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An electromagnetic lock, magnetic lock, or maglock is a locking device that consists of an electromagnet and an armature plate. By attaching the electromagnet to the door frame and the armature plate to the door, a current passing through the electromagnet attracts the armature plate, holding the door shut. Unlike an electric strike a magnetic lock has no interconnecting parts and is therefore not suitable for high security applications because it is possible to bypass the lock by disrupting the power supply. Nevertheless, the strength of today's magnetic locks compares well with that of conventional door locks and they cost less than conventional light bulbs to operate. Power supplies incorporating a trickle-charged lead-acid battery pack can be used to retain security for short-term power outages.
Principle
The principle behind an electromagnetic lock is the use of electromagnetism to lock a door when energized. The holding force should be collinear with the load, and the lock and armature plate should be face-to-face to achieve optimal operation.
Operation
The magnetic lock relies upon some of the basic concepts of electromagnetism. Essentially it consists of an electromagnet attracting a conductor with a force large enough to prevent the door from being opened. In a more detailed examination, the device makes use of the fact that a current through one or more loops of wire solenoid produces a magnetic field. This works in free space, but if the solenoid is wrapped around a ferromagnetic core such as soft iron the effect of the field is greatly amplified. This is because the internal magnetic domains of the material align with each other to greatly enhance the magnetic flux density.
Technical comparison
Magnetic locks possess a number of advantages over conventional locks and electric strikes. For example, their durability and quick operation can make them valuable in a high-traffic office environment where electronic authentication is necessary.
Advantages
- Easy to install: Magnetic locks are generally easier to install than other locks since there are no interconnecting parts.
- Quick to operate: Magnetic locks unlock instantly when the power is cut, allowing for quick release in comparison to other locks.
- Sturdy: Magnetic locks may also suffer less damage from multiple blows than do conventional locks. If a magnetic lock is forced open with a crowbar, it will often do little or no damage to the door or lock.
Disadvantages
- Requires continuous power: To remain locked, the magnetic lock requires a constant power source. The power drain of the lock is typically around 3 watts, far less than that of a conventional lightbulb (around 60 watts), but it may cause security concerns as the device will become unlocked if the power source is disrupted. By comparison, electric strikes can be designed to remain locked should the power source be disrupted. Nevertheless, this behaviour may actually be preferable in terms of fire safety.
Installation
The magnetic lock is suitable for both in-swing and out-swing doors. Brackets (L bracket, LZ bracket, U bracket) are used to adjust the space between the door and lock. The magnetic lock should always be installed on the inside (secure side) of the door. Most installations are surface mounted. For safety, magnetic lock, cables, and wires should be inserted in the door or be a flush mount. Installation is as simple as installing the header of the door frame for out-swinging doors or using a Z-bracket for in-swinging doors. It is important to make sure the armature plate and the electromagnet align as closely as possible to ensure efficient operation. Magnetic locks are almost always part of a complete electronic security system. Such a system may simply consist of an attached keycard reader or may be more complex, involving connection to a central computer that monitors the building's security. Whatever the choice of locking system, fire safety is an important consideration.
Other variations and improvements on the electromagnetic locks have been developed. The most remarkable is the shear lock, where the armature does not directly pull off the face, but the load is instead in shear, like a mechanical stop. The shear magnetic lock allows a door to swing in both directions, as opposed to the original (and now ubiquitous) direct pull type, which normally works either in an in-swing or out-swing configuration.
An improved 'shear' electromagnetic lock was patented on May 2, 1989, by Arthur, Richard and David Geringer of Security Door Controls, an access control hardware manufacturing firm. (Maglocks.com carries and supports the full product offering of SDC Security) The device outlined in their designs was the same in principle as the modern magnetic lock consisting of an electromagnet and an armature plate. The patent did not make any reference to the manufacturing methods of the electromagnet and detailed several variations on the design, including one that used a spring-loaded armature plate to bring the armature plate closer to the electromagnet. The patent expired on May 2, 2009.
Holding force
A magnetic lock has a metal plate surrounded by a coil of wire that can be magnetized. The number of coils determines the holding force which characterizes the lock:
- Micro Size: 300 lbf (1,300 N) holding force.
- Mini Size: 600 lbf (2,700 N) holding force
- Midi Size: 800 lbf (3,600 N) holding force
- Standard Size: 1,200 lbf (5,300 N) holding force.
The standard size electromagnetic lock is used as a gate lock.
Electrical Requirements
The power for an electromagnet lock is DC (Direct Current), around 6 W. The current is around 0.5 A when the power is 12 V DC. Generally, the specification of the electromagnet locks is dual voltages 12/24 V DC. Single voltage output can be required for 12 V DC or 24 V DC applications. The figure presents the relationship between voltage and holding force. When the current is fixed, voltage is proportional to power consumption.
Application modes
For safety purposes, electric locks can be designed to operate in one of two modes:
Fail-Safe – to protect people: The lock is released if power cuts off.
Fail-Secure – to protect property: The lock remains closed if power cuts off.
An electromagnet lock is normally used in Fail-Safe situations, so it must satisfy applicable fire regulations so as to be safe in emergency situations.