Re: [TCML] solid state varaic

[Steve Ward <steve.ward@xxxxxxxxx>]

Scott,

Yes, what you describe can be known as a synchronous-buck converter, which
are popularized by class-D audio amplifiers.  Basically a half or
full-bridge configuration of switches, and a LC output filter.  The PWM
control can come from a variety of techniques from classic controls theory
and standard PWM generating chips, or to something more clever like
hysteretic control laws, or "one cycle control" techniques.  Each control
scheme has advantages, the hysteresis one is my favorite due to the
simplicity and robustness and almost guaranteed stability, the disadvantage
(and lack of presence in industry) is possibly due to the shift in switching
frequency of these control schemes, which can possibly increase the cost of
the EMI filter at the front end to pass emissions, but for us tesla coilers,
we dont care about minor EMI problems so its a good solution.

You may be able to get by with something less elegant or correct, but i
thought i should at least mention solid engineering input on a subject as
esoteric as this.  I dont think you need a processor, a 60hz sine wave input
signal could be generated from a resonant sine wave generator, and scaled
with a simple op amp circuit.  Basically, it would be a big audio amp that
played 60hz at variable volume.  I suggest looking at International
Rectifiers application notes about their (audio amp) control schemes, they
are probably the most approachable solution for someone new to the field of
power electronics and controls.

Steve

On Tue, Jul 27, 2010 at 8:04 PM, Scott Bogard <sdbogard@xxxxxxxxx> wrote:

> 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.
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