[Prev][Next][Index][Thread]

Re: Optimal Quenching

Subject: 
            Re: Optimal Quenching
       Date: 
            Tue, 25 Mar 1997 08:22:39 +1200
       From: 
            "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization: 
            Wellington Polytechnic, NZ
         To: 
            tesla-at-pupman-dot-com


Hi Skip,

> Big Snip
> >     A final note about the MOSFET experiments: anyone (everyone) who
> > uses either transformers with leakage inductance built-in (neons) or
> > near perfect transformers (pigs) with "current-limiting" inductors
> > attached should know that current-limiting applies *only* to limiting
> > the transformer current in the gap. It has exactly the opposite
> > effect when charging the primary cap because the two form a resonant
> > circuit of rather high Q. 
> 
> Malcolm
> 
> Are you saying that if a cap is "matched" to a neon (ie., a .021uf for a
> 15kv-at-120ma neon) then it would be possible to charge the cap to the peak
> neon voltage (or more) more than once each half cycle of the mains. If
> so it seems to me that our poor old neon will be able to deliver
> sustantially more power than the nameplate implies?
> 
> 
> Skip

No, because time is factored in - the circuit is resonant at a low 
frequency. To get quick charge, the L/C value would have to be 
smaller but then it would no longer be resonant at the mains 
frequency unless both L and C were altered. The problem with neons is 
that you cannot vary the charging inductance. You can vary Cp but 
then the thing is off-tune and current-limiting starts to take 
effect (no longer purely resistive).
    The current in a series resonant circuit = AC Voltage applied
in series with it divided by the vectorial sum of the reactances:

I = V/SQRT(R^2 + (Xl-Xc)^2)

You can "tune" the charge time to suit a rotary break speed other 
than 2xmains frequency.
     Here's a phenomenon which demonstrates the resonant nature of 
the charge circuit. I have done this as has Ed Philips and many 
people do it unwittingly. Suppose you choose your cap such that the 
resonant -at- mains condition is met. Then it follows that the circuit 
will accumulate energy (ring up) with each half cycle until the gap 
fires if its Q is greater than 0.5 with dynamic losses taken into 
account. Demonstration: using a static gap set to fire at the 
transformer o/c voltage, turn the variac up part way. The gap will 
fire regularly but at a sub-multiple of the mains due to the 
necessary /half cycle energy accumulation. I think Ed had his gap 
firing at just a 20% variac setting. I have opened the gap on a small 
system such that it fired at fmains rather than 2fmains with a near 
full setting. I could have lost the transformer doing this.

Malcolm