Abstract:
An electric machine includes an outer stator having a plurality of windings, an inner stator having a plurality of permanent magnets and arranged within the outer stator in concentric relationship to the outer stator, and a rotor arranged between the outer stator and the inner stator in concentric relationship to the outer and inner stators and movable relative to the outer and inner stators. The rotor includes a mounting element which is made from a non-magnetic material and has a plurality of recesses, with a magnetically soft segment being arranged in each of the recesses. A first cooling device cools the permanent magnets of the inner stator.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of European Patent Application, Serial No. EP 14163031.9, filed Apr. 1, 2014, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference in its entirety as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an electric machine with permanently excited inner stator. 
         [0003]    The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
         [0004]    Electric machines are known which include an outer stator and an inner stator, between which a rotor is arranged. Electric machines can be designed as synchronous machines, in particular three-phase synchronous machines. Such three-phase synchronous machines can be operated as a motor or as a generator, for example, and are steadily gaining in significance. Permanently excited three-phase synchronous machines are known in the art and include permanent magnets arranged on the rotor or the armature. Permanent magnets have the disadvantage of limited operating temperatures. For this reason, magnets made of rare earths are increasingly used because of their higher operating temperatures in comparison with permanent magnets which are normally used. A disadvantage of rare-earth magnets is their high price. Moreover, cooling of the rotor and/or the permanent magnets, e.g. by water cooling, can only be achieved at high cost. 
         [0005]    It would therefore be desirable and advantageous to provide an improved electric machine, in particular synchronous machine which obviates prior art shortcomings and which is simple in structure and yet reliable in operation in an economical and effective manner. 
       SUMMARY OF THE INVENTION 
       [0006]    According to one aspect of the present invention, an electric machine includes an outer stator having a plurality of windings, an inner stator having a plurality of permanent magnets and arranged within the outer stator in concentric relationship to the outer stator, a rotor arranged between the outer stator and the inner stator in concentric relationship to the outer and inner stators and movable relative to the outer and inner stators, the rotor including a mounting element made from a non-magnetic material and having a plurality of recesses, with a magnetically soft segment being arranged in each of the recesses, and a first cooling device configured to cool the permanent magnets of the inner stator. 
         [0007]    The electric machine can be designed as a three-phase synchronous machine and can be used as a motor or as a generator. The electric machine includes an outer stator, which has a plurality of windings. For this purpose, the outer stator can have corresponding slots, in which the windings are arranged. The rotor is arranged within the outer stator and can be connected to a shaft in a non-rotatable manner. The rotor itself includes a mounting element, which is made from a non-magnetic material. A non-magnetic material cannot usually be influenced by a magnetic field. A plurality of magnetically soft segments is embedded in the mounting element. Finally, the electric machine includes an inner stator, which has a plurality of permanent magnets. It is thereby possible to provide a permanently excited three-phase synchronous machine, in which the permanent magnets are arranged at the inner stator and therefore do not move during operation of the electric machine. Consequently, heat which is produced during operation of the electric machine can be carried away from the permanent magnets more easily than in the case of electric machines in which the permanent magnets are provided in the rotor. 
         [0008]    The electric machine has a first cooling device for cooling the inner stator. Heat produced during operation of the electric machine can therefore be reliably carried away from the permanent magnets of the inner stator. In this way, it is possible to prevent the permanent magnets from becoming demagnetized. Consequently, magnets having lower coercive field strength can be used. This in turn means that permanent magnets having a lower proportion of rare earths can be used. In this way, the remanence can be increased and the costs can be reduced. 
         [0009]    According to another advantageous feature of the present invention, a second cooling device can be provided to cool the outer stator. The second cooling device allows the windings of the outer stator to be prevented from overheating during operation of the electric machine and possibly being damaged as a result. In this way, it is possible to ensure reliable operation of the electric machine. 
         [0010]    According to another advantageous feature of the present invention, the first cooling device and/or the second cooling device has a plurality of cooling tubes for passage of a cooling liquid. For example, water or a water-glycol mixture can be used as a cooling liquid. Suitable cooling tubes can be manufactured easily and economically. Advantageously, the cooling tubes can be arranged in an iron core assembly of the inner stator and/or the outer stator. For example, the cooling tubes of the first cooling device and/or the second cooling device can be provided by corresponding holes which are made in the respective iron core assembly. Such cooling tubes can be manufactured easily and economically. 
         [0011]    According to another advantageous feature of the present invention, the cooling tubes can be arranged uniformly distributed in a circumferential direction along an axial direction of the electric machine. By virtue of such an arrangement of the cooling tubes, heat produced during operation of the electric machine in both the inner stator and the outer stator can be carried away uniformly. 
         [0012]    According to another advantageous feature of the present invention, the permanent magnets of the inner stator can be made from a ferrite. The use of a ferrite has the advantage that economical permanent magnets can therefore be used for the inner stator. 
         [0013]    According to another advantageous feature of the present invention, the permanent magnets of the inner stator can contain iron-neodymium-boron. Such permanent magnets of the inner stator contain iron, neodymium and/or boron, and exhibit high temperature stability. 
         [0014]    According to another advantageous feature of the present invention, the rotor can have a pot-shaped configuration. Such an embodiment of the rotor has the advantage that a rotor with a low mass inertia can therefore be provided. In this way, an electric machine or motor with a high dynamic performance can be provided. 
         [0015]    According to another advantageous feature of the present invention, the rotor can be connected to a shaft which is designed as a hollow shaft. When the electric machine is used for a machine tool in particular, the use of a hollow shaft allows a flexible configuration of the machine tool. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0016]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
           [0017]      FIG. 1  is a cross section of a first embodiment of an electric machine according to the present invention; 
           [0018]      FIG. 2  is a longitudinal section of the electric machine of  FIG. 1 ; 
           [0019]      FIG. 3  is a cross section of a second embodiment of an electric machine according to the present invention; 
           [0020]      FIG. 4  is a longitudinal section of the electric machine of  FIG. 3 ; and 
           [0021]      FIG. 5  is a diagram illustrating a change in magnetic quality of the permanent magnets of an inner stator of the electric machine as a function of the temperature. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0023]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown a cross section of a first embodiment of an electric machine according to the present invention, generally designated by reference numeral  1 . The electric machine  1  includes an outer stator  2 . The outer stator  2  has an iron core assembly  4 , having a plurality of teeth  5  which are separated by slots  6 . Corresponding windings  3  are inserted into the slots  6 . The windings  3  are usually electrically connected to a three-phase network (not shown here). A rotor  7  is arranged within the outer stator  2 . The rotor  7  is arranged concentrically to the outer stator  2  in this case. The rotor  7  has a mounting element  8 , which is made from a non-magnetic material. The mounting element  8  has a plurality of recesses, into which magnetically soft segments  9  are embedded in each case. The electric machine  1  also has an inner stator  10 . The inner stator  10  is arranged concentrically within the rotor  7 . The inner stator  10  has an iron core assembly  11 , comprising a plurality of recesses in which a permanent magnet  12  is arranged in each case. The permanent magnets  12  can contain neodymium-iron-boron, for example. 
         [0024]    In the present exemplary embodiment, the outer stator  2  or its windings  3  have the number of pole pairs pw=4. The inner stator  10  or its permanent magnets  12  have the number of pole pairs pm=6. The rotor  7  here has ten magnetically soft segments  9 . Therefore the number of pole pairs of the rotor is pr=10. The number of externally effective pole pairs in this case corresponds to the number of pole pairs of the rotor. The numbers of pole pairs of the electric machine  1  can generally be combined according to the following formula: 
         [0000]    
       
      
       pr=|pm+/−pw| 
      
     
         [0025]      FIG. 2  shows the electric machine  1  according to  FIG. 1  in longitudinal section. It can be seen here that the rotor  7  is so designed as to be pot-shaped. The pot-shaped rotor  7  is connected to a shaft  13  in a non-rotatable manner. A housing  14  of the electric machine can also be seen in  FIG. 2 . The housing  14  has a first flange  15  on the drive side. The housing  14  has a second flange  16  on the non-drive side. A first bearing  17  is arranged between the first flange  15  and the shaft  13 , and is designed as e.g. a ball bearing. The electric machine  1  also has a supporting element  18 , on which the inner stator  10  is held. A second bearing element  19  is arranged between the supporting element  18  and the rotor  7 , and can likewise be designed as a ball bearing. 
         [0026]      FIG. 3  shows a further embodiment variant of an electric machine  1  in cross section. The electric machine  1  represented in  FIG. 3  differs from that in  FIG. 1  in that the electric machine  1  has a first cooling device  20  for cooling the inner stator  10 . The first cooling device  20  comprises a plurality of cooling tubes  21 , which are arranged within the iron core assembly  11  of the inner stator  12 . The cooling tubes  21  are uniformly arranged in a circumferential direction of the inner stator  12 . The cooling tubes  21  extend along the axial direction of the electric machine  1 . The permanent magnets  12  can be cooled by means of the first cooling device  20 . In the case of the electric machine  1  according to  FIG. 3 , the permanent magnets  12  can be made from a ferrite, for example. 
         [0027]    The electric machine  1  additionally comprises a second cooling device  22 . The second cooling device  22  likewise comprises a plurality of cooling tubes  23  which extend along the axial direction of the electric machine. The cooling tubes  23  of the second cooling device  22  are likewise uniformly arranged in the circumferential direction of the outer stator  2 . A cooling medium, in particular a cooling liquid, can flow through the cooling tubes,  21 ,  23 . The windings  3  of the outer stator  2  can be cooled by means of the second cooling device  22 . By virtue of the first cooling device  20 , the heat which is produced during operation of the electric machine  1  can be carried away from the inner stator  10 . In this way, it is possible to prevent the permanent magnets  9  from overheating and therefore becoming demagnetized. 
         [0028]    In the case of the exemplary embodiment according to  FIG. 3 , the winding system in the outer stator  2  has the number of pole pairs pw=5. The number of permanent magnets  12  in the inner stator  10  gives the number of pole pairs pm=12 for the inner stator  10 . Applying the formula cited above, the number of externally effective pole pairs of the rotor  7  is pr=17. 
         [0029]      FIG. 4  shows the electric machine  1  according to  FIG. 3  in cross section. In this context, it can be seen that the shaft  13  of the electric machine  1  is designed as a hollow shaft in the present exemplary embodiment. In addition, the electric machine  1  in this context has a first bearing element  24  and a second bearing element  25 . The first bearing element  24  is arranged between the first flange  15  and the rotor  7 . The second bearing element  25  is arranged between the second flange  16  and the rotor  7 . The bearing elements  24 ,  25  can be designed as ball bearings in particular. 
         [0030]      FIG. 5  shows a graph  26  which represents the coercive field strength Hc on the x-axis and the remanence Br on the y-axis. Exemplary values for the coercive field strength Hc are represented in kA/m on the x-axis here. Furthermore, exemplary values for the remanence Br are represented on the y-axis. Furthermore, the broken line  27  indicates the minimum value for the coercive field strength Hc. The graph  26  is intended to show the connection between remanence Br, coercive field strength Hc and temperature. The straight line  28  here shows the connection between coercive field strength Hc and remanence Br for a permanent magnet  12  which is made from iron-neodymium-boron. 
         [0031]    Such magnetic qualities with high remanence have a higher proportion of heavy rare earths. In this case, the point  29  represents a first material with high coercive field strength Hc and low remanence Br. When the temperature within the electric machine  1  is increased to a value of 130° C., for example, the coercive field strength Hc and the remanence Br of the first material change as a function of the temperature coefficients of the first material. With regard to the connection between coercive field strength Hc and remanence Br, the connection described by the straight line  30  applies. 
         [0032]    If the permanent magnets  12  are now cooled by means of the first cooling device  20  using a cooling liquid, in particular water, and consequently heated up to a maximal temperature of only 40° C., materials having a lower coercive field strength Hc can be used. This is illustrated here by the straight line  31 . If a second material as indicated here by the point  32  is used in this case, having a high remanence Br and a low coercive field strength Hc, this has the advantage firstly that the remanence Br is clearly increased, e.g. by a value of 20%. This is illustrated here by the arrow  33 . A further advantage is derived in that fewer expensive rare earths need to be used for the permanent magnets  12 . The selection of the first and the second material is based on the requirement that the minimum value for the coercive field strength Hc, as indicated by the line  27 , must be satisfied, because demagnetization would otherwise occur during operation of the electric machine  1 . 
         [0033]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.