Method for winding coils on rotor core

A core includes a ring body and a plurality of teeth. The teeth extend radially outward form the outer circumference of the ring body. The core is formed by assembling a first core member and a second core member. Each core member has part of the teeth the number of which is half the total number of the teeth. Each tooth includes a tooth body about which the coil is wound, and a magnetism converging portion provided at the distal end of the tooth body. The tooth height of each tooth body gradually increases from a distal section to a proximal section of the tooth body. The tooth width gradually decreases from the distal section to the proximal section. The wire is wound about each of the teeth of the first and second core members. Then, the first and second core members are assembled to form the core. Accordingly, a rotor core having a high coil accommodation efficiency and a high coil space factor is obtained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Applications No. 2002-304670, filed Oct. 18, 2002 and 2003-014907, filed Jan. 23, 2003.

BACKGROUND OF THE INVENTION

The present invention relates to a rotor core, a direct-current motor, and a method for winding coils on a rotor core.

Recently, to meet demands for compact and high performance motors, motor cores having an increased coil density and an improved accommodation efficiency are wanted. A rotor core of a direct-current motor has radially extending teeth. A coil is wound about each tooth. A slot between each adjacent pair of teeth, or an accommodation space for a coil, narrows toward the radially inner end and widens toward the radially outer end.

Thus, when winding a coil about each tooth in a concentrated manner, the number of times the coil is wound increases toward the radially outer end. Thus, the measurement of the wound coil along the axial direction of the motor, or the coil height, increases toward the outer end of the teeth, which increases the axial size of the motor.

Japanese Laid-Open Patent Publication No. 9-19095 discloses a rotor core having teeth, in which core the circumferential width of each tooth increases toward the radially outer end, and the height (the measurement along the axial direction of the motor) of each tooth decreases toward the radially outer end. This configuration reduces the coil height at the radially outer sections of the teeth and reduces the size of the motor.

However, in the configuration disclosed in the above publication, the space between the distal sections of each adjacent pair of the teeth widens toward the radially outer end, and each coil is wound about the corresponding tooth using those spaces. Therefore, the space factor of the coils is limited. The distal section of each tooth faces one of magnets provided on a stator. Magnetism converges on the distal section of each tooth. Since the thickness (height) of the distal section, or the magnetism converging section, of each tooth is small, the necessary magnetic flux does not flow through the teeth. This lowers the motor torque.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide a rotor core, a direct-current motor, and a method for winding coils on a rotor core that improve the accommodation efficiency and the space factor of coils.

To achieve the above objective, the present invention provides a rotor core. The rotor core includes a ring body and a plurality of teeth extending radially outward from an outer circumference of the ring body. Each tooth includes a coil winding portion about which a coil is wound. The coil winding portion includes a proximal section and a distal section. The proximal section is coupled to the ring body. The distal section is located radially outward of the proximal section A magnetism converging portion is provided at the distal section of the coil winding portion. The measurement of each coil winding portion with respect to the axial direction of the rotor core gradually increases from the distal section to the proximal section. The measurement of each coil winding portion with respect to the circumferential direction of the rotor core gradually decreases from the distal section to the proximal section. The rotor core includes a plurality of assembled core members. Each core member has part of the teeth the number of which obtained by dividing the total number of the teeth of the rotor core by the number of the core members. The teeth of each core member are spaced at equal angular intervals.

According to another aspect of the invention, a rotor core that includes a ring body and a plurality of teeth is provided. The teeth extend radially outward from an outer circumference of the ring body. Each tooth includes a coil winding portion about which a coil is wound. The coil winding portion includes a proximal section and a distal section. The proximal section is coupled to the ring body. The distal section is located radially outward of the proximal section A magnetism converging portion is provided at the distal section of the coil winding portion. The rotor core includes a plurality of assembled core members. Each core member has part of the teeth the number of which obtained by dividing the total number of the teeth of the rotor core by the number of the core members. The teeth of each core member are spaced at equal angular intervals. In each core member prior to assembly, the magnetism converging section does not exist in a range between the proximal section and the distal section of each coil winding portion with respect to a direction perpendicular to the extending direction of the coil winding portion.

In addition, present invention may be applicable to provide a method for winding coils on a rotor core. The rotor core includes a ring body and a plurality of teeth extending radially outward from an outer circumference of the ring body. Each tooth includes a coil winding portion about which a coils is wound. The coil winding portion includes a proximal section and a distal section. The proximal section is coupled to the ring body. The distal section is located radially outward of the proximal section. A magnetism converging section is provided at the distal section of the coil winding portion. The rotor core includes a plurality of core members assembled to form the rotor core. Each core member has part of the teeth the number of which obtained by dividing the total number of the teeth of the rotor core by the number of the core members. The teeth of each core member are spaced at equal angular intervals. The winding method includes steps of: holding with a jig one of the core members prior to assembly at at least one of the teeth of the, core member; and rotating the core member held by the jig about a rotation axis along the extending direction of at least one of the teeth, thereby winding the wire forming the coil about the one tooth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown inFIGS. 1to2, a direct-current motor1according to this embodiment has a stator2and an armature3.

The stator2includes a yoke4and magnets5provided in the yoke4. The magnets5function as magnetic poles. In this embodiment, the number of the magnets5is six, and the magnets5are fixed to the inner surface of the yoke4at equal angular intervals. The motor1has six poles. The yoke4is shaped as a cylinder with one end closed. An end frame6is provided at the open end of the yoke4.

The armature3includes a rotary shaft7, a rotor core10, and a commutator11. The rotor core10is fixed to an axial center portion of the rotary shaft7. The commutator11is fixed to a portion of the rotary shaft7that is close to one end. The rotary shaft7is rotatably supported with a pair of bearings15. One of the bearings15is provided at a center of the lid of the yoke4, and the other bearing15is provided at a center of the end frame6. The core10is located in the yoke4and surrounded by the magnets5. In the axial direction of the rotary shaft7, a side corresponding to the end frame6(rightward portion as viewed inFIG. 1) is referred to as the lower side, and a side opposite from the end frame6(leftward portion as viewed inFIG. 1) is referred to as the upper side.

The commutator11has a substantially cylindrical insulator12. Segments13, the number of which is twenty-four in this embodiment, are provided on the outer circumference of the commutator insulator12. The commutator insulator12has short circuit members14for short circuiting the segments13to each other. Brushes16are provided in the yoke4. The Brushes16slide on the segments13.

In this embodiment, each short circuit member14short circuits three of the segments13so that the three segments13are at the same potential. The number of the segments13is twenty-four. Each segment13forms a group with two other segments13at intervals of seven other segments13. The three segments13in each group are set at the same potential.

As shown inFIG. 3, the core10includes a ring body21and teeth23. Each tooth23extends radially outward from the outer circumference of the ring body21. In this embodiment, the number of the teeth23is eight, and the teeth23are provided at equal angular intervals. Therefore, eight slots17are created. Each slot17is defined between a pair of adjacent teeth23. The ring body21includes an inner ring portion25having a center hole24and an outer ring portion26located about the inner ring portion25. Each tooth23is located on the outer circumference of the outer ring portion26.

The radially inward portion of the ring body21is dented in the axial direction relative to the radially outward portion. That is, recesses27are formed on the upper and lower surface of the ring body21. The boundary between the inner ring portion25and the outer ring portion26is located inward of the recesses27. The axial measurement of the inner ring portion25is less than the axial measurement of the outer ring portion26. The measurement difference is within a range that ensures a cross-sectional area sufficient as a magnetic path.

As shown inFIGS. 1to3, each tooth23includes a coil winding section, which is a tooth body28, and a wide section, which is a magnetism converging portion29. A coil31is wound about the tooth body28. The tooth body28extends radially outward from the outer circumference of the ring body21. A tooth insulator30is attached to each tooth body28. A coil31is wound about each tooth body28with the corresponding tooth insulator30in between.

Each coil31is formed of a wire31a. Each tooth insulator30has two projections30aprovided at the radially outer end to fix the ends of the corresponding wire31a. The projections30aare located at the same side (at the lower side in this embodiment) with respect to the axial direction of the core10. The ends of each wire31a, which is wound about one of the tooth bodies28, are drawn after being fixed to the projections30a. The ends of the wire31aare then connected to the corresponding segments13with one of the short circuit members14.

The magnetism converging portion29is provided at the distal section of the tooth body28and is formed as a flange that extends in the circumferential and axial directions of the core10. The measurement of each magnetism converging portion29with respect to the axial direction of the core10is substantially the same as the measurement of each magnet5in the yoke4.

The measurement of each tooth body28with respect to the axial direction of the core10will be referred to as tooth height, and the measurement of each coil31with respect to the axial direction of the core10will be referred to as coil height. As shown inFIG. 1, the tooth height gradually increases from a distal section28aof each tooth body28to a proximal section28b. That is, a proximal tooth height Lhb, which is the tooth height of the proximal section28bof each tooth body28, is greater than a distal tooth height Lht, which is the tooth height of the distal section28a.

The coil height has a maximum value (Lct) at a position near the distal section28aof the tooth body28, and gradually decreases from the maximum value Lct to a minimum value (Lcb) at the proximal section28b.

A measurement of each tooth body28with respect to the circumferential direction of the core10will be referred to as tooth width. As shown inFIG. 2, the tooth width gradually decreases from the distal section28aof each tooth body28to the proximal section28b. That is, a proximal tooth width Lwb, which is the tooth width at the proximal section28bof each tooth body28, is smaller than a distal tooth width Lwt, which is the tooth width at the distal section28a.

A cross-section of each tooth body28perpendicular to the extending direction (the direction from the distal section28atoward the proximal section28b) of the tooth body28will be referred to as magnetic flux passing cross-section. The area of the magnetic flux passing cross-section at the distal section28ais substantially equal to the magnetic flux passing cross-section at the proximal section28b. The magnetic flux passing cross-section of each tooth body28is rectangular, and each tooth body28substantially satisfies the following equation. (the distal tooth height Lht×the distal tooth width Lwt)=(the proximal tooth height Lhb×the proximal tooth width Lwb)

Each of the side surfaces28c,28dof each tooth body28is a trapezoid. The upper base of the trapezoid, which is the distal tooth height Lht, is shorter than the lower base, which is the proximal tooth height Lhb. Likewise, each of an upper side surface28eand a lower side surface28fof each tooth body28is a trapezoid. The upper base of the trapezoid, which is the proximal tooth width Lwb, is shorter than the lower base, which is the distal tooth width Lwt. In this embodiment, four sides28g,28h,28i, and28jthat extend along the radial direction of each tooth body28are linear from the proximal section28bto the distal section28a.

As shown inFIG. 4, the core10is formed by assembling a first core member33and a second core member34with each other.

The first core member33includes a first inner ring portion25ahaving a center hole24aand a first outer ring portion26aprovided about the outer circumference of the first inner ring portion25a. Four of the teeth23are provided on the outer circumference of the first outer ring portion26aat equal angular intervals (90°). In this embodiment, the number of the teeth23provided on the first core member33is the half (four) of the total number (eight) of the teeth23of the core10.

The axial measurement of the first outer ring portion26aat the outer circumference is half the proximal tooth height Lhb. An upper surface28eof each tooth body28is connected to the upper surface of the circumferential portion of the first outer ring portion26awithout a step. Therefore, the first inner ring portion25aand the first outer ring portion26aof the first core member33are displaced upward from the center of the teeth23with respect to the axial direction of the first core member33.

The first outer ring portion26ais divided into four sections by four notches41a. Each notch41ais located between each adjacent pair of the teeth23such that the notches41aand the teeth23are alternately arranged at equal angular intervals (45°). The width of each notch41a(the measurement with respect to the circumferential direction of the first core member33) gradually decreases from the outer circumference toward the inner circumference of the first outer ring portion26a. That is, each notch41ais shaped like a wedge.

Four coupling projection44aare provided on the lower surface42of the first outer ring portion26a. Each coupling projection44aextends from the outer edge to the inner edge of the lower surface42of the first outer ring portion26a. The outer edge of each coupling projection44ais connected to the proximal section28bof the corresponding tooth body28.

The measurement of each coupling projection44aalong the axial direction of the first core member33is half the proximal tooth height Lhb. The lower surface of each coupling projection44ais connected to the lower surface of the corresponding tooth23without a step. Therefore, the coupling projections44aof the first core member33are displaced downward from the center of the teeth23with respect to the axial direction of the first core member33.

The width of each coupling projection44a(the measurement with respect to the circumferential direction of the first core member33) gradually decreases from the outer circumference toward the inner circumference of the first outer ring portion26a. That is, each coupling projection44ais shaped like a wedge.

The second core member34includes a second inner ring portion25bhaving a center hole24band a second outer ring portion26bprovided about the outer circumference of the second inner ring portion25b. Four of the teeth23are provided on the outer circumference of the second outer ring portion26bat equal angular intervals (90°). In this embodiment, the number of the teeth23provided on the second core member34is the half (four) of the total number of the teeth23of the core10.

The axial measurement of the second outer ring portion26bat the outer circumference is half the proximal tooth height Lhb. A lower surface28fof each tooth body28is connected to the lower surface of the circumferential portion of the second outer ring portion26bwithout a step.

Therefore, the second inner ring portion25band the second outer ring portion26bof the second core member34are displaced downward from the center of the teeth23with respect to the axial direction of the second core member34.

The second outer ring portion26bis divided into four sections by four notches41b. Each notch41bis located between each adjacent pair of the teeth23such that the notches41band the teeth23are alternately arranged at equal angular intervals (45°). The width of each notch41b(the measurement with respect to the circumferential direction of the second core member34) gradually decreases from the outer circumference toward the inner circumference of the second outer ring portion26b. That is, each notch41bis shaped like a wedge.

Four coupling projections44bare provided on the upper surface45of the second outer ring portion26b. Each coupling projection44bextends from the outer edge to the inner edge of the upper surface45of the second outer ring portion26b. The outer edge of each coupling projection44bis connected to the proximal section28bof the corresponding tooth body28.

The measurement of each coupling projection44aalong the axial direction of the second core member34is half the proximal tooth height Lhb. The upper surface of each coupling projection44bis connected to the upper surface of the corresponding tooth28without a step. Therefore, the coupling projections44bof the second core member34are displaced upward from the center of the teeth23with respect to the axial direction of the second core member34.

The width of each coupling projection44b(the measurement with respect to the circumferential direction of the second core member34) gradually decreases from the outer circumference toward the inner circumference of the second outer ring portion26b. That is, each coupling projection44bis shaped like a wedge.

That is, although the first core member33and the second core member34are placed upside down relative to each other, the structure of the first core member33is identical to that of the second core member34. Further, the shape of each notch41aof the first core member33corresponds to the shape of each coupling projection44bof the second core member34. Likewise, the shape of each notch41bof the second core member34corresponds to the shape of each coupling projection44aof the first core member33.

Each of the first and second core members33,34have the teeth23the number of which is half the total number of the teeth23of the core10. Thus, when the first core member33and the second core member34are engaged with each other, the core10having the eight teeth23is obtained.

In this embodiment, each of the first core member33and the second core member34is formed by compressing magnetic powder such that the first inner ring portion25a(the second inner ring portion25b), the first outer ring portion26a(the second outer ring portion26b), and the four teeth23are integrated.

FIG. 5shows a coil winding apparatus53for winding the wires31aof the coils31to the teeth23. The coil winding apparatus53includes two wire feeders (not shown), two pulleys54, two guiding members55, and two jigs56. The wire31a, which is drawn from each wire feeder, is fed to the first core portion33(the second core member34), which is supported by the jigs56, via the corresponding pulley54and the corresponding guiding member55.

Each jig56is located downstream of the corresponding guiding member55along the feeding direction of the corresponding wire31a. Each jig56includes a teeth holder57and a rotary shaft58. A holding recess57ais formed in each teeth holder57. The holding recess57ais shaped to correspond to the magnetism converging portion29.

The teeth holders57hold two of the teeth23that are spaced by 180° in the first core member33(the second core member34). Thus, the first core member33(the second core member34) is held by two jigs56. The holding recesses57aof the jigs56face each other, and the first core member33(the second core member34) is held between the jigs56.

The rotary shaft58of each teeth holder57extends in a direction away from the holding recess57a. Each rotary shaft58is coupled to the rotary shaft of a drive motor (not shown). The jigs56are rotated in the same direction (direction indicated by arrows F1inFIG. 5) by the drive motors. When the jigs56are rotated, the first core member33(the second core member34) is rotated. Accordingly, the wires31aare simultaneously wound (in a concentrated manner) about the two of the teeth23held by the teeth holder57.

Each guiding member55is located downstream of the corresponding pulley54along the feeding direction of the corresponding wire31a. Each guiding member55guides the wire31athat is fed from the corresponding wire feeder. Each guiding member55is moved along the axial direction of the corresponding rotary shaft58(direction indicated by arrows F2in FIG.5).

The coil winding apparatus53winds the wires31aabout the teeth28. The apparatus53also fixes the wire31ato the projections30aprior to and after winding the wire31ato the tooth body28. During winding of the wire31a, each guiding member55guides the wire31abetween the proximal section28band the distal section28aof the tooth body28about which the wire31ais wound.

When the wire31ais fixed, each guiding member55guides the wire31ato the projections30aof the tooth insulator30.

A method for manufacturing the armature3with the coil winding apparatus53will hereafter be described.

First, the tooth insulator30is attached to each tooth23of the first core member33and the second core member34. The projections30aof the tooth insulator30attached to the first core member33are located at a side (the lower side) of the first core member33where the coupling projections44aare provided. The projections30aof the tooth insulator30attached to the second core member34are located at a side opposite from the side (the upper side) where the coupling projections44bare provided. Then, the jigs56hold two of the teeth23that are on a line perpendicular to the axis of the first core member33.

During winding of the wire31aby the coil winding apparatus53, the wires31athat are drawn from the wire feeders are sent to the first core member33through the pulleys54and the guiding members55. Rotation of the jigs56causes the wires31ato be wound about the two teeth23held by the jigs56. Since the space between each adjacent pair of the teeth23of the first core member33is wide (the teeth23are spaced by 90°), the wires31adrawn from the wire feeders are prevented from contacting the magnetism converging portions29of the teeth23about which the wires31aare not wound.

When winding of the wires31aabout the two teeth23is completed, the coil winding apparatus53performs fixing of the wires31a. At this time, the wires31aare guided to the projections30aby the guiding members55, and the jigs56are rotated in a direction opposite from the direction when winding of the wires31a(in a direction indicated by arrows F3). This fixes each wire31ato the corresponding projection30a. Fixing of the wire31aprior to the winding is performed in the same manner as the fixing of the wire31aafter the winding.

After cutting the wire31ato be a predetermined length, two of the teeth23about which the wires31have not been wound are held by the teeth holders57. Then, winding and fixing of the wires31aare performed. Accordingly, winding of the wires31ato the first core member33is completed. The winding and fixing of the wires31aare performed to the second core member34. In this manner, the core members33,34each having the four coils31are obtained.

Next, the axes of the first core member33and the second core member34are aligned such that the first inner ring portion25aof the first core member33and the second inner ring portion25bof the second core member34overlap each other. Then, the first core member33and the second core member34are assembled to each other such that the teeth23of the first core member33and the teeth23of the second core member34are displaced by 45°. Accordingly, the core10is formed.

Specifically, each coupling projection44aof the first core member33is fitted in one of the notches41bof the second core member34, and each coupling projection44bof the second core member34is fitted in one of the notches41aof the first core member33. Accordingly, the first core member33is coupled to the second core member34.

The first inner ring portion25aand the second inner ring portion25bform the inner ring portion25of the ring body21. The first outer ring portion26a, the second outer ring portion26b, the coupling projections44a, and the coupling projections44bform the outer ring portion26. The inner ring portion25and the outer ring portion26form the ring body21. The teeth23are located on the outer circumference of the ring body21with equal angular intervals (45°)

Thereafter, the core10and the commutator11are fixed to the rotary shaft7, and the ends of each wire31aare connected to the corresponding short circuit member14. Each short circuit member14is connected to the corresponding segments13of the commutator11. The armature3is thus completed.

The relationship between the shape and the area of the magnetic flux passing cross-section of each tooth body28will now be described.

FIG. 6is a graph showing the relationship between a height ratio Kh and a width ratio Kw. In the graph, Lhx represents the tooth height (actual tooth height) of each tooth body28at a given section between the distal section28aand the proximal section28b. The height ratio Kh represents the ratio of the tooth height Lhx to a reference tooth height Lh0. Lwx represents the tooth width (actual tooth width) at a given section between the distal section28aand the proximal section28b. The width ratio Kw represents the ratio of the tooth width Lwx to a reference tooth width Lw0.

In this embodiment, the four sides28gto28jof each tooth body28are linear from the proximal section28bto the distal section28aof the tooth body28. Therefore, the actual tooth height Lhx and the actual tooth width Lwx linearly changes from the distal section28ato the proximal section28b. The relationship between the height ratio Kh and the width ratio Kw satisfies the following equation;
Kh=−α(Kw−1)+1
in which α is an arbitrary coefficient.

On the other hand, if an equation (the reference tooth height Lh0×the reference tooth width Lw0)=(the actual tooth height Lhx×the actual tooth width Lwx) is satisfied at an given section between the distal section28aand the proximal section28bof each tooth body28, that is, if the area of magnetic flux passing cross-section is constant at a given section between the distal section28aand the proximal section28b, an equation Kh=1/Kw is satisfied, and the sides28gto28jof the tooth body28are curved.

For example, suppose that a reference cross-sectional area S0 and the area St of the magnetic flux passing cross-section at the distal section28aare both 50 mm (S0=St=50 mm), the reference tooth height Lh0 and the distal tooth height Lht are both 10 mm (Lh0=Lht=10 mm), and the reference tooth width Lw0 and the distal tooth width Lwt are both 5 mm (Lw0=Lwt=5 mm).

Also, suppose that the area Sb of the magnetic flux passing cross-section at the proximal section28bis 50 mm, which is equal to the area St of the magnetic flux passing cross-section at the distal section28a, the proximal tooth height Lhb is 12.5 mm (Kh=1.25), and the proximal tooth width Lwb is 4 mm (Kw=0.8).

If the area of a given magnetic flux passing cross-section between the proximal section28band the distal section28aof the tooth body28is constant, the combination of Kw and Kh (Kw, Kh) at the given magnetic flux passing cross-section is plotted on a curve1of an inverse proportion (Kh=1/Kw) that passes through a point A (1, 1) and a point B (0.8, 1.25) on the graph of FIG.6.

The area of the magnetic flux passing cross-section at a given section between the distal section28aand the proximal section28bof the tooth body28is represented by an actual cross-sectional area Sx. If the actual cross-sectional area Sx is greater than the reference cross-sectional area S0, the point (Kw, Kh) is plotted above the curve1. If the actual cross-sectional area Sx is smaller than the reference cross-sectional area S0, the point (Kw, Kh) is plotted below the curve1.

In this embodiment, the four sides28gto28jof each tooth body28linearly extend from the proximal section28bto the distal section28aof the tooth body28. Therefore, a point (Kw, Kh) at a given magnetic flux passing cross-section between the proximal section28band the distal section28ais plotted on a straight line m passing through the point A and the point B on the graph of FIG.6. As shown inFIG. 6, the straight line m is above the curve1in a range of 0.8<Kw<1.0.

The area St of the magnetic flux passing cross-section at the distal section28a(which is equal to the area Sb of the magnetic flux passing cross-section at the proximal section28b) is set as the reference cross-section S0. The four sides28gto28jof each tooth body28are linear from the proximal section28bto the distal section28aof the tooth body28. Accordingly, the actual cross-sectional area Sx of the flux passing cross-section at a given section between the distal section28aand the proximal section28bof the tooth body28(point X in the graph) is greater than the reference cross-sectional area S0, or the area St of the flux passing through cross-section at the distal section28a. Therefore, effective flux is not reduced at a given section between the distal section28aand the proximal section28bof the tooth body28.

The above embodiment has the following advantages.

(1) Each tooth23of the core10includes the tooth body28, about which the coil31is wound, and the magnetism converging portion29provided at the distal end of the tooth body28. The tooth width of each tooth body28gradually decreases from the distal section28atoward the proximal section28b.

Accordingly, the space of each slot17between an adjacent pair of the teeth23(the space between the teeth23) widens toward the radially inner end. Therefore, the amount of the coil31accommodated in the slot17is increased.

In a radially outer portion of each slot17, a center portion with respect to the circumferential direction of the core10is a portion that is farthest from the adjacent teeth28and therefore cannot be used for accommodating the coil31. However, in the above embodiment, since the tooth width is great at the distal section28a, it is easy to accommodate the coil31in that space. Accordingly, the coil accommodation efficiency of the slots17is improved.

The tooth height of each tooth28gradually increases from the distal section28ato the proximal section28b. Therefore, since the tooth height of each tooth body28decreases toward the distal section28a, the coil height is suppressed without decreasing the number of winding of the coil31. As a result, the size of the direct-current motor1is reduced.

Further, although the tooth width gradually decreases from the distal section28ato the proximal section28b, the tooth height increases. Thus, a necessary area of the flux passing cross-section is maintained. That is, since the coil accommodation efficiency of the slots17is high, a sufficient space for accommodating the coil31exists in each slot17even if the magnetic path (the cross-sectional area of the tooth body28) is increased to suppress loss of the effective magnetic flux. Thus, the amount of flux can be increased by using strong magnets5to increase the torque generated by the direct-current motor1.

Further, the core10is formed by assembling the first and second core members33,34each having the teeth23of a number that is half the total number of the teeth23of the core10. The tooth height of each tooth28gradually increases from the distal section28ato the proximal section28b. Therefore, the tooth height of the proximal section28b, which receives load due to assembling, is readily increased to improve the durability of the teeth23against the assembling load applied when assembling the first core member33with the second core member34. Thus, deformation and cracking of the teeth23due to assembly of the first core member33and the second core member34are prevented.

(2) In this embodiment, the coil31is wound about each tooth23of the first and second core members33,34prior to assembly of the core members33,34. Thereafter, the first core member33and the second core member34are assembled to form the core10.

Prior to the assembly, the first core member33is separated from the second core member34, and the space between each adjacent pair of the teeth23is wide (the teeth23are spaced by 90°). There is thus little restriction to winding of the coil31. This increases the efficiency of winding of the coil31and, as a result, increases the coil space factor.

(3) The teeth28are formed such that the area of the flux passing cross-section at the distal section28aof each tooth body28is substantially equal to the area of the flux passing cross-section at the proximal section28b, or such that the equation (the distal tooth height Lht×the distal tooth width Lwt)=(the proximal tooth height Lhb×the proximal tooth width Lwb) is substantially satisfied. Thus, the effective flux at the distal section28aand the proximal section28bof each tooth body28is prevented from decreasing. As a result, the torque generated by the direct-current motor1is increased.

(4) The four sides28gto28jof each tooth28linearly extend from the proximal section28bto the distal section28a. This simplifies the shape of each tooth body28, and thus facilitates the forming of the core10.

Further, if the area of the flux passing cross-section at at least one of the distal section28aand the proximal section28bis set as the minimum cross-sectional area (reference cross-sectional area), the area of the flux passing cross-section at a given section between the distal section28aand the proximal section28bof each tooth28is greater than the reference cross-sectional area. Therefore, the effective flux at a given section of the tooth body28is prevented from decreasing. As a result, the torque generated by the direct-current motor1is increased.

(5) The measurement of the magnetism converging portion29of each tooth23with respect to the axial direction of the core10is substantially the same as the measurement of each magnet5in the yoke4. Therefore, the flux of each magnet5flows into the magnetism converging portion29at positions relatively close to the magnet5. As a result, the torque generated by the direct-current motor1is increased.

(6) The recesses27are formed on the upper and lower surface of the ring body21.

Therefore, an axial end of the commutator11and an axial end of the bearing15toward the recesses27are accommodated in the recesses27. Accordingly, the axial size of the armature3is reduced. As a result, the axial size of the direct-current motor1is reduced.

(7) As the jigs56rotate, the first core member33(the second core member34) rotates, and the wires31aare wound about the teeth23. Therefore, compared to a case in which a nozzle is rotated to wind the wire31aabout each tooth23, the wire31ais not wound with complicated actions. Thus, the winding speed of the wire31ais readily increased. This improves the productivity of the direct-current motor1.

Also, since no space for inserting a nozzle is required in each slot17, only the space required for the coil31needs to be created in each slot17. As a result, space that is conventionally incapable of accommodating a coil is used to accommodate the coil31. This increases the space factor of the wire31a, and, in turn, increases the space factor of the direct-current motor1.

(8) Two of the teeth23are each held by the jig56.

Thus, compared to a case where only one of the teeth23is held by the jig56, the core members33,34are steadily rotated. Also, since two of the teeth23that are angularly spaced by 180° are held by the jigs56, it is possible to draw the wires31ain direction perpendicular to the direction along which the teeth23extend. This facilitates the wires31ato be wound about each tooth23in a uniform thickness. Thus, since the wires31aare stably wound, the productivity of the direct-current motor1is further improved.

(9) The guiding members55are moved along the extending direction of the teeth23about which the wires31aare being wound (the direction indicated by arrows F2), thereby guiding the wires31adrawn from the wire feeders. Therefore, by moving the guiding members55while the jigs56are rotated, the wires31aare properly aligned and wound about the teeth23at a uniform thickness. This improves the space factor of the coils31.

(10) When the jigs56are rotated, the first core member33(the second core member34) is rotated. Accordingly, the wires31aare simultaneously wound (in a concentrated manner) about two of the teeth23held by the teeth holders57. Therefore, compared to a case where the wire31ais consecutively wound about one of the teeth23at a time, the speed at which the wires31aare wound about the teeth23of the first core member33(the second core member34) is doubled. Therefore, the productivity of the direct-current motor1is further improved.

(11) The coil winding apparatus53not only winds the wires31aabout the teeth23, but also fixes the wires31ato the projections30a(wire fixing). Therefore, unlike a case where the coil winding and the wire fixing are performed by separate apparatuses, apparatuses need not be replaced between the coil winding and the wire fixing. That is, the coil winding and the wire fixing are continuously performed. This further improves the productivity of the direct-current motor1.

(12) Before the core10is assembled, that is, when the first core member33and the second core member34are still separated, the magnetism converging portion29does not exist in a range from the proximal section28bto the distal section28aof each tooth body28of the cores33,34with respect to a direction perpendicular to the extending direction of the tooth body28.

Thus, the wires31adrawn from the wire feeders do not contact the magnetism converging portions29of the teeth23about which the wires31aare not being wound. Therefore, since restrictions during winding of the wires31aare reduced, the productivity of the direct-current motor1is further improved.

(13) The projections30aof the tooth insulator30attached to the first core member33are provided on the side (the lower side) where the coupling projections44aare provided. The projections30aof the tooth insulator30attached to the second core member34are provided on the side opposite from the side (the upper side) where the coupling projections44bare provided.

Therefore, when the first core member33and the second core member34are assembled, the ends of the wires31aof the coils31of the first core member33and the ends of the wires31aof the coils31of the second core member34are drawn from the same side (the lower side) with respect to the axial direction of the core10. This facilitates connection of the wires31ato the commutator11.

(14) The six magnets5are provided on the inner circumference of the yoke4at equal angular intervals. The eight teeth23are provided on the core10at equal angular intervals. The eight slots17are defined among the teeth23. The twenty-four segments13are provided on the outer circumference of the commutator insulator12of the commutator11. That is, the direct-current motor1has six poles, eight slots, and twenty-four segments.

Therefore, torque vectors of each pair of the slots17that are symmetric with respect to the axis of the core10counteract each other, which suppresses vibration of the armature3functioning as a rotor. As a result, the direct-current motor1with small vibrations is obtained.

(15) The core members33,34are formed by compressing magnetic powder. Therefore, although each tooth23has a complicated shape in which the magnetism converging portion29extends from the distal section28aof the tooth body28in the axial direction of the core10, the teeth23having such a complicated shape are readily formed compared to a case where the core members33,34are formed by laminating metal plates.

As shown inFIG. 7, the ratio between the distal tooth height Lht and the proximal tooth height Lhb (the ratio between the distal tooth width Lwt and the proximal tooth width Lwb) of each tooth body28may be changed such that the coil height Lc of the coil31wound about the tooth body28is constant from the distal section28ato the proximal section28bof the tooth body28.

That is, the teeth28may be formed such that the upper surface and the lower surface of the coils31are substantially parallel. This configuration further improves the space factor of the coils31. As a result, the size of the direct-current motor1is further reduced.

In the above embodiments, the area of the flux passing cross-section at the distal section28aof each tooth body28is substantially equal to that of the proximal section28b, and the four sides28gto28jof each tooth body28linearly extend from the proximal section28bto the distal section28a. However, as indicated by curve 1 for comparison in the graphFIG. 6, the sides28gto28jmay be changed such that the area of the flux passing cross-section is constant in a given section between the proximal section28band the distal section28aof the tooth body28.

That is, as shown inFIGS. 8 and 9, the sides48gto48jmay be curved inward as shown inFIGS. 8 and 9such that an equation (the reference tooth height Lh0×the reference tooth width Lw0)=(the actual tooth height Lhx×the actual tooth width Lwx) is satisfied at the flux passing cross-section at a given section in the extending direction of each tooth body48.

This configuration optimizes the size of each tooth body48in relation to the effective flux. Accordingly, the size of each tooth48is reduced while maintaining the effective flux at the given section in the extending direction of the tooth48. Further, compared to the case where the four sides48gto48jare linear, the space for winding the coil31(the slot17) is enlarged by curving the sides48gto48jinward. Thus, a greater amount of the coil31can be wound about the tooth body48. Also, the coil height of the coils31is reduced.

In the above embodiments, two of the teeth23that are angularly spaced by 180° are each held by the jig56. However, only one of the teeth23may be held by the jig56.

In the above embodiments, the wires31aare simultaneously wound about two of the teeth23of the first core member33(the second core member34). However, the wires31amay be consecutively wound about one of the teeth23at a time.

If each of the first and second core members33,34has an odd-number (for example, three) of the teeth23as shown inFIG. 10, the shape of one of the two jigs56may be changed.

In this case, a jig56ahaving a holding recess57bis prepared. The jig56aholds adjacent ends of a pair of adjacent magnetism converging portions29. The two jigs56,56ahold the three teeth23to hold the first core member33(the second core member34). The wire31ais wound about the tooth23held by the jig56.

In the above embodiments, the guiding members55need not be moved. Further, the guiding members55may be omitted.

In the above embodiments, the projections30aof the tooth insulator30attached to the second core member34are located at a side opposite from the side where the coupling projections44bare located. However, the projections30amay be provided elsewhere. In this case, the ends of the wires31awound about the teeth28of the first core member33may be drawn in the opposite direction from the ends of the wires31awound about the teeth28of the second core member34with respect to the axial direction of the core10.

In the above embodiments, the number of the teeth23, the number of the magnets5, and the number of the segments13are eight, six, and twenty-four, respectively. However, these numbers may be changed. That is, the direct-current motor1may have a configuration other than the configuration of six-pole, eight-slot, and twenty-segments.

In the above embodiments, the first core member33and the second core member34are formed by compressing magnetic powder. However, the core members33,34may be formed by laminating metal plates.

In the illustrated embodiments, the core10is formed by assembling the two core members33,34. However, the core10may be formed by assembling more than two core members. In this case, each core member has teeth the number of which is determined by dividing the total number of the teeth23of the core10by the number of the core members, and the teeth23on each core member are spaced by equal angular intervals.

In the above embodiments, the present invention is applied to the rotor core of the direct-current motor with brushes. However, the present invention may be applied to other types of motors such as a brushless direct-current motor.