How does a variable speed pump drive system save energy, compared to a system that runs across-the-line?
A pump drive system ran across-the-line will operate the motor at its base speed as the speed is determined by the fixed 60 Hz frequency. The flow will be controlled by mechanical restriction (valves, vanes, etc.); yet the motor will continue to consume the maximum required power. A variable speed pump drive system gives you the ability to adjust the flow by varying the speed of the motor. The torque load decreases as a function of the square of the speed (T = n^2) and likewise the power requirement decreases as a function of the cube of the speed (HP = n^3). This is known as affinity laws and it defines the relationship of power consumption to flow or speed. It only applies to variable torque load such as centrifugal loads: pumps (no static head), fans, etc. By lowering the speed, you will also be lowering the pump?s flow as well as the motor?s power requirement; thus, reducing the energy demand.
