Re: calculating safe primary turn-to-turn distance

["Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Tesla list wrote:
> 
> Original poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>
> 
> >
> > I just realized that is true for a helical winding, not a
> > pancake coil.! as the inductance/turn tapers off severely
> > towards the inner turns :(
> >
> 
> Excellent point.... What would a good approximation be? linear? Square law?
> (inductance should vary with the area, right?) So, more inductance/turn on
> the inside turns.. = more voltage drop

Jim, Malcolm and all,

The incremental inductance/turn increases significantly for the
outermost turns, since the inductance increases as a function of the
radius squared and turns squared. 

For example, a 10-turn pancake primary with inner diameter of 10" and a
turn-turn spacing of 0.75" from center-to-center will have approximately
50 uH of total inductance. However, the incremental change in iductance
from turn 1 to 2 only represents about 1% of the total inductance, while
going from turn 9 to 10 contributes almost 20% of the total inductance.
This means that about 20% of the total primary voltage stress will
reside between turn 9 and 10 compared with only 10% if a linear
volts/turn assumption is made - basically twice the voltage stress! As
expected, the innermost turns will see a much lower turn/turn voltage
stress. 
 
      Lpri -at-	     Delta vs	   % of Tank voltage 
Turn  Turn (uH)   Prev. Turn (uH)  Stress to Prev. Turn
 1	0.6		0.0		1.2%  		
 2	2.1		1.6		3.1%
 3	4.6		2.5		4.9%  
 4	8.0		3.4		6.7%
 5	12.3		4.3		8.6%
 6	17.6		5.3		10.6%
 7	24.0		6.4		12.7%
 8	31.5		7.5		15.0%
 9	40.2		8.7		17.4%
10	50.2		10.0		19.9%

Gettin' stressed out in Illinois...

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-com
Web Site: http://www.teslamania-dot-com