Abstract:
Disclosed herein is a dynamoelectric machine rotor. The rotor includes, a plurality of first cavities positioned near a circumferential surface of the rotor, each first cavity receptive of at least one permanent magnet, and a plurality of second cavities positioned substantially between circumferentially adjacent first cavities.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a non-provisional application of U.S. Ser. No. 60/835,811, Aug. 4, 2006, the contents of which are incorporated by reference herein in their entirety. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    Dyanmoelectric machines often use permanent magnets positioned within a rotor that rotates within a central bore of a stator to convert mechanical energy to electrical energy and vice versa. 
         [0003]    Magnetic flux lines extend between poles of opposing polarity within the individual permanent magnets as well as between adjacent permanent magnets. The paths and density of these magnetic flux lines can have a significant effect on the relationship of torque versus rotational angle of the rotor of the dynamoelectric machine. For example, uneven distribution of flux lines around the perimeter of the rotor can result in higher and lower levels of torque, often referred to as torque ripple, experienced during rotation of the rotor in the dynamoelectric machine. Such torque ripple may be undesirable for several reasons, such as, audible noise, loss of efficiency, and increased component wear, for example. 
         [0004]    The paths that the flux lines follow are determined, in part, by materials positioned between and around the opposing poles and the geometry of such materials. Flux lines position themselves preferentially within soft magnetic materials as opposed to hard magnetic materials and material voids. Therefore, rotor design can have a significant effect on the flux line paths generated. 
         [0005]    Accordingly, improvements in the art of rotor design that reduce torque ripple and the side effects associated therewith are desirable in the art. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    Disclosed herein is a dynamoelectric machine rotor. The rotor includes, a plurality of first cavities positioned near a circumferential surface of the rotor, each first cavity receptive of at least one permanent magnet, and a plurality of second cavities positioned substantially between circumferentially adjacent first cavities. 
         [0007]    Further disclose herein is a dynamoelectric machine rotor assembly. The assembly includes, a rotor, a plurality of first cavities formed within the rotor near a circumferential surface thereof, a plurality of permanent magnets, each one of the plurality of permanent magnets being fixedly attached to the rotor within one of the plurality of first cavities, and a plurality of second cavities formed within the rotor, each of the plurality of second cavities being positioned between circumferentially adjacent first cavities. 
         [0008]    Further disclosed herein is a method for minimizing torque ripple of a dynamoelectric machine. The method includes, inhibiting natural flux line formation while a rotor of the dynamoelectric machine is in motion by interrupting selected regions of the rotor prone to flux passage by interpositing one or more cavities in the region, and directing flux lines around the one or more cavities in the rotor. 
         [0009]    Further disclosed herein in a method of making a rotor for a dynamoelectric machine. The method includes, forming a rotor with a plurality of first holes receptive of magnets and a plurality of second holes for sculpting flux lines. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0011]      FIG. 1  depicts a partial cross sectional view of a dynamoelectric machine depicted herein. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring to  FIG. 1 , a partial cross sectional view of a dynamoelectric machine  6  disclosed herein is depicted. A rotor  10  has permanent magnets  14  fixedly positioned within first cavities  18  formed therein. The rotor  10  is located concentrically within a stator  22  and rotates about a rotor axis (not shown). Clearance between an outer circumferential surface  26  of the rotor  10  and an inner circumferential surface  30  of the stator  22  form a radial air gap  34  therebetween. The air gap  34  is intentionally kept small to maximize performance of the dynamoelectric machine  6 . 
         [0013]    The stator  22  includes wound coils  38  fixedly positioned within slots  42  formed therein. The coils  38  are wound from an insulated conducting material such as copper, for example. Electric current is passed through the coils  38  of the stator  22  to generate magnetic fields that react with the magnetic fields of the permanent magnets  14  of the rotor  10  during conversion of energy by the dynamoelectric machine  6 . Such conversion of energy can be from mechanical to electrical or from electrical to mechanical, for example. Performance and efficiency of the energy conversion is partially dependent upon the shape and distribution of flux lines from the permanent magnets  14  of the rotor  10 . 
         [0014]    The magnetic field of the permanent magnets  14  is shaped, in part, by the material and geometry of the rotor  10 . Magnetic flux lines tend to concentrate in soft magnetic materials and tend to avoid hard magnetic materials and material voids, such as air pockets and cavities or cavities with non-magnetic fillers, in the soft magnetic material. The rotor  10  is, therefore, intentionally made of a soft magnetic material, such as silicon steel or powdered metal, for example, to allow the flux lines to be shaped by the geometric shape of the rotor  10 . 
         [0015]    Magnetic flux lines extend between magnetic poles of opposite polarity. For example, flux lines extend between a south (S) pole  46 , of a first magnet  48 , and a north (N) pole  49 , of the first magnet  48 , and simultaneously the leakage flux lines extend between the S pole  46 , of the first magnet  48 , and an N pole  59  of a second magnet  58 . The amount of rotor material located between the adjacent poles  46  and  59  will have an effect on the routing of the flux lines between the poles  46  and  59  and the strength of the magnetic field in that area of the rotor  10 . Consequently, the geometric design of the rotor  10  can influence the strength of the magnetic fields around the perimeter of the rotor  10  resulting in areas with locally stronger and locally weaker magnetic fields. 
         [0016]    Having locally stronger and locally weaker magnetic fields around the perimeter of the rotor  10  can cause variations in torque of the dynamoelectric machine  6  as the rotor  10  is rotated relative to the stator  22 . Such a variation in torque is commonly known as torque ripple. Torque ripple can cause variations in rotational speed of the rotor  10  within each complete rotation of the rotor  10 , for example. Such variations in rotational speed can cause increases a rate of wear of components such as drive belts and bearings for example. Torque ripple can also cause vibration and undesirable audible noise to be emitted from the dynamoelectric machine  6 . Additionally, torque ripple has been shown to have a detrimental affect on efficiency of dynamoelectric machines. Consequently, it is often desirable to decrease variations in the magnetic field around the perimeter of the rotor  10  and thereby decrease torque ripple associated therewith. 
         [0017]    As mentioned above, flux lines tend to avoid cavities formed in a soft magnetic material. As such, careful positioning of cavities in a soft magnetic material can be used to beneficially sculpt magnetic flux lines to optimize energy transfer and minimize torque ripple. A second cavity  64 , disclosed herein, is positioned in an area where flux lines tend to be shorted. More specifically, the second cavity  64  is positioned in the rotor  10  between two adjacent poles  46  and  59 . Such a positioning of the second cavity  64  causes flux lines to route around the second cavity  64  thereby elongating the path length of the flux lines and decreasing the total flux lines that would otherwise be shorted. The second cavity  64  can therefore be used to increase a uniformity of the magnetic field strength around the perimeter of the rotor  10 . Such an increase in uniformity of the magnetic field strength about the perimeter of the rotor  10  can decrease the magnitude of torque ripple and the problems, mentioned above, associated therewith. The second cavity  64  increases uniformity of magnetic strength by reducing a number of flux lines shorted between the adjacent poles  46 ,  59  within the rotor  10 . The cavity  64  forces more flux lines to pass through the air-gap  34  where they link with the flux lines of the magnetic field of generated by the stator coil  38 . Interactions between flux from the permanent magnets  14  and flux from the stator coils  38  is a key factor in efficient electromechanical energy conversion. 
         [0018]    Though the second cavity  64  need not extend fully through the axial length of the rotor  10 , embodiments wherein the second cavity  64  does extend fully through the rotor  10  may be desirable to create axial symmetry of the second cavity  64  relative to the magnets  14 . In the circumferential direction the second cavity  64 , as shown, is symmetrical. Such a design may provide uniformity of magnetic strength regardless of the rotational direction of travel of the rotor  10  and may therefore be preferred for applications where rotation in either direction is desirable. Alternatively, for applications wherein the rotor  10  travels in only a single rotational direction an asymmetrical second cavity may be desirable. Such an asymmetrical second cavity may provide for a more uniform flux line distribution and correspondingly reduced torque ripple in one direction as opposed to the opposite direction. 
         [0019]    Radial positioning of the second cavities  64  within the rotor  10  will also effect routing of flux lines. The second cavities  64  should be positioned as close to the circumferential surface  26  as feasible without actually being connected to the surface  26 , thereby leaving a bridge  68  of soft magnetic material between the second cavities  64  and the surface  26 . The bridge  68  should be so thin that it is saturated with flux lines thereby diverting additional flux lines through the air gap  34  and into the stator  22 . The bridge  68  should be thick enough, however, to maintain structural integrity even subsequent to a machining operation if a machining operation is utilized as discussed below. The presence of the bridge  68 , as opposed to connecting the second cavity  64  to the surface  26 , as a notch or groove, presents a continuous circumferential surface  26 . The continuous nature of the circumferential surface  26  significantly improves the machinability of the surface  26  while extending longevity of cutting tools used thereon. 
         [0020]    Machining of the surface  26  can be desirable to remove local protrusions and depressions that may be present in the surface  26  subsequent to the manufacture of the rotor  10 . Such a machining operation may take place on a lathe, for example, and can improve the concentricity of the circumferential surface  26  with an axis of rotation of the rotor  10 . Such improved rotor concentricity may allow for a smaller air gap  34  and an associated improvement in efficiency. The rotor  10  may be manufactured in different ways, one of which is by stacking and fixing together several individual and substantially identical laminations. Such laminations may be made by stamping them from a sheet of metal, for example. Another method of manufacture involves compression of a powdered metal and subsequent sintering of the powdered metal to form the rotor  10  into a solid stack. The laminations or solid stack, in this embodiment, are made with the first cavities  18  and second cavities  64  formed therein. Axial alignment of the cavities  18  and  64  is, therefore, controlled during the manufacturing process. In the lamination example the lamination stacking process for the rotor  10  controls the alignment of the laminations to one another, which can affect a size of any protrusions or depressions resulting in the circumferential surface  26 . Regardless of the manufacturing method used to fabricate the rotor  10  local protrusions and depressions may be present in a size that would be desirable to be removed by a subsequent machining process. 
         [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.