Patent GB-524329-A (GB)
Title: Improvements in or relating to the control of the supply of electric power to an electrical load
IPC: B23K11/24
Filed: 1939-01-27
Abstract: 524,329. Supply systems. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Jan. 27, 1939, No. 2860. Convention date, Jan. 28, 1938. [Class 38 (iv)] Apparatus for controlling the power supply to a load such as a spot welding system comprises control means between the source and the load and means for supplying from a separate D.C. source impulses of decreasing electrical potential to the control means, the impulse supply means including a condenser and a charging circuit permitting the repeated flow of current impulses to charge the condenser. A welding transformer 7 is supplied from supply conductors 17, 19 through cross-connected discharge devices 21, 23. Each of these devices has an anode 25, a mercury pool cathode 27, and an ignition electrode 29 of boron carbide or silicon carbide. The discharge devices 21, 23 are rendered conductive during half cycles of the source which correspond in number to the interval during which welding current is to flow. When a pushbutton 31 is closed the coil 33 of a relay 35 is connected across D.C. conductors 37, 39 and the relay is energized and closes, at its upper contact 41, a circuit through a reactor 45, a condenser 49 and resistance 51 in parallel, a second condenser 59, and a resistance 61. The lower contact 63 of the relay 35 is normally closed and short-circuits the condenser 53 so that this is completely discharged. When the relay is energized the short-circuit is removed. The condenser 53 has a capacity of the order of ten times that of the condenser 49. The discharge device 59 may be of the hot cathode gaseous type or of the hot cathode high vacuum type. Control potential sufficient to increase the conductivity of the auxiliary discharge device 59 is provided through an auxiliary transformer 67, a phase shift network 69 comprising a resistance 71 and a reactor 73, and a saturable transformer 75 connected as shown. The phase may be adjusted by varying the resistance 71. The secondary winding 85 of the transformer 75 supplies a potential of peaked wave form and adjustably displaced in phase with respect to the potential of the supply conductors 17, 19. A resistance 87 in the control circuit of the discharge device 59 is supplied from the secondary winding 85 through a fullwave copper oxide rectifier 89. The resistance 87 is connected between the cathode 57 and control electrode 65 of the auxiliary discharge device 59 through a grid condenser 95. When the contact 41 closes and an impulse is impressed on the control circuit of the discharge device 59 this is rendered conductive and the condenser 53 is partially charged by an amount limited by the smaller condenser 49. Since the inductance 45 is present the total potential across the two condensers is, at the end of the charging impulse, somewhat greater than that of the D.C. source 37, 39 and the discharge device 59 is extinguished. The potential maintaining the condensers 49, 53 charged is then removed and the condenser 49 discharges through the resistance 51. The potential across the discharge device 59 again becomes positive and the device again becomes conductive, this process continuing until the large condenser 53 is completely charged. At each charging impulse a potential impulse is produced between adjustable taps 105, 107 on resistances 61, 109, this potential being applied to the control electrode 113 of a second auxiliary discharge device 115 of the hot cathode gaseous type or of the high vacuum type. The anode 119 is connected to one output terminal 121 of a bridge network 123 comprising resistances 125, 127, 129, 131, the input terminals 133, 135 being supplied from the secondary winding 137 of a transformer 139 through halfwave rectifiers 145, 147 of the copper oxide type. The cathode 117 of the auxiliary discharge device 115 is connected to an intermediate tap 143 on the secondary winding 137. Current flows through either of the rectifiers 145, 147 in its current passing direction and through the auxiliary discharge device 115 if this is conductive. such current flow unbalancing the bridge. During a number of half-cycles, predeterminable by adjusting the taps 105, 107, potential peaks sufficient to render the device 115 conductive may be impressed between the control electrode 113 and cathode 117. Current flow through the discharge device 115 during these half-cycles unbalances the bridge network 123 and causes potential impulses in the secondary windings 161, 163 of an output transformer 159, the primary winding of which is connected between the output terminals 121, 149. The secondary windings 161, 163 are connected, through brasing sources 173, 175, between the control electrodes 165 and cathodes 167 of ignition discharge devices 169, 171, the anode circuits of which include the ignition electrodes 29 of the main discharge devices 21, 23. When an ignition device 169 or 171 is rendered conductive, current is passed through the corresponding ignition electrode 29 and the corresponding main discharge device 21 or 23 is rendered conductive at the preset point in the half period and current is transmitted through the primary winding 15 of the welding transformer 7. The arrangement in which the large condenser 53 is charged in half-cycle, steps through a small transformer and a discharge device may be replaced by one in which a normally charged condenser is similarly discharged in steps.