Time before demagnetization? 1

[AgainstOdds]

If two neodymium magnets were held north to south with 1cm separating them, how quickly would they demagnetize under the following conditions:
- Average temperatures (say -10 to 130 degrees F at the
extremes)
- Exposed to air
- high pressures (possibly spiking to as much as 400psi)

Here is a link to the exact magnets I wish to use:

http://www.kjmagnetics.com/proddetail.asp?prod=BX041-N50&cat=168

I realize there is no specific answer to this question, but I would just like to know a ballpark figure. If someone could tell me if I'd be looking at 5 years or 5 days before I see a noticeable decrease in the magnets' magnetic fields that would be great. Thanks.

 

[sreid]

Under those conditions, they would never demagnatize.

 

[EdStainless]

If you had a situation where you would get some demagnetization it should happen fairy fast. I would expect readings taken within minutes or hours to be stable. The biggest issue would be the peak temperature. Every the temp went a little hotter you could loose a little more field.

But in your example the magnets are attracting, so there is not demag field at all.

With lower coercivity grades you can see fields slowly decay over months or years.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

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

 

[AgainstOdds]

Awesome, thank you for your replies. I'm assuming if I used the same magnets in a repelling orientation (N-N or S-S) then I would see a decrease in their magnetic fields over time because each magnet would try to reverse the polarity of the other, correct?

 

[sreid]

All Neo magnets I know of are hard to demagnatize at room temperature. Much easier at higher temperatures. They are harder to demagnatize at low temperatures.

At the temperature ranges you are talking about, I don't think you'd see much demagnafication even if the the same poles were in contact.

A test to do; put two magnets in a glass tube and see how far apart they repel. Then clamp them together for a week. Then remeasure the repelling distance in a glass tube. It will remain the same.

 

[AgainstOdds]

Thanks for your help, sreid. Would it make any difference as far as demagnetization is concerned if the magnets were passing through multiple magnetic fields rapidly, say at a rate of up to 10,000 fields per second? (I apologize for my seemingly insane questions.)

 

[MagMike]

AgainstOdds: Given that this is an engineering discussion board (not purely theoretical), I've been curious as to what your application involves? What are you trying to build?

 

[MJR2]

The changing magnetic fields may produce eddy current heating. I would expect it will. That could increase the temperature to a point where demagnetization will take place. At 10,000Hz it would happen very quickly. Rare earth magnets are metallic.

The permeance of the part you refrence is very low and will experience demagnetization much sooner than the basic properties would suggest. Particularly if subjected to N-N orientation and temperature.



Mike

 

[AgainstOdds]

MJR2: Thank you for your reply. How much heat are we talking about producing? Will it likely be more than could be compensated for by a simply cooling system, or could it be managed if the magnets were only operating at 5-6kHz?

MagMike: I do indeed have a project that I am working on. My questions are not purely theoretical, but due to the nature of my project I would rather not disclose exactly what it is. However, if I am in danger of being removed from the forum because of my questions I may be able to provide some information.

 

[MJR2]

You are talking in in basically theoretical terms and so am I. Without details we cannot do much more. It could be a lot of heat. Consider inductive heating as a reference.

Mike

 

[AgainstOdds]

Let's say 70 of the above listed magnets were attached to the outside of a 11.5" O.D. cylinder in a fixed position, and 70 more of the above listed magnets were attached to the inside of a 12" I.D. cylinder which could be rotated around the inner fixed cylinder. If the outer cylinder were spun at 8000rpm each magnet attached to it would pass through approx. 9300 magnetic fields per second. If it were spun at 4000rpm each magnet would pass through approx. 4700 fields per second. Can someone give me an idea of how much heat would be generated in these two situations, and how quickly? Thank you.

I apologize for not giving adequate details concerning my project.

 

[IRstuff]

Now, you're simply talking about the situation inside a motor, and the fields neither demagnetize nor heat up the magnets sufficiently to demagnetize.

TTFN

FAQ731-376

 

[sreid]

IRSTUFF,

I disagree. In a brushless motor, the stator and rotor fields rotate in synchronism.

I don't know how one would go about simulating this. Do any of the magnetic FEA programs model heating in the magnets?

 

[MJR2]

I think Infolytica's software Magnet has modules that can solve this. I don't have them. One might do a step wise solution watching the torque and infer from that a maximum number for heating. But it's against odds that he can get an accurate five minute answer/suggestion from us.

The field will not be very great given the airgaps. But the frequency is high. Eddy current heating can take place in seconds.

AgainstOdds these details you have now provided are generally adequate. You have told us the magnet and the geometry. Plus the important detail of frequency.

Mike