Questions:

Is the welder using AC or DC ?

Is the item he's welding ungrounded -- not Earthed ?

Was the motor fully isolated when the welder did his thing ?

Am I correct that the crane is using DC ?

Are power electronics involved with the crane ?

 

This most often happens when the welder has not connected directly to the work. I have replaced sealtight and wiring a number of times because the EGC and the sealtight were the return path for the welder.

 

Same here, and not just cranes.........

I don't see how it's possible to burn up motor windings while welding. Unless the crane is in operation at the time, the motor windings are isolated.

If the motors burnt up because of single phasing, I'd be looking at the O/Ls and the crane rails. And make sure that the O/Ls do indeed cause the correct contractor to open.

One possibility, though unlikely, is that capacitive/inductive coupling between the welder and the control circuit caused a contractor to pull partially in. Far enough to make 2 of the poles but not all 3.

Pretty far out in left field but I've seen stranger things happen........

What is the supply system? If it's grounded B and the welder ground is connected to the building steel and not the workpiece then I could see the welding current burn up the crane rail brushes or maybe a spot on the rail itself. This would cause the motor to single phase.

Remember that you're dealing with ironworkers here, if they push the button and the crane doesn't move, they'll push the button a bunch of times then hold it in.........you get the idea........

 

DC welder. AC crane. No electronics. Motor rewind guy says single phase was the issue. But I find it hard to believe. I'm trying to get my head around if current could flow through a winding. It should be insulated from ground....

 

I would not expect that welding would cause winding damage. As you said, there should be no path for the welding current to get to the wingings.

 

I have seen bearings weld on a motor when a guy was welding on a direct coupled pump and put the ground on the motor case. Shouldn't affect windings though. I have also seen welder current leakage issues when they were using a crane to suspend a long piece of pipe being welded on a spinner. They used regular grease instead of conductive grease on the rotating ground and the welder grounded through the body of the spinner and the motor controller ground. Destroyed the DC controller on the spinner though, not the crane.

 

NO MORE WELDING ON THE CRANE !
problem fixed !

 

I have worked on a late 50's vintage 10 ton overhead crane in our powerhouse that was AC . NO electronics. I think I can say most of the cranes on our site are AC.

LC

 

Havn't heard a motor

I have heard it a lot that if you weld on something with a motor attached to it, ??

The saying is "weld on a machine and it will have problems" But it usually with controls and drives not the motors. Ground placement and a GOOD ground cable is everything. Welding on a machine was a last resort for us, then an electrician would disconnect allot of stuff and check where the welder placed their ground.

 

We have large wet saws at my work and sometimes have to make repairs with a welder. The company that makes the saw told me to make sure the power to the saw was disconnect and to ground very close to the are I am welding. Or I could risk damage to electronics or motors. Have had to weld near gear boxes and motors many times and as long as I am grounded close by no issues.

 

What it could be is that welding on the crane has caused one or more of the duct-o-bar fingers to arc and pit, so when the motors run, the voltage they are getting is severely unbalanced. That should be easy enough to check.

 

the crane as phase protection on the bus bar. It would also detect an undervoltage. So we know that busbar isn't the issue

 

Let me throw this in the mix .,,

did you check the crane hoist or trolley motor brake to make sure they are isloalted .,,

I know some of crane motors do have spring apply and electrically released braking system in there.,,

Just a thought due some of the motor may have either 120 volt or 480 volt brake coil in there ( ya it may have other voltage too )

and I am thinking of how it got single phasing it may have stray currents from the welder that run thru the motor brake coil and may inducted a small bit of current to get the winding hot and cause a single phasing .,,

Just a thought to check it out.,,

 

Ya potentially.

We were there again today and checked absolutely every 3 phase power connection. NO signs of any arcing or heat signs. But tested the phase monitor system and there we some issues. A transformer in the panel backfeeding caused enough voltage for the phase monitor not to pick it up.... My co workers and I are struggling to find the cause of this.

 

This is a test after signing on. Not directly related but I have a welding shop with steel building. I have a rod in the floor from frame and a lug I connect to it to the building and to island with benches in the floor, also pipe with 120v.
I actually have 2 benches. So, what I did was use isolated recepts and no ground with the AC and electrically grounded it with the welding ground and fed it gfci. I have seen cases involving cranes and even tools sitting on separate benches use circuit ground if welder had work lead from machine and didn't move it. So we hook work permantly to the frame and steel bond it all together leaving no way to do it wrong. Would have to bring an AC circuit from elsewhere to provide a separate pathway, just eliminates any potential for cords from grounded tools laying on benches to become pathways for welding current.