Beginning in the 1990s, the introduction of new VSD technology and inverter duty designs for 3-phase induction motors made it easy and economical to begin providing variable speed packages. Instead of using a blow off valve or starting/stopping, the VSD was used to vary the output frequency of the blower directly. Varying the blower operating speed adjusts the pressure vs. flow curve of the blower, with lower operating speeds displacing less volume flow.
Why is this significant? Slowing down the blower can be significantly more efficient than blowing off air. A blower running at its nominal speed on a VSD compared to a fixed speed will have the same flow output and require a slightly higher amount of power input due to VSD electrical losses. With modern VSD equipment, this loss is typically less than 3 percent. However, with more flow reduction, more energy can be saved using a VSD. A lobe blower operating with VSD has a turndown of 60-70 percent, meaning that the minimum flow is 60-70 percent lower than the maximum flow. For a blower operating with a blowoff valve, it still produces the same amount of flow in the blower, and the only energy savings is potentially a slight decrease in the system pressure. Therefore, a VSD can offer almost 60-70 percent in energy savings in a process requiring 60-70 percent less flow than the blower’s nominal output. For any application with a large, variable flow demand, these savings easily pay for the extra investment cost of the VSD.
An additional performance benefit of VSD is the ability to increase the operating speed beyond the base frequency (60 Hz in the U.S.). Older pieces of equipment can be retrofitted with VSD and a larger motor, if necessary, to increase the performance output of the equipment. The investment costs for this retrofit are considerably lower than purchasing new equipment to get a slightly higher flow output.