Standard transformers in North America are built to the RWMA standard which calls for a 50% duty cycle rating. Based on this a series of steps must be followed, estimated or assumed:
• The primary voltage has to be a known starting point?
• What secondary weld current is required?
• What is the impedance of the circuit? This is a machine function.
• The secondary voltage will be a function of Ohm”s Law of the weld current and impedance.
If one knows the starting primary voltage and calculates the duty cycle and knows the secondary current and can calculate the secondary voltage and turns ratio.
The answer to the original question is:
Know the primary power input.
Know the secondary welds per minute and secondary amperage.
The transformer builder can then determine the product best suited for your application.
Other related articles in this blog are:
Reference: RWMA Resistance Welding Manual 4th Edition
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.
Using the desired current and length the proper MCM (Maximum Continuous Current) cable can be determined for the application.
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.
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.
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
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
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