First, the characteristics of frequency control and analysis of how much the user's water is often changed, so the lack of water supply or excess water supply occurs from time to time. The imbalance between water and water supply is mainly reflected in the pressure of the water supply, that is, there is more water and less water supply, then the pressure is low; if there is less water and more water is supplied, the pressure is greater. Keeping the pressure of water supply constant can maintain a balance between water supply and water use, that is, when there is a large amount of water, the water supply is also large, and when the water supply is low, the water supply is also small, thereby improving the quality of the water supply.
Constant pressure water supply system is very important for some industries or special users. For example, in some production processes, if the tap water supply is under pressure for a short time or water is cut off for a short period of time, the quality of the product may be affected. In severe cases, the product may be scrapped and the equipment may be damaged. In case of fire, if there is insufficient water supply or no water supply, rapid fire extinguishing may occur, which may cause major economic losses and casualties. Therefore, the use of constant pressure water supply systems in certain water use areas has greater economic and social significance.
With the development of power technology, frequency control technology is becoming more and more sophisticated. The intelligent water supply control system with frequency conversion as the core replaces the water supply equipment such as high water tanks and pressure tanks in the past. The start is smooth, and the starting current can be limited within the rated current. This avoids the impact on the grid at start-up; as the average speed of the pump is reduced, the useful life of the pump and the valve can be extended; the water hammer effect at start and stop can be eliminated. Its stable and safe operating performance, simple and convenient operation methods, and complete and thoughtful functions will enable water supply to save water, save electricity, save manpower, and ultimately achieve high-efficiency operation.
Second, constant pressure water supply frequency conversion application is usually the same way in the water supply system, set up more than one commonly used pump, when the water supply is large, more than one pump is fully open, the amount of water supply hours to open one or two. In the use of frequency control for constant pressure water supply, it uses two methods, one is that all pumps are equipped with a frequency converter; the second is that each pump is equipped with a frequency converter. According to the pressure feedback signal, the latter method automatically adjusts the output frequency of the inverter through the PID operation, changes the motor speed, and ultimately achieves the purpose of constant pressure in the pipe network. A closed-loop circuit is relatively simple, but the cost is high. The former method has low cost and poor performance, but the control program is more complicated and is the future development direction. Our company developed the NKL-A series constant pressure water supply control system to realize the function of an inverter to control any number of motors. The principles described below are all systems that use a frequency converter to drive multiple motors.
Third, PID control principle According to the feedback principle: In order to maintain a physical quantity unchanged or basically unchanged, it should lead this physical quantity to compare with the constant value to form a closed-loop system. If we want to keep the water pressure constant, we must introduce the water pressure feedback value and the given value to form a closed-loop system. However, the controlled system is characterized by a non-linear, large-inertia system. Now the combination of control and PID uses fuzzy control when pressure fluctuations are large to speed up the response. PID is used to maintain static accuracy when the pressure range is small. This is achieved through the PLC plus intelligent instrument can be the algorithm, while the programming of the PLC is the current pump frequency and frequency conversion. Practice has proved that using this method is feasible, and the cost is not high.
In order to maintain the pressure of the water supply network unchanged, according to the feedback theorem, a pressure transmitter is installed as a feedback component in the management of the pipe network system. Because the water supply system has a long pipe and a large pipe diameter, the pressure of the pipe network is relatively slow. The system is a large lag system, and it is not easy to directly use PID regulators to control, and PLC is used to control the system to achieve the control system.
Fourth, the frequency conversion control principle uses the frequency conversion velocity modulation to realize the constant pressure water supply, compared with realizes the constant pressure water supply with the adjustment valve, the energy conservation effect is very remarkable (can calculate according to the concrete condition). Its advantages are:
1. Start balance, starting current can be limited within the rated current, thus avoiding the impact on the power grid when starting;
2. As the average speed of the pump is reduced, the service life of pumps and valves can be extended;
3. It can eliminate the water hammer effect when start and stop;
In general, when a single inverter controls a single motor, it is sufficient to match the capacity of the inverter with the actual motor capacity. When a frequency converter controls two motors at the same time, in principle, the motor capacity of the frequency converter should be equal to the sum of the capacities of the two motors. However, if the water consumption at the peak load is much different from the full-speed water supply of the two pumps, consider reducing the capacity of the inverter appropriately, but be careful to leave enough capacity.
Although the pump operates at a low speed, the operating current of the motor is small. However, when the user's water consumption changes frequently, the motor will be in a state of frequent up and down speeds, and the current to increase or decrease may slightly exceed the rated current of the motor, resulting in the motor overheating. Therefore, thermal protection of the motor is necessary. For this kind of temperature rise due to the frequent rise and fall of speed, the electronic thermal protection function in the inverter is difficult to play a protective role, so the thermal relay should be used for thermal protection of the motor.
In terms of main function presets, the highest frequency should be the rated frequency of the motor for the maximum operating frequency of the frequency converter. When the PID regulator is used to raise or lower the speed, the time of ascent and descent should be set as short as possible to avoid affecting the dynamic response process determined by the PID regulator. If the inverter itself has a PID adjustment function, as long as the PID function is set when the preset is valid, the set up speed and the deceleration time will automatically become invalid.
V. Characteristics of constant pressure water supply system 1. Power saving: Optimized energy-saving control software enables the water pump to achieve maximum energy-saving operation;
2. Saving water: Set the pipe network pressure according to the actual water situation, and automatically control the water pump output, reducing the running and leakage of water;
3. Reliable operation: The soft start of the pump is realized by the inverter, so that the water pump realizes the no-impact switching from the power frequency to the frequency conversion, preventing the pipe network from impact, avoiding the pipe network pressure from exceeding the limit, and the pipeline cracking.
4. Networking function: All Chinese industrial control configuration software is used to monitor each site in real time, such as voltage, current, working frequency, pressure and flow of the pipe network of the motor. And can accumulate the electricity consumption of each site, accumulate the water output of each pump, and provide various forms of printed statements to analyze statistics.
5, flexible control: sub-water supply, regular water supply, manual selection of work methods.
6, self-protection function is perfect: If a pump fails, take the initiative to send alarm information to the upper machine, and start the standby pump to maintain water balance. In case of failure of the automatic control system, the user can directly operate the manual system to protect the water supply.
VI. Application scope of the system 1. Waterworks and pressurized pumping houses 2. Residential living areas, hotels and other buildings 3. Enterprise production water 4. Boiler circulating water system 5. Farmland irrigation system
Application of Frequency Converter in Constant Pressure Water Supply