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Re: NST current metering surprises
(is your old email address no longer valid?)
> Original Poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com>
> I finally got around to measuring my NST primary and secondary
>currents, with the help of a hamfest 20A AC ammeter and a
>borrowed 250mA AC ammeter.
> Briefly, my coil uses a 15KV/60mA NST. Until recently, I had been
>using a .012uF cap, selected to be mains-resonant with the NST
>secondary, and I got 51" sparks. My 1.6K/113W protection circuit
> resistors ran hotter than I thought they should. Now I run with a
>.022uF cap with even better 58" sparks, and even hotter resistors,
>but now I'm tripping my 20 Amp circuit breaker after about a minute
>of runtime. This with 88uF of power factor correction caps.
> Today I metered the NST secondary current. This is an unmodified NST
>whose short-circuit current measures 72mA (-at-140VAC). With the
>.022uF cap, it was pushing a whopping 230mA! Excluding power due to
>bypass cap discharging, that accounts for over 84 Watts, finally enough
>to explain the hot resistors. When I went back to my .012uF cap, the
>current fell to 130mA.
Interesting. The question is if the measured 230mA are really
continiously present. What I mean is, this might be a flux,
because your meter canīt follow the true current flowing, esp.
if you are seeing inductive kicking effects mentioned on the
List. The 22nF cap is twice the MRC (mains resonant cap)
size, so I doubt you are seeing a current increase due to XC
cancelling out XL. I suspect that the 230mA current is a (lot of?)
spike(s). This WILL help you charge the (too big) cap, but it
(most likely) isnīt your average rms current. I think you would
need a scope (and TFīs fiber optics probe) to really see what
is going on. Another possibility might be electrostatic charges
building up around the meter itself (due to the high voltage
present), but this doesnīt really explain the difference you saw
in the second experiment (with stable current readings). Are the
meters, you used, frequency independant RMS meters? If they
are simple meters (for pure sinusoidal measurement), any
distortion of the waveform will change their accuracy quite a
> Experimenting with the power factor correction caps, I couldn't tell
>if they had any effect with the .022uF cap as the meter pegged
>both with and without it. Perhaps I should have measured how long
> it took my breaker to trip, with and without the PFC.
> With the .012 uF reso-cap, primary current was significantly less,
>dropping from 14-20 to 11-14 Amps without and with the 88uF
>PFC. Secondary current was unchanged with or without the PFCs
Your secondary current shouldnīt be affected by the (non) presence of a
PFC cap. In a SMPS (ferrit xformer) this CAN be the case, if you are
experiencing resonance from harmonics on the primary side. These just
waste power. If you use a PFC (well really, more like a filter circuit)
in this case, you will see an increase in output power, because the
energy (previously being turned into waste heat) is now actually being
fed into the transformer at a "useable" frequency (i.e: less waste in
the form of heat). However, in the case of a 50/60 cyle powered
"hunk of iron", the PFC wonīt add any real output power. All it does,
is to keep the breaker (who doesnīt care what kind of current your
are pushing through it) from tripping too soon.
P.S: You would need a bigger PFC cap for your setup. A quick
calculation gave me a 171ĩF unit as the best size (for 140V
> I stated the primary currents as ranges because the reading is
>extremely unstable, no doubt due to the chaotic firing of the
>static gap. It's curious that the secondary currents seem so
>much more stable.
Exactly this makes me suspicious of the secondary current
measurement. I would expect it to be the other way around,
because the transformer acts as a step down unit (viewing
hv spikes from the mainsī side), which would help dampen
out the average current flow. Of course, a small current
"swing" on the secondary side will translate into a much
larger swing on the primary side (because of the
transformer turns ratio), but I still canīt imagine the primary
current being very stable AND the secondary current
swinging around wildly.
> One further interesting observation. With the .012uF cap, spark
>output and primary current increased monotonically with variac
>setting. With the .022uF cap, there was a distinct null in current
>and somewhat in performance, at about 60% variac setting, and
>at this point, the primary current became rock-steady. The sound
> of the arc also became much more stable. I'll bet the charging
>was a non-chaotic 60 or 120Hz (didn't have the scope out).
That could be, but what exactly do you mean by the words:
"there was a distinct null in current". Surely the current wasnīt
actually "zero" below 60% of the variac setting.
Coiler greets from germany,