Different pumping energy with VSD (variable frequency drive, frequency inverter) & throttling
One important reason of applying variable speed drives (VSDs, variable frequency drives, VFDs, frequency converters, ac drives) is to save energy, thus to reduce the energy bill.
Savings from variable speed drives (VSDs, variable frequency drives, VFDs, frequency converters, ac drives) come from reduced load of the fan, pump, or driven device. With fans and pumps, energy consumed is proportional to the cube root of shaft speed. If shaft speed is reduced by 10%, flow is reduced by 10%, while power consumption is reduced by 27%. And if shaft speed is reduced by 20%, power is reduced by 49%.
Compared to throttling as a means of flow control, speed reduction provides dramatic energy savings. Throttling to reduce flow in a fan or pump backs the device up on its operating curve, increasing pressure and often increasing power consumption.
The graphs above of a typical circulating pump (75% efficient) demonstrate the difference in energy use between throttling and adjustable speed drive (ASD, adjustable frequency drive, AFD, frequency inverter) to reduce flow by 33%. The operating point for peak conditions is at the intersection of the pump curve and the system head curve (925 gpm at 42-foot head) and draws about 13 horsepower.
Reducing flow through throttling to 600 gpm requires the pump to produce about 45 feet of head, drawing about 9 horsepower. Reducing speed with adjustable speed drive (ASD, adjustable frequency drive, AFD, frequency inverter) reduces the head to less than 20 feet and draws less than 5 horsepower (HP). This results in almost 70% reduction in power consumption.
