Electric motor

Abstract

This electric motor comprises a stator in which teeth that have coils are disposed so as to be separated from each other in the circumferential direction, a rotor in which permanent magnets are provided to portions that correspond to the teeth, and magnetic wedges that are provided between the distal ends of adjacent teeth. The saturation magnetic flux density of the magnetic wedges is equal to or less than the saturation magnetic flux density of the teeth.

Description

electric motor

[0001] The present invention relates to an electric motor.

[0002] JP2000-261998A discloses an electric motor in which magnetic wedges with complex structures, each with a different magnetic permeability at each location, are arranged between the teeth of a stator to increase rotational efficiency.

[0003] In this motor, magnetic wedges provided between the teeth cause leakage flux to constantly flow between the teeth, which causes the motor to have poor torque characteristics when driven at high torque.

[0004] The present invention has been made in view of the above problems, and has an object to provide an electric motor that can improve torque characteristics even when a magnetic wedge is used.

[0005] According to one aspect of the present invention, an electric motor includes a stator having teeth with coils spaced apart in the circumferential direction, a rotor having permanent magnets provided in portions corresponding to the teeth, and magnetic wedges provided between the tips of adjacent teeth, the saturation magnetic flux density of the magnetic wedges being equal to or less than the saturation magnetic flux density of the teeth.

[0006] In this electric motor, the saturation magnetic flux density of the magnetic wedge is equal to or lower than the saturation magnetic flux density of the teeth, and the magnetic wedge provided between the teeth is prone to magnetic saturation.

[0007] Therefore, when the magnetic wedge becomes magnetically saturated during high torque driving, the increase in leakage magnetic flux flowing between the teeth is suppressed, and the decrease in the flow of magnetic flux along the main magnetic path, which contributes to increased torque, can be suppressed.

[0008] Therefore, even when the magnetic wedge is used, the torque characteristics of the electric motor can be improved during high torque driving.

[0009] Fig. 1 is an enlarged view showing a main part of an electric motor according to this embodiment. Fig. 2 is a perspective view showing a magnetic wedge of an electric motor according to this embodiment. Fig. 3 is an enlarged view showing a main part of an electric motor according to a first modified example. Fig. 4 is a perspective view showing a magnetic wedge of an electric motor according to the first modified example. Fig. 5 is an enlarged view showing a main part of an electric motor according to a second modified example.

[0010] <Embodiments> Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0011] Fig. 1 is an enlarged view showing a main part of an electric motor 10 according to this embodiment, and Fig. 1 shows a part of the interior of the electric motor 10. Fig. 2 is a perspective view showing a magnetic wedge 36 of the electric motor 10 according to this embodiment.

[0012] 1 is a so-called inner rotor type electric motor used in, for example, an electric vehicle. The electric motor 10 is mounted on the vehicle to drive the drive wheels provided on the vehicle and also functions as a generator during deceleration.

[0013] A known example of the electric motor 10 is a surface permanent magnet synchronous motor in which permanent magnets 22 are provided on the surface of the rotor 20. Another known example of the electric motor 10 is an interior permanent magnet synchronous motor in which permanent magnets (not shown) are embedded inside the rotor 20.

[0014] The electric motor 10 of this embodiment will be described as an example of a surface magnet type permanent magnet synchronous motor, but the electric motor 10 of this embodiment can also be applied to an embedded magnet type permanent magnet synchronous motor.

[0015] The electric motor 10 includes a stator 34 having teeth 32 with coils 30 arranged spaced apart in the circumferential direction S, and a rotor 20 having permanent magnets 22 provided on opposing surfaces (rotor core surfaces) 20A that correspond to the teeth 32. The electric motor 10 includes magnetic wedges 36 provided between the tips of adjacent teeth 32. The rotor 20 is arranged inside the stator 34. The rotor 20 is rotatable relative to the stator 34.

[0016] (Stator) The stator 34 is fixed to a casing (not shown). The stator 34 is made of, for example, a plurality of stacked circular ring-shaped electromagnetic steel sheets, and the stator 34 is formed in a cylindrical shape. The electromagnetic steel sheets that make up the stator 34 are made of non-oriented electromagnetic steel sheets that have excellent magnetic properties in all directions.

[0017] A plurality of notches are formed at equal intervals in the circumferential direction S on the inner edge 34A of the stator 34. The notches form slots 40 that open inward in the stator 34. Teeth 32 extending inward are formed between adjacent slots 40.

[0018] The teeth 32 have a body portion 32A that extends inward from the ring-shaped outer edge portion 50 of the stator 34. At the tip of the body portion 32A, a widened portion 32B is formed, the width of which increases toward the tip. At the tip of the widened portion 32B, a widened portion 32C is formed, the width of which remains constant until it reaches the tip.

[0019] A wire is wound around the body portion 32A of each tooth 32, and each tooth 32 has a coil 30 made up of the wire. A portion of the coil 30 of each tooth 32 is disposed in a slot 40 formed between the teeth 32.

[0020] A current from a driving device (not shown) is input to each coil 30. Each tooth 32 is excited by the current input to the coil 30.

[0021] (Rotor) The rotor 20 is formed, for example, in a cylindrical shape. A rotation shaft 60 is provided at the rotation center C of the rotor 20. The rotation shaft 60 is supported by a casing (not shown) via a bearing. The rotor 20 is rotatable around the rotation shaft 60.

[0022] The outer peripheral surface of the rotor 20 forms an opposing surface 20A that faces the end surfaces of the teeth 32 of the stator 34. The aforementioned permanent magnets 22 are provided on the opposing surface 20A.

[0023] The permanent magnets 22 form a Halbach array magnetic circuit having a plurality of main magnets 70 (70A, 70B) arranged in the circumferential direction S of the rotor 20 and auxiliary magnets 72 (72A, 72B, 72C, 72D) arranged between each main magnet 70 (70A, 70B) and magnetized in a direction different from the magnetization direction of the adjacent main magnets 70 (70A, 70B). The magnetization directions of the auxiliary magnets 72 (72A, 72B, 72C, 72D) are inclined within a range of 90 degrees or less with respect to the magnetization direction of the main magnets 70 (70A, 70B).

[0024] In the specification and drawings, the first main magnet constituting the main magnet 70 will be described with the reference numeral "70A," which is the addition of "A" to the reference numeral "70" indicating the main magnet. The second main magnet constituting the main magnet 70 will be described with the reference numeral "70B," which is the addition of "B" to the reference numeral "70" indicating the main magnet.

[0025] Furthermore, the first auxiliary magnet constituting the auxiliary magnet 72 will be described using the symbol "72A" which is an addition of "A" to the symbol "72" indicating an auxiliary magnet. The second auxiliary magnet constituting the auxiliary magnet 72 will be described using the symbol "72B" which is an addition of "B" to the symbol "72" indicating an auxiliary magnet. The third auxiliary magnet constituting the auxiliary magnet 72 will be described using the symbol "72C" which is an addition of "C" to the symbol "72" indicating an auxiliary magnet. The fourth auxiliary magnet constituting the auxiliary magnet 72 will be described using the symbol "72D" which is an addition of "D" to the symbol "72" indicating an auxiliary magnet.

[0026] Specifically, the main magnet 70 has a first main magnet 70A magnetized so that the radially outer side R of the rotor 20, which is on the tooth 32 side, is the north pole, and a second main magnet 70B magnetized so that the radially inner side R, which is on the rotating shaft 60 side, is the north pole.

[0027] A first auxiliary magnet 72A and a second auxiliary magnet 72B are arranged in this order on one side of the first main magnet 70A in the circumferential direction S. The second main magnet 70B is arranged on one side of the second auxiliary magnet 72B, and the first auxiliary magnet 72A and the second auxiliary magnet 72B are arranged between the first main magnet 70A and the second main magnet 70B.

[0028] A third auxiliary magnet 72C and a fourth auxiliary magnet 72D are arranged in this order on the other side of the first main magnet 70A in the circumferential direction S. The second main magnet 70B is arranged on the other side of the fourth auxiliary magnet 72D, and the third auxiliary magnet 72C and the fourth auxiliary magnet 72D are arranged between the first main magnet 70A and the second main magnet 70B.

[0029] The first auxiliary magnet 72A is magnetized so that the north pole is inclined toward the adjacent first main magnet 70A as it moves outward in the radial direction R, that is, toward the teeth 32. The second auxiliary magnet 72B is magnetized so that the north pole is inclined toward the adjacent second main magnet 70B as it moves inward in the radial direction R, that is, toward the rotation shaft 60.

[0030] The third auxiliary magnet 72C is magnetized so that the north pole is inclined toward the adjacent first main magnet 70A as it moves outward in the radial direction R, that is, toward the teeth 32. The fourth auxiliary magnet 72D is magnetized so that the north pole is inclined toward the adjacent second main magnet 70B as it moves inward in the radial direction R, that is, toward the rotation shaft 60.

[0031] The tilt direction indicating the magnetization direction of each of the auxiliary magnets 72A, 72B, 72C, and 72D is tilted at 45 degrees with respect to the radial direction R.

[0032] As a result, the rotor 20 is configured so that the magnetic field strength directed from the permanent magnets 22 on the opposing surface 20A toward each tooth 32 of the stator 34 is increased.

[0033] 1 and 2 , the magnetic wedge 36 has a main body 36A disposed between the widened portions 32B of adjacent teeth 32, and a protruding portion 36B protruding from the main body 36A and disposed between the widened portions 32C of adjacent teeth 32. The main body 36A is formed in a trapezoidal shape in side view, with the width dimension narrowing toward the rotor 20. The protruding portion 36B is formed in a rectangular shape in side view. A tip end surface 36C of the protruding portion 36B faces the permanent magnets 22 that form the outer peripheral surface of the rotor 20, with a gap formed between them.

[0034] The magnetic wedge 36 is made of a soft magnetic material, such as a low-permeability magnetic material, such as a sintered mixture of iron oxide as the main component and cobalt, nickel, manganese, and the like.

[0035] The saturation magnetic flux density of the magnetic wedge 36 is set to be equal to or less than the saturation magnetic flux density of the teeth 32 formed on the stator 34, and is set to a value that causes the magnetic wedge 36 to become magnetically saturated during high torque driving. Note that the saturation magnetic flux density of the magnetic wedge 36 is preferably smaller than the saturation magnetic flux density of the teeth 32.

[0036] More specifically, the magnetic wedge 36 is made of a magnetic material having an initial magnetic permeability of 5000 H / m or less at room temperature and a saturation magnetic flux density of 400 mT or more.

[0037] In contrast, the stator 34 having the teeth 32 is formed of a magnetic material having a relative magnetic permeability of about 17,000 H / m at room temperature and a saturation magnetic flux density of 2 T or less.

[0038] The magnetic wedge 36 is made of grain-oriented electromagnetic steel sheets that have excellent magnetic properties in the rolling direction. In this case, the magnetic wedge 36 can be formed, for example, from a plurality of grain-oriented electromagnetic steel sheets that are laminated so that the rolling direction is the circumferential direction S, which is the direction of magnetic flux flowing between adjacent teeth 32. As a result, the magnetic wedge 36 has a higher magnetic permeability in the circumferential direction S, which is the direction of magnetic flux flowing between adjacent teeth 32, than in the radial direction R, which is the direction intersecting the flow direction.

[0039] In this way, by forming the magnetic wedge 36 from a grain-oriented electromagnetic steel plate, it is possible to impart anisotropy to the inductance, which makes it possible for the magnetic wedge 36 to facilitate the passage of magnetic flux between adjacent teeth 32.

[0040] More specifically, the magnetic wedge 36 of this embodiment has a relative magnetic permeability in the circumferential direction S of approximately 5,000 H / m, and a relative magnetic permeability in the radial direction R of 5,000 H / m or less.

[0041] The magnetic wedge 36 may have the same relative permeability in the circumferential direction S and the radial direction R, but the relative permeability in the radial direction R is set to be no greater than the relative permeability in the circumferential direction S. Preferably, the magnetic wedge 36 has a relative permeability in the radial direction R that is smaller than the relative permeability in the circumferential direction S.

[0042] Note that, although the present embodiment will be described taking as an example a case where the magnetic wedge 36 is formed from a grain-oriented electromagnetic steel plate, the magnetic wedge 36 is not limited to this configuration. For example, the magnetic wedge 36 may be configured by alternately stacking plates with high magnetic permeability and plates with low magnetic permeability in the radial direction R, so that the magnetic permeability in the circumferential direction S, which is the direction in which magnetic flux flows between adjacent teeth 32, is higher than the magnetic permeability in the radial direction R, which is the intersecting direction that intersects with the flow direction.

[0043] Furthermore, the magnetic permeability of the magnetic wedges 36 is set to be equal to or less than the magnetic permeability of the teeth 32 formed on the stator 34. It is desirable that the magnetic permeability of the magnetic wedges 36 be smaller than the magnetic permeability of the teeth 32.

[0044] In this embodiment, in which the magnetic wedges 36 are made of grain-oriented electromagnetic steel plates, the magnetic permeability of the magnetic wedges 36 in the circumferential direction S is set to be equal to or less than the magnetic permeability of the teeth 32 of the stator 34, which are made of non-oriented electromagnetic steel plates. In this case, too, it is desirable that the magnetic permeability of the magnetic wedges 36 in the circumferential direction S be smaller than the magnetic permeability of the teeth 32.

[0045] Specifically, the relative permeability in the circumferential direction S of the magnetic wedge 36 in this embodiment is approximately 5,000 H / m, whereas the relative permeability of the stator 34 having the teeth 32 is set to a value of 5,000 H / m or less.

[0046] In this embodiment, an example has been given in which the magnetic wedge 36 is made of directional electromagnetic steel plate and the magnetic permeability of the magnetic wedge 36 in the circumferential direction S is equal to or less than the magnetic permeability of the teeth 32 of the stator 34, but the electric motor 10 is not limited to this configuration.

[0047] For example, in a configuration in which the magnetic wedges 36 are not made of grain-oriented electromagnetic steel plates, the magnetic permeability of the magnetic wedges 36 may be set to be equal to or lower than the magnetic permeability of the teeth 32 formed on the stator 34. In such an electric motor 10, it is also desirable that the magnetic permeability of the magnetic wedges 36 be lower than the magnetic permeability of the teeth 32.

[0048] Modifications of the above embodiment will be described below. In the modifications, parts that are the same as or equivalent to those in the above embodiment will be given the same reference numerals and will not be described again. Only parts that differ from the above embodiment will be described. All modifications fall within the scope of the present invention, just like the above embodiment.

[0049] (First Modification) Fig. 3 is an enlarged view showing a main part of the electric motor 10 according to the first modification. Fig. 4 is a perspective view showing the magnetic wedge 36 of the electric motor 10 according to the first modification. The electric motor 10 according to the first modification has a different configuration of the magnetic wedge 36 compared to the embodiment described above.

[0050] 3 and 4 , the magnetic wedge 36 of the electric motor 10 according to the first modification has a groove 100 extending along the rotation axis direction of the rotation shaft 60 of the electric motor 10 on an opposite surface 36D, which is the surface located opposite to the tip surface 36C, which is the surface facing the rotor 20. The opposite surface 36D is disposed in a slot 40 formed between the teeth 32.

[0051] The groove 100 formed on the opposite surface 36D is located at the center of the opposite surface 36D in the circumferential direction S. This increases the magnetic resistance of the magnetic wedge 36 at the center of the opposite surface 36D in the circumferential direction S where the groove 100 is formed.

[0052] In addition, this modified example will be described using an example in which the groove 100 is provided at the center of the circumferential direction S on the opposite surface 36D, but the groove 100 may also be provided at a position other than the center of the circumferential direction S on the opposite surface 36D.

[0053] The opposite surface 36D is divided into a one-side opposite surface 36D1 on one side in the circumferential direction S, and an other-side opposite surface 36D2 on the other side in the circumferential direction S, with the groove 100 as the boundary. The one-side opposite surface 36D1 faces the end face of the coil 30 provided on the tooth 32 on one side in the circumferential direction S of the slot 40 in which the opposite surface 36D is arranged. The other-side opposite surface 36D2 faces the end face of the coil 30 provided on the tooth 32 on the other side in the circumferential direction S of the slot 40 in which the opposite surface 36D is arranged.

[0054] The groove 100 formed on the opposite surface 36D has a cross section that is arc-shaped.

[0055] Although this modification will be described taking as an example a case where the cross-sectional shape of the groove 100 is arc-shaped, the cross-sectional shape of the groove 100 is not limited to this shape. The cross-sectional shape of the groove 100 may be, for example, triangular or rectangular.

[0056] Here, in the magnetic wedge 36 having the groove 100 with a rectangular cross section, the force generated according to the magnitude of the magnetic flux is likely to concentrate at the corners of the groove 100, and there is a risk of breakage during high torque driving. Therefore, in this modification, the cross section of the groove 100 is made arc-shaped, which suppresses local stress concentration and makes it possible to suppress unexpected breakage of the magnetic wedge 36.

[0057] 5 is an enlarged view showing a main portion of the electric motor 10 according to the second modification. The electric motor 10 according to the second modification is a so-called outer rotor type electric motor, and the positional relationship between the stator 34 and the rotor 20 is different from that of the above-described embodiment.

[0058] 5 , the electric motor 10 according to the second modification includes a stator 34 in which teeth 32 having coils 30 are arranged at intervals in the circumferential direction S, and a rotor 20 in which permanent magnets 22 are provided on opposing surfaces 20A that face the teeth 32. The electric motor 10 according to the second modification also includes magnetic wedges 36 provided between the tips of adjacent teeth 32. The rotor 20 is arranged outside the stator 34. The rotor 20 is rotatable relative to the stator 34.

[0059] The permanent magnets 22 on the opposing surface 20A of the rotor 20 form a Halbach array magnetic circuit having a plurality of main magnets 70 arranged in the circumferential direction S of the rotor 20 and auxiliary magnets 72 arranged between each main magnet 70 and magnetized in a direction different from the magnetization direction of the adjacent main magnets 70. The magnetization direction of the auxiliary magnets 72 is inclined at an angle of 90 degrees or less with respect to the magnetization direction of the main magnets 70.

[0060] (Operations and Effects) As described above, the electric motor 10 of this embodiment includes a stator 34 in which teeth 32 having coils 30 are arranged at intervals in the circumferential direction S, and a rotor 20 in which permanent magnets 22 are provided in portions corresponding to the teeth 32. The electric motor 10 includes magnetic wedges 36 provided between the tips of adjacent teeth 32. The saturation magnetic flux density of the magnetic wedges 36 is equal to or lower than the saturation magnetic flux density of the teeth 32.

[0061] In such an electric motor 10, the saturation magnetic flux density of the magnetic wedges 36 is equal to or lower than the saturation magnetic flux density of the teeth 32, and the magnetic wedges 36 provided between the teeth 32 are prone to magnetic saturation.

[0062] Therefore, when the magnetic wedges 36 become magnetically saturated during high-torque driving, the electric motor 10 can reduce the magnetic permeability of the magnetic wedges 36. This prevents an increase in leakage magnetic flux flowing between the teeth 32, thereby preventing a decrease in the flow of magnetic flux along the main magnetic path, which contributes to increased torque.

[0063] Therefore, even when the magnetic wedge 36 is used, the electric motor 10 can improve the torque characteristics during high torque driving.

[0064] Furthermore, when the motor 10 is driven at low torque in the high-speed region where the rotor 20 rotates at high speed, leakage magnetic flux can be formed between adjacent teeth 32, thereby improving rotational efficiency.

[0065] Therefore, the electric motor 10 can improve rotational efficiency in the high-speed range while suppressing deterioration of torque characteristics during high-torque driving.

[0066] In addition, in this embodiment, the magnetic wedge 36 has a higher magnetic permeability in the circumferential direction S, which is the direction in which magnetic flux flows between adjacent teeth 32, than in the radial direction R, which is the intersecting direction that intersects with the flow direction.

[0067] In this configuration, the magnetic wedges 36 can increase the leakage magnetic flux flowing between the teeth 32 to be greater than the magnetic flux flowing in the radial direction R, which is the intersecting direction. This allows the electric motor 10 to further improve the rotational efficiency at low torque in the high-speed range.

[0068] In this embodiment, the magnetic wedge 36 is made of a grain-oriented electromagnetic steel plate.

[0069] By using directional electromagnetic steel sheets, the electric motor 10 having such a configuration can form magnetic wedges 36 whose magnetic permeability in the circumferential direction S, which is the flow direction, is higher than that in the radial direction R, which is the intersecting direction.

[0070] In this embodiment, the magnetic permeability of the magnetic wedges 36 is equal to or less than the magnetic permeability of the teeth 32 .

[0071] In this configuration, the electric motor 10 can ensure leakage magnetic flux flowing between the teeth 32 by making the magnetic permeability of the magnetic wedge 36 equal to or lower than that of the teeth 32, thereby improving rotational efficiency at low torque in the high-speed range.

[0072] If the magnetic permeability of the magnetic wedge 36 is made smaller than that of the teeth 32, the concentration of magnetic flux flowing in the magnetic wedge 36 can be suppressed compared to when the magnetic permeability of the magnetic wedge 36 is greater than that of the teeth 32. This allows the electric motor 10 to ensure that the magnetic flux flows through the teeth 32, thereby enabling further improvement in torque characteristics.

[0073] In addition, in this embodiment, the permanent magnet 22 forms a Halbach array magnetic circuit having a plurality of main magnets 70 (70A, 70B) arranged in the circumferential direction S of the rotor 20, and auxiliary magnets 72 (72A, 72B, 72C, 72D) arranged between the main magnets 70 (70A, 70B) and magnetized in a direction different from the magnetization direction of the adjacent main magnets 70 (70A, 70B).

[0074] In this configuration, the permanent magnets 22 of the rotor 20 form a magnetic circuit in a Halbach array, making it possible to increase the strength of the magnetic field directed toward the teeth 32 .

[0075] In addition, in this embodiment, the magnetic wedge 36 has a groove 100 extending along the rotational axis direction in which the rotational shaft 60 of the electric motor 10 extends on the opposite surface 36D, which is the surface located opposite the surface facing the rotor 20 (see Figures 3 and 4).

[0076] In this configuration, the magnetic wedge 36 has a groove 100 formed on the opposite surface 36D located on the slot 40 side. This makes it possible to suppress the magnetic flux toward the slot 40 compared to when the opposite surface 36D located on the slot 40 side does not have the groove 100, thereby improving the effect of the magnetic wedge 36 described above.

[0077] In this embodiment, the groove 100 is located at the center in the circumferential direction S, and has an arc-shaped cross section (see FIG. 4).

[0078] In this configuration, the groove 100 formed on the opposite surface 36D of the magnetic wedge 36 has an arc-shaped cross section. Therefore, the electric motor 10 is able to disperse stress generated on the inner surface of the groove 100, compared to when the cross section of the groove 100 is formed, for example, in a rectangular shape, and it is possible to suppress unexpected damage to the magnetic wedge 36.

[0079] The above describes the embodiments and modifications of the present invention, but the above embodiments and modifications merely illustrate some of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments and modifications.

[0080] The electric motor 10 of the present embodiment and each of the modified examples described above has been described using the electric motor 10 used in an electric vehicle as an example, but the electric motor 10 is not limited to this and may be an electric motor 10 used in other devices.

[0081] The electric motor 10 of the present embodiment and each of the modified examples described above has been described using the example of the magnetic wedge 36 having a magnetic permeability lower than that of the teeth 32, but the magnetic permeability of the magnetic wedge 36 does not have to be lower than that of the teeth 32.

Claims

1. An electric motor comprising: a stator having teeth with coils arranged at intervals in the circumferential direction; a rotor having permanent magnets provided in portions corresponding to the teeth; and magnetic wedges provided between the tips of adjacent teeth, wherein the saturation magnetic flux density of the magnetic wedges is equal to or less than the saturation magnetic flux density of the teeth.

2. An electric motor according to claim 1, wherein the magnetic wedge has a higher magnetic permeability in the direction of magnetic flux flowing between adjacent teeth than in the direction intersecting the magnetic flux flowing in the direction of flux flow.

3. An electric motor according to claim 2, wherein the magnetic wedge is made of a grain-oriented electromagnetic steel plate.

4. An electric motor according to claim 1, wherein the magnetic wedge has a magnetic permeability equal to or less than that of the teeth.

5. An electric motor according to any one of claims 1 to 4, wherein the permanent magnets form a magnetic circuit in a Halbach array having a plurality of main magnets arranged in the circumferential direction of the rotor and auxiliary magnets arranged between the main magnets and magnetized in a direction different from the magnetization direction of the adjacent main magnets.

6. An electric motor according to any one of claims 1 to 4, wherein the magnetic wedge has a groove extending along the direction of the rotation axis of the electric motor on a surface located opposite to the surface facing the rotor.

7. An electric motor according to claim 6, wherein the groove is positioned at the circumferential center and has an arc-shaped cross section.

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