On Fri, 8 Jul 2016 20:16:52 +0800, Rheilly Phoull
wrote:

On 8/07/2016 8:01 PM, 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?

Remember the power in these systems comes from a backup battery to make
it fail safe. It would involve major rebuild costs to do that.
Good point! Thank you for the reply.