Patent GB-437727-A (GB)
Title: Improvements in and relating to electric welding
IPC: B23K11/24
Filed: 1934-02-27
Abstract: 437,727. Switching systems for resistance welding. BRITISH THOMSONHOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Lord, H. W.; 1103, Palmer Avenue, Schenectady, New York, U.S.A.) Feb. 27, 1934, No. 6341. Convention date, March 1, 1933. [Class 38 (iv)] The supply of A.C. or pulsating current to a welding transformer 3 is effected through a grid controlled discharge tube 4 preferably of the arc type which is controlled so as to allow current to flow for regulatable parts of a half cycle of the A.C. supply. Closure of a main switch 5 causes application of a " biassing off voltage on the grid of valve 4 through a transformer 6 which is connected (through contacts 25 on a closed contactor 26), across part of a transformer winding 31 supplied from the mains. At the same time rectifiers 59 are energized and build up a D.C. voltage across a potentiometer 44, 45 so as to apply a negative biassing " off " voltage in the grid circuit of the valve 4. During this period a time delay relay 71 is energized from the mains and after a time predetermined by the time taken for the heating up of the cathodes of the valves used, contacts 32 are closed, thereby energizing relay 26 from the mains. Contacts 72 are opened to remove a short circuit around a grid controlled arc discharge valve 37 and connect it across potentiometer 44 in series with a resistance 42 which is in the grid circuit of valve 4, and also to remove the negative bias voltage from the grid of valve 4. Contacts 25 also open, thereby connecting the primary of the grid transformer 16 of valve 4 across a phase-shifting circuit comprising resistance 34 and condenser 33 across the mains. Valve 4 fires at a predetermined point in a half cycle and immediately a voltage is applied to the grid of valve 37 through a transformer 52 in series with a bias voltage from the potentiometer 45. Valve 37 fires and a p.d. appears across resistance 42 which applies a negative bias on the valve 4 so as to prevent its re-ignition after the termination of the half cycle. This p.d. is present as long as the valve 37 is conductive. Initiation of a further spot weld of short duration is caused by operation of a switch 63 which connects a charged condenser 62 across valve 37 to commutate or extinguish it. This removes the bias voltage across resistance 42, causes valve 4 to operate, thereby reigniting valve 37 to produce a p.d. across resistance 42 and extinguish the valve 4 at the end of the half cycle so as to prevent its re-ignition until switch 63 is again operated. In a modification, Fig. 2, valve 4 is provided with an out of phase bias voltage on its grid through a transformer 112 and with a voltage across a resistance 116 which is connected to a transformer 129 the primary of which receives the charging or discharging current of a condenser 121 from a phase shifting circuit 125, 126 through a grid controlled arc discharge valve 127. Valve 127 is controlled partly by a peaked wave-form voltage obtained from a peaky wave transformer 86 the primary 87 of which is connected to a phase shifting circuit 97, 98 across the mains, and partly by the p.d. across a resistance 14 in series with part 137 of a potentiometer resistance 137, 138 connected across D.C. mains 142, 143. An arc discharge valve 136 is connected in series with resistance 144 across a part 138 of the potentiometer and is normally biassed " off " by the p.d. across a resistance 139 in the potentiometer. Closure of the mains switch 104 causes operation of the peaky wave transformer 86 to render valve 127 conductive thereby imparting an impulse to the grid of valve 4 to render it conductive and also to the grid of valve 126 through a winding 147 of transformer 129. Valve 136 becomes conductive and the p.d. across resistance 144 applies a negative bias to the grid of valve 127 to render it non- conductive as long as valve 136 is conductive. Valve 4 does not re-ignite after the end of the half cycle owing to the bias voltage from transformer 112. To initiate a further weld a switch 146 is operated to connect a condenser 145 across valve 136, thereby extinguishing it and causing re-operation of valves 127 and 4 and repetition of the cycle of operations. A time delay switch such as 71 in Fig. 1, may be used to protect the cathodes of the valves. If D.C. bias voltage is used on the valve 4 as in Fig. 1, the output of the transformer 86 may be applied direct to the grid of valve 4 without the use of the valve relay 127. The instant of ignition of the valve 4 may be controlled by varying the resistance 97 or capacity 98 in the phase shifting circuit 97, 98. Reference has been directed by the Comptroller to Specifications 391,232, 416,374, and 434,018.