DC Motor Arcing to Earth & "blowing" Drive Thyristors 2

[sparxicus]

Please bear with me as I am a first time "poster"!.
I have a 49kw ABB Dc Motor which seems to be arcing from the bottom brushes to the motor case. This is causing two of the 3 phase fuses feeding the ABB drive to blow (in one case it actually took out the main 200A Breaker feeding the entire M/C. On two occaisions it has also taken out the thyristors in the ABB Drive (and a cicuit board). We have had the motor tested and reconditioned, and no faults are apparent. An ABB engineer has come in and changed the Thyristors and a PCB on the Drive unit. This morning the M/C Operators reported a massive flash and the motor went down. You can see a big arc spot on the bottom of the motor casing (just underneath the bottom brush holders) You can also see arcing marks on one of these holders. Brushes are new (motor is clean and dry, with no excessive carbon residue). As I said, Motor and Drive have just been serviced!.But drive fuses(2) and Thyristors have gone again. Any ideas/suggestions from any of you Boffins out there would be a Godsend, as all our Technical staff are scratching our heads. Any ideas gratefully accepted ASAP.
Cheers Jason (sparxicus)

 

[waross]

What is the motor driving?
yours

 

[edison123]

Probably, the brush pig tail to ground clearance is insufficient. Try placing some insulation board / paper in between the stator casing / end shield and the brush pig tails.


* Basically, I would like a full-time job on part-time basis *

 

[sparxicus]

Thanks for your replys. But there is sufficient clearance between pigtails and casing. Also the brush holder springs are tight enough to give a good connection with the com (without stretching the pigtails). The motor is a ABB type DMP - 4L 49.9kw Arm 440V/127A Ex 310B/4.25A 1510 rpm driving a twin screw Extruder (for plastic profile).

 

[Skogsgurra]

Take a deep look at the mains voltage. You need a scope and a differential probe. I have had very high "snap off" voltages when there were commutation notches in the mains voltage. The notches were produced by another thyristor controlled rectifier that was run without commutation reactors. It took some time to realise how this happened, but after introducing reactors in the AC branch of the other rectifier, we had reduced notches and no flash over.

Gunnar Englund

http://www.gke.org

 

[DickDV]

You may also be a victim of a floating delta power supply that occasionally has a phase lead go to ground (sometimes called corner grounding).

This will only be a problem if the DC drive does not have an isolation transformer with a grounded wye secondary.

 

[noshorts]

I have seen on a GE DC300, a fault with the field applying SCR package loosing a phase, inturn causing a week feild (or maybe the corect feild only half the time). When this happened under heavy loading it caused a huge current spike in the amature circuit. In the case I worked on, it blew up in between the SCRs for the armature circuit, but I suppose it could blow at the weakest point.

 

[sparxicus]

noshorts & skogsgurra. Thankyou for your replys.
After conducting some more research (although I have not come to a solid conclusion yet!) I remembered another Extrusion machine with a DC Drive had a similar problem a few weeks ago. In this case the operators reported the factory lights dimming for a fraction of a second just before the machine broke down. Two of the 100A (Y & B) 3 phase fuses protecting the drive blew and the main 200A breaker at the Dis Board tripped.
Talking to other Engineers in the area apparently many factories in our area have been having momentory "power dips" on the incoming red phase from the local supply company.
Going back to the original MC I forgot to mention that when the problem occured it also took out the 150A contactor suppling the DC Drive, welding blue and yellow phase contacts together, but leaving the red phase contacts OK.
One more thing, over the last few months the CAN Bus system on this machine has been crashing intermittently, could this be connected to a power dip on one phase (pilz safety relays remain ok, but I suppose they are looking at 50Hz while the PLC and CAN Bus system has a very much higher scan rate and thus would pick up any supply dips no matter how short their duration.
AGAIN ANY SUGGESTIONS VERY GRATEFULLY RECIEVED. THANKS Jason. (sparxicus)

 

[waross]

One thing to consider as a general point. An overcurrent/over voltage on the armature circuit usually causes flashover on the comutator. If the current is flashing from a brush holder or pigtail to ground, you may have a very high common mode voltage to ground rather than a high line to line voltage across the armature.
This may be a direction to point your troubleshooting efforts.
respectfully