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Re: flashing vs. whatever


From: 	Barton B. Anderson[SMTP:mopar-at-mn.uswest-dot-net]
Sent: 	Tuesday, November 25, 1997 10:24 AM
To: 	Tesla List
Subject: 	Re: flashing vs. whatever

Tesla List wrote:

> From:   Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
> Reply To:       bert.hickman-at-aquila-dot-com
> Sent:   Tuesday, November 25, 1997 8:41 AM
> To:     Tesla List
> Subject:        Re: flashing vs. whatever
>
> Bart,
>
> Flat plate caps work very well with heavy duty aluminum foil - flashing
> is not necessary. However, I do question the dielectric thickness and
> capacitance value, especially if you're using the cap with a rotary gap.
> 32 mils of dielectric is pretty thin, especially if you're planning on
> "sizing" the cap to the transformer. When you "size" the tank cap to
> your power transformer, you're actually "tuning" the pair to resonate at
> line frequency. For excessively long gap settings, this can
> significantly increase the voltage stress on the cap and transformer by
> a factor of 3-5X (or more!) over the nominal transformer output voltage!
> Connecting the caps in parallel will NOT increase their voltage handling
> capability, only the effective capacitance. Also, with a tank cap this
> large, you'll need to have a fairly large secondary and toroid so that
> you can bring this system in tune with a reasonable number of primary
> turns.
>
> How big were you planning on making your secondary and toroid, and what
> voltage/current is your power source??

Bert,Yes, it's a large value cap! Here's my project: (100KHz, 17KW)

Items completed:
Secondary: 12.5"OD, 33.2" H. 783 turns. #18 polythermalese magnet wire (1/4
Wave of 2460ft).
Primary: Flat pancake 3/8" soft copper tubing. Currently 8 turns (may be
expanded), 3/8 spaced, 23" OD. Adjustable up/down first winding of sec. by 9
inches each direction.
X-Former: Pole Pig 14.4K, 240, 10KVA, 1 Ph.
Toroid: 8 x 30".

Not completed or attained:
Cap: 0.256uF (Just now starting to seriously think about this design)
Sparkgap:  now at 8 electrodes (should finish this weekend)
Variac (not attained yet but found 3, 30 amp'ers. Have yet to inspect and
measure)
Ballast (not built or attained yet. Either variac modified or welder;
undecided)
Stand-alone breaker box to be put in garage (off incoming).
And a CRAP LOAD of safety!!! (including labels).

Your exactly right about parallel caps NOT increasing their voltage handling.
I think I said "series" in my post (brain must have been turned off last
night) Yes Parallel! What I'm doing is deciding on purchasing vs. building a
cap for this big dog. I have a contact at CSI. He offered to build the cap
(all polypropylene) for $300 (might be the way to go). He said this particular
process takes about 6 weeks to build one of these caps (unlike the paper and
poly type used for wicking the oil).

In my equation I used 2.3 as the dielectric of x-former oil which is near
polyethylene film. However, I see what you mean. 32 mils (total) isn't even
close. I did this calculation pretty fast and with not a lot of digging into
the archives. It looks like it would take more along the lines of 4, 30 mil
sheets on each plate side. I would end up with a distance over a 1/4inch
between plates. This seems extreme and is based on 240V/mil at breakdown. Is
this value correct? If so, I could end up with a huge and maybe impossible to
build cap capable of about 60KV. However, this assumes a linear build and
doesn't account for layering of dielectric. Is there an equation (or good
assumption) for finding voltage handling capability of layered film plate
caps?

Thanks,
Bart