Q7Advanced Electric Drives

Frequency control by power electronic devices is achieved using a variable-frequency drive (VFD), typically consisting of a rectifier stage (converting the fixed-frequency AC supply to DC) followed by an inverter stage (using controlled power semiconductor switches - IGBTs or thyristors - to synthesize a variable-frequency, variable-magnitude AC output from the DC link, most commonly via pulse-width modulation switching patterns). By adjusting the switching pattern and timing of the inverter's semiconductor devices, the output frequency (and correspondingly, via v/f control, the output voltage) can be smoothly varied over a wide range, allowing precise, efficient speed control of AC induction and synchronous motors without the poor efficiency and limited controllability of older methods such as pole-changing or rotor resistance control.