Breakthrough in EV Power Control: New Study Optimizes DC Contactor Performance
The relentless pursuit of efficiency, reliability, and safety in electric vehicles (EVs) and renewable energy systems is driving innovation at every level of engineering, from the battery chemistry to the power management systems. A critical, yet often overlooked, component in this complex ecosystem is the humble DC contactor. These electromagnetic switches are the gatekeepers of high-power circuits, responsible for safely connecting and disconnecting the massive currents that flow between a vehicle’s battery pack and its motor, or within charging infrastructure. When an EV driver presses the accelerator, it is the contactor that must instantly and reliably close the circuit, delivering power. When the vehicle is parked or a fault is detected, it is the contactor that must open, isolating the high-voltage system. The speed and stability of this “closing action” are paramount for system performance and longevity.
A groundbreaking new study from researchers at Xi’an Jiaotong University has shed significant light on the intricate dynamics of a specific type of DC contactor—the parallel double-coil design. This research, published in the prestigious journal Electric Power Engineering Technology, details a sophisticated simulation method that accurately models the entire closing process, providing invaluable insights for engineers designing the next generation of power control systems for EVs and beyond. The work, led by PhD candidate Chengyang Yan, Professor Lijun Wang, and their colleagues WenZhe Zhang, YiFan Huang, and Kai Wang, represents a major step forward in predictive engineering, moving beyond traditional single-software simulations to a more holistic, multi-physics approach.
The significance of this research cannot be overstated. As the global push toward electrification intensifies, the demands on DC contactors are becoming more stringent. They must operate faster, with greater precision, and with minimal wear and tear. One of the most detrimental phenomena in a contactor’s operation is “contact bounce.” This occurs when the moving contact slams into the stationary contact with such force that it rebounds, creating a brief, high-energy arc. This arc, even if it lasts only milliseconds, causes material erosion on the contact surfaces, gradually degrading their performance and shortening the component’s lifespan. In a high-voltage, high-current environment like an EV, this degradation can lead to increased resistance, heat generation, and ultimately, system failure. Therefore, minimizing contact bounce is a primary design goal. However, predicting and mitigating bounce has been a persistent challenge, as it is the result of a complex interplay between electromagnetic forces, mechanical dynamics, and material properties.
Traditional methods of analyzing contactor performance have often relied on either physical testing, which is time-consuming and expensive, or computer simulations using a single software package. These single-domain simulations, while useful, have a critical limitation: they struggle to accurately capture the tight coupling between the electrical, magnetic, and mechanical systems. For instance, a simulation might calculate the electromagnetic force based on a static coil design, but it wouldn’t dynamically account for how the movement of the iron core changes the magnetic circuit, which in turn alters the coil’s inductance and current, thereby changing the force itself. This feedback loop is central to the contactor’s behavior but is difficult to model in isolation.
The research team from Xi’an Jiaotong University has addressed this challenge head-on by pioneering a co-simulation methodology. Their approach seamlessly integrates two powerful engineering software platforms: Ansys Maxwell, a leading electromagnetic field analysis tool, and MSC Adams, a premier multi-body dynamics simulation software. This union of disciplines creates a far more realistic and accurate model of the contactor’s operation. The process begins with Maxwell, which calculates the electromagnetic force generated by the coils at various positions of the moving iron core. This force is not a static value; it is a dynamic function that changes as the core moves closer to the stationary part, altering the magnetic flux path. The researchers then feed this time-varying force data directly into the Adams model as the primary driving input.
The Adams model, in turn, is a detailed mechanical representation of the entire contactor mechanism. It includes the moving and stationary iron cores, the connecting rods, the moving and stationary contacts, and the critical springs—both the return spring that holds the contactor open and the contact spring that ensures a firm connection once closed. The software applies the laws of physics to this system, calculating the acceleration, velocity, and displacement of all moving parts in response to the electromagnetic force, gravity, spring forces, and the impact when the contacts meet. By setting the simulation time to 40 milliseconds and running the calculation, the researchers can observe the entire closing sequence in virtual reality, from the initial energization of the coil to the final settling of the contacts.
The true validation of any simulation lies in its ability to predict real-world behavior. To test their model, the team conducted physical experiments on a prototype DC contactor under low-current conditions. They used high-speed data acquisition equipment to record the voltage across the contacts and the current through the coil with microsecond precision. By analyzing the moment the contact voltage drops to zero, they could determine the exact closing time. The experimental results showed a closing time of approximately 15.6 milliseconds. Strikingly, their co-simulation predicted a closing time of 15.8 milliseconds—a difference of less than 2%. This remarkable agreement between simulation and experiment is a powerful testament to the accuracy and robustness of their methodology. It proves that their virtual model is not just a theoretical exercise but a reliable tool for predicting actual performance.
With a validated simulation platform in hand, the researchers were able to conduct a series of parametric studies, systematically altering key design variables to understand their impact on the closing action and contact bounce. This is where the study delivers its most valuable engineering insights. The first area of investigation was the influence of spring pre-pressures. The team tested a range of pre-loads for both the return spring and the contact spring. The results revealed a nuanced picture. For the return spring, a pre-pressure around 5.1 Newtons was found to be optimal. At this setting, the moving iron core reached its closed position the fastest and most stably. Lower pre-pressures allowed for quicker initial movement but resulted in a slight “bounce-back” before final closure, while higher pre-pressures simply took longer to overcome, increasing the overall closing time. For the contact spring, the findings were even more critical. Within a moderate range (2.7N to 16.7N), the pre-pressure had little effect on the core’s motion. However, when the pre-pressure was increased to very high levels (19.7N and 22.7N), a detrimental phenomenon occurred: the moving iron core would actually move downward slightly before reversing course and moving upward to close. This counter-intuitive “dip” was caused by the initial electromagnetic force being insufficient to overcome the massive spring force, leading to a temporary collapse of the magnetic circuit. This not only dramatically increased the closing time but also introduced significant instability. The clear takeaway for designers is that while sufficient contact pressure is necessary for a low-resistance connection, there is a critical threshold beyond which the spring force becomes counterproductive, and “more is not better.”
The study then turned its focus to the heart of the contactor’s operation: the electromagnetic coils. The parallel double-coil design features a “start” coil and a “hold” coil. The start coil, with fewer turns of thicker wire, is designed to provide a massive initial surge of current and force to rapidly initiate movement. The hold coil, with many more turns of thinner wire, then takes over to maintain the closed position with much lower power consumption. The researchers first examined the effect of the start coil’s number of turns. They found that, as expected, more turns generally led to a faster closing time due to a higher ampere-turn product, which is directly proportional to the magnetic field strength. However, the relationship was not linear. Increasing the turns from 188 to 288 yielded a significant improvement, but further increases up to 438 provided diminishing returns. This suggests that there is an optimal point of diminishing returns where the added copper and electrical resistance of more turns negate the benefits of the stronger initial force. For this specific design, a start coil of 288 turns was deemed the most practical choice.
The impact of the coil’s internal resistance was found to be profound. As the resistance of either the start or hold coil increased, the simulation showed a consistent trend: the driving current decreased, the electromagnetic force weakened, and the closing time lengthened. This is a direct consequence of Ohm’s Law (V=IR); for a fixed supply voltage of 12V, a higher resistance means a lower current. But the implications went beyond just speed. The study revealed a direct link between coil resistance and contact bounce. When the start coil’s resistance was too high, resulting in a very low starting current, the moving iron core approached the closed position with less kinetic energy. Paradoxically, this did not lead to a gentler closure. Instead, the lower energy meant the core was more susceptible to being “pushed back” by the contact spring, leading to larger and more violent bouncing upon impact. This finding is crucial, as it highlights that simply reducing current to save energy can have unintended negative consequences on mechanical stability.
The analysis of the hold coil parameters yielded similarly important results. The researchers discovered that a lower number of turns in the hold coil actually led to a faster closing time. This counter-intuitive result can be explained by the dynamic nature of the system. A coil with fewer turns has lower inductance. When the moving core is far from the stationary part, the magnetic circuit has a large air gap and high reluctance. A low-inductance coil allows the current to rise very quickly in this high-reluctance state, generating a strong initial pull. A high-inductance coil (with more turns) resists this rapid current change, resulting in a slower build-up of force at the beginning of the stroke. While a higher ampere-turn product is generally desirable, the speed of current rise in the initial phase proved to be more critical for minimizing total closing time in this design.
However, the story does not end with the fastest possible closure. The final and perhaps most critical insight from the study concerns the trade-off between speed and stability. While a very high ampere-turn product (achieved with a high number of turns) can generate immense force and speed, it also imparts a large amount of kinetic energy to the moving parts. When this high-energy system comes to a sudden stop at the end of its stroke, the energy must be dissipated. This leads to significant contact bounce, which, as discussed, is highly destructive. The simulations showed that a hold coil with 1300 turns produced the shortest closing time, but it also caused the most severe bouncing. After a comprehensive analysis that balanced closing speed, energy consumption, and mechanical stability, the researchers concluded that a hold coil with 1870 turns was the optimal choice for this application. It provided a fast and reliable closure while keeping the contact bounce within acceptable limits, thus maximizing the overall lifespan of the contactor.
In conclusion, the research conducted by Chengyang Yan, Lijun Wang, WenZhe Zhang, YiFan Huang, and Kai Wang at Xi’an Jiaotong University represents a significant leap in the engineering of DC contactors. By developing and validating a sophisticated co-simulation method, they have created a powerful virtual testbed that can accurately predict the complex, multi-physics behavior of these critical components. Their parametric studies provide clear, data-driven guidance for designers, revealing the optimal settings for springs and coils to achieve a fast, stable, and durable closing action. The findings on the detrimental effects of excessive spring pre-load and the critical balance between electromagnetic force and contact bounce are particularly valuable. As the automotive and energy industries continue to demand higher performance and greater reliability from their power systems, this kind of fundamental, application-focused research is essential. It provides the scientific foundation upon which safer, more efficient, and longer-lasting electric vehicles and renewable energy installations can be built. The work, published in Electric Power Engineering Technology (DOI: 10.12158/j.2096-3203.2024.01.022), is a shining example of how advanced simulation and rigorous analysis can solve real-world engineering challenges.