current voltage characteristic of the plasma arc.
The energy properties of the plasma arc
Plasma arc - the converter of electrical energy into heat. So on the one hand, as an element of an electrical circuit, it is characterized by electrical parameters (current, voltage), and on the other hand, as a source of heat - thermal parameters (temperature, heat content). There is a complex relationship between the parameters of the first and second groups. Structural DC plasma arc can be represented as a number of specific sites consistently along its axis. The plasma arc, for example, direct action (see Fig.) Consists of a cathode region (1), dosoplovogo (2), vnutrisoplovogo (3) and zasoplovogo (4) sections of the column and the anode (5), located almost on the workpiece . We denote them by u 1 - u 5 , respectively. Accordingly, the arc voltage is the sum of voltage drops in these areas. U q = U 1 + U 2 + U 3 + U 4 + U 5 Similarly, the arc voltage is calculated indirect effect, except that the anode region is not included in the sum of voltage drops. At the site vnutrisoplovom column is a cylindrical conductive channel, while at the nozzle exit with increasing distance from a conductive column diameter increases and reaches a value on the product and the temperature and flow velocity of the plasma jet decreases.
Depending on the current and the degree of compression of the arc in the plasma torches with a tungsten cathode, the value of u1 varies from 5-8, but with the zirconium cathode within 10-12 V. The value of U 5 practically does not depend much on the material of the anode, the plasma-forming medium, and the current is 6.5 V. Thus, the plasma arc voltage is determined primarily by the field strength and the length of sections that make up the arc column. Voltage drop across the sites (2) and (3) are approximately equal (with equal length) Low plasma arc find the field intensity of the arc column: E e = U e * l e are counting then the current density by the formula: where k = 0,6 .. 0.9 - filling factor of the plasma channel nozzle. After this we find the conductivity of the plasma in the arc: and the known dependence of the various gases sigma = f (T) to determine the average cross-section of conductive column temperature Tm. For example, in Fig. 1 shows a graph of this dependence for nitrogen. plasma temperature is the heat source parameter of the plasma torches. It varies as the cross section of the arc column, and along its axis. The picture of the temperature distribution in the plasma arc can be obtained rather complicated experimental or calculated. In most cases, sufficient for engineering calculations to determine the average cross-section of conductive temperature of the plasma column as shown above. enthalpy of the plasma arc important thermal parameter of the plasma jet is its specific heat (enthalpy), ie, the amount of heat contained in a unit volume or mass of the jet. I = ST J / g , where C - specific heat of gas at temperature T, J / g ° C. In Fig. Figure 2 shows the number of heat from the gas temperature at atmospheric pressure, which show that the heat content of molecular gas at relatively low temperatures ((4-8) × 10 3 ° C) due to absorption of the energy released during dissociation of molecules reaches high values and an order of magnitude greater than the heat content of monatomic gases. Next heat the threshold of a sharp rise in the plasma occurs at a temperature of approximately 12.10 3 ° K by the absorption of the energy released in the ionization of atoms. vysokoentalpiynyh Using molecular plasma-forming gases in the energy for a more profitable because they are at lower temperatures have the same thermal efficiency as monatomic gases. At the same time reduces the heat loss by radiation in the walls of the plasma torch and the environment (these losses are proportional to the fourth power of temperature). The higher the heat content of the plasma (working) gas, the more power you want to send a unit length of the arc column, the greater, therefore, for a given current strength of the field column E. Thus, the field strength of the column, and hence the voltage of the plasma arc is primarily determined by composition of the plasma (working) gas. Current-voltage characteristics of plasma torches. Influence of working gas in the arc voltage is clearly illustrated by the current-voltage characteristics of plasma torches. representing the relationship between voltage and current of the arc, ceteris paribus (arc length, gas flow, the parameters of the plasma torch, the external environment). At low currents the current-voltage characteristics of plasma torches falling, and with increasing magnitude of the current move in the independent and incremental. At constant composition of the gas tensions of all sections of the plasma arc column increases with the degree of compression. The degree of compression of the arc column increases (up to a point) with a decrease in the diameter of the forming nozzle and increase the working gas flow rate. Studies show that most of the gas passes through the peripheral areas of the post and with increasing flow more intensively cools and compresses the pole. more intense squeezed arc, the lower the current value of its current-voltage characteristic becomes increasing. Thus, the plasma arc voltage depends on the structural dimensions of the plasma torch (d nozzle , l nozzle ) of the arc current, the composition and flow of the working gas and, finally, on the distance from the face of the plasma torch to the workpiece (l 5 ). To determine the operating voltage of the plasma torch of this type of building a family of current-voltage characteristics, each of which is removed at constant composition and gas flow rate Q, the length l of 5 and constant structural dimensions of the plasma torch. Sometimes it is also building the external characteristics of the plasma arc: U d = F (Q) and U d = F (L 5 ) at I d = const. These characteristics are increasing. They can be approximated by a linear and used to create automatic control of the process of welding arc voltage.
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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