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.
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