Abstract:
A method of forming a rotor lamination assembly for an electric machine includes aligning a plurality of laminations to form a rotor lamination assembly. The rotor lamination assembly includes an outer diametric surface. The method also includes mounting a rotor lamination compression sleeve to the outer diametric surface of the rotor lamination assembly. The rotor lamination compression sleeve radially compresses the plurality of laminations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/953,025, filed Nov. 23, 2010, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Exemplary embodiments pertain to the art of electric machines and, more particularly, to an electric machine including a rotor lamination assembly having a rotor lamination compression sleeve. 
         [0003]    Electric machines include a rotor that rotates relative to a stator. Electrical current passing though the stator is influenced by a magnetic field developed in the rotor creating an electro-motive force that causes the rotor to spin. Certain electric motors/generators employ permanent magnets in the rotor. The permanent magnets are mounted in magnet slots formed in the rotor which is typically constructed from a plurality of stacked laminations. Generally, the permanent magnets are mounted near an outside edge of the rotor, as close to the outside edge as possible, in order to maximize torque and minimize flux losses. Mounting the permanent magnets in this manner creates a thin bridge area between the magnet slots and the outside edge of the rotor lamination. 
         [0004]    During high speed operation, centrifugal forces on the rotor create stresses in the thin bridge area. If operated at too high a speed, the stress can exceed a yield strength of the laminations. In such a case, the rotor will fail. Accordingly, there exists a trade off between maximizing torque and operating the electric machine at high speed. Maximizing torque by mounting the permanent magnets as close to the outside edge of the rotor limits the overall operational speed of the electrical machine. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    Disclosed is a method of forming a rotor lamination assembly for an electric machine. The method includes aligning a plurality of laminations to form a rotor lamination assembly. The rotor lamination assembly includes an outer diametric surface. The method also includes mounting a rotor lamination compression sleeve to the outer diametric surface of the rotor lamination assembly. The rotor lamination compression sleeve radially compresses the plurality of laminations. 
         [0006]    Further disclosed is a method of forming a rotor lamination assembly for an electric machine. The method includes aligning a plurality of laminations to form the rotor lamination assembly, and compressing the plurality of rotor laminations with a rotor lamination compression sleeve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0008]      FIG. 1  is a partial, cross-sectional view of an electric machine including a rotor lamination assembly having a rotor lamination compression sleeve; 
           [0009]      FIG. 2  is a plan view of a rotor lamination assembly in accordance with one aspect of the exemplary embodiment provided with the rotor lamination compression sleeve of  FIG. 1 ; 
           [0010]      FIG. 3  is a perspective view of the rotor lamination compression sleeve of  FIG. 1  in accordance with an exemplary embodiment; and 
           [0011]      FIG. 4  is a plan view of a rotor lamination assembly in accordance with another aspect of the exemplary embodiment provided with the rotor lamination compression sleeve of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
         [0013]    Exemplary embodiments provide sleeve member that structurally supports high stress regions of a rotor lamination assembly. The sleeve member extends about and compresses the rotor lamination assembly to support tensile stresses that develop in rotor lamination edge regions. By supporting the edge regions of the rotor laminations, an electric machine may be operated at higher output speeds without subjecting the rotor to high stresses that may lead to premature rotor failure. 
         [0014]    An electric machine is indicated generally at  2  in  FIG. 1 . Electric machine  2  includes a housing  4  having first and second side walls  6  and  7  that are joined by a first end wall  8  and a second end wall or cover  10  to collectively define an interior portion  12 . First side wall  6  includes an inner surface  16  and second side wall  7  includes an inner surface  17 . At this point it should be understood that housing  4  could also be formed to include a single side wall having a continuous inner surface. Electric machine  2  is further shown to include a stator  24  arranged at inner surfaces  16  and  17  of first and second side walls  6  and  7 . Stator  24  includes a body  28  having a first end portion  29  that extends to a second end portion  30  and supports a plurality of windings  36 . Windings  36  include a first end turn portion  40  and a second end turn portion  41 . 
         [0015]    Electric machine  2  is shown to include a shaft  54  rotatably supported within housing  4 . Shaft  54  includes a first end  56  that extends to a second end  57  through an intermediate portion  59 . First end  56  is rotatably supported relative to second end wall  10  through a first bearing  63  and second end  57  is rotatably supported relative to first end wall  8  through a second bearing  64 . Shaft  54  supports a rotor  70  that is rotatably mounted within housing  4 . Rotor  70  includes a hub  74  that is fixed relative to intermediate portion  59  and a rotor lamination assembly  79 . Rotor lamination assembly  79  includes a plurality of laminations, one of which is indicated at  84 . Laminations  84  are stacked and aligned to define an outer diametric surface  87  of rotor lamination assembly  79 . At this point it should be understood that electric machine  2  could also be configured with a rotor rotatably supported to a central shaft by bearings. 
         [0016]    As best shown in  FIG. 2 , lamination  84  includes a body member  104  having an outer diametric edge  106  and an inner diametric edge  108  that defines a central opening  109 . Lamination  84  includes a radial web  110  that extends between outer and inner diametric edges  106  and  108 . A plurality of magnet receiving members  116 - 131  are formed in radial web  110  and extend about lamination  84 . Magnet receiving members  116 - 131  are configured to receive a corresponding plurality of magnets  134 - 149 . Magnets  134 - 149  are rotated relative to stator  24  to generate an electro-motive force in windings  36 . As each magnet receiving member  116 - 131  is similarly constructed, a detailed description will follow referencing magnet receiving member  116  with an understanding that the remaining magnet receiving members  117 - 131  are similarly constructed. Magnet receiving member  116  includes a first end section  153  that extends to a second end section  154 . Second end section  154  defines an interruption zone  160  at outer diametric edge  106 . A first filler  163  is arranged between first end section  153  and magnet  134  and a second filler  164  is arranged between second end section  154  and magnet  134 . First and second fillers  163  and  164  support and/or retain magnet  134  within magnetic receiving member  116 . At this point it should be understood that the above-described structure is provided for illustrative purposes and should not be considered as limiting to the exemplary embodiment which is directed to a rotor lamination compression sleeve  170  positioned upon outer diametric surface  87  of rotor lamination assembly  79 . 
         [0017]    In accordance with an exemplary embodiment illustrated in  FIG. 3 , rotor lamination compression sleeve  170  includes a body member  174  having an outer diametric surface  176  and an inner diametric surface  178  that defines an annular ring  180 . At rest, inner diametric surface  178  defines a first diameter “X” of rotor lamination compression sleeve  170 . As will be discussed more fully below, rotor lamination compression sleeve  170  is formed from a material, such as austenitic nickel-chromium alloys, other high strength alloys, steels and the like, that expands to a second diameter “Y” of rotor  70 . The first diameter “X” of rotor lamination compression sleeve  170  is sized to provide a radial compressive force to rotor lamination assembly  79  when rotor  70  is at rest. That is, in a free state, rotor lamination compression sleeve  170  includes an inner diameter that is smaller than an outer diameter of the plurality of laminations  84 . The smaller, free-state, diameter generates a pre-load on the plurality of laminations  84 . More specifically, at rest, rotor lamination compression sleeve  170  is in tension and the plurality of laminations  84  experience a radial compressive force. The radial compressive force supports the outer diametric edge  106  of each of the plurality of laminations  84 . With this arrangement, an increase of tensile stress at outer diametric edge  106 , or in a bridge area (not separately labeled) between adjacent magnet receiving members  116 - 131  is supported by rotor lamination compression sleeve  170 . Supporting outer diametric edge  106  and/or the bridge area enhances operating characteristics of electric machine  2 . That is, reducing tensile stress allows electric machine  2  to operate at higher speed levels. 
         [0018]    In further accordance with an exemplary embodiment, when rotor  70  begins to experience centrifugal forces rotor lamination compression sleeve  170  gradually expands reducing the first radial compressive force. That is, centrifugal forces cause rotor lamination compression sleeve  170  to gradually expand thereby reducing the first radial compressive force to a second, lower radial compressive force. As the radial compressive force is reduced, tensile stresses in rotor lamination assembly  79  increase. However, while the first radial compressive force decreases, the second radial compressive force still provides external support such that the tensile stresses remain below a critical tensile stress that would lead to rotor failure. At this point it should be understood that while rotor lamination compression sleeve  170  creates a larger air gap between an outer diameter of rotor  70  and an inner diameter of the stator  24  that may reduce performance, any reduction in performance is off-set by the reduction in tensile stresses and an increased overall operational envelope. 
         [0019]    In addition to rotor laminations having open magnet receiving members, rotor lamination compression sleeve  170  may be employed with a wide range of rotor laminations including partially open (not shown) and closed laminations such as shown at  190  in  FIG. 4  wherein like reference numbers represent corresponding parts in the respective views. Lamination  190  includes a plurality of magnet receiving members, one of which is indicated at  194 . Magnet receiving member  194  includes a first end section  196  that extends to a second end section  197 . Second end section  197  is closed thereby defining a bridge region  200 . Bridge region  200  is supported by rotor lamination compression sleeve  170 . In this manner, lamination  190  is more resistant to tensile stresses developed during operation and electric machine  2  is operable at a higher speed ranges than those previously attainable. 
         [0020]    At this point it should be understood that the exemplary embodiments describe a rotor lamination compression sleeve that structurally supports a rotor lamination assembly during operation. The structural support provided to the rotor lamination assembly enables each lamination to better withstand tensile stresses that are developed during operation, particularly, at high speed. In this manner, the rotor lamination compression sleeve enhances an overall operational envelope of the electric machine. 
         [0021]    While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.