Abstract:
The present invention relates to an electrical multipole motor/generator or electrical machine with axial magnetic flux, wherein the machine has a number of stator pole cores being larger than the number of rotor pole shoes. Thus, a motor/generator or electrical machine is provided in which the machine comprises a rotor secured to a shaft with an axis of rotation, where the rotor comprises magnets or means for producing a magnetic field and a number set to N of pole shoes. The machine further comprises a first stator with air gaps formed between the rotor and the first stator, where the first stator comprises a number set to M of separate pole cores or pole legs having corresponding separate coils or set of windings wound on and surrounding said pole cores or pole legs, wherein N and M are larger than one and M is larger than N. N may be an equal number, and M may be equal to 2N, or M may be equal to 3N, or M may be equal to 4N. According to an embodiment of the invention, the electrical machine may further comprise a second stator with air gaps formed between the rotor and the second stator, where the second stator comprises a number set to P of separate pole cores having corresponding separate coils or set of windings wound on and surrounding said pole cores, wherein P is larger than one. P may be larger than N, and P may be equal to 2N, or P may be equal to 3N, or P may be equal to 4N. Thus, according to an embodiment of the invention P may be equal to M.

Description:
FIELD OF INVENTION 
       [0001]    This present invention relates to an electrical machine being a motor or generator, and more particularly to a synchronous machine having a number of stator pole cores being larger than the number of rotor pole shoes. The present invention further relates to an electrical machine with axial magnetic flux. The electrical machine can operate either as a motor or generator, and will just be called generator in the following. 
       BACKGROUND OF THE INVENTION 
       [0002]    Electrical generators may be used in many different fields. When a generator is e.g. used in a wind turbine, one of the more important economic parameters, with respect to the dimensioning of the wind turbine, is the size of the housing. It is therefore of great Importance to be able to minimize the diameter of the wind turbine. In order to minimize the housing one has to minimize the gearbox/gear wheel connecting the wing and the generator. This can be achieved by providing a generator which has a relatively large effect per revolution. 
         [0003]    One way to achieve this is to have a generator with as small a radial extent as possible, since the generator occupies a relative large amount of space in the housing of the wind turbine. 
         [0004]    Another aspect to be considered when implementing generators in wind turbines is that the generator has to be effective both at a low and a high number of revolutions. 
         [0005]    An electrical machine based on a conventional radial flux generator, see  FIG. 1 , is most frequently used. A main problem with generators of this kind in certain situations is that the diameter for a given power output is relatively large, because of the radially built stator construction. A further disadvantage is that the stator surrounds/encircles the rotor, thereby adding to the diameter of the generator. 
         [0006]    Another disadvantage is the relative low induction in the air gap caused by the individual arrangement of the material between the recess  7  and the recess  2  themselves, since only the material  7  carries the flux and only covers about 50% of the free space toward the gap. 
         [0007]    There are many generators of similar kind, which are optimized in one way or another, but they all have a radial flux and thus involve the same problem, i.e. a relatively larger diameter, like the one described above. 
         [0008]    A motor or generator having an axial flux, see  FIG. 2 , is proposed In WO 00/48247, which is hereby included by reference. Here, a motor or generator is provided having a magnetic flux path through one or more pole cores  15  surrounded by current windings  16  and extending in the axial direction. This allows a high density of the magnetic flux to be passed through the pole cores  15 , resulting in a low consumption of material for the pole cores when compared to machines, where for example a large stator diameter may be needed in order to conduct a high magnetic flux. By having the pole cores  15  arranged parallel to the axis of the rotor  10 , the overall diameter of the motor or generator may be reduced, thus providing a solution to some of the above-mentioned problems. 
         [0009]    For the motor or generator described in WO 00/48247 the number of pole cores or pole legs  15  arranged in the stator equals the number of magnets arranged in the rotor, and according to the embodiment illustrated in  FIG. 2 , the motor or generator comprises one rotor  10  and one stator. The rotor  10  has a number of pole shoes  13 , disposed between magnets  12 . The stator comprises a number of pole cores or pole legs  15 , where the number of pole legs  15  equals the number of magnets  12 , which again equals the number of pole shoes  13 . There are two adjacent local magnetic circuits for each given pole core  15 . Two of these are schematically illustrated by the first and second loops  18   a ,  18   b . It is seen that when the pole shoes  13  are facing the pole legs  15 , a magnetic flux path  18   a  includes a first pole leg, a first pole shoe, a magnet, a second pole shoe, and a second pole leg. 
         [0010]    In  FIG. 2 , the density of the magnetic flux in the flux path  18   a  or  18   b  is relatively high, leading to a high resulting axial magnetic force between the rotor  10  and pole legs  15  of the stator. When the stator  10  is rotated so that each magnet  12  is now facing a pole leg  15 , a third magnetic flux path will include only one pole leg  15 , a first pole shoe, a magnet, and a second pole shoe (this situation is illustrated in  FIG. 3 ). The magnetic flux density of the third flux path is lower than for the first and the second flux paths  18   a ,  18   b , leading to a lower resulting axial magnetic force between the rotor  10  and the pole legs  15 , when compared to the rotor position illustrated in  FIG. 2 . So, when the rotor  10  is rotated during use, the resulting axial force between the rotor  10  and the pole legs  15  of the stator will vary between a relatively high and a relatively low force. Such a high, varying axial force may result in several drawbacks including a high wear on the rotor  10  and its axial connection. 
         [0011]    According to another embodiment of the motor or generator described in WO 00/48247 having an axial flux and illustrated in  FIG. 3 , the motor or generator comprises one rotor  301  and two stators  302 ,  303  arranged on opposite sides of the rotor  301 . The rotor  301  has pole shoes  304 ,  305  and a magnet  306 ,  307 ,  308  between each two succeeding pole shoes. The pole shoes  304 ,  305  are crossing the rotor  301 , whereby pole shoes are provided on each side of the rotor  301 . The first stator  302  has pole cores or pole legs  309 ,  310  facing the poles shoes  304 ,  304  of the rotor  301 , while the second stator  303  has pole cores or pole legs  311 ,  312 ,  313  facing the magnets  306 ,  307 ,  308  of the rotor  301 . Here, the pole legs  309 ,  310  of the first stator  302  is displaced compared to the position of the pole legs  311 ,  312 ,  313  of the second stator  303 . 
         [0012]    For the generator of  FIG. 3 , a first magnetic flux path  314  of the rotor  301  and the first stator  302  includes the pole legs  310 ,  309 , the pole shoe  304 , the magnet  307 , and the pole shoe  305 . However, a second magnetic flux path  315  exists corresponding to the flux path  314 . This second magnetic flux path  315  includes only one pole leg  312 , the pole shoe  304 , the magnet  307 , and the pole shoe  305 . It should be understood that as the number of pole legs in the stators  302 ,  303  equals the number of magnets in the rotor  301 , similar corresponding magnetic flux paths exist for the remaining stator pole legs and rotor pole shoes and magnets. 
         [0013]    Here, the density of the magnetic flux in flux path  314  is much higher than the density of the magnetic flux in flux path  315 . Thus, the resulting axial magnetic force between the rotor  301  and the first stator  302  is much higher than the resulting and oppositely directed axial magnetic force between the rotor  301  and the second stator  303 . However, when the rotor  301  is rotated so that the pole shoes  304 ,  305  are now facing pole legs  312 ,  313 , respectively of the second stator  302 , while the magnet  307  is facing pole leg  310 , the magnitudes of the oppositely directed axial magnetic forces between the rotor  301  and the two stators  302 ,  303  changes, so that the force between the rotor  301  and first stator  302  is lower than the force between the rotor  301  and the second stator  302 . 
         [0014]    So, when the rotor  301  is rotating during use, the maximum axial force on the rotor  301  is high, but changes in direction during the rotation. Such a high, varying axial force may result in several drawbacks including a high wear on the rotor  301  and its axial connection. 
         [0015]    Thus, there is a need for a design of a motor or generator having an axial flux, but having only a relatively small variation in the varying axial force on the rotor to thereby reduce the wear of the rotating parts. 
       SUMMARY OF THE INVENTION 
       [0016]    According to a first aspect of the present invention there is provided an electrical machine comprising:
       a rotor secured to a shaft with an axis of rotation, said rotor comprising magnets or means for producing a magnetic field and a number set to N of pole shoes,   a first stator with air gaps formed between the rotor and the first stator, said first stator comprising a number set to M of separate pole cores or pole legs having corresponding separate coils or set of windings wound on and surrounding said pole cores or pole legs, wherein N and M are larger than one and M is larger than N.       
 
         [0019]    It is preferred that the rotor magnets or means for producing a magnetic field are arranged between the pole shoes. According to a preferred embodiment of the present invention, the rotor magnets or means for producing a magnetic field alternate with the pole shoes. Thus, the number N of rotor pole shoes may be equal to the number of magnets or means for producing a magnetic field arranged in the rotor. 
         [0020]    It is preferred that N is an equal number. It is also preferred that M is given by A times N, M=AN, where A is an integral number larger than 1. Thus, it is preferred that M may be equal to 2N, or M may be equal to 3N, or M may be equal to 4N. 
         [0021]    However, the present invention also covers embodiments where M differs from A times N. Here, according to an embodiment of the invention, M may be given by N plus 2 times C, M=N+2C, where C is an integral number larger than or equal to 1. 
         [0022]    According to a preferred embodiment of the present invention, the machine of the invention may further comprise a second stator with air gaps formed between the rotor and the second stator, said second stator comprising a number set to P of separate pole cores having corresponding separate coils or set of windings wound on and surrounding said pole cores, wherein P is larger than one. 
         [0023]    According to an embodiment of the invention, P may be smaller than or equal to N. However, it is preferred that P is larger than N, and P may be given by B times N, where B is an integral number larger than 1. Thus, it is preferred that P may be equal to 2N, or P may be equal to 3N, or P may be equal to 4N. It is also within a preferred embodiment that P is equal to M. 
         [0024]    Also here, the present invention covers embodiments where P differs from B times N, and according to an embodiment of the invention, P may be given by N plus 2 times D, P=N+2D, where D is an integral number larger than or equal to 1. 
         [0025]    It is within a preferred embodiment of the Invention that each separate pole core has a corresponding separate coil or set of windings. It is also within a preferred embodiment that the rotor is arranged so that at least part of the rotor is substantially perpendicular to the axis of rotation 
         [0026]    According to embodiments of the present invention, the pole cores or pole legs may have different orientation in relation to the axis of rotation. However, in a preferred embodiment, at least a portion of one or more of the pole cores or pole legs of the first and/or second stator is arranged at an angle to the axis of rotation, said angle being equal to or greater than 0 degrees and below 90 degrees. Here, the angle between the poles cores or pole legs and the axis of rotation may be equal to or below 45 degrees, such as equal to or below 30 degrees. Preferably, at least a portion of one or more of the pole cores or pole legs may be substantially parallel to the axis of rotation, and it is also within a preferred embodiment that at least a portion of all of the pole cores or pole legs is substantially parallel to the axis of rotation. When at least a portion of one or more of the pole cores or pole legs are substantially parallel to the axis of rotation, then one or more windings or coils may also have their axis substantially parallel to the axis of rotation. 
         [0027]    For the electrical machine of the invention, the first stator may preferably be arranged opposite to and facing a first side of the rotor. For the embodiments of the Invention having two stators, it is preferred that the second stator is arranged opposite to and facing a second side of the rotor. 
         [0028]    When arranging the magnets or means for producing magnetic fields and the pole cores or pole legs, it is preferred that they are arranged so that the pole cores of a stator provide part(s) of one or more magnetic flux paths. Here, a magnetic flux path may include flux paths through two pole cores, and the magnetic flux path may further include two air gaps. Preferably, a magnetic flux path includes two and only two pole cores, and the magnetic flux path may further include two and only two air gaps. 
         [0029]    For the machine of the present invention it is preferred that the rotor is substantially circular. It is also preferred that the first and/or second stator further comprises a magnetic conductive end plate connected to the pole cores, where the end plate(s) may be arranged substantially parallel and opposite to the rotor. 
         [0030]    It is preferred that the magnets or means for producing a magnetic field are arranged in pairs having poles of similar polarity facing each other. When arranging the magnets or means for producing a magnetic field, different arrangement may be used. Thus, the magnets or means for producing a magnetic field may be located radially and equidistantly in the rotor. They may also be located on the first side of the rotor facing ends of the pole cores of the first stator. For embodiments having two stators, magnets or means for producing a magnetic field may be located on the second side of the rotor facing ends of the pole cores of the second stator. However, it is preferred that the magnets or means for producing a magnetic field are located on the outer periphery of the rotor. 
         [0031]    Different outer measures may be used for the pole cores or pole legs arranged in a stator. However, according to an embodiment of the invention it is preferred that the width of a pole core or pole leg is substantially equal to the distance between two successive pole cores or pole legs. It is also preferred that the width of a pole shoe is substantially equal to two times the distance between two successive pole cores or pole legs of the first and/or second stator. 
         [0032]    It should be understood that according to the present invention, the magnets or means for producing a magnetic field may be permanent magnets or electromagnets. 
         [0033]    When producing or forming the windings or coils of the machine of the invention, it is preferred to use a flat concentrated coil. When producing the pole cores, it is preferred that these are made of or assembled of a magnetic conducting material, which magnetic conducting material may be a field oriented soft magnetic lamination. 
         [0034]    The machine according to the embodiments of the present invention may preferably be formed as a synchronous one phase machine. The machine may have the form of a generator, which may be provided with a mechanical force/power via the shaft to generate an electrical power via the windings, or the machine may have the form of a motor, which may be provided with power from an electrical source via the windings to generate a mechanical force/power via the shaft. 
         [0035]    It should be understood that a generator according to an embodiment of the present invention may be well suited to be used in a wind turbine. 
         [0036]    A further object of the present invention is to provide a machine or generator/motor, which may provide a multiple phase output without enlarging the diameter of the generator. The multiple number of phases may be achieved by arranging a corresponding number of one phase machines according to any one of the above mentioned embodiments in series. 
         [0037]    According to a preferred embodiment of the present invention, the pole legs or pole cores may be formed by substantially U-shaped elements. Here, the U-shaped elements may be arranged in the stator so that one pole leg is formed by two adjacent legs of two U-shaped elements. It is preferred that the U-shaped pole legs or pole cores are made of a magnetic conducting material, and that the pole legs are arranged on a stator plate made of a material having a low magnetic conductivity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    The invention will be explained more fully below in connection with some preferred embodiments and reference to the accompanying drawings, in which: 
           [0039]      FIG. 1  shows a sectional view of a part of a prior art generator having a radial magnetic flux, 
           [0040]      FIG. 2  shows a schematic view of a prior art generator having an axial magnetic flux and having one rotor and one stator, 
           [0041]      FIG. 3  shows a perspective view of a prior art generator having an axial magnetic flux and having one rotor and two stators, 
           [0042]      FIG. 4  shows a sectional view of part of the generator of  FIG. 3 , 
           [0043]      FIGS. 5   a  and  5   b  show a sectional view of a first embodiment of a generator/motor according to the invention having one rotor and one stator, 
           [0044]      FIG. 6  shows a sectional view of a second embodiment of a generator/motor according to the invention having one rotor and two stators, 
           [0045]      FIG. 7  shows a sectional view of a third embodiment of a generator/motor according to the invention having one rotor and one stator, and 
           [0046]      FIG. 8  shows a sectional view of a fourth embodiment of a generator/motor according to the invention having one rotor and two stators. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0047]      FIG. 1  shows a sectional view of a part of a generator according to the prior art. The figure shows a stator  1  which has recesses  2  with coils  3  wound in the traditionally manner, i.e. from a given recess to another, depending on the phases of the current generated. Also shown is a rotor  4  with magnets  5  spaced apart from the boundary of the rotor  4 . Between the magnets  5  on the rotor  4  and the stator  1  there is an air gap. 
         [0048]    When the rotor  4  moves via a shaft (not shown) with respect to the stator  1 , the magnets are moved past the coils  3  and current is thus induced in these. 
         [0049]    If current is supplied to the coils  3 , a magnetic field will make the rotor  4  and the shaft move, and the electrical machine functions as a motor. 
         [0050]    The construction according to the prior art has the disadvantages already mentioned in the Background of the Invention. 
         [0051]      FIG. 2  shows a schematic view of an embodiment of a prior art generator having an axial magnetic flux and described in WO 00/48247. The Figure shows a pole wheel  10  which functions as a rotor and is secured to a shaft  11 . A plurality of magnets  12  is disposed radially in such a way that the magnets have poles of similar polarity (N) facing each other and poles of similar polarity (S) facing each other. The magnets  12  are preferably permanent magnets but could also be electromagnets or the like. 
         [0052]    A plurality of pole shoes  13 , preferably made of laminated sheet metal or massive iron, is disposed between the magnets  12 , which concentrate the magnetic flux and have a relatively small remanence/residual magnetism, i.e. they are good magnetic conductors. The pole shoes  13  and the magnets  12  are magnetically Isolated from the shaft  11 . 
         [0053]    Spaced apart from the rotor  10 , a magnetic termination plate/end shield  17  is provided with a plurality of pole legs/pole cores  15  secured to the plate  17  in such a way that only an air gap  14  exists between the rotor  10  and the pole shoes  13 . The plate  17  and the pole cores  15  function as a stator  15 ,  17 . The plate  17  is preferably a circular core using non field orientated laminated iron wrapped in a circular shape using one length of iron. 
         [0054]    The plate  17  functions as a magnetic ‘short circuit’ and conducts the magnetic flux between the relevant pole cores  15  in a given magnetic circuit. Thus, in this embodiment, a closed local magnetic circuit consists of: a magnet  12 , a pole shoe  13 , an air gap  14  (which amplifies the flux), a pole core  15 , the magnetic termination plate  17 , an adjacent pole core, a neighbour air gap, a neighbour pole shoe. 
         [0055]    There are two adjacent local magnetic circuits for each given pole core  15 . Two of these are schematically illustrated by the loops  18   a ,  18   b , and have already been discussed in the background of the invention. 
         [0056]    Electrical windings  16 , e.g. coils, preferably surround each of the pole legs  15 . Preferably, the coils  16  are tightly and closely wound around the pole legs  15 . This arrangement is very efficient with respect to induction in the windings/coils  16 , since the flux is highly concentrated/uniform in the pole cores  15  in this arrangement. The windings  16  are preferably formed by flat concentrated coil, which has a high fill factor. By having the windings  16  concentrated on the pole cores  15  almost all of the coil material is affected, as opposed to the generator shown in  FIG. 1 , since the flux flow affects almost all of the coil material (except of course the material conducting the current away from the generator). 
         [0057]    When the rotor  10  is moved with respect to the stator  15 ,  17 , the magnetic flux in a given pole core  15  changes direction, since the polarity at the air gap  14  changes (from N to S or vice versa), and current is thereby induced in the windings  16 . This induction is very efficient, as mentioned, since the magnetic flux is highly concentrated/uniform in the area surrounded by windings  16 , i.e. in the pole core  15 . 
         [0058]    For stand-alone generators the shaft  11  is preferably rotatably mounted in a bearing or the like (not shown) in the plate  17  to support the shaft  11  additionally and stabilize the rotation of the rotor  10  with respect to the stator  15 ,  17 . For generators used in wind turbines, the rotor  10  is preferably secured on the shaft of the wind turbines and the stator  15 ,  17  is preferably secured to a bearing holding the shaft of the wind turbines. 
         [0059]    In  FIG. 2  the magnets  12  are arranged radially, but as an alternative, they may be arranged on the side of the rotor  10  in such a way that the magnets have poles of similar polarity (N) facing each other and poles of similar polarity (S) facing each other, and in such a way that the magnets  12  are facing the pole legs  15 . Also here, pole shoes  13  may be disposed between the magnets  12 . 
         [0060]      FIG. 3  shows a perspective view of a prior art motor/generator corresponding to another embodiment of a motor/generator described in WO 00/48247 having an axial magnetic flux and having one rotor and two stators. The motor/generator comprises one rotor  301  and two stators  302 ,  303  arranged on opposite sides of the rotor  301 . The rotor  301  has pole shoes  304 ,  305  and a magnet  306 ,  307 ,  308  between each two succeeding pole shoes. The pole shoes  304 ,  305  are crossing the rotor  301 , whereby pole shoes are provided on each side of the rotor  301 . 
         [0061]    The first stator  302  has pole cores or pole legs  309 ,  310  facing the poles shoes  304 ,  305  of the rotor  301 , while the second stator  303  has pole cores or pole legs  311 ,  312 ,  313  facing the magnets  306 ,  307 ,  308  of the rotor  301 . Here, the pole legs  309 ,  310  of the first stator  302  is displaced compared to the position of the pole legs  311 ,  312 ,  313  of the second stator  303 . The pole cores  309 ,  310  of the first stator  302  are secured to a first magnetic termination plate/end shield, while the pole cores  311 ,  312 ,  313  of the second stator  303  are secured to a second magnetic termination plate/end shield. Each of the termination plates is preferably a circular core using non field orientated laminated iron wrapped in a circular shape using one length of iron. 
         [0062]    Similar to the pole legs  15  of  FIG. 2 , then for the pole legs  309 ,  310  of the first stator and  311 ,  312 ,  313  of the second stator, electrical windings (not shown), e.g. coils, surround each of the pole legs. Preferably, the coils are tightly and closely wound around the pole legs. 
         [0063]    The magnetic flux paths  314 ,  315  of the motor/generator of  FIG. 3  have already been discussed in the Background of the Invention. 
         [0064]    It should be understood that the materials used for the motor/generator of  FIG. 3  may correspond to the materials used for the motor/generator of  FIG. 2 . 
         [0065]    In  FIG. 4  is shown a sectional view of part of the motor/generator of  FIG. 3 , and the reference numbers are the same as for  FIG. 3 .  FIG. 4  gives a more detailed view of the arrangement of the poles legs  309 - 313  of the two stators  302 ,  303  in relation to the pole shoes  304 ,  305  and the magnets  306 ,  307 ,  308  of the rotor  301 . It is also seen how the first magnetic flux path  314  includes the pole legs  309 ,  310 , the pole shoe  304 , the magnet  307 , and the pole shoe  305 , while the second magnetic flux path  315  includes the pole leg  312 , the pole shoe  304 , the magnet  307 , and the pole shoe  305 . As the pole leg  312  is not facing any of the poles shoes  304  or  305 , the magnetic flux density of flux path  315  is much lower than for the flux path  314 . 
         [0066]    However, according to the present invention, the resulting high, varying axial force of the prior art motor/generators discussed above may be reduced by having a stator with a larger number of pole cores than magnets or pole shoes arranged in the rotor. This is illustrated in the followings figures, in which  FIG. 5  shows a sectional view of a first embodiment of a generator/motor according to the invention having one rotor  501  and one stator  502 . 
         [0067]    The motor/generator of  FIG. 5  corresponds to the design of the motor/generator of  FIG. 2  and may have the same outer dimensions. The motor/generator of  FIG. 5  has a rotor  501  with pole shoes  503  and a magnet  504  between each two succeeding pole shoes  503 . So, the number of magnets  504  is equal to the number of pole shoes  503 , which number is set equal to N. The stator  502  comprises a number M of pole cores or pole legs  505 , with each pole core or pole leg  505  having a corresponding coil  506 , but in contrast to the motor/generator of  FIG. 2 , M is two times N, whereby the number of pole cores or legs  505  is two times the number of pole shoes  503  or magnets  504  arranged in the rotor  501 . The pole cores  505  of the stator are secured to a magnetic termination plate/end shield  507  and the rotor  501  may be secured to a shaft (not shown). It is preferred that the pole shoes  503  and the magnets  504  are magnetically isolated from the shaft. It is also preferred that the magnets  504  are permanent magnets. 
         [0068]    In a preferred embodiment, the materials used for motor/generator of  FIG. 5  correspond to the materials used for the motor/generator of  FIG. 2 . However, for the motor/generator of  FIG. 5  it is preferred that the width of a pole shoe  503  is substantially equal to the width of a magnet  504 , and the width of a stator pole core or leg  505  may be only half the width of a rotor pole shoe  503  or a rotor magnet  504 . 
         [0069]    In  FIG. 5   a , the rotor  501  is in a position so that each pole shoe  503  and each magnet are directly facing a pole leg  505 . A magnetic flux path  508  is shown, and it is seen that the path  508  includes a first pole leg  505   a , a first pole shoe  503   a , a magnet  504   a , a second pole shoe  503   b , and a second pole leg  505   b . It is also seen that a third pole leg  505   c  in between the first and second pole legs  505   a,b  is not part of the magnetic flux path  508 . 
         [0070]    When comparing the magnetic flux path  508  of  FIG. 5   a  to the magnetic flux path  18   a  of  FIG. 2 , the total area of the two pole legs  505   a ,  505   b  facing the rotor  502  is smaller than the total area of the two pole legs  15  in  FIG. 2  facing the rotor  10  and being part of the magnetic flux path  18   a . Thus, the total magnetic flux in the flux path  508  is smaller than the total magnetic flux running in flux path  18   a , with the result that the maximum resulting axial force between the rotor  501  and the stator  502  in  FIG. 5   a  is smaller than the maximum resulting axial force between the rotor  10  and the stator in  FIG. 2 . 
         [0071]    However, when the machine of  FIG. 5  is a generator then, due to the lower magnetic flux in the pole legs  505   a ,  505   b , the generated electrical output of the coils  506   a ,  506   b  is smaller than the output generated from the coils  16  surrounding the pole legs of the flux path  18   a  in  FIG. 2 . This may be compensated for by having a higher number of wirings in the coils  506   a ,  506   b  when compared to the wirings of the coils  16  in  FIG. 2 . As the circumference of a pole leg  505  in  FIG. 5  is smaller than the circumference of a pole leg  15  in  FIG. 2 , then for the same consumption of coil material (such as copper), a larger number of wirings may be achieved for the coils  506  in  FIG. 5  than for the coils  16  in  FIG. 2 . 
         [0072]    In  FIG. 5   b , the rotor  501  has been rotated when compared to  FIG. 5   a  and is in a position so that each pole leg  505  is directly facing a passage between a pole shoe  503  and a magnet  504 . A magnetic flux path  509  is shown, and it is seen that it includes a pole leg  505   d , a pole shoe  503   c , a magnet  504   b , another pole shoe  503   d , and another pole leg  505   e . It is also seen that in this position it is two neighbouring pole legs  505   d ,  505   e , which are now part of the flux path  509 . It is furthermore seen that when neglecting the effects of leakage or stray flux, the magnetic flux path  508  uses half of the pole legs  505   a  and  505   b , and the magnetic flux path  509  uses half of the pole legs  505   d  and  505   e . Thus, the total magnetic flux running in the flux path  509  of  FIG. 5   b  is substantially equal to the magnetic flux running in the flux path  508  of  FIG. 5   a.    
         [0073]    In  FIG. 5   a  there are two flux paths running through the pole legs  505   a  and  505   b , but there are no flux paths using the pole leg  505   c . Thus, half of the pole legs in  FIG. 5   a  are filled up by two flux paths, while the other half of the pole legs have no flux path. In  FIG. 5   b  a flux path is running through every pole leg, but only half of each pole leg is occupied by a flux path. So, the total magnetic flux between the stator  502  and the rotor  501  in  FIG. 5   a  is substantially equal to the total magnetic flux between the stator  502  and the rotor  501  in  FIG. 5   b . Thus, the change in the resulting axial force when rotating the rotor  501  of the motor/generator of  FIG. 5  will be very small and much smaller than the change in the resulting axial force of the motor/generator of  FIG. 2 . 
         [0074]      FIG. 6  shows a sectional view of a second embodiment of a generator/motor according to the invention having one rotor  601  and two stators  602   a ,  602   b.    
         [0075]    The motor/generator of  FIG. 6  corresponds to the design of the motor/generator of  FIG. 3  and may have the same outer dimensions. The motor/generator of  FIG. 6  has a rotor  601  with pole shoes  603  and a magnet  604  between each two succeeding pole shoes  603 . So, the number of magnets  604  is equal to the number of pole shoes  603 , which number is set equal to N. Both the first stator  602   a  and the second stator  602   b  comprises a number M of pole cores or pole legs  605   a ,  605   b , with each pole core or pole leg  605   a ,  605   b  having a corresponding coil  606   a ,  606   b , but in contrast to the motor/generator of  FIG. 3 , M is two times N, whereby the number of pole cores or legs  605   a  and  605   b  is two times the number of pole shoes  603  or magnets  604  arranged in the rotor  601 . Also here, the pole cores  605   a  and  605   b  of the stators  602   a  and  602   b  are secured to corresponding magnetic termination plates/end shields  607   a ,  607   b  and the rotor  601  may be secured to a shaft (not shown). Again, it is preferred that the pole shoes  603  and the magnets  604  are magnetically isolated from the shaft. It is also preferred that the magnets  604  are permanent magnets. 
         [0076]    In a preferred embodiment, the materials used for motor/generator of  FIG. 6  correspond to the materials used for the motor/generator of  FIG. 3 . However, for the motor/generator of  FIG. 6  it is preferred that the width of a pole shoe  603  is substantially equal to the width of a magnet  604 , and the width of a stator pole core or leg  605  may be only half the width of a rotor pole shoe  603  or a rotor magnet  604 . 
         [0077]    In  FIG. 6 , the rotor  601  is in a position so that each pole shoe  603  and each magnet  604  are directly facing a pole leg  605   a  of the first stator  602   a . A magnetic flux path  608  is shown, and it is seen that it includes a first pole leg  605   aa , a first pole shoe  603   a , a magnet  604   a , a second pole shoe  603   b , and a second pole leg  605   ab . It is also seen that a third pole leg  605   ac  in between the first and second pole legs is not part of the magnetic flux path  608 . For the pole legs  605   b  of the second stator, each pole leg  605   b  is directly facing a passage between a pole shoe  503  and a magnet  504 . A magnetic flux path  609  is shown, and it is seen that it includes a pole leg  605   ba , the first pole shoe  603   a , the magnet  604   a , a second pole shoe  603   b , and another pole leg  605   bb.    
         [0078]    For the magnetic flux paths  608  and  609 , the discussion given above in connection with  FIGS. 5   a  and  5   b  is valid, leading to the result that for the shown position of the rotor  601  and the stators  602   a  and  602   b , the total magnetic flux between the stator  602   a  and the rotor  601  is substantially equal to the total magnetic flux between the stator  602   b  and the rotor  601 . So, the resulting axial force between the rotor  601  and the first stator  602   a  is smaller than the resulting axial force between the rotor  301  and the first stator  302  of  FIG. 3 , while the resulting axial force between the rotor  601  and the second stator  602   b  is higher than the resulting axial force between the rotor  301  and the second stator  303  of  FIG. 3 . 
         [0079]    Thus, the change in the resulting axial force when rotating the rotor  601  of the motor/generator of  FIG. 6  will be very small and much smaller than the change in the resulting axial force of the motor/generator of  FIG. 3 . 
         [0080]    The discussion for the number of wires of the coils  506  given above in connection with  FIG. 5  is also valid for the coils  606   a  and  606   b  of  FIG. 6 . 
         [0081]    It is also within the present invention to provide a generator in which the number M of stator pole cores is four times the number N of the pole shoes. This is illustrated in  FIG. 7 , which shows a sectional view of a third embodiment according to the invention. The motor/generator of  FIG. 7  corresponds to the design of the motor/generator of  FIG. 5  and may have the same outer dimensions, but in  FIG. 7  M is four times N, whereas in  FIG. 5  M is two times N. The motor/generator of  FIG. 7  has a rotor  701  with pole shoes  703  and a magnet  704  between each two succeeding pole shoes  703 . Also here, the number of magnets  704  is equal to the number of pole shoes  703 , which number is set equal to N. The stator  702  comprises the number M of pole cores or pole legs  705 , with each pole core or pole leg  705  having a corresponding coil  706 . The pole cores  705  of the stator may be secured to a magnetic termination plate/end shield  707  and the rotor  701  may be secured to a shaft (not shown). It is preferred that the pole shoes  703  and the magnets  704  are magnetically isolated from the shaft. It is also preferred that the magnets  704  are permanent magnets. 
         [0082]    In a preferred embodiment, the materials used for the motor/generator of  FIG. 7  correspond to the materials used for the motor/generator of  FIG. 5 . Thus, for the motor/generator of  FIG. 7  it is preferred that the width of a pole shoe  703  is substantially equal to the width of a magnet  704 , and the width of a stator pole core or leg  705  may be only one quarter of the width of a rotor pole shoe  703  or a rotor magnet  704 . 
         [0083]    In  FIG. 7 , the rotor  701  is in a position so that each pole shoe  703  and each magnet are directly facing two pole legs  705 . A magnetic flux path  708  is shown, and it is seen that the path  708  includes a first pole leg  705   a , a first pole shoe  703   a , a magnet  704   a , a second pole shoe  703   b , and a second pole leg  705   b . It is also seen that two pole legs in between the first and second pole legs  705   a,b  are not part of the magnetic flux path  708 . 
         [0084]    From  FIG. 7  it is seen that the flux path  708  fully occupies the pole legs  705   a  and  705   b , while the neighbouring pole legs also facing a pole shoe are occupied by another magnetic flux path. For the remaining half of the pole legs, which are facing a magnet, then to a good approximation, no magnetic flux path is running through these pole legs. When comparing the embodiment of  FIG. 7  with the embodiment of  FIG. 5   a , the total magnetic flux running between the rotor  701 ,  501  and the stator  702 ,  502  will be the same for the same dimensions and for the same number N of magnets  704 ,  504  and pole shoes  703 ,  503 . 
         [0085]    When rotating the rotator  701  of  FIG. 7  the width of a pole leg  705  in a clockwise direction, then the situation will be the same as shown in  FIG. 7 , i.e. for each pole shoe  703  two pole legs  705  will bee facing the pole shoe  703 , and for each magnet  704  two pole legs  705  will be facing the magnet  704 . So, the total pole leg area facing a pole shoe will be the same as illustrated in  FIG. 7  with the result that the total magnetic flux running between the stator  702  and the rotor will be substantially the same as in  FIG. 7 . Thus, the same discussion may be used as given in connection with  FIG. 5 , and the change in the resulting axial force when rotating the rotor  701  of the motor/generator of  FIG. 7  will be very small. 
         [0086]      FIG. 8  shows a sectional view of a fourth embodiment according to the invention having one rotor  801  and two stators  802   a  and  802   b . The motor/generator of  FIG. 8  corresponds to the design of the motor/generator of  FIG. 6  and may have the same outer dimensions. The motor/generator of  FIG. 8  has a rotor  801  with pole shoes  803  and a magnet  804  between each two succeeding pole shoes  803 . So, the number of magnets  804  is equal to the number of pole shoes  803 , which number is set equal to N. Both the first stator  802   a  and the second stator  802   b  comprises a number M of pole cores or pole legs  805   a ,  805   b , with each pole core or pole leg  805   a ,  805   b  having a corresponding coil  806   a ,  806   b , but in contrast to the motor/generator of  FIG. 6 , M is four times N, whereby the number of pole cores or legs  805   a  and  805   b  is four times the number of pole shoes  803  or magnets  804  arranged in the rotor  801 . Also here, the pole cores  805   a  and  805   b  of the stators  802   a  and  802   b  may be secured to corresponding magnetic termination plates/end shields  807   a ,  807   b  and the rotor  801  may be secured to a shaft (not shown). Again, it is preferred that the pole shoes  803  and the magnets  804  are magnetically isolated from the shaft. It is also preferred that the magnets  804  are permanent magnets. 
         [0087]    In a preferred embodiment, the materials used for motor/generator of  FIG. 8  correspond to the materials used for the motor/generator of  FIG. 6 . However, for the motor/generator of  FIG. 8  it is preferred that the width of a pole shoe  803  is substantially equal to the width of a magnet  804 , and the width of a stator pole core or leg  805  may be only one quarter of the width of a rotor pole shoe  803  or a rotor magnet  804 . 
         [0088]    In  FIG. 8 , the rotor  801  is in a position in relation to the stators  802   a  and  802   b  similar to the position of the stator  701  and the stator  702  of  FIG. 7 . So, the rotor  801  is in a position so that each pole shoe  803  and each magnet  804  are directly facing two pole legs  805   a  and  805   b  of each stator  802   a ,  802   b . Two magnetic flux paths  808  and  809  are shown, and It is seen that each path  808  and  809  includes two pole legs, two pole shoes, and one magnet. It is also seen that two pole legs, which are arranged in between two pole legs being part of a flux path  808 ,  809 , are not part of a magnetic flux path. 
         [0089]    So, in  FIG. 8  the total pole leg area facing a pole shoe will be the same for the pole legs  805   a  of stator  802   a  as for the pole legs  805   b  of the stator  802   b , with the result that the total magnetic flux running between the stator  802   a  and the rotor  801  will be substantially the same as for the stator  802   b  and the rotor  801 . Thus, the same discussion may be used as given in connection with  FIGS. 5-7 , and the change in the resulting axial force when rotating the rotor  801  of the motor/generator of  FIG. 8  will be very small. 
         [0090]    In should be understood that for electrical machines of the present invention, the stator may in most cases comprise a relatively large number of separate pole cores or pole legs (for example 40 pole cores) and a corresponding number of separate or discrete coils or set of windings. 
         [0091]    Such as large number of discrete and galvanic separated coils gives the opportunity of forming a very large number of combinations of voltages and currents. 
         [0092]    A few examples: 
         [0000]    all coils of one stator may be arranged in series to produce a high voltage;
 
all coils of one stator may be arranged in parallel to obtain the same voltage as of one coil, but a higher current;
 
one, two, three or more coils arranged in parallel may be arranged in series with one, two, three or more coils arranged in parallel, all coils being part of the same stator.
 
         [0093]    While the invention has been particularly shown and described with reference to particular embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention, and it is intended that such changes come within the scope of the following claims.