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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Magnetic door holders question
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 |
#2
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Magnetic door holders question
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#3
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Magnetic door holders question
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#4
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Magnetic door holders question
On Monday, February 10, 2014 11:32:26 AM UTC-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 Ken and Cydrome's idea to put a thin spacer on the disc to make it easier to 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 not release. |
#5
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Magnetic door holders question
On Monday, February 10, 2014 2:06:37 PM UTC-5, wrote:
Ken and Cydrome's idea to put a thin spacer on the disc to make it easier to 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 not 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. |
#6
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Magnetic door holders question
On Mon, 10 Feb 2014 10:32:26 -0700, wrote:
...snip... 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'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. |
#7
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Magnetic door holders question
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 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. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#8
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Magnetic door holders question
Adrian Tuddenham wrote:
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 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. |
#9
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Magnetic door holders question
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#10
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Magnetic door holders question
Cydrome Leader wrote:
Adrian Tuddenham wrote: 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 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. 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 100 years. Perhaps the O/P should contact the manufacturer and suggest they stamp their armatures slightly concave in future. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#11
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Magnetic door holders question
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#13
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Magnetic door holders question
Adrian Tuddenham wrote:
Cydrome Leader wrote: Adrian Tuddenham wrote: 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 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. 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 100 years. Perhaps the O/P should contact the manufacturer and suggest they stamp their armatures slightly concave in future. It might be worth looking into the power source for those devices. Too much voltage will likely cause them to be too strong. |
#14
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Magnetic door holders question
Cydrome Leader wrote:
Adrian Tuddenham wrote: Cydrome Leader wrote: Adrian Tuddenham wrote: 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 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. 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 100 years. Perhaps the O/P should contact the manufacturer and suggest they stamp their armatures slightly concave in future. 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. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#15
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Magnetic door holders question
Each coil draws .015A at 24VDC. Putting them in series would not be practical 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 initiation and indicating circuits. You would not want to run 120V through these. Further, most fire alarm guys are certified in low voltage and although 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 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 |
#16
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Magnetic door holders question
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 "across" the door too much.
On Tuesday, February 11, 2014 5:08:58 PM UTC-5, wrote: Each coil draws .015A at 24VDC. Putting them in series would not be practical 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 initiation and indicating circuits. You would not want to run 120V through these. Further, most fire alarm guys are certified in low voltage and although 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 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 |
#17
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Magnetic door holders question
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#18
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Magnetic door holders question
Adrian Tuddenham wrote: Cydrome Leader wrote: Adrian Tuddenham wrote: Cydrome Leader wrote: Adrian Tuddenham wrote: 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 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. 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 100 years. Perhaps the O/P should contact the manufacturer and suggest they stamp their armatures slightly concave in future. 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. All I've seen in use, use AC to prevent this problem. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge. |
#19
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Magnetic door holders question
Michael A. Terrell wrote:
Adrian Tuddenham wrote: Cydrome Leader wrote: Adrian Tuddenham wrote: Cydrome Leader wrote: Adrian Tuddenham wrote: 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 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. 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 100 years. Perhaps the O/P should contact the manufacturer and suggest they stamp their armatures slightly concave in future. 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. 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. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#20
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Magnetic door holders question
Adrian Tuddenham wrote: Michael A. Terrell wrote: 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. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge. |
#21
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Magnetic door holders question
Michael A. Terrell wrote:
Adrian Tuddenham wrote: Michael A. Terrell wrote: 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. These D.C. magnets with flat pole faces are getting stuck because of residual magnetism - A.C. contactors with laminated pole faces which have become hammered flat, get stuck in exactly the same way. This is not because the contactors have had to do a lot of work in order to develop flat pole faces, it is because the residual magnetism is the same, regardless of whether it resulted from D.C. or the last half-cycle of A.C. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#22
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Magnetic door holders question
Adrian Tuddenham wrote: Michael A. Terrell wrote: Adrian Tuddenham wrote: Michael A. Terrell wrote: 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. These D.C. magnets with flat pole faces are getting stuck because of residual magnetism - A.C. contactors with laminated pole faces which have become hammered flat, get stuck in exactly the same way. This is not because the contactors have had to do a lot of work in order to develop flat pole faces, it is because the residual magnetism is the same, regardless of whether it resulted from D.C. or the last half-cycle of A.C. The old sliding fire doors on slanted rollers that used a rope and weights were pretty much foolproof. They didn't need electricity, and there was little you could do to screw them up. Once they were moving, you couldn't hold them open. Those laminated contactors take thousands and thousands of operations to flatten into a solid face. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge. |
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Magnetic door holders question
wrote in message ... Each coil draws .015A at 24VDC. Putting them in series would not be practical 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 initiation and indicating circuits. You would not want to run 120V through these. Further, most fire alarm guys are certified in low voltage and although 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 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 Sorry to dig this up but need to interject a bit of input. First the problem you are having is unusual. Since this is a commercial fire panel it needs to be worked on by the servicing alarm company to make any adjustment or corrections to the system. This is required by NFPA as well as the AHJ in your jurisdiction. There are procedure that can determine the cause and correction of such issues. Personally I would be checking residual voltage at the mag door holders when power is removed. Also checking the door plates to see if they are somehow magnetized and applying the extra holding force you describe. Depending on the above results there are certain device applications that can correct the problem. Also knowing the mfg, model and the vintage of the Fire Alarm Panel itself could go along way to solving the problem. Again, doing work on a Commercial Fire Panel needs to done by trained professionals. If you are not, don't touch it. Les |
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Magnetic door holders question
On Sunday, February 16, 2014 6:33:37 PM UTC-5, Mick Nowell wrote:
Disconnect the PU from the door. Get a scale and see how much force it takes to open each unit. Before any modification I suggest exercising them. Pop each one open and "brillo" the oxidization/whatever on each surface. Repeat. +1 All the speculation is based on the conclusion that residual magnetism is the problem, and that may be premature. Turn the power off, take an ordinary piece of steel like a cover plate, and see if it sticks to the magnet. If it does (which I doubt) put a multimeter on it and confirm the power is really off. Then use the spring scale to see how hard the steel sticks. Compare that to how hard it is to pull the door open. |
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On Tuesday, March 25, 2014 3:44:44 PM UTC-4, Tim R wrote:
On Sunday, February 16, 2014 6:33:37 PM UTC-5, Mick Nowell wrote: Disconnect the PU from the door. Get a scale and see how much force it takes to open each unit. Before any modification I suggest exercising them. Pop each one open and "brillo" the oxidization/whatever on each surface. Repeat. +1 All the speculation is based on the conclusion that residual magnetism is the problem, and that may be premature. Turn the power off, take an ordinary piece of steel like a cover plate, and see if it sticks to the magnet. If it does (which I doubt) put a multimeter on it and confirm the power is really off. Then use the spring scale to see how hard the steel sticks. Compare that to how hard it is to pull the door open. Er, shut, I mean. |
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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 |
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#28
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Magnetic door holders question
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. |
#29
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Magnetic door holders question
It has been my experience that these beasts fail in three typical ways:
a) they are so seldom checked that dirt accumulates under the leaf and cements the door in place (!!). b) the plate rusts to the magnet. c) see a) above but insert carpet. All due to systematic and systemic negligence. Under my watch, such devices are tested weekly, typically on Friday at office (or school) closing. I have never seen residual magnetism sufficient to hold the door. Neither plate nor magnet has enough mass for that. Peter Wieck Melrose Park, PA |
#30
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Magnetic door holders question
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. |
#31
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Magnetic door holders question
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. Not true. Most of the systems that I have seen will automatically release the doors during a power fail. Since these are fire doors, the default is to close them in any "emergency". A power failure is considered an emergency and the doors are released. As matter of fact, during a fire inspection we will trigger these doors in one of two ways: 1) Fire alarm trip and 2) power failure. In order for the facility to pass the inspection, both methods need to work. Dan |
#32
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Magnetic door holders question
On Fri, 8 Jul 2016 20:16:52 +0800, Rheilly Phoull
wrote: snip 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. For this service, loss of power would invoke the fail safe condition, without a back-up battery. Fail safe is the de-energized state. RL |
#33
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Magnetic door holders question
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 |
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#36
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Magnetic door holders question
Clean hinges with CRC Spray Silicone/\...use the spray as a cleaning hose..
then lube hinges with a drop or 2 of http://www.3inone.com/images/3in1/pr...k-dry-lube.jpg at HD |
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