Variable-frequency screw air compressor lubrication control system and control method

Abstract

The invention discloses a lubricating control system and method for a variable-frequency screw air compressor. The system comprises a compressor, an oil-gas separation barrel, an oil temperature control valve, an electric proportioning valve, a manual bypass valve and a sensor group, wherein the electric proportioning valve and the manual bypass valve are connected in parallel. The controller is configured to adjust the opening degree of the electric proportioning valve through the first PID control based on the real-time difference value between the exhaust temperature and the cooled oil temperature, so that the actual exhaust temperature rise is equal to a set value; when the output frequency of the frequency converter is lower than the critical threshold value, second PID control based on the downstream oil pressure is switched to, and the downstream oil pressure is maintained to be not lower than the lower limit set value; the critical frequency is the sum of the output frequency of the frequency converter and the offset when the exhaust temperature rise cannot reach a set value. The variable-frequency driving technology and intelligent lubrication control are combined, collaborative regulation and control of oil pressure, oil temperature and exhaust temperature are achieved through sensor data fusion, the problem that the lubrication efficiency of a traditional system is insufficient under variable working conditions is effectively solved, and the energy efficiency performance is remarkably improved while equipment safety is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of compressed air, and in particular relates to a lubrication control system and a control method for a variable frequency screw air compressor. Background Art

[0002] As one of the core power sources for industrial production, compressed air plays an irreplaceable role in modern manufacturing. Its applications span key sectors such as equipment manufacturing, automotive manufacturing, metallurgy, electricity, electronics, medical equipment, and textiles. Its high efficiency makes it the second-largest industrial power source after electricity. However, the high energy consumption of compressed air systems has consistently constrained energy efficiency improvements in the industrial sector. Optimizing the energy efficiency of compressed air systems has become a key technical direction for achieving industrial energy conservation and consumption reduction.

[0003] Among the many types of compressors, micro-oil screw air compressors have become the mainstream model for current industrial applications due to their efficient operation and cost advantages. This type of equipment achieves lubrication, cooling, and sealing functions by injecting lubricating oil into the compression chamber. The lubricating oil supply is set at the factory based on the speed parameters under full-load conditions to ensure stable operation of the equipment under rated conditions. However, in the actual application of variable-frequency micro-oil screw air compressors, their characteristic of dynamically responding to gas demand through constant pressure speed regulation technology has exposed a significant technical bottleneck under low-load conditions: when the motor speed decreases, the lubricating oil in the compression chamber cannot be discharged through the oil return system in a timely manner, causing the lubricating oil to accumulate in the compression chamber, cooler, and oil return pipe. This oil accumulation phenomenon significantly reduces the effective working volume of the compression chamber, which in turn leads to a series of problems such as reduced compression efficiency and increased energy consumption under low-load conditions, becoming a key technical obstacle to improving the energy efficiency of variable-frequency air compressors.

[0004] Therefore, how to provide a control method for improving the compression efficiency of a variable frequency micro-oil screw compressor is one of the technical problems that those skilled in the art need to solve urgently. Summary of the Invention

[0005] The main purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and provide a variable frequency screw air compressor lubrication control system and control method, which can control the oil injection amount in the compression chamber according to the load state and exhaust temperature of the variable frequency micro-oil screw machine, so as to achieve the purpose of improving the compression efficiency of the variable frequency micro-oil screw machine.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a variable frequency screw air compressor lubrication control system, comprising:

[0008] The compressor and the variable frequency motor that drives it;

[0009] The oil-gas separation barrel is connected to the compressor exhaust port through an oil-gas mixing pipeline;

[0010] The main oil return pipeline is connected to the oil temperature control valve, oil filter, electric proportional control valve in sequence from the oil outlet of the oil and gas separation barrel, and finally to the oil inlet of the compressor;

[0011] Secondary oil return pipeline, which leads the lubricating oil at the bottom of the oil separator to the compressor air inlet;

[0012] Sensor set, including:

[0013] The exhaust temperature sensor detects the compressor exhaust temperature.

[0014] After cooling, the oil temperature sensor detects the oil temperature at the outlet of the oil cooler.

[0015] The oil pressure sensor before the valve detects the oil pressure at the inlet of the electric proportional control valve.

[0016] The oil pressure sensor after the valve detects the oil pressure at the outlet of the electric proportional control valve;

[0017] The controller is electrically connected to the sensor group, the electric proportional control valve and the variable frequency motor, and is configured to execute multiple closed-loop control strategies:

[0018] Based on the real-time difference between the exhaust temperature and the cooled oil temperature, the opening of the electric proportional control valve is adjusted through the first PID control so that the actual exhaust temperature rise is equal to the set value;

[0019] When the inverter output frequency is lower than the critical threshold, it switches to the second PID control based on the valve downstream oil pressure to maintain the valve downstream oil pressure not lower than the lower limit set value; the critical frequency is the inverter output frequency when the exhaust temperature rise cannot reach the set value plus the offset.

[0020] As a preferred technical solution, it also includes:

[0021] The oil temperature control valve is set at the lubricating oil outlet of the oil-gas separation barrel and is used to select whether to activate the oil cooler according to the lubricating oil temperature;

[0022] The oil cooler is connected to the oil temperature control valve and is used to cool the high-temperature lubricating oil;

[0023] The oil filter is located downstream of the oil cooler and is used to filter impurities in the lubricating oil.

[0024] As a preferred technical solution, the oil temperature control valve is a temperature-sensitive valve. When it is detected that the lubricating oil temperature exceeds a set threshold, the lubricating oil is automatically directed to the oil cooler; otherwise, it flows directly to the oil filter.

[0025] As a preferred technical solution, the oil temperature control valve selects whether to allow the lubricating oil to flow through the oil cooler according to the temperature of the lubricating oil.

[0026] As a preferred technical solution, the following parameters are preset in the controller:

[0027] Exhaust temperature rise constant setting value, exhaust temperature too high alarm value, valve front oil pressure too low alarm value;

[0028] The lower limit setting value of the oil pressure after the valve;

[0029] PID gain parameters for exhaust temperature rise control and oil pressure control;

[0030] The minimum opening value of the electric proportional control valve.

[0031] In a second aspect, the present invention provides a variable frequency screw air compressor lubrication control method, comprising the following steps:

[0032] Real-time collection of exhaust temperature, oil temperature after cooling, oil pressure before valve, oil pressure after valve, electric proportional control valve opening and inverter output frequency;

[0033] If the inverter is not running, adjust the electric proportional control valve to fully open;

[0034] If the inverter is running, the exhaust temperature rise closed loop control is executed:

[0035] Calculate the real-time exhaust temperature rise, where the exhaust temperature rise = exhaust temperature - oil temperature after cooling;

[0036] The opening of the electric proportional control valve is adjusted by the first PID control so that the actual exhaust temperature rise approaches the exhaust temperature rise constant setting value;

[0037] When the inverter output frequency is lower than the critical frequency, it switches to oil pressure closed-loop control:

[0038] The second PID control is used to adjust the opening of the electric proportional control valve so that the oil pressure after the valve is not lower than the lower limit setting value of the oil pressure after the valve;

[0039] The critical frequency is the inverter output frequency plus an offset when the exhaust temperature rise cannot reach the set value.

[0040] As a preferred technical solution, the exhaust temperature rise closed-loop control has a higher priority than the oil pressure closed-loop control; when the inverter output frequency is higher than the critical frequency, the exhaust temperature rise closed-loop control is forcibly enabled.

[0041] As a preferred technical solution, the boundary frequency is determined dynamically:

[0042] In the exhaust temperature rise closed-loop control, if the actual exhaust temperature rise continues to be lower than the set value for more than the set time, the current inverter output frequency is recorded as the reference frequency;

[0043] Critical frequency = reference frequency + 5Hz.

[0044] As an optimal technical solution, the first PID control uses the exhaust temperature rise to control the proportional gain value, integral gain value, and differential gain value; the second PID control uses the oil pressure to control the proportional gain value, integral gain value, and differential gain value, and the two sets of gain parameters are independent of each other.

[0045] As a preferred technical solution, an alarm is triggered and the current control mode is maintained when any of the following conditions are detected:

[0046] The exhaust temperature exceeds the exhaust temperature alarm value;

[0047] The oil pressure before the valve is lower than the low oil pressure alarm value before the valve.

[0048] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0049] The present invention sets up a lubrication control system including a compressor, an exhaust temperature sensor, an after-cooling oil temperature sensor, a valve front oil pressure sensor, a valve rear oil pressure sensor, an electric proportional control valve and a controller. The controller is used to obtain the speed of the variable frequency motor and the exhaust temperature data in real time, and the electric proportional valve opening is dynamically adjusted in combination with the PID algorithm to maintain a constant exhaust temperature, and the electric valve is forcibly limited to the minimum opening at low speeds. At the same time, the system automatically switches the lubricating oil cooling path through the oil temperature control valve, combines the secondary oil return pipeline to recover the residual lubricating oil, and is equipped with a manual bypass valve as a redundant adjustment channel. This solution solves the technical problem of reduced compression chamber volume and decreased compression efficiency caused by lubricating oil accumulation in existing variable-frequency micro-oil screw air compressors under low-load conditions, achieving the following technical effects: 1) Optimizing compression chamber lubrication and sealing balance through dynamic oil injection control, significantly improving compression efficiency at low loads; 2) Based on oil temperature adaptive management and closed-loop PID adjustment, it reduces ineffective lubricating oil circulation, energy consumption and oil loss; 3) The minimum opening protection mechanism is combined with the oil pressure / temperature alarm function to avoid equipment operation due to oil shortage or overheating damage, extending the life of key components; 4) The manual bypass valve and intelligent control work together to ensure system reliability, taking into account both automation and emergency operation needs. Ultimately, while improving the energy efficiency of the variable-frequency air compressor, the comprehensive technical goals of energy saving and consumption reduction, enhanced operational stability and reduced maintenance costs are achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0051] Figure 1 It is a structural schematic diagram of the lubrication control system of the variable frequency screw air compressor of the present invention;

[0052] Figure 2 It is a schematic diagram of the mechanical structure of the variable frequency screw air compressor lubrication control system of the present invention;

[0053] Figure 3 The present invention is a flow chart of a variable frequency screw air compressor lubrication control method.

[0054] 1-Atmospheric inlet; 2-Air filter; 3-Compressor; 4-Oil-gas mixing pipeline; 5-Oil-gas separation barrel; 6-Oil separator; 7-Compressed air outlet; 8-Secondary oil return pipeline; 9-Oil temperature control valve; 10-Oil filter; 11-Electric proportional valve; 12-Exhaust temperature sensor; 13-Main oil return pipeline; 14-Oil cooler; 15-Manual bypass valve; 16-Oil pressure sensor before valve; 17-Oil pressure sensor after cooling; 18-Oil pressure sensor after valve; 19-Frequency conversion motor. DETAILED DESCRIPTION

[0055] In order to enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0056] References to "embodiments" in this application mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described in this application may be combined with other embodiments.

[0057] like Figure 1 、 Figure 2 As shown, this embodiment provides a variable frequency screw air compressor lubrication control system, which is applied to a variable frequency micro-oil screw air compressor, including a controller, an atmospheric inlet 1, an air filter 2, a compressor 3, an oil-gas mixing pipeline 4, an oil-gas separation barrel 5, an oil separator 6, a compressed air outlet 7, a secondary oil return pipeline 8, an oil temperature control valve 9, an oil filter 10, an electric proportional valve 11, an exhaust temperature sensor 12, a main oil return pipeline 13, an oil cooler 14, a manual bypass valve 15, a valve front oil pressure sensor 16, a cooling post oil pressure sensor 17, a valve rear oil pressure sensor 18 and a variable frequency motor 19.

[0058] The compressor 3 is driven by a variable frequency motor 19 and is used to compress air, which enters through the atmospheric inlet 1;

[0059] The oil-gas separation barrel 5 is connected to the outlet of the compressor and is used to separate compressed air and lubricating oil;

[0060] The secondary oil return pipe 8 draws the lubricating oil accumulated in the oil separator 6 back to the air inlet of the compressor 3 to reduce lubricating oil loss.

[0061] The sensor group includes an exhaust temperature sensor 12, a cooled oil temperature sensor 17, a pre-valve oil pressure sensor 16, and a post-valve oil pressure sensor 18; wherein:

[0062] The exhaust gas temperature sensor 12 detects the compressor exhaust gas temperature.

[0063] After cooling, the oil temperature sensor 17 detects the oil temperature at the outlet of the oil cooler 14.

[0064] The oil pressure sensor 16 before the valve detects the oil pressure at the inlet of the electric proportional control valve.

[0065] The post-valve oil pressure sensor 18 detects the oil pressure at the outlet of the electric proportional control valve.

[0066] The oil temperature control valve 9 is connected to the lubricating oil outlet of the oil-gas separation barrel, and selects the lubricating oil to flow to the oil cooler 14 or directly enter the oil filter 10 according to the oil temperature; it is also used to select whether to activate the oil cooler 14 according to the lubricating oil temperature;

[0067] Furthermore, the oil temperature control valve 9 is a temperature-sensitive valve. When it is detected that the lubricating oil temperature exceeds a set threshold, the lubricating oil is automatically directed to the oil cooler 14 ; otherwise, the lubricating oil flows directly to the oil filter 10 .

[0068] The oil filter 10 is located downstream of the oil cooler 14 and is used to filter impurities in the lubricating oil.

[0069] The electric proportional control valve 11 and the manual bypass valve 15 are arranged in parallel downstream of the oil filter to adjust the return flow of the lubricating oil;

[0070] The exhaust temperature sensor 12 is provided at the compressed air outlet 7 and is used to detect the exhaust temperature;

[0071] The oil cooler 14 is connected to the oil temperature control valve 9 and is used to cool the high-temperature lubricating oil;

[0072] The controller is electrically connected to the variable frequency motor 19 , the exhaust temperature sensor 12 , the oil pressure sensor 16 and the electric proportional control valve 11 .

[0073] Please refer again Figure 1, the controller is configured to: based on the real-time difference between the exhaust temperature and the cooled oil temperature, adjust the opening of the electric proportional control valve through the first PID control so that the actual exhaust temperature rise is equal to the set value;

[0074] When the inverter output frequency is lower than the critical threshold, it switches to the second PID control based on the valve downstream oil pressure to maintain the valve downstream oil pressure not lower than the lower limit set value.

[0075] Furthermore, the critical frequency is the inverter output frequency plus an offset when the exhaust temperature rise cannot reach a set value.

[0076] Furthermore, the controller is electrically connected to the variable frequency motor 19, the sensor group, and the electric proportional control valve 11. Specifically, the controller is configured with a constant exhaust temperature rise setting value, an excessively high exhaust temperature alarm value, a low upstream oil pressure alarm value, a downstream oil pressure lower limit setting value, the electric proportional control valve's exhaust temperature rise control proportional gain, integral gain, and differential gain values, as well as the electric proportional control valve's oil pressure control proportional gain, integral gain, and differential gain values. The oil pressure sensor and exhaust temperature sensor are connected to the controller via an analog input module, the frequency converter is connected to the controller via a communication module, and the controller's analog output module is connected to the electric proportional control valve.

[0077] Further, such as Figure 2 As shown, the atmosphere enters the compressor after passing through the air filter, and the lubricating oil enters the compressor through the secondary oil return pipe and the main oil return pipe. The compressor is driven by a variable frequency motor. After the compressor is started, the air is compressed and enters the oil-gas mixing pipe, and then enters the oil-gas separation barrel.

[0078] The oil-gas separator is used to separate compressed air and lubricating oil. It first separates the majority of the lubricating oil through cyclonic separation. After this initial separation, the compressed air passes through the oil separator for secondary separation. A small amount of lubricating oil accumulates inside the oil separator and is drawn back to the compressor's air inlet through the secondary oil return line. After oil-gas separation, the compressed air enters the compressed air cooler and the main pressure line.

[0079] The lubricating oil separated from the oil-gas separation barrel will first enter the oil temperature control valve. The oil temperature control valve will choose whether to let the lubricating oil enter the oil cooler for cooling or directly enter the oil filter without cooling according to the current lubricating oil temperature.

[0080] After the lubricating oil is cooled and filtered, the oil pressure needs to be measured. The lubricating oil flow can be adjusted through an electric proportional control valve or a manual bypass valve before entering the compressor inlet, completing the entire lubrication cycle.

[0081] like Figure 3 As shown, this embodiment provides a variable frequency screw air compressor lubrication control method, comprising the following steps:

[0082] (1) The controller sets the constant setting value of exhaust temperature rise, the alarm value of exhaust temperature too high, the alarm value of oil pressure before valve too low, the lower limit setting value of oil pressure after valve, the proportional gain value, integral gain value, differential gain value of exhaust temperature rise control of electric proportional control valve, and the proportional gain value, integral gain value, differential gain value of oil pressure control of electric proportional control valve.

[0083] (2) After the system is started, use multi-closed-loop control to perform the following steps:

[0084] a) The controller continuously collects the real-time values of exhaust temperature, oil temperature after cooling, oil pressure before valve, oil pressure after valve, electric proportional control valve opening, and inverter output frequency, and calculates the real-time exhaust temperature rise, which is the exhaust temperature value - oil temperature value after cooling;

[0085] b) The controller reads the operating status of the inverter and the motor speed. If the inverter is not running, the opening of the electric proportional control valve is adjusted to the fully open state;

[0086] c) If the inverter is running, the PID control program in the controller adjusts the opening of the electric proportional control valve according to the constant setting value of the exhaust temperature rise, so that the actual exhaust temperature rise value is consistent with the constant setting value of the exhaust temperature rise;

[0087] d) If the inverter output frequency is too low and the exhaust temperature rise cannot reach the constant exhaust temperature rise setting through PID control, the controller records the inverter output frequency value at that moment. When the inverter output frequency is lower than this frequency value + 5Hz, the PID control program in the controller adjusts the opening of the electric proportional valve according to the lower limit setting of the valve downstream oil pressure, so that the actual value of the valve downstream oil pressure matches the lower limit setting of the valve downstream oil pressure, ensuring minimum lubrication in the compression chamber.

[0088] Those skilled in the art will appreciate that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program. The program can be stored in a non-volatile computer-readable storage medium. When the program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, storage, database or other media used in the embodiments provided in this application can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM is available in various forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM).

[0089] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0090] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A variable frequency screw air compressor lubrication control system, characterized in that: include: A compressor (3) and a variable frequency motor (19) for driving the compressor; The oil-gas separation barrel (5) is connected to the compressor exhaust port via the oil-gas mixing pipeline (4); The main oil return pipe (13) is connected to the oil temperature control valve (9), the oil filter (10), the electric proportional control valve (11) from the oil outlet of the oil and gas separation barrel (5) in sequence, and finally connected to the oil inlet of the compressor; A secondary oil return pipe (8) guides the lubricating oil at the bottom of the oil separator (6) to the compressor air inlet; Sensor set, including: The exhaust gas temperature sensor (12) detects the compressor exhaust gas temperature. After cooling, the oil temperature sensor (17) detects the oil temperature at the outlet of the oil cooler (14). The oil pressure sensor (16) before the valve detects the oil pressure at the inlet of the electric proportional control valve. The oil pressure sensor (18) after the valve detects the oil pressure at the outlet of the electric proportional control valve; The controller is electrically connected to the sensor group, the electric proportional control valve (11) and the variable frequency motor, and is configured to execute a multi-closed loop control strategy: Based on the real-time difference between the exhaust temperature and the cooled oil temperature, the opening of the electric proportional control valve is adjusted through the first PID control so that the actual exhaust temperature rise is equal to the set value; When the inverter output frequency is lower than the critical threshold, it switches to the second PID control based on the valve downstream oil pressure to maintain the valve downstream oil pressure not lower than the lower limit set value; the critical frequency is the inverter output frequency when the exhaust temperature rise cannot reach the set value plus the offset.

2. A variable frequency screw air compressor lubrication control system according to claim 1, characterized in that: Also includes: An oil temperature control valve (9) is provided at the lubricating oil outlet of the oil-gas separation barrel (5) and is used to select whether to activate the oil cooler (14) according to the lubricating oil temperature; An oil cooler (14), connected to the oil temperature control valve (9), for cooling the high-temperature lubricating oil; The oil filter (10) is located downstream of the oil cooler (14) and is used to filter impurities in the lubricating oil.

3. The variable frequency screw air compressor lubrication control system according to claim 1, characterized in that: The oil temperature control valve (9) is a temperature-sensitive valve. When it is detected that the lubricating oil temperature exceeds a set threshold, the lubricating oil is automatically directed to the oil cooler (14); otherwise, the lubricating oil flows directly to the oil filter (10).

4. A variable frequency screw air compressor lubrication control system according to claim 1, characterized in that: The oil temperature control valve (3) selects whether to allow the lubricating oil to flow through the oil cooler (4) according to the temperature of the lubricating oil.

5. The variable frequency screw air compressor lubrication control system according to claim 1, characterized in that: The following parameters are preset in the controller: Exhaust temperature rise constant setting value, exhaust temperature too high alarm value, valve front oil pressure too low alarm value; The lower limit setting value of the oil pressure after the valve; PID gain parameters for exhaust temperature rise control and oil pressure control; The minimum opening value of the electric proportional control valve.

6. A variable frequency screw air compressor lubrication control method, characterized in that: The following steps are involved: Real-time collection of exhaust temperature, oil temperature after cooling, oil pressure before valve, oil pressure after valve, electric proportional control valve opening and inverter output frequency; If the inverter is not running, adjust the electric proportional control valve to fully open; If the inverter is running, the exhaust temperature rise closed loop control is executed: Calculate the real-time exhaust temperature rise, where the exhaust temperature rise = exhaust temperature - oil temperature after cooling; The opening of the electric proportional control valve is adjusted by the first PID control so that the actual exhaust temperature rise approaches the exhaust temperature rise constant setting value; When the inverter output frequency is lower than the critical frequency, it switches to oil pressure closed-loop control: The second PID control is used to adjust the opening of the electric proportional control valve so that the oil pressure after the valve is not lower than the lower limit setting value of the oil pressure after the valve; The critical frequency is the inverter output frequency plus an offset when the exhaust temperature rise cannot reach the set value.

7. A variable frequency screw air compressor lubrication control method according to claim 6, characterized in that: The exhaust temperature rise closed-loop control has a higher priority than the oil pressure closed-loop control; when the inverter output frequency is higher than the critical frequency, the exhaust temperature rise closed-loop control is forcibly enabled.

8. The variable frequency screw air compressor lubrication control method according to claim 6, characterized in that: The critical frequency is determined dynamically by: In the exhaust temperature rise closed-loop control, if the actual exhaust temperature rise continues to be lower than the set value for more than the set time, the current inverter output frequency is recorded as the reference frequency; Critical frequency = reference frequency + 5Hz.

9. The variable frequency screw air compressor lubrication control method according to claim 6, characterized in that: The first PID control uses the exhaust temperature rise to control the proportional gain value, integral gain value, and differential gain value; the second PID control uses the oil pressure to control the proportional gain value, integral gain value, and differential gain value, and the two sets of gain parameters are independent of each other.

10. The variable frequency screw air compressor lubrication control method according to claim 6, characterized in that: When any of the following conditions is detected, an alarm is triggered and the current control mode is maintained: The exhaust temperature exceeds the exhaust temperature alarm value; The oil pressure before the valve is lower than the low oil pressure alarm value before the valve.

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