WO2022027961A1

WO2022027961A1 - Novel motor structure

Info

Publication number: WO2022027961A1
Application number: PCT/CN2021/079551
Authority: WO
Inventors: 肖锐
Original assignee: 中山大洋电机股份有限公司; 湖北惠洋电器制造有限公司
Priority date: 2020-08-05
Filing date: 2021-03-08
Publication date: 2022-02-10

Abstract

Disclosed is a novel motor structure. The present invention comprises a rotor assembly and a stator assembly, wherein the stator assembly comprises several E-shaped stator iron cores uniformly distributed in a circumferential direction, the several E-shaped stator iron cores being distributed on the periphery of the rotor assembly; the E-shaped stator iron cores each comprise a yoke portion, and a left pole arm, a middle pole arm and a right pole arm which extend out from the yoke portion, end portions of the left pole arms, the middle pole arms and the right pole arms respectively forming magnetic pole faces, the periphery of the rotor assembly is surrounded by several magnetic pole faces at intervals, and winding parts of coil windings wound on all the E-shaped stator iron cores are the same, i.e. the coil winding being wound on the middle pole arm of each E-shaped stator iron core or the coil winding being wound on both the left pole arm and the right pole arm of each E-shaped stator iron core. The present application uses an E-shaped stator iron core structure, so that a stator notch opening is small, and the distribution density of magnetic pole faces is increased, thereby reducing the rotating pulsation of the motor and increasing the efficiency of the motor.

Description

A new type of motor structure

Technical field:

The present invention relates to a novel motor structure.

Background technique:

At present, there is a structure of a high-speed running motor on the market. It has high rotor speed, low manufacturing cost and simple structure, and is widely used in the field of vacuum cleaners. The stator assembly of the motor with this structure usually includes several stator cores. Each stator The cores are often formed in a c-shaped shape, each c-shaped stator core has a back and two pole arms (which extend from the back) with a pole face at the end of each pole arm, the bobbin is provided at the c On the C-shaped stator cores, the windings are wound around the bobbin, four pieces of the c-shaped stator cores 2 are provided, and the windings 3 are wound around each stator core 2 . Each of the four stator cores and windings is identical, see Chinese Patent No. 201810569772.5, including at least one pair of c-shaped stator cores, each c-shaped stator core having a bobbin and windings wound around each bobbin , wherein the windings on each adjacent pair of the c-shaped stator core are wound in opposite directions, the prior art has defects/or problems: it adopts a C-shaped structure, the slot opening is large, the magnetic pole face distribution density is low, resulting in The torque ripple of the motor is large, resulting in low motor efficiency.

Invention content:

The purpose of the present invention is to provide a new type of motor structure, which can solve the technical problem of using a C-shaped structure in the prior art, the slot opening is large, the torque ripple of the motor is large, and the motor efficiency is low.

The purpose of the present invention is achieved through the following technical solutions.

The first object of the present invention is to provide a novel motor structure, including a rotor assembly and a stator assembly, characterized in that: the stator assembly includes a plurality of E-shaped stator cores evenly distributed in the circumferential direction, and the plurality of E-shaped stator cores are distributed At the periphery of the rotor assembly, the E-shaped stator core includes a yoke and a left pole arm, a middle pole arm and a right pole arm extending from the yoke, and the ends of the left pole arm, the middle pole arm and the right pole arm respectively form magnetic pole faces , the periphery of the rotor assembly is surrounded by a number of magnetic pole faces at intervals, and the winding positions of the coil windings on all E-shaped stator cores are the same, that is, the middle pole arm on each E-shaped stator core is wound with coil windings or on each E-shaped stator core. A piece of E-shaped stator core is wound with coil windings on both the left and right pole arms.

As mentioned above, when the middle pole arm of the E-shaped stator core is wound with the coil winding, the winding directions of the coil windings on the two adjacent E-shaped stator cores are opposite.

As mentioned above, when the coil winding is wound on the left pole arm and the right pole arm on the E-shaped stator core at the same time, when the coil winding is wound on the left pole arm and the right pole arm of the same E-shaped stator core, the winding direction is: are the same; the winding directions of the coil windings wound on two adjacent E-shaped stator cores are opposite.

The above-mentioned several pieces of E-shaped stator cores refer to 2 pieces, 4 pieces or 6 pieces.

There are four E-shaped stator cores mentioned above.

The magnetic pole faces at the ends of the above-mentioned left pole arm, middle pole arm and right pole arm are arc surfaces.

The above-mentioned magnetic pole faces respectively include a left magnetic pole face, a middle magnetic pole face and a right magnetic pole face, and the ratio between the width L1 of the middle magnetic pole face and the width L2 of the middle pole arm of all E-shaped stator cores is in the following range: 1≤L1/L 2≤1.3.

The left pole arm and the right pole arm on all the E-shaped stator cores mentioned above are asymmetrical structures.

The above-mentioned left pole arm and right pole arm are asymmetrical structures, which means that the width L3 of the left pole arm close to the yoke is larger than the width L4 of the right pole arm close to the yoke or the width L3 of the left pole arm close to the yoke is larger than the width L3 of the left pole arm close to the yoke. The width L4 of the pole arm near the yoke is small.

A first air gap, a second air gap and a third air gap are respectively formed between the above-mentioned left magnetic pole face, middle magnetic pole face and right magnetic pole face and the periphery of the rotor assembly. The air gaps are all non-uniform air gaps.

The first air gap, the second air gap and the third air gap mentioned above are all gradual air gaps, and the first air gap, the second air gap and the third air gap are gradually formed by the rotor assembly in the counterclockwise direction along the counterclockwise direction. Big gets smaller.

The structures of the above-mentioned adjacent E-shaped stator cores are the same or different.

There are four E-shaped stator cores mentioned above.

Mounting bosses are protruded from the outer edges of the yoke portion of the left pole arm and the right pole arm.

The above-mentioned width L2 of the middle pole arm is greater than the width L3 of the left pole arm near the yoke, and the width L2 of the middle pole arm is greater than the width L4 of the right pole arm near the yoke.

Compared with the prior art, the present invention has the following effects:

1) The present invention includes a rotor assembly and a stator assembly, and is characterized in that: the stator assembly includes a plurality of E-shaped stator iron cores uniformly distributed in the circumferential direction, and a plurality of E-shaped stator iron cores are distributed on the periphery of the rotor assembly, and the E-shaped stator iron core includes The yoke part and the left pole arm, the middle pole arm and the right pole arm extending from the yoke part, the ends of the left pole arm, the middle pole arm and the right pole arm respectively form magnetic pole faces, and the periphery of the rotor assembly is surrounded by a number of magnetic pole faces at intervals, The winding positions of the coil windings on all E-shaped stator cores are the same, that is, the coil windings are wound on the middle pole arm on each E-shaped stator core or the left pole is wound on each E-shaped stator core at the same time. The arm and the right pole arm are wound with coil windings, adopting the E-shaped stator core structure, the opening of the stator slot is small, and the distribution density of the magnetic pole surface is increased, thereby reducing the rotational motion of the motor and improving the efficiency of the motor;

2) Other advantages of the present invention are described in detail in the embodiment section.

Description of drawings:

1 is a schematic diagram provided by Embodiment 1 of the present invention;

2 is a schematic structural diagram provided by Embodiment 1 of the present invention;

3 is another schematic structural diagram provided by Embodiment 1 of the present invention;

4 is a schematic diagram provided by Embodiment 2 of the present invention;

5 is a schematic structural diagram provided by Embodiment 2 of the present invention;

6 is a schematic structural diagram provided by Embodiment 2 of the present invention;

7 is a schematic structural diagram provided by Embodiment 3 of the present invention;

8 is a schematic structural diagram provided by Embodiment 4 of the present invention;

9 is a schematic structural diagram provided by Embodiment 5 of the present invention;

10 is a schematic structural diagram provided by Embodiment 6 of the present invention;

11 is a schematic structural diagram of Embodiment 7 of the present invention;

Fig. 12 is a partial enlarged view of part A of this Fig. 11;

13 is a schematic structural diagram of the seventh embodiment of the present utility model after dimensioning;

14 is another implementation structure diagram provided by Embodiment 7 of the present invention;

15 is a third implementation structure diagram provided by Embodiment 7 of the present utility model;

16 is a schematic diagram provided by Embodiment 8 of the present invention;

Fig. 17 is a partial enlarged view of B in Fig. 16;

FIG. 18 is a schematic structural diagram of Embodiment 8 of the present invention.

detailed description:

The present invention will be described in further detail below through specific embodiments and in conjunction with the accompanying drawings.

Example 1:

As shown in FIGS. 1 to 3 , this embodiment provides a novel motor structure, including a rotor assembly 3 and a stator assembly, characterized in that: the stator assembly includes a plurality of E-shaped stator cores 10 uniformly distributed in the circumferential direction, Several E-shaped stator cores 10 are distributed around the rotor assembly 3. The E-shaped stator core 10 includes a yoke 11 and a left pole arm 12, a middle pole arm 13 and a right pole arm 14 extending from the yoke 11. The ends of the arm 12 , the middle pole arm 13 and the right pole arm 14 respectively form magnetic pole faces 100 , and the periphery of the rotor assembly 3 is surrounded by a number of magnetic pole faces 100 at intervals. The same, that is, the coil winding 2 is wound around the middle pole arm 13 on each E-shaped stator core 10, and the E-shaped stator core structure is adopted. Rotary pulse, improve motor efficiency.

The middle pole arm 13 of the E-shaped stator iron core 10 is wound around the coil winding 2 , and the winding directions of the coil windings 2 on two adjacent E-shaped stator iron cores 10 are opposite, and the structure arrangement is reasonable. The coil winding 2 can be wound in two different directions, which will be referred to as a clockwise direction (CW for short) and a counterclockwise direction (CCW for short). The coil windings 2 wound on the middle pole arms 13 on the E-shaped stator core 10 are wound in alternate directions, so that the coil windings 2 wound on the middle pole arms 13 on the two adjacent E-shaped stator cores 10 along the Wind in the opposite direction. That is, the magnetic poles generated by the magnetic pole faces 100 of the middle pole arms 13 on two adjacent E-shaped stator cores 10 are opposite.

On the same E-shaped stator iron core 10, when the coil winding 2 wound around the middle pole arm 13 introduces current, when the magnetic pole generated on the magnetic pole face 100 of the middle pole arm 13 is N pole, then on the same E-shaped stator iron The magnetic poles generated by the magnetic pole faces 100 of the left pole arm 12 and the right pole arm 14 of the core 10 are S poles; on the contrary, on the same E-shaped stator core 10, when the coil winding 2 wound by the middle pole arm 13 introduces current, the When the magnetic poles generated by the magnetic pole face 100 of the middle pole arm 13 are S poles, the magnetic poles produced by the magnetic pole faces 100 of the left pole arm 12 and the right pole arm 14 of the same E-shaped stator core 10 are N poles.

The working principle of the present invention is as follows: by introducing periodically varying currents into the coil windings of the four E-shaped stator cores 10 or introducing currents into the coil windings of the four E-shaped stator cores 10 according to different time sequences, a rotating magnetic field is generated, Magnetic coupling is generated with the rotor, and the traction rotor rotates at high speed. The rotor can adopt a permanent magnet rotor structure, that is, an embedded magnetic tile or a surface-mounted magnetic tile structure. The permanent magnet rotor is a well-known technology and will not be described in detail here.

The number of pieces of E-shaped stator iron cores 10 is preferably 4 pieces, the layout is better, the magnetic coupling of the stator and the rotor is more uniform, and the control is simpler.

The magnetic pole faces 100 at the ends of the left pole arm 12 , the middle pole arm 13 and the right pole arm 14 are arc surfaces, which can better match the circular surface of the rotor and make the air gap magnetic field more uniform.

The following table 1 is the analysis and comparison results of the prior art C-shaped stator core structure and the patented technology structure, and four stator cores are used for comparative analysis at the same time:

Table 1

From the above table 1, it can be concluded that the new motor structure provided by the present invention adopts the E-shaped stator iron core structure, which is better than the motor using the C-shaped stator iron core structure. The E-shaped stator iron core structure has a small opening of the stator slot and enlarged magnetic poles. Therefore, the copper consumption is reduced by about 5%, although the iron consumption is increased by about 15%, but the cost of copper is much higher than that of iron, thus obtaining greater cost advantage.

Embodiment 2:

As shown in Figs. 4 to 6, a novel motor structure includes a rotor assembly 3 and a stator assembly, characterized in that the stator assembly includes a plurality of E-shaped stator cores 10 evenly distributed in the circumferential direction, and a plurality of E-shaped stator irons The core 10 is distributed around the rotor assembly 3, and the E-shaped stator core 10 includes a yoke 11 and a left pole arm 12, a middle pole arm 13 and a right pole arm 14 extending from the yoke 11. The left pole arm 12, the middle pole arm 13 and the ends of the right pole arm 14 respectively form magnetic pole faces 100. The periphery of the rotor assembly 3 is surrounded by a number of magnetic pole faces 100 at intervals. A piece of E-shaped stator iron core 10 is wound with coil winding 2 on the left pole arm 12 and the right pole arm 14 at the same time. The E-shaped stator iron core structure is adopted. The opening of the stator slot is small, and the distribution density of the magnetic pole surface is increased, thereby reducing the motor's Rotary pulse, improve motor efficiency.

This embodiment is a modification on the basis of the first embodiment: that is, when the coil winding 2 is wound on the left pole arm 12 and the right pole arm 14 on the E-shaped stator core 10 at the same time, the middle pole arm 13 is no longer wound with the coil winding 2, When the left pole arm 12 and the right pole arm 14 of the same E-shaped stator core 10 are wound with the coil winding 2, the winding directions are the same; The line direction is opposite, and the structure is arranged reasonably. It has a reasonable structure. The coil winding 2 can be wound in two different directions, which will be referred to as a clockwise direction (CW for short) and a counterclockwise direction (CCW for short). The magnetic poles generated by the magnetic pole faces 100 of the left pole arm 12 and the right pole arm 14 on two adjacent E-shaped stator cores 10 are opposite, and the left pole arm 12 and the right pole arm 14 of the same E-shaped stator core 10 The pole faces 100 produce the same poles.

On the same E-shaped stator core 10, when current is introduced into the coil winding 2 wound by the left pole arm 12 and the right pole arm 14, the magnetic poles generated on the magnetic pole faces 100 of the left pole arm 12 and the right pole arm 14 are N When the pole is in the same E-shaped stator core 10, the magnetic pole generated on the magnetic pole face 100 of the middle pole arm 13 of the same E-shaped stator core 10 is the S pole; When the coil winding 2 wound around the pole arm 14 introduces a current, when the magnetic poles generated by the magnetic pole faces 100 of the left pole arm 12 and the right pole arm 14 are S poles, then the same E-shaped stator core 10 is placed in the middle pole arm 13 of the same piece of E-shaped stator core 10. The magnetic pole produced by the magnetic pole face 100 is an N pole.

Embodiment three:

As shown in FIG. 7 , this embodiment is further followed up on the basis of Embodiment 1. There are two pieces of E-shaped stator iron cores 10 , and the winding positions of the coil windings 2 on all E-shaped stator iron cores 10 are the same. That is, the coil winding 2 is wound on the middle pole arm 13 of each E-shaped stator core 10, and the coil winding 2 is no longer wound on the left pole arm 12 and the right pole arm 14, which can reduce the manufacturing cost and meet the needs of different customers. .

Embodiment 4:

As shown in FIG. 8 , this embodiment is a further follow-up on the basis of Embodiment 1. There are 6 pieces of E-shaped stator iron cores 10 , and all the E-shaped stator iron cores 10 are wound with the same winding position of the coil winding 2 . That is, the coil winding 2 is wound around the middle pole arm 13 of each E-shaped stator core 10, and the coil winding 2 is no longer wound around the left pole arm 12 and the right pole arm 14, so as to increase the distribution density of the magnetic pole surface and improve the efficiency. , to meet different customer needs.

Embodiment 5:

As shown in FIG. 9 , this embodiment is further improved on the basis of the second embodiment. There are two pieces of E-shaped stator iron cores 10 , and all the E-shaped stator iron cores 10 are wound around the coil winding 2 on the same winding site. , the middle pole arm 13 on each E-shaped stator core 10 is no longer wound with the coil winding 2, and the coil winding 2 wound on the left pole arm 12 and the right pole arm 14 can reduce the manufacturing cost and meet the needs of different customers.

Embodiment 6:

As shown in FIG. 10 , this embodiment is further improved on the basis of the second embodiment. There are 6 pieces of E-shaped stator iron cores 10 , and the winding positions of the coil windings 2 on all E-shaped stator iron cores 10 are the same. , the middle pole arm 13 on each E-shaped stator core 10 is no longer wound with the coil winding 2, and the coil winding 2 wound on the left pole arm 12 and the right pole arm 14 increases the distribution density of the magnetic pole surface to meet the needs of different customers .

Embodiment 7:

As shown in FIGS. 11 to 15 , this embodiment is further improved on the basis of Embodiment 1 or Embodiment 2. The magnetic pole face 100 includes a left magnetic pole face 121 , a middle magnetic pole face 131 and a right magnetic pole face 141 respectively. All E-shaped stators The ratio of the width L1 of the middle magnetic pole face 131 of the iron core 10 to the width L2 of the middle pole arm 13 is in the following range: 1≤L1/L2≤1.3, the structure satisfies the production process foundation of the motor stator, the structure is simple, the structure layout is reasonable, and the The stator core reduces the difficulty of winding during winding and ensures the qualified rate of the product.

In FIG. 11 , L1=L2; the slot openings of the two adjacent pole arms on the same E-shaped stator core 10 are made wider, so as to facilitate the wire embedding process for the coil winding 2 . The left pole arm 12 and the right pole arm 14 on all E-shaped stator cores 10 are asymmetrical structures, and the width L3 of the left pole arm 12 close to the yoke 11 is smaller than the width L4 of the right pole arm 14 close to the yoke 11, which effectively reduces the Torque ripple to improve efficiency.

In FIG. 14, the width L1 of the middle magnetic pole face 131 of the E-shaped stator core 10 is greater than the width L2 of the middle pole arm 13, and the ratio is in the following range: 1≤L1/L2≤1.3. That is, the width L1 of the middle magnetic pole face 131 is slightly larger than the width L2 of the middle pole arm 13 , but it is ensured that the slot openings of the two adjacent pole arms on the same E-shaped stator core 10 have sufficient inlay width. The adjacent E-shaped stator cores 10 have the same structure, which can effectively reduce torque ripple and improve efficiency.

In FIG. 15 , the structures of adjacent E-shaped stator cores 10 are different, which can effectively reduce torque ripple and improve efficiency.

The left pole arm 12 and the right pole arm 14 on all E-shaped stator cores 10 are asymmetrical structures, which can effectively reduce torque ripple and improve motor efficiency.

The left pole arm 12 and the right pole arm 14 are asymmetrical structures, which means that the width L3 of the left pole arm 12 close to the yoke 11 is smaller than the width L4 of the right pole arm 14 close to the yoke 11, which can reduce torque ripple and improve motor efficiency .

A first air gap 122 , a second air gap 132 and a third air gap 142 are respectively formed between the left magnetic pole face 121 , the middle magnetic pole face 131 and the right magnetic pole face 141 and the periphery of the rotor assembly 3 . Both the second air gap 132 and the third air gap 142 are non-uniform air gaps to improve electromagnetic performance.

The first air gap 122 , the second air gap 132 and the third air gap 142 are all progressive air gaps, and the first air gap 122 , the second air gap 132 and the third air gap 142 take the rotor assembly 3 as the center along the counterclockwise direction. The direction is gradually changed from large to small, ensuring electromagnetic performance, optimizing motor performance, and enhancing the starting reliability of the motor.

As shown in FIG. 14 , the adjacent E-shaped stator cores 10 may be the same; as shown in FIG. 15 , the adjacent E-shaped stator cores 10 may be different.

The width L2 of the middle pole arm 13 is greater than the width L3 of the left pole arm 12 near the yoke 11 , the width L2 of the middle pole arm 13 is greater than the width L4 of the right pole arm 14 near the yoke 11 , the structure is simple and the layout is reasonable.

Embodiment 8:

As shown in FIG. 16 to FIG. 18 , this embodiment is a modification based on the first embodiment. The left pole arm 12 and the right pole arm 14 are asymmetrical structures, and the width L3 of the left pole arm 12 close to the yoke 11 is larger than that of the right pole arm The width L4 of the arm 14 close to the yoke 11 is large, which can reduce the torque ripple and improve the motor efficiency.

The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited thereto, and any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principle of the present invention are equivalent. The replacement modes are all included in the protection scope of the present invention.

Claims

A novel motor structure, comprising a rotor assembly (3) and a stator assembly, characterized in that the stator assembly includes a plurality of E-shaped stator iron cores (10) uniformly distributed in the circumferential direction, and the plurality of E-shaped stator iron cores (10) are distributed At the periphery of the rotor assembly (3), the E-shaped stator core (10) includes a yoke (11) and a left pole arm (12), a middle pole arm (13) and a right pole arm (11) extending from the yoke (11). 14), the ends of the left pole arm (12), the middle pole arm (13) and the right pole arm (14) respectively form magnetic pole faces (100), and the periphery of the rotor assembly (3) is surrounded by a plurality of magnetic pole faces (100) at intervals, The winding positions of the coil windings (2) on all E-shaped stator cores (10) are the same, that is, the coil windings (2) are wound around the middle pole arm (13) on each E-shaped stator core (10). Alternatively, the coil windings (2) are simultaneously wound on the left pole arm (12) and the right pole arm (14) on each E-shaped stator core (10).
A novel motor structure according to claim 1, characterized in that: when the middle pole arm (13) of the E-shaped stator iron core (10) is wound around the coil winding (2), two adjacent E-shaped stator iron cores ( 10) The winding direction of the coil winding (2) is reversed.
A novel motor structure according to claim 1, characterized in that: when the coil winding (2) is wound on the left pole arm (12) and the right pole arm (14) at the same time on the E-shaped stator core (10) , when the coil winding (2) is wound on the left pole arm (12) and the right pole arm (14) of the same E-shaped stator core (10), the winding directions are the same; two adjacent E-shaped stator cores The winding direction of the coil winding (2) on (10) is opposite.
A novel motor structure according to claim 1, 2 or 3, characterized in that: several E-shaped stator iron cores (10) refer to 2, 4, or 6 pieces.
A novel motor structure according to claim 4, characterized in that: there are 4 pieces of E-shaped stator iron cores (10).
A novel motor structure according to claim 4, characterized in that: the magnetic pole faces (100) at the ends of the left pole arm (12), the middle pole arm (13) and the right pole arm (14) are arc surfaces.
A novel motor structure according to claim 1, 2 or 3, wherein the magnetic pole face (100) comprises a left magnetic pole face (121), a middle magnetic pole face (131) and a right magnetic pole face (141) respectively , the ratio of the width L1 of the middle magnetic pole face (131) of all E-shaped stator cores (10) to the width L2 of the middle pole arm (13) is in the following range: 1≤L1/L2≤1.3.
A novel motor structure according to claim 8, wherein the left pole arm (12) and the right pole arm (14) on all E-shaped stator cores (10) are asymmetrical structures.
A novel motor structure according to claim 9, characterized in that: the left pole arm (12) and the right pole arm (14) are asymmetrical structures, which means that the left pole arm (12) is close to the yoke (11). The width L3 is larger than the width L4 of the right pole arm (14) close to the yoke (11) or the width L3 of the left pole arm (12) close to the yoke (11) is larger than the width L3 of the right pole arm (14) close to the yoke (11) L4 is small.
A novel motor structure according to claim 9 or 10, characterized in that: between the left magnetic pole face (121), the middle magnetic pole face (131) and the right magnetic pole face (141) and the periphery of the rotor assembly (3) A first air gap (122), a second air gap (132) and a third air gap (142) are respectively formed, and the first air gap (122), the second air gap (132) and the third air gap (142) are all is an uneven air gap.
A novel motor structure according to claim 11, characterized in that: the first air gap (122), the second air gap (132) and the third air gap (142) are all progressive air gaps, and the first air gap ( 122 ), the second air gap ( 132 ) and the third air gap ( 142 ) gradually change from large to small along the counterclockwise direction with the rotor assembly ( 3 ) as the center.
A novel motor structure according to claim 10, characterized in that the structures of adjacent E-shaped stator iron cores (10) are the same or different.
A novel motor structure according to claim 11, characterized in that: there are 4 pieces of E-shaped stator iron cores (10).
A novel motor structure according to claim 11, wherein the yoke (11) is located on the outer edges of the left pole arm (12) and the right pole arm (14) with mounting bosses (111) protruding.
A novel motor structure according to claim 11, characterized in that: the width L2 of the middle pole arm (13) is greater than the width L3 of the left pole arm (12) close to the yoke (11), and the width L3 of the middle pole arm (13) The width L2 is greater than the width L4 of the right pole arm (14) close to the yoke (11).