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Toroid core to condense magentic flux?

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Barry

Fri Jan 29 2010, 01:01PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

GhostNull wrote ...

what do you mean by light and heavy toroidal core

I expect he's referring the cross-sectional area of the toroid.

What does he mean by "waste most of the power in the air"? I don't understand that part.

As usual, adding iron to the magnetic path will raise inductance. So we design around it with, say, fewer turns in the coil and lower capacitance and higher voltage, to compensate. The tendency toward higher inductance is not a problem as long as you take it into account during the design.

Magnetic saturation is always a factor, as it's relatively easy to accomplish with common parts. This includes the iron projectile itself, where thinner nails and screws have very small cross-sectional area.

Cheers, Barry
An invisible man marries an invisible woman. The kids were nothing to look at either.

WaveRider

Fri Jan 29 2010, 03:28PM

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

Off the top of my head, I think a toroidal coil will not perform as well as the usual solenoid for the following reasons:

Saturation of toroid will mean flux lines will 'escape' the core and not contribute to moving the projectile. By contrast, in a solenoid with a saturable sheath, all flux lines must pass thru the center of the coil. Flux lines that "escape" the solenoid shield can still contribute to moving the profectile.
As Barry noted, do not underestimate mechanical stresses in the toroidal gap. When energised, the gap will tend to close up, possibly breaking your brittle toroid.
Solenoid works better than toroid in concentrating field at the center. This is a feature of the solenoid's cylindrical symmetry. No such field symmetry exists in the proposed toroid geometry.

There are probably other reasons as well....
Cheers!

Dr. Slack

Fri Jan 29 2010, 08:25PM

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

Fundamentally, coil gun is a linear motor.

A motor's efficiency can be measured in several ways.

It is energy efficient if it transforms a higher percentage of input energy into projectile energy. Now you don't see many efficient motors being built with huge air-gaps like a conventional coil gun, and I would expect a coil gun built with a toroid like that to be more energy efficient than the conventional AS LONG AS the energy is kept low enough not to saturate the core, which means keeping it pretty low energy per stage.

It is space efficient, or materials efficient, or cost or time efficient if it produces a big bang for the buck. Whacking a huge amount of energy in from big caps and getting a field in the solenoid well in excess of material saturation is going to give you the most bang for your size, and it uses no expensive heavy magnetic material, and is quick to build, but will have lousy energy efficiency - when people get 5% they boast about it.

If you try to make a coil gun from the cut toroid design, then because of the higher inductance, better magnetic efficiency and saturation problem (to name but 3 issues), most lessons that people have learnt and posted from building solenoidal coil guns will be irrelevant, and you will have to go back to basic physics to design it properly.

GhostNull

Sun Jan 31 2010, 10:24AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

okay thanks for the replies guys so to wrap this up in a summery

Toroid core to condense magentic flux?

Yes:

Better magnetic effeciency (strongest point)

Less stray magnetic fields

Less eddy currents

No:

Easily saturated core means care should be taken in design

Higher inductence means care should be taken in design

Core will be under great strain during pusle and could break, so again care should be taken in design

Solenoid design would concentrate field in center of barrel better

Solenoid design is much easier to construct

Small area for projectile to interact with magnetic field

Solenoid design magnetic has a cylindercial symmetry

Not much experience or knowledge in creating and designing of toroidial design

The toroidial design is a new unelxplored ground and there sure are alot of reasons to not use the preposed toroidial design; mainly the solenoid design would be a much easier alternative but with carefull design the cut toroid design is feasible and holds the hope of much better efficiency

GhostNull

Sun Jan 31 2010, 10:26AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

now all we need is some one to try build this

Russell Haley

Sun Feb 14 2010, 06:23AM

Registered Member #2478 Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47

What about slipping a few ferrite toroid cores around the barrel, on top of the coil, so as to reduce the reluctance in the path of the external flux?

Barry

Sun Feb 14 2010, 04:03PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

GhostNull wrote ...

now all we need is some one to try build this

It sure would be interesting to see one. I am always stymied by the fact it's easier to think of things than to try them.

Russell Haley wrote ...

What about slipping a few ferrite toroid cores around the barrel, on top of the coil, so as to reduce the reluctance in the path of the external flux?

Good idea, I'll remember that one. I'll stay alert for cheap toroid surplus cores for possible use on conventional coilguns.

By the way, be aware of the magnetic field direction here. The current goes in circles and magnetic flux is perpendicular, so the iron path of interest is along the length of the solenoid. The mating faces of two round toroid cores will work against it, even when tightly pressed together. It would be better if ferrite was somehow continuous from one end of the coil to the other; we don't care about small gaps in a circular path around the coil.

Does anyone know a source of ferrite in square or tubular rod shapes?

Barry
A jumper cable walks into a bar. The bartender says, "I'll serve you, but don't start anything."

Russell Haley

Mon Feb 15 2010, 03:10AM

Registered Member #2478 Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47

Noise suppressor beads, but they're way too small.

thehappyelectron

Sun Mar 14 2010, 11:59AM

Registered Member #2738 Joined: Sun Mar 14 2010, 05:31AM
Location:
Posts: 2

i have thought of a similar design in the past, but opted to build a 'traditional' coil gun 6 caps 330v 4300uf ea, in parallel. mainly out of frustration of working with toroid-cores, as a new member i might as well contribute....but i am going to need some help, the most frustrating part of the build was trying to remove the notch and discovering the fragile powdery inards. i contemplated epoxy to seal and strengthen. but after realizing how fragile they were inside it did not make sense to build the design without a proper cradle. so i am relatively new to los angeles... does anyone know of any great surplus stores in the area? (orignially from houston also where my setup is)

GhostNull

Sat Mar 20 2010, 07:10AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

like i said before you could use the core from a toroidal transformer, a laminated toroidal core.

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