[Prev][Next][Index][Thread]

Re: Solid Toroid Beneficial?



Max:

Sorry, no pictures.  I am writing up my research notes, perhaps 200 pages
worth. They will be made available for the cost of photocopying and postage.
I hope to get things finished up in the next 2 or 3 months, but don't hold
your breath.

Gary Johnson

At 08:19 PM 3/15/00 -0700, you wrote:
>Original Poster: "Max Erhard" <max.erhard-at-softhome-dot-net> 
>
>Sounds like a very impressive coil, got any pictures of it?
>
>Tesla List wrote:
>> 
>> Original Poster: "Gary Johnson" <gjohnson-at-ksu.edu>
>> 
>> I have been working on a solid state drive for a Tesla coil for many years.
>> Standard wisdom has been that sparks will be inferior to those produced by
>> conventional Tesla coils (with a spark gap), but explanations given always
>> seemed a bit vague to me.  I have been trying to combine theory and
>> experiment in such a way that they will be readily understood by electrical
>> engineers, even if my sparks are not as long as those of conventional TCs.
>> I think I am getting close to understanding what is going on, but hit
>> something the other day that surprised me.  Perhaps someone can help me past
>> the mental block.
>> 
>> I am testing a variety of coils and toroids, but for purposes of discussion,
>> I will describe one coil and two toroids.  The coil is 14 ga magnet wire
>> spacewound on a polyethylene form, diameter 39.64 cm, winding length 116.62
>> cm, 387 turns.  One toroid is half spun aluminum, 6 inches by 24 inches. I
>> assume it was made by starting with a flat disc of aluminum perhaps 40
>> inches in diameter, and turning the edge down and back in until the outer
>> portion looks like a toroid.  I set it on the coil so the bottom lip is
>> level with the top turn and only an inch or so away.  It would definitely
>> form a shorted turn at the top of the coil.  The other toroid is made from
>> about 15 lengths of quarter inch copper tubing spaced about a quarter inch
>> apart.  The center is open, so there is no place for eddy currents to flow.
>> Size and capacitance are almost identical to the half spun toroid, so
>> resonant frequency is basically the same.
>> 
>> The coil is driven directly at the base by a square wave from an IGBT
>> inverter. Think of an extra coil being driven by a function generator.  The
>> power rating is on the order of 50 kW for short periods.  I have actually
>> seen a peak power of 33 kW at the point of spark initiation.  It has the
>> capability of CW operation, but because of thermal and circuit breaker
>> constraints I mostly use disruptive operation.  Power is applied for say 4
>> to 10 ms for 1 to 10 times per second.  Sparks look like standard disruptive
>> TC sparks, white, thick, up to 3 ft long in air.  A coil this size with a
>> properly sized top load should produce sparks at least 10 ft long.  The key
>> is the peak power applied.  A conventional TC driven by a pig with an
>> average power of 10 kW would be supplying peak powers to the coil of at
>> least 100 kW during discharge.  Other things being equal, a peak power of
>> 100 kW will always produce a longer spark than 33 kW.  It might be possible
>> to build solid state drives capable of pushing hundreds of kW into a TC, but
>> it would be a challenge!  So my inverter is not going to win any spark
>> length competition, but it is nice for making measurements on coils.
>> 
>> I measure the top voltage in two ways.  One is a fiber optic system (good
>> for any voltage) and the other is a capacitive voltage divider good up to
>> 100 kV.  The latter is used to calibrate the former.
>> 
>> Finally, I am ready to ask my question.  If I am not mistaken, the standard
>> wisdom among Tesla coilers is that shorted turns are bad.  Eddy current
>> losses will lower the efficiency of the TC.  However, the current and
>> magnetic field decrease as one goes to the top of a TC, so a shorted turn at
>> the top does not decrease performance a noticeable amount.  The 'big dawgs'
>> do not cut their beautiful toroids with the hope of getting a few more
>> inches of spark.  But I am measuring the opposite effect.  The shorted turn
>> toroid actually improves performance.  I am testing well below spark
>> breakout, with input power in the range of 100 to 800 Watts.  At fixed input
>> voltage, the shorted turn toroid will have a top voltage about 10 to 15
>> percent higher than that of the open turn toroid.  The input current will be
>> 10 to 15 percent higher as well, for the shorted turn toroid.  The trends
>> are consistent: more current = more input power = higher top voltage =
>> longer spark (if voltage is high enough for breakout).  The surprise is that
>> this occurs for the shorted turn toroid rather than the open turn toroid.
>> Has anyone else seen this effect?  Does the effect show up in PSPICE models?
>> Is there a 'simple' explanation?
>> 
>> I have noticed my name on the list lately in regard to some early results I
>> published about input impedance.  Anything I have written about Tesla coils
>> is in the public domain.
>> 
>> Gary Johnson
>
>