Re: [FEMM] alnico5 versus ceramic5 ?

[David Meeker <dmeeker@xxxxxxxx>]

Bengt Dahlberg wrote:

> Hello Keith.
> Thanks for your answer but it leaves me a little bit pussled. The 
> thing is table 2.1 at this webpage 
> http://www.magnetsales.com/Design/DesignG.htm . Looking at that tells 
> me that "alnico5" is almost twice as strong as the  "ceramic8", am I 
> reading this table the wrong way or what. Where I'm coming from 
> (speakerbuilding) alnico is way better than ferrit so this a little 
> bit confusing.
>  
> Best regards. Bengt Dahlberg   Sweden

Energy product isn't the whole story--the resulting flux density depends 
on the magnet's characteristics and the circuit that you are driving.  
The way that people classically think about this is in terms the "load 
line".  There's a nice explanation of load lines at:
http://www.magnetweb.com/Sect4A.htm
The load line is essentially mapping the reluctance of the device 
external to the magnet onto the second-quadrant demagnetization curve 
such that the operating point of the magnet can be obtained via 
graphical means.

An interesting exercise is to plot the demagnetization curves of Alnico 
5 and Ceramic 5 on the same graph, like in the attachment.  If the load 
is relatively low reluctance, the load line is very steep, and the 
alinco produces a high flux density.  If the load has a relatively high 
reluctance, the load line is shallow, and the ceramic produces a higher 
flux density than the alnico.  In general, you have to design your 
magnetic device with the type and grade of magnetic material in mind, so 
that the magnet is presented with a load that is compatible with the 
magnet's properties and dimensions.

Dave.