Variable Speed Drives (VSD)/ Variable Frequency Drives
VSDs find application in fans, pumps, compressors and other equipment where flow or output varies. They take advantage of the affinity law which states that power is proportional to speed cubed. Hence, by reducing speed by half, power consumption is lowered to one-eight. Usually, electric motors which drive pumps, fans, etc. are operated at a constant speed. And if less output is required, a downstream equipment (like damper in a fan installation) would have to block part of the output while the motor is still running at a constant speed, thereby wasting energy. Now, if a VSD was installed, it would adjust the motor speed to a level that would deliver the output required, thereby saving energy. A case study is John Muir Medical Centre in California. They retrofitted electric motors – the largest motors and those with the longest run times – associated with their chiller with variable speed drives (VSDs). By matching the speed on these motors with cooling demand, they recorded substantial reductions in energy consumption. The project cost $258,143 but their calculated annual savings was $54,150 and also qualified for a $63,173 utility rebate and will experience a 3.6-year payback on the project.
