That last idea is an elegant one which would probably work quite well;
however, it has one very big drawback:
The safety of the whole system depends on the doors closing if anything
goes wrong. If the wires were burned through, the control box would
lose contact with the magnets and could not demagnetise them. The
fail-safe aspect of the system would be lost.
The usual method is to insert a non-magnetic shim of some kind, but this
can wear down and fail after a few years. A more subtle way is to
abrade or machine down one of the pole faces so that it is not quite in
the same plane as the other. If these are 'pot' magnets, you could skim
a thou or so off the centre pole if you have access to a lathe.
Unfortunately this would destroy any anti-corrosion plating on the
metal, but that might not matter if the buildings are dry.
An alternative would be to 'dish' the armature plate so that the concave
side was towards the magnet. To do this you would need to take it off
the door and stand it outside on a solid foundation (or an anvil, if you
have one). A supporting ring to back up the outer edge can be
improvised from a piece of hard wood with a hole in it or a short offcut
of steel pipe. To dish it slightly you will need a steel bar or other
hard object with one end slightly rounded - and a sledge hammer to hit
it with. Try to do it in one hit, so as to avoid peening the surface.
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".
co.uk" to reply)
www.poppyrecords.co.uk