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Issues with running a large air coil using a full bridge

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jpsmith123

Tue Jan 17 2012, 02:12AM

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

What would supposedly happen with ferrite? (Your sentence about the ferrite is incomplete).

Patrick

Tue Jan 17 2012, 02:26AM

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

jpsmith123 wrote ...

What would supposedly happen with ferrite? (Your sentence about the ferrite is incomplete).

well it seems to me hes expecting lower currents than an air-core transformer can effectively run at. with SG-TC's arnt there enormous currents sloshing about the tank circuit? seems to me 18 kHz isnt very high either... i could be wrong though, still not sure why he chose this topology over other more proven designs.

Dr. ISOTOP

Tue Jan 17 2012, 02:35AM

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

si2030 wrote ...

Hi there,

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

Yes, but are you driving the primary at the resonant frequency of the secondary + multiplier?

si2030

Tue Jan 17 2012, 02:49AM

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

Here's a little from my friend (English is his third language) on issues of ferrite and building an appropriate transformer...

"We need 50 KV and 2 KW. A big fly back; and a low frequency like 200 to 500 Hz, very low for the CW capacitor series. Then we must increase frequency and we'll have a small fly back. It is perfectly possible to calculate it, so as a TV fly back. But TV fly backs were developed by a long and pocket-painful work for a given power and frequency, without varying both of them. We need something that bears variation of power, frequency and input voltage. One given fly back (with ferrite core) DOES NOT bear such variations. We can build a very good transformer for given values, but megavoltage has surprises in power absorbing and resonance-frequency. Then we can store that transformer in the cupboard and begin to build other.
The ideal solution is a CORELESS coil that bears as frequency as power variations.

So it was suggested that coreless coil would be more practicable for this job.... I now have a perfectly good coil that uses way to much current as per Steve's initial assessment...

Question is.. is there a transformer design I can build that will do this job?

Using the core characteristics of the one I just made I ran a simulation using the following schematic..

1326768367 1571 FT132436 Symetrical Inductor Snubber

Here is the simulation with the current through one of the IGBTs and its average plus the trace for the secondary.. I guess this would also suffer from excessive current owing to the size of the primary...

Patrick

Tue Jan 17 2012, 03:31AM

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

OK with the above reasoning, ill say this...

first, we look at a normal UU or EE core, low magntising current compared to real power/current transferred.

next, we look at an I core, generally, we assume largish magnitising currents due to the broken magnetic circuit. Some times this "waste current" is greater than the transferred current/power.

Finnally, we look at an air-core. Yes it has all the advantages you have cited, however for those benefits you pay dearly with circuits needing ultra high currents, and high frequency and good coupling. so your design approaches more and more that of a tesla coil....

let me clarify your problem: you are seeing large currents through your bridge/primary, But small voltage across it. Is this correct?

So your trying to use a Tesla coil "like" device to drive the voltage multiplier's bottom section. right?

So your trying to build a 50kV Tesla Coil, full bridge driven...

si2030

Tue Jan 17 2012, 03:56AM

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

Yep that sounds pretty much exactly it...

So 50kv peak telsa coil.... is it possible to feed the output of a telsa coil into the Cockroft Walton multiplier and make secondary resonant with the Cockroft Walton Multiplier?

Patrick

Tue Jan 17 2012, 04:04AM

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

si2030 wrote ...

Yep that sounds pretty much exactly it...

So 50kv peak telsa coil.... is it possible to feed the output of a telsa coil into the Cockroft Walton multiplier and make secondary resonant with the Cockroft Walton Multiplier?

OK im glad we cleared that up, however im no TC expert. My experience has led you this far, but i cant take you any further. There are others far more able in this matter...

There are two issues i see...
1) i dont know if you can get a TC to produce a Voltage thats stable at some specified number.
most TC'ers just what as much V as they can get.

2) If you cant keep the TC output stable (IE a slow up-ring form 50 towards 100kV) then for a little variation in the input of the CW, youll get a huge variation in ouput form the top of the CW. (as the small difference at input will be multiplied byt hte stages) thus you may blow stuff out...

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