Controls & Transformers

Questions and Answers

Resistance welding cables are made in different conductor sizes and lengths. Additionally they are manufactured as air cooled or water cooled. The air-cooled version must be attached to water cooled surfaces at both ends. The are generally covered with a protective sheath to prevent abrasion. Select the cable based on the ampacity and length required.
It is important to select the proper size for the job and minimize connection joints and length to reduce resistance in the overall circuit. One must know the expected current that is going to be passed. This with the distance from the transformer to the electrode one has an approximate length.

Cable Air Cooled Sizing

Using the desired current and length the proper MCM (Maximum Continuous Current) cable can be determined for the application.

Cable Water Cooled Sizing

Water cooled cables offer higher conduction capacities with a premium cable if required.

Reference: RWMA Bulletin 16

Duty cycle is a measure of the work that the transformer is doing. Machines are designed not to exceed 50% duty cycle. All components of the machine cables, shunts, transformer, controls and all conductors are designed for this value. If exceeded the life of the machine will be shortened dramatically. The formula shown below is based upon standard AC - 60 cycle welding with a one minute integration time period.

Calculate Duty Cycle

A complete example of the calculation of duty cycle is shown in the article linked below. This example covers calculations for both AC and MFDC equipment.


Reference: RWMA – Resistance Welding Manual Section 19.3

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”

Read more:

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

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