Electric motor and reduction motor

Disclosed is a motor in which a commutator (10) is provided with connecting wires which short-circuit equipotential segments; brushes (21) are constituted by a low-speed brush (21a), a high-speed brush (21b), and a common brush (21c) used in common by the low-speed and high-speed brushes, and are juxtaposed along the circumferential direction. The circumferential brush width (W2) of the high-speed brush is set to be smaller than the circumferential brush width (W1) of the low-speed brush. The high-speed brush and the low-speed brush are formed so that simultaneous sliding contact with equipotential segments (15) can be avoided. Additionally, armature cores (8) are provided such that a plurality of teeth (12) is point-symmetrical about a rotary shaft (3) at equal intervals in the circumferential direction, and the teeth and slots (13) are formed so as to exist alternately at intervals of 90 degrees in the circumferential direction. By virtue of the above configuration, vibration and noise can be reduced while achieving miniaturization and high performance of a motor.

TECHNICAL FIELD

The present invention relates to an electric motor mounted on, for example, a vehicle, and a reduction motor using the same.

Priority is claimed on Japanese Patent Application No. 2008-260987 filed on Oct. 7, 2008, the contents of which are incorporated herein by reference.

BACKGROUND ART

Electric motors with brushes have conventionally often been used as wiper motors for an automobile. In this kind of electric motor, a plurality of permanent magnets is arranged at equal intervals in the circumferential direction at the inner peripheral surface of a cylindrical yoke, and an armature is rotationally supported inside these permanent magnets. The armature has an armature core in which a plurality of teeth is formed in a radial fashion. A plurality of long slots is formed in the axial direction between the respective teeth, and coils are formed by coiling winding wires between the slots with predetermined intervals by an overlapping winding method. The coils are electrically connected to a commutator which is fitted and fitted to the rotary shaft from the outside so as to be adjacent to the armature core.

The commutator has a plurality of segments, which is metal pieces, arranged in the circumferential direction in a mutually insulated state, and a winding starting end and a winding finishing end of a coil are connected to each segment. Additionally, the segments are connected to brushes, respectively, so as to be capable of sliding contact with the segments, and electric power is supplied to the respective coils via the brushes. Also, a magnetic field is formed in a coil to which electric power is supplied, and a rotary shaft is driven by a magnetic attractive or repulsive force which is generated between the permanent magnets of the yoke.

Here, recently, from necessities of small size and high performance of the wiper motor, there is disclosed a technique for achieving multiple poles in which the number of magnetic poles is 4 (the number of pole pairs is 2) and multiple slots, achieving high performance in a motor, arranging four brushes at equal intervals in the circumferential direction, and making the speed of the motor variable (for example, refer to Patent Document 1).

In the motor of Patent Document 1, the amount of an electric current to be supplied to a coil in each mode of a LOW mode, a MID mode, and a HI mode is changed by changing energization patterns to the four brushes. Through this configuration, the rotational frequency of the motor in each mode can be changed while providing a motor in which the number of magnetic poles is 4, and multiple slots are achieved.

Prior Technology Documents

DISCLOSURE OF THE INVENTION

[Problem to be Solved by the Invention]

Meanwhile, in the above-described motor, gaps are respectively formed between four permanent magnets disposed at the yoke. Thus, changes in magnetic flux increase between the permanent magnet side and the gaps with both circumferential ends of each permanent magnet as a border. For this reason, when each tooth passes through both the circumferential ends of each permanent magnet, a magnetic attractive or repulsive force which acts on the tooth changes greatly, and thereby cogging torque is generated. As a result, the vibration and noise of the electric motor increases.

The invention has been made in view of the above-described circumstances, and the object thereof is to provide a variable-speed electric motor and a reduction motor which can reduce vibration and noise while achieving miniaturization and high performance.

[Means for Solving the Problem]

The invention relates to an electric motor including a yoke whose number of pole pairs is 2, and an armature rotationally journalled to the yoke. The armature includes a rotary shaft journalled to the yoke; an armature core attached to the rotary shaft and having a plurality of teeth extending in a radial fashion toward the radial direction and allowing coils to be wound therearound, and a plurality of slots formed between the teeth and extending along the axial direction; and a commutator provided at the rotary shaft so as to be adjacent to the armature core, and having a plurality of segments arranged in the circumferential direction. Brushes which supply electric power to the coils via the segments come into sliding contact with the segments. In a first invention related to the invention, the commutator is provided with short-circuit members which short-circuit equipotential segments. Additionally, the brushes are constituted by three brushes of a low-speed brush, a high-speed brush, and a common brush used in common by the low-speed and high-speed brushes and juxtaposed along the circumferential direction. The circumferential width of the high-speed brush is set to be smaller than the circumferential width of the low-speed brush, and the high-speed brush and the low-speed brush are formed so that simultaneous sliding contact with the equipotential segments can be avoided. Additionally, the armature cores are provided such that the plurality of teeth is point-symmetrical about the rotary shaft at equal intervals in the circumferential direction, and the teeth and slots are formed so as to exist alternately at intervals of 90 degrees in the circumferential direction.

In a second invention related to the invention, the number of the plurality of teeth and the plurality of slots are set to any of 7 times, 9 times, and 11 times the number of pole pairs.

In a third invention related to the invention, the external diameter of the commutator is set within a range of 20 mm or more and 30 mm or less.

In a fourth invention related to the invention, the circumferential widths of the low-speed brush and the common brush are set within a range of 2.5 mm or more and 5 mm or less.

In a fifth invention related to the invention, the circumferential width of the high-speed brush is set to a range which is equal to or more than 1.5 mm or more and smaller than 2.5 mm.

Additionally, a sixth invention related to the invention provides a reduction motor including the electric motor according to any of the above first to fifth inventions, a worm shaft coupled to the rotary shaft of the electric motor, and a worm gear which meshes with the worm shaft.

[Effects of the Invention]

In the invention, the armature cores are provided such that the plurality of teeth is point-symmetrical about the rotary shaft at equal intervals in the circumferential direction, and the teeth and slots are formed so as to exist alternately at intervals of 90 degrees in the circumferential direction. Accordingly, in a case where the number of pole pairs is 2, the relative positional relationship between an N-pole and a tooth and a slot which face this N-pole and the relative positional relationship between an S-pole and a tooth and a slot which face this S-pole can be changed.

For this reason, the generation timing of the cogging torque generated in a tooth (slot) which passes through an N pole, and the generation timing of the cogging torque generated in a tooth (slot) which passes through an S pole can be shifted. Hence, the cogging torque of the whole armature decreases, and it is possible to reduce the vibration and noise of the electric motor.

Additionally, by setting the number of the plurality of teeth and the plurality of slots to any of 7 times, 9 times, and 11 times the number of pole pairs, multiple slots can be formed while reducing the vibration and noise. For this reason, a high-performance electric motor and a reduction motor can be provided.

Moreover, since the brushes are constituted by three brushes of a low-speed brush, a high-speed brush, and a common brush used in common by the low-speed and high-speed brushes, in respect to making the speed variable, the number of parts can be reduced as compared to the related art.

Moreover, the circumferential width of the high-speed brush is set to be smaller than the circumferential width of the low-speed brush, and the high-speed brush and the low-speed brush are formed so as to avoid simultaneous sliding contact with the equipotential segments. For this reason, the circumferential width of the segments is decreased compared to the related art, and miniaturization of the commutator is easily achieved. As a result, miniaturization of the whole electric motor can be achieved.

Here, in a case where the electric motor is rotationally driven at low speed, as the high-speed brush which is not used comes into sliding contact with a segment, a difference may be caused in the number of coils of an equivalent electric circuit and variation may occur in electric currents which flow through the respective coils. Thereby, vibration and noise of the electric motor may increase.

However, the influence of the high-speed brush during low rotational driving can be made small by setting the circumferential width of the high-speed brush to be smaller than the circumferential width of the low-speed brush. For this reason, an electric motor and a reduction motor with less vibration and noise can be provided.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, an embodiment of the invention will be described with reference to the drawings.

As shown inFIGS. 1 to 3, a reduction motor1is used as, for example, a wiper motor of an automobile, and includes an electric motor2, and a reduction mechanism4coupled to a rotary shaft3of the electric motor2.

The electric motor2has a bottomed tubular yoke5, and an armature6rotatably provided within the yoke5.

A tubular portion53of the yoke5is formed substantially in the shape of a cylinder, and four segment type permanent magnets7are disposed at equal intervals in a circumferential direction on the inner peripheral surface of the tubular portion such that magnetic poles become alternate. That is, the number of pole pairs of the permanent magnets7provided at the yoke5is set to 2.

The radial center of a bottom wall (end portion)51of the yoke5is formed with a boss portion19which protrudes axially outward, and a bearing18for journalling one end of the rotary shaft3is provided at the boss portion19. In addition, the external surface of the yoke5is painted in black. Through this black painting, the amount of heat released from the yoke5to the outside can be increased, and the rise in temperature of the electric motor2can be made low.

An opening53aof the tubular portion53is provided with an outer flange portion52. The outer flange portion52is formed with a bolt-hole (not shown). A bolt24is inserted through this bolt-hole, and the yoke5is fastened and fixed to the reduction mechanism4as the bolt is screwed into a bolt-hole (not shown) formed in a gear housing23(which will be described below) of the reduction mechanism4.

The armature6is fitted to the rotary shaft3from the outside, and includes a fixed armature core8, armature coils9wound around the armature core8, and a commutator10arranged at the other end of the rotary shaft3. Each armature core8is formed by laminating (laminated core) plates made of a magnetic material punched by press working or the like in the axial direction or pressure-forming (dust core) soft magnetic powder, and has a substantially annular core body11.

Eighteen teeth12, which are substantially T-shaped in an axial plan view, are provided in a radial fashion at equal intervals along the circumferential direction at an outer peripheral portion of the core body11. Each tooth12is constituted by a winding drum portion31which extends radially and has a winding wire14wound therearound, and a peripheral wall portion32which is provided at the tip of the winding drum portion31and extends so as to become symmetrical with respect to the winding drum portion31. That is, the peripheral wall portion32provided at the tip of the tooth12constitutes the outer peripheral surface of the armature core8, and the peripheral wall portion32is brought into a state where the peripheral wall portion faces a permanent magnet7.

Eighteen dovetail slots13are provided between adjacent teeth12by providing the teeth12in a radial fashion at the outer peripheral portion of the core body11. The slots13extend along the axial direction, and are formed at equal intervals along the circumferential direction.

An enamel-coated winding wire14is inserted through between the slots13, and the winding wire14is wound around the winding drum portion31of the tooth12via an insulator (not shown) which is an insulating material. Thereby, a plurality of armature coils9is formed on the outer periphery of the armature core8.

Here, when the eighteen teeth12are formed at equal intervals along the circumferential direction, all the teeth12and slots13exist point-symmetrically about the rotary shaft3. On the other hand, the teeth12and the slots13exist alternately in a positional relationship with intervals of 90 degrees in the circumferential direction.

That is, since four permanent magnets7(the number of magnetic poles is four) are provided in the present embodiment, the number of pole pairs is two, and the eighteen teeth12(slots13) are provided with respect to this. That is, the number of the teeth12(slots13) is set to 9 times the number of pole pairs.

Additionally, all the teeth12and slots13are point-symmetrical about the rotary shaft3, and the teeth12and the slots13exist alternately in a positional relationship with intervals of 90 degrees in the circumferential direction. Thereby, the relative positional relationship between an N-pole permanent magnet7and each tooth12which faces this magnet and the relative positional relationship between an S-pole permanent magnet7and each tooth12which faces this magnet are brought into the state of having shifted by a distance equivalent to half of the width of a slot13along the circumferential direction.

The commutator10is inserted from the outside into and fixed closer to the other end of the rotary shaft3than the armature core8. Eighteen segments15formed from a conductive material are attached to the outer peripheral surface of the commutator10. The segments15are made of a plate-like metal piece which is long in the axial direction, and are fixed in parallel at equal intervals along the circumferential direction in a state where the segments are insulated from each other. The external diameter D1of the commutator10is set within a range of 20 mm or more and 30 mm or less.

A riser16which is bent in a folded-back fashion to the external-diameter side is integrally formed at the end of each segment15on the side of the armature core8. A winding wire14which becomes a winding starting end and a winding finishing end of an armature coil9is hung around the riser16, and the winding wire14is fixed to the riser16by fusing or the like. Thereby, a segment15and an armature coil9corresponding to this segment are electrically connected to each other.

Additionally, connecting wires40are respectively hung around the risers15corresponding to equipotential segments15, i.e., segments15(every pair of segments15separated from nine positions in the present embodiment) which face each other about the rotary shaft3, and the connecting wires40are fixed to the risers16by fusing or the like (refer toFIG. 5). The connecting wires40are provided to short-circuit the equipotential segments15from each other, and are disposed between the commutator10and the armature core8.

The commutator10configured in this way is brought into a state where the commutator faces the gear housing23of the reduction mechanism4. The gear housing23is constituted by a housing body42which is formed substantially in the shape of a box having an opening42aon one face thereof and houses a gear group41of the reduction mechanism4, and a cover43which blocks the opening42aof the housing body42. A brush housing portion22is formed on the electric motor2side of the housing body42, and the commutator10of the electric motor2faces the brush housing portion.

As shown inFIGS. 2 to 4, the brush housing portion22is concavely formed on the electric motor2side of the gear housing23. A peripheral wall30of the brush housing portion22is formed so as to have a substantially oval cross-section, and is constituted by planar walls30aand arcuate walls30b.

A cover33, which is formed in the shape of a tube having a substantially oval cross-section so as to correspond to the brush housing portion, is provided inside the brush housing portion22. The cover33also has planar walls33aand arcuate walls33b. Moreover, a holder stay34formed so as to correspond to the cover33is provided inside the cover33. The holder stay34is fastened and fixed to the side wall42bof the housing body42by bolts35.

Brush holders36are provided in three places along the circumferential direction at the holder stay34. Brushes21are supported within the brush holders36, respectively, so as to protrude and retract from the brush holders in a state where the brushes are biased via springs S, respectively. Since the tips of the brushes21are biased by the springs S, the tips of the brushes come into sliding contact with the commutator10, and the electric power (not shown) from the outside is supplied to the commutator10via the brushes21.

Brushes21are constituted by a low-speed brush21aand a high-speed brush21b, which are connected to the anode side, and a common brush21cwhich is used in common by the low-speed brush21aor the high-speed brush21b, and is connected to the cathode side. The low-speed brush21aand the common brush21care mutually disposed at an interval of 180° in electrical angle, i.e., at an interval of 90° in the circumferential direction in mechanical angle. Meanwhile, the high-speed brush21bis arranged apart from the low-speed brush21aby an angle α in the circumferential direction. In addition, although the present embodiment describes that the common brush21cis used as the cathode side, and the low-speed brush21aand the high-speed brush21bare used as the anode side, the anode side and cathode side may be reversed.

In the present embodiment, the electric resistance value of the high-speed brush21ais set to be two or more times higher than the electric resistance values of the low-speed brush21aand the common brush21c. Therefore, the current value when an electric current is supplied to the armature coils9from the high-speed brush21acan be lowered. Thereby, when an electric current is supplied to the armature coils9from the high-speed brush21aand the armature6of the electric motor2is rotating at high speed, the current value of a large current (lock current) supplied to the armature coils9can be lowered in a case where the rotation of the armature6is stopped (locked) by an external load. Therefore, any damage to an element for protecting an electric circuit, such as a fuse provided in a motor drive device, can be prevented in advance.

Here, since the equipotential segments15of the commutator10, i.e., the segments15which face each other about the rotary shaft3, are short-circuited by the connecting wires40, it is possible to supply electric power even to the segments with which the brushes21do not come into sliding contact. Accordingly, the high-speed brush21bexists at a position which is advanced by an angle θ with respect to the low-speed brush21a. In addition, in the present embodiment, the angle θ is set to 30 degrees.

By arranging the respective brushes21ato21cin this way, the portions of the cover33and the holder stay34where the brushes21ato21cdo not exist can be cut away. That is, the cover33can be formed so as to have a substantially oval cross-section, and the low-speed brush21aand the common brush21ccan be arranged near connecting portions between the planar walls33aand the arcuate wall33b. On the other hand, the high-speed brush21bcan be arranged at the arcuate wall33bof the cover33opposite to places, in which the low-speed brush21aof the cover33and the common brush21care arranged, about the rotary shaft3. For this reason, the brush housing portion22can be formed so as to have a substantially oval cross-section, and it is possible to flatten the brush housing portion22.

Additionally, as shown in detail inFIG. 3, the circumferential brush widths W1by which the low-speed brush21aand the common brush21ccome into sliding contact with the commutator10are set to be almost the same. In contrast, the circumferential brush width W2by which the high-speed brush21bcomes into sliding contact with the commutator10is set to be smaller than the brush width W1of the low-speed brush21a. Specifically, when the external diameter of the commutator10is set within a range of 20 mm or more and 30 mm or less, the brush widths W1of the low-speed brush21aand the common brush21care set within a range of 2.5 mm or more and 5 mm or less. On the other hand, the brush width W2of the high-speed brush21bis set to a range which is equal to and more than 1.5 mm and smaller than 2.5 mm.

By setting the brush widths W1of the low-speed brush21aand the common brush21c, and the brush width W2of the high-speed brush21bin this way, it is possible to avoid a situation where the low-speed brush21aand the high-speed brush21bcome into sliding contact with the same segment15simultaneously.

That is, for example, the low-speed brush21aexists even at a position point-symmetrical with respect thereto about the rotary shaft3by a connecting wire40connected to the commutator10(refer to a two-dot chain line inFIG. 3). In this case, the spacing between the low-speed brush21aand the high-speed brush21bbecomes almost the same as the spacing between adjacent segments15,15. However, since the brush width W2of the high-speed brush21bis set to be smaller than the brush width W1of the low-speed brush21a, it is possible to avoid a situation where the low-speed brush21aand the high-speed brush21bcome into sliding contact with the same segment15simultaneously.

This is also the same between the high-speed brush21band the common brush21c. That is, the high-speed brush21bexists even at a position point-symmetrical with respect thereto about the rotary shaft3by a connecting wire40connected to the commutator10. However, since the brush width W2of the high-speed brush21bis set to be smaller than the brush width W1of the common brush21c, it is possible to avoid a situation where the high-speed brush21band the common brush21ccome into sliding contact with the same segment15simultaneously.

As shown inFIGS. 1 and 2, the gear group41housed in the housing body42of the gear housing23is constituted by a worm shaft25coupled to the rotary shaft3of the electric motor2, a stepped gear26which meshes with the worm shaft25, and a spur gear27which meshes with the stepped gear26. The worm shaft25has one end coupled to the rotary shaft3and the other end rotationally journalled to the housing body42. The stepped gear26is obtained by integrally forming a worm gear28which meshes with the worm shaft25, and a smaller-diameter gear29which is formed to have a smaller diameter than the worm gear28.

An idler shaft61is press-fitted into the radial center of the stepped gear26. The idler shaft61protrudes to the side opposite to the smaller-diameter gear29, and this protruding end61ais rotationally journalled to the housing body42. On the other hand, the tip of the smaller-diameter gear29which exists at the end opposite to the end61aof the idler shaft61is rotationally journalled to the cover43. In this way, the stepped gear26is brought into a state where both ends thereof are journalled to the housing body42and the cover43.

The spur gear27meshes with the smaller-diameter gear29of the stepped gear26. A boss portion65is formed at the radial center of the spur gear27so as to protrude toward the cover43side. The boss portion65is rotationally supported by the cover43. Additionally, an output shaft62is press-fitted into the boss portion65. The output shaft62protrudes from a bottom wall (end portion)42cof the housing body42. The boss portion63is formed at the part of the bottom wall42cof the housing body42corresponding to the output shaft62so as to protrude outward. The boss portion63is provided with a sliding bearing64for rotationally journalling the output shaft62.

The portion of the output shaft62which protrudes from the housing body42is formed with a tapered portion66which is gradually tapered as it goes to the tip. The tapered portion66is formed with serrations67. Thereby, for example, an external mechanism for driving a wiper or the like and the output shaft62can be coupled together.

In addition, a connector68is provided at the side wall42bof the housing body42so as to protrude along the axial direction of the rotary shaft3. The connector68is provided to supply the electric power from the outside to the electric motor2. A receiving port69of the connector68is provided with a connection terminal70, and the connection terminal70is electrically connected to the brushes21(21ato21c) of the electric motor2. Therefore, the electric power from the outside is supplied to the commutator10via the brushes21.

Moreover, a bolt seat71for fastening and fixing the cover43is formed integrally with an opening edge of the housing body42. An attachment seat73, which has a bolt-hole (not shown) through which the bolt72can be inserted, is integrally formed at the part of the housing body42of the cover43corresponding to the bolt seat71. As the bolt72is inserted through the attachment seat73, and the bolt72is screwed into the bolt seat71of the housing body42. Thereby, the cover43is fastened and fixed to the housing body42.

Additionally, the cover43is provided with a power distribution substrate74for electrically connecting the connection terminal70of the connector68and the brushes21of the electric motor2. The power distribution substrate74is formed with a pattern (not shown) which has a role of a lead wire.

Next, the structure for winding the winding wire14around the armature core8of the armature6will be described with reference toFIG. 5.

FIG. 5is a developed view of the armature6, and a gap between adjacent teeth12is equivalent to the slot13. In addition, in the following drawings, the respective segments15and the respective teeth12will be described with reference numerals given thereto.

As shown in detail in this drawing, the equipotential segments15are short-circuited by the connecting wires40. That is, in the present embodiment, every ninth segment15(for example, a first segment15and a tenth segment15) are short-circuited by the connecting wires40, respectively.

Here, the winding wire14is constituted by a first conductive wire110and a second conductive wire120. In addition, inFIG. 5, the first conductive wire110is shown by solid lines, and the second conductive wire120is shown by broken lines.

The first conductive wire110is wound in the forward direction between every pair of arbitrary slots13and13separated by three positions, and is wound in the backward direction between slots13and13which have shifted by 90 degrees, respectively, in the circumferential direction from the arbitrary slots13and13, thereby forming first to ninth winding wires161to169.

The second conductive wire120is wound in the forward direction between every pair of arbitrary slots13and13separated by three positions, and is wound in the backward direction between slots13and13which have shifted by 90 degrees, respectively, in the circumferential direction from the arbitrary slots13and13, thereby forming other first to ninth winding wires171to179.

The first to ninth winding wires (161to169) and the other first to ninth winding wires (171to179) exist at positions which are respectively point-symmetrical about the rotary shaft3.

That is, the first winding wire161formed by the first conductive wire110and the other first winding wire171formed by the second conductive wire120exist at positions which face each other about the rotary shaft3, and a first winding wire pair is formed by the winding wire161and the winding wire171. Similarly, a second winding wire pair (162,172), a third winding wire pair (163,173), a fourth winding wire pair (164,174), a fifth winding wire pair (165,175), a sixth winding wire pair (166,176), a seventh winding wire pair (167,177), an eighth winding wire pair (168,178), and a ninth winding wire pair (169,179) are formed by the second to ninth winding wires162to169and the other second to ninth winding wires172to179, respectively.

The first to ninth winding wires161to169are connected in a series via nine connecting wires40, respectively. On the other hand, the other first to ninth winding wires171to179are connected in a series via nine connecting wires40, respectively. A winding starting end and a winding finishing end of each of the winding wires161to179are connected between adjacent segments15and15. The first to ninth winding wires161to169and the other first to ninth winding wires171to179, which are formed between the slots13, respectively, in this way, can be wound, for example, using a double flyer type winding machine or the like.

Next, the operation of the reduction motor1will be described.

First, during low rotational driving, in the electric motor2of the reduction motor1, electric power is supplied to the common brush21cand the low-speed brush21a. At this time, magnetic fields are generated in the armature coils9wound around the armature core8, and magnetic attractive or repulsive forces are generated between the magnetic fields and the permanent magnets7provided at the yoke5to drive the rotary shaft3. On the other hand, during high rotational driving, the electric motor2is advanced by the high-speed brush21b, and operates at higher rotational speed than that during low rotational driving.

When the rotary shaft3is driven, the driving is transmitted to the output shaft62via the reduction mechanism4. Since an external mechanism for driving a wiper or the like is coupled to the output shaft62, the external mechanism operates at a low speed or operates at a high speed according to rotation of the output shaft62.

Additionally, all the teeth12and slots13of the electric motor2are point-symmetrical about the rotary shaft3, and the teeth12and the slots13exist alternately in a positional relationship with intervals of 90 degrees in the circumferential direction, i.e., at an interval of 180 degrees in electrical angle. Thereby, the relative positional relationship between an N-pole permanent magnet7and each tooth12which faces this magnet and the relative positional relationship between an S-pole permanent magnet7and each tooth12which faces this magnet are brought into the state of having deviated by a distance equivalent to half of the width of a slot13along the circumferential direction.

For this reason, the generation timing of the cogging torque of a tooth12of the portion corresponding to an N pole and the generation timing of the cogging torque of a tooth12of the portion corresponding to an S pole shift from each other. Therefore, the cogging torque of the whole electric motor2decreases.

Additionally, since electric power is supplied by the common brush21cand the low-speed brush21aduring low rotational driving, electric power is not supplied to the high-speed brush21b, and a non-energized state is brought about. For this reason, when the high-speed brush21bexists so as to straddle between adjacent segments15and15, the segments15and15are short-circuited by the high-speed brush21b. Also, the winding wire14connected to the short-circuited segments15and15only becomes a closed loop. As a result, a difference is caused in the number of coils of an equivalent electric circuit.

At this time, since magnetic flux passes through the winding wire14which becomes a closed loop by the high-speed brush21b, an induced voltage (counter-electromotive force) is generated in the winding wire14due to a change in this magnetic flux. An electric current in a direction opposite to the energized direction flows momentarily due to this induced voltage (counter-electromotive force), and commutation deteriorates. This will increase a torque ripple.

However, in the present embodiment, the brush widths W1of the low-speed brush21aand the common brush21cin the circumferential direction are set to be almost the same as each other, and the circumferential brush width W2of the high-speed brush21bis set to be smaller than the brush width W1of the low-speed brush21a(refer toFIG. 3). For this reason, the time for which the high-speed brush21bshort-circuits adjacent segments15and15can be set, and the electric current which flows into the winding wire14in which a closed loop has been formed by the high-speed brush21b, can be reduced.

On the other hand, since electric power is supplied by the common brush21cand the high-speed brush21bduring high rotational driving, electric power is not supplied to the low-speed brush21a, and a non-energized state is brought about. However, since an induced voltage (counter-electromotive force) which deteriorates commutation is not easily generated in the low-speed brush21a, the low-speed brush21ahas almost no influence during high rotational driving.

Accordingly, according to the above-described embodiment, the number of pole pairs is 2, i.e., the number of magnetic poles are four, and multiple slots are formed (the numbers of slots are 7 times, 9 times, and 11 times the number of pole pairs). Thus, the cogging torque can be reduced even in the variable-speed electric motor2. For this reason, it is possible to reduce the vibration and noise of the electric motor2(reduction motor1).

Particularly, an increase in the torque ripple resulting from the high-speed brush21bcan be reduced during low rotational driving with high use frequency, as compared to during high rotational driving. For this reason, it is possible to further reduce the vibration and noise of the electric motor2during low rotational driving.

Additionally, by setting the circumferential brush width W2of the high-speed brush21bto be smaller than the circumferential brush widths W1of the low-speed brush21aand the common brush21c, it is possible to avoid a situation where the low-speed brush21aand the high-speed brush21bcome into sliding contact with the same segment15simultaneously. For this reason, it is possible to provide a small-sized high-performance electric motor2with multiple slots.

Moreover, as the reduction motor1is constituted by the electric motor2, and the reduction mechanism4coupled to the rotary shaft3of the electric motor2, it is possible to achieve miniaturization and high performance of the reduction motor1.

In addition, it should be understood that the invention is not limited to the above-described embodiment, but various modifications may be made to the above-described embodiment without departing from the scope of the invention.

Additionally, the case where the armature core8of the electric motor2is provided with eighteen teeth12, and the number of the teeth12is set to 9 times the number of pole pairs has been described in the above-described embodiment. However, the number of the teeth12is not limited thereto. In a case where high performance by multiple slots of the electric motor2is taken into consideration, it is desirable to set the number of the teeth12to any of 7 times, 9 times, and 11 times the number of pole pairs.

INDUSTRIAL APPLICABILITY

As described above, according to the invention, it is possible to provide a variable-speed electric motor and a reduction motor which can reduce the vibration and noise while achieving miniaturization and high performance.

DESCRIPTION OF THE REFERENCE SYMBOLS

1: REDUCTION MOTOR

2: ELECTRIC MOTOR

4: REDUCTION MECHANISM

21c: COMMON BRUSH

D1: EXTERNAL DIAMETER