The Impact of Voltage Generation on Harmonic Spectra of Current and Flux Density in the Welding Transformer for a Middle Frequency Resistance Spot Welding System

This paper deals with a middle frequency direct current (MFDC) resistance spot welding system (RSWS). 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 in the input converter, caused by interaction among the asymmetrical design of the transformer, its magnetically nonlinear behavior, and unequal characteristics of the diodes in the output rectifier, can cause over-current protection switch-off of the discussed system. These current spikes can be effectively avoided by the closed- loop control of the load current and flux density in the welding transformer's iron core. The welding transformer supply voltage can be generated either by the pulse width modulation or by the hysteresis controllers. This paper analyzes the impact of voltage generation method applied in the MFDC RSWS on harmonic spectra of current and iron core flux density responsible for acoustic noise.