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Re: Joules - Vrms or Vp (fwd)
---------- Forwarded message ----------
Date: Wed, 05 Aug 1998 14:21:34 -0500
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Joules - Vrms or Vp (fwd)
Bart and all,
Sorry for the delay - I was "on the road" (on vacation) and am wading
through my stack of e-mail now.
Basically I also agree with Robert. Use Vgap (i.e., the breakdown
voltage of the main gap), instead of Vpeak, to calculate "bang size": J
= 1/2 x C x Vgap^2. Vgap is usually adjusted so that it's consistently
less than the maximum charging voltage of the supply (Vpeak) in order to
get consistent firing, and adjustment is relatively easy to do with a
synchronous rotary gap or a properly adjusted multiple-static gap. By
measuring Vgap, Cp, and BPS, one can appropriately characterize the
energy per bang and avreage power being applied to the primary side of
the Tesla Coil.
BTW, if you've "sized" your tank cap to your power source, you'll likely
encounter 50/60 Hz resonant rise. While this can easily occur with a NST
source (indeed, most Tesla Coil design programs tend to size the tank
cap so that this does occur), it can also occur with ballasted pole
pigs. During this condition, Vpeak can be MANY times the nominal peak
voltage that would be calculated from the RMS faceplate voltage of the
HV supply. Now as long as the power source AND the tank cap can
withstand the higher voltage, it's quite possible to set Vgap so that
it's significantly greater than the nominal (faceplate calculated)
Vpeak. However, the 1/2 Cp*Vgap^2 energy equation still holds in the
primary circuit. Since the maximum Vgap can vary substantially in an
asynchronous rotary gap, the usual result is an overvolted tank cap or a
carbon-tracked NST. This probably accounts for many a dead NST, even
those protected with RC or RLC filters, and for killed tank caps with
pole pigs. A pig usually shrugs off this abuse, often only growling a
bit more than usual...
-- Bert --
Tesla List wrote:
>
> ---------- Forwarded message ----------
> Date: Wed, 29 Jul 1998 23:27:57 -0500
> From: "Barton B. Anderson" <mopar-at-uswest-dot-net>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Joules - Vrms or Vp (fwd)
>
> Tesla List wrote: Subject: Re: Joules - Vrms or Vp (fwd)
>
> > > Regarding the calculation for Joules..... J = 1/2 x C x V^2.
> > > Should V = Vp or should V = Vrms? Why? (if you know).
> > V = Vpeak, since that is the energy when the gap fires.
>
> First, thanks to all who responded. I was *patient* to not reply
> until I heard from quite a few on the list on this matter. The
> question was derived from Terry's Coil data for my coil where Vp
> was used to calculate 12.4J vs. what I originally calculated to
> 6J. The difference was of course, I was using Vrms instead of Vp.
> Hence, "ask the List".
>
> It is very apparent, that the joules we calculate for TC's is
> dependent on our reference of time and energy. We could measure
> voltage across the cap when the gap fires initially, or we could
> view it as the energy available across the cap "prior" to the
> firing, and still yet, we could view it in terms of energy
> available minus losses after the bang.
>
> I personally like Robert's and others expression that the energy
> available at the time of firing is what should be determined. If
> our gaps are not synched to peak firing, or our losses are
> extensive, we should term these differently. It appears that the
> V^2 is Vp^2 for all practical purposes. Possibly a separate energy
> delivered rating is needed to separate efficiencies and losses
> into the equation. Jsec.?
>
> Thanks again to Greg, Richard, Stone, Dale, Antonio, Shaun,
> Robert, and Terry for your responses and related responses. John,
> Malcolm, and Bert, I would still be interested in your respected
> thoughts on this subject either on or off the list. I know
> efficiencies have played a major role in discussions in the past.
>
> Bart