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# Re: [femm] Material for pole pieces of Magnet

- To: femm@yahoogroups.com
- Subject: Re: [femm] Material for pole pieces of Magnet
- From: dcm3c@aol.com
- Date: Wed, 29 Aug 2001 00:49:31 EDT

In a message dated 8/27/01 2:06:39 PM Eastern Daylight Time,
c_f_cheuk@yahoo.com writes:

> 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 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 expensive stuff to mild steel. Temperature effect?
> demagnetisation? Can't sleep at night? Am I barking up the wrong or
> the right tree?

I've designed and built this sort of actuator before for motion control
applications. For the reasons that you outlined above (i.e. core is
basically carrying DC flux and there's a big, honking air gap that masks the
core's properties), I have used low carbon steel before (for example 1018).
It's cheap, and the actuators worked just like the were predicted to.
Although materials like Vacoflux-50 (which is a Cobalt Iron--the "50" stands
for 50% cobalt) have a high Curie temperature relative to other types of core
iron, it's hard to imagine that this could be a factor here--the NdFeB magnet
would be the limiting factor as far as the operating temperature of your
machine pretty much regardless of the iron that you pick. In general, it's
best to avoid Vacoflux or Hiperco if you can because of its very high cost
and poor mechanical properties.
Dave.

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