Re: additional transformers [rolled caps]

From: 	Robert W. Stephens[SMTP:rwstephens-at-headwaters-dot-com]
Reply To: 	rwstephens-at-headwaters-dot-com
Sent: 	Thursday, August 07, 1997 12:21 AM
To: 	Tesla List
Subject: 	Re: additional transformers [rolled caps]

> From: 	richard hull[SMTP:rhull-at-richmond.infi-dot-net]
> Sent: 	Friday, August 01, 1997 5:52 PM
> To: 	Tesla List
> Subject: 	Re: additional transformers [rolled caps]
> A
> >> > >
> >> > >Peter,
> >> > 
> >> > I recommended leaving a full 2" edge separation metal to edge of dielectric
> >> > in all rolled capacitors!
> >> > 
> >> > Richard Hull, TCBOR
> >> 
> >> Thanks for the info on that, I will go 2" on my future overlaps. It
> >> seems strange to me that sparks seem to travel much further along the
> >> surface of an insulator, no matter how clean it seems to be. Maybe this
> >> is due to microscopic impurities on the surface of the plastic?
> >> 
> >> Another little trick that you probably have already figured out is to
> >> insulate the input connections to the plates by rolling a few layers of
> >> poly around them.
> >> 
> >> Cheers, Peter E.
> >> 
> >Peter, Richard, All,
> >
> >I had the same problem with a rolled poly cap I made once, and I 
> >*was* using a 2 inch surround from the foil to the edge of the poly.  
> >Dismantling the cap after the failure showed these burns distributed 
> >all along the edge of the foil outwards into the poly.
> >
> >I had a chat with a capacitor engineer at Condesnser Products about 
> >this phenomenon and was told that this is why HV capacitors are 
> >generally not successful when one tries to apply more than 8-10 kV 
> >across a single stage.  He explained that the energy exchange between 
> >the plate and the local surface of the dielectric becomes too violent 
> >and just completely overstresses the dielectric causing it to 
> >break down.  Placing additional thickness of poly does not 
> >significantly reduce this violent surface area exchange so you will 
> >still get failures no matter what you do if you try to run a single 
> >stage cap at 15 or 20 kV.
> >
> >rwstephens
> >
> >
> >
> >Robert,
> This is the very reason I have stated even in tape #3 where the big rolled
> jobs are made that one should never run them at over 10KV.  Series for more
> volts.  In tape #8 where we build another type of poly cap, I note that 7-8
> KV is the max on the single unit.  Nonetheless,  many never headed my
> warning and blew out there caps almost immediately.  Too many coilers rush
> to the 15,000 60ma neons and use a single capacitor unit.  They will lose it
> sooner or later.  
> The two inch boarder is minumum on caps used over 5KV per unit.  Once aged
> and all the air is out I have run my single, big, rolled oil caps with 12KV
> neon transformers and luckily have never lost one.  When working on maggeies
> #1-8 from 1990-1995 I used two of the big rolled jobs in series with a
> 14,400 volt potential transformer and never suffered a break down, but the
> stress was reduced on each unit by series stringing them.
> Since then I have used only professionally produced discharge caps.  The old
> 1990 built rolled caps are still kickin' and are often used in snap setups
> for visitors, etc.
> Concomittant with the use of homemade caps is a sense of power input
> limitation in the form of break rate.  Lots of folks kill these spartan
> units with 15,800 rpm 24 point rotary gaps!!!!  Even 2 KV might blow them at
> that rate of delivery.  Newbies always want the 20,000 volt transformers and
> turbine speed rotaries and wonder at the ease of destruction of their
> capacitors.
> There is rarely a real viable need for break rate over 360/second (maggies
> can handle more).  If a rotary gap has more than 8 points, it will be a bear
> to quench well with out super high speed and attendant cap destruction.
> I have no way of knowing, but I suspect a lot of Good CP caps fell to the
> turbine rotary effect.
> Richard Hull, TCBOR

You have shared some valuable pointers again for newbies here and 
safety reminders for the rest of us.  I can really appreciate how a 
turbine break rate will destroy the best commercial capacitor but I 
just wanted to say in reference to your 360 BPS suggestion that I 
have personally found ~ 402.5 BPS (14 stud rotary wheel on a 1725 RPM 
non-synchronous motor) to be really sweet.  A wheel with 12 studs 
yielding ~345 BPS was significantly less impressive in my MTC unit, all 
things else being identically equal.  I guess there is an ion channel 
lifetime effect difference happening here that is better at the 
slightly higher break rate of 400,  but suspect a supply sinewave 
filling factor or synchronizing benefit also as the break pulses 'walk' through 
the 60 cycle mains phase in a slightly more concentrated manner.

The Twin system which I have recently built really comes alive in 
this  400 PPS regieme, and is noticeably less remarkable at slower 
rates, including attempts to set the non-sync DC rotary break motor 
to run synchronously at 120 BPS (due to large wheel inertia and 
variac control I can successfully ride the crest of the sinewave peaks for a few
seconds at a time).  I subsequently have no intention to operate the 
Twin at 120 BPS synchronous,  nor any need to operate it's break rate 
exceeding 402 PPS.