Pemanent Magnets - a bit confused about BH curves
Pemanent Magnets - a bit confused about BH curves
(OP)
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.
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.





RE: Pemanent Magnets - a bit confused about BH curves
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
RE: Pemanent Magnets - a bit confused about BH curves
RE: Pemanent Magnets - a bit confused about BH curves
Mike
RE: Pemanent Magnets - a bit confused about BH curves
You get useful work out of a magnet by trying to demagnetize it.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Pemanent Magnets - a bit confused about BH curves
I really would not have to worry about knees or saturation unless there were other magnetic sources in the circuit, correct?
RE: Pemanent Magnets - a bit confused about BH curves
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.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Pemanent Magnets - a bit confused about BH curves
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
RE: Pemanent Magnets - a bit confused about BH curves
RE: Pemanent Magnets - a bit confused about BH curves
RE: Pemanent Magnets - a bit confused about BH curves
IsraelKK: Interesting approach.
-Mike
RE: Pemanent Magnets - a bit confused about BH curves
This is the exact approach that Dr. David Meeker uses in his excellent free FEA program FEMM.
RE: Pemanent Magnets - a bit confused about BH curves
RE: Pemanent Magnets - a bit confused about BH curves
RE: Pemanent Magnets - a bit confused about BH curves
RE: Pemanent Magnets - a bit confused about BH curves
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).
RE: Pemanent Magnets - a bit confused about BH curves
RE: Pemanent Magnets - a bit confused about BH curves