OCPD selection for Welder

[SteveO NE]

I suppose my understanding of the Duty Cycle is what confuses me. I thought 100% meant you could use the welder for the full 10 minutes as opposed to 30% which is only 3 minutes in a 10 minute period. But that seems to go against the currents listed. Lower Duty Cycle + higher amp draw

To pile onto what was already said, 100% duty cycle can be non-stop welding, so everything has to be continuous rated for heat soak. 30% duty cycle is non-continuous, so you need to be able to handle the peak draw without a nuisance trip. A time delay 25A fuse is going to handle something in the neighborhood of 50A for 3 minutes, a normal operating fuse may be closer to 25A at that time. You have to consider each component is affected in each scenario and go with the worst of each.

 

[Sparky2791]

You limit the time of use of the welder unit in order not to exceed the build up of heat in the unit, if you intend to avail of higher amp output. The basic equation for heat in welders is Heat energy = I² x R x t x K, t in the equation is the welding time, R being the resistance of the welder and K a constant for the welder setup. Assuming R and K remains the same, the only variables left are t and I (amps) and that to keep the heat energy within a certain level, one can use a higher amp (I) while keeping the time t shorter, hence a smaller duty cycle. E.g. a 200A welder rated at 100% duty cycle could be set to deliver 200A x sqrt(1/0.30) = 365A @ 30% duty cycle! Hope that clears everything.

It does help. My issue is these welders are for a welding shop in a high school where there is really no control over how students end up using these welders. In an effort to protect the welders from being improperly used and protect them from damage as well as protect the circuit because students will be using these I would look to go with what the mfgr. recommends which is of course the conservative approach of 100% duty cycle warranting the lower OCP size. I'll have to discuss this with the teachers.

 

[Sparky2791]

Thanks everyone for the replies. I did a welding lab like 15 years ago and probably had all the same questions back then as well.