B & H Temperature Coefficients

[Clyde38]

I am having difficulty reconciling one of the temperature coefficients for permanent magnet materials. Magnet vendors will supply a 2nd quadrant (sometimes partial 3rd quadrant) B/H curve data for their material at various temperatures. They also provide a table of properties in which they list several parameters including temperature coefficients for Br and HcJ (sometimes listed as ∆Br/Br or ∆Hcj/Hcj) in units of either %/K or %/°C (same thing). If you apply these coefficients to the graph data the Br is pretty equivalent. I can’t reconcile the H quantities and I’m not sure why. I know that some of the curves become non-linear as the temperature increases (decreases for ferrite) so what does this temperature coefficient (HcJ) apply to, or more importantly how can it be used? When modeling magnet materials for motor design within an FEA environment, I typically ignore the non-linearity because I never operate in that area. To easily describe the B/H curve in the simulation a relative permeability and coercivity are used for various temperatures. I have been using the temperature coefficient for Br and calculating the coercivity by using the relative permeability. Shouldn’t I be able to just use the temperature coefficient for the H (at least for the linear portions/temperatures)?

 

[RyreInc]

The temperature coefficient for coercivity is referring to Hcj (or Hci), not Hc; where Hcj is the intrinsic coercivity, vs Hc which is (normal) coercivity. You are correct to calculate the new Hc by taking the temperature-adjusted Br and using permeability as the slope of the normal B-H curve. Since Hcj is shrinking with increased temperatures that non-linear region is working its way up the curve. Example curves

 

[Clyde38]

Hi RyreInc,

Thanks for the reply. I understand that the temperature coefficient for the coercivity is for HcJ/Hci. My problem is understanding why the value does not seem to agree with the plotted values on any of the graphs provided by the manufacturers. I've attached a spread sheet to help describe my question. I've overlaid a calculated graph over a typical B/H curve supplied by a vendor. If you change the temperature in cell "U8" and observe the normal curve as well as the red diamond shaped marker you will see what I mean (I hope). The normal curve calculated from the temperature coefficient of Br agrees with the graph. The red diamond marker that represents the calculated H does not.

 

[RyreInc]

Hmm, I see your issue now, that is indeed very strange... I couldn't get the data to agree using the given temperature coefficient. Calculating the coefficient by looking at the change in Hcj vs. temperature shows a decreasing number ranging from -0.65%/°C to -0.51%/°C.

Temp Coefficient
60 -0.65%
80 -0.63%
100 -0.61%
120 -0.58%
150 -0.55%
180 -0.51%

This may be a question for Arnold (or whoever supplied the datasheet).

 

[Clyde38]

I have not found one datasheet from any vendor that supports this portion of their temperature coefficient data (see attached spread sheet, Ive added a couple of alternate curves from different manufacturers). There must be more to the story. So I guess I will have to ask various suppliers until I get the answer. Thanks for your input.

 

[MagBen]

This is because coefficient data was obtained from 20 to 180C, but obviously, the coefficient of Hci is not constant at different temp ranges. while Br temp coefficient seems to be more of constant. Also, the Br coefficient is much smaller than that for Hci (.5% vs .1%), it is easier to see the Hci difference when the curve scales for both B and H are the same.