Variable Frequency Drives And Its Energy Savings

This paper discuss the speed control of ac motors by using variable frequency drives (VFD). The speed of ac motors is a direct function of frequency, since the stator windings are inductive in nature the reactance varies with frequency. As frequency is increased, winding reactance increases right along with it. This increase in the reactance would result in decrease of stator current if the VFD output voltage remain constant. The ac voltage output by a VFD is made to vary in proportion to the applied frequency, so that the stator current will remain in the good operating limits throughout the speed range of the VFD. This correspondence is called the voltage to frequency ratio, abbreviated as “ “ratio or “ ” ratio.