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RE: [TCML] Variac

I'm not sure that using a Variac does anything to reduce stress on the cap or MMC.  As far as excessive current, a cap doesn't care if it occurs on the charging or discharging cycle; an Amp is an Amp and it doesn't matter which way it's flowing.  When the cap is charging, the current is limited to 30 mA or 60 mA or some similar modest value by NST, and it doesn't matter what you do to the primary of the NST.  In contrast, every time the spark gap fires and discharges the cap, the peak discharge current can be 100's of Amps.  This is where an over current situation could stress the cap, and this is unaffected by the use of a Variac.

A better reason to use a Variac might be that if conditions were such that racing arcs occur on the secondary, slowly ramping the voltage up with a Variac might give one an opportunity to detect that and shut down before the damage is permanent.

It may also be prudent to perform rough primary tap tuning at less than full power, since if tuning is off, primary energy languishes in the primary tank circuit and this is more stressful to the cap.  But truthfully, I'm not sure that I did that.

The best (IMO) reason to use a Variac is to take advantage of the additional voltage boost that most Variacs provide.  And of course there's that sense of power and control that spinning that big honkin' knob gives you!

But seriously, I agree that many do not use a Variac at all with no ill effects.

Regards, Gary Lau

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of DC Cox
> Sent: Tuesday, June 24, 2008 10:49 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Variac
> The usual connection is 120 vac input on #1 and #4, with output on #1
> (common) and #3 (the adjustable wiper).  If unsure, Superior has tech prints
> on their website.
> It's used to slowly bring up the voltage on the nst thus placing less stress
> on the MMC caps if you happen to hit the power switch at the sine wave
> peak.  Many experimenters do not use them at all and have good results.
> Dr. Resonance
> On Tue, Jun 24, 2008 at 5:12 PM, stamsund <stamsund@xxxxxxxxxxxxxx> wrote:
> > I found a variable transformer among my shop goodies.  It's a Superior
> > Electric Powerstat, Model Q116U, 120 volt primary, 0-140 volts out, 1.3 KVA,
> > 9 Amps.  I understand it should be used between AC power and the NST, but
> > I'm not really sure why it's needed and/or recommended and exactly how to
> > connect it.
> >
> > Rob Robertsen, KA7YAK

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