GB2055491A - Automatic regulation of arc welding - Google Patents

Abstract

With underwater arc welding, long leads separating the arc from the welding plant produce a voltage drop, which on a constant voltage supply interferes with the arc voltage regulation. This regulation is restored by keeping the output voltage of the plant equal to the sum of a preset voltage and another voltage, proportional to the short circuit current. Upon starting (contactor 6 closed), circuit 9 memorises the short- circuit current as measured by saturable reactor 8 and adds this to the preset value. The welding plant is then supplied via contactor 5, rather than 6 and choke 7, and the regulation begins. <IMAGE>

Description

SPECIFICATION Equipment for underwater arc welding The invention refers to equipment for underwater arc welding and welding in similar conditions where the power source is installed a considerable distance away from the welding arc, e.g. at sea level, with the welding operator working at the bottom of the sea, at a considerable depth. When welding with consumable electrodes in a shielding gas, the so called MIG process, the power source is usually expected to supply to the arc a constant voltage which together with an electrode fed into the arc at a constant rate of feed results in stable arc conditions.

My British patent specification 1540880 describes a method to overcome the difficulties relating from the fact, that the voltage drop along the cables may result in the requirement of a rising output voltage of the power source in contrast to the constant voltage required at the arc. According to my patent specification pilot leads from the arc to the power source permit the output voltage of the plant being controlled by comparing the arc voltage with the preset voltage of a potentiometer installed in the power source.

Object of the invention is an equipment which permits the described operation without the need of pilot leads. On underwater arc welding equipment according to the invention means are provided for controlling the output voltage of the welding plant so that it remains equal to the sum of two voltages, one of these being preset on a potentiometer at the plant, and the other a voltage proportional to the current and determined by the short circuit current at a measured voltage, as well as means for changing the control of the output voltage from said measured voltage to said sum of two voltages.

An example of the invention is shown in the drawing. The welding arc 1 to be operated at the bottom of the sea, receives its supply over long cables 2 from the welding plant 3 stationed above sea level and connected to the mains 4 over a contactor 5. Another contactor 6 connects the welding plant 3 to the mains 4 over a choke 7.

Before starting the welding operation the welding plane is connected to its supply over the choke 6, with the aim to reduce the voltage at the point of operation to a safe value. The current flowing through the cable is sensed over the D.C. current transformed 8, designed as a D.C. saturated reactor. When the operator touches the workpiece earth, the current through the cables as well as the voltage are sensed and stored in the computing circuit 9. After this storage has been completed the computing circuit gives a signal to the contactor 5 which then connects the plant through the mains, it reconnects the control of the output voltage, and it also starts operating the electrode feed not shown in the drawing.The control circuitry is preferably designed so that the values of voltage and current taken at the initial short circuit are recorded without delay, whereas the controlled output voltage on the plant during welding will receive its signal with a delay so as to avoid hunting of the controls.

A circuit similar to that shown in the drawing can be used where the welding plant is connected to the welding arc immediately at or near its full value i.e. without preliminary series connection of a choke. In that case the short circuit current will be measured and compared with the output voltage of the plant when the arc is struck and the signal for the start of the electrode feed is given.

For an operation described it will be preferable to use a welding equipment described in my British patent specification 947814 where the short circuit current is limited to a defined value by giving the plant a nearly rectangular characteristic.

It is known to compensate the voltage drop in power cables by compounding the supply voltage.

An application of such known methods for the control of welding plant has to account for the special conditions imposed by the characteristics of the arc. The invention makes use of the fact, that the arc produced with a consumable electrode is as a rule preceded by a short circuit current, either for striking the arc itself or for closing a pilot circuit connecting the power source. In both cases the output voltage feeding the welding cables will be equal or very nearly equal to the voltage drop along these cables.

1. An equipment for underwater arc welding over long welding leads with consumable electrodes supplied at a constant rate of feed, with means for controlling the output voltage of the welding plant so that it remains equal to the sum of two voltages, one of these being preset on a potentiometer at the plant, and the other a voltage proportional to the current and determined by the short circuit current at a measured voltage, as well as means for changing the control of the output voltage from said measured voltage to said sum of two voltages.

2. An equipment according to claim 1, where the output voltage is reduced until the operator produces a short circuit, with means for measuring the output voltage and short circuit current.

3. An equipment according to claim 1, incorporating a welding plant with constant or rising output voltage characteristic in the arc voltage range and static limitation of the short circuit current beyond that range.

4. An equipment according to claim 1 to 3, with fast response to the current and voltage signals under short circuit conditions, and delayed response afterwards.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Equipment for underwater arc welding The invention refers to equipment for underwater arc welding and welding in similar conditions where the power source is installed a considerable distance away from the welding arc, e.g. at sea level, with the welding operator working at the bottom of the sea, at a considerable depth. When welding with consumable electrodes in a shielding gas, the so called MIG process, the power source is usually expected to supply to the arc a constant voltage which together with an electrode fed into the arc at a constant rate of feed results in stable arc conditions. My British patent specification 1540880 describes a method to overcome the difficulties relating from the fact, that the voltage drop along the cables may result in the requirement of a rising output voltage of the power source in contrast to the constant voltage required at the arc. According to my patent specification pilot leads from the arc to the power source permit the output voltage of the plant being controlled by comparing the arc voltage with the preset voltage of a potentiometer installed in the power source. Object of the invention is an equipment which permits the described operation without the need of pilot leads. On underwater arc welding equipment according to the invention means are provided for controlling the output voltage of the welding plant so that it remains equal to the sum of two voltages, one of these being preset on a potentiometer at the plant, and the other a voltage proportional to the current and determined by the short circuit current at a measured voltage, as well as means for changing the control of the output voltage from said measured voltage to said sum of two voltages. An example of the invention is shown in the drawing. The welding arc 1 to be operated at the bottom of the sea, receives its supply over long cables 2 from the welding plant 3 stationed above sea level and connected to the mains 4 over a contactor 5. Another contactor 6 connects the welding plant 3 to the mains 4 over a choke 7. Before starting the welding operation the welding plane is connected to its supply over the choke 6, with the aim to reduce the voltage at the point of operation to a safe value. The current flowing through the cable is sensed over the D.C. current transformed 8, designed as a D.C. saturated reactor. When the operator touches the workpiece earth, the current through the cables as well as the voltage are sensed and stored in the computing circuit 9. After this storage has been completed the computing circuit gives a signal to the contactor 5 which then connects the plant through the mains, it reconnects the control of the output voltage, and it also starts operating the electrode feed not shown in the drawing.The control circuitry is preferably designed so that the values of voltage and current taken at the initial short circuit are recorded without delay, whereas the controlled output voltage on the plant during welding will receive its signal with a delay so as to avoid hunting of the controls. A circuit similar to that shown in the drawing can be used where the welding plant is connected to the welding arc immediately at or near its full value i.e. without preliminary series connection of a choke. In that case the short circuit current will be measured and compared with the output voltage of the plant when the arc is struck and the signal for the start of the electrode feed is given. For an operation described it will be preferable to use a welding equipment described in my British patent specification 947814 where the short circuit current is limited to a defined value by giving the plant a nearly rectangular characteristic. It is known to compensate the voltage drop in power cables by compounding the supply voltage. An application of such known methods for the control of welding plant has to account for the special conditions imposed by the characteristics of the arc. The invention makes use of the fact, that the arc produced with a consumable electrode is as a rule preceded by a short circuit current, either for striking the arc itself or for closing a pilot circuit connecting the power source. In both cases the output voltage feeding the welding cables will be equal or very nearly equal to the voltage drop along these cables. CLAIMS

1. An equipment for underwater arc welding over long welding leads with consumable electrodes supplied at a constant rate of feed, with means for controlling the output voltage of the welding plant so that it remains equal to the sum of two voltages, one of these being preset on a potentiometer at the plant, and the other a voltage proportional to the current and determined by the short circuit current at a measured voltage, as well as means for changing the control of the output voltage from said measured voltage to said sum of two voltages.

2. An equipment according to claim 1, where the output voltage is reduced until the operator produces a short circuit, with means for measuring the output voltage and short circuit current.

3. An equipment according to claim 1, incorporating a welding plant with constant or rising output voltage characteristic in the arc voltage range and static limitation of the short circuit current beyond that range.

4. An equipment according to claim 1 to 3, with fast response to the current and voltage signals under short circuit conditions, and delayed response afterwards.