Iron Core Saturation of a Welding Transformer in a Medium Frequency Resistance Spot Welding System Caused by the Asymmetric Output Rectifier Characteristics

This paper deals with a middle frequency resistance spot welding system. It consists of a semiconductor input converter, a single phase welding transformer with one primary coil and two secondary coils, and a full-wave output rectifier connected to the transformer´s secondary coils. The unwanted current spikes often appear in the transformer´s primary coil current during steady state operation, which was proven by measurements. The numerical analysis, based on a nonlinear model of the discussed system, has shown that these current spikes are caused by the interaction among asymmetrical design of the transformer, its magnetically nonlinear behavior, and unequal characteristics of the diodes in the output rectifier. This undesirable phenomenon can be efficiently avoided by the passive or active approach proposed in this work. The passive approach is based on the correct positioning of carefully chosen diodes in the output rectifier, while the active one includes the closed loop welding current control and the closed loop control of flux density in the transformer´s iron core. The consequent employment of the active approach makes possible reduction of the transformer´s iron core size at the same power of the spot welding system.