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Current pulses from my coil gun experiments

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LAN_403

w1vlf

Sun Mar 02 2008, 01:50AM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Folks,
Take a look at these pix.
I took a few pix of the current in my coil gun using a storage scope and 150 amp shunt.
Here are the results.

These pulses were made with a standard 150 amp, 50 mv shunt in the DC line to the inductor.

The vertical scale is +/- 1 volt so each division is .250 volts

3.1 divisions is about 800 mv output from the shunt.

each 50 mv = 150 amps so

16 x 150 = 2400 amps??

By calculation the coil consisting of approx 40' of #14 wire is .102 Ohms.

Ohms law says I = E/R so 275 volts / .102 = 2696 amps

Holy Mackerel..
Where did I go wrong?
Over 2000 Amps?? Peak?
Also note the shape of the pulse with the projectile.. can we assume the bump inthe pulse is a function of the projectile motiuon in the coil?

Being very new to this I am not sure if this is good or bad.
The name of the game is to impart energy to the projectile in the most effecient manner.

Sooooo... Does it seem like the coil could have more turns to iincrease the magnetic field?

I do not yet know how fast the projectile is travelling, just the mass at 25 grams

Thank you
PauLC
W1VLF

WaveRider

Mon Mar 03 2008, 06:26PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

It is quite possible that you have 2000 amps going through your coil. You can also measure the slope of the voltage curve across the capacitors to verify the current (from -C * dV/dt = I, based on the known capacitance value).

Also, the "lump" is the effect of the exiting projectile putting energy back into the electrical circuit (as a result of "suckback"). If you are interested in the theoretical reasons for this, have a look at

Cheers!

w1vlf

Tue Mar 04 2008, 09:07PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Hey Waverider,

Thanks for the reply. I downloaded the article you referenced and read it, but a lot of the math is beyond me.
Is there a way to to use the pulse shape to optimze the energy imparted to the projectile?
I get the feeling that some of this is trial and error.
I will read that article again and see if more of it makes sense.
Thank you
PauLC
W1VLF

WaveRider

Wed Mar 05 2008, 05:17PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Yeah..the article is a bit involved. However, to answer your question in a simple manner: the trick is to switch off the current when the force on the projectile tends to zero as its volume fills the coil. This happens when the armature is completely inside the coil. It is easier to time things well if the armature is somewhat longer or shorter than the coil. A good rule is to switch off the current the moment when the armature volume within the coil no longer increases (armature is fully within the coil).

You want the current in the stator windings to be as small as possible when the armature volume begins to exit the coil. This means quenching the current so it is near zero when the armature volume begins to exit the coil. And, you guessed it, trial and error can help.....but a good optically triggered switch and careful design can guarantee good operation with a minimum of tweaking..

TheMerovingian

Thu Mar 06 2008, 02:02PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

These measures remind me the measures I have taken with my scope and the rogosky coil current measuring system. Same lump on the end of the coilgun waveform. Regarding your measure keep in mind that shunt inductance and skin effect (increased resistance) can false your measures in positive. I would suggest you to try a rogosky coil designed for low frequency, it is less invasive, symple to attach and remove and has a good accuracy and low noise.

Tesladownunder

Thu Mar 06 2008, 04:49PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

TheMerovingian wrote ...

These measures remind me the measures I have taken with my scope and the rogosky coil current measuring system.

Just who I wanted to see. Do you have the circuit diagram of your peak and hold circuit for the Rogowski coil? I can't find it any where on the net or your site. I've wanted it for some time now. I want to do local current measurements with my Rogowski without having to use the CRO all the time particularly as I am starting to use bigger shots and will have to limit current to 200kA or so.

TDU

WaveRider

Thu Mar 06 2008, 06:13PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Hi TDU. With the Rogowski coil, you need an integrator, not sample-and-hold. An RC network with a time-constant about 10 times longer than the firing pulse width will work...or a digital oscilloscope will allow you to do the integration..(because Rogowski coil voltage output is proportional to dI/dt)...

Tesladownunder

Fri Mar 07 2008, 12:39AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

WaveRider wrote ...

Hi TDU. With the Rogowski coil, you need an integrator, not sample-and-hold. An RC network with a time-constant about 10 times longer than the firing pulse width will work...or a digital oscilloscope will allow you to do the integration..(because Rogowski coil voltage output is proportional to dI/dt)...

I have a funtioning Rogowski coil with active integration to measure high currents with can crushing. It works well. I have even determined the current flow within a can during crushing (with a 3 turn coil at 80kA, this is about twice the 40kA current supplied to the coil). I have to read this off the oscilloscope currently, but a simple peak and hold is all I need added to hold the peak reading after the integration.

1204850094 10 FT40348 Cancrushrogowskipeak

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