Abstract:
A primary part of a linear electrical machine includes a first member for production of a first magnetic field, a second member for production of a second magnetic field. The first member and the second member are arranged to realize a superimposition of the first magnetic field with the second magnetic field. Arranged on at least one end face of the primary part is a flux-guiding element to reduce a force ripple. The flux-guiding element is constructed in the form of an end tooth module having at least one permanent magnet.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of European Patent Application, Serial No. 08002960, filed Feb. 18, 2008, 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 primary part of a linear electrical machine, and to a linear electrical machine with a primary part. 
         [0003]    Nothing in the following discussion of the state of the art is to be construed as an admission of prior art. 
         [0004]    Linear electrical machines have a primary part and a secondary part which are arranged in opposite relationship and separated from one another by an air gap. The primary part is intended for electric current to be passed through it. It is possible for both the primary part and the secondary part to have active means for production of magnetic fields. For example, the primary part has a winding through which current can be passed, and the secondary part has permanent magnets. In addition, it is also possible for the primary part to have a plurality of active means for production of magnetic fields, and for the secondary part to be free of such means. 
         [0005]    German patent document DE 10 2004 045 992 A1 discloses an electrical machine with a primary part which has all the magnetic sources of the electrical machine. The primary part has, for example, a winding through which current can be passed and permanent magnets whereas the secondary part is, for example, only in the form of a toothed iron reaction rail. 
         [0006]    Unlike electrical machines which operate by rotation, linear electrical machines have end areas in which the electromagnetic part of the machine ends. If, for example, a linear motor is designed using a short stator construction, i.e. a primary part is sized shorter than a secondary part, the primary part has two end areas which are located in the influence area of the secondary part. The ends of the primary part interact with the secondary part such that it has a significant influence on the active force ripple and the passive force ripple, also referred to as the cogging force. Parasitic cogging forces which occur as a result of the magnetic interaction between the primary part and the secondary part are referred to as passive force ripple. This results in vibration, rough running and drag errors during machining processes. 
         [0007]    Furthermore, the induced voltages, i.e. the electromotive forces (EMF), in the initial and end coils at the end faces of the primary part are generally less pronounced than in the central coils in view of an absent magnetic return path. As a consequence, the electrical machine does not have a symmetrical induced voltage and an additional current-dependent force ripple, which is referred to as active force ripple is experienced in addition to reductions in the force. 
         [0008]    German patent document DE 10 2005 004380 A1 discloses a linear motor with force ripple compensation by using a force ripple compensation tooth of a predefined width, which is separated by an additional air gap from the normal air gap and is at a different distance from the adjacent teeth of the laminated core as the remaining teeth of the laminated core. 
         [0009]    It would therefore be desirable and advantageous to address prior art shortcomings. 
       SUMMARY OF THE INVENTION 
       [0010]    According to one aspect of the present invention, a primary part of a linear electrical machine includes a first member for production of a first magnetic field, a second member for production of a second magnetic field, wherein the first member and the second member are arranged to realize a superimposition of the first magnetic field with the second magnetic field, and a flux-guiding element arranged on at least one end face of the primary part to reduce a force ripple, with the flux-guiding element being constructed in the form of an end tooth module having at least one permanent magnet. 
         [0011]    Currently preferred is the arrangement of a flux-guiding element on each of the two end faces of the primary part. Suitably, the flux-guiding element is designed for production of a magnetic field and may include a member in correspondence to the first or the second member for production of a magnetic field of the primary part. 
         [0012]    The fitting of flux-guiding elements makes it possible to reduce the active and, in particular, the passive force ripple, that is to say the cogging force. The induced voltages in the end teeth, to which windings are flitted, of the primary part are raised, with the aim of this being to ensure that the EMF of the winding of the primary part is as uniform as possible. 
         [0013]    According to another advantageous feature of the present invention, the first member for production of a first magnetic field may be realized in the form of a single-phase or polyphase winding. In particular, the primary part may have a three-phase winding in the form of tooth-wound coils for connection to a three-phase power supply system. The second member for production of a second magnetic field may be formed from permanent-magnet material, in particular in the form of integral permanent magnets or multi-part permanent magnets. 
         [0014]    According to another advantageous feature of the present invention, the primary part can be formed by a predefined number of tooth modules, wherein each tooth module has at least one permanent magnet and one tooth-wound coil. A primary part can be constructed in a modular form by means of individual tooth modules, depending on the desired type and length. The desired number of tooth modules is joined together as appropriate, for example by brackets. Each tooth module is, in particular, formed by electrical laminates as a laminated core, in order to reduce eddy currents in the primary part. 
         [0015]    According to another advantageous feature of the present invention, the flux-guiding element may be constructed in the form of an end tooth module on the end faces of the primary part and have at least one permanent magnet. In terms of its type, the end tooth module corresponds to the other tooth modules of the primary part. It differs only in that no windings are fitted to it, i.e. it has no tooth-wound coil. 
         [0016]    The present invention thus resolves prior art shortcomings by providing at least one, currently preferred two additional tooth modules at both ends of the primary part, the end tooth modules. These end tooth modules have the same basic geometry as the tooth modules which support the winding or coil of the primary part, and they are arranged as a continuation of the active primary part area at both ends. 
         [0017]    The geometry and/or magnetization of the permanent-magnet material or of the permanent magnet in the end tooth module is critical for compensation in particular for the cogging forces. 
         [0018]    According to another advantageous feature of the present invention, the permanent magnets of the primary part may have a predeterminable magnetization and a magnetic field strength, wherein the permanent magnets of the tooth modules and the permanent magnets of the end tooth modules have a same magnetization. Compared to a tooth module, the end tooth module may have a permanent magnet of smaller volume. As a result the permanent magnet in the end tooth module is smaller, and less permanent-magnet material is arranged in the end tooth module. For example, the permanent magnet in the end tooth module is reduced in size by a factor of 2 to 4 in comparison to a permanent magnet in the tooth module, thus making it possible to optimally reduce the cogging force. The extent of the reduction in size of the permanent magnet in the end tooth module is dependent inter alia on a track width of a linear motor. 
         [0019]    According to another advantageous feature of the present invention, the permanent magnets of the primary part may have a predeterminable magnetization, wherein the permanent magnets of the tooth modules have a higher magnetization than the permanent magnets in the end tooth modules. Thus, the permanent magnets in the tooth modules and end tooth modules have a same geometric size and a same volume, but their magnetization differs. In other words, the magnetic material incorporated in the end tooth modules differs from that in the tooth modules. Examples of magnetic materials include neodymium-iron-boron magnets, samarium-cobalt magnets or ferrite magnets. These different magnetic materials have different magnetizations which are characterized, for example, by the magnetic characteristic values such as remanent induction or coercivity field strength. For example, a remanent induction reduced to about 25% results in an electrical machine cogging force reduced by a factor of 4-5 in comparison to the cogging force of a machine without modified end tooth modules. 
         [0020]    The invention is applicable in particular to polyphase linear motors with permanent-magnet excitation in the primary part and with a passive secondary part, for example in the form of a toothed iron structure. As in the case of all linear motors and linear electrical machines, electromagnetic discontinuities result in a cogging force, which interferes with motor operation, at both primary part ends. 
         [0021]    According to another aspect of the present invention, a linear electrical machine includes a primary part including a first member for production of a first magnetic field, a second member for production of a second magnetic field, wherein the first member and the second member are arranged to realize a superimposition of the first magnetic field with the second magnetic field, and a flux-guiding element arranged on at least one end face of the primary part to reduce a force ripple, with the flux-guiding element being constructed in the form of an end tooth module having at least one permanent magnet, and a secondary part constructed absent a production of a magnetic field. 
         [0022]    According to another advantageous feature of the present invention, the secondary part may be implemented in the form of a toothed iron reaction part with a plurality of teeth and slots. The secondary part may be advantageously laminated, i.e. it is formed by a multiplicity of individual electrical laminates, in order to avoid eddy-current loses. Alternatively, however, the secondary part may also be formed by a solid, toothed iron reaction rail. The linear electrical machine is, in particular, a synchronous linear motor. 
         [0023]    An electrical machine designed such as this has the advantage that the secondary part of the electrical machine has no active member for production of a magnetic field. The secondary part is only constructed for guidance of magnetic fields, and is therefore simple and costs little to manufacture. 
         [0024]    The use of end tooth modules which have a modified permanent magnet material in comparison to the other tooth modules to reduce the cogging force results in a number of advantages: 
         [0025]    better characteristics of the electrical machine, in particular of a linear motor, during operation, such as better synchronism, a greater rated force, higher drive dynamics and no “ghost movement” of the switched-off motor resulting from cogging force; 
         [0026]    better characteristics in a drive system (for example the Siemens Sinamics drive system), for example more accurate pole position identification as a result of reduced electromagnetic asymmetries (end effects) of the linear motor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0027]    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: 
           [0028]      FIG. 1  shows a sectional view of a first embodiment of a linear electrical machine according to the present invention; 
           [0029]      FIG. 2  shows a sectional view of a second embodiment of a linear electrical machine according to the present invention; 
           [0030]      FIG. 3  shows a sectional view of a third embodiment of a linear electrical machine according to the present invention; and 
           [0031]      FIG. 4  shows a graphical illustration of the cogging force profile for various linear electrical machines. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0032]    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. 
         [0033]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown a sectional view of a first embodiment of a linear electrical machine according to the present invention, generally designated by reference numeral  10 . The linear electrical machine  10  includes a primary part  2  and a secondary part  3 . The primary part  2  includes six tooth modules  4  and two end tooth modules  11  arranged on each end face of the primary part  2 . Each tooth module  4  is formed by a laminated iron core  5  and has a permanent magnet  6  which is integrated in the tooth module  4 . Of course, the permanent magnet  6  may also be arranged outside the tooth module  4  on the tooth module  4  on the side of the primary part  2  facing the air gap δ. By way of example, a permanent magnet  6  has the remanent induction B R =1.2 T. Furthermore, each tooth module  4  has a tooth-wound coil  7 , wherein the six tooth-wound coils  7  form the winding of the primary part  2 . The winding has three phases, with the phases u, v, w, in such a way that two tooth-wound coils  7  are formed for each phase u, v, w. The tooth-wound coils  7  are located in the slots  9  in the primary part  2 . 
         [0034]    Each end tooth module  11  does not have a tooth-wound coil and has a permanent magnet  12  which is modified in comparison to the permanent magnet  6 . All the permanent magnets  6  and  12  are formed from the same magnetic material and, for example, have a remanent induction B R =1.2 T. However, the permanent magnet  12  has a smaller volume than the permanent magnet  6 , that is to say it is smaller, in order to optimally reduce the cogging force. For a specific linear motor design, the motor cogging force can be very greatly reduced (for example by a factor of 5), for example by reducing the height of the permanent magnet  12  to about 40% of the normal height of the permanent magnets  6 . The permanent magnets  12  are likewise arranged flush with the lower edge of the respective end tooth module  11 , in the same way as the permanent magnets  6  in the other tooth modules  4 . All the permanent magnets  6  and  12  are at the same distance from the air gap δ. 
         [0035]    The tooth modules  4  and the end tooth modules  11  are physically identical, apart from this. The end tooth modules  11 , when compared to the tooth modules  4 , have the same slot pitch τ z  as that of the tooth modules  4 . 
         [0036]      FIG. 2  shows a second embodiment of a linear electrical machine  10  in accordance with the present invention. The design of the electrical machine  10  corresponds essentially to that of the machine  10  shown in  FIG. 2 . The permanent magnets  6  and  12  are formed from the same magnetic material, but the permanent magnets  12  in the end tooth modules  11  have a smaller volume, that is to say they are smaller. 
         [0037]    In comparison to the illustration shown in  FIG. 2 , however, the permanent magnets  12  do not end flush with the lower edge of the end tooth modules  11 , but are arranged offset. In particular, the permanent magnets  12  are withdrawn into the end tooth modules  11 . They are arranged offset upwards, that is to say away from the air gap δ. 
         [0038]    The permanent magnets  12  have the height h PM  and are offset by the height h 0 . In particular, the following applies for h 0 : 0≦h 0 ≦10δ, thus making it possible to finely adjust the force ripple compensation effect. Permanent magnets  12  with the same geometric dimensions, in particular the same height h PM  can thus be used for different physical sizes of linear motors or else for different track widths of linear motors, and the force ripple compensation effect can be appropriately adapted by respective adaptation of the height h 0 . 
         [0039]      FIG. 3  shows a third embodiment of a linear electrical machine according to the invention, generally designated by reference numeral  20 . The primary part  2  has the two additional end tooth modules  11  in addition to the six tooth modules  4 , with one end tooth module  11  being arranged on each end face of the primary part  2 . Each end tooth module  11  has a permanent magnet  13  which is modified in comparison to the permanent magnet  6 , but has no tooth-wound coil. The permanent magnets  6  and  13  have the same geometric dimensions, but different magnetizations. For example, the permanent magnets  6  have a remanent induction B R =1.2 T, while in contrast the permanent magnets  13  have a remanent induction B R =0.3 T. A remanent induction B R  such as this reduced to about 25% results in a cogging force of the linear electrical machine  20  being reduced by a factor of 4-5 in comparison to the cogging force of the machine  1  as shown in  FIG. 1  without end tooth modules. 
         [0040]      FIG. 4  shows an illustration of the cogging-force profile of various electrical machines. The reference symbol  14  denotes the amplitude of the cogging force of an electrical machine  1  as shown in  FIG. 1 , that is to say of a machine without flux-guiding elements for reducing the force ripple. The reference symbol  15 , in contrast, denotes the amplitude of the cogging force of an electrical machine  10  as shown in  FIG. 2  with end tooth modules  11 .  FIG. 4  shows well that only very minor cogging forces occur when corresponding elements are arranged to reduce the cogging force. 
         [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 of the present invention. The embodiments were chosen and described in order to best 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.