Testing magnet characteristics to evaluate new (to me) manufacturers

Clyde38 · Nov 25, 2012

What would be the ideal magnetic characteristics to have an independent test facility check to qualify new suppliers of permanent magnets. I would have them perform a B-H curve test and provide a Hysteresis graph of the sample at the temperatures I'm interested in. Some vendors will provide a representative Hysteresis graph for a single lot of magnets (in small quantities), but I am interested in an unbiased independent test.

I would also have them test the plating of NdFeB magnets to insure that they have a good process. I really don't know how to ask for this other than to slice the material and observe the interface of the layers.

I know that any testing I have done will not necessarily be representative of the repeatability of their product, but I would like to know that they have at least provided the material with the properties that I have ordered.

Any suggestions will be greatly appreciated.

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MagMike · Nov 25, 2012

Obviously you want to be sure the properties match or exceed the published specs. One additional parameter I would recommend checking is the squareness of the BH curve: Hk/Hci. The higher the better.

To check the plating, there are two tests: One is a salt fog test as per ASTM B117. The other is an ASTM scratch/peel test. I don't have the specification number handy now but can dig it up.

Hope this helps.

robertib · Nov 25, 2012

One other factor you will need to check is whether the magnetisation is symmetric across the centre of the magnet. We have had this problem in the past - even with our main supplier.
The way we check for asymmetry is to measure the field in the centre of the magnet, and then at each edge.

EdStainless · Nov 26, 2012

We used to place magnets in a Helmholtz coil and tilt them to see if the field strength increased. Not a good sign.

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Plymouth Tube

Clyde38 · Nov 26, 2012

Thanks MagMike.

robertib, do you use a Hall probe for this test on the surface of the magnet? Should the field be the same across the entire surface (I suspect not)?

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robertib · Nov 26, 2012

We used to use a standard, bought-in gaussmeter, but we use Hall probes now (for ease of repeatability).

The field should be strongest at the centre of the magnet, dropping off towards the edges - but it should drop off in a reasonably uniform manner. The circumstances for using the magnet should determine how uniform the drop-off can be, some applications can tolerate more asymmetry than others.

MagBen · Nov 27, 2012

Robertib,
Agreed the teh center of magnet has strongest field, but I was wondering how you measured the magnetic field strength in the center of a magnet using Hall probes, one cannot insert the probe inside the solid magnet, right? waht i can image is to use a search coil to measure the flux to get Bd (devided by the cross section area) in an open circuit or a specific magnetic circuit.
except for cutting in pieces and testing them individually, it seems really challenging to evalaute the magentic uniformity along the magnetic path for a single magnet.

IRstuff · Nov 27, 2012

I think they mean "center" of the face of the magnet pole

TTFN
faq731-376

robertib · Nov 27, 2012

IRStuff is correct. We just measure on the North face of the magnet - it is sufficient for our applications. Other applications may need to measure more accurately.

MagBen · Nov 28, 2012

believe it or not, for a rectangular magnet, the four corners have the highest field.

ndsram · Dec 14, 2012

Dear Clyde,
Well I'm doing the same as a quality manager. We would check for the BH properties of the magnet and in addition to this, we would also perform "Total Flux Measurement" by Helmholtz Coil and Fluxmeter. This measurement can be correlated with the BH Properties and it is inter dependent (BH properties and Total Flux). In addition to this, Hall probe is used to measure the magnetic surface flux density at the center of the magnet.

For the plating on the surface of the NdFeB magnet, perform pull load and shear load test to conform the planting peel off from the NdFeb magnet surface. This is done by gluing a metal piece and test will be performed.

All the Best
N D Senthil Ram

ndsram · Feb 4, 2013

Dear Clyde,
Well I'm doing the same as a quality manager. We would check for the BH properties of the magnet and in addition to this, we would also perform "Total Flux Measurement" by Helmholtz Coil and Fluxmeter. This measurement can be correlated with the BH Properties and it is inter dependent (BH properties and Total Flux). In addition to this, Hall probe is used to measure the magnetic surface flux density at the center of the magnet.

For the plating on the surface of the NdFeB magnet, perform pull load and shear load test to conform the planting peel off from the NdFeb magnet surface. This is done by gluing a metal piece and test will be performed.

All the Best
N D Senthil Ram

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