Pemanent Magnets - a bit confused about BH curves 4

[BobM3]

I've got a nice brochure from Magnet Applications that talks about making permanent magnets. I need some clarification about how they work in a magnetic circuit. The brochure shows the magnetization curve and states that permanent magnets operate in the 2nd quadrant. I see there is a B(residual) and an H(coersive) that identify where the curve intersects the axes in the second quadrant. I don't believe that curve defines the operating line in a magnetic circuit though, correct?

I believe the H for the "demagnetizing" curve is the H produced by an external magnetizing force in the circuit, correct? In other words, a permanent magnet operating in a magnetic circuit with no other magnetizing forces would have its own BH relationship in the circuit that does not relate to the "demagnetizing" curve? Is that BH relationship (2nd quadrant) readily available from magnet manufactures? I see Br, Hc and Hci listed for various materials but I think I would need more info to know the BH relationship for the magnet in my circuit.

 

[MagMike]

It's a tough thing to describe in a text-based forum like this. However, I'll make an attempt.

A permanent magnet operating in a magnetic circuit has what is called a permeance coefficient, which is defined by the geometry (shape) of the magnet. While there are different ways of calculating the permeance coefficient, it's safe to say a long, slender magnet will have a higher permeance coefficient than a short, stubby one.

The permeance coefficient is used to draw a line that starts at (0,0) and has a slope (in cgs units) equal to that permeance coefficient. Most 2nd quadrant BH curves have numbers along the left and top side of the graphs, these are the permeance coefficient and are there as an aid in drawing the line. The intercept of the line you just drew with the BH curve defines the operating point of that magnet shape. This will tell you the magnetic flux at the center of the magnet.

I hope I didn't muddle things up for you. If so, I'm sure someone with more eloquence than me will chime in.

Mike

 

[BobM3]

That makes sense Mike. The brochure I have shows a line constructed in the second quadrant and calls it the "load line". It is not explained how it is drawn. One point intercepts 0,0 and the other point intercepts the BH demag curve at a point that is refered to as BH(max). Does using the slope of the permeance coefficient always intercept the BH demag curve at the BH(max) point? That point, BH(max), is also given in the brochure for each magent material.

 

[MagMike]

The permeance coefficient does not always intercept the BH demag curve at the BH(max) point. The line you are referring to shows where the maximimum efficiency occurs, but only if you have the luxury of designing to that permeance coefficient. In actual practice, size constrants or other non-magnetic parameters prevent one from working at that point. Most of the time, one is trying to engineer a shape with a permeance coefficient high enough so the line stays above the knee of the BH curve at all times. Note how the knee generally becomes more pronounced as the temperature increases.

Mike

 

[EdStainless]

In design you usually want to work above the knee so that unforeseen events don't cause serious degradation of your field. If there is an electromagnet in the circuit that creates a field in opposition to the magnet then that will lower the load line.
You get useful work out of a magnet by trying to demagnetize it.

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Plymouth Tube

 

[BobM3]

o.k., I'm a bit confuse about a "knee" in the second quadrant. Is there one? If there are no other magnetic sources in my circuit then I believe the maximum B I could obtain would be Br (magnetic short circuit) and my load line would be nearly vertical. If my circuit was almost magnetically "open" I would be operating on a nearly horizontal line and the maximum H would be Hc.

I really would not have to worry about knees or saturation unless there were other magnetic sources in the circuit, correct?

 

[EdStainless]

Your first statement is true.

Sorry, left out a detail, it depends on the grade. Alnico, some ferrites, and some high Br rare earth grades have a knee in the curve.
But in every permanent magnet application there is a fluctuation of the field, that is where the work comes in. You need to analyze at the worst case.

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Plymouth Tube

 

[MagMike]

BobM3: The knees are in the BH curves, not the load lines. They are very pronounced in Alnico materials as well as fully sintered NdFeB at elevated temperature. Many SmCo and hard ferrite (ceramic) materials do not have them.

When I mentioned that you want to operate above the knee of the curve, I'm referring to the intercept of the load line with the BH curve. That intercept should stay above the knee on the BH curve.

As EdStainless said, it's important to analyize at the worst case. If they load line intercepts the BH curve below the knee, the magnet will lose a portion of its magnetism. That lost magnetism can usually be regained by remagnetization, but it's best to avoid it in the first place.

Mike

 

[BobM3]

I think I know why I'm having a tough time with this. I'm use to looking at magnetization curves of electrical steels where there's not much going on in the 2nd quadrant. Permanent magnet materials have the wide loops not the narrow loops. So I think I've got it now. Does the sketch I've linked to make sense?

 

[israelkk]

The trick to use a permanent magnet as though it is a magneto motive force similar to an iron inside the coil amper-turns is to put the axes in the Hc point. Thereby moving the B-H curve of the magnet to the first quadrant. This way you look at the magnet as though is is a coil with a current looped around an iron pole (which is causing the B flux density).

 

[MagMike]

BobM3: The sketch makes perfect sense, that is how it works.

IsraelKK: Interesting approach.

-Mike

 

[israelkk]

MagMike

This is the exact approach that Dr. David Meeker uses in his excellent free FEA program FEMM.

 

[BobM3]

Thanks guys - I think I got it now.

 

[Clyde38]

I realize it's rather late to comment, but I've looked at the Scan.PDF that BobM3 provided. I'm not sure that there is a good understanding of what was ment. I've attached what I hope is a more clear BH Curve explanation.

[link

http://www.linkedin.com/in/clydehancock

]

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[BobM3]

That's a nice sketch Clyde. The lower of the two figures is how I have come to understand (with everyone's help) how a permanent magnet works in a circuit without any other magnetic sources.

 

[Clyde38]

BobM3,
What I was trying to point out is that in your sketch, you have the "Acceptable" load lines drawn below the knee (low permeance coefficients). In order to minimize the chance of demagnetization, the load lines (operating point) should be above the knee (higher permeance coefficients).

[link

http://www.linkedin.com/in/clydehancock

]

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[BobM3]

Oh, you're right! Thanks for pointing that out.

 

[Stingwheels]

The equation n x p = 120 x f, says that the number of poles to increase the speed decreases, thus, to have an external rotor motor with 12 teeth and 16 poles, the division tends to have no cogging, but the number of poles is very large, so I think that the number of poles should be 8-pole, for a stator with 100 mm in diameter, this calculation would be more correct? Linkedin