12. DEALING WITH UNKNOWN TRANSFORMER by Sam Belkin, MSEE
Very often a welding shop has an old transformer with unknown parameters. Do not rush to throw it away. In most cases it is not too difficult to do a little "reverse engineering" and give a new life to the old veteran. For a welding transformer it is usually easy to count secondary turns n2. Then it is necessary to apply small AC voltage (12 - 48 V) Vp to the primary, and measure the voltage across the secondary Vs. The sought number of primary turns may be calculated from the transformer ratio formula:
Next measure the width of the center leg and the stack of the core lamination. Their product is the cross sectional area of the core Ac. The optimal primary voltage of this transformer, assume that the core laminations made from a silicon-iron alloy with the maximal flux density of about 1.6 Tesla, and the operation frequency is 60 Hz, can be found from simple formula:
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