Its control process is when the water reaches the limit, the water level detection send signals to PLC, PLC started rising frequency converter and frequency signal output to the frequency converter, when the frequency converter has reached 50 hz if the water level is still at the lower limit position, PLC signal switch of sewage pump 1 # switch to run the power frequency, delay after a period of time by the frequency converter to start the sewage pump 2 #. https://www.micnodrive.com/applications/water-and-energy-saving.html