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The characteristics of a projectile made of permalloy

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Barry

Sat Sept 13 2008, 06:51PM

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

nicora wrote ...
According to your ideas, if a permeability of a projectile increases unlimitedly, the velocity of the projectile increases unlimitedly.
So I think that there is a supremum (in other words, saturation value) if we use the law of energy conservation. Is this way of thinking right?

Right, the kinetic energy can never exceed the capacitor's potential energy. (If I can figure out a way around that I'll be stinkin rich! lol)

Other factors will prevent us from getting close to 100% energy transfer. The biggest factor seems to be the magnetic coupling between the coil and projectile. Simulations suggest that we'll only get a few % efficiency with a coil in air because of the path taken by magnetic flux. This can be raised by adding external iron to reduce the total magnetic reluctance.

Many other factors are important, too. Considerations such as saturation, eddy current, resistive losses, hysteresis, capacitor ESR, and more can all have significant effects. I'll be happy if I can just get efficiency above 10% in a single stage coilgun. This is a challenging (but fun) prospect.

Cheers, Barry
thinking: what would the flying spaghetti monster do?

nicora

Sun Sept 14 2008, 03:30PM

Registered Member #1681 Joined: Fri Sept 05 2008, 07:39PM
Location: Tokyo, Japan
Posts: 16

>Barry
I see...
So, what do you use as a simulation software?
Is it FEMM?
I understand the effect of a permeability of materials, but I am curious about a coercivity of a material.
I think that if a material has a high coercivity, the efficiency will increase.
How do you say?

Barry

Mon Sept 15 2008, 01:26PM

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

I use FEMM 4.0 by Dr David Meeker. Great stuff with LUA scripting. Some of my simulations are here.

Coercivity? It has to do with magnetizing a substance, and whether it is a hard or soft magnet. I guess haven't looked at it much in coilguns, since I'd rather first improve energy transfer efficiency, and later deal with factors such as losses in the B-H curve. I've been using common iron materials (read: extremely cheap!) so far and might go to exotics later if it seems worthwhile.

I've been assuming that hysterises losses with welding rod iron and transformer core material are negligible in this application. What do you think?

Cheers, Barry
Barry denies everything. Actually, he doesn't.

Dave Marshall

Mon Sept 15 2008, 01:42PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Permalloy would be best suited overall to low powered coilguns, but in this application, it could well offer some fairly significant boosts in efficiency.

The big problem with permalloy, as mentioned above, is it saturates very easily. This means that you must carefully design your system to keep flux density below saturation point.

The only problem with saturation is that once your projectile reaches its saturation point, all of the energy beyond that level that you dump in the coil does little more than turn the whole system into a really pretty looking induction furnace. You aren't imparting any more kinetic energy into the projectile.

Some time ago I found a source for permalloy foil, which was handy for avoiding some of the problems Barry described. By gluing the foil to a non-metallic backing (paper, plastic, rubber, whatever) and then rolling the very flexible result into a cylinder, you could avoid eddy currents, as well as reduce its density somewhat to achieve greater velocity at a similar efficiency.

McMaster may still have some of that permalloy foil, if you're interested, and it wasn't insanely expensive. About $5 per projectile.

Other sources for good projectile material on the cheap include 'cow magnets' (cylindrical 1/4" diameter ALNICO magnets about 2.5" long) and solenoid cores, which can be had from surplus places damn cheap and in a variety of sizes.

Dave

rp181

Mon Sept 15 2008, 11:00PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

silicon steel is byfar the most cost effective projectile.

nicora

Sat Sept 20 2008, 03:33AM

Registered Member #1681 Joined: Fri Sept 05 2008, 07:39PM
Location: Tokyo, Japan
Posts: 16

I hope you will forgive me for being so slow to answer.
>Barry
I think that we can neglect hysterises losses too.
But according to a technical book, the force acting at a projectile is proportional to the magnetic permeability of a projectile.
And it means that the efficiency increases if we use a material which has high coercivity because it has high permeability from start to finish.

>Dave
Thank you for your all the information.
I use neodymium magnet (N40) as a projectile now.
But I discovered the fact that if I used this projectile, the efficiency didn't increase significantly as compared with a projectile made of soft iron.

>rp181
I am trying to buy the material now.

rp181

Sat Sept 20 2008, 04:55PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

well the company said they could provide me with a sample. Im not sure if its foil, but its most likley 1ft x .5in dia rod of hymu 80. Ile email them on monday and ask.

WaveRider

Mon Sept 22 2008, 08:43PM

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

It is no suprise that your neodymium magnet gave worse results than soft iron. The force on the magnet will be linearly related to the coil current whereas the soft iron (when it is unsaturated) will experience a force almost proportional to the square of the coil current. (Altho' as the soft iron saturates, the force rolls off to a nearly a linear function w.r.t. coil current. )

In short, in reluctance coilguns, projectile saturation flux density is a more important parameter than permeability. Mild steel projectiles are very cheap and work well, since saturation flux densities of 1.8-2T are achievable. The trick is to keep induced eddy currents to a minimum in the projectile.

Regarding permeability, it is enough to have a relative permeability around 500-2000 (again, like mild steel). There is little to be gained using permeabilities greater than this, especially since special materials can become quite expensive for an experimenter's budget!

nicora

Tue Sept 23 2008, 04:57PM

Registered Member #1681 Joined: Fri Sept 05 2008, 07:39PM
Location: Tokyo, Japan
Posts: 16

>rp181
Could you tell me the result of the email?

>WaveRider
I have not ever read what you said.
What is the source of the information?
But it is very interesting.
My newest coil-gun's maximum flux density is about 3T(in air).
So does this mean that it is better to use a projectile that saturates at 4T than 2T?

rp181

Wed Sept 24 2008, 02:17AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I just got the hymu-80 rod today, .5in X 1ft :D. I lost all my stronger magnets, and i cant really tell the difference with fridge magnets.

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