Arc Welder output Current Control and DC Rectifier Upgrade

[john_weld]

Dear All,

I have a Arc Welder with 250 Amps output that can work on both single phase and two phase 7.5 KVA ( 230V / 440 V , 50 Hz ) with 250 Amps AC Output and Open Ckt voltage of about 80 Volts.

I was checking the Possibility of upgrading my old welder that use a tapped choke connected to the secondary with 5 different taps to select 5 different output AC welding current. This required manual change of holder cable from one selection output stud to the other. This is very difficult and could not select the correct power required for welding as only 5 output studs are there and the time taken is more for changing from one output stud to the other.

I have seen many schematic on web using either a welding transformer that has a movable core to select the current or using a control circuit in the primary side of the transformer. But to control from the primary side of the transformer in my welder would be difficult as I need to use it on both single phase and two phase power.

So thought of using a variable inductor in the Secondary of the welding transformer to control the welding current .Need your help to check this circuit attached and let me know if you see any problem or if I need any change in this design .

If there are any other schematics that you have seen to control the secondary with out the variable inductor would be good.

I would also like to know how we need to calculate the number of turns and the size of the variable inductor coil to be used for controlling 250 Amps welding current.

Thanks ,

Regards,
John

 

[R Kraft]

You would be far better off , putting your adjustable choke in the primary circuit.
Robert

 

[shenion]

That looks right.

There are ways using saturated inductors. Basically, add a second set of windings with many turns and control the inductance by a DC current in the winding. Requires two inductors though but does give power setting by rheostat. Usually still have high and low taps. Mine works this way with 3 taps. It is very complicated to calculate and design.

Doing PWM on the input will work to a point. 230/400 should not matter as you are controlling a thyrister.

For L1, find your lowest welding current, say 30A. At 80V OCV and a weld voltage of 14V , that would be (80-14)/30 or 2.2 ohm reactance (Z). At 50Hz, Z=2 * pi * 50 * L or L=Z/(2*pi*50) for 2.2/(2*3.14*50)=7mH (millihenries).

That gives you a ball park for the inductance. 14V was off of one of my welder tables.

You can search for calculating inductance for winding inductors.

For Z, I find just make it big. I used 30 turns on a 1.7" core with a 1/8" steel frame around it.

 

[john_weld]

Dear Robert / Shenion ,

Thanks for your kind advice.

I have couple of schematics post below that can be used to control on the Primary side of the single phase 230 volt Welder. Not sure if I can use it for my welder as I need to use it on both single phase and two phase 440 volt AC. So thought of just posting it .May be some one can help me on this final design that is better .Please let me know your feedback.

I have couple of other schematics that is used to contorted on the secondary using electronics circuits ,one has both CC and CV I believe. I will make a documentation on that and post it shortly.

If any one have better schematics can you please post it hope I may will find it useful to check if I can use it for my upgrade.

Thanks,

Regards,
John

 

[shenion]

The last one looks best. All might have issues with stability but the last one puts a small reverse voltage on teh SCR's that will help false triggering.. Could be fixed by caps across the + and gate terminals of the SCR's.

230/415 is only a difference in voltage. A 500k pot for R3 on 415v would probably work for 230.

Yes, the newer version of my Miller has SCR's on the secondary. More to get rid of a mechanical contactor.

A better design would feed the gates from a transformer, that could work easier on 230/415 (primary always connected to 230V winding.)