Permanent magnet motor variable frequency device for centrifugal compressor

Abstract

The utility model relates to centrifugal compressor drive equipment technical field especially relates to a kind of permanent magnet motor frequency conversion equipment for centrifugal compressor, including base, base top surface is sequentially provided with permanent magnet motor, gear box and centrifugal compressor from left to right, between the output shaft of permanent magnet motor and the input shaft of gear box large diameter and between the output shaft of gear box small diameter and the input shaft of centrifugal compressor are all equipped with transmission connection by mechanical connecting mechanism, compared with by adopting steam turbine driving mode, not only need not to provide power by fuel combustion, avoid heat loss phenomenon, energy -efficient, moreover, will not produce a large amount of waste gas, improve the quality of working environment, low carbon environmental protection, through the speed increasing of gear box, the high speed demand of centrifugal compressor is better matched, effectively shorten the start time of centrifugal compressor, support centrifugal compressor frequent start-stop, cancel the complex steam system of steam turbine, reduce the maintenance frequency and maintenance cost of later period.

Description

Technical Field

[0001] This utility model relates to the field of centrifugal compressor drive equipment technology, and in particular to a permanent magnet motor frequency converter for centrifugal compressors. Background Technology

[0002] Centrifugal compressors are core power equipment in chemical processes such as coal chemical engineering, ammonia synthesis, and methanol production. Traditional drive methods often use steam turbines, which present the following problems: 1. Low energy efficiency: Steam turbines rely on boilers for steam supply, resulting in insufficient overall thermal efficiency, and the steam system also suffers from heat loss; 2. Complex startup: Steam turbines require preheating and have a long acceleration time, making it impossible to quickly respond to process requirements; 3. High maintenance costs: Steam turbines require regular maintenance of components such as seals and blades, and rely on a professional operation and maintenance team; 4. High carbon emissions: Steam supply from coal-fired boilers results in high carbon dioxide emissions, which does not meet the requirements of green production. Existing technologies attempt to replace steam turbines with asynchronous motors, but problems such as narrow speed range, large starting current surge, and low efficiency under low load still exist. Therefore, further improvements to existing technologies are needed. Utility Model Content

[0003] The purpose of this invention is to provide a permanent magnet motor frequency converter for centrifugal compressors to solve the problems mentioned in the background art.

[0004] The present invention adopts the following technical solution: a permanent magnet motor frequency converter for a centrifugal compressor, comprising a base, wherein a permanent magnet motor, a gearbox and a centrifugal compressor are arranged sequentially from left to right on the top surface of the base, and the output shaft of the permanent magnet motor and the large-diameter input shaft of the gearbox and the small-diameter output shaft of the gearbox and the input shaft of the centrifugal compressor are both connected by mechanical connection mechanisms for transmission.

[0005] Optionally, the mechanical connection mechanism includes a coupling, which mechanically connects the output shaft of the permanent magnet motor to the large-diameter input shaft of the gearbox and the small-diameter output shaft of the gearbox to the input shaft of the centrifugal compressor, thereby transmitting rotational force.

[0006] Optionally, a high-voltage frequency converter and an intelligent control unit are also provided on the top surface of the base. The high-voltage frequency converter is electrically connected to the permanent magnet motor and the intelligent control unit.

[0007] Optionally, the mechanical connection mechanism includes a flange, which, through the flange and bolts and nuts, mechanically connects the output shaft of the permanent magnet motor to the large-diameter input shaft of the gearbox and the small-diameter output shaft of the gearbox to the input shaft of the centrifugal compressor, thereby realizing the transmission of rotational force.

[0008] Optionally, the mechanical connection mechanism includes a connector one and a connector two. Connector one is a column with a cylindrical groove one on its left side and a plurality of insertion grooves arranged in an array on the right arc surface of connector one. Connector two is a pipe fitting with an insertion block one adapted to the insertion groove on its left side. Insertion blocks two adapted to the size of the keyway on the output shaft or input shaft are provided on the inner walls of both the cylindrical groove one and connector two.

[0009] Optionally, the mechanical connection mechanism also includes a support assembly disposed below connector one and connector two and a top cover disposed above connector one and connector two. The support assembly and the top cover are connected vertically to provide vertical and horizontal limiting support for the output shaft, connector one, connector two and input shaft, so that the output shaft, connector one, connector two and input shaft can only perform rotational movements.

[0010] Optionally, the support assembly includes a bottom cover, with semi-circular grooves symmetrically formed on the top surface and both sides of the bottom cover. Semi-circular grooves 1 and 2 are connected between the semi-circular grooves 1. The length of the semi-circular grooves 2 is adapted to the length of the connectors 1 and 2 after splicing. Several support legs are detachably provided on the bottom surface of the bottom cover.

[0011] Optionally, limit rings are fitted and fixedly installed on the arc surfaces of connector one and connector two, and a semicircular groove three adapted to the limit rings is opened on the arc surface of semicircular groove two.

[0012] Optionally, rectangular sliding grooves are symmetrically provided on both the left and right sides of the bottom cover. The upper end of the support leg can be inserted into the rectangular sliding groove and fixed to the bottom cover with bolts. The lower end of the support leg is fixed to the base with bolts.

[0013] Optionally, both the top cover and the bottom cover are symmetrically slidably provided with arc-shaped plates on their left and right sides. The cooperation of the arc-shaped plates on the same side of the top cover and the bottom cover forms an extended covering area whose extension range can be controlled.

[0014] Compared with the prior art, this utility model has the following advantages: 1. In this application, a permanent magnet motor, a gearbox, and a centrifugal compressor are arranged sequentially from left to right on the top surface of the base. The output shaft of the permanent magnet motor is connected to the large-diameter input shaft of the gearbox, and the small-diameter output shaft of the gearbox is connected to the input shaft of the centrifugal compressor through mechanical connection mechanisms. The centrifugal compressor is driven by the cooperation of the permanent magnet motor, gearbox, and mechanical connection mechanisms. Compared with the existing technology that uses a steam turbine drive, this method not only eliminates the need for fuel combustion to provide power, avoiding heat loss and improving energy utilization, but also saves energy and does not produce a large amount of waste gas, greatly improving the quality of the working environment. It is low-carbon and environmentally friendly, meeting the requirements of green production. Furthermore, the speed increase of the gearbox can better match the high-speed requirements of the centrifugal compressor, effectively shortening the start-up time of the centrifugal compressor and supporting frequent start-ups and shutdowns. Finally, the complex steam system of the steam turbine is eliminated, which can greatly reduce the frequency and cost of maintenance in the later stage.

[0015] 2. In this application, a high-voltage frequency converter and an intelligent control unit are also provided on the top surface of the base. The high-voltage frequency converter is electrically connected to the permanent magnet motor and the intelligent control unit. Through the setting of the high-voltage frequency converter, it can be used as a soft start and adjustment unit to control the permanent magnet motor, reduce the mechanical shock when the permanent magnet motor starts and stops, and extend the service life of the permanent magnet motor. Through the setting of the intelligent control unit, the temperature, vibration, gearbox oil pressure and other parameters of the permanent magnet motor are monitored in real time to realize interlock protection.

[0016] 3. In this application, the mechanical connection mechanism includes a first connector and a second connector. The first connector is a column with a cylindrical groove on its left side and a plurality of insertion grooves arranged in an array on the right arc surface. The second connector is a pipe with an insertion block adapted to the insertion groove on its left side. The rotational force can be effectively transmitted through the cooperation of the first connector and the second connector with the permanent magnet motor, gearbox and centrifugal compressor.

[0017] 4. In this application, the mechanical connection mechanism also includes a support component disposed below connector one and connector two, and a top cover disposed above connector one and connector two. Through the cooperation of the support component and the top cover, not only can the output shaft, input shaft, connector one, and connector two be limited and supported, reducing the load force borne by the output shaft, connector one, connector two, and input shaft due to their own weight when suspended, but it can also effectively ensure the stability of the output shaft, connector one, connector two, and input shaft when rotating. Finally, it can also cover and protect the output shaft, connector one, connector two, and input shaft, preventing them from being exposed, reducing corrosion and avoiding unnecessary impact damage, and improving service life. At the same time, it can also prevent the rotating output shaft, connector one, connector two, and input shaft from causing injury to workers, improving the safety of the working environment and making it more practical. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall structure in Embodiment 1 of this utility model; Figure 2 This is a schematic diagram of the overall structure in Embodiment 3 of this utility model; Figure 3 This is a schematic diagram of the overall structure of the mechanical connection mechanism during assembly in Embodiment 3 of this utility model; Figure 4 This is a schematic diagram of the assembled structure of connector one and connector two in Embodiment 3 of this utility model; Figure 5 This is an exploded view of connector one and connector two in embodiment three of this utility model. Figure 1 ; Figure 6 This is an exploded view of connector one and connector two in embodiment three of this utility model. Figure 2 ; Figure 7 This is an exploded view of the top cover and support assembly in Embodiment 3 of this utility model. Figure 1 ; Figure 8 This is an exploded view of the top cover and support assembly in Embodiment 3 of this utility model. Figure 2 ; Figure 9 This is an exploded view of the mechanical connection mechanism in Embodiment 3 of this utility model.

[0019] In the diagram: 1. Base; 2. Permanent magnet motor; 3. Gearbox; 4. Centrifugal compressor; 5. Coupling; 6. Connector 1; 7. Connector 2; 8. Columnar groove 1; 9. Insertion groove; 10. Insertion block 1; 11. Insertion block 2; 12. Support assembly; 13. Top cover; 14. Bottom cover; 15. Semicircular groove 1; 16. Semicircular groove 2; 17. Support leg; 18. Limiting ring; 19. Semicircular groove 3; 20. Rectangular slide; 21. Arc plate. Detailed Implementation

[0020] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Those skilled in the art should understand that the embodiments described below are only a part of the embodiments disclosed in this utility model, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this utility model without creative effort are within the scope of protection of this utility model.

[0021] Various non-limiting embodiments of this utility model are described in detail below. Any number of elements in the accompanying drawings is for illustrative purposes only and not for limitation, and any naming is for distinction only and has no limiting meaning.

[0022] The principles and spirit of this utility model will be explained in detail below with reference to several representative embodiments.

[0023] Example 1: Please see Figure 1 The present invention will be described in detail below with reference to the accompanying drawings and embodiments: A permanent magnet motor frequency converter for a centrifugal compressor includes a base 1. A permanent magnet motor 2, a gearbox 3 and a centrifugal compressor 4 are arranged sequentially from left to right on the top surface of the base 1. The output shaft of the permanent magnet motor 2 and the input shaft of the gearbox 3 with a large diameter are connected by mechanical connection mechanisms, and the output shaft of the gearbox 3 with a small diameter is connected to the input shaft of the centrifugal compressor 4.

[0024] The centrifugal compressor 4 is driven by the cooperation of permanent magnet motor 2, gearbox 3 and mechanical connection mechanism. Compared with the existing technology that uses steam turbine drive, it not only eliminates the need for fuel combustion to provide power, avoiding heat loss and improving energy utilization, but also saves energy and does not produce a large amount of waste gas, greatly improving the quality of the working environment. It is low-carbon and environmentally friendly, meeting the requirements of green production. Furthermore, the speed increase of gearbox 3 can better match the high speed requirements of centrifugal compressor 4, effectively shortening the start-up time of centrifugal compressor 4 and supporting frequent start-up and shutdown of centrifugal compressor 4. Finally, by eliminating the complex steam system of steam turbine, the frequency and cost of maintenance in the later stage can be greatly reduced.

[0025] Please see Figure 1 The mechanical connection mechanism includes a coupling 5, which mechanically connects the output shaft of the permanent magnet motor 2 to the large-diameter input shaft of the gearbox 3 and the small-diameter output shaft of the gearbox 3 to the input shaft of the centrifugal compressor 4, thereby achieving effective and stable transmission of rotational force.

[0026] The permanent magnet motor 2 adopts neodymium iron boron magnets and a fully enclosed air-cooled or air-water-cooled structure, which is suitable for high-temperature, explosion-proof, and dusty environments in chemical sites.

[0027] The top surface of base 1 is also equipped with a high-voltage frequency converter and an intelligent control unit. The high-voltage frequency converter is electrically connected to the permanent magnet motor 2 and the intelligent control unit. Through the high-voltage frequency converter, it can act as a soft starter and adjustment unit to control the permanent magnet motor 2, reducing the mechanical shock experienced by the permanent magnet motor 2 during start-up and shutdown, and extending the service life of the permanent magnet motor 2. Through the intelligent control unit, parameters such as the temperature, vibration, and oil pressure of the gearbox 3 of the permanent magnet motor 2 are monitored in real time, achieving interlocking protection. The high-voltage frequency converter and intelligent control unit in the above structure are existing technologies and will not be described in detail here.

[0028] Safety redundancy design: The gearbox 3 is equipped with multi-point temperature sensors and vibration probes that communicate with the intelligent control unit. When the limit is exceeded, the PLC is triggered to reduce the load and stop the machine. The windings of the permanent magnet motor 2 are embedded with PTC thermistors that communicate with the intelligent control unit to monitor the insulation status in real time and prevent the risk of demagnetization.

[0029] Example 2: The mechanical connection mechanism includes a flange (not shown), which, through the flange and bolts and nuts, mechanically connects the output shaft of the permanent magnet motor 2 to the large-diameter input shaft of the gearbox 3 and the small-diameter output shaft of the gearbox 3 to the input shaft of the centrifugal compressor 4, thereby achieving effective and stable transmission of rotational force.

[0030] Example 3: The difference from Embodiment 1 or Embodiment 2 is that, please refer to Figure 2-6 The mechanical connection mechanism includes a first connector 6 and a second connector 7. The first connector 6 is a column and has a cylindrical groove 8 on its left side. The diameter of the cylindrical groove 8 is adapted to the diameter of the output shaft of the permanent magnet motor 2 or the gearbox 3. Several insertion grooves 9 are arrayed on the right arc surface of the first connector 6. The second connector 7 is a pipe. An insertion block 10 adapted to the insertion groove 9 is provided on the left side of the second connector 7. The inner diameter of the second connector 7 is adapted to the diameter of the input shaft of the gearbox 3 or the centrifugal compressor 4. Insertion blocks 21 adapted to the size of the keyway on the output shaft or the input shaft are provided on the inner walls of both the cylindrical groove 8 and the second connector 7.

[0031] When installing connector 6 and connector 7, first insert the cylindrical groove 8 of connector 6 into the output shaft of permanent magnet motor 2 or gearbox 3 and insert the plug block 11 into the keyway. Then, fit connector 7 onto the input shaft of gearbox 3 or centrifugal compressor 4 and insert the plug block 11 into the corresponding keyway. Then, adjust the horizontal position of permanent magnet motor 2 and centrifugal compressor 4 (i.e., move closer to gearbox 3) so that the plug block 10 on connector 7 is inserted into the plug groove 9 on connector 6, so that the plug block 10 is in full contact with the plug groove 9. Then, fix permanent magnet motor 2, gearbox 3 and centrifugal compressor 4 on base 1 with bolts to complete the transmission connection between shafts.

[0032] Please see Figure 3 , 7 -9. The mechanical connection mechanism also includes a support assembly 12 disposed below the first connector 6 and the second connector 7 and a top cover 13 disposed above the first connector 6 and the second connector 7. The support assembly 12 and the top cover 13 are connected vertically to provide vertical and horizontal limiting support for the output shaft, the first connector 6, the second connector 7 and the input shaft, so that the output shaft, the first connector 6, the second connector 7 and the input shaft can only rotate.

[0033] The cooperation of the support assembly 12 and the top cover 13 not only provides limiting support for the output shaft, input shaft, connector 6, and connector 7, reducing the load force borne by the output shaft, connector 6, connector 7, and input shaft due to their own weight when suspended, thus improving their service life, but also effectively ensures the stability of the output shaft, connector 6, connector 7, and input shaft during rotation. Finally, it also covers and protects the output shaft, connector 6, connector 7, and input shaft, preventing them from being exposed, reducing corrosion, and avoiding unnecessary impact damage, thus improving their service life. At the same time, it also prevents the rotating output shaft, connector 6, connector 7, and input shaft from causing injury to workers, improving the safety of the working environment and making it more practical.

[0034] Please see Figure 7-8 The support component 12 includes a bottom cover 14. Semicircular grooves 15 are symmetrically provided on the top surface of the bottom cover 14 and the ground sides of the top cover 13. Semicircular grooves 16 are provided between the semicircular grooves 15. The length of the semicircular grooves 16 is adapted to the length of the connectors 6 and 7 after splicing. Several support legs 17 are detachably provided on the bottom surface of the bottom cover 14.

[0035] When the top cover 13 and the bottom cover 14 are overlapped and fixed by bolts and nuts, the semi-circular groove 15 on the top cover 13 and the bottom cover 14 overlap to form a cylindrical groove 2 that is adapted to the size of the output shaft or the input shaft. The semi-circular groove 2 16 forms a cylindrical groove 3 that is adapted to the overall shape and size of the spliced ​​connector 1 6 and connector 2 7.

[0036] Please see Figure 4-8 To further enhance the stability of the output shaft, connector 6, connector 7 and input shaft during rotation, limit rings 18 are fitted and fixedly installed on the arc surfaces of connector 6 and connector 7, and semicircular grooves 19 adapted to limit rings 18 are opened on the arc surface of semicircular groove 16.

[0037] Please see Figure 3 , 7 -9. Rectangular grooves 20 are symmetrically provided on both the left and right sides of the bottom cover 14. The upper end of the support leg 17 can be inserted into the rectangular groove 20 and fixed to the bottom cover 14 by bolts. The lower end of the support leg 17 can be fixed to the base 1 by bolts.

[0038] Please see Figure 2 , 7 -9. In order to further completely cover the rotating part, arc-shaped plates 21 are symmetrically slidably arranged on the left and right sides of the top cover 13 and the bottom cover 14. The cooperation of the arc-shaped plates 21 on the same side of the top cover 13 and the bottom cover 14 forms an extended covering area with controllable extension range, which can meet the needs of different axis lengths.

[0039] During use, firstly, the gearbox 3 is fixed to the center of the top surface of the base 1 with bolts. Then, the output shaft of the permanent magnet motor 2 is aligned with the large-diameter input shaft of the gearbox 3 with a gap. Next, the cylindrical groove 8 of the connector 6 is fitted onto the output shaft of the permanent magnet motor 2, and the insertion block 11 in the cylindrical groove 8 is inserted into the keyway of the output shaft. Then, the connector 7 is fitted onto the input shaft of the gearbox 3, and the insertion block 11 on the connector 7 is inserted into the keyway on the input shaft of the gearbox 3. Then, the permanent magnet motor 2 is moved horizontally so that the insertion block 10 of the connector 7 is fully inserted into the insertion groove 9 of the connector 6. Finally, the permanent magnet motor 2 is stabilized on the base 1 with bolts.

[0040] After connector 16 and connector 27 are connected, the bottom cover 14 is then fitted to the lower arc surfaces of connector 16 and connector 27 from bottom to top, ensuring that the assembled connector 16 and connector 27 are entirely within the semi-circular groove 26. At this time, the limiting rings 18 on connector 16 and connector 27 are within the semi-circular groove 39. Then, the support leg 17 is inserted into the rectangular sliding groove 20 on the bottom cover 14 and fixed with bolts. After the bottom cover 14 is installed, the top cover 13 is then placed on top of the bottom cover 14 from top to bottom and fixed to the bottom cover 14 and top cover 13 with bolts and nuts. At this point, the cylindrical groove 2 formed by the overlap of the upper and lower semi-circular grooves 15 supplies power to the permanent magnet motor 2. The output shaft and the input shaft of gearbox 3 are closely fitted and supported. The cylindrical groove formed by the overlapping of the upper and lower semi-circular grooves 16 provides overall limiting support for the spliced ​​connectors 6 and 7. This not only provides sufficient support for the rotation of the input shaft, connectors 6 and 7, reducing the load force borne by their own weight, but also improves the stability of the input shaft, connectors 6 and 7 during rotation. Furthermore, it prevents the input shaft, connectors 6 and 7 from being exposed to air, reducing corrosion and impact, and also prevents rotating parts from causing injury to workers, improving safety and extending the service life of the device.

[0041] When the bottom cover 14 and the top cover 13 are insufficient to completely cover all the rotating objects, pull the arc plates 21 on both sides of the top cover 13 and the bottom cover 14 until the corresponding arc plates 21 completely cover the rotating objects on the same side, thus completing the installation process of the permanent magnet motor 2 and the gearbox 3.

[0042] After the permanent magnet motor 2 is installed, the centrifugal compressor 4 can be installed by following the same steps as installing the permanent magnet motor 2.

[0043] Based on the above description in this specification, those skilled in the art will also understand that the following terms, such as "upper," "lower," "front," "rear," "left," "right," "inner," and "outer," which indicate orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings of this specification. They are only for the purpose of facilitating the explanation of the present invention and simplifying the description, and do not explicitly or implicitly suggest that the device or element involved must have the specific orientation, or be constructed and operated in a specific orientation. Therefore, the above-mentioned orientation or positional relationship terms should not be understood or interpreted as a limitation on the present invention.

[0044] In addition, in the description of this specification, "multiple" means at least two, such as two, three or more, etc., unless otherwise expressly and specifically defined.

Claims

1. A variable frequency drive for a permanent magnet motor used in a centrifugal compressor, characterized in that: The device includes a base, on which a permanent magnet motor, a gearbox, and a centrifugal compressor are arranged sequentially from left to right on the top surface. The output shaft of the permanent magnet motor is connected to the large-diameter input shaft of the gearbox, and the small-diameter output shaft of the gearbox is connected to the input shaft of the centrifugal compressor through mechanical connection mechanisms.

2. The permanent magnet motor frequency converter for a centrifugal compressor according to claim 1, characterized in that: The mechanical connection mechanism includes a coupling, which mechanically connects the output shaft of the permanent magnet motor to the large-diameter input shaft of the gearbox and the small-diameter output shaft of the gearbox to the input shaft of the centrifugal compressor, thereby transmitting rotational force.

3. The permanent magnet motor frequency converter for a centrifugal compressor according to claim 1, characterized in that: The top surface of the base is also equipped with a high-voltage frequency converter and an intelligent control unit, which are electrically connected to the permanent magnet motor and the intelligent control unit.

4. The permanent magnet motor frequency converter for a centrifugal compressor according to claim 1, characterized in that: The mechanical connection mechanism includes a flange, which, along with bolts and nuts, mechanically connects the output shaft of the permanent magnet motor to the large-diameter input shaft of the gearbox and the small-diameter output shaft of the gearbox to the input shaft of the centrifugal compressor, thereby transmitting rotational force.

5. The permanent magnet motor frequency converter for a centrifugal compressor according to claim 1, characterized in that: The mechanical connection mechanism includes connector one and connector two. Connector one is a column with a cylindrical groove on its left side and several insertion grooves arranged in an array on the right arc surface. Connector two is a pipe with an insertion block one adapted to the insertion groove on its left side. Insertion blocks two adapted to the size of the keyway on the output shaft or input shaft are provided on the inner walls of both the cylindrical groove one and connector two.

6. The permanent magnet motor frequency converter for a centrifugal compressor according to claim 5, characterized in that: The mechanical connection mechanism also includes a support assembly located below connector one and connector two, and a top cover located above connector one and connector two. The support assembly and the top cover are connected vertically to provide vertical and horizontal limiting support for the output shaft, connector one, connector two, and input shaft, so that the output shaft, connector one, connector two, and input shaft can only rotate.

7. The permanent magnet motor frequency converter for a centrifugal compressor according to claim 6, characterized in that: The support assembly includes a bottom cover, with semi-circular grooves symmetrically formed on the top surface of the bottom cover and both sides of the bottom surface. Semi-circular grooves 1 and 2 are connected between the semi-circular grooves 1 and 2. The length of the semi-circular grooves 2 is adapted to the length of the connector 1 and connector 2 after splicing. Several support legs are detachably provided on the bottom surface of the bottom cover.

8. The permanent magnet motor frequency converter for a centrifugal compressor according to claim 6, characterized in that: Both connector one and connector two are fitted with and fixed with limit rings on their arc surfaces, and semicircular groove three is opened on the arc surface of semicircular groove two to match the limit rings.

9. The permanent magnet motor frequency converter for a centrifugal compressor according to claim 7, characterized in that: The bottom cover has symmetrical rectangular sliding grooves on both the left and right sides. The upper end of the support leg can be inserted into the rectangular sliding groove and fixed to the bottom cover with bolts. The lower end of the support leg is fixed to the base with bolts.

10. The permanent magnet motor frequency converter for a centrifugal compressor according to claim 7, characterized in that: The top and bottom covers are symmetrically slidably equipped with arc-shaped plates on both sides. The cooperation of the arc-shaped plates on the same side of the top and bottom covers forms an extended covering area whose extension range can be controlled.

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