Re: Sizing capacitors

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

Thanks Dr. R., Terry, this is great stuff!  I'm
finally beginning to understand "LTR" and "STR" :-)
For my DC (non-resonant-charging) project, as Terry
mentioned, I ought not to have to worry about
transformer/cap resonance, which is goodness, I
suppose!

Now I'm left to play with ballasting the little PT for
200-300VA.  I had originally thought about using some
of the adjustable 5k 100W resistors I have all over
the place, but what a waste!  The problem with using a
PT this small is that a pair of 3kV 30mA NSTs would be
smaller, especially after adding a ballast.  Still,
the little PT looks "cool", so I'll see what I can
come up with :-)

Thanks again,
Aaron


--- Tesla list <tesla@xxxxxxxxxx> wrote:

> Original poster: "Dr. Resonance"
> <resonance@xxxxxxxxxx>
>
>
>
> Another interesting formulation is matching the
> transformer to the
> capacitor size.
>
> Imagine if you have a variac to adjust the voltage
> and another variac
> to adjust the current, ie, an adjustable series
> inductor on the
> primary side of your pole xmfr / PT xmfr.
>
> For initial testing you might set the xmfrs
> impedance equal to the
> impedance of the capacitor bank.  Use a climbing arc
> to set the
> current to a certain value, ie, perhaps 30 Amperes.
> Knowing the 30
> amp current value on the primary side using 220
> volts you can calc.
> the power in the system  The, using the 14,400 volt
> sec xmfr value
> you find the sec. current value.  You then use this
> value to match
> the capacitive reactance value and you have the
> resonant match value
> for the xmfr - cap bank.  Go slightly above this
> value to prevent
> undesireable xmfr to cap resonance, usually 1.3 to
> 1.4 times the
> capacitive match value at resonance.
>
> Now, here is where the interesting part begins.
>
> Set up a current meter on the primary side and
> monitor the primary
> amps. Increase the voltage control variac to 89-90%
> and then adjust
> the current control (series variac) for 25-30 amps.
> As power is
> applied to the coil system, at some point, you will
> notice the
> current actually begin to decrease even as you
> increase the voltage
> control variac.  In ham radio, tuning linear amps,
> this was referred
> to as "dipping the plate current at resonance".  The
> current will
> decrease as the voltage variac (power into the
> system is
> increased).  There is a "tuning dip" on the ammeter
> at this point.
>
> This tuning point is the most efficient operating
> point for a classic
> spark gap Tesla coil system.  As you increase the
> power into the
> system the amps will again start to rise.  Slightly
> above the "tuning
> dip" point is the point at which the sparks will be
> their longest
> with a reasonable minimum of input power for any
> given
> system.  Longer sparks can be produced above this
> current input
> point, however, a lot more power is required and the
> system is not
> operating in an efficient mode, ie, you have
> increased the power
> factor above the unity point and this is an area
> where classic Tesla
> coils do not operate very smooth.  They become
> current hogs and try
> to saturate the current controller and produce audio
> "thumps" in the
> current and voltage variacs.
>
> This is what I refer to as "tuning the power supply
> to the system"
> and will give you a reliable indication of the most
> efficient power
> input (current level especially) for your given
> system.  It's an
> experiment worth doing at least once to determine
> where the "resonant
> current dip" point is for your system.  Then you can
> adjust the
> number of turns on your series power reactor to
> prodvide this proper
> level of current just as your voltage variac reaches
> 100% on it's scale.
>
> Up to approx 35 Amps a single variac provides
> voltage control for
> this experiment while a single variac in series with
> the pole xmfr
> pri provides the current control.  If you are
> running a 0.1 uF to 0.2
> uF cap bank then you will need a dual variac for the
> voltage control
> and another dual variac for the current control to
> determine this
> tuning point.  After this experiment is conducted
> you will know the
> best current for your system and the second current
> control variac is
> eliminated and the turns are adjusted on the primary
> series reactor
> to provide this "dip current" or slightly above (10%
> is usually very
> good for performance).  This is the best way to
> determine the proper
> amount of current your current reactor / pole xmfr
> system should be
> providing to your system.
>
> Then, matching the impedance to determine the
> resonant value
> capacitance, and increase this value by 1.3 x Cres
> and you will have
> a very nice performing system.  Values up to 1.6
> Cres do work but
> they produce sparks with more of a "capacitive
> discharge" and don't
> seem to be as active as the sparks off a toroid at
> 1.3 x Cres value.
>
> Dr. Resonance
>
>
>
>
> >Original poster: "J. Aaron Holmes"
> <jaholmes@xxxxxxxxxxxxxxxxxx>
> >
> >Thanks, Terry!  Very helpful as usual.  I suppose
> my
> >doubts about the transformer <==> capacitor
> >formulation come about because it seems to me that
> a
> >capacitive load on the transformer is going to have
> a
> >crummy power factor, thus rendering the transformer
> VA
> >incompatible with the coil Watts value.
> >
> >Is that true?  In other words, to use 290nF would
> mean
> >200W effective power from the transformer, but the
> VA
> >would be higher, right?  If I'm way off, just shut
> me
> >up! :-))  Otherwise, what I guess I was asking was
> >this:  How would one go about choosing a cap for a
> >given VA, or otherwise so as to avoid exceeding
> some
> >rated peak current.  Again, this is a PT and
> probably
> >can take a few times its rated 200VA...guess I'm
> just
> >interested in "getting the math" instead of
> surviving
> >on "rules of thumb" as I have for years now.
> >
> >Regarding "resonance" and "DC"...duh!  My brain was
> >obviously already in bed when I went off on
> LTR/STR.
> >I'd actually *love* to know how those values were
> >computed for NSTs, etc., but if you recall sending
> it
> >to the list or posting it somewhere, I'll just look
> >harder :-)
> >
> >Good night (for me!) -
> >Aaron, N7OE
> >
> >--- Tesla list <tesla@xxxxxxxxxx> wrote:
> >
> > > Original poster: Vardan
> > > <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
> > >
> > > Hi,
> > >
> > > At 10:14 PM 6/16/2006, you wrote:
> > > >I've built coils before using caps that were
> > > obviously
> > > >"way STR" (e.g., 30nF on a 15kVA pig), and so
> never
> > > >bothered to really understand how one sizes a
> cap
> > > to
> > > >take maximum advantage of one's transformer
> (didn't
> > > >have room in my garage to do that with a pig
> > > anyway!),
> > > >but now I'm working on a coil based on a tiny
> 200VA
> > > PT
>
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