Re: 8 inch coil at 100 amps

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 12/17/00 12:52:31 AM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

> Original poster: "D.Wightman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <
> dwightman-at-mmcable-dot-com>
>  
>  After trying and trying again, it seems that my 8 inch coil will never run 
> at
>  100 amps. 

Dwight,

A lot depends on the length of the secondary.  If it's too short, it will
burn up.  45 amps at 240 volts is about 11kW.  That's a lot of power.
100 amps is 24kW.  Usually with a coil like that, a rather large secondary
should be used.  More below.

>  From burning out the secondary several times, to adding "creepage
>  disks", it just won't digest all that juice! The rsg will severely burn out
>  electrodes and performance stays the same. It is capable of only about 45 
> amps
>  input to the pig and that is it! 

Since your electrodes are burning out, this suggests the gap losses
are high.  You would probably benefit from having a larger inductance
in your secondary and primary.  The electrodes are probably running
too hot too, and not quenching.  they may even be re-firing, which 
wastes a lot of power.  This occurs if the mechanical dwell is long, and 
the rotor speed is slow, or if the cap is small, etc.

> My theory about this is that the wire 
> diameter
>  on the secondary is 22 guage. The electrons want to flow on the outside of 
> the
>  conductor. It seems that the coil is just maxed out. Larger wire diameter 
on
>  the secondary should allow the coil to put out longer sparks. Yes that 
means
>  building a bigger coil.

Basically the coil must be tall enough for the power input.  For instance
Richard Hull used a 45" tall coil for 10kW or so, at about 500 bps.  If 
your bps is lower and the bang size larger, the coil may need to be
taller than that.  I'm guessing that your coil is shorter than 45" esp if
you're using a high bps.  I think for 120 bps, a 1200 watt coil should
be 2 feet tall (the secondary).  So for 4800 watts, it should be 4 feet
tall, and for 19kW it should be 8 feet tall.  (It can be shorter if the bps
is higher because the bang size will be smaller).   So yes, you 
probably need a larger coil and thicker wire.  The wire thickness
is a tradeoff with gap losses, so on a very large coil, the wire should
be made thicker as the power is increased.  I like to simply use
1600 turns.  This automatically gives a good wire size as the
coil is made physically larger and is supplied with more power.

>  Also i've been thinking about Kevin Eldridges coil,
>  which is doing 24 feet. He is using stranded wire which would make one 
think,
> 
>  "more surface area". Ahaaa!!!!! Of course his coil is 24 inches in 
diameter,
>  and is using somewhere around 10 or 12 guage wire. Don't quote me on that 
> wire
>  size, 

The importance of using thick wire with a lot of surface area is 
over-rated.  In reality, other factors are more important.  Of course
the wire should be reasonably thick for the power input.  I would chose
a wire thickness that gives 1600 turns on the secondary.  This will
give nice low gap losses, and make the coil efficient overall.  A wide
coil is good too because it increases the inductance and makes the
frequency lower (helps the gap de-ionize and quench) and it makes
the inductance higher, which results in a higher primary surge Z
which reduces gap losses.

> But regardless of that if he puts 60 amps to his coil he probably gets 
> 15
>  feet of spark, while on the other hand if i put 60 amps to my 8 inch coil, 
I'
> ll
>  get only 10 feet and roast 1/4 inch tungston electrodes in 1 minute on the 
> rsg!

The fact that your are roasting those electrodes probably says 
something is wrong in addition to wire size. 
If your break-rate is very high, this will
make the coil less efficient also, especially if you're using a large
capacitor.  You may need more series gaps in the rotary too at
that power level.  Or some vacuum quench external series static
gaps in addition to the rotary, especially if the rotary has just two
gaps presently.  If your cap is too small, this will limit the spark
output too.  A tesla coil must be balanced among the following;
power input, bps, cap size, toroid size, secondary size, mechanical
dwell, etc.  One must really consider all aspects of the coil when
suggesting a cure.  Everything kind of works together (or fights
one another).

>  This leads me to a question. Is it correct to say that in tesla coil
>  applications, Are the electrons flowing on the outer surface area of the
>  conductor, specificly on the secondary. And if so, is it correct in 
> reasoning
>  to increase wire surface area in order to allow more flow throught the 
> system.
>  And as a result, longer sparks? Thanks everyone!

It is true that the current flows only in the outer part of the wire, and
yes, your wire may be a little thin for your power input.  But I think
there are other problems in your system, such as a too short
secondary, etc.  Does the secondary get hot?  If not, then there
can't be very much loss in it.  

I didn't cover all details here (there are so many).  But they've been
discussed in prior postings.

JOhn Freau
>