A sixty-kilogram fire door became jammed in a half-open state during a simulated power outage test because the electromagnet failed to release immediately.
Many engineers often focus only on whether the static suction force meets the standard when designing access control locks. After disassembling multiple failed cases on-site, we found that the real fatal flaw is usually not insufficient suction force, but rather the electromagnet’s inability to reset smoothly under invisible stresses such as high temperature, high humidity, or mechanical eccentricity.
The core value of electromagnets in safety passages is not “how tightly they hold,” but “whether they can absolutely release when they should.”
Selection Traps for Latching Solenoids vs. Push-Pull Solenoids
Why do latching solenoids, which passed thousands of tests in the lab, frequently fail within half a year after being installed on fire doors? Compared to general industrial automation equipment, security and fire safety systems have almost zero tolerance for electromagnet failures.
Continuous 24-hour power supply is typical for fire door release mechanisms, causing coil temperatures to climb above 80°C. Under these extreme conditions, the resistance of the copper wire increases, leading to a decrease in the starting current of push-pull solenoids, and the suction force may decay by over 30%. While latching solenoids use permanent magnets to maintain their state, they demand extremely high precision in drive circuit pulse width and voltage stability, which represents the design tradeoff between demagnetizing air gap and holding force.
Temperature Rise and Residual Magnetism Issues in Access Control Solenoids Under Prolonged Energization
We must admit that no single electromagnet can simultaneously satisfy infinite lifespan and extremely low cost. When an access control solenoid is energized for an extended period, the core material gradually magnetizes, leading to residual magnetism that is difficult to eliminate.
In the design of fire door release mechanisms, when the ambient temperature reaches 60°C and power is continuously supplied for more than 48 hours, the residual magnetism decay time of the electromagnet will double. At this point, a physical anti-residual magnetism air gap of no less than 0.2 mm must be designed to ensure immediate release upon power disconnection.
| Electromagnet Type | Power Consumption Characteristics | Residual Magnetism Risk | Applicable Scenarios |
|---|---|---|---|
| Push-pull solenoid | High (requires continuous power) | Medium (releases upon power disconnection) | High-frequency access control opening/closing |
| Latching solenoid | Very low (pulse energization only) | High (requires reverse pulse demagnetization) | Fire doors, normally open safety passages |
On-site Measurement: Mechanical Fatigue and Environmental Stress of Access Control Solenoids Under High-Frequency Use
During an acceptance test at a large commercial office building, we found some push-pull solenoids jammed due to lateral force caused by door leaf sagging. When the lateral force exceeds 5 Newtons, the friction between the slider and the guide sleeve sharply increases, preventing the internal spring from pushing the slider back into position. This is like a stretched rubber band losing its elasticity and being unable to return to its original position; even a slight mechanical eccentricity can compromise the entire safety defense.
Addressing the increased friction caused by lateral force requires starting with the slider material and guide sleeve tolerance.
How should procurement personnel evaluate lifespan performance in real environments when reviewing specifications?
Although latching solenoids save power through permanent magnets, their demagnetization pulse width is extremely sensitive to voltage fluctuations.
- Continuously energize in a 60°C environment for 24 hours, measuring coil temperature rise and suction force decay rate.
- Apply a lateral force of 5 Newtons, testing if the electromagnet can still reset smoothly.
- Conduct 1 million cycles of fault-free operation tests, observing the wear condition of the demagnetizing shim.
1 million cycles of fault-free operation is the basic threshold for smart lock solenoids, requiring precise calculation of the magnetic circuit and spring rebound force.
Building Highly Reliable Safety Passages: A Customized Selection Guide for Smart Lock Solenoids
Although standard products can shorten development cycles, customized protection designs are often an uncompromising choice in high-temperature, high-humidity outdoor access control systems. When we disassembled a fire door electromagnet that failed to release due to residual magnetism, we found its demagnetizing air gap was completely worn out. This is precisely why Shih Shin Technology (世僖科技) insists on dynamic simulation testing when assisting clients in planning safety passage systems, ensuring that every electromagnet operates precisely even in extreme environments.
Does your next security project also face similar environmental challenges?
When selecting electromagnets, one should not only consider static specifications but also incorporate temperature rise, residual magnetism, and mechanical lateral force into the overall consideration. Only by conducting rigorous physical boundary tests at the initial design stage can we ensure that safety passages provide their intended protection at critical moments.
It is recommended to provide door weight and expected lateral force data at the initial stage of the project for the technical team to conduct magnetic circuit simulation, thereby shortening post-development debugging time by over 50%.
