Variable Frequency Drive (VFD)
•The VFD control the speed of three phase induction motor by varying the frequency and voltage.
•The difference between synchronous speed and rotor speed is called slip.
•By varying the synchronous speed the rotor speed can be varied.
•Synchronous speed is directly proportional to frequency and inversely proportional to number of poles.
•VFD consist of rectifier stage, inverter stage and control.
•Rectifier is a six set of diode bridge which converts AC to DC.
•Between rectifier and inverter there is a filter which smoothen the DC.
•Inverter is six set of IGBT or GTO or SCR which converts the DC to variable AC with control system.
•Control system maintain the constant ratio of voltage to frequency.
•VFDs are of three types one is Current source inverter (CSI) second is voltage source inverter (VSI) and third is pulse width modulation (PWM) VFD.
•The CSI consists of large and expensive inductors so it gives smooth current.
•VSI VFD have poor power factor.
•So the third one means Pulse Width Modulation (PWM) VFDs are most commonly use because of good power factor and low cost.
•In the control of VFD the PWM method is used.