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Low speed coilgun efficiency (why it's so bad at high accelerations)

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Forty

Mon May 23 2011, 05:26PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I've tried all sorts of pieces of ferrite that will fit into a 30cal coil. they work fine but the problem is that they tend to shatter on impact.
if the concern is magnetic saturation of the armature, wouldn't ferrite be a worse choice than iron? the several google results i looked at seemed to indicate that ferrite saturates between 0.2 T and 0.5 T, while iron saturates at 1.6 T

Ash Small

Mon May 23 2011, 06:03PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Forty wrote ...

I've tried all sorts of pieces of ferrite that will fit into a 30cal coil. they work fine but the problem is that they tend to shatter on impact.
if the concern is magnetic saturation of the armature, wouldn't ferrite be a worse choice than iron? the several google results i looked at seemed to indicate that ferrite saturates between 0.2 T and 0.5 T, while iron saturates at 1.6 T

I understand that ferrite is more suitable for high frequency applications, although they saturate at a lower T, due to the fact that eddy currents can't circulate, because they are powdered and in a resin matrix. Maybe someone can clarify this?

BigBad

Mon May 23 2011, 06:15PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

Yes, the lower saturation will make the magnetic pressure lower. On the upside ferrites usually have a lower density, so the acceleration may not be a lot different, if you pick the right ferrite.

Still, it looks like trying to lower eddy currents is the polar opposite of what you want to do.

BenB

Mon May 23 2011, 06:36PM

Registered Member #3785 Joined: Sat Mar 26 2011, 03:37PM
Location: Surrey, UK
Posts: 16

Regarding eddy currents, I could have sworn some hobbyists have had success with slotting projectiles longitudinally to improve efficiency by reducing eddy currents in the projectile? Or would you say that this would have a contrary effect?

BigBad

Mon May 23 2011, 09:35PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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It depends on what you do with the magnetic field around the projectile.

Theoretically, you could have a saturation field that pulls the projectile along, and then sit a sweeping AC magnetic field on top of that, that pushes it as well via the eddy currents.

Forty

Mon May 23 2011, 09:56PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

what about lamination to reduce eddy currents at the surface? since the pulse is so short, shouldn't the eddy currents only occur on the outside?

BigBad

Mon May 23 2011, 10:21PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Then your coilgun would go slower and be less efficient.

I think that, given the choice between using your energy to saturate iron, and giving you eddy currents, you would probably want to use it all on eddy currents.

klugesmith

Tue May 24 2011, 02:49AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

BigBad wrote ...
In a coil gun at low speeds the projectile is being pulled along by magnetic pressure. If you look up magnetic pressure, you'll probably find that it goes as a square law on the magnetic field strength.

Excellent point. So far, so good.

BigBad wrote ...
Well... but actually that's not entirely true. What it goes as is BH, where the B is applied field and H is the reaction of the object (or its own field if it's a permanent or electromagnet). For a superconductor, it can create a complete equal and opposite, proportional H field, so the pressure goes as B^2. Air has no H so no pressure. Metal gives you a pressure only when it has its own field. etc. etc.

Great observation about the importance of BH product, and the fact that it changes from square law to linear when core material saturates.

But, no offense indended,

1) I think you have B and H backwards-- at least, I think it's easier to explain if B is the material response to H.
Current flowing in a coil (or moving charges at the atomic level) is a source of H. H is measured in amperes per meter (SI) or in oersteds.
B is proportional to H in vacuum or, for practical purposes, in air. B is measured teslas (SI) or gausses.
Ferromagnetic materials have much larger B for a given H, until they saturate-- at which point B is constant while H can increase without limit.
As you say, a superconductor can be treated as a perfectly diamagnetic material, with B=0 inside.

and 2) In a reluctance coilgun or electromechanical solenoid,
the place where BH product is important is not in the core material, but in the adjacent air.
BH-product-wise, air is good because large H for a given B.
Iron -is- important to concentrate the B in air, and avoid wasting H; as a line of flux crosses a material boundary |B| is continuous while |H| can change dramatically.

As a rule of thumb, the magnetic pressure in air at B = 1 tesla is about 400 kPa (60 psi).
As you say, it's a square law, so the air gap energy at 2 teslas is about 1600 kPa (240 psi).
Good reference for figuring order-of-magnitude torque of electric motors or pull of solenoids.
But that's about the limit for B guided by iron, because of saturation.
Derivation:
Pressure (pascal, or N/m^2) is another name for energy density (J/m^3).
Magnetic energy density is BH/2.
In an air gap with B = 1 T, H = 1/u0 = 1e7/4pi = 800 kA/m, BH/2 = 400 kPa.
In iron with relative permeability 1000 at the same B, H = 800 A/m and BH/2 = 400 Pa (0.06 psi).
Think of a CG projectile or solenoid plunger moving in a direction to reduce the total magnetic energy of the system (most of which is in air). Same as moving in a direction to increase the inductance of the system.

Let's illustrate with a quantitative example similar to a coilgun:
an electromechanical solenoid with enough current to generate B = 1 tesla in the metal and the air gap.
Say the coil is surrounded by external iron. Give one end a flat pole piece, and the other end a pole piece with hole for a plunger of 1 cm cross-sectional area. The plunger is 1 mm away from touching the flat pole piece & closing the magnetic circuit.
The complete magnetic path length (through plunger in the coil and returning through iron outside the coil) is 10 cm.

How much coil current will that need?
Integrating H around the whole magnetic path:
In the iron, 800 A/m * 0.099 m = 80 A.
In the gap, 800 kA/m * 0.001 m = 800 A.
Total 880 A (ampere-turns) to generate 1 tesla of flux.

We already figured the magnetic pressure at 1T is 400 kPa, for plunger-pulling force of 40 N (9 pounds). If we let the plunger be pulled in all the way, while maintaining B = 1 T (by reducing the current), the mechanical work will be 40 N * 1 mm m = 40 mJ.
Original BH/2 energy was 40 mJ in air + 4 mJ in iron;
final BH/2 energy is just the 4 mJ in iron. Delta BH/2 energy is 40 mJ.

With no air gap, the 1 T flux level needs only 80 ampere-turns. If we maintained the original current, the iron would be saturated -- H=8800 A/m but B not much over 2 T. Plunger pulling force would increase along the stroke, and you could still figure the total work from reduction in total BH/2 energy (air + iron).

BigBad

Tue May 24 2011, 04:39AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Yes, you're right I did have B-H around the wrong way.

The thing is 2T is already saturated, so you won't get 4 times the pressure at that applied field strength. 1.6 T is about the limit for the square law- after that it goes up linearly.

You can look at this different ways. One good way to look at it, is that the individual dipoles in the metal are all lined up at saturation and attracted (and repelled) to the currents in your coils.

Increasing the currents in the coils fairly obviously increases the field linearly.

Navrit Bal

Tue May 24 2011, 11:43AM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Sweet

I've never even heard of magnetic pressure before now. Thanks everyone for clearing up various concepts like the B and H fields, I'm sure that I'm not the only one learning form this thread.

Would it be fair to conclude that saturation of the projectile is one of the major problem with the high current and acceleration coilguns that we all seem to build?

What would the effect of using a superconducting projectile be?

Also, is there any sort of saturation effect via induction, if so what is it?

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