Re: JF efficiency theory (again)

["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 2/18/01 10:15:28 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

> > 
>  > I was wondering why some coilers get such good results when completely
>  > disregaurding such details. For instance, Ross Overstreet's coil:
>  > 6"x24" wound with #22 for a total of ~880 turns, a primary of approx
>  > 7-8 turns (from the photo) and ~2.2kVA input along with a static gap.
>  > He gets 6-7 ft sparks which is just about right from 1.7(sqrt(input
>  > power)). However, he's only using half the reccommended number of
>  > turns on both the primary AND secondary. Is it possible that it is not
>  > the NUMBER of turns, that maybe it's simply proportionate on some
>  > level?
>  
>  I agree with your conclusion. It is in fact the reactance nee surge 
>  impedance that matters. Obviously, many different coils having 6-7 
>  turns can be wound with large variations in inductance. Likewise, a 
>  single 6-7 coil will have a wide range of reactance depending on what 
>  frequency it is run at.
>   

Malcolm, Mike, Ross, all,

Although the inductance will vary with the coil's diameter, most
coils are built along similar design proportions.  Generally, as
a coil gets larger it will remain in proportion for the most part.

Consider these coils:  (I'm using Arial font)

Pri uH  turns    cap uF      reactance        kHz
                                        (ohms)
100       10       .15              30                50    (large coil)
100                 .05              45                70     (large coil)
100       20      .015             90               160    (small coil, with
1600 turn secondary.)
400       20      .15               60                 25     (large coil, 
with
1600 turn secondary.)

The 50 kHz coil above may have few primary turns because
of the large coil diameter.  However, the cap is large, so
the reactance or Zsurge is relatively low.  To make the 
Zsurge high, it would be necessary to use a lot more turns
on both the primary and secondary.  My guess is that if
this was done, the 50 kHz coil would give about 10% longer
sparks, and run at 25kHz.  This is the kind of reasoning that
I am using.  The 25kHz coil is an example of this.  The Zsurge
rises to 60, which is still lower than my small coil.  It seems 
to be rather difficult to get a high Zsurge on a large coil, 
because of the large capacitors that are used.  Especially at
low bps.  To me this makes it even *more* important to use many 
turns in both the primary and secondary of a large coil.  Note:  the 
figures I used above are approximate, from a chart, but they 
show the trend.

Regarding large coils with more than 1000 turns, it's my guess 
that none have ever been built.  Does anyone know of one?

Alan Jones built a medium-large coil using 1300 turns, and it 
seems to be very good, but the input power is not known for 
sure at present, but Alan will be measuring it when he can.  
I'm not sure if his caps are low-loss types either.  The coil gives 
95" sparks or so, and runs at 120 bps.  This is the only coil 
I've heard of that approaches my suggestion of using 1600 turns
on a large coil.  As far as I know, there is no data at all on a 
large coil (8 - 100kW), using 1600 turns at 120 bps.

In any case it's a trade-off between gap losses and secondary
losses, as Malcolm often said.  I think it is there, that the
1600 turns idea for large coils will sink or swim.  Certainly,
secondary losses are less important than once believed, and
gap losses are more important than once believed.  It is this
kind of thinking that makes me optimistic about the probable
benefits of using 1600 turns on a large coil.  Again, I only
expect about 10% longer sparks.  Although they should be
brighter and more thrilling too.  Careful measurements are 
needed here.

Cheers,
John Freau