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.
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Reference: RWMA Resistance Welding Manual 4th Edition