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Re: spiral secondary ?
Hi Mike and all,
On Thu, 15 Oct 1998, Tesla List wrote:
> Original Poster: wwl-at-netcomuk.co.uk (Mike Harrison)
>
> Just a strange thought that floated through my mind the other day -
> would a flat (or angled) spiral *secondary* be practical ? I'm sure
> the change in inductance per turn would have some effect - maybe good,
> maybe bad?
> Maybe too many volts per turn to insulate on the outer turns? What
> shape should the primary be?
> Shouldn't be too impossible to wind a flat one - plastic insulated
> wire would be fairly easy, magnet wire would be a bit trickier, but
> should be possible using a plastic sheet covered with double-sided
> sticky film, so it sticks down as you wind it.
> Any takers?
Yup!
> Feel free to shoot it down in flames....!
No, quite the opposite. Mike, your timing is amazing! We're currently
constructing a new prototype tesla coil using this same design and
construction technique - We've never built a coil like this before, so
we're covering new ground. The theory looks interesting, but, the
practicalities will tell most of the story...we can hardly wait! :-)
In fact, Tesla noted this type of design (or similar) in a few of his
patents (refer patent numbers 593,138 (1897), 645,576 (1900) - Power
transmission by Wire and Wireless transmission of electricity,
respecitively).
Bill Wysock refers (providing I have interperated the story correctly) to
a similar design he proposed for Model 14M Magnifier in his Model 13M
story (found at http://www.ttr-dot-com/story.html)
-- Mike, this should answer your questions...
(...please, if may I quote from your inspiring story Bill...)
"The original design for the Master Oscillator, (Tesla primary and
secondary coils), was to be a multi-layered archimedian flat spiral that
would be 20 feet in diameter. The framework supporting these two coils was
to be made of fiberglass and form a framework that could be bolted
together on location, and would elevate the horizontal plane of the
primary and secondary coils 20 feet high above ground. The electrical and
physical advantages of this design over Tesla's 49.25 foot diameter
helical-wound primary and secondary coils as used in 1899, are well known
Even Tesla had previous practical experience with this form of r.f.
transformer winding technique, as witnessed from photographs in his
Houston Street laboratory prior to the Colorado Springs development work,
and again at Wardenclyffe; Tesla used flat spiral transformer designs.
These advantages are the ability to control the voltage gradient
distribution along the secondary winding, and an improved coupling
coefficient between the primary and secondary, where the primary is wound
about the circumference of the outer diameter of the secondary coil, which
is the ground point of the secondary. This places the largest amount of
secondary coil inductance in a closer magnetically coupled influence of
the primary winding, thereby increasing mutual coupling between the two
windings, producing higher energ y transfer efficiency.
In a high frequency, high voltage transformer, E-field or voltage gradient
control becomes a serious concern. Corona losses need to be eliminated or
minimized in order to achieve maximum transformer efficiency. In a flat
spiral secondary construction where the low voltage or ground point is at
the periphery, the high voltage point will be at the center of the spiral.
The electromagnetic fields set up in this type of winding produce a
Faraday shielding effect on successive turns, reducing stray corona
leakage. In Tesla's early development work, thisform of control was
experimented with, as well as other methods, such as enclosing the entire
transformer in oil. It is interesting to note that the modern-day
automotive ignition coil (for which Tesla received a patient,) combined
the attributes of a primary winding around concentric layers of secondary
windings, the high voltage end of which is at the center of the winding,
and is electrically connected to a silicon steel laminated core. This core
material increases the permeability of both windings and, along with the
whole assembly being contained in oil, provides a design of smallest size
and highest efficiency.
In a Tesla transformer of much higher input power and size, it becomes
impractical to employ oil or any ferrous core material. However, the
fore-mentioned coupling efficiency of the concentric layer approach with
the high voltage end at the center of the construction, becomes a very
attractive design feature." -- Bill Wysock, Tesla Technology Research.
This story gave me the inspiration to traverse this avenue of coiling -
Thanks heaps Bill! :-)
Cheers,
Rod
Tesla Technology Australia
http://www.tesla-technology-dot-com.au