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Issues with running a large air coil using a full bridge

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Dr. ISOTOP

Mon Jan 16 2012, 01:44PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

So it looks like you're trying to rectify a CW SSTC here...is your secondary tuned to your drive frequency? If not, you'll have very little energy transfer to it, and your primary current will be tremendous.

si2030

Mon Jan 16 2012, 01:52PM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

Hi there,

Well the idea here was to have the CW and the coil's secondary work at resonance....

ScotchTapeLord

Mon Jan 16 2012, 02:23PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

I think it would behoove us to use the full terms, continuous wave or Cockroft-Walton multiplier, rather than the abbreviation "cw" to clear up some confusion...

The multiplier will have an optimal frequency but it is much more important to have you air coils tuned. If you want to use such a low primary impedance then I think a capacitor and spark gap would be your best bet.

si2030

Mon Jan 16 2012, 02:34PM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

So are you suggesting that I run the primary in resonance as well as the secondary where they resonate at the same frequency?

Pinky's Brain

Mon Jan 16 2012, 03:34PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

A primary tank cap will have the power supply and IGBTs see only a small part of the current ... but the copper in the primary would still see all of it, not a realistic solution. Why did you chose to make your own air coil over using a TV flyback as a transformer BTW?

PS. on second thought, looking at the wire gauge in the picture the primary might be able to take it ... you'd need a robust cap though.

jpsmith123

Mon Jan 16 2012, 04:22PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Your voltage multiplier was designed for an input of 50 kv peak? If so you must have a lot of money tied up in that beast!

Anyway, although I've never tried it myself, I've seen in the literature where a ferrite transformer was used to drive a series resonant circuit where the capacitance of the multiplier is part of the capacitance of the LC resonator.

You've already got a high-inductance coil insulated for high voltage; maybe that could be your inductor, then you just resonate it with a HV capacitor across which your multiplier is connected?

You'd have to experiment with it in spice to see how it would work under varying loads etc.

Alternatively, being that you apparently have no qualms about using a tank of oil, you could build a big ferrite transformer using, say, four large cores mechanically configured as a "cross" (with each core having a one square inch cross-section), with four primary windings in parallel and a secondary which gives you about 100 volts (peak) per turn at, say, 30 khz.

Pinky's Brain

Mon Jan 16 2012, 05:24PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

BTW, isn't this thing a loop antenna?

Sulaiman

Mon Jan 16 2012, 07:19PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I would seriously consider the zvs/Royer/CFPR inverter for this application because it will produce the least stresses on all components/load EXCEPT the two required high voltage transistors.

When I run cfpr on 12V to 50V dc all is as expected,
when I tried full-wave rectified mains (230Vac Europe) I couldn't protect 1500V transistors from eventually exploding, gave up.
Maybe you will not have this problem, but 115 Vac would be much easier.

Run the cfpr at a Q of about 5 to 10 (VAR in the primary resonant circuit/output power) and you should have no problems, the voltage multiplier will be a light loading.
(it will drop the resonant frequency of the inverter a little but the load will not be resonant)

At 18 kHz 20uH = j2.26 Ohms which should give some voltage
My first thought is an inter-turn short in your secondary (DRAT !)
You can check by measured inductance << calculated
or low Q with a test capacitor across either primary or secondary.

The inductance that resonates with the multiplier is actually the leakage inductance, which you can measure.

si2030

Tue Jan 17 2012, 12:55AM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

Hi Guys,

So I have the Cockcroft Walton tower already built... it has 120kV rated capacitors and 150kV avalanche rated diodes... and yes it did cost me a fair bit... here's a picture..

THe air coil was suggested by a friend in Argentina who said that using ferrite would cause

I also have the larger oil tank it sits in ready and the whole thing will stand around 1.8 mtres high..

So I do need some driver system that will run 50kV peak and run the Cockcroft stack at resonance with a coil.

I had used spice to obtain a working system and recently (a couple a weeks ago) I discovered another way to run flyback using capacitor and coil snubber system LT Spice simulation attached.

]symetrical_inductor_snubber.zip[/file]

Wondering you might have a look at this circuit and tell me if this might be a better option...

In this I would be very happy to do the work but its like passing a camel through the eye of the needle.... I need the secondary to be resonant but 50kV is quite a significant voltage..

I look forward to your replies.

Simon

Patrick

Tue Jan 17 2012, 01:52AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

si2030 wrote ...

Simon

AH! i see you are a fan of the Acrylic, like me, thats a great build! i like the cell-cast stuff best.

i think youll need to include ferrite, im just not sure an air core will do what you want.

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