AC systems were the go-to systems for many years for almost all spot welding. They were consistent, reliable, available and lasted forever. DC systems were large and costly. With electronics, MFDC emerged, and DC became economical and transformer size for DC was reduced. It became possible to use more payload on the end of a robot arm and MFDC became the means to increased automation. It was more power with less weight than an AC transformer could deliver. All automation systems are pretty much MFDC today.
The cost for the mfdc weight advantage is a more costly system and probably a shorter total life span. That may operate with 20% less power usage? AC equipment is very reliable for a very long time.
For a complete understanding of MFDC vs AC read the article:
COMPARE MFDC VS AC IN RESISTANCE WELDING?
The ability to control at the millisecond level can improve quality control. This is where real money can be saved by fewer rejects or rework.
In today’s market estimates of new systems are:
Automotive:
MFDC 95%
AC 5%
Commercial Goods:
MFDC 50%
AC 50%
Both MFDC and AC are good reliable systems. MFDC is the choice for automation. In other applications it comes down to what will work best in your plant with your people.
Reference: RWMA – Resistance Welding Manual 4th Edition
AWS – Welding Journal, Q & A July 2019