Motor having noise limiter circuit

[0002] This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-278267 filed on Sep. 13, 2000 and Japanese Patent Application No. 2000-374763 filed on Dec. 8, 2000.

[0003] 1. Field of the Invention

[0004] The present invention relates a motor having a noise limiter circuit for restraining generation of electromagnetic noise between brushes and a commutator.

[0005] 2. Description of Related Art

[0006] In general, electromagnetic noise is generated between brushes and a commutator during rotation of a direct current motor. When power supplied to the brushes includes a relatively large amount of noise, a relatively large amount of electromagnetic noise is generated. Thus, some previously proposed motors have a noise limiter circuit that includes capacitors, choke coils and the like to smooth the electric power supplied to the brushes.

[0007] To reduce a size (thickness) of such a motor, it has been proposed to use an oblate cylindrical housing having a smaller profile in comparison to a cylindrical housing. In order to further reduce the size of the motor having the oblate cylindrical housing, effective arrangement of the electronic components, such as the choke coils, within the housing has been demanded.

[0008] Thus, it is an objective of the present invention to provide a motor that allows a reduction in its size by effective arrangement of its components.

[0009] To achieve the objective of the present invention, there is provided a motor including an electrically conductive yoke housing and a noise limiter circuit. The yoke housing is configured in an oblate cylindrical cup form having two opposed flat side walls and two opposed arcuate side walls, each of which joins corresponding opposed lateral edges of the flat side walls together. The yoke housing has an open end at one end and receives a plurality of brushes and a commutator. The noise limiter circuit is arranged at the open end of the yoke housing and smoothes electric power to be supplied to the plurality of brushes to restrain generation of electromagnetic noise between the plurality of brushes and the commutator. The noise limiter circuit includes at least one of the following components: a plurality of helically wound cylindrical choke coils and a plurality of capacitors. Each choke coil is arranged adjacent to a corresponding corner of the yoke housing located between a corresponding one of the flat side walls and a corresponding one of the arcuate side walls in such a manner that each choke coil extends substantially parallel to a central axis of the yoke housing. The capacitors are arranged adjacent to a center of one of the flat side walls.

[0010] The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:

[0011] FIG. 1 is a partial cutaway plan view of a motor according to an embodiment of the present invention;

[0012] FIG. 2 is a cross-sectional view along line II—II in FIG. 1:

[0013] FIG. 3 is a partial enlarged cross-sectional view of FIG. 2;

[0014] FIG. 4 is a schematic enlarged view of a brush holder of the motor;

[0015] FIG. 5 is a cross-sectional view along line V—V in FIG. 3;

[0016] FIG. 6 is a schematic view showing a molding operation of a choke coil receiving portion of the brush holder;

[0017] FIG. 7 is a cross-sectional view along line VII—VII in FIG. 3;

[0018] FIG. 8 is a partial exploded perspective view showing capacitors to be installed in the brush holder;

[0019] FIG. 9 is a cross-sectional view along line IX—IX in FIG. 3;

[0020] FIG. 10 is a circuit diagram of a noise limiter circuit of the embodiment;

[0021] FIG. 11 is a partial exploded perspective view showing a modification of FIG. 8; and

[0022] FIG. 12 is a circuit diagram showing a modification of the noise limiter circuit shown in FIG. 10.

[0023] One embodiment of the present invention will be described with reference to the accompanying drawings.

[0024] FIG. 1 shows a motor having a speed reducing mechanism according to the present embodiment. The motor 1 includes a motor unit 2 acting as a drive source of the motor 1. The motor unit 2 includes a yoke housing 3, a rotor 5, magnets 6 and a brush holder 8. The yoke housing 3 is made of an electrically conductive metal material. The rotor 5 includes a rotatable shaft 4. The brush holder 8 is made of a dielectric resin material and supports brushes 7a, 7b.

[0025] The yoke housing 3 is configured in an oblate cylindrical cup form having an open end at one end and a closed end at the other end. As shown in FIGS. 2 and 3, the yoke housing 3 includes two opposed parallel flat side walls 3aand two opposed arcuate side walls 3b,each of which joins corresponding opposed lateral edges of the flat side walls 3atogether. A bearing 9 is secured to a base 3cof the yoke housing 3 and rotatably supports a base end (top end in FIG. 1) of a rotatable shaft 4. A resin housing 10 made of a resin material is connected to the open end 3dof the yoke housing 3 with screws 11 and receives a portion of the rotatable shaft 4 which protrudes from the open end 3dof the yoke housing 3.

[0026] The resin housing 10 has a predetermined interior shape that allows the portion of the rotatable shaft 4, a worm wheel 12 and the like to be received therein. Two bearing 13, 14 are secured within the resin housing 10 and rotatably support a middle portion and a distal end portion (lower end in FIG. 1) of the rotatable shaft 4, respectively. A worm 4ais provided in the rotatable shaft 4 between the two bearings 13, 14. The worm 4ais meshed with the worm wheel 12, so that a rotational force of the worm 4ais transmitted in a direction perpendicular to a central axis L1 of the rotatable shaft 4.

[0027] The brush holder 8 is held between the open end 3dof the yoke housing 3 and a securing portion 10aof the resin housing 10 to which the yoke housing 3 is secured. The brush holder 8 has an engaging portion 8athat is fitted within the open end 3dof the yoke housing 3. Each brush 7a, 7bsupported by the brush holder 8 slidably engages against a commutator 5athat is provided at an open end 3dside portion of the rotator 5.

[0028] Each brush 7a, 7bis shaped in a rectangular parallelepiped form. As shown in FIGS. 2 and 3, each brush 7a, 7bis held in such a manner that a longitudinal axis L3 of each brush 7a, 7bextends in a direction perpendicular to the central axis L1 of the yoke housing 3 (rotatable shaft 4) and coincides with a straight line L2 that extends through a center of each arcuate side wall 3b. With this arrangement, in the yoke housing 3, a relatively large space is provided in a widthwise direction (direction perpendicular to the straight line L2 in FIGS. 2 and 3) of the brushes 7a, 7b, so that it is still possible to accommodate larger brushes 7a, 7bwithin the yoke housing 3 without increasing the size of the yoke housing 3.

[0029] The brush holder 8 includes a power supply unit 15 integrally formed therein. The power supply unit 15 is exposed from the resin housing 10. In this case, the power supply unit 15 extends out of the resin housing 10 along the straight line L2 and then extends in a direction parallel to the central axis L1 of the yoke housing 3. That is, the power supply unit 15 is configured to extend in a direction (hereinafter referred to as a flat direction of the yoke housing 3) parallel to a plane of each flat side wall 3aof the yoke housing 3. The flat direction of the yoke housing 3 is also the flat direction of the motor 1, so that size (thickness) of the motor 1 can be reduced by making the power supply unit 15 to extend in the flat direction of the yoke housing 3.

[0030] Furthermore, power source terminals 16a, 16bare insert molded within the power supply unit 15 and are disposed within a connection recess 15ato allow connection between the power source terminals 16a, 16band an external connector (not shown). The connection recess 15ahas an open end that opens in the direction of the axis L1 at its top end in FIG. 1. That is, the connection recess 15ais equally connectable to both the external connector extending from a front side of the connection recess 15atoward the connection recess 15aalong a direction perpendicular to the plane of FIG. 1 and the external connector extending from a rear side of the connection recess 15atoward the connection recess 15aalong the direction perpendicular to the plane of FIG. 1. Thus, with this construction of the power supply unit 15, there is achieved a higher degree of freedom in terms of connecting direction of the motor 1. The power supply terminals 16a, 16bof the power supply unit 15 are electrically connected to the brushes 7a, 7b, respectively, to supply the electrical power.

[0031] With reference to FIGS. 2 and 3, the brush holder 8 includes a circuit board (not shown) integrated therein. Two capacitors 21, 22, two choke coils 23, 24, a circuit breaker (PTC: Positive Temperature Coefficient) 25 and the like are arranged on the circuit board. As shown in FIG. 10, the capacitors 21, 22 and the choke coils 23, 24 constitute a noise limiter circuit 20 for limiting or restraining generation of electromagnetic noise between the brushes 7a, 7band the commutator 5a. The noise limiter circuit 20 is arranged between the brushes 7a, 7band the power supply terminals 16a, 16bthat supply the electrical power to the brushes 7a, 7b.

[0032] That is, the brush 7ais connected to the power supply terminal 16athrough the choke coil 23, and the brush 7bis connected to the power supply terminal 16bthrough the choke coil 24 and the circuit breaker 25. The brush 7ais electrically connected to the yoke housing 3 through the capacitor 21, and the brush 7bis electrically connected to the yoke housing 3 through the capacitor 22. The capacitors 21, 22 and the choke coils 23, 24 smooth the power to be supplied to the brushes 7a, 7band thereby restrain a large voltage change while the brushes 7a, 7bslidably engage against the commutator 5a. Thus, the capacitors 21, 22 and the choke coils 23, 24 restrain generation of electromagnetic noise between the brushes 7a, 7band the commutator 5a.

[0033] As shown in FIGS. 1 to 3, each choke coil 23, 24 is helically wound into a cylindrical form and centrally receives a core secured therein. Each choke coil 23, 24 has a length greater than its diameter. One end of each choke coil 23, 24 is solder-bonded to the circuit board integrated in the brush holder 8 to make electrical connection to the corresponding power source terminal 16a, 16b, and the other end of each choke coil 23, 24 is connected to the corresponding brush 7a, 7bthrough a pig tail 7c. In this case, the other end 23a, 24aof each choke coil 23, 24 is positioned adjacent to the corresponding brush 7a, 7bbut is spaced away from the commutator 5a. Specifically, as shown in FIG. 3, two central lines L4, L5 extend through the center of each choke coil 23, 24 in crisscross fashion. Among these central lines L4, L5, one central line L4 extends parallel to the straight line L2 through the center of the choke coil 23, 24, and the other central line L5 extends perpendicular to the one central line L4 through the center of the choke coil 23, 24. The other end 23a, 24aof each choke coil 23, 24 is positioned in such a manner that the other end 23a, 24aof each choke coil 23, 24 is located on the same side of the one central line L4 as the corresponding brush 7a, 7band is also located on an opposite side of the central line L5 with respect to the commutator 5a. That is, each pig tail 7cconnected to the other end 23a, 24aof each choke coil 23, 24 is spaced away from the commutator 5aand is prevented from contacting the commutator 5ato restrain short-circuiting therebetween. Each choke coil 23, 24 is arranged in such a manner that an axial line L6 of the choke coil 23, 24 extends parallel to the central axis L1 of the yoke housing 3 (rotatable shaft 4) at a corresponding corner of the yoke housing 3 between the corresponding flat side wall 3aand the corresponding arcuate side wall 3b.

[0034] That is, the choke coils 23, 24 are the relatively large electronic components in comparison to the capacitors 21, 22 and are arranged at the corners of the yoke housing 3 where a relatively large space can be provided between each choke coil 23, 24 and the commutator 5a. In this way, effective use of the internal space of the yoke housing 3 can be achieved. Furthermore, since the choke coils 23, 24 having the length greater than its diameter are arranged to extend parallel to the central axis L1 of the yoke housing 3 (rotatable shaft 4), the choke coils 23, 24 occupy a minimum portion of a cross-section of the yoke housing 3 that is located in a plane perpendicular to the central axis L1 of the yoke housing 3.

[0035] Furthermore, as shown in FIG. 4, the brush holder 8 includes a couple of choke coil receiving portions 8bfor receiving and positioning the corresponding choke coils 23, 24. As shown in FIG. 5, each choke coil receiving portion 8bhas an axial depth that is greater than the axial length of the main body (cylindrical portion) of the corresponding choke coil 23, 24, so that the main body of the choke coil 23, 24 received within the choke coil receiving portion 8bdoes not protrude from the choke coil receiving portion 8b. In this way, contact between each choke coil 23, 24 and the other component (e.g., the yoke housing 3, the commutator 5aor the like) is prevented, thereby improving the insulation of each choke coil 23, 24.

[0036] Each choke coil receiving portion 8bincludes a resilient piece 8dand an outer lateral wall 8cto be engaged with the yoke housing 3. A protrusion 8eis formed in an axial center portion of an inner peripheral surface of the outer lateral wall 8cwhich opposes the resilient piece 8d. When each choke coil 23, 24 is received within the corresponding choke coil receiving portion 8bbefore solder-bonding the choke coil 23, 24, the protrusion 8eengages between adjacent outer ridges of the choke coil 23, 24. The resilient piece 8dkeeps the engaged state of the protrusion 8eby exerting an urging force against the choke coil 23,24 and urging the choke coil 23, 24 against the outer lateral wall 8c, as shown in FIG. 5. Thus, before solder-bonding each choke coil 23, 24, the choke coil 23, 24 is prevented from moving out of the choke coil receiving portion 8bby the protrusion 8eand the resilient piece 8d, thereby allowing easier solder-bonding operation of the choke coil 23, 24.

[0037] With reference to FIG. 6, the brush holder 8 is molded by an upper die 30 and a lower die 31. Each choke coil receiving portion 8bis molded by a corresponding portion of the upper die 30 and a corresponding portion of the lower die 31. In the lower die 31, a projection 31ais provided. The projection 31aextends in an axial direction of the choke coil receiving portion 8band penetrates through a bottom surface of the choke coil receiving portion 8b, so that a through hole 8fis formed through the bottom surface of each choke coil receiving portion 8bof the molded brush holder 8. In the present embodiment, the protrusion 8eis molded between a distal end surface of the projection 31aand the upper die 30.

[0038] As shown in FIG. 8, each capacitor 21, 22 includes a main body 21a, 22aof a rectangular parallelepiped shape and two wire-shaped legs 21b, 21c, 22b, 22cextending from the main body 21a, 22a. One leg 21b, 22bof each capacitor 21, 22 acts as a brush-side terminal that is connected to the corresponding brush 7a, 7b, and the other leg 21c, 22cof each capacitor 21, 22 acts as a ground-side terminal. The two wire-shaped legs 21b, 21c, 22b, 22cof each capacitor 21, 22 are arranged along a direction (hereinafter referred to as a flat direction of the main body 21a, 22a) parallel to a plane of a flat surface portion 21e, 22eof the main body 21a, 22a.

[0039] As shown in FIG. 4, the brush holder 8 also includes a capacitor receiving portion 8gfor receiving and positioning the capacitors 21, 22 therein. As shown in FIG. 7, an axial depth of the capacitor receiving portion 8gis greater than an axial length of the main body 21a, 22aof each capacitor 21, 22, so that the capacitors 21, 22 received in the capacitor receiving portion 8gdo not protrude from the capacitor receiving portion 8g. In this way, contact between the capacitors 21, 22 and the other components (e.g., the yoke housing 3, the commutator 5aor the like) is prevented, thereby improving insulation of the capacitors 21,22.

[0040] The one leg (brush-side terminal) 21b, 22bof each capacitor 21, 22 is connected to a corresponding connector element extending from the circuit board integrally formed in the brush holder 8. The capacitors 21, 22 are substantially arranged at the center of the flat side wall 3athat is located on the opposite side of the commutator 5awith respect to the choke coils 23, 24.

[0041] That is, the capacitors 21, 22 are the relatively small electronic components in comparison to the choke coils 23, 24. Thus, effective use of the internal space of the yoke housing 3 is advantageously achieved by arranging the capacitors 21, 22 in a small space located between the center of the flat side wall 3aand the commutator 5a. Furthermore, the capacitors 21, 22 are arranged in such a manner that the flat direction of the main body 21a, 22aof each capacitor 21, 22 is substantially parallel to a plane of the flat side wall 3a, and the legs 21b, 21c, 22b, 22cof each capacitor 21, 22 are arranged along the direction parallel to the plane of the flat side wall 3a. Thus, an increase in the accommodating space for accommodating the capacitors 21, 22 in the cross-section of the yoke housing 3 located in the plane perpendicular to the central axis L1 of the yoke housing 3, is substantially restrained.

[0042] Furthermore, as shown in FIG. 8, the other legs (ground-side leg) 21c, 22cof the capacitors 21, 22 are commonly solder-bonded to a single connector plate 26. The connector plate 26 has a connector element 26a. The connector element 26ais disposed outside of the engaging portion 8aand is electrically connected to an inner side of the flat side wall 3aof the yoke housing 3. Thus, when the engaging portion 8aof the brush holder 8 is fitted within the yoke housing 3, the connector plate 26 (connector element 26a) is electrically connected to the yoke housing 3, i.e., the legs (ground-side terminals) 21c, 22cof the capacitors 21, 22 are electrically connected to the yoke housing 3.

[0043] As shown in FIGS. 2 and 3, the circuit breaker 25 is solder-bonded to the circuit board integrally formed in the brush holder 8 and is generally positioned at the center of the flat side wall 3alocated on the other side of the commutator 5awith respect to the capacitors 21, 22. The circuit breaker 25 is the electronic component that is shaped in a flat rectangular parallelepiped form having a flat portion 25a. Thus, the circuit breaker 25 can be arranged in a small space defined between the center portion of the flat side wall 3aand the commutator 5ain such a manner that a plane of the flat portion 25aof the circuit breaker 25 extends substantially parallel to the plane of the flat side wall 3a, thereby improving the efficiency of the internal space of the yoke housing 3.

[0044] As shown in FIG. 4, the brush holder 8 includes a circuit breaker receiving portion 8hfor receiving and positioning the circuit breaker 25 therein. As shown in FIG. 9, an axial depth of the circuit breaker receiving portion 8his greater than an axial length of a main body of the circuit breaker 25, so that the circuit breaker 25 received in the circuit breaker receiving portion 8hdoes not protrude from the circuit breaker receiving portion 8h. In this way, contact between the circuit breaker 25 and the other components (e.g., the yoke housing 3, the commutator 5aor the like) is prevented, thereby improving insulation of the circuit breaker 25.

[0045] As described above, the present embodiment provides the following advantages.

[0046] (1) Each choke coil 23, 24, which constitutes the noise limiter circuit 20, is arranged in such a manner that the axial line L6 of the choke coil 23, 24 extends substantially parallel to the central axis L1 of the yoke housing 3 at the corresponding corner of the yoke housing 3 between the corresponding flat side wall 3aand the corresponding arcuate side wall 3b. Since the rotor 5 (commutator 5a) is arranged at the center of the yoke housing 3, the corner between the flat side wall 3aand the arcuate side wall 3bis the place where the relatively large space can be provided between the rotator 5 (commutator 6a) and the corresponding choke coil 23, 24. Thus, since the choke coils 23, 24 are the relatively large components with respect to the capacitors 21, 22, it is possible to improve the efficiency of the internal space of the yoke housing 3 by positioning each choke coil 23, 24 at the corresponding corner between the corresponding flat side wall 3aand the arcuate side wall 3b. Thus, a size of the cross-section of the yoke housing 3 (an opening size of the open end of the yoke housing 3) located in the plane perpendicular to the central axis L1 of the yoke housing 3 can be reduced, thereby allowing a reduction in the size of the motor 1. Furthermore, since this type of choke coil 23, 24 is generally cylindrical and has the axial length greater than its diameter, it is possible to substantially restrain an increase in the accommodating space for accommodating the choke coils 23, 24 in the plate perpendicular to the central axis L1 of the yoke housing 3 by arranging the choke coil 23, 24 parallel to the central axis L1 of the yoke housing 3.

[0047] (2) The capacitors 21, 22, which constitute the noise limiter circuit 20, are arranged adjacent to the center of the flat side wall 3aof the yoke housing 3. As described above, the rotor 5 (commutator 5a) is arranged in the center of the yoke housing 3, so that the center of the flat side wall 3aof the yoke housing 3 is the narrow portion where the space between the center of the flat side wall 3aand the rotor 5 (commutator 5a) is small. Thus, since the capacitors 21, 22 are the relatively small electronic components in comparison to the choke coils 23, 24, it is possible to improve the efficiency of the internal space of the yoke housing 3 by arranging the capacitors 21, 22 at the center of the flat side wall 3a, thereby allowing a reduction in the size of the motor 1.

[0048] (3) The circuit breaker 25 is generally arranged at the center of the flat side wall 3athat opposes the flat side wall 3aat which the capacitors 21, 22 are arranged. Similar to the space between the capacitors 21, 22 and the rotor 5 (commutator 5a), this is also the narrow portion where the space between the circuit breaker 25 and the rotor 5 (commutator 5a) is small. However, since the circuit breaker 25 has the flat rectangular parallelepiped form, it is possible to improve the efficiency of the internal space of the yoke housing 3 by positioning the circuit breaker 25 in this narrow portion, thereby allowing further reduction in the size of the motor 1.

[0049] (4) Since the choke coils 23, 24 are received in the choke coil receiving portion 8bprovided in the brush holder 8, it is not required to provide a dedicated separate component for supporting the choke coils 23, 24, thereby allowing reduction in the number of the components and also reduction in the number of assembling steps. Since the choke coils 23, 24 are received in the choke coil receiving portions 8b, the choke coils 23, 24 can be appropriately positioned. In this way, contact between each choke coil 23, 24 and the other component can be prevented, thereby restraining damage to the choke coils 23, 24 and improving the insulation of each choke coil 23, 24.

[0050] (5) Each choke coil receiving portion 8bincludes the protrusion 8e, which engages between adjacent outer ridges of the choke coil 23, 24, and the resilient piece 8d, which extends in the axial direction of the choke coil 23, 24 on the opposite side of the choke coil 23, 24 with respect to the protrusion 8e. Thus, before solder-bonding each choke coil 23, 24 received in the corresponding choke coil receiving portion 8b, the protrusion 8eengages between the adjacent outer ridges of the choke coil 23, 24, and this engagement of the protrusion 8dis maintained by the resilient force of the resilient piece 8d. Thus, before solder-bonding each choke coil 23, 24, the choke coil 23, 24 is prevented from moving out of the choke coil receiving portion 8bby the protrusion 8eand the resilient piece 8d, thereby allowing easier solder-bonding operation of the choke coil 23, 24.

[0051] (6) The capacitors 21, 22 are received within the capacitor receiving portion 8gprovided in the brush holder 8, so that it is not required to provide a dedicated separate component for supporting the capacitors 21, 22, thereby allowing reduction in the number of the components and also reduction in the number of assembling steps. Since the capacitors 21, 22 are received within the capacitor receiving portion 8g, the capacitors 21, 22 can be appropriately positioned. In this way, contact between each capacitor 21, 22 and the other component is prevented, thereby restraining damage to each capacitor 21, 22 and improving the insulation of each capacitor 21, 22.

[0052] (7) The leg (ground-side terminal) 21c, 22cof each capacitor 21, 22 is electrically connected to the yoke housing 3 through the connector plate 26 when the brush holder 8 is installed in the open end of the yoke housing 3. Thus, a separate step of electrically connecting the leg 21c, 22c(ground-side terminal) of the capacitor 21, 22 to the yoke housing 3 is not required.

[0053] (8) Each brush 7a, 7bis arranged in such a manner that the longitudinal axis L3 of each brush 7a, 7bextends perpendicular to the central axis L1 of the yoke housing 3 (rotatable shaft 4) and generally coincides with the straight line L2 that passes through the center of each arcuate side wall 3bof the yoke housing 3. For example, if each brush 7a, 7bis designed to be positioned at the corner of the yoke housing 3 between the flat side wall 3aand the arcuate side wall 3band needs to be increased in the size in the widthwise direction of the brush 7a, 7b, a sufficient space cannot be provided between the flat side wall 3aand the flat side wall 3aside of the brush 7a, 7b. Thus, it is difficult to allow the required increase in the size of the brush 7a, 7b, for example, by changing the holding position of each brush 7a, 7b. However, in the present embodiment, since a relatively large space can be provided in the widthwise direction of the brush 7a, 7b, it is relatively easy to meet the required increase in the size of the brush 7a, 7b.

[0054] The present embodiment can be modified as follows.

[0055] In the above embodiment, as shown in FIGS. 1 to 3, although the cylindrical choke coils 23, 24 are used, each choke coil 23, 24 needs not be completely cylindrical. For example, each choke coil 23, 24 may be helically wound into a tube form having an ellipsoidal cross-section.

[0056] In the above embodiment, although the choke coils 23, 24 are arranged at the positions depicted in FIGS. 1 to 3, the positions of the choke coils 23, 24 are not limited to those depicted in FIGS. 1 to 3. For example, the choke coils 23, 24 can be arranged at diagonally opposed corners of the yoke housing 3.

[0057] In the above embodiment, the main body 21a, 22aof each capacitor 21, 22 is shaped in the rectangular parallelepiped form. Furthermore, the legs 21b, 21c, 22b, 22cof each capacitor 21, 22 are shaped in the wire form and are arranged along the flat direction of the main body 21a, 22a. However, it is not essential to use such a capacitor 21, 22. For example, each capacitor may have a main body shaped in a cylindrical form. Also, each leg of the capacitor may be shaped in a plate form. Furthermore, it is not essential to arrange the legs 21b, 21c, 22b, 22cof each capacitor 21, 22 along the flat direction of the main body 21a, 22a.

[0058] In the above embodiment, the capacitors 21, 22 are arranged in such a manner that the flat direction of the main body 21a, 22aof each capacitor 21, 22 is substantially parallel to the plane of the flat side wall 3aof the yoke housing 3. However, the present invention is not limited to this arrangement.

[0059] In the above embodiment, although the other leg 21c, 22cof each capacitor 21, 22, which acts as the ground-side terminal, is electrically connected to the yoke housing 3 through the connector plate 26, the other leg 21c, 22ccan be directly electrically connected to the yoke housing 3. For example, as shown in FIG. 11, the other leg (ground-side terminal) 21c, 22cof each capacitor 21, 22 may be exposed outside of the engaging portion 8athrough a corresponding slit 8iarranged in the engaging portion 8a, and a distal end portion 21d, 22dof the leg 21c, 22cof each capacitor 21, 22 may be received in a corresponding receiving groove 8jthat continuously extends from the slit 8i. The distal end portion 21d, 22dprotrudes slightly outwardly from an outer side surface of the engaging portion 8a. Thus, when the engaging portion 8aof the brush holder 8 is inserted within the yoke housing 3, the distal end portion 21d, 22dof the leg 21c, 22cof each capacitor 21, 22 is resiliently engaged against the inner side surface of the flat side wall 3aof the housing 3. Even with this arrangement, the leg 21c, 22cof each capacitor 21, 22 can be electrically connected to the yoke housing 3 when the brush holder 8 is received in the open end of the yoke housing 3. Thus, the step of electrically connecting the leg 21c, 22cof each capacitor 21, 22 to the yoke housing 3 is not required. Furthermore, a dedicated separate component for electrically connecting the leg (ground-side terminal) 21c, 22cof each capacitor 21, 22 to the yoke housing 3 is not required, thereby allowing reduction in the number of components and reduction in the number of the assembling steps. In this case, the receiving grooves 8jis not essential and can be eliminated.

[0060] In the above embodiment, each one of the choke coils 23, 24 and the capacitors 21, 22 is received and supported in the corresponding receiving portion 8b, 8g, 8hof the brush holder 8. The shape of each receiving portion 8b, 8g, 8his not limited to the above described shape. For example, the choke coil receiving portion 8bmay have an inner diameter that is substantially the same as the outer diameter of the corresponding choke coil 23, 24, and the protrusion 8emay be eliminated. The choke coil 23, 24 received in this receiving portion 8bmay be retained in place by the urging force of the resilient piece 8dacting against the choke coil 23, 24 toward the outer lateral wall 8c. Furthermore, the choke coils 23, 24 and the capacitors 21, 22 may be supported by any component, such as the resin housing 10, other than the brush holder 8.

[0061] In the above embodiment, although the noise limiter circuit 20 includes the two choke coils 23, 24 and the two capacitors 21, 22, any other arrangement can be used for the noise limiter circuit 20. For example, a resistor may be added to the noise limiter circuit 20. Alternatively, it is possible to eliminate the choke coils 23, 24 or the capacitors 21, 22 from the noise limiter circuit 20.

[0062] Furthermore, the noise limiter circuit 20 shown in FIG. 10 may be modified as shown in FIG. 12. In the noise limiter circuit 20 shown in FIG. 12, the circuit breaker 25 is eliminated. Also, the one leg 21b, 22bof each capacitor 21, 22 is connected to a node N1, N2 located between the choke coil 23, 24 and the brush 7a, 7b, and the other leg 21c, 22cof each capacitor 21, 22 is connected to the yoke housing 3.

[0063] In the above embodiment, although the brushes 7a, 7bare arranged in such a manner that the longitudinal axis L3 of each brush 7a, 7bextends perpendicular to the central axis L1 of the yoke housing 3 (rotatable shaft 4), the longitudinal axis L3 of each brush 7a, 7bmay be inclined with respect to the central axis L1 of the yoke housing 3 (rotatable shaft 4). That is, the longitudinal axis L3 of each brush 7a, 7bonly needs to intersect the central axis L1.

[0064] In the above embodiment, although the connection recess 15aof the power supply unit 15 has the opening that opens in the direction of the central axis L1 of the yoke housing 3, the opening of the connection recess 15amay open in any other suitable direction.

[0065] In the above embodiment, although the power supply unit 15 extends in the flat direction of the yoke housing 3, the power supply unit 15 may extend in any other suitable direction.

[0066] In the above embodiment, although the power supply unit 15 is integrally formed in the brush holder 8, the power supply unit 15 may be formed separately from the brush holder 8.

[0067] In the above embodiment, although the present invention is implemented in the motor 1 having the speed reducing mechanism, the present invention can be implemented in any other motor of a different structure, such as a motor having no speed reducing mechanism. Furthermore, the present invention may be implemented in a motor having more than two brushes.

[0068] Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore, not limited to the specific details, representative apparatus, and illustrative examples shown and described.