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Adrian Tuddenham[_2_]

Leif Neland 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.

Would it be possible to feed the electromagnets with 24V AC?

That should prevent the doors to get permanently magnetized.
[...]

A.C. magnets can also get stuck because of residual magnetism, depending
on whereabouts in the cycle the current is broken.

I occasionally have to file down areas of the pole faces of A.C.
contactors that have become polished through frequent use and have
started randomly sticking in the "on" position (highly embarrassing if
they control motors).

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