AC controllers use "phase angle firing" to control current.
Expulsion can be due to the firing angle setting...
Here is how: The average weld current is what your meter reads.
This is the effective current you are putting into the weld nugget.
But, with an AC welder, this current has peaks and valleys. The valley
is the off time, or "inter-cycle cooling time". You can see in the following
illustration, the current doesn't flow at all, until the SCR is turned on.
Then the current rises to a peak, and falls rapidly back to zero.
This illustration shows firing at 96% heat, and the resultant weld current
is about 10,000 amps, a pretty normal setting for most sheet metal.
But the peak current is much higher, over 16,000 amps.
So how long do you think your weld can take 16 kA before it overheats
and blows up with expulsion? Probably less than a half cycle (8 milliseconds)
Now, consider the situation when we have a larger transformer, to allow for line voltage drops and stepper boost.
The next illustration uses a transformer with more secondary voltage, so to get the 10kA current it is set at 62 percent.
This causes the firing point to be at 74 degrees. Now the inter-cycle cooling time is longer, so the current must be
higher to equal the same 10kA average to make the weld. But look at the peak current at 25kA.
It is almost impossible to apply 25kA to the weld without getting expulsion...
Of course DC welding has no peak current, you would just set it at 10kA, and that's it, no big peak current !
So now, I hope you can see why DC welding is so much better to use for quality welding.