RU2175595C2 - Power source of direct electric current welding arc - Google Patents

Abstract

FIELD: electric arc welding in production plants and home appliance devices. SUBSTANCE: power source includes mains rectifier 1, bridge type voltage converter having semiconductor switches 2,3,4,5, output transformer 6, output rectifier 12. Capacitor 9 is connected in parallel to output terminal of bridge type rectifier through alternating current switch 10. The last is controlled by means of electric current pickup 8 connected in series with primary winding of output transformer 6. Output terminal of pickup 8 is connected through threshold device 11 with input of switch 10. Capacitor 9 that is connected at high load electric current values, allows to provide resonance switching mode of semiconductor switches 2,3,4,5 in range of large electric currents and to increase by 1.5 times outlet electric current value. EFFECT: enhanced specific characteristics of power source. 2 dwg

Description

 The invention relates to welding equipment, in particular to small-sized devices for welding with a direct current electric arc, which use a link with a frequency increased relative to the power supply network.

 There are known sources of power for the welding arc [1, 2], in which a high frequency link on thyristors is used to reduce weight and dimensions. Due to the relatively low frequency (units of kHz) due to the low speed of existing thyristors, the mass of the output transformer and reactive elements remains high, which limits the use of these devices in portable welding machines.

 A known power source for loads of electric arc type, in which the high frequency link is a bridge voltage converter on transistors [3]. Transistors, having better frequency properties than thyristors, can increase the conversion frequency to several tens of kilohertz and, thereby, reduce the size and weight of the power supply relative to the thyristor device. The disadvantage of transistor converters is the low power of power sources based on them. This is due to the fact that when working on an inductive load in the form of a welding transformer, it is necessary to reduce the average value of the current within the transistor's open state by 2–3 times to prevent the transistor from overloading with pulsed power at the time it is turned off.

 Of the known power sources, the closest in technical essence and the achieved result to this invention is a power source for a DC welding arc, containing a network rectifier, a bridge transistor converter with a load in the form of a series-connected output transformer and a limiting reactor, an output rectifier [4].

 The converter in this device operates in a self-oscillating (resonant) mode, or in a mode close to resonant. In this mode, the transistors are turned off at a bullet voltage on them, which allows to obtain more power at the output of the converter than the analogue presented above [3]. The disadvantage of this source also remains a low value of the output power and welding current and, as a result, low weight and size characteristics.

 The invention is aimed at the propagation of the resonant mode of switching transistors in the region of maximum collector currents, which leads to an increase in the output current and to improve the overall dimensions of the power source of the DC arc.

 This is achieved by the power source of the DC welding arc, containing a network rectifier, a bridge voltage converter on semiconductor switches, a restrictive inductor, an output transformer, an output rectifier, the output of a network rectifier connected to the input diagonal of the bridge converter, to the output diagonal of which an output transformer is connected in series with the limit a choke, and an output rectifier is connected to the secondary winding of the output transformer. In addition, unlike the prototype [4], a capacitor is connected to the AC output of the bridge converter through an AC key, and a current sensor is connected in series with the input winding of the output transformer, and the output of the current sensor through a threshold device is connected to the control input of the AC key. Thanks to this connection of the capacitor at high load currents, it is possible to reduce the dynamic loads on the semiconductor switches of the bridge converter when turned off, thereby increasing their load in the average current.

 The authors and the applicant are not aware of the power sources of the DC welding arc, containing such a combination of elements and with their connection as in the claimed source, which allows to obtain a result that is not achieved by other technical solutions.

 In FIG. 1 shows a block diagram of a power source; in FIG. 2 - diagrams of currents and voltages at characteristic points of the circuit.

 The power source of the DC welding arc contains a network rectifier 1, to the DC voltage output of which a bridge voltage converter of four semiconductor switches 2, 3, 4, 5 is connected. A series-connected output transformer 6 (primary winding) is connected in parallel to the AC voltage output of the bridge converter, a restrictor inductor 7 and a current sensor 8, as well as a series-connected capacitor 9 and an alternating current switch 10. The output of the current sensor 8 is connected to the control input of the AC key 10 through the threshold device 11. The secondary winding of the output transformer 6 is connected to the output rectifier 12, the DC voltage output of which is connected to the load. In FIG. 1 also shows a diagram of a transistor switch 3 with a point 13 on the collector of the transistor.

 The power source of the welding arc operates as follows.

 Using a control circuit (not shown in Fig.), Semiconductor switches 2 and 5 located on one diagonal of the bridge are opened for one half-cycle of the output voltage of the bridge converter, and keys 3 and 4 on the second diagonal of the bridge are opened with the keys closed for the second half-cycle of the output voltage 2 and 5. At the output of the bridge converter, we have an ac voltage of a rectangular shape (plot 1, Fig. 2), which, through the output transformer 6 and the output rectifier 12, is supplied to the load.

 To clarify the essence of the invention, we consider the mode of turning off the transistor of the key 3 of the bridge converter under the load created by the output transformer 6 and the restrictive inductor 7. In FIG. 2 curves 1 and 2 show the voltage and current at point 13 on the collector of the transistor of switch 3.

где C_Э - эквивалентная емкость нагрузки, пересчитанная параллельно транзистору ключа (см. фиг. 1), либо емкость специально подключенного внешнего конденсатора;
U_m - максимум напряжения на коллекторе транзистора;
I_m1 - максимальное значение тока коллектора транзистора перед его выключением.The rise time of the voltage across the collector τ _f1 is determined from the known expression of the charge of the capacitance with direct current on the switching interval

where C _E is the equivalent load capacity, counted in parallel with the switch transistor (see Fig. 1), or the capacity of a specially connected external capacitor;
U _m is the maximum voltage at the collector of the transistor;
I _m1 - the maximum value of the collector current of the transistor before turning it off.

For reliable operation of the transistor in the key, it is required that the collector current decay time t _SP , which is determined mainly by the transistor parameters, be less than or equal to the value of τ _ф1 defined by expression (1). Otherwise, when t _SP > τ _{f1, the} final stage of turning off the transistor will occur at the maximum voltage on the collector (diagrams 1, 2 of Fig. 1), which is unacceptable for transistors.

Увеличить значение I_m1 в соответствии с формулой (2) можно только путем увеличения C_Э, так как величина t_СП определяется параметрами транзистора, а U_m определяется напряжением входной сети и не могут быть произвольно изменены. Верхнее значение величины C_Э определяется возможностью работы устройства на холостом ходу. Для этого случая из формулы (1) максимальное значение C_Э
C_ЭМ = U_m•I_mXX• τ_ф1 ,
где I_mXX - величина тока нагрузки на холостом ходу, определяемая током холостого хода выходного трансформатора.For the case t _SP = τ _ф1 from expression (1) we have the limiting amplitude of the collector current

Increasing the value of I _m1 in accordance with formula (2) is possible only by increasing C _E , since the value of t _SP is determined by the parameters of the transistor, and U _m is determined by the voltage of the input network and cannot be arbitrarily changed. The upper value of the value of C _E is determined by the possibility of the device idling. For this case, from formula (1), the maximum value of C _e
C _EM = U _m • I _mXX • τ _f1 ,
where I _mXX is the value of the load current at idle, determined by the idle current of the output transformer.

From diagram 1 it follows that the maximum value of τ _ph1 cannot be greater than T / 4, where T is the period of the output voltage, since this will lead to the disappearance of the flat part on the collector voltage diagram, and in practice, to failure of the converter vibrations.

Then, for τ _ф1 ≤ T / 4, the previous expression will have the form
With _EM = U _m • I _mXX • T / 4. (3)
Expressions (2) and (3) determine the limiting possibilities for the amplitude of the current in the collector of the transistor switch, implemented in a known scheme.

The inventors have proposed to introduce a capacitor 9 in the power source of the welding arc parallel to the AC output of the bridge converter, performing the function C _E only at high load currents during the closed state of the AC key 10, with the AC key 10 controlled by the current sensor 8 and a threshold device eleven.

The signal of the current sensor 8 is supplied to the threshold device 11 and, when the threshold value of the sensor signal 8 (threshold U _P ) is exceeded (plot 3 of FIG. 2), it closes the alternating current switch 10. Thus, a capacitor 9 is connected to work, which in this case determines the rate of increase of voltage on the key collector 3 τ _f2 (plot 5 of Fig. 2). In this case, short current pulses flow through the capacitor 9, the duration of which is equal to the rise time of the voltage across the collector of the transistor (plot 4 of Fig. 2). The condition τ _ph2 > t _SP is fulfilled and it becomes possible to additionally increase the current through the keys 2, 3, 4, 5 of the bridge converter in accordance with expression (1). At low currents, the threshold device 11 does not work, the capacitor 9 does not connect, and the operation of the proposed source does not differ from the work of the known one.

 The inventive device is tested on a sample of the welding machine ASPT-60, commercially available by the Ryazan Instrument Plant according to the patent of the Russian Federation N 2018424. A node from a current sensor on a K20x12x6 ferrite ring, a threshold device on KU112 thyristors, a K78-2 capacitor and an alternating current key is introduced into the welding machine on the transistor KT878A without heat sink. The mass of the introduced node is 0.12 kg. With the same KT847A transistors in the keys of the brain transducer of the welding machine, the output current was brought from 60 A to 90 A. To the existing mass of ASPT-60 4.5 kg was added 0.12 kg of the introduced unit and 0.8 kg from the installation of more powerful power elements, rated for current 90 A.

 Thus, in the proposed power source of the DC welding arc, an increase in the output current by 1.5 times and specific characteristics in the output current from 13.3 A / kg to 16.6 A / kg, i.e. by 25%.

Sources of information
1. Copyright certificate N 15300367, cl. B 23 K 9/00 B.I. N 47, 1989

 2. RU N 2080221, class B 23 K 9/00 B.I. N 15, 1997

 3. Copyright certificate N 1825681, cl. B 23 K 9/00, NOGN 71177, B.I. N 15, 1997

 4. RU N 2018424, cl. B 23 K 9/00 B.I. N 16, 1991

 5. Sources of secondary power supply of electronic equipment E.M. Romash M., Radio and Communications, 1981, p. 139 - 144.

Claims (1)

 A power source for a DC welding arc, containing a network rectifier connected to the input diagonal of the bridge voltage converter with semiconductor switches, a threshold device, an output transformer and a limiting choke connected in series, the output of a bridge voltage converter connected to an output transformer whose secondary winding is connected to output rectifier, and restrictive choke, characterized in that it is equipped with a capacitor, a sensor t it is connected with an alternating current key, and the capacitor is connected through an alternating current key to the output of the bridge voltage converter, the current sensor is connected in series with the primary winding of the output transformer, and the output of the current sensor through a threshold device is connected to the input of the alternating current key.

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