When the switch value is “TRUE” the contact will be in open state and Auto mode will be selectedĪ positive pulse NO contact is used for the Start switch, when this switch is activated a short pulse of “TRUE” value is received and drops back to low. When the switch value is “FALSE” the contact will be in close state and Manual mode will be selected. The NO contact of Auto/Manual switch is used for Auto mode. The NC contact of Auto/Manual switch is used for Manual mode. The NO contact of this switch is used in logic, whenever well level is more than low limit the limit switch contact closes and gives “TRUE” value and whenever well level drops below low limit the limit switch contact opens and gives “FALSE” value. Well Low Level Switch (Input)- W L L SW.The NC contact pulse of this switch is used in logic, whenever overhead tank level is more than low limit the limit switch contact opens and gives “FALSE” value and whenever overhead tank level drops below low limit the limit switch contact closes and gives a momentary pulse of “TRUE” value. Overhead tank Low Level Switch (Input) – OT L L SW.The NC contact of this switch is used in logic, whenever the overhead tank level reaches high limit the limit switch contact opens and gives “FALSE” value and in normal condition the limit switch contact will remain closed and give “TRUE” value. Overhead tank High Level Switch (Input) – OT H L SW.List of Inputs and Outputs with its abbreviations Master Start/Stop has also been included to start the process when mode is in manual.īelow shown is the PLC program in Ladder logic to Control water Level of an Overhead tank, along with program explanation and run time test cases.When these inputs are received in PLC, output to the motor which is connected to the water pump needs to be powered ON/OFF in such a way that overhead tank water level can be maintained as mentioned in control description.To detect low level of the well, a level switch is used at the bottom of the well which provides output in digital terms as well.To detect high and low levels of overhead tank, two level switches are placed, one at the bottom of overhead tank and other one at the top of the overhead tank, which gives output in digital terms that is when corresponding levels are detected.Whenever well water level is low, the water pump should not be allowed to start in both Manual and Auto Mode.Ĭontrol Diagram: Overhead Water tank level control diagram This water pumps inlet line is connected to a well and discharge line is connected to the overhead tank. When high level of overhead tank reaches, then the water pump should stop. In this mode when the water level of the overhead tank reaches the low level, the water pump should start and it should run until water level in the overhead tank reaches high level. In this mode water pump can be started or stopped manually. There are two modes of operation involved, manual mode and Auto mode as explained below Let us consider one overhead tank installed on top of a building of which water level needs to be controlled.
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