Variable frequency drives (VFDs), also known as variable speed drives (VSDs), are used to modulate the speed of motors in various applications, from hydronic water pumping to VAV fan units to vacuum systems.
VFDs allow machinery to match the demand of a system by modulating speeds and therefore capacity, but the benefits hardly end there. They allow engineers and designers to exploit what may seem like a loophole in physics. The fan laws and pump laws, also known as affinity laws, describe how VFDs can modulate capacity non-linearly relative to power input. For example, a fan motor moving 1,000 cfm at full speed will move 500 cfm at 50% but will only consume approximately 12.5% of the power used at full speed. The power use is proportional to the cube of the speed, as expressed below: