Re: SIZE of RF chokes?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi JT,

I think I understand your confusion here, so let me take a stab.

The variac is a very efficient transformer that can reduce or 
increase the line voltage presented to the load.  Assuming a 
resistive load to keep things simple, the power draw is the output 
voltage squared / ouput load resistance.  Given a fixed load, the 
power draw will be higher at 140Vac output than at 120Vac output.  It 
is totally possible to extract more power (and/or current) from the 
variac than it is rated for.  If your variac is rated at 4 amps and 
your load draws 4 amps at 120Vout,  then you will exceed the variac 
current rating by running the variac at 140V.

Another element that is confusing (and sometimes even to me) is the 
current transformation.  Lets forget specs for a moment.  If the 
variac is set to 140Vac out and the load draws 4 amps, what will be 
the current draw from the 120V line.  If one assumes no loss in the 
variac then the current should be 140/120 * output current (derived 
from Pout = Pin).  Not unlike a two coiled ideally 
coupled  transformer.  A variac is an autotransformer where all turns 
are highly coupled to other turns.  This is what I believe to be 
correct based on energy conservation but I haven't gone thru the math 
on it yet.  Others may comment.

Gerry R.


> Original poster: "JT Bowles" <jasotb@xxxxxxxxxxx>
>
> but, my variac outputs, or will only take, 4 amps of current, at 
> 120VAC. So wont hiring out voltage to 140 VAC, output only 3.33 
> amps? The power throughout will still be exactly equal
>
>
> > From: "Tesla list" <tesla@xxxxxxxxxx>
> > To: tesla@xxxxxxxxxx
> > Subject: Re: SIZE of RF chokes?
> > Date: Sat, 26 Nov 2005 17:00:11 -0700
> >
> > Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
> >
> > Hi Gary,
> >
> > I agree,  if you leave your safety gap setting based on 120Vac then 
> > no more voltage (I take full advantage of 140Vac and set the safety 
> > gap based on 140V - others may not).  140Vac may run the NST closer 
> > to saturation and thus the current could be higher but not the 
> > voltage if the safety gap is set at 120Vac.  I have never heard of 
> > an NST failure due to current only voltage.
> >
> > Gerry R.
> >
> > > Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>
> > >
> > > Although intuition suggests that running an NST at 140VAC is more
> > > stressful than at 120VAC, I can't think of an actual reason why.
> > >
> > > I say crank it to 140V and enjoy!
> > >
> > > Regards, Gary Lau
> > > MA, USA
> > >
> > > > Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
> > > >
> > > > Hi JT,
> > > >
> > > > There are never guarantees with using NST's in TC service.  Many if
> > > > not most members in this group run the input of their 120Vac rated
> > > > NSTs at 140Vac. Do what you are comfortable with.  If you have a NST
> > > > that is rated at 120Vac_in 12KV_out, then this NST will generate
> > > > 14KV_out with 140Vac_in. Remember, these are rms voltages.  If you do
> > > > plan on running your NST at 140Vin, then set your safety gap with
> > > 140Vin.
> > > >
> > > > Gerry R.
> > > >
> > > >
> > > > >Original poster: "JT Bowles" <jasotb@xxxxxxxxxxx>
> > > > >
> > > > >So, you're saying i can run my transformer at 140VAC in, and as long
> > > > >as my safety gap is set correctly, I dont have to worry about
> > > destroying it?
> > > > >
> > > > >sweet. Except ,it is a linear relationship right? if i increase my
> > > > >input voltage by 1/6 previous voltage,
> > > > >I should get in increase of 1/6 voltage on the secondary right?
> > > > >
> > > > >That means if i wire my NST for 140VAC in, 14KVAC comes out, correct?
> > > > >
> > > > >
> > > > >>From: "Tesla list" <tesla@xxxxxxxxxx>
> > > > >>To: tesla@xxxxxxxxxx
> > > > >>Subject: Re: SIZE of RF chokes?
> > > > >>Date: Thu, 24 Nov 2005 10:38:59 -0700
> > > > >>
> > > > >>Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
> > > > >>
> > > > >>Hi JT,
> > > > >>
> > > > >>Excessively high voltage is what usually kills NSTs.  Use LTR cap
> > > > >>value to avoid resonant charging and a properly set safety gap to
> > > > >>avoid these high voltages.  Yes 140Vac will probably shorten the
> > > > >>life of an NST to some extent.  If you have a properly set safety
> > > > >>gap and an LTR cap value,  you probably shouldn't  worry about this
> > > > >>too much as you wont enjoy the hobby as much.  I run my 15 KV (21
> > > > >>KVpeak) NST's at 25 KVpeak with a SRSG and haven't lost one yet.
> > > > >>
> > > > >>Gerry R.
> > > >
> > > >
> > > >