Permanent Magnet Material Specs 1

[dgallup]

I have recently inherited a number of sensor designs, mostly variable reluctance but some Hall effect. They all use a permanent magnet of some variety. I have always worked with soft magnetic materials before. A lot of the drawings use company specific trade names for the material like Magnequench MQ2-E which is obsolete. I want to get away from proprietary specifications.

I have a copy of the MMPA standard No 0100-00 but find it lacking. There are no tolerances on the magnetic properties, they don't even say if they are nominal, typical, minimum or what. Nor does it really say how to properly specify a material. I am interested in international standards such as IEC 60404-8 but have to spend $110 to see if it is really useful. I have seen lots of ISO standards that were not worth the electrons they were downloaded on.

Anybody have any experience with IEC 60404-8? Does it have useful grades and magnetic property limits? If not, what standard do you use to specify permanent magnets?

 

[MagMike]

I'm not familar with the IEC standard you mentioned.

The most effective method I've found is to call out the generic name (NdFeB, SmCo, etc) along with the relevant magnetic properties.

Usually I specify a nominal BHmax, a nominal remanence value, and a minimum intrinsic coercivity. If necessary, I specify a tolerance on the remananece (+/-5%).

Avoid specifying anything but a minimum on the intrinsic coercivity. It's usually harmless if the coercivity is too high, as long as it meets the BHmax & remanence specification.

 

[janas21]

I am a new employee and I don't have so much experience in this matters, but unfortunately the data sheets with the specifications are all half and they try to hind things...that usually are provided to customers and so on.

As concerning the international standards, they are too general, that at the end you can not gain any information from there.

But as I mentioned before I am a new employ and this is my filling up to now. What MagMike has mentioned, is what you should ask always when you want to have the right specific informations about magnets.

 

[EdStainless]

I haven't looked at the MMPA stuff in years, back in the 80's I was writing some of it.
The properties listed in the MMPA and in company lit are almost always nominal.
Typically we worked to minBr, minEP, and minHc. The ranges are really a function of the material and manufacturing process. The better higher performance materials tend to have tighter ranges.
It depends on what your suppliers will commit to.

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[israelkk]

The MMPA 0100-00 standard gives the following in paragraph 9.1 page 5 I think you missed it.

9.1 Performance Testing Approach--Magnetic
Characteristics: The principal characteristics--Br, Hc,
Hci, and (BH)max of a magnetic material are used to
identify a specific subgrade within a material class.
Generally, individual manufacturers can hold unit
magnetic property tolerances of ± 5% for residual flux
density, Br and ± 8% for coercive force, Hc. The range
for the energy product, (BH)max is ± 10%. Intrinsic
coercive force, Hci, is generally specified as minimum
value only.
The

 

[EdStainless]

Does 9.1 fall into a section that addresses a specific material?
These do look like reasonable values.

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Rust never sleeps
Neither should your protection

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[dgallup]

Thanks Israelkk, I guess I did miss that. Ed - Those tolerances appear to be generic for all materials. While I have little doubt that a given manufacture can hold those tolerances, it would seam to prohibit dual or multiple sourcing as each manufacture seams to have somewhat different ranges.

The current spec for the particular magnet purchasing wants to find a new supplier for is better defined than most of them:

Br 10500 G +/-10%
Hc 9000 Oe +/-10%
Hci 11000 OE min
BHmax 26 MGOe (no tolerance or other indication)

Is there any problem with specifying only minimum values for Br, Hc, Hci and BHmax, at least for use in a variable reluctance sensor? It is no problem to find different sources for magnets specified that way. Otherwise, the quoted proprietary blends falls out of the specification someplace. I would still rather use an industry standard but it doesn't seem like suppliers use the MMPA 0100-00 grades.

There will be PPAP and qualification testing of the sensor so I'm confident we will find any problems.

 

[MagMike]

dgallup:
One very important item you didn't list in your latest posting is generic magnet material. Either SmCo or NdFeB can be made to that specification, so it is somewhat important to clarify.

The only other thing I would recommend is not calling out the +/- spec on the Hc value, make it a minimum value.

 

[RMWS]

It is important to look at the specific magnetic requirements in your sensor before specifying anything. In a variable reluctance sensor, where the operating point (load line) range of the magnet is probably pretty well defined, you will want to get a minimum Br which largely will define the flux in your gap, and an Hci that is high enough so the magnet does not become demagnetized at its most severe operating load line and operating temperature extremes. BHmax is probably not as important for you as Br and Hci. The more things you specify, the higher the cost of your material will be as each property must be controlled (or sorted) and verified. If Hci becomes too high you might have trouble magnetizing the material. Higher Hci materials require a higher magnetizing field to saturate. If you are buying the magnets already magnetized this would not be a factor for you - it's your supplier's problem!

Magnetic Instrumentation, Inc.

http://www.maginst.com

 

[MJR2]

We have given up on generic standards. Each vendor is qualified and every material/vendor has their own part number.

Make sure you include a complete BH curve (all relevant temperatures) in your internal spec.

Keep a sample from each vendor.

Inspect every lot.

Mike