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Re: variac

Subject: 
        Re: variac
  Date: 
        Mon, 17 Mar 1997 17:49:47 -0600 (CST)
  From: 
        c604313-at-showme.missouri.edu
    To: 
        Tesla List <tesla-at-pupman-dot-com>
    CC: 
        tesla-at-poodle.pupman-dot-com


> > 
> > Hello to all!
> > 
> > It sure would be nice to hook up a variac transformer on the primary
> > side
> > of some step up transformers. My question is by how much can one skimp
> > on
> > the power rating of such devices??
> > 
> > Consider banked neons totaling 15 kv at 150 ma = 2250 W impedance
> > matched
> > with the primary caps in an equidrive setup. Certainly one wouldn't need
> > a
> > 20 amperer variac running off of a 110 line. Could one get by with just
> > a
> > 10 amperer variac if running a coil for only short periods of time  (10
> > minutes) ?? What have you all been able to get by with safely?
> > 
> > I've been enjoying all the discourse and expertise of the list!
> > 
> >                              -Bert S.
> 
> Bert,
> 
> The actual primary current will be a function of your tank capacitance
> and whether you use Power Factor Correction (PFC) capacitors. If your
> tank caps are "sized" so that they're close to drawing the faceplate
> current rating from the neon secondaries at the incoming mains
> frequency, you'll pull significantly more current than the values you've
> calculated above. For example, although I'm using a pair of 15KV 60MA
> neons, I actually will pull over 35 Amperes of primary current - and
> even adding PFC capacitors only drops this down to about 25 Amps. With
> the secondaries simply short-circuited and with 240 uF of PFC, the
> maximum primary current draw is only about 12 Amps.
> 
> Although a variac can be overloaded to some extent, be careful you don't
> overheat it to the point where the winding loosens and the rotor brush
> no longer slides smoothly across the winding. Installing an inexpensive
> AC ammeter in series with the variac will allow you to easily monitor
> the variac's current level versus its rating. After you've overloaded
> the variac a bit (never more than 200% of max - the brush will "cook" as
> well!), turn power off and feel for the degree of temperature rise in
> the winding and core - if it's getting too hot to touch, or the
> insulation's starting to smell a bit, back off and let it cool down to
> prevent permanent damage. 
> 
> BTW, adding non-polarized (mylar, or oil-paper) PFC capacitors across
> the primary winding of the neons will help reduce overall current draw
> and may permit you to squeeze a little bit more out out your undersized
> variac.
> 
> Safe coilin' to you!
> 
> -- Bert --
> 
> -- Bert --
> 
> 
Bert, Thanks for the information!

I was not aware of the actual current draw by the transformers!
I would have guessed that the average draw would have been less
due to the short period of time that it takes to charge the caps
and the current limiting of the transformers during the time when the
spark gap fires.

I have not yet found any variacs of such size within my budget and
am mostly just gathering clues as to what to buy before i lay down 
the big bucks. Using Power factor correction caps as you have
suggested would then be a good idea if I find a good variac/capacitor
combo. No doubt that the larger value of capacitance on the cap would be
better than a smaller value if the price is right.

Still, I'm curious as to what percent I may be able to overdrive a
variac. Maybe this is too general of a question as it probably depends
on
the make of the variac and other factors.

Thanks for the help!

-Bert S.