DC Motor flashover

[ditchy]

We recently have had 3 flashovers over a period of about 2 years on a 330kW 750volt DC motor on a Centre roll drive on a paper machine, that has been in service for about 20 years. The Centre roll is in a group of 3 drives, one of which is a master drive with an encoder controlling the armature volts of the group via a single quadrant thyristor converter. The Centre roll motor's armature is connected across this armature voltage and we control the load on this motor by adjusting the field current, the back emf then controls its armature current. The motor will be working fine, no sparking, steady current, steady speed, and then suddenly flashover between brush arms. As well as flash damage, the interpole windings get distorted, which causes us a lot of down time. The spare is fitted, and this can then work fine for months and then out of the blue, bang, another flashover. The other motor in the group, a 290kW never suffers this problem although its control is the same. We suspect the problem maybe caused if the field controller goes faulty and we get an increase in field current turning the motor into a generator and getting the flashover. We have changed field controllers but still had the problem! Has anyone got any idea's

 

[aolalde]

My personal opinion is that control by field is very convenient but could easily turn unsteady. When for whatever reason the excitation current is drastically reduced the motor counter emf is too low demanding a large armature current. That current could be out of the commutating capacity leading to a flashover. An expensive but steady alternative is to keep the field constant and control by armature voltage.

 

[sreid]

I'd carefully check all the power wiring. One way of doing this is thermal imaging to see if any connections are loose and heating up. Intermittant connections could cause the problem you describe.

After 20 years, you might consider a total rewire.

 

[Skogsgurra]

Is this a Slitter/Rewinder?

A rebuild is easier and lower cost than you may think. I am doing that on a Jagenberg right now and the material costs no more than around USD 5000 for each drive. You keep the DC motors and get modern digital four-quadrant rectifiers. You can do the rebuild in steps. Changing the problem drive first. And possibly stay there.

Your next flash-over and subsequent production loss will probably cost more than the rebuild.

Gunnar Englund

http://www.gke.org

 

[UKpete]

It does sound like a field failure problem. Usually it's a momentary loss of field that causes a flashover, not a sudden increase in field. DC traction motors constantly see large steps in field voltage and armature voltage (e.g. during "gaping") with no problems. You don't mention a field failure unit - do you have one?

 

[ditchy]

Thanks for all the replies. It looks as though the motor field variation is the most likely cause. We do have a field failure relay in the circuit, and it has never operated, and we also have changed the field failure relay in case it was faulty. We put some monitoring equipment on, and at the last flashover we did see the field current rise, not fall. It is our plan now to convert to a current controlled drive using its own converter and encoder, I was just curious if my idea's were sound

So thanks once again for your thoughts

 

[waross]

With a fast rise in field current, the motor will become a generator and back feed. An overloaded generator will also flash over similar to a motor.
My answer to students question,
"What's the difference between a D.C. Generator or a D.C. motor?"
Answer
"Often only a few volts or a few RPM."
We would then set up the equipment in the shop and verify the statement.

 

[jktwn]

Ref thread 237-32685