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Voltage Reversal of Pulse Caps
Hello,
I sent another message this morning answering, as best I could, the questions
from Bob Thaden concerning % reversal of pulse discharge capacitors and their
life expectancy. I have since talked to an engineer at Condenser Products about
this. I have some new info to give the group.
First, this message concerns mosty those that are involved in the group purchase
of some Tesla pulse discharge capacitors from Condenser Products that I
arranged. It will apply to other capacitors somewhat. It gives some idea of
the point of critical strain on these types of capacitors. Here are the specs
on the caps we purchased.
Tesla tank circuit pulse discharge capacitors
Part number TC253-34-300
.025 uF
20000 VAC RMS rated
dimensions: 4-1/2" x 13-1/2", not including studs
studs: 1/2" - 13 x 1"
Here is the first message I sent.
------------------------------------------
>Did anyone ever supply those requested specs (dc rating and shot
>life)?
No, I didn't. But here it is as written by Condenser Products. These are VERY,
VERY conservative ratings.
1. The DC rating by safe allowable voltage stress across margins of capacitor
sections is 60 KV.
2. By standard DC stress level per mil of dielectric is 136 KV. The
approximate hours of life at:
90% reversal 1,200 PPS 500 hours
90% reversal 400 PPS 1500 hours
80% reversal 1,200 PPS 2500 hours
80% reversal 400 PPS 7,500 hours
These figures are based on 15 KV RMS.
Now, someone posted much higher figures on the shot life ratings of these caps.
I don't remember who it was. It could have been Ed Sonderman. Was it you Ed?
These shot life figures are open for interpretation. I am awaiting a call from
Condenser Products now to discuss this matter further. We don't really use them
the way that this test indicates. I'll let you know of my findings.
>I still don't understand the '100% reversal' comment.
The 100% reversal refers to the capacitor being charged 100% full on one side,
being pulsed, and then being charged 100% in the opposite polarity, and then
pulsed. This is typically not the case. With a rotary gap at high speed, the
cap is pulsed many times on one side during the first 180 degrees of the input
waveform. Then it is reversed during the second 180 degrees of the input
waveform. So you get maybe 5-15 pulses in one polarity and then it reverses.
But, again this is true only of rotary gaps.
>I don't even
>understand the significance of the 'pulses per second'. These are running at
>100khz. Does this mean you can't use a static gap? Obviously, I'm still
>missing some theory, so be kind:)
Pulses per second refers to how many times it is charged in the tank circuit
---------------------------------------
Ok, % reversal applies to the point when the capacitor reverses polarity. What
was its last charging voltage seen at the point of reversal?
First, with a static gap it will be 21000 VAC peak (15000 VAC RMS). That means
that the above figures won't apply. The figures above are meant for that number
of reversals per second show at full 21000 peak. A static gap should fire 120
times per second with 120 reversals. Since there are chokes and other such
resistances in the system and the time the discharge ends will be past peak
voltage, the 90% figure can be used for an estimation. This would mean a 18900
VAC peak to peak reversal. Just by taking either of the 90% figures above and
converting them to 120 PPS, it gives us a life of 5000+ hours run time, or
2,160,000,000 reversals and pulses.
Now consider a rotary gap. The gap is breaking the sine wave up a particular
number of times per second. For the sake of this conversation, I'll choose 1200
breaks per second. Calculating how many breaks can occur per half cycle, I get
a maximum of 10. Of course some of the breaks will occur at a point when the
instantaneous voltage potential is too low to cause an arc. This will depend
upon the gap distance. Here I'll take a guess that no arcing will occur much
less than 5000 volts from 0, since we have set our gaps up for our peak to peak
voltage of 21000, which is 10500 from zero. So, if we are breaking our sine
wave up fast, like 1200 BPS, we will get many different charging voltages into
the tank circuit and capacitor. As the sine wave drops below 5000, where I
assume no arcing can occur, no charging will occur until the sine wave gets
below 5000 volts negative. That is the point of reversal we have to concern
ourselves with when estimating capacitor life. So, as you can see, the voltage
at time of reversal may be as low as 10000 volts peak to peak, not 21000. This
is a little less than a 50% reversal voltage. Keep in mind that the above
analysis uses hypothetical figures. However, I'm trying to use realistic
voltages in this analysis.
So, let's assume that our reversal will occur at this value of 50% of our peak
voltage of 21000 peak to peak. This gives is a 50% reversal figure. Since the
AC here in the US is 60 Hz, we will always get no more than 120 reversals per
second. So, with the same 15000 VAC RMS power with a high speed rotary gap, our
shot life goes up, over that of the static gap. It will be 32,000,000,000
reversals, or 7,400+ hours of operation, based on a 50 % reversal. I am told
that a more accurate estimate of this based on empirical data provided by
Condenser Products is 7,600+ hours of operation. This is again a conservative
figure.
Let's take the worst case of 5000 hours of operation with a static gap. Suppose
you ran it every day for 15 minutes, which you will probably never do. This
means that after 54.8 years you will finally have to replace the capacitor. If
you ran it continuous and had spark gaps that didn't deteriorate and everything
was ozone-proof, you could run it for 208.3 days before blowing the capacitor.
If you are working with 50 Hz AC, you can multiply all of the above life
expectancy figures by 6/5.
An additional note: If you are using these caps with a 21000 DC voltage supply,
and never reversed polarity, the expected shot life is 200,000,000,000. With a
gap rotating at 1200 BPS, you can run for 46,000+ hours. That comes up to 504
years of operation at 15 minutes per day. Continuous it would run for 5.25
years.
But whether you are using a static gap system or a rotary, you will get a long
life from one of these caps. I hope I have answered this sufficiently.
Scott Myers