MODELING OF THE CONTACT MATERIAL PROPERTIES & COMPOSITION INFLUENCES ON THE ELECTRICAL ARC EROSION PHENOMENON

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On the way to AgCdO substitution

A large number of researchers have been studying over the years in order to find out the specific material compositions that will produce a contact material having optimal performance. Nowadays, following the trend in the European market place to manufacture ‘green’ devices, another constraint that makes the problem even more complex is to develop contact materials that are Cadmium free. Years of development by contact manufacturers have produced many suitable replacement combinations of silver tin oxide, zinc oxide, indium oxide and mixtures of oxide. A large part of this effort has focused on materials based on silver tin oxide in the belief that the semi-refractory tin oxide particles would provide performance properties, especially resistance to contact welding and arc erosion, comparable to those of cadmium oxide; comparable but not the same and not as good in terms of overall behaviour.
Differences in erosion mechanisms and contact properties are mainly due to the considerably higher thermal stability of tin oxide and to the fact that CdO is wetted more easily by the silver melt than is SnO2. As a consequence, Silver/Cadmium oxide materials exhibit a somewhat more favorable overtemperature behavior than Silver/Tin oxide materials. Furthermore, many alternative CdO free contacts, particularly silver tin oxide (Ag/SnO2), are much more difficult to fabricate because of poor metallurgical properties. An earlier review by Shen [5] about the manufacturing difficulties and higher cost of CdO free contacts is still true today. Silver cadmium oxide with up to 15 % CdO can be made very efficiently by internal oxidation or powder metallurgy to include a range of physical, mechanical, metallurgical and electrical properties. However, both internal oxidation and powder metallurgy processes are complicated by the brittleness of many non-CdO compositions. The most successful CdO substitution has been based on silver tin oxide with various additives [6]. It has been also found that the contact resistance and the temperature rise of Ag/SnO2 are higher than those of Ag/CdO in the same conditions, so its applications have been limited [7]. But AgSnO2, with different additives to make up for its weaknesses, remains the best applicant to substitute AgCdO [8], [6]. Consequently, although silver/cadmium oxide (Ag/CdO) has been the preferred material for contacts due to its excellent performance properties and ease of manufacture into many configurations, silver/tin oxide contact materials have begun to replace silver/cadmium oxide materials in switching devices such as contactors, circuit breakers or relays. But power switching device manufacturers are still waiting for a better substitute. So, to end up with directions to design a good AgCdO substitute constitutes one of the main challenges of this research work.

Test Condition and Testing Procedure

The five contacts have all been tested under the same operating conditions: Supply current of 400 A, Circuit voltage of 28 V DC, Ambient: air, room temperature.
The only difference between these contacts lies in the number of breaking operations they have been subjected to. The purpose being to study and understand the effects of an increasing number of electric arcs on the contact material, we decided to make the first AgCdO sample undergo one “half-cycle” (only one break operation); the second one, two; the third one, three; the fourth one, ten and finally the fifth one, a hundred half-cycles. For every arc discharge, the contact resistance was measured.
Several reasons can explain the decision to work with half-cycles. The most important one is that we want to control the number of electric arcs that hit the contact material, and operating the contact only during the opening cycle allows us not to deal with the bounces of contacts induced while closing. An uncontrolled number of very destructive short arc discharges depending on the number of bounces is then one of the main effects of these contact bounces while closing, and we want to avoid these because they are unpredictable. Another reason for using this “half cycle” testing procedure is to eliminate mechanical erosion that may occur during the closing of contacts. Indeed, this study intends to study only the contact material erosion due to the arcing phenomenon.

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AgCdO Surface Dynamics

This section is focused on the study of the evolution of the contact surface with the number of electrical arcs under the operating conditions above mentioned. The following pictures correspond to the anode behavior on the moving electrode after a certain number of electrical arcs.

AgCdO Layer Dynamics

This section is devoted to the study of the contact material layer dynamics of the same samples above analyzed. The following pictures also correspond to the anode behavior on the moving electrode after a certain number of electrical arcs.

AgSnO2 Layer Dynamics

This section is devoted to the study of the contact material layer dynamics of the same samples above analyzed. The following pictures also correspond to the anode behavior on the moving electrode after a certain number of electrical arcs.

Experimental results

The same methodology as the one applied to study AgCdO layer dynamics has been used. Therefore, all the samples have been cut after experiments according to a vertical plan located at 1 mm from their periphery, polished and then analyzed
The initial configuration of tin oxide particles within the contact material layer before arcing can be seen in Figure 3.2-1.
The tin oxide particles are homogeneously and finely distributed within the layer. The AgSnO2 sample having been subjected to one electrical arc is shown in Figure 3.2-2.
The formation of a few tin oxide clusters located close to the contact surface can then be observed.
Opened and enclosed cavities of tin oxide granules can be noticed in Figure 3.2-4, a picture representing the impacted surface layer after three electrical arcs.
Some bigger tin oxide particles and some clusters of tin oxide can be seen in the surrounding of the electrode surface having been subjected to ten arcs in Figure 3.2-5. explained by the fact that, under gravity effects, since silver density is about 10.49 g/cm3 and tin oxide density is about 7.01 g/cm3, oxide particles move up within the molten bath to the contact surface.

Table of contents :

CHAPTER 1 INTRODUCTION
1.1. Arc erosion: breaking arc and material transfer
1.1.1. Metallic phase
1.1.2. Gaseous phase
1.2. Silver Metal Oxide based contact materials
1.2.1. AgCdO issue
1.2.2. On the way to AgCdO substitution
1.3. Methodology & outline
CHAPTER 2 AgCdO ARC EROSION BEHAVIOR
2.1. Arc erosion experiments
2.1.1. Sample Definition
2.1.2. Test Condition and Testing Procedure
2.1.3. Experimental Set Up
2.1.4. Experimental apparatii
2.2. AgCdO Surface Dynamics
2.2.1. Experimental results
2.2.2. Discussion
2.3. AgCdO Layer Dynamics
2.3.1. Experimental results
2.3.2. Discussion
CHAPTER 3 AgSnO2 ARC EROSION BEHAVIOR
3.1. AgSnO2 Surface Dynamics
3.1.1. Experimental results
3.1.2. Discussion
3.2. AgSnO2 Layer Dynamics
3.2.1. Experimental results
3.2.2. Discussion
3.3. Conclusion and comparison with AgCdO
CHAPTER 4 COMPLETE MACROSCOPIC ARC EROSION MODEL AND EXPERIMENTAL VALIDATION
4.1. Arc erosion model architecture
4.1.1. Arc Energy Transport Model
4.1.2. Thermal Model
4.1.3. Magneto-hydrodynamic Model
4.2. Arc erosion experiments
4.2.1. Sample Definition
4.2.2. Test Condition and Testing Procedure
4.2.3. White Light Interferometry
4.2.4. Comparison between model and experimental results
4.3. Conclusion
CHAPTER 5 MODELING OF THE CONTACT MATERIAL PROPERTIES & COMPOSITION INFLUENCES ON THE ELECTRICAL ARC EROSION PHENOMENON
5.1. Contact material properties influence
5.1.1. Force contributions
5.1.2. Ab initio
5.1.3. Contact material properties study
5.2. Tin oxide composition influence
5.2.1. Methodology
5.2.2. Results
5.3. Conclusion
CHAPTER 6 CONCLUSION
REFERENCES

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