CONTROL METHOD FOR POWER UNIT OF PLASMA ARC FURNACE

PURPOSE:To eliminate the break a generated arc by turning off the pulse of the arm connected to an omitted plasma electrode and applying the gate pulses to four remaining arms (eight arms in all) in a phase relation similar to that secured when each arm is actuated as a 3-phase oure bridge circuit. CONSTITUTION:When an electrode 6B is omitted, the gate pulses of arms V1, Y1, U2 and X2 of the rectifiers 4A and 4B connected to the electrode 6B are turned off. Meanwhile the gate pulses are applied to the eight remaining rams U1, W1, X1, Z1, V2, W2, Y2 and Z2 in a phase relation equal to that secured when each of these arms is working as a 6-pulse rectifier (3-phase pure bridge circuit). Thus the current waveforms ir1 and is2 are identical with a 120 deg. conduction form. At the same time, a zero period is never produced to the waveform of an electrode current iR (= is2 - ir1) of an arc furnace 5 that is obtained from the addition/subtraction of currents. As a result, a stable arc is maintained.