On Sat, 12 Feb 2011 13:52:48 -0800, "Artemus"
wrote:


"Bob Engelhardt" wrote in message
...
I just made a 2-MOTE (MOT electromagnet) brake as an evaluation of the idea:

--------

I made it small to keep the time, effort, and materials small. But it
has an 8-1/2" capacity, which would be enough for a lot of the stuff I
do (e.g. electronics project boxes).

Unfortunately the results are very disappointing. I ran the MOTE's on
250v (AC into the bridge) and could not quite finish the bend on .040
(about 20 ga) steel. 18ga (.050) started, but only got to about 45
degrees before the clamping bar started sliding back. .030 bent very
nicely and although too light for a lot of stuff, boxes in that guage
would be useful.

So I have definitely ruled out MOTE's for the 24 inch brake that I'd
like to make. I'll be thinking about what it would take to wind the
coil for that. I expect that it would be painful, but the challenge
would be interesting.

Bob

Very nice Bob.
An observation & comments.
The bar you have welded to the front (hinge side) legs of the MOTs
will short out a lot of the flux if the MOTs are wired out of phase.
This is happening right where the most flux is needed to effectively
clamp the work piece. The steel angle piece the MOTs are sitting on
will also short flux. Fixing that is trivial - wire them in phase.

Is your top clamping bar too thin? A thin bar will saturate and not
provide the full force the MOTs are capable of delivering.

Adding a block of steel between the center legs and another across
the back legs will distribute the external flux more evenly across the
unit. This will provide better hold down force especially with a
saturating top bar. Don't add any steel which narrows the gap between
the center and outer legs of the MOT as this will reduce the external
flux available.
Art


A workmanlike effort - I'm surprised that it's not satisfactory.
A few possible thoughts

You've welded the Es to the fixed bar. As Art has pointed out
it's essential that these are both N (or both S) Poles. The weld
also needs to be machined dead flat - even a few thou gap reduces
the holding force.

You haven't shown the keeper bar that needs to be placed over the
workpiece. This again needs to be flat and at least as thick as
the outer arm of the E.

With the two old primaries parallel connected and fed from a
bridge rectifier The DC component of the rectified AC will be the
MEAN value of the supply voltage i.e. 0.9 x RMS value.
About 1000uf across the primaries will raise it to near the
supply voltage peak value - about 1.4 x RMS.

As a final desperate move feed the primaries from a voltage
doubler.

Leave the rectifier + and - outputs connnected to the windings.

Link the two rectifier AC inputs together.

Connect the + side of a 1000uf electrolytic to the two joined AC
inputs.

Connect one side of the supply to the minus end of the winding

Connect the other side of the supply to the minus side of the
capacitor.

This should give you close to 1.8 x RMS supply voltage

A further 1000uF connrcted across the winding would raise this to
near 2.8 x RMS supply voltage.

good luck!

Jim