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A Duty Cycle Reality Check

 

A Duty Cycle Reality CheckWe hear a lot of talk when it comes to duty cycle, including a lot of misinformation about compact integrated inverters (the type with a built-in wire feeder) as not being true "industrial" welding machines. This article attempts to add clarity to the conversation and provide two critical reality checks.

The rated output or "size" of a welding machine is measured using amps, volts and duty cycle. Duty cycle is the amount of time it may be operated at a given output without exceeding the temperature limits of its components, and it is measured using a 10-minute cycle.

In our example, the welding machine has a duty cycle of 40% at when MIG welding at 285 amps/28 volts. It can weld continuously for 4 minutes at 285 amps/28 volts and will then need to cool for the remaining 6 minutes. Technically, what happens is that the welding machine has thermocouples inside it that measure temperature and, to protect the components, shut the machine down when necessary. You may actually be able to weld before the end of the 6-minute cooling cycle if the components are cool enough, but not at full duty cycle.

There are three additional sub-factors to consider:

  • Duty cycle and welding output are inversely proportional. When welding at lower outputs, duty cycle increases. Using the same example from above, the duty cycle will increase to 60% when MIG welding at 220 amps/25 volts.
  • Duty cycle is evaluated at a specific ambient temperature. The "gold standard" for industrial equipment manufacturers is 104o F. If the ambient temperature is cooler, duty cycle increases. For an example, when welding at 70o F, you could experience close to a 100% percent duty cycle (specifics will vary by machine and actual parameters, but welding in cooler temperatures definitely boosts duty cycle).
  • In a multi-process, multi-voltage welding system, duty cycle and welding output will vary by process and input power, as the example below show.

Rated Welding Outputs - Integrated MIG/Stick/TIG Inverter

Process/

Rated Output

Duty Cycle, 1 phase

(230VAC Primary)

Duty Cycle, 3 phase

(230VAC Primary)

Total Range

5 - 300A

5 - 350A

MIG

285A/28V @ 40%

285A/28V @ 50%

Stick

260A/30V @ 40%

285A/31V @ 40%

TIG

285A/21V @ 40%

285A/21V @ 50%

So, is the inverter described above a full "industrial" or just a "light industrial" machine? This debate occurs on bulletin boards all the time, we honestly feel it distracts operators from discussing more meaningful topics - such as what diameter electrode are you using?

Electrode Capacity

A Duty Cycle Reality CheckIn most applications, the current carrying capacity of the welding wire dictates the maximum welding output. In turn, the wire selected needs to match requirements for welding process, weld bead profile, metallurgy, mechanical properties and acceptable heat input.

For MIG welding production work on metal 1/4-in. and thinner (the focus of this magazine), .035 and .045-in. diameter mild steel wires meet many of the needs. The recommended parameters for short circuit MIG welding on 1/4-in. steel fall in the range of 180 - 190 amps and 21 - 22 volts. For spray transfer welding, the recommended parameters would be around 200 - 210 amps and 24 - 25 volts.

In production situations, our inverter has plenty of power. In fact, it can produce more welding output than a .035-in. welding wire can physically carry. While a .045-in. wire can be used for spray transfer at up to 350 amps with a 92% argon/8% CO 2 gas mix, that much heat would be unsuitable for the metal thickness at hand.

Work Flow

The nature of most welding projects requires numerous work stoppages, especially with the TIG process. Think about all the things that require an operator to stop welding:

  • Change wire spools and Stick electrodes
  • Sharpening the tungsten
  • Grind welds/remove slag and silica islands
  • Complete the joint
  • Reposition the body
  • Stop to prevent hand fatigue
  • Moving around the weldment to prevent warping
  • Clamp and tack another part in place
  • Adjust fixtures

A Duty Cycle Reality CheckEvery time the operator stops welding, the welding machine cools down and the duty cycle gets a chance to reset. Even in industrial settings, the actual "arc on" time rarely exceeds 25%, with mid- to high-teens being common.

Please note that we are not remotely suggesting a 285-amp, 40% duty cycle inverter as the best solution for most industrial applications. What we are saying is that the conversation needs to change. For MIG welding with a .035- or .045-in. diameter wire using conventional processes and welding on the metal thicknesses used by MetalForming readers, you could pull the MIG gun trigger and the only reason the system would stop welding is that the wire spool ran out. Welding machine duty cycle won't be an issue.

 

 

This article originally appeared in the January 2018 issue of MetalForming Magazine. It is reprinted here with permission of the Precision Metalforming Association.

Posted in Welding Process , Tagged with Welding

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