Phil,
This is something that I have wondered about. Lets say that you have your break rate set at 200 BPS (4 bangs per cycle) and the firing angles are at 0, 90, 180, and 270 degrees. Assuming that the gap fires every time, there will be no voltage reversal due to firing phase angle. I am excluding voltage reversals due to the primary tank circuit oscillation in this discussion. Now lets assume that you have adjusted the phase angle so that the firing points are at 45, 135, 225, and 315 degrees. In this scenario, two of the firing points are effectively cancelled due to voltage reversal on the capacitors resulting in an effective 100 BPS firing rate. This is what I assume you are doing to simulate a 100 BPS firing rate from what is normally a 200 BPS firing rate. The potential problem, as I see it, is the following 2 charging intervals:
1. 135 degrees to 225 degrees
2. 315 degrees to 45 degrees
During these two charging intervals, the charging voltage on the capacitors passes through zero and ranges between positive and negative equal value voltages, effectively cancelling the charge on the capacitors during these two intervals. This appears to me to represent a 100% voltage reversal to the capacitors. This voltage reversal will occur twice per cycle or 100 reversals per second for 50 Hz power. We all know that pulse capacitors do not like large voltage reversals. I wonder if this is what is damaging your capacitors? This also leads me to wonder if a ARSG is more stressful on the capacitors than a SRSG for the same reason.
A mitigating factor is the fact that the rise time of the voltage reversal at the power line frequency is about 1000 times slower than than the voltage reversal due to tank circuit oscillation. This is much less stressful on the capacitors but I don't know how much. I haven't seen any capacitor specs about the effect of voltage reversal rise time on capacitor life.
Steve White
Cedar Rapids, Iowa
----- Original Message -----
From: "phil" <pip@xxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Friday, August 24, 2018 8:11:30 AM
Subject: [TCML] Watch your Caps!
All,
The Cornell-Dubilier's 942C20P15K-F capacitors have long become
something of a stalwart for Tesla Coil use, however they are not totally
immune to damage!
https://www.flickr.com/photos/33962508@N03/
This came about by running my 200bps synchronous coil with the phase
setting badly out - this was being done to simulate running as 100bps.
The cap string was rated at 36KV, and while that voltage rating has been
adequate for the last 6 years or so when running at 200bps, it most
likely proved too low for my latest 100bps antics; the experiment being
the 'final straw that broke the camel's back'. Lesson learnt.
Regards Phil
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