Patent GB-833210-A (GB)
Title: Electrical power unit
Inventor: DRACOPOLI JOHN LAWRENCE
IPC: B23K9/073
Filed: 1955-07-26
Abstract: 833,210. Supply systems for welding arcs. HIVOLT Ltd. Feb. 1, 1956 [July 26, 1955], No. 21604/55. Class 38 (4). Relates to an A.C. arc-welding system for aluminium in which a highvoltage starter pulse is automatically replaced by arc-maintaining surges of lower voltage as soon as the arc has struck on both half-cycles, a capacitor C15 for reducing or preventing the passage of direct-current through the welding transformer being short-circuited during this starting period. In this manner, the capacitor does not discharge at the end of the starting period and so cause the passage of a heavy current through the arc, such heavy discharge being detrimental when thin gauge metal is being worked. As shown, the capacitor C15, shunted by resistor R26 and in series with choke CH2, is connected to the welding electrode C. An arc-voltage-responsive relay D/4 is connected in series with rectifier M3 and is actuated when the arc strikes on both half-cycles to open contacts D2 and to de-energize relay E/1 As a result, contacts E1 are opened and capacitor C15 is brought into operation to reduce or prevent the flow of D.C. through the welding transformer T4. The anode circuit of a surge generator valve V3 includes a surge capacitor C12 and ignition coil T3 whilst relay D/4 is energized. The high voltage developed by the coil T3 breaks down a spark gap K and so reaches the electrode C. The valve V3 is rendered conductive once every cycle of the current derived from transformer T4 by means of a timing valve V2 which has a grid connected to the transformer and its anode connected to the control grid of the valve V3. When relay D/4 is energized, this grid of the valve V2 is connected to earth through capacitor C9 so that the discharge of capacitor C12 occurs when the arc voltage is at its peak. As soon as the relay D/4 is de-energized, contacts D3 open the anode circuit of the valve V3, whilst contacts D4 close to short-circuit the primary of the ignition coil. At the same time, contacts D1 open to disconnect capacitor C9 so that the operation of valves V2, V3 is advanced whereby voltage surges from valve V1 pass through resistors R23, R24, and choke coil CH2 to the work, and now occur at the instant when the welding-current has dropped to zero. Before a switch S4 in the handle of the welding torch is closed, relay A/3 is energized and the anode lead from valve V3 to valve V1 is opened so that there is no surge injection. The closure of switch S4 to commence a welding operation energizes relay B/4, whereby current passes through a gas valve operating solenoid T2 admitting argon to the welding torch. At the same time, relay A/3 is disconnected from its high-voltage supply, and, when capacitors C2, C3 are discharged, the relay operates to complete the anode circuit of valve V3 and start the surge injection of the ignition coil. Subsequently, when switch S4 is opened at the end of a welding operation, the surge injection is stopped before the supply of argon is discontinued. Indicator lamps are provided to show what parts of the unit are in circuit. Switch S2 is operated by pressure of cooling water so that switch S4 is isolated if water is not flowing. A switch S6 removes the capacitor C15 when it is not required, when welding stainless steel, for example. A ganged switch S5 adapts the power unit to different welding transformers. Specification 705,164 is referred to.