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Feasibility Analysis on Application of Water Pump Frequency Converter in Water Supply Plant
ccording to relevant data, more than 70% of the total electricity generated in China is lost to electric motors, and pumps and fans account for 40% of the total consumption. By replacing the original flow regulation with motor frequency conversion, an average of 30 to 40 billion kilowatts can be saved per hour. In the waterworks, the frequency converter is mainly used for pumping water and water supply. In addition to saving electricity, it can also adjust the flow rate of smooth water intake and the pressure of water delivery to meet the requirements of water supply technology and water production.

First, the principle of frequency conversion and energy saving of water purification plants

    In most of the rebuilt and newly built automated water pumping stations, inverters are equipped to further reduce energy consumption while improving the process. In this process, the inverter control mode is basically the same as the PLC and the manual operation mode, and can be switched on-site through the control box. In the water pumping room of the water plant, through the PLC automatic control system and PID adjustment, not only can the inverter automatically adjust the speed, but also ensure the constant pressure water supply, even in the unattended pump house, can further improve the water plant production efficiency With security. In the water taken by the inverter, the PID and PLC adjustment can not only allow the inverter to achieve automatic speed control, but also allow the clean water to be in a stable state all the time, and provide high-quality and efficient water supply in the unattended pump room. In the automatic water pump house control design, usually the water level controls the pressure control of the pump room and the frequency of the inverter, and the water pressure is affected by changing the working status of the water pump room converter. The time lag of the clear water tank is especially prominent, and it also flocculates. The impact of water flow in the pool, advection tank and filter tank is also great. Therefore, using the water level of the clear water tank to save energy in the pump house unit has no satisfactory effect on energy saving. It is the main characteristic of the water pump that the load torque and the square of the speed are presented in a positive proportion relationship. At the same time, the shaft power is proportional to the cubic amount of the speed. Q: Qo=n:no, H:Ho=(n:no)2, P:Po=(n:no)3, where no is the rated reference rotational speed, n is the operating rotational speed, Ho is the head of no, and Qo is The flow of no, H is the head of n, Po is the power of no, and P is also the power of n. For the actual pump load, there is a head that has actual contact with the height difference. Special attention must be paid to the frequency conversion operation or the speed regulation.

    In actual work, the run point is generally blocked by the pipeline, and the intersection point between H and Q also plays an important role. For example, more than 80% of the operating points will not be in the C area but in the D area. At the same time, shaft power must take into account similar problems, which means that the operating point cannot cross the cubic curve directly. That is to say, at the same speed, the greater the lift, the greater the proportion of flow reduction. When the speed range is reduced to a certain range, the energy-saving effect will be reduced.

    In the speed energy-saving effect control and valve control, if the waterworks flow rate is converted from 1.0 to 0.5, in the valve control, using the small valve to make the impedance of the correlation curve from R1 to R2, the working point at this time will also be from the original Point A moves to point B; if speed control is used, R1 will move directly from point A to point B in the same impedance curve. When the P-to-Q axis power characteristic curve is constant, 100% of the valve speed will directly move from point A to point B. In the speed control, it will move from point A to the control power of the actual head. Point D, compared with the valve control results, it can get the equivalent of BD saving. If you are using variable frequency speed control, since the actual lift is relatively small, the shaft power will be closer and closer to the cubic curve, and the energy saving effect of speed regulation will be even greater.
    For the water pump of the waterworks, since the water supply system needs to be replenished at any time, the water consumption varies greatly in different time zones and different seasons, and the system and flow pressure to be supplemented also differ greatly. The traditional energy-saving method is to adjust the head pressure and the water-saving flow, which not only wastes resources, but also has a troublesome process, requires higher costs, and also causes a lot of mechanical losses.

Second, the water plant pump frequency converter energy-saving technology requirements

    In the energy-saving transformation of water pump inverters in waterworks, inverters generally use a combination of remote control and local control. The remote control includes pressure negative feedback and frequency control, and the general frequency adjustment is between 0 and 50 Hz. The direct frequency control and pressure negative feedback remote control are all PLC-selected. In the process of operation, it can also be selected according to the specific control mode of the inverter. The local control is the control of the panel, and the control mode is adjusted to LOCAL. In the frequency adjustment, manual frequency control is used to ensure the factory pressure and flow. In the transformation of the waterworks converter, in order to ensure the quality of the transformation, it is necessary to meet the basic energy saving requirements in accordance with the energy saving target. In this process, the price is reasonable; it is important that the pump must achieve high-strength continuous and reliable operation, and then the protection system From the power frequency to frequency conversion, and from frequency conversion to power frequency conversion, once the instantaneous power outage occurs, the system can be operated normally through automatic restart.

    In a water plant pump inverter, the system has three pumps, and each motor capacity can reach 75KW, one as a spare, and the other two normal work. The specific conditions are as follows: In the three pumps, they are operated at fixed speed and variable speed respectively, and the inverter is intelligently used as a power source for a motor. Therefore, the stopping and starting of each motor will correspond to each other. Through this kind of logical circuit control, not only the circuit switching can be guaranteed, but also the energy saving goal can be achieved.

    In the external wiring and frequency converter selection, the general use of fan, pump special inverter, using PLC control. In the inverter wiring, the pipe pressure in the operation of the pump will directly affect the operation status of the pump. If the pipe pressure is higher than 0.8, one set of constant speed, variable speed, and reserve are used; when the pipe pressure is lower than 0.64, two sets are used for backup, one for variable speed or fixed speed; when the pipe pressure is lower than 0.52, two sets are used for backup and one is used as Speed change. These three conditions are based on the pipeline flow and the pressure sensor signal, which is converted into a control signal by the function generator, and then the valve and the motor are started. For an operating motor, the pressure signal is generally switched at three different speeds.

    In the inverter wiring, there is no direct relationship between the motor rotation and the phase sequence connection of the power supply. Generally, the motor steering is achieved according to the motor connection method. In this process, it should be noted that: The motor side and the power supply side must not be reversed, otherwise it will damage the inverter. For the power switch, KM1 contactor, QF capacity is selected according to the specific requirements of the water plant pump inverter. Through the installation of power meters, voltmeters, power factor meters, ammeters and other analog-type meters on the inverter, the motor is carefully monitored; for relay connections such as CM, FWD, and C1V1, a high-sensitivity miniature dual-contact relay is usually used. . In the frequency setting, it can be manually switched according to the flow rate and pressure. In order to convert the power factor, the DK coil must be installed to set the alarm circuit. Once a fault occurs, the alarm effect can be achieved through the sound and light.

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