Transformer conversion


Thread Starter

ravi chandran

Dear sirs,

currently i am working on a old type of welding machine. It seems that the transformer of the welding machine uses 415 Vac ( two phase only )
The primary side we have several coil to be selected by selector switch but anyway the input voltage is same. The secondary output is at 11~12 Vac with max current at 1200 A. The problem is, i was instructed to modify this ac welding machine to give dc output voltage. Is there any possibilities to convert it with most cheapest cost. I thought of using a rectifier at the
output. Will it help and is there any rectifier in market which as the capability to handle 1200 A. Please advice

Thanks in advance.

ravi chandran
Hi Ravi:

To my knowledge, there are a couple of ways you can go about this. First is to make your own rectifier stack. You can get a pair of rectifiers per power leg and basically form a "full wave bridge". You may want to double up on the diodes per position to help handle the nominal current loads you will be introducing (and this helps to handle the spikes quite nicely). A rectifier stack of this size will definately need some sort of active cooling (water
flow) to help keep it from going into thermal.meltdown.

The other possibility is to call a welding transformer manufacturer (like Roman) and talk over your application with them. They *may* sell a DC conversion kit. Be prepared to discuss the application in detail with them and they will be able to help determine if DC will help in your situation. In all the welders I built/installed over the years, I only did 1 DC welder. It was
necessary due to the large quantity of metal that was in the center of the weld field. Going to DC helped us in that situation to prevent the weld-field from forming it's own transformer with the part, shunting the current away from the weld.

The basics here relate to the forming of an "iron core inductor". Basically, if you remember your AC circuits class, an inductor resists changes in current. Current is exactly what you need in a welding situation so anything that resists the flow of current = bad idea.

Hoping this helps.

Ron Gage - Saginaw, MI
([email protected])

David McConnell

There is a point being missed here. Yes, it is possible to build a rectifier. Use a full wave bridge with the proper heat sinking and you have
DC for sure. Obviously the bridge must be on the output side of the transformer. There may be need for a fan, which is probably already a part of the AC machine. Look inside any AC powered DC output welder and this is what you will find (plus an inductor... read on). There should be no need for water cooling.

The issue is that with the rectifier you will have pulsating DC. Each time the voltage crosses zero, the arc is going to go out! To solve this, you must have a series inductor. As the current (voltage) goes toward zero, the Ldi/dt characteristic of the inductor will hold the voltage (current) above the point of arc extinguishment.

Establishing the size of the inductor, obtaining it, and finding a place to put it is going to be the trick. Perhaps you may find a workable inductor in a junked welding machine somewhere. I did such a conversion on my 230 Amp home welding machine and found a scrapped inductor after finding that I could not strike and hold an arc without it. Unfortunately, I bypassed the
effort of figuring out what inductance I needed so I cannot begin to give you a suggestion! Perhaps the firm mentioned by Mr. Gage can be of help.

Good luck,

Dave McConnell
Responding to David's reply:

The application data is insufficient to go into a detailed discussion on welder design. That is, DC vs AC, arc in air or inert gas blanket, etc.

That said, if a three phase source is used the DC voltage never goes "thru" zero. Hence, the currrent will not "pulse". The resultant "ripple" that is much lower for a three phase full-wave bridge configuration, and even lower for a double-star connection.

The former requires a single step-down transformer. The latter configuration can be accomplished using two step down transformers where the secondary of one is "offset" 90º from the other.

Phil Corso, PE
Trip-A-Larm Corp
To List,

Following is a correction to my Mar 15, 2000, 4:34pm E-mail on the subject.

The last sentence, should have read:

"... of one has a 60º (not 90º) phase-shift with respect to the other.

Phil Corso, PE
Trip-A-Larm Corp