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The power factor is a very important process variable in ac resistance spot welding (RSW) operation. Because of the variation of welding load during the welding process, the power factor is a time-varying variable. The power factor of RSW cannot be obtained online because the value of power factor angle cannot be directly measured in real time. After analyzing the electrical structure of RSW, a new algorithm for measuring the power factor angle in real time is developed. Then, an integrated algorithm for calculating the power factor on a practical discrete system in real time is proposed. Experiments were conducted to validate the effectiveness of the proposed method. The experimental results showed that the proposed algorithm can be used to obtain more accurate values of the power factor angle than using other methods over a very large operation range. The final results of power factors obtained from the proposed algorithm are very close to the results from the numerical simulation.