Abstract:
A permanent-field armature for an electrical machine includes an armature body having internal magnet pockets, and a plurality of permanent magnets having a magnetization for generating a magnetic field and disposed in the magnetic pockets in one-to-one correspondence. The armature body guides the magnetic field between the magnet pockets and magnetic poles of the armature on an armature surface. Each magnet pocket is formed for guiding the magnetic field in a material which has at least one area having a magnetic anisotropy with an easy axis and a hard axis of magnetizing capability, with the easy axis being aligned in parallel relation to or at least at an angle of less than 25° to a magnetization direction of the magnetization of the permanent magnet disposed in the magnet pocket.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of European Patent Application, Serial No. 14185613.8, filed Sep. 19, 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 a permanent-magnetic field armature, in short permanent-field armature for an electrical machine and also to an electrical machine with such an armature and to a method for manufacturing the armature. 
         [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]    Permanent magnet-field electrical machines have a rotor structure in which the magnets are disposed inside the rotor, which is also referred to as IPM construction (IPM—Interior Permanent Magnet). To this end slots or magnet pockets are provided in the rotor laminated core in which the permanent magnets are disposed. Rotors or this type are especially robust mechanically because the internal permanent magnets are protected from mechanical influences. 
         [0005]    In an armature with interior magnet pockets, i.e. a rotor for example, the problem arises that, to guide the magnetic flux between the armature surface and the interior magnet pockets a soft-magnetic material should be used in the armature body. However a magnetic short circuit within the armature body can also be produced by such a soft magnetic material, so that the magnetic excitation of the permanent magnets cannot be used for driving the armature. This magnetic short circuit is also referred to as leakage. 
         [0006]    To address this problem, the provision of so-called flux inhibitors in the rotor body has been proposed. These flux inhibitors can be embodied between the interior magnet pockets and be hollow spaces filled with air for example. The disadvantage here is that such hollow spaces destabilize the rotor body mechanically. 
         [0007]    It would be desirable and advantageous to provide an improved permanently excited armature of an electrical machine to obviate prior art shortcomings and to reduce a magnetic short circuit of the magnetic field of the permanent magnets within the armature body. 
       SUMMARY OF THE INVENTION 
       [0008]    According to one aspect of the present invention, a permanent-field armature for an electrical machine includes an armature body having internal magnet pockets, and a plurality of permanent magnets having a magnetization for generating a magnetic field and disposed in the magnetic pockets in one-to-one correspondence, the armature body being configured to guide the magnetic field between the magnet pockets and magnetic poles of the armature on an armature surface, each magnet pocket being formed for guiding the magnetic field in a material which has at least one area having a magnetic anisotropy with an easy axis and a hard axis of magnetizing capability, with the easy axis being aligned in parallel relation to or at least at an angle of less than 25° to a magnetization direction of the magnetization of the permanent magnet disposed in the magnet pocket. 
         [0009]    In accordance with the present invention, a permanent-field armature for an electrical machine is involved as armature. The electrical machine can for example be a linear machine or a rotation machine. In the latter case the armature is embodied as a rotor. The armature according to the invention has a number of permanent magnets with a magnetization for creating a permanent magnetic field. The permanent magnets are disposed in an armature body. For this purpose the armature body has interior magnet pockets in which one of the permanent magnets is disposed in each case. The term “magnet pockets” is a term known in the art and relates to slots or wells inside the armature body. The armature body is designed to guide the magnetic field between the magnet pockets in the magnetic poles of the armature on its armature surface. In accordance with the prior art, the armature body can be embodied for this purpose from a soft-magnetic material and have the described flux inhibitors between the magnet pockets. 
         [0010]    In contrast thereto, in accordance with the present invention, for guiding the magnetic field each magnet pocket is embodied in a material which, at least in some areas or completely, has a magnetic anisotropy with an easy axis and a hard axis of magnetizability. The easy axis is also referred to as the preferred direction. The material can also have more than one easy axis with the largest value of relative permeability and/or more than one hard axis with the smallest value of relative permeability. The provision of a single easy axis is advantageous in order to avoid the flux guidance jumping around. The easy axis of the material is aligned in parallel to a magnetization direction of the magnetization of the permanent magnet disposed in the magnet pocket. The parallel alignment does not have to be perfect in such cases. The easy axis can be aligned at least at an angle of less than 25°, preferably less than 15° to the magnetization direction of the magnetization of the permanent magnet. The described alignment of the easy axis can be especially realized in an edge area adjoining the magnet pocket. The anisotropy of the material is especially effected by a magnetocrystalline anisotropy. 
         [0011]    The advantage produced by the invention is that the leakage of the magnetic field of the permanent magnets within the armature body is low, so that a higher usable flux is made possible, i.e. a magnetic flux which exits from the armature (north pole) and enters (south pole) through the magnetic poles. A leakage means a deviation of the magnetic flux from the magnetization direction of the permanent magnets within the armature body. It is reduced by the alignment of the easy axis. 
         [0012]    In the event of the armature being embodied as a rotor for a rotation machine, it is advantageous when, for each magnet pocket in an edge area adjoining the magnet pocket, the easy axis is aligned in the radial direction. This enables pole gaps to be provided between the magnetic poles which have an especially low magnetic interactivity, so that a small magnetic voltage drop is provided at the q inductance (i.e. along the q axis), which in its turn makes a high maximum power possible. 
         [0013]    According to another advantageous feature of the present invention, the armature body can be formed from a number of segments such that the easy axis at each of the segments is aligned separately from the other ones of the segments. Thus, in each segment, the alignment of the axes of the anisotropy is embodied separately from the other segments. In other words, for each segment, the alignment of the axes of the anisotropy has been set before the assembly of the armature body. This is advantageous because an inhomogeneous anisotropy field can be set in the armature. With a rotor in particular the alignment of the easy axis in a radially-interior area can be different to the alignment of the easy axis in a radially-exterior area, i.e. on the armature surface. 
         [0014]    According to another advantageous feature of the present invention, at a contact surface between two of the segments, the easy axis of one of the two segments and the easy axis of the other one of the two segments can be disposed in parallel or at least at an angle of less than 25° to one another. The easy axes are thus preferably aligned tangentially or laterally. This enables the magnetic circuit inside the armature to be closed with an especially low loss. 
         [0015]    According to another advantageous feature of the present invention, the material can be arranged contiguously between neighboring magnet pockets. In other words, in the case the rotor, no flux inhibitor in the form of a hollow space is formed along the circumferential direction between the magnet pockets, but the anisotropic material is provided contiguously, of which the easy axis can be aligned radially for example. By eliminating the need for flux inhibitors in the form of hollow spaces, the armature becomes mechanically especially stable. 
         [0016]    A further advantage in dispensing with flux inhibitors is the especially dense arrangement of permanent magnets disposed with opposing poles. Accordingly the invention provides for designs of the armature in which a number of the magnetic poles can amount to four or six or eight or more than eight. The larger the number of magnetic poles is, the larger the drive force, especially the torque, can be. 
         [0017]    It has proved especially suitable for a relative magnetic permeability of the material along the easy axis to have a value greater than 15, especially greater than 20 and along the hard axis to have a value less than two, especially less than 1.6. The field-guidance characteristic of the armature body can be combined especially well with the suppression of the leakage field, i.e. the magnetic short-circuit. In order to obtain the described magnetic characteristics, the alloying, the tempering process in the embodiment of the armature body and/or of its segments as well as the magnetic field with which the anisotropy is set are available as parameters during the manufacture of the armature. Simple tests make the parameterization of the manufacturing process possible here, in order to obtain the described values for the relative magnetic permeability. 
         [0018]    According to another advantageous feature of the present invention, the material can have a magnetic saturation which lies at a flux density of more than 0.8 Tesla, preferably at at least 1.0 Tesla. The advantage produced by this is that the stator field can be used especially efficiently for driving the armature. 
         [0019]    In order to realize all the described magnetic properties, according to another advantageous feature of the present invention, the material can be neodymium-iron-boron, NdFeB. This material is obtainable in alloys in which the relative magnetic permeability can be set to the described values and through a coercitive field strength of less than 10 kiloamperes per meter the described field-guiding properties can be embodied in the armature body. The material is thus not a permanent magnet, but has flux-guiding properties. 
         [0020]    According to another aspect of the present invention, an electrical machine includes a stator having magnetic coils, and an armature comprising an armature body having internal magnet pockets, and a plurality of permanent magnets having a magnetization for generating a magnetic field and disposed in the magnetic pockets in one-to-one correspondence, said armature body being configured to guide the magnetic field between the magnet pockets and magnetic poles of the armature on an armature surface, each said magnet pocket being formed for guiding the magnetic field in a material which has at least one area having a magnetic anisotropy with an easy axis and a difficult axis of magnetizing capability, said easy axis being aligned in parallel relation to or at least at an angle of less than 25° to a magnetization direction of the magnetization of the permanent magnet disposed in the magnet pocket. 
         [0021]    In accordance with the present invention, an electrical machine has a stator with magnetic coils for creating a magnetic stator field. Through the magnetic interaction of the stator field with the magnetic poles on the armature surface a dynamoelectric effect is produced in a manner known per se. The electrical machine can also be embodied as a generator in which a voltage is induced in the magnetic coils. The electrical machine according to the invention has the advantage that it can be operated especially efficiently, since the stray flux in the armature is low. In addition a high regulation dynamic is made possible, since the delay time during current buildup, because of the low inductivity along the q axis, i.e. in the area of the pole gaps, is low. 
         [0022]    According to still another aspect of the present invention, a method for manufacturing an armature includes providing an armature body with interior magnet pockets, disposing a permanent magnet in each of the magnet pockets, and, to guide the magnetic field of the permanent magnets through the armature body on its armature surface, providing a magnetic anisotropy in a material of the armature body such that an easy axis of the anisotropy is aligned, for each magnetic pocket, in an edge area adjoining each magnetic pocket in parallel relation to or at least at an angle of less than 25° to a magnetization direction of a magnetization of the permanent magnet disposed in the magnet pocket. 
         [0023]    According to another advantageous feature of the present invention, the armature body, for aligning the easy axis, can be formed from segments, and each segment can be formed from a blank which has a predetermined magnetic anisotropy. In other words, the armature body is thus composed of components or segments of which each has its own suitable alignment of the easy axis corresponding to the position of the segment in the armature body. The advantage produced by this is that in each segment individually a suitable alignment of the easy axis can be imprinted or implanted without the other segments being influenced by this. 
         [0024]    According to another advantageous feature of the present invention, implanting of the alignment can be set from outside for the individual segments or also for the complete armature body, for example by means of a magnetic field which penetrates the material during the manufacturing of the armature body or of a segment thereof. The advantage produced by this is that the anisotropy field, i.e. the course of the easy axis can be predetermined through a correspondingly selected field course of the magnetic field implanted from outside. 
         [0025]    According to another advantageous feature of the present invention, the armature body or individual segments of the armature body can be formed by pressing a powder and/or a granulate which contains the material in each case, wherein at the same time this powder and/or granulate has a magnetic field applied to it. By using a powder and/or granulate the shape of the armature body and/or of a segment thereof can expediently be set through a corresponding press shape for the function of the armature and at the same time the easy axis can be aligned by means of the magnetic field during pressing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0026]    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: 
           [0027]      FIG. 1  shows a schematic longitudinal section of an electric machine according to the present invention; 
           [0028]      FIG. 2  shows a schematic cross-section of one embodiment of a rotor for the electrical machine of  FIG. 1 ; 
           [0029]      FIG. 3  shows a schematic cross-section of another embodiment of a rotor for the electrical machine of  FIG. 1 ; 
           [0030]      FIG. 4  shows a schematic diagram of a segment of the rotor of  FIG. 3  with a parallel alignment of the easy axis of a magnetic anisotropy, 
           [0031]      FIG. 5  shows a schematic diagram of a segment of the rotor with a radial alignment of an easy axis of the anisotropy, 
           [0032]      FIG. 6  shows a schematic diagram of a segment of the rotor with a lateral alignment of an easy axis of a magnetic anisotropy, 
           [0033]      FIG. 7  shows a schematic cross-section of still another embodiment of a rotor with eight magnetic poles for the electrical machine of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0034]    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. 
         [0035]    The exemplary embodiments explained below involve preferred embodiments of the invention. In the exemplary embodiments however the described components of the embodiments each represent individual features of the invention to be considered independently of one another, which also develop the invention independently in each case and thus are also to be seen, individually or in a combination other than that shown, as a component of the invention. Furthermore the described embodiments are also able to be supplemented by further of the already described features of the invention. 
         [0036]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown a schematic longitudinal section of one embodiment of an electric machine according to the present invention, generally designated by reference character E. The electrical machine E can involve a synchronous motor for example. In  FIG. 1 , the axis of rotation A also represents the axis of symmetry of the diagram. The electrical machine E includes a stator St, in which windings W of electrical coils are disposed, wherein only one of the windings W is shown in  FIG. 1 . The windings W can be powered alternately by an alternating current source C, through which inside the stator St a magnetic rotating field arises in an air gap L of the electrical machine E. The alternating current source C can for example be an inverter or a fixed-frequency electrical supply network. The electrical machine E can also be embodied as a generator, wherein the alternating current source C can be dispensed with or can be replaced by a rectifier. 
         [0037]    A rotor  10 , which can be connected in a torsion-proof manner to a shaft D, can be located inside the stator St. The shaft D is supported rotatably in the stator St, able to be rotated around the axis of rotation A. The rotor  10  can be one embodiment of an armature according to the invention. 
         [0038]      FIG. 2  shows the rotor  10  only in cross-section. Of the rotor  10 ,  FIG. 2  shows an armature body  12  in which magnet pockets  14  are formed. For sake of clarity, only a few of the magnet pockets  14  are provided with a reference character. The magnet pockets  14  can for example be embodied as wells, of which the longitudinal axis is aligned in parallel to the axis of rotation A, i.e. in parallel to the longitudinal extent of the shaft D. A permanent magnet  16  can be disposed in each of the magnet pockets  14 . For sake of clarity, again only a few permanent magnets  16  are provided with a reference character. The permanent magnets  16  can have a magnetization with the magnetization direction  18  which is illustrated by a direction arrow in  FIG. 2  and also in the further figures. Again only a few magnetization directions  18  are provided with a reference character. Through the magnetization directions  18  north poles N and south poles S are produced in each case on the permanent magnets  16 . Through the magnetic field emerging from the permanent magnets which penetrates the armature body  12 , magnetic poles  22  are produced on an armature surface  20  which once again in accordance with the permanent magnets  16  disposed under the armature surface  20 , are north poles N or south poles S.  FIG. 2  illustrates how through a leakage flux  24  within the armature body  12 , a magnetic flux  26  emerging from a permanent magnet  16  can enter into a neighboring permanent magnet  16  as a leakage flux  24  or can reach the magnet without getting into a vicinity  28  of the armature  10 , i.e. into the air gap L. 
         [0039]    With the rotor  10  however the leakage flux  24  is especially small, so that the efficiency of the electrical machine E is not significantly adversely affected. Also no flux inhibitors are needed in the rotor  10  for this, which can adversely affect the mechanical stability of the rotor  10  through hollow spaces. 
         [0040]    The following description explains with reference to  FIGS. 3 to 7  as to how the stray flux  24  can be reduced. 
         [0041]      FIG. 3  shows a further embodiment of the armature according to the invention in the form of rotor  10  which can have the same functionality as the armature  10  of  FIG. 2 . Parts corresponding with those in  FIG. 2  are denoted by identical reference numerals and not explained again. The description below will center on the differences between the embodiments. In this embodiment, the rotor  10  in  FIG. 3  has only one permanent magnet  16  per magnet pole  22  in cross section, wherein the rotor  10  of  FIG. 2  has two permanent magnets  16  per magnet pole  22 . The following description relating to  FIG. 3  also equally applicable to the rotor  10  depicted in  FIG. 2  however. 
         [0042]    To reduce or suppress in the rotor  10  a crossover of the magnetic flux of a permanent magnet  16  transverse to a q axis  30  within the armature body  12 , the armature body  18  has a material  32  with a magnetic anisotropy. Advantageously, the magnetic anisotropy is embodied such that a single easy axis  34  and a single hard axis aligned at a right angle to the easy axis  34  is formed. 
         [0043]    In  FIG. 3  a direction in which the easy axis  34  extends or runs is illustrated by a respective double-ended arrow. The easy axis  34  is aligned in each case in the area of the magnet pockets  14  preferably in parallel to the magnetization direction  18 , as is illustrated in  FIG. 3 . The parallel alignment of the magnetization  18  is provided especially in the area of the magnet pockets  14  along a d axis  36 . 
         [0044]    In order to make possible for the rotor  10  the different alignments of the easy axis  34  being produced along the circumference of the rotor, the rotor  10  can be formed from segments  38 . The rotor  10  can then be formed from anisotropic soft-magnetic material  32  which is divided into the segments  38  with the number  2   p,  wherein p is the number of pole pairs. With the four poles shown in  FIG. 3  four segments  38  are thus produced. The segments  38  are divided alternately into north pole N and south pole S. In each magnetic segment  38  the slots of the magnet pockets  14  for the permanent magnets  16  are present. The armature body  12  can be created from the segments  38  through material-to-material connection of the segments  38 . In this case the alignment of the easy axis  34  can be set during manufacture of the entire armature body  12  or also during manufacture of the individual segments  38 . The material-to-material bond can be achieved for example by gluing, sintering and/or soldering. 
         [0045]    The material  32  is advantageously characterized by the following values for the relative permeability: Along the easy axis  34 , it has a value of advantageously greater than 15, especially greater than 20. Along the hard axis aligned at a right angle thereto (not shown), the relative permeability advantageously has a value of less than two, especially less than 1.6. The saturation flux density preferably amounts to more than 0.8 Tesla, especially to more than one Tesla. 
         [0046]    On surfaces  40  at which the segments  38  touch each other, an alignment of the easy axis  34  of the anisotropic material  32  can be set differently. In this connection, reference is made to  FIGS. 4 to 6  hereinafter. 
         [0047]      FIGS. 4 to 6  show each a single segment  38 . In  FIG. 4 , the alignment of the easy axis  34  over the entire segment  38  is parallel. This makes the manufacturing of an individual segment especially simple and thus low-cost. 
         [0048]    In  FIG. 5  the alignment of the easy axis  34  for the segment  38  is radial in relation to the axis of rotation A. The parallel alignment of the easy axis  34  to the magnetization  18  is produced here along the d axis  36 . In the edge area of the permanent magnet  16  the magnetization  18  and the easy axis  34  establish an angle  42 , which is advantageously less than 25 degrees. In  FIG. 6 , in a radially interior area  44  in relation to the permanent magnet  16 , the easy axis  34  is aligned in parallel to an easy axis  34 ′ of the respective neighboring segment. At the boundary surfaces  40  the abutting easy axes  34 ,  34 ′ are aligned in parallel to one another. Through this, the magnetic circuit  46  in the radially interior area  44  can be especially easily closed, so that a leakage flux in the area of the shaft D is reduced. The easy axes  34 ,  34 ′ are thus aligned laterally in the radially interior area  44 . 
         [0049]      FIG. 7  shows a rotor  10  with eight magnetic poles, i.e. with a pole pair number p equal to four, in cross-section. The permanent magnets  16  are disposed comparatively close to one another in this arrangement. In other words, a respective pole gap  48  between neighboring segments  38  can be embodied smaller than with a conventional permanent magnet field rotor. The reason for this is that in the area of the pole gaps  48  no flux inhibitor in the form of a hollow space has to be provided between the between the permanent magnet  16 . The anisotropic material  32  can be disposed there, so that the rotor  10 , even with a pole number of eight, remains mechanically stable. 
         [0050]    In the shown exemplary embodiment, the advantages of an IPM structure are combined with the advantages of a massive armature body  12 , i.e. an armature body without hollow spaces as flux inhibitors. In addition, a small inductance is produced along the q axes  30 , i.e. in the pole gaps  48 . Thus a lower voltage drop is produced at the q inductance, which in turn makes possible a high maximum power. In addition the delay time during current buildup in the stator St is low, which makes possible a high regulation dynamic of the electrical machine E. 
         [0051]    To manufacture the armature body  12  or an individual segment  38 , a powder or granulate can be employed for example which has an alloy with neodymium-iron-boron (NdFeB), wherein with this alloy, in a manner known per se, a coercitive field strength in a range between ten and one thousand, especially between 50 and 500 Amperes per centimeter is set. A remanence is set in the range of between zero and two is advantageous, especially in a range between zero and 1.5 Tesla. 
         [0052]    The powder can be pressed into the shape of the armature body  12  or into the shape of an individual segment  38 . Here the power can have a magnetic field applied to it which for example can be created by a magnetic coil. Through the magnetic field the alignment of the easy axis  34  is defined during pressing of the powder and/or granulate. Thereafter the pressed powder and/or granulate can be strengthened for example by sintering. 
         [0053]    Overall it is shown by the examples how, through the invention, a permanent magnet machine with internal magnets and with low armature feedback can be provided by anisotropic soft-magnetic material. 
         [0054]    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. 
         [0055]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: