GB1221106A - Apparatus for plasma arc metal machining - Google Patents

Abstract

1,221,106. Automatic current control. V.R. LEPP, J.S. SEVIEV, I.S. SHAPIRD, B.D. BEIDER and L.M. PERSITS. April 8, 1968, No.16723/68. Heading G3R. In apparatus including a plasma arc torch (not shown), the torch is supplied from a source a, b, c by way of a contactor 19, a transformer 20 and a magnetic amplifier 22 having control windings 23, 24, and, in operation, the arc current is maintained at a desired value by the corrective regulation of the control current in the winding 24 in dependence upon an error signal representative of arc-current variations; this error signal is derived from a resistor 31 to which a reference voltage is applied at terminals/, g in opposition to an arc-current dependent signal obtained from a current transformer 28 associated with a rectifier 29, variations in the error signal caused by fluctuations and ripple in the arc current being smoothed by an integrating circuit 30, 32. The error signal is applied to the gate electrode of a thyristor 33 the firing point of which is thus controlled, in dependence upon arc-current variations, to regulate the control current in the winding 24 which is supplied from the secondary winding 21 of the transformer 20 by way of the thyristor 33. The control winding 23 is associated with an RC circuit 27 which allows the initial arc current to build up gradually to the working value when the apparatus is first switched on; the apparatus may be remotely controlled Fig. 1 (not shown), and as a higher operating voltage is then required a further RC circuit 26, associated with the winding 24, co-operates with the circuit 27 to effect the initial current build-up, the circuits 26, 27 being controlled by a relay carrying the arc current; the secondary windings 21 of the transformer 20 are connected in star to provide the higher operating voltage required for remote control.