Patent GB-1173134-A (GB)
Title: Improvements relating to Controlled Transfer Arc Welding
Inventor: NEEDHAM JAMES CHRISTOPHER
IPC: B23K9/09
Filed: 1966-03-29
Abstract: 1,173,134. Welding by fusion. WELDING-INSTITUTE. 28 March, 1967 [29 March, 1966], No. 13904/66. Heading B3R. [Also in Division H2] Arc-welding apparatus of the type in which controlled transfer from a consumable electrode to a workpiece is effected by supplying an arcmaintaining current cyclically enhanced to produce the transfer, comprises a detector for sensing the detachment of a droplet from the consumable electrode and means responsive to the detector for terminating the enhancement of the current immediately after or a predetermined period after the detachment. Such control of the pulse current duration in response to changes in pulse amplitude provides improved operation due to the fact that the pulse current and duration are related by the equation: where (1) I is the pulse amplitude; (2) T is the effective duration of the pulse (time between start of pulse and the droplet detachment);  (3) B and n are coefficients depending on the electrode material and the size of the droplet to be detached ; and (4) the pulse waveform is substantially square or rectangular in shape. The detector may be reponsive to the arc voltage increase occurring when a detachment takes place or responsive to the rate of change of arc voltage to sense the instant of detachment. Alternatively, a photo-electric arrangement responsive to detachment of the droplet may be used. In Fig. 1, which shows one phase of a three-phase supply for providing a D.C. output 18, 22, a detector 33 is connected across the electrode and workpiece and comprises the circuit shown in Fig. 3, including a differentiating network R3, C3 responsive to the pulse of arc voltage occurring at detachment of the droplet, Fig. 2 (not shown). The output of ths network constitutes a signal applied to a time duration control unit 24 to switch off (immediately or after a predetermined time) the gating signal for silicon controlled rectifiers 19, 25 supplying the enhancement current pulse to the arc-maintaining current supplied by diodes 20, 26. Mal-operation due to voltage pulses other than those caused by droplet detachment is prevented by a bias voltage (20V) ; by circuit members R1, Cl, R2, C2 and diodes D; or by making it impossible to terminate the pulse within a predetermined time of its application. A discussion is given of the operation of various parts of the circuit of Fig. 3 in response to the various voltage fluctuations and transients occurring during an enhancement current pulse. The relationship between pulse amplitude and duration and depth of weld penetration is also discussed, Fig. 4 (not shown). Fig. 5 shows an arrangement which is used when the electrode material is such that a voltage pulse is not formed at droplet detachment. In this arrangement the detachment hides or reveals a light source 40 which co-operates with a photo-cell 41 and amplifier 42 to control the unit 24. The output 43 from the amplifier 42 provides pulse termination and the output 45 initiates a further pulse when conditions are satisfactory for the detachment of another droplet. The source 40 may be a helium-neon gas laser. Various relationships between the droplet and the light source are discussed in connection with the control of arc length and with cold starting under various welding conditions. Modifications to Fig. 1 are also discussed, e.g. provision of separate sources to supply arc-maintaining and pulse currents, provision of an additional source for cold starting, variation of the pulse frequency and the replacement of the silicon controlled rectifiers by gated ON/OFF transistors.