Abstract:
A flux concerning electrical machine has a stator and a rotor, the stator including on its face a series of notches housing a series of stator coils, and further including a series of housings for excitation means, and the rotor including a plurality of flux commutator teeth for selectively making pairs of closed magnetic circuits through the stator coils. In accordance with invention the stator includes a plurality of independent stator coils defining as many phases and located in a series of individual cells angularly distributed on its circumference and the rotor teeth are equi-angularly spaced so that the angular positions of the various pairs of teeth have predetermined phases relative to the various individual cells. The invention is applicable in particular to brushless alternators for motor vehicles.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is generally concerned with rotating machines such as motor vehicle alternators. 
     A conventional motor vehicle alternator is a polyphase generator generally including a stator within which turns a rotor provided with an excitation coil. The coil is energised via brushes in contact with two collector rings on a projecting part of the rotor shaft. 
     Using brushes has disadvantages, including the need for a relatively great axial length of the alternator, a set of brushes and collectors that increase the unit cost and malfunctions due to faulty contact between the brushes and the collector rings, in particular as a result of wear. 
     There exist in the prior art certain proposals aimed at providing rotating machines that can be used as motor vehicle alternators that have no brushes. 
     In one prior art alternator the claws of the two pole wheels of the rotor, which normally interpenetrate, are truncated to provide space in a transverse plane for a support for a fixed excitation coil disposed inside the pole wheels. However, this approach is detrimental to the efficiency of the machine, because the areas of the air gap are then very significantly reduced. What is more, for a given output/speed curve, a machine of the above type is significantly heavier than a conventional machine, which is particularly disadvantageous in the case of vehicle alternators. 
     In another prior art alternator two pole wheels with interleaved claws are mounted cantilever fashion at one axial end of the machine between an internal fixed excitation coil and external stator coils. 
     This prior art solution also has disadvantages, in particular the fact that the axial dimension and the weight of the machine are necessarily increased. Also, the enlarged air gaps in the rotor reduce the efficiency of the machine. 
     What is more, in both cases referred to above, the design of the machine makes it essential to take into account electromagnetic phenomena not only in the plane perpendicular to the rotation axis, i.e. in two dimensions, but also in the direction of the axis, i.e. in the third spatial dimension. 
     This necessity for three-dimensional design of the machine makes it extremely difficult and time-consuming to model and to optimise the various parameters. 
     A flux commutating machine with no brushes, known in particular from document EP-0 707 374, has the advantage of being easily modelled and optimised in two dimensions only. 
     This prior art machine nevertheless has the drawback of being restricted to single-phase operation, although three-phase machines can be desirable in a large number of applications, in particular in terms of electromagnetic efficiency and in terms of the simplicity and economy of the associated rectifier and smoothing means. 
     BRIEF SUMMARY OF THE INVENTION 
     The Applicant has found that it is possible to use the flux commutation technique to produce various polyphase machines with great flexibility while retaining two-dimensional design and two-dimensional optimisation. 
     Accordingly, the present invention proposes a flux commutating electrical machine including a stator and a rotor, the stator having on its inside face a series of notches housing a series of stator coils and further including a series of housings for excitation means and a rotor including a plurality of flux commutator teeth adapted selectively to establish pairs of closed magnetic circuits through the stator coils, characterised in that the stator includes a plurality of independent stator coils defining the same number of phases and situated in a series of individual cells angularly distributed around its circumference and in that the teeth of the rotor are equi-angularly spaced so that the angular positions of the various pairs of teeth have predetermined phases relative to the various individual cells. 
     Preferred but non-limiting aspects of the machine in accordance with the invention are as follows: 
     each individual cell has a stator structure defining a pair of spaced notches for two portions of a stator coil, said notches being delimited laterally by two stator teeth, and excitation means adapted to establish in one or other of the stator teeth within said stator coil a magnetic field varying in accordance with the mutual angular position of the rotor teeth and the stator teeth; 
     the individual cells are separated from each other by gaps; 
     the individual cells are separated from each other by decoupling permanent magnets the field orientation of which is opposite to that of the excitation means; 
     the excitation means of each cell include a permanent magnet disposed between two stator elements having a generally U-shaped profile defining said notches and said stator teeth; 
     the excitation means of each cell include an excitation coil disposed in two notches of a single stator element one of which is substantially half-way between two stator coil notches also formed in said stator element; 
     the excitation coil is disposed in two notches in the inside surface and in the outside surface of the stator element to generate an essentially tangential magnetic field at said coil; 
     the excitation coil is disposed in two notches both in the inside surface of the stator element to generate an essentially radial magnetic field at said coil; 
     the various excitation coils are connected in parallel to the same source of current; 
     the machine comprises pairs of cells formed in a common structural element housing two stator coils corresponding to two different phases, a magnet for exciting one of the stator coils and an excitation coil for exciting the other stator coil; 
     the stator coils are angularly equidistant over all of the inside circumference of the stator, the angular gap between the various cells being substantially equal to the angular dimension of a notch receiving one run of a stator coil; 
     the portions of the stator coils in the same cell or the same group of cells are separated by a constant angular gap and the angular gap between portions of successive stator coils in two separate cells or groups of cells is different from said constant angular gap; 
     said constant angular gap is identical to the angular gap between two adjacent rotor teeth; 
     the geometry of the rotor and stator structures of the machine satisfies the following condition: 
     
       
         N M.q.[(N   c +1).Δθ R +(m/q).Δθ R ]=k.Δθ R =2π 
       
     
      where 
     N M  is the number of individual machines on the circumference of the stator, 
     q is the number of phases of each individual machine, 
     Nc is the number of excitation means, such as magnets, per phase of each individual machine, 
     m is a positive or negative integer representing the value of the phase difference between two individual machines and is in the range −(q−1) to +(q−1), 
     Δθ R  is the constant angular gap between two adjacent rotor teeth, or rotor pitch, 
     Δθ S  is the constant angular gap between two adjacent runs of stator coils, or stator pitch, and 
     k is an integer; 
     the rotor pitch and the stator pitch satisfy the following condition: 
     
       
         (⅞).Δθ S ≦Δθ R ≦(5/4).Δθ S   
       
     
     the stator pitch is equal to the rotor pitch and in that the geometry of the rotor and stator structures of the machine satisfies the following condition: 
     
       
         N m   .q . (N c +1)+N M   .m=k   
       
     
     the machine comprises a single individual machine with three regularly distributed individual cells respectively accommodating the stator coils of three phases and the rotor includes a number of rotor teeth equal to 4, 5, 7 or 8 and preferably equal to 5; 
     the machine comprises two individual machines each with three regularly distributed individual cells respectively accommodating the stator coils of three phases and the rotor includes a number of rotor teeth equal to 8, 10, 14 or 16 and preferably equal to 10; 
     the machine comprises a single stator frame including at least two series of internal notches adapted to accommodate respectively the stator coils and the excitation magnets or the excitation coils. 
     As indicated above, a rotating machine in accordance with the invention is advantageously used as a brushless alternator for motor vehicles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, aims and advantages of the present invention will become more apparent on reading the following detailed description of preferred embodiments of the invention given by way of example and with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic linear representation of the rotor and stator structure of a rotating machine in accordance with the present invention, 
     FIGS. 2 through 9 are diagrammatic cross-sectional views of eight embodiments of the rotor and stator structure of a rotating machine in accordance with the present invention, 
     FIG. 10 is a diagrammatic view in cross-section showing one possible generalisation of the rotor and stator structure of a machine in accordance with the invention, and 
     FIG. 11 is a view in cross-section of one concrete embodiment of a rotor and stator structure in accordance with the invention, without the associated coils and magnets. 
     Note that the following description is given with reference to a generator mode of operation. The skilled person will obviously realise on reading the description how such machines can operate as motors. 
     Note also that as far as possible components or parts that are identical or similar in different figures are designated by the same reference symbols. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring first of all to FIG. 1, for explanatory purposes part of a stator and part of a rotor of a flux commutating electrical machine in accordance with the invention are represented in a linear fashion. 
     The rotor  100  has along an edge a plurality of rotor teeth  101  which are preferably regularly spaced at a pitch Δθ R . 
     The stator  200  comprises a plurality of cells  210  each including two preferably identical stator elements  211  with a U-shaped profile defining on the side facing towards the rotor a notch receiving a stator coil  213 . Accordingly each element  211  defines two teeth  212  on respective opposite sides of the notch. An excitation permanent magnet  214  is placed between the two elements  211  with its N/S orientation directed from one stator element  211  to its neighbour, as shown here. Each cell  210  is separated from the adjoining cell by a gap  300 , the width of which is preferably equal to that of the magnets  214 . In this case, the pitch Δθ S  of the various stator elements  211  is constant. 
     As described in particular in document EP-A-0 707 374, a necessary condition for a machine equipped with a rotor and a stator as described above is that the pitch Δθ R  is close to the pitch Δθ S . 
     In this case, when they are in line with two stator teeth  212  delimiting both an excitation magnet  214  and a stator coil  213 , the pairs of rotor teeth  101  apply a maximal excitation magnetic flux to the stator coil. Rotation of the rotor therefore generates an alternating current in the coil. 
     The idea on which the present invention is based is to design the geometry of the rotor and of the stator so that the positions of pairs of teeth of the rotor relative to the stator elements is different from one cell to another, which is achieved by appropriate choice of the value of Δθ R  for a given value Δθ S . 
     In particular, if a three-phase machine is required, i.e. a machine having three independent stator coils in three stator cells  210 , it can be shown that the following equation must be satisfied: 
     
       
         2.Δθ S =2.Δθ R +(m/3).Δθ R   (1) 
       
     
     with mε{−2,−1,1,2}. 
     In the remainder of the description m=−1. 
     Given the above hypothesis, i.e. the use of three cells  210  each including two elements  211 , in other words six elements  211  regularly distributed around the circumference of a circular stator: 
     
       
         Δθ S =2π/6=π/3 
       
     
     Equation (1) above therefore gives: 
     
       
         Δθ R =2π/5 
       
     
     Thus by choosing a rotor with five equi-angularly spaced teeth  101  at its periphery a three-phase flux commutating rotating machine is obtained. 
     The values m=−2, m=1 and m=2 would respectively yield a rotor with 4, 7 and 8 teeth  101 . 
     Clearly the above reasoning can easily be applied to any number of phases. 
     FIG. 2 is a schematic representation of the machine obtained as described above. 
     This figure shows that, because of the necessary differences between the values of Δθ R  and Δθ S , the alignments between the rotor teeth  101  and the stator teeth  212  adjoining the notches are never optimal. The resulting flux losses are largely acceptable, however. 
     The above approach can naturally be applied to a stator including a number of cells  210  equal to an integer multiple of the number of phases. 
     For example, if the stator  200  has six cells  210 , the above calculation yields a rotor having ten equiangularly distributed rotor teeth  101 . 
     FIG. 3 shows this embodiment. In this case the stator coils of the three phases are arranged sequentially: phase  1 , phase  2 , phase  3 , phase  1 , phase  2 , phase  3 ; in this specific example the two coils for the same phase are therefore in two diametrically opposite cells  210 . 
     Depending on whether the output voltage or the current is to be emphasized, the two stator coils  213  of the same phase are connected either in series or in parallel. 
     FIG. 4 shows a variant of the rotor and stator structure from FIG. 2 in which additional permanent magnets  215  are inserted into the gaps  300  between pairs of stator cells  210 . 
     The magnets  215  have the opposite N/S orientation to the excitation magnets  214 , as shown. 
     The magnets  215  reinforce the excitation flux and improve the decoupling between the various phases in that they provide obstacles to the exit of the flux lines at the angular limits of each cell. 
     FIG. 5 shows the same variant but applied to the rotor and stator structure shown in FIG.  3 . 
     Excitation using permanent magnets as shown in FIGS. 2 to  5  can be replaced by excitation using coils. 
     Thus FIG. 6 shows a three-phase structure with three stator cells  210  and five rotor teeth, analogous to FIG. 2, in which excitation is provided by coils interleaved radially and circumferentially with the stator coils. 
     Three stator coils  213 ,  213 ′ and  213 ″ are shown, corresponding to the three phases, with three excitation coils  216 ,  216 ′ and  216 ″ wound in the directions indicated. The three stator coils and the three excitation coils are preferably formed in twelve regularly spaced notches on the inside face of a single stator frame  211 ′. 
     Each cell or phase  210  is delimited as indicated in chain-dotted line. 
     In the above type of embodiment each excitation coil  216  produces an essentially radial magnetic flux that flows in one of the two stator teeth  212  that it circumscribes as soon as the latter tooth is in magnetic contact with one of the teeth  101  of the rotor and the phenomenon observed is similar to that obtained with excitation by permanent magnets. 
     To have an excitation voltage that is as high as possible when the machine is connected to an onboard network of a motor vehicle, the three excitation coils  216 ,  216 ′ and  216 ″ are advantageously connected in parallel to the terminals of an excitation input EXC, as shown. 
     FIG. 7 shows a rotor and stator structure with six cells  210  separated by gaps  300  in a manner analogous to FIG.  3 . In this case the machine is of the mixed excitation type, however. Three first cells  210  therefore have an excitation magnet  214  between two adjacent U-shaped stator elements  211 . The other three cells  210 ′ each have a single rotor frame element  211 ′ defining two internal notches for the stator coil  213  and, halfway between the notches in the circumferential direction, an excitation coil  217  in an axial-radial plane received in two other notches respectively formed in the inside face and the outside face of the frame part  211 ′. 
     For balanced electrical behaviour when the rotor is rotating, the cells  210  and  210 ′ are preferably disposed in an alternating fashion. Accordingly each phase has a cell  210  excited by magnets and a cell  210 ′ excited by coils. 
     FIG. 8 shows an embodiment that differs from the one from FIG. 7 in three respects: 
     first, two adjacent cells  210 ,  210 ′ are grouped together on the same frame element  211 ′; 
     secondly, the cells  210 ′ excited by coils no longer have a coil lying in an axial-radial plane but a coil  216  analogous to that from FIG. 6, i.e. in a tangential plane; note that in this regard each coil has a run extending inside the phase winding  213  concerned and a run extending between the two phase windings of the same group of two cells; 
     thirdly, the frame members  211 ″ are separated by inverse decoupling and flux reinforcing magnets  215 , in a similar manner to what is shown in FIGS. 4 and 5. 
     Note that in all the preceding embodiments the rotor pitch Δθ R , i.e. the angular distance between two rotor teeth  101 , is not equal to the stator pitch Δθ S , i.e. the angular difference between two homologous stator teeth  212  or two adjacent stator members  210 . 
     An embodiment will now be described with reference to FIG. 9 which, whilst retaining the polyphase, and in particular three-phase, nature of the machine, provides for equal rotor and stator pitches Δθ R  and Δθ S , respectively. In FIG. 9, portions of the stator coils in the same cell  210  are separated by a constant angular gap (Δθ S ) and the angular gap between successive stator coils is not equal to the constant angular gap (Δθ S ). 
     In this particular example a rotor  100  has seven teeth spaced by a rotor pitch Δθ R  equal to 2π/7. 
     Three stator cells  210  are provided, each including, as in FIG. 2, two stator elements  211  receiving a stator winding  213  between two teeth  212  and separated by an excitation permanent magnet  214 . The stator pitch Δθ S  in each cell is also equal to 2π/7, as indicated hereinabove. 
     To assure an electric phase difference of 2π/3 between the first and second cells  210  it is therefore necessary and sufficient for the angular offset Δθ C  between the two cells to be equal to: 
     
       
         2π/7+2π/7+(1/3).2π/7=2π/3 
       
     
     which produces three cells  210  regularly spaced at 2π/3. 
     What is more, the teeth  212  preferably have the same angular dimension Δθ D  as the notches for the stator windings  213  and the excitation magnets  214 . In this specific case, this angular dimension is (2π/7)/4, that is to say 
     
       
         Δθ D =π/14. 
       
     
     In the above case, to optimise the coupling between the rotor teeth  101  and the stator teeth  212 , said rotor teeth  101  also have an angular dimension of π/14. 
     The angular dimension of the gaps  300  between the adjoining cells  210  is in this case equal to: 
     
       
         2π/3−(7.π/14)=π/6 
       
     
     How to generalise the present invention to a polyphase machine having any number of cells per phase will now be explained with reference to FIG.  10 . 
     The following notation will be used: 
     N M : the number of individual machines on the stator circumference 
     q: the number of phases of each individual machine 
     N c : the number of excitation means, such as magnets, per phase of each individual machine 
     m: an integer representing the value of the phase shift between two individual machines. 
     In the present example, this phase comprises a set of N c +1 “U”-shape stator elements  211  separated from each other by N c  magnets. 
     To determine the angular offset of the successive phases each phase is deemed to occupy an angular range equal to (N c +1).Δθ S , incorporating a fictitious (N c +1)th magnet  214 ′ shown in chain-dotted line in the figures. 
     The successive phases having to be spaced by (m/q).Δθ R  by definition, a machine with q phases will therefore occupy an angular range equal to 
      q.[(N c +1).Δθ R +((m/q).Δθ R ] 
     A machine with N M  individual machines will therefore occupy on the circumference of the stator a range equal to 
     
       
         N M.q.[(N   c +1).Δθ R +(m/q).Δθ R ]=2π  (2) 
       
     
     With the hypothesis of a stator with regularly distributed stator elements  211 : 
     
       
         k.Δθ S =2π  (3) 
       
     
     where k is an integer. 
     The number k is necessarily greater than 6 assuming a polyphase machine with a plurality of individual machines. 
     To assure proper commutation of the flux by the teeth  101  of the rotor, Δθ R  and Δθ S  are preferably chosen to be as close together as possible, and a condition of the following type is preferably chosen: 
     
       
         (7/8).Δθ S ≦(5/4).Δθ S   
       
     
     Finally, note that a positive sign is preferably applied to the value of  m , knowing that a negative sign could lead to untimely overlapping of two successive phases or to insufficient decoupling due to an excessively high proximity between phases. 
     The optimum machines obtained from the above developments will now be briefly described, respectively in the case of a two-phase machine and in the case of a three-phase machine, taking as hypothesis Δθ S =Δθ R . In this case the number k obtained is the number of teeth  101  on the rotor. 
     In the case of a two-phase machine (q=2 and m=±1), from equations (2) and (3): 
     
       
         k=2N M .[(N c +1)±1/2] 
       
     
     For example: 
     for m=+1, N M =2 and N c =1 
     
       
         k=4.[(1+1)+1/2]=10 
       
     
     for m=+1, N M =2 and N c =2 
     
       
         k=4.[(2+1)+1/2]=14 
       
     
     for m=−1, N M =2 and N c =1 
     
       
         k=4.[(1+1)−1/2]=6 
       
     
     for m=−1, N M =2 and N c =2 
     
       
         k=4.[(2+1)−1/2]=10 
       
     
     For a three-phase motor, q=3 and mε{−2;−1,1,2}. Adopting the same simplifying hypothesis as previously, i.e. Δθ S =Δθ R : 
     
       
         k=3N M .[(N c +1)+m/3] 
       
     
     For example: 
     for m=+1, N M =1 and N c =1 
     
       
         k=3.[(1+1)+1/3]=7 
       
     
     for m=+1, N M =1 and N c =2 
      k=3.[(2+1)+1/3]=10 
     for m=−1, N M =2 and N c =3 
     
       
         k=3.[(3+1)+1/3]=13 
       
     
     We have attempted to determine the optimum number of rotor teeth when the following parameters of the machine are imposed: 
     its outside radius R ext ; 
     its air gap radius R ent ; 
     it air gap size E. 
     It can be shown that the optimal number of teeth k opt  is obtained from the following formula: 
     
       
         K opt =π/(4.[(E/R ent ).((R ext /R ent )−(1+(E/2.R ent ))]) 
       
     
     Finally, FIG. 11 shows one concrete embodiment of the stator structure and of the rotor structure of a rotating machine in accordance with the invention embodying in particular the solution shown in FIG. 8 (machine with excitation by coils and by magnets and also provided with decoupling magnets). 
     Note that each rotor tooth  101  and each stator tooth  212  has a respective slightly enlarged root  101   a  and  212   a.    
     Note also that the stator includes a ferromagnetic frame element in which twenty-four notches spaced by 15° are formed. In this case the stator pitch Δθ S  is 30°. 
     The notches are divided into three groups with different depths depending on whether they receive phase coils, excitation coils or magnets. 
     To be more precise, the deepest notches  220  receive the excitation magnets  214  and the decoupling magnets  215  (not shown), the intermediate depth notches receive the excitation coils  216  (not shown) and, finally, the shallowest notches  222  receive the phase coils  213  (not shown). 
     Finally, six recesses  223  are provided on the outside periphery of the stator  211 ′ for welding tags of stacked laminations constituting the frame of the stator, in a manner that is conventional in itself. 
     The rotor  100  has thirteen teeth  101  regularly spaced in pairs by an angle Δθ R  equal to 2π/13. 
     Of course the present invention is not limited to the embodiments described and shown and the skilled person will know how to vary or modify them within the spirit of the invention.