Duty Cycle and Rated Current for Welding Equipment

during welding process, the machine will be running with a ON-OFF-ON-OFF duty cycle

at 50% duty, you will have 50% ON time, 50% OFF time,

at 60%, you have 60% ON and 40% OFF

The end result could be closer to (250/60)x35 = 145.8A

You are correct in that the rated current is inverse to the duty cycle.

Practical the machine is designed with a capacity of cooling which is deciding on the duty cycle.

During operation the machine develops heat due to the I^2 R of the machine.

If the rate at which the heat is removed is smaller than this the machine will heat up till it eventually fails.

The idling time between welds is when the cooling takes place.

It is common sense that the higher the welding current the more heat develops and the longer it takes for cooling in between welds therefor reducing the duty cycle.

I hope this answers your question.

Thanks Johan. I agree with your suggestion. The higher the current, the higher the I^2 R loss. So, it is a must to cool down the machine for longer time.

but during on time at 35% duty cycle, how many current we can get from this type of machine.

this current may be higher than the rated current of the machine. Is it possible? i could not believe and understand it. Pls help me to solve this problem.

Thanks and regards,

Kotuu2.

Hi all,

I absolutely agree with the meaning of duty cicle: It's the maximum continuous time percentage the machine can be "ON" without damage.

Another question is rated current. As in any other electrical machine my opinion is that rated current is the maximum current selectable. Of course it has some relatonship with the duty cycle, but: Does anybody think that in the example machine if you reduce the "on" time to 6% you can rise the current to 2500A?

The maximum current that can be drawn from a welding transformer depends on the amount of iron in the core.

As the current increases so does the flux density and as the flux density increases more energy per unit is required untill the core is saturated.

If the transformer is a 5 kilowatt transformer and the secondary voltage is 50 Volts then the maximum current that can be drawn will be 100 ampere.

Normally the transformer has either tappings or a shunt to control the energy (amperage) of the welder.

It still rings true that the more current you draw the shorter the usable period (duty cycle) but it does not hold true that the current is inversely proportional to the duty cycle ad infinitem.

Excellent! You nailed it precisely as the NEC and the manufacurers define it.

Dear Kottu2,

Your calculations are OK. As you have rightly pointed out:

desired duty cycle in % * desired current ² = rated duty cycle in % * rated current ²

In your calculations, you have correctly shown that at 35% duty cycle, the current could be 327 amps which you feel is not practical for a 250 amps welding machine.

Everything is fine except for the interpretation of the results.

It should be concluded that, a 250 amps, 60 % duty cycle machine could be used at maximum current possible for the machine ( with 3½ minutes 'on' cycle) without the danger of damaging the insulation if the duty cycle is reduced to 35 % i.e. the insulation will never get damaged as heat generated is low at that cycle.

Similarly, for same machine the desired current should be 193 amps at 100 % duty cycle. The interpretation again is, a max current of 193 amps should be used for continuous welding (say GMAW process) without the danger of damaging the insulation.

Trust the details are clear to you.

One way to reduce heat gain is to add fan capacity for more efficiant cooling.........even chilled air.

Reality is the machine is too small for his indicated HIGH AMP/TIME use.

MR. GUY

I would like to express my thanks to all in CR4 forum for giving advice me, spending your precious time on this topic and teaching me.

I could see the light because of you all!!!

Thks and Rgds,

Kotuu2.