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Understanding magnetic saturation.

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Yanom

Wed Dec 19 2012, 02:44AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158

So coilguns are supposed to not saturate the projectile for best efficiency.
here's a B-H curve:
Mag18

So, apparently, if my coil is less than 3000 Amp-Turns per Meter, the projectile will not get saturated. Correct? So even if I leave the projectile in there overnight, with the coil powered on, I'm not going to saturate it? Time is not a factor in oversaturation?

2Spoons

Wed Dec 19 2012, 05:37AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

no, time is not a factor.

klugesmith

Wed Dec 19 2012, 07:11AM

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

Magnetization of material is not an instantaneous process, but the time involved is negligible at the frequencies
of coilguns and transformers. Including datacomm isolation transformers with ferrite cores at > 1 Gb/s.

For electrically conductive materials like solid steel coilgun slugs, changes in magnetization are slightly retarded in time as you move from the surface toward the core. This is a manifestation of eddy current ampere-turns opposing the external ampere-turns. Another word for it is skin effect. Not the same as the BH response of microscopic metal crystals.

Nice chart, Yanom. But I hope the curves are meant to be qualitative and not quantitative.
Problem is, if they are qualitative and you put real units on the axes, then quantitatively some of them are very wrong.

BigBad

Wed Dec 19 2012, 01:15PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Actually, what you said, that you don't want to saturate, is wrong, do want to saturate, you just don't want to go a long, long way past saturation.

And it's just one trade off. If you only have one coil, then running well past saturation is probably optimal, you'll get better speed, it's just that the efficiency plummets; at some point you're spending more on capacitors than you are on coils and drivers.

WaveRider

Wed Jan 09 2013, 09:27AM

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

A few years ago, I studied the influence of saturation and eddy currents on coilgun performance. I include a few links that may be helpful.
Cheers,
Bill

Saturation effects on force
Comprehensive modeling of saturation and eddy currents
Notes on eddy current losses

Barry

Wed Jan 09 2013, 04:47PM

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

You do want to reach saturation, at which point increasing input energy will have decreasing returns. How far you want to go beyond saturation depends on whether your goal is energy efficiency or exit velocity.

Really great articles, Bill, thanks for posting!

Barry
The letter J does not appear anywhere on the periodic table of the elements

Yandersen

Mon Feb 11 2013, 11:42PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Just had some thoughts to share about the saturation.

The highest efficiency coilgun can possibly show is exactly at the saturation point. Here is the explanation...
Imagine some coil of the multistage coilgun. To maximize the efficiency we need to maximize the increase of energy bullet gets from passing that coil, and we need to minimize the heat dissipated while bullet was passing through energized coil.
As we know, below saturation, force is proportional to the square of the coil' current. So while below that, doubling the current leads to increase in force 4 times (so the resulting energy increase is 4 times higher), as well as power dissipation - it is 4 times higher too. From the first glance, efficiency should stay the same then. BUT! Higher force results in higher average speed, so it takes less time for bullet to pass that coil, decreasing the time of heat dissipation. As a result, the amount of energy dissipated will be less than 4 times higher comparing to the not-doubled current. In other words, increase in energy results in higher efficiency.
But when we go over the saturation, doubling the current will just double the force (so as increase in Ekinetic), while power dissipation will be 4 times higher anyway. With this huge disproportion decrease in time of coil' passing will not give sufficient benefit. So over the saturation efficiency lowers with increase of energy.
As the result, the highest efficiency is at the saturation point, making CG' efficiency pretty much material-bound property.

Barry

Tue Feb 12 2013, 02:59AM

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

wrote ...
The highest efficiency coilgun can possibly show is exactly at the saturation point.

I believe the maximum efficiency will occur be near the point of maximum permeability. This is close to initial rolloff into the knee region of the B-H curve, and it sounds like what you're trying to describe.

Here's a general graph to compare permeability (Î¼) as a function of applied H field, taken from Wikipedia: Saturation (magnetic). The maximum permeability is Î¼max, beyond which additional applied energy will have diminishing returns.

1360637348 90 FT148180 Permeability Of Ferromagnet By Zureks

I would be mildly surprised if the maximum efficiency is exactly at the Î¼max point. Because if you drive the coil just a little bit harder, then the firing time is shorter and also the time that energy is lost to heat in resistance is therefore shorter.

Also, as BigBad correctly pointed out, this is just one trade-off in the bigger picture of getting the most of whatever you want to get out of your investment in time, money and effort.

Cheers, Barry
Autocorrect has become my worst enema

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