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Re: [TCML] Asynchronous versus synchronous gaps, was Old Post

Hello all,

Several years ago I used a Maxwell application note to compute the projected lifetime of my capacitor bank. The bank consists of 6 Maxwell 37667 capacitors in a series-parallel arrangement. Each capacitor has the following specs:

* 30 nf
* 35 KV
* 15% VR
* 500 PPS
* 3 x 10^8 shots

The total capacitance of the array is 45 nf and rated for 70 KV. I operate at 240 PPS. I bought the capacitors used so I had to make a guess on the remaining number of shots. I assumed the lifetime was only half used. I think I assumed that I would be operating the coil for one hour per month. My SRSG coil operates at 14.4 KV RMS so I have a maximum voltage headroom of 3.5 to 1. Doing all the calculations as specified in the application note, I arrived at a remaining lifetime of about 40 years! I will be long-gone before those capacitors fail. The moral of the story is that 7.5 exponent voltage relationship to life time is extremely powerful.

Steve White
Cedar Rapids, Iowa

----- Original Message -----
From: "jimlux" <jimlux@xxxxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Tuesday, September 15, 2020 8:31:27 AM
Subject: Re: [TCML] Asynchronous versus synchronous gaps, was Old Post

On 9/14/20 5:03 AM, David Rieben wrote:
> Hi Steve,
> To be perfectly honest, since I have no formal training in electrical engineering, I’m afraid your inquiry is beyond my collective expertise. 🤪 I will say that I’ve heard that the voltage reversal  vs shot life issues can be reasonably dealt with by choosing a cap whose maximum voltage rating is a minimum of 3x the peak input AC voltage (rms x 1.41), assuming that it is properly designed with a low loss dielectric system. My transformer can produce up to 16.8 kvAC so 16800 x 1.41=23688, and 23688 x 3 = 71,064. My primary tank capacitor is rated at 75 kV, so I should be ‘good’.
> David
> Sent from my iPhone
>> On Sep 13, 2020, at 12:38 PM, Steve White <steve.white1@xxxxxxxxx> wrote:
>> Hi David,
>> I've always wondered if a ARSG has some negative effect on capacitor life. With a ARSG essentially firing at slow-rolling random times it seems as if many of those firing events would be during capacitor voltage reversals during the charging process. Of course the frequency of the 60 Hz charging voltage reversals is much lower than the voltage reversals experienced in the primary coil tank circuit. Since percent voltage reversal is a key capacitor lifetime parameter, it seems like something to consider.
>> That potential problem can't happen with a SRSG because the firing points are nominally at 0 and 180 degrees for a 120 PPS system or at 0, 90, 180, and 225 degrees for a 240 PPS system.
>> Maybe the effect is negligible.
>> Thoughts?
>> Steve White
>> Cedar Rapids, Iowa
>> --

Pulse discharge operation, particularly with underdamped (ringing) 
discharges, places large internal stresses on components. Empirical 
expressions have been developed to predict the life of a pulse discharge 
capacitor under conditions other than the nominal design. For instance, 
in an article by K. Salisbury, S. Lloyd, and Y.G. Chen at Maxwell 
Laboratories, "A transportable 50 KA Dual Mode Lightning Simulator", the 
following equation is given.

Lx = Lref * (Qref/Qx)^1.6 * (Vref/Vx)^7.5

x subscript refers to the application
ref subscript refers to the reference data
L is the expected life (in shots),
Q is the discharge waveform Q
V is the capacitor charge voltage.

Note that capacitor voltage is the most important life determinant, with 
a 7.5 exponent. A little reduction in voltage leads to a huge increase 
in life.

For large-ish Q
Q = sqrt( 1 + 1 /( 2 /pi*ln(VR))^2)

VR = exp( - pi/2 * sqrt(1/(q^2-1)))
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