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Waupaca WI? If that is an open invite I'll be making my way there for sure (MN here).

No coils yet but I can bring my largest VDG :)


> On Feb 21, 2017, at 5:09 PM, Stan Gray <wsmg@xxxxxxx> wrote:
> Bert, You always amaze me with your knowledge. I wish I had it. I also miss the DC Teslathon with all you guys, Terry, Steve, and Jeff and more I have trouble with names. Is there any chance that you could get together with the gang to come up to Waupaca for a Teslathon? If so I'll arrange it.
> Come on guys lets make it a true memorial, it's been awhile. I'll make sure the cannon's ready to go and have plenty of power to fire up coils.
> Stan
>      From: Bert Hickman <bert@xxxxxxxxxxxxxxxxxxxxx>
> To: Tesla Coil Mailing List <tesla@xxxxxxxxxx> 
> Sent: Tuesday, February 21, 2017 2:02 PM
> Subject: Re: [TCML] MMC
> Hi Doug,
> Tesla Coil caps see a very nasty oscillatory, high-current, high 
> rep-rate, high-voltage environment that is extremely challenging to a 
> capacitor's dielectric and metalization system. Fortunately, the 
> self-healing feature of these particular caps allows you to overstress 
> them without suffering immediate catastrophic failure. It allows you to 
> trade off capacitor run-time life versus initial cost for the capacitor 
> bank. Excessive voltage stress on your self-healing caps results in the 
> eventual death of your tank cap, and the greater the overstress, the 
> shorter the expected life.
> For TC use, moderately conservative design practice suggests that each 
> MMC string should have a total DC rating no less that 2.5X - 3X your HV 
> source RMS face plate voltage rating (30 - 36 kVDC for your 12 kV NST). 
> However, you CAN choose to use a lower factor - with more risk and 
> reduced lifetime. Some MMC design charts even show a factor as low as 
> 1.33X (i.e., only 8 caps in series for a 12 kV NST!). Using fewer caps 
> in each MMC string significantly increases the voltage stress on each 
> cap. In TC caps, overvolting is most often caused by voltage reversals 
> in the ringing tank circuit. The dielectric system of a cap that rapidly 
> reverses polarity "sees" a voltage stress that's 2X as high as the 
> initial capacitor voltage.
> Initial symptoms of overvolting are typically silent, showing up as 
> small sparks (partial discharges) along the boundary between the 
> capacitor's metalization and adjacent dielectric. When the dielectric 
> fails, a short-circuit and self-healing event occurs. Self-healing 
> events near the outside tape layer can sometimes be seen as flashes of 
> light. Once this destructive process begins, it progressively chews at 
> the polypropylene dielectric, causing large numbers of 
> short-circuit/self-healing clearing events. These progressively damage, 
> and eventually destroy, your tank caps. Depending on the degree of 
> overstress, this can take minutes, hours, or days. By using more caps in 
> each string to reduce voltage stress, the degradation process can be 
> avoided and the usable lifetime of the caps can be extended indefinitely.
> Looking at the physics and of the internal structure of these 
> self-healing capacitors, I would recommend using a factor no less that 
> 2.5x to 3x Vsupply(RMS), or 15 to 18 caps/string for your 12 kV NST.
> If you only need a few hours of run-time life, you can further reduce 
> the number in each string to perhaps 2X or less. I wouldn't, but you 
> can...  :)
> Bert
> doug wrote:
>> I have 3 MMC’s each consisting of 15 .15u X 2 Kv Caps. [10nf X 30Kv] I can connect them P or  S. Which would give me the best setup using a 12X30 NST.
>> Doug
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