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by Sam Belkin, MSEE
Welding transformers are intermittent in operation. They deliver a current only for the duration of the welding time and are idle between the welds. The amount of time the transformer supplies the current, in ratio to idle time, is called the duty cycle (D.C.). Usually the duty cycle is determined for one minute. For example a welding machine is making 4 welds per minute and each weld is 9 cycles long. With a 60 Hz AC line it gives 60 x 60 = 3600 cycles per minute. The 4 welds (9 cycles each) will last 4 x 9 = 36 cycles. Therefore the duty cycle D.C. = 36 / 3600 = 0.01 or 1 %. The transformers of resistance welding machine's usually relate at 50 % duty cycle. In order to express the kVA rate of the welding machine at 50 % duty cycle the following equation may be used for: kVA50% = kVA x Ö (D.C./50) when kVA is the power rate of the transformer and D.C. is the duty cycle of the machine. © Digiweld 1999 |