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The PLC system needs to connect with various I/O devices. PLC control circuit wiring is shown as Figure 9. It demonstrates that how a PLC system is connected to the external devices such as input push buttons and the coil of the pumps.As shown in above Figure 9, SW1 is an HOA switch used to change operation between manual mode and automatic mode. PB1, PB3, PB5, and PB7 are normally open pushbuttons used to manually start the four pumps respectively while PB2, PB4, PB6 and PB8 are normally closed pushbuttons used to manually stop the four pumps respectively. OL1 to OL4 are the normally closed contacts of overload relays.
These contacts can open the pump power circuits under the condition of overload to provide protection to the pumps. FC1 to FC4 are the fault contacts on the pumps. When the pumps are energized but not running for any reasons, the fault contacts shall tell the PLC that the pumps have some problems to start. Then the PLC will send out some alarms to alert the operator. VFD fault is a normally closed contact of the VFD. When the VFD has a fault and stops running, the VFD fault contacts will tell this situation to the PLC.
Then the PLC can make the corresponding action. FS1 and FS2 are float switches used to offer signals of low and high water tank level. Reset is a push button used to reset the horn alarm. Pressure Feed Back is the pressure value that the pressure sensor send back to the PLC. M0 to M8 are contactors which are used to control the four pumps to run at variable speeds or full speed.Lite1 to Lite 8 indicated that the pumps are running at full speeds or variable speeds. The horn gives a sound alarm while Lite9 gives a visual alarm for a VFD fault. Lite10 to Lite13 indicates faults of the four pumps. Lite14 and Lite15 offer a visual alarm for low and high water tank level respectively.


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