DC Inductor for an Arc Welder Conversion

[GlynnHeeswijk]

Hello,

I want to make a choke for a AC to DC welder conversion.

The welder is a 50hz single phase oil cooled unit with constant current output upto 110A. I will bridge rectify the output to give rectified AC output.

I need to smooth the output and as I need a constant current output, I'll use a choke rather than capacitors.

I believe or low frequency you generally use laminated electrical steel. However sourcing electrical steel is difficult in low quantities. Thus I can either buy an old core or make one from mild steel.

For a DC choke how significant are eddy current losses, hysteresis etc?

Thank you

 

[Paul Colby]

You might supply a schematic of your idea. I can't see how blocking 50Hz with an inductor would leave you any current?

 

[GlynnHeeswijk]

Hello,

The choke is post-rectifier. The choke resists changes in current and hence smooths the output.

Attached is a schematic from Google but it is very similar to what I'm doing (most old arc welder work in this way).

 

[Baluncore]

The choke will follow the rectifier so it will keep the DC arc running. It does not need to smooth the ripple on the DC, it only needs to continue the arc at high frequencies. For that reason the core needs to be an iron powder or ferrite. Very high voltages will be present on the output side of the choke.

I would use two chokes in series.
First, the core removed from a microwave oven transformer, stripped of windings, with the magnetic shunts knocked out. Then wind about 30 turns of thick copper insulated strip onto the core. That will fill in the gaps while the bridge is zero crossing.

Second, a smaller iron powder core with about 10 turns to provide the RF re-strike.

 

[GlynnHeeswijk]

Thanks. From what I understand MOTs tend to be a bit too small for the task. I was thinking of making a bigger choke but can't source the electrical steel, not sure how critical this is for a DC choke.

I have a spare welding transformer which is fair bit larger than a MOT that might be ok. I will gap it to avoid saturation.

Not planning HF start just yet but might do in the future.

 

[Baluncore]

Not planning HF start just yet but might do in the future.

The HF choke is for welding with iron powder rods or TIG.
I do not suggest you build a spark unit yet.
A MOT is sufficient, just use less turns to avoid saturation.
The choke can be quite small.
You should use an adjustable gap choke welder to limit the AC current before the rectifier, otherwise you will need some big open wire resistors in the circuit.

 

[Tom.G]

The purpose of the choke (called "reactor" in the welding industry) is to supply a phase shift. If the voltage and current cross Zero at the same time, the arc will extinguish. The reactor supplies the current during the voltage Zero crossing, maintaining the arc.

 

[Baluncore]

From what I understand MOTs tend to be a bit too small for the task.

Unlike an AC arc, a DC arc is difficult to extinguish. It takes less than 1 mH of reactance to keep the current flowing through the phase diodes that are about to turn off, while the other phase diodes are turning on. You do not need a big choke on a DC welder.