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by Sam Belkin, MSEE
There are three basic sources of electrical welding current that are used today. These are the battery powered, the capacitor discharge, and transformer units. The battery powered source contains the battery (usually lead-acid) and a charger that recharges that battery. This type of a welding current source can provide DC up to several hundred amperes. The advantages are the low cost and the low current consumption from the AC line. The disadvantages are the heavy weight, difficulties with commutation high DC, and providing a high welding current at the level of thousands of amperes. The capacitor discharge sources utilizes the ability of the electrical capacitor to accumulate and save energy. Energy stored in the capacitor may be found as
where C is capacitance in Farad and V is voltage across the capacitor's terminals in Volt. The energy is directly proportional to capacitance and voltage squared. If the capacitance is doubled, the energy stored is doubled and if voltage is doubled, the energy is quadrupled. In the real capacitor discharge type welding machines the voltage may be as high as a few hundred Volts, which can be dangerous. This type of machines have a relatively low size and weight and are widely used for stud welding and the joining of small parts. The most common source of the welding current is a step-down transformer which can provide high power with a safety voltage. The ideal welding transformer lowers the AC line voltage and increases the current while maintaining constant volt x amps product. For example, such a transformer will transform 15 A from a 120 V AC line to 150 A at 12 V, or to 600 A at 3 V, or to 1500 A at 1.2 V. Note that the power (volt x amps) is constant and equals 1800 VA. From this the maximal available current for a given line voltage may be found. For instance, a portable benchtop welding machine needs to be designed to operate from standard 120 v AC line, and the required secondary voltage is 1 V. Due to a 20 A per receptacle current limitation the theoretical maximal welding current will be about 20 x 120 / 1 = 2400 A for continuous operation. For a real value, the efficiency of the machine should be considered. © Digiweld 1999 |