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 _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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