Abstract:
An interior permanent magnet motor includes a housing, a ring-shaped stator fixed in the housing and having a coil which generates a magnetic field when a voltage is applied, a rotor being disposed for rotation within, and relative to the ring-shaped stator. The rotor includes a shaft rotatably supported by the housing, a magnetic plate pair disposed about an outer circumference of the rotor. A triangular member is disposed between the magnetic plate pair and the shaft. The triangular member having a flat surface mated to each inner end of each magnetic plate of the magnetic plate pair. The triangular member directs flux produced by rotation of the rotor toward the stator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This patent application claims priority to U.S. Provisional Patent Application Ser. No. 62/081,214, filed Nov. 18, 2014, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to permanent magnet motors that include interior permanent magnets in a rotor. 
         [0003]    Permanent magnet brushless (PMBLDC or PMSM) motors may exhibit relatively high torque densities and are therefore useful in industrial drives for high performance applications. Permanent magnet (PM) motors with buried magnets are used in variable speed drives. 
         [0004]    The placement of magnets inside the magnet pockets of interior permanent magnet (IPM) motors with rectangular bar magnets is an issue due to the manufacturing tolerances of both magnet bars and magnet pockets. This magnet placement creates ripple torque depending on the slot/pole combination of the motor. For high performance applications, torque ripple is an important challenge for PM motors as it creates vibration and speed pulsation. Moreover, cogging torque minimization in IPM motors is more challenging compared to surface permanent magnet (SPM) motors. IPM motors allow for smaller air gaps and linear skewing. Shaping of the magnet presents design difficulties due to the rectangular shape of the permanent magnets. 
         [0005]    Various techniques have been attempted to minimize the cogging torque. Conventional techniques tend to add to the complexity and can negatively impact output torque. In addition, in motors employing sintered magnets, the increased complexity can contribute significantly to cost. 
         [0006]    Magnet pole shaping, skewing of rotor magnets or stator structures, step-skewing of rotor magnets, combining slots and poles, magnet shaping, and incorporation of notches in the stator teeth have been employed to minimize cogging torque in PM motors. Unfortunately, however, these conventional techniques cause additional design challenges. For example, the use of segmented stators, while bringing about improvements in slot fill and manufacturing time of the motor, have also given rise to certain undesirable harmonics, such as a large ninth order harmonic attributed to the gaps disposed between stator segments. 
         [0007]    Accordingly, it is desirable to have an improved rotor design and techniques for imbedding magnets in rotors of IBPM. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect of the invention, an interior permanent magnet motor comprises a housing, a ring-shaped stator fixed in the housing and having a coil which generates a magnetic field when a voltage is applied, a rotor being disposed for rotation within, and relative to the ring-shaped stator, the rotor comprises a shaft rotatably supported by the housing a magnetic plate pair disposed about an outer circumference of the rotor, wherein each magnetic plate of the magnetic plate pair has opposing sides that extend from the outer circumference toward the shaft, the opposing sides are bounded by an inner end of each magnetic plate, and a triangular member disposed between the magnetic plate pair and the shaft, the triangular member having a flat surface mated to each inner end of each magnetic plate of the magnetic plate pair, the triangular member directs flux produced by rotation of the rotor toward the stator. 
         [0009]    In another aspect of the invention, an interior permanent magnet rotor comprises a rotor being disposed for rotation within, and relative to the ring-shaped stator, the rotor comprises a shaft rotatably supported by the housing; a magnetic plate pair disposed about an outer circumference of the rotor, wherein each magnetic plate of the magnetic plate pair has opposing sides that extend from the outer circumference toward the shaft, the opposing sides are bounded by an inner end of each magnetic plate; a triangular member disposed between the magnetic plate pair and the shaft, the triangular member having a flat surface mated to each inner end of each magnetic plate of the magnetic plate pair, the triangular member directs flux produced by rotation of the rotor toward the stator. 
         [0010]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0012]      FIG. 1  shows a motor in accordance with the invention; 
           [0013]      FIG. 2  shows a rotor in accordance with the invention; 
           [0014]      FIG. 3  illustrates a magnetic plate pair of the rotor in accordance with the invention; 
           [0015]      FIG. 4  illustrates specific geometries of the rotor in accordance with the invention; 
           [0016]      FIG. 5  shows a relationship for torque constant (K t ) saturation by comparing results with conventional sintered magnets versus exemplary designs in accordance with the invention; and 
           [0017]      FIG. 6  shows an exemplary relationship for average torque by comparing results with conventional sintered magnets versus exemplary designs in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring now to the Figures, where the invention will be described with reference to specific embodiments without limiting the same,  FIG. 1  illustrates a cross-sectional view of an IPM motor  100 . As shown in  FIG. 1 , the IPM motor  100  comprises a housing  102 , a ring-shaped stator  104  fixed in the housing  102  and a rotor  106 . The ring-shaped stator  104  may have a coil suitable for conducting an electrical current. In this embodiment, the coil of the stator  104  is formed by plurality of cores  108 . The rotor  106  includes a shaft  110  rotatably attached to the housing  102 . The electrical current in the coil of the ring-shaped stator  104  may cause rotation of the shaft  110  relative to the ring-shaped stator  104 . The IPM motor  100 , including the ring-shaped stator  104  and the rotor  106 , may be cylindrically shaped or disk shaped, in some embodiments. 
         [0019]      FIG. 2  illustrates the rotor  106  in accordance with some embodiments of the invention. In addition to the shaft  110 , the rotor  106  comprises at least one magnetic plate pair  202 . The magnetic plate pair  202  may be disposed about an outer circumference  203  of the rotor  102 . In this embodiment, the outer circumference  203  is spaced inward toward the shaft  110 , leaving a space between the outer surface of the rotor  106  and an outer end of a magnetic plate of the magnetic plate pair  202 . 
         [0020]    In the embodiment shown in  FIG. 2 , a plurality of magnetic plate pairs  204  are circumferentially spaced about the rotor. Although six magnetic plate pairs are illustrated as the plurality of magnetic plate pairs  204  for purposes of description, any number of magnetic plate pairs may exist in the rotor  106 , such as three, four, ten, etc. 
         [0021]    Adjacent magnetic plate pairs may alternate in magnetic polarity. For example, a first magnetic plate pair may have a north magnetic polarity, where second magnetic plate pair may have a south magnetic polarity. The alternation of magnetic polarity of the plurality of magnetic plate pairs may continue throughout the rotor. Furthermore, adjacent magnetic plate pairs may be spaced by a pitch defined by a distance P. As shown in  FIG. 2 , the plurality of magnetic plate pairs  204  are approximately equidistantly spaced about the rotor  106 , so the pitch P is approximately equal between magnetic pairs. 
         [0022]    In some embodiments, the plurality of magnetic plate pairs  204  are anisotropic injected molded magnets. The rotor  106  can be manufactured by using powder metal, a casting process, or any other suitable metal. 
         [0023]      FIG. 3  illustrates the magnetic plate pair  202  of the rotor  106  in more detail. The magnetic plate pair  202  has magnetic plates  304 ,  305 . In this embodiment, magnetic plates  304 ,  305  each have opposing convex sides  306 ,  308  that extend from the outer circumference toward the shaft  110 . The opposing convex sides  306 ,  308  of magnetic plates  304 ,  305  are bounded by the outer ends  310 ,  311  and an inner ends  312 ,  313  of the respective magnetic plates  304 ,  305 . 
         [0024]    In some embodiments, the magnetic plates  304 ,  305  may be injection-molded, or filled by using an injection molding process. The invention is not limited to an injection molding process. In addition, in some embodiments, the magnetic plates  304 ,  305  may be compressed magnets. The magnetic plates  304 ,  305  may represent any magnetic plates of the plurality of magnetic pairs. 
         [0025]    In this embodiment, the magnetic plates  304 ,  305  are oriented to form an angle α between magnetic plates of the magnetic plate pair. The angle α may increase as a radial distance from the shaft  110  increases (e.g. distance from the inner end toward the outer end of the magnetic plate pair). 
         [0026]    As shown in  FIG. 3 , the rotor  106  may further comprise a plurality of triangular members. In this embodiment, a triangular member  314  of the plurality of triangular members is disposed between the magnetic plate pair  202  and the shaft. The triangular member  314  has a flat surface mated to inner ends  312 ,  313  of the magnetic plates  304 ,  305  of the magnetic plate pair  202 . Accordingly, the flat surface of the triangular member  314  may physically contact each the magnetic plates  304 ,  305  of the magnetic plate pair  202 . 
         [0027]    The flat surface of the triangular member  314  may be bounded by a second side and third side of the triangular member. The second side and third side of the triangular member may be adjacent to one another, and extend from the flat surface toward the shaft  110 , forming an apex of the triangular member  314 . The apex of the triangular member may extend to the shaft, or as shown in  FIG. 2 , the apex may be spaced from the shaft. The spacing of the apex from the shaft leaves a space formed by inner circumference to the shaft. 
         [0028]    The plurality of triangular members may be made of any non-magnetic material including but not limited to plastic, aluminum, and/or glue. Alternatively, the plurality of triangular members may be an air gap formed by the rotor  106  and the inner ends  312 ,  313  of the magnetic plate pair  202 . The composition of triangular members with the rotor  106  may vary within the rotor  106 , or be consistent within the rotor  106 . 
         [0029]    The plurality of triangular members are configured to decrease flux leakage by directing flux away from the shaft  110 . Thus, the flux is concentrated radially outward, while softening torque pulsations of the motor. 
         [0030]      FIG. 4  illustrates specific geometries of the rotor  106 . A magnet inner arc diameter (IAD), magnet outer arc diameter (OAD) are defined. A minimum distance between the magnet and the outer rotor radius is defined by WEB. An outer magnet thickness (OMT), inner rib thickness (IRT), and an angular distance in between two plates of a single magnetic pole is defined by α. These parameters shape the non-magnetic material, reducing cogging and ripple torque, those parameters are also shown below. An outer non-magnetic thickness (ONMT), an inner non-magnetic thickness (INMT) and non-magnetic width (NMW) may define a triangular member that decreases flux leakage, concentrating the flux radially outward while softening torque pulsations of a motor. 
         [0031]      FIG. 5  shows relationships for torque constant (K t ) saturation by comparing results with conventional sintered interior permanent magnets versus exemplary designs in accordance with the invention. In an exemplary embodiment, the injection molded IBPM shows greater K t  relative to sintered interior permanent magnet motors. 
         [0032]      FIG. 6  shows relationships for average torque (T avg ) by comparing results with conventional sintered interior permanent magnets versus exemplary designs in accordance with some embodiments of the invention. In an exemplary embodiment, the injection molded IBPM shows greater average torque relative to sintered interior permanent magnet motors. 
         [0033]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.