KR101405963B1 - motor - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a motor, and more particularly, to an outer type motor, which minimizes a leakage magnetic flux to improve efficiency and improve vibration and cogging torque performance.

In addition, the present invention relates to a motor that can be applied to a direct-coupled type washing machine and a washing machine using the same.

According to the present invention, there is provided a motor including a stator and a rotor rotatably provided outside the stator in the radial direction, the rotor comprising: a base; A side wall portion extending perpendicular to the base portion; The inner circumferential surface and the outer circumferential surface of the inner circumferential surface of the side wall portion are provided with a permanent magnet having the same radius of curvature so that the radial thickness decreases from the center to the circumferential direction. / RTI >

Motor, magnetic flux, permanent magnet

Description

Motor {motor}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a motor, and more particularly, to an outer type motor, which minimizes a leakage magnetic flux to improve efficiency and improve vibration and cogging torque performance.

In addition, the present invention relates to a motor that can be applied to a direct-coupled type washing machine and a washing machine using the same.

Generally, the motor transmits the rotating force of the rotor to the rotating shaft, and the rotating shaft drives the load. For example, the rotary shaft may be connected to the drum of the washing machine to drive the drum, and the fan may be connected to the fan of the refrigerator to supply cool air to the required space.

On the other hand, in such a motor, the rotor is rotated by electromagnetic interaction with the stator. To this end, a coil is wound on the stator, and as the current is applied to the coil, the rotor rotates with respect to the stator.

Hereinafter, a conventional motor will be described with reference to FIGS. 1 to 5. FIG. For convenience of explanation, the motor is applied to a drum washing machine as an example.

Referring to FIG. 1, the structure of a conventional direct-type drum washing machine will be briefly described.

1 is a longitudinal sectional view showing the structure of a conventional direct-coupled drum type washing machine. The drum type washing machine comprises a tub 2 provided inside the cabinet 1, a drum 3 provided inside the tub 2, And a bearing (not shown) provided on the outer circumferential surface of both ends of the washing shaft 4 to transmit the driving force of the motor 5 to the drum 3.

A door 21 is installed in front of the cabinet 1 and a gasket 22 is installed between the door 21 and the tub 2.

A hanging spring 23 for supporting the tub 2 is installed between the inside of the upper surface of the cabinet 1 and the upper side of the outer surface of the tub 2 and the inside of the lower surface of the cabinet 1, (2) A friction damper 24 is provided between the lower side of the outer circumferential surface to damp the vibration of the tub 2, which occurs upon dewatering.

The motor 5 includes a stator 7 fixed to the rear wall 200 of the tub and a rotor 6 enclosing the stator 7 so that the driving force of the rotor is directly transmitted to the washing shaft 4) to the drum (3). That is, the drum 3 is driven in a direct manner without the need for power transmission through a separate pulley or belt arrangement.

The construction of a conventional motor, particularly a direct-coupled motor, will be described with reference to FIGS. 2 to 3. FIG.

Figures 2 and 3 show the stator and rotor of the motor shown in Figure 1, respectively.

The motor 5 is composed of a stator 7 and a rotor 6 and generates a rotating magnetic field between the coil 8 wound around the stator and the permanent magnet 16 of the rotor when the power is applied, Is rotated.

The stator 7 includes a stator core 10 including a coil 8 and a plurality of teeth 9 on which the coils are wound and an insulator 12 for insulating the coils from the stator core . A bolt or the like is inserted into the coupling hole 11 on the rear wall side of the tub 2 to be fixedly coupled to the stator 7.

The stator core 10 may be formed by laminating a steel sheet, and may be formed by connecting spiral-shaped or circumferentially-divided sections.

 The rotor 6 is connected to the washing shaft 4 which surrounds the stator 7 and is connected to the drum 3 through the tub. That is, the rotation of the rotor rotates the drum through the washing shaft.

Generally, a bearing housing (not shown) is provided between the tub rear wall 200 and the stator to rotatably support the washing shaft.

The rotor 6 of the motor 5 will be described in more detail. The rotor includes the side wall portion 13 and the base portion 14. Here, the rotor frame 15 including the side wall portion 13 and the base portion 14 may be referred to as a rotor frame 15. A through hole is formed in a central portion of the base portion 14 so that the washing shaft can be connected to directly drive the drum. The rotor includes a permanent magnet (16) disposed in a circumferential direction inside the side wall portion (13). A plurality of the permanent magnets 16 may be provided along the circumferential direction.

The connector 30 connecting the washing shaft 4 and the rotor frame 15 is integrally formed with the rotor frame 15 or is coupled to the through hole through a separate coupling means.

A serration 31 is formed at the center of the connector to transmit the driving force of the rotor to the washing shaft by serration coupling with the washing shaft 4.

A plurality of cooling fins 16, a cooling hole 17 and a drain hole 18 are formed in the base portion 14 of the rotor frame in the circumferential direction to prevent the stator from overheating, .

In order to enhance the rigidity of the rotor frame 12, an embossed portion 19 is formed in the base portion 14 of the rotor frame. In order to support the permanent magnet, an inner circumferential direction of the side wall portion 13 The bending portion 20 is formed.

However, in the conventional rotor, as shown in FIG. 4, the magnetic flux from the permanent magnet 16 does not enter the stator 7 but flows into the nearby rotor 6 to generate a magnetic flux leaked. That is, there is a problem that the efficiency of the motor is lowered due to the leakage magnetic flux.

More specifically, the side wall 13 and the base 14 of the rotor 6 are formed of a magnetic material, and the magnetic flux generated by the permanent magnet may be applied to the side wall 13 or the base 14 There is a problem that leakage occurs. Particularly, when the bending portion 20 for fixing the permanent magnets is formed, the magnetic flux may leak directly through the bending portion.

In addition, as shown in FIG. 4, the problem of the leakage magnetic flux becomes more pronounced when the height of the permanent magnet is larger than the stack height of the stator facing the permanent magnet. That is, as shown in FIG. 4, when the lower end portion of the permanent magnet 16 is closer to the rotor 60 than the stator 7, particularly to the base 14 and the bending portion 20, there is a problem.

5 is a diagram showing a magnetization waveform of a conventional direct-coupled motor permanent magnet.

Conventionally, the permanent magnet is formed into a plurality of pieces and then joined to the inner surface of the inner wall of the rotor in the circumferential direction. The permanent magnet is magnetized such that N and S poles are alternately formed along the circumferential direction.

As shown in FIG. 4, the permanent magnet 16 has a curved inner surface, and the gap (gap) with the stator is constant along the circumferential direction. This type of permanent magnet is generally called a C type permanent magnet.

Therefore, as shown in FIG. 5, the magnetization waveform of the permanent magnet appears as a square wave shape in accordance with the angle, or in a trapezoidal shape, and a point where the magnetic pole changes abruptly along the circumferential direction of the rotor is formed. In other words, the magnetic pole suddenly changes at a point where the magnetic flux passes from a specific permanent magnet to a neighboring permanent magnet.

Generally, a conventional washing machine motor is a BLDC motor (Brushless DC motor), and the rotational speed of the rotor is controlled by an inverter. That is, after converting the AC voltage, which is a commercial power source, into a DC voltage, the AC voltage is converted into a three-phase (u, v, w) AC voltage through the inverter and then applied to the motor.

The voltage applied to the motor is a PWM (Pulse Width Modulation) waveform. The magnitude of the duty ratio is adjusted to adjust the magnitude and frequency of the voltage applied to the motor.

The driving method of the inverter can be divided into a square-wave driving and a sinusoidal driving, which are divided according to whether the inverter has a 180-degree conduction angle or a 120-degree conduction angle in six switching sequences of a three-phase inverter.

Here, when the conduction angle is 120 degrees, since the harmonic component is small, it is closer to the sinusoidal wave, and generally, sinusoidal wave driving is used in order to drive the motor of the washing machine.

However, in the motor having the sinusoidal wave drive and the motor having the spherical wave magnetization type, the cogging torque or the like is generated due to the spherical waveform of the motor, and due to such a problem, the torque A torque ripple is generated and the efficiency of the motor is lowered, and vibration and noise are increased.

That is, the cogging torque is generated due to the abrupt change of the interaction between the permanent magnet 16 and the slot between the teeth 9 wound around the coil 8 by the rotation of the motor, There is a problem that it becomes larger.

Therefore, in general direct-current type motors for washing machines that drive sinusoidal waves, measures are taken to solve the problems of cogging torque which is increased due to the spherical-wave-like shape of the permanent magnet, and vibration and noise caused thereby.

An object of the present invention is to solve the problem of the conventional motor described above.

More specifically, it is an object of the present invention to provide a motor having an increased efficiency by minimizing a magnetic flux leaked to a rotor.

Another object of the present invention is to provide a motor that reduces vibration and noise by reducing the influence of cogging torque and harmonics, thereby improving efficiency.

Another object of the present invention is to provide a motor capable of simplifying the manufacturing process of the motor and facilitating mass production.

According to an aspect of the present invention, there is provided a motor including a stator and a rotor rotatably disposed outside the stator in a radial direction, the rotor including: a base portion formed of a magnetic material; A side wall portion formed integrally with the base portion and extending perpendicularly to the base portion; And a permanent magnet provided on the inner side of the side wall part, wherein a distance from an inner surface of the base part to a lower end of the permanent magnet is larger than a radial thickness of the permanent magnet.

Here, the base portion and the side wall portion of the rotor may be integrally formed by pressing an iron plate. That is, the rotor frame constituting the rotor can be integrally formed.

The height of the permanent magnets may be greater than the stack height of the stator facing the permanent magnets.

The side wall portion may include a bending portion for reinforcing the strength. At this time, the bending portion may be formed such that the inner diameter of the side wall portion (second side wall portion) spaced from the base portion is larger than the inner diameter of the side wall portion (first sidewall portion) close to the base portion.

In addition, the distance between the inner surface of the bending portion and the lower end of the permanent magnet may be greater than the radial thickness of the permanent magnet. In this case, the radial thickness of the permanent magnet may be greater than the distance between the first sidewall portion and the inner surface of the second sidewall portion due to the bending portion. That is, the radial thickness of the permanent magnet may be greater than the radial width of the bending portion. The distance between the inner surface of the bending portion and the lower end of the permanent magnet may be smaller than the radial thickness of the permanent magnet.

The plurality of permanent magnets may be provided along the circumferential direction of the rotor, and may be fixed to the side wall portion through an adhesive interposed between the permanent magnets and the side wall portion.

Preferably, the radius of curvature of the circumferential surface of the permanent magnets is smaller than the radius of curvature of the inner surface of the side wall portion.

Preferably, the inner circumferential surface of the permanent magnet is formed as a curved surface having a predetermined radius of curvature so that the thickness in the radial direction decreases from the center to the circumferential direction. In addition, the outer circumferential surface of the permanent magnet may be formed as a curved surface having a predetermined curvature radius so that the radial thickness decreases from the center to the circumferential direction.

Of course, the radius of curvature of the inner circumferential surface and the inner circumferential surface of the permanent magnet may be the same. In this case, since the inside and the outside of the permanent magnet are not distinguished from each other, the permanent magnet can be easily fixed to the rotor.

According to another aspect of the present invention, there is provided a motor including a stator and a rotor rotatably disposed outside the stator in a radial direction, the rotor including: a base; A side wall portion extending perpendicular to the base portion; The inner circumferential surface and the outer circumferential surface of the inner circumferential surface of the side wall portion are provided with permanent magnets having the same radius of curvature so that the radial thickness decreases from the center to the circumferential direction. to provide.

At this time, other features of the motor may include all of the features described in the above embodiments.

According to the present invention, the problem of the conventional motor can be solved.

More specifically, it is possible to provide a motor in which the magnetic flux leaked to the rotor is minimized to increase the efficiency.

In addition, it is possible to reduce the influence of cogging torque and harmonics to reduce vibration and noise, thereby providing a motor with improved efficiency.

In addition, it is possible to provide a motor capable of simplifying the manufacturing process of the motor and facilitating mass production.

According to the present invention, it is possible to provide a drum washing machine with improved efficiency and reduced vibration and noise as a whole by improving the performance of the motor, thereby maximizing the satisfaction of the user.

The motor according to the present invention may be a motor applied to a relatively bulky household appliance, such as a washing machine, a dryer, or a dishwasher, but the application is not necessarily limited thereto.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First, an embodiment of the present invention will be described with reference to FIG.

Fig. 6 is a cross-sectional view schematically showing a part of the motor, particularly, a cross-sectional view showing the side wall portion of the rotor in detail.

As shown in FIG. 6, the rotor 60 of the motor according to the present embodiment includes a base portion 140, and a side wall portion 130. The base portion 130 and the side wall portion 140 are formed of a magnetic material.

In addition, the motor includes a permanent magnet 160 disposed inside the side wall portion. The magnetic flux generated from the permanent magnet flows to the stator core through the stator 70, particularly the tooth 71, which is also formed of a magnetic material.

On the other hand, the magnetic flux flows through magnetic material. As described above, the side wall portion 130 and the base portion 140 of the rotor are also formed of a magnetic material. Therefore, in order to maximize the efficiency of the motor, the magnetic flux generated from the permanent magnet 160 must flow to the stator 70, and the magnetic flux leaked to the rotor 160 needs to be minimized.

For this purpose, in this embodiment, the distance L1 from the inner surface of the base 140 to the lower end of the permanent magnet 140 is preferably larger than the radial thickness T of the permanent magnet. That is, it is desirable to maximize the distance between the inner surface of the permanent magnet and the base 140. This is because there is a certain limit in reducing the gap g which is the gap between the inner surface of the permanent magnet and the stator. Therefore, by further increasing the separation distance L1 instead of reducing the gap, the magnetic flux generated in the permanent magnet can be prevented from leaking to the rotor as much as possible.

On the other hand, the present embodiment does not form the bending portion 20 (see Fig. 4) described in the conventional motor. That is, the side wall part 130 is vertically extended without a step from the base part 140 to the part where the permanent magnet 160 is fixed. That is, the bending portion 20 is not formed in the portion "A ".

The bending portion particularly reinforces the rigidity of the side wall portion 130 of the rotor. When the thickness of the side wall portion is equal to or greater than a predetermined value, satisfactory rigidity can be obtained without forming a bending portion.

Further, the bending portion 20 is formed by press working, which is not machined, and therefore the dimensions thereof are not very precise. Therefore, as shown in FIG. 4, when the permanent magnet is positioned through the bending portion 20, a deviation in the fixed height of the permanent magnet may occur. Since the motor is preferably structurally and dynamically symmetrical in the radial direction due to its own characteristics, there is a fear that vibration and noise are increased when the rotor rotates when there is such a symmetry deviation. Therefore, the bending portion for fixing the position of the permanent magnet can be omitted for fixing the permanent magnet more precisely. In addition, it can be fixed to the side wall part 130 with high accuracy through a method described later.

Meanwhile, it is preferable that the above-described permanent magnet 160 is formed to have a height h of the stator, that is, a height higher than the stack height. This is because the height of the permanent magnet can be further increased to further increase the magnetic torque due to the permanent magnet. Of course, as the height of the permanent magnet is increased, a leakage magnetic flux may be generated in the lower portion of the permanent magnet as shown in FIG. 6, but this can be minimized by making L1 larger than T as described above.

Hereinafter, another embodiment of the present invention will be described with reference to FIG.

7, the distance L1 between the inner surface of the base portion and the lower end of the permanent magnet is greater than the radial thickness T of the permanent magnet 160, as in the above- . Therefore, leakage of the magnetic flux from the lower end of the permanent magnet to the base portion 140 can be minimized.

The present embodiment may include the bending unit 200, unlike the above-described embodiment. That is, the side wall part 130 may further include a bending part 200 for reinforcing the strength of the side wall part. In other words, the bending portion 200 is formed at the portion "B ".

Here, the sidewall 130 is divided into a lower first sidewall 131 and an upper second sidewall 132 about the bending portion 200. In addition, the inner diameter of the second sidewall portion, that is, the sidewall portion spaced apart from the base portion is larger than the inner diameter of the sidewall portion adjacent to the first sidewall portion, that is, the base portion 140, through the bending portion 200 .

Here, unlike the conventional bending portion, the bending portion 200 is not a structure for fixing or positioning the permanent magnet 160, but merely a structure for reinforcing the strength of the side wall portion 130. Therefore, it is preferable that the permanent magnet 160 is fixed with a distance L2 from the bending part 200. [

As shown in FIG. 7, in this embodiment, the distance L2 from the inner surface of the bending portion to the lower end of the permanent magnet is formed to be smaller than the radial thickness T of the permanent magnet.

First, the lower end of the permanent magnet is separated from the bending portion to prevent the magnetic flux from leaking through the bending portion 200. Therefore, the permanent magnet is not fixed through the bending part 200 as described above. However, the separation distance L2 must be sufficiently secured. That is, in relation to the thickness T of the permanent magnet. This is because as the thickness of the permanent magnet increases, the amount of leakage of the magnetic flux from the lower end of the permanent magnet to the bending portion becomes larger.
On the other hand, when the spacing distance L2 is formed to be infinitely large, the height of the side wall part 130 becomes long, and therefore the thickness of the entire motor must be considerably increased. Therefore, it is preferable that the inner surface of the bending portion 200 and the lower end of the permanent magnet are spaced apart from each other, and the distance L2 is smaller than the radial thickness T of the permanent magnet.

In addition, it is not preferable that the side wall part 130 due to the bending part 200 is close to the base part 140 in terms of the strength reinforcement. That is, when L2 is excessively increased, the bending portion is excessively close to the base 140. The L2 is formed at a predetermined distance and is smaller than the radial width L3 of the bending part 200, that is, the distance between the inner surface of the first sidewall part 131 and the inner surface of the second sidewall part 132 The radial thickness T of the permanent magnet can be made larger. This type is shown in Fig.

That is, by making the width L3 of the bending portion smaller than the thickness T of the permanent magnet, it is possible to minimize the leakage of the magnetic flux in the lower end of the permanent magnet through the bending portion 200 and the first side wall portion 131 have.

The thickness of the permanent magnet may be enlarged by a distance between the inner surface of the sidewall 130 where the permanent magnet is located and the inner surface of the permanent magnet. This is because the permanent magnet may not completely come into close contact with the inner surface of the side wall portion. That is, an adhesive for fixing the permanent magnets can be interposed between the outer surface of the permanent magnet and the inner surface of the side wall portion. Here, if a sufficient amount of adhesive is used, a gap due to such an adhesive can be formed. Therefore, in any case, it is preferable that the inner diameter formed by the permanent magnet, that is, the inner diameter formed by the inner surface of the permanent magnet is smaller than the inner diameter formed by the side wall portion 130. Here, the inner diameter formed by the side wall portion means the inner diameter of the first side wall portion formed by the first side wall portion and the second side wall portion 132, as described above. That is, the smallest inside diameter of the side wall part 130. [ Therefore, the magnetic flux can be prevented from leaking through the side wall portion 130 of the rotor 60 by separating the inner side surface and the side wall portion of the permanent magnet 160.

Hereinafter, an embodiment of the present invention for reducing the cogging torque will be described with reference to Figs. 8 to 10. Fig. Of course, the present embodiment can be independent of the above-described embodiments and can be applied together.

According to the present embodiment, it is preferable that the permanent magnet 160 fixed to the rotor 60 is formed so that a portion facing the stator 70 has a convex surface as shown in Fig. That is, the permanent magnet shown in FIG. 4 may have a concave surface, but may have a thickness smaller than the center thickness t1 of the permanent magnet. In other words, the thickness t1 at the stimulation center can be made larger than the thickness t2 at the outermost stimulation pole. Here, t1 and t2 may be a radial thickness, particularly at the center line of the permanent magnet. This means that the gap g at the center of the permanent magnet becomes minimum and the gap g at the outermost pole of the stimulus becomes maximum.

Therefore, the density of the magnetic flux flowing to the stator decreases from the magnetic pole center to the outermost magnetic pole, and the magnetic flux density is similar to the sine wave magnetic flux density as shown in FIG. 9 due to the density variation. In other words, the point where the magnetic flux density changes abruptly is not generated, so that cogging torque and vibration can be minimized. This can be more clearly shown in FIG.

As shown in Fig. 10, when the conventional C-type permanent magnet is used, the cogging torque in the case of using the permanent magnet according to the present embodiment (R-type permanent magnet for convenience of explanation) (B) .

On the other hand, when the R type permanent magnet is used, the material ratio of the permanent magnets can be reduced as the gap g is partially enlarged. In addition, if the material ratio is the same as that of the conventional C-type permanent magnet, the height of the permanent magnet can be increased correspondingly, so that a stronger magnetic flux is generated and the magnetic torque performance can be further improved.

Here, the inner curved surface of the R permanent magnet has a predetermined radius of curvature R. Of course, the center of the R will be radially outward of the permanent magnet. The outer curved surface of the permanent magnet may be formed in close contact with the side wall 130 of the rotor. In this case, the permanent magnet can be more stably fixed to the side wall part 130.

However, the plurality of permanent magnets may be provided along the circumferential direction of the rotor. For example, when the permanent magnet has 48 poles, 48 permanent magnet pieces may be provided in the rotor. That is, one permanent magnet may be formed to have one magnetic pole. This is because, as described above, it is easy to form such that the thickness in the radial direction from the center of one magnetic pole is reduced to both sides. That is, when the permanent magnet shown in FIG. 8 is formed to have two magnetic poles, the magnetic flux intensity at the magnetic pole center will be different due to the difference in thickness. Therefore, it is preferable to have one magnetic pole per one permanent magnet, particularly one R-type permanent magnet.

In this case, when the inner shape and the outer shape of the permanent magnet are distinguished from each other, there is a problem that the inner side and the outer side of the permanent magnet must be distinguished each time the permanent magnet is fixed. If a large number of motors are produced, the rise in manufacturing costs due to such a large number of motors can not be ignored.

Therefore, it is preferable that the curvature radii of the inner circumferential surface and the outer circumferential surface of the permanent magnet are the same. That is, both the inner side and the outer side can be formed to have the same radius of curvature R. In this case, since it is not necessary to distinguish between the inner side and the outer side when fixing the permanent magnet, the manufacturing cost can be reduced. However, in this case, since the outer surface of the permanent magnet is not in close contact with the side wall portion 130 of the rotor, it may be difficult to fix the permanent magnet.

Hereinafter, a method for easily fixing the permanent magnet to the rotor will be described with reference to FIG. The present method can be applied to all the embodiments described above.

First, the permanent magnet is seated on the jig 400 on which the permanent magnet 160 can be seated. The permanent magnets are seated so that the side fixed to the side wall of the rotor faces outward. Here, the jig may be formed to match the shape of the permanent magnet so that the permanent magnet can be stably mounted on the jig, and may be manufactured as an electromagnet as required. Therefore, even when the inner and outer sides of the permanent magnet are formed to have the radius of curvature R, the permanent magnet can be seated on the jig without deviating from the center.

On the other hand, an adhesive 420 is applied to the inside of the side wall of the rotor. That is, the adhesive is applied in advance to the portion to which the permanent magnet is attached. In some cases, the adhesive may be applied to the permanent magnet placed on the jig.

Thereafter, the jig is inserted into the rotor. Here, the fixed position of the permanent magnet can be adjusted very precisely without any deviation by adjusting the length of insertion of the jig. Therefore, this method can fix the permanent magnet to the rotor very precisely, not only when the radius of curvature of the inner side surface and the outer side surface of the permanent magnet is the same, but also when there is no bending portion.

1 is a longitudinal sectional view showing the structure of a conventional direct-coupled drum washing machine;

FIG. 2 is a perspective view of the stator shown in FIG. 1; FIG.

FIG. 3 is a partially cutaway perspective view of the rotor shown in FIG. 1; FIG.

Figure 4 is a partial cross-sectional view of the rotor shown in Figure 3;

FIG. 5 is a graph schematically showing the magnetic flux density of the rotor shown in FIG. 3;

6 is a partial cross-sectional view of a rotor according to an embodiment of the present invention;

7 is a partial cross-sectional view of a rotor according to another embodiment of the present invention;

8 is a perspective view of a permanent magnet applied to a rotor according to an embodiment of the present invention;

FIG. 9 is a graph schematically showing the magnetic flux density of a rotor according to an embodiment of the present invention; FIG.

10 is a graph comparing cogging torque of an embodiment of the present invention with a conventional motor; And

11 is a schematic view showing a method of fixing a permanent magnet according to an embodiment of the present invention.

Claims (11)

1. A motor comprising a stator and a rotor rotatably disposed radially outward of the stator, The rotor may include: A base portion; A side wall portion extending perpendicular to the base portion; And Wherein the inner circumferential surface and the outer circumferential surface are convexly formed with the same radius of curvature so that the thickness of the permanent magnet formed in the circumferential direction becomes smaller toward the both sides in the circumferential direction from the center, , ≪ / RTI > The side wall portion includes a bending portion formed to have a larger inner diameter of a side wall portion (second side wall portion) spaced from the base portion than an inner diameter of a side wall portion (first side wall portion) Wherein the radial thickness at the center of the permanent magnet is greater than the distance between the first sidewall portion and the inner surface of the second sidewall portion due to the bending portion. The method according to claim 1, Wherein the base portion and the side wall portion are integrally formed of a magnetic material. 3. The method of claim 2, Wherein the base portion and the side wall portion are formed by pressing an iron plate. The method according to claim 1, Wherein a distance from an inner surface of the base portion to a lower end of the permanent magnet is formed to be greater than a radial thickness at a center of the permanent magnet. The method according to claim 1, Wherein a radius of curvature of an outer circumferential surface of the permanent magnet is smaller than a radius of curvature of an inner surface of the side wall portion. delete The method according to claim 1, Wherein a distance from an inner surface of the bending portion to a lower end of the permanent magnet is smaller than a radial thickness at a center of the permanent magnet. delete delete 8. The method according to any one of claims 1 to 5 and 7, Wherein the plurality of permanent magnets are fixedly attached to the side wall portion through an adhesive interposed between the permanent magnets and the side wall portion in a state of being mounted on the jig. delete

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