The concept of duty cycle is to be clearly understood before operating any machine. Unlike the arc welding power sources, the resistance welding power sources are designated for 1 minute integrating period. Suppose a machine is rated for 50 % duty cycle, it means that after operating the machine for ½ min. at full load, the machine has to be cooled for the next ½ min. The duty cycle at loads different to the full load can be calculated from the formula.Duty cycle at Y kVA = (rated kVA / Y kVA) x rated KVAThis formula is applicable straight in the case of under loads but while overloading, we should not overlook the constraint imposed by the maximum capacities of the circuit elements e.g. diodes, contacts, etc., though the machine is going to be overloaded to that extent only for a very brief period allowed by this formula. In effect, this formula takes care of the heating of the winding only.