Spot Welder Short Circuit Voltage. Trying to calculate the current

[multifrag]

My Clamp meter only goes to 600A... below of what the spot welder is capable of. I'm trying to figure out the current difference between welding pins that are very close to the transformer(20cm) and welding pen that is 80cm away(25mm2). In terms of performance the difference is immense. I've done some Short Circuit Voltage readings and am a bit confused with the results. Why are Pins have lower short circuit voltage?

Pulses(5ms)	Pen	Pins
2P	0.57V	0.47V
4P	0.9V	0.68V
10P	1.4V	1.07V
18P	1.93V	1.45V

According to the the formula I found on this website (I = U * diameter [mm2] / (0.0175 * length [m])) these are the currents I should be expecting(transformer wire length 0.6m), which I don't think are right:

Pulses(5ms)	Pen (1.4m)	Pins (0.8m)
2P	581A	839A
4P	918A	1214A
10P	1428A	1911A
18P	1939A	2589A

My unit should be capable of 800A Max and 3.2kW pulse power draw...  Datasheet to the unit

[amyk]

If the pulse is fast enough, the inductance of the wiring will also have an effect as well as resistance.

[multifrag]

If the pulse is fast enough, the inductance of the wiring will also have an effect as well as resistance.

There is a voltage drop through the wires, but I consistently record higher numbers for the pen. The cable is made to work with 170A continuous so it should be fine with 800A for maximum of 0.09 of a second. Wire Resistance Calculator is giving me 0.94 mOhm for 1.4m 25mm2(Including transformer winding) copper wire. At 1.93V the wires resistance is giving me up to 2000A. While the pins have a calculated resistance of 0.54 mOhm and current of up to 2700A at 1.45V. So in theory I'm loosing 700A through potential current capability, but 2000A is still way higher than 800A max that this spot welder should be able to produce. I'm probably mixing something up.

I'm just trying to produce similar welding results that I have at the pins with the welding pen.

[unitedatoms]

You can try to model the circuit in LTSpice. Consider that inductance of 1cm thick wire 80cm long is 800 nH, some ohmic resistance, known source pulse shape, series resistance of source, leakage inductance of secondary coil of transformer, etc. Also environment can matter, say if surface of table on which wires go over is sheet steel, so it adds up an inductance.

The calculator for inductance of straight wire:

http://www.consultrsr.net/resources/eis/induct5.htm

Well, the return wire is cancelling the inductance, except the area of long narrow loop. So the inductance can be may be 10 times less. It should be possible to measure inductance with LCR meter.

Edit: Oh. And it can possibly be also a skin effect. At 60 Hz the depth is limited to 8.5mm. With 5ms pulses the major energy is at 200Hz. So skin effect worsens.

[multifrag]

You can try to model the circuit in LTSpice. Consider that inductance of 1cm thick wire 80cm long is 800 nH, some ohmic resistance, known source pulse shape, series resistance of source, leakage inductance of secondary coil of transformer, etc. Also environment can matter, say if surface of table on which wires go over is sheet steel, so it adds up an inductance.

The calculator for inductance of straight wire:

http://www.consultrsr.net/resources/eis/induct5.htm

Well, the return wire is cancelling the inductance, except the area of long narrow loop. So the inductance can be may be 10 times less. It should be possible to measure inductance with LCR meter.

Edit: Oh. And it can possibly be also a skin effect.

I have LCR-t4 laying around. Tested the wires(From pen and pins) and they come up as 1Ohm resistor even though with a multimeter it's a dead short... Just for the sake to see if the LCR-t4 working, grabbed an inductor 33μH and it showed correctly.  With the calculator you provided I should be seeing 1.735μH. LCR-t4 minimum Inductance: 0.01mH. So that's probably why i get a resistor value...

In terms of Skin effect Im in UK, so 50Hz and with the calculator I get about 9.2mm. In terms of pulses. Minimum about of pulses is 2. Not sure the spot welder just combines the pulse time or runs one after the other. It could be 10-90ms, it could be 2-18 separate 5ms pulses. If they combined at 90ms I should have 11Hz which would give me a depth of 19.65mm.

I mean I can look into upgrading to thicker wiring if that will fix the problem, It's just that I already upgraded from original 16mm2 to 25mm2. I saw an improvement in performance at low levels, between old cable and new, but nothing close to the pins.