Electromagnetic Lock Basics

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Created: April 1, 2007

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When the door is open and the shear electromagnetic lock is energized, the door can move to the closed position without correct alignment of the spring loaded strike plate to the magnet. When the electromagnetic lock is powered, the open door's shear locator posts keep this special strike plate far enough away that the electromagnet cannot gain sufficient holding power on the strike plate. Once the shear locator posts are able to enter the recesses in the electromagnetic lock, the armature is drawn into contacting the magnet face and locking the door. The shear locator post mechanism eliminates the need of timers or sensors, permitting the door to close before the electromagnetic lock is energized.

The shear type design permits locking of the door while maintaining the continuity of the door and frame. There are no exposed magnets or plates. In addition, the shear type locking mechanism can be used with both single and double swinging doors. Because the magnet and the strike assembly are both concealed, a door can swing in both directions. Shear locks are normally mounted into the header or threshold of a sliding or swinging door.

The Securitron MM15 E-Mag is a hybrid design electromagnetic lock that provides 4, 000 pounds of holding force, according to Securitron. The compact MM15 is equipped with two spring loaded, hardened steel clamps (mandibles) that extend from the lock housing. They are designed to place additional holding force onto the modified armature in the event an attempt is made to pull open the door. When this type of attempted forced entry occurs, the mandibles clamp down onto the edges of the armature adding a significant amount of holding force. Only when the force applied to pull open the door is eliminated do the mandibles release the armature. When the current is turned off, the MM15 releases and the door can be opened. The MM15 is designed for indoor applications only.

Another application for electromagnetic locks is to control access of manual and automatic gates. The lock is installed onto the fence post and the armature is mounted onto the swinging or sliding gate. The size of the electromagnetic lock is determined by the size and weight of the gate. For specific applications, contact the electromagnetic lock manufacturer.

Gate operators are designed to close gates. Many of these motorized units do not have speed adjustments. To keep the gate from slamming closed, shock absorbing strike mount kits are available. These kits are designed to absorb the excessive impact between the electromagnetic lock and the armature.

To accommodate these different applications, electromagnetic lock manufacturers have developed a number of different shape mounting brackets and fillers. These metal components ensure that the electromagnetic lock is solidly mounted. The fillers are used to compensate for the different styles of jamb stops.

Since there are no moving parts in an electromagnetic lock, there is no easy way to determine if the door is secured. For this reason, when electromagnetic locks are used for security applications rather than traffic control, order the locks with monitoring capabilities. The monitoring options include a bond sensor, signal sensor, door position switch, anti-tamper sensor and visual status indicators.

A bond sensor can monitor a portion or the entire surface of the armature. Should there be any variation beyond the established parameters, a signal will be initiated indicating the electromagnetic lock is not generating full holding potential.

A lock status sensor is designed to indicate when the door is secure, not just closed.

A door position switch (DPS) can provide a remote signal indicating if the door is opened or closed. This DPS is normally used for auto relock with an access control system.

An anti-tamper switch will indicate if the splice chamber cover has been tampered with or has been removed. This type of sensor is normally an alarm point on the panel or part of an access control system.

The LED visible from the rear of the electromagnetic lock gives a visual status of the lock. Check with the lock manufacturer to determine the operating colors for your specific magnet.

Surface/header mount electromagnetic locks are designed for swinging doors that open out from the secured side. Smaller electromagnetic locks can be used to secure office furniture doors/drawers and display cases. These surface mount locks can be used on doors where the armature is mounted onto the interior face of the door. Surface mount electromagnetic locks can be header mounted or surface mounted. They are designed for the access control of hollow metal, wood, aluminum and glass door openings.