Abstract:
A rotor of an electric machine is disclosed that resists expansion of the rotor components even at high rotational speed. The rotor includes first and second pluralities of laminations having slots to accept rotor bars. A support disk, also having slots, is placed between the laminations. The support disk, into which the rotor bars are slid, restrains the rotor bars from bending outwardly at high rotational speeds of the rotor. The rotor bars are further restrained at the ends by end rings, which have apertures into which ends of the rotor bars are placed. In some embodiments, containment rings are placed over axial extension of the end rings to prevent outward bowing at high speeds. In some embodiments, the rotor includes a stiffener sleeve to provide additional resistance to expansion during high rotational speeds.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation in part of U.S. application Ser. No. 12/791,832, filed 1 Jun. 2010, and which is incorporated herein. This application claims benefit of U.S. provisional application Ser. No. 61/217,674 filed 3 Jun. 2009. 
     
    
     FIELD 
       [0002]    This disclosure relates to the field of electric motors and more specifically to rotors of such motors that contain magnetic field reactive elements suitable for high speed operations. 
       SUMMARY 
       [0003]    Particularly challenging aspects in the design of the rotor of an electric motor that has the capability to be driven at speeds exceeding 100,000 rpm concern the prevention of centrifugal forces from expanding the rotor elements such that they become separated from the shaft to which they are attached. In the case of an induction motor, it is important to prevent expansion of the rotor elements to avoid coming into contact with the stator element. 
         [0004]    Electric motor rotors disclosed herein are suitable for use in turbochargers and other environments where motors may be required to operate at significantly high speeds exceeding 100,000 rpm. Typically, electrically controlled turbochargers employ a high speed electrical motor to rotate the turbo shaft which exists between the oppositely mounted compressor and turbine. The embodiments disclosed herein provide a center supporting disk on the rotor to provide additional support to the rotor bars to minimize their outward deformation during high speed operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIGS. 1A and 1B  are exploded views of components included in a rotor of an electric induction motor. 
           [0006]      FIG. 2  is a cross-sectional plan view along the axis of an induction motor rotor assembly comprising the components shown in  FIGS. 1A and 1B . 
           [0007]      FIG. 3  is a plan view of a lamination taken along section line in  FIG. 2 . 
           [0008]      FIG. 4  is an enlarged view of a lamination aperture from  FIG. 3 , containing a rotor bar. 
           [0009]      FIG. 5  is a plan view of a center support ring taken along section line V-V in  FIG. 2 . 
           [0010]      FIG. 6  is an enlarged view of a portion of  FIG. 5  of a center support ring aperture containing a rotor bar. 
           [0011]      FIG. 7  is a cross-sectional plan view along the axis of an induction motor rotor assembly mounted directly on a shaft. 
           [0012]      FIGS. 8 and 9  illustrate embodiments by which a rotor may be assembled. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    In  FIG. 1A  the major components of a rotor  200  of an electric induction motor include an assembled rotor element  210 , containment rings  204  and  206  and stiffener sleeve component  202  for mounting on a rotor shaft  240  ( FIG. 2 ). 
         [0014]    In  FIGS. 1B and 2 , rotor element  210  is shown to include two end rings  212  and  214  (sometimes referred to as “balance” rings) having a plurality of apertures  112  and  114 , a plurality of (19) rotor bars  218 , and a plurality of (65) steel laminations in sets  216   a  and  216   b  arranged in axially aligned stacks. A central supporting disk (also referred to herein as an anti-expansion disk)  226  is centrally located between laminations sets  216   a  and  216   b . Rotor bars  218  slide through apertures in steel lamination sets  216   a  and  216   b , through apertures in supporting disk  226 , and through apertures in end rings  212  and  214 . The purpose of central supporting disk  226  is to minimize the effects of centrifugal forces from distorting the rotor bars  218  during high speed operations. 
         [0015]    Steel laminations  216  can be formed of high-strength electrical steel, such as Hyperco 50™, heat treated to provide maximum strength, and oxide coated to prevent electrical current losses between laminations. Rotor bars  218  can be made from a high strength-to-density ratio (specific modulus) and high electrical conductivity alloy, such as 2219 Al. 
         [0016]    During assembly, rotor lamination sets  216   a  and  216   b  are coaxially arranged in stacks on either side of lamination supporting disk  226 . Rotor bars  218  are inserted into (or molded in) slots  217  ( 217   a - 217   s ) and  227  ( 227   a - 227   s ). End rings  212  and  214  are installed on each end and the ends of rotor bars are received into apertures  112  and  114  of the balance rings  212  and  214 , respectively. The assembly is then clamped together axially to compress the laminations together. Rotor bars  218  are then welded to end rings  212  and  214 . Such welding may employ an electron beam process or any other process that provides effective high strength welding for such metals. Heat sinks are attached to the rotor during this process to minimize the distortional effects of welding. After welding, rotor  210  is machined on all outside surfaces and the ID to improve concentricity of the inside diameter, ID, and outside diameter, OD, as well as balance. 
         [0017]    Following machining, the rotor assembly  210  is slid onto the stiffener sleeve  202 . The assembly is then balanced and the stiffener sleeve  202  is press fitted onto shaft  240 . While there may be some tolerance between the stiffener sleeve  202  and the ID of the laminations to prevent pre-stress in the laminations, the end rings  212  and  214  and central support disk  226  are press fitted onto the sleeve  202  to secure the rotor assembly  210  to shaft  240  under extremes in operational circumstances. 
         [0018]    Rotor  210  can alternatively be injection molded in a high-pressure injection molding process where the rotor laminations  216   a  and  216   b  are placed in a mold and molten aluminum is injected into slots  217  and  227  to form the rotor bars  218 . In the same process, end rings  212  and  214  and central support disk  226  are also formed. 
         [0019]    End rings  212  and  214  are preferably fabricated from the same or similar alloy used to fabricate the rotor bars  218  and serve to minimize expansion of the rotor ends during high speed operations. Furthermore, central support disk  226  may be fabricated from the same or similar alloy as used for end rings  212  and  214  and rotor bars  218 . 
         [0020]    To further mitigate the effects of centrifugal forces generated at high rotational speeds, the end rings  212  and  214  can include axial extensions  213  and  215 . Extensions  213  and  215  are smaller in diameter than the main body of the end rings  212  and  214 . By making end-ring extensions  213  and  215  smaller in diameter, the extensions experience much less centrifugal force and therefore retain their press fit onto the stiffener  202  and shaft  240  throughout the broad range of operating speed. 
         [0021]    In some embodiments, containment rings  204  and  206 , formed of high strength steel, are clamped around the end rings  212  and  214  to further ensure the integrity of the press fit between end rings, stiffener sleeve  202  and shaft  240 . In  FIGS. 1A and 2 , containment rings  204  and  206  are located on end ring extensions  213  and  215 . 
         [0022]    When employed in an electrically-controlled turbocharger design, motor rotors are typically elongated. There is a concern that longer rotor bars, such as 218 in  FIGS. 1B and 2  may be subjected to large centrifugal forces at high rotational speeds that act on the central portions of the rotor bars forcing them outwardly in a radial direction sufficient to affect the motor-to-stator air gap. If distortion of the rotor bars is too great, the rotor contacts the stator. In some embodiments, the individual laminations are provided with an oxide coating to prevent shorting between adjacent laminations and to prevent shorting between the surfaces of the slots formed in the laminations to the rotor bars. If large outward forces act upon the laminations the oxide coating on the surfaces of slots  217  could wear and eventually lead to shorting between laminations and the rotor bars. End rings  212  and  214  as well as central support disk  226  restrain expansion of the rotor rods  218 . In the embodiment shown in  FIG. 2 , central support disk  226  is shown between lamination stacks  216   a  and  216   b . In an embodiment in which it is advantageous to have a particularly long rotor, at least one additional support disk is provided between additional sets of lamination stacks. Thus, in one alternative embodiment, there are three sets of lamination stacks with a first support disk provided between first and second sets of the lamination stacks and a second support disk provided between second and third set of the lamination stacks. 
         [0023]    In  FIG. 2 , a first protuberance  222  and a second axial protuberance  223  extend outwardly in a radial direction from stiffener sleeve  202 . The space between the two protuberances  222  and  223  is of a smaller diameter. This smaller diameter portion provides a shoulder for a tool to grab onto the stiffener sleeve  202  for disassembly. 
         [0024]      FIG. 3 , a cross-sectional view of lamination  216   a  taken along section line in  FIG. 2 , shows the distribution of the 19 slots  217   a - 217   s . In this view, the stiffener  202  is shown surrounding the rotor shaft  240 . Rotor bars  218   a - 218   s  are inserted into the corresponding slots  217   a - 217   s.    
         [0025]    As can be seen in  FIG. 4 , the enlarged view of slot  217   a  in lamination  216   a  is radially oriented. Rotor bar  218   a  is inserted into the slot  217   a . When stationary, as shown in  FIG. 4 , slot  217   a  is slightly larger than rotor bar  218   a . An air gap  219  exists between slot  217   a  and rotor bar  218   a . At high rotational speeds, rotor bar  218   a  expands more than lamination set  216   a  and thus more than slot  217   a . Therefore, at high speed, air gap  219  is taken up by the expanded rotor bar  218   a . An air gap opening  220  provides a separation so that poles are formed in the adjacent teeth (the radial portions of the laminations between the adjacent slots). In  FIG. 5 , a cross-sectional view through central support disk  226  taken along section line V-V in  FIG. 2  shows the distribution of the 19 slots  227   a - 227   s . Central support disk  226  surrounds stiffener  202  which is press fitted to the rotor shaft  240 . Rotor bars  218   a - 218   s  are inserted into the corresponding slots  217   a - 217   s . There is no air gap between slot  227   a  of central support disk  226  and rotor bar  218   a  as can be seen in detail in the embodiment shown in  FIG. 6  because rotor bar  218   a  and central support disk  226  are made of materials with similar expansion characteristics. Further, by avoiding an air space, support disk restrains rotor bar  218   a  from outward movement. 
         [0026]      FIG. 7  illustrates a rotor assembly  300  that is mounted directly on rotor shaft  340  that can be used in environments where a stiffening component is not included. Rotor assembly  300  includes two end rings  312  and  314 , a plurality of rotor bars  318  (only one of which is shown in this cross section) and steel lamination sets  316   a  and  316   b  that are axially aligned stacks. A central support disk  326  is centrally located and has slots through which rotor bars  318  are inserted. Central disk  326  provides stiffening to minimize the distortion of the rotor bars at high rotational speeds. Extensions  313  and  315 , extending axially from end rings  312  and  314  are smaller in diameter than the main body of the end rings  312  and  314  to reduce the mass surrounding the press fit to shaft  340 . 
         [0027]    The procedure to assemble the rotor assembly onto the shaft, according to one embodiment, is illustrated in  FIG. 8 . In  800 , laminations stacks  216   a  and  216   b  are arranged on either side of supporting disk  226 . The slots are aligned so that in  802  the rotor bars  218   a - s  can be inserted in the slots through  216   a ,  216   b , and  226 . In  804 , end rings  212  and  214  are slid onto rotor bars  218   a - s  with apertures in the end rings engaging with the rotor bars. In  806 , the assembly is clamped together to compress the laminations axially and a heat sink is attached prior to welding the end rings  212  and  214  to the rotor bars  218 . The welding process may be an electron beam process. In blocks  800  through  808 , rotor assembly  210  is formed, designated as  810  in  FIG. 8 . After rotor assembly  210  is welded, it is machined to improve its concentricity and balance. In embodiments that include containment rings  204  and  206 , they are fit onto extensions  213  and  215 , respectively, in  814 . Rotor assembly  210  is press fit onto stiffener sleeve  202  in block  816 . In one embodiment, only the end rings  212  and  214  and central support disk  226  are press fit on the stiffener sleeve. Lamination sets  216   a  and  216   b  are slightly oversize, with respect to the inner diameter, to avoid cracking the laminations during assembly. In block  818 , stiffener sleeve  202  is press fit onto rotor shaft  240 . 
         [0028]    In one embodiment, the end rings, the central support disk, and the rotor bars are made of the same material, e.g., an aluminum alloy, and these are produced by injection molding. In such embodiment, the manufacture begins with stacking laminations sets  216   a  and  216   b , as illustrated in  FIG. 9 , starting at 900. Lamination sets  216   a  and  216   b  are placed in an injection mold and secured in place during molding, at  902 . Also in  902 , a die is centrally located within the laminations so that aluminum is not injected into the space reserved for rotor shaft  240  and stiffener sleeve  202 . The molten aluminum alloy is injected into the mold, in  904 . The end rings, central support disk, and rotor bars are one integral part, which, of course, cannot be disassembled from the laminations. The combination of parts forms the rotor assembly. The rotor assembly is cooled,  906 , before being ejected from the mold,  908 . In  908 , the die is removed from the rotor assembly. Group 910 designates the processes to form the rotor assembly. In  912 , the rotor assembly is machined to remove artifacts from the mold process. Additionally, the machining may improve the dimensional accuracy and hence balance and fit of the rotor assembly. In embodiments that include containment rings  204  and  206 , they are fit onto extensions of the end rings, respectively, in  914 . Rotor assembly is press fit onto stiffener sleeve  202  in block  916 . In one embodiment, only the end rings and central support disk  226  are press fit on the stiffener sleeve. Lamination sets  216   a  and  216   b  may be slightly oversize, with respect to the inner diameter, to avoid cracking the laminations during assembly. In block  818 , stiffener sleeve  202  is press fit onto rotor shaft  240 . In embodiments in which a stiffener sleeve  202  is not used, the rotor assembly is press fit directly onto rotor shaft  240 . 
         [0029]    The embodiments shown here are exemplary in nature and shall not be considered to be a restriction on the scope of the claims set forth herein.