A magnetic lock is a simple locking device that consists of an electromagnet and 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 compare well with conventional door locks and cost less than conventional light bulbs to operate.

History

The magnetic lock was first patented on May 2, 1989, by Arthur, Richard and David Geringer. The device outlined in their designs was the same in principle as the modern magnetic lock consisting of an electromagnet and 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 is still current and is set to expire on May 2, 2009.

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 sufficiently large enough to prevent the door from being opened. In more detailed examination, the device makes use of the fact that a current flowing 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.

Equations

Using the Biot-Savart law, it can be shown that the magnetic flux density $B$ induced by a solidnoid of effective length $l$ with a current $I$ flowing through $N$ loops is given by the equation:

$B\; =\; \backslash frac\{\backslash mu\_0\backslash mu\_rIN\}\{l\}$

The force $F$ between the electromagnet and the armature plate with surface area $S$ exposed to the electromagnet is given by the equation:

$F\; =\; \backslash frac\{B^2S\}\{2\backslash mu\_0\}$

In both equations, $\backslash mu\_0$ represents the permeability of free space and $\backslash mu\_r$ represents 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. Nevertheless in a number of applications economic or security reasons can lead to conventional locks or electric strikes being a better choice.

Advantages

Easy to install

Magnetic locks are generally easier to install than other locks given there are no interconnecting parts.

Quick operation

Magnetic locks unlock instantly when the power is cut allowing for quick operation in comparison to other locks.

Durable

The lack of interconnecting parts means the lock is far more durable than other locks especially when used in high traffic areas.

Competitive with conventional locks

Despite common misperceptions, today's magnetic locks are competitive with conventional locks. For example, the ASSA Heavy Duty High Security Modular Lockcase is designed to withstand an end load of 5,000 N. In comparison, the Securitron 1200 Pound Magnetic Lock is designed to withstand a load of 5,300 N and there are higher power models available. Magnetic locks may also suffer less damage from multiple blows.

Disadvantages

Requires continuous power when locked

To remain locked the magetic lock requires a constant power source. At around 3 watts, the power drain of the lock is typically 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. In comparison, electric strikes can be designed to remain locked should the power source be disrupted. Nevertheless, this behaviour can actually be preferable in terms of fire safety.

Cost

The Securitron 1200 Pound Magnetic Lock costs around $340 USD for the lock alone. It must then be connected to an electronic access-control system (for example a keycard reader). In comparison, conventional locks can be purchased for under $40 USD. Annual operation of the magnetic lock will also cost around $3 USD in terms of electric power consumption.

Installation

The magnetic lock is generally placed on the inside of the door frame to prevent tampering. 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.

References

1. Geringer A. Geringer R. Geringer D. Electromagnetic Door Lock Device, United States Patent Number 4826223, May 2, 1989.
2. Sadiku, M. Elements of Electromagnetics (3rd edition), Oxford University Press, 2001 (ISBN 019513477X).
3. Performance Specifications, http://www.assa.co.uk/resources/Services/docs/PSG%20Word.rtf, ASSA Limited (Last updated 7th, May 2003)
4. Securitron Magnetic Locks, http://www.nokey.com/secmagloc.html, The Keyless Lock Store (Accessed 12th, September 2005)
5. Purchase Rates, http://www.westernpower.com.au/home/products_services/renewable_energy_buyback/purchase_rates.html, Western Power (Accessed 12th, September 2005) (assumes 3 W power consumption)
6. The Complete Book of Locks and Locksmithing (4th edition), Bill Phillips, McGraw-Hill Inc. 1995.

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