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Re: exploding capacitors



Original poster: "Steve Zeitler" <zeitler-at-verizon-dot-net> 


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, March 02, 2004 3:00 PM
Subject: Re: exploding capacitors


 > Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>
 >
 > Tesla list wrote:
 > >Original poster: "Steve Zeitler" <zeitler-at-verizon-dot-net>
 > >I'm reading all these postings about caps blowing up. Being the keeper of
 > >a medium size coil in a commercial setting, naturally i don't want this
to
 > >happen to me (caps blowing up)
 > >The latest post mentions an asyncronous rotary gap being used  causing
the
 > >failure. I've not heard of this before and the coil I mention has one. Is
 > >this bad and why?
 > >Also, the "cap" in this coil consists of 4 .1uf 30kv plastic cap brand 2
 > >in series on each side of the primary giving .025uf  at 120kv. An old Ham
 > >radio manual I have shows using resistors in parallel with caps when they
 > >are hooked in series to equalize the voltage each cap sees. I'm not doing
 > >this right now should I? I'm running 11-12kv after ballasting and I'm in
 > >the process of upgrading to 2 potential transformers instead of 2
homemade
 > >ones. The voltage is about the same but the delivered current is higher.
 > >This allows me to run the rotary gap much faster ...more PPS. comments?
 > >                                                    Steve Z
 > >                                                     Philly
 > >
 > >.
 >
 > Steve,
 >
 > An async gap can cause unexpectedly high voltages to develop across the
the
 > HV transformer and tank cap under certain operating conditions. This
 > particular problem can easily be alleviated by making sure you connect a
 > properly set static spark gap (similar to a safety gap) directly across
the
 > async gap, and that you also use a front end "Terry filter" and safety
gaps.
 >
 > Plastic Capacitors makes both Polypropylene and Mylar capacitors, and they
 > normally recommend using type BNZ for Tesla Coil applications. These
 > phenolic-cased caps have been known to fail (and may begin leaking oil),
 > but I have not heard of any violent failures. Which model are you using in
 > your system?
 >
 > Although connecting resistors across the capacitor chain will help bleed
 > off any residual voltages after the system is shut down, they'll have
 > virtually no influence on equalizing the voltages developed across the
 > chain. For pulse applications, voltage sharing is governed entirely by the
 > relative values of the capacitors in the chain. Since your caps are all
 > approximately equal and of the same type (i.e., similar ESR), the voltage
 > sharing should be approximately equal.
 >
 > Equalizing resistors are often necessary for proper voltage sharing in
 > steady state DC/filter applications due to variations in capacitor leakage
 > current. For your "equidrive" tank configuration, I'd recommend adding
them
 > to prevent a possible shock hazard since otherwise the equidrive
 > configuration can leave the primary winding somewhat  "hot" long after the
 > system has been shut down.
 >
 > I think I remember seeing your system while visiting my daughter in Philly
 > a couple of years back - if I remember correctly there was also a Marx
 > Generator nearby...?
 >
 > Good luck!
 >
 > -- Bert --
 >
 >  hello bert
  these Plastic caps are in tin cans. The exact designation on the cans I'll
have to get tomorrow. I can install an extra gap across the rotary to avoid
those unusual high voltages
I've never had any cap trouble but I have had trouble with trannys. The
original tranny was a 15k -at-120 NST. Fried it. I depotted it and the short
was inside one of the windings...bummer
One of the homemade trannys had an arcrover during some full power
experiments. I was able to fix it but always ran the coil at a lower power
thereafter. I have always used filtering on both HV leads consisting of a 1K
100w res + a 3/4" X long 8" piece of plex tubing wound with a layer of #34
wire, inductance unknown . the museums insistence on running with a wire
pointer on top  arcing to a ground wire as a "lightning rod" doesn't help
things.
The 2 -1.5kva 70:1 potential trannies were donated by Philly electric co.
Theyre tested to 110kv impulses so I don't expect any more trouble here
especially with the extra gap suggested  and safety gaps too. Other jobs get
in the way of finishing the fix up but hey...how many people get to play
with a TC on company time?
Yes there are 2 Marx generators around but thats off topic.
                                                             Steve Z