how dose calculate of resistance welding transformer power(KVA)?

-- Hossein hashemy (hosshash@yahoo.com), October 19, 2003

Answers

kVA Rating at 50% Duty Cycle - ‘kVA 50%’ The thermal rating of the conversion equipment (transformer) represented by the load which can be applied without exceeding the allowable temperature rise of the materials used in construction of the device is referred to as the kVA rating. Since resistance welding equipment is used intermittently, the rating is based on a 50 percent duty cycle over a one minute (60 second) integrating period, which means a load equivalent to the kVA rating at 50% duty cycle can be applied for a maximum of 30 seconds followed by a rest period of 30 seconds. This is also the nameplate rating. kVA demand - ‘kVA dem’ The actual load applied to conversion equipment during welding. This is the product of the no-load output voltage times the welding current.

Weld Time" - ‘t’ The duration of welding current through the workpiece in making a weld expressed in cycles of the power line frequency.

Production Rate - ‘N’ The number of welds made in one minute.

Frequency - ‘f’ Frequency is the power line frequency expressed in cycles per second or Hertz(Hz). Domestic frequencies are usually 60 Hz.

Duty Cycle - ‘D.C.’ "Duty Cycle" is expressed in percent during the integrating period (60 sec). The duty cycle may be calculated using the following formula:

D.C. = [(N´ t)/(60´ f)]´ 100

Frequently it is required to relate the kVA demand and duty cycle under which the conversion equipment is operating to its kVA rating at 50 percent duty to assure that the conversion equipment is operated within safe limits. The relationship is expressed by the following formula:

kVA 50% = kVA dem ´ Sqaure Root of(D.C./50)

This calculation must yield a result which is less than or equal to the nameplate rating of the conversion equipment.

Cautionary Note: The nameplate rating is based on the highest secondary voltage. When operating on a lower voltage tap, the nameplate rating must be reduced proportionally to the reduction in secondary voltage. See the link at the top of the page for a spreadsheet useful for computing KVA ratings at different tap settings.

"RWMA Manual", Fourth Edition.

-- Kurt Tolliver (kurt@unitekmiyachi.com), October 19, 2003.