Permanent Magnetic Field

[James08]

If you magnetize a ferrous material with solenoid, would the magnetic strength of the material be stablized already right after it being magnetized or would the magnetic strength of the material be fluctuated and then stablized over a period of time?

 

[EdStainless]

If there are no temperature changes, and no applied fields, then it will stablize very quickly, like within seconds.

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Corrosion never sleeps, but it can be managed.

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[James08]

Would the mangetic strength of the ferrous material still be stabilzed fairly fast if you magnetize and then de-magnetized with a video tape eraser serveral times?

 

[MagMike]

Can you please clarify your question? If you are magnetizing and de-magnetizing the material repeatedly you are not talking about a stable system. Or are you describing one cycle: Magnetize once and then demagnetize once?

Are you describing a situation where you want to fully/partially magnetize something and then quickly fully/partially demagnetize it?

Is the video tape eraser demagnetizing the entire volume of material or just a partial volume?

If you have an application in mind, it would be helpful to describe what you are trying to do.

 

[James08]

The ferrous material is located in a housing that consists other metals. When magnetizing the ferrous material, the housing consitings the ferrous material and other metals is put into the magnetic field produced by the solenoid. The caliberation process is to magnetize the ferrous material to have a centain mangetic strength. There are times when the ferrous material is over magnetized and it needs to be de-magnetized. Thhis whole process is repeated until the magnetic strength of the ferrous material is in a certain range. So would the magnetic strength of the ferrous material be stabilized fairly fast after undergoing that caliberation process?

 

[MagMike]

Thanks for the explanation. It sounds like a very tricky assembly you have there.

One key question is: What is the ferrous material that you are magnetizing/stabilizing? I'm guessing it is Alnico.

I've been in similar situations, trying to make an assembly of permanent magnets & steel produce a very specific gauss range. It's always quite a challenge.

Assuming you are dealing with permanent magnets, the magnetic strength of your assembly is not likely to be affected by the speed of your calibration process.

The best way to calibrate/stabilize is to fully magnetize the components and then "knock it down" by either:
1) Applying a weaker, opposing magnetic field or
2) Heating the system to partially thermally demagnetize.

I'm not sure what equipment you have, but if you have enough control on that video tape eraser, I believe you will have an easier time fully magnetizing the part and then knocking it down step by step (i.e. incrementally increasing the power to your video tape eraser).

Hope this helps.

 

[EdStainless]

If you want it the most stable you need to fully satureate before you 'calibrate' it.
You can knock the field level back using applied field or temperature, or you can do it like the microwave guys do, add shunts. That way everything is still fully saturated and most stable. You add small pieces of plain steel (or alloys if you are looking for some temperature effects) to short circut a small amount of the field.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

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[UKpete]

James08, magnetization has some time dependence because of eddy currents in the material being magnetized. What happens is that when a changing magnetizing field is applied to a ferromagnetic material, it will induce eddy currents which (according to Lenz's law) produce a field which opposes the original change in field. As the magnetizing field stabilizes to a steady value, the eddy currents reduce and full magnetization can be achieved.

The practical result is that the ferromagnetic material will magnetize fully at its surface first, and with time the magnetization penetrates to centre. This may take several milli-seconds, according to the electrical conductivity of the material. Laminated materials and non-conductive materials such as soft-ferrites can be magnetized at a much faster rate.

The phenomonon causes problems when magnetizing rare earth magnets. These require very high magnetizing forces which are usually provided by high current pulses in the magnetizing coil (kA magnitude, usually supplied by capacitor discharge). The problem is, because of the eddy current it takes a finite time for the field to penetrate to the centre of the magnet hence the pulse must be of sufficient duration, which requires more capacitors. Time-stepping finite element analysis is used in more demanding applications for magnetizing fixtures.

In your application you say you also have other metals (non ferrous?) around the ferromagnetic material. Unfortunately, these will also have eddy currents in them depending on how conductive they are, and these currents will oppose the original field whilst it is changing in magnetude. This is the principle of electromagnetic screening. In effect the system responds like a low-pass filter, the higher the conductivity of the components in the field, the lower the roll-off frequency.

 

[hacksaw]

how are your controlling the waveshape?

 

[EdStainless]

In pulse mag you need to balance the peak current against the pulse width. We had some coils that worked fine for very small magnets but because of short pulse width it wouldn't saturate lager ones.
You can build a coil with more turns. It will have higher inductance which will spread the peak, and lower it. The increased resistance will also reduce the peak and cause more heating.
Oh, watch your fingers. Half a million Amps can really slam stuff around. I still have fragments of RE magnest imbedded in my finger tips after they exploded during manetization.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

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