Patent Publication Number: US-2004056550-A1

Title: Stator for an electric machine and method for producing the same

Description:
BACKGROUND OF THE INVENTION  
       [0001] The invention relates to an electric machine and a stator for an electric machine and a method for manufacturing same.  
       BACKGROUND OF THE INVENTION  
       [0002] Such electric machines and stators for them are known in the state of the art in the most different configurations.  
       [0003] From EP 1 039 616 A2 an electric machine with an internal rotor and a stator is known which comprises several grooves arranged adjacent to one another. Stator windings are built from conductor bars arranged in the grooves as well as from conductor portions forming winding overhangs, with the conductor bars being riveted at their ends to the conductor portions.  
       [0004] From DE 43 21 236 C1 a multi-phase electric machine is known with a winding from flat conductor formed parts which are inserted in grooves. At the transition from a groove formed in the stator into a winding overhang, an increase in the conductor cross-section towards the groove width takes place. Joints all of which are located at the winding overhang end faces are formed by welding or brazing.  
       [0005] The term “electric machine” as used herein is to be understood as covering both an electric motor and an electric generator, regardless of whether the electric machine is designed as a rotary machine or, for example, as a linear motor.  
       [0006] An essential factor which contributes significantly to the efficiency of an electric machine is the space factor of the (stator) winding (i.e. the ratio of the winding wire volume to the total volume of the winding chamber). Another factor which contributes considerably to the efficiency is the design of the winding overhangs (axially protruding beyond the stator body) of the stator winding. Due to the fact that, in particular, the winding portion axially protruding beyond the stator body is electromagnetically not effective, it has been endeavoured to design this portion of the windings as space-saving as possible. This is of particular importance with multi-phase machines, because in this case the spatial design of the winding overhangs is particularly expensive.  
       [0007] In order to increase the space factor of the (stator) winding it is known to install conductor bars with a rectangular cross-section into the winding chambers, which are connected via corresponding conductor portions in the coil ends with the respective windings.  
       [0008] Problem on Which the Invention is Based  
       [0009] The joints of the conductor bars in the winding chambers with the conductor portions in the coil ends are an essential factor for the reliability of the respective machine. This holds true all the more as the spatially very limited conditions in the area of the winding overhangs exclude a number of known joining techniques.  
       [0010] All known concepts have in common that with a sufficiently compact construction, the reliability is no longer achieved in large-scale production applications. Moreover, known manufacturing approaches are very expensive.  
       [0011] Spot welding, for example, is also to be excluded, at least with machines in the power range below one megawatt, because the risk of the welding electrodes adhering to the parts to be welded is very high.  
       OBJECT OF THE INVENTION  
       [0012] It is the object of the resent invention to provide a compact electric machine which can economically be manufactured in large-scale production as well as a method for manufacturing same.  
       [0013] Inventive Solution  
       [0014] In order to eliminate the drawbacks of the state of the art, the invention teaches an electric machine which is defined by the characteristics of claim 1, as well as a method for manufacturing such an electric machine.  
       [0015] Construction and Developments of the Inventive Solution  
       [0016] According to the invention, the object is solved by an electric machine with a stator which comprises several grooves arranged distributed about its circumference and forming winding chambers for accommodating at least one stator winding, with each stator winding being constructed of conductor bars of an essentially rectangular cross-section in the winding chambers and conductor portions forming winding overhangs, with the conductor bars being connected at their ends with the conductor portions in such a manner that each of the conductor portions comprises an essentially U-shaped end section with two opposite legs whose inner sides facing each other are joined with corresponding side faces of an end section of one of the conductor bars.  
       [0017] This solution enables a particularly reliable and economic manufacture of the stator windings with excellent electric and mechanic properties, even under large-scale production conditions.  
       [0018] In a first preferred embodiment, the joint between the end section of the conductor bar and the end section of the conductor portion comprises a layer of brazing solder, preferably silver brazing solder, tin brazing solder, or the like. Alternatively, the joint between the end section of the conductor bar and the end section of the conductor portion may comprise a layer of high-temperature soft solder, preferably with a melting point of at least 380° C.  
       [0019] In order to prevent the occurrence of a thickening (with the associated space problem) in the area of the winding overhangs, preferably the end section of the conductor bar is tapered by at least approx. the wall thickness of the essentially U-shaped end section of the conductor portion. This is provided for all side faces at which the end section of the conductor portion is in contact with the end section of the conductor bar.  
       [0020] If the end section of the conductor portion is in contact and in (an integral) connection with only two (e.g. opposite) side faces of the end section of the conductor bar, the packing density of the winding layers in the winding overhang can be maintained equal to that on the winding groove.  
       [0021] Preferably each of the opposite legs comprises a projection on its inner face facing the end section of the conductor bar, which makes contact with the corresponding side faces of the end section of the conductor bar. This facilitates a defined sequence of the integral joining process.  
       [0022] The integral joint can be effected in a particularly simple manner by means of electric impulse welding.  
       [0023] Especially with multi-phase electric machines, it is necessary that the winding overhangs are laterally offset relative to one another. This can be achieved most easily in that the conductor portion protrudes beyond the end section of the conductor bar and is cranked.  
       [0024] The invention also relates to a method for manufacturing an electric machine with a stator which comprises several grooves arranged distributed about its circumference and forming winding chambers for accommodating at least one stator winding, as defined above, with the steps: inserting an essentially rectangular conductor bar into a winding chamber so that an end section of the conductor bar protrudes beyond at least one end face of the stator, integrally joining of a conductor portion with the protruding end section of the conductor bar, with the conductor portion comprising an essentially U-shaped end section with two opposite legs whose inner sides facing each other are joined with corresponding side faces of an end section of one of the conductor bars.  
       [0025] The two opposite legs thereby encompass the respective side faces of the end section of the conductor bar and are pressed against these.  
       [0026] Simultaneously with this pressing operation or at a later time, electric contact electrodes are applied to the conductor bar and the conductor portion, through which a predefined electric power impulse is then sent which is sufficient to melt the material at the joint(s).  
       [0027] An essential aspect is that the places where the electric contact electrodes are applied to the conductor bar and the conductor portion are different from the pressure-subjected places. This avoids bonding or melting of the contact electrodes with the parts to be joined.  
       [0028] According to the invention the power of the impulse is determined to be such that in the joint area essentially no heat dissipates to the environment. This is achieved primarily by introducing the power into the parts to be joined during a time interval as short as possible. This causes the melting process to take place so rapidly that hardly any energy is dissipated to the environment prior to the completion of the joining process. 
     
    
    
     [0029] Further characteristics, properties, advantages, and possible modifications will become obvious for those with skill in the art from the following description in which reference is made to the accompanying drawings.  
     [0030]FIG. 1 schematically illustrates the plan view of a development of a stator for an electric motor according to the invention with sectioned stator windings.  
     [0031]FIGS. 2 and 3 schematically illustrate in a perspective view how the conductor bar of a winding according to FIG. 1 is to be joined with a conductor portion forming the winding overhang.  
     [0032]FIG. 4 illustrates in a schematic plan view how the conductor bar and the conductor portion from FIGS. 2, 3 are pressed together and supplied with an electric power impulse.  
     [0033]FIG. 5 shows an alternative embodiment of a conductor portion in a schematic perspective view. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0034]FIG. 1 shows a plan view of a portion of a development of a stator  10  of an internal rotor motor machine (not shown in detail), with the invention being also applicable to an external rotor machine. In the present embodiment the stator  10  is built from stacked laminations, but could also consist of iron particles pressed to the respective shape.  
     [0035] The stator  10  has grooves  12  arranged adjacent to each other, which form the winding chambers for the respective stator coil windings  14 . In the illustrated embodiment the winding chambers have an essentially rectangular cross-section, with slots  16  being provided in the side facing towards the rotor (not shown). Thus, teeth  18  are formed between two each slots  16 .  
     [0036] Each stator winding  14  is formed from conductor bars  20  with an essentially rectangular cross-section, which have been inserted into the winding chambers and joined with conductor portions  22  which form winding overhangs (see FIG. 2). The conductor bars  20  are integrally and electrically connected at their end sections  26  with the conductor portions  22 . This is realised in that each of the conductor portions  22  comprises an essentially U-shaped end section  30  with two opposite legs  32 ,  34  whose inner sides  32   a ,  34   a  facing each other are joined with corresponding side faces  26   a ,  26   b  of the end section  26  of one of the conductor bars  20 . FIGS. 2 and 3 only show the joint between one end of a conductor bar  20  and one half of a (otherwise mirror image) conductor portion  22 .  
     [0037] In order to produce this joint, a layer of (silver) brazing solder is applied to the side faces  26   a ,  26   b  of the end section  26  of the conductor bar  20 .  
     [0038] As shown in the embodiment of FIG. 3, the end section  26  of the conductor bar  20  is tapered by approx. the wall thickness of the essentially U-shaped end section  30  of the conductor portion  22 . It is thereby achieved that the space conditions in the area of the winding overhangs are not too restricted or that the winding overhangs can be arranged very compact so that the electromagnetically not effective part of the stator coil winding is relatively small. Due to the fact that the conductor bars  20  and the conductor portions  22  are joined with each other via the two side faces  26   a ,  26   b  or via the inner faces  32   a ,  34   a , respectively, a very large interface and thus a mechanically and electrically very reliable connection is achieved.  
     [0039] At the inner faces  32   a ,  34   a  facing each other, of the opposite legs  32 ,  34 , a projection  38  in the form of a cone is provided with is embossed from the outside. Due to the fact that the joint between the conductor bar  20  and the conductor portion  22  is made be electric impulse welding as will be explained in more detail below, this projection  38  achieves a reproducible electric contact making in the welding process and thus a defined melting process of the material to be joined.  
     [0040] In the manufacture of an electric machine with the above described stator  10 , it is to be proceeded as follows: First, a stator  10  (see FIG. 1) is provided which has the appropriate grooves  12 . Into these grooves  12  the rectangular conductor bars  20  are inserted which are dimensioned in such a manner that at both end faces of the stator  10  one each end section  26  of the conductor bar  20  is protruding. By using conductor bars  20  whose shape is matched to the shape of the grooves  12 , the packing density (i.e. the space factor) can be considerably increased compared to the standard coil windings from wire rod.  
     [0041] Subsequently, a conductor portion  22  is joined with the protruding end section  26  of the conductor bar  20 . For this purpose, the two opposite legs  32 ,  34  of the end section  30  of the conductor portion  22  are pressed against the respective side faces  26   a ,  26   b  of the end section  26  of the conductor bar  20  by means of two press jaws  40 ,  42  (see FIG. 4). Contrary to conventional tools for electric impulse welding, no current flows through this press jaws  40 ,  42 . Rather, these are merely moved towards each other by the forces F, so that the two opposite legs  32 ,  34  of the end section  30  of the conductor portion  22  are pressed against the respective side faces  26   a ,  26   b  of the end section  26  of the conductor bar  20 . The tips of the projections  38  thereby come into contact with the side faces  26   a ,  26   b  of the end section  26 . Simultaneously with the pressing together and the associated contact between the projections  38  and the side faces  26   a ,  26   b , electric contacts are applied at each of the conductor bar  20  and the conductor portion  22 , through which a predefined electric power impulse flows which is sufficient to melt the material at the joint(s).  
     [0042] As is illustrated in FIG. 4, the places where the electric contacts are applied to the conductor bar  20  and the conductor portion  22  are different from places where the press jaws  40 ,  42  press the two opposite legs  32 ,  34  of the end section  30  of the conductor portion  22  against the respective side faces  26   a ,  26   b  of the end section  26  of the conductor bar  20 . In the illustrated embodiment the electric power impulse is introduced through two contact pads  50 ,  52  which, on the one hand, are applied to a middle web  56  (see FIGS. 2, 4) of the conductor portion  22  and, on the other hand, to an end face  26   c  of the conductor bar  20 . It goes without saying that—dependent on the spatial conditions—other places for the introduction of the electric power impulse can be used at the conductor portion  22  or the conductor bar  20 , respectively. That which is decisive is that the contact points for the electric power impulse are different from the places where the conductor portion  22  is joined (welded) to the conductor bar  20 . Due to the fact that no electric current flows through the places of the introduction of force, bonding of the press jaws  40 ,  42  to one of the parts to be joined is avoided.  
     [0043] In order to carry out the joining process as efficiently as possible, in an embodiment (not shown in detail) of the two contact pads  50 ,  52  a soft solder (e.g. in the form of a soft solder wire through a duct) is fed to their interface with the respective part (conductor portion  22  or conductor bar  20 ). Upon the introduction of the electric power impulse, the soft solder will also melt. While the press jaws  40 ,  42  are still urging the two legs  32 ,  34  of the conductor portion  22  against the end section  26  of the conductor bar  20 , the contact pads  50 ,  52 —as long as the soft solder is still liquid—are lifted off the conductor portion  22  or the conductor bar  20 , respectively. Only afterwards, the two press jaws  40 ,  42  are withdrawn. This ensures that none of the contact pads  50 ,  52  adheres to the conductor portion  22  or the conductor bar  20 .  
     [0044]FIG. 5 shows a further embodiment of the conductor portion  22  or of the conductor bar  20 , respectively, where the conductor portion  22  contacts the side faces  26   a ,  26   b  of the end section  26  of the conductor bar  20  only with the two opposite legs  32 ,  34  of its end section  30 . This embodiment is advantageous in that the distance to the neighbouring conductor bar  20  in the same winding chamber is not affected by the joint of the conductor bar  20  with its respective conductor portion  22 . Here, the contact pad  50  to be applied to the conductor portion  22  can be applied to the middle piece  56  overlapping the end face  26   c  of the conductor bar  20 , and the contact pad  52  to be applied to the conductor bar  20  can be applied to the opposite end of the conductor bar  20 .  
     [0045] An essential advantage of this embodiment is that the end section  26  need not comprise any tapering in order to realise a space-saving joint between the conductor bars  20  and the conductor portion  22  in the critical orientations (in particular relative to the neighbouring conductor bars).  
     [0046] The ratios of the individual parts and sections therefrom and their material thicknesses shown in the figures are not to be understood as being limiting. Rather, individual dimensions can differ from those shown.