Frequency conversion cycle switching constant pres

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In the water supply pump station of the waterworks, the water supply system is generally composed of several pumps with similar head. The traditional method of adjusting the water pressure and flow is to manually control the number of water pumps according to the expected output water pressure and flow. For example, in a waterworks with a water supply capacity of 10000 tons/day, there are many pump configuration schemes, one of which is a feasible scheme consisting of three 160kW and one 90kw pumps. When the system works, the traditional method is that if the water supply is large, obviously, the flow and pipe water pressure can not meet the requirements. At this time, it is necessary to manually input the water pump. According to the on-site pipe water pressure, the workers decide whether to input the 160kW water pump or the 90kw water pump; If the water supply decreases, the pipe pressure will rise, and then the pump needs to be cut off manually. At night, the water consumption is small, and a 90kw water pump is considered for energy conservation. Due to the large flow of the pump, in order to avoid the "water hammer" effect, when manually switching on and off, the pump should follow the "start first, then open the valve". When switching off the pump, the operation procedure of "close the valve first, then stop" should be followed. For low-power water pumps, ordinary check valves are installed on the outlet side of the water pump, which can automatically ensure the above operating procedures, but there is still a "water hammer" effect at the moment before the check valve is closed during shutdown. If the "micro resistance slow closing check valve" is installed, there is basically no "water hammer" effect during shutdown

second, the control scheme of variable-frequency constant pressure water supply

because the consumption of urban tap water changes with the season, and changes with the time of day. In order to keep the water pressure of water supply constant, the most common method is to adopt the variable-frequency constant pressure water supply system, that is, the pressure transmitter is installed on the main pipe to detect the pipe pressure signal, and then the pressure signal is sent to the analog signal input port of the frequency converter (PLC), thus forming a pressure closed-loop control system. The constant pipe pressure depends on the regulation and control of the frequency converter. For the case of multiple pumps, there are two different control system schemes. One is "sequential control scheme". The system diagram is shown in Figure 1

Figure 1 sequential control scheme system diagram

in the figure: BP1 - frequency converter; Bu1 ~ bu4 - soft starter, Pt - pressure transmitter. It can be seen from Figure 1 that the frequency converter is connected to the first water pump motor. When the pump needs to be added or reduced, the output signal of the frequency converter ro1~ro3 ports starts or stops other water pumps. At this time, the starting of the water pump adopts the self coupling decompression starting device or soft starter. The characteristic of this scheme is that the pump motor does not need to be converted. It switches between the rotating speed and power frequency of the electromechanical system; The first water pump is always connected to the frequency converter, and there is no voltage loss during the switching process; Since all pumps except the variable frequency pump have soft starters, there is no need to make a standby system. When the frequency converter fails, the M2 ~ M4 water pumps can be manually started with soft starters to ensure that the water supply will not be interrupted; Each motor has a starter, and the initial investment is large. The other is "cycle switching" "The scheme and system diagram are shown in Figure 2.

Figure 2 frequency conversion constant pressure water supply cycle switching scheme system diagram

in the figure: BP1 - frequency converter, bu1 - soft starter, Pt - pressure transmitter, zj1, zj2 - used for the start/stop and automatic/manual conversion of the control system. It can be seen from Figure 2 that the frequency converter is connected to the first water pump motor. When it needs to add pump, the frequency converter stops running, and the output port RO1 ~ ro3 of the frequency converter outputs the signal No. to PLC, and PLC controls the switching process. At the beginning of switching, the frequency converter, but the modified material such as Geon bio is their core business unit, stops output (the frequency converter is set to free stop), uses the inertia of the water pump to switch the first water pump to power frequency operation, and the frequency converter is connected to the second water pump to start and operate. As such, the second water pump is switched to power frequency operation, and the frequency converter is connected to the third water pump to start and operate; Need 2 When there is a load phenomenon and the pump needs to be reduced, the system stops the first water pump, and the second water pump with fairy ball pattern printed on the two-way elastic cloth stops. At this time, the frequency converter is connected to the third water pump. When the pump needs to be added again, switch to the third pump for circulation. This way ensures that one pump will always operate with frequency conversion, and any one of the four pumps may operate with frequency conversion. In this way, the pipe pressure can be kept basically constant no matter how the water consumption changes, and the operation time of each pump is basically the same, which brings convenience to maintenance and overhaul. Therefore, most water supply manufacturers are fond of the circulating switching scheme. However, this scheme also has shortcomings, that is, in the process of only one frequency converter running and switching to power frequency, it will cause short-term voltage loss of the pipe, which should be paid full attention to in the design. In addition, a set of standby system must be set, and the soft starter in the figure is used as standby. When the frequency converter or PLC fails, the soft starter can be used to manually start each pump to operate and supply water in turn

III. working process of cyclic switching

as we all know, the output end of the frequency converter cannot be connected to the power supply, nor can it be switched off with load during operation. The switching process should be carried out according to the following procedures. When the circulating switching constant pressure water supply system is put into operation, when the output frequency of the frequency converter has reached 50Hz or 52hz, whether the upper limit frequency of the frequency converter can be set to 52hz depends on whether the pump motor is overloaded when running at 52hz. The water pressure of the pipe does not reach the given value after operating for 60s at 50Hz frequency. At this time, the pump needs to be switched to power frequency operation. The switching process is: first close the electric valve of the water pump, and then stop the frequency converter (the parking mode is set to free parking). The water pump motor operates inertia. Considering the residual voltage in the motor, the motor cannot be switched to power frequency immediately, but delay for a period of time until the residual voltage in the motor drops to a smaller value, which ensures that the switching current impact caused by the difference between the power supply voltage and the residual voltage is small, The switching time of 160kW water pump motor in a water plant is 600ms. The air switch connected in the power frequency circuit of the motor has a capacity of 400A. After on-site commissioning, the current impact during the switching process is small, and each switching is 100% successful. Shut down after closing the valve, and the water pump motor is basically in no-load operation. At 600ms, the speed of the motor does not drop much, making the current impact small during switching. After switching, open the electric valve; The stopped frequency converter is switched to another water pump to start and operate, and then the electric valve is opened. When cutting off the power frequency pump, close the valve first and then stop, so that there is no "water hammer" phenomenon. These

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