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Theory behind suckback

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LAN_403

iamelephant

Sat Apr 02 2011, 03:39AM

Registered Member #3802 Joined: Sat Apr 02 2011, 03:20AM
Location:
Posts: 2

Hello,
I have made a coilgun for a school assignment, and although I knew that no current should be in the circuit past half way through the solenoid to avoid suck-back, I never really understood why.

On every solenoid field diagram i find, the field goes right through the solenoid and doesnt indicate that the projectile is attracted to the center of the solenoid only. Ive searcehd the forum and the wiki, however I never really found why the projectile is attracted to the center of the solenoid.
Does it have anything to do with Lenz;s Law?

Thanks, and I must say I very much like this community

MArked One

Sat Apr 02 2011, 03:37PM

Registered Member #3688 Joined: Mon Feb 14 2011, 07:39PM
Location: Europe
Posts: 38

Greetings.

As far as I know, (and somebody should correct me if I'm wrong) the pulse you receive from the capacitors will cause the coil to pull the projectile towards its center; and any more energy will pull the projectile back. This decreases efficiency, and is a large problem for some large coilguns.

To put this in theoretical terms, the coil wants to see the most flux change when the projectile is inserted. The point of maximal flux change occurs at the center of the coil. The more energy you put in, the more 'energetically' will the coil pull the projectile towards that point.

hope this helps.

Kind Regards,
Ivan Kozlov.

Turkey9

Sun Apr 03 2011, 01:35AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Get your hands on FEMM and simulate a solenoid without a projectile, with it partially in, and with it at the center. This will help you visualize it much better.

When the projectile is part way into the coil (the tip has not yet reached the center), the magnetic field will be very distorted. However when the projectile is centered, it will become symmetrical again.

I think this might be the physics behind it:
The reason that iron is attracted to a magnet is that it becomes magnetized when in the presence of a magnetic field. When iron is close to the north pole of a magnet, it can be thought of as having a south pole facing it. As a projectile passes through a coil, it's north and south poles want to be as close as possible to the opposite poles of the solenoid. This ends up happening at the center of the solenoid. That is why the projectile is attracted to the center of the coil.

Suckback is a little bit different however. The current flowing through solenoid from a capacitor will oscillate. This means that if you look at the field strength (not polarity), it will get very large and then go to zero when current flows one way. It will then grow very large again as the current flows the other way. In a coilgun, the projectile is attracted to the center of the solenoid during the first pulse from the magnetic field and then exits as the field drops back down to zero. If the coilgun isn't tuned properly, the projectile might not be out of the coil yet when the second pulse starts due to the oscillations. This is when it feels a force wanting to suck it back into the coil. This will of course slow the projectile or even stop it.

Yet another situation for suckback....: The pulse is far too long. If the pulse is too long, the projectile will reach the center of the coil before the pulse has died out. It's momentum will counter act the force of the magnetic field and allow it to pass the center, but as long as the pulse has not died out, it will feel a force wanting to suck it back to the center. This force will decrease it's velocity and rob energy. Sometimes the pulse will not act enough to stop the projectile, only slow it.

Coilguns are finicky and have to be tuned properly before they will work at all. You have to make sure the pulse is not too short or too long or "suck back" will kill your efficiency or sometimes even stop the projectile from going anywhere.

An interesting not, you can tune a coilgun so that the projectile will shoot backwards out of the coil with a good amount of force.

iamelephant

Sun Apr 03 2011, 06:17AM

Registered Member #3802 Joined: Sat Apr 02 2011, 03:20AM
Location:
Posts: 2

Thanks for the replies guys!

Ill take what you said into consideration, thanks for taking the time to write out such thought-out responses

QQQ9.0

Fri Apr 08 2011, 04:31AM

Registered Member #3746 Joined: Mon Mar 07 2011, 02:39AM
Location:
Posts: 1

I disagree entirely with the post above.

Coilguns work like a "sling shot", they pull the projectile towards the centre, and (ideally) when the projectile meets the centre of the field, the field shuts off and the projectile flies through. Suck back occurs when the field lasts too long, and the field is still on while the projectile passes through its centre. This means now the projectile is again being attracted to the centre, but now in the opposite direction since its on its way out.

Yes, capacitors do oscillate mildly (or moreso if you dont have an anti parralell diode) but not enough to hurt the speed of the projectile. (from my findings).

qqq

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