GB2095489A

GB2095489A - Arc starting circuit for welding apparatus

Info

Publication number: GB2095489A
Application number: GB8207724A
Authority: GB
Original assignee: FOSTER WHEELER AUTOMATED WELDI
Current assignee: FOSTER WHEELER AUTOMATED WELDI
Priority date: 1981-03-20
Filing date: 1982-03-17
Publication date: 1982-09-29
Also published as: GB2095489B

Abstract

The invention relates to arc initiation in arc welding apparatus and seeks to overcome the problems inherent in the use of high frequency a.c. arc initiating systems. A welding arc is generated by the application of an high d.c. voltage (e.g. 5KV) in an initiation circuit which is isolated from the welding machine 2 by means of a diode 6 in the welding circuit which prevents the initiation current delivered to the weld gap 4 passing to the welding machine. The initiation voltage can be constant d.c. Alternatively high voltage d.c. pulses may be generated by a circuit 12 at a rate which is equal to or a multiple of the a.c. mains supply frequency, circuit 12 being powered by a step-up transformer 8 and a full-wave rectifier 10. In a further embodiment, (Fig. 2 not shown), the output of transformer 8 is fed to the weld gap 4 via a current limiting resistor (16) and a half-wave rectifier (18). The arc-initiation circuit may be turned off automatically by a time switch or by a current-sensitive switch in the welding circuit. <IMAGE>

Description

SPECIFICATION Arc starting system for welding apparatus The invention relates to arc welding apparatus and to an arc initiation circuit therefor. Particularly, it seeks to mitigate problems inherent in the use of high frequency alternating current in such systems.

A principal disadvantage of high frequency a.c. arc initiation systems is that high frequency is impedance senstive, and any inductance or capacitance in the weld cables will cause the high frequency not to appear or be severely attenuated. Another negative point against high frequency is that it has the habit of "leaking", such that bad joints etc., will cause tracking to neighbouring equipment, causing malfunction (especially to low level electronics). Radiation and pickup on chart recorders etc., can also be a problem, as high frequency has the nature of being akin to Radio Frequency. (Radio Frequency interference has and is legislated against in many jurisdictions.) The present invention is directed to the use of direct current in an arc starting system.The use of d.c. is such that it is non-impedance sensitive, and may be used to initiate arcs through weld cables of theoretically infinite long lines. In practice, this cable length is limited by voltage drop due to the welding set (say 1,000 metres go and return; i.e.

2,000m).

According to the invention, arc welding apparatus has a welding circuit for connecting a power source to a weld gap at a weld station, which circuit includes a diode blocking the flow of current from the weld station to a said source; and an arc initiation circuit coupled to the welding circuit and a source of high voltage, for ionizing the arc gap to permit passage of the welding current. The arc initiation circuit normally generates an high d.c. voltage for ionizing the arc gap, and this voltage may be applied as a sequence of pulses, either at supply frequency, or at an integral multiple thereof.

There is an important safety aspect in the use of direct current according to the invention. The high voltage can be limited to deliver a current of approximately 1-2 mA.

Hence, fatalities etc., cannot occur as 10 mA is needed to cause fibrillation and at least 20 mA for fatalities to occur. The current is fixed, and not limited by the operator resistance. If the operator is of low resistance (for example very sweaty) the current drawn by him is still only that determined by the apparatus.

Two embodiments of the invention will now be described by way of example and with reference to the accompanying drawing in which each of Figs. 1 and 2 shows a circuit diagram for apparatus according to the invention.

In both circuits shown in the drawing, the welding current is provided from a welding machine set output or back-up supply indicated at source 2 to maintain an art at a welding gap 4. This welding circuit includes an high current, high voltage blocking diode 6 which prevents the flow of current from the welding gap to the source 2.

To strike the arc for the welding circuit to maintain, an arc initiation circuit is provided for applying an high voltage to ionize the gap 4. In the circuit of Fig. 1, the initiation circuit comprises a step up transformer 8 connected to a mains supply (typically 110 v a.c.) for generating a voltage of say 5 Kv. This is connected through a full-wave rectifier 10 to a pulse generator 12. The generator 12 delivers a pulsed, high voltage, low direct current to the arc gap 4. The generator can be adapted to provide a pulse frequency equal to or a multiple of the supply frequency (normally 50 Hz or 60 Hz) at a voltage selected up to 5 Kv (normally 1-5 Kv depending on the size of the gap 4) delivering a current of 1 to 2 mA.

In operation of the apparatus illustrated in Fig. 1, the arc is struck by closing a switch 14 which connects the mains supply. This generates current in the initiation circuit applying an high voltage to, and ionizing the welding gap 4. Before, as, or while the initiation circuit is operative, the welding circuit can be completed; the welding current voltage (of the order of 60 V) will not significantly affect the operation of the initiation circuit.

The diode 6 allows the weld current to pass, but stops the high arc initiation voltage from reaching the weld M/C set or back-up supply.

As soon as the arc gap 4 has been ionized, the open circuit weld M/C volts (- 60V) causes the weld current to flow, and as soon as the weld current flows the mains supply to the initiation circuit is turned off. Disconnection of the initiation circuit can be automatic; e.g., by a current sensitive switch in the welding circuit, or by a time switch. The time required to strike an arc can be accurately predicted.

There are two particular practical implementations of the circuit illustrated in Fig. 1.

Firstly, the d.c. high voltage can be pulsed at the mains frequency (i.e., 50 Hz, 60 Hz etc.) or at a multiple of; e.g., twice the mains frequency, (i.e., 100 Hz at 50 Hz, 120 Hz at 60 Hz etc.). This provides bursts of d.c. with all its attendant advantages such as operation from non-regulated supplies, and synchronous operation without need for high voltage switches. Constant d.c. can also be applied.

Secondly, the system can have an internal control-rectifier unit for providing the backup supply, thus allowing the use of low voltage arc sets (- 20V), providing enhanced operator safety. The unit may be transfigured for + ve or - ve earth merely by reversing the polarity of the HV diode.

The circuit of Fig. 2 operates in the same way as that of Fig. 1, but the initiation circuit is somewhat simplified. In this case, the high a.c. voltage (e.g. 3 Kv) generated at the transformer 8 is applied to the weld gap 4 through a resistor 16 and an half wave rectifying diode 18. The resistor 16 is typically a 1 MQ 2W resistor, producing of the order of 4.5 Kv at the weld gap 4 with a current of around 2.5 mA.

Apparatus of the invention offers a number of advantages. They can be used with long weld cables without deterioration of arc strike capability. They are safe regarding current limit (below 10 mA and normally around 1-2 mA), and simple to use (no active switching required). Interface to any weld set is easy, and the system can be adapted for A.C.

operation without difficulty, by the use of two diodes. Operation from non-regulated supplies (a.c. or d.c.) is possible and there is no risk of interference to outside equipment such as Radio Frequency interference.

Claims

1. Arc welding apparatus having a welding circuit for connecting a power source to a weld gap at a weld station, which circuit includes a diode blocking the flow of current from the weld station to a said source; and an arc initiation circuit coupled to the welding circuit and a source of high voltage, for ionizing the arc gap to permit passage of the welding circuit.

2. Apparatus according to Claim 1 wherein the arc initiation circuit comprises a transformer connectable to a mains supply for producing an high voltage; and a resistor in the circuit to maintain the current in the circuit at a safe level.

3. Apparatus according to Claim 1 or Claim 2 wherein the arc initiation circuit includes a full wave rectifier and a pulse generator.

4. Apparatus according to any preceding Claim wherein the arc initiation circuit is adapted to apply direct current to the arc gap.

5. Apparatus according to Claim 4 wherein said direct current is pulsed at the mains frequency.

6. Apparatus according to Claim 4 wherein said direct current is pulsed at an integral multiple of the mains frequency.

7. Arc welding apparatus substantially as described herein with reference to Fig. 1 or Fig. 2 of the accompanying drawing.