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JUST AN IDEA: Hybrid coil/gauss plus Lorentz/Thompson

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LAN_403

Sulaiman

Mon Apr 06 2015, 12:04PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Whilst thinking about Signification's post
............................................. .......
I have been wondering about something for a very long time now an have had no luck in finding an answer...
If a ferromagnetic slug (initially UN-magnitised) is placed into an external magnetic field (the 'H' field) that is strong enough to -almost- saturate it, how long does it take the magnetic domains to all get aligned to the extent corresponding to that external H field? Is this 'alignment' process instantaneous or is there some sort of time constant to consider here? If there is some sort of significant delay--I would assume that the delay will shorten as a function of the H-field strength. In other words, the slug's internal (B) field would be reached more quickly when the H field is stronger. Perhaps VERY fast if the H field is MUCH stronger than the value of the saturated B field of the ferrous slug.
............................................ .........
it occurred to me that a hybrid Gauss/Lorentz gun may be practical,
(e.g. electrically; d.c. supply, coil, transistor as in a flyback circuit)

ON . energy supplied to coil, magnetic flux buildup, projectile 'sucked' into coil
OFF . energy stored in coil RAPIDLY removed (e.g. very high flyback voltage),
Lenz/eddy-current/Lorentz repulsion of projectile due to high dB/dt, projectile 'spat' out of coil.

imagine the projectile as iron wrapped in copper,

during the 'on' phase current will build up in the coil roughly as V.t/L
the magnetic field will increasingly attract the iron to the centre of the coil
there will be a 'Lenz/Eddy' repulsion due to the copper projectile sheath, but much smaller force than the main attraction.

as soon as the projectile reaches the centre of the coil the transistor turns off and allows a very high voltage flyback discharge
. the coil current drops rapidly so the magnetic field does not suck back the projectile
. the rapidly changing magnetic field causes a 'Lenz/Eddy' repulsion due to the copper projectile sheath.

The circuit diagram would look normal, trigger/gate/base drive signal to transistor driving the coil,
the transistor is key#1, low on voltage, fast dv/dt turn off, high voltage avalanche rated.
Maybe not suitable for large coils, but for smaller coils there may be a significant efficiency increase.

LOTS of experimenting with core/jacket/coil/timing/transistor combinations and some luck required
I'm too lazy but someone may want to try which is why I'm posting this.

ON period projectile acts mainly as a gauss/coil gun
OFF period projectile acts mainly as a Lorentz/Thompson coil gun
Transistor could be bjt like crt tv/monitor flyback, mosfet, igbt etc.
just an idea if anyone wants to try.

EDIT; exact opposite is to have a diode across the coil and have a short ON time to establish peak current in coil
coil turned off before the projectile gets near the centre of the coil
and a long current decay time to continue 'sucking' the projectile into the coil
there will be a 'tail' current causing some suck-back
but this slight loss should be more than compensated for by the lower electrical input energy (short on-time)

DerAlbi

Mon Apr 06 2015, 02:15PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Smileytime and Blockquote abuse! Let the fun start.

it occurred to me that a hybrid Gauss/Lorentz gun may be practical

energy stored in coil RAPIDLY removed

How?

e.g. very high flyback voltage

Aha.

So....

as soon as the projectile reaches the centre of the coil the transistor turns off and allows a very high voltage flyback discharge

And this seems still

practical

Put the Transistor in the Explosion-Chamber and you are at least sure you use all the Energy. That is then

practical

Seriously: has it never occured to you why no one shuts down a Coil

RAPIDLY

..when this is actually something everybody here wishes to be able to do?

so the magnetic field does not suck back the projectile

How did you come to say this is

practical

again?

Sulaiman

Mon Apr 06 2015, 04:33PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I just want to say here that no matter how high the voltage spikes, the current keeps flowing until the energy in the megnatic field is transfered to somewhere else. In your case you have some imaginary transistor which must sustain all the Voltage*Current during turnoff..
Or in other word: every single Joule left in the Coil at turn-off... will just explode your Transistor. Thats what the diode prevents...
..........................
reply;
of course, the current will flow whilst integral(Von.dt) > integral(Voff.dt)
so higher Voff means shorter Toff.
All the little Joules stored in the coil will be dissipated in the transistor during avalanche breakdown,
this is why it is for smaller coils.
If you want more power use series-parallel TVSs to dump the energy instead of the transistor avalanche.

Seriously: has it never occured to you why no one shuts down a Coil

RAPIDLY

..when this is actually something everybody here wishes to be able to do?

so the magnetic field does not suck back the projectile

How did you come to say this is

practical

again?

...........................
think about it ....
slowest .dI/dt = coil with anti-parallel diode
moderate -dI/dt = half-bridge recovering energy into the capacitor/supply
Fastest energy dissipation (reduced suck-back duration)
followed by extremely rapid -dI/dt (to cause Lorentz force in copper jacket)

This method DUMPS energy stored in the coil (either in the transistor or TVSs)
Whilst 'dumping' the coil current will be maintained at the peak current before turn-off
.... maximum current maintained as projectile approaches the coil centre ....
.... very rapid collapse of magnetic field
....... reducing suck-back
....... inducing repulsion as the projectile leaves coil centre

DerAlbi

Mon Apr 06 2015, 04:42PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

You really underestimate the "dumping".
With a didode, the current decays slowly.. BUT the energy is disipated within the copper and heats quite a considerable ammount of mass.
You can not start to think that you can dispate that kind of energy in a Junction which is measured in MILLIGRAMS.
Please do the math: an Energy that would heat a 100g Copper-Coil by 0.5Â°C would heat a 0.01g Junction of silicon... how much?

Again i feel the urge to ask you: why do you assume its "practical" to switch off a coil "rapidly" ???
Its one of the biggest problems with Coilguns.. if this is so easy to come by and "just do it"...................... there SO much effort done in figuring out the right timing and stuff.. WHY.. when you "just can turn it of RAPIDLY"

Sry dude.. thats because: you cant. ...well.. you can.. but its NOT PRACTICAL

My quoting massacre above was intendend to show you how your own text can be used to point out the concept problems. You wrote stuff that inherently excludes each other.

Sulaiman

Mon Apr 06 2015, 06:49PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

this is only for small/tabletop coils, not WMD

maybe not as practical as I imagined?

DerAlbi

Mon Apr 06 2015, 08:01PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Well.... IGBTs wont handle excessive voltages. They just die. Mosfets can be avalance rated. (thats what you need).
The avalanvce energy is in the mJ-Range... and thats the maximum allowed Energy after the shot in your Coil.
Tabletop... well.. yeah. ultra small table

BigBad

Tue Apr 07 2015, 08:30AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Not sure what circuit you're using. Are you using an H-bridge? That would stick the energy back into the electrolytic. You could run PWM. You won't get voltages higher than the cap (ignoring the switching transients due to strays). And you can drain the coil quicker, because you're putting negative volts across it.

Uspring

Tue Apr 07 2015, 09:36AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Sulaiman wrote:

. the coil current drops rapidly so the magnetic field does not suck back the projectile
. the rapidly changing magnetic field causes a 'Lenz/Eddy' repulsion due to the copper projectile sheath.

For me it looks like these are conflicting statements. The force on a magnetic moment is m * dB/dx. There needs to be some B to cause a force. Not only a dB/dt. The basic idea is to invert m, as it is passing through the center of the coil, i.e. the eddy current caused m has to exceed the m by magnetisation.

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