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Need 30 kilovolts!

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Oneironaut

Wed May 16 2012, 06:29PM

Registered Member #3616 Joined: Fri Jan 14 2011, 08:47PM
Location:
Posts: 34

In the link posted (see video), I ran the pole pig out of the oil constantly without any issue. I could hear it hissing, but no arcs ever occurred in the coils.

Just had another crazy idea.

Run a pair of series MOTs with 240 rather than 120. In "theory", I would get an output of around 8-10kv. Add a CW, and I may be able to pump out 20-30kv from it. It may not work, but for the cost of the parts, I would be willing to give it a try.

I am going to the yard to take photos of those pole pigs and maybe someone here knowledgeable in power transmission hardware could advise as to their possible output.

Brad

Ben Solon

Wed May 16 2012, 07:56PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

that "hissing" is corona discharge. had you run it any longer, the insulation would have been burned through by the corona, then a nice hot 14kva arc would have been ignited inside the coil.

Oneironaut

Wed May 16 2012, 09:04PM

Registered Member #3616 Joined: Fri Jan 14 2011, 08:47PM
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Posts: 34

ben123324 wrote ...

that "hissing" is corona discharge. had you run it any longer, the insulation would have been burned through by the corona, then a nice hot 14kva arc would have been ignited inside the coil.

I ran it that way for several months, charging the five 50kv caps about once every 10 seconds for hours at a time. The only arc issues I had was on the first version of my hand wound inductors. Heat was also never any issue whatsoever. Ok, the stove coils (ballast) became warm after some use, but never the caps or transformer. Other than blowing up the diode array and the vessel (multiple times), never had any problems.

Brad

teravolt

Thu May 17 2012, 02:06AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

how fast do you want to charge? why not use a cuple of 15kv neons in parallel with a dublier circuit. the neon transformer would do the current limitting for the diodes. how much capacitance are you charging?

klugesmith

Thu May 17 2012, 02:30AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Oneironaut wrote ...
Run a pair of series MOTs with 240 rather than 120. In "theory", I would get an output of around 8-10kv. Add a CW, and I may be able to pump out 20-30kv from it. It may not work, but for the cost of the parts, I would be willing to give it a try.

Hi Brad.
That's the second time you've talked about operating power transformers at 2x their rated voltage.
Now I generally applaud anyone who reports experimental results here.
But here is a small-scale teaching experiment about core saturation.
It may save you some time and expense, before you try to double-volt a MOT or pole pig.

Take a small 120V-input transformer, such as one made for low voltage lighting / lawn sprinkler control / doorbell etc.
Know its rated maximum primary current.
Then with its secondary unloaded, measure and chart the primary current
as you progressively increase the primary voltage beyond the nameplate rating.

If you are in the US and have split 240V electricity, but no 240V variac,
you could cover the range of interest with a 120V variac on one side of a 240V circuit.

Magnetic flux in the core depends on voltage and frequency, not current.
At rated voltage, the magnetizing current (no-load primary current) is a tiny fraction of
full-load current. (Well, a significant fraction in the case of MOT's and XRT's).
Approaching saturation, the I vs V curve turns upward like an exponential hockey stick.
Usually before 150% of rated voltage, it will surpass the rated full-load current.
If you put 240 VAC (unballasted) on a 120 VAC winding, it's like putting DC on the winding.
Unless you blow a fuse, trip the breaker, or melt the lead-in wire,
the primary will burn out in a matter of seconds, if not sooner.

[edit] Of course you can limit the 240V current to a tolerable value
with a ballast device in series. Then you will get much less than
240V RMS across the primary -- even with no load on the secondary.
The voltage waveforms will be highly distorted (not sinusoidal).
On the brighter side:
Compared to sinusoidal operation, at a given large value of true-RMS primary current:
double-V supply with resistive ballast can get maybe 20% more RMS voltage
and maybe 2x the peak voltage. Maybe that's just what you need!
This can be explored with your small 120V transformer,
on 240V circuit with light bulbs for ballast.

Have fun!

Dr. Slack

Thu May 17 2012, 07:21AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Another idea would be to acquire 2 of the pole transformers and run their 240 sides in parallel, tying the 14kv outputs in series to obtain 28kv. I have done this with MOTs, so I know that works, but that's a lot of weight on my wooden shop floor!

What just about works with MOTs doesn't necessarily with higher voltages. It depends how you series those 14kV outputs.

If you put both their grounds to ground, antiphase the primaries, and get 28kV balanced out of the two hot terminals, then it's perfectly feasible. However, you can get exactly the same output voltage from a rectifying charge pump whether you use one or both secondaries arranged like this, just different amunts of output current, so it's not worth doing.

If you lift the ground of the second secondary, and connect it to the hot of the first, to get 28kV with respect to ground on the second output, then you are asking for trouble. You can lift the ground of a MOT secondary, because the standard winding to core insulation that has to be there anyway for mechanical and transient protection is good enough to stand off several kV. The winding to core insulation of your second secondary may not be able to stand off the full peak 20kV from the first. It might, like the MOT, be only good enough for several kV, maybe even 10? You could try it and see, but stand well back.

+1 with Kluge's observations that if you current limit an over-volted primary, the sharp edges that you get as the primary goes in and out of saturation may give you 2x voltage spikes at the output, which may be all you need for a cap charger. However, you could get away with this in a MOT, 2x overvoltage on 3kV is only 3kv extra for the hot to core insulation. You may run into trouble if you subject your secondaries to an extra unexpected 20kV peak, or you might be OK. Stand well back if you try it.

Oneironaut

Thu May 17 2012, 02:28PM

Registered Member #3616 Joined: Fri Jan 14 2011, 08:47PM
Location:
Posts: 34

Thanks again for all the advice. This community is a great source of information.

The caps I want to charge will be in a Marx configuration - 5 of them rated at 1uF and 50Kv.

I am going to take a few photos of the internals of those scrap yard polepigs in hopes someone will be able to identify their type, but so far I am swaying towards a 6 stack MOT now. Found a few great threads on the, here.

Thanks.
Brad

Tetris

Fri May 18 2012, 09:36PM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

connect 2 15kv nsts in series

Thomas W

Fri May 18 2012, 09:38PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

HVC, putting NSTs in series is a dangerous idea, to do so means that both cases will be live and they will die from corona issues quickly from what i know.

Oneironaut

Sat May 19 2012, 02:32AM

Registered Member #3616 Joined: Fri Jan 14 2011, 08:47PM
Location:
Posts: 34

I went back to the scrap yard to have a look at those polepigs again. There is a nice little guy in the back no larger than a garbage can with a single HV terminal. No doubt, a single residential unit rated at 14.440kv input. There were also a bunch of big piggies standing about 4 feet tall with markings of 7200x14400 on the cans. What would the 7200 part mean?

Piggie2

I may just grab the 2 smaller ones as the cases even look repairable. You can never have enough pole pigs in the garage!

Brad

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