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Re: [TCML] Cap Discharge Questions



On 1/20/11 2:07 AM, Phil Tuck wrote:
Hello Dave,
Thank you for the reply. Yes, I was referring to the individual caps. My old MMC of 216 caps had its bleed resistors twisted with the cap's lead wires, so when I rebuilt the MMC I had to cut the connections away, leaving too short a lead on the old cap for a new bleed resistor. The result is I now have 12 strings of 18 in series with no bleed resistors. I was going to use a resistive stick to discharge, and then keeping the stick in place, apply a link across the MMC to leave on for when not in use.
So I presume (actually  I am hoping) your reply of  "No DC path from the 'inner' caps, round the loop, to equalize." Applies only if whatever was shorting the MMC (HV winding or stick etc.) is removed.
Let's say I have just three hypothetical, fully discharged, caps in series, like this:

connection 'a' ---  [ CAP 1] ---  connection 'b'  ---  [CAP 2] ---  connection 'c' ---  [CAP 3] ---  connection 'd'

  I then apply 1000v just across CAP 2 using connection points 'b'&  'c'. Once the charge is removed I then connect, and leave connected, a resistance across all three caps at points a'&  'd' Will not the presence of the resistor and the outer caps provide a path for the middle cap to discharge?   (I have never fully understood the mechanism that allows a cap to regain some of its charge.)


OK.. so what happens is that the charge on Cap 2 (Cap 1 and 3 have no charge) will redistribute across all three such that the voltage on on Cap 1 and Cap 3 is equal to the voltage on Cap 2. (that is, Vcap1 = Vcap3 = 1/2 Vcap2, at the end)

Neglecting resistance.. the charge that moves out of Cap2 has to be distributed between Cap1 and Cap3, so, if they were all equal values, you can calculate what the end situation is. (I haven't finished my morning coffee yet, so I can't do it in my head.. left as an exercise for the reader) (Q2 = 1000 * C2 at start.

If you were to then remove the shorting wire from a to d, you'd have the situation where the "whole string" has zero volts, but the individual caps have voltage on them/charge stored within.

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