JP2010068593A - Electric motor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric motor which reduces magnetic sound by attaining size reduction and high output, and reduction in cogging torque. <P>SOLUTION: A magnet 22 includes an outer circumferential portion 52 opposing the inner circumferential surface of a yoke 21, an inner circumferential portion 53 formed in an arc-shape as seen on a flat surface in the axial direction and opposed to the tip surface of teeth 50, and a second flat portion 55 formed on the circumferential both ends of the inner circumferential portion 53. The relation among these portions is set: W1≥D×0.4 and W2≥W1×0.05, wherein the outer diameter of an armature core 11 is D, the width of the outer circumferential portion is W1 and the width of the flat portion is W2. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electric motor used for, for example, a starter motor for starting an engine for a motorcycle.

  Generally, an electric motor with a brush is often used as a starter motor for starting an engine of a motorcycle. This type of electric motor has a configuration in which a plurality of magnets are disposed on the inner peripheral surface of a yoke having a cylindrical portion, and an armature around which an armature coil is wound is rotatably supported. . The armature has an armature core that is externally fixed to the rotating shaft. In the armature core, teeth for winding the winding are formed radially, and a long slot is formed between the teeth in the axial direction.

  A winding is wound around each tooth to form an armature coil. The armature coil is electrically connected to a plurality of segments of the commutator attached to the rotating shaft. Each segment can be slidably contacted with a brush, and a current is supplied to the armature coil by applying a voltage from the brush to the segment. At this time, a magnetic field is formed in the armature coil, and the rotating shaft is driven by a magnetic attractive force or repulsive force generated between the magnet and the yoke.

  Here, in the electric motor, cogging torque is generated in the armature by the magnetic force of the magnet disposed on the yoke, and magnetic noise (noise) is generated due to this. In addition, in response to the recent demand for higher performance of electric motors, when the number of electric motors is increased, the frequency of rotating sound is high, and when the electric motor is stopped, the rotating sound of the armature is between And the natural frequency of the yoke may overlap and resonate.

Therefore, for example, as shown in Patent Document 1, the angle extending around the rotation axis of the magnet, that is, the magnet angle, the number of occurrences of cogging is an integral multiple of 2 or more of the least common multiple of the number of slots and the number of magnetic poles. In order to reduce the cogging torque, a technique has been proposed.
JP 2003-134772 A

By the way, in the electric motor used for the motorcycle described above, there is a high need for weight reduction of the vehicle including the vehicle body, a small motor is desired, and further higher output is desired.
Therefore, in order to reduce the size and increase the output of the electric motor, for example, a configuration in which the number of magnet poles is increased can be considered. In addition, as a constituent material of a magnet that replaces a conventionally used ferrite magnet, a configuration using a neodymium rare earth magnet having a higher magnetic flux density than a ferrite magnet can be considered.

  However, when a rare earth magnet is used as a constituent material of the magnet, it is possible to achieve a small size and high output, but since the magnetic attractive force and repulsive force between the armature coil and the magnet become strong, compared with the case where a ferrite magnet is used. This increases the cogging torque. As a result, there is a problem that the magnetic noise of the electric motor is increased.

  Accordingly, the present invention has been made in view of the above-described circumstances, and provides an electric motor that can reduce magnetic noise by reducing cogging torque while achieving a small size and high output. It is.

In order to solve the above-described problems, the invention described in claim 1 includes a yoke having a cylindrical portion, a plurality of magnets arranged in parallel along the circumferential direction on the inner peripheral surface of the yoke, and the yoke. In the electric motor comprising a rotating shaft provided rotatably inside and an armature core attached to the rotating shaft and having a plurality of teeth extending radially outward, the magnet is disposed on an inner peripheral surface of the yoke. The outer peripheral part which opposes, it forms in an arc shape by the axial direction planar view, the inner peripheral part which opposes the front end surface of the said teeth, and the circumferential direction both ends of the said inner peripheral part form flat along the width direction of the said magnet If the diameter of the armature core is D, the width of the outer peripheral portion is W1, and the width of the flat portion is W2, W1 ≧ D × 0.4 and W2 ≧ W1 × 0. Set to 05 It is characterized by that.
According to this structure, since the flat part is formed in the circumferential direction both ends, the plate | board thickness of a magnet becomes thin gradually as it goes to the circumferential direction both ends. For this reason, when the teeth pass through the inner peripheral portion by the rotation of the armature core and reach the flat portion, the distance between the magnet and the teeth gradually increases. As a result, the magnetic flux density generated between the magnet and the teeth gradually weakens, so that the magnetic attractive force by the magnet gradually weakens.
On the other hand, when the teeth approach the magnet adjacent to the magnets, the distance between the magnets and the teeth gradually decreases as the teeth approach the flat part due to the rotation of the armature core. The magnetic flux density generated between the two increases gradually. Therefore, the magnetic attractive force by the magnet gradually increases.
Thereby, since the change of the magnetic flux density which arises when teeth pass between adjacent magnets can be made gentle, the cogging torque at the time of teeth passing between adjacent magnets can be reduced.

Here, when the diameter of the armature core is D, the width of the outer peripheral portion is W1, and the width of the flat portion is W2, it is preferable to set W1 ≧ D × 0.4 and W2 ≧ W1 × 0.05. .
On the other hand, when W1 is smaller than D × 0.4 (W1 <D × 0.4), the magnetic flux is reduced because the magnetic flux at the peak is reduced and the cogging torque is reduced. Reducing the magnetic flux is not preferable because the output of the motor is reduced and high output cannot be achieved. On the other hand, if W2 is smaller than D × 0.05 (W2 <0.05), the width W2 of the flat portion is too narrow, and it is difficult to reduce the cogging torque when the magnetic flux is switched, and the magnetic sound is increased. It is not preferable.

The invention described in claim 2 is characterized in that the magnet is made of a rare earth magnet.
According to this configuration, the magnetic flux density can be increased by using a rare earth magnet as the magnet as compared with the conventional ferrite magnet.

According to the first aspect of the present invention, since the change in magnetic flux density generated when the teeth pass between the adjacent magnets can be moderated, the cogging torque when the teeth pass between the adjacent magnets can be reduced. Can be reduced. Accordingly, since the fluctuation range of the cogging torque, that is, the torque ripple can be reduced, the magnetic noise of the motor generated due to the cogging torque can be reduced.
Further, by setting the width W1 of the outer peripheral portion and the width W2 of the flat portion to W1 ≧ D × 0.4 and W2 ≧ W1 × 0.05, the magnetic flux can be switched smoothly, and the motor can be reduced in size. High output and cogging torque reduction can be combined.

  According to the second aspect of the present invention, since the magnetic flux density can be increased by using the rare earth magnet as the magnet as compared with the conventional ferrite magnet, the electric motor can be downsized and the high size can be reduced. Output becomes possible.

Next, embodiments of the present invention will be described with reference to the drawings.
As shown in FIGS. 1 to 3, the electric motor 1 is used as, for example, a two-wheel starter motor, and includes a front bracket 2 (left side in FIG. 1) fixed to the vehicle body and a front bracket 2. A cylindrical stator 3 to which one end is fixed, a rear bracket 4 (right side in FIG. 1) that closes the other end of the stator 3, and disposed in the stator 3, rotate to the front bracket 2 and the rear bracket 4. And an armature 5 that is freely supported.

  The armature 5 has a rotating shaft 10 connected to the shaft of the engine, and an armature core 11 and a commutator 12 are fixed to the rotating shaft 10. The armature core 11 is manufactured by laminating iron core pieces, and has a plurality (for example, 32 in this embodiment) of teeth 50 extending radially about the rotation shaft 10 (see FIG. 3). The teeth 50 are formed in a substantially T shape in a plan view in the axial direction, and a dovetail slot 51 that is long in the axial direction is formed between the teeth 50. A coil 13 is wound around the tooth 50 via a slot 51.

  The commutator 12 is a so-called disk type commutator having a disk shape. The commutator 12 has a plurality of segments 14 (for example, 32 in this embodiment) arranged radially around the rotation shaft 10, and between the segments 14, the insulation between the segments 14 is ensured. A slit (not shown) is formed. One end of a winding forming the coil 13 is fixed to the segments 14.

  The front bracket 2 is provided with a hole 15 through which the rotary shaft 10 penetrates in the center, and a bearing 16 that rotatably supports one end portion of the rotary shaft 10 is press-fitted and fixed in the hole 15. Furthermore, an oil seal 17 is disposed on the outer side in the radial direction than the bearing 16. Further, an O-ring 18 used for mounting on the engine case is attached to the outer periphery. The outer edge portion of the front bracket 2 is fixed to the stator 3 with bolts 19.

  The stator 3 includes a yoke 21 formed in a substantially cylindrical shape having a cylindrical portion 21A. On the inner peripheral side of the yoke 21, a magnet 22 formed in the shape of a roof tile for field is fixed. The magnets 22 are made of neodymium rare earth magnets, and are arranged at equal intervals along the circumferential direction of the yoke 21 at every angle θ (for example, every 60 degrees). That is, in the present embodiment, the electric motor 1 is a 6-pole 32-slot motor in which the number of slots 51 (tooth 50) is 32 and the six magnets 22 are provided. Further, by using a rare earth magnet for the magnet 22 as in the present embodiment, the magnetic flux density can be increased compared to the ferrite magnet, and the electric motor 1 can be miniaturized and the output can be increased. Become.

As shown in FIGS. 1 and 4, the rear bracket 4 is fixed with a bolt 23 at the other end of the yoke 21 where the magnet 22 is disposed.
The rear bracket 4 has a bottomed cylindrical shape, and the other end portion of the rotary shaft 10 of the armature 5 is inserted into a boss portion 25 protruding from the center of the bottom portion 4A. An oilless metal 26 is pressed into the boss portion 25 as a bearing, and the armature 5 is rotatably supported via the oilless metal 26.
The rear bracket 4 is made of a conductive material such as aluminum die casting.

  A brush holder 30 is fixed inside the bottom portion 4A of the rear bracket 4. The brush holder 30 is a substantially annular resin molded product, and protrudes so that the central portion 30 </ b> A surrounds the boss portion 25. Further, four holder portions 31 are integrally provided so as to surround the boss portion 25. The holder portion 31 extends in the axial direction, and one conductive brush 32 slidably contacting the segment 14 of the commutator 12 of the armature 5 is slidably received in each holder portion 31.

  The brush 32 is urged toward the commutator 12 by a spring 33 disposed in the holder portion 31. A pigtail 34 is attached to the brush 32 by integral molding. The pigtail 34 is pulled out from a slit (not shown) formed in the holder portion 31, and the pigtail 34 extending from two of the four brushes 32 (plus-side brushes) is connected to a power supply terminal via a power supply plate 35. 36. Pigtails 34 extending from the remaining two brushes 32 (minus side brushes) are connected to a ground terminal (not shown).

  The power supply terminal 36 is formed of a bolt member in which the head 41 is disposed in the brush holder 30. Each of the brush holder 30 and the rear bracket 4 is provided with terminal insertion holes 42 and 43 through which the power supply terminal 36 is inserted. An O-ring 44 is inserted into the terminal insertion hole 43, and the power supply terminal 36 is inserted from the inside of the brush holder 30 toward the outside of the rear bracket 4. The head 41 of the power supply terminal 36 is electrically connected to the power supply plate 35. The power feeding plate 35 is bent in a substantially L shape, and the above-described two pigtails 34 are fixed thereto by spot welding or the like.

  Outside the rear bracket 4, a nut 47 is tightened after passing the resin bush 45 and the stopper washer 46 through the power supply terminal 36, and the brush holder 30 and the rear bracket 4 are sandwiched between the power supply terminal 36 and the nut 47. Yes. A power supply wiring (not shown) is connected to the nut 47 and is electrically connected to the battery. An insulating cover 48 is attached to the outside of the rear bracket 4 so as to surround the nut 47.

  Under such a configuration, the electric motor 1 is fixed to a vehicle body of a motorcycle, such as an engine cover. At this time, the rotary shaft 10 extending from the front bracket 2 is connected to a one-way clutch attached to the crankshaft. The electric motor 1 is grounded by the vehicle body. When a voltage is applied to the power supply terminal 36, a current flows from the commutator 12 that is in sliding contact with the brush to the coil 13, and the armature 5 rotates. The rotation of the armature 5 rotates the crankshaft via the one-way clutch and starts the engine.

As shown in FIGS. 2 to 4, the magnet 22 described above is formed in an arc shape in an axial plan view, and includes an outer peripheral portion 52, an inner peripheral portion 53, and these outer peripheral portion 52 and inner peripheral portion 53. It is comprised by the 1st flat part 54 and the 2nd flat part 55 which were formed between (refer FIG. 4).
The outer peripheral portion 52 is formed with a radius of curvature equivalent to the radius of curvature of the inner peripheral surface of the cylindrical portion 21A, and the magnet 22 is attached to the yoke by fixing the outer peripheral portion 52 and the inner peripheral surface of the cylindrical portion 21A. 21 is fixed.

The inner peripheral portion 53 is formed to be smaller than the radius of curvature of the outer peripheral portion 52, and is opposed to the above-described distal end surface 57 (a radially outer surface) of the tooth 50 with a gap. That is, the change in the magnetic flux density between the inner peripheral portion 53 and the tip surface 57 of the tooth 50 is the change in the magnetic flux density between the magnet 22 and the tooth 50.
The first flat portion 54 is formed flat along the thickness direction of the magnet 22 at both circumferential ends of the outer peripheral portion 52.

  The second flat portion 55 is a flat surface formed so as to be orthogonal to the first flat portion 54 at both ends in the circumferential direction of the inner peripheral portion 53, and both the end portions in the circumferential direction of the inner peripheral portion 53 and the second flat portion 55. Is formed as an inflection point 56 of the magnet 22. Therefore, the arc angle β of the inner peripheral portion 53 (the angle between both inflection points 56) is formed smaller than the arc angle of the outer peripheral portion 52. That is, the magnet 22 is formed so that the plate thickness gradually decreases from the inflection point 56 toward both ends in the circumferential direction.

Here, if the width of the outer peripheral portion 52, that is, the length of the string of the outer peripheral portion 52 is W1, and the outer diameter D of the armature core 11 (see FIG. 3), W1 is formed to be D × 0.4 or more. Is preferable (W1 ≧ D × 0.4). On the other hand, when W1 is smaller than D × 0.4 (W1 <D × 0.4), the magnetic flux is reduced because the magnetic flux at the peak is reduced and the cogging torque is reduced. Reducing the magnetic flux is not preferable because the output of the motor is reduced and high output cannot be achieved.
On the other hand, by setting W1 ≧ D × 0.4, it is possible to obtain a small and high-output motor characteristic utilizing the characteristic of the rare earth magnet (magnet 22).

Further, when the width of the second flat portion is W2, W2 is preferably formed to be W1 × 0.05 or more (W2 ≧ W1 × 0.05). On the other hand, if W2 is smaller than D × 0.05 (W2 <0.05), the width W2 of the flat portion is too narrow, and it is difficult to reduce the cogging torque when the magnetic flux is switched, and the magnetic noise increases. It is not preferable.
On the other hand, by setting W2 ≧ W1 × 0.05, the magnetic flux can be switched smoothly, so that the cogging torque can be reduced while maintaining a high output.
Therefore, when the diameter of the armature core is D, the width of the outer peripheral portion is W1, and the width of the flat portion is W2, by setting W1 ≧ D × 0.4 and W2 ≧ W1 × 0.05, Switching can be performed smoothly, and it is possible to coexist with miniaturization and high output of the motor and cogging torque reduction.

Next, the process of generating cogging torque will be described with reference to FIG. In the following description, the above-described magnets 22 are indicated by a magnet 22A and a magnet 22B adjacent to the magnet 22A in order to distinguish them.
First, in the state shown in FIG. 5, four teeth 50 (hereinafter referred to as opposing teeth 50A) are in a state of facing the inner peripheral portion 53 of the magnet 22A. The distance between is equal. In this section, that is, the section in which the opposing teeth 50A pass through the range of the angle β of the magnet 22 (see FIG. 4) (inner peripheral portion 53), the magnets 22 cause the three opposing teeth 50A to be magnetically attracted to the same extent. It is sucked by force. For this reason, even if the armature 5 is rotated clockwise (in the direction of the arrow in FIG. 5), for example, little cogging torque is generated.

  Subsequently, when the armature 5 is rotated clockwise, among the four opposing teeth 50A, the opposing teeth 50A located at the rotation direction side end pass through the inner peripheral portion 53 of the magnet 22A, and the inner peripheral portion 53 and the 2 An inflection point 56 with the flat portion 55 is reached. Then, the opposing teeth 50A pass through the inflection point 56, and the opposing teeth 50A pass through the second flat portion 55 after facing the second flat portion 55. At the same time, the teeth 50B located on the opposite side to the rotation direction of the opposing teeth 50A are attracted to the magnet 22A.

  Here, when the opposing teeth 50A pass through the second flat portion 55 through the inflection point 56 of the magnet 22A, as the armature 5 rotates, that is, as the opposing teeth 50A move away from the inflection point 56 in the circumferential direction, the opposing teeth The distance between the tip surface 57 of 50A and the magnet 22 (second flat portion 55) gradually increases. Then, as the distance between the tip surface 57 of the magnet 22A and the opposing tooth 50A increases, the magnetic flux density generated between the magnet 22A and the opposing tooth 50A gradually decreases. That is, the magnetic attractive force by the magnet 22A gradually weakens. As a result, the force in the rotational direction (clockwise direction) acting on the armature 5 gradually weakens, so that the cogging torque generated by the change in magnetic flux density can be reduced.

  When the opposing tooth 50A reaches the magnet 22B adjacent to the magnet 22A, it first passes through the second flat portion 55 and then passes through the inflection point 56 and the inner peripheral portion 53. At this time, the distance between the tip surface 57 of the opposing tooth 50A and the magnet 22B (second flat portion 55) is such that the opposing tooth 50A approaches the inflection point 56 from the circumferential end of the second flat portion 55 of the magnet 22B. As it gradually shrinks. Then, as the distance between the magnet 22B and the tip surface 57 of the counter teeth 50A decreases, the magnetic flux density generated between the magnet 22B and the counter teeth 50A gradually increases. That is, the magnetic attractive force by the magnet 22B gradually increases. After that, when the opposing tooth 50A passes through the inflection point 56 of the magnet 22B and opposes the inner peripheral portion 53, the distance between the opposing tooth 50A and the magnet 22B becomes constant again. Sucked in. When the armature 5 rotates, these behaviors are repeated.

Next, the sound pressure (dBA) with respect to the frequency (Hz) was compared between the electric motor 1 using the magnet 22 of the present embodiment and the electric motor using the conventional magnet.
First, a conventional magnet will be described. In the following description, the same reference numerals are given to the same components as those of the magnet 22 (see FIG. 4) of the present embodiment, and the description thereof is omitted.
As shown in FIG. 7, the conventional magnet 122 includes an outer peripheral portion 52, a first flat portion 54 formed flat in the thickness direction of the magnet 122 at both circumferential ends of the outer peripheral portion 52, and an outer peripheral portion 52. The inner peripheral portion 153 is smaller than the radius of curvature and formed to be equal to the arc length of the outer peripheral portion 52. That is, the plate thickness of the magnet 122 is formed substantially uniformly along the circumferential direction (range of the angle α). In addition, a curved portion 155 (for example, a radius of 0.5 mm) is formed between the inner peripheral portion 153 and the first flat portion 54 to bend from both ends in the circumferential direction of the inner peripheral portion 153 to the first flat portion 54. ing.

FIG. 6 is a graph showing sound pressure (dBA) generated by cogging torque with respect to frequency (Hz) in the above-described electric motor of the present embodiment (solid line in FIG. 5) and the conventional electric motor (broken line in FIG. 5). is there.
As shown in FIG. 6, it can be seen that the electric motor 1 of the present embodiment has a small sound pressure in the entire frequency range as compared with the conventional electric motor. Further, it can be seen that the electric motor 1 of the present embodiment has a smaller fluctuation range of the sound pressure, so-called torque ripple, than the conventional electric motor.

This is probably because the electric motor 1 of the present embodiment has a gentler change in magnetic flux density that occurs when passing between adjacent magnets 122 than the conventional electric motor. That is, in the conventional magnet 122, since the thickness of the magnet 122 is substantially uniform in the circumferential direction, until the teeth pass the inner peripheral portion 153 of the magnet 122 (angle α in FIG. 7), the teeth A constant magnetic flux density is always generated between the magnet 122 and the magnet 122. Therefore, a constant magnetic attractive force can be obtained until the magnet 122 passes.
However, when the teeth pass through the magnet 122, the magnetic flux density rapidly decreases between adjacent magnets 122. Then, when attracted by the adjacent magnet 122, the magnetic flux density rapidly increases and a force directed in the rotation direction acts on the armature 5. As a result, when passing between adjacent magnets 122, the change in magnetic flux density increases, and the sound pressure increases rapidly. Thereby, it is considered that the fluctuation range of the sound pressure, that is, the torque ripple is also increased.

  On the other hand, in the electric motor 1 of the present embodiment, the magnetic flux density generated when the teeth 50 pass between the adjacent magnets 22 by forming the second flat portions 55 at both circumferential ends of the magnets 22. Change can be moderated. Therefore, it is considered that the torque ripple can be reduced by reducing the cogging torque generated when the teeth 50 pass between the adjacent magnets 22.

Therefore, according to the above-mentioned embodiment, since the 2nd flat part 55 is formed in the circumferential direction both ends of the magnet 22, the plate | board thickness of the magnet 22 becomes thin gradually as it goes to the circumferential direction both ends from the inflection point 56. FIG. Therefore, when the opposing teeth 50A pass through the inner peripheral portion 53 and reach the second flat portion 55 by the rotation of the armature 5, the distance between the magnet 22A and the opposing teeth 50A gradually increases. As a result, the magnetic flux density generated between the magnet 22A and the opposing tooth 50A gradually weakens, so that the magnetic attractive force by the magnet 22A gradually weakens.
On the other hand, when the opposing tooth 50A approaches the magnet 22B adjacent to the magnet 22A, when the opposing tooth 50A reaches the second flat portion 55, the distance between the magnet 22B and the opposing tooth 50A gradually decreases. The magnetic flux density generated between the magnet 22B and the opposing tooth 50A gradually increases. Therefore, the magnetic attractive force by the magnet 22B gradually increases.

Thereby, since the change in magnetic flux density generated when the teeth 50 pass between the adjacent magnets 22A and 22B can be moderated, the cogging torque when the teeth 50 pass between the adjacent magnets 22A and 22B can be reduced. Can be reduced.
Therefore, it is possible to reduce the generation of cogging torque and to reduce the fluctuation range of the cogging torque, that is, the torque ripple, while reducing the size and the output of the electric motor 1 by using a rare earth magnet as the magnet 22. . Therefore, the magnetic sound of the electric motor 1 generated due to the cogging torque can be reduced.

It should be noted that the technical scope of the present invention is not limited to the above-described embodiments, and includes those in which various modifications are made to the above-described embodiments without departing from the spirit of the present invention. In other words, the configuration described in the above-described embodiment is merely an example, and can be changed as appropriate.
For example, in the above-described embodiment, a so-called 6-pole / 32-slot motor having 32 slots and 6 magnets has been described. However, the present invention is not limited to this, and the design can be changed as appropriate.
Moreover, in the above-mentioned embodiment, although the outer peripheral part of the magnet was also formed in the arc shape by the axial direction planar view, it is not restricted to this, You may form an outer peripheral part flat. Thereby, the magnet is not processed according to the shape of the inner peripheral surface of the yoke. Therefore, the processing cost can be reduced and the manufacturing cost can be reduced.

It is a partial cross section figure of the electric motor in the embodiment of the present invention. It is a top view of the stator in the embodiment of the present invention. It is sectional drawing which follows the AA line of FIG. It is a top view of the magnet in the embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line AA in FIG. 1 and is an explanatory diagram for explaining the operation of the present embodiment. It is a graph which shows the sound pressure (dBA) with respect to a frequency (Hz). It is a top view of the conventional magnet.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electric motor 10 Rotating shaft 11 Armature core 21 Yoke 21A Tube part 22, 22A, 22B Magnet 50, 50A, 50B Teeth 52 Outer part 53 Inner part 55 Second flat part (flat part)

Claims (2)

A yoke having a cylindrical portion;
A plurality of magnets arranged side by side along the circumferential direction on the inner circumferential surface of the yoke;
A rotating shaft provided rotatably inside the yoke;
In an electric motor comprising an armature core attached to the rotating shaft and having a plurality of teeth extending radially outward,
The magnet has an outer peripheral portion facing the inner peripheral surface of the yoke, an arc shape in an axial plan view, an inner peripheral portion facing the tip end surface of the teeth, and both circumferential end portions of the inner peripheral portion. A flat portion formed flat along the width direction of the magnet,
When the diameter of the armature core is D, the width of the outer peripheral portion is W1, and the width of the flat portion is W2,
An electric motor characterized in that W1 ≧ D × 0.4 and W2 ≧ W1 × 0.05 are set.   The electric motor according to claim 1, wherein the magnet is a rare earth magnet.

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