Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Patent
1993-01-04
1995-03-21
Stephan, Steven L.
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
323351, 219383, G05F 140, H05B 718
Patent
active
053999576
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention concerns a direct current (dc) arc torch power supply. Direct current 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 to generate 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.
The arc torch, in this application, is required to generate plasma over long periods of time. It has, however, proved difficult to maintain the arc reliably over such periods of time using conventional power supplies.
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 so that 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.
BACKGROUND ART
A known power supply employs a thyristor, or a silicon controlled rectifier (SCR), in each phase of an alternating current main supply. At least two of the thyristors are always ON at any given time to conduct current to an inductor which stores energy and smooths the output. The other 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 three phase supply). It follows that 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. For example, a 50 kW arc torch consuming 200 Amps will need an inductor of 20 mH, which would weigh several tons, to limit current ripple to less than 50 Amps. 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
an input port for connection to a source of direct current and an output port for connection to the electrodes of an arc torch.
A controlled switch and an inductor are connected in series between the input port and the output port.
A free-wheeling diode is connected such that, in use, it is reverse biased when the switch is ON, and forward biased when the switch is OFF to maintain direct current flow through the arc and the inductance.
A feedback circuit has a current sensor to sense the instantaneous value of current flowing through the arc, and a control terminal connected to the switch. The feedback circuit, in use, provides a control signal at the control terminal to turn the switch ON when the instantaneous value reaches a first level and OFF when the instantaneous value reaches a second level to maintain the instantaneous value substantially between the first and second levels.
This circuit uses a direct current input and controls it to provide the required current to the ar
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Supplementary European Searle Report (2 pages) Apr. 26, 1993.
Database WPIL, Week 338, Derwent Publications Ltd. London, GB; AN 88-233345 33] & SU-A-1 368 128 (Gorki Poly) 23 Jan. 1988 (abstract).
Patent Abstracts of Japan, vol. 7, No. 178 (E-191) 6 Aug. 1983 & JP-A-58 084 415 (Tetsushin Kogyo) 20 May 1983 (abstract).
Stephan Steven L.
The University of Sydney The Electricity Commission of New South
To Ed
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