By the nature of current plasma torches have the highest diversity of options. Therefore, we consider only the major ones. The vast majority of plasma torches for metal processing is performed on a direct current straight polarity (Scheme 1). This is due, above all, a physical feature of the arc, namely, that at the anode of the arc stands out more heat than at the cathode. The thermal power released in the electrode of the plasma torch, in contrast to the consumable electrode arc welding is not only useless but harmful. The smallest heat load is the electrode is the cathode. Suffice it to note that the maximum current load lantanirovanny tungsten electrode with alternating current is about twice, and the reverse polarity when using the DC is ten times lower than in straight polarity. Therefore, the DC plasma torches have the highest efficiency of capacity utilization. A pillar of intensely compressed arc must be rigidly stabilized on the axis of the electrode and the plasma torch nozzle. If you change the polarity of the electrode, this stabilization is disrupted, so the arc AC squeeze harder than the arc DC. An important advantage of direct current plasma generators compared to the AC plasma torches is more stable arc. The passage of current through zero may cause the extinction of the arc, so it is usually open-circuit voltage alternating current power source is not less than twice the operating voltage of the arc. When powered by direct current plasma torches can be achieved ratio ud / uxx equal to 0.8-0.9. Consequently, in the same installed power capacity of the arc and the dimensions of the DC current is less than the power and size of the AC power source. In addition, the constant current source provides a uniform loading of the three-phase network. Rectifying the problem existed before the DC is now practically solved by creating a broad and powerful production of electrotechnical small semiconductor valves. Initial costs and operating costs of the plasma units AC and DC are approximately equal. Therefore, in view of the above benefits for most of the processing of materials appropriate to apply a DC plasma torches. Plasma torches are used alternating current, in some cases because of the technological requirements of the process. For example, plasma welding of aluminum alloys is necessary to conduct an alternating current, as in periods of reverse polarity at the expense of the effect of cathode sputtering destroyed refractory oxide film of aluminum, which hinders the process of fusing metal. When the plasma melting in the case of parallel operation of multiple high-power DC plasma torches for a total bath, the anode occurs trudnoustranimoe magnetic interaction between the arcs. Therefore, in these conditions can be used in the AC plasma torches. In this case it is advisable to use torches in multiples of three, which ensures uniform loading of the three-phase network. We consider three basic schemes AC plasma torches. The diagrams 2 and 3 plasma torches are powered by a single-phase transformer. In Figure 3 implemented Gated Switching current so that the electrode operates
only as a cathode (at halftime straight polarity), and nozzle - as the anode (reversed polarity at halftime). In such a scheme is ensured by high resistance of the tungsten electrode. However, with increasing current deteriorating condition of the nozzle, and when working at currents below a certain limit (~ 150 a) is broken arc stability. Scheme 4 plasmatron is powered by a three-phase transformer. In this case, a high stability of the arc, but the electrode and nozzle are relatively severe conditions. In addition, much more complicated structure of the plasma torch. In order to improve the stability of the arc AC plasma torches perform some combo. In this case, the main arc of the AC direct stabilize the auxiliary low-power DC arc, burning, or between the electrode and the nozzle (Figure 5), or between two nozzles (Figure 6), one of which (the cathode) is formed. The second nozzle (anode) is performed with a large inner diameter. Due to their smaller that it is subject to thermal effects of the main pillars of the arc. Along with the arc plasmatron operating on alternating current of industrial frequency, recently developed high-frequency (RF) and microwave (MW) plasma torches [7, 8]. The principle of operation of high-frequency inductive or electrodeless plasma torch (Figure 7) consists in heating the gas to a plasma state in an electromagnetic field coil. For this purpose the cavity inductor fed by high-frequency generator (frequency 1-40 MHz, the voltage to 10 kV and up to 50 kW), fits the tube of heat-resistant insulating material such as quartz. In the plasma-forming gas is fed up and briefly, a metallic or graphite rods. The latter is heated by the field coil and causes the initial heating and ionization of the surrounding gas. When the electrical conductivity of the gas increases to a certain size, it begins an intense heating and ionization of the eddy currents created by the field of the inductor. After the development of self-discharge ring rod is removed from the cavity of the tube. Purge gas through the tube, passing through the annular discharge heated and ends in the form of a plasma jet, which reaches a temperature of 15 000-20 000 ° K, and the outflow velocity is ten times smaller than the velocity of the plasma jet arc plasmatron. High-frequency power is relatively expensive, high-frequency generators are complex and have limited cardinality. It is very clear (in the absence of electrodes) and a soft flare induction plasma torch found a use for the treatment of high purity refractory powders, single crystal growth and other specialized processes not requiring high performance. In the microwave (MW), or e-plasma torches, gas is also heated by electromagnetic field produced by the emitter electrode (Figure 8). Plasma torch high occurs in the form of a pointed electrode at the electrode and the high field strength near it. In the high-voltage and microwave electric field, free electrons are accelerated and acquire a kinetic energy that in a collision with gas particles cause their dissociation and ionization. The electrode is connected to the magnetron plasma torch generator frequency 2000-3000 MHz and a power of 2.5 kW. The plasma torch of the electronic plasma torch is interesting because there is no thermal equilibrium the electron temperature is an order of magnitude higher than the temperature of ions and free atoms. For example, at a temperature of 3500 ° K flame temperature reaches 35 E 000 ° C. This high temperature allows the electrons in the plasma jet chemical reaction synthesis of some special materials.
See also:
Plasma Welding
Plasma Welding Introduction
Plasma Welding Technique
Microplasma Welding
Gases for plasma processing of materials
Separation of plasma jet cutting
Compression of the arc
The energy properties of the plasma arc
Rationalization of plasma welding
Plasma welding and spraying
The plasma melting and remelting
Plasmatron. Requirements for plasmatron
Plasmatron. Schemes, classification
Classification by type of electrode plasma torches
Classification of torches by the nature of the current
Structure of the plasmatron basic units
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