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Re: Not enough pimary inductance?

> Tesla List wrote:
> >
> > Original Poster: Esondrmn-at-aol-dot-com
> >
> > In a message dated 11/16/98 12:27:34 PM Pacific Standard Time,
> > tesla-at-pupman-dot-com writes:
> >
> > <<
> >  Not enough pimary inductance?  You don't necarcarly have to go and build
> >  a whole new coil, why don't you just put a couple of loops in the
> >  circuit somplace to add inductance and lower the freq?  Sounds like a
> >  good idea to me.
> >
> >  Chris, via the inter-thingy
> >   >>
> >
> > Chris,
> >
> > If you are going to add a couple of loops somewhere, I would suggest
adding
> > them to the primary.  It is usually possible to add more outside turns.
> > Adding wire or loops anywhere else in the primary circuit would be off
axis,
> > very lossy and probably noisy RF wise - that power has to go somewhere.
 From
> > an efficiency standpoint, it would be like setting a maximum of 80% on the
> > variac.
> >
> > Ed Sonderman
 
Ed, All,

I have to jump in here and officially attempt to dispell a myth about
off-axis inductance!  There is virtually _no_ wasted energy in an off
axis coil wired in series with the working tank primary coil if the
off-axis coil is wound with conductor hardware as high quality as the
primary coil itself.

Some of you may remember my twin TC which I discussed almost 2 years
ago. It had two flat helix primaries in series via 3/4 inch copper water
pipe transmission line.  Each secondary was tuned with topload for an Fo
of 100kHz.  The TOTAL primary circuit had to tune for 100 kHz as well
(actually a bit less to compensate for streamer loading but that is not
the point I'm trying to make here).  Therefore I ended up with only half
the L in each primary that I would have had to supply if this had been a
single tower coil system. At ~7 kVA this beauty would produce 10 foot
connecting streamers between toploads.

Here's where it gets interesting.  One day I decided to just simply
disconnect the ground connection from the bottom of one of the two HV
resonators and physically remove it from its perch completely.  Now the
system had exactly half the primary L in a coil out in the woods coupled
to nothing deliberate.  Now I turned on the system and started dialing
up the power on the variac.  By the time I had hit merely 3 kVA
(slightly less than half the normal power for this system) the remaining
coil was already exceeding its output streamer length as compared to the
usual twin operation at 7 kVA!  FOLKS...I NEVER CHANGED ANY TUNING AT
ALL!  Even though 50% of the total primary circuit inductance was now
completely off-axis, there was no practical RF power loss!  The magnetic
field that is produced in that freewheeling primary coil re-introduces
energy back into the tank circuit when it collapses because of its own
mutual inductance. The only real losses are I^2R resistance losses of
this coil.

I had heard the well circulated wive's tale that off axis L would waste
energy.  This test graphically proved that it does not.

The practical conclusion is that it IS technically acceptable to place a
high quality air core tuning coil in series with the Tesla coil primary
coil in order to achieve 'fine tuning' without throwing away hard won RF
tank circuit energy!

Real valuable knowledge gleaned by the actual doing. 


Robert W. Stephens