Magnetics Business & Technology - Summer 2016 - (Page 30)
SPONTANEOUS THOUGHTS
a Column by Dr. Stan Trout
Permanent Magnet Mistakes, Part Two
This is the second column in a series, describing the many types of
mistakes made with permanent magnets. Again, my intent is to help
engineers in the future avoid the mistakes made in the past, and
not to embarrass anyone.
Everyone's experience is different, so please feel free to add your
insights in the comments section below.
The second group of mistakes on my list are:
1. Failing to protect from corrosion
2. Failing to optimize the design to use the least amount
of material
The issue of corrosion tends to come up with NdFeB magnets. Our
friends who make and use ferrite magnets can relax here. Since
their product is already fully oxidized, it seems unlikely that any
further chemical reaction is possible, under normal operating conditions. Occasionally corrosion comes up with samarium cobalt or
alnico, but it is usually not as great a concern.
At Magnetics 2004, I made a presentation about the corrosion of
sintered NdFeB magnets.[1] It recounted some of the history and
cures that people have applied to the subject of corrosion. In the
early days of NdFeB corrosion was a huge problem because people
were not doing their homework on the subject. At first, we were
all somewhat tone deaf about corrosion. However, over time the
compositions of the alloys have gradually changed to reduce the
tendency to oxidize and the coating used are now superior to what
we had before. Today many people use double or triple coatings,
such as nickel with copper, to provide the optimum protection. This
"belt and suspenders" approach is good practice in demanding applications, because it does not take much of a pinhole in a coating
to allow a tiny hydrogen atom from dissociated water to penetrate
the coating and start the corrosion process.
As long as engineers consider corrosion early in the design process
and do the proper evaluating and confirming tests of the coatings,
they are likely to be OK with their project.
Someone designing a magnetic device for the first time has a few
basic choices on how to proceed.
1.
2.
3.
4.
They can find a good existing design and duplicate it.
They can purchase a software package and take time to
learn it.
They can hire a consultant who specializes in magnetic
circuit design.
Or they can simply try a variety of magnets: different materials and different dimensions, until they find something that
works satisfactorily.
Depending on the situation, any of the first three above approaches
may make sense. For something relatively simple, the first approach
may work. If the project is a bit unusual and unique, the third approach may be fine. And finally, if you want to be in the design
business, then you really should buy the software and devote an
employee or two to mastering it; that's the second approach.
But occasionally we run into people who have used the fourth approach. By the time they have found me, they are usually frustrated,
and with good reason. The fundamental truth they have discovered is that there are typically many possible magnetic solutions.
In other words, there isn't just a single unique solution to any magnetic design problem. Solutions found by trial and error often use
substantially more magnet material than is actually necessary to
do the job, with a corresponding increase in magnet cost. Such a
magnetic design is very wasteful. So when raw material prices shoot
up, as they did for rare earths a few years ago, people who used
the "cut and try" approach to design find themselves at a very clear
disadvantage compared to their competitors who have used any of
the other approaches. In addition, often it is too late in the design
cycle to make any changes. Much to their chagrin, they realize that
they have locked themselves into a poor and uncompetitive design.
Our recommendation is to use one of the first three methods described above to design magnetic devices and be sure that your
design is optimized to use the minimum amount of material. Do not
design by trial and error.
As Ronald Reagan said, "trust but verify." We need to be vigilant
about corrosion.
Four down, thirteen to go.
Magnetic design is a difficult process. The fundamental math is
onerous, even for people who are relatively good mathematicians.
Today there are have many excellent software packages to help
us design magnetic circuits. But they cost money, normally involve
a significant learning curve and often require some mathematical
sophistication to obtain the maximum benefit.
[1] www.spontaneousmaterials.com/Papers/CorrosionPresentation.pdf
www.spontaneousmaterials.com/Papers/Corrosion.pdf
About the Author - Dr. Stan Trout has more than 40 years' experience in the permanent magnet and rare earth industries.
Dr. Trout has a B.S. in Physics from Lafayette College and a Ph.D. in Metallurgy and Materials Science from the University of
Pennsylvania. Stan is a contributing columnist for Magnetics Business & Technology magazine. Spontaneous Materials, his
consultancy, provides practical solutions in magnetic materials, the rare earths, technical training and technical writing. He
can be reached at strout@ieee.org.
30
Magnetics Business & Technology * Summer 2016
www.MagneticsMagazine.com
http://www.spontaneousmaterials.com/Papers/CorrosionPresentation.pdf
http://www.spontaneousmaterials.com/Papers/Corrosion.pdf
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Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2016
Editor's Choice
Extending the Limits of the Sm2Co17 System
Advances in Manufacture of Low or No Heavy Rare Earths NdFeB magnets
Research & Development
New Measurement Technique Characterizes Permanent Magnets
Electromagnetics
Industry News
The Ambition to Get Highest Flux Densities,Is It Always Reasonable
Marketplace / Advertising Index
Spontaneous Thoughts: Permanent Magnet Mistakes, Part 2
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