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NST Max Ratings and Mains Resonance (fwd)
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From: Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: Thursday, February 26, 1998 1:58 PM
To: Tesla List
Subject: Re: NST Max Ratings and Mains Resonance (fwd)
HI Alfred,
I meant to reply to this several days ago and left it
sitting forgotten in my "unread" folder :(
> From: Alfred C. Erpel [SMTP:aerpel-at-op-dot-net]
> Sent: Tuesday, February 24, 1998 6:31 PM
> To: 'List, Tesla'
> Subject: Re: NST Max Ratings and Mains Resonance (fwd)
>
>
> *******************
> I am replying to this message late because of email problems.
> *******************
>
>
>
> >From: Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
> >Sent: Sunday, February 15, 1998 6:50 PM
> >To: Tesla List
> >Subject: Re: NST Max Ratings and Mains Resonance (fwd)
> >
> >Hi Alfred,
> >
> >> Date: Sat, 14 Feb 1998 11:24:38 -0500
> >> From: "Alfred C. Erpel" <aerpel-at-op-dot-net>
> >> To: Tesla List <tesla-at-pupman-dot-com>
> >> Subject: Re: NST Max Ratings and Mains Resonance (fwd)
> >>
> >
> >>
> >> >
> >> >
> >> >---------- Forwarded message ----------
> >> >Date: Thu, 12 Feb 1998 15:54:09 -0500
> >> >From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
> >
> >> >Subject: NST Max Ratings and Mains Resonance
> >> >
> >> >Bill, Malcolm, and other interested coilers,
> >> >Besides the resonant rise that Malcolm mentions, there is also
> >> >the fact that when the main cap and the transformer are set to
> >> >resonate at the mains frequency, the transformer is capable
> >> >of providing current levels that are several times the usual
> >> >'current-limited' value. If the wire in the secondary is too
> >> >thin, then you can actually burn out the secondary winding
> >> >under these mains-resonant conditions.
> >> >
> >> >Thus, with the proper resonant conditions a 15KV 60 MA
> >> >NST can charge the mains cap up to voltages in excess of
> >> >40KV, and at a rate that is much greater than the 60 MA
> >> >rating would suggest. Note that both the extra voltage and
> >> >extra current can contribute to the NST failing prematurely.
> >> >
> >> >Hope this helps.
> >> >Fr. Tom McGahee
> >>
> >>
> >
> >> Hello,
> >>
> >> It is my understanding that in a series resonant circuit, the
> >> capacitive reactance and inductive reactance exactly cancel out, leaving
> >> only the pure resistance (ohms) as the total circuit impedance.
> >> Therefore,(at resonance) the current (I) flowing in the circuit is
> >> determined by I=E/R where E = input voltage (to the series
> >> resonant circuit) and R = ohms resistance of the circuit.
> >>
> >> My point is, I can see how power (EI) is increased in an inductive
> >> circuit because voltage is increased (and of equal value) measured across
> >> the capacitor and inductor, but the current (I) flowing in the circuit
> has
> >> not changed.
> >> What resonant conditions allow/cause an increased current flow?
> >
> >If the voltage across a component (resistive or reactive) increases,
> >surely the current through it must have increased if the frequency and
> >component values haven't changed?
> >
> >Malcolm
> >
>
>
> The current flow in a series resonant circuit is determined by the
> voltage applied and the pure resistance (not reactance's because they are
> cancelled out) I=E/R. The voltage across each reactive component (capacitor
> and inductor) is determined by E=IR after having calculated I, above. (R is
> the components reactance, and to keep this simple, I am assuming ideal
> components with no resistance).
> So where is the current increase?
> If I am wrong, please show me where.
It is all very well to say that the reactances cancel out but they
*are* individual components and it is across those individual
components that you get that voltage rise (the voltages being out of
phase with one another). The ratio of an individual reactance to the
circuit resistance determines how high that voltage gets in
conjunction with circuit current which as you rightly point out, is
governed by circuit resistance. The voltage across and individual
component = I.X, not I.R. Circuit I = Vsource/R. I think the
difficulty is that we are not dealing with a voltage source (the NST
is current limited). With each half cycle, the circuit rings up as its
circulating current increases. I have measured Rwindings on a 12kV
60mA transformer as being around 5kOhms (I can't remember whether
that was per side - let's say it was). Then 12000V/10000Ohms =
1.2Amps!! (RMS to boot). There is the current increase.
I hope that clarifies the issue. There is no doubt whatsoever
that the circuit rings up to a value permitted by its Q (or spark gap
setting). I have measured it doing this. I would not like to claim
however that a shunted transformer can deliver an instantaneous
current in excess of its short circuited value. I have not measured
this.
Malcolm