Abstract:
An electric motor driven by three-phase electric power includes armature windings that are connected in series, via an armature-winding connecting line, for each of a plurality of groups of three or N circumferentially-adjoining poles to thereby provide three-phase armature windings, wherein N is an arbitrary number equal to a multiple of three. The armature-winding connecting line connects in series the adjoining armature windings in such a way as to not substantially straddle a relatively great part of the outer periphery of any of the adjoining armature windings.

Description:
FIELD OF THE INVENTION 
   The present invention relates to electric motors and electric power steering apparatus equipped with such electric motors. 
   BACKGROUND OF THE INVENTION 
   As well known, the electric power steering apparatus are steering assisting apparatus which are constructed to activate an electric motor (steering assisting motor) as a human driver manually operates a steering wheel of a motor vehicle, to thereby assist the driver&#39;s manual steering effort. In such electric power steering apparatus, the steering assisting motor, which provides steering assist force or torque, is controlled on the basis of a steering torque signal generated by a steering torque detection section detecting steering torque that is produced on the steering shaft by driver&#39;s operation of the steering wheel and a vehicle velocity signal generated by a vehicle velocity detection section detecting a traveling velocity of the vehicle, so as to reduce manual steering force to be applied by the human driver. 
   For example, Japanese Patent Application Laid-Open Publication No. 2001-275325 discloses an electric power steering apparatus, where the steering torque applied to the steering wheel is delivered to an output shaft of a rack and pinion mechanism while the steering assist torque produced by the electric motor in accordance with the steering torque is delivered to a pinion shaft via a frictional transmission mechanism and worm gear mechanism. Thus, road wheels of the vehicle are steered via the rack and pinion mechanism. The disclosed electric power steering apparatus is designed to: impart a good steering feel by minimizing adverse effects of steering torque variation by the motor when the vehicle should travel straight with the motor kept deactivated; and enhance the controllability of the vehicle by efficiently enhancing the output performance of the motor. For these purposes, the electric motor comprises an annular outer stator including armature windings wound on nine or N (N is an arbitrary number equal to a multiple of nine) circumferentially-arranged poles, and an inner rotor located inwardly of the outer stator and including circumferentially-arranged permanent magnets of eight poles. 
   In the electric motor of the disclosed electric power steering apparatus, the armature-winding connecting line, connecting in series the armature windings, comes out of one of the armature windings, then extends to the next armature winding, adjoining the one armature winding, where it arcuately extends around (i.e., substantially straddles) a considerable or relatively great part of the outer periphery of the armature winding to reach a point of the next armature winding remote from the one armature winding (rather than a point of the next armature winding close to the one armature winding), and then connects to the further next armature winding that does not adjoin the one armature winding. The extra length substantially straddling the considerable part of the outer periphery of the next armature winding as noted above would considerably increase the total length of the armature-winding connecting line. Further, in a case where the armature-winding connecting line should connect from one armature winding of a given phase to another armature winding of the same phase that is spaced from the one armature winding with other armature windings of other phases interposed therebetween, the total length, per phase, of the armature-winding connecting line would inevitably have to be increased further due to an additional length straddling parts of the armature windings of the other phases. 
     FIG. 10  shows an armature-winding connecting line and a neutral-point connecting line in the electric motor of the disclosed electric power steering apparatus, and  FIG. 11  is a conceptual view representatively showing salient poles of one of three phases (U phase in the illustrated example) and respective numbers of turns of the armature windings  100   a - 100   c . The armature windings  100   a ,  100   b  and  100   c  of adjoining three poles are connected in series to provide a U-phase winding unit, armature windings  100   d ,  100   e  and  100   f  of other adjoining three poles are connected in series to provide a V-phase winding unit, and the armature windings  100   g ,  100   h  and  100   i  of the other adjoining three poles are connected in series to provide a W-phase winding unit. 
   In the conventional electric motor, as shown in a plan view of  FIG. 10 , the armature-winding connecting line  110 , connecting in series the adjoining armature windings  100   a - 100   c ,  100   d - 100   f ,  100   g - 100   i , comes out of one of the armature windings, then arcuately extends around (i.e., substantially straddles) a considerable part of the outer periphery of the next armature winding adjoining the one armature windings, then intersects with another armature-winding connecting line  110  to connect to the further next armature winding, as depicted within an oval in the figure. Further, the neutral-point connecting line  120  is led or positioned on the same side as the armature-winding connecting lines  110 , so that layout or placement of the electric motor tends to be difficult and human operators tend to confuse the armature-winding connecting lines  110  and neutral-point connecting line  120  during assembly operations. 
   As further illustrated in  FIG. 11 , the armature-winding connecting lines  110  would inevitably intersect around the middle salient pole  101   b  if the armature windings are to be wound to the same number of turns on the adjoining salient poles  101   a - 101   c . In  FIG. 11 , the armature windings are each shown as wound to five turns on the corresponding pole, as depicted by “(5)”. For the reasons stated above, there has been a demand for an improved electric motor where the armature-winding connecting lines  110  do not intersect and which has a winding structure that can be laid with increased ease and that can be reduced in size. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, there is provided an improved electric motor driven by three-phase electric power, which comprises: a rotor including a rotation shaft and a plurality of permanent magnets arranged circumferentially on the outer periphery of the rotation shaft; and armature windings disposed adjacent to the outer periphery of the rotor, the armature windings being connected in series, via an armature-winding connecting line, for each of a plurality of groups of three or N circumferentially-adjoining poles, to thereby provide three-phase armature windings, wherein N is an arbitrary number equal to a multiple of three. The armature-winding connecting line, connecting in series the armature windings, is positioned so as not to arcuately extend around (or substantially straddle) a part of any of the adjoining armature windings. Preferably, the armature-winding connecting line connects the adjoining armature windings of each of the three phases on one longitudinal side of the rotation shaft. 
   Namely, in the motor of the present invention, the armature-winding connecting line, connecting in series the adjoining armature windings, is led in such a way as to not arcuately extend around (or substantially straddle) a part of any of the adjoining armature windings. Therefore, the armature-winding connecting line can be reduced in length and hence can have reduced electrical resistance, which should contribute to enhancement in the torque output performance of the motor. 
   Preferably, the electric motor of the present invention further comprises a neutral-point connecting line that connects respective neutral points of the three phases, and the armature-winding connecting line and the neutral-point connecting line are positioned on opposite longitudinal sides of the rotation shaft. 
   According to another aspect of the present invention, there is provided an electric power steering apparatus, which comprises: an electric motor driven by three-phase electric power to provide steering assist force to a steering system; a steering torque detection section for detecting steering torque applied to a steering wheel; and a control device for controlling the electric motor in accordance with at least a signal generated by the steering torque detection section. In the electric power steering apparatus, the electric motor comprises: a rotor including a rotation shaft and a plurality of permanent magnets arranged circumferentially on an outer periphery of the rotation shaft; and armature windings disposed adjacent to an outer periphery of the rotor, the armature windings being connected in series, via an armature-winding connecting line, for each of a plurality of groups of three or N circumferentially-adjoining poles, to thereby provide three-phase armature windings, wherein N is an arbitrary number equal to a multiple of three. The armature-winding connecting line, connecting in series the armature windings, is positioned so as not to straddle a part of any of the adjoining armature windings. 
   With the motor capable of achieving enhanced torque output performance, the electric power steering apparatus of the invention can impart more appropriate steering assist force and can significantly improve the steering feel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Certain preferred embodiments of the present invention will hereinafter be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a view showing an overall setup of an electric power steering apparatus equipped with an electric motor of the present invention; 
       FIG. 2  is a view showing mechanical and electric arrangements of the electric power steering apparatus; 
       FIG. 3  is a sectional view taken along the  3 - 3  line of  FIG. 2 ; 
       FIG. 4  is a sectional view taken along the  4 - 4  line of  FIG. 3 ; 
       FIG. 5  is a sectional view taken along the  5 - 5  line of  FIG. 4 , which shows a sectional construction of the motor; 
       FIGS. 6A and 6B  are schematic views showing armature windings in the motor of  FIG. 5 ; 
       FIG. 7  is a view schematically showing respective wound directions of armature windings and a neutral-point connecting line in the motor; 
       FIG. 8  is a view conceptually showing U-phase salient poles and respective numbers of turns of the armature windings shown in  FIG. 7 ; 
       FIG. 9  is a graph comparatively showing relationship between the electrical angle and torque in the motor of the present invention and in a conventional motor; 
       FIG. 10  is a view showing armature-winding connecting line and a neutral-point connecting line in the conventional motor; and 
       FIG. 11  is a conceptual view illustratively showing U-phase salient poles and the numbers of turns of the armature windings in the conventional motor of  FIG. 10 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First, with reference to  FIGS. 1 to 4 , descriptions will be given about a general setup, specific mechanical and electrical arrangements and layout of electronic components of an electric power steering apparatus equipped with an electric motor of the present invention. 
     FIG. 1  is a view showing the overall setup of the electric power steering apparatus  10 , which is constructed to impart steering assist force (steering assist torque) to a steering shaft  12  connected to a steering wheel  11  of a motor vehicle. 
   The steering shaft  12  has an upper end connected to the steering wheel  11  and a lower end connected to a pinion gear (or pinion)  13 . The pinion gear  13  meshes with a rack gear  14   a  formed on a rack shaft  14 . The pinion gear  13  and rack gear  14   a  together constitute a rack and pinion mechanism  15 . Tie rods  16  are provided at opposite ends of the rack shaft  14 , and a front road wheel  17  is connected to the outer end of each of the tie rods  16 . 
   The electric motor  19 , which is, for example, a brushless motor, generates rotational force (torque) for assisting or supplementing the steering torque, and the thus-generated rotational force is transmitted via a power transmission mechanism  18  to the steering shaft  12 . 
   Steering torque detection section  20  is provided on the steering shaft  12 . The steering torque detection section  20  detects steering torque applied by a human driver of the vehicle operating the steering wheel  11 . 
   Reference numeral  21  represents a vehicle velocity detection section for detecting a traveling velocity of the vehicle, and  22  represents a control device implemented by a computer. On the basis of a steering torque signal T output from the steering torque detection section  20  and vehicle velocity signal V output from the vehicle velocity detection section  21 , the control device  22  generates drive control signals SG 1  for controlling rotation of the motor  19 . Rotational angle detection section  23 , which is implemented, for example, by a resolver, is attached to the motor  19 . Rotational angle signal SG 2  output from the rotational angle detection section  23  is fed to the control device  22 . The above-mentioned rack and pinion mechanism  15  is accommodated in a gearbox  24  ( FIG. 2 ). 
   As the driver operates the steering wheel  11  during travel of the vehicle, rotational force based on the steering torque applied to the steering shaft  12  is converted via the rack and pinion mechanism  15  into axial linear movement of the rack shaft  14 , which, via the tie rods  16 , changes a direction of the front road wheels  17 . During that time, the steering torque detection section  20 , attached to the steering shaft  12 , detects the steering torque applied by the driver via the steering wheel  11  and converts the detected steering torque into an electrical steering torque signal T, which is then input to the control device  22 . The vehicle velocity detection section  21  detects the velocity of the vehicle and converts the detected vehicle velocity into an electrical vehicle velocity signal V, which is also input to the control device  22 . 
   The control device  22  generates motor currents Iu, Iv and Iw for driving the motor  19  on the basis of the steering torque signal T and vehicle velocity signal V. Specifically, the motor  19  is a three-phase motor driven by the A.C. motor currents Iu, Iv and Iw of three phases, i.e. U, V and W phases. Namely, the above-mentioned drive control signals SG 1  are in the form of the three-phase motor currents Iu, Iv and Iw. The motor  19  is driven by such motor currents Iu, Iv and Iw to generate steering assist force (steering assist torque) that acts on the steering shaft  12  via the power transmission mechanism  18 . With the electric motor  19  driven in this manner, steering force to be applied manually by the driver to the steering wheel  11  can be reduced. 
     FIG. 2  is a view showing mechanical and electric arrangements of the electric power steering apparatus  10 . 
   The rack shaft  14  is accommodated in a cylindrical housing  31  extending in a widthwise (left-and-right direction of  FIG. 2 ) of the vehicle, and the rack shaft  14  is axially sidable within the cylindrical housing  31 . Ball joints  32  are screwed onto opposite end portions of the rack shaft  14  projecting outwardly of the housing  31 . The left and right tie rods  16  are coupled to the ball joints  32 . The housing  31  has brackets  33  by which the housing  31  is attached to a body of the vehicle, and stoppers  34  provided on its opposite ends. 
   In  FIG. 2 , reference numeral  35  represents an ignition switch,  36  a vehicle-mounted battery, and  37  an A.C. generator (ACG) attached to an engine (not shown) of the vehicle. The vehicle engine causes the A.C. generator  37  to start generating electric power. Necessary electric power is supplied to the control device  22  from the battery  36  or A.C. generator  37 . The control device  22  is attached to the motor  19 . 
     FIG. 3  is a sectional view illustratively showing specific constructions of a steering-shaft support structure, steering torque detection section  20 , power transmission mechanism  18  and rack and pinion mechanism  15 , as well as layout of the electric motor and control device  22 . 
   In  FIG. 3 , the steering shaft  12  is rotatably supported, via two bearings  41  and  42 , in a housing  24   a  forming the gearbox  24 . The rack and pinion mechanism  15  and power transmission mechanism  18  are accommodated in the housing  24   a , and the steering torque detection section  20  is attached to an upper portion of the housing  24   a.    
   The pinion  13 , provided on a lower end portion of the steering shaft  12 , is located between the two bearings  41  and  42 . The rack shaft  14  is guided by a rack guide  45  and normally pressed against the pinion  13  by a pressing member  47  that is in turn resiliently urged by a compression spring  46 . 
   The power transmission mechanism  18  includes a worm gear  49  fixedly mounted on a transmission shaft  48  coupled to the output shaft of the motor  19 , and a worm wheel  50  fixedly mounted on the pinion shaft  12 . 
   The steering torque detection section  20  includes a steering torque sensor  20   a  positioned around the steering shaft  12 , and an electronic circuit section  20   b  for electronically processing a steering torque detection signal output from the steering torque sensor  20   a.    
     FIG. 4  shows detailed constructions of the power transmission mechanism  18  and motor  19 . 
   The motor  19  includes an inner rotor  52  having a plurality of permanent magnets fixedly mounted on a rotation shaft  51 , and annular outer stators  54  and  55  positioned adjacent to and around the outer periphery of the inner rotor  52  and having armature windings  53  wound thereon. 
   The rotation shaft  51  is rotatably supported via two bearings  56  and  57 . One end portion of the rotation shaft  51  forms the output shaft  19   a  of the motor  19 . The output shaft  19   a  of the motor  19  is coupled to the transmission shaft  48  via a torque limiter  58  so that the rotational force of the motor can be transmitted to the transmission shaft  48  via the torque limiter  58 . 
   The worm gear  49  is fixedly mounted on the transmission shaft  48  as noted above, and the worm wheel  50  meshing with the worm gear  49  is fixedly mounted on the steering shaft  12 . 
   The above-mentioned rotational angle detection section (rotational position detection section)  23  for detecting a rotational angle (rotational position) of the inner rotor  52  of the motor  19  is provided at a rear end portion of the rotation shaft  51 . The rotational angle detection section  23  includes a rotating element  23   a  fixed to the rotation shaft  51 , and a detecting element  23   b  for detecting a rotational angle of the rotating element  23   a  through magnetic action. For example, the rotational angle detection section  23  may comprise a resolver. 
   The motor currents Iu, Iv and Iw, which are three-phase A.C. currents, are supplied to the armature windings  53  of the outer stators  54  and  55 . The above-mentioned components of the motor  19  are positioned within a motor case  59 . 
   As illustrated in  FIG. 5 , which is a sectional view taken along the  5 - 5  line of  FIG. 4  and shows a sectional construction of the motor  19 , the outer stator  54  has nine salient poles  62   a - 62   i  extending radially from an outer peripheral surface of a cylindrical portion  61  at equal pitches. The armature windings  53   a - 53   i  are wound on the nine radial poles  62   a - 62   i , to provide the U-, V- and W-phase winding units; in the illustrated example, each of the U-, V- and W-phase winding units is provided by connecting in series a different group of circumferentially-adjoining three of the poles  62   a - 62   i.    
   The inner rotor  52  is a rotational member having eight permanent magnets  52   a - 52   h  arranged along a circumferential direction thereof. These eight permanent magnets  52   a - 52   h  together constitute an annular or ring-shaped member that is magnetized radially (i.e., in a direction passing through a thickness (between inner and outer surfaces) of the ring-shaped member), and the permanent magnets  52   a - 52   h  are arranged in such a manner that N and S poles alternate in the circumferential direction. 
     FIGS. 6A and 6B  are schematic diagrams showing the armature windings of the motor  19 . 
   Specifically,  FIG. 6A  shows the three-phase (U-, V- and W-phase) winding units each provided by serially connecting the armature windings  53   a - 53   c ,  53   d - 53   f  or  53   g - 53   i  wound on adjoining three of the salient poles  62   a - 62   i . Namely, the U-phase winding unit is provided by serially connecting the armature windings  53   a ,  53   b ,  53   c  of first adjoining three poles  62   a ,  62   b ,  62   c , the V-phase winding unit is provided by serially connecting the armature windings  53   d ,  53   e ,  53   f  of second adjoining three poles  62   d ,  62   e ,  62   f , and the W-phase winding unit is provided by serially connecting the armature windings  53   g ,  53   h ,  53   i  of third adjoining three poles  62   g ,  62   h ,  62   i.    
   As illustrated in  FIG. 6B , one terminal U o , V o  or W o  of each of the U-, V- and W-phase winding units is connected to the battery  36 , while the other terminal is connected, at a neutral point N o  of a reference potential, to a neutral-point connecting line Na. Wound directions of the individual armature windings  53   a - 53   i  are illustrated in  FIG. 7 . 
   Referring to  FIG. 7 , in the U-phase winding unit, the armature winding  53   a  is wound in the counterclockwise direction, the armature winding  53   b  in the clockwise direction, and the armature winding  53   c  in the counter-clockwise direction. In order to prevent an armature-winding connecting line  63 , connecting between the armature windings  53   a  and  53   b , from intersecting with the neutral-point connecting line Na near one end portion (upper end portion in the illustrated example of  FIG. 7 ) of the armature winding  53   b  located on one longitudinal (axial) side (upper side in the illustrated example of  FIG. 7 ) of the rotation shaft  51  (see  FIG. 5 ), the armature-winding connecting line  63  is drawn out from a predetermined end portion (lower end portion in  FIG. 7 ) of the armature winding  53   a  located on the other longitudinal side of the rotation shaft  51  and led to a lower end portion of the armature winding  53   b . Similarly, the armature-winding connecting line  63 , connecting between the armature windings  53   b  and  53   c , is drawn out from a lower end portion of the armature winding  53   b  and led to a lower side of the armature winding  53   c.    
   More specifically, the neutral-point connecting line Na, connected with the neutral points N o  of the individual winding unit terminals, is led above the upper end portions of the armature windings  53   a - 53   i , while the armature-winding connecting line  63  is led below the lower end portions of the armature windings  53   a - 53   i . Therefore, the electric motor  19  can be placed or laid with increased ease and significantly reduced in size. Further, it is possible to reduce a possibility of human operators confusing the armature-winding connecting line  63  and neutral-point connecting line Na, since the lines  63  and Na are not located on the same side. 
     FIG. 8  schematically shows, by way of example, the U-phase salient poles  62   a - 62   c  and respective numbers of turns of the armature windings  53   a - 53   c  on the poles  62   a - 62   c.    
   The armature windings  53   a - 53   i  are wound on the respective poles in the directions illustrated in  FIG. 7 . Portion of the armature winding  53   a  adjacent to the armature winding  53   b  has four turns as depicted at ( 4 ) in  FIG. 8 , and the armature winding  53   b  has six turns as depicted at ( 6 ). Portion of the armature winding  53   b  adjacent to the armature winding  53   c  has six turns as depicted at ( 6 ), and the armature winding  53   c  has four turns as depicted at ( 4 ). Portion of the armature winding adjacent to the winding unit of another phase has five turns as depicted at ( 5 ). Alternatively, the portion of the armature winding  53   a  adjacent to the armature winding  53   b  may have six turns, and the armature winding  53   b  may have four turns; in this case, the portion of the armature winding  53   b  adjacent to the armature winding  53   c  has four turns, and the armature winding  53   c  has six turns. 
   It should be understood that the numbers of turns mentioned above are just for illustrative purposes and differ from those actually employed in the motor  19 . 
     FIG. 9  is a graph comparatively showing relationship between the electrical angle and torque in the motor  19  of the present invention and in the conventional motor. Specifically, in the figure, a solid line represents the relationship between the electrical angle and torque in the conventional motor, while a broken line represents the relationship between the electrical angle and torque in the motor  19  of the present invention. 
   In the motor  19  of the present invention, the armature-winding connecting line  63  is led in such a way as to not form an extra length arcuately extending around or substantially straddling a part of any of the adjoining armature windings and to not intersect with the neutral-point connecting line Na. Therefore, the armature-winding connecting line  63  can be reduced in length and hence can have reduced electrical resistance, which should contribute to enhancement in the torque output performance of the motor  19 . 
   However, there might arise effects due to the numbers of turns of the armature windings  53   a - 53   i  that differ among the adjoining portions of the windings  53   a - 53   i . Thus, an experiment was conducted, through which the results of  FIG. 9  were obtained. The results show that the motor  19  of the present invention, having effects of the shortened armature-winding connecting line  63  and the numbers of turns of the armature windings  53   a - 53   i  differing among the adjoining portions of the windings  53   a - 53 , as a whole can achieve far better performance than the conventional motor. 
   Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.