Re: Fan Controller and Variac differences

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Dan, Jason
Fan controller is basically a light dimmer (phase controlled triac) that
has been modified to run on an inductive load by addition of an RC
"snubber" network around the triac, and in some units a small RF choke.
If you look at waveform on an oscilloscope, the power is controlled by
the phase delay of triggering the device on both sides of the AC waveform.
A fan controller (or light dimmer) can be used to adjust the voltage on
HV transformers, but the AC wave is distorted and there are core
saturation effects that can occur.  Bidirectional SCR's used as high power
triacs have been used successfully in commercial broadcast transmitters and
induction heating for three phase voltage control of HV power supplies
since mid to late 1960's.

A variac is an acronym for VARIable AC Transformer, which is a fully
adjustable transformer that allows stepless voltage output control with NO
waveform distortion.  Originally was invented in 1930's by employees of
General Radio (now GenRad), and other manufacturers used
contructional modifications to get around GR patent issues.  Key players now
in variac buiness include Staco, Superior Electric, and Technipower (now
owner of Variac trademark).  The no waveform distortion is a compelling
advantage of this technology, as well as being mechanically and electrically
robust and a relatively simple and mature device.  But the additional
insertion loss in power supplies negatively effects the regulation (how
"stiff" the power supply is to load changes).  These effects were seen
on a large 4kW LVDC regulated power supply that I built for an R&D
experimental effort.  So bottom line, both technologies are similar in
application
but different in operation and theory.

Regards
Dave Sharpe TCBOR
Chesterfield, VA. USA


Tesla list wrote:

> Original poster: "Jason by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jasonp-at-btinternet-dot-com>
>
> Dan,
>
> I am not exacly sure of the nature of fan controllers, but I imagine it is
> very much like a variac. If you have no problem running it on an OBIT then I
> would imagine that with an NST it would still be fine. An NST is just a
> slightly higher powered OBIT really, shunted in the same way.
>
> Regards,
> Jason