On 2011-02-04, Winston wrote:
Bob Engelhardt wrote:
[ ... ]
Well, the MOT electromagnets have both poles in the same plane, so you
wouldn't have to alternate. Like you would with magnets with poles on
the opposite ends of a bar or cylinder. Bob
I'm confused.
Let's say we have two identical MOT electromagnets
connected to the same pulse - D.C. source in the same way.
We discover that the outer 'ears' of the E cores are
both at magnetic 'south'.
Aligned side-by-side, wouldn't these magnets repel
each other?
Yes -- but if properly clamped down, that would not do anything
bad.
Followup question: Would that affect the ability of
the magnets to attract the clamping bar, for better
or worse?
It would weaken the clamping attraction in that area I believe.
If I were making a long electromagnet using Microwave Oven Transformers,
I would probably strip off the windings entirely, butt the cores against
each other as follows (letters are core identifiers, '+' are outer
poles, '-' are inner poles, three cores shown here, but I would use as
many as needed for the length to be covered:
+-----+ +----------+ +-----++-----+ +----------+ +-----++-----+ +----------+ +-----+
| | | | | || | | | | || | | | | |
| | | | | || | | | | || | | | | |
| A+ | | A- | | A+ || B+ | | B- | | B+ || C+ | | C- | | C+ |
| | | | | || | | | | || | | | | |
| | | | | || | | | | || | | | | |
+-----+ +----------+ +-----++-----+ +----------+ +-----++-----+ +----------+ +-----+
And would wind them in a figure eight pattern as follows (numbers are
half turns)
1111111 222222222222 11111111111111 222222222222 11111111111111 22222222222 1111111
+-----+1 2+----------+2 1+-----++-----+1 2+----------+2 1+-----++-----+1 2+----------+2 1+-----+1
| | 1 2 | | 2 1 | || | 1 2 | | 2 1 | || | 1 2 | | 2 1 | |1
| | 1 2 | | 2 1 | || | 1 2 | | 2 1 | || | 1 2 | | 2 1 | |1
| A+ | 1 | A- | 1 | A+ || B+ | 1 | B- | 1 | B+ || C+ | 1 | C- | 1 | C+ |2
| | 2 1 | | 1 2 | || | 2 1 | | 1 2 | || | 2 1 | | 1 2 | |2
| | 2 1 | | 1 2 | || | 2 1 | | 1 2 | || | 2 1 | | 1 2 | |2
+-----+2 1+----------+1 2+-----++-----+2 1+----------+1 2+-----++-----+2 1+----------+1 2+-----+2
222222 11111111111111 22222222222222 11111111111111 22222222222222 11111111111 2222222
N S N S N S N
2 becomes 3 and follows 1, then becomes 4 for the trip back, etc. Since
the end poles of a typical core are half the width of the center core
this would make equal area poles except at the very end, and alternating
poles fairly close together for maximum grip. How many turns would be
needed would be fun to calculate, of course. :-)
I've marked below the poles the alternating North and South
poles of a momentary status. (Or, if you use DC, this could be a stable
status as long as the current is flowing.)
Ideally, you would want to cut apart the core from another
transformer and use it to double the size of the end poles from the
above drawing.
I can envision both possibilities:
The attraction to the clamping bar would be improved
because the resulting field distortion would tend to
push the lines of flux outwards from each electromagnet.
The attraction to the clamping bar would be hindered
because of magnetic 'phase cancellation' between the
magnets.
The center of each pole -- or combined pole -- would have less
flux than the edges, so smaller poles are better -- up to the point
where the flux loops back before it significantly penetrates the
workpiece to reach the clamp bar.
Enjoy,
DoN.
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