Patent GB-598292-A (GB)
Title: Improvements in or relating to electric capacitor control systems
IPC: B23K11/26
Filed: 1944-02-24
Abstract: 598,292. Control systems for consumption apparatus; condenser arrangements. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Feb. 24, 1944, No. 3480. Convention date, Feb. 12, 1943. [Class 38 (iv)] An electric control system comprising a capacitor, a normally open discharge circuit connected across the capacitor, means for effecting closure of the discharge circuit until the capacitor is discharged and then re-opening the circuit, and a voltage source from which the capacitor is charged at variable rate through controlled valves, is characterized by (1) control means which cause the valves to permit, while the capacitor voltage is below a preselected magnitude, an increase in charging current until a predetermined magnitude of current is reached, the control means being responsive to the capacitor voltage and causing the valves gradually to reduce the magnitude of the charging current as the voltage increases above the preselected magnitude and (2) auxiliary means which prevent conduction of the valves for a predetermined time following closure of the discharge circuit. The Figure shows a capacitor spot welding system for effecting a single or succession of welds by discharging a main condenser 5 through a welding transformer 187. Each weld involves the following cycle of automatic operations:-(1) charging the condenser to a predetermined voltage from a source 3 through a three-phase Graetz bridge including controlled rectifiers 13, 15, 17 and uncontrolled rectifiers 19, 21, 23 ; (2) moving the welding electrode 215 into engagement with the work 219 placed on the other electrode ; (3) discharging the condenser into the transformer by firing an ignitron 189 connecting the two in series, which operation also starts a timing circuit 257 for preventing the controlled rectifiers from conducting until the condenser has had time to discharge; and (4) the starting of a " hold " timing circuit 357 controlling the time during which the electrodes are maintained in contact with the work after the discharge. An " off " timing circuit 403 determines the time between successive cycles when carrying out a number of welds. To avoid unidirectional magnetization of the transformer core, reversing contactors 191, 193 cause successive discharges of the condenser to pass through the primary winding in opposite directions, whilst reversal of the condenser polarity to any great extent during discharge is prevented by shunting the transformer and condenser by an ignitron 221 which conducts upon small reversal of polarity. Over-voltage protection is provided by a discharge resistor 181 which is shunted across the condenser by thyratron relay 149, 147 when the voltage across points, 127, 171 of a potentiometer 125 shunting the condenser, rises to a value sufficient to cut off anode current. Charging of condenser 5 to definite value after each discharge is controlled by a hard valve 113 to the grid of which is applied a positive bias from a source 129, a negative bias derived from the condenser over the potentiometer 125 and any further bias (which will be negative) provided by the voltage drop in a resistor 121 when a normally non-conducting thyratron 369 in timing circuit 257 conducts. The amplified variations of bias voltage due to changing condenser volts appearing across the load resistor 107 of valve 113 are applied to the grid a further hard valve 97 having a load resistor 55 which provides negative bias in the control circuit of the rectifiers 13, 15, 17. The unidirectional controlled bias for the rectifiers thus varies with the condenser volts. Until the condenser volts reach a certain value a more or less constant positive bias obtained in the grid circuit of valve 113 and little bias is therefore applied to the rectifiers : the charging rate is, accordingly, high. At a certain value of the condenser volts the positive bias from source 129 is overcome by a negative bias from tapping 123 and the anode current of valve 113 gradually falls, increasing the voltage drop in resistor 55 and thus reducing, and eventually arresting, the flow of charging current. When the condenser is to be discharged an impulse transformer 273 is energized by the discharge of an auxiliary condenser 289 and thyratron 269 fires, additional negative bias for valve 113 thereby being provided by the flow of anode current through resistor 121 for a time depending upon the value of a capacitor 259, so that rectifiers 13, 15, 17 are maintained non-conducting during the discharge period. Subsequently heavy charging surges are avoided by the gradual reduction of negative bias by the slow discharge of 259. The cycle of operations is initiated as follows:-upon closure of the main circuit breakers 7, 85 condenser 5 is charged up, the controlled rectifiers firing later in the A.C. cycle as the condenser potential increases and finally becoming non-conducting. Relay coil 277 is also energized over conductors 275, 281 and a pressure head switch 278 thereby short circuiting the primary of the welding transformer at contacts 283 and charging the auxiliarv condenser 289 over contacts 285 and dry rectifier 287. Upon closure of the starting button 291 relay 299 is energized from conductors 275, 281, over contacts 293 of relay 295, and picks up, its contacts 301, 303 then establishing a holding circuit around the,starting button as well as energizing relay 311 which closes its contacts 191 to connect the welding transformer in circuit with the main condenser. Contacts 313 also close upon pick up of 299, thereby energizing the winding 315 of hydraulic valve 317 which opens and applies pressure to the ram 321 carrying the movable welding electrode 215. When the electrode engages the work, pressure builds up in the head switch and its contacts 276 open to deenergize 279, thereby removing the short circuit from primary of the transformer and connecting condenser 289 so that it discharges through the impulse transformers 207, 273. As a consequence, ignitron 189 is rendered conducting and condenser 5 discharges into the welding transformer. Operation of the timing circuit 257, which prevents the rectifiers from conducting during this period, is also initiated. As the polarity of the main condenser reverses during discharge a tube 225 conducts thereby firing ignitron 221 which effectively short circuits both the condenser and welding transformer and allows the series ignitron 189 to become non-conducting. Thyratron 325 fires at the same time due to the voltage drops in 325 and at 327, thereby energizing time delay relay 345. Thyratron 371 of the " hold " time circuit has an auxiliary condenser 363 normally negatively charged by grid-cathode conduction : when relay 345 actuates, however, the closure of its contacts 375 completes the anode circuit of this thratron and stops charging of condenser 363. At the end of the '' hold " time, i.e. when condenser 363 has discharged through resistor 379, the thyratron conducts and relay 295 in its anode circuit picks up, interrupting the energizing circuit of relay 299 over contacts 293 whilst providing an additional bypass of the starting button 291 over contacts 387. The circuit to the impulse transformers and the energizing circuits of relay 311 and hydraulic valve 397 are also opened. Closing of the " head " switch 276, release of electrode pressure and short circuiting of the welding transformer by the contacts of relay 279 follow,' whilst the charging circuit of auxiliary condenser 289 is re-established. The circuit 257 times out just after the series ignitron has become non-conducting and initiates charging of the main condenser. " Off " time circuit 403 is similar to the " hold " timing circuit 357 and includes a relay 337 in the anode circuit of a thyratron 417 which is energized over contacts 427 upon pick up of relay 295. After a delay imposed by the R.C. circuit 407, 431 the thyratron conducts and relay 337 is energized, its contacts 335, 377 opening to de-energize relays 345, 295. The apparatus is now back in its initial condition and the cycle of operations will repeat as long as the starting button remains pressed. A stepping magnet 307, for a changeover switch 305 is energized over contacts 390 each time relay 295 picks up, whereby reversing relays 309, 401 are energized alternately with successive discharges of the main condenser.