Capacitor Discharge
A capacitor discharge welder is known to discharge its full output very quickly into the weld joint. The weld may be a ½ cycle or so. My knowledge of CD equipment is minimal. I can’t offer any comments on life or maintenance. I understand they can be more expensive to purchase as compared to AC.
AC
In comparison a standard AC welder makes a projection weld in several cycles 5 to 20 or more cycles. This is a much slower weld and may impart a different heat affect than the CD welder which is short and fast.
AC equipment is robust can operate for a very long time with little maintenance. Force, follow-up and alignment are the most important issues to keep an eye on.
PROJECTION WELD PART
MFDC
Recently the MFDC segment of the market has been developing transformers with a “Fast Rise” to duplicate the Capacitor discharge. Their goal is to produce vary large weld currents in 10, 20 or 30 milliseconds. MFDC generates a DC output so the current can flow longer than an AC ½ cycle. They have been successful. This a promising new development.
Reference: RWMA Resistance Welding Manual 4th Edition