Industrial electric heating furnaces – Plasma furnace device – Direct furnace
Patent
1992-11-20
1994-09-20
Reynolds, Bruce A.
Industrial electric heating furnaces
Plasma furnace device
Direct furnace
21912111, 21912154, H05B 700
Patent
active
053496051
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention concerns a direct current (dc) arc torch power supply. dc arc torches employ an electrical discharge arc to heat a working gas and generate a plasma which is then passed through a nozzle comprising the hollow anode of the torch. The plasma may be used to ignite combustible fuel, such as pulverized coal, in a steam raising boiler generating electrical power. The plasma may also be used to warm the combustion chamber prior to ignition, and to ensure stable combustion of the fuel.
Such an arc torch may require a voltage in the range of 0 to 1,000 volts and a current range of from 100 to 300 Amps, that is electrical power in the range from 0 kW to 300 kW.
BACKGROUND ART
One of the particular problems that arises, in generating an electrical discharge arc in a dc arc torch, is that the arc has a large voltage drop from anode to cathode with high levels of voltage fluctuations. The arc will also, normally, have an inverse voltage-current relation and as current rises the voltage drop across the arc will fall. As a result, it is necessary for the power supply to react to a fall in voltage by limiting the arc current.
A known power supply employs a thyristor, or a silicon controlled rectifier (SCR), in each phase of an alternating current mains supply. At least two of the thyristors are ON at any given time, and conducting current to an inductance which stores energy and smooths the output. The thyristors are sequentially turned ON, to control the average current flow, by means of a predictive control circuit, which attempts to predict the current demand over the following cycle. The thyristors are turned OFF by the next current zero to arrive.
This supply has a number of disadvantages. The first is that control is only exercised over the current at the times when the thyristors are being turned ON. This implies an average delay in the current control of a third of a period of the supply (when a thyristor is used in each phase of a third phase supply). It follows there is a maximum rate at which current can be controlled. As a result the inductance must be large enough to limit current ripple at higher rates. This is essential because current zeros extinguish the arc, and high current peaks lead to electrode degradation. This adds greatly to the expense of the power supply.
A second disadvantage arises from the fact that the switching control is predictive, and results from a calculated guess rather than being absolutely determined from the current actually flowing at any given time.
SUMMARY OF THE INVENTION
According to the present invention there is provided a dc power supply for a dc arc torch comprising:
a controlled switch and an inductance connected in series, in use, in circuit with a source of direct current and an arc torch;
a free-wheeling diode connected, in use, to be reverse biased when the switch is ON, and forward biased when the switch is OFF to maintain current flow through the arc and the inductance;
a switch driver to cycle the switch ON and OFF; and
a feedback circuit having a current sensor associated with the circuit and connected to the switch driver to control the duty ratio of the switch in accordance with the current flowing through the arc.
This circuit uses a direct current input and controls it to provide the required current to the arc. It has the advantage that the current produced is independent of the arc voltage waveform, and it is determined by a feedback circuit operating in real time, rather than a predictive controller; this makes the control more accurate and sensitive. Furthermore, current ripple can be reduced from that of a given conventional supplies by operating the switch at higher frequencies than those readily available from a 50 Hz supply. A reduction in the size of inductance can also be achieved if higher operating frequencies are used.
A further advantage of using rectified dc is that it is possible to maintain the voltage level even while supplying high current, and several torches can be run from a single source of direct cu
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Hoang Tu
Reynolds Bruce A.
The University of Sydney
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