Page 36 - MetalForming-Jan-2018-issue

Welding Well
By Doug Smith
A Duty-Cycle Reality Check
When it comes to arc-welding power supplies, we hear a lot of talk about duty cycle—and plenty of misinformation related to compact integrated inverters (the type with a built-in wire feeder) as not being true “industrial” welding machines. This article addresses that topic, and provides two critical reality checks.
We measure rated output or “size” of a welding power supply using amps, volts and duty cycle. Duty cycle, eval- uated using a 10-min. cycle time, rep- resents the amount of time a machine can operate at a given output without exceeding the temperature limits of its components. For example, a power supply with a duty-cycle rating of 40 percent when gas-metal-arc welding (GMAW) at 285 A/28 V can weld con- tinuously for 4 min. at 285 A/28 V. It then must cool for the remaining 6 min. Thermocouples in the power supply measure temperature; to protect the components, they will shut the machine down when necessary. You may actually be able to weld before the end of the 6-min. cooling cycle if the components are cool enough, but not at full-duty cycle.
Three factors to consider:
• Duty cycle and welding output are inversely proportional. When welding at lower outputs, duty cycle increases. For example, when GMA-welding at 220 A/25 V, duty cycle increases to 60 percent.
Doug Smith, product business manager--arc equipment, ESAB Weld- ing & Cutting Products, shares his arc-welding insights in MetalForming magazine’s new Welding Well column every-other month.
Doug Smith
Product Business Manager- Arc Equipment, ESAB Welding & Cutting Products
dsmith@esab.com
• We evaluate duty cycle at a specific ambient temperature—typically 104 F. In cooler ambient temperatures, duty cycle increases. For example, when welding at 70 F, you could experience close to a 100-percent duty cycle— specifics will vary by machine and actu- al parameters, but welding in cooler temperatures definitely boosts duty cycle.
• With a multi-process, multi-voltage power supply, duty cycle and welding output will vary by process and input power (see the chart).
Other Considerations: Electrode Capacity
In most applications, the current- carrying capacity of the welding wire or electrode dictates maximum welding output. In turn, the electrode selected must match the requirements of the welding process, as well as the weld- bead profile, metallurgy, mechanical properties and acceptable heat input.
For GMAW of steel 1⁄4 in. and thin- ner, 0.035- and 0.045-in.-dia. mild-steel electrodes often suffice. The recom- mended parameters for short-circuit GMAW on 1/4-in. steel: 180-190 A, 21- 22 V. For spray transfer: 200-210 A, 24- 25V.
In production situations, the invert- er referenced above has plenty of power. In fact, it can produce more welding output than a 0.035-in. elec- trode can handle. While a 0.045-in.
electrode can be used for spray transfer 350 A with a 92 Ar-8 CO2 shielding gas, that much heat would be unsuitable for the sheet thickness at hand.
Work Flow
The nature of most welding projects requires numerous work stoppages, especially during gas-tungsten-arc welding (GTAW ). Think about all of the things that require an operator to stop welding: change rods, grind welds and remove slag and silica islands, com- plete the joint, reposition the body, rest to prevent hand fatigue, etc. Every time the welder pauses, the power sup- ply cools down and the duty cycle resets. Even in industrial settings, actual arc-on time rarely exceeds 25 percent.
Please note that we do not recom- mend a 285 A, 40-percent duty-cycle inverter as the best solution for most industrial applications. However, the conversation needs to change. For GMAW with a 0.035- or 0.045-in.-dia.
wire using con- ventional process- es, a welder could pull the gun trigger and the only rea- son he might stop welding would be to change the wire spool. Duty cycle will not be the issue. MF
Rated Welding Outputs—Multi-Process Inverter
Process/ Rated Output
Duty Cycle, 1 phase (230VAC Primary)
Duty Cycle, 3 phase (230VAC Primary)
Total Range
5 – 300A
5 – 350A
GMAW
285A/28V @ 40%
285A/28V @ 50%
SMAW
260A/30V @ 40%
285A/31V @ 40%
GTAW
285A/21V @ 40%
285A/21V @ 50%
34 MetalForming/January 2018
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