阅读说明：本技术 感应电动机 (Induction motor ) 是由 三须大辅 小山泰平 松下真琴 长谷部寿郎 于 2018-11-02 设计创作，主要内容包括：本发明提供一种降低由转子的旋转引起的噪音的感应电动机。本实施方式的感应电动机具备产生磁场的定子以及被来自该定子的磁场驱动的转子。转子具备设置于旋转轴的铁芯。多个导体棒插入到设置于铁芯的多个槽内。第一环连接到多个导体棒的一端。第二环连接到多个导体棒的另一端。在铁芯与第一环之间以及铁芯与第二环之间具有间隙。多个导体棒从多个槽跨越间隙连接到第一环或者第二环。导体棒的相对于长度方向的垂直剖面是具有位于转子的外周侧的第一面、位于转子的内周侧的第二面、位于第一面与第二面之间的多个侧面、以及至少在间隙中在第一面与多个侧面之间相对于该第一面以及多个侧面倾斜的倾斜面的多边形。(The invention provides an induction motor which reduces noise caused by rotation of a rotor. The induction motor of the present embodiment includes a stator that generates a magnetic field and a rotor that is driven by the magnetic field from the stator. The rotor includes a core provided on the rotating shaft. The conductor bars are inserted into slots provided in the core. The first ring is connected to one end of the plurality of conductor bars. The second ring is connected to the other end of the plurality of conductor bars. There are gaps between the core and the first ring and between the core and the second ring. A plurality of conductor bars are connected to the first ring or the second ring across the gap from the plurality of slots. The conductor bar has a polygonal shape in a vertical cross section with respect to the longitudinal direction, the polygonal shape having a first surface located on the outer peripheral side of the rotor, a second surface located on the inner peripheral side of the rotor, a plurality of side surfaces located between the first surface and the second surface, and inclined surfaces inclined with respect to the first surface and the plurality of side surfaces at least in the gap between the first surface and the plurality of side surfaces.)

1. An induction motor comprising a stator for generating a magnetic field and a rotor driven by the magnetic field from the stator,

the rotor is provided with:

an iron core provided to the rotating shaft;

a plurality of conductor bars inserted into a plurality of slots provided in the core;

a first ring connected to one end of the plurality of conductor bars; and

a second ring connected to the other ends of the plurality of conductor bars,

there is a gap between the core and the first ring and between the core and the second ring,

the plurality of conductor bars are connected to the first ring or the second ring from the plurality of slots across the gap,

a cross section of the conductor bar perpendicular to the longitudinal direction is a polygon having a first surface located on an outer peripheral side of the rotor, a second surface located on an inner peripheral side of the rotor, a plurality of side surfaces located between the first surface and the second surface, and inclined surfaces inclined with respect to the first surface and the plurality of side surfaces between the first surface and the plurality of side surfaces at least in the gap,

the inclined surface is inclined at an angle of 10 to 40 degrees with respect to the first surface,

the width of the inclined surface in the direction parallel to the first surface is equal to or less than a width from a slot opening portion provided on an outer peripheral side of the core and communicating with the slot to a corner portion of the slot in the direction parallel to the first surface.

2. The induction motor of claim 1,

the width of the inclined surface in the direction parallel to the first surface is 0.5mm or more.

3. The induction motor according to claim 1 or 2,

the inclined surface is provided at a portion exposed in the gap and is not provided at a portion inserted into the groove.

4. The induction motor according to any one of claims 1 to 3,

the inclined surface is provided in both a portion exposed in the gap and a portion inserted into the groove.

5. The induction motor according to any one of claims 1 to 4,

the groove has a polygonal vertical cross section in the longitudinal direction, and the polygonal shape has a first inner surface located on the outer peripheral side of the rotor, a second inner surface located on the inner peripheral side of the rotor, a plurality of inner surfaces located between the first inner surface and the second inner surface, and inner inclined surfaces inclined with respect to the first inner surface and the plurality of inner surfaces between the first inner surface and the plurality of inner surfaces.

6. The induction motor of claim 5,

the inner inclined surface of the groove is inclined with respect to the first inner surface at an angle substantially equal to an inclination angle of the inclined surface of the conductor bar with respect to the first surface.

7. The induction motor according to claim 5 or 6,

the inner inclined surface of the groove is inclined at an angle of 10 to 40 degrees with respect to the first inner surface.

8. The induction motor according to any one of claims 5 to 7,

the shape of the groove in a vertical cross section with respect to the longitudinal direction is substantially similar to the shape of the conductor bar in a vertical cross section with respect to the longitudinal direction, and is slightly larger than the shape of the conductor bar in a vertical cross section with respect to the longitudinal direction.

Technical Field

Embodiments of the present invention relate to an induction motor.

Background

As an induction motor, a so-called cage type induction motor is known. A cage-type induction motor includes: a stator in which a stator coil is disposed on a substantially cylindrical stator core; and a rotor provided radially inside the stator and rotatably provided with respect to the stator.

The rotor has a core fixed to the rotating shaft, and a plurality of teeth extending in the radial direction are radially arranged on the core. A groove into which the conductor bar is inserted is formed between circumferentially adjacent teeth. The conductor bars are connected by short-circuiting rings at the axial ends of the rotor core.

In general, in order to weld the conductor bars and the short ring, a gap is provided between the end of the core and the short ring. However, since the conductor bars are exposed in the gap, wind noise accompanying the rotation is generated in the conductor bars themselves or in the space between adjacent conductor bars due to the rotation of the rotor. This is associated with an increase in noise of the induction motor.

Disclosure of Invention

Technical problem to be solved by the invention

Provided is an induction motor capable of reducing noise caused by rotation of a rotor.

Means for solving the problems

The induction motor of the present embodiment includes a stator that generates a magnetic field and a rotor that is driven by the magnetic field from the stator. The rotor includes a core provided on the rotating shaft. The conductor bars are inserted into slots provided in the core. The first ring is connected to one end of the plurality of conductor bars. The second ring is connected to the other end of the plurality of conductor bars. There are gaps between the core and the first ring and between the core and the second ring. A plurality of conductor bars are connected to the first ring or the second ring across the gap from the plurality of slots. The conductor bar has a polygonal shape in a vertical cross section with respect to the longitudinal direction, the polygonal shape having a first surface located on an outer peripheral side of the rotor, a second surface located on an inner peripheral side of the rotor, a plurality of side surfaces located between the first surface and the second surface, and inclined surfaces inclined with respect to the first surface and the plurality of side surfaces at least in the gap between the first surface and the plurality of side surfaces. The inclined surface is inclined at an angle of 10 to 40 degrees with respect to the first surface. The width of the inclined surface in the direction parallel to the first surface is not more than the width from a slot opening provided on the outer peripheral side of the core and communicating with the slot to a corner portion of the slot in the direction parallel to the first surface.

Drawings

Fig. 1 is a sectional view showing a configuration example of an induction motor according to a first embodiment.

Fig. 2 is a perspective view showing a structural example of the rotor.

Fig. 3 is a side view showing a structural example of the rotor core.

Fig. 4 is a diagram showing a part of a vertical section of the rotor with respect to the axial direction.

Fig. 5 is a sectional view showing a configuration example of the conductor bar 30 according to the first embodiment.

Fig. 6 is a perspective view showing a configuration example of one end portion of the conductor bar 30.

Fig. 7 is a diagram illustrating a relationship between the inclination angle of the inclined surface 31 and wind noise generated by rotation of the rotor 3.

Fig. 8 is a sectional view showing a configuration example of the conductor bar 30 according to the second embodiment.

Fig. 9 is a perspective view showing a configuration example of one end portion of the conductor bar 30 according to the second embodiment.

Fig. 10 is a sectional view showing a configuration example of the conductor bar 30 and the slot S according to the third embodiment.

Detailed Description

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present embodiment is not intended to limit the present invention. The drawings are schematic or conceptual, and the ratio of each portion and the like is not necessarily the same as the real ratio. In the description and the drawings, the same reference numerals are given to the same elements as those described above with respect to the conventional drawings, and detailed description is omitted as appropriate.

(first embodiment)

Fig. 1 is a sectional view showing a configuration example of an induction motor 1 according to a first embodiment. In fig. 1, only a structure of a single-side half of the induction motor 1 along the center axis C is shown. The induction motor 1 is a motor used for driving a railway vehicle (not shown), for example.

The induction motor 1 includes a stator 2, a rotor 3 provided rotatably with respect to the stator 2, and a casing 4 supporting the stator 2 and the rotor 3. The stator 2 is fixed to the housing 4. The rotor 3 is configured to be rotatable about a central axis C with respect to the stator 2. The induction motor 1 supplies current to a stator (primary side) 2, and generates an induced current in a rotor (secondary conductor) 3 by a magnetic field generated by the stator 2. The rotor obtains a rotational torque and is rotationally driven by the magnetic field from the stator.

In the following description, the direction of the central axis C is simply referred to as the axial direction, the direction of rotation about the central axis C is simply referred to as the circumferential direction (rotational direction), and the direction orthogonal to the axial direction and the circumferential direction is simply referred to as the radial direction (radial direction).

The stator 2 has a substantially cylindrical stator core 5. The stator core 5 is obtained by laminating a plurality of electromagnetic steel sheets 6 in the axial direction. The electromagnetic steel sheet 6 is a thin steel sheet produced by adding silicon to iron, for example.

A plurality of stator teeth 7 protruding toward the central axis C are provided on the inner circumferential surface side of the stator core 5. The stator teeth 7 are arranged at substantially equal intervals in the circumferential direction. Stator slots 8 are provided between circumferentially adjacent stator teeth 7. The stator coil 9 is wound around each stator tooth 7 through each stator slot 8. The stator coils 9 are provided to be suspended from both axial ends of the stator core 5 toward the axial outside. For example, dc power supplied from an overhead wire via a pantograph (both not shown) is converted into ac power and supplied to the stator coil 9.

Stator core pressing pieces 10 are provided at both axial ends of the stator core 5, respectively. The stator core presser 10 presses and holds the plurality of electromagnetic steel sheets 6 stacked to constitute the stator core 5 from both axial sides so as not to separate the plurality of electromagnetic steel sheets 6. The stator core presser 10 is formed in a substantially annular shape by a metal such as iron, and has an outer diameter larger than that of the stator core 5. The inner diameter of the stator core holder 10 is set to such a degree that the stator core holder 10 does not contact the stator coil 9. The stator core 5 and the stator core holder 10 are integrated by welding or the like.

The housing 4 includes: a pair of cylindrical mirror covers 11, 12 disposed on both sides of the stator 2 in the axial direction; and a pair of bearing brackets 13, 14 integrated with the mirror caps 11, 12, respectively. The mirror covers 11 and 12 are disposed in a state where the openings 11a and 12a face the stator core 5 side, respectively. Outer flange portions 15 and 16 are formed on the outer peripheral edges of the openings 11a and 12a of the mirror covers 11 and 12, respectively.

The outer diameters of the outer flange portions 15, 16 are set to be almost the same as the outer diameter of the stator core holder 10. Thereby, the stator core presser 10 and the outer flange portions 15 and 16 of the mirror caps 11 and 12 overlap in the axial direction. The stator core holder 10 and the outer flange portions 15 and 16 of the mirror covers 11 and 12 are fastened and fixed by bolts and/or nuts, not shown. Thereby, the stator 2 is supported by the mirror covers 11, 12.

Openings 11c and 12c are formed in the radial centers of the bottom portions 11b and 12b of the mirror covers 11 and 12, respectively. In order to close the openings 11c, 12c, corresponding bearing holders 13, 14 are provided. The bearing brackets 13, 14 are integrated with the corresponding mirror caps 11, 12, respectively.

The bearing holders 13, 14 are each formed in a substantially truncated cone shape, and are disposed so that the projecting direction thereof faces the stator 2 side. The insertion holes 13a and 14a through which the rotary shaft 21 can be inserted extend through the radial centers of the bearing brackets 13 and 14 in the axial direction. The bearing housing portions 13b and 14b are recessed axially outward from the radial centers of the bearing holders 13 and 14, respectively. The bearings 17 and 18 are provided in the bearing housing portions 13b and 14b, respectively. The rotary shaft 21 is rotatably supported by the bearing holders 13 and 14 through bearings 17 and 18. The housing 4 is fixed to, for example, the underfloor of a railway vehicle (neither is shown).

The rotor 3 has a rotary shaft 21 supported rotatably about a central axis C. Rotor core 22 having a substantially cylindrical shape (hereinafter simply referred to as core 22) is fitted around and fixed to rotating shaft 21. The outer diameter of the core 22 is set so that a small gap is formed between the outer peripheral surface 22a of the core 22 and the stator teeth 7 of the stator 2.

The core 22 is also formed by laminating a plurality of electromagnetic steel sheets 23 in the axial direction. A through hole 24 is provided at the radial center of the core 22. The rotary shaft 21 extends through the entire axial through hole 24, and rotates around the central axis C integrally with the core 22. When the rotary shaft 21 is inserted into the core 22, the core 22 and the rotary shaft 21 are integrated by press fitting, shrink fitting, or the like.

Rotor core holders 25 (hereinafter, also simply referred to as core holders 25) having a substantially disk shape are provided at both ends of the core 22 in the axial direction. The core presser 25 is also formed of a metal such as iron, and has a through hole 25a formed at the radial center. The rotary shaft 21 extends through the entire axial through hole 25a, and is integrated with the core 22 to rotate about the central axis C. The core presser 25 has a function of holding the stacked electromagnetic steel plates 23 constituting the core 22 so as not to be separated from each other and so as not to be displaced in the axial direction with respect to the rotary shaft 21.

Fig. 2 is a perspective view showing a structural example of the rotor 3. Fig. 3 is a side view showing a structural example of the iron core 22. Fig. 4 is a diagram showing a part of a vertical section of the rotor 3 with respect to the axial direction.

The rotor 3 includes a rotary shaft 21, a core 22, conductor bars 30, a first short ring 41, a second short ring 42, and a core presser 25.

As shown in fig. 2, the rotary shaft 21 has an elongated shape along the central axis C and is rotatable about the central axis C. The core 22 is fixed to the rotating shaft 21 and has a plurality of teeth T protruding radially outward. As shown in fig. 4, a groove S is provided between a plurality of teeth T adjacent in the circumferential direction Dr. The teeth T are arranged at substantially equal intervals in the circumferential direction. The circumferential widths of the plurality of teeth T are substantially equal to each other. Accordingly, the grooves S are also arranged at substantially equal intervals in the circumferential direction, and the widths of the grooves S in the circumferential direction are also substantially equal to each other. As shown in fig. 2 and 3, the slot S is a space extending in the axial direction between the teeth T, and into which the conductor bar 30 can be inserted. As shown in fig. 4, the slot S communicates with a slot opening OP provided on the outer peripheral side of the core 22. The width in the circumferential direction Dr of the slot opening OP is narrower than the width in the circumferential direction of the slot S so that the conductor bar 30 does not come off the slot opening OP. As shown in fig. 2 and 3, core holders 25 are provided at both axial ends of the core 22 so as to prevent the stacked magnetic steel sheets 23 constituting the core 22 from being separated from each other.

The conductor bar 30 is inserted into the slot S. The conductor bar 30 has an elongated shape in the axial direction like the slots S, and the conductor bar 30 is longer than the slots S. Therefore, the conductor bars 30 protrude from both ends of the iron core 22 in the axial direction. One end of the protruding conductor bar 30 is connected to the first short ring 41, and the other end thereof is connected to the second short ring 42. Thereby, the plurality of conductor bars 30 are connected and electrically connected by the first shorting ring 41 and the second shorting ring 42. The conductor rod 30 is made of a non-magnetic material such as copper or aluminum.

As shown in fig. 2 and 3, the first shorting ring 41 is connected to one end of the plurality of conductor bars 30 by, for example, welding. The second short ring 42 is connected to the other ends of the plurality of conductor bars 30 by, for example, welding. The first short ring 41 and the second short ring 42 are made of a non-magnetic material such as copper or aluminum.

Here, in order to connect the first and second short rings 41 and 42 to the conductor bar 30 by welding or the like, as shown in fig. 3, gaps (cavities) 50 are provided between the core 22 (teeth T) and the first short ring 41 and between the core 22 and the second short ring 42. The cavity 50 is provided between the plurality of conductor bars 30 adjacent in the circumferential direction at both axial end portions of the conductor bars 30. The conductor bar 30 is connected from the slot S across the cavity 50 to the first short circuit ring 41 or the second short circuit ring 42. The cavity 50 is necessary for manufacturing the rotor 3, but as described above, it causes wind noise during normal operation. This results in an increase in noise of the induction motor.

Fig. 5 is a sectional view showing a configuration example of the conductor bar 30 according to the first embodiment. Fig. 5 shows an enlarged view of a cross section of the broken-line box B of fig. 4. The core 22 is provided with a slot S extending in the axial direction (the direction perpendicular to the paper surface of fig. 5). The conductor bar 30, which likewise extends in the axial direction, is inserted into the slot S.

As shown in fig. 5, the shape of the vertical cross section of the groove with respect to the longitudinal direction is substantially similar to the shape of the vertical cross section of the conductor bar 30 with respect to the longitudinal direction, and is slightly larger than the shape of the vertical cross section of the conductor bar 30 with respect to the longitudinal direction. Thereby, the conductor bar 30 can be inserted into the slot S. In a state where the conductor bar 30 is inserted into the groove S, a minute gap is formed between the grooves S. Therefore, the width of the conductor bar 30 in the circumferential direction is almost the same as or slightly smaller than the width of the slot S in the circumferential direction.

The conductor bar 30 inserted into the slot S is fixed in the slot S by forging, caulking, adhesive, or the like, for example, via the slot opening OP.

On the outer peripheral side of the core 22, the end of the conductor bar 30 is chamfered and has an inclined surface 31. In the present embodiment, the inclined surface 31 is provided at a portion exposed in the cavity 50 and is not provided at a portion inserted into the groove S.

Fig. 6 is a perspective view showing a configuration example of one end portion of the conductor bar 30. The conductor bar 30 includes a conductor portion 30a exposed from the cavity 50 and a conductor portion 30b inserted into the slot S. The vertical section of the conductor bar 30 with respect to the longitudinal direction (axial direction) includes: a first surface F1 located on the outer circumferential side of the rotor 3; a second surface F2 located on the inner circumferential side of the rotor 3; and a plurality of side surfaces (third and fourth surfaces) F3, F4 located between the first surface F1 and the second surface F2. Further, in the conductor portion 30a exposed by the cavity 50, the conductor bar 30 has two inclined surfaces 31 between the first surface F1 and the side surfaces F3 and F4. The inclined surface 31 is inclined with respect to the first surface F1 and the side surfaces F3, F4. In other words, the inclined surface 31 is inclined with respect to the circumferential direction (rotational direction) as well as the radial direction. Thus, the conductor bar 30 has a substantially hexagonal vertical cross section with respect to the axial direction. In the conductor portion 30b hidden in the groove S, a vertical cross section of the conductor bar 30 with respect to the axial direction is a substantially rectangular shape including the first surface F1, the second surface F2, and the side surfaces F3 and F4. Further, the inclined surface 31 may be a flat surface, but may also be a curved surface. The inclined surface 31 may have some degree of unevenness.

In this way, the inclined surfaces 31 are provided by chamfering both ends of the first surface F1 on the outer peripheral side of the conductor bar 30 exposed in the cavity 50, thereby reducing wind noise of the conductor bar 30 and/or the cavity 50 caused by the rotation of the rotor 3. This can suppress noise of the induction motor 1.

Fig. 7 is a diagram illustrating a relationship between the inclination angle of the inclined surface 31 and wind noise generated by rotation of the rotor 3. The inclination angle of the inclined surface 31 indicates the inclination angle of the inclined surface 31 with respect to the first surface F1 or the rotation direction. That is, the inclination angle of the inclined surface 31 is an angle represented by θ in fig. 6. Further, the graph is a result of calculating noise generated in the cavity 50 by thermal fluid analysis.

When the inclination angle θ is too small, the inclined surface 31 is almost coplanar with the first surface F1 (circumferential direction or rotational direction), and thus the wind noise (sound pressure level) is hardly reduced. On the other hand, when the inclination angle θ is too large, the inclined surface 31 comes close to the side surfaces F3, F4 (radial direction), and thus the wind noise (sound pressure level) is not reduced.

For example, in order to make the sound pressure level 115dB or less, the inclination angle θ is preferably about 10 degrees to about 40 degrees. In particular, the sound pressure level is less than 110dB at an inclination angle θ of about 30 degrees. Therefore, it can be said that, more preferably, the inclination angle θ is about 30 degrees. By setting the inclination angle of the inclined surface 31 in this way, wind noise of the conductor bars 30 and/or the cavity 50 caused by rotation of the rotor 3 is further reduced.

As shown in fig. 5 or 6, the width of the inclined surface 31 in the direction parallel to the first surface F1 is W31, and as shown in fig. 5, the width from the end of the groove opening OP to the corner of the groove in the direction parallel to the first surface F1 is Ws. In this case, the width W31 is preferably equal to or less than the width Ws. This is because the conductor bar 30 can be reliably fixed in the groove S when the conductor bar 30 is forged. In addition, the end of the stub 30 is generally rounded at a radius of about 0.5 mm. Therefore, the width W31 of the inclined surface 31 is 0.5mm or more.

The inclined surfaces 31 are provided at both ends of the first surface F1 on the outer peripheral side of the conductor bar 30, but may be provided at both ends of the second surface F2 on the inner peripheral side of the conductor bar 30.

(second embodiment)

Fig. 8 is a sectional view showing a configuration example of the conductor bar 30 according to the second embodiment. In the second embodiment, the entire end of the conductor bar 30 is chamfered and has the inclined surface 31 on the outer peripheral side of the core 22. That is, in the second embodiment, the inclined surface 31 is provided to both the conductor portion 30a exposed in the cavity 50 and the conductor portion 30b inserted into the groove S.

Fig. 9 is a perspective view showing a configuration example of one end portion of the conductor bar 30 according to the second embodiment. Referring to fig. 9, the inclined surface 31 is provided on both the conductor portion 30a and the conductor portion 30 b. The shape and size of the inclined surface 31 may be the same as those of the inclined surface 31 of the first embodiment. In addition, other configurations of the second embodiment may be the same as the corresponding configurations of the first embodiment.

In this way, the inclined surface 31 may be provided along the extending direction of the conductor bar 30 as a whole. The second embodiment can obtain the same effects as the first embodiment. Further, by providing the inclined surface 31 over the entire conductor bar 30, the conductor bar 30 can be processed relatively easily. That is, in the manufacture of the conductor bar 30, the conductor portion 30a and the conductor portion 30b need not be processed separately, but may be processed in the same manner. Thereby, the manufacturing cost is reduced. For example, the conductor bar 30 may be chamfered in the longitudinal direction by pressing or the like in the manufacturing stage.

(third embodiment)

Fig. 10 is a sectional view showing a configuration example of the conductor bar 30 and the slot S according to the third embodiment. In the third embodiment, the conductor bar 30 of the second embodiment is used. Further, in the third embodiment, the vertical cross section of the slot S with respect to the longitudinal direction is substantially similar to the shape of the vertical cross section of the conductor bar 30 with respect to the longitudinal direction, and is slightly larger than the shape of the vertical cross section of the conductor bar 30 with respect to the longitudinal direction.

For example, a vertical section of the groove S with respect to the longitudinal direction (axial direction) includes: a first inner surface FS1 located on the outer circumferential side of the rotor 3; a second inner surface FS2 located on the inner peripheral side of the rotor 3; and a plurality of inner side surfaces FS3, FS4 located between the first inner surface FS1 and the second inner surface FS 2. Further, the slot S has an inner inclined surface 33 between the first inner surface FS1 and the inner side surfaces FS3, FS 4. The inner inclined surface 33 is inclined with respect to the first inner surface FS1 and the inner side surfaces FS3, FS 4. In other words, the inner inclined surface 33 is inclined with respect to the circumferential direction (rotational direction) as well as the radial direction. Thus, the groove S has a substantially hexagonal cross section perpendicular to the axial direction. Further, the inner inclined surface 33 may be a flat surface or a curved surface. The inner inclined surface 33 may have some degree of unevenness.

The inner inclined surface 33 of the groove S is inclined with respect to the first inner surface FS1 at an angle substantially equal to the inclination angle θ of the inclined surface 31 of the conductor bar 30. That is, the inner inclined surface 33 of the groove S is preferably inclined at an angle of about 10 degrees to about 40 degrees with respect to the first inner surface FS1 (circumferential or rotational direction). In particular, when the inclination angle θ of the inclined surface 31 is about 30 degrees, the inclination angle of the inner inclined surface 33 of the groove S is also preferably about 30 degrees. This makes it possible to easily insert the conductor bar 30 into the slot S without generating an excessive gap between the conductor bar 30 and the slot S. Therefore, the conductor bar 30 becomes easily fixed in the groove S.

The other structures of the third embodiment may be the same as the corresponding structures of the second embodiment. Therefore, the third embodiment can obtain the same effects as the second embodiment.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in other various ways, and various omissions, substitutions, and changes can be made without departing from the spirit of the invention. These embodiments and modifications are included in the scope and gist of the invention, and are also included in the scope of the invention described in the claims and the equivalent scope thereof.