1.5T magnet 4

[magnetonico]

Hello:
I am considering to build a 1.5T magnet for my lab but, I would like to avoid dealing with a power supply & wiring considerations. I found very interesting the discussions posted on threads 340-132216 and 133735. However, I wonder which are the reasons for not using magnetic circuits based on permanent magnets (rare earths, alnico, etc.). The question I have is how difficult and/or expensive may be to reach 1.5T in a 25mm gap using this approach instead the classical electromagnet design. I will be really thankful in getting some feedback on this. Thank you very much in advance for any comment!.

 

[MagMike]

Hello magnetonico,

Obviously the primary consideration is cost. Secondary considerations include:
1) Active volume (you mentioned 25mm gap, but across how large an area?)
2) Field uniformity
3) Ability to adjust/reverse the field (difficult but not impossible with a permanent magnet dipole)

Your magnetic field requirement is pretty challenging for a permanent magnet dipole. I've been involved in a bunch over the years. Just a few months ago we built and shipped one that produced 0.95 Tesla across a 35mm gap. About 8 years ago we made one that produced 1 Tesla across a 50mm gap.

Achieving 1.5 Tesla across 25mm gap in a permanent magnet would be challenging, but possible. A lot depends on any additional requirements.

 

[MJR2]

There is also a safety factor to consider. An electromagnet can be shut off.

What are the other dimensions of your air gap?

What kind of open access do you need to the air gap.

It's relitively easy to obtain a 1.5T field with electromagnets.

It can also be done with PM's as MagMike says.

 

[magnetonico]

Hello MagMike and MJR2:
many thanks for your answer!. The diameter of polar faces should be at least 5" in order to reach a reasonable uniformity within a volume space of about 3ml. There is no need to reverse the field. Open access is not critical since a 7" lenght and about 1" probe will be introduced in the field. I know that, after some work, should be relatively easy to get 1.5T with an electromagnet. However, the main problem I see is that a good (I mean stable enough) power supply has an additional cost and a considerable amount of work (and time) to reach success. Someone told me that 1.5T was possible using permanent magnets, but I´m affraid that this person is minimizing the problem. He mentioned a 40kg magnet weight. Would this be possible? (even against difficulties).

 

[Clyde38]

Magnetonico,
Search the web for information about "Halbach" effect.

This will get you were you need to be.

 

[prex]

As 1.5 T is presently an upper limit for B[sub]r[/sub] of good quality rare earth magnets, I can't see how a field of 1.5 T could be achieved in a gap of 25 mm, as that field would at most be obtained at zero gap (and of course no magnet arrangement, including Halbach arrays, may outperform a closed circuit).

prex

http://www.xcalcs.com

Online tools for structural design

 

[MagMike]

Hello prex,

Sorry to disagree with you, but it most certainly is possible to build assemblies that exceed the Br of the material. As long as the coercivity of the magnet is sufficient, superposition can be used to produce assemblies where the field in the gap is larger than Br. Most often these assemblies are made with SmCo or high-coercivity NdFeB.

Granted they are not easy to assemble, and the gap length is rather small, but they have been made. I'll dig up some references and post a little later.

 

[MagMike]

Found the link I was thinking of. A research group in Japan was able to produce >4 Tesla:

http://www.nirs.go.jp/report/nene/h13/01/08.htm

 

[magnetonico]

Thank you Guys!. The discussion is very helpful (specialy due to my inexperience in the field). I still wonder about the weight. We will need to move the magnet from one application to other over several meters. This is also a reason to look for a permanent-magnet-based solution and not going there and here with a power supply. As far as I know, rare earths should be favourable also in this sense (at least compared to Alnico, even when probably alnico is not applicable in this case). Is this OK?. Which should be better or easier to handle?, SmCo or NdFeB?.

 

[MJR2]

Permanent magnet design is possible. As to using samarium or neo magnets that washes out in the econmics of the design. One uses whatever achieves the goal at least cost. However given your desire to have a pole face of 5 inches diameter and 1 inch air gap my first 'guess' is that 40kg is not possible.

However I understand your request is for a 1 inch (25mm) air gap and round pole. With fairly uniform field. The uniformity of field is important to describe. It could cost you plenty for a uniform field you don't need.

What would be the duty cycle for the magnet. A smaller electromagnet could be made if only sort duration occassional use is necessary.

I must emphasis the safety issue with a powerful PM.

Coil weight might be 200 pounds alone plus the steel circuit. Designing the electromagnet is a bit quicker than the PM. 2000 watts of power.

 

[prex]

MagMike,
I suppose that magnetonico is looking for a real world solution, not a demonstration project for researchers.
Of course you can get more than B[sub]r[/sub], the way is to concentrate the flux by means of pole expansions. However this is not only difficult because of inevitable losses, but the new limit is the saturation of iron somewhere below 2 T. As you can see in the link you provided, the iron was cooled down to -50 °C to achieve the 4 T; also compare the mass of the assembly to the tiny gap obtained.
All this is certainly not what magnetonico is looking for. My advice to him is abandon the idea of PM, if he really needs 1.5 T, and turn back to electromagnets.

prex

http://www.xcalcs.com

Online tools for structural design

 

[MagMike]

Hello Prex,

My company has made permanent magnet dipoles where the flux in the gap exceeds the Br of the material. We are not the only company to make & sell them either. I posted the link to the Japanese article because I did not want to violate the rules of EngTips regarding promoting & selling my company.

We don't make them very often but they do produce 2+ Tesla, at room temperature. Granted, the air gap was only 1 millimeter, but the unit was small enough to hold in your hand

 

[magnetonico]

Well, at first I would like to thank you all for your valuable comments. It is clear now to me that:
1-An electromagnet should be easier to handle. Since duty cycle is high, coils disipation will have to be considered in detail. However, the counterpart is the the need of a high stable power supply, weight and system complexity.
2-PM desing is possible. But, I will have to check the the possibility of dealing with limiting conditions for field uniformity and air-gap. Since I can manage to reduce the proble this may be possible.
Let´s see.....
Thank you all once again!.

 

[EdStainless]

How important is over all size? A large elctromagnet reduces many of the design limitations. As long as you get the required NxA it will work. I would rather build with fewer turns and more amps. I have built some rather large units over the years.

http://www.dynapower.com/

I have used 10,000A dyanpower's without any problems.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[magnetonico]

In fact the original idea was to deal with low weight, no power supply (or a small one) and a resonable gap/field uniformity for the applications in mind. Nice problem!.

 

[sreid]

I "Scoped" the magnet size required.

Neo magnet, Br=1.2 Tesla

Air gap 1 inch

Tapered iron pole pieces, 5 in. dia to 8.2 in dia.

Magnet dia. 8.25 inches (area ratio 2.7:1)

Magnet length 2.7 inches

This operates the magnet at a PC of 1 (Maximum MgOe)

The average field in the gap = 1.6 Tesla

 

[magnetonico]

Thank you sreid!. I will check ....I need to learn and convince myself before starting doing. But this forum was of an extremely valuable help. Thanks again!.

 

[MJR2]

I had a little spare time so I ran sreid's design through Infolytica's Magnet FEA software.

The length of the tapered poles were 2 inches each side of the 1 inch airgap. In addition I used a C-frame circuit of 3 inches thick and 10 inches wide. I tried 2 inches but it saturated. The weight of the 1018 steel used in the circuit is 350 lbs. No provision was made for the force pulling the C-frame together of over 700 lbs with N40M or 840 lbs with N50M.

The field was very uniform over about a 3 inch diameter circle in the center of the 5 inch diameter pole nose. With N50M it drops to about 9700 gauss center of gap on the 5 inch circumference.

Field on centerline, center of gap will be
N40M = 10092 gauss Br~1.25T
N50M = 11100 gauss Br~1.4T

Field on the axial centerline varies little from pole face to pole face.

I must emphasize once again the danger of working with this circuit. You can and people have lost fingers and crushed hands with permanent magnet stacks of much less strength than this. I don't think I would want to work with this. I know my shop guys would not be pleased given the task. And they know what they are doing. Electromagnetic is not all that complicated considering the dangers involved with PM's.

Merry Christmas and a healthy and prosperous New Year to all.

 

[sreid]

The cautions about continous high magnetic fields are real. One place that has continuous high magnetic fields is the superconducting coils of a medical MRI imager. There are many warmings given yet there are still instances of flying scaples and needles. There was one known incident of a flying oxygen cylinder and there was once a fork lift that got sucked in.

But the dangers of high magnetic fields are not limited to using Rare Earth Magnets. Neo magnets have become cheap, no power is used and no heat is generated. They are not easily demagnitized.

Large magnets are assembled from smaller magnets. As long as fixturing and/or keepering is used during the assembly, the dangers are minimal.

 

[Skogsgurra]

I do not understand this concern that a DC power supply should add to the "complexity" of the device. I think the opposite. I have been using rare earth magnets to deflect electron beams and I have used electro magnets. The gap was a little more than 25 mm and the length was around 300 mm (end of a linear accelerator horn).

The PM solution was hell to work with. The DC magnet version was smooth and easy to handle. Mind you, this is personal experience - not the universal truth.

Gunnar Englund

http://www.gke.org