Abstract:
A rotor includes a shaft and a plurality of permanent magnets which are arranged around the shaft in a circumferential direction for permanent excitation. At least one of the permanent magnets is attached to the shaft by a material joint or formfit. A flux conducting device for conducting a magnetic flux of the permanent magnets has a plurality of separate soft-magnetic flux conducting elements, with each flux conducting element being mounted between two of the permanent magnets and fixed thereto so as to be indirectly held in place on the shaft. At least one of the flux conducting elements has at least one contact area sized to cover an outer edge of one of the permanent magnets in the radial direction.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of European Patent Application, Serial No. 12159915, filed Mar. 18, 2013, pursuant to 35 U.S.C. 119(a)-(d), the content 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 rotor with permanent excitation, and to an electric machine having such a rotor, and to a manufacturing method for the rotor. 
         [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]    Rotors can be provided to form a soft-magnetic base body composed of laminated metal sheets as the flux conducting device. The laminated metal sheets are stacked up in layers to form a laminated core, with an electrically insulating layer being provided between the individual sheets in order to avoid eddy currents. A hole is punched out in the center of the individual sheets so that following its completion the laminated core will have a passage aperture through which the shaft of the rotor is inserted. The laminated core can thus be joined to the shaft by means of an interference fit. Furthermore, cutouts are also punched out on the outer edge of the laminated sheets such that pockets are produced in the finished laminated core in the circumferential direction, into which pockets permanent magnets for producing a permanent excitation of the rotor can be embedded. The term “circumferential direction” has hereby its usual meaning, i.e. in relation to an axis of rotation of the rotor, the circumferential direction is established in a plane perpendicular to the axis of rotation of the rotor, which corresponds to the direction in which a circle runs around the axis of rotation in this plane. 
         [0005]    It would be desirable and advantageous to provide an improved rotor with permanent excitation, improved electric machine having such a rotor, and improved method for manufacturing a rotor to obviate prior art shortcomings. 
       SUMMARY OF THE INVENTION 
       [0006]    According to one aspect of the present invention, a rotor includes a shaft, a plurality of permanent magnets arranged around the shaft in a circumferential direction for permanent excitation, at least one of the permanent magnets being attached to the shaft by a material joint or formfit, and a flux conducting device for conducting a magnetic flux of the permanent magnets, the flux conducting device having a plurality of separate soft-magnetic flux conducting elements, with each flux conducting element being mounted between two of the permanent magnets and fixed thereto so as to be indirectly held in place on the shaft, at least one of the flux conducting elements having at least one contact area sized to cover an outer edge of one of the permanent magnets in the radial direction. 
         [0007]    The present invention resolves prior art problems by mounting the permanent magnets on the shaft itself, i.e. they are not accommodated in pockets of a soft-magnetic laminated core. For example, the permanent magnets are secured to the shaft by an adhesive bond. A flux conducting device is provided to conduct the magnetic flux of the permanent magnets. The flux conducting device includes a plurality of separate soft-magnetic flux conducting elements, for instance individual laminated cores, each of which forming a segment of the rotor. The individual flux conducting elements are each mounted between two of the permanent magnets and secured thereto. Thus, the soft-magnetic flux conducting elements are held in place only indirectly on the shaft and not secured on the shaft by an interference fit for example. 
         [0008]    A rotor according to the invention has the advantage that it can be assembled very easily around the shaft by first fixing the permanent magnets to the rotor shaft, subsequently arranging the flux conducting elements in the spaces between the permanent magnets and then attaching them to the permanent magnets. 
         [0009]    According to another advantageous feature of the present invention, the at least one of the permanent magnets may be attached to the shaft by soldering or welding. When the material joint is a soldered joint, a solder containing silver and/or tin can be used. A soldered joint is on the one hand strong enough to hold the permanent magnets securely on the shaft against the centrifugal forces acting during a rotation of the rotor. On the other hand the solder of the soldered joint is soft enough not to break in the event of vibrations in the rotor. A further possibility of attaching the permanent magnets to the shaft is a welded joint. In this case, however, it is important to take into account that the magnetization of the permanent magnets is not lost due to the heat during the welding operation. There is considerably less risk of this in the case of a soldered joint. Possible examples of formfitting connections are fixing the permanent magnets by means of a screwed connection or forming a dovetail joint. 
         [0010]    In order to obtain an even more regular, sinusoidal curve for a flux conducting element, a step or undercut can be provided by which the flux conducting element bears on an outside edge of a permanent magnet such that the outside edge is covered by the step of the flux conducting element. The term “outside edge” relates in this context to an edge of the permanent magnet located on the outside in the radial direction. A flux conducting element can furthermore be aligned particularly easily with respect to the mentioned enveloping circle by a step in the flux conducting element. 
         [0011]    Accordingly, a permanently excited electric machine can also be provided at particularly low cost on the basis of the rotor according to the invention. The rotor according to the invention is particularly suitable for producing a synchronous machine, a servomotor and a stepper motor. 
         [0012]    The term “shaft” within the context of the invention denotes the entire internal region of the rotor which does not form in any significant way a magnetically active part for generating a torque for driving the rotor or for generating electrical voltage. In other words, the shaft is that part of the rotor which has only a supporting function and serves for mechanically transmitting a torque. In contrast thereto, the magnetically active part is formed by the permanent magnets and the flux conducting elements. The shaft can therefore be a bar made of steel for example or else also a hollow cylindrical body (hollow shaft). In order to increase the size of an outer circumference of the rotor in the region of the permanent magnets, the shaft can have e.g. spokes and, supported by the spokes, a ring to which the permanent magnets are then attached. 
         [0013]    According to another advantageous feature of the present invention, at least one of the permanent magnets may have a metal coating in a contact area on the shaft in order to facilitate producing the connection, in particular a soldered or welded joint. Likewise, the shaft can have a coating in the contact area that is made of a different metal from the metal of which the shaft itself is made. A coating including nickel or copper has proved particularly beneficial for a soldered joint in this case. 
         [0014]    In the contact area, the shaft and the permanent magnets beneficially also correspond to each other in terms of their shape. Depending on the size and type of the permanent magnets, it can be provided that a permanent magnet has a concavely curved surface. Then it is possible to use a normal cylindrical shaft which does not have to be specially adapted in its shape in order to provide the rotor according to the invention. If, however, it is more cost-effective to embody the shaft specifically and accordingly to be able to use cuboidal and therefore easy-to-shape permanent magnets, then a shaft is preferred whose surface is flat in the contact area of the permanent magnet. This enables easy-to-shape permanent magnets to be used, which can therefore be manufactured at reasonable cost. In other words a shaft having a polygonal contour in cross-section is used if all of the permanent magnets in the contact area have a planar surface. 
         [0015]    According to another advantageous feature of the present invention, at least one of the flux conducting elements can be adhesively bonded to at least one permanent magnet. This is particularly cost-effective to manufacture results. Advantageously, the adhesive bond may include a silicone adhesive. Like the soldered joint for the permanent magnets, this has the necessary strength for holding the flux conducting elements secure against the centrifugal forces on the one hand, and on the other hand has sufficient elasticity for compensating or even damping vibrations in the rotor. 
         [0016]    According to another advantageous feature of the present invention, the rotor has perpendicular to its axis of rotation a cross-section in which the permanent magnets can be arranged in a star shape on the shaft, and the flux conducting elements can be arranged in a wedge shape, i.e. cylinder segments, between the permanent magnets. The thus resulting rotor is very robust and nonetheless particularly easy to manufacture. 
         [0017]    With regard to the permanent magnets themselves, these can be rare earth magnets for example. Currently preferred are ferrite magnets as permanent magnets since they are significantly cheaper than rare earth magnets, even though ferrite magnets have a lower magnetic field strength. However, that is not a problem because the magnetic field of ferrite magnets can be concentrated to such an extent that a magnetic field having the field strength that is usual for permanently excited electric machines can be generated in an air gap between the rotor and a stator of an electric machine. For that purpose, the magnetizations of the permanent magnets should simply point in the circumferential direction, i.e. not radially away from the shaft, but tangentially to the circumference of the shaft. The flux conducting elements located between the permanent magnets ensure that the magnetic field lines are diverted outward in the radial direction and emerge radially from the rotor on its outer circumference. As a result of the diversion, a flux concentration is established such that the field of the ferrite magnets is stronger concentrated in the air gap than at the surface of the ferrite magnets themselves. Rare earth magnets can of course also be used in this case, enabling an even stronger magnetic field to be generated. 
         [0018]    An even greater flux concentration can be achieved when the magnetizations of two adjacent permanent magnets are aligned in opposite directions to each other. 
         [0019]    An additional aim in electric machines is also to generate in the air gap between rotor and stator a magnetic field whose field strength (not direction) extends sinusoidally around the rotor in the circumferential direction. This can be achieved in the case of the rotor according to the invention by a particular configuration of the outer surface of the flux conducting elements, i.e. this surface of each flux conducting element which bounds the latter (in relation to the shaft) in the radial direction outward toward the rotor exterior. The surface advantageously has a curvature in the circumferential direction of the rotor with a curvature radius which is smaller than a radius of an enveloping circle which describes the maximum outer circumference of the rotor in the circumferential direction. Advantageously, the curvature radius may range between 50 and 100 percent of the enveloping circle radius. 
         [0020]    The refinement described in connection with the rotor according to the invention also constitutes refinements of the electric machine. 
         [0021]    According to another aspect of the present invention, a method for manufacturing a rotor includes attaching permanent magnets to a shaft by a material joint or formfit, inserting flux conducting elements in spaces between the permanent magnets such that external surfaces of all the flux conducting elements are tangentially touched by an enveloping circle and steps of the flux conducting elements bear on contact edges of the permanent magnets, with the contact edge of each of the permanent magnets being formed by an edge of the permanent magnet pointing to an outside face of the rotor, and securing the flux conducting elements to the permanent magnets. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0022]    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: 
           [0023]      FIG. 1  is a schematic cross-sectional view of one embodiment of a rotor according to the invention; 
           [0024]      FIG. 2  is a schematic view of a connection of a permanent magnet to a shaft, as produced during the manufacture of a rotor according to the invention; 
           [0025]      FIG. 3  is a schematic cross-sectional view of another embodiment of a rotor according to the invention; and 
           [0026]      FIG. 4  is a schematic view of contact edges of permanent magnets which are covered by flux conducting elements in a rotor according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0027]    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. 
         [0028]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown a cross-section of a rotor according to the invention, generally designated by reference numeral  10  and having permanent magnets  14  attached to a shaft  12  with flux conducting elements  16  inserted between them in each case. Only a section of the rotor  10  is shown in  FIG. 1 , as indicated by broken lines. The rotor  10  can be installed in an electric machine, for example a synchronous machine or a servomotor. The electric machine can for example be a drive for an electric vehicle. 
         [0029]    An axis of rotation  18  of the rotor  10  extends perpendicularly to the image plane of  FIG. 1 . The rotor  10  has a cylindrical basic shape overall, the cylinder axis coinciding with the rotor axis  18 . In the example shown, the shaft  12  can be formed from a solid body made of steel. The permanent magnets  14  can have an elongate, cuboidal basic shape, the longitudinal dimension extending parallel to the axis of rotation  18 . The flux conducting elements  16  likewise are elongate bodies, each having the basic shape of a cylinder segment, bodies likewise being aligned with their longitudinal direction extending parallel to the axis of rotation  18 . All in all, the permanent magnets  14  in the cross-section shown in  FIG. 1  formed perpendicularly to the axis of rotation  18  are arranged in a star shape on the shaft  12 . The flux conducting elements  16  have a wedge shape in cross-section and are arranged between the permanent magnets  14 . 
         [0030]    The permanent magnets  14  can be ceramic magnets, ferrite magnets for example. The permanent magnets  14  are each attached to the shaft  12  by a soldered joint  20 . For clarity of illustration reasons, the position of the soldered joint  20  is shown for only one of the permanent magnets  14  in  FIG. 1 . With the permanent magnets  14 , a respective magnetization  24  points along a circumferential direction  26  (the circumferential direction  26  being indicated here by a double arrow, because in this case the orientation is irrelevant). The magnetic field generated by the permanent magnets  14  is in this case redirected outward in a radial direction  28  by means of the flux conducting elements  16  and concentrated in the process. 
         [0031]    For this purpose the flux conducting elements  16  are produced from a soft-magnetic material. They can each be formed for example by means of a laminated core. The flux conducting elements  16  are not mounted directly on the shaft  12 . Each flux conducting element  16  is fixed in place by means of an adhesive bond  22  on those permanent magnets  14  between which it is located. In the case of the adhesive bonds  22 , too, only the adhesive bonds  22  for one flux conducting element  16  are labeled with a reference sign in  FIG. 1  for clarity of illustration reasons. 
         [0032]    In the following it is explained with reference to  FIG. 2  how a rotor, the rotor  10  of  FIG. 1  for example, can be manufactured. To make navigating the drawings easier, the same reference sign as in  FIG. 1  is used in each case in  FIG. 2  and also in the following figures for elements corresponding to elements from  FIG. 1 . 
         [0033]      FIG. 2  shows once more in a magnified view a shaft  12  to which a permanent magnet  14  has been attached. The permanent magnet  14  has a cuboidal basic shape such that a surface  30  of the permanent magnet  14  in a contact area  32  of the permanent magnet  14  on the shaft  12  is flat. A surface  34  of the shaft  12  is shaped correspondingly in the contact area  32 , which is to say that it too is flat. Overall, therefore, the shaft  12  has a polygon shape in a cross-section perpendicular to the axis of rotation  18  if the contact surfaces of all of the permanent magnets  14  are flat. Alternatively thereto, the shaft  12  can also be round if the permanent magnets  14  have a correspondingly round contour on the side facing toward the shaft  12 , that is to say if they are each curved concavely there toward the inside of the permanent magnets  14 . 
         [0034]    In order to attach the permanent magnet  14  to the shaft  12 , a solder  36 , e.g. in the form of a solder paste, was applied and heated. The solder  36  can be for example a soft solder, for example a solder including silver or tin. In order to obtain a strong, material-to-material bond by means of the solder  36 , in the example shown in  FIG. 2  both the permanent magnet  14  and the shaft  12  are coated with a layer  38 , in particular a metal layer, such as a layer composed of nickel and/or copper for example. 
         [0035]    After the permanent magnet  14  and further permanent magnets (compare  FIG. 1 ) are mounted on the shaft  12  in the manner shown in  FIG. 2 , flux conducting elements  16 , in other words for example sheet-metal segments, are inserted between the permanent magnets  14  and fixed to the permanent magnets  14  e.g. by means of an adhesive, for example a silicone glue. 
         [0036]    Referring to  FIG. 3 , it is explained in the following how an external magnetic field can be generated by means of permanent magnets  14  in the case of a segmented rotor  10 , said magnetic field having a maximally sinusoidal curve of the field strength along a circumferential direction  26 . The rotor  10  shown in  FIG. 3  can be for example the rotor of  FIG. 1 . For clarity of illustration reasons only a few permanent magnets  14  and flux conducting elements  16  are labeled with reference signs in  FIG. 3 .  FIG. 3  again shows a section through a rotor  10  perpendicular to an axis of rotation  18 . Also shown in  FIG. 3  for greater clarity of illustration is an enveloping circle  40  which describes the maximum outer circumference of the rotor  10  starting from the axis of rotation  18 . The enveloping circle  40  has an enveloping circle radius  42 . 
         [0037]    The individual flux conducting elements  16  have external surfaces  44  which are curved along the circumferential direction  26 . By external surface  44  is meant in this context that surface of a flux conducting element  16  which limits the respective flux conducting element  16  in the radial direction  28 . For clarity of illustration reasons corresponding components are labeled with a reference sign for one flux conducting element  16  only for the explanations in  FIG. 3 . In this case the external surfaces  44  of the flux conducting elements  16  each have a curvature radius  46  which is smaller than the enveloping circle radius  42 . 
         [0038]    In  FIG. 4  it is shown how it can be achieved with the aid of undercuts or steps  48  that during gluing the flux conducting elements  16  are guided only precisely so far between the permanent magnets  14  to ensure that all the external surfaces  44  are tangentially touched by the enveloping circle  40 . Toward that end the flux conducting elements  16  are inserted so far between the permanent magnets  14  until the steps  48  bear on contact edges  50  of the permanent magnets  14 . By contact edge  50  of a permanent magnet  14  is meant in this context an edge of the permanent magnet  14  pointing toward an outside face of the rotor  10 . Furthermore, by covering the contact edges  50  of the permanent magnet  14  by means of the steps  48  of the flux conducting elements  16 , the curve of the field strength of the outer magnetic field of the rotor  10  is better adapted to the desired sinusoidal curve. Field peaks of the magnetic field exiting or entering the flux conducting elements  16  are additionally avoided by means of tapers in the region of the steps  48 . 
         [0039]    The examples shown furthermore have the advantage that the permanent magnets  14  and flux conducting elements  16  can be fixedly connected to the shaft in such a way that no additional external braces are necessary to stabilize the flux conducting elements  16  and the permanent magnets  14  against centrifugal forces. 
         [0040]    It is shown overall by means of the examples how a combination of the bonding techniques of soldering and gluing can be used to enable a robust and at the same time easily realized manufacturing method for a permanent-magnet-excited rotor with internal magnet in flux concentration. The use of metallized ferrite magnets for soldering on a metallized shaft is also shown. Steps in flux conducting elements, such as laminated core segments for example, allow precise positioning of the individual flux conducting elements between the permanent magnets. 
         [0041]    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. 
         [0042]    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: