H value on BH curve

[tails009]

Hi there.

I am a mechanical engineer and am just starting to get into the world of magnetism which is both fascinating and confusing to me. Essentially I'm trying to understand what is happening in the second quadrant in a BH curve with no external field applied.

I have attached a BH curve and applied a load line for a permeance co-eff of 1.0. I believe this tells me that the the flux density of this magnetic material at 20°C is 5.75kG and roughly 4.75kG at 200°C. Am I right so far?

then projecting vertically down I get a value for H. This is what I'm not sure about. I think I understand about the coercive force etc during the magnetisation process but I am interested in the magnets properties after it has been magnetised - i.e. no external field.

Upon reading "Magnet Materials: fundamentals and device applications" by Nicola Spadin (page 128) I am left thinking that this demagnitization of 5.75kOe is due to a quantity of the flux passing back through the magnet rather than the air surrounding it. This makes sense in as much that and infinately long magnet has infinate permeance and therforeB=Br.

This further confuses me however, because I dont know the strength of the magnet? Or is this not the purpose of the curve?

Any help in clearing this misunderstanding up would be much appreciated.

 

[MagMike]

You are mostly correct. The flux density at the center of the magnet with permeance coefficient of 1.0 and at 20°C is 5.75 kG.

The actual magnet output, which is what you would measure if you had a gaussmeter probe, depends on the shape of the part. A rod with a permeance coefficient of 1.0 would have a different output than a rectangular block of permeance coefficient. The shape of a magnet plays an important (if somewhat complicated) role in determining its output.

The curves describe how a given material would behave at different temperatures. One has to go through a secondary calculation to include the shape effects, but this isn't too bad for simple shapes. The curves are also handy if the magnet is immersed in an opposing magnetic field (such as a neighboring permanent magnet). Another important use is to ensure that the designer stays above the knee of the curve at the expected operating temperature.

Hope this helps,
Mike

 

[electricpete]

I dont know the strength of the magnet? Or is this not the purpose of the curve?

It is the purpose of the curve.
Think of the 2nd quadrant of B vs H as 1st quadrant of Flow rate vs Dp for a pump... or current vs voltage for a voltage source. The magnet is a source and it's curves are shown. Intersection of that source curve with the load curve gives the operating point.

=====================================
(2B)+(2B)' ?

 

[tails009]

Thanks for your help guys.

What really is still confusing me is that from the graph, this magnet will have (in the centre of the magnet) a flux density of 5.75kG, but a field strength of -5.75kOe.

It's this negative number that confuses me. Is this a field strength of 5.75kOe with the -ive denoting direction?

If the permeance is increased by changing the geometry of the magnet (stretching it thinner and longer?) This new magnet will have more flux in the centre but less field strength.

The way I picture things in my head is that for an increase in flux density the field strength should increase. In a similar way that if flow rate is increased so does the differential pressure on a flow rate vs DP plot.

 

[EdStainless]

The signs are by convention, just like current flow or polarity.
The only way to use a magnet and get work out is by trying to demagnetize it. Hence the field is called negative.

Where did your blue lines come from? Unless you have altered the magnet it should be on the red curve for room temp. Where on the curve depends on the shape factors.
A long thin magnet will have less self demagnetizing field, a short wide one will have more.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[tails009]

The blue line is the load line where I have assumed the permeance coefficient is 1.0.

The two blue dashed lines are just projections to the axis for the point on the red normal curve of B=5.75, H=5.75 @20°C.

Thanks for your help all. I think I'm slowly getting the idea of it!

 

[electricpete]

Here is a sequence of scenarios that I hope explains the logic for the sign conventions.

In scenario 1 ("magnetization") , assume that our PM material is shorted from end-to-end with a C-shaped infinitely-permeable soft iron piece. Wrap wire around the iron piece and energize to create very large DC excitation H in the defined positive-direction to push the PM far into saturation. The PM is magnetized with B in the same positive direction as applied H. H is positive and B is positive during scenario 1.

Scenario 2 ("PM opposes external negatively-excited circuit") is achieved by starting with scenario 1 and then switching the DC excitation to the opposite direction (negative direction) but only a small value (far less than Hc). Which direction does flux Phi =B*A flow in the PM? Still in the positive direction because domains aligned during the original large positive excitation and the subsequent negative excitation is not enough to realign them. H is negative and B is positive during scenario 2.

Scenario 3 ("PM drives flux thru external reluctance") – How was the iron interacting with the external circuit in the previous scenario? The PM was overcoming the external negative applied H to keep B flowing in the positive direction. It should be obvious that the external circuit in scenario 2 was acting as a load in the magnetic circuit and the PM was acting like a source. So we can replace everything outside the PM with a reluctance (load) that give the same flux density as scenario 2. This gives us the final scenario 3 where the PM is overcoming the H drop of the external reluctance to keep flux flowing in the positive direction. H is negative B is positive. This is the scenario in your attachment.

=====================================
(2B)+(2B)' ?

 

[electricpete]

To recap, the previous scenario started with B in same direction as H during magnetization of the PM (scenario 1) and ended with B and H in opposite direction when the PM is driving the external magnetic circuit (scenario 3).

If we wanted to redefine our conventions such that we ended up with B and H in the same direction during scenario 3, we would have had to defined B and H in opposite directions during scenario 1 (magnetization). That would not be a logical convention because the domains that create B~mu0*M during magnetization align with the external field H (same polarity as H).

=====================================
(2B)+(2B)' ?

 

[tails009]

Thanks for that.

Appreciate all your help!

 

[EdStainless]

The curves are only the second quadrant of the entire picture, however all magnets in the real world with no electromagnetic coils in the circuit will be in the second quadrant. This is the real world.

Since you have temp curves it is worth noting that if you have a magnet and at it max working point it sees a rise in temp, and then you reduce the field load it will recoil to a point dictated by where it was at temp. It will demagnetize further than if it had not been hot.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[electricpete]

Someone somewhere needs to apply a field to magnetize this material initially, and at that time the material will be in the 1st quadrant. Understanding this concept helps us understand why we view the magnet in the 2nd quadrant after the excitation is removed. Also PMs are used in applications where there are other sources of mmf.

=====================================
(2B)+(2B)' ?