On Sat, 05 Feb 2011 07:02:32 -0800, Winston
wrote:

wrote:
On Fri, 04 Feb 2011 13:44:05 -0800, Winston
wrote:

wrote:

(...)

If you're building from scatch there is no point in usiing
silicon steel laminations. These are only needed to reduce eddy
current loss when using AC excitation.

With pure DC or rectified AC exicitation this is unnecessary -
solid soft iron or mild steel is just as good.

I was thinking in terms of relative permeability.
Steel at ca. 100 isn't quite as nice as silicon steel
at ca. 4000.

http://en.wikipedia.org/wiki/Magnetic_permeability#Values_for_some_common_mater ials


The simplest arrangement is to mill a long slot all the way along
one side of a rectangular bar of mild steel to leave an long U
section.

It can be excited by a single winding round the bottom of the U
or, slightly more efficiently (shorter mean turn length) by a
pair of windings - one on each vertical limb.

I like the double sets of pole pieces as shown in the Magnabend ad.
Lots of flux near the 'bending point' is good. Two concentrations
of flux tends to resist part yaw, too.

Er. Make that three concentrations vs two concentrations.


--Winston


The permeability figures quoted in your reference are for
quenched 0.9% carbon steel. This is not a mild steel but a high
carbon steel - file hard when quenched.

Mild steel is typically 0.1% carbon and magnetically pretty
similar to soft iron.

Jim

Ah! Good catch!

Still, at least a 2.5:1 advantage to soft iron WRT mild steel:

http://www.microwaves101.com/encyclo...cmaterials.cfm

2.5 times better performance for only 20% more money is probably
worth it. See for example McMaster 89175K27 at $169.71 each vs
8910K487 at $142.31 each. I agree that paying ca. $680 for core
material for each prototype would be beyond the budget for most
hobbyists, though!

--Winston -- Like me, for example!


The 2.5:1 permeability difference is only significant at
extremely small air gap and medium flux densities.

With 5"iron length and .005" residual air gap the working
permeability of the iron circuit drops to 877 for silicon iron
and 667 for mild steel.

The above assumes that the iron is working somewhere near its
maximum permeability flux density. Typical electromagnets work at
higher flux densities where the permeability is starting to drop.

Kay and Laby "Physical and Chemical Constants" shows how this
varies for both 3% oriented silicon steel and mild steel.

Excitation Oersteads 10 50 500

Mild steel flux density 14,000 17,000 21,000

Silicon steel flux density 17,800 19,000 20,300

Jim