Variable-frequency drives (vfd’s) convert ac input voltage into dc in a rectifier and switch the dc voltage to the output in a manner that simulates a sine wave of voltage to the motor. The electronics controlling the switching permit the output voltage to be varied in frequency, thus controlling the speed of a standard ac motor, which is proportional to frequency. For a detailed explanation of drive and motor operation, refer to my August 1999 column (page 72).
Power quality impacts can occur on both the input and output sides of the vfd. We will start this month with the input side, which has the greater potential to affect other equipment.