Cydrome Leader wrote:

Adrian Tuddenham wrote:
wrote:

I work on commercial fire alarm systems mostly in apartment houses. Many
of these buildings in the common hallways employ electromagnetic units
mounted on the walls and an iron disk on the back corner of the doors.
When the doors are opened and the disks are mated with the electromagnet
the doors are held open in place. These doors all have pneumatic closers
on them as well which are always applying a force in the opposite
direction to try to close the door.

When the alarm is activated the 24VDC is removed from the coils and the
doors are supposed to be automatically pulled closed by the force of the
pneumatic unit. This doesn't always work because in spite of the opposing
force applied by the pneumatic unit, in many cases the electromagnets seem
to hold enough residual magnetism to keep disks from releasing and the
doors from closing. It often becomes necessary to increase the opposing
pneumatic force tremendously in order to overcome this.

I have discussed this with various manufacturers of these electromagnetic
units and in all but one instance have received the same bull**** answer
that they've "never heard of this".

The one exception was one tech who ventured that perhaps momentarily
reversing polarity on alarm before DC drop out might work, however he had
never tried it. Does anyone have any ideas about this? Thanks, Lenny

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.

The problem with the heavy handed methods is you're then deliberately
altering a safety device. A little tape here and there could have been
done by anybody who just didn't know better.

Granted, it's unlikely to cause the building to catch on fire, with the
doors stuck open killing everybody inside, but these are not really
devices to mess with. The door might just slam into somebody's face- you
never know.

While I may have done electrical work before, if it's conduit painted red
(at least here in Chicago this is common) or marked Life/Safety or L/S or
something similar, the rule is don't open it, don't touch it.

It is a sad situation where a safety device could be made more reliable
by a simple modification but this is prevented by the legislation which
is there to make things safer.

My biggest worry would be how such a badly-designed "safety" system with
an easily-recognised fault came to be made by a manufacturer who
presumably specialises in that field. Add to that, the inspection and
checking processes which must all have failed to pick up the problem.
It isn't as though D.C. electromagnetism is a new and unknown field full
of unsuspected effects - this problem has been well-understood for over
100 years.

Perhaps the O/P should contact the manufacturer and suggest they stamp
their armatures slightly concave in future.

--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk