Active thermal management system power transformer

Abstract

The utility model relates to a power transformer of an active thermal management system. The power transformer comprises a transformer body, a cooling and dehumidifying device, a driving device, a wind pressure adjusting device and a heat exchanger. The cooling and dehumidifying device comprises a shell, a cooling cavity, a snakelike cooling pipe, a centrifugal impeller, a water collector and a cold air chamber. Air is cooled through the snakelike cooling pipe, then air humidity is reduced through combined separation of the centrifugal impeller and the water collector, and then linkage adjustment is carried out with the heat exchanger.

Description

Technical Field

[0001] The utility model relates to the technical field of transformers, in particular to a power transformer with an active thermal management system. Background Art

[0002] Transformers, a common type of electrical equipment, are categorized into two types: oil-immersed and dry-type. Dry-type transformers are widely used in the power sector due to their advantages, such as fewer accessories, no oil conservator, no safety airway, no seals, and light weight and compact size.

[0003] Dry-type transformers typically dissipate heat naturally through their metal casings, or employ internal fans for auxiliary cooling. However, because the cooling system cannot be actively adjusted, the transformer's body temperature can rise or fall significantly when the load increases or decreases.

[0004] There is an urgent need on the market for a transformer whose heat dissipation device can actively adjust to achieve no obvious temperature change when the transformer load increases or decreases. Utility Model Content

[0005] The purpose of the present invention is to provide a power transformer with an active thermal management system to solve the above-mentioned defects caused by the prior art.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] An active thermal management system power transformer includes a transformer body, a cooling and dehumidifying device, a driving device, an air pressure regulating device and a heat exchanger;

[0008] The cooling and dehumidifying device is arranged on the outside of the transformer body. The cooling and dehumidifying device and the transformer body are connected through an air duct, and the air pressure regulating device is arranged on the air duct.

[0009] The cooling and dehumidification device includes a shell, a cooling chamber, a serpentine cooling pipe and a cold air chamber. The serpentine cooler is arranged in the cooling chamber and is connected to the heat exchanger through an oil pipe. The cold air chamber is arranged below the cooling chamber. The cold air chamber and the cooling chamber are separated by a horizontal partition with a circular hole in the middle of the partition.

[0010] The driving device is vertically arranged in the middle of the cooling chamber, including a variable frequency motor, an oil pump and a vertical shaft. One end passes through the oil pump upward and is connected to the output shaft of the variable frequency motor. The variable frequency motor is electrically connected to the temperature control unit. The oil pump, serpentine cooling tube and heat exchanger are connected in sequence through oil pipes. A flow regulating valve is set at the cooling medium inlet end of the heat exchanger, and the flow regulating valve actuator is connected to the oil pump through an oil pipe.

[0011] Preferably, the centrifugal impeller and the water collector are horizontally arranged in the middle of the cold air chamber, the water collector is arranged below the centrifugal impeller, and one end of the vertical shaft is fixedly connected to the center of the centrifugal impeller.

[0012] Preferably, the centrifugal impeller is composed of an upper clamping plate, a lower clamping plate and arc-shaped partition blades. A cam is fixedly provided at the center of the upper surface of the upper clamping plate, and the outer surface of the cam is rotatably sealed with the circular hole in the center of the partition plate.

[0013] Preferably, the wind pressure regulating device is arranged in the air duct of the cooling and dehumidifying device and the air outlet of the transformer. The wind pressure regulating device on the air duct is two safety valves. When the wind pressure in the transformer body is higher than the set value, the two safety valves automatically open to release air. The wind pressure regulating device at the transformer air outlet is two safety valves. When the air pressure is lower than the set value, the two safety valves automatically open to replenish air.

[0014] Preferably, the water collector is funnel-shaped, and the funnel mouth of the water collector evenly surrounds the upper and lower plates of the centrifugal impeller. The diameter of the funnel mouth of the water collector is smaller than the diameter of the cold air chamber, which can meet the passage of cooling air. The edge of the funnel mouth of the water collector is higher than the upper plate of the centrifugal impeller and bends inward, maintaining a certain distance between it and the partition, which can meet the passage of cooling air.

[0015] Preferably, the upper clamping plate and the lower clamping plate of the centrifugal impeller are both flat thin plates, and the outer diameter of the upper clamping plate is smaller than that of the lower clamping plate.

[0016] Compared with the existing technology, this active thermal management system power transformer has the following beneficial effects:

[0017] (1) The utility model adopts a serpentine cooling pipe to cool the air first, and then combines with a centrifugal impeller and a water accumulator to separate the moisture in the cooled air, thereby reducing the humidity of the air and reducing the erosion and corrosion of the insulation equipment in the transformer body, thereby achieving the beneficial effects of significantly improving the heat dissipation efficiency of the transformer, extending the life of the transformer and improving the safety of the transformer.

[0018] (2) The utility model adopts a drive device vertically arranged in the middle of the cooling chamber, including a variable frequency motor, an oil pump and a vertical shaft. One end of the vertical shaft is fixedly connected to the center of the centrifugal impeller, and the other end passes through the oil pump upwards and is connected to the output shaft of the variable frequency motor. The variable frequency motor is electrically connected to the temperature control unit. The oil pump, the serpentine cooling pipe and the heat exchanger are connected in sequence through the oil pipe. A flow control valve is set at the cooling medium inlet end of the heat exchanger. The flow control valve actuator is connected to the oil pump through the oil pipe. The oil pressure change of the oil pump controls the opening of the cooling medium inlet flow control valve of the heat exchanger. Active linkage adjustment achieves the beneficial effects of high adjustment sensitivity, fast adjustment speed and small temperature change of the transformer body. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present invention, and together with the description, serve to explain the principles of the present invention.

[0020] Figure 1 This is a cross-sectional view of a power transformer with an active thermal management system according to the present utility model;

[0021] Figure 2 This utility model is an active thermal management system power transformer Figure 1 Middle AA section;

[0022] Figure 3 This is a schematic diagram of the three-dimensional structure of a centrifugal impeller of a power transformer in an active thermal management system of the utility model.

[0023] In the figure, 1. Transformer body; 101. Transformer low-voltage winding; 102. Transformer high-voltage winding; 103. Transformer air outlet; 2. Frequency conversion motor; 201. Fixed bracket; 3. Oil pump; 301. Oil pipe; 4. Cooling and dehumidifying device; 401. Housing; 402. Cooling chamber; 403. Snake cooling pipe; 404. Water collector; 405. Cold air chamber; 406. Air duct; 407. Air inlet; 408. Plate; 409, centrifugal impeller; 4091, upper splint; 4092, lower splint; 4093, arc-shaped partition blade; 4094, cam; 4100, drain pipe; 4101, drain valve; 5, vertical shaft; 6, air duct; 7, heat exchanger; 8, air pressure regulating device; 801, safety valve; 9, H-shaped fixing bracket; 10, temperature control unit; 11, inlet flow regulating valve; 12, temperature sensor; 13, driving device. DETAILED DESCRIPTION

[0024] Among them, the drawings are only used for illustrative purposes and represent only schematic diagrams rather than actual pictures, and should not be understood as limiting this patent; in order to better illustrate the embodiments of the utility model, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0025] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "inside", "outside" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting this patent. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0026] In the description of this utility model, unless otherwise expressly specified or limited, when the term "connection" or the like appears to indicate a connection relationship between components, such term should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be internal communication between two components or an interaction relationship between two components. For those skilled in the art, the specific meanings of the above terms in this utility model can be understood according to the specific circumstances.

[0027] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more precise definition of the protection scope of the present invention.

[0028] Example 1

[0029] like Figure 1-Figure 3 As shown, an active thermal management system power transformer includes a transformer body 1, a cooling and dehumidification device 4, a driving device 13, an air pressure regulating device 8, and a heat exchanger 7.

[0030] The cooling and dehumidifying device 4 is arranged outside the transformer body 1 . The cooling and dehumidifying device 4 and the transformer body 1 are connected via an air duct 6 . The air pressure regulating device 8 is arranged on the air duct 6 .

[0031] The drive unit 13 consists of a variable frequency motor 2, an oil pump 3, a vertical shaft 5, an oil pipe 301, and a temperature control unit 10. One end of the vertical shaft 5 is rotatably connected to the housing 401 of the cooling and dehumidifying device 4. The vertical shaft 5 then extends upward through the shaft of the oil pump 3 and is connected to the output shaft of the variable frequency motor 2. The vertical shaft 5 is the shaft of the oil pump 3. The variable frequency motor 2 is fixedly connected to the housing 401 of the cooling and dehumidifying device 4 via a fixing bracket 201. The variable frequency motor 2 is electrically connected to the temperature control unit 10.

[0032] The transformer's low-voltage winding 101 draws high line current. Before active thermal management is implemented, the temperature of the transformer's low-voltage winding 101 is typically higher than that of the transformer's high-voltage winding 102. To achieve faster and more uniform cooling, this utility model employs a cooling and dehumidifying device 4 fixed to the outer wall of the transformer's low-voltage winding 101 via an H-shaped mounting bracket 9. The outer casing 401 is an elongated cube. The interior of the transformer body 1 communicates with the cold air chamber 405 within the cooling and dehumidifying device 4 via a circular air duct 406, which can be square or other shaped.

[0033] The cooling and dehumidifying device 4 includes a housing 401, a cooling chamber 402, a serpentine cooling pipe 403, and a cold air chamber 405. The cooling chamber 402 is located in the cooling and dehumidifying device 4 and is arranged at the upper part of the cooling and dehumidifying device 4. It is a vertical cylindrical chamber. The serpentine cooling pipe 403 is arranged in the cooling chamber 402 and is connected to the heat exchanger 7 through the oil pipe 301. The cold air chamber 405 is a vertical cylindrical chamber. The cold air chamber 405 is arranged below the cooling chamber 402. The cold air chamber 405 is separated from the cooling chamber 402 by a horizontal partition 408, and a circular hole is provided in the middle of the partition 408. A drain pipe 4100 is provided on the lower bottom plate of the cold air chamber 405, and a drain valve 4101 is provided on the drain pipe 4100.

[0034] The air inlet 407 of the cooling chamber 402 is connected to the transformer air outlet 103 through an air duct. The air inlet 407 is set on the top side of the cooling and dehumidification device 4, and there are four of them, which are symmetrically distributed around the circumference. It can also be set to two or more symmetrically distributed. The cooling chamber 402 and the cold air chamber 405 are separated by a horizontal partition 408, and a circular hole is set in the middle of the partition 408. The serpentine cooling pipe 403 is horizontally set in the cooling chamber 402, and can be set to multiple layers or a single layer. The oil inlet of the serpentine cooling pipe 403 is connected to the outlet of the oil pump 3 through the oil pipe 301, and the oil outlet is connected to the oil inlet of the heat exchanger 7 through the oil pipe 301.

[0035] An air pressure regulating device 8 is provided in the air duct 406 of the cooling and dehumidifying device 4 and the transformer air outlet 103. The air pressure regulating device 8 on the air duct 406 is composed of two safety valves 801. When the air pressure in the transformer body 1 is higher than the set value, the two safety valves 801 automatically open to release air. The air pressure regulating device 8 at the transformer air outlet 103 is composed of two safety valves 8. When the air pressure is lower than the set value, the two safety valves 801 automatically open to replenish air.

[0036] The oil outlet port of the serpentine cooling tube 403 is connected to the oil inlet port of the heat exchanger 7 via the oil pipe 301, and the oil inlet port of the serpentine cooling tube 403 is connected to the oil output port of the oil pump 3 via the oil pipe 301. The oil output port of the heat exchanger 7 is connected to the oil inlet port of the oil pump 3 via the oil pipe 301. A flow control valve 11 is provided at the cooling medium inlet end of the heat exchanger 7. The opening of the cooling medium inlet flow control valve 11 of the heat exchanger 7 is controlled by the output oil pressure of the oil pump 3. The greater the oil pressure, the greater the opening. When the accumulated water in the water collector 404 reaches the set height, the drain valve 4101 will automatically open to drain the water, and the dust in the air will also be mixed with the accumulated water and discharged from the water collector 404.

[0037] Example 2

[0038] An internal fan is used to assist in heat dissipation. In summer, the air temperature is high and the humidity is high, and the dust in the air continuously washes the surface of the insulation equipment inside the transformer body, causing accelerated corrosion and aging of the surface of the insulation equipment inside the transformer body, affecting the life of the transformer and the safety of the insulation equipment.

[0039] like Figure 1-Figure 3 As shown, the cooling and dehumidifying device 4 includes a housing 401, a cooling chamber 402, a serpentine cooling pipe 403, a centrifugal impeller 409, a water collector 404, and a cold air chamber 405. The serpentine cooling pipe 403 is disposed within the cooling chamber 402 and is connected to the heat exchanger 7 via an oil pipe 301. The cold air chamber 405 is disposed below the cooling chamber 402 and is separated from the cooling chamber 402 by a horizontal partition 408 having a circular hole in the middle. The centrifugal impeller 409 and the water collector 404 are disposed horizontally in the middle of the cold air chamber 405; the water collector 404 is disposed below the centrifugal impeller 409. The driving device 13 is vertically disposed in the middle of the cooling chamber 402, with one end of the driving device 13 fixedly connected to the center of the centrifugal impeller 409.

[0040] The centrifugal impeller 409 is horizontally arranged below the serpentine cooling tube 403. The centrifugal impeller 409 is composed of an upper clamping plate 4091, a lower clamping plate 4092, and an arc-shaped partition blade 4093. The upper clamping plate 4091 and the lower clamping plate 4092 are both flat thin plates. The outer diameter of the upper clamping plate 4091 is smaller than the outer diameter of the lower clamping plate 4092. A cam 4094 is provided on the upper surface of the upper clamping plate 4091. The cam 4094 is concentric with the circular hole in the center of the upper clamping plate 4091 and the partition 408. The outer surface of the cam 4094 is rotatably sealed with the circular hole in the center of the partition 408. The upper surface of the lower clamping plate 4092 and the inner circle of the cam 4094 form a vertical cylindrical cavity. A plurality of arc-shaped partition blades 4093 are vertically clamped between the parallel upper clamping plate 4091 and the lower clamping plate 4092, and are fixedly connected to the upper clamping plate 4091 and the lower clamping plate 4092. One end of the arcuate partition blade 4093 is inscribed inwardly with the outer circumference of the lower clamping plate 4092, and the other end is circumscribed to the inner circumference of the upper clamping plate 4091. An arcuate closed air passage is formed between the upper clamping plate 4091, the lower clamping plate 4092, and two adjacent arcuate partition blades 4093, with the width of the partition blades as the height. This arcuate air passage gradually widens from the inside of the centrifugal impeller 409 to the outside.

[0041] One end of the vertical shaft 5 is fixedly connected to the center of the lower clamping plate 4092 of the centrifugal impeller 409. The vertical shaft 5 extends vertically upward through the center of the hole in the upper clamping plate 4091 of the centrifugal impeller 409, upward through the center of the cam 4094, through the serpentine cooling tube 403, and vertically through the housing 401 of the cooling and dehumidifying device 4. The vertical shaft 5 is rotatably connected to the housing 401 of the cooling and dehumidifying device 4. The vertical shaft 5 then extends upward through the oil pump 3. The other end of the vertical shaft 5 is drivingly connected to the output shaft of the frequency conversion motor 2. The vertical shaft 5 serves as the shaft of the oil pump 3. The frequency conversion motor 2 is fixedly connected to the housing 401 of the cooling and dehumidifying device 4 via a fixed bracket 201.

[0042] The water collector 404 is funnel-shaped and is vertically arranged in the middle of the cold air chamber 405. The funnel mouth of the water collector 404 evenly surrounds the upper splint 4091 and the lower splint 4092 of the centrifugal impeller 409. The diameter of the funnel mouth of the water collector 404 is smaller than the diameter of the cold air chamber 405, which can meet the passage of cooling air. The edge of the funnel mouth of the water collector 404 is higher than the upper splint 4091 of the centrifugal impeller 409 and bends inward, maintaining a certain distance between it and the partition 408, which can meet the passage of cooling air.

[0043] Working principle:

[0044] After the frequency conversion motor 2 is started, it drives the vertical shaft 5 to rotate at the output end. The rotation of the vertical shaft 5 drives the oil pump 3 to work, and the cooling oil in the serpentine cooling tube 403 gradually begins to circulate and accelerate. The rotation of the vertical shaft 5 also drives the centrifugal impeller 409 to rotate. The centrifugal impeller 409 rotates, and the air in the arc-shaped air channel is squeezed and thrown out of the centrifugal impeller 409. A low-pressure area of air is formed in the cylindrical cavity area surrounded by the lower clamping plate 4092 and the cam 4094, forming a suction force to draw the air from the air inlet 407 into the cooling chamber 402. After the air enters the cooling chamber 402, it passes through the relatively low-temperature serpentine cooling tube 403, where heat is absorbed and the temperature is lowered. The water saturation in the cooled air decreases, and the air precipitates and aggregates with dust in the air to form small droplets. These are sucked in by the rotating centrifugal impeller 409 and then ejected at an accelerated rate. They collide with the inner wall of the funnel opening of the water collector 404 and are then turned back. Because the inertia of the water droplets is much greater than that of the air, most of the water droplets remain on the inner wall of the water collector 404 and, under the influence of gravity, flow downward along the inner wall of the funnel of the water collector 404 into the water collector 404. The turned-back air flows along the upper edge of the funnel of the water collector 404 back to the outer wall of the cam 4094, then turns back again, passes through the gap between the edge of the funnel opening of the water collector 404 and the partition 408, and then passes through the gap between the funnel opening of the water collector 404 and the inner wall of the cold air chamber 405, entering the cold air chamber 405. The cold air entering the cold air chamber 405 passes through the air duct 406 and then enters the transformer body 1 to participate in cooling the transformer body 1. The air passing through this process not only has its temperature significantly reduced, but also has moisture and dust in the air removed.

[0045] When the transformer's load increases, the heat generated within the transformer increases. At this point, the temperature sensor 12 within the transformer transmits the measured temperature value to the temperature control unit 10. The temperature control unit 10 actively adjusts the speed of the variable-frequency motor 2 based on the magnitude of the transformer's temperature increase, increasing the speed of the variable-frequency motor 2. The increased speed of the variable-frequency motor 2 drives the speed of the oil pump 3, which in turn increases the cooling oil circulation rate in the serpentine cooling tube 403. This increased speed also increases the cooling oil pressure, which actively increases the opening of the coolant inlet flow control valve 11 of the heat exchanger 7. This increases the flow rate of the cooling medium, allowing the cooling oil to be removed more quickly per unit time. Consequently, the cooling oil in the serpentine cooling tube 403 removes more heat from the cooling air per unit time. The increased speed of the variable-frequency motor 2 also increases the flow rate of the cooling air per unit time. This increased flow rate also removes more heat from the transformer body 1, effectively regulating the internal temperature of the transformer body 1. When the transformer's load decreases, the heat generated within the transformer body 1 decreases, causing the temperature within the transformer body 1 to drop rapidly. At this time, the temperature sensor 12 installed in the transformer body 1 transmits the measured temperature value to the temperature control unit 10. The temperature control unit 10 compares the temperature transmitted by the temperature sensor 12 with the set temperature. After calculation, the temperature control unit 10 actively adjusts the speed of the variable frequency motor 2 according to the extent of the transformer temperature reduction, reducing the speed of the variable frequency motor 2. The reduced speed of the variable frequency motor 2 will drive the speed of the oil pump 3 to decrease. The reduced speed of the oil pump 3 will reduce the oil circulation rate of the cooling oil system of the serpentine cooling pipe 403. The reduced speed of the oil pump 3 will also reduce the pressure of the cooling oil. The reduced cooling oil pressure will actively close the opening of the cooling medium inlet flow control valve 11 of the heat exchanger 7. The reduced cooling medium flow rate will reduce the amount of heat removed by the cooling oil per unit time. The cooling oil in the serpentine cooling pipe 403 will also remove less heat from the cooling air per unit time, and the temperature of the cooling air will tend to increase. The slower speed of the variable frequency motor 2 will also reduce the flow rate of the cooling air per unit time. The reduced cooling air flow will also remove less heat from the transformer body 1, achieving the effect of rapid and active regulation of the internal temperature of the transformer.

[0046] Those skilled in the art will readily envision other embodiments of the present invention after considering the specification and practicing the disclosure herein. This application is intended to cover any variations, uses, or adaptations of the present invention that follow the general principles of the present invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered as exemplary only, with the true scope and spirit of the present invention being indicated by the claims.

[0047] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from the scope thereof. The scope of the present invention is limited only by the appended claims.

Claims

1. An active thermal management system power transformer, characterized in that: It includes transformer body, cooling and dehumidifying device, driving device, wind pressure regulating device and heat exchanger; The cooling and dehumidifying device is arranged outside the transformer body, the cooling and dehumidifying device and the transformer body are connected through an air duct, and the air pressure regulating device is arranged on the air duct; The cooling and dehumidifying device includes a shell, a cooling chamber, a serpentine cooling pipe, and a cold air chamber. The serpentine cooler is arranged in the cooling chamber and is connected to the heat exchanger through an oil pipe. The cold air chamber is arranged below the cooling chamber. The cold air chamber and the cooling chamber are separated by a horizontal partition, and a circular hole is provided in the middle of the partition. The driving device is vertically arranged in the middle of the cooling chamber, and includes a variable frequency motor, an oil pump, and a vertical shaft. One end of the driving device upwardly penetrates the oil pump and is connected to the output shaft of the variable frequency motor. The variable frequency motor is electrically connected to the temperature control unit. The oil pump, the serpentine cooling tube, and the heat exchanger are connected in sequence through an oil pipe. A flow regulating valve is provided at the cooling medium inlet end of the heat exchanger. The flow regulating valve actuator is connected to the oil pump through an oil pipe. The centrifugal impeller and the water collector are horizontally arranged in the middle of the cold air chamber, the water collector is arranged below the centrifugal impeller, and one end of the vertical shaft is fixedly connected to the center of the centrifugal impeller.

2. The active thermal management system power transformer according to claim 1, characterized in that: The centrifugal impeller is composed of an upper clamping plate, a lower clamping plate, and arc-shaped partition blades. A cam is fixedly arranged at the center of the upper surface of the upper clamping plate, and the outer surface of the cam is rotatably sealed with the circular hole in the center of the partition plate.

3. The active thermal management system power transformer according to claim 1, characterized in that: The wind pressure regulating device is arranged in the air duct of the cooling and dehumidifying device and the transformer air outlet. The wind pressure regulating device on the air duct is composed of two safety valves. When the wind pressure in the transformer body is higher than the set value, the two safety valves automatically open to release air. The wind pressure regulating device on the transformer air outlet is composed of two safety valves. When the air pressure is lower than the set value, the two safety valves automatically open to replenish air.

4. The active thermal management system power transformer according to claim 1, characterized in that: The water collector is funnel-shaped and uniformly surrounds the upper and lower plates of the centrifugal impeller. The diameter of the water collector is smaller than the diameter of the cold air chamber, which can meet the passage of cooling air. The edge of the funnel mouth of the water collector is higher than the upper plate of the centrifugal impeller and bends inward, maintaining a certain distance between it and the partition, which can meet the passage of cooling air.

5. The active thermal management system power transformer according to claim 1, characterized in that: The upper clamping plate and the lower clamping plate of the centrifugal impeller are both flat thin plates, and the outer diameter of the upper clamping plate is smaller than that of the lower clamping plate.

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