DC welders

[Kolo]

Hi,
I hope somebody can help.

I have a 240 V 3 phase motor which turns a DC gen and would like to control the start/stop of the moter when I touch the welding rod to the metal to be welded. The problem I am faced with is that I need to test for continuity but to do so would require equipment that can handle 230 amp max.

If anyone has any circuit sugestions to control the motor thus the dc gen and finally the welder output power I would greatly appreciate it.

Thank you

 

[moturcu]

Kolo
You can put a transformer to the system such that its secondary winding will be your cable + generator winding + welding rod. Its primary side will be supplied with an appropriate AC voltage.
As you touch the welding rod to the metal you would short the secondary of the transformer. Than you can measure primary current. By using this short circuit information you can trigger DC motor. As loong as you weld, short condition will exist and motor will not stop.

 

[Kolo]

Thank you moturcu you have somewhat confirmed my ideas

I am still a little uncertain. I though about using a transformer circuit to couple the ac test power into the output. However, correct me if I am wrong but as I understand it you are suggesting that the transformer be inserted in series with the dc gen. This would mean that at max operation the secondary winding of the transformer would have 230 amps dc through it, which would mean that I would need a transformer rated for 37.5 kva @ 120 volts on the primary side. I believe that the secondary voltage should be small as to not interfere with dc welding.

Please verify or correct me.

Thank you

 

[BJC]

It's not uncommon in fact very common for welders to have a high frequency ac current on top of a dc current. It's usually around 2Khz ( yeah I know not really high frequency).
You probably get the same effect with a rotary generator.

 

[moturcu]

Kolo
We do not want to transfer power with the transformer. It will act almost like a current transformer. So its power rate does not need to be that high.

The only concern can be the saturation of the transformer core due to high DC current. But this can be eliminated by introducing air gap into the flux path of the transformer and/or by using low-permeability magnetic material.

 

[Kolo]

moturcu

What type of transformer would you suggest for this application?

 

[Kolo]

I must apologize moturcu I am still a bit confused. Am I correct is saying that the secondary winding of the transformer is connected in series with the dc gen and that the winding needs to handle 230 amps dc?