A first line, which extends through a circumferential center of a tooth, and a second line, which extends through a circumferential center of an undercut between two segments adjacent to the tooth, intersect at an angle C. A third line, which extends through a magnetic-flux-free region of a magnet, and a fourth line, which extends through a circumferential center of a power supply brush, intersect at an angle D. The third line and a fifth line, which extends through a circumferential center of one of magnetic poles of the magnet, intersect at an angle E. A reference line of a stator and a sixth line, which extends through a reference point of a brush holder, intersect at an angle F. A shift angle S, which is obtained by C+D−E+F, is set such that a phase difference between an armature torque and a cogging torque is 180°±80°.

TECHNICAL FIELD

The present disclosure relates to a direct-current motor.

BACKGROUND

For example, vehicles are equipped with control brakes such as ABS (anti-lock braking system), and a direct-current motor is used as a drive source of the ABS. One previously proposed direct-current motor, which is used as the drive source of the ABS, includes: an armature and a commutator installed to a shaft; and a plurality of power supply brushes configured to slide along a plurality of segments of the commutator. When a deviation in the timing of commutation occurs among the power supply brushes, an imbalance in a magnetic field is generated at coils of the armature, resulting in vibration and noise during rotation of the direct-current motor.

SUMMARY

According to the present disclosure, there is provided a direct-current motor that includes a rotor, a stator and a brush holder. The rotor includes a shaft, an armature core and a commutator. The armature core has: a plurality of teeth, which are arranged in a circumferential direction about a central axis of the shaft; and a plurality of slots, each of which is formed between corresponding adjacent two of the plurality of teeth. The commutator has a plurality of segments which are placed adjacent to the armature core and are arranged in the circumferential direction about the central axis. The stator includes a plurality of field magnets which are arranged in the circumferential direction about the central axis. The plurality of field magnets form a plurality of magnetic poles, and each of the plurality of field magnets has corresponding two or more magnetic poles among the plurality of magnetic poles. The brush holder holds a plurality of power supply brushes which are arranged in the circumferential direction about the central axis. Each of the plurality of power supply brushes is configured to contact the plurality of segments. A relationship between a number A of the plurality of slots and a number B of the plurality of magnetic poles is A=n×B where n is a natural number.

A first imaginary line, which radially extends about the central axis through a circumferential center of one of the plurality of teeth, and a second imaginary line, which radially extends about the central axis through a circumferential center of an undercut located between corresponding adjacent two of the plurality of segments placed adjacent to the one of the plurality of teeth, intersect each other at an angle C. A third imaginary line, which radially extends about the central axis through a magnetic-flux-free region of one of the plurality of field magnets, and a fourth imaginary line, which radially extends about the central axis through a circumferential center of one of the plurality of power supply brushes placed adjacent to the third imaginary line, intersect each other at an angle D. The third imaginary line and a fifth imaginary line, which radially extends about the central axis through a circumferential center of one of the corresponding two or more magnetic poles of the one of the plurality of field magnets, intersect each other at an angle E. A reference line of the stator, which radially extends about the central axis, and a sixth imaginary line, which radially extends about the central axis through a reference point of an assembling position of the brush holder for assembling the brush holder to the stator, intersect each other at an angle F. A shift angle S, which is obtained by the angle C+the angle D−the angle E+the angle F, is set to result in that a phase difference between an armature torque and a cogging torque is in a range of 180°±80°.

DETAILED DESCRIPTION

For example, vehicles are equipped with control brakes such as ABS (anti-lock braking system), and a direct-current motor is used as a drive source of the ABS. One previously proposed direct-current motor, which is used as the drive source of the ABS, includes: an armature and a commutator installed to a shaft; and a plurality of power supply brushes configured to slide along a plurality of segments of the commutator. When a deviation in the timing of commutation occurs among the power supply brushes, an imbalance in a magnetic field is generated at coils of the armature, resulting in vibration and noise during rotation of the direct-current motor.

At the direct-current motor described above, the resonance of the armature is reduced by improving the winding balance of the windings, which form the coils of the armature, in order to reduce the vibration and the noise at the direct-current motor. However, as the drive source of vehicle has shifted from an internal combustion engine to an electric motor in recent years, there is a growing demand for reducing vibration and noise of components of the vehicle to improve passenger comfort. Therefore, it is necessary to achieve even smaller vibration and noise for the direct-current motor installed at the vehicle.

According to the present disclosure, there is provided a direct-current motor including:a rotor that includes:a shaft;an armature core that has: a plurality of teeth, which are arranged in a circumferential direction about a central axis of the shaft; and a plurality of slots, each of which is formed between corresponding adjacent two of the plurality of teeth; anda commutator that has a plurality of segments which are placed adjacent to the armature core and are arranged in the circumferential direction about the central axis;a stator that includes a plurality of field magnets which are arranged in the circumferential direction about the central axis, wherein the plurality of field magnets form a plurality of magnetic poles, and each of the plurality of field magnets has corresponding two or more magnetic poles among the plurality of magnetic poles; anda brush holder that holds a plurality of power supply brushes which are arranged in the circumferential direction about the central axis, wherein each of the plurality of power supply brushes is configured to contact the plurality of segments, wherein:a relationship between a number A of the plurality of slots and a number B of the plurality of magnetic poles is A=n×B where n is a natural number; andin a view taken from an output side of the shaft, with reference to the central axis, the direct-current motor is configured as follows:a first imaginary line, which radially extends about the central axis through a circumferential center of one of the plurality of teeth, and a second imaginary line, which radially extends about the central axis through a circumferential center of an undercut located between corresponding adjacent two of the plurality of segments placed adjacent to the one of the plurality of teeth, intersect each other at an angle C;a third imaginary line, which radially extends about the central axis through a magnetic-flux-free region of one of the plurality of field magnets, and a fourth imaginary line, which radially extends about the central axis through a circumferential center of one of the plurality of power supply brushes placed adjacent to the third imaginary line, intersect each other at an angle D;the third imaginary line and a fifth imaginary line, which radially extends about the central axis through a circumferential center of one of the corresponding two or more magnetic poles of the one of the plurality of field magnets, intersect each other at an angle E;a reference line of the stator, which radially extends about the central axis, and a sixth imaginary line, which radially extends about the central axis through a reference point of an assembling position of the brush holder for assembling the brush holder to the stator, intersect each other at an angle F; anda shift angle S, which is obtained by the angle C+the angle D−the angle E+the angle F, is set to result in that a phase difference between an armature torque and a cogging torque is in a range of 180°±80°.

At the direct-current motor described above, the timing of commutation can be adjusted among the power supply brushes by adjusting the shift angle S that is obtained based on the composite of the angles C, D, E, F, which relate to the phase of the armature torque and the phase of the cogging torque. According to the verification performed by the inventor of the present application, by taking the output characteristic of the direct-current motor also into consideration, it is confirmed that when the shift angle S is set such that the phase difference between the armature torque and the cogging torque is in a range of 180°±80°, the vibration and the noise of the direct-current motor can be reduced more than ever. It is further preferred that the shift angle S is adjusted such that the phase difference is in a range of 180°±60°.

At the direct-current motor of the present disclosure, it is desirable that the angle C is in a range of 0°±3°; the angle D is in a range of 30°±3°; the angle E is in a range of 30°±3°; the angle F is in a range of 0°±3°; and the shift angle S is in a range of 0°±3°. In a case where the number B of the plurality of magnetic poles is six, and the natural number n is three, it is desirable that each of a plurality of windings is wound around each corresponding set of three teeth among the plurality of teeth at equal intervals of 120° and thereby forms three coils. The direct-current motor of the present disclosure is used at a control brake.

According to the present disclosure, there is provided the direct-current motor that can reduce the vibration and the noise more than ever before.

Hereinafter, an embodiment of the present disclosure will be described with reference to the accompanying drawings. In order to facilitate understanding of the description, the same components are indicated by the same reference signs as much as possible in each drawing, and redundant descriptions are omitted.

FIG.1is an exploded perspective view schematically showing a structure of a direct-current motor10of the present embodiment. The direct-current motor10is a direct-current motor that is incorporated into a drive source of a control brake, such as an ABS (anti-lock braking system). Specifically, a rotational drive force, which is generated through rotation of the direct-current motor10, is converted into a linear reciprocating motion of a piston of an oil pump (not shown) to control an oil pressure, so that a braking force of a brake disc (not shown) is controlled.

The direct-current motor10includes a rotor20, a stator30and a brush device40. The rotor20includes: a shaft21which functions as an output shaft; an armature core22which is fixed to the shaft21; a commutator23which is placed adjacent to an output side of the shaft21and is fixed to the shaft21; and a plurality of windings24which are wound on the armature core22. An end portion (i.e., the output side) of the shaft21projects from the brush device40to the outside of the direct-current motor10and is coupled to the driven device (not shown). Thereby, the shaft21transmits the rotational drive force to the driven device.

FIG.2is a side view of the rotor20viewed from the output side of the shaft21. InFIG.2, only one of the windings24is indicated for the sake of easy understanding. With reference toFIGS.1and2, the armature core22includes: an annular portion22awhich is shaped in a cylindrical tubular form and is securely press-fitted to the shaft21; and a plurality of teeth22bwhich radially outwardly project from an outer peripheral surface of the annular portion22a. A plurality of slots22care formed at the armature core22such that each of the slots22cis formed between corresponding adjacent two of the teeth22b. The armature core22is made of a magnetic material.

The teeth22bare arranged at equal intervals in a circumferential direction about a central axis X of the shaft21. In a view taken from the output side of the shaft21, each of the teeth22bis shaped into a T-shape. According to the present embodiment, in the armature core22, the number of the teeth22bis eighteen, and these eighteen teeth22bare arranged at 20° intervals in the circumferential direction about the central axis X. Therefore, the number of the slots22cis eighteen, and these eighteen slots22care arranged at equal intervals in the circumferential direction about the central axis X and are formed at the outer peripheral surface of the armature core22.

The plurality of windings24are wound on the armature core22by distributed winding. Here, each of the plurality of windings24is wound around each corresponding set of three teeth22bamong the plurality of teeth22bat equal intervals of 120° and thereby forms three coils25. Specifically, as shown inFIG.2, one winding24is wound a plurality of times around a corresponding first set of three teeth22bamong the plurality of teeth22bto form a first coil25. This winding24is then wound a plurality of times around a corresponding second set of three teeth22b, which are spaced from the first set of three teeth22bby 120° in the circumferential direction about the central axis X to form a second coil25. This winding24is further wound a plurality of times around a corresponding third set of three teeth22b, which are spaced from the second set of three teeth22bby 120° in the circumferential direction about the central axis X to form a third coil25.

In the present embodiment, since the armature core22has the eighteen teeth22b, the creation of the three coils25by each corresponding one of the windings24is repeated eighteen times while shifting the set of three teeth22bin the circumferential direction. That is, more coils25are stacked on top of the already formed coils25. Thus, at the armature core22, the total number of the coils25is fifty-four (i.e., 3×18=54). Although not depicted in the drawings, each of two ends of each winding24is electrically connected to a corresponding one of a plurality of segments23aof the commutator23.

The commutator23is securely press-fitted to the shaft21, and thereby the commutator23is fixed to the shaft21. The commutator23has the plurality of segments (commutator segments)23aarranged at equal intervals in the circumferential direction about the central axis X. The number of the segments23ais eighteen that corresponds to the number of the teeth22band the number of the slots22c. Each adjacent two of the segments23aare electrically insulated from each other, and an undercut23b, which is a groove, is formed between the adjacent two of the segments23a. Although not depicted in the drawings, each of the segments23ais electrically connected with the corresponding end of the corresponding one of the windings24.

FIG.3is a side view of the stator30viewed from the output side of the shaft21. InFIG.3, indication of the rotor20and the brush device40is omitted. With reference toFIGS.1and3, the stator30includes a yoke housing31.

The yoke housing31has: a bottom31awhich is shaped in a circular form; a peripheral wall31bwhich is shaped in a cylindrical tubular form and extends from an outer peripheral edge of the bottom31a; an opening31cof the peripheral wall31bwhich is opposite to the bottom31a; and a flange31dwhich is perpendicular to the peripheral wall31band extends outward from an outer peripheral edge of the opening31c. The rotor20is received at an inside of the yoke housing31, and a bearing member (not shown), which rotatably supports one end portion of the shaft21of the rotor20, is installed at the bottom31a. A central axis of the peripheral wall31b, which is shaped in the cylindrical tubular form, coincides with the central axis X of the shaft21. The brush device40is installed to the opening31c.

A plurality of field magnets32are fixed to an inner peripheral surface of the peripheral wall31b. In the present embodiment, the number of the field magnets32is three, and these three field magnets32are curved along the inner peripheral surface of the peripheral wall31band are arranged at equal intervals of 120° in the circumferential direction. Each field magnet32is a one-piece magnet that forms a magnetic-flux-free region (a neutral region that is not magnetized to have a magnetic polarity)32a, and the magnetic-flux-free region32ais formed at a circumferential center of the field magnet32in the circumferential direction about the central axis X of the shaft21. Furthermore, each field magnet32has a plurality of magnetic poles. Specifically, each field magnet32has one N-pole magnet (N-magnetic pole)32N and one S-pole magnet (S-magnetic pole)32S placed on two circumferentially opposite sides, respectively, of the magnetic-flux-free region32a. Alternatively, each field magnet32may be a field magnet that has the N-pole magnet32N and the S-pole magnet32S which are formed separately.

Each field magnet32is placed on the radially outer side of the teeth22band are opposed to the teeth22b. An electromagnetic force is generated at the armature core22based on the electric current conducted through the coils25, and this electromagnetic force becomes the torque that rotates the rotor20. In the direct-current motor10of the present embodiment, the three field magnets32form the six magnetic poles, so that the direct-current motor10has the six magnetic poles. That is, when a natural number n is 3, a relationship between the number (A) of the slots22cand the number (B) of the magnetic poles is (A)=n×(B), which results in 18=3×6.

Returning toFIG.1, the brush device40has a brush holder41that is installed to an inside of the opening31cof the yoke housing31by, for example, press-fitting to close the opening31c. The brush holder41has an opening41awhich receives the other end portion (the output side) of the shaft21of the rotor20. A bearing member (not shown), which rotatably supports the other end portion of the shaft21, is installed at an inside of the opening41a.

FIG.4is a side view of the brush holder41viewed from a side that is opposite to the output side of the shaft21. InFIG.4, indication of the rotor20is omitted. With reference toFIGS.1and4, a plurality of brush boxes42are held by an inner surface of the brush holder41, and a corresponding one of a plurality of power supply brushes43is supported at each of the brush holders41. In the present embodiment, the number of the brush boxes42is six, and these six brush boxes42and the six power supply brushes43respectively received in the brush boxes42are arranged at equal intervals (60° intervals) in the circumferential direction about the central axis X of the shaft21. With reference toFIGS.3and4, each of the power supply brushes43is placed at a circumferential center, i.e., a magnetic pole center of a corresponding one of the N-pole magnet32N and the S-pole magnet32S of a corresponding one of the field magnets32.

Each power supply brush43is a sintered body made from a material that mainly includes graphite powder and copper powder, and each power supply brush43is connected to a power supply member (not shown) installed to the corresponding brush holder41. Thereby, the electric current is supplied from an external power source to each power supply brush43through the corresponding power supply member. The brush holder41and the brush boxes42are made of a dielectric material, such as a dielectric resin material.

Each power supply brush43is placed on a radially outer side of the segments23aof the commutator23and is urged against the segments23ato contact the segments23ain the radial direction of the direct-current motor10by, for example, a coil spring (not shown) which is received in the corresponding brush box42in a manner that allows reciprocation of the power supply brush43. Each power supply brush43is slid along the segments23ain response to the rotation of the rotor20, so that the power supply brush43can supply the electric current to the corresponding winding24, i.e., the corresponding coil25electrically connected to the corresponding segment23a.

FIG.5is a side view of the direct-current motor10viewed from the output side of the shaft21. With reference toFIGS.1and5, a reference line RL, which radially extends through the central axis X of the shaft21, is defined at the yoke housing31. The reference line RL is a line that radially extends through a center of one of three openings31eformed at the flange31dof the yoke housing31. The three openings31eare arranged at equal intervals of 120° in the circumferential direction about the central axis X. In the present embodiment, the reference line RL coincides with the magnetic-flux-free region32aof a corresponding one of the field magnets32installed at the inside of the yoke housing31.

Three blind holes41bare formed at an outer surface of the brush holder41, and these three blind holes41bare arranged at equal intervals of 120° in the circumferential direction about the central axis X. A center of one of the blind holes41bfunctions as a reference point RP of an assembling position of the brush holder41(i.e., the brush device40) at the time of assembling the brush holder41(i.e., the brush device40) to the yoke housing31. Specifically, the assembling position of the brush holder41relative to the yoke housing31in the circumferential direction about the central axis X is defined by adjusting the position of the reference line RL of the yoke housing31and the position of the reference point RP of the brush holder1.

Now, torque fluctuations of the direct-current motor, which are generated due to the structure of the direct-current motor in general, will be described. Factors, which cause the torque fluctuations, include: armature torque which is generated according to an electromagnetic force generated at the armature core22at the time of energizing the coils25; and cogging torque which is generated under an influence of magnetic field lines applied from the field magnets32to the armature core22. The armature torque and the cogging torque change depending on the rotational position of the rotor20. Specifically, a change in a magnitude of each of the armature torque and the cogging torque in response to the rotational angle is expressed by a smooth sine curve.

The sum of the armature torque and the cogging torque is so called the torque ripple. Therefore, when a phase of the sine curve of the armature torque coincides with a phase of the sine curve of the cogging torque, the torque ripple increases, resulting in increased vibration and noise of the direct-current motor. The present disclosure reduces the torque ripple and thereby reduces the vibration and the noise of the direct-current motor10by adjusting the phases of the armature torque and the phase of the cogging torque, i.e., by shifting and offsetting the phase of the armature torque and the phase of the cogging torque relative to each other.

In an attempt to reduce the torque ripple, the inventor of the present application has focused on adjusting the timing of commutation among the plurality of power supply brushes43by adjusting a shift angle S, which is obtained based on a composite of the following angles which relate to the phase of the armature torque and the phase of the cogging torque.

As a first angle among the above-described angles, with reference toFIG.2, in a view taken from the output side of the shaft21, with reference to the central axis X, a first imaginary line (first line) L1, which radially extends about the central axis X through a circumferential center of one of the teeth22bof the armature core22, and a second imaginary line (second line) L2, which radially extends about the central axis X through a circumferential center (i.e., a circumferential center of the undercut23b) between corresponding adjacent two of the segments23aplaced adjacent to the one of the teeth22b, intersect each other at an angle C. A size of this angle C is adjusted in a range of 0°±3° to account for tolerances.

As a second angle among the above-described angles, with reference toFIG.4, in the view taken from the output side of the shaft21, with reference to the central axis X, a third imaginary line (third line) L3, which radially extends about the central axis X through the magnetic-flux-free region32aof one of the field magnets32, and a fourth imaginary line (fourth line) L4, which radially extends about the central axis X through a circumferential center of one of the power supply brushes43(e.g., a circumferential center of the power supply brush43which corresponds to the N-pole magnet32N) placed adjacent to the third imaginary line L3, intersect each other at an angle D (FIG.4indicates the angle D viewed from the opposite side that is opposite to the output side of the shaft21). A size of this angle D is adjusted in a range of 30°±3° to account for tolerances. The fourth imaginary line L4may be defined by a line, which extends through a circumferential center of the brush box42instead of the power supply brush43described above.

As a third angle among the above-described angles, with reference toFIG.3, in the view taken from the output side of the shaft21, with reference to the central axis X, the third imaginary line L3, which radially extends about the central axis X through the magnetic-flux-free region32aof the one of the field magnets32, and a fifth imaginary line (fifth line) L5, which radially extends about the central axis X through a circumferential center of one of the two magnetic poles of the one of the plurality of field magnets32placed adjacent to the third imaginary line L3(e.g., the circumferential center, i.e., a magnetic pole center of the N-pole magnet32N described above with reference to the angle D), intersect each other at an angle E. A size of this angle E is adjusted in a range of 30°±3° to account for tolerances.

As a fourth angle among the above-described angles, with reference toFIG.5, in the view taken from the output side of the shaft21, with reference to the central axis X, the reference line RL of the stator30(i.e., the reference line RL of the yoke housing31), which radially extends about the central axis X, and a sixth imaginary line L6, which radially extends about the central axis X through the reference point RP of the assembling position of the brush holder41for assembling the brush holder41to the stator30(i.e., the yoke housing31), intersect each other at an angle F. InFIG.5, this angle F is set to 0°, so that the reference line RL and the sixth imaginary line L6coincide with each other. A size of this angle F is adjusted in a range of 0°±3° to account for tolerances.

In the present embodiment, the shift angle S, which is set and is obtained by the angle C+the angle D−the angle E+the angle F, is set to adjust the timing of commutation. In order to set the shift angle S, the inventor has carried out analytical simulation to verify the shift angle S. The analytical simulation is carried out to check a relationship between the shift angle S and the torque ripple (the armature torque and the cogging torque) when the rotor20is rotated at a no-load frequency of 1500 rpm at the direct-current motor10of the present embodiment.

FIG.6is a graph showing a relationship between the shift angle S and the torque ripple based on the result of the analytical simulation. The direct-current motor10used in the analysis has the eighteen teeth22band the six magnetic poles. Therefore, a vertical axis on the right side of the graph shows an analytical value (Nm) of the torque ripple in the 18th order vibration mode. Furthermore, a vertical axis on the left side of the graph shows an average value of an actual measured value (m/s2) of the 18th order vibration of the direct-current motor10. The analytical value of the torque ripple and the actual measured value of the 18th order vibration both indicate similar results. Specifically, these values are reduced the most when the shift angle S is set to 0°, and these values are increased when the shift angle S is increased or decreased from 0°.

FIG.7is a graph showing a relationship between the shift angle S and the phase of the torque ripple (the phase of the armature torque and the phase of the cogging torque) based on the result of the analytical simulation. It is clearly understood from the graph that in the case where the shift angle S is 0°, i.e., a difference between the phase of the armature torque and the phase of the cogging torque is 180°, the phase of the sine curve of the armature torque and the phase of the sine curve of the cogging torque can be canceled each other at maximum. When the shift angle S is increased or decreased from 0°, the difference between the phase of the armature torque and the phase of the cogging torque is increased or decreased from 180°.

According to the result of the analytical simulation, it is confirmed that in the case where the shift angle S is set to 0°, the torque ripple is most reduced, and thereby the vibration and the noise are reduced. On the other hand, it is common technical knowledge that a reduction in the torque ripple leads to a reduction in the output characteristic of the direct-current motor10. Therefore, when the output characteristic of the direct-current motor10is considered together with the torque ripple, it is desirable that the shift angle S is set in a range of 0°±4°, so that the phase difference falls in a range of 180°±80°. Furthermore, in order to further reduce the vibration and the noise of the direct-current motor10, it is further desirable that the shift angle S is set in a range of 0°±3°, so that the phase difference falls in a range of 180°±60°.

The present embodiment has been described above with reference to the specific examples. However, the present disclosure is not limited to the above specific examples. Appropriate design changes made by those skilled in the art to the above specific examples are also included in the scope of the present disclosure as long as they have the features of the present disclosure. Each element included in the specific examples described above, and its arrangement, conditions, shape, etc., are not limited to those illustrated and can be changed as appropriate. As long as there is no technical contradiction, the combination of the elements included in the specific examples described above can be changed as appropriate.