Above shown is a booster pump control panel, that is controlled using PID control. PID Stands for proportional, integral, derivative. The PID signal comes from a 0 – 10 Volts pressure sensor that is connected along the pipe supplying water to the pressure tank, when the pressure drops below a certain value, the PLC will command the VSD to turn on, thus running the pump. 

The unique part about this system is that, by varying the FC value of the VSD, we are able to control the frequency that the VSD is running at according to the pressure sensor reading. Example, if we set the FC value of the VSD at 30% and say the cut in pressure is 2.5 bar and cut out pressure is 3.5 bar. The VSD will run at 50 Hz (Highest) if the pressure is at 2.5 – 2.6 bar while the VSD will slowly throttle its frequency to 30Hz when the pressure is at 3.3 – 3.4 bar. 

In this scenario, the VSD “saves” energy by running the pump according to the pressure requirement of the system, also reducing frequency of start-stop of the pump. Increasing the lifespan of the equipment. There is no point of putting a VSD in the system, where there is no PID control. This is because the VSD only start and run from 0 – 50 Hz, and always run at maximum speed. This is not ideal for energy efficient buildings and defeats the purpose of VSD.