Power Sources and Equipment
Constant-current characteristic
SMAW needs a drooping, constant-current output so that arc-length changes produce small current changes. A constant-voltage machine built for GMAW will not hold a stable manual arc. The main families are transformer (AC only), transformer-rectifier (AC and DC), inverter (light, stable, efficient) and engine-driven (site work).
Duty cycle
The rating is a percentage of a 10-minute period. "300 A at 60%" means 6 minutes of welding at 300 A followed by 4 minutes cooling. Continuous work requires derating: allowable continuous current = rated current x square root of (rated duty / required duty). The thermal cut-out protects the machine, but repeated tripping signals you are outside the rating.
Matching electrode to machine
E6010 needs a high open-circuit voltage (often 70-80 V) and DC; many basic inverters cannot run it without a dedicated cellulosic mode. E7018 needs a stable, low-spatter arc and runs on AC or DCEP. Check cable size against current and length - undersized work leads cause voltage drop, a soft arc and erratic penetration.
Worked example: sizing for continuous work
The job needs 230 A for a two-hour run. Machine A is rated 250 A at 60%: continuous capacity = 250 x square root of 0.6 = 194 A, too small. Machine B is rated 300 A at 60%: 300 x 0.775 = 232 A, just adequate. Machine B, with correctly sized leads, is the choice.
Traps that cost marks
- Selecting a CV (wire-feed) machine for stick welding.
- Forgetting that long extension leads reduce arc stability.
- Changing polarity or cable connections with the machine switched on.
- Reading duty cycle as a time limit per weld rather than a 10-minute average.