GB2096418A

GB2096418A - Arc discharge apparatus

Info

Publication number: GB2096418A
Application number: GB8207629A
Authority: GB
Original assignee: GOODWIN ENGINEERING DEVELOPMEN
Current assignee: GOODWIN ENGINEERING DEVELOPMEN
Priority date: 1981-03-24
Filing date: 1982-03-16
Publication date: 1982-10-13

Abstract

In arc or discharge apparatus, eg a plasma torch, a first circuit containing electrodes 3 is connected to a source 1, 2 of electrical supply, and a second circuit is connected across the power lines 1, 2 of the first circuit for generating a pulse of voltage across the electrodes 3 to initiate the arc or discharge, said second circuit having a capacitor C, current control means 9, 10, a switching device 11, and including the primary winding 8 of a pulse transformer 5, the secondary winding of which is connected to the electrodes 3, the current control means being such as to cause the switching device 11 to turn on when the voltage on the capacitor 6 reaches a predetermined level to thereby discharge the capacitor 6 through the primary 8. The current control means may comprise a Zener diode 9 and a resistor 10, or a variable resistor (12) or a variable potentiometer (13), (Figs 2, 3, not shown). A tuning capacitor 16 may be provided for the transformer and the transformer secondary may be connected to the electrodes 3 via a spark gap 14. The supply 1, 2 may be D.C., or A.C., a diode 15 being provided in the case of an A.C. supply. The circuit may be arranged so that the pulses cease once an arc or discharge has been created across electrode 3, or so that pulses are produced for as long as the supply 1, 2 is maintained. <IMAGE>

Description

SPECIFICATION Plasma arc apparatus This invention relates to apparatus requiring the initiation of an arc or discharge between the electrodes of the apparatus, and is particularly but not exclusively concerned with plasma arc apparatus.

In the normal way plasma arc apparatus has a torch within which is situated an electrode, and a nozzle secured to the torch constituting a second electrode. Normally at the on-set of use of the equipment an arc is struck between the electrode and the nozzle, which ionizes gas passing through the nozzle to produce a short jet of conductive plasma. That short jet of conductive plasma can be brought into close proximity with a workpiece to effect the work required, or, with the workpiece connected in the circuit, the workpiece itself can serve as an electrode for an arc that may also be struck between the electrode and the workpiece itself.

In orderto initiate an arc say between an electrode and a nozzle, it is usual to superimpose a high frequency supply on to the power lines to the electrodes. Normally the high frequency voltage is blocked such that it appears at the electrodes to cause a spark to jump the gap to initiate an arc. Once an arc has been formed, this normally operates with a power supply having a "drooping characteristic".

The object of the invention is to provide an arc or discharge initiation means that is relatively simple to produce and install, and which is powered off the lines to the electrodes.

According to the present invention, means for initiating an arc or discharge between electrodes comprises a first circuit containing the electrodes and connected to a source of electrical supply and a second circuit connected across the power lines of the first circuit, for generating a pulse of voltage across the electrodes to initiate the arc or discharge, said second circuit having a capacitor, current control means, and a switching device, and including the primary winding of a pulse transformer, the sec ondarywinding of which is connected to the electrodes, the current control means being such as to allow the switching device to operate when the voltage on the capacitor reaches a predetermined level.

Thus, in the absence of an arc or discharge and with the electrical supply switched on, the voltage in the capacitor builds up until it can be discharged through the primary winding of the transformer causing an oscillating output from the secondary winding to generate a series of, e.g., sparks across the gap between the electrodes. The control means may be set such that once an arc or discharge is created, the generation of, e.g., sparks across the electrodes ceases, or can be set such that there is the continuous generation of, e.g., sparks for so long as power is supplied to the electrode lines. Thus, in the former case, with a power supply with a drooping characteristic, the trigger voltage for the switching device can be set so as to be below the open circuit voltage and above the arc or discharge voltage.

Once the arc or discharge is created the voltage on the lines and hence the voltage on the capacitor is below the trigger voltage, the capacitor is prevented from discharging. In the latter case, the trigger voltage for the switching device is set below the arc or discharge voltage, and there is therefore the continuous provision of, e.g., sparks irrespective of the presence or absence of an arc or discharge.

The control means in the pulse generating circuit may be such as to preventthe passage of current until an operating voltage is reached. Thus, a zener diode can be provided in the line connecting the capacitor to the switching device. When the capacitor voltage reaches the trigger voltage, current flows through the zener diode to switch on a switching device, such as a thyristor, and allow the capacitor to be discharged through the primary winding of the pulse transformer. The capacitor then re-charges from the power lines until the trigger voltage is again reached and when the capacitor is again discharged. Alternatively the control means may be such as to allow the passage of current dependent upon the voltage in the capacitor, a switching device such as a thyristor being set to operate at a predetermined threshold current.Thus, a resistive net-work or circuit can be provided between the capacitor and the switching device such as a thyristor. Here again, only when the capacitor voltage reaches the trigger voltage is a current provided at or above a threshold level to activate the switching device and hence allow the capacitor to discharge through the primary winding of the pulse transformer.

In addition to its simplicity of construction and ease of assembly, the means of the invention has the major advantage of being completely automatic in operation when required. So long as the control means, whether it be a zener diode or a resistive net-work or circuit, is set such that the switching device can only be triggered when the line voltage is greater then the trigger voltage, a series of, e.g., sparks will automatically break the gap across the electrodes in the absence of an arc or discharge, and for so long as an arc or discharge is present the production of, e.g., sparks across that gap is prevented.

Instead of the secondary winding of the pulse transformer being connected directly to the electrodes, a spark gap can be provided in the line from the secondary winding to the electrodes. This spark gap is broken down whereby high frequency pulsed supply can be provided to the electrodes themselves.

The means of the invention can operate with either AC or DC supply. If an AC supply is provided, a diode may be required in the circuit such that the means of the invention operates on a half-cycle supply.

It may also be advantageous to tune the pulse transformer with one or more capacitors.

In its application to plasma arc equipment the two electrodes referred to above can be provided The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

one by an electrode within a torch of the plasma arc apparatus and the other by the nozzle of the torch.

Thus, once an arc is struck between the electrode and the nozzle, gas passing through the nozzle is ionized to produce a jet of conductive plasma. It will however be understood that the invention is equally applicable to any other apparatus requiring the generation of pulses across a gap between electrodes.

Three examples of the invention are shown by way of example only in the accompanying drawings, in which: Figure 1 is a circuit diagram showing means of initiating an arc or discharge in accordance with the invention; Figure 2 corresponds to Figure 1 but shows a mod ifiedform of the invention; and Figure 3 corresponds to Figure 1 but shows a still further modified form of the invention.

In Figure 1, there is shown a positive rail 1 and a negative rail 2 connecting a source of electrical supply to the electrodes 3 of an arc or discharge apparatus. In the live rail 1 is a blocking choke 4, and across the rails is connected a pulse transformer 5 and associated capacitor 6 and resistor 7 whereby high frequency voltage is caused to appear at the electrodes 3 to cause a spark to jump the gap. In the line connecting the capacitor 6 to the primary winding 8 of the transformer, current control means is provided in the form of a zener diode 9 and associated gate current limiting resistor 10. The control means are connected to a switching device in the form of a thyristor 11.Thus, when the voltage in the capacitor re-charges from the power lines up to the trigger voltage set by the zener diode, current flows to the thyristor to switch it on to allow the capacitor to discharge through the primary winding of a pulse transformer 5, to provide a pulse of high frequency voltage from the secondary winding of the transforme to the electrodes 3. The capacitor then re-charges until the trigger voltage is again reached.

Alternatively, as is shown in Figures 3 and 4, in place of the zener diode 9 and resistor 10 of Figure 1, a resistive net-work or circuit can be provided between the capacitor 6 and the thyristor 11. Thus in Figure 2 a variable resistor 12 is provided and in Figure 3 an adjustable potential divider 13. With both constructions, a trigger level of voltage is set and only when the capacitor voltage reaches the trigger voltage is a current provided at or above a threshold level to switch on the thyristor and allow the capacitor to the discharge.

With all three forms of the invention exemplified in the drawings, the production of a spark can be automatic and continuous at the onset of power supply. Equally they can be set such that a spark is only produced when an arc or discharge is not present, simply by setting thetriggervoltage higherthan the arc or discharge voltage.

As is shown by Figure 1, a spark gap 14 can be provided in the line from the secondary winding of the transformer 5 to the arc electrodes, and to allow the invention to operate with either AC or DC supply a diode 15 can be provided in the line from the live rail to the capacitor. It can also be advantageous to provide a tuning capacitor 16 for the pulse transformer 5.

Claims

1. Means for initiating an arc or discharge between electrodes comprising a first circuit containing the electrodes and connected to a source of electrical supply and a second circuit connected across the power lines of the first circuit, for generating a pulse of voltage across the electrodes to initiate the arc or discharge, said second circuit having a capacitor, current control means, and a switching device, and including the primary winding of a pulse transformer, the secondary winding of which is connected to the electrodes, the current control means being such as to allow the switching device to operate when the voltage on the capacitor reaches a predetermined level.

2. Means as in Claim 1, wherein the current control means are set such that once an arc or discharge is created the generation of a pulse of voltage across the electrodes ceases.

3. Means as in Claim 1, wherein the current control means are set such that there is the continuous generation of pulsed voltage across the electrodes for so long as power supply is maintained.

4. Means as in Claim 1 or Claim 2, wherein with a power supply with a drooping characteristic a trigger voltage for the switching device is set to be below the open circuit voltage and above the arc or discharge voltage.

5. Means as in Claim 1 or Claim 3, wherein the trigger voltage for the switching device is set below the arc or discharge voltage.

6. Means as in any of Claims 1 to 5, wherein the current control means is set as to prevent the passage of current until an operating voltage is reached.

7. Means as in claim 6, wherein the current control means is a zener diode.

8. Means as in any of Claims 1 to 5, wherein the current control means is such as to allowthe passage of current dependent upon the voltage in the capacitor, the switching device being set to operate at a predetermined threshold current.

9. Means as in Claim 8, wherein a resistive network or circuit is provided between the capacitor and the switching device,

10. Means as in any of Claims 1 to 9, wherein the switching device is a thyristor.

11. Means as in any of Claims 1 to 10, wherein the secondary winding of the pulse transformer is connected to the electrodes via a circuit gap.

12. Means as in any of Claims 1 to 11, wherein for AC supply, a diode is provided in the circuit containing the capacitor.

13. Means as in any of Claims 1 to 12, wherein a tuning capacitor is providedforthe pulse transformer.

14. Means for initiating an arc or discharge between electrodes substantially as hereinbefore described with reference to Figure 1.

15. Means for initiating an arc or discharge between electrodes substantially as hereinbefore described with reference to Figures 2 or 3.