Motors drawing excessive currents 3

[Parchie]

I have a problem with long travel motors of an overhead crane where one of the two motors was rewound after being burnt out. When the motor came back after being rewound, both motors began to draw excessive currents, beyond their full load amps! Any idea where one should look at to solve this problem?

 

[itsmoked]

Your hardware is having a problem with keeping the ends of the beam parallel. As soon as they start to go out of parallel the forces will sky-rocket and the motors/drives will be immediately overloaded.

In all things like this you actually have multiple problems:
1) There should be something to detect the gantry starting to skew that stops the system.
2) The drives should be set to protect the motors so an overload burnout is impossible.
3) The drives should trip each other whenever one of them trips.
4) There should be a feedback system to keep the drives (and the ends of the gantry) synced together so skews don't even start to occur.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Did you check the direction of rotation of rewound motor?

Muthu

http://www.edison.co.in

 

[waross]

What edison says;
Or
The rewound motor has been rewound for a different speed.
Or
You have a serious mechanical problem which led to the burnout in the first place.
For instance you may have an undriven wheel which has seized or is dragging enough that the skew is overloading the drive motors.
Check all the wheels and the gear boxes and the alignments for binding.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

The motors should be a matched set, they were but they aren't any more. A slight change is parameters changes the slip speed for a given load and now the two motors run a different speeds.

 

[waross]

A change in slip speed shouldn't make that much difference if the base speed is the same. eg: one motor at 1760 RPM versus the other motor at 1750 RPM.
BUT
The motors will run at the same slip RPM but one motor will be further down the torque curve and will be developing more torque and pushing harder.
That may be causing skewing of the bridge and that in turn may be causing the excess current.
Good call David.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Yes, skew is a huge problem if not controlled, because when the bridge skews, the wheels run up against the rails and increase the friction, often dramatically. Then in my experience, it's VERY difficult to control skew if the motors are mismatched (as they now are). I've tried it with VFDs and had to resort to long range laser sensors to measure the distances between corners (because as the bridge becomes a parallelogram, the corners are no longer equidistant), then feed the delta into a PID loop controlling the VFD speeds to try to keep it straight. Very complex, very expensive, it would have been much much cheaper to just replace both motors with a matched set every time one went bad... but that wasn't my decision, I just implemented it.

Or it's the reverse rotation issue, that one would be kind of funny...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

DC or AC motors?
If DC, there are a lot of subtle things. And a few very gross.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Parchie]

Seems I got a lot to do! Thank you all, Will feed back if it gets resolved.

 

[HamburgerHelper]

I don't got any answers to your problem but the title felt like clickbait. I couldn't not click to find out what was long and was giving you problems.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[Parchie]

@HamburgerHelper (Electrical),
Long travel motors are those motors that drive the overhead crane along its "long" rails. Others call it the bridge drive. The other movement is called "crab" travel, where your load is moved along the bridge.

 

[waross]

Anecdote warning.
One of our customers had a very long bridge crane that had a tendency to skew. The crane had been in service for years and the slight skewing was not causing undue wear but it did cause a squealing noise whenever the bridge traveled.
A new guy in the office misdiagnosed a lubrication problem.
He had some arrangement made up to grease the rails.
That was done with one pass of the crane and got rid of the noise.
It sure did. The crane sat in every low spot in the track spinning it's wheels.
It took a couple of days to get all the grease off of the rails and wheels before the crane could be used again.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

A little sand would have taken care of that faster...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[crshears]

Hmmm...would the sand have stuck in the grease for good, or would regular re-sanding have been necessary?

Also, OP doesn't state what types of loads this crane moves; site contamination with sand could be problematic in this situation, but Parchie would have to tell us that.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Don't forget that the guy who called for the grease in the first place also planned the cleanup.
Most of the travel was over an unpaved outside yard. Sand may have worked.
I was just an observer, thank goodness.
I think it was a case of;
"Put it back like it was."

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Parchie]

@crshears,

Also, OP doesn't state what types of loads this crane moves; site contamination with sand could be problematic in this situation, but Parchie would have to tell us that.

It's an overhead crane like any other. It moves bags of product unto a loading area where the products gets picked up by forklifts for storing to adjacent product building. No sand around. One of the bridge drive motor got burnt and then rewound. The problem started when the rewound motor was re-installed. My hunch is that these two drive motors were not matched, skewing the bridge--> ever since the start of operation. It was only now that production crew requested an investigation after one motor got burnt. We are verifying if those motors are really matched.

 

[waross]

Have you watched the operators?
Crane motors are pretty rugged but repeated jogging can fry the best of motors. Also, reversing to stop. With a wound rotor motor you can slow down with a low speed in the opposite direction but if the motors kick into high speed when the brdge is going in the opposite direction it will shake the whole building.
Are these multi speed motors and are they both on the same speed?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MikeHalloran]

Some crane rails are flat on top; I have no idea how they work.
Most, AFAIK, are crowned, amounting to the center being ~.012" higher than the edges, or about a 5" radius.
That works with slightly tapered wheels to make the bridge center and align itself, without the flanges ever touching the rail.
If the rails or wheels are very old and worn, that all goes to hell.
Careful inspection of rails and wheels might produce clues about the original burnout.





Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

Most of the bridge crane rails that I have seen had flanges on both sides of the wheels. I am familiar with the principle of tapered wheels. On a long bridge the taper is not as effective as when the rails are nearer together.

Bill
--------------------
"Why not the best?"
Jimmy Carter