The energy saving potential when a frequency converter is used depends on the type of load being driven and the optimisation of the efficiency of the pump or the drive by the frequency converter, as well as how much time the system operates under partial load. Domestic water and wastewater systems are designed for rarely-occurring peak loads, so they are usually operated under partial load.
Centrifugal pumps and fans offer the largest potential for energy savings. They fall in the class of fluid flow machines with variable torque curves, which are subject to the following proportionality rules. The flow increases linearly with increasing speed (rpm), while the pressure increases quadratically and the power consumption increases cubically.
The decisive factor for energy savings is the cubic relationship between rpm and power consumption. A pump running at half its rated speed, for example, needs only one-eighth of the power necessary for operation at its rated speed. Even small reductions in speed thus lead to significant energy savings. For example, a 20% speed reduction yields 50% energy savings. The main benefit of using a frequency converter is that speed control does not waste power (unlike regulation with a throttle valve or damper, for example), but instead adjusts the motor power to match exactly the actual demand.
Additional energy savings can be achieved by optimising the efficiency of the pump or drive with frequency converter operation. The voltage control characteristic (V/f curve) supplies the right voltage to the motor for every frequency (and thus motor speed). In this way, the controller avoids motor losses resulting from excessive reactive current.
Article by Gregers Geilager