TWI Industrial Member Report Summary 717/2000
C N Ribton and O Nello
This work is aimed at improving the reliability and performance of HV power supplies particularly for lower power (<6kW) EBW machines. Gun flashovers are a severe drawback in EBW, especially where the workpiece has a high value. The defects produced can be difficult and expensive to repair. A low stored energy power supply will address the two main issues arising from flashovers, ie gun electrode roughening and HV recovery by the power supply following a flashover.
Gun electrode roughening occurs because the energy stored by the power supply is dissipated into the flashover. This can occur either during gun conditioning or whilst welding. The high current density and consequent heat from the arc evaporates some material from the electrode surface (anode or gun cartridge) leaving a small pit. Reducing the energy stored in EB power supplies will reduce the damage to the gun electrodes should a flashover occur.
Both diode and triode guns can suffer from flashover but the mechanism for the breakdown, and the effect on the workpiece will be different. The beam current from a triode gun is controlled by a grid electrode. Flashover between the grid and cathode causes a surge of beam current to be produced by the gun to a power level in excess of its maximum rating, normally invoking a power supply trip. In contrast, the beam current from diode guns is controlled by the temperature of the cathode, and there is no grid. If a flashover occurs the beam current reduces as the accelerating potential diminishes. This characteristic is responsible for the more manageable flashover defects produced by diodes ( ie voids or cold shuts) compared with triodes ( ie large diameter and deep holes).
Although TWI has developed and applied diode guns at high powers, most industrial EBW machines use triodes. Consequently, the cost of repair of flashover defects can be high.
If a flashover occurs during welding then it is necessary to switch off the accelerating potential to ensure the arc quenches before re-establishing the high voltage and continuing welding. This must be very rapid if defects, due to the weld pool freezing, are to be avoided. Recovery of the HV requires the capacitance of the power supply to be recharged and the higher the stored energy the higher the peak power required during the recharge. The power supply's stored energy characteristic will limit the rate at which the HV can be recovered.
In addition, an HV cable (which itself will store appreciable energy) is not required if the power supply can be made suitably compact so that it may be fitted to the gun column without impeding mobility. Elimination of the HV cable, terminations and sockets will improve the reliability of the supply.
This work aimed to meet the requirements for a low stored energy power supply matching the above characteristics, compact enough to be integrated with a low power EB gun column.
- Review high voltage power supply requirements and select an optimum approach for integration with an EB gun column.
- Design a compact high voltage power supply.
- Test the power supply and assess performance.