Re: tank circuit of VTTC

["Herwig Roscher" <herwig.roscher-at-gmx.de> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Bert Hickman wrote to the list:

> The real problem is that, because of skin effect, the skin depth of 
your copper conductors is only to be about 0.15 mils (0.004 mm) 
for the operating frequency of your coil (about 300 kHz). 
 
Bert,

Yes, I failed to take into account the skin effect. According to my 
collection of formulas the intrusion depth (at which the current is 
reduced to 37 percent of the current at the surface), for copper is 
calculated to be	
					65.6/SQRT frequency

depth in µm ( = mm/1000), frequency in MHz

For a frequency of 300 kHz this depth will be about 0.2 mm.

Assuming a conductor of  6 mm cross section (very sorry for using 
these unusual units of measure), which is between AWG 10 and 
AWG 9, the area is reduced from 6.0 to 4.4 mm squared e.g. to 
about 70 percent.

> Because of skin effect, a circular #10 AWG conductor that has 
almost 8200 square mils of conductor area at DC is reduced to 
about 48 square
- Which formula did you use?

> I'd suggest beefing up the tank circuit by going to larger diameter 
copper tubing or flat copper strap, and possibly silver plating it as 
well.
- The problem is:
I can get 25 m lenghts of solid AWG 10, but in the local home 
depots there is no chance for solid AWG 7 (next available size). 
Stranded AWG 7 is offered by the m, but tapping would be the 
problem now. Another solution would be the use of 1/4" tubing, but 
then the coil becomes very large.

So, there are 2 choices:
1. to give AWG 10 a try.
2. to use stranded AWG 7 without tapping and to try tuning the 
tank circuit with different caps.

> However, I have no doubt that a properly constructed MMC could safely
> handle this task.
I admit, that I looked for the peak current only. I remembered 
Terrys tests but forgot that the measuring current was only 3 A.

As far as I have solved the tank coil problem I'll measure the 
inductivity and redesign the MMC.

>You may be able to squeeze more out of the MMC by blowing air 
across them
- This was my intention.

> beefing up the leads
- of course!

> or immersing the unit in oil
- I built vertically stacked poly caps which were immersed in oil, 
but now I try to avoid this mess.

> You MAY also be able to get by with a bit lower design voltage 
on the MMC - perhaps going to 25-30 kV - to save on cost. 
- That's always welcome.

> Hope this helped, and good luck in your design!
- It does - thank you very much.

Regards

Herwig