Abstract:
A method of assembling an induction rotor includes inserting a plurality of conductor bars into a stack of disks whereby distal ends of the conductor bars project from respective axial ends of the stack, placing first and second end rings onto the respective axial ends of the stack so that the ends of the conductor bars fit into slots in the respective end rings, and compressing the slots against the conductor bars by impacting at least one of the end rings. The method may include selectively adjusting an amount of end ring material being compressed by varying heights of respective areas of a top surface of the end ring and/or selectively adjusting an amount of end ring material being compressed by varying heights of respective areas of an impacting surface.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to structural improvement of induction type electric machines and, more particularly, to a method of assembling an induction rotor. 
         [0002]    An induction motor is an asynchronous electric machine powered by alternating current (AC), where such power is induced in a rotor via electromagnetic induction. For example, polyphase AC currents may be provided to stator windings structured to create a rotating magnetic field that induces current in conductors of a rotor, whereby interaction between such induced currents and the magnetic fields causes the rotor to rotate. Induction motors may have any number of phases. An induction motor may operate as a generator or traction motor, for example, when driven at a negative slip. 
         [0003]    Rotors of induction motors may conventionally include a cage such as a squirrel cage having parallel axial or skewed conductor bars of copper or aluminum extending between opposite rotor ends and positioned at radially outward locations along the circumference of the rotor. The rotor may have a substantially cylindrical iron core formed as a stack of individual laminated disks, for example disks of a silicon steel material. Each core disk may have axial slots for passing the copper or aluminum bars there-through when the slots are in alignment with one another in a lamination stack. Distal ends of individual conductor bars may be structurally supported and in electrical communication with one another by connection of the respective bar ends to one or more end rings disposed at each rotor end. 
         [0004]    Due to the high costs associated with permanent magnet electric motors, electric machines for many different applications are being redesigned to utilize induction rotors. However, conventional induction rotors may have a reduced number of applications due to poor mechanical properties of the chosen material and/or due to inconsistent assembly methods, especially when structural weakness is exacerbated by the size and speed of the rotor. When an induction motor is utilized in a given application such as automotive, the rotor must tolerate high speed rotation and associated large centrifugal force. In addition, high temperatures, potential metal fatigue, and other factors may aggregate to cause structural breakdown resulting in damage or deformation of the rotor. For example, an induction rotor generates higher temperatures within the rotor itself, further reducing mechanical and structural integrity. 
         [0005]    There are various conventional techniques that may be used for assembling induction rotors. For example, conventional induction machines may utilize varying grades of aluminum or copper in die-casting the end rings/plates and the conductor bars of the cage as an integral unit. However, conventional die-cast induction rotors may have a reduced number of applications due to poor mechanical properties of the chosen die-cast material and due to problems related to manufacturing. Depending on the grade, the cast material strength may vary significantly. Another conventional induction rotor assembly technique may include forming individual conductor bars, forming two end rings having slots/channels corresponding to the axial slots of the lamination stack, inserting the bars through the axial slots, positioning the respective end rings at the opposite axial ends of the rotor so that the conductor bars pass through the end rings, pressing the end rings axially toward one another, and then welding the end portions of the conductor bars to the end rings. Such welding of conductor bars may produce inconsistent results and poor contact between the end rings and the conductor bars. A further conventional technique for assembling induction rotors may substitute a heading operation for the welding of conductor bars. In such a heading process, the protruding ends of the conductor bars are compressed and flattened against the respective exterior axial surfaces of the end rings. Structural problems may result from a heading operation. After being impacted in an axially inward direction, the compressed conductor bars become self-biasing in an axially outward direction and, over time, such conductor bars expand and become loose with respect to the end ring slots. 
       SUMMARY 
       [0006]    It is therefore desirable to obviate the above-mentioned disadvantages by providing a method of assembling an induction rotor that provides consistent contact and secure engagement between conductor bars and end rings. The disclosed embodiments yield improved electrical and mechanical characteristics, reducing or eliminating loose fitting engagement by preventing length contraction and expansion of rotor conductor bars. 
         [0007]    According to an embodiment, a method of assembling an induction rotor includes providing a plurality of conductor bars in a rotor core so that distal ends of the conductor bars project from respective axial ends of the core, placing first and second end rings onto the respective axial ends of the core so that the ends of the conductor bars fit into slots in the respective end rings, and impacting at least one of the end rings, thereby locking the end ring to the conductor bars. 
         [0008]    According to another embodiment, a method of assembling an induction rotor includes providing a plurality of conductor bars in a rotor core so that distal ends of the conductor bars project from respective axial ends of the core, placing first and second end rings onto the respective axial ends of the core so that the ends of the conductor bars fit into slots in the respective end rings, and reducing the volume of the slots, thereby locking the end ring to the conductor bars. 
         [0009]    According to a further embodiment, apparatus for assembling an induction rotor includes a housing for retaining a rotor core and two end rings in axial alignment, the rotor core and end rings each having a plurality of longitudinally extending slots containing conductor bars. Such apparatus also includes a press for providing an impacting force, and an impacting surface for transferring the impacting force to one of the end rings to thereby compress the respective end ring slots against the conductor bars. 
         [0010]    The foregoing summary does not limit the invention, which is defined by the attached claims. Similarly, neither the Title nor the Abstract is to be taken as limiting in any way the scope of the claimed invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0011]    The above-mentioned aspects of exemplary embodiments will become more apparent and will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a schematic view of an induction type electric machine. 
           [0013]      FIG. 2  is a top plan view of an induction motor rotor lamination stack; 
           [0014]      FIGS. 3A-3D  show simplified cross sectional views of a portion of an induction rotor, illustrating an exemplary rotor assembly method; 
           [0015]      FIGS. 4A-4D  show simplified cross sectional views of a portion of an induction rotor, illustrating a rotor assembly method according to an exemplary embodiment; 
           [0016]      FIG. 5  is a partial top plan view of an induction rotor showing relative positions of conductor bars and rectangular impact target areas interposed therebetween, according to an exemplary embodiment; 
           [0017]      FIGS. 6A-6D  show simplified cross sectional views of a portion of an induction rotor, illustrating a rotor assembly method according to an exemplary embodiment; 
           [0018]      FIGS. 7A-7D  show simplified cross sectional views of a portion of an induction rotor, illustrating a rotor assembly method according to an exemplary embodiment; 
           [0019]      FIGS. 8A-8H  are respective top plan views of exemplary conductor bar slot shapes that may be adapted for use with the disclosed embodiments; and 
           [0020]      FIG. 9  is a partial top plan view of an induction rotor assembly according to an exemplary embodiment. 
       
    
    
       [0021]    Corresponding reference characters indicate corresponding or similar parts throughout the several views. 
       DETAILED DESCRIPTION 
       [0022]    The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of these teachings. 
         [0023]      FIG. 1  is a schematic view of an induction type electric machine  1  such an induction motor/generator. In an exemplary embodiment, electric machine  1  may be a traction motor for a hybrid or electric type vehicle. Electric machine  1  has a stator  2  that includes a plurality of stator windings  3  typically disposed in an interior portion thereof. Stator  2  may be securely mounted in a housing (not shown) having a plurality of longitudinally extending fins formed to be spaced from one another on an external surface thereof for dissipating heat produced in the stator windings  3 . For example, stator  2  may have a non-magnetic, electrically non-conductive bobbin (not shown) wound with separate phase coils. A rotor  4  has a center shaft  5  and is concentrically mounted within stator  2  so that rotor  4  rotates circumferentially respecting a longitudinal axis of shaft  5 . Rotor  4  has a front end ring portion  6  and a rear end ring portion  7  respectively disposed at opposite axial ends of rotor  4 , each being formed by a process that includes die-casting. When a voltage from an external power source (not shown) is supplied to the stator windings, stator  2  produces a rotating magnetic field. In operation, voltage is impressed on rotor  4  as an induced voltage. The inductive interaction of the rotating magnetic field with longitudinally extending conductive bars  8  of rotor  4  causes rotor  4  to rotate. 
         [0024]      FIG. 2  is a top plan view of an induction motor rotor lamination stack  30  formed by stacking individual laminations, each typically made of steel sheet metal and generally shaped as a ring or disk. The laminations may be formed, for example, by a stamping operation. When assembled, lamination stack  30  has a generally columnar shape around central longitudinal axis  10 . The laminations are each formed so that assembled lamination stack  30  has a uniform center aperture  33  within which shaft  5  and associated structure may be positioned. Annular inner surface  32  may include a slot (not shown) for engaging a hub, shaft, or other structure of rotor  4 . Spaces/holes  34  are typically formed around the periphery of each lamination so that when the laminations are placed in registration with one another by forming lamination stack  30 , such spaces form corresponding continuous passages or slots  36  each extending in a generally lengthwise direction through lamination stack  30  proximate the radially outward exterior surface  31 . Such slots  36  may be substantially parallel with central longitudinal axis  10  of rotor  4  or they may be skewed. An assembly of laminations may be formed/stacked as a spiral. 
         [0025]    In order to reduce vibration, magnetic noise, and unwanted linear and radial movement of the laminations, and/or to reduce adverse effects of variations in dimensions (e.g., thicknesses) of individual laminations, lamination stack  30  may be formed with incremental variations in the shapes of individual laminations. In addition, for example, the laminations may be arranged in groups prior to stack assembly and such groups may include slight variations in shapes of individual teeth  35 , whereby a particular resonance is avoided. Lamination stack  30  may be formed with structure physically attached to individual laminations or to stack  30  in order to modify the corresponding electromagnetic profile. An assembly of lamination stack  30  may include bolting, riveting, welding, brazing, bonding, clamping, or staking, whereby mass distribution, elastic distribution, damping, and electromagnetic profile are affected. The electromagnetic structure may also be affected, for example, by selection of the particular interference fit used for staking adjacent laminations, and by the amount of force used by a staking punch for radially compressing a boss (not shown) of a lamination within a hole of an adjacent lamination. 
         [0026]      FIGS. 3A-3D  each show a simplified cross sectional view of a portion of an induction rotor, and are provided for illustrating an exemplary assembly method. As shown in  FIG. 3A , a lamination stack  30  is assembled and placed into a retaining fixture (not shown). Conductor bars  8  are inserted into lamination channels  36  and through lamination stack  30 , whereby conductor bar ends  9  extend axially outward of the top surface  38  of lamination stack  30 . Similarly, the opposite ends (not shown) of conductor bars  8  extend below a bottom surface (not shown) of lamination stack  30 . An end ring  6  has a plurality of clearance holes/slots  11  formed around a circumference thereof and aligned with corresponding channels  36  and conductor bars  8 . End ring  6  is placed so that conductor bars  8  pass through slots  11 , and is seated when axially inward facing surface  12  of end ring abuts top surface  38  of lamination stack  30 . When conductor bars  8  are inserted into end ring  6 , there are gaps/voids  19  in slots  11  that provide clearance between a given conductor bar  8  and slot  11 . Portions of conductor bars  8  may be tapered or shaped in any appropriate manner. 
         [0027]    When end ring  6  has been seated and is flush with lamination stack  30 , a punch  13  impacts conductor bar ends  9  as shown in  FIG. 3B . Conductor bars  8  are formed of a ductile material such as copper that deforms and does not significantly fracture when the force of the impacting from punch  13  is applied. The applied force exceeds the yield strength of conductor bars  8 , whereby the copper or other material is compressed into end ring holes  11  to significantly improve contacting between end ring  6  and individual conductor bars  8 . The amount of force and velocity thereof are adjusted to optimize the filling of end ring holes  11  with the compressed copper of conductor bars  8 .  FIGS. 3C and 3D  each show a cross section of a portion of the resultant headed assembly  14  after the impacting. Compressed/flattened conductor bar heads  15  become approximately flush with axially outward facing end ring surface  16 , and a flange  17  or similar portion may be formed for a given conductor bar  8 , depending on various factors such as the amount of conductor bar  8  being compressed into end ring hole  11 , the force (e.g., velocity) of punch  13 , the amount of clearance between conductor bar  8  and end ring hole  11  prior to the impacting, the ambient and/or applied heat, the ductile properties of conductor bar  8  and end ring  6 , and others. The deformed end ring material optimally completely fills the corresponding clearances between end ring  6  and circumferential portion(s)  18  of conductor bars  8  that existed prior to the impacting, so that portions  18  are in tight abutment with compressed surfaces of end ring holes  11 , whereby electrical resistance is reduced and rotor efficiency is increased. 
         [0028]    The induction rotor assembly method illustrated in  FIGS. 3A-3D  is problematic because the impacting causes a compression of conductor bars  8  in the lengthwise or axial direction. When conductor bars  8  subsequently expand in the opposite lengthwise or axial direction, they may become loose, which results in reliability problems for rotor  4 . This lengthwise expansion of rotor bars  8  may also be different for various rotor bars  8 , whereby the respective tension, tightness, and integrity of mechanical engagement, and the associated performance of individual conductor bar portions may be inconsistent. Such problems are exacerbated by vibration, heat, and other operational conditions experienced by electrical machine  1 , resulting in a reduction of machine life. In addition, when heat is applied during the conventional rotor assembly process, ancillary rotor parts may be affected and additional manufacturing operations may be necessitated, such as those involving ovens, venting, safety and personnel protections, environmental protections, costs, cooling times and cooling areas, fixturing, and others. 
         [0029]      FIGS. 4A-4D  each show a simplified cross sectional view of a portion of an induction rotor, and are provided for illustrating an assembly method according to an exemplary embodiment. As shown in  FIG. 4A , a lamination stack  30  is assembled and placed into a retaining fixture (not shown). Conductor bars  8  are inserted into lamination channels  36  and through lamination stack  30 , whereby conductor bar ends  9  extend axially outward of the top surface  38  of lamination stack  30 . Similarly, the opposite ends (not shown) of conductor bars  8  extend below a bottom surface (not shown) of lamination stack  30 . An end ring  6  has a plurality of clearance holes/slots  11  formed around a circumference thereof and aligned with corresponding channels  36  and conductor bars  8 . 
         [0030]      FIG. 4B  shows end ring  6  placed so that conductor bars  8  pass through slots  11 , whereby end ring  6  is seated when axially inward facing surface  12  of end ring  6  abuts top surface  38  of lamination stack  30 . When conductor bars  8  are inserted into end ring  6 , there are gaps/voids  20  in slots  11  that provide clearance between a given conductor bar  8  and slot  11 . When end ring  6  has been seated and is flush with lamination stack  30 , a punch  21  impacts the top surface  22  of end ring  6 . Punch  21  has cavities/indented portions  23  that are aligned with conductor bars  8  so that when punch  21  impacts end ring  6 , conductor bars  8  are not directly impacted. Instead, contacting surface(s)  24  of punch  21  strikes end ring  6  and compresses end ring material into voids  20 . End rings  6 ,  7  and conductor bars  8  are formed of a ductile material such as copper that deforms and does not significantly fracture when the force of the impacting from punch  21  is applied. The applied force exceeds the yield strength of end ring  6 , whereby the copper or other material is compressed into end ring voids  20  to significantly improve contacting between end ring  6  and individual conductor bars  8 . The amount of force and velocity thereof are adjusted to optimize the filling of end ring voids  20  with the compressed copper of end ring  6 . 
         [0031]      FIGS. 4C and 4D  each show a simplified cross section of a portion of the resultant assembly  25  after the impacting. The compressed top surface  26  includes compressed portions  27  between adjacent conductor bars  8 . For example,  FIG. 5  is a top plan view of a portion of assembly  25  showing impact areas  28  that are targeted by contacting surfaces  24  of punch  21 . Such contacting surfaces  24  may be structured as rectangles for striking end ring top surface  22  between adjacent ones of end ring slots  11 . When top surface  22  of end ring  6  is substantially flat prior to the impacting, impact areas  28  may become concave surfaces having the same shape as the corresponding rectangular contacting surfaces  24 . When top surface  22  has protrusions in impact areas  28  prior to the impacting, then the impacting from punch  21  may result in compressed end ring surface  26  becoming substantially flat. The surface area and shape of the contacting surface(s)  24  of punch  21  may optionally be the same as that of top surface  22  of end ring  6 . For example, contacting surface  24  may be a round disk having indented cavities  23  aligned with and shaped the same as corresponding ones of end ring slots  11 , and a plan view of such an impacting surface may be substantially similar to that shown in  FIG. 2 . In such a case, compressed top surface  26  may become uniform and substantially flat. The impacting of end ring  6  compresses the relatively soft metal (e.g., copper) so that the metal is pressed around conductor bars  8  without substantially impacting conductor bar ends  9 . Metal of end ring  6  is pressed against conductor bar  8  to greatly reduce resistivity along main body portion  37  of conductor bar  8 . Very slight gaps/voids  32  may remain at given portions of the end ring/conductor bar interface, depending on the choice of contacting surfaces  24 , pre-impact end ring surface shape(s), and parameters such as ambient and/or applied heat, impact force, deformation volume, initial gap volume, conductor bar/end ring slot shapes, punch force (e.g., velocity), ductile properties, and other related factors. As shown in  FIG. 4D , the compressing forces end ring material  29  against main body portion  37 , whereby main body portion  37  may then have a smaller cross sectional profile area compared with that of conductor bar end  9 . The axial length of conductor bar  8  may be chosen so that conductor bar end  9  is below end ring surface  22  prior to the compressing, and then becomes flush with compressed top surface  26 . As a result of the compressing, main body portion  37  is in tight abutment with the surface of end ring holes  11 , whereby electrical resistance is reduced and rotor efficiency is increased. 
         [0032]    The contiguous relation of enlarged end ring material  29  in the middle of end ring slot  11  and corresponding non-narrowed portions  71  of conductor bar  8  acts to lock conductor bar  8  in place because the axially outward bias of end ring  6  presses the corresponding surfaces together. Thereby, the problem of compressed conductor bars having “memory” and being biased for returning to pre-compression length is obviated by minimizing compression of conductor bars  8  and by compressing end ring material  29  against main body portion  37 . The disclosed compression of lamination stack  30  and end rings  6 ,  7  causes end ring  6  to be biased axially outward and into further engagement with respective conductor bars  8 . 
         [0033]      FIGS. 6A-6D  each show a simplified cross sectional view of a portion of an induction rotor, and are provided for illustrating an assembly method according to an exemplary embodiment. Lamination stack  30  is assembled and placed into a retaining fixture (not shown). Conductor bars  8  are inserted into lamination channels  36  and through lamination stack  30 , whereby conductor bar ends  9  extend axially outward of the top surface  38  of lamination stack  30 . Similarly, the opposite ends (not shown) of conductor bars  8  extend below a bottom surface (not shown) of lamination stack  30 . An end ring  40  has a plurality of clearance holes/slots  11  formed around a circumference thereof and aligned with corresponding channels  36  and conductor bars  8 . As shown in  FIG. 6A , end ring  40  has top projections  41  and bottom projections  42  respectively extending from end ring top surface  44  and end ring bottom surface  12  and formed about each end ring slot  11 . 
         [0034]      FIG. 6B  shows end ring  40  placed so that conductor bars  8  pass through slots  11 , whereby end ring  40  is pre-seated when bottom projections  42  of end ring  40  abuts top surface  38  of lamination stack  30 . When conductor bars  8  are inserted into end ring  40 , there are gaps/voids  39  in slots  11  that provide clearance between a given conductor bar  8  and slot  11 . There are also spaces  48  created by bottom end ring projections  42  offsetting end ring surface  45  away from lamination stack top surface  38 . When end ring  40  has been pre-seated, a punch  43  impacts top surface  44  of end ring  40 . Punch  43  has cavities/indented portions  46  that are aligned with conductor bars  8  so that when punch  43  impacts end ring  40 , conductor bars  8  are not directly impacted. Instead, contacting surface(s)  47  of punch  43  strikes end ring  40  and compresses end ring material, including material of projections  41 ,  42  into voids  39 . Bottom end ring projections  42  become compressed, thereby making end ring surface  45  flush with lamination stack top surface  38 . End ring  40  and conductor bars  8  are formed of a ductile material such as copper that deforms and does not significantly fracture when the force of the impacting from punch  43  is applied. The applied force exceeds the yield strength of end ring  40 , whereby the copper or other material is compressed into voids  39  to significantly improve contacting between end ring  40  and individual conductor bars  8 . The amount of force and velocity are adjusted to optimize the filling of end ring voids  39  with the compressed copper of end ring  40 . 
         [0035]      FIGS. 6C and 6D  each show a simplified cross section of a portion of the resultant assembly  49  after the impacting. The compressed top surface  50  includes compressed portions  51  between adjacent conductor bars  8 . The compressed top surface  50  and compressed bottom surface  52  are now flattened and the material of projections  41 ,  42  is now contiguous with conductor bar  8  at top end ring/conductor bar interface  53  and bottom end ring/conductor bar interface  54 . In addition, the compressed main body portion  55  about a given end ring slot  11  and the compressed main body  56  of conductor bar  8  each has a more consistent cross section shape when viewed axially along the conductor bar portion from lamination stack  30  to conductor bar end  9 . 
         [0036]      FIGS. 7A-7D  each show a simplified cross sectional view of a portion of an induction rotor, and are provided for illustrating an assembly method according to an exemplary embodiment. Lamination stack  30  is assembled and placed into a retaining fixture (not shown). Conductor bars  8  are inserted into lamination channels  36  and through lamination stack  30 , whereby conductor bar ends  9  extend axially outward of the top surface  38  of lamination stack  30 . Similarly, the opposite ends (not shown) of conductor bars  8  extend below a bottom surface (not shown) of lamination stack  30 . An end ring  6  has a plurality of clearance holes/slots  11  formed around a circumference thereof and aligned with corresponding channels  36  and conductor bars  8 . 
         [0037]      FIG. 7B  shows end ring  6  placed so that conductor bars  8  pass through slots  11 , whereby end ring  6  is seated when axially inward facing surface  12  of end ring  6  abuts top surface  38  of lamination stack  30 . When conductor bars  8  are inserted into end ring  6 , there are gaps/voids  20  in slots  11  that provide clearance between a given conductor bar  8  and slot  11 . When end ring  6  has been seated and is flush with lamination stack  30 , a punch  55  impacts the top surface  22  of end ring  6 . Punch  55  has cavities/indented portions  56  that are aligned with conductor bars  8  so that when punch  55  impacts end ring  6 , conductor bars  8  are not directly impacted. Instead, contacting surface(s)  57  of punch  55  strikes end ring  6  and compresses end ring material into voids  20 . Contacting surface(s)  57  has protrusions that have a contoured shape. In  FIG. 7B , the contoured shape has a raised portion, although contacting surface(s)  57  may be formed in any appropriate shape. For example, contacting surface(s)  57  may have a raised portion that approximates the shape of a target impact area  28  (e.g.,  FIG. 5 ), may include portions of various sizes and heights, may include spiked portions, may include portions structured to impact protrusions formed in end ring  6 , may include alternating raised and lowered portions, may include a series of protrusions, may include protrusions that outline at least a portion of a slot  11 , may be structured for compressing an end ring slot  11  having any shape, and others. A given end ring slot  11  may have a shape such as those shown by way of non-limiting example in  FIGS. 8A-8H , and corresponding contacting surface(s)  57  may, for example, have portions structured for impacting at least a portion of end ring material surrounding such slot. 
         [0038]      FIGS. 7C and 7D  each show a simplified cross section of a portion of the resultant assembly  58  after the impacting. The compressed top surface  59  includes compressed and indented portions  60  between adjacent conductor bars  8 . For example, indented portion  60  may have a shape of impact area  28  shown in  FIG. 5 . When top surface  22  of end ring  6  is substantially flat prior to the impacting, indented portions  60  may become concave surfaces having the same shape as the corresponding contacting surface(s)  57 . Indented portions  60  may partially or completely surround respective ones of slots  11 . Compressed material of end ring  6  fills spaces  20  so that such material of end ring  6  is contiguous with respective conductor bars  8 . The compressed main body portion  62  about a given end ring slot  11  and the compressed main body  63  of conductor bar  8  are pressed together to form a contiguous interface along the conductor bar portion from lamination stack  30  to conductor bar end  9 . The compressing forces the end ring material laterally toward conductor bar  8  at upper portions  61 , whereby the end ring/conductor bar interface  64  has reduced resistivity. As with other embodiments, the amount of impacting force and velocity are adjusted to optimize the filling of spaces  20  with the compressed copper of end ring  6 . 
         [0039]    Any features of the disclosed embodiments may be practiced in combination with one another, depending on a particular application. The impacting of a given end ring may be performed in a single strike, in multiple strikes, in association with other induction rotor processing, or in a series of strikes as part of concurrent or independent processes. The impacting may be imposed at any angle relative to the top surface of a given end ring. For example, a contacting surface of a punch may include individual spike-shaped portions (not shown) that strike an end ring top surface in a direction toward a slot  11 , such striking being at an angle with respect to a longitudinal axis of the induction rotor. In an exemplary embodiment, end rings  6 ,  7  are pressed toward one another prior to the compressing step, thereby axially biasing lamination stack  30  against end rings  6 ,  7 . As a result, a consistent and substantially uniform axially outward biasing further maintains the integrity of the induction rotor after the compressing operation because a slight axially outward biasing of lamination stack  30  strengthens the interface of conductor bars  8  and compressed slots  11 . 
         [0040]      FIG. 9  is a partial top plan view of an exemplary embodiment of an induction rotor  64  having an end ring  66  with a top surface  69  that includes impacting target area  67 . Target area  67  has protrusions/convex portions  65  that are separated by concave portions  68 , whereby the impacting compresses portions  65  into portions  68 , in addition to compressing end ring material against conductor bars  8 , thereby selectively modulating the filling of gaps/voids between slots  11  and conductor bars  8 . For example, since the space is relatively small between the outer radial perimeter of end ring  66  and the outer radial portion of conductor bar  8 , a series of concave portions  68  may be spaced apart along this portion of end ring  66 , whereby the impacting of target area  67  forces end ring material into concave portions  68  to provide relief and avoid excessive pressure and material at a relatively fragile portion of end ring  66 . Other portions of top surface  69  may include convex portions  65  and/or portions that are neither concave nor convex. In this manner, control over compression of material may include selectively adjusting the amount of material being compressed at specific locations. Concave-convex portions may be defined in any axis, whereby relief and compression modulation may be provided for lateral as well as axial material movement. For example, partitions between convex portions  65  and concave portions  68  may be angled, and may include variations in axially extending portions of end ring slots  11  and/or protruding portions of top surface  69 . In an exemplary embodiment, an axially extending groove  70  is formed in end ring  66 , whereby the impacting of adjacent convex portions  65  forces end ring material in a substantially axial direction. Concave-convex portions may include at least one groove in an interior bore portion of at least one of the respective slots  11  and at least one corresponding projection aligned with the groove, whereby the impacting forces material of the projection into the groove. Concave-convex portions may include alternating concave and convex portions formed about respective slots  11 . The terms “concave” and “convex” are used herein to describe voids and protrusions having any shape or profile and are not limited to curved or otherwise regular surfaces. Conductor bars  8  may include tapered portions. 
         [0041]    Contoured impact surfaces of a punch impacting surface may each have a protrusion volume distribution in proportion to a corresponding void space between a conductor bar and slot. Similarly, a given end ring may include protrusions surrounding respective ones of slots  11 , the protrusions each having a volume distribution in proportion to voids between conductor bar  8  and slot  11 . Convex portions may be provided on both top and bottom surfaces of either end ring. End rings  6 , 7  may be impacted simultaneously for locking conductor bars  8  at both axial ends of rotor  4 . 
         [0042]    In various embodiments, conductor bar ends  9  may be brazed to end ring  6 . For example, a brazing alloy or filler metal may be melted and distributed by capillary action at conductor bar ends  9  to thereby further improve bonding of conductor bars  8  with end ring  6 . 
         [0043]    While various embodiments incorporating the present invention have been described in detail, further modifications and adaptations of the invention may occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.