Re: Modeling Secondary Inductance

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Jeff,

On 9 Sep 2001, at 19:58, Tesla list wrote:

> Original poster: "Janet Johnson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jpjmassage-at-earthlink-dot-net>
> 
> Hi everyone,
> I was using WinTesla to design my coil and started playing with the Spice
model
> provided.  A book I have (The Tesla Coil Book by Brent Turner) describes the
> secondary as a series resonant ckt but the Spice model from WinTesla
shows the
> self capacitance plus the toroid cap in parallel.  This makes sense to me
since
> self-capacitance comes from inter-winding and planar, both of which are in
> parallel with the coil inductance.  Is the book wrong or am I missing
> something? 

Consider any LC circuit. Whether it is a parallel or series circuit 
is defined by the method of connection to it. If you look into the 
base of the coil, you see a low impedance. If you look across it, you 
see a high impedance.

 Another point of confusion for me is how the secondary is treated
> both as a quarter-wave resonator and an LC resonator.  The first is a
> transmission line concept (i.e. distributed inductance and capacitance) while
> the second is a lumped element concept.  Is this like wave/particle
duality of
> light or something?  

The reality is that for a component to be truly lumped, it must 
occupy an infinitely small portion of space. In that sense, no 
component is truly lumped but many come close (e.g. a small 
capacitor). A magnetic field is generated whenever there is a current 
flow and shows there is an inductive component wherever current 
flows. Likewise, an electric field is present whenever a potential 
difference is present.
    You can usefully treat the coil as a lumped component circuit for 
a specific range of purposes such as winding to a design resonant 
frequency. However, the fact that this is not an accurate description 
of its operation is shown by the fact that the current through the 
coil is not uniform and that there is a propagation delay associated 
with it. So the lumped view is more of a recipe approach. 
     Suggested reading - papers on the modelling project on the TSSP 
website. 

> Something else I noticed about the model was that when the spark gap switch
> closed, the coil output spiked up and rang down.  I thought the secondary
> oscillations built over successive firings until the voltage reached a
> threshold and blasted out sparks.  I read an analogy with lasers somewhere...

The only time a secondary can ring up and up is with a CW driver. It 
cannot do that with a gap for a number of reasons. I think it was a 
fond hope of Teslas that it might do exactly that when he operated at 
thousands of breaks per second. However, once the breakrate gets that 
high, the streamers at the output become a continuous arc, clamping 
output voltage severely. A mechanical gap cannot match the phasing 
and timing requirements for a disruptive coil to mimic a CW driven 
coil. Experiments have been done using solid state drives in various 
forms which clearly show the problems of a mechanical system.
     Suggested reading - Greg Leyh's excellent post of a a few weeks 
ago on the considerations behind LOD's largest feasible coil proposal.

> Anyway, if some of you theoriticians out there wouldn't mind setting me
> straight on the concepts, I would be most grateful.  I am an engineer so I
> can't help having to know exactly how these things work!
> Thanks in advance
>  
> Jeff Johnson
> Salt Lake City

Me too.

Regards,
malcolm