resistance spot welder

[djelectric]

Hi, I have a resistance spot welder to hook up and need help making sure my calculations are correct.
Welder name plate
250 kva
480 volt
50% duty cycle (.71 630.31(A)(2))
single phase
250000/480x.71=369.8 amps

The welder has a 250 amp thermal overload and the paper work is calling for 400 amp fuses,I think I am missing something or not understanding why there is such a difference between my calculated amp draw and the 250 amp thermal.

Thank's for any help.

 

[dbuckley]

Because it will take time for the thermal overload to trip as its only "loaded" to 148% of its rating. If the user stays within the duty cycle then the thermal O/L wont trip, as it'll cool down between loadings. If the user goes at it too hard, it'll open. So the fuses protect against catastrophic faults, and the thermal O/L enforces the duty cycle.

 

[Pham Van Hung]

Hi, I have a resistance spot welder to hook up and need help making sure my calculations are correct.
Welder name plate
250 kva
480 volt
50% duty cycle (.71 630.31(A)(2))
single phase
250000/480x.71=369.8 amps

The welder has a 250 amp thermal overload and the paper work is calling for 400 amp fuses,I think I am missing something or not understanding why there is such a difference between my calculated amp draw and the 250 amp thermal.

Thank's for any help.

Djelectric,
250kVA based on 50% duty cycle is per international standard. When you get max weld current, it has only about 5-6% of duty cycle. That explains why 250A thermal overload used. You also need to check with the thermal overload characteristic.
HungPV