Application in Air Compressors
Application of Inverters in Air Compressors
Description
1. There are various types of air compressors, including piston air compressors, screw air compressors, and centrifugal air compressors; however, their gas supply control methods predominantly rely on the load/unload control method. Although this gas supply control method is conceptually simple and operationally convenient, it presents numerous issues such as excessive energy consumption, susceptibility to damage in the air intake valve, and unstable gas pressure. With the advancement and progression of society, efficient and low-consumption technologies have gained increasing significance. Therefore, exploring the feasibility of implementing variable-frequency speed regulation technology in the domain of air compressor gas supply to conserve electricity while enhancing performance and improving gas supply quality has become a topic of considerable concern.
2. The Operational Mechanism of an Air Compressor
The current practice in air compressors involves the utilization of either two-point control (upper and lower limit control) or start-stop control (for small air compressors). This means that when the pressure inside the gas cylinder of an air compressor reaches the upper limit of the set value, it automatically shuts off its intake valve based on internal pressure or oil pressure, thereby causing a small air compressor to cease operation.
When the pressure drops to the lower limit of the set value, the air compressor initiates the opening of the intake valve and restarts the small air compressor. Conventional control methods are susceptible to causing power grid impacts and potential damage to the air compressor itself, particularly in situations where there are frequent fluctuations in air consumption.
The air receiver receives compressed air during normal operation. The main control microcontroller is responsible for monitoring and controlling various parameters of the air compressor, such as air temperature, pressure, screw temperature, cooling water pressure and temperature, oil pressure, and oil temperature.
When the outlet pressure of the air compressor reaches the upper limit of the set value, the intake port is closed via the oil pressure branch valve and the internal circulation pipe is opened, enabling it to operate in a closed loop. Meanwhile, continuous utilization of compressed air by end-users persists.
When the pressure drops below the set lower limit, the oil pressure branch valve will close the circulation pipe and open the air inlet, allowing filtered air to be compressed into the air storage tank through the filter. This process occurs during static conditions, original starting method (Y-△), and loading and unloading. However, it can have a significant impact on grid power supply and distribution equipment as well as screw performance, resulting in serious energy waste.
The main motor speed decreases, resulting in a significant reduction in shaft power and offering substantial potential for energy savings. The impact of frequency conversion on energy efficiency is particularly noteworthy, especially for systems and equipment with extensive adjustment ranges. Practical applications have demonstrated that even slight changes in speed (frequency) can lead to considerable variations in shaft power. Due to this characteristic, the method of frequency conversion speed regulation (energy saving) has emerged as a prominent trend and is increasingly being adopted across various industries and fields of adjustment.
3. The Issues with the Gas Loading and Unloading Control System
3.1 Energy Consumption Analysis: It is well-known that the load/unload control method induces pressure fluctuations in compressed gas between Pmin and Pmax. Here, Pmin represents the minimum pressure required for normal operation. Generally, the relationship between Pmin and Pmax can be mathematically expressed as CPmax = (1 + δ)Pmin, where δ is a percentage ranging from approximately 10% to 25%. By employing variable-frequency speed regulation technology for continuous gas supply adjustment, pipeline pressure can be effectively maintained at a level close to meeting the gas supply requirements, i.e., near Pmin. Consequently, it becomes evident that compared to air compressors controlled by variable-frequency systems, energy wastage in load/unload gas supply control methods primarily occurs in two aspects.
a) The energy consumed when the compressed air pressure exceeds Pmin will be subsequently dissipated once the pressure returns to Pmin. Under the original control method, the pressure will continue to rise (until reaching Pmax) thereafter. This process also entails energy consumption.
b) The energy consumed during unloading due to improper regulation of the discharge method typically occurs when the pressure reaches its maximum limit. In such cases, the air compressor reduces pressure by employing the following method: closing the intake valve to idle the motor and simultaneously releasing excess compressed air from the separator tank through the vent valve. This regulatory approach results in significant energy wastage.
3.2 The other limitations
a) By utilizing mechanical means to adjust the air intake valve, it becomes challenging to achieve continuous adjustment of the air supply. Consequently, when there are constant fluctuations in air consumption, the resulting variations in air supply pressure inevitably become more pronounced. As a result, the precision of the air supply fails to meet process requirements. Furthermore, frequent adjustments of the air intake valve accelerate its wear and tear while also increasing maintenance workload and associated costs.
b) The durability of the vent valve cannot be guaranteed by frequent opening and closing of the vent valve.
4. The Design of a Control Scheme for Constant Pressure Gas Supply
After analyzing the aforementioned issues with the current gas supply control method, we have implemented variable-frequency speed regulation technology for maintaining constant pressure in gas supply. The pressure transmitter collects the actual pressure P and transmits it to the PID intelligent speed regulator. This value is then compared with the set pressure P0, and based on their difference, a predetermined PID control mode calculation is performed to generate a control signal that is sent to the VVVF variable-frequency drive. By utilizing this drive, both frequency and speed of the motor are controlled, ensuring that the actual pressure P remains consistently close to the desired set pressure P0.
Meanwhile, the scheme enhances the capability to switch between rated frequency and variable frequency while preserving the integrity of the original control and protection system. Moreover, by implementing this scheme, the air compressor motor can be smoothly started from a stationary state using an inverter, thereby achieving a soft start and mitigating both the starting impact current and mechanical stress on the air compressor.
5. The Enhancement of Technical Indicators and Configuration
The field-oriented vector control enables complete decoupling of motor variables and closed-loop current regulation. By leveraging the cutting-edge 32-bit motor control DSP from Texas Instruments, this product achieves high-speed execution of complex and precise control algorithms, making it the pioneering application in China.
Speed regulation accuracy: 0.01HZ
Range of speed regulation: 0.5-600.00HZ
Impact load: 180% of the motor's rated torque, with no tripping occurring within a duration of 10 seconds.
Low-frequency torque: 0.5Hz, with an output of 150% rated torque.
The acceleration and deceleration are performed at 150% of the rated torque.
An integrated, multi-functional digital PID regulator combination.
The device is equipped with an integrated standard 485 data interface.
Programmable Switched Input/Output Ports: 7 Inputs and 1 Output.
Programmable Relay Outputs: 2 Channels with selectable Open/Closed settings.
Programmable Analog Input/Output Ports: 2 Input Channels and 2 Output Channels.
Independent Airflow, Soft-Start Switch without physical contacts, Low-Inductance DC Bus Bar for enhanced reliability design.
6. The Effect of Transformation
(1) The entire retrofit system maintains the original control principle of the air compressor, ensuring that the original protection devices remain effective. Frequency conversion/variable frequency switching is achieved through electrical and mechanical dual interlocking, significantly enhancing system safety and reliability.
(2) After the completion of the compressed air system upgrade project, the initial trial run proved successful, demonstrating a stable operation with significantly reduced levels of vibrations and noise.
(3) The buffer cylinder pressure increases by 0.2kg at certain frequencies, while ensuring that the oil pressure, oil temperature, and data from all points are optimized within safe numerical values.
(4) After frequency conversion, the air compressor starts up smoothly, eliminating any impact current phenomenon during operation and significantly reducing mechanical stress on the equipment itself.
(5) While ensuring a steady supply of pipeline gas, there is a significant reduction in current consumption, minimizing instances of full-load operation which typically occurs around 40Hz. Compared to previous methods, energy savings exceed 30%, allowing for investment recovery within approximately 10 months.
(6) Due to stable gas supply resulting from this transformation, maintenance requirements for both air compressors and associated electrical and mechanical equipment are significantly reduced, leading to evident comprehensive benefits.
(7) Following this conversion process, safe and reliable operation is ensured while simultaneously meeting all necessary process requirements for utilizing gas.
