The air compressor is a device that can convert the mechanical energy of the motive force into the pressure energy of the gas and feed the compressed gas into the gas storage tank to maintain a certain pressure. It is the main body of the gas source device. As an aerodynamic supply device, it is widely used in many basic industries such as metallurgy, mining, machinery manufacturing, tobacco, chemical industry, etc. It belongs to the large-scale Electrical Equipment (such as water pump, fan, boiler, air). Among the presses, etc., due to structural and technical applications, the common air compressors are in no-load operation for most of the time. Therefore, the energy-saving potential of the economic operation of the air compressor is huge, and the social benefits are considerable. In this paper, we use a common screw air compressor GA55CP-7.5, (0.7/0.75MP, 2.2 air compressor working principle of our company to take a common screw air compressor (GA55CP-7.5) of our company as an example. Its working principle is that it uses a pair of male and female rotors that are parallel to each other to make a relatively high-speed rotary motion in the body, so that a volume of "V"-shaped teeth is formed between the tooth gap of the male and female rotors and the body, which is continuously generated with the rotation of the rotor. Volume change, air is along the line of emulsion explosives production line, which mainly uses gas equipment for 102 emulsification preparation workshop, one Tpt, two KP charging machines, 103 finished packaging workshops, 2 automatic packing machines; 101 workshop, process pipeline residual liquid Cleaning, considering that the old production line has a good 55kw air compressor can be used, no new air compressor is purchased. At the beginning of the commissioning phase, the air consumption is not large (minimum air pressure 0.45MPa), the air compressor is running normally (the sound is relatively stable) After normal production, the minimum pressure is set to 0.62 MPa. During operation, the sound of the air compressor is periodically changed, and the pressure gauge of the pipe network fluctuates (0.63-0.7 MPa). The flow card table detects that the motor current changes from 40A to 110A, and repeats in about 1 minute. The maintenance time of 110A current (loading) is 6-10s, and the maintenance time of 40A-50A (unloading) is 158L/S, 55KW, 2967rpm. Speed ​​transformation is taken as an example to illustrate. 2 Classification and working principle of air compressor 2.1 Classification of air compressor 2.1.1 According to the working principle, it can be divided into two categories: volumetric and speed type A, volumetric air compressor: according to the motion mode, it can be divided into reciprocating and rotating. Reciprocating type includes piston type and diaphragm type; rotary type is divided into sliding type, screw type and rotor type. B. Speed ​​air compressors can be divided into axial flow type, centrifugal type and rotor type. 2.1.2 According to the output gas pressure P, the height is divided into five kinds of 0.2MPa); the low-pressure air compressor rotor axis direction is transported from the suction end to the output end to enter the gas storage tank (generally to limit the large fluctuation of the pipe network pressure) To achieve the suction, compression and exhaust process of the air compressor. When the user's air consumption is less than the air compressor's exhaust volume, the pipe network pressure rises to reach the set upper limit value Pmax (the table is set to 0.72 MPa). The control device closes the intake valve and the air compressor enters the no-load (unloading) During the operation phase; as the user continues to use the gas, the gas pressure of the gas storage tank will drop. When the pressure reaches the set lower limit value Pmin (actual setting 0.62 MPa), the pressure controller “triggers†the solenoid valve to open the intake valve. The air compressor re-enters the loading operation phase, thus repeating the working state of adding and unloading. 3 Air compressor operation status and existing problems 3.1 Air compressor operation status In 2011, the company built a new one with an annual output of 12000T limbs for 30s, and the remaining 20 seconds were excessive. This shows that the air compressor has frequent intake valves. action. 3.2 Problems in operation 3.2.1 Starting surge current is large, can not be frequently turned on and off. Although the Y-△ step-down start is adopted, the starting current is still as high as 420A, which has a large impact on the power supply system and sometimes affects the normal operation of other equipment. Work, especially in places where the voltage fluctuation range is small, so frequent starts are not allowed. 3.2.2 The no-load running time is long, and the power is wasted seriously. Because the air compressor can't start frequently, when the air consumption is small or some equipment can't work normally, when the system pressure reaches the set upper limit, the air compressor will be inevitably closed. Intake valve unloading) Long-term no-load operation. In this state, the air compressor is not exhausted, and the external output energy is almost zero. The motor no-load is doing useless work, and the motor no-load energy consumption accounts for 25% of the rated capacity— - about 50%, so there is a serious waste of electricity. According to our actual investigation, the workshop has two-shift production, working hours are 15-16 hours, but the air compressor has 7-8 hours of idle operation, only 2-3 hours of full load operation, and the remaining one third is Light load over-running phase. 3.2.3 Power frequency operation High speed, high noise, increased maintenance The motor's speed is 2967rpm, high speed, high vibration and noise, affecting the service life of bearings, especially intake valves and other consumable parts, increasing maintenance the amount. 3.2.4 The air pressure of the pipe network is unstable, affecting the normal operation of the pneumatic components. Because the air compressor is in the state of adding and unloading, the response to the change of air pressure is slow, especially the later 101 workers Wang Hui (1985-), Anhui Yinzhou, Male, undergraduate, mechanical and electrical department of Anhui Leiming Kehua Co., Ltd., engaged in engineering power supply and distribution design management. The technology entrepreneurial house adopts liquid ammonium nitrate instead of solid bagged ammonium nitrate fertilizer. When the liquid ammonium nitrate is discharged, the gas volume increases, and the pipe network pressure fluctuates greatly, which affects the action success rate of the high-precision pneumatic components of the 103 workshop product packaging (such as the robot movement In place) 4 classification, speed control principle and control mode of the inverter 4.1 Classification of the inverter The inverter is a device capable of converting a fixed voltage and a fixed frequency alternating current into an alternating current whose voltage and frequency are continuously adjustable, according to the The working principle, control method and use are divided into the following categories. AC-DC-AC (inverter) has only one transformation link, which converts AC voltage of constant voltage and constant frequency (CV\CF) into AC power of variable voltage and frequency conversion (VVF); according to its phase number, circulation and output waveform Differently include single and three phases; with and without circulation; square wave, sine wave, etc. The device converts alternating current (AC) into direct current (DC) and converts (DC) into alternating current (AC) with adjustable voltage and frequency. According to the energy storage mode of the DC link, it can be divided into two types: voltage type (capacitor energy storage) and current type (reactance energy storage). According to different voltage regulation methods, it can be divided into pulse amplitude modulation and pulse width modulation. a. Pulse amplitude modulation (PAM) is a method of changing the amplitude of the power supply voltage (Ed) or current (Id) for output control. Therefore, only the frequency is controlled in the inverter section, and the rectification section only controls the output voltage or current. b. Pulse width modulation (PWM) is the output voltage level that is achieved by changing the duty cycle of the output pulse. It is the most widely used sinusoidal pulse width modulation method that varies according to the sine law. 4.1.2 According to the control method: voltage-frequency ratio, slip ratio, vector, direct torque four A, voltage-to-frequency ratio control U/F: The characteristic is to control the output voltage and frequency of the inverter at the same time. Next, by keeping the U/F constant, the motor obtains the required torque characteristics, the control cost is low, and it is used for a general-purpose inverter with low precision requirements. B. Slip control SF is an improved control method based on U/F. The motor and speed sensor form a speed feedback closed-loop speed control system. The output frequency of the inverter is determined by the actual speed and slip frequency of the motor. The sum is automatically set to achieve speed control while the output torque is controlled. Since the mode is closed-loop control, the speed regulation accuracy and torque performance are better than U/F, but it is necessary to install a speed sensor on the motor shaft, and adjust the slip according to the motor characteristics, and the versatility is not good. C. Vector control VC, which mainly decomposes the stator current of the asynchronous motor into a current component (excitation current) that generates a magnetic field, and a current component (torque current) that generates a torque perpendicular thereto, and respectively controls the same. The magnitude and phase of the stator current, that is, the vector of the control current, is controlled to compensate for the lack of starting torque reduction in the low frequency state. Controlled as a control amount. 4.2 Inverter speed control basic principle (DC) - AC (AC) power conversion principle, integrated electrical, electronic, microcomputer control and other technologies in a comprehensive electrical product, it can be continuously adjusted AC frequency according to the needs of work (pressure), to achieve the purpose of speed regulation. From the relationship between the speed and frequency of the asynchronous motor, n=60fX(1-s)/P, the motor speed n and the frequency f are linearly proportional, in which the n-motor speed rpm (rev/min), f-motor power frequency HZ (Hertz), S-slip, (that is, the difference between the rated speed of the asynchronous motor and the synchronous speed and the ratio of the synchronous speed) is generally 1% - 2%, the P-motor pole pair is constant, with the motor determine. Under normal circumstances, the AC power frequency (50HZ) power supply frequency does not change, the normal motor speed (if not changed P) is fixed, not adjustable; however, if the power supply frequency f is changed, the speed n changes accordingly To achieve stepless continuous speed regulation, this is the theoretical basis and basic principle of frequency conversion speed regulation. 4.3 Inverter speed control mode 4.3.1 Constant proportional control mode When the frequency converter adjusts the frequency, the output voltage is adjusted to maintain U". When f is high, the stator voltage drop of the motor can be neglected. To achieve constant flux, it must be pressed. E/f=constant value, because the induced potential E of the motor is difficult to measure and control, an approximate constant flux method is actually adopted, and the constant proportional control is adopted when the frequency is high, and the low frequency compensation measure is introduced when f is low. That is, the output voltage of the variable frequency power supply is appropriately increased through the control link to compensate the voltage drop of the stator winding, and the magnetic flux is maintained substantially unchanged. When the speed regulation frequency exceeds the rated frequency, if it is still controlled by constant ratio or constant flux, the motor running voltage will exceed the rated value, which is not allowed; at this time, constant power control is adopted, that is, when f1>f1n, Keep U1 = U1N without voltage coordination control. As the frequency increases, the air gap flux will be less than the rated flux, resulting in a decrease in torque T. When the frequency f increases, the speed n will increase, by the power P=TXn/ It can be seen from 9550 that the power reduction P is maintained constant when the multiple of T reduction is equal to the multiple of n increase (or its product remains unchanged). 5 frequency control scheme selection 5.1 constant pressure gas supply working mode variable frequency speed regulation constant pressure gas supply control principle block diagram as shown. When the gas consumption changes, the pressure sensor converts the pressure change of the gas storage tank into an electric signal to the PLC through the A/D, and sets the pressure parameter through the programmed income, and outputs it to the inverter (VF) through D/A conversion. The PID regulator compares with the set value and performs calculation according to the setting program according to the magnitude of the difference, and generates a control signal to control the frequency and voltage of the inverter (VF) to change the theoretical research speed of the variable motor; The displacement of the air compressor is linearly proportional to the speed. Therefore, when the system pressure is low (compared with the set value), the speed control device increases the inverter output frequency f and increases the air compressor speed n. The amount of exhaust increases; vice versa. In short, the actual displacement of the air compressor can be adjusted according to the pressure change of the system pipe network, and always maintains above the given minimum air pressure. The system pressure does not fluctuate significantly, ensuring the constant pressure supply of the pipe network. 5.2 Guaranteed power frequency and frequency conversion two sets (interlocking) working mode to increase the reliability of air compressor air supply to ensure normal production, the system's original power frequency operation mode (open loop) is retained, and the frequency conversion speed control mode is operated under normal conditions. (closed loop), once there is a problem with the pressure sensor or other faults in the system, it can be switched to the power frequency operation mode (open loop) without affecting normal production. The two sets of control loops are independent (interlocking with each other) to ensure the normal operation of the air compressor. 5.3 Other matters needing attention 5.3.1 The inverter must be selected according to the characteristics of the load. Generally, the load characteristics (output power) of the fan and the pump are not linearly proportional to the rotational speed (three-squared, squared, respectively), but The output power and speed of the air compressor are a linear relationship. It belongs to the constant torque load. The speed should not be too low when starting. Generally, it should not be lower than 30% of the rated speed. Considering the actual situation of the air compressor, choose the domestic GD300― 75kw vector inverter. 5.3.2 Keep the pressure of the gas storage tank stable, and the fluctuation range has no static difference control. Because the gas storage tank can keep the system air pressure change relatively stable, the insufficiency will lead to a large increase in the inertia of the system feedback, so that the pressure change feedback sensitivity is somewhat improved. To reduce (such as serious overshoot, large amplitude of oscillation, unstable system, etc.), special large inertia PID regulator must be used to overcome the above deficiencies and increase the reliability of system operation. System (S7-200), touch screen (TH765), realizes time-phase pressure control; 5.3.5 has perfect safety protection function, easy to operate and easy to maintain. Such as zero gas automatic shutdown, fault (overcurrent, over temperature, under voltage, etc.) self-diagnosis, display, query, alarm and so on. 5.3.6 Considering the electromagnetic interference of variable frequency speed regulation, it is necessary to take shielding measures (adding reactors and wiring). 6 Energy-saving effect 6.1 The power consumption before the transformation is considered to be 15 hours per day. Consider the three-hour period of full load, over 4.5 hours, and 7.5 hours of empty load (converted 140 pages). The observation equation measured by position and velocity is observed as the difference between the solution amount and the quantity. According to the PWCS observability theory, the rank of the upper TOM is the same as the rank of the extracted observable matrix (SOM) as shown in the following equation. The zero space corresponding to the continuous system observable matrix Qy is included in the F/corresponding continuous system coefficient matrix. The change of the observability of the system before and after the rotation can be directly analyzed by equation (9). The observable matrices 2 and Q2 before and after the rotation are respectively calculated, and the ranks are all 11, that is, at least three variables in the system are unobservable. In fact, we can transform the last item of the first equation in equation (1) as follows: and select the state variable as X! =TTT, forming new system equations and ordinal matrix. The linear transformation of F is multiplied by the reversible matrix cb for the 10th column to the 12th column. Since it is an elementary transformation, the rank of the observability matrix is ​​not changed. Obviously, after the elementary transformation, the differential equation of equation (1) becomes basically the same as the platform differential equation, and the same observability conclusion can be obtained, that is, the equivalent northward gyro drift, the equivalent gyro drift can be observed, etc. There is a coupling between the eastward gyro drift and the heading error. However, the X-gyro and Y-gyro drifts installed horizontally on the carrier will be mainly projected to the east and north directions, respectively, and the equivalent drift in only one direction in the north direction is observable. Therefore, X-gyro and Y-gyro are often unobservable. Using the same method, it is not difficult to obtain that the X accelerometer and the Y accelerometer bias mounted horizontally on the carrier are also unobservable. Consider the extraction and observation in the total process before and after the rotation (upper page 138) 7.5=170.25kwh (P2=P1XI3/I1 rated power X no-load average working current/rated current) After commissioning, after stable operation, the normal speed is displayed at 14901510rpm (Running for about 12 hours a day), the enthalpy is running at 890,900 rpm (about 3 hours), the power is P=TXn/9550, T is basically constant, and the energy consumption of 6.2.1 normal working speed 1 14901510 rpm can be obtained) W1 at this time the average value of the motor 900rpm) W2 motor's actual work 6.2.3 after the total daily power consumption W = W1 + W2 = 333.6 + 51 = 384.6kwh 6.3 years of electricity and reduce electricity costs 6.3.1 Annual electricity consumption = 132.25kwh, 24 working days per month, considering November, annual electricity saving at least 24X11X132.25=34914wkh6.3.2 years of saving electricity expenses to calculate the comprehensive industrial electricity price of 1.2 yuan / kWh, annual electricity saving 1.2 X34914=41896.8 yuan, the investment can be recovered in less than one year. 7 Conclusion Air compressor frequency conversion speed regulation energy-saving transformation, technical reliability (running for half a year), simple installation, easy operation, low failure rate, considerable economic benefits, it is worthy of promotion and application.
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Analysis on Energy-saving Reconstruction of Air Compressor Frequency Control