Motor

A motor includes a tubular housing having flat side walls along each side of rectangular joined with each other by joining sections, a magnet, having magnet poles in the jointing section, disposed along the inner peripheral surfaces of the housing, a shaft inserted along the axis of the housing, an armature fixed to the shaft in such a position as to be opposite to the magnet, a commutator mounted on the shaft, a carbon brush in sliding contact with the outer peripheral surface of the commutator, and electrical components provided within the housing. The electrical components in the longer direction are arranged such that they are not parallel with all the sides of the housing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2009-147736, filed on Jun. 22, 2009, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a motor and more particularly to a motor having brushes for power feeding within a housing.

2. Description of the Related Art

A DC motor (hereinafter referred to simply as “motor” also) is used in all fields and applications ranging from electrical equipment for automobiles and the like, audio and video equipment to household electrical appliances, toys and models.

For the motors in such applications, it is desired that electrical noise occurring during their operation be reduced. There are some known arts to reduce such noise by connecting electrical elements, such as capacitors or choke coils, to the circuit constituting the motor. For example, Reference (1) in the following Related Art List discloses a motor having a plurality of choke coils arranged in parallel with the direction of the rotational axis of the motor. Also, Reference (2) discloses a motor having a pair of choke coils aligned with each other.

2. Related Art List

However, the arrangement of choke coils in the motor as described above may inevitably lead to larger sizes of the motor in the axial and radial directions. Moreover, shortening the length of the choke coil in order to downsize it may result in a loss of the effect for reducing electrical noise.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoing circumstances, and a purpose thereof is to provide a technology for downsizing the motor without a loss of reliability.

To resolve the foregoing problems, a motor according to one embodiment of the present invention comprises: a tubular housing having four flat side walls along each side of rectangular, the side walls joined with each other by joining sections; a magnet disposed along inner peripheral surfaces of the housing, the magnet having magnetic poles in the joining sections; a shaft inserted along an axis of the housing; an armature fixed to the shaft in such a position as to be opposite to the magnet; a commutator mounted on the shaft; a brush in sliding contact with an outer peripheral surface of the commutator; and an electrical component provided within the housing. The electrical component in a longer direction thereof is arranged such that it is not parallel with all side walls of the housing.

The “tubular” meant here indicates that the shape of the housing may be a polygon having a plurality of side walls along each side. And the joining sections that connect the side walls may be chamfered or curved (rounded). From the point of view of downsizing the motor, it is desirable that the plurality of side walls form a regular polygon. The length of the electrical component is preferably greater than the length from the outer periphery of the commutator to the side wall.

By employing this embodiment, the housing is so shaped that the cross section thereof is larger than that in the case where the shape of the housing is cylindrical. That is, according to this embodiment, the area of the joining sections is larger than that in the case where the shape thereof is cylindrical. Accordingly, it is easy to place the electrical components in such a manner that the electrical components in a longer direction thereof can be arranged such that they are not parallel with all sides of the housing. As a result, the degree of freedom in the layout of the electrical components increases.

The electrical components in the longer direction thereof may be arranged such that the electrical components intersect with two adjacent side walls with a joining section positioned therebetween. Thus, the area of the joining sections is used effectively, which in turn increases the degree of freedom in the kind or the number of electronic components mounted. As a result, the motor is downsized and also highly reliable.

Also, according to this embodiment, a column-shaped carbon brush is biased in the longer direction and slides along the commutator. In other words, the carbon brush is used as a power feeding brush to the armature, so that the motor can be used for high current and high output as compared with a case where a metal brush is used in the motor.

The electrical components may be arranged such that a pair thereof are in a plane perpendicular to the axis of the housing and the electrical components are arranged around the shaft such that the electrical components in the longer direction thereof intersect with each other.

The electrical component may be a choke coil. Where the electrical component is a choke coil, the choke coil is arranged such that the two adjacent side walls with a joining section positioned therebetween and the axial direction of the choke coils intersect with each other. Thus, the sufficient coil length is assured. As a result, the motor is downsized and is highly reliable with the minimized occurrence of electrical noise. Also, a pair of choke coils is arranged nonparellely with each other and therefore the effect for suppressing the electrical noise is enhanced.

The brush may be a carbon brush. The carbon brush may be placed on a diagonal connecting the joining sections. In an attempt to downsize by providing the magnetic poles in the joining sections of a regular-polygonal housing, the carbon brushes are placed on the diagonals where the distance from the commutator can be the longest. As a result, the carbon brushes may be used for a longer period of time, and the life of the motor can be lengthened also.

DETAILED DESCRIPTION OF THE INVENTION

Hereinbelow, the embodiments will now be described with reference to drawings. Note that in all of the Figures the same structural components and members are given the same reference numerals and the repeated description thereof is omitted as appropriate. Moreover, the embodiments given are for illustrative purposes only and does not limit the scope of the present invention.

FIG. 1is a side view of a DC motor according to an embodiment of the present invention.FIG. 2is an exploded perspective view of a DC motor according to the embodiment as seen from a metal case side.FIG. 3is an exploded perspective view of a DC motor according to the embodiment as seen from a brush holder side.

As illustrated inFIG. 1toFIG. 3, a DC motor (hereinafter referred to as “motor”)10according to the present embodiment is so configured that a rotor14is housed in a tubular housing12. The housing12is an assembly of a bottomed tubular metal case16and a securing holder19that secures a tubular resin brush holder18to the metal case16. The metal case16also functions as a yoke forming a magnetic circuit. The metal case16has a tubular field magnet (hereinafter referred to as “magnet”)20fixed to the inner peripheral surface thereof and, together with it, forms a stator. In the center of the bottom of the metal case16is a boss17having a through-hole17athrough which a shaft22penetrates. The boss17, formed in such a manner as to slightly protrude outward, houses a bearing to be discussed later.

The rotor14, which is disposed on an end half of the rotary shaft22, is comprised of an armature24, a commutator26, a detecting magnet28, and the like. Disposed in the brush holder18is a pair of carbon brushes30placed opposite to the commutator26. The brush holder18is provided with a connecting terminal32for electrical connection to a circuit board27. The circuit board27has electronic components, such as inter-terminal capacitors and grounding capacitors, mounted thereon. The brush holder18, connected to the circuit board27by the connecting terminal32, is inserted and fitted to the metal case16. After the brush holder18is fitted into an opening section of the metal case16as described above, an opening side of the metal case16is sealed by the securing holder19.

An internal space of the housing12is thus defined by the metal case16and the brush holder18. The armature24includes a core46press-fitted on the shaft22and windings48wound around the core46(seeFIG. 4). The core46is placed opposite to the magnet20with a predetermined clearance (magnetic gap) between the outer periphery thereof and the inner periphery of the magnet20. The details of the magnetic pole configuration by the magnet20and the core46will be discussed later.

The commutator26, which is cylindrical in shape, is press-fitted on the shaft22in such a position that it is placed opposite to the carbon brushes30with the connecting terminal32fitted into the metal case16. The brush holder18has tubular carbon holders31fixed thereto, and the carbon brushes30are inserted into and held by the carbon holders31.

The detecting magnet28, which is a cylinder in shape a little smaller in outside diameter than the commutator26, is inserted into the shaft22in such a manner as to abut on the commutator26in the direction of axis line. The circuit board27has a Hall element (not shown) mounted thereon, and the Hall element, when the circuit board27is fitted into the metal case16, is placed opposite to the detecting magnet28.

The detecting magnet28is magnetized in two poles such that as it rotates, the north pole and the south pole appear alternately on the outer periphery thereof. The Hall element detects the switching (boundary) of the magnetic poles along with the rotation of the detecting magnet28and outputs a pulse signal therefor. The rotational speed (number of revolutions) of the motor10can be detected by counting the number of pulse signals in a predetermined period of time.

FIG. 4is a cross-sectional view taken along arrows A-A inFIG. 1. The metal case16, with a square-shaped cross section, has four flat side walls, and the adjacent side walls are joined with each other by a curved (rounded) corner section (joining section). In this manner, the motor10as a whole is downsized with the corner sections of the square rounded. The magnet20has the outer peripheral surface thereof bonded to the inner peripheral surface of the metal case16with an adhesive and is of such1Q shape that it is thicker in positions corresponding to the corner sections of the metal case16and thinner in positions corresponding to the side walls thereof. The magnet20is so configured that the center of the hypothetical circle formed by the inner peripheral surface thereof coincides with the shaft center of the armature24, and it has magnetic poles formed substantially in the thicker sections thereof.

That is, the magnet20is a 4-pole field magnet of a tubular body having a wall of uneven thickness, with a magnetic pole60(north pole), a magnetic pole61(south pole), a magnetic pole62(north pole), and a magnetic pole63(south pole) provided at circumferentially equal intervals. And the center of each magnetic pole in the thicker wall sections is located in each of the four corner sections of the metal case16. The magnet20may be formed, for instance, by integrally molding a magnetic material into a tubular body having a wall of uneven thickness, fixing it inside the metal case16, and then magnetizing it from outside the metal case16using a magnetic field generator. The magnetizing technology is publicly known and therefore the detailed description thereof is omitted here.

On the other hand, the core46has six magnetic poles64to69which extend radially from the central cylinder portion press-fitted on the shaft22, and the magnetic poles64to69each have a winding48wound therearound. It is to be noted that there is an application of a coating powder for insulation between the core46and the winding48.

Now a brush holder incorporating a PTC (Positive Temperature Coefficient) device will be described in detail.FIG. 5is an elevational view of a brush holder according to the present embodiment.FIG. 6is an elevational view of a brush holder according to the present embodiment as seen from a securing holder side.FIG. 7is a cross-sectional view taken along line B-B inFIG. 5.FIG. 8is a cross-sectional view taken along line C-C inFIG. 5.

As illustrated inFIG. 5toFIG. 8, the brush holder18has a tubular shape with a bottom, and the corner sections of the square cross section are rounded the same way as with the metal case16. Provided in the center of the brush holder18is a circular hole into which the commutator26and the detecting magnet28can be inserted in the direction of axis line. Around the hole, various functional parts are optimally arranged, thus making an effective use of the space. Note that the direction of axis line of the brush holder18is equal to the direction of the axis of rotation of the shaft22which is inserted in the brush holder18.

Fixed in the top left and right corner sections of the brush holder18as shown inFIG. 5is a pair of tubular carbon holders31disposed at 90 degrees apart along the circumferential direction. And slidably held in the carbon holders31are carbon brushes30. The carbon holders31, each having a long tube-like body of a square cross section and made of an electrically conductive metal, are so arranged as to extend in the radial direction from the axis center of the brush holder18. It should be appreciated that use of the carbon brushes30as the brushes for power feeding to the armature24allows the DC motor10to be used in higher current and higher output applications than with metal brushes.

The carbon brush30, which is a long prismatic column with a rectangular cross section, is inserted and slidably held in the carbon holder31. Thus, when the metal case16, the rotor14, the brush holder18, the circuit board27and the securing holder19are fitted together, the carbon brushes30are so arranged as to extend on the lines connecting the axis center of the commutator26to the respective corner sections of the brush holder18. In the present embodiment, the lines connecting the axis center to the corner sections are also on the diagonals of a square formed by connecting the flat parts of the brush holder18, and therefore they are the positions where the space in the radial direction is the longest of the circumferential positions within the brush holder18. This means that the carbon brushes30are placed in positions where they can be the longest. Accordingly, in an attempt to downsize by providing magnetic poles in the corner sections of a regular-polygonal housing, the carbon brushes are placed on the diagonals where the distance from the commutator can be the longest. As a result, the carbon brushes may be used for a longer period of time, and the life of the motor may be lengthened also.

A torsion spring70is disposed in the space between a corner section and a side wall of the brush holder18. The torsion spring70, which is comprised of a coil portion71, a first end73and a second end72, is so configured that the coil portion71thereof is inserted and held in a boss portion92located close to a side wall of the brush holder18and the first end73extending from the coil portion71is in contact with the rear end face of the carbon brush30. The second end72extending from the coil portion71is fixed to the upper portion of the carbon holder31. The torsion spring70applies a biasing force to a commutator26side with the first end73thereof communicating the biasing force accumulated in the coil portion71to the carbon brush30.

A not-shown slit in a stroke direction of the carbon brush30is provided in the surface of the carbon holder31opposite to the torsion spring70. Therefore, even when the carbon brush30moves in a stroke toward the commutator26, the first end73of the torsion spring70gets displaced along the slit, thereby applying a biasing force to the carbon brush30constantly. This means that the carbon brush30can maintain stable contact with the commutator26even when it is worn short over years of use. On the other hand, if the coil portion71of the torsion spring70is placed away from the extended line of the stroke of the carbon brush30, it will be possible to choose the longest possible carbon brush30in advance. Then the life of the carbon brush30itself and further the life of the motor10will be lengthened.

A PTC device78is disposed on the end face of the brush holder18on a metal case side, that is, on the side opposite to the armature24, near each of the bottom left and right corner sections where the carbon holders31are not disposed. Thus, the DC motor10as a whole can be downsized by making effective use of the corner sections of the brush holder18where the carbon brushes30are not provided.

On the other hand, as shown inFIG. 6, two choke coils76are disposed on the end face of the brush holder18on a securing holder19side. The choke coils76, which are the elements for erasing electrical noise, are disposed in the bottom left and right corner sections respectively as shown. The two choke coils76, in the longer direction, are each so arranged as to intersect with the two adjacent flat side walls18aof the brush holder18. Also, the two choke coils76, in the longer direction, are so arranged as to intersect with each other. Note that, as shown inFIG. 8, the brush holder18according to the present embodiment is provided with a grounding wire77having a contact portion77a, which comes into contact with the inner side of the metal case16when it is fitted into the metal case16, and an end portion77b, which is soldered to the circuit board27.

FIG. 9is a circuit diagram of the DC motor according to the present embodiment. By employing such a circuit configuration as this, if two choke coils76are mounted on the motor, the effect for suppressing the electrical noise will be enhanced as compared with a case where they are placed in parallel with each other. Also, the choke coil76is placed in a position on a side opposite to the end face where the carbon holder31is provided. This arrangement allows the choke coil76to be placed in a position where it can be the longest. That is, this arrangement allows the carbon brush30to be placed in a position where it can be the longest. Also, the choke coil76is provided in a plane perpendicular to the axis line of the housing12. This arrangement saves the space occupied by the brush holder18in the axial direction and consequently the overall length of the motor10is downsized.

A detailed description is now given of the PTC device78according to the present embodiment. The PTC device78according to the present embodiment is a thermistor having a positive temperature coefficient. In the present embodiment, a resistor element formed of a polymer-based material is exemplified but it may be formed of a ceramic-based material instead, for instance. As the polymer-based PTC device, known is one in which conductive particles such as carbon black and nickel are dispersed into a polymer having a low melting point. Its mechanism is as follows. That is, as the polymer melts, the contacts among the conductive particles are severed and thereby the electric resistance increases. As a result, the current is less likely to flow and, therefore, overcurrent and overheating are suppressed. More specifically, excellent characteristics are achieved when the conductive particles such as carbon black are uniformly dispersed into a crystalline polymer such as polyethylene. On the other hand, a ceramic, in which additives are added to barium titanate, or the like is known as a ceramic-based PTC device. In such PTC devices, the overcurrent and overheating are suppressed using the properties that the electric resistance rises sharply at near the Curie temperature of barium titanate.

FIG. 10is a cross-sectional view schematically showing a state where a PTC device according to the present embodiment is held between electrode terminals. The PTC device78is comprised of a resistor element80having a positive temperature coefficient, a first electrode82disposed on one face of the resistor element80, and a second electrode84disposed on the other face of the resistor element80. The PTC device78detects the overcurrent in a live circuit or a sharp rise in surrounding temperature, and interrupts the circuit if it is detected.

The brush holder18is provided with a pair of electrode terminals88and90between which the PTC device78is held. The electrode terminal88has a protrusion88athat partially presses the first electrode82. Since the first electrode82is thicker than the resistor element80, the first electrode82has a certain degree of strength and therefore the deformation is less likely to occur in the first electrode82even though it is partially pressed by the protrusion88aof the electrode terminal88. This structure provides a space between the first electrode82and the electrode terminal88, thus improving the heat radiation properties.

Also, the PIC device78are sandwiched between the electrode terminals88and90with each stacking surface80abetween the resistor element80and each electrode being parallel to an axis line X of the housing12. If the stacking surfaces80aof the PTC device78are to be placed perpendicular to the axis line X of the housing12, they must be so placed as not to interfere with the shaft22of the DC motor10. In such a case, the size of the housing in the radial direction will be inevitably made larger in order to increase the area of the stacking surface80aof the resistor element80to lower the resistance of PTC device78. In contrast thereto, the stacking surface80ais so arranged as to be in parallel with the axis line of the housing12, as described above, so that the stacking surface can be made wider without increasing the diameter of the housing12and also without interfering with the shaft22of the DC motor10.

As described above, the DC motor10according to the present embodiment includes the tubular housing12, having a plurality of flat side walls, whose cross section is polygonal, the magnets12, disposed along the inner peripheral surfaces of the housing12, each having magnetic poles in the corner sections, the shaft22inserted along the axis line of the housing12, the armature24fixed to the shaft22and arranged in such a position as to be opposite to the magnet20, the commutator26mounted on the shaft22coaxially with the armature24, the carbon brushes30, each having a column-shaped main body, the main body in a longer direction being provided along the radial direction of the commutator26in the housing, the tip surface thereof being in sliding contact with the outer peripheral surface of the commutator26, the carbon holder31, provided within the housing, which supports the carbon brush30in such a manner that the carbon brush30can move in a stroke toward the radial direction of the commutator26, the torsion spring70, provided within the housing, which biases the carbon brush30to a commutator side, the PTC device78, provided within the housing, which electrically conducts to the armature24and controls the current flowing through the armature24, and the electrode terminals88and90that hold the PTC device78therebetween.

Since the circuit board27is provided on a securing holder19side of the brush holder18shown inFIG. 6, a pair of choke coils76is so mounted on the brush holder18as to cover from the securing holder19side (SeeFIG. 2andFIG. 3). Various circuit elements such as a capacitor for removing the electrical noise, a diode for use in circuit protection and a Hall element are mounted on the circuit board27. Also, the choke coils76are provided in the positions opposite to the armature24with the brush holder18positioned therebetween. Thus, even if the choke coil76, which should be normally fixed tightly, gets somehow loose, the choke coil7will not come in contact with the armature24.

As described above, by employing the motor10according to the present embodiment, the layout of the electrical components such as the choke coils is made ingeniously, so that the motor10can be downsized and the reliability of the motor can be improved. By way of the following six exemplary embodiments, a description will be given of other layouts using the choke coil as an example with reference toFIG. 11toFIG. 18. Note that in the following description of the six exemplary embodiments, each constituting member is simplified in each Figure.

FIG. 11is a schematic illustration showing a positional relationship between a brush holder and a choke coil of a motor according to a first exemplary embodiment. The first exemplary embodiment differs from the embodiment shown inFIG. 6. That is, though a pair of choke coils76is used in the embodiment shown inFIG. 6, a single choke coil94is used in this first exemplary embodiment. The choke coil94, in its longer direction Y, is arranged nonparellely with all the side walls18a. Also, the choke coil94, in the longer direction Y, is so arranged as to intersect with two adjacent side walls18awith a corner section positioned therebetween. Such an arrangement enables a larger choke coil to be mounted on one face of the brush holder18vertical to the axis line of the housing.

FIG. 12is a schematic illustration showing a positional relationship between a brush holder and choke coils of a motor according to a second exemplary embodiment. A pair of choke coils96in the second exemplary embodiment, in their respective longer directions Y, is arranged such that it is not parallel with all the side walls18a. Also, each of the choke coils96, in the longer direction Y, is so arranged as to intersect with two adjacent side walls18awith a corner section positioned therebetween. Also, the pair of choke coils96is arranged in such positions that they are parallel to each other with an axis line X of the housing and are symmetrical with respect to the axis line X as the center. Such an arrangement enables a plurality of larger choke coils to be mounted on one face of the brush holder18vertical to the axis line of the housing.

FIG. 13is a schematic illustration showing a positional relationship between a brush holder and choke coils of a motor according to a third exemplary embodiment. Note that in the second exemplary embodiment the pair of choke coils96shown inFIG. 12is arranged almost line-symmetrically with respect to the axis line X of the housing as the center. Differing from such an arrangement shown inFIG. 12, a pair of choke coils98shown, inFIG. 13, according to the third exemplary embodiment is arranged almost point-symmetrically with respect to the axis line X of the housing as the center. Also, the choke coil96, in the longer direction, is disposed at an angle less than 45 degrees relative to one of two adjacent side walls18aand is also disposed at an angle greater than 45 degrees relative to the other thereof. This arrangement enables the choke coils96of a larger size to be mounted on one face of the brush holder18.

FIG. 14is a schematic illustration showing a positional relationship between a brush holder and choke coils of a motor according to a fourth exemplary embodiment. In the brush holder18shown inFIG. 14, a choke coil100c, in a longer direction, which is so arranged as to intersect with a choke coil100aand a choke coil100bis arranged in addition to a pair of choke coils100aand100b, disposed parallel to each other, as shown inFIG. 13. Accordingly, the choke coils100a,100band100cof approximately the same size can be arranged on one face of the brush holder18. It is to be noted here that a similar arrangement enables four choke coils to be mounted on one face of the brush holder18.

FIG. 15is a schematic illustration showing a positional relationship between a brush holder and choke coils of a motor according to a fifth exemplary embodiment.FIG. 16is a side view of the brush holder shown inFIG. 15. A pair of choke coils102according to the fifth exemplary embodiment, in their respective longer directions Y, is arranged such that it is not parallel with all the side walls18a. Also, each of the choke coils102, in the longer direction Y, is so arranged as to intersect with two adjacent side walls18awith a corner section positioned therebetween. Also, the pair of choke coils102is arranged in such positions that the longer directions Y intersect with each other. The carbon brushes104are also arranged on a face of the brush holder18opposite to the face thereof where the choke coils102are arranged. A pair of the carbon brushes104is disposed on diagonals Z connecting the corner sections. This arrangement enables the choke coils and the carbon brushes to be arranged without any interference with each other.

FIG. 17is a schematic illustration showing a positional relationship between the brush holder, choke coils, and carbon brushes of a motor according to a sixth exemplary embodiment.FIG. 18is a side view of the brush holder shown inFIG. 17. A pair of choke coils106according to the sixth exemplary embodiment, in their respective longer directions Y, is arranged such that it is not parallel with all the side walls18a. Also, each of the choke coils106, in the longer direction Y, is so arranged as to intersect with two adjacent side walls18awith a corner section positioned therebetween. Also, the pair of choke coils106is arranged in such positions that the longer directions Y intersect with each other. The carbon brushes108are also arranged on one of the faces of the brush holder18where the choke coils106are arranged. A pair of the carbon brushes108is disposed on the diagonals Z connecting the corner sections. This arrangement enables the choke coils and the carbon brushes to be arranged on the same face of the brush holder18without any interference with each other. This further enables the axial length of the motor to be made shorter.

The present invention is not limited to the above-described embodiment and exemplary embodiments only, and it is understood by those skilled in the art that various modifications such as changes in design may be added to the embodiment and the exemplary embodiments based on their knowledge and the embodiment and the exemplary embodiments added with such modifications are also within the scope of the present invention.

In the above-described embodiment, an exemplified structure is such that choke coils76are arranged on one face of the brush holder18and carbon brushes30on the other face thereof. However, a motor according to a modification may be so configured that the choke coils76and the carbon brushes30are arranged together on one face of the brush holder18. More specifically, a pair of choke coils76may be so arranged around the shaft that the axial directions of the respective choke coils intersect with each other (in a gable fashion) as described previously, and the carbon brushes30may be so arranged that the stroke directions are in parallel with the axial directions of the choke coils76. In other words, the pair of choke coils76arranged such that they are not parallel with each other are closer to each other at one end and wider apart from each other at the other end thereof. Thus it is possible to place the carbon brushes30in the regions between the other ends of the respective choke coils76. In this manner, a pair of choke coils76and carbon brushes30can be arranged together on one face of the brush holder18by utilizing the space efficiently. As a result, the choke coils76can be accommodated without the extension of the axial length of the motor10.

In the above-described embodiment, an exemplified housing12(metal case16and brush holder18) of the motor10is quadrangular in shape having four flat surfaces and rounded corner sections. In a modification thereof, the housing may be of octagonal or other polygonal shape, for instance. The corner sections may be chamfered round or flat or may be without any chamfer.

In the above-described embodiment, an exemplified carbon brush30is a prismatic column with a rectangular cross section. However, the carbon brush30may be a cylindrical or other long column in shape. In such a case, the shape of the carbon holder which holds the carbon brush may be determined accordingly.

In the above-described embodiment, an exemplified 4-pole magnet20is integrally molded. In a modification thereof, however, four magnets each having a thick-walled portion and thin-walled portions may be prepared separately in advance, and they may be fixed to the four corner sections of the metal case16, respectively. That is, the magnets constituting the respective magnetic poles, which are separated from each other at the boundary (thin-walled part) between the neighboring magnetic poles of the magnet20, may be formed separately.

In the above-described embodiment, an exemplified structure is such that the circuit board27with an element for circuit protection mounted thereon is disposed between the brush holder18and the securing holder19. In a modification thereof, however, the motor may be of a simplified structure without the installation of such a printed-circuit board with circuits mounted thereon, depending on the application of the motor.