The precautions for the inverter application are as follows:
1) The signal line and control line should be shielded, which is good for preventing interference. When the line is long, for example, the distance jumps 100 m, the wire cross section should be enlarged. Do not place the signal wires and control wires in the same cable trench or bridge as the power cables to avoid mutual interference. It is best to place them in a tube. This is more appropriate.
2) The transmission signal is mainly based on the selection of the current signal, because the current signal is not easily attenuated and is not susceptible to interference. In practical applications, the signal output by the sensor is a voltage signal, and the voltage signal can be converted into a current signal by the converter.
3) The closed-loop control of the inverter is generally positive, that is, the input signal is large and the output is also large (for example, when the central air-conditioning is working and the general pressure, flow, temperature, etc. are controlled). However, it is also counterproductive, that is, the input signal is large, and the output is relatively small (for example, the central air conditioner is used for heating and the heating water pump of the heating station). Closed loop control is shown in Figure 1.
4) If the pressure signal can be selected during closed-loop control, do not select the flow signal. This is because the pressure signal sensor is low in price, easy to install, small in workload, and easy to debug. However, if the process has flow ratio requirements and the requirements are accurate, then the flow controller must be selected, and the appropriate flow meter (such as electromagnetic type, target type) should be selected according to actual pressure, flow, temperature, medium, speed, etc. , vortex street, orifice plate, etc.).
5) The built-in PLC and PID functions of the inverter are suitable for systems with small signal fluctuation and stable. However, since the built-in PLC and PID functions only adjust the time constant during operation, it is difficult to obtain a satisfactory over-process requirement, and the debugging is time consuming.
In addition, this adjustment is not intelligent, so it is not often used, but an external intelligent PID regulator is used. For example, Japan Fuji PXD series, Xiamen Anton, etc., is very convenient. When using SV (upper limit value), there is PV (running value) indication during operation, and it is intelligent, ensuring the best transition process conditions, and it is ideal to use. Regarding PLC, according to the nature of control quantity, number of points, digital quantity, analog quantity, signal processing and other requirements, various brands of external PLC, such as Siemens S7-400, S7-300, S7-200, etc., are selected.
6) The signal converter is also frequently used in the peripheral circuit of the inverter, and is generally composed of a Hall element plus an electronic circuit. According to signal transformation and processing, it can be divided into voltage variable current, current variable voltage, DC to AC, AC to DC, voltage to variable frequency, current to variable frequency, one input and multiple output, multiple input and output, signal superposition, signal splitting, etc. Various converters. For example, the Shenzhen St. CE-T series of power isolation sensors / transmitters, the application is very convenient. There are quite a few similar products in China, and users can choose their own applications as needed.
7) When the inverter is applied, it is often equipped with peripheral circuits. The methods are often as follows:
(1) A logic function circuit composed of control elements such as self-made relays;
(2) buy off-the-shelf unit external circuits (such as Mitsubishi Corporation of Japan);
(3) Select simple programmable controller LOGO (both foreign and domestic);
(4) When using different functions of the inverter, a function card (such as Japan Sancha inverter) can be selected;
(5) Select small and medium programmable controllers.
8) There are two common types of frequency conversion technology transformation schemes for multiple pumps connected in parallel with constant pressure water supply (for example, clean water pumps, medium and large water pump stations, hot water supply center stations, etc.).
According to the experience of use, the scheme (1) saves the initial investment, but the energy saving effect is poor. Start the inverter to 50 Hz after starting, then start the power frequency, and then switch to energy-saving control. In the water supply system, only the water pump driven by the frequency converter is used, the pressure is slightly smaller, and the system has turbulence and loss.
The scheme (2) has a large investment, but it is 20% more energy-efficient than the scheme (1). The pressure of the pump is the same, and there is no turbulence loss, and the effect is better.
9) When multiple pumps are connected in parallel with constant pressure water supply, only one sensor is used in the signal series mode, and the advantages are as follows.
(1) Cost savings. Just a set of sensors and PIDs, as shown in Figure 4.
(2) Since there is only one control signal, the output frequency is the same, that is, the same frequency, so the pressure is also consistent, and there is no turbulence loss.
(3) During constant pressure water supply, when the flow rate changes, the number of pumps that are started is changed by PLC control. At least one set at a time, two at a moderate amount, and three at a larger amount. When the inverter is not working, the circuit (current) signal is in the path (the signal flows in, there is no output voltage, frequency).
(4) More beneficially, because the system has only one control signal, even if the three pumps are different, the operating frequency is the same (ie, synchronous), and the pressure is also the same, so that the turbulence loss is zero, that is, the loss is the smallest, so the power is saved. The best results.
10) Reducing the substrate (basic frequency) is the most effective way to increase the starting torque. The principle analysis is as follows.
(2) Why is it better to reduce the base frequency and increase the starting torque? The details are listed in Table 1.
As shown in Table 1, because the starting torque is greatly improved, some equipments that are difficult to start, such as extruders, washing machines, dryers, mixers, coating machines, mixers, large fans, pumps, Roots blowers All of them can be started smoothly. This is more effective than the usual increase in the starting frequency. Using this method, combined with the heavy load to light load measures, to improve the current protection to the maximum, almost all equipment can be started. Therefore, it is most effective and the most convenient way to reduce the base frequency to increase the starting torque.
(3) The base frequency reduction does not have to be reduced to 30 Hz when this condition is applied. It can be stepped down every 5 Hz, and the frequency of the drop can be as long as the system can be started.
(4) The lower limit of the substrate frequency should not be lower than 30 Hz. From the torque point of view, the lower the lower limit, the greater the torque. However, it is also necessary to consider that the voltage rises too fast and the IGBT is damaged when the dynamic du/dt is too large. The actual result is that this torque boosting measure can be safely and safely used when the temperature drops from 50 Hz to 30 Hz.
(5) Some people are worried, for example, that the voltage has reached 380 V when the substrate frequency is lowered at 30 Hz. Then, when normal operation may need to reach 50 Hz, whether the output voltage jumps 380 V, so the motor can not stand, the answer is that this phenomenon will not happen.
(6) Some people worry that if the falling substrate is 30 Hz, the voltage has reached 380 V. Then the normal operation may need to reach 50 Hz when the output frequency reaches 50 Hz. The answer is that the output frequency can of course reach 50 Hz.
(7) The above two (5) (6) are determined by the software writing process. The use process has been confirmed, these two points can be assured.
11) The relationship between dynamic pressure, static pressure and total pressure is as follows:
(1) Static pressure is the pressure required to reach the highest point of the pump outlet pressure (head), generally 1 kg water pressure per 10 m high water column.
(2) Dynamic pressure is the pressure drop caused by the resistance caused by the difference between the flow rate of the liquid and the pipe wall, the valve (regulating valve, the check valve, the pressure reducing valve, etc.) and the different layers of the same section during the flow of water. Part of the calculation is very difficult.
(3) Full pressure = (static pressure + dynamic pressure) = 1.2 static pressure.
(4) The pump must set the lower limit frequency to about 30 Hz, otherwise it is easy to evacuate the water in the closed tube. Because a large amount of air is dissolved in the water, when the water pump is to be started, the air chamber is easily generated, posing a high voltage danger.
12) The empirical and economic values ​​are as follows:
It is feasible to use a frequency converter to save power for various devices. This has been confirmed by many practical success stories.
(1) The empirical value is more conservative, and it has greater wealth, is not the most economical, and has the potential to be tapped. When the empirical value is used, it shall be arranged according to the actual situation on site, and the working condition parameters shall be used with certain changes, so as not to affect the normal use as the lower limit condition. This is a prerequisite for achieving energy savings.
(2) The economic value is based on the principle of meeting the lower limit of the system, the empirical value is moderately reduced, and the potential is tapped to achieve energy-saving effects. If the working condition parameters are unchanged, what is the energy saving? Moreover, the frequency converter itself is not an energy generating device (generator, battery, solar energy), its own efficiency is very high, from 97% to 98%, but there is always loss, from 2% to 3%.
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