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Michael Pritula,
Founder & CEO Research Developer
With a professional career spanning over three decades, I have held various positions in both the public and private sectors. My strengths in analysis and research highlight my innovative and technical skills. Currently, I work as a media consultant in the field of political strategy and analysis, providing expert assessments on military and political developments for international media. In the field of science and research, I hold two patents. One patent involves the development of a new Stirling engine and the other is related to the development of liquid metal contacts for vacuum switches.
The Technical Physics of Elastic Contacts
Mykhailo Prytula
In the beginning, there were contacts. And the creator divided the contacts into movable and stationary, and the electric arc flew between them, creating flickers of light and heat. It is the properties of the contacts and the processes that occur between them at different voltages and currents that determine the design of modern switching apparatus.
The title "The Technical Physics of Elastic Contacts" is a humorous parody of Ragnar Holm’s classic work "The Technical Physics of Electrical Contacts". It highlights the key idea of our research: using elastic contacts to reduce electrodynamic forces, which is the main conclusion of this work. The main formula that determines electromagnetic forces indicates the advantages of elastic contacts in reducing these forces, making them more effective compared to traditional rigid contacts.
So let us open our sacred book, "Die technische Physik der elektrischen Kontakte" (1) by Ragnar Holm. The most important characteristics are the contact resistance between closed contacts and the electrodynamic forces that act during the closing and opening of contacts and their operation in the closed state.
The contact resistance determines how much heat will be generated at the contact transition when the nominal current flows. And this heat needs to be dissipated from the contacts.
P=I2RThe power lost at the contacts is determined by the formula:
P=260020.000050=338 WattWhen the nominal current reaches hundreds and thousands of amps, a significant amount of heat is generated. Let’s consider a serial OLTC with a regulation current of 2600 Amps and a contact resistance of 50 microohms:
Thus, when regulating the three-phase current on the OLTC, at least 1 kilowatt of power will be constantly released in a closed space. Another example: a vacuum interrupter from EATON for operation in a nominal current mode of 4000 Amps requires additional cooling (2,3). Let’s look at the microscopic structure of the most advanced composite contact materials, which combine very hard tungsten carbide and copper. Before the first activation, the contacts are polished and fit well to each other (Fig.1)
The contact resistance, especially in a vacuum, is small, and everything is fine. But after the first switches, the copper from the top layer is carried away by the arc discharge, and all subsequent switches occur through current bridges, randomly scattered grains of tungsten carbide (Fig.2).
In fact, the contacts close not by the entire visible area of the contacts (S), but only by a certain part, protruding by fractions of a millimeter, and the actual area of the contacts (So) is much smaller than the visible one. How much smaller? No one knows exactly, but everyone tries to compress the contacts harder to increase the actual area, thereby reducing the contact resistance. How hard? Thousands of newtons, the higher the current, the higher the pressure.
For such pressure, different methods are used, the most common being compression using powerful spring systems. The entire design thought is to compress the contacts harder mechanically and then solve this problem. «However, while the use of compression springs may effectively provide the force necessary to close the contacts, these springs detrimentally delay the opening of the contacts when needed. That is, the compression springs must first be uncompressed before the contacts are capable of moving to provide an adequate gap for circuit interruption. This delay in the opening of the contacts is particularly disadvantageous in medium voltage direct current (DC) circuits requiring ultra-fast switching capability» (4).
Therefore, compressing the contacts with springs interferes but is necessary not only to reduce contact resistance but also to balance the electrodynamic forces:
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Basic formula for electrodynamic forces
2. Formula for contact surface area
3. Modified formula for electrodynamic forces
These formulas and concepts are detailed in Ragnar Holm's work "Die technische Physik der elektrischen Kontakte" (1958), which is a fundamental source in the field of electrical contacts.
We look at that mysterious point G (Druckfestigkeit, compressive strength) and make a simple conclusion: the harder the contacts, the greater the electrodynamic forces, the greater the pressure needed on the contacts, and the larger the springs needed. Thus, the traditional approach of increasing the hardness of the contacts and, consequently, the compression forces is flawed. It is based on the assumption that increasing the pressure on the contacts will reduce the contact resistance. However, the formula So=P/G shows that high compressive strength (G) leads to a reduction in the actual contact area, increasing contact resistance and electrodynamic forces. This, in turn, requires even greater efforts to compress the contacts, creating a vicious cycle of problems.
Oh Great Ragnar Holm, they are still trying to increase the hardness of the contacts and then increase the compression force of these contacts, and then they complain that it hinders them!
Another conclusion to be drawn from looking at this formula is that to reduce the electrodynamic forces, and hence the compression forces, the hardness of the contact material should be reduced.
What? Soft contacts? But what about contact resistance? What about inertial rebound? And compression in tons? What about the doctoral and professorial titles earned on compressing hard contacts? What about the thousands of patents?
Concept of Elastic Contacts
Elastic contacts in their essence and mechanics are typical wire mesh vibration dampers (Fig.3) made of refractory metals, impregnated with low-melting alloys that provide contact through the liquid phase.
In early literature, the term composite liquid-metal contacts is used, but this is not a defining characteristic of this specific type of contact, as the liquid phase exists as a thin layer on the surface of the refractory wire, while the important characteristics, namely anti-vibration and contact over the entire visible area, are achieved thanks to the properties of the knitted damper.
That is why these elastic contacts do not have inertial rebound, cannot weld, do not have, in the usual sense of the word, contact resistance, and, as will be shown later, do not experience electromagnetic separation. But if these contact materials have such outstanding properties, why are they not yet widely used in electrical engineering?
Main Problems of Elastic Contact Materials:
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Production Technology: Until now, the technology for manufacturing and applying elastic contact materials required very expensive equipment, complex thermochemical processes in a hydrogen atmosphere, and, accordingly, specially trained personnel. The problem is that Gallium and its alloys have poor adhesion to tungsten and other refractory metals.
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Lack of Consolidated Information: Existing information on research into such elastic contacts has not been consolidated into one source and is not well known to specialists.
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Lack of Systematic Research: Systematic studies that would allow engineers to apply such materials have not been conducted so far, despite the outstanding properties of elastic contacts.
Manufacturing Elastic Contacts
The first problem, the technological one, was solved by the author in April 2024, who developed a simple method for the manufacture and application of elastic contact materials (patent application PCTIB2024/054125). The proposed method of manufacturing and applying elastic contacts is simpler and, in most cases, more economical than the manufacture and application of traditional rigid contacts in vacuum sw itching equipment.Manufacturing elastic contacts boils down to a few simple operations:
Making the Damper: A damper is made from knitted wire, usually tungsten, the same wire that was recently used for incandescent lamp filaments, or stainless steel, in special cases - molybdenum, niobium, rhenium, and their alloys. The technology is widely used to manufacture knitted dampers, as noted, vacuum chamber manufacturers receive ready-made dampers from manufacturers.
Soldering the Dampers: These dampers are soldered to the conductors in the same way as rigid contacts.
Impregnation with Low-Melting Alloy: Dampers are impregnated with a low-melting alloy, which remains liquid under operating conditions. These are usually various eutectic alloys of gallium, indium, and tin with the addition of alloying elements, such as silver, to lower the melting point. And that’s end.
Conducted Research and Testing of Elastic Contacts
In this work, the author collects all available information on the research of elastic contacts, including those that have not yet been published and the experiments conducted by the author.
Heating Test
1. First Experimental Vacuum Arc Quenching Chamber:
The first experimental vacuum arc quenching chamber (4) with elastic contacts was made in 2001 (6).
Elastic contacts with a diameter of 22 mm and a thickness of 5 mm were made from a damper of tungsten wire with a thickness of 50 microns, impregnated with an indium-gallium-tin eutectic alloy in a hydrogen atmosphere at about 1000 degrees Celsius (7).
The resulting contacts were mechanically (without soldering) fixed in the conductors of a serial vacuum chamber for a nominal current of 240 Amps and tested in the corresponding serial contactor.
The vacuum chamber was tested for heating with a current of 300-500 amps, in series with a serial vacuum chamber for comparison. Both chambers were inserted into a serial vacuum contactor. The tests showed that the heating losses on elastic contacts are 20 times lower than those on serial molybdenum contacts, and a simple replacement of contacts in a serial vacuum chamber with elastic ones doubled the nominal current of the serial switching equipment while maintaining geometric dimensions.
Second Experimental Vacuum Chamber: The second experimental chamber with elastic contacts with a diameter of 20 mm was tested on August 21, 2003, and showed a heating of 34 degrees at a current of 900 Amps (8).
Research on the Electrodynamic Stability of Elastic Contacts
The research on the electrodynamic stability of elastic contacts was conducted using the setup shown in Figure 5.
Here, A and B are the terminals to which a direct electric current is supplied. Terminal A is connected to a movable contact system I, carrying the movable contact 2.
Opposite it is the stationary contact 3, fixed on the contact cup 4.
Investigation of Electrodynamic Stability During Switching
In the off state, the movable contact system, connected to rod 9, was held by latch 10. When switched on, a signal was sent to the electromagnet coil 11, rod 9 was released from the latch, and the contacts 2 and 3 were closed under the weight of the movable system and, if necessary, additional weight 12. The investigated elastic contact with a surface area of 700 mm² was placed between the copper contacts 2 and 3.Tests were conducted at currents up to 45 kA. During the research, the current and voltage drop on contacts 2 and 3 were recorded using an oscilloscope.In Figure 6, a characteristic oscillogram obtained when switching a constant current of I = 42.5 kA with elastic contacts is shown.
The oscillogram shows that the voltage drop on the contacts remains constant (straight line 2) during the time the current (curve I) increased to the maximum value of I = 42.5 kA. Therefore, no contact bounce was observed. Vibrations and welding of the elastic contacts were also absent.
For comparison, Figure 7 shows a characteristic oscillogram of the switching process with copper contacts. In this case, the switching was accompanied by an almost instantaneous bounce of the copper contacts at a current of I = 3 kA, as evidenced by the increase in voltage drop between these contacts (curve 2) followed by their welding.
In elastic contacts, no contact bounce occurs during switching because the soft elastic base prevents the occurrence of elastic deformations in the solid electrodes when they collide with each other through this base. In addition, in elastic contacts, the actual contact surface area is very close to the apparent contact surface area. Therefore, according to the formula,the electrodynamic forces in elastic contacts are extremely small, hence there is no electrodynamic bounce of the contacts.
Investigation of Electrodynamic Stability at Short-Circuit Currents
The pressure on the contacts was applied using weight 12 (5) and was 50 N.Figures 8 and 9 show characteristic oscillograms of the process of passing short-circuit current through elastic contacts (Fig. 8) and copper contacts (Fig. 9) with compensation of the electrodynamic forces of the setup circuit. Here, curve I shows the increase in short-circuit current passing through the tested contacts, while line 2 corresponds to the voltage drop across these contacts. The bounce of the elastic contact was not observed throughout the range of investigated currents (up to 45 kA), while the bounce of copper contacts occurred at a current of 10 kA (Fig .9, line 2) followed by their welding.
Switching Tests of Elastic ContactsSwitching tests of elastic contacts (Fig.10) for durability were conducted in a pre-production vacuum contactor specially designed to work with elastic contacts. During the tests, 200,000 switching cycles at 250A in AC4 mode, current 600 amperes, and voltage 690 volts were successfully completed (10).Switching tests for overvoltages showed that overvoltages were 2-3 times lower than the norm (11).CONCLUSIONThese contacts, based on elastic damping elements made of refractory metals and impregnated with a fusible eutectic, can be used in vacuum switching apparatus, especially in systems requiring the switching of large currents (e.g., electrolyzers for hydrogen and metal production) or high-speed switching (e.g., medium voltage direct current). They are also suitable for instantly increasing the switching capacity of existing systems (e.g., safe boosting of OLTC for wind turbine transformers). By using elastic contacts, the limitations on the nominal current magnitude caused by the quadratic increase in compression force are removed. Additionally, new systems can be smaller and cheaper.
Further research and inclusion in standards based on these studies are necessary for implementation.
Mykhailo Prytula
References
1. Holm, Ragnar. "The Technical Physics of Electrical Contacts." (1958).
2. Eaton. (2018). "Vacuum Interruption (EVI) Technology Product Guide." Retrieved from official Eaton documentation, page 25.
3. Eaton. (2011). "Vacuum Interrupter (EVI) Technology Product Guide." Retrieved from official Eaton documentation, page 15.
4. Patent US11152174B2, owned by Eaton Intelligent Power Ltd.
5. International Patent Application PCTIB2024054125 - Prytula, M. (2024).
6. "Elektrotechnika" Journal #2, 2002 ISSN 0013-5860 - Elektrotechnika. (2002).
7. Smirnov, Y. I. (1985). "Method for Producing Liquid Metal Contacts with a Refractory Framework." Patent SU1325590A1.
8. Vacuum Tube Test Report PIJZ 640140.901. (2003).
9. Beljajew, Vladimir Lvovich. "Liquid Metal Composite Contacts and Their Use in High Current Electrical Devices." Dissertation for the Degree of Doctor of Technical Sciences, Scientific Supervisor - Honored Scientist of the RSFSR, Doctor of Technical Sciences, Professor O.B. Bron. Leningrad, 1983.
10. Test Report of Prototype Vacuum Contactors Type KM17R-33VM with Composite Liquid Metal Main Contacts for Switching Wear Resistance at 660V - PIJZ. (2007).
11. State Makeevka Research Institute for Occupational Safety in Mining (MakNII) - MakNII. (2006). Overvoltage Test Protocol No. 1021-A.
12. Ukrainian Utility Model No. 20612 — Y. Smirnov, M. (2010). "Modular Vacuum Contactor with Liquid Metal Contacts." Ukrainian Utility Model No. 20612.
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