Motor Generator Welder Troubleshooting

Down at the Steubenville Works of Wheeling Pittsburgh Steel I found an old motor generator welder today. I wired it up and turned it on and the AC side works fine. Comes right up to speed no problem. However I do not get anything from the DC side. It reads only a couple of volts at most, and the exciter at the brushes is reading only 1 to 2 volts.

What might be the problem with the welder? What should I be checking? I assume that the problem may be in the exciter, not putting out any voltage to the DC generator. What provides the excitation for the exciter?

I don't know who made this welder. The tag is aluminum and so heavily corroded that I cannot make out the name. Its at least 50 years old by my guess.

Thanks

Maybe the generator lost what is called residual magnetism. This is what gets the exciter excited and energizes the generator field,
I think you have to re-magnetize the exciter. I think the procedure is to briefly energize the exciter field with a battery.
Other board readers may know the procedure better then I described it here.

Try this...

Rick- try this trick:

Get a big magnet, and slap it on the side of the exciter frame, start the welder, and apply a good load... set it to low output, and short the leads.... it SHOULD start generating... and once you've left it 'working' for a little bit, it'll re-establish it's own self-excitation.

OK I'll give it a try on Thursday when I am back down at the mill. I found out that it is a Lincoln Shield Arc, and then found a wiring diagram online. Looks like there are just two leads to the exciter and they go to the brushes. I'll try your magnet suggestion first, as I have a powerful rare earth magnet that I can use.

Google "polarizing a generator". We had a bunch of motor generators and diesel and gas driven generator welding machines while we were in the contracting business. Had one gas powered Hobart that would invariably have to be re-polarized if it sat unused for a month or more. The rest of them could seemingly sit forever without losing their polarization. Its a real simple process to re-polarize one, but I don't think it'll happen by placing a magnet in the vicinity.

Ye, it just need to be flashed. One one end there will be a smaller generator. You can flash it one of two ways. Take a wire and short the brushes together. If there is any residual at all there should be a small spark and the exciter will come up. If not take a battery charger or something like it and touch it to the two terminals. I am not sure whats what since I dont know what kind of machine you have.

Same result...

Slapping a magnet on it does essentially the same thing as flashing... it establishes a field with which it will all 'start'. It does work nicely- I do it on DC automotive generators, welders and AC generator systems all the time. I use a big 250lb pull job my dad bought from Edmund Scientific in 1982.

The catch to flashing, or for that matter, ANY attempt to rebuild gauss, is that if you flash it, then get results... and then shut the machine down shortly thereafter without putting a fair load on it, it won't re-orient the magnetic property of the exciter or generator frame.

SO... once it's going, put a load on it, and let it work for a while. On generators, use resistive loads, avoid inductive loads... it'll build better, stronger, and faster, without ever messing with wiring.

I got one to go using a aa battery out of my maglight once (was to hand at the time). The funny part was i got the welder operator convinced, i had charged it from the battery. He then proceded to explain said chargeing method to the forman. Forman clearly thought welder was nuts, but didn't know what to make of things. What amazed me was how little it clearly needed to get it excited - start building its on charge. It just sorta snow balls. That said a magnet on the side of it is probaly a lot easyer for every day use, if it losses its charge quickly. The one i was messing with had gone at least 3 years that we knew of unused as it soposedly wouldn't work.

We unstuck the brushes and the welder now works. However its at full power at all times. I turned a 5/32 rod into a puddle of molten metal in about 5 seconds! The controls are quite xidized, so we'll be cleaning them up next. Otherwise I think its going to work.

the old bomb shell welders will not arc if phase is reversed
they will come up but just spin and make no arc

Something wrong with the reactor assembly.

What part is the reactor?

Welde

Rick
Clean the voltage adjustment pots with some abrasive paper and blow out all the crud. The reactor is a passive device that has no adjustment. It is a coil of wire wound around an iron core and smooths out the current flow. The big pots have a spring loaded brush that contacts the resistance wire that is cast into the ceramic body.

JRW

This machine has a rheostat and another device which has an iron core and a heavy coil of wire, with a wiper on the inside. One controls voltage and the other controls current. There is a lot of green oxidation and we'll clean that up and then try it again.

Probably...

The reactor is that big core with a big wire, and small wire, with small wire going to rheostat. The small wire circuit is open.

The way that works, Rick, is that the reactor will carry full flow current, as long as the small wire is high resistance, or in your case, open circuit.

When a current load is placed on the small wire, it 'sucks' down the magnetic ability of the reactor, hence, limiting the current output. As you noted, clean it out, and it should be back amongst the living.

One problem with flashing these MG welders is flashing it the wrong direction. Another problem is that your battery should have a rather hefty silicon diode is series so that when the voltage comes up the battery will not have 25 to 80 volts across it - bad for a 12-volt or 1.5-volt battery. To overcome the voltage drop of the diode you would need at least 3 dry cell batteries in series.

When I was at Carnegie-Mellon University in 1981 I bought a Hobart MG welder that they had set up to provide 24 to 32 VDC for experiments. One thing that I did not get with it was the variable resistor for setting the amperage. After I moved in with grandmother in 1982 I contacted the Hobart dealer in Akron, Ohio and they were able to have Hobart send me an electrical schematic. The DC generator was a 2-pole machine with 3 brushes. I later found out from a book at the John Carroll University is that the purpose of the 3rd brush was to provide a negative feedback so that voltage droops as current is drawn from it. Essentially, these generators operate partly on the principle of an Amplidyne cross field generator that was at one time used as a servo amplifier for leveling elevators. I eventually shipped this to a friend in Connecticut.

Also, the AC leads did not have any numbers on them the person who unwired it somehow made the tags go away. Since it probably needs new bearings and brushes it would have to be taken to an industrial motor rewind shop for that kind of work and they could solder in modern rubber insulated leads with numbers on them. They also somehow threw away the Ac and DC junction boxes but it would be a single matter to drill and cut some holes in the back of a metallic or nonmetallic boxes.

Another thing that can go wrong with any DC machine is that when the brushes are about 50% worn that impedes current flow. During the Summer of 1993 I installed a 400 ton Minster punch press plus the steel coil straightener that came with it. The DC motor in the steel coil straightener had stalling problems until in Summer 1997 I put new brushes into its motor-generator set and its drive motor. I sanded the brushes to fit the commutators and then chalkstoned the brushes. Work like champ afterwards.

Mike Cole,

Columbus, Ohio