The residential community has nearly a thousand users. The current water supply system experiences severe water shortages and pressure drops during periods of high water consumption, significantly impacting residents' quality of life. The current system uses a constant-speed pump control method, adjusting flow and pressure by changing valve sizes to maintain a constant water pressure. This method has the following problems in operation.
1. Manual operation results in adjustment lag, poor overall system stability, and low automation, causing the overflow pipe to drain water frequently, leading to resource waste.
2. Running the water pump at a constant speed not only wastes electrical energy, but also easily damages the bearings due to long-term high-speed operation, affecting the service life of the pump. In addition, the standby water pump has been found to have rusted and seized.
3. During peak water usage periods in summer each year, water pressure cannot be guaranteed. In the event of a sudden power grid failure, insufficient water supply will cause inconvenience to residents.
In order to provide a constant pressure water supply, it is very important to renovate the water supply system of residential communities.
System requirements:
1. Original water supply system
The original water supply system used two (one as a backup) 7.5KW motors to control the water level in the water tower. The flow rate and pressure were adjusted by changing the size of the valves to regulate the water pressure supply. The motors in the system used hard start, and there was only one pressure in the water supply.
2. Requirements for the renovated water supply system
(2) When the water pump is working, it can switch from variable frequency operation to power frequency operation, or it can switch from power frequency operation to variable frequency operation. There is an interlock control between power frequency operation and variable frequency operation.
(3) When the motor switches from frequency conversion operation to power grid operation and from power grid operation to frequency converter operation, there must be a certain delay before the contactor automatically closes after the speed stabilizes, so as to prevent the power electronic devices from being damaged by operating overvoltage and induced electromotive force generated by the high speed of the motor.
(4) It features automatic operation, stability, energy saving, and economy.
The transformation plan was determined and the system composition was determined.
1. Renovation Plan
(1) Based on the requirements of energy saving and economy of the system, three 10KW motors are selected to control three water pumps for water supply. One of them is on standby. During peak water consumption, two water pumps are running to meet the water pressure requirements, and during off-peak water consumption, one water pump is running to achieve the purpose of energy saving.
(2) PLC, frequency converter, pressure sensor and time control switch are selected as control units to realize automatic control of water outlet network pressure, automatic switching between water pumps and automatic opening and closing of valves, etc., which greatly improves the automation level and reliability of the equipment.
2. System Composition
The frequency converter is the core device for speed control. Its main function is to achieve stepless speed regulation of the motor and water pump by changing the frequency of the output power supply, thus automatically adjusting the motor and pump speed according to changes in water consumption and maintaining constant pressure in the pipe network. In addition, the system provides comprehensive protection functions such as overvoltage, overcurrent, and overheating through hardware and software, and enables hot start of the motor. Each water pump has frequency conversion and manual operation functions. Therefore, by adjusting the motor speed according to the required water pressure, pressure is controlled, achieving energy saving.
System upgrade
1. Component selection
(1) The LT311A time control switch is selected. It is a single-chip microcomputer programmable time controller with 24 preset time programs, which can be arranged in sequence to form 12 pairs of timer switches. Among them, programs 1 to 8 form 4 pairs of timer switches as the first output, programs 9 to 16 form 4 pairs of timer switches as the second output, and programs 17 to 24 form 4 pairs of timer switches as the third output. If the three outputs are combined into one, the timer control of turning the switches on and off 12 times a day can be realized.
(2) A PS4 type pressure sensor is selected. This sensor has both digital and analog output functions. The measurement range of the sensor is 0.3 ~ 0.8 MPa, and its pin configuration is shown in Figure 4. Its digital output can be preset by pressure. When the detected pressure reaches the preset pressure, the switch closes.
(3) PLC selection: Select according to the number of input and output points of the system. The time control switch has three time control output signals, plus start, stop, manual control of three motors and six solenoid valves, etc., for a total of 22 input points and 22 output points; in addition, the whole system does not have special function instructions for the PLC, so the economical and small FXON-60MR PLC can be selected.
(4) Select the inverter according to the power of the motor: If the motor power is 10KW, you can choose the FR-A540-10KW inverter from Mitsubishi Corporation of Japan. The inverter itself has a braking function and does not require an external braking element.
2. System-wide control
(1) Time pressure matching control
① Time setting of the time control switch: 0 : 00~5 : 00 , 11 : 00~14 : 00 , 21 : 00~24 : 00 are time programs 1~6, output from the first channel; 5 : 00~8 : 00 , 14 : 00~17 : 00 are time programs 8~11, output from the second channel; 8 : 00~11 : 00 , 17 : 00~21 : 00 are time programs 17~20, output from the third channel. Then connect to terminals X10, X11, and X12 of the PLC input.
② The pressure settings are 0.38MPa , 0.5MPa , 0.47MPa , 0.52MPa , 0.48MPa , 0.53MPa , and 0.42MPa . This can be achieved by controlling the input voltage at terminals 2 of the frequency converter using the switching signals output from the PLC.
③ After the inverter provides a pressure for a certain period of time, it compares the pressure signal detected by the pressure sensor with the pressure signal fed back to the inverter's 4 terminals. The inverter's output frequency can be adjusted at any time to make the actual pressure equal to the given pressure, thus realizing the inverter's PID control.
(2) Switching between variable frequency operation and power frequency operation
When the frequency of the motor's variable frequency drive (VFD) power supply rises to the mains frequency, the PLC controls the relay to disconnect the VFD's power supply, and the power is directly supplied from the grid. When the water supply pipe pressure still does not meet the requirements, the second motor starts operating with VFD power. Based on actual field measurements, the following characteristics are observed: when the pressure reaches 0.5 MPa , the VFD output frequency is the mains frequency, at which point it is necessary to switch from VFD operation to grid-powered operation. This is achieved by setting the pressure sensor to 50Hz and controlling the PLC through the sensor's on/off output to switch between VFD and grid operation.
5) The inverter program settings are as follows:
Pr79=2; (External operation mode selected)
Pr128=20; (PID control action selection: negative feedback)
Pr129 = 300%; (PID proportional range)
Pr130 = 180s; (PID integral time)
Pr134 = 3s; (PID derivative time)
Pr1 = 10Hz; (Lower limit frequency)
Pr2 = 50Hz; (Upper frequency limit)
Pr904 = 0mA; 0Hz; (Sensor output calibration)
Pr905 = 16mA; 50Hz; (Sensor output calibration)
Conclusion:
The constant pressure variable frequency water supply system controlled by this PLC can certainly guarantee the water supply for users in the community, while greatly reducing the labor intensity of maintenance and extending the service life of equipment. It achieves true automatic control, not only solving the problems existing in the current water supply system, but also showing significant energy-saving effects. Therefore, it has great value in terms of energy saving and improving users' living conditions.
