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shenion
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On AC, the voltage drops to 0 on each cycle (sine wave). The arc will go out and need to be restarted. Square wave switches quickly between a + and - voltage, the arc is more stable and on some it does need HF start to maintain the arc.
wyn
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When the welder is being used for AC the sine wave is changed into a more square sort of shape. So instead of taking a 1/4 of the cycle to get from 0 up to the peak of the wave form . It goes straight from + to - or vice versa and stays at the maximum + or - for longer. This is supposed to give more time cleaning and more time heating.
Thanks guys. Managed to find a bit more detail on this subject as follows :-
AC power source
For materials such as aluminium, which has a tenacious oxide film on the surface, AC power must be employed. By switching between positive and negative polarity, the periods of electrode positive will remove the oxide and clean the surface.
The figure shows current and voltage waveforms for (sine wave) AC TIG welding.
Disadvantages of conventional, sine wave AC compared with DC are:
the arc is more diffuse
HF is required to reignite the arc at each current reversal
excessive heating of the electrode makes it impossible to maintain a tapered point and the end becomes balled
Square wave AC, or switched DC, power sources are particularly attractive for welding aluminium.
By switching between polarities, arc reignition is made easier so that the HF can be reduced or eliminated. The ability to imbalance the waveform to vary the proportion of positive to negative polarity is important by determining the relative amount of heat generated in the workpiece and the electrode.
To weld the root run, the power source is operated with the greater amount of positive polarity to put the maximum heat into the workpiece.
For filler runs a greater proportion of negative polarity should be used to minimise heating of the electrode. By using 90% negative polarity, it is possible to maintain a pointed electrode. A balanced position (50% electrode positive and negative polarities) is preferable for welding heavily oxidised aluminium.
AC power source
For materials such as aluminium, which has a tenacious oxide film on the surface, AC power must be employed. By switching between positive and negative polarity, the periods of electrode positive will remove the oxide and clean the surface.
The figure shows current and voltage waveforms for (sine wave) AC TIG welding.
Disadvantages of conventional, sine wave AC compared with DC are:
the arc is more diffuse
HF is required to reignite the arc at each current reversal
excessive heating of the electrode makes it impossible to maintain a tapered point and the end becomes balled
Square wave AC, or switched DC, power sources are particularly attractive for welding aluminium.
By switching between polarities, arc reignition is made easier so that the HF can be reduced or eliminated. The ability to imbalance the waveform to vary the proportion of positive to negative polarity is important by determining the relative amount of heat generated in the workpiece and the electrode.
To weld the root run, the power source is operated with the greater amount of positive polarity to put the maximum heat into the workpiece.
For filler runs a greater proportion of negative polarity should be used to minimise heating of the electrode. By using 90% negative polarity, it is possible to maintain a pointed electrode. A balanced position (50% electrode positive and negative polarities) is preferable for welding heavily oxidised aluminium.
More on the diferent AC waveforms here (pg14) and here
As well as not needing continuous HF inverter squarewave is more efficient. With sinewave AC the arc spends most of it's time rising or falling, very little time at 'peak'. Squarewave, with more time spent at peak needs less electrode positive (EP) time for cathodic cleaning so you can bias towards more EN for work heating. Inverter squarewave also offers different AC frequencies (transformers restricted to mains frequencies) which can be used to 'shape' the arc- higher AC frequencies 'tighten' the arc cone improving focus and arc stiffness/stability
Some powersources also have independant current selection i.e. EN and EP amperages are indepentantly selectable.
As well as not needing continuous HF inverter squarewave is more efficient. With sinewave AC the arc spends most of it's time rising or falling, very little time at 'peak'. Squarewave, with more time spent at peak needs less electrode positive (EP) time for cathodic cleaning so you can bias towards more EN for work heating. Inverter squarewave also offers different AC frequencies (transformers restricted to mains frequencies) which can be used to 'shape' the arc- higher AC frequencies 'tighten' the arc cone improving focus and arc stiffness/stability
Some powersources also have independant current selection i.e. EN and EP amperages are indepentantly selectable.
