Document 8QqZ22rGd4Xdm1QKjJX8oJ8e

WELDING AND CUTT1NC carbon arc cutting and cutting with heavily coated electrodes With small diameter electrodes used on thin sheets for manual arc welding, current values vary from 10 to 50 amp. For most manual welding, because the welder must withstand the heat, current values should not exceed 500 or 600 amp. Automatic machine arc welding may use current values up to 2,000 amp or even higher on special applications Such opera tions employ AC only for more than 500 to 600 amp because with AC "are blow" (swerving of the arc path caused by mag netic forces) is almost completely absent Arc blow is especially detrimental when higher current values are used Converters Commercial AC and DC power supplies are not suitable for arc welding Conse quently, to convert commercial power for arc welding, a rectifier must be used for DC welding, and a transformer for AC welding A motor generator may also be used to produce alternating or direct current. Since these welding converters are sub ject to abuse and are given little expert attention as compared to ordinary electri cal equipment, they should be well protect ed, both mechanically and electrically. Often the machines are moved about, so convenient primary power receptacles should be pro vided about the shop or construction opera tion Plugs and receptables should meet the requirements of Underwriters' Laborato ries, Inc. AC transformers for welding are usually air-cooled with two-winding primaries The transformers should be built m accordance with the high standards of electrical con struction used on power or distribution transformers A typical industrial-rated AC transform er-type welding unit should be equipped with a voltage-reducing control which auto matically reduces the open-circuit secondary voltage to 30 volts during idling This reduc tion of the voltage is delayed two to three sec onds from the instant the arc is broken to pre vent interference with normal welding op erations. Contact of the electrode to the work automatically restores normal welding voltage. Other safety features are (a) cir cuit breaker, integrally mounted on the unit, for providing thermal overload pro tection and serving as a convenient discon nect switch, (b) circuit breaker of the automatic type or a fused safety switch mounted on the wall and rated for use with the unit and the power supply circuit, (c) three-conductor power supply cable, one conductor of which is for grounding pur poses, between the wall mounted breaker and the unit Large welding machines are usually in stalled as stationary units Since primary line switches are not normally provided as part of the equipment, they should be in stalled by the person making the installa tion The smaller machines, which may be used as portable units, customarily have primary switches built into them and are provided with primary cables and attachment plugs of proper capacity. Welding transformers should not be attached to lighting circuits under any circumstance. If a gasoline-powered welding generator is used inside a building or in a confined area, the engine exhaust should lead to the outside atmosphere. Otherwise, carbon mon oxide and other toxic gases may accumulate. Voltages The voltage across the welding arc varies from 15 to 40 volts, depending on the type and size of electrode used. The welding cir cuit must supply somewhat higher voltage to strike the arc. This voltage is called the open circuit or "no load" voltage After the arc is established, the open circuit volt age drops to a value about equal to the arc voltage plus the lead voltage drop The open circuit voltages on DC welding ma chines should be less than 100 volts For AC transformer welding machines, American Standard Z491, Safety m Weld ing and Cutting prescribes a maximum open circuit voltage of 80 volts Some standard heavy duty industrial AC welding machines have open circuit volt ages of 65 volts Reliable control equip ment is available which automatically re duces the open circuit voltage to about 30 volts during the "off" arc period, at which time the welder may be exposed to the open circuit voltage. This control equip ment Is commonly used where welders are working outdoors on wet steel or m con fined quarters or under other severe condi- 29-16