Patent GB-1529726-A (GB)
Title: Control circuit for automatic electric arcwelder
IPC: H02P3/12
Filed: 1975-10-31
Abstract: 1529726 Automatic control LINCOLN ELECTRIC CO 31 Oct 1975 [13 Nov 1974] 45506/75 Heading G3N [Also in Division H2] The electrode E in an arc welding apparatus is fed by a D.C. motor M having an armature MA fed with a D.C. voltage of constant polarity and a field winding MF fed with current in one or other direction by SCR101 or SCR102, the armature being shunted by a diode D103 so that the motor may be rapidly braked, during a cold start operation of the apparatus, by simultaneously de-energizing the armature and reversing the field current to pass the motor EMF through the diode. Operation of the apparatus may be in either one of the following modes:- Cold start. Inch down switch IDS is closed to energize an optical switch OCI302 and apply a voltage from transformer T301 to the electrode E and workpiece W and to an oscillator QU301, C302, R302-308 and another optical switch OCI301 via respective rectifier bridges. Oscillator operation charges capacitor C306 via transistor Q302, and a further transistor Q303 is turned on by the switch OCI301 to apply a signal VTS to a logic circuit LC. The latter emits a signal MRS to cause circuit MC to energize the armature MA and also a signal MDS to cause SCR101 to be fired and so rotate the motor to feed the electrode E towards W. When members EW touch the switch OCI301 turns off transistor Q303 to remove signal VTS and cause circuit LC to remove signals MRS, MDS, thus deenergizing armature MA, firing the SCR102 and braking the motor as described above. Start switch STS is then closed to produce signal PSV for causing power to be applied to E, W and also signal MRS to cause re-energization of armature MA and movement of E away from W to strike the arc. Formation of the latter re-energizes the oscillator and member OCI301 to produce signals VTS and MDS and so reverse the motor to move E towards W. Thereafter a weld-current sensing switch CAS closes to produce signal WSV which turns off transistor Q301 and allows the signal ASS from capacitor C306 (responsive to oscillator frequency and hence to the arc voltage) to be applied to circuit MC, whereby the motor speed is varied to maintain the arc voltage substantially constant. Hot start. Switch STS is closed to produce signal PSV and hence apply power to members EW. The voltage therebetween energizes the oscillator and member OCI301 to produce signals VTS, MRS and MDS as before, although the motor speed is limited to a value less than that during welding by a pre-set control in circuit MC. When members EW touch the signal MRS is maintained and the signal MDS may remain, or disappear and then reappear as during the cold start, according to the contact time of members E, W and how soon the arc is established. Thus the motor may continue the electrode advance or temporarily reverse it, before the arc stabilizes and switch CAS closes to produce the arc-responsive motor speed control. When switch CAS closes members DZ301, Q301 function to limit the rate of change of motor speed from the value corresponding to the pre-set control of circuit MC to that determined by the arc voltage when transistor Q301 is turned off. Members R317, D311 allow slow speed (inching) operation when the voltage between members E, W is insufficient to operate the oscillator.