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Time response of: ferrous slug B-field vs coil barrel H-field

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LAN_403

Signification

Sun Apr 05 2015, 04:20PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I have been wondering about something for a very long time now an have had no luck in finding an answer...
If a ferromagnetic slug (initially UN-magnitised) is placed into an external magnetic field (the 'H' field) that is strong enough to -almost- saturate it, how long does it take the magnetic domains to all get aligned to the extent corresponding to that external H field? Is this 'alignment' process instantaneous or is there some sort of time constant to consider here? If there is some sort of significant delay--I would assume that the delay will shorten as a function of the H-field strength. In other words, the slug's internal (B) field would be reached more quickly when the H field is stronger. Perhaps VERY fast if the H field is MUCH stronger than the value of the saturated B field of the ferrous slug.

Here is an idea that I hope some readers will find useful: I plan on attempting to estimate the strength of the 'saturating' H strength of any particular slug since the value, ur, is too many times unspecified--surprisingly, even for metal rods from McMaster C. I would simply 'tie' the slug inside my coil (with a small string for example) opposing the magnetic pull, and measure the force applied to the slug as the coil current is increased to the point where the force on the ferrous slug is no longer significantly amplified by the huge ur value of the slug metal, assuming I can get a strong enough constant field. Otherwise, I will consider a smaller slug, assuming the magnetic pull force is proportional to the slug volume in a uniform field--is it???? Also, other parameters can be derived in this manner --ie, the slug position, as well as the sensor position, can be varied to compare field values 'with and without' the slug present, to correct for errors such as those caused by the 'slug presence'. The setup would be very simple, consisting of a string holding the slug in any place in the coil 'barrel' while a simple scale measures the pull vs current. The H field can also be measured by a simple tiny Hall sensor held at a desired position in the coil.

An interesting note: during an MIT lecture concerning magnetic saturation, a famous physicist (anyone heard of MIT's Walter Lewin) stated that there are three 'secret' things that he is not allowed to tell, one being how to measure the (this) field strength (B) INSIDE a metal. FYI: one of the other 'secrets' (during a lecture on fluid dynamics) was stated to be the pressure/depth our modern submarines could withstand/obtain. I don't know the third subject.

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BUT--my number one goal is to know about that time response of a neutral ferromagnetic slug's internal field to a quickly applied external field.
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BigBad

Mon Apr 06 2015, 01:58AM

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

Signification wrote ...

I have been wondering about something for a very long time now an have had no luck in finding an answer...
If a ferromagnetic slug (initially UN-magnitised) is placed into an external magnetic field (the 'H' field) that is strong enough to -almost- saturate it, how long does it take the magnetic domains to all get aligned to the extent corresponding to that external H field? Is this 'alignment' process instantaneous or is there some sort of time constant to consider here?

It depends on the projectile; and the magic words are 'skin depth'.

If the field has been present for 1ms, then the dominant frequency is about 1/0.001 is about 1000hz (give or take a factor of 2 PI) and skin depth will form according to that.

If the projectile is made of powder core or ferrite, then the skin depth will be (unless it's ridiculously, enormously thick) the whole projectile, if it's solid iron, only the surface will saturate for quite a few milliseconds.

DerAlbi

Mon Apr 06 2015, 10:09AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

..what you are looking for is the complex permeability.
Its the same like the complex permittivity. It can tell you how fast molecules can respond to a changing electric field (like in a microwave) and where the materials eigenfriequency is and so on.

BigBad is not quite correct. He is right, that skineffect and stuff changes the geometry response due to eddy-currents and so on but not the materials reponse.
The propagation and magnetisation time and how the H-Field is really linked to the B-Field is described by the complex permeability. There is a magic value named "loss tangent" to every material (exists for dielectric loss and magnetic loss) and the loss tangent does change with frequency

if you go into this.. your Âµr shall be considered as Âµr = Âµr' + i*Âµr'' from now on.

But since this seems to be projectile-related: yes there is a theoretical limit how fast a projectile can get before coils become more and more useless around it because the interaction time does not allow for magnetisation anymore... but this is afaik far into the super-sonic.. which is quite far away for us hobbyists. I know from ferrite transformer cores that the complex permeability is given in the datasheet.. (goohhhogle for "ferroxcube datasheet").. and if you really get into this stuff and find data for iron... and maybe let us know what timeconstant it has... i never thought this is a practical relevant value.. but one better know

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