Exploring DC Contactor Contact Forces Through Field Discussions
What is a high voltage DC contactor?
👉 Learn what a high-voltage DC contactor is? Check out this article.
During recent technical discussions with clients conducting short-circuit tests on DC contactors, one recurring topic emerged — contact bounce under fault current conditions. This phenomenon, where mechanically closed contacts suddenly separate when exposed to high current, sparked in-depth conversations with both engineers and users.
Why does this happen? The answer lies in what we now technically identify as the dc contact repulsion force—a critical contributor to the operational safety and reliability of high voltage DC contactors (HVDC contactors). Understanding this force is essential for improving performance under fault conditions and ensuring long-term durability.
Case Insight – Identifying Contactors Exposed to Short-Circuit Testing
During these conversations, we shared how to identify whether a DC contactor has undergone short-circuit testing based on the contact surface condition. When a contactor is closed and exposed to a short-circuit current above 4kA, a strong electric repulsion force acts on the contact bridge, often causing a momentary bounce or separation.
This separation results in an intense arc at the contact point, leaving behind a visible molten mark or glow spot on the contact surface—evidence of an arc pool caused by sudden contact separation due to extreme dc contact repulsion force. This is a telltale sign that the contactor has experienced high fault current, and should be flagged for inspection or replacement after testing.
However, it’s important to note that DC arcing can also occur during the normal breaking process of a contactor—even at relatively lower currents such as 500A, 600A, or 1kA. In these cases, although arcs are present during the contact separation, the resulting damage differs significantly from that caused by short-circuit conditions.
Specifically, arcing from large but non-fault current interruption tends to create rougher and more eroded contact surfaces, rather than the localized melting or glow spot seen in short-circuit-induced failures. This difference in surface morphology allows engineers to visually distinguish between damage caused by normal switching arcs and short-circuit electric repulsion events in HVDC contactors.
Such visual diagnostics, combined with knowledge of the contactor’s test history, provide an effective means to evaluate dc contactor contact force resistance and overall durability in high-voltage applications.
What is Electric Repulsion in HVDC Contactors?
When two conductive contacts carry a high current, a magnetic field forms around them. This field, interacting with the current itself, produces a repulsive force—commonly called the Holm force—between the contacts. This phenomenon is not just theoretical; in contactors under high short-circuit currents, this force can grow so strong that it physically separates the contacts, causing contact failure.
This effect is more than just a magnetic side note; it’s a crucial design challenge in dc contactor contact force optimization.
Simulating Contact Repulsion Force: The Ansoft Maxwell Approach
To quantify and better understand the electric repulsion force in HVDC contactors, the research team employed finite element analysis (FEA) using Ansoft Maxwell. Through electromagnetic field simulation under various contact geometries and current conditions, they were able to extract detailed data regarding:
- Current density across the conductive bridge (Refer to Figure 1 below)
- Magnetic field intensity near contact points (Refer to Figure 2 below)
- Resulting force vectors on both static and moving contacts (Refer to Figure 3 below)
For example, under a 6000A short-circuit, the repulsive force peaked at 26N, confirming the danger zone where contact separation may occur. This approach provided valuable insight into how dc contact repulsion force behaves under different scenarios and laid the foundation for correlating simulation results with real-world performance.
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As shown in Figure 1, current density concentrates heavily near the contact bridge, forming the foundation of repulsion force generation.
Figure 2 reveals how the magnetic field intensity builds around the current path, driving the Lorentz interactions.
In Figure 3, the simulation confirms that a short-circuit current of 6000A causes a repulsive force of up to 26N—highlighting the critical zone where contact separation becomes inevitable.
Factors That Influence DC Contactor Contact Force
Through simulation and physical testing, they identified several variables that dramatically impact the dc contact repulsion force:
Contact Height
Increasing the contact height concentrates the Holm force near the contact spot but doesn’t significantly affect the overall vertical force on the contact spring. However, it reshapes the internal force distribution, which in turn influences mechanical stability.
As shown in Figure 4, increasing contact height has a diminishing effect on Holm force while stabilizing overall contact force distribution.
Contact Gap
Wider gaps slightly reduce the Holm force due to reduced magnetic field intensity but increase the spring’s Lorentz force—raising the overall repulsion and necessitating stronger spring counterforces. Table 4 summarizes how increasing the contact gap slightly reduces Holm force but leads to a continuous rise in Lorentz force, affecting the required contact pressure.
Spring Structure
They analyzed six variations of moving contact springs. Figure 6 and Figure 7 illustrate how different spring geometries influence the overall dc contact repulsion force, enabling us to identify structurally optimized designs. Longer springs or those with altered width-to-height ratios changed the Lorentz force acting on the contact bridge. The optimal design reduced the net repulsion force while maintaining electrical continuity.
[Data Source Description]
The above image data is sourced from publicly available online data.
Real-World Testing of Electric Repulsion in HVDC Contactors
The research team extended their investigation beyond simulation by conducting short-circuit current tests on a prototype high voltage DC contactor. During testing, the contactor maintained a stable connection when exposed to a fault current of 5960A. However, once the current reached 6000A, contact oscillation occurred—leading to a sudden drop in current due to momentary separation of the contact surfaces.
Interestingly, the finite element simulation had predicted 5820A as the maximum tolerable current. The discrepancy between the simulated and experimental values was just 2.35%, providing strong validation for the accuracy and reliability of the simulation model used to analyze dc contact repulsion force in HVDC systems.
Conclusion: From Theory to Design Practice
This article confirmed that electric repulsion, particularly under high fault current, is not just a secondary concern—it defines whether a contactor survives or fails. With precise simulation and careful design of contact geometry and spring structures, we can drastically improve the performance of HVDC contactors in critical systems like electric vehicles and energy storage.
Discover High-Performance HVDC Contactors from BSB
The insights gained from analyzing dc contact repulsion force and validating simulation results directly inform the engineering and design of BSB’s high-voltage DC contactors. With over a decade of experience, BSB has developed a comprehensive range of contactors that deliver exceptional stability, arc resistance, and performance under extreme fault currents.
Whether you’re working on electric vehicles, energy storage systems, or photovoltaic inverters, BSB’s HVDC contactors are designed to meet your toughest application demands.
👇 Explore the full BSB DC Contactor series below:
