RU2062192C1 - Method to control erosion of plasmatron cathode - Google Patents

Abstract

FIELD: plasma cutting, method to control EROSION of plasmatron cathode. SUBSTANCE: method to control erosion of plasmatron cathode, that uses vortex stabilization of arc, provides for connection of cathode and nozzle by resistor and in process of treatment EROSION parameter is registered. Registered parameter is compared with base parameter and cathode is considered eroded as level of erosion parameter exceeds level of base parameter N times as much with N is preliminary given number. Amplitude of alternating component of leakage current running through connecting cathode and nozzle resistor is used as parameter of cathode EROSION.. Amplitude of permanent component of leakage current running through the resistor is taken as base parameter, and in the case N = 3 +/- 0.5. Value of electrical resistance of the resistor is selected so, that in case of non-eroded cathode amplitudes of alternating and permanent components of leakage current running through resistor are equal. EFFECT: simplified method of plasmotron cathode EROSION degree definition. 2 cl, 7 dwg

Description

 The invention relates to the field of plasma cutting, and more particularly to methods of wear of the plasma torch cathode.

 A known method of controlling the wear of the plasma torch cathode, in which the wear parameter is recorded during processing and compared with the basic parameter, which is used as the sound accompanying the cutting process.

 However, this method also does not have sufficient strength to control cathode wear, since the dependence of the leakage current on the degree of cathode wear is not monotonic.

 The aim of the invention is to increase the accuracy of control of the wear of the cathode of the plasma torch.

This goal is achieved by the fact that in the method of controlling the wear of the plasma torch cathode with vortex stabilization of the arc, in which the cathode and the plasma torch nozzle are connected by a resistor and during the processing, the wear parameter proportional to the leakage current through the resistor connecting the cathode and the nozzle is recorded, compare it with the basic parameter and the cathode is considered worn when the level of the wear parameter is exceeded N times the level of the base parameter, where N
a predetermined number, in accordance with the claimed solution, the amplitude of the variable component of the leakage current through the resistor is used as the wear parameter, and the amplitude of the DC component of the leakage current through the same resistor is used as the base parameter, with N 3 ± 5, and the value of the electrical resistance of the resistor choose such that, with an unworn cathode, the amplitudes of the variable and constant components of the leakage current through the resistor are equal.

 Improving the accuracy of diagnostics is achieved due to the fact that the alternating and constant components of the leakage current respond differently to changes in the depth of the erosion hole of the cathode (the alternating component changes about an order of magnitude more), but equally responds to random changes in technological parameters: plasma-forming gas pressure and operating current .

 Using the amplitude of the variable component as the wear parameter allows us to increase the signal value of the wear parameter in comparison with the prototype, and using the amplitude of the constant component as the base parameter allows weakening the influence of process parameters on the accuracy of the cathode wear control.

 The choice of the electrical resistance of the resistor connecting the cathode and the nozzle, such that the amplitudes of the variable and constant components of the leakage current through the resistor are equal when the cathode is not worn out, simplifies signal processing and also improves the accuracy of the result.

 Figure 1 presents the equivalent electrical circuit of the arc in the plasma torch.

 Here: 1 arc power source; 2 cut sheet of metal; 3 plasma torch nozzle; 4 cathode of the plasma torch; 5 arc resistance between the cathode and the nozzle; 6 arc resistance between the nozzle and the cut sheet; 7 - resistance of the gas layer between the surfaces of the arc and nozzle; 8 - a resistor connecting the cathode and the nozzle; 9 voltmeter; 10 block of the automatic cathode wear control system (cathode wear sensor).

 During plasma torch operation, the cathode undergoes erosion wear. In this case, a hole is formed in the zone of arc attachment on the cathode in the plasmatron with vortex stabilization of the arc, the depth and diameter of which increases with time.

 The arc column in the plasma torch rotates with the gas vortex (figure 2). In this case, the cathode spot of the arc moves along the lateral surface of the hole, so the trajectory of the arc does not coincide with the axial line of the plasma torch. And this mismatch is the stronger, the larger the size (depth and diameter) of the hole.

 Deviation from the nozzle axis means the approach of the arc axis to the nozzle wall, i.e. a decrease in the thickness of the interlayer relative to cold gas between the surfaces of the nozzle and the arc and, accordingly, a decrease in the electrical resistance of this interlayer.

где

постоянная составляющая напряжения дуги между катодом и листом;
R_р сопротивление резистора 8;
R_г сопротивление 7 газовой прослойки между поверхностью дуги и сопла;
R_сд сопротивление 6 между соплом и деталью.When the plasma torch is operating, the operating arc current (Fig. 1), initiated by the power source 1, flows through resistances 5 and 6 between the cathode 4 and the cut sheet 2. In the presence of resistance 8 of the resistor connecting the cathode and the nozzle, a so-called leakage current in resistors 6 , 7, 8 between the cathode and the cut part. The DC component of the leakage current is determined by the following ratio:

Where

DC component of the arc voltage between the cathode and the sheet;
R _{p the} resistance of the resistor 8;
R _g resistance 7 of the gas layer between the surface of the arc and the nozzle;
R _sd resistance 6 between the nozzle and the part.

где

переменная составляющая напряжения дуги между катодом и листом.The magnitude of the variable component of the leakage current is determined by the following ratio:

Where

AC voltage component between the cathode and the sheet.

A comparison of formulas (1) and (2) shows that the alternating and constant components of the leakage current equally depend on the sum of the resistances in the denominator, as well as on the resistance R _{g of the} gas layer between the surfaces of the arc and nozzle.

 The formation of the variable component of the arc voltage between the cathode and the nozzle occurs as follows.

The arc trajectory is determined by the rapid rotation of its part located in the nozzle, together with the gas vortex and the slow movement of the cathode zone along the surface of the erosion hole. Such a trajectory does not have strict axial symmetry, which causes a periodic change in the length of the arc between the cathode and the nozzle and, as a result, a periodic change in the arc voltage, for example, between the cathode and the nozzle, the cathode and the cut part, these periodic changes occur with a gas rotation frequency ν in the plasma torch nozzle. For the PMR-74 plasmatron, the rotation frequency is approximately n 10 kHz. There is also a network frequency n _c k • 50 Hz.

. Увеличивается также размах поперечных колебаний дуги внутри сопла и соответственно уменьшается средний зазор между стенкой сопла и дугой и величина сопротивления R_г.With an increase in the size of the cathode erosion well, the amplitude of the arc voltage fluctuations increases, i.e. value increases

. The magnitude of the transverse oscillations of the arc inside the nozzle also increases and, accordingly, the average gap between the nozzle wall and the arc and the resistance value R _g decrease.

тока утечки обусловлено увеличением

и уменьшением R_г при износе катода.Thus, the increase in the variable component

leakage current due to increase

and a decrease in R _g during cathode wear.

практически оказывает влияние только уменьшение R_г при износе катода.Constant component

practically only the decrease in R _g with cathode wear affects.

и

по-разному зависят от степени износа катода: возрастание переменной составляющей

происходит быстрее, чем постоянной составляющей

.therefore

and

differently depend on the degree of cathode wear: an increase in the variable component

faster than constant

.

переменная составляющая тока утечки, кривая

постоянная составляющая тока утечки. Из представленных графиков видно, что при увеличении глубины лунки катода от

до h_л 3,5 мм переменная составляющая

тока утечки изменяется от 0,16 A до 0,6 A, а постоянная

от 0,16A до 0,2A (кривые получены при R_р=25 Ом). При этом зависимости составляющих токов утечки от глубины лунки не являются монотонными. Подъемы чередуются со спадами. Эти спады и подъемы могут быть объяснены циклическим характером эрозии лунки, т.е. при эрозии вместо равномерного увеличения размеров лунки происходит чередование процессов выгорания середины лунки и ее краев. Поскольку процесс эрозии однозначно связан с катодным пятном, то можно сделать вывод о цикличности изменения расстояния от оси лунки до катодного пятна, тела дуги, а также о цикличности изменения R_г и составляющих тока утечки через резистор. Однако из графиков (фиг.3.) видно, что процесс цикличного изменения R_г однонаправленно воздействует на составляющие тока утечки.A typical dependence of the leakage current on the depth of the cathode well is shown in FIG. 3. Here is the curve

AC leakage current component

DC component of leakage current. From the presented graphs it is seen that with increasing depth of the cathode well from

up to h _l 3.5 mm variable component

leakage current varies from 0.16 A to 0.6 A, and the constant

from 0.16A to 0.2A (curves obtained at R _p = 25 Ohms). In this case, the dependences of the component leakage currents on the depth of the well are not monotonic. Rises alternate with downturns. These ups and downs can be explained by the cyclical nature of hole erosion, i.e. with erosion, instead of a uniform increase in the size of the hole, the processes of burning out the middle of the hole and its edges alternate. Since the erosion process is unambiguously associated with the cathode spot, it can be concluded that the distance from the axis of the hole to the cathode spot, the arc body is cyclical, as well as the cyclical changes in R _g and components of the leakage current through the resistor. However, from the graphs (figure 3.) it is seen that the process of cyclic changes of R _g unidirectionally affects the components of the leakage current.

и

зависят также примерно одинаковым образом от расстояния Δ_ск ск между катодом и соплом, давления P₀ рабочего газа, величины I₀ рабочего тока дуги, степени износа Δd_c сопла (Δd_c увеличение диаметра сопла при его износе). Чтобы исключить влияние всех указанных факторов на точность контроля износа катода, в качестве параметра износа используют соотношение

, которое не зависит от Δ_ск, P₀, I₀, Δd_c. Использование этого соотношения позволяет также уменьшить влияние цикличности изменения R_г на точность контроля.In addition, it was experimentally established that

and

depend also approximately the same way the distance Δ _{ck ck} between the cathode and the nozzle pressure P ₀ of the working gas, the working value I ₀ of the arc current, the wear of the nozzle Δd _c (Δd _c increase in diameter of the nozzle when it is worn). To exclude the influence of all these factors on the accuracy of cathode wear control, the ratio

which is independent of Δ _ck , P ₀ , I ₀ , Δd _c . The use of this ratio also reduces the influence of the cyclical changes of R _g on the accuracy of control.

. При неизношенном катоде величина этого отношения равна

. При изношенном катоде 3 ±0,5(h_л.изн. 3,5±0,5 мм).Figure 4 shows the dependence of this relationship on the depth of the hole, i.e., the curve

. With an unworn cathode, this ratio is

. With a worn cathode of 3 ± 0.5 (h _{L. life.} 3.5 ± 0.5 mm).

 From a comparison of the form of any of the amplitude curves of the components of the leakage current (Fig. 3) with the curve of the ratio of these amplitudes (Fig. 4), it can be seen that the latter reflects the degree of wear of the cathode much better and can be taken as the basis for controlling its wear.

When the cathode is worn, the quality of cutting sharply decreases (the width of the cut increases, the drift of the edges, gratification is formed). For cathodes of the studied type, the permissible depth of the hole is equal to h _l.s. = 3.5 ± 0.5 mm.

 To implement the inventive method for monitoring cathode wear, a cathode wear sensor 10 (FIG. 1) is used, the structural diagram of which is shown in FIG. 5.

Here: 1 unit for extracting the variable component of the signal;
2 block selection of the DC component of the signal;
3 cathode normal signal conditioning unit;
4 cathode emergency signal conditioning unit;
5 indicator (LED) of the normal state of the cathode;
6 indicator (LED) of the emergency state of the cathode;
7 power supply.

 The process of monitoring cathode wear using a wear sensor is as follows.

и постоянной

. Очевидно, что указанные составляющие напряжения связаны с составляющими тока утечки через резистор следующим образом:

;

(фиг. 6, 7).After selecting the resistance value R _p between the cathode and the nozzle by experimentally turning on the cutting process, an electric signal carrying information about the cathode wear parameter and an electric signal carrying information about the basic parameter are removed from the resistor R _p installed between the cathode and the plasma torch nozzle, form of voltage components: variable

and constant

. It is obvious that these voltage components are connected with the components of the leakage current through the resistor as follows:

;

(Fig. 6, 7).

тока утечки через резистор R_р. В блоках 3 и 4 происходит сравнение амплитуд составляющих сигнала. При этом блок 3 выдает сигнал на индикатор 5 в случае, если отношение

3 + 0,5, а блок 4 выдает сигнал на индикатор 6 в случае, если отношение

.At the same time, the variable component of the signal is allocated by block 1 (regardless of the frequency of the variable components, i.e., the transmission range of block 1 in frequency covers all frequencies that are really important for monitoring wear), the constant component of the signal is allocated by block 2 (Fig. 5). At the output of blocks 1 and 2, signals are generated proportional to the amplitudes of the components

leakage current through the resistor R _p . In blocks 3 and 4, the amplitudes of the signal components are compared. In this case, block 3 gives a signal to indicator 5 in case the ratio

3 + 0.5, and block 4 gives a signal to indicator 6 in case the ratio

.

 The cutting process is switched off to replace the cathode in the plasma torch by an operator-plasma cutter by the signal of indicator 6.

 The selection and introduction into the sensor of a specific value of N from the range of possible values from 2.5 to 3.5 is carried out by the plasma cutting operator, depending on the type of plasma torch according to the results of the pilot operation of the sensor.

 The implementation of the system for controlling the wear of the plasma torch cathode on plasma cutting machines based on the claimed method allows each plasma cutter to service 2 3 machines simultaneously. Thus, the economic effect of the implementation is defined as a reduction in the necessary costs for the wages of plasma cutters. In the USSR there are approximately 600 cars. With a two-shift operation, this corresponds to the number of plasma cutters, equal to 1200 people. YYY2 YYY4 YYY6

Claims (2)

1. A method for controlling the wear of the plasma torch cathode, in which the wear parameter is recorded during processing and compared with the basic parameter, characterized in that when controlling the plasma torch cathodes with vortex stabilization, the cathode and nozzle are connected by a resistor, and the variable current component is used as the wear parameter leakage I ^≃ through the resistor, and as a basic parameter use the value of the DC component of the leakage current I ⁼ . 2. The method according to claim 1, characterized in that they use a resistor with a value of electrical resistance, which ensures equal equality of the variable and constant components of the leakage current when the cathode is not worn, and the cathode is considered worn when the ratio I ^≃ / I ⁼ = 3 ± 0.5.

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