An electromagnetic lock, magnetic lock, or maglock is a locking device that consists of an electromagnet and an armature plate.
Operation
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.
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 (known as a 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.
Theory
Using Ampère's circuital law, it can be shown that the magnetic flux density induced by a solenoid of effective length with a current through loops is given by the equation:
The force between the electromagnet and the armature plate with surface area exposed to the electromagnet is given by the equation:
In both equations, represents the permeability of free space and the relative permeability of the core.
Although the actual performance of a magnetic lock may differ substantially due to various losses (such as flux leakage between the electromagnet and the conductor), the equations give a good insight into what is necessary to produce a strong magnetic lock. For example, the force of the lock is proportional to the square of the relative permeability of the magnetic core. Given the relative permeability of a material can vary from around 250 for cobalt to around 5000 for soft iron and 7000 for silicon-iron, the choice of magnetic core can therefore have an important impact upon the strength of a magnetic lock. Also relevant is the choice of current, number of loops and effective length of the electromagnet.
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
Remote operation: Magnetic locks can be turned on and off remotely by adjusting the power source 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. There are no moving parts in an electromagnetic lock to break.
Disadvantages
Requires a constant power source in order to be secure
Can de-energize in the event of a power outage, disabling security
Expensive in comparison to mechanical locks
Requires additional hardware for safe operation
Installation
The magnetic lock is suitable for both in-swinging and out-swinging doors. Brackets (L bracket, LZ bracket, U bracket) are used to orient the armature for use with both applications. Filler plates are also used to provide a large, flat mounting area on the door frame when the electromagnet is larger than the available mounting space on the door frame due to the frame's geometry.
The magnetic lock should always be installed on the secure side of the door. Most installations are surface mounted. For safety, magnetic lock, cables, and wires should be routed through the door frame or flush mount with wire moulding.
Installation is straightforward. With in-swinging applications, the electromagnet is typically installed in the opening corner of the door at the door's header. Maglocks can also be installed vertically in the door opening when they are furnished with a full length housing. With this configuration the armature is through-bolted through the door and oriented to mate with the face of the electromagnet. The armature plate and electromagnet must touch in order to provide locking holding force.