1. Robin Kang frequency converter is widely used in air compressors, energy saving and environmental protection. The dedicated inverter for air compressors has received great welcome from the market. There are many types of air compressors, including piston air compressors, screw air compressors, and centrifugal air compressors, but almost all of their air supply control methods use loading and unloading control methods. Although this air supply control method is simple in principle and easy to operate, it has many problems such as high energy consumption, easy damage to the intake valve, and unstable air supply pressure. With the development and progress of society, high-efficiency and low-consumption technologies have attracted more and more attention. Whether the frequency conversion speed regulation technology can be applied in the field of air compressor air supply to save power while improving air compressor performance and air supply quality has become a topic of our concern.

2. Working principle of the air compressor At present, the air compressor adopts two-point control (upper and lower limit control) or start-stop control (small air compressor), that is, when the pressure in the compressed gas cylinder reaches the upper limit of the set value , The air compressor closes the intake valve by its own air pressure or oil pressure, and the small air compressor stops.

When the pressure drops to the lower limit of the set value, the air compressor opens the intake valve, and the small air compressor starts again. The traditional control method is easy to cause an impact on the power grid, and it also has certain damage to the air compressor itself, especially when the air consumption fluctuates frequently.

Under normal working conditions, air is compressed into the air tank. The detection of various points of the air compressor (including compressed air temperature, pressure, screw temperature, cooling water pressure, temperature and oil pressure, oil temperature, etc.) and overall control are controlled by the main control single-board computer.

When the outlet pressure of the air compressor reaches the upper limit of the set value, the air inlet is closed through the oil pressure bypass valve, and the internal circulation pipeline is opened at the same time for self-circulation operation. At this time, the gas consumption unit continues to use gas.

When the pressure drops to the lower limit of the set value, the oil pressure bypass valve closes the circulation pipeline, opens the air inlet, and the air is compressed into the air tank by the filter. In the static state, the original starting mode (Y-△), and the loading and unloading will cause great impact on the power supply and distribution equipment and the rods of the power grid. Especially the serious waste of energy.

The main motor speed drops, the shaft power will drop a lot. The energy saving potential is considerable. ) The effect of variable frequency energy saving is very significant, especially for systems and equipment with a large adjustment range. Through practical applications, it can be intuitively seen that as long as the speed (frequency) is changed slightly when the flow changes, the shaft power will have a greater degree The above changes, because of this feature, the frequency conversion speed regulation (energy saving) method has become a trend and is continuously applied to various industries and various adjustment fields.

3. Problems in the control method of loading and unloading air supply

3.1 Energy consumption analysis We know that the load and load control methods make the pressure of the compressed gas change back and forth between Pmin and Pmax. Pmin is the lowest pressure value, that is, the lowest pressure that can guarantee the user's normal work. In general, the relationship between Pmin and Pmax can be expressed by the following formula: CPmax=(1+δ)Pmin is a percentage, and its value is roughly between 10% and 25%. And if the frequency conversion speed regulation technology can be used to continuously adjust the air supply, the pressure of the pipe network can always be maintained at the pressure that can meet the air supply, that is, near Pmin. It can be seen that compared with the air compressor under the control of the frequency conversion system, the energy wasted in the air compressor under the control mode of loading and unloading air supply is mainly in two parts:

(1) When the compressed air pressure exceeds Pmin, the energy consumed after the pressure reaches Pmin, the original control method determines that the pressure will continue to rise (until Pmax). This process is also an energy-consuming process.

(2) The energy consumed by the unreasonable adjustment method during unloading. Under normal circumstances, when the pressure reaches Pmax, the air compressor reduces the pressure and unloads by the following method: close the intake valve to make the motor idling, and at the same time remove the excess in the separation tank The compressed air is vented through the vent valve. This adjustment method will cause a lot of energy waste.

3.2 Other shortcomings

(1) The intake valve is adjusted mechanically, so that the air supply cannot be continuously adjusted. When the air consumption is constantly changing, the air supply pressure will inevitably produce large fluctuations. The accuracy of gas consumption cannot meet the process requirements. Coupled with frequent adjustment of the intake valve, it will accelerate the wear of the intake valve, increase the amount of maintenance and repair costs.

(2) Open and close the vent valve is frequently used, and the durability of the vent valve cannot be guaranteed.

4. Design of constant pressure gas supply control scheme

In view of the many problems existing in the original air supply control method, after the above analysis, the frequency conversion speed regulation technology is used for constant pressure air supply. The actual pressure P is collected by the pressure transmitter and sent to the PID intelligent speed controller, compared with the pressure setting value P0, and calculated according to the established PID control mode according to the magnitude of the difference, and the control signal is generated and sent to the variable frequency speed controller VVVF , The working frequency and speed of the motor are controlled by the inverter, so that the actual pressure P is always close to the set pressure P0.

At the same time, this scheme can increase the power frequency and frequency conversion switching function, and retain the original control and protection system. In addition, after adopting this scheme, the air compressor motor can be started by the frequency converter from stationary to rotating, realizing a soft start. Avoid the start-up shock current and the mechanical shock caused by the start-up to the air compressor.

5. Technical indicators and configuration

Field-oriented vector control, motor variables are completely decoupled, and current is closed loop. It adopts the latest high-performance 32-bit motor control dedicated DSP from American TI company, completes complex and accurate control algorithms at high speed, and is the first domestic product application.

Speed control accuracy: 0.01HZ

Speed adjustment range: 0.5-600.00HZ

Impact load: 180% motor rated torque, no tripping within 2 seconds.

Low frequency torque: 0.5Hz, 150% rated torque output.

150% rated torque acceleration and deceleration.

Built-in multifunctional combined digital PID regulator.

Built-in standard 485 data interface.

Programmable switch input port: 8 bits, output port: 2 bits.

Programmable relay output port: 1 way, normally open/normally closed optional.

Programmable analog input port: 4 channels, output port: 1 channel.

Voltage can be set power supply: 1 way.

Terminal control power: 1 way.

Independent air duct, non-contact soft start switch, low inductance DC bus bar high reliability design.

6. Modification effect

(1) The whole set of retrofit devices does not change the original control principle of the air compressor, which means that the original air compressor system protection device is still effective. And the power frequency/frequency conversion switch adopts electrical and mechanical double interlocking, which greatly improves the safety and reliability of the system.

(2) After the installation of the air compressor renovation project, the first test run was successful, the operation was stable, and the air compressor vibration and noise were greatly reduced.

(3) Except that the pressure of the buffer cylinder increases by 0.2 kg at some frequencies, the oil pressure, oil temperature and the detection data of each point are all optimized within safe values.

(4) After the frequency conversion transformation, the starting is a soft start, there is no unloading and loading surge current phenomenon during operation, and the mechanical impact of the air compressor itself is greatly reduced.

(5) Under the condition of ensuring the gas supply of the pipe network, the current is greatly reduced, and there is basically no full load phenomenon, generally around 40Hz. Compared with the previous, the power saving rate is more than 30%, and the investment can be recovered in about 10 months.

(6) Air compressors, power supply and distribution equipment, and mechanical equipment have a stable air supply, greatly reducing the maintenance volume, and the overall benefits are obvious.

(7) The operation of the air compressor after the transformation is safe and reliable, and at the same time it meets the technological requirements for gas use.