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Re: [TCML] DRSSTC tuning - primary current modes

This is somewhat similar to an interesting phenomonia I accidently
discovered in 1969.

While tuning a classic RSG type coil using a variac to control voltage and a
second variac in series to control current, I noted the current value going
into the power transformer suddenly hits a notch, and goes lower in value
even as the voltage is increased. After the current decreases approx 5
Amperes as the primary tuning tap is adjusted, then it starts rising again.
A definite notch point.

As a ham radio operator, I noted this is very similar to tuning a
transmitter to an antenna circuit.  At resonance the plate current dips a
bit, so in the case of a Tesla coil, it is also possible to set the series
power reactor at a value that sets the incoming current at this dip value.

As the current controller variac slowly increased the current a dip in the
input current was noted.  Tuning the current value into this "dipped" area
seemed to indicate most efficient operation of the Tesla coil with regard to
total input power.  I also found that operating the circuit just past this
dip point seemed to produce the longest and fattest sparks without adding
excessive power.

In a ham radio transmitter a capacitor is adjusted to hit the dip point
(plate current decreases 10%).  In the case of Tesla coils the primary
inductor (fixed capacitance) is adjusted to hit the resonance point.

In later experiments, I also monitored the circuit's overall power factor,
and noted as the power factor was corrected to near unity (1.0 value) this
coincided with the xmfr's primary current dip point.  Usually a power factor
of around 90% seemed to provide best overall balance of efficient operation
and excessive power input.  Longer sec sparks can be "forced" of course by
slamming in excessive current, but best efficiency of operation occurs at
the "dip point".

Dr. Resonance

On Sun, Feb 22, 2009 at 2:23 AM, David Kronstein

> Hi All,
> I was working on tuning my DRSSTC (coil details at
> http://4hv.org/e107_plugins/forum/forum_viewtopic.php?63284), and I notice
> that the primary current goes very suddenly into a "notch" mode when moving
> the tap point. I've taken a series of scope shots of this and made a couple
> of animated GIFs.
> Condtions: 100VDC bus, single shot, some breakout into air,  ~1' sparks.
> 200A/div current, interrupter output shown on scope.
> This one shows a relatively wide range of primay turns, ranging from 3
> 11/12 to 4 9/12 turns, 1/12 turn per step:
> http://4hv.org/e107_files/public/1235287411_347_FT63284_wide_range.gif
> There's a very sudden change early on. I tried to take more steps, shown in
> the shot below, but the transition point is somewhere in the middle of the
> primary support, so I can't take extra shots in between. 4 2/36 to 4 7/36
> turns in 1/36 turn steps.
> http://4hv.org/e107_files/public/1235287411_347_FT63284_narrow_range.gif
> Why do these sudden changes occur with changing tap point? And what would
> you generally recommend for setting the best tap point? I've heard people
> say to tune the primary about 10% lower than the secondary.
> Thanks,
> David
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