Controls & Transformers

Questions and Answers

Resistance welding transformers are manufactured with very little space between windings and between the primary and secondary coils. To provide electrical integrity they must be separated by insulation. This must be maintained throughout the life of the transformer. This insulation is very thin but can last for a very long time if not abused by heat, moisture or physical damage.



Transformer with Labels


As transformers age or a machine is being recommissioned the condition of the transformer should be considered. Is it in good shape and ready for another long model run? The question can be evaluated and tested. A transformer manufacturer has the proper test equipment to do this and can evaluate and potentially repair, if necessary.

If one is considering evaluating a transformer in house then the following path would be suggested:

Resistance welding transformers require integrity of the insulation between windings in order to operate correctly. The resistance value of the insulation cannot be measured with a typical multi-meter. It must be tested with a Megohmmeter. When resistance welding transformers are new the insulation resistance between primary and secondary winding, between primary winding and case, and between secondary winding and case will typically be greater than 500 MΩ. In fact, many Megohmmeters will show readings of infinity when testing new transformers. As transformers age and are exposed to typical contaminants found in factories (water, weld flash, hydraulic fluids, etc.) the insulation resistance degrades.

When evaluating a transformer to be reinstalled or for continued service the insulation resistance should be measured. You should consider replacing a transformer if any of the insulation resistance readings are found below 50 MΩ. However, it is always best to contact the transformer manufacturer to determine the minimum insulation resistance at which a transformer should be left in service.

Reference: RWMA Resistance Welding Manual Sections - 19 & 25                 

                   AWS Standard J1.3 Guide to Installation and Maintenance of Resistance Welding Machines

To calrify this issue attached is a previous article which shows the sequence of changes from the initial three phase 60 Hz AC input through several combinations of frequency and waveform changes to the final MFDC 1000 Hz output.

The ripple in the MFDC plot represents the 1000Hz frequency. The entire MFDC curve is on the positive side of the sine wave. MFDC does have a rise and decay of 3-8 milliseconds before and after full current is reached. This appears in the plot as an upslope or downslope. This should clear up the any questions. AN ARTICLE ON MFDC RISE AND DECAY IS ALSO AVAILABLE.


How to convert ac to dc to ac to dc

Inverter Control Transformer Schematic

Schematic of Power Conversion In Control and Transformer of Mid Frequency Inverter

The advantage of MFDC is that the weld current has no zero cross overs so it heats the part quickly. Also, it is DC so there are no inductive power losses or problems with magnetic material in the machine throat.
Generally the plant power requirement is reduced substantially with the 3 phase input. Another advantage is robot payload. Higher frequency in the transformer allows the iron core to be smaller. This is a weight reduction which is significant on the end of a robot arm.

Reference:  ENTRON Controls, LLC

In recent years MFDC has become the system of choice for new resistance welding machinery in North America. AC is widely used but for new equipment the preference is MFDC. Automotive and robotic lines use MFDC to take advantage of the weight savings not to mention power reductions and in house primary feed reductions, power factor improvements and others. MFDC does cost more and it is not forgiving if pushed beyond its rated limit.

Its electrical features are excellent in that it heats quickly and does not produce the sine way stitching with the zero cross over cooling. Since it is DC it has virtually no inductance loss when magnetic material enters the throat.

The electrical plot looks as shown below:

MFDC Rise and Decay


Depending upon the equipment the initial period from initiation of the current till the full current flow is reached takes 3-8 milliseconds (ms) in MFDC. This initial period is called “Rise Time”. When the power is turned off the same curve occurs and is called “Decay Time”.

For additional information about rise time and decay time see the article in this forum:


Reference: RWMA – Resistance Welding Manual 4th Edition
                  AWS – Welding Journal, Q & A, July 2019

Hanging transformers are used with portable weld guns. They generally have long conductors to the guns. The transformers tend to be large to drive the power over these long cables.

Portable Gun


The question is what type of core material is suitable for a hanging transformer. This is a machine design question and is beyond the scope of this forum.

Consult a transformer manufacturer for additional information.

Resistance welding transformer primaries are wound with many turns in order to produce the desired turns ratio. This in turn produces the conversion of the high voltage and low amperage in the primary to the desired low voltage and high amperage in the secondary. The winding material is copper and is required to conduct both current and heat at the amperage the transformer is designed for.

Transformer with Labels


Calculation of the winding gauge size is a manufacturing question and is beyond the scope of this forum.

Contact a resistance welding transformer manufacturer for additional assistance.

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