Abstract:
A flux barrier type synchronous reluctance motor in which a plurality of flux barriers are provided in a rotor, of which a plurality of ribs form the end portion of each of the flux barriers, wherein assuming that both ends of a single flux barrier are a first and a second rib, when the first rib faces the center of a slot of a stator, the second rib faces the center of a teeth of the stator.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a synchronous reluctance motor having a plurality of flux barriers, and more particularly, to a barrier type synchronous reluctance motor with an improved flux barrier that is capable of reducing torque ripple. 
     2. Description of the Background Art 
     Generally, two axes are used for controlling a motor: one is a D axis, that is, a boundary of a magnetic pole, and the other is a Q axis, that is, a center of the magnetic pole. The D axis has a high magnetic permeability, and the Q axis has a low magnetic permeability. A torque is generated due to an inductance difference between the D axis and the Q axis. 
     FIG. 1 is a plan view of a synchronous reluctance motor having flux barriers in accordance with the conventional art. 
     For the convenience of explanation, an example is taken where there are four poles and 24 stator slots. 
     As shown in FIG. 1, a stator  10  includes a body  11 , a plurality of slots  13  and teeth  14  formed at the inner circumferential surface  12 . A rotor  20  includes a body  21 , a plurality of flux barriers  22  and a plurality of ribs. 
     As to the ribs, there are provided real ribs  23  (demonstrated by a circle), that is, the end portion of the flux barrier and virtual ribs  24  (demonstrated by ‘x’)., and the interval therebetween (including the actual rib  23  and the virtual rib  24 ) is the same as the pole pitch of the rotator slot. 
     The body  21  of the rotor  20  is made of magnetic substance, and the plurality of flux barriers  22  is made of a non-magnetic substance, which are radially arranged having the Q axis as a center and D axis as a boundary. 
     The operation of the synchronous reluctance motor having the flux barriers in accordance with the conventional art will now be described. 
     First, an inductance is generated by a current applied to the winding coil of the stator  10 , and a torque is generated due to the inductance difference between the D axis and the Q axis owing to the flux barrier  22 , and the rotor is rotated by the torque. 
     Torque ripple is generated due to the relative position of the flux barrier and the slot opening  13   a  of the stator  10 . In other words, the torque ripple is generated due to a variation of the relative position of the slot opening  13   a  of the stator  20  and the rotor rib  23 . 
     FIGS. 2 and 3 are plan views of the synchronous reluctance motor in the case where the rotor  20  is placed at specific positions when the motor of FIG. 1 is operated. FIG. 2 shows the case where the ribs  23  and  24  of the rotor  20  face the slot opening  13   a  of the stator  10 , and FIG. 3 shows the opposite case to that of FIG. 2, that is, the case where the rib  23  of the rotor  20  faces the teeth  14  of the stator  10 . 
     The magnetic resistances which develops in each of the first and the second cases are much different to each other, and due to the magnetic resistance, variations a substantial torque ripple is generated when the rotor  20  is rotated. 
     For that reason, the torque ripple is substantial when the number of the rotor ribs (the actual rib  23  and the virtual rib  24 ) is the same as that of the stator slots and when the interval between the ribs is the same as that of the slots. 
     Since the torque ripple is determined by the arrangement structure of the flux barrier, to reduce the torque ripple, various techniques for designing the structure of the flux barrier have been developed and disclosed, which will now be described. 
     FIG. 4 shows a flux barrier type synchronous reluctance motor having a plurality of flux barriers in accordance with a conventional art, in which the number of the real ribs of the flux barrier  22  is greater than that of the slots  13 . 
     In this case, even though some of the ribs face the slots  13 , since the remaining other ribs face the teeth  14 , the average value of the overall magnetic resistance is lowered although the magnetic resistance at specific portions remains quite large. 
     However, as to this kind of rotor having the plurality of flux barriers, since the structure of the rotor is complicated, its mechanical strength is weakened as compared to a rotor having a few flux barriers, and its fabrication is also difficult. This problem becomes more serious as the size of the rotor is reduced. 
     FIG. 5 is a plan view of a flux barrier type synchronous reluctance motor where D and Q axes are mechanically unbalanced in accordance with the conventional art. 
     Unlike the D axis of a general motor, the D axis is slanted toward one side as much as a predetermined angle θ, so that the D and Q axes are mechanically unbalanced. 
     With this structure, since the spacial intervals of the ribs of the flux barriers  22  and the intervals of the slots  13  are not the same, the torque ripple is reduced. However, since the D axis and the Q axis are unbalanced, it is difficult to judge electric D axis and Q axis when the motor is controlled, and the inductance values Ld and Lq according to the D and Q axes are influenced, causing a reduction of the overall torque output. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a flux barrier type synchronous reluctance motor by which the average value of the magnetic resistance generated according to the rotation of a rotor is constant regardless of the position of the rotor. 
     Another object of the present invention is to provide a flux barrier type synchronous reluctance motor of which the intervals between each rib, that is, the end portion of the flux barrier, are not uniform as a whole. 
     Still another object of the present invention is to provide a flux barrier type synchronous reluctance motor in which the rotor has a simple structure. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a flux barrier type synchronous reluctance motor in which a plurality of flux barriers are provided in a rotor, of which a plurality of ribs form the end portion of each of the flux barriers, wherein assuming that both ends of a single flux barrier are a first and a second ribs, when the first rib faces the center of a slot of a stator, the second rib faces the center of a teeth of the stator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     In the drawings: 
     FIG. 1 is a plan view of a synchronous reluctance motor having flux barriers in accordance with the conventional art; 
     FIGS. 2 and 3 are plan views in case that a rotor is placed at a specific position when a motor of FIG. 1 is rotated in accordance with the conventional art; 
     FIG. 4 is a plan view of a flux barrier type synchronous reluctance motor having a plurality of flux barriers in accordance with the conventional art; 
     FIG. 5 is a plan view of a flux barrier type synchronous reluctance motor having a structure that D and Q axes are mechanically unbalanced in accordance with the conventional art; 
     FIG. 6 is a plan view of a synchronous reluctance motor having flux barriers in accordance with the present invention; 
     FIG. 7 illustrates arrangements of the flux barriers of FIG. 6 in accordance with the present invention; 
     FIGS. 8 and 9 illustrate an instant position of a rotor when the motor is rotated in accordance with the present invention; and 
     FIG. 10 illustrates a magnetic flux pattern according to a numerical analysis for the structure of FIG. 9 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     As mentioned above, the interval between the ribs (including all of the actual rib  23  and the virtual rib  24 ) is the same as the pole pitch of the stator slot in the conventional art. 
     But, in the present invention, the intervals between the ribs is not the same as the pitch of the stator slot, and the intervals of ribs are not uniform. 
     That is, assuming that the both ends of the flux barrier are a first rib and a second rib, when the first rib is facing the stator slot, the second rib is facing the teeth of the stator. 
     FIG. 6 is a plan view of a synchronous reluctance motor having flux barriers in accordance with the present invention. 
     For the convenience of the explanation, an example is given in which there are four poles and 24 stator slots. 
     As shown in FIG. 6, the stator  30  includes a plurality of slots  33  and teeth  34  formed in a body  31  and its inner circumferential surface  32 . 
     The rotor  40  includes a body  41 , a plurality of flux barriers  42  each having predetermined width and length and a plurality of ribs  43  and  44 . The ribs includes the actual rib  43  (indicated by a circle) making the end of the flux barrier and a virtual rib  44  (indicated by ‘x’). 
     The number of the flux barriers  42  per pole of the rotor  40  can be designed by the following two methods. 
     One method is that the number Nb of the flux barriers per pole of the rotor is designed by the following equation as expressed below:              Nb   =       Ns     2      P       -   1             (   1   )                                
     Where P indicates the number of poles of the rotor, and Ns indicates the number of the slots of the stator. 
     In case that the equation (1) is applied to the embodiment as shown in FIG. 6, Ns is 24 and P is 4, so that Nb becomes 2. 
     That is, the number of the flux barriers  42  per pole of the rotor  40  centering around the Q axis, that is, the center of the magnetic pole, is 2. 
     In this respect, in case the slots per pole is 6, the flux barriers are 2 in number, so that actual ribs  43 , that is, the both ends of the flux barriers are 4 in number. 
     According to the other method, the number Nb of the flux barriers per pole of the rotor is expressed in the following equation:              Nb   =     Ns     2      P               (   2   )                                
     The number Nb of the flux barriers computed by the equation (2) is 3, one more than the number Nb of the flux barriers computed by the equation (1), which is not shown in the drawing. 
     In this case, when the slots per pole are 6 in number, the flux barriers are 3 in number, so that the actual ribs  43  making the both ends of the flux barriers is 6 in number. 
     The flux barriers may be selectively used in case according to the above two methods. 
     Meanwhile, an important thing is the relative position with respect to the ribs  43  and  44  of the flux barriers and the stator slots (or teeth). 
     The ribs of the flux barriers shown in FIG. 6 are slanted somewhat inwardly to the position facing the slot of the stator on the Q axis that is the central axis of the magnetic pole. 
     In other words, compared to that of the conventional art, the ribs  43  and  44  of the flux barriers are somewhat slanted to the Q axis that is the central axis of the magnetic pole. 
     Such position of the flux barriers will now be described in detail with reference to FIG.  7 . 
     FIG. 7 illustrates arrangement of the flux barrier of FIG. 6, of which reference numerals R 1 ˜R 6  are positions of the rotor corresponding to the central positions of the stator slots  33 , which are also the same positions of the ribs (including the actual rib and the virtual rib) of the flux barriers in accordance with the conventional art. 
     Reference numerals r 1 ˜r 6  are the positions of the ribs of the flux barriers in accordance with the present invention, which are notably somewhat slanted toward Q axis compared to the positions R 1 ˜R 6 . 
     In this respect, the positions r 3  and r 4  correspond to the positions corresponding to the virtual ribs according to the equation (1), and corresponds to the positions corresponding to the actual ribs according to the equation (2). 
     The distance θ corresponds to one pitch (pole pitch) of the stator slot. The changed positions r 1 ˜r 6  are shifted toward Q axis as long as θ/4 of the pole pitch from the previous positions R 1 ˜R 6 . 
     The first and the sixth positions r 1  and r 6 , the second and the fifth positions r 2  and r 5 , the third and the fourth positions r 3  and r 4  makes the both ends of the flux barriers by pairs, and since the third and the fourth positions r 3  and r 4  are virtual ribs according to the equation (1), there do not exist flux barriers corresponding to them. 
     The description that the both ribs is shifted toward the center as long as ¼(θ/4) of the pole pitch can be also expressed in that one rib is shifted as long as half (θ/2) of the pole pitch. 
     Assuming that the angle between the Q axis and the position of the rib is αm, the space angle α m  can be expressed by the following equation (3):                  α   m     =         (       2      m     -   1     )        θ     -     θ   2         ,     m   ≤     Ns     2      P                 (   3   )                                
     Where, θ indicates a pole pitch, m indicates the order of the ribs distanced from the Q axis, as integers, for which m value for the positions r 3  and r 4  is 1, m value for the positions of R 2  and r 5  is 2, and m value for the positions r 1  and r 6  is 3. 
     When the equation (3) is applied to the case of 4 poles and 24 slots, the space angle α 1  between the nearest rib from the Q axis and the Q axis is 7.5°, the space angle α 2  is 37.5°, and the space angle α 3  is 67.5°. 
     FIGS. 8 and 9 illustrates instant positions of the rotor when the motor is rotated, of which FIG. 8 illustrates a state where the positions of the flux barrier is are balanced on the Q axis, and FIG. 9 illustrates a state where the ribs r 1  and r 2  at one side of the flux barrier are facing the center of the slot, the ribs r 5  and r 6  at the other side of the flux barrier are facing the center of the teeth. 
     By this design, when the rotor is rotated, one rib out of two ribs comprising the two ends of the flux barrier is facing the center of the stator slot, and the other rib is facing the center of the tooth of the stator. 
     FIG. 10 shows a flux pattern according to the numerical analysis for the structure of FIG.  9 . As shown in the drawing, the overall average value of the magnetic reluctance is low. 
     As described above, in a synchronous reluctance motor according to the present invention, while the rotor is being rotated, the average of the magnetic reluctance becomes constant regardless of the positions of the rotor, and thereby the torque ripple is reduced, resulting in a stably driven motor. Since the structure of the rotor is simple, it is easy to fabricate, and thus, the production cost can be reduced. 
     As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof. It should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalents of such meets and bounds are therefore intended to be embraced by the appended claims.