Rotor and motor including the same

Abstract

The rotor includes a shaft and a cylindrical rotor core. The shaft extends along a central axis. The rotor core is disposed on the outside in the radial direction of the shaft and extends in the axial direction. The rotor core has a magnet group and a hole portion. The magnet groups are composed of first magnet parts and second magnet parts, and a plurality of magnet groups are arranged in the circumferential direction. When viewed in the axial direction, there are two first magnet parts in pairs, and the first magnet parts are separated from each other in the circumferential direction from the radially inner side toward the radially outer side. The second magnet portion is disposed between the pair of first magnet portions. A plurality of holes are disposed on the outside in the radial direction of the magnet group, are recessed in the axial direction from one axial end surface, and are disposed in the circumferential direction.

Description

Technical Field

[0001] The present invention relates to a motor having a rotor including the rotor. Background Art

[0002] Conventional rotors include an axis and a cylindrical rotor core. The axis extends along a central axis. The rotor core is disposed radially outward of the axis and extends axially. The rotor core has a magnet group that is composed of a first magnet portion and a second magnet portion and is circumferentially arranged in a plurality of portions. When viewed axially, the first magnet portions are two and paired, and the surfaces opposite to the outer peripheral surface of the rotor core are separated as they move radially outward. The second magnet portion is disposed between a pair of first magnet portions (for example, refer to patent document 1). Prior art literature Patent Literature

[0003] Patent Document 1: International Publication No. 2011 / 002043 Summary of the invention

[0004] However, in conventional motors, part of the magnetic flux extending radially outward from the magnet group may be directed toward the magnet group adjacent in the circumferential direction. As a result, the total harmonic distortion (THD) of the induced voltage generated when current is applied to the coil of the stator radially opposite to the rotor may increase. As a result, when the rotor rotates, vibration and noise may increase.

[0005] An object of the present invention is to provide a rotor capable of reducing the generation of vibration and noise, and a motor including the rotor.

[0006] An exemplary rotor of the present invention comprises an axis and a cylindrical rotor core. The axis extends along the central axis. The rotor core is arranged on the radially outer side of the axis and extends in the axial direction. The rotor core has a magnet group and a hole portion. The magnet group is composed of a first magnet portion and a second magnet portion and is arranged in a plurality in the circumferential direction. When viewed in the axial direction, the first magnet portions are two and paired, and the surfaces opposite to the outer peripheral surface of the rotor core are separated from each other in the circumferential direction as they go from the radial inside to the radial outside. The second magnet portion is arranged between a pair of first magnet portions. The hole portion is arranged on the radially outer side of the magnet group, and is recessed axially from one end surface in the axial direction and is arranged in a plurality in the circumferential direction.

[0007] According to the exemplary present invention, it is possible to provide a rotor capable of reducing the generation of vibration and noise, and a motor including the rotor. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 It is a longitudinal sectional view schematically showing a motor according to a first embodiment of the present invention. Figure 2 This is a cross-sectional view showing a part of the motor according to the first embodiment of the present invention. Figure 3 This is an enlarged cross-sectional view showing a magnet group of the motor according to the first embodiment of the present invention. Figure 4 It is a cross-sectional view showing an enlarged view of a magnet group of a motor according to a second embodiment of the present invention. DETAILED DESCRIPTION

[0009] Hereinafter, an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. In addition, in this specification, the rotation axis of the motor 1 is referred to as the "center axis", and the direction parallel to the center axis C is referred to as the "axial direction". In addition, the directions orthogonal to the center axis C of the motor 1 are referred to as "radial directions", and the directions along the arc centered on the center axis C of the motor 1 are referred to as "circumferential directions". Moreover, in this application, the shape and positional relationship of each part are described with the axial direction as the up and down direction. In addition, the up and down direction is only a name used for the convenience of description, and does not limit the actual positional relationship and direction in the motor 1.

[0010] <First embodiment> (1. Motor structure) A motor 1 according to an exemplary embodiment of the present invention will be described. Figure 1 is a longitudinal sectional view schematically showing a motor 1 according to an embodiment of the present invention, Figure 2 is a cross-sectional view showing a part of the motor 1. Figure 2 In the figure, the housing 21, the shaft 100 and the coil 23 are omitted.

[0011] The motor 1 has a rotor 10 and a stator 20. The stator 20 has a housing 21, a stator core 22, and a coil 23. The housing 21 is formed in a cylindrical shape and accommodates the stator core 22. The housing 21 has a bottom plate 21a and a top plate 21b. The bottom plate 21a is arranged on the axial lower side of the stator core 22 and holds the lower bearing 23a. The top plate 21b is arranged on the axial upper side of the stator core 22 and holds the upper bearing 23b.

[0012] The lower bearing 23 a and the upper bearing 23 b rotatably support a shaft 100 to be described later on the housing 21 .

[0013] The stator core 22 is formed by stacking multiple layers of annular electromagnetic steel sheets in the axial direction. The stator core 22 has a core back 221 and teeth 222. That is, the stator 20 has a core back 221 and teeth 222. The core back 221 is annular, arranged on the radial outer side of the rotor 10 and surrounds the central axis C. The teeth 222 protrude radially inward from the core back 221 and are arranged in plurality in the circumferential direction.

[0014] The plurality of teeth 222 are arranged at equal intervals in the circumferential direction around the central axis C. In the present embodiment, twelve teeth 222 are arranged.

[0015] The tooth 222 has a base portion 222a and an umbrella portion 222b (see Figure 2 ). The base 222a extends radially inward from the core back 221. The umbrella-shaped portion 222b protrudes from the radial inner end of the base 222a to both sides in the circumferential direction. The circumferential length of the umbrella-shaped portion 222b is greater than the circumferential length at the radial inner end of the base 222a.

[0016] The radially inner surface of the umbrella-shaped portion 222b is a curved surface along the circumferential direction. When viewed in the axial direction, the radially inner surface of the umbrella-shaped portion 222b is formed into an arc shape centered on the central axis C. The radially inner surface of the umbrella-shaped portion 222b is opposite to the outer peripheral surface of the rotor core 110 described later in the radial direction with a gap therebetween. The circumferentially adjacent umbrella-shaped portions 222b are arranged in an array with the grooves 223 therebetween.

[0017] The coil 23 is formed by winding a conductive wire around the teeth 222 via an insulator (not shown). Thus, the coil 23 is inserted into each slot 223. In the present embodiment, the coil 23 is formed by centrally winding the conductive wire around the teeth 222. Thus, the stator 20 can be miniaturized and the manufacturing cost of the motor 1 can be reduced.

[0018] (2. Rotor structure) The rotor 10 includes a shaft 100 and a rotor core 110. The shaft 100 is a columnar metal member extending along a central axis C extending in the vertical direction. The rotor core 110 is arranged radially outside the shaft 100 and extends in the axial direction. The rotor core 110 is formed into a cylindrical shape by stacking annular electromagnetic steel sheets in the axial direction.

[0019] The rotor core 110 has magnet groups 120N and 120S, a hole 130, and an axial hole 140. A plurality of magnet groups 120N and 120S are arranged in the circumferential direction. In the present embodiment, a total of eight magnet groups 120N and 120S are arranged, and the magnet groups 120N and the magnet groups 120S are arranged alternately in the circumferential direction. The magnet group 120N forms an N pole on the outer circumferential surface of the rotor core 110, and four magnet groups 120S are arranged. The magnet group 120S forms an S pole on the outer circumferential surface of the rotor core 110, and four magnet groups 120S are arranged.

[0020] Magnet group 120N and magnet group 120S have the same structure except that the magnetic poles on the outer circumference of rotor core 110 and the circumferential positions are different. The total number of magnet groups 120N and 120S is not limited to eight, and may be, for example, 2×n (n is an integer).

[0021] Each magnet group 120N, 120S is composed of a pair of first receiving holes 121, a second receiving hole 122, and gaps 125, 126 (see Figure 3), a pair of first magnet parts 123 and second magnet parts 124. The first receiving hole 121, the second receiving hole 122 and the gaps 125 and 126 penetrate the rotor core 110 in the axial direction. The first magnet part 123 is accommodated in the first receiving hole 121, and the second magnet part 124 is accommodated in the second receiving hole 122. In addition, the first receiving hole 121, the second receiving hole 122 and the gaps 125 and 126 may also be concave shapes that do not penetrate in the axial direction.

[0022] The hole portion 130 is arranged radially outside each magnet group 120N, 120S at the radial outer end of the rotor core 110. The hole portion 130 is axially recessed from one axial end surface of the rotor core 110 and multiple holes are arranged in the circumferential direction. In this embodiment, when viewed in the axial direction, the hole portion 130 is in the shape of a slit extending linearly toward the outer peripheral surface of the rotor core 110 and penetrates in the axial direction.

[0023] The gaps 125, 126 and the hole 130 are less likely to allow magnetic flux to pass through than in the rotor core 110 where the gaps 125, 126 and the hole 130 are not formed. Therefore, by providing the gaps 125, 126 and the hole 130, the flow of magnetic flux can be guided to a predetermined direction. For example, by forming the hole 130 into a slit shape, the flow of magnetic flux can be directed to the direction in which the hole 130 extends. In addition, a non-magnetic part such as a resin part may be arranged inside the gaps 125, 126 and the hole 130.

[0024] The shaft hole 140 extends in the axial direction and the shaft 100 is pressed into the inside. Thus, the shaft 100 is fixed to the rotor core 110. In addition, the shaft hole 140 and the shaft 100 can be fixed by spline connection. In addition, a resin material or a rubber material can also be interposed between the shaft hole 140 and the shaft 100. Thus, the insulation and vibration resistance of the rotor 10 are improved.

[0025] In the motor 1 having the above structure, when a drive current is supplied to the coil 23, a radial magnetic flux is generated in the stator core 22. The magnetic field generated by the magnetic flux of the stator core 22 and the magnetic field generated by the magnet groups 120N and 120S work together to generate torque in the circumferential direction of the rotor 10. The rotor 10 rotates around the central axis C by the torque.

[0026] <3. Detailed Structure of Magnet Group 3> Figure 3 FIG. 1 is an enlarged cross-sectional view showing the magnet group 120N. Figure 3 , the coil 23 is omitted. In the magnet group 120N, when viewed in the axial direction, a pair of first receiving holes 121 are arranged symmetrically around the magnetic pole center line d in the circumferential direction. The magnetic pole center line d is an imaginary line passing through the circumferential center of the second magnet portion 124 and the central axis C in each magnet group 120N, 120S.

[0027] When viewed in the axial direction, the first receiving hole 121 and the second receiving hole 122 are rectangular, and the gaps 125 and 126 are triangular. The gap 125 is arranged on the circumferential outer side of the first receiving hole 121, and the gap 126 is arranged on the circumferential inner side. The second receiving hole 122 is arranged adjacent to the first receiving hole 121 in the circumferential direction across the gap 126. In addition, the first receiving hole 121 of the magnet group 12N and the first receiving hole 121 of the magnet group 12S, which are adjacent in the circumferential direction, are arranged at a predetermined interval in the circumferential direction.

[0028] When viewed in the axial direction, surfaces of the pair of first housing holes 121 that face the outer peripheral surface of the rotor core 110 are separated from each other in the circumferential direction as they go from the radially inner side toward the radially outer side.

[0029] The second receiving hole 122 is disposed at one position between each pair of first receiving holes 121. When viewed in the axial direction, the second receiving hole 122 is orthogonal to the magnetic pole center line d and is linearly symmetrical with respect to the magnetic pole center line d.

[0030] The first magnet part 123 and the second magnet part 124 are in the shape of a cuboid in the present embodiment, and the types are not particularly limited. The first magnet part 123 and the second magnet part 124 can be, for example, a neodymium magnet or a ferrite magnet. In addition, the first magnet part 123 and the second magnet part 124 can also be composed of a plurality of split magnets. At this time, when viewed in the axial direction, the split magnets can also be arranged at a predetermined interval.

[0031] In each magnet group 120N, 120S, the radially outer magnetic poles of the first magnet portion 123 and the radially outer magnetic poles of the second magnet portion 124 are the same. Also, the radially inner magnetic poles of the first magnet portion 123 and the radially inner magnetic poles of the second magnet portion 124 are the same.

[0032] The first magnet portion 123 is arranged adjacent to the gaps 125 and 126 in the circumferential direction in the first receiving hole 121. The second magnet portion 124 is arranged adjacent to the gap 126 in the circumferential direction in the second receiving hole 122. In the present embodiment, the first magnet portion 123 and the second magnet portion 124 are not in contact in the circumferential direction. That is, the second magnet portion 124 is adjacent to the first magnet portion 123 across the gap 126 arranged on both sides of the circumferential direction. Thus, the flow of magnetic flux from one radial surface of the first magnet portion 123 and the second magnet portion 124 to the other radial surface can be suppressed. As a result, the driving efficiency of the motor 1 can be improved while the torque of the rotor 10 is improved.

[0033] In the present embodiment, the first magnet portion 123 and the second magnet portion 124 are of the same shape. When viewed in the axial direction, the first magnet portion 123 and the second magnet portion 124 are rectangular. There are two first magnet portions 123 in a pair, and the surfaces opposite to the outer peripheral surface of the rotor core 110 are separated from each other in the circumferential direction as they move from the radial inner side to the radial outer side. The second magnet portion 124 is arranged between the pair of first magnet portions 123.

[0034] This can suppress the flow of magnetic flux from the first magnet portion 123 toward the circumferentially adjacent magnet groups 120N and 120S. This can suppress the total harmonic distortion (THD) of the induced voltage generated when current is applied to the coil 23. Therefore, the vibration and noise of the rotor 10 can be reduced.

[0035] When viewed in the axial direction, a pair of first magnet portions 123 are arranged symmetrically with respect to the magnetic pole center line d. When viewed in the axial direction, the second magnet portion 124 is arranged so that the long side is orthogonal to the magnetic pole center line d. Thus, the flow of magnetic flux from the second magnet portion 124 toward the radial outside can be directed in a direction parallel to the magnetic pole center line d. Thus, the flow of magnetic flux from the second magnet portion 124 toward the circumferentially adjacent magnet groups 120N and 120S can be suppressed. As a result, the total harmonic distortion (THD) of the induced voltage generated when current is applied to the coil 23 can be suppressed.

[0036] In this embodiment, both radial side surfaces of the first magnet portion 123 are in axial contact with the inner surface of the first receiving hole 121. The circumferential inner side surface of the first magnet portion 123 is exposed to the gap 126. The circumferential outer side surface of the first magnet portion 123 is exposed to the gap 125.

[0037] Both radial side surfaces of the second magnet portion 124 are in contact with the inner surface of the second receiving hole 122 in the entire axial direction. In addition, both circumferential side surfaces of the second magnet portion 124 are exposed to the gap 126 .

[0038] When viewed in the axial direction, the holes 130 extend parallel to the magnetic pole center line d and are arranged at equal intervals in the circumferential direction. In the present embodiment, when viewed in the axial direction, the holes 130 are formed to be longer in the radial direction as they approach the magnetic pole center line d.

[0039] By providing the hole 130, the flow of magnetic flux from the first magnet portion 123 and the second magnet portion 124 toward the circumferentially adjacent magnet groups 120N and 120S can be further suppressed. In addition, the hole 130 is in the shape of a slit extending linearly toward the outer circumferential surface of the rotor core 110, which can further suppress the flow of magnetic flux from the first magnet portion 123 and the second magnet portion 124 toward the circumferentially adjacent magnet groups 120N and 120S. As a result, the total harmonic distortion (THD) of the induced voltage generated when current is applied to the coil 23 can be suppressed.

[0040] Furthermore, by forming the hole 130 parallel to the magnetic pole center line d, the flow of the magnetic flux from the first magnet portion 123 and the second magnet portion 124 to the radially outer side can be directed in a direction parallel to the magnetic pole center line d. Thus, the flow of the magnetic flux from the first magnet portion 123 and the second magnet portion 124 to the circumferentially adjacent magnet groups 120N and 120S can be further suppressed. As a result, the total harmonic distortion (THD) of the induced voltage generated when the current is applied to the coil 23 can be suppressed.

[0041] In addition, the radius of curvature of the outer shape of the rotor core 110 facing each magnet group 120N, 120S is smaller than the distance from the center to the radial outer end of the rotor core 110. As a result, the gap W between the teeth 222 and the rotor core 110 increases as it moves away from the magnetic pole center line d in the circumferential direction. Therefore, the flow of magnetic flux from the first magnet part 123 and the second magnet part 124 toward the circumferentially adjacent magnet groups 120N, 120S can be further suppressed.

[0042] <Second embodiment> Next, a second embodiment of the present invention will be described. Figure 4 FIG. 1 is an enlarged cross-sectional view showing the magnet group 120N. Figure 1 to Figure 3 The same reference numerals are used for the same parts as those in the first embodiment. In the second embodiment, the arrangement of the holes 130 is different from that in the first embodiment. The other parts are the same as those in the first embodiment.

[0043] The circumferential spacing of the hole portions 130 arranged in the area radially opposite to the first magnet portion 123 is smaller than the circumferential spacing of the hole portions 130 arranged in the area radially opposite to the second magnet portion 124. In the present embodiment, the hole portion 130 is not provided on the magnetic pole center line d in the area radially opposite to the second magnet portion 124.

[0044] When the hole portion 130 is provided in an area radially opposite to the second magnet portion 124, the flow of magnetic flux from the second magnet portion 124 toward the radial outside may be skewed near the radial ends of the hole portion 130. In the present embodiment, the area of ​​the hole portion 130 is reduced in the area radially opposite to the second magnet portion 124, thereby reducing the skewness of the flow of magnetic flux of the second magnet portion 124. Therefore, it is possible to reduce the situation where the flow of magnetic flux from the second magnet portion 124 toward the radial outside is suppressed by the hole portion 130. Thus, it is possible to suppress the total harmonic distortion (THD) of the induced voltage generated when current is applied to the coil 23.

[0045] Furthermore, when viewed in the axial direction, the hole 130 may be disposed only in the region opposite to the first magnet portion 123 in the extending direction of the magnetic pole center line d. This can further reduce the suppression of the flow of magnetic flux from the second magnet portion 124 toward the radial outside by the hole 130.

[0046] <4. Others> The above is a description of the embodiments of the present invention. In addition, the scope of the present invention is not limited to the above-mentioned embodiments. The present invention can be implemented by adding various changes without departing from the main purpose of the invention. Moreover, the above-mentioned embodiments can be appropriately combined arbitrarily. In the above-mentioned embodiment, the hole portion 130 is formed in a slit shape, but it can also be formed in a circular shape when viewed in the axial direction. By arranging the circular hole portions 130 in a prescribed configuration, the flow of magnetic flux can be guided to a prescribed direction. Thereby, the total harmonic distortion (THD) of the induced voltage generated when current is applied to the coil 23 can be suppressed.

[0047] Furthermore, in the above-described embodiment, the first receiving hole 121 and the second receiving hole 122 are arranged in the circumferential direction with the gap 126 interposed therebetween, but the first receiving hole 121 and the second receiving hole 122 may be arranged in the circumferential direction without the gap 126 interposed therebetween. By filling the gap 126 with the core material of the rotor core 110, the strength of the rotor core 110 is improved.

[0048] <5. Notes> As described above, a rotor 10 of one embodiment of the present invention comprises: an axis 100 extending along a central axis; and a cylindrical rotor core 110 arranged radially outside the axis and extending axially, the rotor core having: a magnet group 120N, 120S, which is composed of a first magnet portion 123 and a second magnet portion 124 and is arranged in plurality in the circumferential direction, wherein when viewed in the axial direction, the first magnet portions are two and paired, and the surfaces opposite to the outer peripheral surface of the rotor core are separated from each other in the circumferential direction as they go from the radial inside to the radial outside, and the second magnet portion is arranged between a pair of the first magnet portions; and a hole portion 130, which is arranged radially outside the magnet group, is recessed axially from one end surface in the axial direction, and is arranged in plurality in the circumferential direction (first structure).

[0049] Furthermore, in addition to the above-mentioned first structure, the second magnet portion may be arranged to be orthogonal to a magnetic pole center line d passing through the circumferential center of the second magnet portion and the central axis when viewed in the axial direction (second structure).

[0050] Furthermore, in addition to the above-mentioned first or second structure, the hole portion may be in the shape of a slit extending linearly in the radial direction when viewed in the axial direction (third structure).

[0051] Furthermore, in any one of the first to third structures, the hole portions may extend parallel to the magnetic pole center line when viewed in the axial direction and be arranged at equal intervals in the circumferential direction (fourth structure).

[0052] In addition, on the basis of any one of the above-mentioned first to fourth structures, it is also possible to construct that, when observed along the axial direction, the circumferential spacing of the hole portion arranged in the area radially opposite to the first magnet portion is smaller than the circumferential spacing of the hole portion arranged in the area radially opposite to the second magnet portion (fifth structure).

[0053] Furthermore, based on any one of the first to fifth structures, the curvature radius of the outer shape of the rotor core facing each magnet group may be smaller than the distance from the center to the radial outer end of the rotor core (sixth structure).

[0054] Furthermore, in any one of the first to sixth structures, the hole portion may be arranged only in a region facing the first magnet portion in the extending direction of the magnetic pole center line when viewed in the axial direction (seventh structure).

[0055] In addition, in any one of the first to seventh structures, the second magnet portion may be adjacent to the first magnet portion via the gap 126 disposed on both sides in the circumferential direction (eighth structure).

[0056] In addition, it can also be set to a structure of motor 1, which comprises: a rotor of any one of the above-mentioned first to eighth structures; and a stator 20, wherein the stator has a core back 221, teeth 222 and a coil 23, the annular core back is arranged on the radial outside of the rotor and surrounds the center axis, the teeth protrude radially inward from the core back and are arranged in plurality in the circumferential direction, and the coil is formed by winding the conductive wire concentratedly around the teeth (ninth structure). Industrial Applicability

[0057] The present invention can be used, for example, in electric equipment equipped with a motor, automobiles, ships, aircraft, trains, electric-assisted bicycles, wind power generators, and the like. Explanation of symbols

[0058] 1 motor; 10 rotor; 20 stator; 21 housing; 21a bottom plate; 21b top plate; 22 stator core; 23 coil; 23a lower bearing; 23b upper bearing; 100 shaft; 110 rotor core; 120N, 120S magnet group; 121 first receiving hole; 122 second receiving hole; 123 first magnet part; 124 second magnet part; 125, 126 gap; 130 hole; 140 shaft hole; 221 core back; 222 teeth; 222a base; 222b umbrella part; 223 slot; C center axis; W gap; d magnetic pole center line.

Claims

1. A rotor, characterized in that: have: a shaft extending along a central axis; and a cylindrical rotor core, the rotor core being arranged radially outward of the shaft and extending in the axial direction, The rotor core has: a magnet group, the magnet group being composed of a first magnet portion and a second magnet portion and being arranged in a plurality in the circumferential direction, wherein when viewed in the axial direction, the first magnet portions are two and form a pair, and the surfaces opposite to the outer peripheral surface of the rotor core portion are separated from each other in the circumferential direction as they go from the radial inside to the radial outside, and the second magnet portion is arranged between the pair of the first magnet portions; and The hole portion is arranged on the radially outer side of the magnet group, is recessed in the axial direction from one axial end surface, and is arranged in plurality in the circumferential direction.

2. The rotor according to claim 1, characterized in that When viewed in the axial direction, the second magnet portion is arranged to be orthogonal to a magnetic pole center line passing through a circumferential center of the second magnet portion and the central axis.

3. The rotor according to claim 1 or 2, characterized in that: When viewed in the axial direction, the hole portion has a slit shape extending linearly toward the outer peripheral surface of the rotor core.

4. The rotor according to claim 3, characterized in that When viewed in the axial direction, the hole portion extends parallel to the magnetic pole center line.

5. The rotor according to claim 4, characterized in that When viewed in the axial direction, the circumferential intervals of the holes arranged in a region radially opposed to the first magnet portion are smaller than the circumferential intervals of the holes arranged in a region radially opposed to the second magnet portion.

6. The rotor according to claim 1 or 2, characterized in that: The curvature radius of the outer shape of the rotor core facing each of the magnet groups is smaller than the distance from the center to the radially outer end of the rotor core.

7. The rotor according to claim 4, characterized in that When viewed in the axial direction, the hole portion is arranged only in a region facing the first magnet portion in the extending direction of the magnetic pole center line.

8. The rotor according to claim 1 or 2, characterized in that: The second magnet portion is adjacent to the first magnet portion via gaps disposed on both sides in the circumferential direction.

9. A motor, characterized in that: It comprises: a rotor as claimed in claim 1 or 2; and a stator, wherein the stator has a core back, teeth and coils, the annular core back is arranged on the radially outer side of the rotor and surrounds the central axis, the teeth protrude radially inward from the core back and are arranged in plurality in the circumferential direction, and the coils are formed by winding a conductive wire concentratedly around the teeth.

Images