Power supply for single-phase resistance welders

THE rapidly increasing use of a-c contact welders in metal fabricating plants imposes single-phase low-power-factor fluctuating loads on plant power systems. The loads may range from a few kilo volt-amperes for a few cycles per weld, as in the case of a small spot welder, up to several thousand kilovolt-amperes for five to ten seconds per weld in the case of a large butt welder. Where the plant is supplied from a small private generating station, single-phase welding loads not only cause troublesome voltage fluctuations but may damage the damper windings of the generators. If power is purchased from a utility, voltage fluctuations may still be present, not only in the plant but often in the entire adjacent area. To eliminate such disturbances it has been necessary in numerous instances to install motor generators to serve as phase converters to supply single-phase power. It is the purpose of this paper to discuss the more common applications for single-phase welding generators and the electrical problems involved in each type of application.