magnetization

[ishkabidle2]

Apparently magnetization methods are fairly secret since I never see any specifics in the books I've been reading. Can anyone tell me if a magnet material has to be magnetized all at once, or can it be done cumulatively?

To put it in perspective, a 2x meter magnet was made for some particular project that was used as an example in a book, and I wonder if they had to make equipment to magnetize the entire length at once, or could they make a small unit that would work its way along the length without significant problems.

 

[MagMike]

It depends.

It depends on the type of material being magnetized. It's very easy to magnetize Alnico and Hard Ferrite material. SmCo and NdFeB take much more energy to magnetize.

It also depends on the size of the part. The poles of an electromagnet or the inside diameter of an air core solenoid will limit the size that can be magnetized during one pulse. If the final assembly is very large, then usually it is divided into smaller chunks, magnetized and assembled while magnetized.

It also depends on how much energy your magnetizing equipment can produce. Alnico and Hard Ferrite can usually be fully magnetized with only a few kiloJoules of energy. SmCo and NdFeB usually require much more (15 kJ and up).

It also depends on the magnetic pattern in the assembly. You didn't mention if the 2x meter magnet was all magnetized in the same direction, or if it contain alternating poles.

A 2x meter large magnet was probably made up of smaller chunks that were magnetized and then assembled while magnetized.

 

[ishkabidle2]

I'll try to find the reference again, but my impression was that it was a single pole-pair neo. No mention was made of assembling pieces to build it. It seems like my question wasn't clear, so I'll try again.

For reference sake, consider a single neo magnet 15 ft x 2 ft x 1 ft. Desired magnetization producing a single NS pole pair for the entire magnet, in the 2 ft thickness direction. Can you magnetize the first foot of the 15 foot length, move the magnetizing assembly along the length and magnetize the next foot, etc? Will this produce a uniform field strength or a sinusoidal set of fields? What if the magnetizing field is not moved in discrete units, but kept on and simply moved along the 15 foot length at an appropriate speed?

 

[MJR2]

It is quite unlikely that you will be able to magnetize a neo circuit as you describe. You will probably need to build it from magnetized blocks.

This will be an extremely dangerous magnet to build.

The 2 meter wide magnets you describe can if built with ferrite be charged in steps or continuously depending on the charger available. The poles would be charged through one at a time. The magnetically charged poles then assembled.



Mike

 

[MagMike]

A single Neo magnet of that size would have to be constructed from smaller pieces that are magnetized individually and then assembled to form that size (while magnetized). It wouldn't be easy, but it can be done.

The magnetizing fields necessary to magnetize NdFeB generally require air core solenoids connected to a high energy capacitor bank. The part to be magnetized is positioned at the center of the bore during magnetization. The part has to be inside the coil.

 

[ishkabidle2]

The specific difficulties mentioned are why it would be advantageous (if possible) to build a solenoid of sufficient diameter and relatively short length, and progressively or iteratively magnetize such a large magnet with a matching graduated power demand.

I don't need this info for an existing project, I'm simply trying to define existing boundaries for myself. With that in mind I'm more interested in general possibilities for this category of situation. Is the size the only limiting factor? If you wanted to run a solenoid with a very short length (field producing length) along, for example, a 2" x 1" x 1" n38, does it HAVE to be done all at once?

 

[MJR2]

2 inch x 1 inch x 1 inch neo magnets are charged all at once. Neo magnets of 15 feet x 2 feet x 1 foot cannot be charged in place.

You need to generate fields in excess of 30,000 gauss to charge most neo. You need to be careful about generating eddy currents in the magnet as you charge it. You need to control the orientation of the flux lines in the magnet. Think about how difficult that will be to achieve.

Mike

 

[ishkabidle2]

MJR2, you say that the small n38 is charged all at once. That is not what I asked for. I asked if it is POSSIBLE to do it progressively or iteratively. I want to understand what can and cannot be done, not how difficult it would be, not what already is done.

As for the large magnet, is there anything to show if the eddy currents could be kept below a problem-causing level if the progressive magnetization is slow enough, or the flux lines kept fairly homogeneous with iterative magnetization?

 

[EdStainless]

No, you can't do it a section at a time. You will be creating fields in adjacent portions that demagnetize them.

Aside from the limitations on the physical size of a magnet that can be made in a single piece, the problems of controlling alignment and keeping it from self destructing during magnetization (from heat and or internal forces) are not trivial.
The largest rare earth magnets that I have seen that had reasonable properties were about the size of bricks. They were pressed (iso) and sintered. and then ground to size. Then they were magnetized and assembled.

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