Variable frequency drives (VFDs) have become omnipresent in the building automation world. This proliferation of VFD’s is due to two main factors. First, when properly applied to centrifugal fans and pumps, VFDs conserve a tremendous amount of energy. The Affinity Laws, sometimes referred to as the “fan laws,” state that the horsepower required to drive a centrifugal fan or pump drops as the cube of the speed of said fan or pump. Second, VFD usage is all but required in ASHRAE standard 90.1.¹
Serial communications with modern VFDs have been available since 1994. The advent of the BACnet communications protocol standard has only helped to increase the availability of direct digital communications with devices such as VFDs. Modern VFDs and electronic bypasses have over 100 points of information available from the VFD and bypass. This article will discuss some of the main advantages and usage of the information now available.