On Fri, 08 Jul 2016 08:01:14 -0400, Pat wrote:

On Thu, 7 Jul 2016 22:43:12 -0700 (PDT), wrote:

On Monday, February 10, 2014 at 11:02:26 PM UTC+5:30, 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

I know this is an old post, but I find it interesting. Would using AC
on the electromagnets instead of DC solve the problem?

The OP already had multiple advice that the remanence was highly
unlikely to be the issue - so a different coil or drive method would
be unlikely to solve it, in an application where the installation has
been previously vetted. Fire doors have varying construction,
involving metalic or other heat resistant materials, but once
adjusted, performance should be repeatable.

It's much harder to get predictable performance from a pneumatic door
mechanism and the door it's controlling - without considering any
involvement from various external hold/release mechanisms. This is
complicated if staff are fiddling with the available adjustments at
their extremes, rather than examining other issues such a cleanliness,
lubrication or possible obstructions.

I understand his frustration, but safety critical features are best
addressed by those responsible for their installation and servicing.

This hardware operates on the principles of an electromagnet, as is
more commonly evident in solenoids and relays, so advice on their
operation is relevant.

RL