Answer Posted / ravi teja
The main purpose of VFD is to control the speed of the motor
in general if u take an induction motor its speed can be
changed either by voltage or by frequency if we made voltage
alone variable also speed will change but only up to certain
point on the other hand if speed control is achieved by
frequency speed control is only possible up to certain
limits beyond that the flux will be saturated
The best method is to control the speed by means both
voltage and frequency but keeping their ratio constant
this can be achieved by means of scr (semi controlled
rectifier) vfd is a closed loop system it takes the feedbsck
of speed from the motor (Tacho genrator or Current Fb)and
adjusts the firing of scr accordingly
Now a days in modern technology Converter or inverter
circuit is 6 pulse or 12 pulse depending on the application
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