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The basic principle of frequency control water supply
Aug 17, 2018

At present, the application of variable frequency speed regulation water supply in building water supply is more and more, the main reasons are:


1. The water supply pressure of the variable frequency speed control water supply is adjustable, which can conveniently meet the needs of various water supply pressures. In the design phase, the accuracy of the calculation of the water supply pressure can be reduced, because the water supply pressure can be easily changed at any time. However, when selecting the pump, it should be noted that the lift of the pump should be larger, because the frequency control of the Zui is limited by the pump. The minimum use pressure should not be too small, because the pump does not allow long-term overload operation under low lift and large flow, otherwise the capacity of the inverter and pump motor should be increased to prevent overload.


2. At present, the inverter technology is very mature. There are many inverters of domestic and foreign brands in the market, which provides sufficient technical and material basis for variable frequency speed regulation water supply. The frequency converter has been widely used in various sectors of the national economy. Any brand of frequency converter and variable frequency water supply controller can achieve multi-pump parallel constant pressure water supply. Because of the wide application of building water supply, some inverter design manufacturers directly use the variable frequency water supply controller in the water supply special frequency converter; this kind of frequency converter has the advantages of good reliability and convenient use.


3. Variable frequency speed regulation constant pressure water supply has excellent energy saving effect


According to the water supply principle of the water pump, the water supply flow can be adjusted. In principle, there are two methods; one is the throttling regulation, the large water supply valve is opened, the flow rate is increased; the small water supply valve is closed, and the flow rate is decreased. The second method of regulating the flow rate is speed regulation, the water pump speed is increased, the water supply flow rate is increased, the speed is decreased, and the flow rate is reduced. For occasions where the water flow rate changes frequently (such as domestic water), the speed regulation is used to adjust the flow rate, which has excellent energy saving. effect. China's State Science and Technology Commission and the State Economic and Trade Commission listed the speed control technology of pumps and fans in the "China Energy Conservation Technology Policy Outline" as an energy-saving technology project promoted by the National Ninth Five-Year Plan. It should be pointed out that the energy-saving effect of variable-frequency constant-pressure water supply mainly depends on the change of water flow rate and the reasonable matching of water pump. In order to make the variable-frequency constant-pressure water supply have excellent energy-saving effect, the variable-frequency constant-pressure supply should not adopt the multi-pump parallel water supply mode. The multi-pump parallel constant-voltage variable-frequency water supply theory can be used to know that the multi-pump is connected in parallel with the constant-pressure water supply. As long as one of the pumps is a variable-frequency pump, the rest are all power-frequency pumps, which can realize constant-pressure variable water supply. When the variable frequency constant pressure variable water supply, the flow rate of the variable frequency pump is changed. When the variable frequency pump is large in each parallel pump, the constant pressure water supply can be ensured. The multi-pump parallel constant pressure water supply can be designed to achieve the same efficiency under constant pressure conditions and the power frequency pump (due to the working conditions), and make it work in the high efficiency zone. The flow rate of the variable frequency pump is variable. Its work efficiency changes with the flow. Because the multi-pump parallel constant pressure water supply is adopted, the power of the variable frequency pump is reduced, thereby reducing the energy consumption of the multi-pump parallel variable frequency constant pressure water supply system and improving the energy saving condition.


When the multi-pump parallel constant pressure water supply system adopts the frequency converter with automatic sleep function, when the water flow rate is close to zero, the variable frequency pump can automatically sleep and stop the magnetic circuit, so that the automatic pump stop without water consumption without energy loss can be achieved. Energy saving effect.


The working mode of multi-pump parallel variable frequency constant pressure variable water supply is usually as follows: when the water flow rate is less than the flow rate of one pump under the power frequency constant condition, the constant pressure water supply is regulated by a variable frequency pump; when the water flow rate increases, the frequency conversion The speed of the pump rises automatically; when the speed of the variable frequency pump rises to the power frequency, the water flow is further increased, and the frequency conversion water supply controller automatically controls a power frequency pump to draw in. The flow rate provided by the power frequency pump is constant. (The flow rate under the constant voltage of the power frequency), the other parallel power frequency pumps are put into the same principle.


In the case of multi-pump parallel variable frequency constant pressure variable water supply, when the water flow rate decreases, the speed of the variable frequency speed control pump decreases (the inverter power supply frequency decreases); when the frequency drops to zero flow, the variable frequency water supply controller issues an instruction. Automatically shut down one power frequency pump to exceed the parallel water supply. In order to reduce the impact of the automatic input of the power frequency pump (hydraulic or current impact). When input, the speed of the lead pump automatically drops, and then slowly rises to meet the requirements of constant pressure water supply. The above frequency automatically rises and the drop is controlled by the water supply inverter controller.


Another type of variable frequency water supply mode is usually called a constant pressure variable cycle start and first open and stop mode. In this water supply mode, when the water supply flow rate is less than the flow rate of the variable frequency pump at the constant voltage power frequency, the variable frequency pump automatically adjusts the water supply. When the water flow rate increases, the speed of the variable frequency pump increases. When the speed of the variable frequency pump rises to the power frequency, the variable frequency water supply controller controls the pump to be directly powered by the power frequency grid (not powered by the inverter). The inverter is switched on and a parallel pump is started. As the water flow rate increases, the remaining and associated pumps are soft-started in the same manner as described above. This is the way the loop soft start is invested.


When the water flow rate is reduced, each parallel power frequency pump is exceeded in order, and the order in which the pump is exceeded is controlled by the single-board computer of the inverter controller according to the principle that the pump is exceeded first.


It can be seen from the above that there are usually two working modes for the constant pressure variable feed water of the frequency converter, one is the fixed mode of the variable frequency pump, and the other is the soft start working mode of the frequency conversion cycle. In the variable frequency fixed mode, each parallel water pump is automatically input or exceeded according to the power frequency mode. Because the variable frequency pump is fixed, when the water flow rate changes, the variable frequency pump is always in the running state, so the running time of the variable frequency pump is long. In order to balance the running time of each pump, for the fixed operation mode of the variable frequency pump, it can be designed as the timing rotation operation mode of the variable frequency pump. That is, when a certain variable frequency pump runs for a certain period of time, the variable frequency pump is controlled by the variable frequency controller to automatically perform the rotation. For example: 1 pump frequency conversion at the beginning, 2-3 pump power frequency, when 1 pump frequency conversion operation T time (T can be set in order) automatically rotate to 2 pump frequency, 3-1 pump power frequency, run in this state After T time, it will automatically rotate to 3 pump frequency conversion, 1-2 power frequency, ... so repeat the timing rotation.


Obviously, the variable frequency constant pressure variable water supply system with the automatic rotation control of the variable frequency pump, the variable frequency pump is changed periodically, that is, any parallel pump may become the variable frequency pump. According to the theory of variable frequency constant pressure variable water supply, in order to ensure constant pressure water supply, the variable frequency pump must be the largest of the parallel pumps. For this reason, for the water machine with variable frequency constant pressure water supply and automatic variable frequency pump, the size of each parallel water pump should be the same to ensure constant pressure water supply.


According to the working principle of the inverter, the inverter in operation is not allowed to switch at its output; otherwise, some electronic components in the inverter will be subjected to high current impact during the switching process to reduce its life. In the automatic rotation process of the lead pump, it is necessary to switch at the output end of the inverter; in order to protect the inverter, the inverter should be stopped before automatic switching. Switching at its output with the drive stopped. After switching, restart the inverter and resume normal operation. Therefore, the circuit of the automatic rotation control is complicated, which increases the cost of the inverter control cabinet and reduces the reliability of its use.


When the variable frequency constant pressure variable water supply system has the automatic rotation function of the variable frequency pump, the advantage is that each parallel pump can be periodically rotated to the frequency conversion operation, so that the wear of each parallel pump is balanced. However, in the case of any sand frequency conversion operation, in case the pump failure may cause the inverter protection to trip and stop working. Each parallel pump is controlled by the inverter; when the inverter trips, all parallel pumps must be stopped and the water supply is interrupted.


Therefore, when the reliability of the water ocean is certain, the water supply reliability of the variable frequency constant pressure water supply machine with automatic rotation control function will be lower than that of the variable frequency constant pressure water supply machine without the automatic rotation control function. The author believes that the reliability of water supply is the main contradiction. Therefore, we do not advocate the use of variable frequency constant pressure water supply system with automatic rotation control. The multi-pump parallel, cyclic soft-start variable-frequency constant-pressure water supply system also has the shortcomings of the above-mentioned variable-frequency constant-voltage automatic rotation working mode. In order to ensure constant pressure water supply, the size of each parallel pump is also required to be the same.


In summary, in order to ensure the reliability of water supply, the author does not advocate the use of automatic rotation and variable frequency cycle soft start working mode. Tsinghua Unisplendour Group Automation Engineering Department said in its "ABB Constant Pressure Water Supply System User Manual" that "circular soft start! This is a dangerous temptation. Many people who are engaged in constant pressure water supply are keen to develop this technology, but we recommend whether "We agree with the above-mentioned academic insights of the Automation Engineering Department of Tsinghua Unisplendour Group, and are not keen on engaging in variable frequency circulating soft start water supply.


It can be known from the water supply principle of the water pump that when the throttling loss is equal to zero, the water supply system has a good energy saving effect. At this time, the water supply head of the water pump is completely consumed in the water supply height and the water flow resistance loss. This variable frequency adjustment water supply is called a state-of-the-art variable pressure variable water supply. The head-flow curve of this water supply system and the flow resistance of the pipeline system-flow curve coincide. In the ideal variable pressure variable water supply system, the water head has a constant head and is a constant pressure water supply. In actual buildings, there are many points of water use, not one place, so it is difficult to determine where the constant pressure water point is. The variable pressure variable water supply system is not versatile and is rarely used in engineering. A practical variable pressure variable water supply system is called a quasi-variable pressure variable water supply system; in a quasi-variable variable water supply system, its constant pressure value increases with the increase of water flow rate. For example, multi-pump parallel constant pressure water supply, when a pump works, its constant pressure value will automatically become P1 + ΔP1; when two, three, four pumps are put, its constant pressure value will automatically become P1 + ΔP2, P1 + ΔP1 + ΔP2 + ΔP3, ΔP4... P1ΔP1ΔP2ΔP3, ΔP4... can be set as needed; therefore, the water supply characteristics of the quasi-variable pressure variable system (equipment) can be very close to the ideal variable pressure variable water supply characteristics, with excellent energy saving effect, and the water supply system (equipment) has universal Sex. For example, the internationally renowned ABB water supply dedicated frequency converter has the above-mentioned quasi-variable variable water supply control function.


In fact, in the construction water supply, the quasi-variable variable water supply mode is rarely used because it is difficult to give specific parameters in actual use.


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