JP2010246197A - Magnetic pole core and dc motor using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce torque ripple including cogging torque and current torque ripples, by superimposing not only the third-order or fifth-order harmonic components as well as required optional higher harmonic components more simply by devising the shape of a magnetic pole core. <P>SOLUTION: An electrical angle A between two magnetic poles equivalent to an electrical angle of 360° is θ, a mechanical angle B, which corresponds to the third-order harmonics to be superimposed to obtain a magnetic field intensity distribution superimposed on fundamental wave components, is set to θ/3, and the mechanical angle B is made equal to the open angle of a core being an angle made by both outer ends of a vane part. The order of harmonics to be superimposed is an odd number N of five or more, a mechanical angle C corresponding to the Nth-order harmonics is set to θ/N, and a radial distance from the axis of the core is shortened as it goes outward in its circumferential direction from the position of the mechanical angle C on the peripheral plane of the arm part. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention uses a magnetic pole core and a magnetic pole core that enable torque ripple to be reduced by superimposing not only the third harmonic or fifth harmonic component but also any desired higher harmonic component. The present invention relates to a direct current motor.

  Conventionally, as a motor used for a disk rotation drive device such as an optical disk or a magneto-optical disk, a brushless motor that can be stably operated for a long time without generating noise has been used. However, a brushless motor is generated. Torque ripples that vary periodically depending on the rotation angle occur in the torque.

  The change in output torque with respect to the rotation angle at the motor output shaft is referred to as torque ripple, and it is desirable that it be small because it causes rotation unevenness, vibration, and noise. In this torque ripple, both the cogging torque and the current torque ripple that appears when the armature current flows are combined and appear as torque ripple.

  When a three-phase 120-degree energization that is a typical energization method of a brushless motor is performed, a torque ripple of 13.4% p-p occurs. In order to reduce this torque ripple, it is known that when the armature flux linkage is distorted by superimposing a fifth harmonic on the sine wave of the fundamental wave, the resultant torque ripple is reduced. (See Non-Patent Document 1, page 93). FIG. 7 is a diagram for explaining the torque ripple correction method disclosed in Non-Patent Document 1. As shown in the figure, when the amplitude of the fundamental wave is set to 100%, the torque ripple is reduced from the theoretical value of 13.4% to 2.5% when the fifth harmonic having the amplitude of 7.2% is superimposed. The superposition of the fifth harmonic is performed by superimposing the fifth harmonic component on a magnet having a magnetic field intensity distribution in which the fundamental wave component is magnetized.

  However, if the magnet material is, for example, ferrite, a magnetizing method in which harmonics are superimposed as described above is possible, but for example, it is known as a high-performance (maximum energy product etc.) rare earth magnet. When an Nd—Fe—B based magnet material (neodymium Nd magnet) is used, such a heavy magnetization method cannot be used.

  Patent Document 1 discloses a multi-layer magnetization method similar to Non-Patent Document 1, but Patent Document 1 discloses not only the fifth harmonic component but also the third harmonic in the sinusoidal magnetic field strength distribution of the magnet. The wave component is superimposed and magnetized. Thereby, rotation unevenness can be reduced.

  Furthermore, Patent Document 2 discloses a technique for forming sinusoidal irregularities for 1.5 periods per pole in order to obtain a third harmonic component. Patent Document 2 discloses a superposition magnetization method similar to that described above, but also obtains an induced voltage having a fundamental wave component and a third harmonic component by making the inner peripheral surface of the annular magnet uneven. Disclose technology. However, the machining of the shape of the inner peripheral surface of such a magnet makes it practically impossible to cope with higher harmonic components of the fifth or higher order because the shape becomes too complicated.

  As described above, all of the conventional techniques described with reference to Non-Patent Document 1, Patent Document 1, and Patent Document 2 are not only fundamental wave components but also third harmonics by devising magnet magnetization or magnet shape. A magnetic field intensity distribution having a wave and a fifth harmonic component is to be obtained. As a result, torque ripple can be reduced. However, as described above, the magnet shape becomes complicated in order to cope with neodymium Nd magnets or to cope with higher harmonic components. There is a problem of passing. Further, the effect of integrating the magnetic flux of the core with respect to the magnetic flux change is small, and the radial gap type brushless motor has a drawback that the change of cogging becomes large.

  In contrast, Patent Document 3 discloses a stator magnetic pole core shape for reducing cogging torque. The brushless motor, whether an inner rotor type or an outer rotor type, is usually provided on the rotor side where the magnet rotates, while the magnetic pole core and the winding wound on it are usually provided on the stator side. However, Patent Document 3 devises the shape of the stator magnetic pole core.

  FIG. 8A is a radial sectional view of the brushless motor disclosed in Patent Document 3, and FIG. 8B is an enlarged view of one of the blade portions shown in FIG. In the figure, the stator magnetic pole core is composed of an annular yoke, twelve arm portions arranged in the radial direction on the yoke, and a blade portion projecting left and right from the tip of the arm portion. A slot is formed between adjacent arm portions, and a stator winding (not shown) is wound around the slot.

  The blade portion shown in detail in FIG. 8B is composed of a central arc portion that is formed in an arc shape centered on the shaft center of the shaft, and flat slope portions on both sides thereof. As a result, the magnetic flux density of the central arc portion is substantially flat, and the gap between the rotor magnet and the rotor magnet gradually increases toward the end on the flat slope portion, so that the magnetic flux density also changes gently. For this reason, the change in magnetic field energy is small, and the cogging torque can be reduced.

  However, simply changing the gap gradually is not sufficient to reduce cogging torque and torque ripple. For this reason, in Patent Document 3, the number of magnet poles P and the number of stator magnetic poles M have a relationship of P: M = 6n-2: 6n (where n is an integer of 2 or more). For this reason, it cannot be applied to a basic brushless motor having a relationship of P: M = 8: 6, 12: 9, 16:12, or the like.

Japanese Patent No. 3738984 Japanese Patent No. 3372747 JP 2000-209829 A

Koji Kanno, “How to use a brushless DC motor” Ohmsha, July 10, 2003, pp92-94

  As described above, by devising magnet magnetization or magnet shape on the rotor side, torque ripple can be obtained by obtaining magnetic field intensity distribution having not only fundamental wave components but also third harmonic components and fifth harmonic components. A technique for reducing the above is well known. Further, a technique for reducing cogging torque and torque ripple by devising the shape of the stator magnetic pole core so as to gradually change the air gap on the stator side is known. Conventionally, the third harmonic component and / or the fifth harmonic component (or both) are used as the harmonic component superimposed on the fundamental wave component, but the higher harmonics are not used. This is due to limitations based on the superimposing means. If a simpler superimposing means is found, the torque ripple can be increased by superimposing not only the third harmonic component and the fifth harmonic component, but also the seventh harmonic component or even higher harmonic components. Can be reduced.

  By devising the magnetic pole core shape, the present invention more easily simplifies the cogging torque by superimposing not only the 3rd harmonic or 5th harmonic component, but also any desired higher harmonic component. The object is to reduce torque ripple including both current torque ripple. As a result, the torque ripple can be reduced while using, for example, a neodymium Nd magnet known as a high-performance magnet as the rotor magnet.

  A magnetic pole core according to the present invention is a DC motor including an armature that generates a predetermined magnetic field and a magnet that is disposed to face the armature via a predetermined gap, and configures the armature and radiates about the core axis. It consists of a plurality of extending arm portions and a blade portion formed at the tip of the arm portion and facing the magnet. The vane portion is formed by a first portion in which the gap is substantially constant along the circumferential direction and a pair of second portions that are located on both sides of the first portion and that widen as the gap moves away from the first portion in the circumferential direction. Is done. C represents the angle formed between the core axis and both ends in the circumferential direction on the end surface facing the gap of the first portion, and C represents the angle formed between the core axis and both ends in the circumferential direction on the end surface opposed to the gap of the second portion. Let B be a mechanical angle corresponding to an electrical angle of 360 °, θ, N an odd number of 5 or more, and B = θ / 3 and C = θ / N.

  In addition, the magnetic pole core of the DC motor of the present invention includes an arm portion that extends radially from the central yoke portion and blade portions that extend from the tip of the arm portion to both sides in the circumferential direction. On the outer peripheral surface of the blade part facing the magnet through the air gap, a pair of bending points forming a predetermined angle at a symmetrical position with respect to the center of the blade part is set, and the central part outer peripheral surface located between the pair of bending points is A circular arc shape centered on the shaft axis, with a gap between the magnets at a constant interval, and a shape that is bent in a taper shape so that it is away from the magnet on the outer side in the circumferential direction from each of the bending point pairs To. The mechanical angle B corresponding to the third harmonic to be multiplied to obtain the magnetic field intensity distribution superimposed on the fundamental wave component is defined as θ / 3, where θ is the mechanical angle A between the two magnetic poles corresponding to an electrical angle of 360 °. The mechanical angle B is set to coincide with the core opening angle, which is an angle formed by a line connecting the shaft axis and both outer ends of the blade portion. In order to obtain the magnetic field strength distribution superimposed on the fundamental wave component, the order of the higher harmonics to be superimposed is an odd number N of 5 or more, and the mechanical angle C corresponding to the Nth harmonic is set to θ / N. The mechanical angle C is made to coincide with a predetermined angle of the bending point pair.

  Multiple pairs of bending points are set according to the number of harmonics to be superimposed, and the outer peripheral surface of the central part between the pair of bending points corresponding to the highest order harmonic is formed into an arc shape, and the harmonic order As the height decreases, the bending point pair is bent at a larger angle on the outer side in the circumferential direction and at a larger angle so as to be further away from the magnet.

  According to the present invention, by devising the shape of the magnetic pole core, the motor can be more easily superimposed by superimposing not only the third harmonic component or the fifth harmonic component but also any desired higher harmonic component. The torque ripple including both the cogging torque and the current torque ripple can be reduced regardless of the drive, configuration, number of poles, and the like. In the present invention, high-order harmonics overlap in the gap of the torque ripple, and the torque ripple fills each gap, and the torque ripple generated by the combined torque is further reduced. By superposing a plurality of higher-order harmonics, it becomes closer to a DC change (direct current change).

It is sectional drawing which shows the 1st example of the brushless motor incorporating the stator magnetic pole core which actualized this invention. (A) is a figure which shows the arrangement | positioning relationship of the stator magnetic pole core illustrated in FIG. 1, and a rotor magnet, (B) is a figure which expands and shows the X section in (A), (C) It is a figure which takes out and shows the blade | wing part in order to demonstrate a bending angle. FIG. 3 is a diagram showing another example different from FIG. 2, (A) is a diagram showing the positional relationship between the stator magnetic pole core and the rotor magnet illustrated in FIG. 1, and (B) is an X in (A). It is a figure which expands and shows a part. It is a wave form diagram which shows the measurement result of a cogging ripple. It is a wave form diagram which shows the measurement result of a harmonic torque ripple. It is a graph which shows the magnetic flux waveform after the 13th harmonic addition by a core shape. It is a figure explaining the torque ripple correction method disclosed in Non-Patent Document 1. (A) is radial direction sectional drawing of the brushless motor disclosed by patent document 3, (B) is a figure which expands and shows one of the blade | wing parts shown to (A).

  Hereinafter, the present invention will be described based on examples. FIG. 1 is a cross-sectional view showing a first example of a brushless motor incorporating a stator magnetic pole core embodying the present invention. The illustrated brushless motor is illustrated as an outer rotor type brushless motor having three-phase 16 poles (the number of rotor magnet poles) and 12 stator magnetic poles (slots). The basic configuration is applicable to the phase, and the basic configuration of the brushless motor is that the number of magnet poles P and the number of stator poles M are P: M = 8: 6, 12: 9, 16:12. It is applicable to all brushless motors including Furthermore, the present invention can be applied not only to brush motors but also to all DC motors including brushed DC motors. Brushed DC motors are also required to reduce cogging torque and torque ripple.

  In FIG. 1, on the fixed side of the brushless motor, a bearing holder that is attached to a metal mounting plate and has a bearing therein, and a stator magnetic core that is wound around the outer periphery of the bearing holder (stacked) Core) is arranged. In the illustrated example, the stator magnetic pole core around which the winding is wound is fixed to the outer periphery of the bearing holder fixed to the mounting plate. The bearing holder is provided with a hollow cylindrical oil-impregnated bearing, and at the center bottom thereof is a thrust receiver that supports the tip of the shaft inserted into the hollow oil-impregnated bearing, and a groove formed on the shaft tip side. Contains a fitted retaining washer.

  The rotation side is composed of a rotor case fixed to the shaft at the center and a rotor magnet attached to the rotor case. The rotor magnet has a cylindrical shape so as to face the stator magnetic pole core from the outside in the radial direction through a gap. The illustrated brushless motor includes an electronic rectifier circuit. The electronic rectifier circuit detects the rotational angle position of the rotor using a Hall element, and controls the current flowing through each of the plurality of windings based on the detection signal. Furthermore, the illustrated motor shows an example applied to a disk rotation drive device such as a CD changer. An optical disk or the like is placed on the upper end of the rotor case, and is further fixed by a clamper using a magnet from above. (Not shown). An attraction magnet is provided to attract the rotor case to the mounting plate side. When the magnetic pole core, which is a feature of the present invention, is applied to a brushed DC motor, the magnetic pole core is different from the brushless motor illustrated in FIG. 1 in that the magnetic core is located on the rotating side. However, any DC motor has an armature that generates a predetermined magnetic field (which generates a magnetic field for interacting with a field magnet to obtain torque (rotational force)) and this armature through a predetermined gap. Oppositely disposed (field) magnets are provided.

  2A is a diagram showing the arrangement relationship between the stator magnetic pole core and the rotor magnet illustrated in FIG. 1, and FIG. 2B is an enlarged view of an X portion in FIG. ) Is a view showing the blade portion taken out for explaining the bending angle. The stator magnetic pole core includes an annular central yoke portion, an arm portion extending radially from the central yoke portion, and a blade portion extending in the circumferential direction from the tip of the arm portion. Between the arm portions, as many slots as the number of arms (the number of magnetic poles) are formed, and windings are formed here. The rotor magnet has a cylindrical shape having a predetermined radial thickness, and the outer peripheral side and the inner peripheral side thereof are on concentric circles centering on the shaft axis. The outer peripheral surface of the blade portion is formed so as to open a predetermined interval (gap) from the inner peripheral surface of the magnet. As shown in FIG. 2B, which is an enlarged view of the portion X, the shape of each blade portion is symmetric with respect to a center line extending in the radial direction from the shaft axis. The shaft axis coincides with the core axis. A pair of seventh and fifth bending points, which are paired in pairs, are located symmetrically with respect to the center line on the end face of the blade facing the gap. The outer peripheral surface of the arm part located between two 7th-order bending point pairs is on an arc centered on the shaft axis, and this is indicated as a central arc part in the figure. In this arc part, the space | gap between rotor magnets is a fixed space | interval. In each of the seventh bending point pairs, the outer peripheral surface of the blade portion is formed into a shape that is bent in a taper shape (angle α1) so as to be away from the rotor magnet, in other words, the blade portion is thinned. As a result, when the seventh bending point is passed outward in the circumferential direction, the distance (distance) between the rotor magnet and the rotor magnet increases linearly. That is, the radial distance from the core axis is shortened. And it reaches the fifth bending point on both sides in the circumferential direction. At the fifth-order bending point, the blade portion outer peripheral surface is bent into a taper shape at an angle α2 larger than the angle α1. As a result, when the fifth bending point is passed to the outer side in the circumferential direction, the distance (distance) between the rotor magnet and the rotor magnet increases linearly to the outer end of the blade portion at a larger rate, and conversely, the core axis The radial distance between is reduced.

  Since the illustrated stator magnetic pole is exemplified as 12 poles, as shown in the figure, when the mechanical angle A between two magnetic poles corresponding to an electrical angle of 360 ° is θ, the mechanical angle is θ = 60 °. This mechanical angle A of 60 ° corresponds to the fundamental wave component. The mechanical angle corresponding to the third harmonic is 60 ° (θ) / 3 = 20 ° (B), and this angle B is determined between the shaft axis and the blade part (at the blade end face facing the air gap). It is made to correspond to the core opening angle which is an angle formed by a line connecting the outer ends. The mechanical angle corresponding to the fifth harmonic is 60 ° (θ) / 5 = 12 ° (C), and the mechanical angle corresponding to the seventh harmonic is 60 ° (θ) /7=8.57° (D). These correspond to the fifth and seventh bending points, respectively. As a result, the third harmonic, the fifth harmonic, and the seventh harmonic are formed into a continuous core shape, thereby forming a core on which harmonics are superimposed. Furthermore, when superimposing higher-order, for example, 13th-order harmonics, set the 13th-order bending point at a smaller mechanical angle of 60 ° / 13 = 4.6 ° corresponding to the 13th-order harmonics. The taper is bent away from the magnet from that position.

  Considering that the magnetic flux from the arm part branches in both directions of the blade part at the tip of the arm part, it is appropriate that the width W of the blade part is W = 1/2 * E as the arm part width E. It is. Furthermore, the tip side thickness W1 (FIG. 2C) of the blade portion is about half the thickness W of the central portion, which is the limit of magnetic saturation. From the angle conversion of the width W1, α2 is desirably 30 ° or less. Further, the counter electromotive force Ec needs to be devised so as to give a certain maximum energy while reducing cogging and torque ripple. The back electromotive force Ec change represents a change in magnetic flux. This affects the torque and unloaded rotational speed. For this reason, it is desirable that the magnetic resistance changes at a constant rate from the center of the blade portion to the end portion. Therefore, the bending angle α1 is desirably about 5 ° to 15 °, and α2 is desirably about α1 + (5 ° to 15 °). The bending angles α1 and α2 are angles formed with the tangent at the center of the outer peripheral surface of the arm part.

  FIG. 3 is a diagram illustrating another example different from the blade portion illustrated in FIG. It differs from FIG. 2B only in the shape of the outer peripheral surface of the blade portion. In the stator magnetic pole core illustrated in FIG. 3, only the fifth-order bending point is set. The central arc portion shown in FIG. 3B, which is an enlarged view of the X portion, corresponds to the outer peripheral surface of the arm portion between the fifth bending points. The central arc portion is on an arc centered on the shaft axis (core axis). In this arc part, the space | gap between rotor magnets is a fixed space | interval. In each of both of these fifth-order bending points, the outer peripheral surface of the blade portion is formed into a tapered shape (angle α) so as to be away from the rotor magnet (approaching the core axis). Accordingly, when the fifth bending point is passed to the outer side in the circumferential direction, the distance (distance) between the rotor magnet and the rotor magnet increases linearly to the outer end of the blade part.

  In FIG. 3, as in FIG. 2, if the stator magnetic pole is 12 poles and the distance between the two poles (360 ° in electrical angle) is θ (A), the mechanical angle A is θ = 60 °. This mechanical angle A of 60 ° corresponds to the fundamental wave component. The mechanical angle B corresponding to the third harmonic is 60 ° (θ) / 3 = 20 °, and this angle B is defined as the core opening angle (the angle formed by the line connecting the shaft axis and the outer ends of the blades). ) To match. The mechanical angle C corresponding to the fifth harmonic is 60 ° (θ) / 5 = 12 °, and this is made to correspond to the fifth bending point. As a result, a core in which harmonics are superimposed is established by making the core shape in which the third harmonic and the fifth harmonic are continuous. For the same reason as in the above example, the folding angle α is preferably about 5 ° to 30 °.

  The present invention converts the order of each harmonic (odd order: 2n + 1, n: natural number) into a mechanical angle, and changes the magnetoresistance (gap distance) at the converted angle, thereby corresponding harmonic components. Is introduced. Depending on the number of poles of the motor and the dimensions of the core outer shape, the effective order is 2n + 1 = 3 to 21st. A combination of harmonics can be used alone or in combination. Since the magnetic flux distribution can be represented by an odd function of the Fourier series, the variation in the order of harmonics during mass production is converted to an odd function. For example, the 6th harmonic is converted to the 5th or 7th order closer.

A brushless motor having the stator magnetic pole core shown in FIGS. 1 and 2 was manufactured, and various characteristics were measured. All the characteristics were good.
(1) Cogging ripple FIG. 4 is a waveform diagram showing the measurement result of cogging ripple. The horizontal axis represents time. It can be seen that the cogging ripple is suppressed to Kpp: 0.196 g · cm.
(2) Torque Ripple FIG. 5 is a waveform diagram showing measurement results of harmonic torque ripple. The vertical axis represents the torque during one rotation of the motor. It can be seen that the torque ripple is suppressed to 0.47 mNm.
(3) Noise measurement results The noise measurement results and the back electromotive force Ec measurement results were also good. Although the present invention utilizes partial magnetoresistance changes, the partial harmonic flux is integrated into a sine wave. As a result, according to the present invention, a pure tone (pure sound) by the sine wave back electromotive force Ec can be realized, and thereby a low noise (38.80 dB) can be achieved.

FIG. 6 is a graph showing the magnetic flux waveform after superposition of the 13th harmonic due to the core shape. The magnetic flux waveform of the brushless motor in which the third harmonic is introduced by the core opening angle and the thirteenth harmonic is introduced by the core shape is shown. In addition, the magnetic flux waveform after superimposing the 13th harmonic is shifted upward in the graph for the purpose of easy viewing. Compared with the first embodiment, ripples are improved by making the magnetic flux waveform more sinusoidal.

Claims (9)

In a DC motor including an armature that generates a predetermined magnetic field and a magnet that is disposed to face the armature via a predetermined gap, a plurality of arm portions that configure the armature and extend radially about a core axis; A magnetic pole core formed at the tip of the arm portion and composed of a blade portion facing the magnet,
The blade portion is positioned on both sides of the first portion where the gap is substantially constant along the circumferential direction, and a pair of second widening as the gap moves away from the first portion along the circumferential direction. Formed with parts,
An angle formed between the core axis and the both ends in the circumferential direction of the end surface of the first portion facing the gap is C, and both ends in the circumferential direction of the core shaft and the end surface of the second portion facing the gap. The angle formed by the part is B,
The mechanical angle corresponding to 360 ° in electrical angle is θ, and the odd number of 5 or more is N.
A magnetic pole core, wherein B = θ / 3 and C = θ / N. The magnetic pole core according to claim 1, wherein an outer peripheral surface of the first portion is formed in an arc shape. The angle B corresponds to the third harmonic that is multiplied to obtain the magnetic field strength distribution superimposed on the fundamental wave component, and the angle C is obtained to obtain the magnetic field strength distribution superimposed on the fundamental wave component. The magnetic pole core according to claim 2, which corresponds to an Nth-order harmonic wave that overlaps with. The magnetic pole core according to claim 3, wherein the harmonic order N to be multiplied is 5 to 21. The outer peripheral surface of the arm portion is bent at a mechanical angle position corresponding to the Nth harmonic, and the bending angle is 5 ° to 30 ° as an angle formed with the tangent at the center of the outer peripheral surface of the arm portion. The magnetic pole core according to claim 3. A DC motor using the magnetic pole core according to claim 1. In a DC motor having a magnet arranged so as to face through a gap with respect to a magnetic pole core wound with a winding,
The magnetic pole core has an arm portion extending radially from the central yoke portion in a radial direction, and a blade portion extending from the tip of the arm portion to both sides in the circumferential direction,
On the outer peripheral surface of the blade portion facing the magnet through a gap, a bending point pair having a predetermined angle is set at a symmetrical position with respect to the center of the blade portion, and the outer peripheral surface of the central portion located between the pair of bending points Is formed into an arc shape centered on the shaft axis, the gap between the magnets is set at a constant interval, and the outer circumferential direction from each of the bending point pairs is tapered so as to be away from the magnet. Into a bent shape,
The mechanical angle B corresponding to the third harmonic to be multiplied to obtain the magnetic field intensity distribution superimposed on the fundamental wave component is defined as θ / 3, where θ is the mechanical angle A between the two magnetic poles corresponding to an electrical angle of 360 °. And set this mechanical angle B to coincide with the core opening angle, which is the angle formed by the line connecting the shaft axis and both outer ends of the blade part, and
In order to obtain the magnetic field strength distribution superimposed on the fundamental wave component, the order of the higher harmonics to be superimposed is an odd number N of 5 or more, and the mechanical angle C corresponding to the Nth harmonic is set to θ / N. A DC motor comprising the mechanical angle C matched with a predetermined angle of the bending point pair. Multiple pairs of bending points are set according to the number of harmonics to be superimposed, and the outer peripheral surface of the central part between the pair of bending points corresponding to the highest order harmonic is formed into an arc shape, and the harmonic order The DC motor according to claim 7, wherein the bending point pair is bent at a larger angle in a tapered shape so as to be further away from the magnet as the bending point pair is further away from the magnet. First and second bending point pairs are set corresponding to the seventh harmonic and the fifth harmonic, respectively, and the outer peripheral surface of the central portion between the first bending point pairs is formed into an arc shape, and the second folding point is set. 8. The DC motor according to claim 7, wherein the bending point pair is formed so as to be circumferentially outward from the first bending point and bent at a larger angle with a taper so as to be further away from the magnet.

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