Material for pole pieces of Magnet

["C F Cheuk" <c_f_cheuk@xxxxxxxxx>]

Hi All,

I am in the process of designing a voice coil actuator (re-inventing 
the wheel really). I have always been advised to use advanced 
magnetic material such as Vacoflux 50 for the outer pole piece . The 
magnet I am using is a NdFeB with Hc of 1000000 A/m. The load line is 
above the knee to avoid temperature de-mag effect. The air gap need 
to be of the order of 3 to 4mm to accommodate the coil.

I have been paying £6000/m for that polepiece material which makes 
the actuator financially unattractive. I use Dave's program to 
simulate the magnet/air gap configuration and I evaluate the integral 
of H along flux lines with the following results:

NI drop in inner polepiece (DTD iron) = 99 A
NI drop in outer polepiece (the expansive stuff) = 10 A
NI drop in air gap = 1210 A

What this is telling me is that the B in the air gap is controlled by 
the air gap itself and the polepiece materials have very little 
effect!!
I then change the Vacoflux 50 in the model with 430 Stainless which 
has much worse magnetic properties. Dave's program is great in this 
aspect. It is so easy to change material properties. The resultant B 
in the air gap has scarcely changed.

The conclusion I try to draw is that as long as the polepiece 
material does not exceed its satuation B, the choice of material is 
immaterial! So what if the B in the polepiece is above the knee. It 
does not make any sufficient change in the gap B to justify the high 
cost.

This agrees with most text book when they talk about volume of magnet 
etc as the iron loss is insignificant when compared with the air gap 
loss. But this must be wrong because it contradicts with my guru in 
magnetic (namely the salesman of the expansive stuff). 

I think I should make a polepiece out of mild steel and test out the 
hypothesis. 

Can anyone please enlighten me. What should I watch out for if I 
change the expansive stuff to mild steel. Temperature effect? 
demagnetisation? Can't sleep at night? Am I barking up the wrong or 
the right tree?



C F Cheuk
England