Abstract:
Permanent magnet machines including doubly salient machines having one or more permanent magnets located at least partly and preferably entirely within the stator teeth, thereby avoiding weakening of the stator structure while reducing acoustic noise. The magnets may be located in only a subset of the stator teeth, thereby lowering magnet material and manufacturing costs, and all such magnets may have north poles directed toward an interior of the machine, resulting in reduced cogging and negative torques with improved torque densities. The permanent magnets may also extend within the stator teeth on an angle or diagonal, thereby allowing use of magnets which are wider than the teeth themselves to produce greater flux. Further, a magnetizing device having a single coil may be used to simultaneously magnetize all the stator magnets with a common polarity.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to electric machines, and more particularly to doubly salient machines having stator teeth with permanent magnets therein.  
         BACKGROUND OF THE INVENTION  
         [0002]    A variety of permanent magnet machines, including doubly salient machines, are known in the art in which permanent magnets are positioned within the frame or back iron of a stator. This is typically done to accommodate relatively large magnets capable of producing significant flux. As recognized by the inventor, however, positioning magnets in the stator frame tends to weaken the stator structure, including along those portions where the stator teeth meet the stator frame, and often leads to acoustic noise problems. In many designs, permanent magnets are associated with each stator pole, often with an alternating polarity for each adjacent stator tooth. As recognized by the inventor, however, such a configuration requires an excessive number of permanent magnets as well as a relatively complex device for magnetizing the stator magnets, thus increasing the complexity and manufacturing cost of the permanent magnet machine.  
         SUMMARY OF THE INVENTION  
         [0003]    In order to solve these and other needs in the art, the inventor hereof has succeeded at designing and developing permanent magnet machines, including doubly salient machines, having one or more permanent magnets located at least partly and preferably entirely within the stator teeth, thereby avoiding weakening of the stator back iron structure while reducing acoustic noise. In one embodiment, permanent magnets are located in only a subset of the stator teeth, thereby lowering magnet material and manufacturing costs. In another embodiment, all such magnets are oriented with their north poles facing inwardly (i.e., toward an interior of the machine), resulting in reduced cogging and negative torques with an improved torque density. The permanent magnets may also extend within the stator teeth on an angle or diagonal, thereby allowing use of magnets which are wider than the teeth themselves to produce greater flux.  
           [0004]    In accordance with one aspect of the present invention, a stator for use in a permanent magnet machine includes a frame having an outer peripheral edge and an inner peripheral edge extending about a central axis, a plurality of stator teeth extending from the frame&#39;s inner peripheral edge toward the central axis, and at least one permanent magnet located entirely within one of the stator teeth.  
           [0005]    In accordance with another aspect of the present invention, a stator for use in a permanent magnet machine includes a frame having an outer peripheral edge and an inner peripheral edge extending about a central axis, a plurality of permanent magnets, and a first plurality of stator teeth extending from the frame&#39;s inner peripheral edge toward the central axis. Each of the first plurality of stator teeth have one of the permanent magnets located at least partly therein. The stator also includes a second plurality of stator teeth extending from the frame&#39;s inner peripheral edge toward the central axis. The second plurality of stator teeth each have no permanent magnets located at least partly therein.  
           [0006]    In accordance with still another aspect of the present invention, a stator for use in a permanent magnet machine includes a frame having an outer peripheral edge and an inner peripheral edge extending about a central axis, a plurality of permanent magnets each having inwardly facing north poles, a first plurality of stator teeth extending from the stator frame&#39;s inner peripheral edge toward the central axis, and a second plurality of stator teeth extending from the frame&#39;s inner peripheral edge toward the central axis. The first plurality of stator teeth are each positioned directly between two of the second plurality of stator teeth. Each of the first plurality of stator teeth have one of the permanent magnets located entirely therein, while the second plurality of stator teeth each have no permanent magnets located therein.  
           [0007]    In accordance with a further aspect of the invention, a permanent magnet machine includes a stator of the type described herein.  
           [0008]    While some of the principal features and advantages of the invention have been described above, a greater and more thorough understanding of the invention may be attained by referring to the drawings and the detailed description of preferred embodiments which follow.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a cross-sectional view of a doubly salient machine according to one embodiment in which permanent magnets are positioned within the stator teeth.  
         [0010]    [0010]FIG. 2 illustrates torque curves for the doubly salient machine of FIG. 1.  
         [0011]    [0011]FIG. 3A illustrates a prior art switched reluctance motor having no stator teeth.  
         [0012]    [0012]FIG. 3B illustrates a torque curve for the conventional switched reluctance motor of FIG. 3A.  
         [0013]    [0013]FIG. 4 is a cross-sectional view of a doubly salient machine having permanent magnets in only every other stator tooth.  
         [0014]    [0014]FIG. 5 illustrates torque curves for the doubly salient machine of FIG. 4.  
         [0015]    [0015]FIG. 6 illustrates an exemplary stator coil for the doubly salient machine of FIG. 4.  
         [0016]    [0016]FIG. 7A illustrates a device for magnetizing the stator of FIG. 4.  
         [0017]    [0017]FIG. 7B is a cross-sectional view of the magnetizing device of FIG. 7A.  
         [0018]    [0018]FIG. 8 is a cross-sectional view of a doubly salient machine having north magnets in several adjacent stator teeth.  
         [0019]    [0019]FIG. 9 is a cross-sectional view of a doubly salient machine having permanent magnets extending diagonally within stator teeth.  
         [0020]    [0020]FIG. 10 illustrates one alternative to the machine of FIG. 9 in which the permanent magnets extend into the stator frame.  
         [0021]    [0021]FIG. 11 illustrates another alternative to the machine of FIG. 9 in which notches are formed adjacent to one side of each magnet-bearing stator tooth.  
         [0022]    [0022]FIG. 12 illustrates preferred profiles for the magnet-bearing and non-magnet-bearing stator teeth of the machine of FIG. 11.  
         [0023]    FIGS.  13 A-D compare torque curves for the machine of FIG. 11 with those of a prior art switched reluctance motor.  
         [0024]    Corresponding reference characters indicate corresponding features throughout the several views of the drawings. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]    A doubly salient machine according to one embodiment of the present invention is illustrated in FIG. 1 and indicated generally by reference character  100 . As shown in FIG. 1, the machine  100  includes a rotating member/rotor  102  mounted for rotation about a central axis  104 , and a stationary member/stator  106  extending about the rotor  102  in a magnetically coupled relation. The stator  106  includes a frame  108  (also referred to as the “back iron”) having an outer peripheral edge  110  and an inner peripheral edge  112  extending about the central axis  104 . The stator  106  also includes several salient stator poles/teeth  114 - 136  which extend from the inner peripheral edge  112  toward the central axis  104 , and which are spaced at equal angular intervals around the central axis  104  to form an equal number of slots spaced at equal angular intervals around the central axis  104 . The rotor  102  includes several salient rotor poles/teeth  140 - 154  which extend outwardly relative to the central axis  104 , and which are spaced at equal angular intervals about the central axis  104 , as shown in FIG. 1. The machine  100  is doubly salient in the sense that salient teeth are provided on both the rotor  102  and the stator  106 .  
         [0026]    Similar to the prior art, the stator teeth  114 - 136  each have a permanent magnet  156 - 178  associated therewith, with the permanent magnets alternating between north magnets (i.e., magnets having their north poles oriented toward the central axis  104 ) and south magnets (i.e., magnets having their south poles oriented toward the central axis  104 ) for each successive stator tooth, as shown in FIG. 1. Exemplary permanent magnet  156 , like the other permanent magnets  158 - 178 , is oriented at a generally perpendicular angle relative to a radial axis  179  along which its corresponding stator tooth  114  extends, and is preferably located adjacent to a distal end  180  of its corresponding stator tooth  114 . Thin short circuit regions  181 ,  183  are preferably formed in the stator tooth  114  adjacent to ends  182 ,  184  of exemplary magnet  156 . As is known, these regions  181 ,  183  are sufficiently thin (in a direction of rotation of the rotor  102 , for the embodiment of FIG. 1) so as to minimize leakage flux and shorting of the exemplary magnet  156 .  
         [0027]    Unlike the prior art, the permanent magnets  156 - 178  of FIG. 1 are located entirely within the stator teeth, in contrast to being partially or entirely located within the frame  108 . Positioning the permanent magnets  156 - 178  entirely within the stator teeth  114 - 136 , rather than in the frame  108 , strengthens the stator teeth where they meet the frame, especially in the case where the stator  106  employs a laminate construction. Positioning the permanent magnets in the stator teeth also raises their operating lines, especially when the salient poles of the rotor do not align with those of the stator, thus allowing the magnets to operate without demagnetizing, and provides additional coil space as well.  
         [0028]    The machine  100  of FIG. 1 operates using reluctance torque for rotating the rotor  102 , as is known. Although there are preferably no permanent magnets or coils on the rotor  102 , appropriately energizing the stator coils (not shown in FIG. 1) induces a magnetic orientation in the stator  106  and in the rotor  102  (which is a ferromagnetic material) so as to maximize their flux condition, thereby causing alignment of stator and rotor poles via rotation of the rotor  102 . A representative stator coil  600  is shown in FIG. 6, and is preferably excited such that it induces flux in the same direction as does its corresponding magnet (which, for the exemplary magnet shown in FIG. 6, is toward the central axis). A similar coil is preferably provided for each of the magnet-bearing stator teeth  114 - 136  shown in FIG. 1. These stator coils may be electrically connected in series, in parallel, or in a series-parallel arrangement as desired for any given application of the invention. The machine  100  can advantageously employ a unipolar drive (i.e., a drive employing only non-negative currents).  
         [0029]    [0029]FIG. 2 illustrates a series of torque curves for the machine  100  shown in FIG. 1 when driven by unipolar currents of zero, two and six amps. As shown in FIG. 2, the machine  100  produces significant negative torque at each current level, which obviously detracts from average torque levels. Nevertheless, average torque levels approximate those produced by a conventional switched reluctance motor of comparable size, shown generally in FIG. 3A, for which torque curves are provided in FIG. 3B.  
         [0030]    [0030]FIG. 4 illustrates a doubly salient machine  200  according to another embodiment of the invention. Similar to the machine  100  shown in FIG. 1, the machine  200  includes a stator  206  having a frame  208  from which several salient stator teeth  214 - 236  extend, with north magnets  256 - 266  located entirely within every other stator tooth  214 ,  218 ,  222 ,  226 ,  230 ,  234 . However, no magnets are positioned in the remaining stator teeth  216 ,  220 ,  224 ,  228 ,  232 ,  236 . In other words, the machine  200  of FIG. 4 includes N stator teeth, and M magnets, where M and N are both integers, and where M&lt;N and, more specifically, where M=N/2. Thus, the machine  200  contains only half the magnets of the machine  100  of FIG. 1, thereby reducing costs and simplifying construction. Further, by eliminating the south magnets  158 ,  162 ,  166 ,  170 ,  174 ,  178  employed in the machine  100  of FIG. 1, the machine  200  of FIG. 4 has a reduced cogging torque and reduced negative torque, thereby yielding an improved torque density. The machine  200  of FIG. 4 is also more readily magnetized, as further explained below. Like the machine  100  of FIG. 1, the machine  200  can advantageously employ a unipolar drive.  
         [0031]    [0031]FIG. 5 illustrates a series of torque curves for the machine  200  of FIG. 4 when driven by unipolar currents of zero, two and six amps. As shown in FIG. 5, the machine  200  produces average torque levels which are higher than those produced by the conventional switched reluctance motor of FIG. 3A, for which torque curves are provided in FIG. 3B. The machine  200  also produces significantly less negative torque than the machine  100  of FIG. 1, thereby enhancing average torque levels. As can be seen in FIG. 5, the machine  200  also produces significantly more positive torque than the machine  100  of FIG. 1.  
         [0032]    [0032]FIGS. 7A and 7B illustrate a preferred device  700  for magnetizing the stator  206  of FIG. 4. As best shown in FIG. 7B, the device  700  includes a preferably cylindrical post  702  and a single robust coil  704  extending about a central axis  706  and through a non-magnetic region  705  of the device  700 . The device  700  preferably also includes a support surface  708  for supporting the stator  206  (shown in phantom), as well as an outer peripheral wall  710 . To magnetize the magnets  256 - 266  in the stator  206 , the stator is first positioned relative to the device  700  with the stator&#39;s central axis  104  generally parallel and, more preferably, coaxial with the central axis  706  of the coil  704 , and with the post  702  extending adjacent to the stator teeth  214 - 236 , preferably with the stator  206  supported by the support surface  708 , as best shown in FIG. 7B. The single coil  704  is then energized, thus inducing flux to pass up through the post  702 , across air gaps  714 - 736 , through the stator teeth  214 - 236  (including through the magnets  256 - 266 ), down through the outer peripheral wall  710 , and back to the single coil  704 , as indicated generally by arrows  740 ,  742  in FIG. 7B (assuming the coil  704  is energized so as to effectively render the post  702  a north pole; otherwise, the direction of arrows  740 ,  742  would be reversed). In this manner, the magnets  256 - 266  in the stator  206  can be readily and simultaneously magnetized with a common polarity. The device of FIG. 7 may also be used to magnetize stators according to other embodiments of the invention, including those described below with reference to FIGS.  8 - 11 .  
         [0033]    [0033]FIG. 8 illustrates a machine  300  according to another embodiment of the invention in which only north magnets  356 - 366  are employed. As shown therein, three adjacent stator teeth  314 - 318  each have north magnets  356 - 360  located entirely therein, followed by three stator teeth  320 - 324  having no permanent magnets, and so on. Though suitable for certain applications, the machine  300  of FIG. 8 produces more torque ripple, a higher cogging torque and less torque density than the machine  200  of FIG. 4.  
         [0034]    [0034]FIG. 9 illustrates a doubly salient machine  400  according to another embodiment of the invention. The machine  400  is much like the machine  200  of FIG. 4, in that north magnets  456 - 466  are positioned entirely within every other stator tooth  414 ,  416 ,  422 ,  426 ,  430 ,  434  with no magnets provided in intervening stator teeth  416 ,  420 ,  424 ,  428 ,  432 ,  436 . However, exemplary magnet  456 , like the other permanent magnets  458 - 466 , has its north pole oriented at an oblique (i.e., non-perpendicular) angle relative to the radial axis  479  along which its corresponding stator tooth  414  extends. This allows wider magnets to be employed in the stator teeth, as compared to the magnets employed in the machine  200  of FIG. 4. Thus, as shown in FIG. 9, exemplary magnet  458  has a width  492  that is greater than a width  494  of the corresponding stator tooth  418  in which it is located. Consequently, the magnets  456 - 466  employed in the machine  400  of FIG. 9 have more surface area than the magnets  256 - 266  employed in the machine  200  of FIG. 4, allowing them to produce more flux. At the same time, the magnets  456 - 466  are still positioned entirely within the stator teeth  414 ,  418 ,  422 ,  426 ,  430 ,  434 , as shown in FIG. 9, so as to avoid the disadvantages associated with magnets extending in the frame. With reference to another exemplary magnet  460 , note that its ends  468 ,  469  are generally parallel to sides  470 ,  471  of its corresponding stator tooth  422 , with thin short circuit regions  472 ,  473  provided between the ends  468 ,  469  and the sides  470 ,  471 .  
         [0035]    [0035]FIG. 10 illustrates a doubly salient machine  500  according to another embodiment of the invention in which magnets  556 - 566  extend from within stator teeth  514 ,  518 ,  522 ,  526 ,  530 ,  534  into frame  508  such that the surface area of the magnets  556 - 566  is further increased, as compared to the magnets  456 - 466  of FIG. 9, with a further increase in produced flux. Note that, in FIG. 10, one end  570  of exemplary magnet  558 , which extends into the frame  508 , is bent so as to form a thin short circuit region  571  between that end  570  of the magnet and the inner peripheral edge  512  of the stator  506 . The other end  572  of the exemplary magnet  558  is also bent so as to form a short tip  573  which extends from the bend toward the central axis  104 . This magnet configuration incrementally increases the surface area of the magnets  556 - 566 , again increasing the amount of flux produced. However, because the magnets  556 - 566  of FIG. 10 extend partly into the frame  508 , the strength of the stator teeth  514 ,  518 ,  522 ,  526 ,  530 ,  534  where they meet the frame  508  is reduced, and the amount of acoustic noise generated is increased, as compared to the machine  400  of FIG. 9.  
         [0036]    [0036]FIG. 11 illustrates a doubly salient machine  600  according to still another embodiment of the present invention. This machine  600  incorporates the advantages of the machine  400  of FIG. 9, in which angled magnets are positioning entirely within the stator teeth, and the advantages of the machine  500  of FIG. 10, which employs magnets having greater surface areas than those of FIG. 9 at the expense of having such magnets extend into the stator frame  508 . As shown in FIG. 11, notches  668 - 678  are provided adjacent to one side of each stator tooth  614 ,  618 ,  622 ,  626 ,  630 ,  634  in which a north magnet  656 - 667  is positioned. The notches  668 - 678  extend from the inner peripheral edge  612  of the stator  606  into the frame  608 , and essentially increase the length of such stator teeth  614 ,  618 ,  622 ,  626 ,  630 ,  634  on one side thereof. This allows the magnets  656 - 667 , which have a greater width than those of FIG. 9, to be employed while at the same time preventing such magnets from extending into the frame  608 . Thus, the magnets  656 - 667  of FIG. 11 produce more flux than those of FIG. 9, but the machine  600  of FIG. 11 produces acoustic noise levels below those of the machine  500  of FIG. 10. Note that exemplary notch  668  forms a thin short circuit region  679  adjacent one end  680  of the exemplary magnet  656 , which would otherwise (i.e., in the absence of the exemplary notch  668 ) extend into the stator frame  608 .  
         [0037]    Note that torque curves for the machines of FIGS.  9 - 11  are shaped generally the same as those shown in FIG. 5 for the machine  400  of FIG. 4. However, the positive and negative torque levels for the machines of FIGS.  9 - 11  are somewhat greater than those of FIG. 5 due to the increased magnet areas employed in FIGS.  9 - 11 .  
         [0038]    With further reference to FIG. 11, the magnet-bearing stator teeth  614 ,  618 ,  622 ,  626 ,  630 ,  634  and the non-magnet-bearing stator teeth  616 ,  620 ,  624 ,  628 ,  632 ,  636  are preferably shaped differently to further reduce noise. This is shown more clearly in FIG. 12, which depicts a profile  750  for one of the magnet-bearing stator teeth of FIG. 11 superimposed over a profile  760  (shown in phantom) for one of the non-magnet-bearing stator teeth of FIG. 11. As shown in FIG. 12, the profile  750  for the magnet-bearing stator teeth extends in a generally circumferential direction relative to the central axis  104 , including along its end regions  752 ,  754 . In contrast, the profile  760  for the non-magnet-bearing stator teeth has end regions  762 ,  764  which taper inwardly toward the inner peripheral edge  612 . As a result, air gaps formed between the end regions  762 ,  764  of the non-magnet-bearing stator teeth and the rotor teeth are greater than air gaps formed between the end regions  752 ,  754  of the magnet-bearing stator teeth and the rotor teeth. As discovered by the inventor, this markedly reduces the acoustic noise generated by the machine  600 . The non-magnet-bearing stator teeth in other embodiments of the invention, including those shown in FIGS. 4, 8,  9  and  10 , may also embody end regions which taper inwardly toward the inner peripheral edge of the stator frame to a greater extent than do end regions of the magnet-bearing-stator teeth (if at all) so as to provide a different air gap profile, which reduces noise.  
         [0039]    FIGS.  13 A-D compare torque curves for the machine  600  of FIG. 11 and a comparably sized conventional switched reluctance motor when the motors are driven at unipolar currents of 2.2, 4.5, 5.5 and 6.4 amps, respectively. As shown in FIGS.  13 A-D, the machine  600  of FIG. 11 produces significantly more torque at each current level. The machine  600  of FIG. 11 also has an improved torque density as compared to a conventional switched reluctance motor of comparable size, thus providing more torque for a given motor size, and an improved torque density for a given speed. Further, the machine of FIG. 11 (as well as other embodiments of the invention) has a wide speed range/constant power range. Accordingly, the teachings of the present invention are applicable to a substantial number of electric motor applications, including but not limited to those requiring wide speed ranges such as, e.g., electric vehicles.  
         [0040]    Although not shown in FIGS. 4, 8 and  9 - 11 , the magnet-bearing stator teeth shown therein each preferably employ a stator coil of the type shown in FIG. 6, and these coils are all preferably excited in such a manner as to align each coil with its corresponding magnet&#39;s polarity.  
         [0041]    The exemplary embodiments of the invention described herein and shown in the drawings all employ block magnets, which are generally less expensive than, e.g., arc magnets. It should be understood, however, that a wide variety of magnets may be employed without departing from the scope of the invention. Further, while these exemplary embodiments all employ a stator having twelve salient stator poles/teeth and a rotor having eight salient rotor poles/teeth, the invention is not so limited, as will be apparent to those skilled in the art.  
         [0042]    As noted above, the exemplary embodiments of the invention described herein can each be driven with unipolar currents, in contrast to bipolar currents such as those used in most brushless permanent magnet (“BPM”) machines. With many embodiments of the invention, employing a unipolar drive ensures the the flow of current is always in a direction to assist the permanent magnets, thereby minimizing or eliminating demagnetization issues.  
         [0043]    When introducing features of the present invention or the preferred embodiments thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more such features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional features beyond those noted.  
         [0044]    As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.