Controls & Transformers

This blog is a place to discuss the resistance welding processes, engineering and equipment used. We do not get into specific equipment, engineering, design or sales. This question is better handled through a local Machinery builders or Sales Representative.

This is beyond the scope of this blog.

References: AWS / RWMA - Resistance Welding Manufacturing Alliance

• resistance
• surface resistance

MFDC stands for “Mid Frequency Direct Current”. The output could be direct current at 400-4000 hertz frequency. The input in North America is 60 hertz AC. A combination of control and transformer changes allow for the conversion of the current from AC to DC at the higher frequency. This is explained in another article in this blog:

WHY or HOW DO MFDC CONTROLS CHANGE CURRENT FROM AC TO DC TO AC TO DC?

The use of an MFDC transformer offers many advantages which has made its use very popular in the automation applications. This is discussed in the following article:

COMPARE MFDC VS AC IN RESISTANCE WELDING?

 MFDC is found in most new automation applications. AC continues to have applications in the area of job shops and general applications.

References: Entron Controls

• DC Current
• MFDC Controls
• MFDC Transformer

When the operator actuates the foot switch, a signal is sent to the resistance weld control to actuate the weld cycle. On a typical simple weld cycle the control would initiate:

SQUEEZE – the time for force to build up and contain the work piece

WELD – time of current flow/heating and weld nugget generation

HOLD - nugget cooling, containment & forging

Current does not flow until the squeeze time has been completed. The time from foot initiation to current flow depends upon the length of the squeeze time. The foot pedal has nothing to do with the current initiation.
The welder control initiates the current at the proper predetermined time in the weld sequence. If the squeeze time is short the current initiates quickly. If longer, current initiation is delayed. Squeeze should be adequate to allow force build up to prevent/reduce expulsion.

References: RWMA Resistance Welding Manual 4th Edition

• control sequences
• hold
• squeeze
• weld

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

This question was recently addresed in another article:

HOW MUCH ELECTRICITY WILL MFDC SAVE VS AC?

Do read the article because it varies by machine and set up. There are many savings advantages and differences between AC and MFDC besides electricity to consider. A simple accepted estimate of power savings might be 20%.

Reference: RWMA - Resistance Welding Manual 4th Edition