Re: variac

To: tesla-at-poodle.pupman-dot-com
Subject: Re: variac
From: Tesla List <tesla-at-stic-dot-net>
Date: Sun, 16 Mar 1997 22:14:23 -0600
Approved: tesla-at-stic-dot-net
Subject: 
            Re: variac
       Date: 
            Sat, 15 Mar 1997 23:41:58 -0800
       From: 
            Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: 
            Stoneridge Engineering
         To: 
            Tesla List <tesla-at-pupman-dot-com>
 References: 
            1


Tesla List wrote:
> 
> Subject:
>         variac
>   Date:
>         Sat, 15 Mar 1997 01:33:15 -0600 (CST)
>   From:
>         c604313-at-showme.missouri.edu
>     To:
>         Tesla listserver <tesla-at-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 --