Help with WR Motor Recurrent Failure

[OhioAviator]

Hope someone can give some ideas of where to look. We have a 4,000 HP GEC Alstom WR motor, 4.16 kV with liquid rheostat that has now failed twice in two weeks in the same way. The motor is driving an automobile shredder and was first commmissioned in 1998. In both failures we had two rotor leads (same rotor phase) severely overheat at the point where they attach to the collector ring assembly. The rotor windings not yet checked with a whetstone bridge but are in process of doing so. Rheostat has not been investigated yet, but suspect no problems with it. We plan to repair the motor sufficiently so that we can run it in order to record rotor current in all three phases. Any other ideas/courses of action we should be taking?
Thanks.

 

[jbartos]

Suggestion: There appear to be more than one cause of the malfunction, feasible. E.g.
1. Power supply quality
2. Inter-turn shorts in the affected phase of the rotor winding.
3. The rotor liquid rheostat may be malfunctioning, e.g. contact resistances with liquid are somewhat off.
4. Inter-turn shorts in the stator winding(s)5. A bad contact at some terminal causing voltage dip
6. Etc.

 

[electricpete]

I think there is very high probability that you are right about the resistor circuit. Lift the brushes and check for rheostat resistive balance from there.

 

[OhioAviator]

Thanks JBartos & Electricpete. Your advice was helpful. We measured the individual phase resistances of the rotor with a milliohmeter and found all within spec and balanced. Also inspected the liquid rheostat and found no problems. We would have liked to measure the rotor current with the motor at idle using three clamp-ons, but Alstom says that the effort would be meaningless since the rotor current is non-sinuosoidal and has a significant DC component. We have decided to put the motor back into production and very closely monitor the situation using infrared photography to document where the heating is occuring and hopefully determine its cause and corrective action.

Any other suggestions?

Again, thanks.

 

[DougMSOE]

First off,
If you want to measure the WRotor current do not use the normal clamp-on ampmeter. You need to use a Hall effect type clamp on. FW Bell or Fluke.
OR as I did, use a shunt like you would use for a dc circuit.
I used this method when I worked for a consultant in the scrap industry.
I think I know who you work for since I was in that business for a while.
If you need to 920.453.2195
Doug

 

[DougMSOE]

PS Alstom is wrong about the dc component.
Further If you want it fixed right give me a call. Above post.

 

[rbulsara]

I would look at the liquid rheostat. They are not the most relaibale of all as its resistance can change dramatically by a little change on concentration of electrolyte.

Also look at the shorting contacts of the rheostat, which could lose tension.

 

[rbulsara]

Also investigate some history, if the operators were jogging the motor on and off too often or repeatedly subjected to too heavy a load.