Magnet shape with respect to calculating intrinsic flux density 2

[Clyde38]

Excerpt from LakeShore Measuring Permanent Magnet Characteristics with a Fluxmeter and Helmholtz Coil

"Deriving Intrinsic Flux Density
The magnetic moment of interest in permanent magnet testing is generally
defined as the intrinsic flux density per unit volume of magnet. Based on the
magnetic moment measurement and measured magnet volume, intrinsic
flux density, Bdi, can be calculated.
Moment/Volume"

How does the shape of the magnet influence this "Volume", or does it (arc segment, square or rectangular, cylinder, etc.) And if it does, how is this accounted for?


Clyde Hancock
Design & analysis of electric motors and generators

http://www.motorconsultants.com

 

[MagBen]

Helmholtz Coil here serves only a pickup coil. When the magnetized magnet goes through the coil, fluxmeter will collect all electrical signal generated by the change of flux, so the magnetization = the integral of fluxmeter reading /volume. because this is an open circuit, the measured magentization is not Br, but rather the Bd which depends on self-demag factor, which is related to L/D, and this is shape related.
The higher the L/D, the higher the Bd. if you use a hysteresisgraph in which case is a close circuit, L/D is infinite, and Bd=Br, and now you are measuring the intrinsic material properties, nothing to do with shape.

 

[Clyde38]

Thanks MagBen. So no difference between the shapes, right? L/D is length/diameter, correct? In the case of rectangles or arc segments what constitutes the diameter?

Clyde Hancock
Design & analysis of electric motors and generators

http://www.motorconsultants.com

 

[Clyde38]

It appears that from the LakeShore article you can calculate the intrinsic flux density from magnet vendor data. Is this correct?

Clyde Hancock
Design & analysis of electric motors and generators

http://www.motorconsultants.com

 

[EdStainless]

For regular shapes (cylinder or ring mag along axis or rectangles) using a HHC is a very good approach.
If you are working with a high Hc alloy then the Bd will actually be fairly close to Br.
For non-symmetrical shapes (arcs with radial mag) you really need to test in a fixture.
Typically, these would have pole pieces that match the magnet geometry. Then someplace in the return path of the steel there will be a controlled air gap with a Hall Effect probe in it to measure the flux. Changing the air gap changes the operating slope of the magnet.

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P.E. Metallurgy, consulting work welcomed

 

[EdStainless]

1. Published data is an interpretation of reality. Some people will publish curves that are roughly average values while others will publish curves that are excessively optimistic. So your millage may vary.
2. L/D for a rect is easy, convert area to equivalent diameter and divide by L. For an arc it becomes a series of approximations.
Start with the area at the mid-radius.
With any non-round shape the longer and narrower they are the more they will not exactly behave as expected. This is because the self demagnetization at the far corners will be greater than average for the magnet.

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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

and so to clarify, the so called intrinsic B is the material property, has nothing to do with shape, that is why it is called intrinsic. very easily to be confused.

there are three kinds of B-H curves: normal curve, intrinsic curve, operating/load curve. vendor data is normally based on intrinsic curve.

HHC tests Bdi, not Bd. Bd is proportion to L/D. At (BH)m point, Bd is normally much smaller than Br regardless of Hc (I bet Ed Stainless meant Bdi, NOT Bd, is close to Br when Hc is high)

L/D is an approximation itself. if the magnet does not have a constant cross-section, it is almost meaningless to estimate permeance coefficient based on L/D. Note B=flux/S, small area section will have higher B....

 

[Clyde38]

I know that I should know this but I have to admit that I don't. Where does the +1 come from (Bd/Hd +1)? What is the explanation for the +1?

Clyde Hancock
Design & analysis of electric motors and generators

http://www.motorconsultants.com

 

[EdStainless]

You need to go back and look at how Bd, Bdi, Hc, and Hci are all defined.

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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

originally from H = 4πM + B. when magnetized, the magnet itself generates an internal field which is opposite to the external field. in the air H=B.