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Re: Top Toroid

Tesla List wrote:
> 
> > Subject: Re: Top Toroid
> 
> Subscriber: sgreiner-at-mail.wwnet-dot-com Fri Jan 31 23:10:25 1997
> Date: Thu, 30 Jan 1997 14:39:43 -0800
> From: Skip Greiner <sgreiner-at-mail.wwnet-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Top Toroid
> 
> Tesla List wrote:
> >
> > Subscriber: MALCOLM-at-directorate.wnp.ac.nz Wed Jan 29 21:02:26 1997
> > Date: Thu, 30 Jan 1997 11:02:08 +1200
> > From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
> > To: tesla-at-pupman-dot-com
> > Subject: Re: Top Toroid
> >
> > Hi all,
> >          Thought I might add a brief note to yesterday's diatribe
> > fyi.....
> >
> > >      I think we are generally agreed that the output voltage is
> > > going to be a function of the energy that gets to the secondary at
> > > the end of the first primary ringdown, and the total secondary
> > > capacitance including top load. From first principles, this turns out
> > > to be:  Vo = Vcap*SQRT(Cp/Cs).  As Bert Hickman has showed however,
> > > this ideal will never be reached because of significant losses in the
> > > gap. I think in some instances, around 80% of the energy gets
> > > transferred. Papers by others use the formula: Vo = Vcap*SQRT(Ls/Lp).
> > > This is true because Ls*Cs = Lp*Cp   (basic tuning requirement).
> >
> > <snip>
> > Worth noting that if the prim and sec coil geometries are identical
> > (same height and diameter), the ratio SQRT(Ls/Lp) reduces to a
> > straight turns ratio (Ns/Np). If you want convincing, do the algebra
> > on Wheeler's inductance formula. Note also that this does not take
> > k into account either. It can only be true for a lossless case (no
> > gaps or other losses in either coil), no matter what k is set at.
> > A key point from this is that the turns ratio doesn't count in typical
> > Tesla Coils. A cruel irony of this is that you can make Ls/Lp
> > arbitrarily high, but the lower the surge impedance of the primary,
> > the higher the gap losses.
> >
> > Malcolm
> >
> Hi Malcolm and all
> 
> I have been following with great interest the various posts on gap
> losses but not always accepting the conclusions put forth. I assume from
> your answer above that as the surge impedance goes down then the surge
> current increases. Are you then saying that gap losses go up due to this
> increased current? Doesn't this assume that the "resistance of the gap
> while it fires remains constant?
> 
> My experience with synchronous gaps leads me to a different conclusion.
> I feel quite sure that the current in a firing gap is a function of the
> voltage in the cap as well as other factors. I note that the contacts in
> my gaps run much cooler when the gap is firing near the peak of the
> mains voltage. When firing ahead of the peak (by adjusting the gap
> position with respect to the motor shaft) I notice that the gap pins get
> much hotter. I attribute this increase in gap heating to increased
> resistance in the gap due to a lower current/voltage. I believe that the
> gap resistance is dynasmic and definitely increases at lower currents
> which can definitely be controlled when using a synchronous gap. My
> primaries usually are 3 to 7 turns and I think on the low impedance side
> and therefore have low surge impedance. Still at 1800va input I have no
> trouble using .25" brass contacts in the rotary and they give very long
> life.
> 
> Flames and comments will be appreciated
> 
> Skip


Skip -

  What might be happening is that early firing on the 60Hz waveform
leaves magnetic energy stored in the transformer's core. Hence, you
are not only dumping the capacitor's energy, you are also dumping
the residual energy in the transformer as well. If the gap fires
late, then cap voltage goes down, but the transformer is now 'cooling
off' with less energy available.

- Brent (bturner-at-apc-dot-net)