Compared to traditional water tanks and towers, variable frequency constant pressure water supply has the advantages of stable water supply pressure, high energy-saving rate, good reliability, and no pollution to water resources. This article takes the example of a water supply plant using a frequency converter to drive three pumps to achieve constant pressure water supply, and provides a detailed introduction to a series of dedicated frequency converters for water supply and their application in constant pressure water supply systems.
Introduction
The water supply system is an indispensable and important part of national production and life. The traditional water supply method occupies a large area and is prone to water pollution. The main disadvantage is that the water pressure cannot be maintained constant, resulting in some equipment not working properly. Variable frequency speed regulation technology is a mature AC motor stepless speed regulation technology. In the increasingly urgent demand for safety production and water supply quality, the solution of constant pressure water supply through variable frequency speed regulation is becoming more and more recognized by the public.
On site application equipment and pictures
The on-site application is for a water supply plant in Shenzhen. One 4kW frequency converter is required to control three water pumps to achieve constant pressure water supply. The frequency converter is required to automatically adjust the speed according to the actual water pressure situation and determine whether the second and third pumps need to be put into operation to participate in water supply.
Control Theory
control scheme
The water supply system is equipped with pressure sensors on the official water supply website, which convert the water pressure signal into a 4-20mA current type signal acceptable to the frequency converter. When the system is working, the pressure sensor detects the water pressure in real time, and the frequency converter receives the pressure signal. The difference between the actual pressure signal and the given pressure signal is adjusted to form a pressure closed-loop system. The frequency converter adjusts the speed of the water pump motor by changing the frequency through PID output. However, the upper and lower limits of the operating speed of the variable frequency pump limit the ability of a single pump to cope with peak and off peak periods of water consumption. The CT110 constant pressure water supply dedicated frequency converter developed by Shenzhen Dongli Sci Tech Innovation Technology Co., Ltd. combines the above situation to expand the water supply dedicated logic. Through two auxiliary relays and the number of activated units, it automatically adjusts the peak and valley water consumption to ensure constant pressure in the water supply network and meet the water needs of users.
CT110 Water Supply Logic Explanation
The CT110 series dedicated frequency converter has built-in water supply specific logic and optimized PID control to ensure constant water pressure. At the same time, it automatically processes the logic of adding and subtracting pumps, and automatically adjusts the frequency during the pump addition and subtraction stages to ensure that the water pressure remains stable and controllable during the pump addition and subtraction process.
The water supply logic is explained as follows
1. Pump addition logic: When the water pressure continues to be lower than the set pressure, the frequency converter accelerates and runs. When the frequency converter accelerates to the pump addition frequency point (F13.01), if the water pressure is still lower than (set water pressure percentage) (pump addition pressure tolerance percentage F13.02), it is considered that the current number of water pumps is not enough to use, and more water pumps need to be added. After the pump addition delay time is reached, the auxiliary relay will act, and the pump will run.
2. Pump auxiliary logic: The newly added pump is a power frequency pump, which may cause a rapid increase in water pressure during the pumping process. Therefore, the variable frequency pump will automatically reduce its frequency during the pumping process to avoid excessive water pressure. The deceleration time of the variable frequency pump at this time is determined by F08.01.
3. Pump reduction logic: When the water pressure continues to be higher than the set pressure, the frequency converter runs at a reduced speed. When the frequency converter decelerates to the pump reduction frequency point (F13.04), if the water pressure is still lower than (set water pressure percentage)+(pump reduction pressure tolerance percentage F13.05), it is considered that the current number of water pumps is too many and the pump operation needs to be reduced. After the delay time of the pump reduction is reached, the auxiliary relay will act, and the pump will run at this time.
4. Pump reduction auxiliary logic: The newly reduced pump is a power frequency pump, which may cause a rapid drop in water pressure during the pump reduction process. Therefore, during the pump reduction process, the variable frequency pump will automatically increase the frequency to avoid low water pressure when adding the pump. The acceleration time of the variable frequency pump at this time is determined by F08.00.
5. Sleep function logic: When the auxiliary pumps have stopped and the water pressure is still high, the frequency converter will run at a reduced speed. When the frequency of the frequency converter is lower than the pump reduction frequency point, the frequency converter will automatically sleep and the keyboard will display the "SLEEP" status.
6. Sleep and wake-up logic: In the sleep state of the frequency converter, when the water pressure is low, the set frequency calculated by the PID is higher than the wake-up frequency setting, and the current pressure is lower than (set water pressure percentage) - (wake-up pressure tolerance percentage F13.02), it is considered that the frequency converter pump needs to run. After a wake-up delay, the frequency converter pump will sleep and wake up.
7. Priority of water pump control: The priority of water pump participation in operation is: variable frequency pump>auxiliary pump 1>auxiliary pump 2. That is, when it is necessary to add a pump, first add a variable frequency pump, then auxiliary pump 1, and finally auxiliary pump 2; When it is necessary to reduce the pump, first reduce auxiliary pump 2, then reduce auxiliary pump 1, and finally reduce the frequency converter to sleep and standby.
Introduction to Dongli Sci Tech CT100 Water Supply Special Frequency Converter
The CT110 frequency converter is based on a DSP control system and adopts domestically leading PG free vector control technology, combined with multiple protection methods, which can be applied to asynchronous motors and provide excellent driving performance. The product has greatly improved customer usability and environmental adaptability in terms of air duct design, hardware configuration, and software functionality.
Technical Features
◆ Water supply dedicated logic: Based on on-site working conditions, the water supply logic provides more stable constant pressure control performance.
◆ Accurate motor parameter self-learning: Accurate self-learning of rotating or stationary motor parameters, easy debugging, simple operation, providing higher control accuracy and response speed.
Vectorized V/F control: automatic stator voltage drop compensation, VF control can also ensure excellent low-frequency torque characteristics.
◆ Software current and voltage limiting function: Good voltage and current control effectively reduces the number of protection times for the frequency converter.
◆ Multiple braking modes: Provides multiple braking modes for quick parking.
High reliability design: With a higher overall overheating point and good protection level, it is more suitable for the usage environment of the water supply industry.
◆ Speed tracking restart function: Achieve smooth and impact free starting of the rotating motor.
◆ Automatic voltage adjustment function: When the grid voltage changes, it can automatically maintain a constant output voltage.
Comprehensive fault protection: protection functions for overcurrent, overvoltage, undervoltage, overtemperature, phase loss, overload, etc.
Conclusion
Applying frequency conversion control technology to the field of constant pressure liquid supply, a dedicated control module is added to provide an optimized constant pressure water supply solution. Using this dedicated frequency converter to assemble a water supply automatic control system has the advantages of low investment, high automation, complete protection functions, reliable operation, easy operation, significant water-saving and energy-saving effects, especially for water quality without causing secondary pollution, and has excellent cost-effectiveness.