[Home][2017 Index] Re: [TCML] MMC [Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


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.

      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...  :)


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
Tesla mailing list

Tesla mailing list