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Effects of projectile saturation

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Erwin

Thu Apr 17 2008, 09:26PM

Registered Member #1447 Joined: Thu Apr 17 2008, 08:28PM
Location:
Posts: 1

Hey everyone,

I just wanted to check something. On the vast amount of coilgun information on the internet, some information seems a little confusing: the effects of saturation on the projectile.

James Paul writes on his site about the saturation that it's of limited consequence:"IMHO saturation of the projectile is of limited consequence, insofar as it doesn't place a limit on the attractive force. I do think that running a coilgun with a saturated projectile will be less efficient."

Wikipedia writes on this subject: "One of the greatest limitations to the coil gun is the rate at which the ferromagnetic projectile becomes fully saturated by the magnetic field and the rate at which it loses its magnetic saturation. Once a ferromagnetic object becomes completely saturated the amount of force in which it can be attracted stops increasing."

Barry writes on his site "The magnetic saturation point puts a limit on the maximum effective strength to drive your coil. There will be no benefit at all if you exceed this, only wasted heat."

And, to confuse me even more, he writes: "From Bs and R you can compute the maximum flux your coil should generate. But don't worry: this upper limit is going to be huge! My initial calculation found it will be millions of ampere-turns, using any air-core coil that we've seen so far!" But looking in his Magnetic Materials table, the saturation flux level for iron is at 21500 Gauss, 2,15 Tesla.

Using this simulator, this 'upper limit' of 2,15T is very easy to exceed.

So, my question is, can anyone explain me if it is wise to exceed the 2,15T field strength?

Greetings from the Netherlands,
Erwin

PS: My co-builder tells me not to wonder so much about the theory, and just see what happens... Maybe this is the best idea after all, cause there's so much to consider beforehand, and not so much experience in our heads.

Dr. Shark

Thu Apr 17 2008, 10:52PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Erwin, welcome to the forum! You must be trying hard to make a good first impression here, at least is seems you put at lot of research into your post!

I am not an expert in reluctance coilguns myself, but from my experience with electromagnetic launcher in general it is the change in inductance that drives the projectile through the coil. So the question could be rephrased as: Is the inductance, i.e. the amount of magnetic field generated per amp of current, going to change after the projectile saturates?
By the definition of saturation, more current will not lead to more magnetic field, so I think that the force on the projectile is not going to increase any more after you have reached saturation.

We have at least two experts on magnetics here, WaveRider and Sulaiman, so maybe they will be able to explain this better. In the meantime I hope my explanation goes in the right direction!

PS: Regarding your friend, no, he is wrong! There are so many misconception about coil guns that are holding back the hobby, it is about time someone sits down and does the math how these things actually work. I am trying to make my own small contribution here

, but the father of coilgun theory is really WaveRider with his stuff here

WaveRider

Fri Apr 18 2008, 08:02PM

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

Hi Erwin,
Welcome to the forum.

To briefly answer your question about saturation:

1) When the armature is far from saturation, the force is roughly proportional to the square of the coil current.
2) When the core is strongly saturated, the force is nearly linearly proportional to coil current.

In between, it is a mixture.. See Figure 3 in this paper for an illustration of how the force per unit area changes as the armature saturates.

In short, even when the armature is saturated, force still increases with coil current, albeit at a slower rate. Where the diminishing returns come in is with the losses. When the armature is not saturated, force and coil loss follows roughly the same I^2 law. When the core is saturated, armature force goes as I and loss still as I^2...so as you increase coil current beyond a certain point, efficiency degrades when energy losses outstrip energy transfer to the armature. This is why multistage launchers are always advantageous....

Welkom bij 4HV!!!! Vriendelijke groeten uit het diepste zuiden! (BelgiÃ«)..

Bjørn

Fri Apr 18 2008, 11:00PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I have one way to view it that might be intuitive for some.

Iron contains a large number of small magnetized areas with random orientation so the sum of magnetization is zero. When the iron is placed in a magnetic field the small areas start to align themselves with the magnetic field.

The stronger the field the more areas align and the stronger a magnet the iron becomes. So the force rises with the square of the current because the field gets stronger and the iron becomes a better magnet.

When all available areas are aligned the iron is said to be saturated and can't become a better magnet. The force then continues to rise linearly.

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