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Re: Cap safety & spark gaps
Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
To Mike and others,
After hearing many (seemingly) to not understand how you can get more
voltage out of a transformer than its rated output, I thought it
would be useful for the newcomers or those that maybe dont have a
full grasp yet of electrical circuits to give a primer on some of the
basics of the effect.
The transformer can be distilled down to a simple model that is a
voltage source (open circuit secondary voltage) with a series
resistance (R) and a series (L) feeding the TC primary. This
simplifed model will suffice for purposes of understanding most of
what happens in TC service. NSTs (current limited) and PIGs (not
current limited) will be used to bracket the transformer type you use.
NST's have shunts in them to allow some of the flux (leakage flux)
generated by the primary to bypass the secondary winding. The effect
this has is to increase the leakage flux that adds more series
inductance to the simplified model. This is how NST's current limit
their output for short circuit conditions. The effective inductance
can be determined by using either the nameplate data or the measured
secondary open circuit voltage (Vs_oc) and the secondary short
circuit current (Is_sc). The transformer impedance (mostly inductive
- XL) is Vs_oc/Is_sc expressed in ohms. The equivalent inductance is
XL/(2*pi*line_freq). Resistance has been neglected here as for most
of these transformers, the resistance is small compared to the
inductive impedance (reactance). It will suffice to remember that
there is both primary and secondary winding resistance that shows up
as the R in the model.
PIGs, by their very nature, are designed to keep the leakage
inductance and winding resistance as small as possible. If one were
to short circuit the PIGs output, the winding resistance and leakage
inductance will not be sufficient to adequately current limit the
transformer. Use of PIGs in TC service will require a "ballast"
inductor (prefered) in series with the primary of the PIG to limit
the current when the spark gap fires. This primary inductance can be
transfered to the secondary for the model by increasing its value by
the turns ratio squared.
When we add the TC primary load to the transformer's output and look
at the resulting model for charging purposes only, we have a series
RLC circuit (the R and L come from the transformer and the C comes
from Cp of the primary). The TC primary inductance is very small
compared to the transformer's inductance so it is neglected here.
This series RLC circuit is very underdamped (R is too small to
prevent the RLC from having its own resonance). The consequence of
this is the RLC will have a resonance of its own where its resonant
frequency is dependent on the values of R, L, and C (yes - R effects
this frequency to some extent).
The transformer is a frequency source of either 50 or 60 Hz depending
on where you live. If the RLC circuit also has a resonant frequency
of the same (worst case senerio). It will amplify the Vs_oc voltage
out of the transformer by the factor of Q.
Q = sqrt(L/C) / R
If R is zero, (Q is infinite) you would theoretically get infinite
voltage out or the RLC circuit (across the capacitor). Typically for
NST's (I believe) the Q is about 5 or so. PIGs have much lower R
resulting in higher Q's. This means that if you use a 10KV NST, you
would have 50KV on the capacitor if operated at resonance with no
safety gap and nothing else fails. With no capacitive load (open
circuit), the output of the transformer would be it's Vs_oc. For Cp
values between 0 and Cp=Cres (where RLC resonates with the line
frequency), there is some voltage gain (we call resonant rise). With
Cp=Cres you get the largest rise. Once Cp becomes larger than Cres
(LTR - larger than resonance), the voltage gain starts
decreasing. With a sufficiently large Cp, the voltage gain decreases
back to unity. With Cp values even larger, the voltage gain becomes
less than unity (the transformer can not charge the Cp to full voltage).
The above description is with the spark gap (SG) out of the
picture. When the SG fires, a transcient is created that adds to the
above steady state response. If the SG fires at say 10KV and the SG
perfectly shorts the power source, the transcient response starts at
-10KV (so the sum of steady state response and transcient responce is
0V). When the SG quenches and opens, another transcient response is
started. Any
lingering transcient response will "beat" with the line frequency and
can also result in voltages that can go higher than steady state.
A third mechanism that is associated with SRSG's can also "pump" up
the voltage. This mechanism is what allows SRSGs to effectively
charge a larger LTR cap to full voltage (Vs_oc_peak) than for static
gaps. The details of this mechanism is described in Richie Burnett's site:
http://www.richieburnett.co.uk
Gerry R.
Original poster: "MIKE HARDY" <MHARDY@xxxxxxxxxx>
Due to the recent discussions on safety gap, and air breakdown
voltage, I need some clarification. Are you guys saying that if one
has sufficient energy, and a resonant , or str cap, then charging
voltage is dependent on spark gap length? This explains why output
gets bigger as one opens the gap. So is this why a 35kV maxwell can
blow with a 14.4 pig or PT? How does one keep there caps healthy in
a ballast controled situation, where output is somewhat unlimited? I
realize most ballasted coil configs use rotaries, but right now I'm
using a sucker gap with my PT.