Electronic water pump and vortex loss suppression method thereof

Abstract

This invention discloses an electronic water pump and its eddy current loss suppression method, comprising a housing, a sealing sleeve, a rotor impeller assembly, and a PCBA board. The PCBA board is located between a heat sink base and a lower cover. The stator windings on the stator assembly are electrically connected to the PCBA board. The sealing sleeve is made of metal. The rotor impeller assembly includes a rotor body and an impeller assembly. The rotor body includes a rotor core and magnets fixed outside the rotor core. The rotor core and magnets are covered with a plastic layer. Magnetic guide sheets are embedded within the plastic layer on the upper and / or lower sides of the magnets. The surface of the magnetic guide sheets covers the magnets and the rotor core. This suppression method uses magnetic guide sheets to close the leakage magnetic flux from the rotor core end face through the magnetic guide sheets, thereby suppressing eddy current losses and improving the performance of the electronic water pump.

Description

Technical Field

[0001] This invention relates to an electronic water pump, and also to a method for suppressing eddy current losses in the metal sealing sleeve of the electronic water pump. Background Technology

[0002] An electric water pump is a type of electrically driven water pump, commonly used in automotive cooling systems. It differs from a traditional mechanical water pump, which relies on the mechanical movement of the engine for operation. An electric water pump uses an electric motor or other electrical device as its power source, and its operating status is regulated by an electronic control system.

[0003] When a changing magnetic flux passes through a conductive material, an induced current is generated both in the external conductive circuit and inside the conductive material through which the flux passes. This latter type of induced current is called eddy current. Ferromagnetic materials are all conductive, and when an alternating magnetic flux passes through them, eddy currents always exist within them. The presence of eddy currents causes power loss within the ferromagnetic material; this loss is called eddy current loss Pe, which causes the core to heat up and its temperature to rise. To reduce eddy current loss, alternating magnetic circuits are typically constructed using laminated silicon steel sheets, with an insulating layer generally applied between the sheets to minimize eddy current losses. The applicant previously applied for the basic structure of an electronic water pump, with authorization announcement number CN220956083U. The basic structure of this electronic water pump includes a stator assembly and a rotor assembly, and a sealing spacer is set inside the housing. In order to improve the overall sealing performance and enhance thermal conductivity, the sealing spacer can be set as a metal sealing spacer. In this way, the metal sealing spacer can be integrally formed and fixed between the housing and the top cover to reduce liquid leakage. However, since the sealing spacer itself is also a conductive material, the water pump will generate significant eddy current losses during operation, resulting in a decrease in the performance of the water pump.

[0004] Patent application CN119276025A discloses a stator assembly for an electronic water pump and the same assembly. This application optimizes the stator by constructing a top, middle, and bottom portion of the stator core along the stacking direction of the stator laminations. The thickness of the stator laminations at the top and bottom is less than the thickness of the laminations at the middle portion. This structure optimizes the eddy current loss of the stator. However, the eddy current loss of the rotor in the electronic water pump still requires further optimization. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an electronic water pump in which a magnetic conductive sheet is embedded in the end of the magnet of the rotor body to suppress rotor eddy current loss.

[0006] Another technical problem to be solved by the present invention is to provide an eddy current suppression method for an electronic water pump, wherein a magnetic conductive sheet is embedded in the end of the magnet of the rotor body, thereby suppressing rotor eddy current loss and improving the performance of the electronic water pump.

[0007] To solve the above-mentioned technical problems, the technical solution of the present invention is: an electronic water pump, including a housing, an upper cover at the upper end of the housing, a lower cover at the lower end of the housing, a sealing sleeve inside the housing, a rotor assembly mounting cavity inside the sealing sleeve, a rotor impeller assembly rotatably mounted in the rotor assembly mounting cavity, a fluid chamber formed between the sealing sleeve and the upper cover, an impeller of the rotor impeller assembly located in the fluid chamber, an inlet and an outlet on the upper cover, a stator assembly mounting cavity formed between the sealing sleeve and the housing, and a stator assembly mounted in the stator assembly mounting cavity. The stator assembly includes a heat dissipation base at the lower end of the housing, a PCBA board fixed to the back of the heat dissipation base, the PCBA board located between the heat dissipation base and the lower cover, the stator windings on the stator assembly being electrically connected to the PCBA board, the sealing spacer being a metal sealing spacer, and the rotor impeller assembly including a rotor body and an impeller assembly. The rotor body includes a rotor core and magnets fixed to the outside of the rotor core. The rotor core and magnets are covered with a plastic layer, and magnetic conductive sheets are embedded in the plastic layer on the upper and / or lower side of the magnets. The surface of the magnetic conductive sheets covers the magnets and the rotor core.

[0008] As a preferred embodiment, the outer diameter of the magnetic conductive sheet is greater than or equal to the outer diameter of the circumference of the magnet, and the inner diameter of the magnetic conductive sheet is equal to the inner diameter of the rotor core.

[0009] As a preferred embodiment, the thickness of the magnetic conductive sheet is 0.5mm-1.5mm.

[0010] As a preferred embodiment, the number of magnetic conductive sheets located on the upper side of the magnet is one or more, and the number of magnetic conductive sheets located on the lower side of the magnet is one or more.

[0011] As a preferred embodiment, the peripheral wall of the metal sealing sleeve is provided with several outwardly protruding axial arches. These axial arches are formed by the outward pressing of the peripheral wall of the sealing sleeve. Each axial arch is located in the stator slot of the corresponding stator core, and the length of the axial arch is greater than or equal to the axial length of the stator core.

[0012] As a preferred embodiment, the axially arched cross-sectional shape is arc-shaped.

[0013] As a preferred embodiment, the number of axial arches is equal to the number of stator slots and is evenly distributed around the circumference, with each axial arch corresponding to a specific stator slot.

[0014] After adopting the above technical solution, the effect of the present invention is as follows: Since the sealing sleeve is a metal sealing sleeve, the rotor impeller assembly includes a rotor body and an impeller assembly. The rotor body includes a rotor core and a magnet fixed outside the rotor core. The rotor core and the magnet are covered with a plastic layer. A magnetic guide sheet is embedded in the plastic layer on the upper side and / or lower side of the magnet. The surface of the magnetic guide sheet covers the magnet and the rotor core. Therefore, by setting the magnetic guide sheet, the magnetic lines of force overflowing from the end face of the rotor core will be closed by the magnetic guide sheet after reaching it, so that most of the end leakage flux is closed inside the magnetic guide sheet, which significantly reduces the generation of eddy currents in this area.

[0015] Furthermore, since the outer diameter of the magnetic conductive sheet is greater than or equal to the outer diameter of the circumference of the magnet, and the inner diameter of the magnetic conductive sheet is equal to the inner diameter of the rotor core, and the thickness of the magnetic conductive sheet is 0.5mm-1.5mm, by optimizing the thickness and size of the magnetic conductive sheet, more end leakage flux can be allowed to complete the closure within the ceramic sheet, thereby further reducing rotor eddy current losses.

[0016] Furthermore, since the peripheral wall of the metal sealing sleeve has several outwardly protruding axial arches, these axial arches are formed by the outward compression of the peripheral wall of the sealing sleeve. Each axial arch is located within the stator slot of the corresponding stator core, and the length of the axial arch is greater than or equal to the axial length of the stator core. Therefore, by setting axial arches on the peripheral wall of the sealing sleeve, the axial arches change the local conductive cross-section of the sealing sleeve. Annular eddy currents flow circumferentially within the peripheral wall of the sealing sleeve. The axial arches increase the local resistance in this area, changing the local magnetic field distribution in the stator slot area. The magnetic flux that originally concentrated through the sealing sleeve directly above the slot is partially diverted to both sides of the axial rib after encountering it, resulting in a reduction in the peak magnetic flux density passing through the sealing sleeve wall, further suppressing local eddy current losses on the sealing sleeve.

[0017] Furthermore, since the cross-sectional shape of the axial arch is arc-shaped, the use of arc-shaped axial arches can reduce stress concentration, make molding easier, and have less impact on fluid flow.

[0018] Furthermore, since the number of axial arches is equal to the number of stator slots and is evenly distributed around the circumference, and the axial arches are located one-to-one within the stator slots, the stator slots provide space for the axial arches to protrude. Therefore, by setting one axial arch in each stator slot, the number of axial arches is increased without affecting the air gap between the sealing spacer and the stator core as much as possible, thereby maximizing the local resistance and further suppressing the local eddy current loss on the sealing spacer.

[0019] To solve the second technical problem mentioned above, the technical solution of the present invention is: a method for suppressing eddy current losses in an electronic water pump. This eddy current suppression method uses the structure of the electronic water pump described above. In this method, a magnetic conductive sheet is embedded in the plastic layer of the rotor body above and / or below the magnet. The thickness of the magnetic conductive sheet is greater than or equal to the thickness of the silicon steel sheet of a single rotor core. Partial leakage flux from the end face of the rotor core completes the closure through the magnetic conductive sheet, thereby suppressing eddy current losses.

[0020] Preferably, the peripheral wall of the metal sealing sleeve is provided with several outwardly protruding axial arches. These axial arches are formed by the outward compression of the peripheral wall of the sealing sleeve. Each axial arch is located in the stator slot of the corresponding stator core. The length of the axial arch is greater than or equal to the axial length of the stator core. The axial arches change the local conductive cross section of the metal sealing sleeve and increase the local resistance of the area. The annular eddy current flows circumferentially in the cylindrical wall of the sealing sleeve. After encountering the axial arch, it is partially diverted to both sides of the axial arch, thereby reducing the peak value of the magnetic flux density passing through the wall of the sealing sleeve and suppressing local eddy current loss.

[0021] Preferably, one or more magnetic conductive sheets are provided on both the upper and lower sides of the magnet, and the total thickness of the magnetic conductive sheets is 0.5mm-1.5mm. When two magnetic conductive sheets are provided on the upper and lower sides of the magnet, the two magnetic conductive sheets are of equal thickness and are stacked on top of each other.

[0022] After adopting the above technical solution, the effects of this invention are as follows: This suppression method can suppress eddy currents in electronic water pumps and improve pump performance. Firstly, a magnetic guide plate is embedded in the rotor. The magnetic guide plate can be located on the upper or / and lower side of the rotor's magnets, preferably on both the upper and lower sides. This guides the leakage flux from the rotor magnet end face to the inside of the magnetic guide plate, preventing leakage flux from penetrating the sealing sleeve and generating additional eddy currents. Secondly, an axial arch is provided on the peripheral wall of the sealing sleeve, increasing the local resistance of the sealing sleeve and optimizing the eddy current distribution path. This allows the eddy currents to be diverted to both sides of the axial arch during circumferential flow, reducing local losses. Attached Figure Description

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] Figure 1 This is a perspective view of an electronic water pump according to an embodiment of the present invention;

[0025] Figure 2 It is a bottom view of the sealing spacer and the stator core in cooperation;

[0026] Figure 3 It is a three-dimensional view of the sealing spacer;

[0027] Figure 4This is the front view of the sealing spacer;

[0028] Figure 5 This is a three-dimensional sectional view of an electronic water pump;

[0029] Figure 6 It is a 3D electromagnetic simulation cloud diagram of the original scheme;

[0030] Figure 7 This is a three-dimensional electromagnetic simulation cloud map of conductive sheets with different numbers and thicknesses arranged in an embodiment of the present invention.

[0031] Figure 8 This is a comparison graph of the unloaded back EMF curves of the original scheme and the embodiments of the present invention;

[0032] Figure 9 This is a comparison graph of eddy current loss curves between the original scheme and the embodiment of the present invention;

[0033] In the attached diagram: 1. Outer casing; 2. Top cover; 21. Inlet; 22. Outlet; 3. Bottom cover; 4. Rotor impeller assembly; 41. Sealing sleeve; 411. Axial arch; 42. Impeller assembly; 43. Rotor body; 431. Rotor core; 432. Magnet; 433. Magnetic sheet; 5. Stator assembly; 51. Stator core; 511. Stator slot; 52. Stator winding. Detailed Implementation

[0034] The present invention will be further described in detail below through specific embodiments.

[0035] like Figures 1 to 5As shown, an electronic water pump includes a housing 1, an upper cover 2 at the upper end of the housing 1, a lower cover 3 at the lower end of the housing 1, a sealing sleeve 41 inside the housing 1, a rotor assembly mounting cavity inside the sealing sleeve 41, a rotor impeller assembly 4 rotatably mounted in the rotor assembly mounting cavity, a fluid chamber formed between the sealing sleeve 41 and the upper cover 2, an impeller of the rotor impeller assembly 4 located in the fluid chamber, an inlet 21 and an outlet 22 on the upper cover 2, a stator assembly 5 mounting cavity formed between the sealing sleeve 41 and the housing 1, a stator assembly 5 mounted in the stator assembly 5 mounting cavity, the stator assembly 5 including a stator core 51 and a stator winding 52, and a heat dissipation base at the lower end of the housing 1. In this embodiment, the heat dissipation base is integrally formed with the housing, but it can also be separate. A PCBA board is fixed to the back of the heat dissipation base. The PCBA board is located between the heat dissipation base and the lower cover 3. The stator winding 52 on the stator assembly 5 is electrically connected to the PCBA board. The sealing sleeve 41 is a metal sealing sleeve 41. The rotor impeller assembly 4 includes a rotor body 43 and an impeller assembly 42. The rotor body 43 includes a rotor core 431 and a magnet 432 fixed to the outside of the rotor core 431. The rotor core 431 and the magnet 432 are covered with a plastic layer. A magnetic conductive sheet 433 is embedded in the plastic layer on the upper side and / or lower side of the magnet 432. The surface of the magnetic conductive sheet 433 covers the magnet 432 and the rotor core 431.

[0036] The rotor shaft is fixed inside the sealing sleeve 41, and the rotor body 43 is rotatably mounted on the rotor shaft. The impeller assembly 42 is located above the rotor body 43 and within the fluid chamber. The impeller assembly 42 includes a lower impeller and an upper impeller plate. The lower impeller is formed together with the plastic layer of the rotor body 43 by injection molding. Several blades are provided on the lower impeller, and the upper impeller plate is welded onto the blades to complete the final forming.

[0037] Among them, such as Figure 5 As shown, the outer diameter of the magnetic sheet 433 is greater than or equal to the outer diameter of the circumference of the magnet 432, and the inner diameter of the magnetic sheet 433 is equal to the inner diameter of the rotor core 431. In this way, the magnetic sheet 433 can completely cover the magnet 432 and the rotor core 431.

[0038] The thickness of the magnetic conductive sheet 433 is 0.5mm-1.5mm, and preferably, the thickness of the magnetic conductive sheet 433 in this embodiment can be 0.5mm or 1.0mm.

[0039] Among them, such as Figure 5As shown, in this embodiment, there is one magnetic conductive sheet 433 on the upper side of the magnet 432, with a thickness of 1.0 mm, and one magnetic conductive sheet 433 on the lower side of the magnet 432, with a thickness of 1.0 mm. Of course, there are two magnetic conductive sheets 433 on the upper and lower sides of the magnet 432, each with the same specifications and a thickness of 0.5 mm. The two sheets are stacked on top of each other, with a total thickness of 1 mm. Of course, if three magnetic conductive sheets 433 are stacked, the total thickness can be 1.5 mm.

[0040] like Figures 2 to 4 As shown, the metal sealing sleeve 41 has several outwardly protruding axial arches 411 on its peripheral wall. These axial arches 411 are formed by the outward pressing of the peripheral wall of the sealing sleeve 41. Each axial arch 411 is located within a corresponding stator slot 511 of the stator core 51. The length of each axial arch 411 is greater than or equal to the axial length of the stator core 51. Preferably, the cross-sectional shape of each axial arch 411 is arc-shaped. The number of axial arches 411 is equal to the number of stator slots 511, and they are evenly distributed circumferentially, with each axial arch 411 corresponding to a specific stator slot 511.

[0041] This embodiment discloses a method for suppressing eddy current losses in an electronic water pump. The eddy current suppression method uses the structure of the electronic water pump described above. The eddy current suppression method embeds a magnetic conductive sheet in the plastic layer of the rotor body on the upper and / or lower side of the magnet. The thickness of the magnetic conductive sheet 433 is greater than or equal to the thickness of the silicon steel sheet of a single rotor core. Part of the leakage magnetic flux from the end face of the rotor core is closed through the magnetic conductive sheet to achieve the suppression of eddy current losses. In addition, the peripheral wall of the metal sealing sleeve 41 is provided with several outwardly protruding axial arches 411. These axial arches 411 are formed by the outward extrusion of the peripheral wall of the sealing sleeve 41. Each axial arch 411 is located in the stator slot of the corresponding stator core 51. The length of the axial arch 411 is greater than or equal to the axial length of the stator core 51. The axial arches 411 change the local conductive cross section of the metal sealing sleeve 41 and increase the local resistance of the area. The annular eddy current flows circumferentially in the cylindrical wall of the sealing sleeve 41. After encountering the axial arches 411, it is partially diverted to both sides of the axial arches 411, thereby reducing the peak value of the magnetic flux density passing through the sealing sleeve wall and suppressing local eddy current loss.

[0042] The magnet has one or more magnetic conductive sheets 433 on its upper and lower sides. The total thickness of the magnetic conductive sheets 433 is 0.5mm-1.5mm. When two magnetic conductive sheets 433 are provided on the upper and lower sides of the magnet, the two magnetic conductive sheets 433 have the same thickness and are stacked on each other. Of course, multiple magnetic conductive sheets 433 can also be stacked.

[0043] like Figure 6 and Figure 7 As shown, this embodiment selected an electronic water pump of the same specifications for three-dimensional electromagnetic simulation. Among them, Figure 6 The electronic water pump uses a 316L metal sealing septum 41, while Figure 7 a is a schematic diagram of Scheme 1. In Scheme 1, the electronic water pump has a single 0.5mm magnetic conductive sheet 433 on one side of the magnet 432. Figure 7 b is a schematic diagram of Scheme 2. In Scheme 2, the electronic water pump has two 0.5mm magnetic conductive plates 433 on one side of the magnet 432, and the two plates are stacked on top of each other. Figure 7 c is a schematic diagram of Scheme 3. In Scheme 3, the electronic water pump has a single 1mm magnetic conductive sheet 433 on one side of the magnet 432. Figure 7 d is a schematic diagram of Scheme 4. In Scheme 4, the electronic water pump has a single 1mm magnetic conductive sheet 433 on both sides of the magnet 432.

[0044] from Figure 6 and Figure 7 It can be observed from the eddy current loss density table of the original scheme that the minimum eddy current loss value is 1.026E+2; while the minimum eddy current loss value in Scheme 1 is 1.31E+1; the minimum eddy current loss value in Scheme 2 is 5.511E; the minimum eddy current loss value in Scheme 3 is 4.493E; and the minimum eddy current loss value in Scheme 4 is 2.012E. Therefore, it can be found that the scheme in Scheme 4, which sets a single 1mm magnetic conductive sheet 433 on both the upper and lower sides of the magnet 432, is the best.

[0045] Based on Scheme 4, Scheme 5 is formed by providing several outwardly protruding axial arches 411 on the peripheral wall of the sealing spacer 41.

[0046] like Figure 8 As shown, Figure 8 The diagram illustrates a comparison of the no-load back EMF curves in the original scheme and schemes four and five. Figure 8 'a' is a graph of the no-load back EMF of the original scheme, where the three different colored curves represent the curves of three-phase A, B, and C, respectively; Figure 8 The effective value of the back electromotive force in a is 12.9V; where Figure 8 b is the curve of the no-load back EMF in Scheme 4, and the effective value of the back EMF in the figure is 12.2V; Figure 8 Figure c shows the curve of the no-load back EMF in Scheme 5, and the effective value of the back EMF in the figure is also 12.2V. Obviously, compared with the other two schemes, the effective value of the back EMF in Scheme 4 and Scheme 5 is reduced by 5.4%, which is not significant.

[0047] And such Figure 9 As shown, Figure 9The diagram illustrates the eddy current loss curves in the original scheme and schemes four and five. Figure 9 a is a curve of eddy current loss of the original scheme. From this curve, it can be found that the average eddy current loss is 25.7W. Figure 9 a is a curve of eddy current loss for Scheme 4. From this curve, it can be found that the average eddy current loss is 21.85W. Figure 9 c is the curve of eddy current loss for Scheme 5. The curve shows an average eddy current loss of 19.6 W. Comparing the above results, Scheme 5 is the optimal solution. Compared to the original scheme, Scheme 5 reduces eddy current loss by 23.7%, back EMF by only 5.4%, and total loss by 5.4%.

[0048] Based on the above solutions, the key parameters of the motors in the original solution, solution four, and solution five are compared in the following table:

[0049]

[0050] As can be seen from the above key parameters, the eddy current loss of the present invention is suppressed and the efficiency of the electronic water pump is improved compared with the original solution.

[0051] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications and alterations made to the technical solutions of the present invention without departing from the spirit of the present invention shall fall within the protection scope defined by the claims of the present invention.

Claims

1. An electronic water pump, comprising a housing, an upper cover at the upper end of the housing, a lower cover at the lower end of the housing, a sealing sleeve inside the housing, a rotor assembly mounting cavity inside the sealing sleeve, a rotor impeller assembly rotatably mounted in the rotor assembly mounting cavity, a fluid chamber formed between the sealing sleeve and the upper cover, an impeller of the rotor impeller assembly located within the fluid chamber, an inlet and an outlet on the upper cover, a stator assembly mounting cavity formed between the sealing sleeve and the housing, a stator assembly mounted within the stator assembly mounting cavity, a heat dissipation base at the lower end of the housing, a PCBA board fixed to the back of the heat dissipation base, the PCBA board located between the heat dissipation base and the lower cover, and stator windings on the stator assembly electrically connected to the PCBA board, characterized in that: The sealing sleeve is a metal sealing sleeve. The rotor impeller assembly includes a rotor body and an impeller assembly. The rotor body includes a rotor core and a magnet fixed outside the rotor core. The rotor core and the magnet are covered with a plastic layer. A magnetic guide sheet is embedded in the plastic layer on the upper side and / or lower side of the magnet. The surface of the magnetic guide sheet covers the magnet and the rotor core.

2. The electronic water pump as described in claim 1, characterized in that: The outer diameter of the magnetic conductor is greater than or equal to the outer diameter of the circumference of the magnet, and the inner diameter of the magnetic conductor is equal to the inner diameter of the rotor core.

3. An electronic water pump as described in claim 1, characterized in that: The thickness of the magnetic conductive sheet is 0.5mm-1.5mm.

4. An electronic water pump as described in claim 3, characterized in that: The number of magnetic conductive sheets located on the upper side of the magnet is one or more, and the number of magnetic conductive sheets located on the lower side of the magnet is one or more.

5. An electronic water pump as described in claim 1, characterized in that: The metal sealing sleeve has several outwardly protruding axial arches on its peripheral wall. These axial arches are formed by the outward pressing of the peripheral wall of the sealing sleeve. Each axial arch is located in the stator slot of the corresponding stator core. The length of the axial arch is greater than or equal to the axial length of the stator core.

6. An electronic water pump as described in claim 5, characterized in that: The axially arched cross-sectional shape is arc-shaped.

7. An electronic water pump as described in claim 6, characterized in that: The number of axial arches is equal to the number of stator slots and is evenly distributed around the circumference. The axial arches are located one-to-one within the stator slots.

8. A method for suppressing eddy current losses in an electronic water pump, characterized in that: The eddy current suppression method uses the structure of the electronic water pump as described in claim 1. The eddy current suppression method embeds a magnetic guide sheet in the plastic layer of the rotor body on the upper and / or lower side of the magnet. The thickness of the magnetic guide sheet is greater than or equal to the thickness of the silicon steel sheet of a single rotor core. Part of the leakage flux from the end face of the rotor core completes the closure through the magnetic guide sheet, so as to suppress eddy current loss.

9. The method for suppressing eddy current losses in an electronic water pump as described in claim 8, characterized in that: The metal sealing sleeve has several outwardly protruding axial arches on its peripheral wall. These axial arches are formed by the outward compression of the peripheral wall of the sealing sleeve. Each axial arch is located in the stator slot of the corresponding stator core. The length of the axial arch is greater than or equal to the axial length of the stator core. The axial arches change the local conductive cross section of the metal sealing sleeve and increase the local resistance of the area. The annular eddy current flows circumferentially in the cylindrical wall of the sealing sleeve. After encountering the axial arch, it is partially diverted to both sides of the axial arch, which reduces the peak value of the magnetic flux density passing through the sealing sleeve wall, thereby suppressing local eddy current loss.

10. The method for suppressing eddy current losses in an electronic water pump as described in claim 9, characterized in that: The magnet has one or more magnetic conductive sheets on its upper and lower sides. The total thickness of the magnetic conductive sheets is 0.5mm-1.5mm. When two magnetic conductive sheets are provided on the upper and lower sides of the magnet, the two magnetic conductive sheets are of equal thickness and are stacked on top of each other.

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