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Signification

Mon Mar 04 2019, 12:43PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I am looking for a few reliable methods to stop the current in the solenoid(s) of a coilgun
when the projectile reaches it's centered position. I have a way of detecting this position
and now would like to hear some of your techniques of killing the magnetic field. I assume
it would greatly help if most of the capacitor's energy is discharged at this time.

2Spoons

Mon Mar 04 2019, 09:01PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Applying a large reverse voltage is the fastest way to stop the current.
If you drive your solenoids with a partial bridge ( two transistors, two diodes) you can return the solenoid energy to your storage capacitor - which will improve the overall efficiency of a multi coil system.

Signification

Tue Mar 05 2019, 03:52AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Thanks,
I was also thinking of a center-tap on the solenoid, and when the projectile had traveled to the cut-off point, then just shorting the latter half via the center tap and far end...sound feasible?

DerAlbi

Tue Mar 05 2019, 08:11AM

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

Oh Signification.. you and your creationist mind...

If i get your drift correctly, you want to build a transformer and compensate the magnetic field by short circuiting the secondary. A center tap is the wrong way to go in that case. In a normal transformer you have a core, so separated windings are coupled very close to 1. The leakage inductance will always be there and prevent a true instantaneous magnetic field shut down. The leakage inductance is therefore something you want to minimize... bifilar winding would do that way better than a center tap.

Now please, sit down and ask yourself why this idea is bad.
1) You want to add turns to your coil that will not contribute to accelration. What does this to your power density?
2) If you care so much about turning off the coil... do you equally care about turning on the coil? If not, why not?
3) How do you think, a magnetized projectile will react to a short circuited coil?
4) What is your overall goal? Reducing suck-back? Boosting efficiency? Why ask specifically about turning off? Can your actual goal maybe reached differently?

Signification

Tue Mar 05 2019, 10:12AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

@ Albi:
How would YOU handle suckback and reverse electrolytic voltage?

DerAlbi

Tue Mar 05 2019, 12:20PM

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

..by avoiding or even tolerating it.
Depends on your topology. Mostly its a timing isue.

Is it your only priority, or do you also want to maximize energy transfer and/or effiiency?

You have to understands that your single sentence is representing one key issue when building a coilgun. You can not isolate the problem nor solve it in isolation. Any solution comes with trade-offs and you have to evaluate them to find an optimum.
A suck-back-less coilgun with never negative capacitor voltage is easily buildable by means of an asym. half bridge - basically what Spoon said.
A SCR-Design with over-damping or beefy diodes can do the job too. Having suck-back may not be bad, if your overall energy transfer is better. This is why i question your motive behind the generalized-beyond-usefulness formulated topic.
Without boundary conditions, the optimal (concerning efficiency) halfbridge waveform is an infinitly short, infinitly high (dirac) current pulse that requires infinitisimal small inductance. Congratulations..

Signification

Wed Mar 06 2019, 10:00AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I have only seen electrolytic caps used in coilguns...what is a pulse capacitor were used? I am thinking perhaps most of the energy would be discharged, accelerating the projectile, and perhaps would be mostly discharged by the time the projectile is centered in the coil. With calculations and trial-and-error, of course.

@Albi: I do agree with your number (3), in this case the projectile would undergo braking.

DerAlbi

Wed Mar 06 2019, 11:39AM

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

Yandersen (inactive forum member now) had a breakthrough with foil capacitors used in coilguns here and afaik hit an efficiency record for the time. He basically used the LC circuit with a SCR and totally tolerated the reverse polarity on his bipolar capacitors. This also shaped the current pulse to the typical sinus-halfwave you would expect, and mathmatically leaves him with a partially charged (altough negative voltage) capacitor which is an equivalent to the halfbridge recouperation achieved with the simplicity of the SCR circuit. That was real out of the box thinking.
The energy density of foil capacitors sucks of course, but i was a great idea. Trde-offs as usual.

Signification

Wed Mar 06 2019, 01:26PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I found on ebay some plastic energy discharge caps that were to be used in drones--I think. They are 'dry', rated at 500uF @ 1440v (acid test shows they hold over 3kv before failing begins). They weigh between 15oz. and 17 oz. about 11 square inches. At full charge, that's about 520J (47J/sq in.).
I also have some oil-filled metal energy caps: about 200uF @ 2300v (~530J) about the same mass. I know this is not great energy density comparatively. But there may be some advantage to the faster discharge and the fact that the energy is proportional to the SQUARE of the voltage. Compared to 450v, the energy ratio is about 26:1 in favor of pulsars. Capacitance ratio~10:1 in favor of electrolytics (I hope I did this right--didn't check). Setup may require differently wound coils among other things (like triggering circuits).

2Spoons

Wed Mar 06 2019, 09:17PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

If you run through the math, for a given quantity of any particular dielectric it doesn't matter if you go low voltage/high C or high voltage /low C the energy storage is the same in J/kg. The speed with which you can extract that energy will be different, however.

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