The source of power for butt and spot welders should have the same general characteristics as for arc welding; that is, they should deliver the current at constant voltage regardless of load. In most places current can be purchased from a public service corporation and a transformer, to give current at the desired voltage, will be furnished to power users. City distribution circuits are usually operated at 2200 volts in order to reduce the amount of copper required for the lines and, since alternating current is required for butt and spot welding, it is easy to get current at proper voltage by using a transformer. Most welders are provided with their own transformers wound to operate on a 220-volt circuit, although they can be made for use on any voltage up to 550 if necessary. Consequently, the line transformer should have a 220-volt secondary winding. The welding transformer will step the voltage down to the proper amount for welding and at the same time increase the current to the required amount. When it is necessary for the user of a welder to furnish his own power, it is best to use a motor-generator set, as such a machine will give better regulation than a rotary converter or a synchronous transformer. Engine-driven alternating-current generators may be used, if adequate means are provided to maintain constant speed under all conditions of load, and this is important for spot welders especially, because of the rapid and wide fluctuations of load in service.
Fig. 98. 24 -1nch Drop Arm Spot Welder Showing Different Positions of Universal Points.
Courtesy of National Electric Welder Company.
The transformer does its work through magnetic action and entirely without moving parts. It consists of a laminated iron core, or body, with two sets of copper coils or windings so arranged that the alternating current in the line, or primary side, coils magnetises the iron core and sets up a magnetic field which causes current to flow in the welding, or secondary side, coils. The wires and number of turns in the two sets of coils are so proportioned that the secondary coils produce a high current at low voltage, when the primary coils are energized by a low current at high voltage. For most welding operations the voltage is from 3 to 5 volts, and the power required will vary from about 1 horsepower for small work up to 200 horsepower for large work. The power will vary inversely as the time required to make the weld, rapid work demanding more power than slow work.
Fig. 99. Details of Spot Welder Courtesy of Toledo Electric Welder Company.