Abstract:
A rotor for use in an electrical motor which includes an anti-expansion ring centrally mounted on the rotor to restrict the extent rotor elements mounted on a rotatable shaft may expand outward due to centrifugal forces generated when the motor is operated. Such motors are suited for use in high rotational speed environments such as electrically controlled turbochargers.

Description:
PRIORITY 
     This application claims benefit of U.S. provisional application Ser. No. 61/217,674 filed Jun. 3, 2009. 
    
    
     RELATED APPLICATION 
     This application is related to PCT/US10/2070 filed Jan. 21, 2010, which is incorporated herein by reference. 
     FIELD 
     This disclosure relates to the field of electric motors and more specifically to the area of rotors of such motors which contain magnetic field reactive elements suitable for high speed operations. 
     SUMMARY 
     Particularly challenging aspects in the design of the rotor of an electric motor that has the capability to be driven at high speeds approximating 100,000 rpms, concern the prevention of centrifugal forces from expanding the rotor elements in such a way as 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 from coming into contact with the stator element or from allowing the rods to cause shorting to the laminations. In the case of a permanent magnet motor, similar concerns apply and rotor expansion needs to be restricted. 
     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 in the range of approximately 100,000 rpms and above. Typically, electrically controlled turbochargers employ a high speed electrical motor to rotate the turbo shaft which exists between the oppositely mounted compressor and turbine. In addition to the containment rings and stiffener improvements described in the above-referenced PCT/US10/2070 application to prevent rotor element detachment and reduce vibration effects on the shaft at high speeds, the embodiments disclosed here provide a center supporting ring on the rotor to provide additional restrictions to the rotor bars that minimize their outward deformation during high speed operations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are exploded views of components included in a rotor of an electric induction motor. 
         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 . 
         FIG. 3  is a plan view of a lamination taken along section line III-III in  FIG. 2 . 
         FIG. 4  is an enlarged view of a lamination aperture from  FIG. 3 , containing a rotor bar. 
         FIG. 5  is a plan view of a lamination and center supporting ring taken along section line V-V in  FIG. 2 . 
         FIG. 6  is an enlarged view of a lamination aperture from  FIG. 5 , containing a rotor bar. 
         FIG. 7  is a cross-sectional plan view along the axis of an induction motor rotor assembly mounted directly on a shaft. 
         FIG. 8  is a cross-sectional plan view along the axis of a rotor assembly of a permanent magnet motor mounted on a shaft stiffener. 
         FIG. 9  is a cross-sectional plan view along the axis of a rotor assembly of a permanent magnet motor mounted directly on a shaft. 
     
    
    
     DETAILED DESCRIPTION 
     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 ). 
     In  FIGS. 1B and 2 , rotor element  210  is shown to include two balance rings  212  and  214  (also referred to herein as “end rings”) having a plurality of apertures  112  and  114  (not visible in  FIG. 2 ), a plurality of ( 19 ) rotor bars  218  ( 218   a - 218   s ), and a plurality of ( 65 ) steel laminations in sets  216   a ,  216   b  and  226  arranged in axially aligned stacks. A central supporting ring (also sometimes referred to herein as an anti-expansion ring)  220  is centrally located over lamination set  226  to minimize the effects of centrifugal forces from distorting the rotor bars  218  during high speed operations.  FIG. 2  cross-sectional view is taken along the plane indicated with the dashed line “A” in  FIG. 3  and the dashed line “C” in  FIG. 5 . 
     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. 
     During assembly, rotor lamination sets  216   a  and  216   b  are coaxially arranged in stacks on either side of lamination set stack  226  which has the central supporting ring  220  located to surround the periphery of stack  226 . Rotor bars  218  are inserted into (or molded in) slots  217  ( 217   a - 217   s ) and  227  ( 227   a - 227   s ). Balance rings  212  and  214  are installed on each end and the end of a rotor bar is received into each aperture  112  and  114  of the balance rings  212  and  214 . The assembly is then clamped together axially to compress the laminations together. Rotor bars  218  are then welded to balance 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. 
     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 balance rings  212  and  214  are press fitted onto the sleeve  202  in order to secure the rotor assembly  210  to shaft  240  under all operational circumstances. 
     Rotor  210  can also possibly be made in a high pressure die casting where the rotor laminations  216  and  226  are placed in a die and molten aluminum is injected into the slots  217  and  227  to form the rotor bars  218 , as well as end rings  212  and  214 . 
     The 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. 
     To further mitigate the effects of centrifugal forces generated at high rotational speeds, the end rings  212  and  214  can be extended axially  213  and  215  from each end of the rotor. Extensions  213  and  215  are much smaller in diameter than the main body of the end rings. 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 speed operations. 
     For extra security, containment rings  204  and  206  formed of high strength steel may be clamped around the balance rings  212  and  214  to ensure the integrity of the press fit between balance 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 . 
     When employed in an electrically controlled turbocharger design, motor rotors are usually elongated. There is a concern that longer rotor bars, such as those designated as  218  in  FIGS. 1A ,  1 B and  2 , will be subject to large centrifugal forces at high rpm operating speeds that could cause the central portions of the rotor bars to be forced radially outward sufficiently to distort the motor/stator air gap. If such distortion occurs, and the outer diameter of the rotor is allowed to expand too much, it could cause contact with the stator. In addition, large outward forces could wear against the isolated coating on the insides of slots  217  and eventually cause shorting between laminations and rotor bars. 
     While the end rings  212  and  214  serve to restrain expansion of the ends of rotor rods  118 , an anti-expansion ring  220  that is centrally located on the rotor restricts the outward movement of the remainder of the rotor rods  218 .  FIGS. 1A ,  1 B and  2  illustrate the central location of anti expansion ring  220  with respect to the other elements. 
     In  FIG. 3 , a plan view of a lamination  216  taken along section line III-III in  FIG. 2  shows the circular distribution of the nineteen slots  217   a - 217   s . Lamination  216  is identical to all other laminations in lamination sets  216   a  and  216   b . In that 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.    
     As can be seen in  FIG. 4 , the enlarged view of a slot  217  in lamination  216  is radially oriented and slightly truncated towards the axis. Due to the effect of anti-expansion ring  220  pressing against the central portion of rotor bars, the rotor bars in lamination sets  216   a  and  216   b  are compressed towards the lower end of slot  217  closest to the rotor axis. This compression leaves an air space  219  between the outer rotor bar surface and outer end of the slot  217 . A conventional air gap  215  is shown as being spaced away from the rotor bar  218 . 
     In  FIG. 5 , a plan view of a lamination  226  taken along section line V-V in  FIG. 2  shows the distribution of the 19 slots  227   a - 227   s . Lamination  226  is identical to all other laminations in the central lamination set below anti-expansion ring  220 . Lamination  226  surrounds stiffener  202  which is press fitted to the rotor shaft  240  (or to the shaft  340  in  FIG. 7 ). Rotor bars  218   a - 218   s  are inserted in to the corresponding slots  217   a - 217   s . Lamination  226  and all others in the central lamination set are somewhat different than those in  FIGS. 3 and 4 , due to the placement and accommodation of the anti-expansion ring  220 . As can be seen in the enlarged view of a slot  227  in  FIG. 6 , the outer diameter of the lamination has been reduced by the thickness of the anti-expansion ring  220  as indicated by dashed line “B” in  FIG. 4 . The air gap portion ( 215  shown in lamination  216  of  FIG. 4 ) is removed so that the anti-expansion ring  220  is in direct contact with rotor bar  218 . In this configuration, the combined OD of lamination  226  and anti-expansion ring  220  is the same as lamination  216  in lamination sets  216   a  and  216   b.    
     Anti-expansion ring  220  is preferably formed of high strength steel that is selected to also have a low coefficient of thermal expansion in order to minimize expansion of the rotor assembly  200 . 
       FIG. 7  illustrates a rotor assembly  300  that is mounted directly on rotor shaft  340 , in such environments where a stiffening component is not required to control vibration at operational speeds. Rotor assembly  300  shown to include two balance rings  312  and  314 , a plurality of rotor bars  318  and a plurality of steel lamination sets  316   a ,  316   b  and  326  arranged in axially aligned stacks. A central supporting ring  320  is centrally located over lamination set  326  to minimize the effects of centrifugal forces from distorting the rotor bars  318  during high speed operations. As in the earlier described embodiment, extensions  313  and  315  are much 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 . 
       FIG. 8  illustrates the principles noted above applied to the rotor  400  of a permanent magnet motor. In this embodiment, a stiffener component  402  is press fit to a rotor shaft  440 . A high strength steel cylindrical sleeve  420  extends from and between end rings  412  and  414  to surround the magnetic material  416 . In this manner, the sleeve acts to limit the amount of expansion that the rotor will incur due to centrifugal forces imparted at high operational speeds. End rings  412  and  414  employ the reduced mass extensions  413  and  415  described above. For extra security, containment rings  404  and  406  formed of high strength steel may be clamped around the end rings  412  and  414  to ensure the integrity of the press fit between end rings  412  and  414 , stiffener sleeve  402 , and shaft  440 . Containment rings  404  and  406  are located on end ring extensions  413  and  415 . 
       FIG. 9  illustrates a rotor assembly  500  that is mounted directly on rotor shaft  540 , in such environments where a stiffening component is not required to control vibration at operational speeds. In this embodiment, a high strength steel cylindrical sleeve  520  extends from and between end rings  512  and  514  to surround the magnetic material  516 . In this manner, the sleeve acts to limit the amount of expansion that the rotor will incur due to centrifugal forces imparted at high operational speeds. End rings  512  and  513  employ the reduced mass extensions  513  and  515  described above. Containment rings  504  and  506 , preferably of high strength steel are located on end ring extensions  513  and  515 . 
     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.