Plasma arc cutter and method of controlling the same

Details Plasma arc cutter and method of controlling the same Plasma arc cutter and method of controlling the same

1989-10-18

1991-07-30

Paschall, M. H.

Electric heating

Metal heating

By arc

21912154, 21912155, 21912159, B23K 900

Patent

active

050361767

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. Technical Field
The present invention relates to a plasma arc cutter for cutting workpieces with high precision and high efficiency and a method of controlling the same.
2. Background Art
A conventional plasma arc cutter used in cutting metals has a configuration such as the one shown in FIG. 1. A commercial alternating current 1 is converted into a direct current by a rectifying circuit 2 composed of a diode. The DC output from this rectifying circuit 2 is converted into an alternating current having a predetermined frequency as transistors 5, 6 are alternately switched by base currents alternately output from a control circuit 8 to the bases of the transistors 5, 6 of an inverter circuit 7 composed of capacitors 3, 4 and the transistors 5, 6. Furthermore, an AC output of the inverter circuit 7 is transformed by a transformer 9 and is then converted into a DC output capable of stably maintaining an arc discharge by means of a serial circuit 12 composed of a rectifying circuit 10 and a smoothing reactor 11.
Upon starting of an arc discharge, a contact 13 is closed, and an operating gas is supplied to a gas passage 16 between an electrode 14 and a nozzle 15 of a plasma torch 18. When the operating gas flows, the inverter circuit 7 is actuated, and a voltage which sets the electrode 14 to minus and the nozzle 15 and the workpiece 17 to plus is applied to the plasma torch 18. At this stage, however, although a voltage (no-load voltage) is produced between the electrode 14 and the nozzle 15, dielectric breakdown has not occurred and there is no current flow. Then, a high-frequency generating circuit 19 is operated, and a high-frequency high voltage is generated at both ends of the secondary coil (electrode-side connection 21) of a coupling coil 20. This high-frequency high voltage is applied by a bypass capacitor 22 between the electrode 14 and the nozzle 15 in such a manner as to be superposed on the aforementioned no-load voltage. Thus, dielectric breakdown occurs due to a high-frequency discharge, followed by the generation of a pilot arc and then an arc discharge.
At this time, the rise of a current supplied from the serial circuit 12 composed of the rectifying circuit 10 and the smoothing reactor 11 is delayed by the action of the smoothing reactor 11. Accordingly, a compensation circuit 28 composed of a capacitor 23 and a resistor 24 is provided in parallel with the serial circuit 12. The capacitor 23 is charged up to a no-load voltage produced at the opposite ends of the rectifying circuit 10 at the time when the inverter circuit 7 begins to operate. If the impedance between the electrode 14 and the nozzle 15 declines due to the dielectric breakdown caused by the high-frequency discharge, the capacitor 23 discharges via the resistor 24 and a resistor 25 so as to compensate the serial circuit 12 whose current rise is slow, thereby allowing a pilot arc to be generated positively. In this case, the current value of the pilot arc is determined autonomously in such a manner that the voltage of the pilot arc and a voltage drop in the resistor 25 due to the current of the pilot arc are brought into equilibrium with the voltage of the serial circuit 12 thanks to the current and voltage characteristics of the serial circuit 12. A current detector 26 is adapted to detect a pilot arc current and stop the high-frequency generating circuit 19. When electrical conductance is secured between the electrode 14 and the workpiece 17 by being led by a pilot arc, the capacitor 23 discharges in a main arc circuit constituted by the electrode 14 and the workpiece 17 by means of the resistor 24, and then the current of the serial circuit 12 is supplied, thereby continuing the discharge. If the supply of the current from the serial circuit 12 is confirmed by the current detector 27, the contact 13 is opened, to stop the pilot arc, and a complete main arc discharge follows. The main arc discharge is maintained at a fixed level as a signal corresponding to that current is supplied to the control circuit 8 by the c

REFERENCES:
patent: 4017707 (1977-04-01), Brown et al.
patent: 4839499 (1989-06-01), Kotecki et al.

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