Analysis and Application of Energy Saving and Environmental Protection Technology for Permanent Magnet Frequency Conversion Screw Air Compressor
2023-11-19
In the whole life cycle of the air compressor, the cost mainly covers procurement, routine maintenance and energy consumption costs. Especially the late use of the process of energy consumption costs occupy a huge proportion, this proportion usually accounts for 1/5 ~ 1/4 of the total power consumption of the factory, to pneumatic tools-based factories and mines, a few even up to 1/2, so the energy saving of the air compressor is particularly important and necessary. In terms of screw air compressor type, there are two kinds of energy-saving means, respectively, ordinary frequency conversion type and permanent magnet variable speed type. Ordinary frequency conversion type is to use external frequency converter to regulate the speed of the motor, so as to realize the purpose of energy saving. However, ordinary frequency conversion type is often not full load operation state. Therefore, the energy-saving optimization design or reconstruction of air compressors provides a lot of space for the realization of energy saving and consumption reduction. According to incomplete statistics, the installed capacity of screw air compressors reaches about 65% of the total installed capacity. This study mainly analyzes the energy-saving and environmental protection technology and application of PM VSD screw air compressor（permanent magnet variable speed screw air compressor），from the working principle (working process) of screw air compressor, the type of installed motor of screw air compressor, the energy saving mechanism of permanent magnet synchronous motor of screw air compressor, and energy saving technology analysis of permanent magnet frequency conversion screw air compressor（permanent magnet variable speed screw air compressor）.
1. Introduction to the working principle (working process) of the screw air compressor
Figure 1 shows the working principle (working process) of the screw air compressor. Screw compressor body inside a pair of parallel arrangement and at the same time engaged screw to do rotary movement. Yin and Yang rotor to drive the inlet air rotation, and then compressed volume changes, mainly contains 4 processes：suction process, compression process, compressed gas and oil injection process, as well as the exhaust process.
The whole working process is briefly described as follows: the permanent magnet synchronous motor drives the screw rotor through the coupling, and the outside air fills up the space between the teeth groove of the master and slave screws, and when the inlet side of the screw rotor turns away from the inlet port, the gas inside the space is sealed by the space between the master and slave screws and the body, so as to complete the suction.
The compressed air is further reduced as the sealing space between the screw rotor and the body is rotated at an angle with the screw rotor, showing a state of “helical movement”, and this process is called the compression process. The volume of the compressed air decreases as it is transported along the screw helix to the outlet. As the gas is constantly compressed, the temperature of the gas also rises, at the same time, the lubricant sprayed into the compressed oil is also atomized, play on the compressed air cooling seal and lubricate the mechanical structure of the function. And then when the screw rotor sealing tooth peak to the body of the exhaust port, the compressed gas began to discharge, and then complete the exhaust process. At the same time, the inlet gas continues to repeat the above process, so that the gas from the inlet is continuously compressed and transported to the compressor outlet.
In the system where compressed air is regulated, the air to be compressed passes through the filter at the inlet port to remove such impurities and dust in the air, and the filtered air flows into the suction port of the compressor through the inlet valve, and the incoming gas is mixed into the lubricating oil in the process. Secondly, the compressed oil-gas two-phase mixture enters the gas-oil separator through a check valve for primary and secondary separation respectively, and then the oil-gas after the second separation passes through a minimum pressure valve, a cooler and a gas-water separator before it can be transported to the compressed gas storage system.
The relationship between the pressure p and the volume V of the gas in the compression process is shown in the figure 2. The 3′-1 segment is the isothermal compression process curve. If single-stage compression is used, the power consumed in the compression process is the region contained in 6-1-3″-4-6; when two-stage compression is used, the power consumed in the compression process is the sum of the regions contained in 6-1-2-5-6 and 5-2′-3-4-5. It follows that the power saved is equivalent to the region contained in 2′-2-3″-3-2′. It is easy to see that the power consumption is still the sum of the regions 2′-3′-3″-2 and 1-2-2′-1 compared to the isothermal compression process.
2. Permanent magnet variable speed air compressor energy saving technology principle analysis
2.1 Air compressor installed motor type analysis
Generally speaking, the screw air compressor driven by general-purpose industrial-frequency motor is called industrial-frequency screw air compressor ; and when it is equipped with inverter motor controlled by frequency converter, it is called inverter screw air compressor. When the motor is a permanent magnet synchronous motor with asynchronous start, it is called permanent magnet screw air compressor; and when the motor is a permanent magnet synchronous motor, it is called permanent magnet frequency conversion screw air compressor (PM VSD screw air compressor). The permanent magnet synchronous motor is highly integrated with the frequency converter, the rotor is made of the strongest and most stable permanent magnet, and the stator winding is made of vacuum pressure impregnated paint and epoxy resin for vacuum potting.
2.2 Analysis of energy-saving mechanism of permanent magnet synchronous motor of screw air compressor
From the efficiency accounting formula of permanent magnet motor, it can be seen that the total loss of permanent magnet motor consists of iron loss, copper loss, mechanical loss and other stray loss. And in the total loss, iron and copper loss accounts for more than 4/5. Therefore, the energy-saving requirements of permanent magnet synchronous motors for screw air compressors mainly consider the following three points.
2.2.1 High efficiency η.
The magnetic field of a screw air compressor permanent magnet synchronous motor is generated by permanent magnets. The use of permanent magnets reduces the energy dissipation associated with the excitation current. Permanent magnet synchronous motors have a wide high efficiency region, especially when the motor is running at low speeds. This characteristic is much higher than that of Y-series cage asynchronous motors. By comparing the rotational speed n-efficiency η curve diagrams of Y-series asynchronous motors and permanent magnet synchronous motors (see Fig.3), it can be seen that although the maximum efficiencies of the two types of motors are relatively similar, however, n (rotational speed) is in the range of 0.15n1~ n1 (n1 is the synchronous rotational speed), and the efficiency of permanent magnet synchronous motors is always close to the maximum efficiency η max. However, in the same range of rotational speeds, the efficiency η of the Y-series caged induction motor is distributed in the range of 0.20n1~ n1. The efficiency η of Y series cage asynchronous motor is distributed in the range of 0.20~0.95. It can be seen that the permanent magnet synchronous motor has very high efficiency η, and the use of permanent magnet synchronous motor makes the energy-saving effect especially obvious.
2.2.2 Large starting torque T.
Generally speaking, the ratio of the starting torque T of permanent magnet synchronous motor to the rated torque T0 is more than 3. Comparatively speaking, the value of Y-series cage asynchronous motor under the same conditions is only about 1.5. The characteristic of large starting torque is more suitable for the use of screw compressor and other heavy-duty machinery.
2.2.3 High and stable power factor cos α.
Since the magnetic field of permanent magnet synchronous motor does not rely on the excitation winding, there is no reactive power loss due to large impedance of air gap between stator and rotor. Therefore, the power factor cosα of PM synchronous motors can reach a maximum of 0.96. At the same time, the number of poles of PM synchronous motors does not cause a change in the power factor cosα, so the power factor cosα is relatively stable. Especially at lower operating speeds, the power factor cosα of Y-series cage asynchronous motors is only about 0.9 in comparison. In addition, the power factor cosα of the Y series cage asynchronous motor decreases with the increase of the number of poles of the motor. From the point of view of power factor cosα, permanent magnet synchronous motors are also very suitable for installation in screw compressors to meet the energy saving requirements.
2.3 Energy saving technology analysis of permanent magnet frequency conversion screw air compressor
Purely from a technical point of view, the reason why the PM VSD screw air compressor (permanent magnet variable speed screw air compressor) has energy saving and environmental protection is mainly because of the use of frequency converter drive inverter motor will make the amount of gas generated and energy consumption is a linear change to reduce energy consumption; the reasonable configuration of the frequency converter and air compressor auxiliary devices can improve the operating performance of the air compressor; the start-up current of the air compressor driven by the inverter motor is small, the energy consumption is reduced, and the impact on the power grid is small, which in turn Improve the reliability and stability of the whole air compressor. Due to the use of frequency converter for infinitely variable speed regulation, the screw air compressor starts gently and adjusts the response speed quickly, so the air pressure of the air compressor is relatively stable. Based on the soft start characteristics of the frequency converter, it can ensure that the starting current is not greater than 1.6 times the rated current (Ie). This is compared with the case of 6 times the rated current (Ie) during startup under industrial frequency operation, the startup impact is weakened a lot, which greatly reduces the impact of the startup process on the entire power grid, and at the same time reduces the mechanical impact inside the air compressor. The flow control of permanent magnet frequency conversion screw air compressor（permanent magnet variable speed screw air compressor）is variable, which can achieve the purpose of small differential pressure loss and small no-load loss. This is because the frequency converter can adjust the motor speed to control the displacement of the air compressor through the real-time air consumption. In addition, through the optimization of the corresponding control strategy, the energy saving effect can be further improved. The use of modulation technology can make the motor more adaptable to the AC voltage. For example, when the AC voltage is slightly lower, the system is still able to output sufficient driving torque; when the AC voltage is slightly higher, the application of frequency conversion technology is also able to drive normally. Permanent magnet frequency conversion motor in the frequency converter regulation, the speed of the air compressor for the variable value, so that you can control the air compressor gas production, this control is the most direct and the most energy-saving form of control. Using the closed-loop control of the frequency converter, screw air compressor rotation can be realized by the pressure of the gas supply system to automatically regulate the speed, the pressure fluctuations of the exhaust pipeline system is small, high stability. This is of great significance to the stability of the production process system outside the air compressor, realizing the energy saving of the process system, reducing the compressor power consumption, and playing a practical energy-saving effect.
The use of variable frequency technology makes the screw air compressor system changes, the root cause is that the variable frequency air compressor is used according to the air volume adjustment. The lower the operating frequency, the lower the speed of the compressor, the lower the corresponding bearing loss. For example, the air inlet valve can be optimized to meet the use by only retaining the normally closed function, and the capacitive solenoid valve and the corresponding regulating mechanism can also be optimized and removed. In addition, to reduce the screw air compressor failure frequency and reduce the amount of maintenance in the process of use, can be optimized for the oil-injected screw air compressor air pressure injection frequency gear oil pump for lubricant injection, so that you can achieve both reliable and efficient results.
Concluding Remarks
This study introduces the working characteristics and working principle of screw air compressor, and analyzes the application and advantages of permanent magnet permanent magnet frequency conversion technology in screw compressor for energy saving and environmental protection. Energy saving and environmental protection is an inevitable requirement for the screw air compressor industry to improve its energy consumption level. The use of permanent magnet frequency conversion motor and frequency conversion technology in screw air compressors has enriched the product range of screw air compressors and promoted the development of the air compressor industry. In view of the unique performance of permanent magnet synchronous motor, in the process of variable operating conditions, the energy-saving effect of screw air compressor with permanent magnet frequency conversion technology is particularly obvious, and at the same time, the energy-saving technology of permanent magnet frequency conversion screw air compressor（permanent magnet variable speed screw air compressor） has a particularly good potential for deep excavation. Permanent magnet frequency conversion screw air compressor（permanent magnet variable speed screw air compressor） in the use of high efficiency and energy saving, will contribute to energy saving more power. Read more.