Michael A. Terrell wrote:

Adrian Tuddenham wrote:

Cydrome Leader wrote:

Adrian Tuddenham wrote:
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.

It might be worth looking into the power source for those devices. Too
much voltage will likely cause them to be too strong.

It's the residual magnetism that is causing the problem; if the main
magnetisation is above a certain minimum level, the residual is not much
affected by any extra energisation. The limit to the power source is
more likely to be the temperature rise of the energising coils.

I don't know how the O/P's system is arranged, but I think the UK ones
have the magnets in series around the building, so that the circuit can
be broken near any doorway and all the doors on that circuit will shut.
In that case the PSU would either be constant-current or would have a
transformer with adjustable tappings which can be set according to how
many magnets are in circuit.


All I've seen in use, use AC to prevent this problem.

....except that A.C. doesn't prevent it. I've seen enough stuck
contactors with A.C. coils to be quite certain of that.


--
~ Adrian Tuddenham ~
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