Abstract:
A permanent magnet machine is provided with a rotor positioned at least partially within a stator. The rotor includes first and second ring segments oriented axially around a central axis. The rotor defines first and second configurations in the first and second ring segments, respectively. The first configuration is sufficiently different from the second configuration such that torque ripple may be minimized. A first layer of slots, defining a slot outer edge, may be formed in the rotor. In one embodiment, a stator-to-slot gap varies between the first and second ring segments. In another embodiment, a stator-rotor gap varies between the first and second ring segments. In another embodiment, a bridge thickness varies between the first and second ring segments. Thus the rotor exhibits axial asymmetry.

Description:
TECHNICAL FIELD 
     The present invention relates generally to electric machines, and more particularly, to the configuration of an interior permanent magnet machine. 
     BACKGROUND 
     An interior permanent magnet machine generally includes a rotor having a plurality of magnets of alternating polarity around the outer periphery of the rotor. The rotor is rotatable within a stator which generally includes a plurality of windings and magnetic poles of alternating polarity. Permanent magnet machines may produce undesirable torque ripple, resulting in unwanted vibration and noise. Traditionally, the configuration of the rotor in interior permanent magnet machines is axially symmetric. 
     SUMMARY 
     A permanent magnet machine is provided with a rotor positioned at least partially within a stator. The rotor includes first and second ring segments oriented axially around a central axis. The rotor defines a first configuration in the first ring segment and a second configuration in the second ring segment. The first configuration is sufficiently different from the second configuration such that torque ripple may be minimized. The rotor may include a third ring segment defining a third configuration. The third configuration may be different from both the first and the second configurations. Thus the rotor exhibits axial asymmetry. 
     The rotor defines a rotor outer profile while the stator defines a stator inner profile. A stator-rotor gap is defined between the rotor outer profile and the stator inner profile. The stator-rotor gap may vary between the first and second ring segments. A rotor radius is defined between the central axis of the rotor and the rotor outer profile. The rotor radius may vary between the first and second ring segments. 
     A first layer of slots, defining a slot outer edge, is formed in the rotor. A stator-to-slot gap is defined between the slot outer edge and the stator inner profile. The stator-to-slot gap may vary between the first and second ring segments. 
     A bridge thickness is defined between the slot outer edge and the rotor outer profile. The bridge thickness may vary between the first and second ring segments. In one embodiment, the bridge thickness varies while the air gap is uniform between the ring segments. In one embodiment, the bridge thickness is uniform while the air gap varies between the ring segments. 
     A first and a second layer of slots may be formed in the rotor. The first layer of slots is located at a first distance from the center axis in the first ring segment and at a second distance from the center axis in the second ring segment. The second layer of slots is located at a third distance from the center axis in the first ring segment and at a fourth distance from the center axis in the second ring segment. The first, second, third and fourth distances may be different from one another. 
     The detailed description and the drawings or figures are supportive and descriptive of the invention, but the scope of the invention is defined solely by the claims. While some of the best modes and other embodiments for carrying out the claimed invention have been described in detail, various alternative designs and embodiments exist for practicing the invention defined in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic partial perspective view of an interior permanent magnet machine showing a rotor but not a stator; 
         FIG. 2  is a schematic partial cross-sectional view through the rotor shown in  FIG. 1  and including a stator; 
         FIG. 3  is a schematic partial cross-sectional view along axis  3 - 3  of the machine shown in  FIG. 2 , in accordance with a first embodiment; and 
         FIG. 4  is a schematic partial cross-sectional view along axis  3 - 3 , in accordance with a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the Figures, wherein like reference numbers refer to the same or similar components throughout the several views,  FIG. 1  is a schematic partial perspective view of an interior permanent magnet machine  10  having a rotor  12  arranged around a center axis  14 .  FIG. 2  is a schematic partial cross-sectional view through the rotor  12 . Referring to  FIG. 2 , the rotor  12  is rotatable within a generally annular stator  16  (not shown in  FIG. 1 ) having a plurality of windings  18 .  FIGS. 3-4  are schematic partial cross-sectional views through axis  3 - 3  shown in  FIG. 2 , in accordance with two embodiments of the present disclosure. Referring to  FIGS. 2-4 , the stator  16  defines a stator inner profile  20  while the rotor  12  defines a rotor outer profile  22  and a rotor inner profile  23 . An air gap  24  (between the stator  16  and rotor  12 ) is defined between the rotor outer profile  22  and the stator inner profile  20 . 
     Referring to  FIGS. 1-4 , the rotor  12  is formed with a plurality of slots  26  that extend into the rotor  12  and define a three-dimensional volume having any suitable shape. All or a portion of the slots  26  may be filled with permanent magnets  28 . Referring to  FIG. 1 , the rotor  12  includes a plurality of poles  30 , defined by respective pole axes, one of which is generally indicated by reference numeral  32 . The slots  26  may be configured to be symmetric relative to the pole axis  32 . Each pole  30  is formed at least in part by the magnets  28  in the slots  26 . In the embodiment shown, the rotor  12  has ten poles  30 , however, it may be formed with any number of poles or slots. 
     Referring to  FIGS. 2-4 , the slots  26  may be arranged in a radially-outer first layer  34  and a radially-inner second layer  36 . The first and second layers  34 ,  36  may be formed with any number of slots  26 . Referring to  FIG. 2 , each of the slots  26  in the first layer  34  defines a first centerline  42 . Each of the slots  26  in the second layer  36  defines a second centerline  44 . Referring to  FIGS. 2-3 , the first layer  34  is located at a first radial distance  46  from the first centerline  42  to the center axis  14 . The second layer  36  is located at a second radial distance  48  from the second centerline  44  to the center axis. 
     Referring to  FIG. 2 , the slots  26  in the first and second layers  34 ,  36  define a slot outer edge  50  near the rotor outer profile  22 . A bridge thickness  52  is defined between the slot outer edge  50  and the rotor outer profile  22 , as shown in  FIGS. 2-4 . The bridge thickness  52  may be the same for the first and second layers  34 ,  36 . Referring to  FIGS. 2-4 , a stator-to-slot gap  54  is defined between the slot outer edge  50  and the stator inner profile  20 . A rotor radius  56  is defined between the rotor outer profile  22  and the central axis  14 , as shown in  FIGS. 2-4 . 
     Referring to  FIGS. 1 ,  3  and  4 , the rotor  12  may divided into at least two ring segments oriented axially around a central axis  14 . For illustrative purposes, the rotor  12  is shown with first, second and third ring segments  58 ,  60  and  62 ; however any number of ring segments may be used. In other words, the rotor  12  may have ‘n’ ring segments, where ‘n’ can be any integer. As shown in  FIG. 1 , the ring segments  58 ,  60  and  62  are shaped in the form of an annulus or ring. The second ring segment  60  is axially adjacent to the first and third ring segments  58 ,  62 . The rotor  12  is configured to have an axially asymmetric configuration, i.e., a different configuration in at least two of the ring segments  58 ,  60 ,  62 . The axially asymmetric configuration is described with respect to two embodiments: a first embodiment shown in  FIG. 3  and a second embodiment shown in  FIG. 4 . 
     The bridge thickness  52  (defined between the slot outer edge  50  and the rotor outer profile  22  and shown in  FIG. 2 ) may be different in the first, second and third ring segments  58 ,  60 ,  62 . In the first embodiment shown in  FIG. 3 , the first ring segment  58  has a first bridge thickness  70 . The second ring segment  60  has a second bridge thickness  72  which is different from the first bridge thickness  70 . The third ring segment  62  has a third bridge thickness  74 , which may be different from both the first and second bridge thicknesses  70 ,  72 . In one example, the first, second and third bridge thicknesses  70 ,  72 ,  74  are 1.2 mm, 1.5 mm and 1.6 mm, respectively. In the embodiment shown in  FIG. 3 , the air gap  24  (defined between the rotor outer profile  22  and the stator inner profile  20 ) may be uniform along the ring segments  58 ,  60 ,  62 . 
     Referring to  FIG. 2-3 , positions of the first and second layers  34 ,  36  of slots  26  relative to the central axis  14  (through the first and second radial distances  46 ,  48  described above) may be different in the first, second and third ring segments  58 ,  60 ,  62 . In one example, the first radial distance  46  is 95 mm, 94 mm and 93 mm in the first, second and third ring segments  58 ,  60 ,  62 , respectively. In one example, the second radial distance  48  is 92 mm, 90 mm and 88 mm in the first, second and third ring segments  58 ,  60 ,  62 , respectively. 
     The air gap  24  (defined between the rotor outer profile  22  and the stator inner profile  20  and shown in  FIG. 2 ) is configured to vary between the first, second and third ring segments  58 ,  60 ,  62 . Referring to  FIG. 4 , the second embodiment shows a machine  110  where the first ring segment  58  has a first air gap  170 . The second ring segment  60  has a second air gap  172  which is different from the first air gap  170 . The third ring segment  62  has a third air gap  174 , which may be different from both the first and second air gaps  170 ,  172 . In one example, the first, second and third air gaps  170 ,  172 ,  174  are 1.0 mm, 0.6 mm and 0.8 mm, respectively. 
     Stated in another way, the rotor radius  56  (shown in  FIG. 2 ) may be configured to vary between the first, second and third ring segments  58 ,  60 ,  62 . In one example, the rotor radius  56  is 99.8 mm, 100.2 mm and 100.4 mm in the first, second and third ring segments  58 ,  60 ,  62 , respectively. In the embodiment shown in  FIG. 4 , the bridge thickness  52  (defined between the slot outer edge and the rotor outer profile  FIG. 2 ) may be uniform along the first, second and third ring segments  58 ,  60 ,  62 . 
     Referring to  FIGS. 2-4 , in both embodiments, the stator-to-slot gap  54  may be configured to vary between the first, second and third ring segments  58 ,  60 ,  62 . As shown in  FIGS. 2-4 , the stator-to-slot gap  54  (defined between the slot outer edge  50  and the stator inner profile  20 ) is a sum of the air gap  24  (defined between the rotor outer profile  22  and the stator inner profile  20 ) and the bridge thickness  52  (defined between the slot outer edge  50  and the rotor outer profile  22 ). In one example, the stator-to-slot gap  54  is 1.3 mm, 1.5 mm and 1.7 mm in the first, second and third ring segments  58 ,  60 ,  62 , respectively. 
     In summary, the rotor  12  defines a first configuration in the first ring segment  58  and a second configuration in the second ring segment  60 . Each configuration may be defined by parameters such as, but not limited to, bridge thickness  52 , air gap  24 , rotor radius  56 , stator-slot gap  54 , and first and second radial distances  46 ,  48  (position of slots  26  in the first and second layers  34 ,  36 ). The first and second configurations are sufficiently different from one another in order to minimize torque ripple. The torque pulsation created by the first ring segment  58  may be reduced by the counter torque pulsation created by the second ring segment  60 , thereby minimizing torque. A third configuration may be defined in the third ring segment  62 , which may be different from both the first and second configurations. 
     The parameters described above such as bridge thickness  52 , air gap  24 , rotor radius  56 , stator-slot gap  54 , and first and second radial distances  46 ,  48  (of the first and second layers  34 ,  36  of slots  26 ) may be optimized to obtain the desired level of averaging for torque ripple reduction. The parameters may be optimized in any combination, that is, some parameters kept constant and some parameters kept as variables. This optimization may be performed empirically or through conventional computer modeling methods known in the art. By way of example only, Design of Expebridgeents (DOE) is a methodology for setting up a set of virtual or physical expebridgeents in which input variables are varied in a systematic manner, for the purpose of determining the correlation between input variables and to predict results or output, as opposed to the one-factor-at-a-time method. For example, the bridge thickness  52  in the rotor  12  may be varied and the output or torque ripple produced observed for resultant changes. In one example, the optimization may be set up with the objective that the torque ripple is between 2 and 5 Newton-meters (“Nm”). The torque ripple may be defined as the difference between the minimum and maximum torque generated during one cycle or revolution. Optionally, the optimization may be set up with the constraint that the minimum average torque generated by the machine is at least 100 Nm. Another constraint may be that the total energy loss in the system is less than or equal to 100 kilo Joules. Another constraint may be that the electromotive force or induced voltage is greater than or equal to 30 Volts. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.