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# Using second quadrant B/H curve to determine demagnetization

## Using second quadrant B/H curve to determine demagnetization

(OP)
Using the magnet thickness and airgap thickness to construct a load line (Bd/Hd) to determine the operating point on the normal curve, and then adding 1 (Bd/Hd +1) I should be able to have a line parallel to this to intersect where the intrinsic curve drops 10% and read the reverse field required to demagnetize to this point.

What I don’t understand is that if I increase the airgap and go through the same procedure it appears that it would take less reverse field to demagnetize the magnet. What is wrong with my thought process?

Clyde Hancock
Design & analysis of electric motors and generators
www.motorconsultants.com

### RE: Using second quadrant B/H curve to determine demagnetization

Increasing the air gap lowers the operating slope and makes the magnet easier to demagnetize.

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P.E. Metallurgy, consulting work welcomed

### RE: Using second quadrant B/H curve to determine demagnetization

(OP)
That is exactly what I thought however as I was explaining this to a friend and they asked "so if I increase the air gap to 100 times the existing air gap then how would it be easier to demag the magnet". Then he said how would it be easier to demag with an air gap that effectively removes magnet from the reverse field. I'm sure that I'm not explaining myself.

Clyde Hancock
Design & analysis of electric motors and generators
www.motorconsultants.com

### RE: Using second quadrant B/H curve to determine demagnetization

you donot physically "demag" your magnet. refer to "recoil curve" principle.

### RE: Using second quadrant B/H curve to determine demagnetization

(OP)
MagBen,

Sorry for being so "thick". What do you mean when you say "you do not physically demag your magnet"? I have researched "recoil curve" and the answer is not obvious to me. I feel a little embarrassed because I can't explain why a larger air gap makes it easier to demag. It is obvious that it lowers the operating slope and I can see that you are easily below the knee with very little reverse current. I just can't explain to this person why you can't take this to an extreme by having a very large airgap (much larger than the thickness of the magnet) that it is easier to demag the magnet. His comment "so does that mean that if I introduce a reverse field across the room it would be easier to demag the magnet?" leaves me without a response other than you know that's not true.

Clyde Hancock
Design & analysis of electric motors and generators
www.motorconsultants.com

### RE: Using second quadrant B/H curve to determine demagnetization

When you apply an opposing field it pushes the operating point down (and to the left) on the curve.
And then when that field is removed the magnet recoils back parallel and ends up at a lower operating point.
Yes, this can be looked at partial demagnetization.
But where it ends up can be fairly complex to figure out.
Just be assured that it will be at lower strength and lower field.
And if this is lower then you can do less work which weakens the output of your device.

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P.E. Metallurgy, consulting work welcomed

### RE: Using second quadrant B/H curve to determine demagnetization

(OP)
EdStainless,

I understand what you are saying. What I'm struggling with is explaining to a colleague why it is easier to demag with a larger air gap. They argue that if the reverse field is further away from the magnet that it should have to be stronger to affect the magnet.

Clyde Hancock
Design & analysis of electric motors and generators
www.motorconsultants.com

### RE: Using second quadrant B/H curve to determine demagnetization

The magnet only sees the conditions at the operating point.
So when you shift it lower you are choosing to work where the magnet is weaker.
And what makes them think that the applied field is any weaker?
We presume that it will couple very efficiently with your magnetic circuit and apply a very similar demagnetizing effect.

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P.E. Metallurgy, consulting work welcomed

### RE: Using second quadrant B/H curve to determine demagnetization

Note the internal demag for the magnet itself Hd =-4πMs. "-" means direction is opposite to the external field. when air gap is extreme, it is open circle, with a weakest external field the magnet offers.
where/why does your de-magnetized field come from to de-mag/reverse the mag direction of your magnet?!

### RE: Using second quadrant B/H curve to determine demagnetization

(OP)
I should explain that this discussion stems from a paper by James Gollhardt when he was with Allen-Bradley (see attached). If you reference Fig. 13 you'll see what I mean. You can see that the peak armature field is the difference between the load line and the parallel load line. Now if you increase the air gap (leaving the magnet the same length) you can see that the slope leans further to the left. In doing so, the distance for peak armature field is reduced to get to the same point on the curve.

Clyde Hancock
Design & analysis of electric motors and generators
www.motorconsultants.com

### RE: Using second quadrant B/H curve to determine demagnetization

That is one way to think about it, but if the distance is less then you can resist less armature field.
And this paper is concerned with ferrite magnets, so you have specific curve shapes to deal with.

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P.E. Metallurgy, consulting work welcomed

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