Magnetic door holders question

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.

feed them less than 24 volts, or maybe put a layer of packing tape over the plate to add some space, or get somebody to "accidentally" paint over the mating surfaces, although this is likely to make them stick as well.

On Monday, February 10, 2014 11:32:26 AM UTC-6, snipped-for-privacy@gmail.com wrot e:

of these buildings in the common hallways employ electromagnetic units moun ted 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 w ell which are always applying a force in the opposite direction to try to c lose the door. When the alarm is activated the 24VDC is removed from the co ils and the doors are supposed to be automatically pulled closed by the for ce 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 electromagn ets seem to hold enough residual magnetism to keep disks from releasing and the doors from closing. It often becomes necessary to increase the opposin g pneumatic force tremendously in order to overcome this. I have discussed this with various manufacturers of these electromagnetic units and in all b ut one instance have received the same bullshit answer that they've "never heard of this". The one exception was one tech who ventured that perhaps mo mentarily reversing polarity on alarm before DC drop out might work, howeve r he had never tried it. Does anyone have any ideas about this? Thanks, Len ny

Ken and Cydrome's idea to put a thin spacer on the disc to make it easier t o release sounds like a good idea. My experience has always been that the magnetism was not strong enough to hold the door open, not that it would no t release.

I've never seen this and I have a fair amount of experience with these doors in hospitals.

It is more commonly hard to get them to stay open.

I would suspect a door closer problem first.

I've always found that having ferrous metal in a DC field touching each other is not a good idea. Add a 3 mil layer of kapton tape between the two. the extra distance will prevent the 'residual' magnetism from having enough strength to hold and keep the doors open. More likely difficulty keeping them open.

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.

Shoot video of the failure, and send it to them. Tell them if they can't fix the problem that you will post the video and details online, in case of a fire. Put it some place like Youtube or Vimeo, where it is sure to be seen by people and asked the OEM what it will do to their reputation and their insurance costs.

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

Perhaps the O/P should contact the manufacturer and suggest they stamp their armatures slightly concave in future.

snipped-for-privacy@gmail.com:

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

That should prevent the doors to get permanently magnetized. I don't know if that'll make the doors/magnets hum though.

Leif

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).

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.

Each coil draws .015A at 24VDC. Putting them in series would not be practic al especially in a large building. There are systems that use 120VAC coils but the circuits have to have the correct cable, (Romex) pre wired for them . Most buildings are wired using 18 or 16 AWG. FPLR low voltage cable for i nitiation and indicating circuits. You would not want to run 120V through t hese. Further, most fire alarm guys are certified in low voltage and althou gh I wired my own house and I could certainly wire a 120V circuit, we are n ot electricians. For this reason all the door holders I've run into are the 24VDC variety.

I do like Michael's idea of video taping the failure and sending it to the manufacturer. Perhaps that will get them off their asses to come up with a solution. Lenny

Any chance the door closer arms can be relocated to have a better angle to act on the door? Maybe when the door is fully open, they're pulling "acros s" the door too much.

On Tuesday, February 11, 2014 5:08:58 PM UTC-5, snipped-for-privacy@gmail.com wrot e:

ical especially in a large building. There are systems that use 120VAC coil s but the circuits have to have the correct cable, (Romex) pre wired for th em. Most buildings are wired using 18 or 16 AWG. FPLR low voltage cable for initiation and indicating circuits. You would not want to run 120V through these. Further, most fire alarm guys are certified in low voltage and alth ough I wired my own house and I could certainly wire a 120V circuit, we are not electricians. For this reason all the door holders I've run into are t he 24VDC variety.

e manufacturer. Perhaps that will get them off their asses to come up with a solution. Lenny

The panel and other components are likely UL listed. I would set a video to UL. May not help your immediate problem but could prevent similar problems elsewhere in new installs.

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.

They are not contactors. Some hold the door open for years between operations. Contactors get beat to death by repeatedly slamming the pole piece into the metal on the moving contacts.

You're comparing apples to water melons.