A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. VFD also known as variable speed drives (VSD), adjustable speed drive and  adjustable frequency drive.

Energy Conversation Principle of Pump VFDs

The pump flow and pump (3 phase asynchronous motor) speed is proportional, the pump head is proportional to the square of pump speed, the pump shaft power is equal to the product of flow and head, so the water pump shaft power is proportional to the three cubed pump speed. From these formulas, we can get to know that changing the pump speed is to changing the pump power.

The follow basic formula is:

In the above formula, Q stands for flow, N stands for pump speed, H stands for head, and KW stands for the shaft power.

For example: if we change the supply frequency from 50Hz to 45Hz, so P45 / P50 = (45/50) 3 = 0.729, P45 = 0.729 P50. If we change the supply frequency from 50Hz to 40Hz, so P40 / P50 = (40/50) 3 = 0.512, P40 = 0.512 P50.

The pump water supply system is designed according to the maximum needs in practical conditions, but the actual use of water supply system do not necessarily reaches the maximum amount of water, so the valve regulating system increases the resistance to the throttle. In case like this, it will cause a lot of loss of electrical power. The use of pump VSDs allows the system to work smoothly and stably by changing the pump speed to regulate the water supply, so that is the reason why constant pressure water supply system is of high energy efficiency and low cost.