. Railway mechanical engineer . arc to reduce the voltage to the desired value. Thisresistance serves a double purpose in that it cuts down thearc voltage and, at the same time, prevents a heavy shortcircuit current when the electrode is held in contact withthe object previous to drawing the arc. Water barrels orrheostats may be used for this purpose, as shown in Grid resistances are superior to water barrels in that they are more portable and may be worked hard withoutgiving trouble such as is experienced due to boiling overof water rheostats. For the occasional job, however, thewater
. Railway mechanical engineer . arc to reduce the voltage to the desired value. Thisresistance serves a double purpose in that it cuts down thearc voltage and, at the same time, prevents a heavy shortcircuit current when the electrode is held in contact withthe object previous to drawing the arc. Water barrels orrheostats may be used for this purpose, as shown in Grid resistances are superior to water barrels in that they are more portable and may be worked hard withoutgiving trouble such as is experienced due to boiling overof water rheostats. For the occasional job, however, thewater rheostat is as satisfactory as the expensive motorgenerator set, except that with its use the cost for currentis greater. The wiring diagram illustrated in Fig. 2 showsthe connections of an electric welding outfit using grid re-sistances. MOTOR GENERATOR SETS If alternating current alone is available, a motor gen-erator set is a necessity. Such a set is also of value wherethe direct current is at 115 volts or higher and where a. Fig. 2—Diagram Showing the Connections for a Welding OutfitUsing Grid Resistances considerable volume of work is handled, as there is con-siderable power lost in reducing the voltage from 230 to50 volts by resistance. The purpose of a motor generatorset is to transform the available power and deliver it inthe form best suited for the welding. In a straight directcurrent set the only object is to save power. The justificationof the investment depends upon the amount of weldingwork done, the character of the work, the voltage of thesupply and the cost of power. Where the only availablepower is alternating current, a motor generator set is neces-sary since arc welding with alternating current can be doneonly with the greatest difficulty. Two distinct types of motor generator sets are availablefor arc welding, different manufacturers championing dif-ferent systems. In one type the current is delivered at anapproximately constant pressure of 75 volts and a
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectrailroadengineering
