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Re: new transformer - performance
Robin,
Your system and new transformer are working exactly as they should!
The new transformer has a much higher turns ratio than your previous
one, and you're still using the same size tank cap and running at the
same breakdown voltage as before via your synchronous gap. Your tank cap
is recharged by the available current (not voltage!) coming from your HV
transformer, and this in turn is limited by your LV-side ballast
inductance. However, in order to supply the same tank capacitor charging
current as you did before, you'll need to provide significantly greater
current on the low-voltage side of your HV transformer. Let's look at
the numbers for your system...
The "load" seen by your transformer (prior to the gap firing) will be
about 35.4 kohms reactive (capacitive) at 50 Hz. This will be
"reflected" back into the primary side of the HV transformer by the
turns ratio squared:
Tank Capacitor Reactance:
Zc = 1/(2*Pi*C*Fm) = 1/(2*3.14*9e-8*50)
= 35.4 kohms (reactive)
Previous transformer:
Turns ratio (old) = 13800/240 = 57.5
Zin (old) = 35000/(57.5*57.5)
= 10.7 ohms capacitive
New transformer:
Turns ratio (new) = 33000/240 = 137.5
Zin (new) = 35000/(137.5*137.5)
= 1.9 ohms capacitive
So in effect, the same MMC tank cap now "looks" (at least to the pig's
primary circuit!) to be about 5.7X the size it was with the previous
transformer - this is why your primary current (with no gap firing)
seems to be much higher than before.
Since you're finding that you need to run with a variac voltage setting
that's much higher than you'd expect in order to begin firing your sync
gap, the existing ballast inductance is limiting the maximum recharge
current to a lower level than the capacitor "wants" to see, especially
near HV zero-crossings. Under these conditions, you're most likely
charging your tank cap in a more linear fashion instead of applying a
(heavier) co-sinusoidal current. If your ballast inductance would allow
you to supply sufficient primary-side current, you'd find the gap would
indeed fire at a much lower variac setting. At a high setting, you are
able to charge the tank cap so that it's just sufficient to fire the
gap. Once this occurs, the bang size and breakrate remain unchanged, and
the system otherwise behaves as it did before.
Some options:
1. Reconfigure your tank cap for a higher voltage rating and lower
capacitance, and then add enough TC primary inductance to bring the
system back into tune. Halving the capacitance and doubling the voltage
rating combined with increasing the gap's firing voltage to about 28 kV
would keep your bang size about the same. However, this may introduce
other problems because of the higher voltage stress on the primary and
gap.
2. Reduce the amount of ballast inductance and live with the higher
current demand off the 240 volt mains. This will also permit you to
increase the real power going to your system. However, you run an
increased risk of overvolting your tank caps!
3. Drive the new system off a 120 volt power source (if available) and
keep the ballast inductance the same.
Hope this helped!
-- Bert --
--
Bert Hickman
Stoneridge Engineering
http://www.teslamania-dot-com
Tesla List wrote:
>
> Original Poster: "Robin Copini" <rcopini-at-merlin-dot-net.au>
>
> HI all,
>
> Some weeks ago, I had the opportunity to exchange my 13.8Kv / 5kva pole
> pig for a 33kv / 15 kva pole pig at no charge. It arrived about 2 days ago
> and I reset my coil up with the new pig and sat down to enjoy the show.
>
> Now I had expected that with the higher voltage the gap would begin to
> fire at a lower current setting so I adjusted everything and powered up -
> nothing!, I slowly increased the current setting with the voltage set at
> about 20kv, finally the gap began to fire at 60 AMPS!! I almost got a nose
> bleed when I saw the meters move.
>
> I've tried all sorts of settings right up to 270V input from the voltage
> variac, (35640V out) and different current settings on the current control
> variac and no difference - no gap activity until at least 50 amps is on
> it's way into the pig.
>
> Coil specs -
>
> 0.09uF MMC, (33 caps in series, 9 strings)
> 200BPS sync rotary 4 stationary electrodes
> all tank cables are 0 or 1 guage multi-strand welding cables - I've checked
> everything top to bottom and nothing has changed since the last time I ran
> it with the old pig.
>
> Questions ;
>
> a) Why does the gap require so much current before firing? - When firing,
> the sound is clean like when used with the old pig.
>
> b) The transformer was totally refurbished before I got it so I am fairly
> certain of the output - why does the current draw increase when voltage is
> increased?
>
> c) Why doesn't the increased voltage cause the gap to fire earlier
> independent of current setting?
>
> Coil performance is on par with the older transformer only now the voltage
> is much higher and the current is higher also.
>
> Thanks, and best regards
>
> Robin Copini
>
> p.s. based on the discussion on coil performance based on streamer
> length/input power mine now has to be the most 'inefficient' coil on the
> list :-)).