When a massive lightning surge current floods into a relay, the contact system is often the first to fail. At this moment, a significant Holm force is generated on the contact surfaces. This electromagnetic force violently repels the contacts, causing the circuit—which should remain closed—to open abnormally, leading to equipment failure.
Conventional relays seem insufficient to address this difficulty. Recent technical innovations ingeniously employ the Lorentz force to mitigate the Holm force, so attaining a stabilising effect of "using force to counteract force."
The threat of lightning to relays lies at its core in the instantaneous, extremely high current. When lightning current (which can reach tens of kiloamperes) passes through the relay's narrow contact area, two key forces are generated between the contacts.
The first is the Holm force (also known as contact repulsion force). It originates from the severe constriction effect of the current at the contact points (actually multiple microscopic asperities).
The current lines are intensely squeezed at these microscopic asperities, generating a powerful electromagnetic force that attempts to push the contacts apart. Its magnitude is proportional to the square of the current. During a lightning strike, this value is sufficient to overcome the normal contact pressure of the contacts.
The other is the Lorentz force. This is the force exerted on a conductor carrying an electric current within a magnetic field. In a relay, when current traverses a uniquely configured conductor, it encounters a force inside the magnetic field it generates. The orientation of this force adheres to the left-hand rule.
The crucial aspect is that via innovative design, the orientation of the Lorentz force can be directed to oppose the Holm force, thus mitigating its detrimental impact.
For a long time, improving a relay's lightning withstand capability primarily relied on "strengthening" approaches such as increasing contact material hardness or enhancing contact pressure. However, facing the quadratically increasing Holm force, these methods quickly reach their limits.
A particularly prominent contradiction arises in attempts to utilize the Lorentz force. In DC relays, to extinguish the arc generated during high-current interruption, "magnetic blowout arc extinction" technology is often used, which involves placing permanent magnets around the contacts.
However, this magnetic field causes the current-carrying moving spring (armature) to experience an additional, often downward, Lorentz force. This force not only fails to help resist the repulsive force but may also cancel out part of the contact pressure, making the contacts more likely to be repelled under lightning current, effectively reducing the product's short-circuit withstand capability.
This precisely corroborates the core challenge described in the user project: "The Lorentz force is easily utilized in small-gap scenarios, but its utilization becomes very difficult with large contact gaps." How to precisely control the direction and magnitude of the Lorentz force, making it an "ally" rather than an "enemy," is the greatest design difficulty.
To break through the aforementioned困境, it is necessary to move beyond localized optimization thinking and undertake integrated innovative design of the relay's contact system. Cutting-edge technical solutions mainly revolve around two core ideas.
Aiming at the side effects caused by the magnetic blowout arc extinction field, an innovative solution is to reconstruct the spatial magnetic field distribution by changing the number, arrangement, and polarity of the permanent magnets.
For example, a patented technology from Xiamen Hongfa Electric Control adopts a three-magnet layout: two of these magnets are located on either side of the moving spring's width direction, corresponding to one set of moving and stationary contacts; the third magnet is located on one side of the moving spring's length direction, corresponding to another set of contacts.
The key design lies in the fact that the magnetic pole face of the third magnet is approximately perpendicular to the magnetic pole faces of the first two magnets. This specialised magnetic circuit design allows the net Lorentz force acting on the moving spring within the magnetic blowout field to be nearly zero, so fundamentally mitigating adverse side effects and maintaining consistent contact pressure.
To actively generate a beneficial Lorentz force to resist the Holm force, the key lies in designing a specific current-carrying conductor loop. A sophisticated method involves implementing a design that incorporates "flexible multi-strand soft copper wires linked to a rigid moving spring" alongside a "U-shaped" configuration.
Force Generation: When lightning current flows through the "U-shaped" moving spring and its connected flexible wires, based on their winding direction, it can generate a Lorentz force on the two parallel arms of the U-shaped structure. This force is directed parallel and perpendicular to the contact plane.
Force Transmission and Transformation: This force exerts influence on the entire mobile spring assembly. The adaptable linkage of the multi-strand soft copper wires is essential, as it guarantees the effective transmission of the Lorentz force, converting it into beneficial pressure on the contact surface, instead of inducing stiff distortion or displacement of the structure.
Dynamic Compensation: When the Holm force attempts to repel the contacts, the pre-configured Lorentz force synchronously increases, forming a dynamic, adaptive compensation mechanism that "presses" the contacts together. The U-shaped configuration enhances mechanical stability and leverage.
This design adeptly addresses the issue of "challenges in employing the Lorentz force with substantial contact gaps.""Through flexible connections and overall structural design, it achieves efficient collection and utilization of the Lorentz force on current-carrying conductors over a wider range beyond the small-gap effect (i.e., the current constriction zone).
Achieving the aforementioned precise force control is inseparable from advanced design tools and materials.
Multiphysics coupling simulation has become an indispensable R&D tool in this field. In high-reliability areas such as aerospace, the analysis of sealed electromagnetic relays has adopted a multi-body dynamics-electromagnetic finite element bidirectional interactive coupling calculation model.
This method can calculate in real-time the interaction between transient mechanical torque, electromagnetic forces, and deformation parameters, accurately simulating the complex dynamic process at the moment of lightning strike. It controls the calculation error of key parameters like holding force to an average level of 2.21%.
This far surpasses conventional methods reliant on static data sheets, delivering precise predictions for optimising factors such as U-shaped structural dimensions, flexible wire length, and stiffness.
Regarding materials, the moving spring is typically made of high-elasticity, high-conductivity alloys like beryllium copper or phosphor bronze to withstand repeated force and thermal shock. The multi-strand soft copper wires necessitate exceptional purity and flexibility to guarantee dependable current conduction and force retention following prolonged vibration.
Insulation materials and housing structure also need to be reconsidered to meet the high-voltage insulation requirements potentially brought by lightning strikes.
This project’s primary novelty is its comprehensive analysis of the process by which Holm force induces contact failure in relays during lightning surge currents, together with its inventive design philosophy that effectively mitigates this issue by the application of Lorentz force. The project successfully designed a key mechanism combining a "U-shaped" moving spring structure with a shaft and flexible multi-strand copper wire connections, efficiently transforming the generated Lorentz force into positive pressure that stabilizes the contacts. This design, precisely verified and optimized through multiphysics coupling simulation technology, achieves a fundamental technological breakthrough from traditional "passive endurance" to "active dynamic counteraction," significantly enhancing the reliability and lifespan of relays in extreme lightning environments.
