A VFD is a controller which alters the rotational speed of an AC motor. The voltage is varied along with the frequency to the motor. This gives the AC motor a soft start when there is a demand for water. The VFD then controls the motor to whatever RPM is needed to maintain a constant pressure. This is done through a transducer which senses the line pressure. The end product is a better system for the consumer with less pressure fluctuations and typically a savings on power consumption. Other benefits include: low voltage protection, high voltage, high current and loss of prime (run dry or dry well). The need for bladder tanks is also minimized with a VFD. Regardless of the flow or horsepower of the pump only a single bladder tank is needed. In some instances that alone can pay for the VFD.