Irreversible Losses

[Clyde38]

The attached picture is typical information supplied by a magnet vendor for a particular material’s property. The published “Operating Temperature” is 150º C. From the graphs provided, is it correct to expect an irreversible loss of about -5% at ?55ºC with the magnet in an “open circuit” condition?

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[MagMike]

It'll depend on the shape of the magnet. Short, stubby magnets will lose more than long, slender ones.

If you post the dimensions (and magnetic orientation), someone can quickly let you know.

Mike

 

[Clyde38]

Mike,
The magnet that I'm referring to is a disc with a 4mm diameter and 1.2mm thickness. It is magnetized through the diameter. How does one determine the susceptibility to demagnetization with respect to size? I have always tried to approximate the permeance coefficient and place this information on the graph of the B/H curve for the material I'm interested in. If the operating slope shows the magnet to be below the knee I know there can be a problem. This magnet has a PC of about .76 (Evershed) and I think that it will subject to demagnetization at about 100º C. On the other hand, the magnet is used in a totally open circuit condition and I’m not sure how this will affect the resistance to demag. The chance of the magnet being subjected to an external field is minimal.


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[MagMike]

Hello Clyde,

The size & orientation are used to calculate the permeance coefficient, which you conveniently provded at .76.

Using this value, it appears that your analysis is correct, the magnet will start to lose some magnetization above 100°C. A 5% reduction seems reasonable.

The open circuit permeance coefficient is 0.76. If you had this magnet on a piece of steel, or other ferromagnetic material, the effective permeance coefficient would increase, and the magnet could tolerate somewhat higher tempertures without demagnetizing. There is an upper limit to this, the best one can do is to double the permeance coefficient.

 

[Clyde38]

Hi Mike,
Thanks for the reply. I made a mistake when calculating the PC. I neglected the orientation (magnetized through the diameter). It turns out the PC is not ~.76. It is really ~3.9. So it appears that the situation is much better than I first thought. Thanks again for your thoughts, and jogging mine.


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