Subject: CAPACITOR RATINGS
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Tue, 21 Mar 1995 00:21:00 GMT
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Quoting Ed Sonderman:
ES> If I should decide to buy a commercial Tesla tank circuit capacitor,
ES> what working voltage rating do I want to specify? I might want to go
ES> up to 15kv some day, maybe even 20kv. At 15kv the peak voltage is
ES> about 21kv but I would guess the voltage the capacitor actually sees
ES> is 2 or 3 times that.
If you inquire into a commercial capacitor, these questions should
be directed to the company that makes them. There needs to be a clear
understanding with the capacitor manufacturer that the cap is specifically
intended for use in a Tesla tank circuit. The voltage rating of the cap-
acitor needs to be stated in terms of "xx KVAC PULSE", which means it is
rated for AC input and spark/pulse excitation. The capacitor manufacturer
is probably going to want to know what the intended duty cycle the cap-
acitor will be expected to pulse at, which is usually 5% or less.
Condenser Products and NWL capacitors both have extensive experience in
Tesla tank circuit capacitance. Condenser Products for one has a "TC"
series of pulse discharging capacitors with the appropriate ratings and
duty cycles. BTW, you are quite correct in your deduction that the cap-
acitor dielectric sees two or three times the waveform peak voltage, the
commercial KVAC Pulse rating takes this into account.
ES> BTW, what am I likely to pay for a .02mfd capacitor like this?
I am going to guess about $150.00 for a 35KVAC Pulse rated .02 mfd,
but it has been some time since I called around for bids.
ES> If I am able to find a non shunted step up transformer, will I need
ES> to use a current limiting system like I saw in your video? I think
ES> you were using part of an old AC arc welder. Can you explain how
ES> this works. You must be limiting the primary current of your pole
ES> pig. Why is this necessary? Don't you control its input voltage
ES> with your variac? Thanks, Ed Sonderman
If you use a non-shunted step-up xfmr, it is highly advisable that some
type of current limiting be placed in the supply circuit. I have used
both resistive and inductive "ballast" to current limit the step-up
xfmr primary, and the best setups use a combination of both.
The secondary of the step-up transformer sees nothing but a dead short
when it is supplying the Tesla tank circuit. This is true even if the
capacitor and the power supply are well matched. Since the supply
transformer "sees" a dead short while charging the Tesla tank circuit
capacitor, it follows that when you hook the step-up xfmr primary up to
the low voltage mains the lights will dim and the breaker may trip. It
is quite possible to dim the neighbors house as well.
This has nothing to do with variacs limiting the voltage; it has every-
thing to do with the step-up transformer pulling loads of current which
the variac is not designed to regulate. Variacs are designed to pass
current efficiently, so some other circuit element must be placed in
series to control the current. Resistance, inductance, or a combination
of both are commonly used to "ballast" or current limit the step-up
transformer on high powered Tesla coils.
In my video I used a standard arc welder in series with my variac. An
arc welder has core modifications which allow the user to limit the
amount of energy passing through the core. Without these core modifi-
cations the arc welder would dim the lights and pull the power grid
through the welding rod; but with these core modifications the arc
can be current limited and controlled to the point that precision
welding can be acomplished. An arc welder is therefore simply a
current-limited step-down transformer. If you short the secondary of
an arc welder out, the current drawn from the supply mains are still
limited by the user controlled power settings (taps, sliders, etc)
that control the energy passed through the core.
The secondary of the arc welder was shorted out, and the primary was
placed in series with my power cabinet variacs. By changing the core
settings I was able to precisely control the current passing through
the variacs. By changing the variac settings I was able to control the
voltage as well.
The same effect can be achieved by placing resistance in series with
the step-up primary. The only disadvantage is that resistance gets hot.
A very smooth current control can be achieved by placing oven elements
in parallel and placing the high energy resistor in series with the variac.
Regardless of current limiting, some voltage drop is normal. When I go
shopping for non-shunted transfomers I look for about 10% higher voltage
ratings to compensate for the line voltage drop caused by the current
... If all else fails... Throw another megavolt across it!
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