Re: Input power measurement

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

Original poster: "Barton B. Anderson by way of Terry Fritz <teslalist-at-qwest-dot-net>" <classictesla-at-netzero-dot-com>

Hi Gary,

You know, your right. I was trying to talk about the resonant condition 
just a little, but mainly to point to Richie's site. I should not have 
stated that this was the cause to the high current draw. I mainly wanted to 
point out the maximum current "characteristic" of a resonant circuit (which 
of course is true). Actually, what I said in that first line is just 
opposite of what it should say. When the cap is resonant, we cancel 
reactance's to a large degree and by doing so, we actually "reduce" the 
current draw required to charge a cap to x-voltage, thus decreasing the 
charge time. It's the maximum current characteristic of resonance that 
allows this. As we move off resonance (either direction), reactance's feed 
back into the line. This opposes and we need more current if we are to 
charge the cap in the same time frame. So if Skip moved off a resonant cap 
size, he will probably see a larger current draw.

I agree that the face plate rating is not the limit and really isn't for 
most transformers unless poorly designed. That point probably vary's for 
each NST manf., size, and model. Skip had 17A, which for a modified NST, 
isn't all that surprising. It's the removal of shunts that is the probable 
cause for the higher current. Can't be the cap size. If he were to move 
towards LTR, the draw would increase, however, he may be near saturation now.

Thanks and take care,
Bart

Tesla list wrote:

>Original poster: "Lau, Gary by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><Gary.Lau-at-hp-dot-com>
>
>I don't think that the cap being mains-resonant results in the maximum 
>current draw.  With my 15/60 NST, I've noted OK performance with a 
>mains-resonant 0.01uF cap, better-still performance (and comparably higher 
>mains current) with a 0.02uF cap, and unstable performance with 
>breaker-popping current draw with a 0.03uF cap.
>
>It appears that when driving a Tesla Coil load, the faceplate rating on an 
>NST is irrelevant, grossly understating the maximum possible VA draw.  I 
>"think" it has to do with a near-resonant mains condition of the NST 
>secondary and the tank cap.  If you have a LTR cap trading energy to the 
>NST, a larger cap will dump more energy into the NST, resulting in a 
>higher secondary current, saturating the current shunts.
>
>Gary Lau
>MA, USA
>
>  -----Original Message-----
>From:     Tesla list [mailto:tesla-at-pupman-dot-com]
>Sent:    Sunday, March 09, 2003 1:32 PM
>To:    tesla-at-pupman-dot-com
>Subject:    Re: Input power measurement
>
>Original poster: "Barton B. Anderson by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <classictesla-at-netzero-dot-com>
>
>Hi Skip,
>
>Resonant mode running is causing the high charging current. The cap value
>is designed to be resonant with the transformer at mains frequency
>(charging frequency). At resonance, LC reactances are ideally removed and
>leave only the winding resistance in the circuit. Thus, a maximum current
>characteristic is achieved (which is the purpose of running in a resonant
>mode). Richie Burnett has a good write up of this (sometimes ok, sometimes
>not).
>
>http://www.richieburnett.co.uk/resonant.html#resonant
>
>Take care,
>Bart
>
>Tesla list wrote:
>
> >Original poster: "Skip Greiner by way of Terry Fritz
> ><teslalist-at-qwest-dot-net>" <sgreiner-at-wwnet-dot-com>
> >
> >Hi all
> >I am using a 9kv at 120ma NST. In operation I use an iron vane ammeter
> >(known to be within 5% accuracy) to monitor current. My mains voltage is
> >nominally 117V. Interestingly the current into the NST is about 17A when
> >running in resonant mode. It is my understanding that an iron vane meter
> >reads true input current regardless of waveform. Therefore my input power to
> >the NST is nearly 2000 watts.
> >
> >I would appreciate comments from anyone who can help explain the discrepancy
> >between the 1080va rating of the "current limited NST and the measured input
> >in resonant mode operation.
> >Skip
> >
> >
> >
> >
>
>
>
>