet of rotor metal sheets is important in order, on one hand, to reduce the moment of inertia of the rotor  11  and, on the other hand, to be able to guarantee obtaining the maximum torque wanted for the machine. 
     As can be seen in  FIGS. 6   a  and  6   b , showing two different forms of embodiment, the bridge BR is defined dimensionally by its height H and by its length L. 
     Such a bridge BR is necessary for reasons of mechanical resistance of the rotor  11 . In the forms of embodiment in  FIGS. 6   a  and  6   b , the bridge BR is continuous over the entire axial length of the rotor  11 , such that the slot E 3  does not emerge at any point of the circumferential surface of the rotor  11 . In the form of embodiment of  FIG. 6   b , the bridge BR is set back in relation to the circumferential surface of the rotor and a wider gap is obtained in the bridge BR between the stator  10  and the rotor  11 . In other forms of embodiment of the invention not represented, the bridge BR can be made discontinuous by letting the slot E 3  emerge on the exterior, at the circumferential surface of the rotor  11 , for example, one metal sheet in two. 
     It will be noted that the form of embodiment of  FIG. 6   b  with a bridge BR set back and the forms of embodiment indicated above with a discontinuous bridge BR can be interesting in certain applications, in order to reduce iron losses by a reduction of harmonics in the magnetomotive force. In the forms of embodiment described here, the variable gap between the rotor  11  and the stator  10 , visible in  FIGS. 1 ,  6   a  and  6   b , also contributes to a reduction of harmonics and therefore in iron losses. 
     With reference to  FIGS. 7 and 8 , the tests carried out by the inventive entity show that a good compromise between a maximum torque (Cmax) and a minimum inertia (I) will be obtained for a height H of the bridge BR which must remain less than approximately 1 times the width l a  (1×l a ) of the permanent magnets PM. 
     As shown in  FIG. 7 , the torque Cmax is achieved for a height H of 1×l a . Moreover, as can be seen in  FIG. 8 , a height H greater than 1×l a  will only result in increasing the inertia I without any gain of torque. 
     On a practical level, bearing in mind the rules for cutting metal sheets indicated earlier in the description, the height H of the bridge BR will be between approximately 1 times the thickness of the metal sheet and approximately 1 times the width of the magnet l a . Thus, for example, with metal sheets having a thickness of 0.35 mm and magnets having a width of 5 mm, the height of the bridge BR according to the invention will here be between approximately 0.35 mm and approximately 5 mm. 
       FIGS. 9 and 10  show curves of the torque Cmax and inertia I as a function of the length L of the bridge BR. The tests carried out by the inventive entity show that the best compromise of performance between a maximum torque and a minimum inertia will be obtained for a length L of the bridge BR of less than approximately 1.5 times the width l a  (1×l a ) of the permanent magnets PM. On a practical level the minimum length L which the bridge BR can take will be of the order of 1.5 times the thickness of the metal sheet, namely 0.520 mm approximately for a metal sheet thickness of 0.35 mm. 
     The invention has been described here in the context of particular forms of embodiment. The invention will find significant applications in electric traction engines used in cars, for electric vehicles and hybrid vehicles. However, it must be clear that the invention will also find applications in fields other than the automotive field.