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Name: John
Status: student
Grade: 9-12
Location: WI 
Country: N/A
Date: 6/22/2005


Question:
How do you calculate how much current that you can put through a voice coil actuator before it will demagnetize a neodymium magnet structure?


Replies:
John-

It sounds possibly doable depending on the magnet structure. But the structure could be different than I imagine by guessing. It could be all magnet and no iron, or enormous magnets with just enough iron, or little magnets with long tubes of iron.

I was under the impression that neodymium/iron/boron magnets were very difficult to de-magnetize unless hot, compared with, say, alnico magnets. And that the manufacturer had to impose current while hot to cause good magnetization. But I am sure all such things are not absolute.

Most magnet structures have a passive iron pieces, which concentrate roughly 3000 gauss at magnet material 6 units of area broad, into 12,000 gauss across a narrow air-gap only 1 unit of area broad. The magnet material requires something like 5000 gauss to de-magnetize, and in that case the concentrator is working in reverse, requiring over 30,000 gauss at the coil, and the iron can conduct only 20,000 gauss. Any field over that might as well be in air, and it will not be guided straight to the magnets as lower fields would.

I would probably take one brand-new, known-perfect unit, measure its force/current ratio to 1% precision with pulses of a safe low test current at known low (room) temperature, then overdrive it with big pulses until the force/current ratio at the test current decreases by 3-10%, and call that a threshold. Perhaps your unit would still be considered usable. Even that result would not be a hard usage limit. The next reduction by the same percentage would be harder to achieve. That is, if I am right. It might be difficult to do more without burning up the coil. Due to skin-depths, the shortest effective pulse must be at least 0.1 second.

I have the idea that there can be a current level which will temporarily, reversibly overwhelm the magnet material or saturate the nearest iron. Normal magnetism pops right back when you stop applying this current. If so, there is a leveling off in the force-vs current curve. Beyond this current level, there is little additional force generated, so no point in using or allowing higher currents.

If you must calculate, you will need to learn the number of turns in your coil, and the magnetic field in the air-gap, and the effective concentration ratio of the iron parts, a more advanced set of magnetic units (one for coercion, such as ampere-turns, and one for polarization flux), and, sorry, I'm running out of steam....

Jim Swenson



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