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phinsil6

Tue Jan 13 2009, 01:58PM

Registered Member #1914 Joined: Wed Jan 07 2009, 06:50PM
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Posts: 17

hi, i am trying to build kind of an uber solenoid. I'm using 50' of marine tinned 10AWG cable about a small pvc pipe. here is a link of the cable i got.

This cable is wrapped around a 5" pvc pipe and the coils are almost perfect, but for some reason i cant get any magnetic field from it. the little power supply im using is a 12v 2A power supply which works well with a little 24AWG solenoid. Any ideas as to why this coil just wont make a field?
thanks in advance for all suggestions

Dr. Slack

Tue Jan 13 2009, 03:51PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I suspect it is making a field, but it's probably only making the field it does, and not the field you were hoping for.

50ft wound on 5" is only 38 turns. Carrying 2A (I assume your PSU is a constant current limit, if it's foldback limiting, the short circuit current (which is what your coil is) will be very much less than 2A) that's 76 Ampere turns. The copper dia is about 0.1", so I'll guess overall dia as 0.2", so 38 turns is a winding length of about 7". We could use published formulae for field from a solenoid, but it's a bit more ethnic to go from first principles, and it's within spitting distance of the right answer anyway. The length of the mean magnetic field line loop from a 5" dia coil 7" long is of the order of 24" (just add all the dimensions round the loop), or 0.6m in real units. That's an H field of 76/0.6 = 126 A/m. In air, that H field will produce a magnetic field of H*u0 = 126 * 4pi * 1e-7 = 160uT (micro Tesla).

As a comparison, Earth's magnetic field varies from about 30uT to 60uT depending on location. Perhaps the uber-solenoid doesn't rip your arm off when you cautiously approach it holding a spanner, but does it deflect a compass needle?

Your 24AWG solenoid will have a few more turns on it, and is probably smaller, more AT and less field loop length => much more H field. Not only that, smaller dimensions mean a larger field *gradient*, which is what pulls the spanner into the coil.

Incidentally, 50ft of 10awg wire is about 50mOhm resistance. If you can find a 12v supply that doesn't limit (auto battery?), then you will do a little better with the 240A, but not much, and only briefly before the PVC burns off. To get higher fields, you need more layers (to increase the ampere turns without significantly increasing the length of the loop), iron in the return path (to reduce the length of the loop in air), thin PU rather than thick PVC insulation (called magnet wire) (to increase the ampere turns without increasing the length of the loop).

Good luck. Every journey starts with a single step. There's no such thing as a failed experiment, as long as you learn from it.

Dr. Dark Current

Tue Jan 13 2009, 03:51PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Is the current from your power supply limited?
If not,
1) it may have turned off to protect itself from overcurrent
2) the fuse in the PSU may have blown
3) it is toasted

Make sure the PSU works, then limit the current (e.g. with a properly sized resistor, in your case 6 ohms 25 watts), then repeat your experiment.

Edit: Dr. Slack beat me to it. If you want bigger field, use more turns.

phinsil6

Tue Jan 13 2009, 05:17PM

Registered Member #1914 Joined: Wed Jan 07 2009, 06:50PM
Location:
Posts: 17

Thank you for your very detailed responses. I guess i should tell you what im really trying to do with this. im trying to make a coil gun. so i want the maximum magnetic field built with wire that can withstand ~450v from a capacitor with 1500uF capacitance. Thats why i picked the 10AWG, however maybe that wire isnt the best option. So probably magnet wire is the best path to take for a project like this. But can magnet wire withstand that kind of power from a capacitor before mentioned?

phinsil6

Tue Jan 13 2009, 07:56PM

Registered Member #1914 Joined: Wed Jan 07 2009, 06:50PM
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Posts: 17

would 14awg magnet wire be better?

Dr. Dark Current

Tue Jan 13 2009, 08:11PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Coilguns are pretty complex to calculate. Measuring the magnetic field while passing a direct current through the coil tells you absolutely nothing about how it will perform in a coil gun.

phinsil6

Tue Jan 13 2009, 08:15PM

Registered Member #1914 Joined: Wed Jan 07 2009, 06:50PM
Location:
Posts: 17

okay, so would 14 awg magnet wire be the better option? or to be more specific, square magnet wire, for better coils?

Dr. Dark Current

Tue Jan 13 2009, 10:00PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

phinsil6 wrote ...

okay, so would 14 awg magnet wire be the better option? or to be more specific, square magnet wire, for better coils?

edit: square magnet wire should theoretically be better if you can wind it nicely.

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