The main factor affecting weld quality of spot welding is net voltage fluctuation and the change of secondary loop impedance. The change of the power factor angle of load is produced with the change of secondary loop impedance. Theoretically the net voltage fluctuation and the change of secondary loop impedance is shown by the effective value of welding current, and by compensating the constant current control is realized. A new constant current control system of spot welding is designed, which uses the control mode of constant quantity of heat output, by checking the effective value of net voltage before welding, compensating the net voltage fluctuation and the power factor angle of load and adjusting the fire angel of SCR. The result shows that this system overcomes shortcoming, i.e. the half-wave spot welding cannot output constant current, because it is too late for welding current feedback to be adjusted. It is also suitable for multi-wave spot welding.%影响点焊电流恒定输出的主要因素是网压波动和二次回路阻抗的变化，二次回路阻抗变化会导致负载功率因数角变化。理论分析表明：焊接电流有效值反映了网压波动和负载功率因数角的变化情况，对二者进行补偿，可实现恒流控制。本文所设计的恒流控制系统采用了恒热量输出的控制方式，利用焊接前检测网压有效值的方法，对网压波动和负载功率因数进行补偿，不断调整可控硅触发角，以保证恒流输出。实验结果表明：该控制系统既克服了因焊接电流来不及参与反馈控制调节而不能保证半波点焊恒流输出的缺陷，同时也适用于多波点焊的恒流控制。
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