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Ken[_6_]

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

Couldn't you place something over the metal disks to reduce the magnetic
coupling with the magnet? I would think you could find a point where it
would still hold the door when needed, and release it when 24v is removed.