Magnetic pulse compression / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Magnetic pulse compression

1 2 3 next

Move Thread

LAN_403

Dr. Shark

Mon Jul 16 2007, 02:18PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

"Pulse compression" and "Magnetic Switch" are terms that come up quite a lot in pulsed power papers, but it seems that this area of magnetics is mostly ignored in the amateur community. I don't know whether this is for lack of understanding or because this has no relevance to our toys like can crushers and coilguns, so I would like to investigate a bit.

Alledgedly "Much has been written concerning the design of magnetic switches in Melville line pulse compression networks. In these networks, magnetic switches are used to discharge one capacitor into another. Pulse compression is achieved by discharging each capacitor at a faster rate than it was charged."
Yet, it is not very clear to myself how this magnetc pulse compression / magnetic switching works. A reasonable but incomplete explanation can be found here:

There is a lot more on the IEEE website (e.g.

), but I guess most people don't have a subscrition for that.
If I get it, an iron core inductor is "charged up" from a capacitor until it saturates, at which point the "switch closes", and inductance drops by a factor of 1000 or so. This in turn allows huge currents to flow out of the inductor, and it discharges a lot faster than it was charged.

What I _don't_ get is how the inductor know it is supposed to keep it's inductance low after the discharge starts. Shouldn't the the core be immediately back out of saturation once a tiny bit of current flows out of it?

In a similar device, the "saturable reactor", a DC bias on a separate winding on the core is used to control the degree of saturation (see

), which makes a lot more sense, but I don't think this is done in pulsed power applications.

It would be cool if somebody could clear up the theory a bit, but the interesting bit is of course the practical side.
Wound't it be neat if an affordable, off-the-shelf 'lytic+SCR combo could be boosted with a couple of gapped MOT cores to provide coin-shrinking power like those big Maxwells most of us cannot afford?

Eric

Mon Jul 16 2007, 09:01PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

joe wrote ...

What I _don't_ get is how the inductor know it is supposed to keep it's inductance low after the discharge starts. Shouldn't the the core be immediately back out of saturation once a tiny bit of current flows out of it?

I read that mag inc doc a while ago and was a little confused too. The saturable reactor is not just acting as a switch, after all a switch wouldn't double the voltages. In the first phase the reactor has high inductance and energy from the cap is both charging the next cap and 'charging' the reactor. In the second phase the reactor is charged/saturated and it goes to low inductance and any remaining energy from the first cap enters the second until the voltages start to equalize. In the third the dying current pulls the reactor out of saturation (it's high inductance again) and it produces the big voltage as the field/current decays to continue charging the second capacitor.

The problem I see is that you need special iron to make them work, very square BH curves and very thin well insulated laminations as well as a high Bsat. Also, since the reactors have to store a good portion of the energy to be transferred at saturation, they would have to be big to handle 100s/1000s of joules.

Dr. Shark

Tue Jul 17 2007, 08:17AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Thanks Eric, I am not sure I understand you correctly, but anyway I found the following:

Magnetic pulse compression is a hoax!

A compression stage will never give you a smaller pulse than you would get by directly discharging the capacitor in that compression stage. The whole point of the exercise seems to be the "switching" part, you can e.g. get away with a 10kA switch but your load will see 100kA or whatever. The price you pay is that you _still_ need the expensive pulse capacitors, you just don't switch them directly.

Aparently this has some merit for industrial applications where high repetition rates are required and the initial cost of the aparatus does not matter much. For our purposes, pulse compression is completely useless.

That's it from me, while I still don't know how this stuff works, I do know that I don't care

Steve Conner

Tue Jul 17 2007, 09:46AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It's not a hoax. All it does is make the pulse shorter and sharper: it compresses it in time. If it made it bigger, they would call it magnetic pulse amplification or something.

The inductor "knows to keep its inductance low" just because of the physics of the thing. Think of the discharge as a half cycle of a LC resonant circuit. If the inductor didn't saturate, the current would be a half sinusoid of low frequency and relatively low amplitude. It starts off that way. But the inductor then saturates while the current is still increasing through it, on its way to the 90 degree point of the sine wave.

The LC oscillation now continues at a higher frequency because the inductor is much smaller. The current keeps increasing, because it's not 90 degrees yet. It peaks out at a much higher current due to the lower surge impedance of the new resonant circuit with its smaller inductance. (Though as you noticed, the peak current is still limited by the ESR of everything, just as if you discharged the capacitor to the load directly.) It then falls back down until the inductor comes out of saturation. At which point, the half-cycle completes at the original low frequency.

So the overall waveform is more or less a half sinusoid with another much sharper half sinusoid sticking out the middle. (IIRC.)

To recap, if the capacitor were discharged directly to the load, you might get an equally high peak current, but it would have a slow, ugly exponential tail. A magnetic switch sharpens the pulse, and presumably conserves the energy that would have been wasted in the tail.

The Mag-Inc explanation is pretty poor, because it implies that the cascade circuit would work with non-saturating inductors, which it won't. The cascade only works if each "switch" can hold off "firing" until the previous stage has done its job.

Madgyver

Tue Jul 17 2007, 11:59AM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

Wow that paper is realy good. You know a few other good ones that you care to post?

Deathray Labs

Mon Jan 14 2008, 05:43PM

Registered Member #1227 Joined: Mon Jan 14 2008, 04:49PM
Location:
Posts: 8

Hi,

This is my first post and have been searching for some time for someone that could have an intelligent conversation with me about pulse compression at my level. So first, thanks for bringing it up and the informative follow up posts.

I would like to suggest that the forum open a thread concerning electrolysis since so many people are interested in hydrogen production and the only thing that could make high voltage or pulse compression more fun would be adding water and hydrogen.

I have been exploring the possibility that Stan Meyer may have used pulse compression without really knowing he was. (He called it rubber band effect.) Is there such a thing as magnetic DEcompression?

Anyway, I am interested in building a 50-100 watt pulse compression circuit for electrolysis testing and would very much value any advice or references you may have. There very few papers on the subject available and this is a terribly overlooked area especially with energy costs what they are right now.

I have a few related pdf's but they do not get far past the block diagram. Does any one have a design calculator sort of tool or something? I am not an RF person but I am trainable.

Mates

Mon Jan 14 2008, 06:02PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Deathray Labs wrote ...

I have been exploring the possibility that Stan Meyer may have used pulse compression without really knowing he was. (He called it rubber band effect.) Is there such a thing as magnetic DEcompression?

Stan Meyer - isn't it the guy who used water for powering his car? I'm afraid the guys here will not like you with this kind of bullshit

Deathray Labs

Mon Jan 14 2008, 06:33PM

Registered Member #1227 Joined: Mon Jan 14 2008, 04:49PM
Location:
Posts: 8

Mates wrote ...

Stan Meyer - isn't it the guy who used water for powering his car? I'm afraid the guys here will not like you with this kind of bullshit

Huh... Electrolysis is a subject well studied by Michael Faraday and I doubt too many people have a problem with him. Not a snappy dresser but still a pretty good scientist for his day.

I read somewhere that magnetic pulse compression proved Stephan Hawkings theory there is a space less empty than empty space since MPC compresses energy in time. I doubt too many people have a problem with Hawking... Especially now that he is dead and picking on a dead gimp is pretty much socially off limits anywhere these days...

and since all my experiments with electrolysis indicate electron packing on the anode is THE problem and an accelerator may help break that barrier by beating the H2O molecule's unusual snails pace ability to reorder...

Oh silly me. I feel like such an idiot. Please forgive me for my out of order request for information regarding pulse compression. It was my first post and I must have just wanted to jump right in there into your open source like community of forum contributers to learn and share information.

Your post was most helpful "Mates". Please allow me to share something in return. It is a free pdf you can download off the web that really gets to the basics of magnetic pulse compression.
] 1200335628_1227_FT28498_pulse_compression_circuit. pdf[/file]

Bjørn

Mon Jan 14 2008, 08:02PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Elelctrolysis is fine. Pulse compression is fine.

We don't want to hear about any pseudo science breakthroughs in energy creation (electrolysis or otherwise) until you have received your Nobel Prize. The rules state clearly:

I. Free energy, electrogravitics, and all other types of pseudoscience are not allowed on any part of this site, nor are links to sites that deal primarily with these subjects. If this is what you seek, you can google many, many other sites and forums that will accept such topics. Discussion of such topics will only be allowed if someone has undeniable, 100% solid proof that this isn't pseudoscience. If this is the case, we'd also like to hear about when and where you will be accepting your Nobel Prize.

Stephen Hawking seems to be still alive so that is news to us.

Deathray Labs

Mon Jan 14 2008, 08:28PM

Registered Member #1227 Joined: Mon Jan 14 2008, 04:49PM
Location:
Posts: 8

I stand corrected, Hawking is alive, this site is dead.

C-ya

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob