Int. Cf. H 05 B 7/18 DEVICE FOR CONTROLLING AN ELECTRIC ARC INSTALLATION This invention relates to electromechanical devices, in particular to the systems of automatic control for electric arc plasma installations, which are applicable in such spheres as plasma metallurgy, plasma chemistry. A conventional device for controlling operational modes of an electric arc installation includes a plasma generator of indirect action, a rectifier connected with an energy source and with the cathode and. the anode of the said plasma generator, a source of plasma-forming gas, a control block, which controls a starting mode of operation and excitation of an arc discharge, and the outlet of the said control block is connected into the arc discharge excitation circuit and the inlet of the said control block is switched into the general circuit of the rectifier (E.M. Esibyan. Plasma Arc Apparatuses, Kiev, "Tekhnika" Publishers, 1971, p. 137-140). The main disadvantages of the above mentioned installation lie in its inability to ensure stability of an arc discharge mode and adequate adjustment of operational modes, and also in a short service life of electrodes in the plasma generators of indirect action. A conventional device (a prototype), which in its technical essence and results attained bears the most close resemblance to the present invention, is the one for controlling the operational modes of electric arc installations, which incorporates a plasma generator of indirect action, a rectifier connected with a cathode and an anode of the said plasma generator, a source of plasma-forming gas, a control block, which controls a starting mode of operation and excitation of an arc discharge, the inlet of the said control block is switched into the general circuit of the rectifier, and, according to the present invention, the said control block, which controls a starting mode of operation and excitation of an arc discharge, is supplied with coils of automatic switches incorporating delay circuits, which via control contacts and thermal relays are connected with the arc discharge excitation circuit of the said plasma generator, and wherein the rectifier comprises a semi-automatic three-phase diode thyristor rectifier, each phase of which is supplied with an additional diode, and with a resistor connected in parallel with the thyristor (Ukrainian Patent No. 21208, Int. Cl. H 05 B 7/18, Appl. date 11.05.94. Publ. 16.10.2000).
However, the above mentioned installation fails to ensure stability in the operation of the plasma generator and adequate adjustment of its operation modes due to the following reasons: - under conditions of operation, high current fluctuations (pulsations) result in damage to thyristors; - the excitation block fails to ensure reliable and stable starting the plasma generator; - the device fails to control electrical reliability of gaps between the electrodes of the plasma generator, and that results in the rapid burning-up of electrodes; - the device does not ensure temperature control and thus, the electrodes of the plasma generator get overheated. The primary object of the present invention is to provide the means for controlling operation of electric are installations and, due to introduction of alternative elements and units into the control circuit, to minimise fluctuation amplitudes in current and voltage pulsations, to ensure precise control over electrical reliability of gaps between the electrodes, to provide temperature control means for the electrodes of the plasma generator, and to ensure control over their wear, and thereby to ensure reliable initiation of an arc discharge, to expand the range of stable operation of the plasma generator, and to increase the reliability of operation of the system as a whole. According to the invention the set objective is achieved by the provision of a device for control over the operation of an electric arc installation, which includes a plasma generator, a source of plasma-forming gas, a control block, which controls a starting mode of operation and excitation of an arc discharge, and the said device also includes a high frequency step-up transformer, a semi-automatic three-phase diode thyristor rectifier connected with the cathode and the anode of the plasma generator, and each phase of the rectifier is supplied with an additional diode, and a resistor connected in parallel with the thyristor, and an electric circuit breaker, and, according to the present invention, in the semi-automatic three-phase diode thyristor rectifier connected in series with the thyristor, on the anode side, there is provided a resistor, and the high-frequency transformer is assembled on a number of individual closed-circuit magnetic lines, the primary windings of which are connected in series, and the secondary winding envelops all the magnetic lines and is switched in series into the cathode circuit; and the control block, which controls starting modes of operation and arc charge initiation, incorporates means for temperature control of cooling water in the electrodes of the plasma generator, such as, for instance, thermocouples with comparator units; and control means to control electrical reliability of gaps between the electrodes of the plasma generator, each of the said control means comprises a high-frequency transformer the primary winding of which 'via a capacitor is connected with the electrodes, and its secondary winding is connected 'with comparator units; a means for control over wear of electrodes, which represents a tubular electrode mounted inside the cathode cavity of the plasma generator and capable of longitudinal movement inside the cavity, the said electrode is isolated from the abut swirler of the cathode and is switched on to an additional adjustable voltage source either of negative or positive polarity, and the said electrode is connected with a comparator unit, and also, via a flowmeter, it is connected to the source of plasma forming gas, therein the control winding of the electric circuit breaker is switched on via series-connected contacts of comparator units. Introduction of an additional resistor in series with the thyristor in the circuit of the semi-automatic three-phase diode thyristor rectifier has ensured a reliable operation of the rectifier and thereby has expanded the range of stable operation of the plasma generator, made the generator starting more reliable, improved stability of arc discharge burning within the range of operational current fluctuations due to lower pulsation amplitudes in current and voltage. Due to provision of the means for controlling electrical reliability of gaps between the electrodes of the plasma generator, it has become feasible to ensure permanent control over electrical reliability of gaps between the electrodes, to sustain plasma flow parameters over time, and to exclude probable burning-up of the electrodes. Introduction of the control means for temperature control of cooling water in the electrodes of the plasma generator has ensured a reliable control over temperature and eliminated any risk of electrode overheating. The installation has been also provided with the means for control over wear of electrodes, thereby ensuring reliable and stable burning of an arc discharge, and in case wear in the electrode exceeds a critical extent, the said means automatically shuts the plasma generator off. If any departure from the set mode parameters occurs in any above described devices, the control system automatically operates and shuts them off and thereby preventing all kinds of emergency. The picture shows a basic diagram of the control means for an electric arc installation; which includes a semi-automatic three-phase diode thyristor rectifier I switched on to a power transformer (not shown). The rectifier 1 is comprised of a three phase diode bridge assembled on the diodes 2 and the additional diodes 3. Connected in series with the diodes 3, in each phase of the anode power supply circuit of the electric arc installation are the thyristors 4 with series-connected resistors 5. In each phase, the by-pass resistors 6 are connected in parallel with the thyristors 4. The cathode circuit of the installation incorporates the electric circuit breaker 7. The high-frequency transformer 8 is assembled on a number of individual close-circuit magnetic Lines, wherein the primary windings of all the magnetic lines are connected in series, and the secondary winding envelopes all the magnetic lines and is switched to the cathode 8. The capacitor 10 protects the rectifier I from the action of high-frequency (r.f.) current. The circuit, between the anode II and an intermediate electrode 12, incorporates a limiting resistor 13 with a circuit release 14. The cooling channels for the electrodes 9, 11 and 12 accommodate the control means 15 for control over the temperature of cooling water, such as, for example, thermocouples, which are connected with the comparator units 16; and, mounted between the electrodes 9 - 12 and 11- 12; there are the means for controlling electrical reliability of gaps between the electrodes, each of the said control means comprises a high-frequency transformer the primary winding 17 of which via the capacitor 18 and the electric circuit breaker 19 is connected with the electrodes, and the secondary winding is connected with the comparator unit 20. The cathode chamber of the plasma generator accommodates the means for control over wear of the electrodes, which represents the tubular electrode 21 mounted in such a manner that it is capable of longitudinal movement inside the cavity, the said electrode is isolated from the abut swirler 22 of the cathode 9. The electrode 21 is connected with an additional adjustable voltage source 23 either of negative or positive polarity and with comparator unit 24, and also via flowmeter 25 it is connected to the source 26 of plasma forming gas. The control winding 27 of the circuit breaker 7 is switched on through the series connected contacts of the comparator units 16, 20 and 24. The device operates in the following manner: Alternating voltage is supplied to the semi-automatic three-phase diode thyristor rectifier 1. The angle of thyristor cut-off is set to the value 1800 by a the power regulator (not shown); the thyristors 4 are closed. Cooling water and plasma forming gas are supplied to the plasma generator and switch on the electric circuit breaker 7. Rectified current is supplied to the plasma generator, as well as to the terminals of the control block, which controls the starting mode of operation and initiation of an arc discharge. When current flows through the system of the thyristor and two resistors, the scheme actually operates as a voltage divider; and when voltage values from all the phases are added up, the pulse amplitude of resulted operating current is decreased. The current from the control winding 27 of the electric circuit breaker 7 flows through the series-connected contacts of the comparator unit 16, which controls the temperature of cooling water, through the control means 20 for controlling electrical reliability of gaps between the electrodes, and through the control means 24, which controls wear of the electrodes. In case any deviations from the set modes of operation occur in the above mentioned devices, the control winding 27 of the electric circuit breaker 7 operates and shuts off the power source from the plasma generator. With the overall dimensions in the plasma generator and those in the power source being the same, it has become feasible to increase the values of operating current from 200-220 A to 500-600 A, those of operating voltage from 600 W to 1,200 W, and that, in turn, has resulted in the power increase from 150.. .200 kW to 600-700 kW of the plasma generator. At present, relevant documentation has been developed for a pilot installation; trial runs have been performed, and the positive results have been obtained.