Application of Frequency Inverter in Constant Pressure Water Supply
Abstract: This article introduces the structure and working principle of constant pressure water supply system based on frequency inverter. The system adopts frequency conversion speed regulation mode, automatically adjusts the speed of pump motor, maintains constant water supply pressure. The system has the advantages of energy saving, reliability , high degree of automation, and improves the quality of water supply.
PID Key words: variable frequency speed regulation, constant pressure water supply, PID
Introduction：With the improvement of people's quality of life, the quality of life in the water quality requirements are also getting higher and higher. At the same time, due to the requirements of the factory process, the quality of water supply has also put forward higher requirements. Variable frequency constant pressure water supply with its environmental protection, energy saving and high quality of water supply in the water supply industry has been more and more recognized. In the development of urban residential areas, the use of community or community as a unified overall water supply program, will make the equipment utilization and energy saving ratio greatly improved, and reduce the initial investment and land area.
一、The principle of work
Variable frequency constant pressure water supply system uses setting pressure (external or internal setting) and the actual feedback pressure (pressure sensor feedback for 4~20mA or 0-10V or remote pressure gauge feedback),which send a water increase or reduce the signal after comparison with the pressure set value, PID operation to change motor speed by the frequency inverter control. If in a certain delay time, the pressure is still insufficient or too large, and then to switches the constant pressure water supply controller for power frequency / frequency conversion to realize the actual pipe network pressure is consistent with the setting pressure. In addition, with the reduction of water consumption, the pressure of the pipe network is increased, and the frequency converter automatically reduces the output frequency when the pressure is set, and the purpose of energy saving is achieved.
二、The advantages of variable frequency constant pressure water supply to replace the traditional water supply
1. Variable frequency constant pressure water supply can automatically maintain constant pressure and start or stop the pump in accordance with the feedback of the pressure signal with stepless pressure adjustment and good quality of water supply. Compared with the traditional water supply, it will not cause the pipe rupture and the resonance phenomenon when the water tap is opened.
2. It avoids frequent start and stop of pump ,and the soft motor start mode reduces the motor water pump's start impact, increased the motor water pump's service life, also avoided the water hammer phenomenon in the traditional water supply.
3. Water tank, designed in traditional water supply ,not only is a waste of funds, occupies a larger space, does not meet the health standards with water pollution, and the water pressure is not stable. But such problems will be smoothly done or easily solved by the variable frequency constant pressure water supply.
4. In variable frequency constant pressure water supply ,the system can automatically detect the pressure of the pipe network according to the actual usage of the users, and control the speed of the pump to achieve the energy saving effect. It avoids pump running phenomenon when the water tower unattended, which saves manpower and material resources.
5. Variable frequency constant pressure water supply can automatically realize the function of multi pump circulation, so that the pump has a similar use of time and wear, which reduced the long-term operation of the main pump and wear and tear and the long-term shutdown of the pump and the problem of corrosion, extended the service life of the motor pump.
6. Variable frequency constant pressure water supply system protection function is complete with reliable operation, functions of less pressure, over current, overload, overheating, lack of phase, short circuit protection and so on.
The frequency conversion constant pressure water supply has the advantages of high efficiency, energy saving, stable pressure, reliable operation, simple operation, convenient installation, small occupation area, low noise, no pollution, low investment and high efficiency. Especially suitable for:
1. Water supply and hot water heating system for hotels, office buildings, apartments, residential quarters and other places;
2. Fire fighting water supply system of high building and large civil building;
3. Industrial and mining enterprises;
4. All kinds of water plant;
四、Energy Saving Principle
It can be known by the working principle of the pump: Pump flow is proportional to pump (motor) speed,pump head is proportional to the square of pump (motor) speed, pump shaft power is equal to the product of the flow and head. Therefore, the pump shaft power and pump (motor) speed is proportional to the cube. The motor speed is n=60f (1-s) /p; (n: motor speed, f: power frequency, s: slip, p: motor pole number, motor speed and power is proportional to the frequency). Pump shaft power and speed (motor speed, motor power frequency) is proportional to the three cube. According to the above principle, it can be known that changing the speed of the pump can change the power of the pump.
The basic formula: flow Q∝n, pressure (lift) H∝n2, power P=Q*H∝n3.
For example: The power supply frequency 50Hz down to 45Hz, P45/P50= (45/50) 3= 0.729, that is P45=0.729 *P50;the power supply frequency is reduced from 50HZ to 40HZ, then P40/P50=（40/50）3= 0.512,that is P40=0.512* P50. The traditional water supply pump is generally designed according to the water supply system in the maximum operating conditions. But the water system may not be able to reach the maximum amount of water in a lot of time of the actual use. It generally adjusts the system resistance to throttle by valve, which results in motor power loss. By changing the speed to regulate the supply of water, the inverter can not only make the system stable and steady state, but also recover the investment by reducing the speed of energy saving. From the image below, we can see the three kinds of comparison of the inverter control, input valve control and output valve control methods.
Three kinds of flow control method of the power consumption of 100kW motor measured comparison table:
|Inverter shaft power||Output valve control shaft power||Output valve control shaft power||Ideal shaft power|
From the above table, we can find that the energy saving effect of frequency inverter is very obvious when the flow is low, and the actual flow is rarely achieved the design of the maximum flow in practical applications. Many motor driving equipment has problems like a large margin, low efficiency, large power consumption, large starting current, big noise and other, which constantly impact on the economic efficiency of enterprises. Inverter investment can fundamentally solve these problems, and can quickly recover the investment, the general recovery period is not more than 12 months.
五、Application of ACI inverter in constant pressure water supply
Variable frequency constant pressure water supply system of Zhufeng Electric is the perfect solution to the existing water supply problem.It has the features of energy saving, high efficiency, reliability, high automation and high intelligence.
A clean water supply in Dongguan with our one to four water supply project as an example:
The system is mainly configured as follows:
Intelligent constant pressure controller
ACI frequency inverter
System main circuit
The system is mainly composed of constant pressure controller, frequency inverter, four water pumps and pressure sensors; Frequency inverter is the core part of the whole water supply system with constant frequency and constant pressure. The pump motor is the output link, the speed is controlled by the frequency converter to realize constant flow and constant pressure control. The frequency changer accepts the signal of the constant pressure controller to carry on the speed control to the water pump, pressure sensor detects the water pressure of pipe network and signal it to constant pressure controller. And then through the PID operation to adjust the frequency of the inverter to control the speed of the pump,which realizes a closed loop control system. When the system is started, the first pump with frequency conversion from 0Hz to start running, after the 50Hz, is switched to power frequency operation if the pressure is still less than the set pressure and start a pump from 0Hz at the same time. If it run to 50Hz without the set pressure value, then it switch to the power frequency operation and start the next pump at the same time. If the pressure exceeds the set value, the system will reduce the frequency or reduce the pump, in order to reach a constant pressure in the set value. When the useless water pressure reaches the set value, the system will automatically enter the dormant state, which also automatically wake up.
Because the ACI inverter has the PID function, it can not use an external voltage controller in one to one with smooth adjustment.
System will be based on the "first stop" principle to switch to the water pump, which avoids a long-term operation of a pump serious wear and the rust of other pump ,which prolongs the service life of the pump.
The system also has the function of fault self diagnosis, the pump will have alarm for overload, lack of phase, sensor disconnection, sensor short circuit, high water pressure, low water level, frequency converter fault. Some faults such as high water pressure and low water level system can automatically process and resume operation, which is highintelligent automation.
Variable frequency constant pressure water supply has lower cost and higher quality of water supply than the traditional water supply mode，which
gradually got the recognition of the industry, and is the future direction of development. Our constant pressure water supply system has been widely used in various industrial and mining enterprises, which achieves the standardization, serialization, and promotes the development of the industry.
【1】ZhangYanbin Application of Variable Frequency Speed Control. Beijing: Mechanical Industry Press, 2000.
【2】Intelligent Constant Pressure Controller Instructions. Zhufeng Electric