Controls & Transformers

Questions and Answers

Throat size refers to the area of the welder enclosed by the path of the current from the transformer, through the conductors, electrodes then conductors and back to the transformer. A schematic is shown below. A robotic transgun and press welder all have a throat area. They are just different sizes depending upon the part being welded.

  Throat Area

                      THROAT AREA

Size is determined by the job requirements. For power efficiency one should generally not size the throat area larger than required for clearance of the product. This is especially true for AC equipment where inductance and impedance are a factor. Large throat areas can dictate increased large transformer and control (power supply packages). This is dictated by the impedance losses. A job shop may weld many different parts on the same machine. This will dictate a larger power supply.

The science behind this is:   “OHM’s LAW”

The transformer turns ratio in resistance welding reflects the number of secondary coil turns versus the number of primary coil turns. In AC transformers, there are many turns in the primary. The secondary has one turn. If there are 50 turns then the ratio is 50/1. Therefor the primary voltage will drop from 220 V and 100 amp input to 220/50 = 4 volts in the secondary. The equations for this are:


Vpri = Voltage on the primary                Tpri = Turns in the primary coil

Vsec= Voltage on the secondary          Tsec = Turns in the secondary coil

 Transformer Labeled Sketch

Power into Transformer equals power out of Transformer

Frequently transformers have tap switches or are built with parallel circuits. In this case a series bar can be installed to change the turns ratio or the tap can be changed to a different turns ratio.
To measure the actual turns ratio one must take the input output readings as shown above for the different taps or with the bar in or out to learn which is the turns ratio desired. This will depend upon the type of transformer you have.
Of course first look at the transformer drawing which will show the schematic with this information included.

Another article in this blog on this subject is:


Reference: RWMA Resistance Welding Manual, Chapter 19


This question is very involved. It depends upon what type of transformer (three phase, single phase, AC, DC). This blog does not get involved with machine design and build subjects. Contact your local machine sources or distributors for information and assistance.

Reference: RWMA Resistance Welding Manual 4th Edition

This question gets into the design of machine components. Design is beyond the scope of this blog. There are several articles describing the components of a transformer and their function in resistance welding. One for MFDC is shown below:

What is MFDC welding?

Inverter Control Transformer Schematic

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


Reference: RWMA Resistance Welding Manual 4th Edition

Many resistance welding controls but not all have the ability to operate in more than one language. All controls are different and methods to change the language will vary.

  Control AC

            AC CONTROL

Consult you control manual first. Then go to the control manufacturer, a local distributor or equipment dealer for assistance. Information will be available to answer this question.

This blog does not get into the operation of individual machines components.

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