Before the contacts of a relay open or close, enough energy can be present to cause an electric discharge across the gap. This phenomenon is called an arc. Arcs can damage and shorten the life of a relay. Each time an arc ignites, there is some level of contact erosion; how much is determined by the severity and duration of the arc. Severe arcing can also cause contact welding, a condition where the two sides are welded together, resulting in a perpetually closed condition.
Because of the dangers of arcs, understanding the mechanism of arc discharge and developing a safe means of arc interruption is a major goal for manufacturers of power relays. Omron recently developed an arc simulation technology to learn more about arc behavior.
Traditionally, high speed cameras and electrical waveform measurements have been the primary methods of researching arc behavior. In order to analyze temperature fields, pressure fields, and flow fields, which are factors that affect arc behavior, Omron turned to computer-aided engineering (CAE) to simulate the conditions that might ignite an arc. The CAE-based simulation technology combined thermal fluid analysis and electromagnetic field analysis.
The simulation assumes a DC arc using the test circuit shown in Figure 1 for the simulation. This circuit is the basis for the analysis model shown in Figure 2.
Figure 1: Circuit for simulation
Figure 2: Analysis model
The arc interruption simulates the method that DC arc interruption is typically done, where a magnet is used to elongate and quench the arc by Lorentz force (magnetic arc quenching). The simulation includes a uniform magnetic flux field to enable arc interruption.
Figure 3 shows the CAE simulated arc (top) compared to a physical arc, as captured by a high speed camera (bottom). These results show that the behavior of the simulated arc compares well to that of an arc discharge in the real world.
Figure 3: Arc shape during magnetic arc quenching
Factors that are difficult to characterize using electrical measurements and high speed cameras are temperature, pressure, and flow fields; however, the CAE-based simulation enabled visualization of the three-dimensional behavior of all three (Figure 5).
Figure 5: Contour diagrams of the temperature, pressure, and flow fields
The simulation also allowed analysis to be performed at four levels of magnetic flux density (20 mT, 40 mT, 70 mT, and 85 mT), to determine the effect of different magnetic fields on the arc discharge. Results showed that arc length and diameter decrease with an increase in magnetic flux density. When an arc is elongated, the arc resistance increases. As resistance increases, current decreases, eventually reaching zero, leading to arc interruption.
This finding contradicts real-world findings, however, which show that an increase in magnetic flux density actually decreases arc length. The engineers at Omron turned back to the simulation to see what was happening.
Arc resistance, Rarc is expressed by:
Where σ stands for electrical conductivity [S/m], l is arc length [m], and S is arc cross-sectional area [m2]. This equation suggests that the increase in arc resistance may be due to the decrease in the arc cross-sectional area. Figure 6 illustrates the effect of increasing the magnetic flux density, along with the temperature field. The low temperature air may have induced a thermal pinch, which reduced the cross-sectional area of the arc, followed by the increase in arc resistance that led to arc interruption.
Figure 6: Effect of magnetic flux density on arc diameter
The full results of Omron’s study are detailed at https://www.omron.com/global/en/technology/omrontechnics/vol52/002.html
Some of the discoveries have been incorporated into Omron power relays, such as the G9KB .
For more information on the design of relay contacts, including the arcing phenomenon, take a look at Tech Spotlight: How Do You Make The Most of Renewable Energy?