Prahlad:
I presume that by single cycle welding you are referring to a single AC voltage half cycle. I also presume that you fire the switching element some time into the welding cycle so that the next zero crossing of the AC cycle can be used to shut off the switching element.

Is it possible for you, when a single cycle weld is requested, to perform the weld as follows:

1) Command a first cycle with a very long firing angle (ie close to 180 degrees in phase) to create a very low energy pulse. The idea is to use this as a dummy weld pulse that is just high enough to measure but low enough so as to not significantly effect the actual weld desired. This measurement can give you "actual data" to seed your control loop.

2) Run the actual single cycle weld pulse on the next AC cycle using feedback from the "dummy cycle".

It may actually be possible to run multiple cycles of the trial pulses.

The soft-start concept mentioned by some one earlier is actually an extension of this concept wherein the weld is started slowly over a number of cycles by ramping the pulse energy to the target value gradually. In welding this soft start may not even be feasible because one attribute of metallic fusing is the rapid rise time of the energy. I am also familiar with the fact there there may be a characteristic of weld joint annealing that relates to a gradual taper off of the energy being delivered to the weld tips.

Michael Karas