I have a welder with a big air gapped transformer. Input specs: 220V, 3200 W active, 25 A total (cos Phi 0.56). Output specs: at full load 110 A welding current. I haven't checked whether output is DC pulsed (simple rectification) or is AC, but the electrode connector is marked as "ground" and the clip ("pinza") as such, so I guess it's rectified. The welder is rated for "intermittent" use for 3.2 mm electrodes, and continuous for 2.5 mm electrodes.
When I switch it on, if the current is set to more than 50% of full scale, the overload switch in the garage trips.
Also, at full power the total input current (25 A) poses a strain on the standard plug that is provided (16 A power plug, I wonder… '70s safety I guess).
I could add a capacitor of about 150-200 uF to improve the average power factor during use and reduce strain on the plug and wires, but what would happen to the inrush current?
Since in the transient no magnetic field is present and the transformer is like a plain wire, I would expect no change in the inrush current, therefore requiring a soft-start circuit.
Best Answer
Transformer inrush typically has little to do with the load on the secondary. So the capacitor would have to go on the primary to have any effect. On the primary it will undoubtedly increase inrush current, though probably for a shorter period than the transformer. That's because at the point the device is turned on there will be an instantaneous voltage applied to the capacitor, making for a dramatic rate of change from zero. Capacitor impedance falls as rate of voltage change increases, so it will be close to zero in this case. The only thing limiting the current then is the series resistance.
The best way to reduce transformer inrush (which is caused by the delay establishing a corresponding magnetic field) is with an inrush current limiter. The three basic inrush current limiters are, in rough order of complexity: a fixed resistor with a time-delayed relay across it to short it out after the inrush period; a NTC thermistor; which automatically falls in resistance over time due to heat; and an electronic soft-starter.