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Technical Explanations - The Full Story
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Technical Explanations - The Full Story - 08.19.2008, 03:42 AM

How a motor can become demagnetized

Demag happens when the magnetic force applied to the magnet exceeds the coercive strength of the magnet. In other words, if the strength of an external magnetic field (like the field winding of an electric motor) exceeds a certain level, the magnet will be demagnetized (or remagnetized in a different orientation.) So any field of sufficient strength will demag a magnet.

It just so happens that the field strength at which demag happens is also affected by temperature. Look at this picture:

This is for an N44SH magnet (fairly common magnet type.)

If the field strength exceeds the coercive force of the magnet (at any temperature) the magnet will fail. It's just MUCH more sensitive to demag at high temperature (see the sharp downward part of the diagonal curve (at 140C)? That's the demag point. And the higher the temp, the lower the power required to demag.) On the chart, B is the field strength of the magnet itself, and H is the applied field trying to demag the magnet (field from windings in a motor...)

Curie temperature is the temperature where the magnet will fail, just by being at that temperature (think about the atoms moving enough that they start jumping around -- and out of alignment with each other.)

SO -- you can still demag a magnet if the applied field is very very strong, even at low temperatures (for this type of magnet, you would have to apply a field so strong that it crosses the knee on the -B side of the graph, which isn't shown.) And at high temperatures, it takes much less field strength to demag a magnet. This is why BOTH the temperature grade of the magnet, AND the field strength of the magnet are important. The higher the field strength of the magnet, the more resistant it is to demag at lower than max operating temperatures with very strong fields. The higher the temperature grade of the magnet, the more resistant it is to demag at elevated temperatures. IF you operate above the "knee" in the diagonal line on the graph, the demag is temporary, below the "knee" is a permanent demag.

Patrick del Castillo
President, Principle Engineer
Castle Creations

Last edited by BrianG; 08.25.2008 at 10:51 AM.
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