GB2201051A - Ignition device for a plasma or welding torch - Google Patents

Abstract

A triac module 7 delivers voltage pulses from a mains supply 1, 2 to cause high frequency oscillations in a parallel resonant L-C circuit 12, 14, the inductor of which forms the primary winding of a step-up transformer 13, so that a spark gap 19 across the secondary 15 repeatedly breaks down to apply high voltage transient signals across the secondary which are coupled to the torch connection terminals 25, 35 via capacitors 23, 26. An inductor 30 blocks the passage of these signals to the D.C. supply 32. A car iginition coil may be used as the transformer 13. The device may provide bursts of pulses at 50m sec intervals with a ringing frequency of 200 KHz and an output voltage of 40 KV which is sufficient to ignite an arc in conditons of higher gas pressure or gas velocity than is possible with prior devices. <IMAGE>

Description

IGNITION DEVICE This invention relates to an ignition device for supplying high voltage, high frequency signals to a plasma or welding torch.

In a plasma or welding torch, an arc is struck between two electrodes: commonly, one electrode is part of the torch whereas the other electrode is the workpiece to be cut or welded. The voltage required to maintain this arc, once established, is considerably less than that required to initially break down the gaseous mixture separating the electrodes.

Consequently, an ignition device is used in order to supply high voltage, high frequency signal pulses to one of the electrodes in order to break down the electrode gap prior to the establishment of the arc.

For such a high frequency signal, the voltage required to break down the electrode gap decreases as the period of the signal is reduced below the positive ion transit time between electrodes; frequencies of approximately 2MHz have been found to be appropriate in certain circumstances.

The deficiencies of known ignition devices are, however, considerable: typically, the voltage output of such devices is less than 3KV. This results in a potential gradient between electrodes which is insufficient, even for closely spaced electrodes, to cause the break down of the gaseous mixture separating the electrodes for pressures of the gaseous mixture which are substantially greater than atmospheric pressure. Further, the output circuits of known ignition devices are susceptible to-detuning by stray capacitances of the leads connecting the device to the torch. A further problem with known devices is that they are not easily portable. This often neccessitates the use of long leads from the ignition device to the torch, with a consequent severe attenuation of the high voltage signal.

In accordance with the present invention, an ignition device for supplying high voltage, high frequency signals to a plasma or welding torch comprises an oscillatory circuit, resonant at the said high frequency: mains voltage supply means triggerable to initiate an oscillation in the oscillatory circuit; step-up transformer means having a primary winding energised by said oscillatory circuit, and a secondary winding operable to apply a voltage across a spark gap sufficient to cause repeated break down and produce associated high voltage transient signals across said secondary winding: a coupling capacitor connected to couple the high voltage transient signals to the torch: and an inductor connected to be in series with the D.C.

supply to the torch to isolate the supply from the transient signals.

The output voltage of this device ean be approximately 40KV. Such a voltage is sufficient to cause the breadkown of the gaseous mixture separating the electrodes, even for gas pressures substantially higher than atmospheric pressure. Further, the output circuit is not susceptible to detuning by any stray capacitances of the leads connecting the device to the torch.

According to further features of the invention, the oscillatory circuit is non-inductively connected to the mains voltage supply means, and the coupling capacitor is further connected to a non-inductive means to supply the high voltage, high frequency signals to the torch.

According to another feature of the invention, the transformer means comprises a car ignition coil.

This ignition device is, therefore, considerably lighter and more portable than known devices since it includes, in particular, only a single transformer.

The use of a standard automobile component, capable of delivering the required output voltage, further contributes to the relatively lower production costs of the present device.

An example of the present invention will now be described with reference to the accompanying drawings in which Figure 1 is a circuit diagram of an ignition device for supplying a high voltage, high frequency signal to a plasma or welding torch.

Referring now to Figure 1, a 240V mains voltage is applied to the input terminals 1 and 2 of the ignition device. A pair of microswitches are connected to, respectively, terminal 2 and, via a 5 amp fuse 3, to terminal 1. It will be apparent that a microswitch assembly may be replaced by other switching devices such as a footswich or a switch operated when the gas or cooling water flow to the torch is initiated.

Microswitch 5 is connected to one terminal of a Triac module 7 with a variable phase lag control. The other terminal of the Triac module 7 is connected via current limiting, 12 Ohm, 50 Watt resistors 8, 9 and 10, to one terminal of the primary winding 12 of a car ignition coil 13. The other terminal of the primary winding 12 is connected via a connection 16 to microswitch 4.

A 0.1 micro Fd, 1500 Volt capacitor 14 is connected between the two terminals of the primary winding 12. The secondary winding 15 of the ignition coil 13 has one end connected to a connection 17, itself connected to connection '16, and the other end to a terminal 18. A spark gap 19 is connected to terminal 18 from one side and to connection 16, via a terminal 20, from the other side. A 1 MOhm resistor 21 is connected from terminal 20 to terminal 22, there being a 0.1 micro Fd, 1500 volt capacitor 23 connected between terminal 22 and connection 17. An output terminal 25, for connection to the positive terminal of the torch and to earth, is connected to terminal 22.

A 1000 pF, EFIT capacitor 26 is connected at one end to a terminal 18 and at its other end to a junction 28. An inductor, such as an FX3845, is connected to a junction 28 and, via a connection 31, to a terminal 32 for connection to the negative terminal of a D.C. power supply for the torch. A 0.1 micro Fd, 1500 volt capacitor 33 is connected between line 31 and the earthed enclosure 34 of the ignition device.

Junction 28 is further connected to an output terminal 35 for connection to the cathode of the torch.

The operation of the circuit is as follows: when the microswitches 4 and 5 are closed, the 240 volt mains voltage applied across the Triac module 7 causes the Triac to fire, delivering a series of voltage pulses to the oscillator circuit comprising the primary winding 12 in parallel with capacitor 14. Each voltage pulse causes high frequency oscillations in this circuit, resulting in the generation of high frequency pulses in the secondary winding. The high frequency pulses result in, at appropriate times, the repeated break down of the spark gap 19. This repeated break down results in high voltage transient signals across the secondary winding which is coupled, via EHT capacitor 26 to junction 28. In addition, this coupling capacitor prevents the D.C. power supply from shorting to earth. Further, inductor 30 constitutes a very high impedance to the high frequency signals, thereby preventing the power supply from shunting the signals. The arrangement disclosed can generate an output voltage of approximately 40KV. Consequently, output terminal 35 supplies the high voltage, high frequency signals to the cathode of the torch. It will be apparent that the ringing frequency can he varied by altering the magnitude of the inductance of inductor 30, or the capacitance of capactor 26. Further, hy varying the phase shift of the Triac module 7 or the distance separating the electrodes of the spark gap 19, the output voltage appearing on terminal 35 can be greatly varied. Routine modifications of the present components has been found to allow a variation in output voltage of up to approximately 17 KVolts. With the present components, bursts of pulses occur at 50ms intervals, with a ringing frequency of 200kHz.

The present example of the ignition device has been found to be capable of causing the breakdown of the gaseous mixture separating electrodes of a torch which are up to 5mm apart for gas pressures of up to 28 atmospheres and for gas velocities of up to lOOm/s for argon.

Claims (6)

CLAIMS:

1. An ignition device for supplying high voltage, high frequency signals to a plasma or welding torch, comprising: an oscillatory circuit, resonant at the said high frequency: mains voltage supply means trigger able to initiate an oscillation in the oscillatory circuit; step-up transformer means having a primary winding energised by said oscillatory circuit, and a secondary winding operable to apply a voltage across a spark gap sufficient to cause repeated break down and produce associated high voltage transient signals across said secondary winding: a coupling capacitor connected to couple the high voltage transient signals to the torch; and an inductor connected to be in series with the high voltage D.C..

supply to the torch to isolate the supply from the transient signals.

2. An ignition device as claimed in Claim 1 wherein the oscillatory circuit is non-inductively connected to the mains voltage supply means.

3. An ignition device as claimed in Claim 1 or Claim 2 having a non-inductive means to supply the high voltage, high frequency signals from said coupling capacitor.

4. An ignition device as claimed in any preceeding claim wherein the mains voltage supply means comprises a Triac.

5. An ignition device as claimed in any preceeding claim wherein said transformer means comprises a car ignition coil.

6. An ignition device substantially as hereinbefore described with reference to the accompanying drawing.