Abstract:
A shorting ring support structure of a rotor of an electric machine has a radially outer ring and a member extending radially inwardly from the outer ring and defining a radial length, the radially extending member being spaced axially inwardly from opposing axial surfaces of a die-cast shorting ring whereby the radially extending member is fully embedded in the shorting ring for a substantial majority of its radial length. Rotor conductor bars and the shorting ring are formed of an integrally cast material that secures the shorting ring support structure to the rotor body. A method of manufacturing includes providing a shorting ring support structure and a rotor body, and then casting conductor bars in slots of the rotor body and a shorting ring on the rotor body to form a rotor, where an outer ring of the support structure defines the outer radial limit of the shorting ring.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to electric machines and, more particularly, to structure of and a method of making a die-cast 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, for example when driven at a negative slip. 
         [0003]    Rotors of induction motors may conventionally include a cage such as a squirrel cage having axially parallel or skewed bars of copper or aluminum extending between opposite rotor ends and positioned at radially outward locations along the circumference of the rotor. Distal ends of individual bars may be provided with structural support and be in electrical communication with one another by connection of the respective bar ends to one or more continuous shorting ring disposed at each rotor end. The rotor may have a substantially cylindrical iron core formed as a stack of individual laminated disks of a silicon steel material. Each core disk may have slots or axial through holes for passing the copper or aluminum bars therethrough. 
         [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 die-cast induction rotors may have a reduced number of applications due to poor mechanical properties of the chosen die-cast material, 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 with forces acting in a radial outward direction and those acting in an axial direction to cause structural breakdown resulting in damage or deformation of the cast shorting rings of a rotor. For example, an induction rotor generates higher temperatures within the rotor itself, further reducing mechanical and structural integrity of shorting rings. 
       SUMMARY 
       [0005]    It is desirable to obviate the above-mentioned disadvantages by providing a rotor for an induction motor, the rotor having a structure that enables a high speed operation in a high temperature ambient environment. The disclosed embodiments provide a method and structure for retaining the die-cast material of an induction rotor, specifically in shorting ring portions of the rotor. The disclosed embodiments also provide a method and structure that improve efficiency of an induction rotor, that minimize electrical losses in shorting ring portions of an induction rotor by maximizing the proportion of die-cast copper or other conductive material in the shorting ring portions, while still radially and axially retaining such die-cast material. The disclosed embodiments further provide a method and structure whereby die-cast shorting ring material is placed proximate a periphery of a shorting ring in specific locations chosen to optimize rotor performance, while still being radially and axially retained. By implementing such retention structure, the structural limitations of the die-cast material are greatly reduced. 
         [0006]    According to an embodiment, an electric machine having a stator includes a rotor operably coupled with the stator, the rotor having a rotor body and defining a rotational axis. A plurality of conductor bars are supported on the rotor body and extend between axial ends of the rotor body. At least one shorting ring provides electrical communication between separate ones of the plurality of conductor bars. The electric machine includes at least one shorting ring support structure, the shorting ring support structure having a radially outer ring member and at least one member extending radially inwardly from the outer ring member and defining a radial length, the radially extending member being spaced axially inwardly from opposing axial surfaces of the shorting ring whereby the radially extending member is fully embedded in the shorting ring for a substantial majority of its radial length. The conductor bars and the at least one shorting ring are formed of an integrally cast material that secures the shorting ring support structure to the rotor body. 
         [0007]    According to another embodiment, a method of manufacturing an electric machine having a stator includes providing a rotor body defining a rotational axis and having at least one slot extending between axial ends of the rotor body. The method includes providing a shorting ring support structure that includes an outer ring member having an axial end surface facing the rotor body, the axial end surface having a radially inner edge and a radially outer edge, the axial end surface being spaced apart from the rotor body at the radially inner edge and being engaged with the rotor body proximate the radially outer edge. The method also includes casting both at least one conductor bar in the slot and a shorting ring on the rotor body to thereby form a rotor wherein the outer ring member defines the outer radial limit of the shorting ring, whereby cast material secures the shorting ring support structure to the rotor body. 
         [0008]    According to a further embodiment, a method of manufacturing an electric machine having a stator includes providing a rotor body defining a rotational axis and having a plurality of conductor slots extending between axial ends of the rotor body. The method includes providing first and second shorting ring support structures, each shorting ring support structure having an inner ring member, an outer ring member and a plurality of spokes radially extending between the inner and outer ring members. The method includes respectively positioning the first and second shorting ring support structures proximate opposite axial ends of the rotor body, and casting a plurality of conductor bars in corresponding ones of the plurality of conductor slots and casting first and second shorting rings at the opposite axial ends of the rotor body to thereby form a rotor, wherein the first and second shorting ring support structures define inner and outer radial limits of the first and second shorting rings respectively and wherein the spokes of each of the first and second shorting ring support structures are spaced axially inwardly from opposing axial surfaces of the first and second shorting rings whereby each of the spokes is fully embedded in one of the first and second shorting rings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0009]    The above mentioned and other features of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawing figures, wherein: 
           [0010]      FIG. 1  is a schematic view of an electric machine; 
           [0011]      FIG. 2  is a perspective view of an induction rotor lamination stack; 
           [0012]      FIG. 3  is a top plan view of a retention structure for a rotor of an induction motor, according to an exemplary embodiment; 
           [0013]      FIG. 4  is a perspective view of the retention structure of  FIG. 3 ; 
           [0014]      FIG. 5  is a cross section view taken along the line  5 - 5  of  FIG. 3 ; 
           [0015]      FIG. 6  is a perspective view of a retention structure mounted to an induction rotor lamination stack, according to an exemplary embodiment; 
           [0016]      FIG. 7  is a perspective view of a die-cast material geometry that includes rotor bars and shorting rings of a rotor of an induction motor, according to an exemplary embodiment; 
           [0017]      FIG. 8  is a perspective view of a rotor of an induction motor after a die-cast operation, according to an exemplary embodiment; 
           [0018]      FIG. 9  is a cross-section view of a portion of the rotor of  FIG. 8 , taken along the line  9 - 9 , with a hub portion being added for illustration purposes; 
           [0019]      FIG. 10  is a perspective view of a completed rotor of an induction motor after hub assembly and O.D. machining, according to an exemplary embodiment; 
           [0020]      FIG. 11  is a top plan view of a retention structure for a rotor of an induction motor, according to an exemplary embodiment; and 
           [0021]      FIG. 12  is a cross section view taken along the line  12 - 12  of  FIG. 11 . 
       
    
    
       [0022]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the illustrated embodiments show several forms of the invention, such embodiments are exemplary and are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms or applications disclosed. 
       DETAILED DESCRIPTION 
       [0023]      FIG. 1  is a schematic view of an electric machine  1  such as 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 shorting ring portion  6  and a rear shorting 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  3 , 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. Such conductor bars are formed along the outer circumference of rotor  4  and may be axial or skewed. The above-mentioned electric-kinetic energy flow is reversible because mechanical torque induced on rotor  4  will generate electricity. 
         [0024]      FIG. 2  shows an induction motor rotor lamination stack  30  formed by stacking individual laminations  31 , for example silicone steel sheet metal in a general shape of a ring or disk. When assembled, lamination stack  30  has a generally columnar shape around a central longitudinal axis. Laminations  31  are typically formed of silicone steel material to minimize electromagnetic losses being generated in lamination stack  30  of rotor  4  during operation. Laminations  31  are each formed so that assembled lamination stack  30  has a uniform center aperture  32  where a shaft and associated structure may be positioned. Spaces  33  are formed around the periphery of each lamination  31 , so that when laminations  31  are placed in registration with one another by forming lamination stack  30 , such spaces form corresponding passages each extending in a generally lengthwise direction through lamination stack  30  proximate the radially outward exterior surface  34  of lamination stack  30 . Laminations  31  may be formed, for example by a stamping operation, so that when a number of laminations  31  are stacked, the resultant passages are continuous. Such passages may be substantially parallel with the central longitudinal axis of rotor  4  or they may be skewed. An assembly of laminations  31  may be formed/stacked as a spiral. Variations in the thickness of laminations  31  may alter the rotation of the assembled rotor  4  and, therefore, it may be necessary to maintain tight mechanical tolerances for individual laminations  31 , especially because a die-casting operation typically involves temperatures too high to allow laminations  31  to be bonded together with epoxy, or other suitable adhesive, prior to the die-casting. When exposed to such high temperature, many conventional epoxy materials will burn and out gas contaminants into adjacent materials. 
         [0025]    In order to reduce vibration, magnetic noise, and unwanted linear and radial movement of laminations  31 , and/or to reduce adverse effects of variations in dimensions (e.g., thicknesses) of individual laminations  31 , lamination stack  30  may be formed with incremental variations in the shapes of individual laminations  31 . In addition, for example, laminations  31  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 or a receptance distribution is altered. Lamination stack  30  may be formed with structure physically attached to individual laminations  31  or 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  31 , and by the amount of force used by a staking punch for radially compressing a boss (not shown) of a lamination  31  within a hole of an adjacent lamination  31 . 
         [0026]    A die-casting operation, as discussed further below, involves copper, aluminum, or other electrically conductive casting material being poured or otherwise injected into a mold, whereby a cage such as a squirrel cage is formed to have rotor bars in the passages created by axial registration of spaces  33  of lamination stack  30 , and is formed to have shorting rings electrically shorting the ends of the rotor bars together. Shorting rings at each longitudinal end of rotor  4  may be formed, generally, as plates or wheels that are coaxial with lamination stack  30 . 
         [0027]      FIG. 3  and  FIG. 4  show an exemplary shorting ring retention structure  10 , and  FIG. 5  shows a cross section taken along the line  5 - 5  of  FIG. 3 . Retention structure  10  may be formed of cast stainless steel, or other suitable material that is essentially non-magnetic to avoid generating losses in the shorting ring portion of rotor  4 . Casting of shorting ring retention structure  10  allows implementation of various geometries not easily obtained by other processes such as stamping, though other methods such as fabrication or forging may be used to create the structure. In one embodiment, an outer ring  11  may have an outer diameter the same as or slightly less than an outer diameter of outer surface  34  of lamination stack  30 . An inner ring  12  is concentric with outer ring  11  and may be formed with an annular flange portion  13  extending radially inward from an axially extending portion  14  of inner ring  12 . A first annular interior space  15  is formed radially inward of an interior face  16  of flange portion  13 . As shown in FIG.  5 , a second annular interior space  17  is coextensive with first space  16  and has a radius larger by the width of flange  13  compared to the radius of first space  16 . A plurality of radially extending spokes  18  connect inner ring  12  and outer ring  11 . Spokes  18  may be formed to have cross section profiles that are round, square, rectangular, or in any other shape. Spokes  18  may be formed so that each spoke  18  is positioned axially inward of respective axially outward surfaces  21 ,  22  of outer ring  11  and inner ring  12 , and is positioned axially outward of respective axially inward surfaces  23 ,  24  of outer ring  11  and inner ring  12 . For example, spokes  18  may be positioned to be parallel with a plane that includes surfaces  23 ,  24  and may be positioned to bisect respective axially extending portions of inner and outer rings  12 ,  11 . The thicknesses of outer ring  11  and inner ring  12  may cause either structure, by itself, to be insufficiently strong enough to maintain structural integrity of a die-cast end plate, but spokes  18  greatly improve the strength of outer ring  11  by their connection to inner ring  12 . For example, spokes  18  retain outer and inner rings  11 ,  12  in their proper relative positions by being embedded within the cast material, thereby maintaining the proper position and integrity of retention structure  10  on rotor  4 . As a result, outer ring  11  is able to withstand the hoop stresses caused by resistance to outward radial forces being imposed on outer ring  11 , by virtue of its embedded spoked attachment to integrally formed inner ring  12 . An annular chamfer  26  is formed between radially inward surface  27  and axially inward surface  23  of outer ring  11 . 
         [0028]    A pre-casting structure is shown in  FIG. 6 . Shorting ring retention structures  10  (see  FIG. 5 ) each have essentially the same outside diameter as lamination stack  30 , so that outer perimeter faces  19  are essentially flush with outer surface  34  of lamination stack  30 . In an exemplary embodiment, a cylindrical mold, having an inner diameter equal to or slightly greater than the diameter of retention structure  10  and lamination stack  30 , and having an essentially planar bottom surface that is orthogonal to the center axis of such cylinder, may be used for die-casting rotor  4 . In such a case, a first shorting ring retention structure  10  is placed so that surfaces  21 ,  22  abut the bottom mold surface and outer perimeter face  19  of retention structure  10  fits snugly against the inner walls of the mold. Next, lamination stack  30  is placed so that the planar outer surface of the endmost lamination  31  is in abutment with surfaces  23 ,  24  of retention structure  10 . The width or diameter  20  of each spoke  18  may be formed to be less than or equal to the widths of teeth  35  formed between each space  33  of lamination  3 , whereby axially extending passages  36  are not covered when lamination stack  30  is placed onto retention structure  10 . Lamination stack  30  and retention structure  10  may each have a keyed structure and/or angular locators (not shown), whereby spokes  18  are aligned with and overlie teeth  35 . A second shorting ring retention structure  10  is then placed on top of lamination stack  30  and similarly aligned so that spokes  18  are atop respective ones of teeth  35 . By such alignment, spokes  18  of the top retention structure  10  may also be circumferentially offset with respect to spokes  18  of the bottom retention structure  10 , for example by being placed to bisect the arc  25  between adjacent spokes  18  of bottom retention structure  10 . Typically, retention structures  10  may simply be aligned and held in place, although stakes may optionally be formed to align/secure retention structure  10  to an outermost lamination  31  at end(s) of lamination stack  30 . After assembly, top and bottom retention structures  10  are in fluid communication with one another via the plurality of parallel passages  36 . Passages  36  are typically not in communication with lamination stack outer surface  34 . Pre-casting structure  40  may be formed so that no peripheral surface is exposed, whereby the die-cast material will remain within structure  40 . 
         [0029]    After the pre-casting structure  40  has been assembled, a beveled mandrel is inserted into the top retention structure  10 , whereby annular center aperture  32  is completely covered. Similarly, any appropriate other areas and cavities are masked prior to die-casting. Any appropriate apparatus for tightening the assembly  40  may be employed, such as a use of opposed balance rings and fasteners, and various jigs known in the art, including one or more spacers and/or sleeve portions that may be inserted into first and/or second annular spaces  15 ,  17  and that may axially extend between top and bottom retention structures  10 . Such sleeve portions may be chosen to snugly fit within laminate stack  30  and thereby align individual laminates  31  with one another, improving uniformity of annular center aperture  32 . Sleeve portions may include guide pins, grooves, and the like. The mandrel may be tightened by a screwing apparatus or may transfer an external tightening and pressing force (e.g., hydraulic) to assembly  40 . Similarly, a hub extension (not shown) may be utilized in a known stacking process that includes striking and thereby bending such extension with a tool. In various embodiments, a top retention structure  10  may have different structural shapes and features compared with a bottom retention structure  10 , or they may be identical. The bottom surface of the mold may be separate and removable from a tubular mold portion and may be adjustable. The mold may include a double cylinder, such as for applying localized pressure. 
         [0030]    The die-casting process typically includes melting aluminum at approximately 660-700° C., melting copper at approximately 1086-1100° C., or melting an appropriate electrically conductive alloy beyond its associated melting temperature. The molten metal is typically injected into the mold structure at a high flow rate and a high pressure. Assembly  40  may be gated at one end and vented at an opposite end, and the die-casting may utilize any number of air vents. When the bottom surface of the mold is horizontal, the cavity between outer ring  11  and inner ring  12  of bottom retention structure  10  fills with molten metal, each of the plurality of passages  36  then fill at essentially the same rate, and finally the cavity between outer ring  11  and inner ring  12  of top retention structure  10  fills. The mold may extend above surfaces  21 ,  22  of retention structure  10  (see, e.g.,  FIG. 5 ) so that the casting metal may be injected to completely cover surfaces  21 ,  22 . The die-cast material may alternatively be injected to form a surface substantially coplanar with surfaces  21 ,  22 , or it may be injected to form an outer surface that is axially inward of surfaces  21 ,  22 . The mandrel is removed after the cast material has solidified. 
         [0031]      FIG. 7  shows an exemplary copper die-cast squirrel cage  38  having a plurality of axially extending conductor bars  46 . The  FIG. 7  view removes lamination stack  30  and retention structures  10  for purposes of illustration, and shows squirrel cage  38  prior to any post-casting machining. Distal ends of conductor bars  46  are electrically shorted together by opposed shorting rings  50 . Such shorting rings  50  may have the same or different axial thicknesses. For example, shorting rings  50  may be dimensioned so that they produce the same dynamic stresses, even if such a requirement mandates different thicknesses. As discussed in the preceding paragraph, an axial end surface  29  of annular shorting ring portion  50  is typically formed during die-casting to be coplanar with surfaces  21 ,  22  of retention structure  10  so that additional machining of surface(s)  29  is not required after such die-casting. Chamfered portions  41  of respective shorting rings  50  extend outwardly from respective outer peripheral surfaces  43 , whereby a larger volume of copper of the shorting ring  50  is in communication with conductor bars  46 . Spoke volumes  44  extend radially outward from inner peripheral surface  45  to outer peripheral surface  43 . The width or diameter  20  of each spoke  18 , and corresponding diameter of each of respective spoke volumes  44 , is typically formed to be as small as possible to maximize the current carrying capabilities and efficiency of shorting rings  50 . As such diameters become smaller, resistance decreases in shorting ring portions where current flows around spokes  18  and the capacity of induced currents, and rotor efficiency, increases. Similarly, the number of spokes  18  may be minimized provided that the desired structural support, retention strength, and shorting ring durability are achieved. Spoke volumes  44  are offset with respect to opposite shorting rings  50  of rotor  4 , so that electromagnetic resonance (e.g., high frequency noise) is avoided by reducing occurrences of asymmetric poles aligning with one another. 
         [0032]      FIG. 8  shows rotor  4  after a die-casting process. Copper or other electrically conductive cast material is contained between a respective outer ring  11  and a respective inner ring  12  of each retention structure  10  (see, e.g.,  FIG. 5 ), and within passages  36  of lamination stack  30  (see, e.g.,  FIG. 2 ). The copper completely covers spokes  18  of retention structures  10  and forms respective axially outer surfaces  29  at distal ends of rotor  4 . Surfaces  29  are typically the only exposed copper portions of rotor  4  after die-casting when subsequent machining of rotor  4  is specified. 
         [0033]      FIG. 9  shows a cross section of a representative portion of an exemplary rotor  4 , taken along the line  9 - 9  of  FIG. 8 . For ease of description, the hub  9  of  FIG. 10  is also shown in  FIG. 9 . Hub  9  is typically formed of steel having appropriate strength to transfer torque and may also include bearings and other structure for accepting a drive shaft  5 . Hub  9  or portions thereof may be inserted into center aperture  32  either before or after die-casting. For example, hub  9  may be inserted into lamination stack  30  after the mandrel used in the die-casting process is removed, prior to machining. Hub  9  may be staked to inner ring  12  of retention structure  10 . For such staking, hub  9  has an axially extending portion  39  that snugly fits in abutment with the annular interior  47  of lamination stack  30  and with radially inward surface  16  of inner ring  12 . The axially outward end portion of axially extending portion  39  is bent radially outward and then axially inward so that an annular bent holding portion  48  of hub  9  is in abutment with an axially outward face  53  of flange  13 . Annular holding portion  48  of hub  9  secures inner ring  12  and to prevent axial movement of retention structure  10 . Inner ring  12  may optionally be formed to have more than one flange, for example having stepped annular portions, and spacers may be installed between hub  9  and inner ring  12 . Flanges of inner ring  12  may be formed as a series of composite steps that allow laminate stack  30  to be accurately positioned onto hub  9 . Flange  13  may optionally be staked with hub  9 . 
         [0034]    A shorting ring portion  50  is die-cast to be integral with conductor bars  46 . Retention structure  10  is axially supported by the die-cast copper which is typically over-molded to completely enclose spokes  18 . The essentially non-magnetic property of retention structure  10  prevents losses being generated in shorting ring  50 . A chamfered annular edge  41  increases the efficiency of rotor  4  by including more of the die-cast copper in a shoulder region  45  between shorting ring portion  50  and integrally-formed die-cast copper conductor bars  46 . The electrical current path through shoulder portion  45  thereby has a larger cross-sectional area and a higher current carrying capacity. Similarly, the spoke type wheel architecture of retention ring  10  allows outer ring  11  to be thinner because radial movement is restrained by spokes  18  and by inner ring  12 . The thinner outer ring  11  also enables a larger volume of copper to be placed near the periphery of shorting ring  40  where distal ends of conductor bars  46  terminate, which further increases rotor efficiency. In addition, the radially outermost portion of rotor  4  may be machined to still further reduce the thickness of outer ring  11 . Thickness of shorting ring  50 , and corresponding axial lengths of inner and outer rings  11 ,  12  typically depend on desired motor size and speed. 
         [0035]    Such machining may remove portions of rotor  4  that are radially outward of outside diameter (O.D.) machining line  28 . The O.D. machining removes substantial portions of the teeth  35  of lamination stack  30 , removes a radially outward portion  37  of outer ring(s)  11 , and removes small radially outward portions of each of the conductor bars  46  so that conductor bars  46  are exposed along the exterior surface of rotor  4  as shown in  FIG. 9 . The O.D. machining typically removes radially outward portion  37  without removing chamfered edge  41  of outer ring  11 , so that outer ring  11  is as thin as possible without exposing the copper of shoulder portion  45 . In some cases, exposing a small circumferential stripe of the copper of shoulder  45  may not appreciably affect either motor  1  performance or durability of shorting ring  50 . As a result of the O.D. machining, the diameters of respective peripheral surfaces  42  of outer rings  11  of retention structures  10  are made to be the same as the outside diameter of lamination stack  30 , for example 180 mm. After the die-casting, the only exposed copper is typically at end surface(s)  29 , and the O.D. machining typically further exposes only conductor bars  46 . By comparison, an induction rotor having exposed but structurally supported shorting rings is disclosed, for example, in co-pending U.S. application Ser. No. ______, entitled, “Induction Rotor Shorting Ring Support Device,” incorporated herein by reference in its entirety. By maximizing copper mass of shorting rings  50  in proximity to conductor bars  46  while still preventing axial and radial movement/deformation of die-cast shorting rings  50 , the current conduction path is maximized, and rotor efficiency and structural integrity are increased. As a result, rotor  4  of electric machine  1  is able to operate at higher rotational speeds. Respective radial thicknesses of inner and outer rings  12 ,  11  may be the same or different. For example, outer ring  11 , after machining, may be thicker to withstand greater centrifugal force, although this will depend on the number of spokes  18  being used. Typically retention structure  10  is balanced so that hoop stress, stress concentration, and tension forces are balanced. For example, when a given portion of retention structure  10  has a significantly lower stress relative to other portions, the corresponding mass of material in the respective stress volume may be reduced. By reducing mass, the inertia of rotor  4  is reduced and rotor  4  is able to speed up or slow down more easily. 
         [0036]    Retention structure  10  in combination with lamination stack  30  acts as an ersatz single-use casting tool/mold in that die-casting a traditional shorting ring requires tooling for defining radially inward and outward surfaces and one or more surfaces for mating with a hub, whereas the disclosed embodiments eliminate such tooling and associated costs. By use of the retention structure, the structural significance of the die-cast material is greatly reduced. 
         [0037]      FIG. 11  shows another embodiment of an exemplary shorting ring retention structure  49 , and  FIG. 12  shows a cross section taken along the line  12 - 12  of  FIG. 11 . Retention structure  49  may be formed of a same material and in the same manner as retention structure  10 , except that spokes  58  are arranged in a semi-tangent pattern, where spokes extend outward from an inner ring  52  at a chosen angle α away from the radii of inner ring  51  and outer ring  52 . The higher the value of angle α, the more spoke  58  becomes tangential to inner ring  52 . In the exemplary embodiment, spokes  58  may be formed to extend from any axial position  56  along the radially outward surface  59  of inner ring  52  to any axial position  57  along the radially inward surface  60  of outer ring  51 . In a given embodiment, spokes of at least one of first and second shorting ring retention structures  49  may respectively extend in a direction which is non-perpendicular to the rotational axis. Retention structure  49  has an annular flange  54  extending radially inward from an axially inward portion of inner ring  52 . Although flange portion  54  is shown by example as extending radially inwardly from an axial inward surface  55  of inner ring  52 , flange portion  54  may alternatively be offset from surface  55  in the axially outward direction. Flange  54  provides an annular surface for receiving bent holding portion  48  of hub  9 . Typically, outer ring  51  may have an outer diameter the same as or slightly less than an outer diameter of outer surface  34  of lamination stack  30 , and inner ring  52  is concentric with outer ring  51 . Similarly, the inner diameter of flange  54  is typically the same as the diameter of center aperture  32  of lamination stack  30 . An annular chamfer  61  is formed between radially inward surface  60  and axially inward surface  62  of outer ring  51 . 
         [0038]    Retention structure  49  may be placed at each axial end of lamination stack  30  prior to die-casting. After a die-casting process, copper or other electrically conductive cast material is contained between outer ring  51  and inner ring  52  of each retention structure  49 , and within passages  36  of lamination stack  30 . Typically, the copper completely covers spokes  58  and distal axial ends of rotor  4  are the only exposed copper portions of rotor  4 . Subsequent O.D. machining of rotor  4  typically removes the radially outward portion of outer ring  51  until all or nearly all of surface  62  is removed. 
         [0039]    While various embodiments 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.