In a constant pressure water supply system, the pipe network water pressure is kept at a constant value no matter how much water demand. Therefore, the water demands of different users in the network can be satisfied, and motor idling is avoid. In order to meet the above requirement, VFD needs to regulate the speed of the motor based on the given and the feedback pressure signals, so that the pipe network pressure can be controlled at a constant value. Figure 6 illustrates the schematic diagram of a constant water supply system.
The system is mainly composed of four pumps with motor, a VFD, PLC and pressure sensors. PLC controls and monitors the operation of the four pumps. It can start or stop the motors, and vary the motor speed. It can also determine to put which motor or motors into operation. PLC employs the built-in PID function to regulate the VFD output which will control the motor speed. The VFD controls a pump motor at a time. The pumps are designed to operate at two frequencies: power frequency (60 Hz) and variable frequency (0 to 60 Hz). When the speed of the pump controlled by the VFD reaches it full speed at 60Hz and the system pressure is still lower than the set constant value, the PLC will control the motor to run at power frequency of 60Hz and put a spare pump into operation to increase the network pressure. The new pump will be controlled by the VFD to run at a variable speed.
Three phases contactor M1, M2, M3 and M4 control pump1, pump2, pump3 and pump4 to run at 60 Hz respectively. M0, M5, M6, M7 and M8 coordinate to control one of the four pumps to run at variable speeds. MCP0 to MCP4 are pump motor circuit protector for turn on or turn off the four pumps and automatically disconnect the pumps from the power source in the event of over current. X is a three phase reactor to limit the circuit current. There is an overload relay for one pump motor. Usually the overload relay is installed with the contactor. The auxiliary contacts of the contactor and the overload relay are connected at certain logic to form the control circuit. The control circuit can determine which pump or pumps are to be connected or disconnected.
One of the four pumps is a redundant pump which is not used under normal condition. It is only used in the event of a fault and sometimes to replace a normal pump during the maintenance. There will is a control method to automatically switch to the redundant pump from the malfunctioning pump.