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Continued Problems (fwd)
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From: fxphoto [SMTP:fxphoto-at-centuryinter-dot-net]
Sent: Friday, May 08, 1998 4:09 PM
To: Tesla List
Subject: Re: Continued Problems (fwd)
Bart, Thanks for the help!
>Date: Tue, 05 May 1998 02:00:16 -0500
>From: "Barton B. Anderson" <mopar-at-mn.uswest-dot-net>
>Bill and all,
>Bill, I think what your are doing here is calculating max current based on
the rated max V and KVA and then calculating C for max capacity based off of
the transformers impedance at max V.
Yes, that is what I was doing.
>This doesn't mean one should necessarily stick with this
>capacitance. For instance, I first tried matching my 10kva xfmr impedance
to the
>cap, which resulted in a .128uF cap at 13.27 joules which some list members
>thankfully warned me of a melt-down (so to speak) as based on the overall
size of
>the coil. I half'd the capacitance to 0.064uF which pulled the joules to
6.64 which
>I felt relatively comfortable to start with.
Bart, would that be the equivalent of around 1600 Amps? 13.27 joules input
60 times per second at one polarity and 60 times at the opposite polarity?
I do see this as a major difference between choosing a cap for a neon sign
transformer and a pole pig. You guys with pole pigs have so much power, you
don't have to be as efficient with it. <VBG>
>> I am sorry that I do not understand the effect of the current control
>> method that you are using has on Xc but it seems that Xc would change as you
>> change the current output. If this is so, the capacitance would need to
>> change according to the current output. Perhaps someone can jump in and
>> explain this to me.
This is where my brain went dead. I was trying to ask about Xl and was
typing Xc. Restated, I was trying to find out what effect the additional
inductance of the current control devices (i.e. welder, variac, chokes etc.)
would have on the secondary's inductance. Upon further investigation, it
looks like there would be absolutely no affect on the Xl of the secondary
since these devices control the current before it reaches the primary.
>The 0.025uF will be at 2kva+ (based on the impedance dictated by current
match of
>the cap to the transformer), but these pole xfrmrs are capable of exceeding
their
>ratings and I think Ed is pulling it up to 8kva as measured from line input and
>current draw without having the safety gap fire and without killing his caps.
>
If my understanding is correct, the current is controlled on the primary
side of the pole pig and would have no effect on the inductance of the
secondary. Current in the tank circuit is controlled by the size of the tank
cap to limit the total joules resonating in the tank circuit at any one
time. There is a large surplus of current available (8kva ! whew!) and the
only way for it to pass into the system is for the cap to be charged to it's
capacity and then discharged across the gap into the primary coil many ,
many times during a
half cycle (assuming 60Hz single phase). A cap is most sensitive to
overvoltage conditions when it is fully charged and least sensitive when
discharged and it seems that a capacitor under these conditions would be
sensitive during most of the 60 Hz cycle.
I picture a capacitor in a tesla system as a bucket in a bucket brigade. A
very small cap would be like trying to put out a fire with a thimble. It
fills with water (current) quickly but the people (the system i.e. wiring,
connections,stray capacitance and inductance,and especially any
connections(resistance)) have to move it very rapidly for it to do any good
at all, and even then it does not do much to put out the fire. It is a
strain on the system as there is a lot of effort used to move that small bit
of water.
On the other hand, too large a cap is like using a 55 gal drum. It takes
forever to fill and when it is filled it puts the system under a real strain
to even try to move it. However if the system can deliver it, it will put
that fire out.
The best sized cap is like the best sized bucket. It can be filled
reasonably quickly, can be moved by the brigade efficiently and quickly and
does not put undue strain on the brigade.
On some systems, this will be a thimble. On others it will be a 55 gal
drum. and on the majority, somewhere in between.
If you see a brigade of scrawny looking guys...thats my tesla coil system,
but they are all scheduled for weightlifting lessons <VBG>
Ed,
What is your main spark gap set at? This would be the total of the rotary
gap and the static gap. My guess now would be that you have too much gap.
My suggestion would be to record all your parameters ( i.e. primary tap
point, gap distances, rotary speed , current settings (amperage), ) so you
can get back to where you started from . Then close down your main gaps to
the minimum safe distance. I am not sure if your rotary gap is adjustable,
but if it is, I am sure that there needs to be enough clearance for
mechanical vibration. I would probably bypass the static gap in order to
start at the minimum gap distance.
I would then begin tuning the coil as if it were a brand new coil . I would
find the best primary tap point. At this point you may need a ground point
only a few inches from the toroid to tell (not having worked with a pole
pig, I don't know what to expect at this level). Tune it for maximum power
throughput without changing the gap length. When max output is reached,
record the settings. The safety gap on your cap should not fire at all
during this tuning. Open the main gap a couple thousandths of an inch and
retune for max throughput. Record the settings. At no point should the total
distance of the main gap (rotary and static combined) exceed the distance of
the capacitor safety gap or the safety gap will fire.
By doing this you will get a good idea of how your coil operates at
different voltage levels (your main gap controls the voltage at which the
capacitor discharges). You should find a point on the way where the max
output is reached without firing the safety gap.
If the safety gap is firing at a very low voltage, the first thing I would
check is too much resistance (including impedence) between the main gap and
the cap. If it is not that, then I would consider puting some chokes on
either side of the cap (chokes designed for large current i.e. out of 3/8"
tubing) to stop the high voltage spike reaching your cap.
I hope this helps.
Bill Turbett
quote: "I am a patient man....a Belvue patient man!"