[TCML] solid state varaic

[Paul Bidmead]

Scott,

this is how most large ups's are built the difference from them to vfd's 
is that the output V/F is adjustable

first u need to rectify the incoming voltage to a dc - this will 
determine the max output - then like u said a PWM with its value varying 
for the F/V u select then a hbridge to output folled by some filtering 
and viola a VFD

this being simple in theory the practical implication can be quite 
unstable / expensive in comparison the the cost of a simple variac... 
not saying it cant be done but for the general hobbyist it can be quite 
daunting

also there is the issue of stepping up the voltage to get +110v ... 
unless you have 240v / 440v available

hope this helps enlighten this subject

Regards
Paul B
> Greetings all,
>     So my research into VFDs has indicated that to get the winding to 
> believe it is being fed a sine wave of X voltage and Y frequency, they 
> use a PWM with a variable duty cycle across the sine wave.  So I'm 
> wondering if it would be possible to make a solid state variac in a 
> similar manner, the advantage would be light weight and I would assume 
> it would be much cheaper when dealing with high powered systems, the 
> disadvantage is I don't think it would be possible to get the 140/280V 
> output like you can with the real thing.  Basically all one would need 
> would be an analog to digital converter being fed by 5VAC off of a 
> potentiometer to make it 0-5VAC, this would feed a PWM driven full 
> bridge IGBT array, at some point you would need a computer like an 
> arduino or some such to logic the digital signal into a duty cycle for 
> the bridge.  The intricacies are beyond me, but my research suggests 
> this has actually been done in some form (for DRSSTCs anyway) if 
> anybody has any experience on how they accomplished this (and if it 
> would work with a transformer) please do share.
>
> Scott Bogard.