Abstract:
A stepping motor  10  having a configuration which includes: a rotor  14  constituted by a rotor magnet  14   b  magnetized circumferentially; a stator  13  constituted by a plurality of phases of stator yokes  16   a  to  16   h  arranged side by side in an axial direction of the stepping motor so as to surround the rotor, and coils  17   a  to  17   d  wound in coil winding portions defined by the stator yokes; an armor member  11, 12  for holding the stator yokes with respect to the axial direction of the stepping motor; and each phase of the stator yokes being constituted by a pair of stator yokes on which magnetic pole teeth of plural phases arranged annularly are combined alternately; wherein the stator yokes are arranged so that every two phases of the stator yokes are paired to form an even number of sets of stator yokes; wherein two phases a and b and c and d in each set of the stator yokes are reverse to each other; and wherein coils  17   a  and  17   b  and  17   c  and  17   d  provided on the two phases of stator yokes in each set are connected to generate magnetic fields in series and in reverse to each other.

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention relates to a stepping motor, and particularly relates to a stepping motor comprising a stator provided with stator yokes each having magnetic pole teeth. 
     2. Related Art 
     Heretofore, a stepping motor is configured, for example, in such a manner as shown in FIGS. 8 and 9. 
     That is, as shown in FIG. 8, a stepping motor  1  includes a hollow cylindrical housing  2  having an open end, a bracket  3  for closing the open end of the housing  2 , a stator  4  provided along the inner circumferential surface of this housing  2 , and a rotor  5  supported rotatably by bearing portions  2   a  and  3   a  provided in the housing  2  and the bracket  3  respectively. 
     The above-mentioned housing  2  is made of metal material and provided with the bearing portion  2   a  in a position near the center of the closed end surface of the housing  2 . 
     The above mentioned bracket  3  is made of metal material and provided with the bearing portion  3   a  in a position near the center of the bracket  3  in the same manner as in the housing  2 . 
     As shown in FIG. 9, the above-mentioned stator  4  is constituted by a substantially cylindrical yoke unit  6  in which four stator yokes  6   a  to  6   d  each provided with a ring-like flange portion having an outer diameter substantially equal to the inner diameter of the housing  2  so as to be stored in the housing  2  are integrally molded with a bobbin  7  of resin, and coils  8   a  and  8   b  wound on coil winding portions defined by the stator yokes  6   a  and  6   b  and by  6   c  and  6   d  respectively. The stator  4  is stored and fixedly held in the housing  2  after the coils  8   a  and  8   b  have been wound. 
     The above-mentioned rotor  5  is constituted by a rotating shaft  5   a  rotatably supported by the above-mentioned bearing portions  2   a  and  3   a , and a hollow cylindrical rotor magnet  5   b  fixedly held on the rotating shaft  5   a . This rotor magnet  5   b  is formed to have an outer diameter slightly smaller than the inner diameter of a central hole of the yoke unit  6 . The rotor magnet  5   b  is magnetized circumferentially. 
     Here, the above-mentioned stator yokes  6   a  to  6   d  are configured, for example, as shown in FIG.  10 . 
     Since all the stator yokes  6   a  to  6   d  have the same shape, only the stator yoke  6   a  is shown in FIG.  10 . 
     In FIG. 10, the stator yoke  6   a  is made of conductive metal material, and constituted by a ring-like flange portion  6   e  and a large number of magnetic pole teeth  6   f  provided to erect vertically from an inner circumferential portion of this ring-like flange portion  6   e.    
     The four stator yokes  6   a  to  6   d  are combined so that the stator yokes  6   b  and  6   c  are put back to back with each other, and the magnetic pole teeth  6   f  of one pair of the stator yokes  6   a  and  6   b  are shifted in phase by a predetermined value from those of the other pair of the stator yokes  6   c  and  6   d . In this state, the stator yokes  6   a  to  6   d  are stored in a mold for molding the bobbin  7 , so that the bobbin  7  of resin is integrally molded around the stator yokes  6   a  to  6   d . The yoke unit  6  is completed thus. 
     In the illustrated case, the stepping motor  1  has a flange  9  on the closed end side of the housing  2  so as to be attached by means of screws or the like to an electronic equipment utilizing the stepping motor  1 . 
     According to the stepping motor  1  configured thus, when a driving current is made to flow into the respective coils  8   a  and  8   b  of the stator  4 , magnetic fields generated in the coils  8   a  and  8   b  interact with the magnetic field of the rotor magnet  5   b  through the respective stator yokes  6   a  to  6   d  of the yoke unit  6 . Then, the rotor  5  is driven and rotated intermittently by the effect of the magnetic pole teeth  6   f  of the respective stator yokes  6   a  to  6   d.    
     However, the stator yokes  6   a  and  6   d  located on the opposite ends are in tight contact with the end surface of the housing  2  or the surface of the bracket  3  while the stator yokes  6   b  and  6   c  located inside are in back-to-back contact with each other. Therefore, magnetic flux leaks out of the magnetic circuits constituted by the pair of stator yokes  6   a  and  6   b  and the pair of stator yokes  6   c  and  6   d  together with the coil  8   a  and  8   b  respectively so that magnetic interference is generated between the magnetic circuits. 
     Here, in the case where the stepping motor  1  is driven by so-called single-phase excitation, and when, for example, the rotor  5  stops sequentially at rotor stop positions shown by the symbols S 1 , S 2 , S 3 , S 4  . . . with respect to the magnetic pole teeth  6   f  of the stator yokes  6   a  to  6   d  as shown in FIG. 11, theoretically, the magnetic center is switched as shown by the symbols A, C, B and D. 
     However, when the above-mentioned leakage of magnetic flux is generated, for example, at the stop position S 2 , the magnetic flux of the stator yoke  6   c  slightly leaks also to the stator yoke  6   b  so that the magnetic center is slightly shifted from the position C toward the position B. Accordingly, the position where the rotor  5  stops actually is also shifted to the position B side. 
     Similarly, at the stop position S 3 , the magnetic flux of the stator yoke  6   b  slightly leaks also to the stator yoke  6   c  so that the effects of excitation of the yokes  6   b  and  6   c  are canceled with each other. Accordingly, the magnetic center is slightly shifted from the position B toward the position C. Accordingly, the position where the rotor  5  stops actually is also shifted to the position C side. 
     In order to reduce such shift of the stop position of the rotor  5  due to the leakage of magnetic flux, heretofore, there are a method in which the dimensions of the comb-like magnetic pole teeth  6   f  in each phase are changed to balance the magnetic force among the respective pole teeth portions  6   f ; a method in which the angle with which the stator yokes  6   b  and  6   c  are combined is slightly shifted from a theoretical angle in order to correct the stop position; and so on. However, since scattering of the magnetic force among the respective magnetic pole teeth  6   f  changes in accordance with the combination of the condition of winding of the coils, the intensity of the magnetic force of the rotor, and so on, it is difficult to correct the displacement of the stop position perfectly. 
     On the other hand, in the case where the stepping motor  1  is driven by so-called two-phase excitation, and when, for example, the rotor  5  stops sequentially at rotor stop positions shown by the symbols S 1 , S 2 , S 3 , S 4  . . . with respect to the magnetic pole teeth  6   f  of the stator yokes  6   a  to  6   d  as shown in FIG. 12, theoretically, the positions D and A are excited at the stop position S 1  so that the magnetic center coincides with the middle position between the positions D and A, that is, the stop position S 1 . Similarly, the positions A and C are excited when the rotor  5  stops at the stop position S 2 , the positions C and B are excited at the stop position S 3 , and the positions B and D are excited at the stop position S 4 , so that the magnetic centers in the respective cases theoretically coincide with the middle positions between the excited positions, that is, the stop positions S 2 , S 3  and S 4 , respectively. 
     However, when the above-mentioned leakage of magnetic flux is generated, for example, at the stop position S 2 , the magnetic force of the housing  2  is given to the stator yoke  6   a  so that the magnetic force in the position A becomes intensive more than that in the position C. As a result, the magnetic center is slightly shifted toward the position A. Accordingly, the position where the rotor  5  stops actually is also shifted on the position A side. 
     Similarly, at the stop position S 4 , the magnetic force of the bracket  3  is given to the stator yoke  6   d  so that the magnetic force in the position D becomes intensive more than that in the position B. As a result, the magnetic center is shifted toward the position D. Accordingly, the position where the rotor  5  stops actually is also shifted to the position D side. 
     Further, in the stepping motor  1  configured thus, the stator yokes  6   a  to  6   d  are generally made of material having a thickness t of 1.0 or 0.8 mm as shown in FIG.  13 (A). 
     In the case where the step angle, that is, the interval between the above-mentioned stop positions is made small, if the tooth width w of each of the magnetic pole teeth  6   f  of the stator yokes  6   a  to  6   d  is made narrow, that is, not larger than the thickness t, not only the stator yokes  6   a  to  6   d  are difficult to be finished, but also the accuracy of finishing becomes low. 
     It was therefore difficult to practically manufacture a stepping motor which was small both in size and in step angle, for example, a stepping motor in which the diameter was not larger than 50 mm and the step angle was not larger than 1.8 degrees. 
     In addition, when the thickness t of each of the stator yokes  6   a  to  6   d  was made thinner than the tooth width w of each of the magnetic pole teeth in order to improve the accuracy of finishing, the saturated magnetic capacities of the stator yokes  6   a  to  6   d  were lowered so that the magnetic force of the stator  4  as a whole was lowered. Therefore, there was a problem that the torque of the stepping motor  1  was lowered. 
     Further, in order to prevent the torque of the stepping motor  1  from being lowered, it is general that the length L of each of the magnetic pole teeth is made to be, for example, 1.5 times as large as an usual length, while the tooth width w is not changed, so that the area where each of the stator yokes  6   a  to  6   d  is opposite to the rotor  5  is increased. 
     However, when the length L of each of the magnetic pole teeth is increased, the magnetic inertia of magnetism emitted from the respective stator yokes  6   a  to  6   d  to the rotor  5  becomes high. As a result, when excitation is changed over at a high frequency, magnetic responsibility becomes low. Therefore, there was a problem that the high-speed rotation characteristic of the stepping motor  1  deteriorated. This is because the step angle is small so that the motor rotation speed becomes too low when the stepping motor  1  is driven at a low frequency. 
     SUMMARY OF INVENTION 
     Taking the foregoing problems into consideration, it is an object of the present invention to provide a stepping motor having a simple configuration, which is small both in size and in stepping angle, large in torque and capable of high-speed rotation. 
     In order to achieve the above object, according to the present invention, as stated in claim  1 , there is provided a stepping motor comprising: a rotor constituted by a rotor magnet magnetized circumferentially and a rotating shaft attached to a center of the rotor magnet; a stator constituted by a plurality of phases of stator yokes arranged side by side in an axial direction of the stepping motor so as to surround the rotor, and coils wound in coil winding portions defined by the stator yokes; an armor member for holding the stator yokes with respect to the axial direction of the stepping motor; and each phase of the stator yokes being constituted by a pair of stator yokes on which magnetic pole teeth of plural phases arranged annularly are combined alternately; wherein the stator yokes are arranged so that every two phases of the stator yokes are paired to form an even number of sets of stator yokes; wherein two phases in each set of the stator yokes are reverse to each other; and wherein coils provided on the two phases of stator yokes in each set are connected to generate magnetic fields in series and in reverse to each other. 
     According to the above-mentioned configuration stated in the present invention, coils wound on two phases of stator yokes constituting each set are made reverse so that currents flow in the coils reversely to each other. Thus, the mutual magnetic interference can be reduced to a low level. 
     Therefore, any stator yoke of any phase in one set is subjected to magnetic interference from phases adjacent to the stator yoke in question, but magnetic interference is given to the stator yoke in question merely from another set adjacent to the one set in question. Accordingly, the magnetic interference from the adjacent phases can be reduced. 
     Since magnetic interference given to any stator yoke in any phase by stator yokes adjacent to the stator yoke in question is reduced in such a manner, the position accuracy of the step angle is improved. 
     In addition, even if the material forming the stator yokes is thin, the stator yoke material is high in electric resistance and low in core loss so that the high-frequency magnetic responsibility of the stator yoke material is improved. Accordingly, the high-frequency responsibility of the motor is also improved. In addition, the saturated magnetic capacity can be ensured because two phases of stator yokes are used as one set. It is therefore possible to prevent the driving torque from being lowered. 
     As a result, if the stator yoke material is made thin, the workability is not spoiled even if the tooth width of each of magnetic pole teeth is made smaller than that in the background art. It is therefore possible to form a stepping motor having a small step angle. 
     Further, since the stator is constituted by a plurality of sets of four-phase stator yokes, the total magnetic force of the rotor is also divided into four phases, so that the detent torque is reduced. It is therefore possible to reduce driving noise at the time of continuous driving. In this case, however, the number of stator yokes is required to be twice as large as that in the background art. However, since the stator yokes may be made thin, for example, to be half as thick as that in the background art, it is possible to restrain the material cost to be substantially equal to that in the background art. 
     In addition, since the structure of the stepping motor as a whole is substantially similar to that in the background art, the steps of assembling the stepping motor are similar to those in the background art. Therefore, the stepping motors of the present invention can be manufactured by using parts and manufacturing lines which have been used for stepping motors in the background art. It is therefore unnecessary to make an investment in additional equipment. 
     In the stepping motor stated in the present invention, the stator yokes are arranged so that in every adjacent two sets of the stator yokes, one of the adjacent two sets are shifted in phase by 90 degrees from the other one of the adjacent two sets. 
     In configuration described above, leakage of magnetic flux from one stator yoke to another stator yoke is generated only between the stator yokes  16   d  and  16   e  which are shifted in phase by 90 degrees from each other. The other stator yokes are all arranged to be reverse in phase to one another so that there is no fear that leakage of magnetic flux, that is, magnetic interference is generated among the stator yokes. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic sectional view showing a configuration of an embodiment of a stepping motor according to the present invention; 
     FIG. 2 is a partially enlarged front view showing the state in which magnetic pole teeth are arranged in stator yokes in the stepping motor in FIG. 1; 
     FIG. 3 is a partially sectional view showing the relationship between an electric current and a magnetic field with respect to one set of stator yokes in the stepping motor in FIG. 1; 
     FIG.  4 ( a ) and ( b ) are schematic perspective view and a wiring diagram showing the state in which respective coils of a stator are connected with one another in the stepping motor in FIG. 1; 
     FIG. 5 is a graph showing the step angle accuracy of the stepping motor according to the present invention shown in FIG.  1  and that of a background-art stepping motor in FIG. 8, in the case of two-phase excitation; 
     FIG. 6 is a graph showing the step angle accuracy of the stepping motor according to the present invention shown in FIG.  1  and that of the background-art stepping motor in FIG. 8, in the case of single-phase excitation; 
     FIG. 7 is a graph showing the torque characteristic of the stepping motor according to the present invention shown in FIG.  1  and that of the background-art stepping motor in FIG. 8; 
     FIG. 8 is an exploded perspective view showing a configuration of an example of a stepping motor in the background art; 
     FIG. 9 is a schematic sectional view of the stepping motor in FIG. 8; 
     FIG. 10 is an enlarged perspective view of a stator yoke in the stepping motor in FIG. 8; 
     FIG. 11 is a view showing leakage of magnetic flux in the case of the stepping motor driven with single-phase excitation in FIG. 8; 
     FIG. 12 is a view showing leakage of magnetic flux in the case of the stepping motor driven with two-phase excitation in FIG. 8; 
     FIG. 13 are views showing stator yokes in the stepping motor in FIG.  8 : (A) is a partially enlarged sectional view; (B) is an enlarged view of magnetic pole teeth viewed from the inside; and (C) is an enlarged view of magnetic pole teeth the whole length of each of which is made large. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be described in detail below on the basis of embodiments shown in the drawings. 
     FIG. 1 shows a configuration of an embodiment of a stepping motor according to the present invention. 
     In FIG. 1, a stepping motor  10  is constituted by a hollow cylindrical housing  11  having an open end, a bracket  12  for closing the open end of the housing  11 , a stator  13  provided along the inner circumferential surface of this housing  11 , a rotor  14  rotatably supported by bearing portions  11   a  and  12   a  provided in the housing  11  and the bracket  12  respectively, and a flange  15  attached to the closed end of the housing  11 . 
     The above-mentioned housing  11  is made of metal material and provided with the bearing portion  11   a  at a position near the center of the closed end surface of the housing  11 . 
     The above-mentioned bracket  12  is made of metal material and provided with the bearing portion  12   a  at a position near the center of the bracket  12  in the same manner as in the housing  11 . 
     The above-mentioned stator  13  is constituted by a substantially cylindrical yoke unit  16  and coils  17   a  to  17   d . In the yoke unit  16 , eight stator yokes  16   a  to  16   h  each provided with a ring-like flange portion having an outer diameter substantially equal to the inner diameter of the housing  11  are integrally molded with resin (not shown) in the same manner as in the background art, so that the yoke unit  16  can be stored in the housing  11 . The coils  17   a  to  17   d  are wound on coil winding portions defined by the stator yokes  16   a  and  16   b ,  16   c  and  16   d ,  16   e  and  16   f  and  16   g  and  16   h , respectively. The stator  13  is received and fixedly held in the housing  11  after the coils  17   a  to  17   d  are wound. 
     The above-mentioned rotor  14  is constituted by a rotating shaft  14   a  rotatably supported by the above-mentioned bearing portions  11   a  and  12   a , and a hollow cylindrical rotor magnet  14   b  fixedly held on the rotating shaft  14   a . This rotor magnet  14   b  is formed to have an outer diameter slightly smaller than the inner diameter of a central hole of the yoke unit  16 . The rotor magnet  14   b  is magnetized circumferentially. 
     Here, each of the above-mentioned stator yokes  16   a  to  16   h  is made of conductive metal material, for example, in the same manner as in the stator yoke  6   a  shown in FIG. 10, but formed to be thinner than the background-art stator yoke  6   a . Each of the stator yokes  16   a  to  16   h  is constituted by a ring-like flange portion and a large number of magnetic pole teeth provided to erect vertically from an inner circumferential portion of this ring-like flange portion. 
     Each of the above-mentioned stator yokes  16   a  to  16   h  may be made of material which is as thick as that of a stepping motor in the background art. 
     The stator yokes are combined so that the stator yokes  16   b  and  16   c ,  16   d  and  16   e , and  16   f  and  16   g  are put back to back with each other. The pairs of the stator yokes  16   a  and  16   b ,  16   c  and  16   d ,  16   e  and  16   f , and  16   g  and  16   h  constitute phases a, b, c and d respectively, as shown in FIGS. 1 and 2. 
     Further, the stator yokes are arranged so that the stator yokes  16   a  to  16   d  of adjacent two phases a and b constitute a first set and the stator yokes  16   e  to  16   h  of adjacent two phases c and d constitute a second set, and so that the magnetic pole teeth of the stator yokes  16   a  to  16   d  of the first set are shifted in phase by 90 degrees from the magnetic pole teeth of the stator yokes  16   e  to  16   h  of the second set, respectively. 
     Since the respective magnetic pole teeth are arranged thus, leakage of magnetic flux from one stator yoke to another stator yoke is generated only between the stator yokes  16   d  and  16   e  which are shifted in phase by 90 degrees from each other. The other stator yokes are all arranged to be reverse in phase to one another so that there is no fear that leakage of magnetic flux, that is, magnetic interference is generated among the stator yokes. 
     The eight stator yokes  16   a  to  16   h  are integrally molded with resin in a condition that they are combined as mentioned above. The yoke unit  16  is completed thus. 
     As for the coils  17   a  to  17   d  wound on the coil winding portions of the stator yokes of the phases a to d respectively, the coils  17   a  and  17   b  are connected in series and in reverse to each other, and similarly the coils  17   c  and  17   d  are connected in series and in reverse to each other, as shown in FIG.  2 . 
     As a result, in each set of stator yokes and coils, for example, in the first set of the stator yokes  16   a  to  16   d  and the coils  17   a  and  17   b  as shown in FIG. 3, currents I flow through the coils  17   a  and  17   b  in the phases a and b reversely to each other, so that the directions of magnetic fields generated in the phases a and b respectively are reverse to each other as shown by the arrows B in FIG.  3 . Therefore, since magnetic flux passing between the stators  16   b  and  16   c  is generated in the same direction, the magnetic fields do not cancel each other, so that no magnetic interference is not generated. 
     Here, winding terminals of the respective coils  17   a  and  17   d  are led to a board  18  (see FIGS.  4 ( a ) and ( b )) provided in the outside of the motor, so that the wire connections of the respective coils  17   a  to  17   d  can be made on this board  18  as shown in FIG.  4 ( a ) and ( b ). The coils  17   a  and  17   b  and the coils  17   c  and  17   d  are driven as phases A and A′ and phases B and B′ respectively by a driving circuit which is similar to that in a stepping motor having a two-phase structure in the background art. 
     In the stepping motor  10  configured thus according to the embodiment of the present invention, driving currents are applied from a not-shown driving circuit to the coils  17   a  to  17   d  wound on the coil winding portions, respectively, of the yoke unit  16  of the stator  13  so that magnetic fields generated in the coils  17   a  to  17   d  respectively interact with the magnetic field of the rotor magnet  14   b  through the respective stator yokes  16   a  to  16   h  of the yoke unit  16 . Then, the rotor  14  is driven to rotate intermittently by the effect of the magnetic pole teeth of the respective stator yokes  16   a  to  16   h.    
     While each stator yoke constitutes a phase in the background-art stepping motor, two phases of stator yokes constitute a set in the stepping motor  10  according to the embodiment of the present invention. It is therefore possible to reduce magnetic interference between stator yokes in adjacent phases. Accordingly, the position accuracy of the step angle (static angle accuracy) is improved. 
     For example, in the static angle accuracy in the case where the stepping motor  10  is driven with two-phase excitation, a characteristic curve shown by the symbol P in FIG. 5 is exhibited. On the other hand, in the static angle accuracy in the background-art stepping motor driven with two-phase excitation, a characteristic curve shown by the symbol Q in FIG. 5 is exhibited. It is therefore understood that the stepping motor  10  according to the embodiment of the present invention is higher in the static angle accuracy. 
     In addition, in the static angle accuracy in the case where the stepping motor  10  has a single-phase structure, a characteristic curve shown by the symbol P in FIG. 6 is exhibited. On the other hand, in the static angle accuracy in the background-art two-phase-driven stepping motor, a characteristic curve shown by the symbol Q in FIG. 6 is exhibited. It is therefore understood that the stepping motor  10  according to the embodiment of the present invention is higher in the static angle accuracy also in the case of the single-phase-driven stepping motor. 
     In addition, even if the material forming the stator yokes is thin, two phases of stator yokes are used as one set, so that the saturated magnetic capacity can be ensured. Accordingly, there is no fear that the driving torque is lowered. 
     Further, when material for the stator yoke is made thin, the electric resistance becomes high and the core loss becomes low, so that the high-frequency magnetic responsibility of material for the stator yoke is improved. Accordingly, the high-frequency responsibility of the stepping motor  10  is also improved. 
     For example, the torque to frequency in the stepping motor  10  takes a sufficiently high value up to a comparatively high frequency as shown by the symbol P in FIG.  7 . However, the torque to frequency in the background-art stepping motor drops at a comparatively low frequency as shown by the symbol Q in FIG.  7 . It is therefore understood that the stepping motor  10  according to the embodiment of the present invention is larger in the driving torque at a high frequency. 
     Thus, even if the width of each of the magnetic pole teeth of the stator yokes  16   a  to  16   d  is made smaller than that in the background art, the workability is not spoiled by making the material for the stator yokes  16   a  to  16   d  thin. It is therefore possible to form a stepping motor having a small step angle. 
     Further, in the illustrated case, the yoke unit  16  is constituted by the stator yokes  16   a  to  16   h  of four phases which are grouped into two sets, so that the total magnetic force of the rotor  14  is also divided into four phases. Accordingly, the detent torque is reduced so that driving noise at the time of continuous driving can be reduced. 
     Here, the yoke unit  16  requires stator yokes twice as many as in the background-art yoke unit. However, the thickness of each stator yoke may be made thinner. For example, if the thickness is reduced to half , the material cost can be kept substantially equal to that in the background art. 
     In addition, since the structure of the stepping motor as a whole is substantially similar to that in the background art, the same assembling process as in the background art may be used. Therefore, the stepping motor  10  can be manufactured by using parts and a manufacturing line for the background-art stepping motor as they are. Therefore, an investment in an additional equipment is unnecessary. 
     Although the yoke unit  16  has the stator yokes  16   a  to  16   h  of four phases which are grouped into two sets in the above-mentioned embodiment, it is apparent that the present invention is not limited to this, but applicable to a stepping motor having an even number of phases, more than three sets of stator yokes. 
     As has been described above, according to the present invention, coils wound on two phases of stator yokes constituting each set are made reverse so that currents flow in the coils reversely to each other. Thus, the mutual magnetic interference can be reduced to a low level. 
     Therefore, any stator yoke of any phase in one set is subjected to magnetic interference from phases adjacent to the stator yoke in question, but magnetic interference is given to the stator yoke in question merely from another set adjacent to the one set in question. Accordingly, the magnetic interference from the adjacent phases can be reduced. 
     Since magnetic interference given to any stator yoke in any phase by stator yokes adjacent to the stator yoke in question is reduced in such a manner, the position accuracy of the step angle is improved. 
     In addition, even if the material forming the stator yokes is thin, the stator yoke material is high in electric resistance and low in core loss so that the high-frequency magnetic responsibility of the stator yoke material is improved. Accordingly, the high-frequency responsibility of the motor is also improved. In addition, the saturated magnetic capacity can be ensured because two phases of stator yokes are used as one set. It is therefore possible to prevent the driving torque from being lowered. 
     As a result, if the stator yoke material is made thin, the workability is not spoiled even if the tooth width of each of magnetic pole teeth is made smaller than that in the background art. It is therefore possible to form a stepping motor having a small step angle. 
     Further, since the stator is constituted by a plurality of sets of four-phase stator yokes, the total magnetic force of the rotor is also divided into four phases, so that the detent torque is reduced. It is therefore possible to reduce driving noise at the time of continuous driving. In this case, however, the number of stator yokes is required to be twice as large as that in the background art. However, since the stator yokes may be made thin, for example, to be half as thick as that in the background art, it is possible to restrain the material cost to be substantially equal to that in the background art. 
     In addition, since the structure of the stepping motor as a whole is substantially similar to that in the background art, the steps of assembling the stepping motor are similar to those in the background art. Therefore, the stepping motors of the present invention can be manufactured by using parts and manufacturing lines which have been used for stepping motors in the background art. It is therefore unnecessary to make an investment in additional equipment. 
     Thus, according to the present invention, there is obtained an extremely superior stepping motor having a simple configuration, which is small both in size and in stepping angle, large in torque and capable of high-speed rotation. 
     Further, leakage of magnetic flux from one stator yoke to another stator yoke is generated only between the stator yokes  16   d  and  16   e  which are shifted in phase by 90 degrees from each other. The other stator yokes are all arranged to be reverse in phase to one another so that there is no fear that leakage of magnetic flux, that is, magnetic interference is generated among the stator yokes.